Structural deformation and detailed architecture of accretionary wedge in the northern Manila subduction zone

NASA Astrophysics Data System (ADS)

Gao, J.; Wu, S.; Yao, Y.; Chen, C.

2017-12-01

The South China Sea (SCS) which located at the southeast edge of the Eurasian plate, is heavily influenced by the Philippine Sea plate and the Indo-Australian plate. As eastern boundary of the SCS, Manila subduction zone was created by the northwestern movement of the Philippine Sea plate, recorded the key information on formation and evolution of the SCS and often triggered off earthquakes and tsunami in the East and South Asia. Using high resolution multi-channel seismic data across the northern Manila subduction zone, this study analyzed sedimentary characteristics of oceanic basin and trench, and fine described features of structural deformation and architecture of accretionary wedge and magmatism to discuss the time of subduction inception, thrust motion and influence of seamount subduction on the geometry of the Manila trench. Results show that lower slope of accretionary wedge mainly consist of imbricated thrusts with blind thrust as the frontal fault and structural wedge whereas upper slope was obscure for intensely structural deformation and magmatism. All the thrust faults merged into a detachment fault/surface which may root in Lower Miocene or even older strata, cut off the Miocene near buried seamount and extended the Pliocene upward, suggesting that this detachment fault was obviously influenced by buried seamount and basement high below the accretionary wedge. Magmatism began to be active from late Miocene and continued to be intense during Pliocene and Quaternary in the oceanic basin, trench and accretionary wedge. Based on characteristics of sedimentary and structural deformation, this study proposed that accretionary wedge of the northern Manila subduction zone formed before 16.5 Ma and propagated to the SCS through piggyback propagation thrusting when seafloor spreading of the SCS was still ongoing before 15 Ma. Subduction of extended continental crust in the northeastern SCS created a significantly concaving eastward to geometric shape of the northern Manila trench. With the subducting of fossil ridge of the SCS to the Manila trench and ridge/trench collision happening in the future, the convexly westward arc feature of Manila trench was changed to flat and will be even concave eastward.

Electrical resistivity structures and tectonic implications of Main Karakorum Thrust (MKT) in the western Himalayas: NNE Pakistan

NASA Astrophysics Data System (ADS)

Shah, Syed Tallataf Hussain; Zhao, Junmeng; Xiao, Qibin; Bhatti, Zahid Imran; Khan, Nangyal Ghani; Zhang, Heng; Deng, Gong; Liu, Hongbing

2018-06-01

We discovered a conductive zone along Main Karakoram Thrust which could be an indication of flat subduction of Kohistan island arc beneath the Eurasian plate. Kohistan island arc collided with the Karakoram Block of the Eurasian Plate in the Early Cretaceous. However, according to findings of many researchers, the subduction ceased about 75 Ma ago. The presence of the conductive zone is an indication of current magmatism or hydrothermal fluids. Maximum low-frequency band data from Fourteen sites with recording periods of 10-2-103 s was acquired along a profile crossing MKT. Our results reveal the existence of multiple low resistivity zones beneath the region extending from shallow to the depths of more than 100 km. These low-resistivity zones might be a signature of the ongoing magmatic activities or hydrothermal fluids along the Shyok Suture Zone. In addition, we discovered another large conductive body towards the south of the study area which could be a result of uprising magmatic plumes generated by the subducting Indian plate along the Indian suture zone and their entrapment in the overlying Kohistan block.

Early history and reactivation of the rand thrust, southern California

NASA Astrophysics Data System (ADS)

Postlethwaite, Clay E.; Jacobson, Carl E.

The Rand thrust of the Rand Mountains in the northwestern Mojave Desert separates an upper plate of quartz monzonite and quartzofeldspathic to amphibolitic gneiss from a lower plate of metagraywacke and mafic schist (Rand Schist). The Rand thrust is considered part of the regionally extensive Vincent/Chocolate Mountain thrust system, which is commonly believed to represent a Late Cretaceous subduction zone. The initial direction of dip and sense of movement along the Vincent/Chocolate Mountain thrust are controversial. Microfabrics of mylonites and quartzites from the Rand Mountains were analyzed in an attempt to determine transport direction for this region, but the results are ambiguous. In addition, the southwestern portion of the Rand thrust was found to have been reactivated as a low-angle normal fault after subduction. Reactivation might have occurred shortly after subduction, in which case it could account for the preservation of high-pressure mineral assemblages in the Rand Schist, or it could be related to mid-Tertiary extension in the western United States. In either event, the reactivation might be responsible for the complicated nature of the microfabrics. The Rand Schist exhibits an inverted metamorphic zonation. Isograds in the schist are not significantly truncated by the reactivated segment of the Rand thrust. This indicates that other segments of the Vincent/Chocolate Mountain thrust should be re-evaluated for the possibility of late movement, even if they show an apparently undisturbed inverted metamorphic zonation.

Late Cretaceous-Cenozoic subduction-collision history of the Southern Neotethys: new evidence from the Çağlayancerit area, SE Turkey

NASA Astrophysics Data System (ADS)

Akıncı, Ahmet Can; Robertson, Alastair H. F.; Ünlügenç, Ulvi Can

2016-01-01

Evidence of the subduction-collision history of the S Neotethys is well exposed in the frontal part of the SE Anatolian thrust belt and the adjacent Arabian continental margin. The foreland succession in the study area begins with Eocene shelf carbonates, ranging from shallow marine to deeper marine, without sedimentary input from the Tauride continent to the north. After a regional hiatus (Oligocene), sedimentation resumed during the Early Miocene with terrigenous gravity-flow deposition in the north (Lice Formation) and shallow-marine carbonates further south. Clastic detritus was derived from the Tauride continent and oceanic accretionary material. The base of the overriding Tauride allochthon comprises ophiolite-derived debris flows, ophiolite-related mélange and dismembered ophiolitic rocks. Above this, the regional-scale Bulgurkaya sedimentary mélange (an olistostrome) includes blocks and dismembered thrust sheets of metamorphic rocks, limestone and sandstone, which include Late Cretaceous and Eocene foraminifera. The matrix is mainly strongly deformed Eocene-Oligocene mudrocks, hemipelagic marl and sandstone turbidites. The thrust stack is topped by a regionally extensive thrust sheet (Malatya metamorphic unit), which includes greenschist facies marble, calcschist, schist and phyllite, representing Tauride continental crust. Beginning during the Late Mesozoic, the S Neotethys subducted northwards beneath a backstop represented by the Tauride microcontinent (Malatya metamorphic unit). Ophiolites formed within the S Neotethys and accreted to the Tauride active margin. Large-scale sedimentary mélange developed along the Tauride active margin during Eocene-Oligocene. On the Arabian margin, a sedimentary hiatus and tilting (Oligocene) is interpreted to record initial continental collision. The Early Miocene terrigenous gravity flows represent a collision-related flexural foreland basin. Southward overthrusting of the Tauride allochthon took place during Early-Middle Miocene. Associated regional uplift triggered large-scale alluvial deposition. The foreland folded and faulted in response to suture zone tightening (Late Miocene). Left-lateral strike slip characterised the Plio-Pleistocene.

Chance findings about early holocene tidal marshes of Grays Harbor, Washington, in relation to rapidly rising seas and great subduction earthquakes

USGS Publications Warehouse

Phipps, James B.; Hemphill-Haley, Eileen; Atwater, Brian F.

2015-06-18

The puzzles posed by these findings include: (1) How did the marshes manage to endure centuries of relative sea-level rise that likely approached 1 cm/yr on average? (2) Did the marshes also endure subsidence that accompanied great thrust earthquakes on the Cascadia Subduction Zone? (3) Was their eventual drowning triggered by a Cascadia earthquake of unusually large size, or can the drowning be explained by sea-level rise that included a jump from drainage of glacial Lake Agassiz?

A Thick, Deformed Sedimentary Wedge in an Erosional Subduction Zone, Southern Costa Rica

NASA Astrophysics Data System (ADS)

Silver, E. A.; Kluesner, J. W.; Edwards, J. H.; Vannucchi, P.

2014-12-01

A paradigm of erosional subduction zones is that the lower part of the wedge is composed of strong, crystalline basement (Clift and Vannucchi, Rev. Geophys., 42, RG2001, 2004). The CRISP 3D seismic reflection study of the southern part of the Costa Rica subduction zone shows quite the opposite. Here the slope is underlain by a series of fault-cored anticlines, with faults dipping both landward and seaward that root into the plate boundary. Deformation intensity increases with depth, and young, near-surface deformation follows that of the deeper structures but with basin inversions indicating a dynamic evolution (Edwards et al., this meeting). Fold wavelength increases landward, consistent with the folding of a landward-thickening wedge. Offscraping in accretion is minimal because incoming sediments on the lower plate are very thin. Within the wedge, thrust faulting dominates at depth in the wedge, whereas normal faulting dominates close to the surface, possibly reflecting uplift of the deforming anticlines. Normal faults form a mesh of NNW and ENE-trending structures, whereas thrust faults are oriented approximately parallel to the dominant fold orientation, which in turn follows the direction of roughness on the subducting plate. Rapid subduction erosion just prior to 2 Ma is inferred from IODP Expedition 334 (Vannucchi et al., 2013, Geology, 49:995-998). Crystalline basement may have been largely removed from the slope region during this rapid erosional event, and the modern wedge may consist of rapidly redeposited material (Expedition 344 Scientists, 2013) that has been undergoing deformation since its inception, producing a structure quite different from that expected of an eroding subduction zone.

Yakataga fold-and-thrust belt: Structural geometry and tectonic implications of a small continental collision zone

NASA Astrophysics Data System (ADS)

Wallace, Wesley K.

Collision of the Yakutat terrane with southern Alaska created a collisional fold-and-thrust belt along the Pacific-North America plate boundary. This southerner fold-and-thrust belt formed within continental sedimentary rocks but with the seaward vergence and tectonic position typical of an accretionary wedge. Northward exposure of progressively older rocks reflects that the fold-and-thrust belt forms a southward-tapered orogenic wedge that increases northward in structural relief and depth of erosion. Narrow, sharp anticlines separate wider, flat-bottomed synclines. Relatively steep thrust faults commonly cut the forelimbs of anticlines. Fold shortening and fault displacement both generally increase northward, whereas fault dip generally decreases northward. The coal-bearing lower part of the sedimentary section serves as a detachment for both folds and thrust faults. The folded and faulted sedimentary section defines a regional south dip of about 8°. The structural relief combined with the low magnitude of shortening of the sedimentary section suggest that the underlying basement is structurally thickened. I propose a new interpretation in which this thickening was accommodated by a passive-roof duplex with basement horses that are separated from the overlying folded and thrust-faulted sedimentary cover by a roof thrust with a backthrust sense of motion. Basement horses are ˜7 km thick, based on the thickness between the inferred roof thrust and the top of the basement in offshore seismic reflection data. This thickness is consistent with the depth of the zone of seismicity onshore. The inferred zone of detachment and imbrication of basement corresponds with the area of surface exposure of the fold-and-thrust belt within the Yakutat terrane and with the Wrangell subduction zone and arc farther landward. By contrast, to the west, the crust of the Yakutat terrane has been carried down a subduction zone that extends far landward with a gentle dip, corresponding with a gap in arc magmatism, anomalous topography, and the rupture zone of the 1964 great southern Alaska earthquake. I suggest that, to the east, detachment and imbrication of basement combined with coupling in the fold-and-thrust belt allowed the delaminated dense mantle lithosphere to subduct with a steeper dip than to the west, where buoyant Yakutat terrane crust remains attached to the subducted lithosphere. According to this interpretation, the Wrangell subduction zone is lithosphere of the Yakutat terrane, not Pacific Ocean lithosphere subducted beneath the Yakutat terrane. The Pacific-North America plate boundary would be within the northern deformed part of the Yakutat terrane, not along the boundary between the undeformed southern part of the Yakutat terrane and oceanic crust of the Pacific Ocean. The plate boundary is an evolving zone of distributed deformation in which most of the convergent component has been accommodated within the fold-and-thrust belt south of the northern boundary of the Yakutat terrane, the Chugach-St. Elias thrust fault, and most of the right-lateral component likely has been accommodated on the Bagley Icefield fault just to the north.

Effective strength of incoming sediments and its implications for plate boundary propagation: Nankai and Costa Rica as type examples of accreting vs. erosive convergent margins

NASA Astrophysics Data System (ADS)

Kopf, Achim

2013-11-01

The location of the seaward tip of a subduction thrust controls material transfer at convergent plate margins, and hence global mass balances. At approximately half of those margins, the material of the subducting plate is completely underthrust so that no accretion or even subduction erosion takes place. Along the remaining margins, material is scraped off the subducting plate and added to the upper plate by frontal accretion. We here examine the physical properties of subducting sediments off Costa Rica and Nankai, type examples for an erosional and an accretionary margin, to investigate which parameters control the level where the frontal thrust cuts into the incoming sediment pile. A series of rotary-shear experiments to measure the frictional strength of the various lithologies entering the two subduction zones were carried out. Results include the following findings: (1) At Costa Rica, clay-rich strata at the top of the incoming succession have the lowest strength (μres = 0.19) while underlying calcareous ooze, chalk and diatomite are strong (up to μres = 0.43; μpeak = 0.56). Hence the entire sediment package is underthrust. (2) Off Japan, clay-rich deposits within the lower Shikoku Basin inventory are weakest (μres = 0.13-0.19) and favour the frontal proto-thrust to migrate into one particular horizon between sandy, competent turbidites below and ash-bearing mud above. (3) Taking in situ data and earlier geotechnical testing into account, it is suggested that mineralogical composition rather than pore-pressure defines the position of the frontal thrust, which locates in the weakest, clay mineral-rich (up to 85 wt.%) materials. (4) Smectite, the dominant clay mineral phase at either margin, shows rate strengthening and stable sliding in the frontal 50 km of the subduction thrust (0.0001-0.1 mm/s, 0.5-25 MPa effective normal stress). (5) Progressive illitization of smectite cannot explain seismogenesis, because illite-rich samples also show velocity strengthening at the conditions tested.

The 2009 Samoa-Tonga great earthquake triggered doublet

USGS Publications Warehouse

Lay, T.; Ammon, C.J.; Kanamori, H.; Rivera, L.; Koper, K.D.; Hutko, Alexander R.

2010-01-01

Great earthquakes (having seismic magnitudes of at least 8) usually involve abrupt sliding of rock masses at a boundary between tectonic plates. Such interplate ruptures produce dynamic and static stress changes that can activate nearby intraplate aftershocks, as is commonly observed in the trench-slope region seaward of a great subduction zone thrust event1-4. The earthquake sequence addressed here involves a rare instance in which a great trench-slope intraplate earthquake triggered extensive interplate faulting, reversing the typical pattern and broadly expanding the seismic and tsunami hazard. On 29 September 2009, within two minutes of the initiation of a normal faulting event with moment magnitude 8.1 in the outer trench-slope at the northern end of the Tonga subduction zone, two major interplate underthrusting subevents (both with moment magnitude 7.8), with total moment equal to a second great earthquake of moment magnitude 8.0, ruptured the nearby subduction zone megathrust. The collective faulting produced tsunami waves with localized regions of about 12metres run-up that claimed 192 lives in Samoa, American Samoa and Tonga. Overlap of the seismic signals obscured the fact that distinct faults separated by more than 50km had ruptured with different geometries, with the triggered thrust faulting only being revealed by detailed seismic wave analyses. Extensive interplate and intraplate aftershock activity was activated over a large region of the northern Tonga subduction zone. ?? 2010 Macmillan Publishers Limited. All rights reserved.

The 2009 Samoa-Tonga great earthquake triggered doublet.

PubMed

Lay, Thorne; Ammon, Charles J; Kanamori, Hiroo; Rivera, Luis; Koper, Keith D; Hutko, Alexander R

2010-08-19

Great earthquakes (having seismic magnitudes of at least 8) usually involve abrupt sliding of rock masses at a boundary between tectonic plates. Such interplate ruptures produce dynamic and static stress changes that can activate nearby intraplate aftershocks, as is commonly observed in the trench-slope region seaward of a great subduction zone thrust event. The earthquake sequence addressed here involves a rare instance in which a great trench-slope intraplate earthquake triggered extensive interplate faulting, reversing the typical pattern and broadly expanding the seismic and tsunami hazard. On 29 September 2009, within two minutes of the initiation of a normal faulting event with moment magnitude 8.1 in the outer trench-slope at the northern end of the Tonga subduction zone, two major interplate underthrusting subevents (both with moment magnitude 7.8), with total moment equal to a second great earthquake of moment magnitude 8.0, ruptured the nearby subduction zone megathrust. The collective faulting produced tsunami waves with localized regions of about 12 metres run-up that claimed 192 lives in Samoa, American Samoa and Tonga. Overlap of the seismic signals obscured the fact that distinct faults separated by more than 50 km had ruptured with different geometries, with the triggered thrust faulting only being revealed by detailed seismic wave analyses. Extensive interplate and intraplate aftershock activity was activated over a large region of the northern Tonga subduction zone.

Structure of crust and upper mantle beneath NW Himalayas, Pamir and Hindukush by multi-scale double-difference seismic tomography

NASA Astrophysics Data System (ADS)

Bhatti, Zahid Imran; Zhao, Junmeng; Khan, Nangyal Ghani; Shah, Syed Tallataf Hussain

2018-08-01

The India-Asia collision and subsequent subduction initiated the evolution of major tectonic features in the Western Syntaxis. The complex tectonic structure and shallow to deep seismicity have attracted geoscientists over the past two decades. The present research is based on a 3D tomographic inversion of P-wave arrival time data to constrain the crustal and upper mantle structure beneath the NW Himalayas and Pamir-Hindukush region using the Double-difference tomography. We utilized a very large multi-scale dataset comprising 19,080 earthquakes recorded at 397 local and regional seismic stations from 1950 to 2017. The northward dipping seismic zone coinciding with the low velocity anomaly suggests the subduction of the Indian lower crust beneath the Hindukush. The extent of the northward advancing Indian slab increases from east to west in this region. We observed no signs of northward subduction of the Indian plate under the Hindukush beyond 71°E longitude. The Indian plate overturns due south after interacting with the Asian plate beneath the southern Pamir, which correlates with the counter-clockwise rotation of the Indian plate. The Asian plate is also imaged as a southward subducting seismic zone beneath the southern Pamir. In the NW Himalayas, the northward subducting Indian plate appears as a gently dipping low velocity anomaly beneath the Karakoram Block. The stresses caused by the collision and subduction along the Shyok Suture and Indus Suture are translated to the south. The crustal scale seismicity and high velocity anomalies indicate an intense deformation in the crust, which is manifested by syntaxial bends and thrust faults to the south of the Main Mantle Thrust.

Subducting seamounts control interplate coupling and seismic rupture in the 2014 Iquique earthquake area

PubMed Central

Geersen, Jacob; Ranero, César R.; Barckhausen, Udo; Reichert, Christian

2015-01-01

To date, the parameters that determine the rupture area of great subduction zone earthquakes remain contentious. On 1 April 2014, the Mw 8.1 Iquique earthquake ruptured a portion of the well-recognized northern Chile seismic gap but left large highly coupled areas un-ruptured. Marine seismic reflection and swath bathymetric data indicate that structural variations in the subducting Nazca Plate control regional-scale plate-coupling variations, and the limited extent of the 2014 earthquake. Several under-thrusting seamounts correlate to the southward and up-dip arrest of seismic rupture during the 2014 Iquique earthquake, thus supporting a causal link. By fracturing of the overriding plate, the subducting seamounts are likely further responsible for reduced plate-coupling in the shallow subduction zone and in a lowly coupled region around 20.5°S. Our data support that structural variations in the lower plate influence coupling and seismic rupture offshore Northern Chile, whereas the structure of the upper plate plays a minor role. PMID:26419949

A reevaluation of the age of the Vincent-Chocolate Mountains thrust system, southern California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jacobsen, C.E.; Barth, A.P.

1993-04-01

The Vincent-Chocolate Mountains (VCM) thrust superposes Mesozoic arc plutons and associated Precambrian country rock above subduction-related Pelona-Orocopia schist. The thrust is disrupted in many areas by postmetamorphic deformation, but appears to be intact in the San Gabriel Mountains. Two Rb-Sr mineral-isochron ages from Pelona Schist and mylonite in the San Gabriel Mountains led Ehlig (1981) to conclude that the original thrusting event occurred at c. 60 Ma. However, biotite K-Ar ages determined by Miller and Morton (1980) for upper plate in the same area caused Dillon (1986) to reach a different conclusion. The biotite ages range mainly from 74--60 Mamore » and increase structurally upward from the VCM thrust. Dillon (1986) inferred that the age gradient was due to uplift and cooling of the upper plate during underthrusting of Pelona Schist. This would indicate that the VCM thrust was at least 74 Ma in age. An alternative to the interpretation of Dillon (1986) is that the biotite age gradient largely predates the VCM thrust. Upward heat flow, leading to older ages at higher structural levels, could have resulted from either static cooling of Cretaceous plutons or uplift and erosion induced by crustal thickening during possible west-directed intra-arc thrusting at c. 88--78 Ma (May and Walker, 1989). Subsequent underthrusting of Pelona Schist would establish a cold lower boundary to the crust and cause the closure of isotopic systems in the base of the upper plate. A 60 Ma time of thrusting is also suggested by two amphibole [sup 40]Ar/[sup 39]Ar ages from the Pelona Schist of the San Gabriel Mountains. Peak metamorphic temperature in this area was below 480 C and amphibole ages should thus indicate time of crystallization rather than subsequent cooling. Four phengite [sup 40]Ar/[sup 39]Ar ages of 55--61 Ma from Pelona Schist and mylonite indicate rapid cooling from peak metamorphic temperatures, consistent with subduction refrigeration.« less

Formation of an active thrust triangle zone associated with structural inversion in a subduction setting, eastern New Zealand

NASA Astrophysics Data System (ADS)

Barnes, Philip M.; Nicol, Andrew

2004-02-01

We analyze a thrust triangle zone, which underlies the continental shelf of Hawke Bay, eastern New Zealand, within the Hikurangi subduction margin. This triangle zone differs from many other examples in that it is active, 90 km from the leading edge of the overriding plate, and formed due to polyphase deformation involving opposed dipping thrust duplex and backthrust, with the later structure forming in response to inversion of an extensional graben. The component structures of the zone mainly developed sequentially rather than synchronously. High-quality marine seismic reflection lines, tied to well and seabed samples, reveal the three-dimensional structure of the zone, together with its 25 Myr evolution and late Quaternary activity. The triangle zone occurs in the lateral overlap between a stack of NW dipping blind thrusts, and a principal backthrust, the Kidnappers fault. The NW dipping thrusts initiated in the early-middle Miocene during the early stages of subduction, with subsequent thrust duplex formation producing major uplift and erosion in the late Miocene-early Pliocene. The active backthrust formed during the late Miocene to early Pliocene as a thin-skinned listric extensional fault confined to the cover sequence. Structural inversion of the extensional fault commenced in the early-middle Pliocene, produced the backthrust and marks the formation of the thrust triangle zone. The thrust duplex and backthrust accrued strain following inversion; however, the later structure accommodated most of the surface deformation in the Quaternary. Section balancing of the triangle zone together with a detailed analysis of reverse displacements along the backthrust reveal spatial and temporal variations of strain accumulation on the two principal components of the zone. Although the formation of the triangle zone is strongly influenced by regional tectonics of the subduction system, these variations may also, in part, reflect local fault interaction. For example, high Quaternary displacement rates on the backthrust accounts for ˜70% of the displacement loss that occurs on the southern segments of the overlapping, Lachlan fault. Understanding the tectonic evolution of such complex, polyphase thrust triangle zones requires the preservation of growth strata that record sequential deformation history. In the absence of such data, synchroneity of opposed dipping thrusts in triangle zones cannot be assumed.

Disparate Tectonic Settings of Devastating Earthquakes in Mexico, September 2017

NASA Astrophysics Data System (ADS)

Li, J.; Chen, W. P.; Ning, J.

2017-12-01

Large earthquakes associated with thrust faulting along the plate interface typically pose the highest seismic risk along subduction zones. However, both damaging earthquakes in Mexico of September 2017 are notable exceptions. The Tehuantepec event on the 8th (Mw 8.1) occurred just landward of the trench but is associated with normal faulting, akin to the large (Ms 8) historical event of 1931 that occurred about 200 km to the northwest along this subduction zone. The Puebla earthquake (on the 19th, Mw 7.1) occurred almost 300 km away from the trench where seismic imaging had indicated that the flat-lying slab steepens abruptly and plunges aseismically into the deep mantle. Here we show that both types of tectonic settings are in fact common along a large portion of the Mexican subduction zone, thus identifying source zones of potentially damaging earthquakes away from the plate interface. Additionally, modeling of broadband waveforms made clear that another significant event (Mw 6.1) on the 23rd, is associated with shallow normal faulting in the upper crust, not directly related to the two damaging earthquakes.

Andean subduction orogeny: feedbacks between tectonics, relief evolution and global climate

NASA Astrophysics Data System (ADS)

Lacassin, Robin; Armijo, Rolando; Coudurier-Curveur, Aurélie; Carrizo, Daniel

2016-04-01

The Andean subduction margin, largest tectonic relief on the Earth (13 km vertically from the trench to the Altiplano) has a stepped morphology, which results of the evolution over the past 50 Myr of two parallel flat-ramp thrust systems, at the - previously unidentified - West Andean Thrust (WAT), and at the subduction interface. The evolution of those thrusts appears concomitant with increasing aridity in the Atacama Desert, which keeps a large-scale record of interplaying tectonics and Cenozoic climate change. The coastal morphology is dominated by the Atacama Bench, a giant uplifted terrace at 1-2km asl. Geomorphic and climatic data, numerical experiments of drainage formation are consistent with the development of a flat Atacama morphology close to sea level, interrupted at ≤10 Ma by tectonic uplift prevailing to the present. This suggests recent trench-ward relief growth by incorporation of the coastal Atacama Bench to the Andes reliefs. Thrust splay structures and other complexities above the subduction interface may explain this relief growth, as well as the distribution of asperities under the oceanward forearc, and the down-dip segmentation of coupling and seismicity on the megathrust. Combining those results with geological knowledge at the scale of the whole Central Andes, we show that the Andean orogeny results from protracted processes of bivergent crustal shortening in a wide region squeezed between the rigid Marginal Block and the S America Plate. The overall growth curve of Andean orogeny over the past 50 Myr appears synchronous with the onset of the "ramp-shaped" temperature decrease since the Early Eocene climatic optimum. Andean growth and global cooling may have operated under the same forcing mechanism at plate-scale, involving viscous flow in the mantle. But Andean growth appears modulated by climatic feedbacks causative of stepwise reductions of erosive power over the Andean margin. The first of such events is coeval with Late Eocene cooling and promoted the eastward propagation of deformation towards the continent interior. The second one, coeval with Late Miocene cooling, is associated with the establishment of hyper-aridity in the Atacama Desert, and is responsible of a tectonic "freezing" which promoted since the triggering of subduction of the Brazilian craton, the Andean bivergent growth, and rapid uplift throughout the Andes-Altiplano. Armijo R., Lacassin R., Coudurier-Curveur A., Carrizo D., Coupled tectonic evolution of Andean orogeny and global climate, Earth Science Reviews, 143, 1-35, doi:10.1016/j.earscirev.2015.01.005, 2015.

Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009.

PubMed

Beavan, J; Wang, X; Holden, C; Wilson, K; Power, W; Prasetya, G; Bevis, M; Kautoke, R

2010-08-19

The Earth's largest earthquakes and tsunamis are usually caused by thrust-faulting earthquakes on the shallow part of the subduction interface between two tectonic plates, where stored elastic energy due to convergence between the plates is rapidly released. The tsunami that devastated the Samoan and northern Tongan islands on 29 September 2009 was preceded by a globally recorded magnitude-8 normal-faulting earthquake in the outer-rise region, where the Pacific plate bends before entering the subduction zone. Preliminary interpretation suggested that this earthquake was the source of the tsunami. Here we show that the outer-rise earthquake was accompanied by a nearly simultaneous rupture of the shallow subduction interface, equivalent to a magnitude-8 earthquake, that also contributed significantly to the tsunami. The subduction interface event was probably a slow earthquake with a rise time of several minutes that triggered the outer-rise event several minutes later. However, we cannot rule out the possibility that the normal fault ruptured first and dynamically triggered the subduction interface event. Our evidence comes from displacements of Global Positioning System stations and modelling of tsunami waves recorded by ocean-bottom pressure sensors, with support from seismic data and tsunami field observations. Evidence of the subduction earthquake in global seismic data is largely hidden because of the earthquake's slow rise time or because its ground motion is disguised by that of the normal-faulting event. Earthquake doublets where subduction interface events trigger large outer-rise earthquakes have been recorded previously, but this is the first well-documented example where the two events occur so closely in time and the triggering event might be a slow earthquake. As well as providing information on strain release mechanisms at subduction zones, earthquakes such as this provide a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone, where slip between the plates is predominantly aseismic.

The Gibraltar subduction: A decade of new geophysical data

NASA Astrophysics Data System (ADS)

Gutscher, M.-A.; Dominguez, S.; Westbrook, G. K.; Le Roy, P.; Rosas, F.; Duarte, J. C.; Terrinha, P.; Miranda, J. M.; Graindorge, D.; Gailler, A.; Sallares, V.; Bartolome, R.

2012-10-01

The Gibraltar arc, spans a complex portion of the Africa-Eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity. The seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold-and-thrust belt, have led to competing geodynamic models (delamination and subduction). Geophysical data acquired in the past decade provide a test for these models and support a narrow east-dipping, subduction zone. Seismic refraction studies indicate oceanic crust below the western Gulf of Cadiz. Tomography of the upper mantle reveals a steep, east-dipping high P-wave velocity body, beneath Gibraltar. The anisotropic mantle fabric from SKS splitting shows arc-parallel "fast directions", consistent with toroidal flow around a narrow, westward retreating subducting slab. The accompanying WSW advance of the Rif-Betic mountain belt has constructed a thick pile of deformed sediments, an accretionary wedge, characterized by west-vergent thrust anticlines. Bathymetric swath-mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided. Subduction has slowed significantly since 5 Ma, but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge. Three possible origins for this deformation are discussed; gravitational spreading, overall NW-SE convergence between Africa and Iberia and finally a WSW tectonic push from slow, but ongoing roll-back subduction. In the absence of arc volcanism and shallow dipping thrust type earthquakes, evidence in favor of present-day subduction can only be indirect and remains the object of debate. Continued activity of the subduction offers a possible explanation for great (M > 8.5) earthquakes known to affect the area, like the famous 1755 Great Lisbon earthquake. Recent GPS studies show SW motion of stations in N Morocco at velocities of 3-6 mm/yr indicating the presence of an independent block, a "Rif-Betic-Alboran" microplate, situated between Iberia and Africa.

Progressive deformation of the Chugach accretionary complex, Alaska, during a paleogene ridge-trench encounter

USGS Publications Warehouse

Kusky, Timothy M.

1997-01-01

The Mesozoic accretionary wedge of south-central Alaska is cut by an array of faults including dextral and sinistral strike-slip faults, synthetic and antithetic thrust faults, and synthetic and antithetic normal faults. The three fault sets are characterized by quartz ± calcite ± chlorite ± prehnite slickensides, and are all relatively late, i.e. all truncate ductile fabrics of the host rocks. Cross-cutting relationships suggest that the thrust fault sets predate the late normal and strike-slip fault sets. Together, the normal and strike-slip fault system exhibits orthorhombic symmetry. Thrust faulting shortened the wedge subhorizontally perpendicular to strike, and then normal and strike-slip faulting extended the wedge oblique to orogenic strike. Strongly curved slickenlines on some faults of each set reveal that displacement directions changed over time. On dip-slip faults (thrust and normal), slickenlines tend to become steeper with younger increments of slip, whereas on strike-slip faults, slickenlines become shallower with younger strain increments. These patterns may result from progressive exhumation of the accretionary wedge while the faults were active, with the curvature of the slickenlines tracking the change from a non-Andersonian stress field at depth to a more Andersonian system (σ1 or σ2 nearly vertical) at shallower crustal levels.We interpret this complex fault array as a progressive deformation that is one response to Paleocene-Eocene subduction of the Kula-Farallon spreading center beneath the accretionary complex because: (1) on the Kenai Peninsula, ENE-striking dextral faults of this array exhibit mutually cross-cutting relationships with Paleocene-Eocene dikes related to ridge subduction; and (2) mineralized strike-slip and normal faults of the orthorhombic system have yielded 40Ar/39Ar ages identical to near-trench intrusives related to ridge subduction. Both features are diachronous along-strike, having formed at circa 65 Ma in the west and 50 Ma in the east. Exhumation of deeper levels of the southern Alaska accretionary wedge and formation of this late fault array is interpreted as a critical taper adjustment to subduction of progressively younger oceanic lithosphere yielding a shallower basal de´collement dip as the Kula-Farallon ridge approached the accretionary prism. The late structures also record different kinematic regimes associated with subduction of different oceanic plates, before and after ridge subduction. Prior to triple junction passage, subduction of the Farallon plate occurred at nearly right angles to the trench axis, whereas after triple junction migration, subduction of the Kula plate involved a significant component of dextral transpression and northward translation of the Chugach terrane. The changes in kinematics are apparent in the sequence of late structures from: (1) thrusting; (2) near-trench plutonism associated with normal + strike-slip faulting; (3) very late gouge-filled dextral faults.

Seismicity near a Highly-Coupled Patch in the Central Ecuador Subduction Zone

NASA Astrophysics Data System (ADS)

Regnier, M. M.; Segovia, M.; Font, Y.; Charvis, P.; Galve, A.; Jarrin, P.; Hello, Y.; Ruiz, M. C.; Pazmino, A.

2017-12-01

The temporary onshore-offshore seismic network deployed during the 2-years period of the OSISEC project provides an unprecedented, detailed and well-focused image of the seismicity for magnitudes as low as 2.0 in the Central Ecuadorian subduction zone. Facing the southern border of the Carnegie Ridge, a shallow and discrete highly-coupled patch is correlated to the subduction of a large oceanic relief. No large earthquake is known in this area that is experiencing recurrent seismic swarms and slow slip events. The shallow and locked subduction interface shows no evidence of background seismicity that instead occurred down dip of the coupled patch where it is possibly controlled by structural features of the overriding plate. We show a clear spatial correlation between the background microseismicity, the down dip extension of the locked patch at 20 km depth and the geology of the upper plate. The dip angle of the interplate contact zone, defined by a smooth interpolation through the hypocenters of thrust events, is consistent with a progressive increase from 6° to 25° from the trench to 20 km depth. Offshore, a seismic swarm, concomitant with a slow slip event rupturing the locked area, highlights the reactivation of secondary active faults that developed within the thickened crust of the subducting Carnegie Ridge, at the leading edge of a large oceanic seamount. No seismicity was detected near the plate interface suggesting that stress still accumulates at small and isolated asperities

Structural characteristics around the frontal thrust along the Nankai Trough revealed by bathymetric and seismic reflection survey

NASA Astrophysics Data System (ADS)

Yamashita, M.; Nakanishi, A.; Moore, G. F.; Kodaira, S.; Nakamura, Y.; Miura, S.; Kaneda, Y.

2016-12-01

Great earthquakes with tsunamis with recurrence intervals of 100-200 years have occurred along the Nankai Trough near central Japan where the Shikoku Basin is subducting with thick sediments on the Philippine Sea plate. To predict the exact height of the tsunami on the coast region generated by these large ruptures, it is important to estimate the vertical deformation that occurs on the seaward end of the rupture area. Recent drilling results have also yielded evidence not only of splay faults that generate tsunamigenic rupture, but also new evidence of tsunamigenic rupture along the frontal thrust at the trench axis in the Nankai Trough. In order to understand the deformation around the frontal thrust at the trench axis, we conducted a dense high-resolution seismic reflection survey with 10-20 km spacing over 1500 km of line length during 2013 and 2014. Clear seismic reflection images of frontal thrusts in the accretionary prism and subducting Shikoku Basin, image deformation along the trench axis between off Muroto Cape and off Ashizuri Cape. The cumulative displacement along the frontal thrust and second thrust are measured from picked distinct reflectors in depth-converted profiles. The average value of cumulative displacement of the frontal thrust is more than 100 m within 2 km depth beneath the seafloor. The location of highest displacement of 300 m displacement agree with the seaward end of slip distribution of the 1946 Nankai event calculated by numerical simulations. We also evaluate the seaward structure for understanding the future rupture distribution. The protothrust zone (PTZ) consisting of many incipient thrusts is identifiable in the portion of trough-fill sediments seaward of the frontal thrust. In order to emphasize the characteristics of frontal thrust and PTZ, we construct the detailed relief image for focusing on the lineated slope of the PTZ at the trough axis. Although our surveys covered a part of Nankai seismogenic zone, it is important to evaluate the initiation and evolution of frontal thrust at the toe of Nankai trough axis. This study is part of "Research project for compound disaster mitigation on the great earthquakes and tsunamis around the Nankai Trough region" funded by the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

Dynamics of subduction, accretion, exhumation and slab roll-back: Mediterranean scenarios

NASA Astrophysics Data System (ADS)

Tirel, C.; Brun, J.; Burov, E. B.; Wortel, M. J.; Lebedev, S.

2010-12-01

A dynamic orogen reveals various tectonic processes brought about by subduction: accretion of oceanic and continental crust, exhumation of UHP-HP rocks, and often, back-arc extension. In the Mediterranean, orogeny is strongly affected by slab retreat, as in the Aegean and Tyrrhenian Seas. In order to examine the different dynamic processes in a self-consistent manner, we perform a parametric study using the fully coupled thermo-mechanical numerical code PARAFLAM. The experiments reproduce a subduction zone in a slab pull mode, with accretion of one (the Tyrrhenian case) and two continental blocks (the Aegean case) that undergo, in sequence, thrusting, burial and exhumation. The modeling shows that despite differences in structure between the two cases, the deformation mechanisms are fundamentally similar and can be described as follows. The accretion of a continental block at the trench beneath the suture zone begins with its burial to UHP-HP conditions and thrusting. Then the continental block is delaminated from its subducting lithosphere. During the subduction-accretion process, the angle of the subducting slab increases due to the buoyancy of the continental block. When the oceanic subduction resumes, the angle of the slab decreases to reach a steady-state position. The Aegean and Tyrrhenian scenarios diverge at this stage, due naturally to the differences of their accretion history. When continental accretion is followed by oceanic subduction only, the continental block that has been accreted and detached stays at close to the trench and does not undergo further deformation, despite the continuing rollback. The extensional deformation is located further within the overriding plate, resulting in continental breakup and the development of an oceanic basin, as in the Tyrrhenian domain. When the continental accretion is followed first by oceanic subduction and then by accretion of another continental block, however, the evolution of the subduction zone is different. The angle of the subducting slab increases again, following the arrival of the second continental block. The first continental block is now disconnected from the trench and is strongly heated by the asthenosphere that rises to just below the Moho. The locus of extension, originally in the overriding plate, moves to the first continental block, resulting in the development of metamorphic core complexes, as in the Aegean domain. Simultaneously, the second continent undergoes burial to UHP-HP conditions, thrusting and exhumation.

Subduction-Related Structure in the Mw 9.2, 1964 Megathrust Rupture Area Offshore Kodiak Island, Alaska

NASA Astrophysics Data System (ADS)

Krabbenhoeft, A.; von Huene, R.; Klaeschen, D.; Miller, J. J.

2016-12-01

Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occurred in Alaskan subduction zones. To better understand rupture processes and their mechanisms, we relate seafloor morphology from multibeam and regional bathymetric compilations with sub-seafloor images and seismic P-wave velocity structures. We re-processed legacy multichannel seismic (MCS) data including shot- and intra-shotgather interpolation, multiple removal and Kirchhoff depth migration. These images even reveal the shallow structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage (1994) wide angle dataset reveals the P-wave velocity distribution as well as the deep structure of the subducting plate to the ocean crust Moho. The subducting oceanic crust morphology is rough and partly hidden by a thick sediment cover that reaches 3 km depth at the trench axis. Bathymetry shows two major contrasting upper plate morphologies: the shallow dipping lower slope consists of trench-parallel ridges that form the accreted prism whereas the steep rough middle and upper slopes are composed of competent older rock.Thrust faults are distributed across the entire slope, some of which connect with the subducted plate interface. A subtle change in seafloor gradient from the lower to the middle slope coincides with a thrust fault zone marking the boundary between the margin framework and the frontal prism. It corresponds to the most prominent lateral increase in seismic P-wave velocities, 25 km landward of the trench axis.Major thrusts in several MCS-lines are correlated with bathymetric data, showing their > 100 km lateral extent, which might also be tsunamigenic paths of earthquake rupture from the seismogenic zone to the seafloor.

Testing Earthquake Links in Mexico From 1978 to the 2017 M = 8.1 Chiapas and M = 7.1 Puebla Shocks

NASA Astrophysics Data System (ADS)

Segou, Margarita; Parsons, Tom

2018-01-01

The M = 8.1 Chiapas and the M = 7.1 Puebla earthquakes occurred in the bending part of the subducting Cocos plate 11 days and 600 km apart, a range that puts them well outside the typical aftershock zone. We find this to be a relatively common occurrence in Mexico, with 14% of M > 7.0 earthquakes since 1900 striking more than 300 km apart and within a 2 week interval, not different from a randomized catalog. We calculate the triggering potential caused by crustal stress redistribution from large subduction earthquakes over the last 40 years. There is no evidence that static stress transfer or dynamic triggering from the 8 September Chiapas earthquake promoted the 19 September earthquake. Both recent earthquakes were promoted by past thrust events instead, including delayed afterslip from the 2012 M = 7.5 Oaxaca earthquake. A repeated pattern of shallow thrust events promoting deep intraslab earthquakes is observed over the past 40 years.

Testing earthquake links in Mexico from 1978 up to the 2017 M=8.1 Chiapas and M=7.1 Puebla shocks

USGS Publications Warehouse

Segou, Margarita; Parsons, Thomas E.

2018-01-01

The M = 8.1 Chiapas and the M = 7.1 Puebla earthquakes occurred in the bending part of the subducting Cocos plate 11 days and ~600 km apart, a range that puts them well outside the typical aftershock zone. We find this to be a relatively common occurrence in Mexico, with 14% of M > 7.0 earthquakes since 1900 striking more than 300 km apart and within a 2 week interval, not different from a randomized catalog. We calculate the triggering potential caused by crustal stress redistribution from large subduction earthquakes over the last 40 years. There is no evidence that static stress transfer or dynamic triggering from the 8 September Chiapas earthquake promoted the 19 September earthquake. Both recent earthquakes were promoted by past thrust events instead, including delayed afterslip from the 2012 M = 7.5 Oaxaca earthquake. A repeated pattern of shallow thrust events promoting deep intraslab earthquakes is observed over the past 40 years.

Subduction and exhumation of a continental margin in the Scandinavian Caledonides: Insights from ultrahigh pressure metamorphism, late orogenic basins and 3D numerical modelling

NASA Astrophysics Data System (ADS)

Cuthbert, Simon

2017-04-01

The Scandinavian Caledonides (SC) represents a plate collision zone of Himalayan style and scale. Three fundamental characteristics of this orogen are: (1) early foreland-directed, tectonic transport and stacking of nappes; (2) late, wholesale reversal of tectonic transport; (3) ultrahigh pressure metamorphism of felsic crust derived from the underthrusting plate at several levels in the orogenic wedge and below the main thrust surface, indicating subduction of continental crust into the mantle. The significance of this for crustal evolution is the profound remodeling of continental crust, direct geochemical interaction of such crust and the mantle and the opening of accommodation space trapping large volumes of clastic detritus within the orogen. The orogenic wedge of the SC was derived from the upper crust of the Baltica continental margin (a hyper-extended passive margin), plus terranes derived from an assemblage of outboard arcs and intra-oceanic basins and, at the highest structural level, elements of the Laurentian margin. Nappe emplacement was driven by Scandian ( 430Ma) collision of Baltica with Laurentia, but emerging Middle Ordovician ages for diamond-facies metamorphism for the most outboard (or rifted) elements of Baltica suggest prior collision with an arc or microcontinent. Nappes derived from Baltica continental crust were subducted, in some cases to depths sufficient to form diamond. These then detached from the upper part of the down-going plate along major thrust faults, at which time they ceased to descend and possibly rose along the subduction channel. Subduction of the remaining continental margin continued below these nappes, possibly driven by slab-pull of the previously subducted Iapetus oceanic lithosphere and metamorphic densification of subducted felsic continental margin. 3D numerical modelling based upon a Caledonide-like plate scenario shows that if a continental corner or promontory enters the subduction zone, the continental margin descends to greater depths than for a simple orthogonal collision and its modelled thermal evolution is consistent with UHP metamorphic assemblages recorded in the southern part of the SC. Furthermore, a tear initiates at the promontary tip along the ocean-continent junction and propagates rapidly along the orogen. The buoyant upthrust of the subducted margin can then lead to reversal of the motion vector of the entire subducting continent, which withdraws the subducted lithospheric margin out of the subduction channel ("eduction"). Because of the diachroneity of slab failure, the continent also rotates, which causes the eduction vector to change azimuth over time. These model behaviours are consistent with the late orogenic structural evolution of the southern SC. However, during the final exhumation stage the crust may not have acted entirely coherently, as some eduction models propose: There is evidence that some inboard Baltica crust experienced late, shallow subduction before detaching as giant "flakes" that carried the orogenic wedge piggyback, forelandwards. Eduction and flake-tectonics could have operated coevally; the model system does not preclude this. Finally, the traction of a large educting (or extruding) mass of continental margin against the overlying orogenic wedge may have stretched and ruptured the wedge, resulting in opening of the late-orogenic Old Red Sandstone molasse basins.

Evidence for large-scale imbrication during Eocene syn-orogenic exhumation of the Hellenic subduction channel (Cyclades, Greece)

NASA Astrophysics Data System (ADS)

Grasemann, Bernhard; Huet, Benjamin; Schneider, David; Rice, Hugh; Lemonnier, Nicolas; Tschegg, Cornelius

2017-04-01

In the Cyclades, Miocene post-orogenic back-arc extension overprinted the exhumed syn- orogenic Eocene subduction channel. Whereas the exact geometry and kinematics of the syn-orogenic exhumation are still controversial, but must have involved a floor thrust and an apparent normal fault at the roof, the post-orogenic extension, leading to the exhumation of Cordilleran-type metamorphic core complexes, is well constrained by several major detachment systems. On the island of Milos, which is part of the South Aegean Volcanic Arc, minor outcrops of schist occur. New data indicate that these witnessed Eocene blueschist facies metamorphism at 8.5 kbar and 400°C, but escaped the Miocene extensional overprint, as they lie in the hanging wall of the West Cycladic Detachment System. In contrast, eclogite pebbles in "Green Lahars" on Milos yield metamorphic conditions of 19.5 kbar at 550°C. Both high-pressure units belong to the Cycladic Blueschist Unit and can only have been juxtaposed by thrusting. This indicates that two nappes, the newly defined Cycladic Blueschist Nappe and the overlying Cycladic Eclogite Nappe, both comprising rocks of the Cycladic Blueschist Unit, exist on Milos. These nappes probably also form the other Cycladic islands, separated by a syn-orogenic thrust, which we name the Trans Cycladic Thrust. The Trans Cycladic Thrust, which traces the orientation of the syn-orogenic exhumation channel, is partly offset by the post-orogenic Miocene extensional detachment systems. As a result of the Mid- to Late Miocene clockwise crustal block rotation, the syn-orogenic channel, and hence the Trans Cycladic Thrust, bends through 90° at Milos, changing from a W-E trending to a N-S trending extrusion-related stretching lineation. Restoration of the Miocene block-rotation and extension results in syn-orogenic thrusting kinematics (top-SSW) in the Cycladic Blueschist Nappe and along the Trans Cycladic Thrust and syn-orogenic apparent normal faulting kinematics (top-NNE) at the roof of the Cycladic Eclogite Nappe, consistent with the Eocene extrusion of the high-pressure rocks in the Cyclades.

a Revision to the Tectonics of the Flores Back-Arc Thrust Zone, Indonesia?

NASA Astrophysics Data System (ADS)

Tikku, A. A.

2011-12-01

The Flores and Bali Basins are continental basins in the Flores back-arc thrust zone associated with Eocene subduction of the Indo-Australian plate beneath the Sunda plate followed by Miocene to present-day inversion/thrusting. The basins are east of Java and north of the islands of Bali, Lombok, Sumbawa and Flores in the East Java Sea area of Indonesia. The tectonic interpretation of these basins is based on seismic, bathymetry and gravity data and is also supported by present-day GPS measurements that demonstrate subduction is no longer active across the Flores thrust zone. Current thinking about the area is that the Flores Basin (on the east end of the thrust zone) had the most extension in the back-arc thrust and may be a proto-oceanic basin, though the option of a purely continental extensional basin can not be ruled out. The Bali Basin (on the west end of the thrust zone) is thought to be shallower and have experienced less continental thinning and extension than the Flores Basin. Depth to basement estimates from recently collected marine magnetic data indicate the depth of the Bali Basin may be comparable to the depth of the Flores Basin. Analysis of the marine magnetic data and potential implications of relative plate motions will be presented.

Along-strike complex geometry of subduction zones - an experimental approach

NASA Astrophysics Data System (ADS)

Midtkandal, I.; Gabrielsen, R. H.; Brun, J.-P.; Huismans, R.

2012-04-01

Recent knowledge of the great geometric and dynamic complexity insubduction zones, combined with new capacity for analogue mechanical and numerical modeling has sparked a number of studies on subduction processes. Not unexpectedly, such models reveal a complex relation between physical conditions during subduction initiation, strength profile of the subducting plate, the thermo-dynamic conditions and the subduction zones geometries. One rare geometrical complexity of subduction that remains particularly controversial, is the potential for polarity shift in subduction systems. The present experiments were therefore performed to explore the influence of the architecture, strength and strain velocity on complexities in subduction zones, focusing on along-strike variation of the collision zone. Of particular concern were the consequences for the geometry and kinematics of the transition zones between segments of contrasting subduction direction. Although the model design to some extent was inspired by the configuration along the Iberian - Eurasian suture zone, the results are also of significance for other orogens with complex along-strike geometries. The experiments were set up to explore the initial state of subduction only, and were accordingly terminated before slab subduction occurred. The model wasbuilt from layers of silicone putty and sand, tailored to simulate the assumed lithospheric geometries and strength-viscosity profiles along the plate boundary zone prior to contraction, and comprises two 'continental' plates separated by a thinner 'oceanic' plate that represents the narrow seaway. The experiment floats on a substrate of sodiumpolytungstate, representing mantle. 24 experimental runs were performed, varying the thickness (and thus strength) of the upper mantle lithosphere, as well as the strain rate. Keeping all other parameters identical for each experiment, the models were shortened by a computer-controlled jackscrew while time-lapse images were recorded. After completion, the models were saturated with water and frozen, allowing for sectioning and profile inspection. The experiments were invariably characterized by different along-strike patterns of deformation, so that three distinct structural domains could be distinguished in all cases. Model descriptions are subdivided accordingly, including domain CC, simulating a continent-continent collision, domain OC, characterized by continent-ocean-continent collision and domain T, representing the transition zone between domain CC and domain OC. The latter zone varied in width and complexity depending on the contrast in structural style developed in the two other domains; in cases where domain OC developed very differently from domain CC, the transition zone was generally wider and more complex. A typical experiment displayed the following features and strain history: In domain CC two principal thrust sheets are displayed, which obviously developed in an in-sequence foreland-directed fashion. The lowermost detachment nucleated at the base of the High Strength Lithospheric Mantle analogue, whereas the uppermost thrust was anchored within the "lower crust". The two thrusts operated in concert, the surface trace of the deepest dominating in the west, and the shallowest in the east. The kinematic development of domain CC could be subdivided into four stages, including initiation of a symmetrical anticline with a minute amplitude and situated directly above the velocity discontinuity defined by the plate contact (stage 1), contemporaneous development of the two thrusts (stage 2) and an associated asymmetrical anticline (stage 3) with a central collapse graben in the latest phase (stage 4). It is noted that the segment CC as seen in a clear majority of the experiments followed this pattern of development. In contrast, the configuration of domain OC displayed greater variation, and included north and south-directed subduction, folding, growth of pop-up-structures and triangle zones. In the "ocean crust" domain, stage 1 was characterized by the growth of a fault-propagation anticline with an E-W-oriented fold axis, ending with the surfacing of a north-vergent thrust. In stage 2, the contraction was concentrated to the south in the oceanic domain, again ending with the surfacing of a thrust, here with top-south transport. By continued movement (stage 3), the thrust fault propagated towards the east, crossing into the "continental" domain and linking with the fault systems of the segment CC. The structure of domain T is dominated by the interference of faults propagating westwards from the domain CC and eastwards from the domain OC, respectively. The zone of overlap in the experiment was significant, and its central part had the geometry of a double "crocodile structure" (sensuMeissner 1989), separating the two areas of northerly and southerly subduction. Hence, its development is less easily subdivided into stages. Reference: Meissner,R., 1989: Rupture, creep lamellae and crocodiles: happenings in the continental crust. Terra Nova, 1, 17-28.

Investigating Along-Strike Variations of Source Parameters for Relocated Thrust Earthquakes Along the Sumatra-Java Subduction Zone

NASA Astrophysics Data System (ADS)

El Hariri, M.; Bilek, S. L.; Deshon, H. R.; Engdahl, E. R.

2009-12-01

Some earthquakes generate anomalously large tsunami waves relative to their surface wave magnitudes (Ms). This class of events, known as tsunami earthquakes, is characterized by having a long rupture duration and low radiated energy at long periods. These earthquakes are relatively rare. There have been only 9 documented cases, including 2 in the Java subduction zone (1994 Mw=7.8 and the 2006 Mw=7.7). Several models have been proposed to explain the unexpectedly large tsunami, such as displacement along high-angle splay faults, landslide-induced tsunami due to coseismic shaking, or large seismic slip within low rigidity sediments or weaker material along the shallowest part of the subduction zone. Slow slip has also been suggested along portions of the 2004 Mw=9.2 Sumatra-Andaman earthquake zone. In this study we compute the source parameters of 90 relocated shallow thrust events (Mw 5.1-7.8) along the Sumatra-Java subduction zone including the two Java tsunami earthquakes. Events are relocated using a modification to the Engdahl, van der Hilst and Buland (EHB) earthquake relocation method that incorporates an automated frequency-dependent phase detector. This allows for the use of increased numbers of phase arrival times, especially depth phases, and improves hypocentral locations. Source time functions, rupture duration and depth estimates are determined using multi-station deconvolution of broadband teleseismic P and SH waves. We seek to correlate any along-strike variation in rupture characteristics with tectonic features and rupture characteristics of the previous slow earthquakes along this margin to gain a better understanding of the conditions resulting in slow ruptures. Preliminary results from the analysis of these events show that in addition to depth-dependent variations there are also along-strike variations in rupture duration. We find that along the Java segment, the longer duration event locates in a highly coupled region corresponding to the location of a proposed subducting seamount. This correlation is less clear along the southern Sumatran segment. One longer duration event is located within the high slip area of the Mw=8.4 2007 rupture, while another is located in the weakly coupled region of the 1935 Mw=7.7 rupture area.

Lateral Moho variations and the geometry of Main Himalayan Thrust beneath Nepal Himalayan orogen revealed by teleseismic receiver functions

NASA Astrophysics Data System (ADS)

He, Ping; Lei, Jianshe; Yuan, Xiaohui; Xu, Xiwei; Xu, Qiang; Liu, Zhikun; Mi, Qi; Zhou, Lianqing

2018-05-01

The lateral Moho variations and the geometry of the Main Himalayan Thrust under the Nepal Himalayan orogen are investigated to determine a new crustal model using a large number of high-quality receiver functions recorded by the HIMNT and HiCLIMB portable seismic networks. Our new model shows an evident and complicated lateral Moho depth variation of 8-16 km in the east-west direction, which is related to the surface tectonic features. These results suggest a non-uniformed crustal deformation, resulted from the splitting and/or tearing of the Indian plate during the northward subduction. Our migrated receiver function images illustrate a discernible ramp structure of the Main Himalayan Thrust with an abrupt downward bending close to the hypocenter of the 2015 Gorkha Mw 7.8 earthquake. The distribution of the aftershocks coincides with the present decollement structure. Integrating previous magnetotelluric soundings and tomographic results, our results suggest that the ramp-shaped structure within the Main Himalayan Thrust could enhance stress concentration leading to the nucleation of the large earthquake. Our new crustal model provides new clues to the formation of the Himalayan orogen.

Structural evidence for northeastward movement on the Chocolate Mountains thrust, southeasternmost Calfornia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dillon, J.T.; Haxel, G.B.; Tosdal, R.M.

1990-11-10

The Late Cretaceous Chocolate Mountains thrust of southeastern California and southwestern Arizona places a block of Proterozoic and Mesozoic continental crust over the late Mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive Orocopia Schist. The Chocolate Mountains thrust is interpreted as a thrust (burial, subduction) fault rather than a low-angle normal (exhumation, unroofing, uplift) fault. The Chocolate Mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds. Fabric relations indicate that these folds are an integral part of and coeval with the thrust zone. On a lower hemisphere equal-area plot representing the orientation and sensemore » of asymmetry of 80 thrust zone folds from 36 localities, spread over an area 60 by 10 km, Z folds plot northwest of and S folds plot southeast of a northeast-southwest striking vertical plane of overall monoclinic symmetry. The only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast. Paleomagnetic data suggest that the original sense of thrusting, prior to Neogene vertical axis tectonic rotation related to the San Andreas fault system, was northward. The essential point is that movement of the upper plate of the Chocolate Mountains thrust evidently was continentward. Continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern California. Alternative tectonic models involving subduction of the Orocopia Schist eastward beneath continental southern California circumvent the suture problem but are presently not supported by any direct structural evidence.« less

Structural controls on the hydrogeology of the Costa Rica subduction thrust NW of the Osa Peninisula (Invited)

NASA Astrophysics Data System (ADS)

Bangs, N. L.; McIntosh, K. D.; Silver, E. A.; Kluesner, J.; Ranero, C. R.

2013-12-01

Three-dimensional seismic reflection data from the Costa Rica margin NW of the Osa peninsula have enabled us to map the subduction megathrust from the trench to ~12 km subseafloor beneath the shelf. The subduction thrust has a large, abrupt downdip transition in seismic reflection amplitude from very high to low amplitude 6 km subseafloor beneath the upper slope. This transition broadly corresponds with an increase in concentration of microseismic earthquakes potentially due to a significant increase in plate coupling (Bangs et al., 2012, AGU Fall Meeting, T13A-2587), thus linking seismic reflection amplitude to fluid content and mechanical coupling along the fault. A detailed look at the overriding plate reflectivity shows numerous high-amplitude, continuous seismic reflections through the upper plate, many of which are clearly reversed-polarity from the seafloor reflection and are thus likely active fluid conduits through the overriding margin wedge, the slope cover sediment, and the seafloor. Broadly, the structural grain of the margin wedge trends E-W and dips landward across the lower slope and onto the shelf, presumably due to stress imparted by subducting ridges. However, directly above the abrupt high-to-low plate-boundary reflection amplitude transition, structures within the overlying margin wedge reverse dip, steepen, and change strike to an ESE direction. Within this zone we interpret a set of parallel reflections with small offsets and reverse-polarity as high-angle reverse faults that act as fluid conduits leading directly into shallow fluid migration systems described by Bangs et al., 2012 (AGU Fall Meeting, T13A-2587) and Kluesner et al. [this meeting]. The coincidence between the plate-boundary reflection amplitude patterns and the change in structure implies that the fluid migration pathways that drain the plate interface are locally disrupted by overriding plate structure in two possible ways: 1) by focusing up dip fluid migration along the plate interface into a thinner but richer fluid zone along the subduction thrust, or 2) by creating a more direct, nearly vertical route along high-angle reverse faults through the overlying margin wedge to the seafloor (possibly shortened by a factor of two) and draining deeper portions of the plate-boundary more efficiently.

Valemount strain zone: A dextral oblique-slip thrust system linking the Rocky Mountain and Omineca belts of the southeastern Canadian Cordillera

NASA Astrophysics Data System (ADS)

McDonough, Michael R.; Simony, Philip S.

1989-03-01

The Valemount strain zone (VSZ), a narrow zone of high orogen-parallel (OP) strain in pebble conglomerate of the Late Proterozoic Miette Group, is the footwall expression of a thrust fault on the western edge of the Rocky Mountain belt, marking the eastern limit of a wide zone of OP fabrics distributed through the Omineca crystalline and western Rocky Mountain belts of the southeastern Canadian Cordillera. Kinematic indicators from the VSZ and the adjacent Bear Foot thrust zone show that both thrust and dextral displacement are associated with folding and thrust motion in the Rocky Mountains, thereby linking the southern Rocky Mountain belt to the Omineca belt by an oblique-slip thrust regime that is tectonically unrelated to the Southern Rocky Mountain Trench. Transverse shortening of thrust sheets and subsequent distribution of OP shear are invoked to explain the parallelism of stretching lineations and fold axes. Strain and kinematic data and the thrust-belt geometry of the VSZ suggest that OP lineations are a product of a large amount of transverse shortening during slightly oblique A-type subduction. Thus, OP lineations are not representative of relative plate motions between North America and accreted terranes, but probably are a function of footwall buttressing of thrust sheets, a mechanism that may be widely applicable to the internal zones of collisional orogens.

Rapid Geodetic Shortening Across the Eastern Cordillera of NW Argentina Observed by the Puna-Andes GPS Array

NASA Astrophysics Data System (ADS)

McFarland, Phillip K.; Bennett, Richard A.; Alvarado, Patricia; DeCelles, Peter G.

2017-10-01

We present crustal velocities for 29 continuously recording GPS stations from the southern central Andes across the Puna, Eastern Cordillera, and Santa Barbara system for the period between the 27 February 2010 Maule and 1 April 2014 Iquique earthquakes in a South American frame. The velocity field exhibits a systematic decrease in magnitude from 35 mm/yr near the trench to <1 mm/yr within the craton. We forward model loading on the Nazca-South America (NZ-SA) subduction interface using back slip on elastic dislocations to approximate a fully locked interface from 10 to 50 km depth. We generate an ensemble of models by iterating over the percentage of NZ-SA convergence accommodated at the subduction interface. Velocity residuals calculated for each model demonstrate that locking on the NZ-SA interface is insufficient to reproduce the observed velocities. We model deformation associated with a back-arc décollement using an edge dislocation, estimating model parameters from the velocity residuals for each forward model of the subduction interface ensemble using a Bayesian approach. We realize our best fit to the thrust-perpendicular velocity field with 70 ± 5% of NZ-SA convergence accommodated at the subduction interface and a slip rate of 9.1 ± 0.9 mm/yr on the fold-thrust belt décollement. We also estimate a locking depth of 14 ± 9 km, which places the downdip extent of the locked zone 135 ± 20 km from the thrust front. The thrust-parallel component of velocity is fit by a constant shear strain rate of -19 × 10-9 yr-1, equivalent to clockwise rigid block rotation of the back arc at a rate of 1.1°/Myr.

Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone

USGS Publications Warehouse

Clarke, S.H.; Carver, G.A.

1992-01-01

Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater.

Late holocene tectonics and paleoseismicity, southern cascadia subduction zone.

PubMed

Clarke, S H; Carver, G A

1992-01-10

Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater.

Seismicity of the Earth 1900–2010 Himalaya and vicinity

USGS Publications Warehouse

Turner, Bethan; Jenkins, Jennifer; Turner, Rebecca; Parker, Amy; Sinclair, Alison; Davies, Sian; Hayes, Gavin P.; Villaseñor, Antonio; Dart, Rirchard L.; Tarr, Arthur C.; Furlong, Kevin P.; Benz, Harley M.

2013-01-01

Seismicity in the Himalaya region predominantly results from the collision of the India and Eurasia continental plates, which are converging at a relative rate of 40–50 mm/yr. Northward underthrusting of India beneath Eurasia generates numerous earthquakes and consequently makes this area one of the most seismically hazardous regions on Earth. The surface expression of the plate boundary is marked by the foothills of the north-south trending Sulaiman Range in the west, the Indo-Burmese Arc in the east, and the east-west trending Himalaya Front in the north of India. Along the western margin of the India plate, relative motions between India and Eurasia are accommodated by strike-slip, reverse, and oblique-slip faulting resulting in the complex Sulaiman Range fold and thrust belt, and the major translational Chaman Fault in Afghanistan. Beneath the Pamir‒Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Further north again, the Tian Shan is a seismically active intra-continental mountain belt defined by a series of east-west trending thrust faults thought to be related to the broad footprint of the India-Eurasia collision. Tectonics in northern India are dominated by motion along the Main Frontal Thrust and associated thrust faults of the India-Eurasia plate boundary, which have resulted in a series of large and devastating earthquakes in (and prior to) the 20th century. The Tibetan Plateau to the north of the main plate boundary is a broad region of uplift associated with the India-Eurasia collision, and is cut by a series of generally east-west trending strike-slip faults. These include the Kunlun, Haiyuan, and the Altyn Tagh faults, all of which are left-lateral structures, and the Kara-Koram right-lateral fault. Throughout the plateau, thrust faults accommodate the north-south compressional component of crustal shortening associated with the ongoing collision of India and Eurasia, while strike-slip and normal faults accommodate east-west extension. To the east, The Longmen Shan thrust belt marks the eastern margin of the Tibetan Plateau separating the complex tectonics of the plateau region from the relatively undeformed Sichuan Basin. Further south, the left-lateral Xiangshuihe-Xiaojiiang, right-lateral Red River and right-lateral Sagaing strike-slip fault systems accommodate deformation along the eastern margin of the India plate. Deep earthquakes have also occurred in the Indo-Burmese Arc region, thought to be an expression of eastward-directed subduction of the India plate, though whether subduction is ongoing is still debated.

Cyclic stressing and seismicity at strongly coupled subduction zones

USGS Publications Warehouse

Taylor, M.A.J.; Zheng, G.; Rice, J.R.; Stuart, W.D.; Dmowska, R.

1996-01-01

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

Gravity modeling of the Muertos Trough and tectonic implications (north-eastern Caribbean)

USGS Publications Warehouse

Granja, Bruna J.L.; Muñoz-Martín, A.; ten Brink, Uri S.; Carbó-Gorosabel, Andrés; Llanes, Estrada P.; Martín-Dávila, J.; Cordoba-Barba, D.; Catalan, Morollon M.

2010-01-01

The Muertos Trough in the northeast Caribbean has been interpreted as a subduction zone from seismicity, leading to infer a possible reversal subduction polarity. However, the distribution of the seismicity is very diffuse and makes definition of the plate geometry difficult. In addition, the compressive deformational features observed in the upper crust and sandbox kinematic modeling do not necessarily suggest a subduction process. We tested the hypothesized subduction of the Caribbean plate's interior beneath the eastern Greater Antilles island arc using gravity modeling. Gravity models simulating a subduction process yield a regional mass deficit beneath the island arc independently of the geometry and depth of the subducted slab used in the models. This mass deficit results from sinking of the less dense Caribbean slab beneath the lithospheric mantle replacing denser mantle materials and suggests that there is not a subducted Caribbean plateau beneath the island arc. The geologically more realistic gravity model which would explain the N-S shortening observed in the upper crust requires an overthrusted Caribbean slab extending at least 60 km northward from the deformation front, a progressive increase in the thrusting angle from 8?? to 30?? reaching a maximum depth of 22 km beneath the insular slope. This new tectonic model for the Muertos Margin, defined as a retroarc thrusting, will help to assess the seismic and tsunami hazard in the region. The use of gravity modeling has provided targets for future wide-angle seismic surveys in the Muertos Margin. ?? 2010 Springer Science+Business Media B.V.

Isolated intermediate-depth seismicity north of the Izu peninsula, Japan: implications for subduction of the Philippine Sea Plate

NASA Astrophysics Data System (ADS)

Nakajima, Junichi

2018-01-01

The subduction of the Philippine Sea (PHS) Plate toward the north of Izu peninsula, Japan, is of great interest because intraslab seismicity is absent where the buoyant Izu volcanic arc has been subducting over the past 15 Myr. This study analyzes 42 earthquakes in an isolated seismic cluster that occurred 100 km north of Izu peninsula at depths of 40-90 km and discusses seismogenesis in the context of plate subduction. We picked P- and S-wave arrival times of earthquakes to produce a complete hypocenter catalogue, carried out double-difference event relocations, and then determined focal mechanism solutions of 7 earthquakes from P-wave polarity data. Based on the focal mechanism solution, the largest earthquake (M3.1) is interpreted as a thrust earthquake along the upper surface of the PHS Plate. Locations of other earthquakes relative to the largest event suggest that most earthquakes occur within the subducting PHS Plate. Our results suggest that the PHS Plate north of Izu peninsula has temperatures low enough to facilitate thrust and intraslab earthquakes at depths of 60-90 km. Earthquakes are likely to occur where pore pressures are locally high, which weakens pre-existing faults. The presence of the intermediate-depth seismic cluster indicates the continuous subduction of the PHS Plate toward the north of Izu peninsula without any disruption.[Figure not available: see fulltext.

Thermal State, Slab Metamorphism, and Interface Seismicity in the Cascadia Subduction Zone Based On 3-D Modeling

NASA Astrophysics Data System (ADS)

Ji, Yingfeng; Yoshioka, Shoichi; Banay, Yuval A.

2017-09-01

Giant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3-D time-dependent thermomechanical model that incorporates an up-to-date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3-D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid-rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500-700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3-D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia.

Bulk rheology and simulated episodic tremor and slip within a numerically-modeled block-dominated subduction melange

NASA Astrophysics Data System (ADS)

Webber, S.; Ellis, S. M.; Fagereng, A.

2015-12-01

We investigate the influence of melange rheology in a subduction thrust interface on stress and slip cycling constrained by observations from an exhumed subduction complex at Chrystalls Beach, New Zealand. A two-phase mélange dominated by large, competent brittle-viscous blocks surrounded by a weak non-linear viscous matrix is numerically modeled, and the evolution of bulk stress are analysed as the domain deforms. The models produce stress cycling behaviour under constant shear strain rate boundary conditions for a wide range of physical conditions that roughly corresponds to depths and strain rates calculated for instrumentally observed episodic tremor and slip (ETS) in presently-deforming subduction thrust interfaces. Stress cycling is accompanied by mixed brittle plastic-viscous deformation, and occurs as a consequence of geometric reorganisation and the progressive development and breakdown of stress bridges as blocks mutually obstruct one another. We argue that periods of low differential stress correspond to periods of rapid mixed-mode deformation and ETS. Stress cycling episodicities are a function of shear strain rate and pressure/temperature conditions at depth. The time period of stress cycling is principally controlled by the geometry (block distribution and density through time) and stress cycling amplitudes are controlled by effective stress. The duration of stress cycling events in the models (months-years) and rapid strain rates are comparable to instrumentally observed ETS. Shear strain rates are 1 - 2 orders of magnitude slower between stress cycling events, suggesting episodic return times within a single model domain are long duration (> centennial timescales), assuming constant flow stress. Finally, we derive a bulk viscous flow law for block dominated subduction mélanges for conditions 300 - 500°C and elevated pore fluid pressures. Bulk flow laws calculated for block-dominated subduction mélanges are non-linear, owing to a combination of non-linear matrix viscosity and development of tensile fractures at rapid shear strain rates. Model behaviour, including the generation of mixed-mode deformation, is highly comparable to the exhumed block-dominated melange found within the Chrystalls Beach Complex.

Development of a Shallow Decollement Along the South-Central Chile Margin from 2D Seismic Reflection Data

NASA Astrophysics Data System (ADS)

Olsen, K.; Bangs, N. L.; Arnulf, A. F.; Trehu, A. M.; Contreras Reyes, E.

2017-12-01

In January and February, 2017, we acquired approximately 5,000 km of deep-penetrating 2D seismic reflection data along the Chile trench between 30° - 44°S as a part of the 2017 Crustal Examination from Valdivia to Illapel to Characterize Huge Earthquakes (CEVICHE) project, on the R/V Langseth. We used a 6,600 in3 airgun source to shoot every 50 m and recorded shots on a 15,100 m, 1212 channel streamer. This survey targeted the structure of this subduction zone across the slip regions of the 2015 Illapel (Mw 8.3), the 2010 Maule (Mw 8.8), and 1960 Valdivia (Mw 9.5) earthquakes. Two dip lines between 37.5°S and 39°S, within the overlapping slip areas of the Maule and Valdivia earthquakes, show a range in the style of initial thrust faulting at the deformation front. At 37.5°S, just south of the Arauco Peninsula, protothrusts at the deformation front are typical of many well-sedimented trench sections in subduction zones worldwide. Here we observe incipient landward-dipping thrusts consisting of 15 faults with typical horizontal spacing of 750 m that can be seen to extend down through the entire 2.5 km thick sediment sequence to the top of the subducting ocean crust. Some form conjugate fault pairs, but all have small offsets of 10-50 m. These thrusts appear to sole into a proto-decollement located just above the top of the ocean crust; however, farther landward beneath the lower slope, a thick, 2.5 km, sequence of layered sediment can be traced > 20 km into the subduction zone. The position of the primary decollement appears to be located near the top of the trench sediment sequence, well above the proto-decollement, allowing subduction of the entire trench sequence. A second line at 39°S across the deformation front shows no frontal thrusts or apparent deformation within the 1.5 km thick section of trench sediment. All of the incoming sediment appears to be subducting beneath a stable decollement that we can image near the top of the trench sediment sequence. The decollement along the northern line may be currently stepping down and transitioning from minimal accretion, typical of this segment of the Chile margin, to accretion of the entire trench section. Alternatively, the initial deformation at the toe may cease and allow slip to shift upward to the shallow decollement and continue to subduct the entire trench sediment section.

Structural anatomy of a dismembered ophiolite suite from Gondwana: The Manamedu complex, Cauvery suture zone, southern India

NASA Astrophysics Data System (ADS)

Chetty, T. R. K.; Yellappa, T.; Nagesh, P.; Mohanty, D. P.; Venkatasivappa, V.; Santosh, M.; Tsunogae, T.

2011-08-01

Detailed geological and structural mapping of the Manamedu ophiolite complex (MOC), from the south-eastern part of the Cauvery suture zone (CSZ) within the Gondwana collisional suture in southern India reveals the anatomy of a dismembered ophiolite succession comprising pyroxenite actinolite-hornblendite, hornblendite, gabbro-norite, gabbro, anorthosite, amphibolite, plagiogranite, mafic dykes, and associated pelagic sediments such as chert-magnetite bands and carbonate horizons. The magmatic foliation trajectory map shows inward dipping foliations and a variety of fold structures. Structural cross-sections of the MOC reveal gentle inward dips with repetition and omission of different lithologies often marked by curvilinear hinge lines. The succession displays imbricate thrust sheets and slices of dismembered ophiolite suites distributed along several localities within the CSZ. The MOC can be interpreted as a deformed large duplex structure associated with south-verging back thrust system, consistent with crustal-scale 'flower structure'. The nature and distribution of ophiolitic rocks in the CSZ suggest supra-subduction zone setting associated with the lithospheric subduction of the Neoproterozoic Mozambique Ocean, followed by collision and obduction during the final stage of amalgamation of the Gondwana supercontinent in the end Precambrian.

Complex thrusting at the toe of the Nankai accretionary prism, NanTroSEIZE Kumano transect

NASA Astrophysics Data System (ADS)

Moore, G. F.; Park, J.; Kodaira, S.; Kaneda, Y.

2009-12-01

Seismic reflection data collected over the past 10 years by the Institute for Research on Earth Evolution (IFREE) of Japan Agency for Marine Earth Science and Technology (JAMSTEC) image a zone of complex thrusting at the toe of the Nankai accretionary prism south of Kii Peninsula, Honshu, Japan. The frontal part of the Nankai prism west of Shionomisaki Canyon (SC) at ~136° E, including the Muroto and Ashizuri Transects off Shikoku, is generally formed of imbricate thrusts with spacing of ~ 1-3 km that dip ~25-35° landward and sole into a prominent décollement. Out-of-sequence thrusts (OOSTs) are usually restricted to the landward margin of this imbricate thrust zone. East of SC, in the Kumano Transect area, the imbricate thrust zone is bounded on its seaward edge by a frontal thrust block that is ~5-6 km wide and consists of several OOSTs. The frontal thrust dips ~5-10° under this ~2-4 km thick block, emplacing this thrust sheet over the trench floor. The number and character of thrusts within the frontal thrust block vary laterally along strike. The 2006 Kumano 3D seismic data set images details of one segment of this complex frontal thrust block. Out-of-sequence faulting has led to underplating of several smaller thrust slices and movement along oblique ramps has led to a complex pattern of faulting that cannot be recognized in even closely-spaced 2D seismic lines. The frontal thrust block is further modified by subduction of seamounts and ridges that have caused large slumps of material from the block.

Three-dimensional splay fault geometry and implications for tsunami generation.

PubMed

Moore, G F; Bangs, N L; Taira, A; Kuramoto, S; Pangborn, E; Tobin, H J

2007-11-16

Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.

The 2006-2007 Kuril Islands great earthquake sequence

USGS Publications Warehouse

Lay, T.; Kanamori, H.; Ammon, C.J.; Hutko, Alexander R.; Furlong, K.; Rivera, L.

2009-01-01

The southwestern half of a ???500 km long seismic gap in the central Kuril Island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007. The nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate, trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries. The intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades (notably an MS 7.2 event on 16 March 1963), leading to speculation that the interplate fault was seismically coupled. This issue was partly resolved by failure of the shallow portion of the interplate megathrust in an MW = 8.3 thrust event on 15 November 2006. This event ruptured ???250 km along the seismic gap, just northeast of the great 1963 Kuril Island (Mw = 8.5) earthquake rupture zone. Within minutes of the thrust event, intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture, with the larger events having normal-faulting mechanisms. An unusual double band of interplate and intraplate aftershocks developed. On 13 January 2007, an MW = 8.1 extensional earthquake ruptured within the Pacific plate beneath the seaward edge of the Kuril trench. This event is the third largest normal-faulting earthquake seaward of a subduction zone on record, and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture. The 13 January 2007 event produced stronger shaking in Japan than the larger thrust event, as a consequence of higher short-period energy radiation from the source. The great event aftershock sequences were dominated by the expected faulting geometries; thrust faulting for the 2006 rupture zone, and normal faulting for the 2007 rupture zone. A large intraplate compressional event occurred on 15 January 2009 (Mw = 7.4) near 45 km depth, below the rupture zone of the 2007 event and in the vicinity of the 16 March 1963 compressional event. The fault geometry, rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations. The occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault. This great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events. Future seismic failure of the remainder of the seismic gap appears viable, with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention. Copyright 2009 by the American Geophysical Union.

Focal mechanisms and tidal modulation for tectonic tremors in Taiwan

NASA Astrophysics Data System (ADS)

Ide, S.; Yabe, S.; Tai, H. J.; Chen, K. H.

2015-12-01

Tectonic tremors in Taiwan have been discovered beneath the southern Central Range, but their hosting structure has been unknown. Here we constrain the focal mechanism of underground deformation related to tremors, using moment tensor inversion in the very low frequency band and tidal stress analysis. Three types of seismic data are used for two analysis steps: detection of tremors and the moment tensor inversion. Short-period seismograms from CWBSN are used for tremor detection. Broadband seismograms from BATS and the TAIGER project are used for both steps. About 1000 tremors were detected using an envelope correlation method in the high frequency band (2-8 Hz). Broadband seismograms are stacked relative to the tremor timing, and inverted for a moment tensor in the low frequency band (0.02-0.05 Hz). The best solution was obtained at 32 km depth, as a double-couple consistent with a low-angle thrust fault dipping to the east-southeast, or a high-angle thrust with a south-southwest strike. Almost all tremors occur when tidal shear stress is positive and normal stress is negative (clamping). Since the clamping stress is high for a high-angle thrust fault, the low-angle thrust fault is more likely to be the fault plane. Tremor rate increases non-linearly with increasing shear stress, suggesting a velocity strengthening friction law. The high tidal sensitivity is inconsistent with horizontal slip motion suggested by previous studies, and normal faults that dominates regional shallow earthquakes. Our results favor thrust slip on a low-angle fault dipping to the east-southeast, consistent with the subduction of the Eurasian plate. The tremor region is characterized by a deep thermal anomaly with decrease normal stress. This region has also experienced enough subduction to produce metamorphic fluids. A large amount of fluid and low vertical stress may explain the high tidal sensitivity.

Core-Log-Seismic Integrative Study of a Subduction Zone Megasplay Fault -An Example from the Nobeoka Thrust, Shimanto Belt, Southwest Japan

NASA Astrophysics Data System (ADS)

Hamahashi, M.; Tsuji, T.; Saito, S.; Tanikawa, W.; Hamada, Y.; Hashimoto, Y.; Kimura, G.

2016-12-01

Investigating the mechanical properties and deformation patterns of megathrusts in subduction zones is important to understand the generation of large earthquakes. The Nobeoka Thrust, a fossilized megasplay fault in Kyushu Shimanto Belt, southwest Japan, exposes foliated fault rocks that were formed under the temperature range of 180-350° (Kondo et al., 2005). During the Nobeoka Thrust Drilling Project (2011), core samples and geophysical logging data were obtained recovering a continuous distribution of multiple fault zones, which provide the opportunity to examine their structure and physical properties in various scales (Hamahashi et al., 2013; 2015). By performing logging data analysis, discrete sample physical property measurements, and synthetic modeling of seismic reflections along the Nobeoka Thrust, we conducted core-log-seismic integrative study to characterize the effects of damage zone architecture and structural anisotropy towards the physical properties of the megasplay. A clear contrast in physical properties across the main fault core and surrounding damage zones were identified, where the fault rocks preserve the porosity of 4.8% in the hanging wall and 7.6% in the footwall, and P-wave velocity of 4.8 km/s and 4.2 km/s, respectively. Multiple sandstone-rich- and shale-rich damage zones were found from the drilled cores, in which velocity decreases significantly in the brecciated zones. The internal structure of these foliated fault rocks consist of heterogeneous lithology and texture, and velocity anisotropy ranges 1-18% (P-wave) and 1.5-80% (S-wave), affected by structural dip angle, foliation density, and sandstone/mudstone ratio. To evaluate the fault properties at the seismogenic depth, we developed velocity/earth models and synthetic modeling of seismic reflection using acoustic logs across the thrust and parameterized lithological and structural elements in the identified multiple damage zones.

Fault structure, properties and activity of the Makran Accretionary Prism and implications for seismogenic potential

NASA Astrophysics Data System (ADS)

Smith, G. L.; McNeill, L. C.; Henstock, T.; Bull, J. M.

2011-12-01

The Makran subduction zone is the widest accretionary prism in the world (~400km), generated by convergence between the Arabian and Eurasian tectonic plates. It represents a global end-member, with a 7km thick incoming sediment section. Accretionary prisms have traditionally been thought to be aseismic due to the presence of unconsolidated sediment and elevated basal pore pressures. The seismogenic potential of the Makran subduction zone is unclear, despite a Mw 8.1 earthquake in 1945 that may have been located on the plate boundary beneath the prism. In this study, a series of imbricate landward dipping (seaward verging) thrust faults have been interpreted across the submarine prism (outer 70 km) using over 6000km of industry multichannel seismic data and bathymetric data. A strong BSR (bottom simulating reflector) is present throughout the prism (excluding the far east). An unreflective décollement is interpreted from the geometry of the prism thrusts. Two major sedimentary units are identified in the input section, the lower of which contains the extension of the unreflective décollement surface. Between 60%-100% of the input section is currently being accreted. The geometry of piggy-back basin stratigraphy shows that the majority of thrusts, including those over 50km from the trench, are recently active. Landward thrusts show evidence for reactivation after periods of quiescence. Negative polarity fault plane reflectors are common in the frontal thrusts and in the eastern prism, where they may be related to increased fault activity and fluid expulsion, and are rarer in older landward thrusts. Significant NE-SW trending basement structures (The Murray Ridge and Little Murray Ridge) on the Arabian plate intersect the deformation front and affect sediment input to the subduction zone. Prism taper and structure are apparently primarily controlled by sediment supply and the secondary influence of subducting basement ridges. The thick, likely distal, sediment section in the west produces a prism with a simple imbricate structure. As basement depth is reduced over the Little Murray Ridge, the accretionary prism structure (fault spacing and deformation front position) changes. In the east, proximity to the Murray Ridge and triple junction is expressed through a reduction in prism width and reduced fault activity. The resulting prism structure and morphology can ultimately be used to assess likely sediment properties and hence seismic potential at the plate boundary.

Structure and kinematics of the Sumatran Fault System in North Sumatra (Indonesia)

NASA Astrophysics Data System (ADS)

Fernández-Blanco, David; Philippon, Melody; von Hagke, Christoph

2016-12-01

Lithospheric-scale faults related to oblique subduction are responsible for some of the most hazardous earthquakes reported worldwide. The mega-thrust in the Sunda sector of the Sumatran oblique subduction has been intensively studied, especially after the infamous 2004 Mw 9.1 earthquake, but its onshore kinematic complement within the Sumatran subduction, the transform Sumatran Fault System, has received considerably less attention. In this paper, we apply a combination of analysis of Digital Elevation Models (ASTER GDEM) and field evidence to resolve the kinematics of the leading edge of deformation of the northern sector of the Sumatran Fault System. To this end, we mapped the northernmost tip of Sumatra, including the islands to the northwest, between 4.5°N and 6°N. Here, major topographic highs are related to different faults. Using field evidence and our GDEM structural mapping, we can show that in the area where the fault bifurcates into two fault strands, two independent kinematic regimes evolve, both consistent with the large-scale framework of the Sumatran Fault System. Whereas the eastern branch is a classic Riedel system, the western branch features a fold-and-thrust belt. The latter contractional feature accommodated significant amounts (c. 20%) of shortening of the system in the study area. Our field observations of the tip of the NSFS match a strain pattern with a western contractional domain (Pulau Weh thrust splay) and an eastern extensional domain (Pulau Aceh Riedel system), which are together characteristic of the tip of a propagating strike-slip fault, from a mechanical viewpoint. For the first time, we describe the strain partitioning resulting from the propagation of the NSFS in Sumatra mainland. Our study helps understanding complex kinematics of an evolving strike-slip system, and stresses the importance of field studies in addition to remote sensing and geophysical studies.

The rigid Andean sliver hypothesis challenged : impact on interseismic coupling on the Chilean subduction zone

NASA Astrophysics Data System (ADS)

Metois, M.

2017-12-01

Convergence partitioning between subduction zones and crustal active structures has been widely evidenced. For instance, the convergence between the Indian and Sunda plates is accommodated both by the Sumatra subduction zone and the Great Sumatran strike-slip fault, that defines a narrow sliver. In Cascadia, small-scale rotating rigid blocks bounded by active faults have been proposed (e.g. McCaffrey et al. 2007). Recent advances in geodetic measurements along the South-American margin especially in Ecuador, Peru and Chile and the need for precise determination of the coupling amount on the megathrust interface in particular for seismic hazard assessment, led several authors to propose the existence of large-scale Andean slivers rotating clockwise and counter-clockwise South and North of the Arica bend, respectively (e.g. Chlieh et al. 2011, Nocquet et al. 2014, Métois et al. 2013). In Chile, one single large Andean sliver bounded to the west by the subduction thrust and to the east by the subandean fold-an-thrust belt active front is used to mimic the velocities observed in the middle to far field that are misfitted by elastic coupling models on the megathrust interface alone (Métois et al. 2016). This rigid sliver is supposed to rotate clockwise around a Euler pole located in the South Atlantic ocean, consistently with long-term observed rotations detected by paleomagnetism (e.g. Arriagada et al. 2008). However, recent GPS data acquired in the Taltal area ( 26°S, Klein et al. submitted) show higher than expected middle-field eastward velocities and question the first-order assumption of a rigid Andean sliver. Mis-modeling the fore-arc deformation has a direct impact on the inverted coupling amount and distribution, and could therefore bias significantly the megathrust rupture scenarios. Correctly estimating the current-day deformation of the Andes is therefore required to properly assess for coupling on the plate interface and is challenging since crustal active structures are often hidden by the intense seismic activity of the subduction zone. Here we discuss the validity of the rigid Andean sliver hypothesis based on GPS velocities, present alternative models for both coupling and sliver kinematics along the Chilean margin, and discuss the relationship between upper plate long and short-term deformation.

Splay Fault Branching from the Hikurangi Subduction Shear Zone: Implications for Slow Slip and Fluid Flow

NASA Astrophysics Data System (ADS)

Henrys, S. A.; Plaza-Faverola, A. A.; Pecher, I. A.; Klaeschen, D.; Wallace, L.

2016-12-01

Seismic reflection data along the East Coast of the New Zealand North Island are used to map the offshore character and geometry of the central Hikurangi subduction thrust and outer wedge in a region of short term ( 2-3 weeks duration) geodetically determined slow-slip events (SSEs). Pre-stack depth migration of line 05CM-38 was used to derive subducting slab geometry and upper crustal structure together with a Vp image of the crust that is resolved to 14 km depth. The subduction interface is a shallow dipping thrust at < 7 km deep near the trench and steps down to 14 km depth along an approximately 18 km long ramp, beneath Porangahau Ridge. This bend in the subducted plate is associated with splay fault branching and coincides with the zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. We infer that the step down in the décollement transfers slip on the plate interface from the top of subducting sediments to the oceanic crust and drives underplating beneath the inner margin of central Hikurangi margin. Low-velocity subducting sediments (LVZ) beneath the plate interface, updip of the plate interface ramp, are interpreted as being capped with a low permeability condensed layer of chalk and interbedded mudstones. We interpret this LVZ as fluid-rich overpressured sediments that have been displaced and later imbricated by splay faults in a region that may mark the up-dip transition from seismic to aseismic behavior. Further, we hypothesize that fluids derived from the overpressured sediment are channeled along splay faults to the shallow sub-seafloor near Porangahau Ridge where seafloor seepage and an upwarping of the gas hydrate Bottom-Simulating Reflector have been documented.

An Examination of Seismicity Linking the Solomon Islands and Vanuatu Subduction Zones

NASA Astrophysics Data System (ADS)

Neely, J. S.; Furlong, K. P.

2015-12-01

The Solomon Islands-Vanuatu composite subduction zone represents a tectonically complex region along the Pacific-Australia plate boundary in the southwest Pacific Ocean. Here the Australia plate subducts under the Pacific plate in two segments: the South Solomon Trench and the Vanuatu Trench. The two subducting sections are offset by a 200 km long, transform fault - the San Cristobal Trough (SCT) - which acts as a Subduction-Transform Edge Propagator (STEP) fault. The subducting segments have experienced much more frequent and larger seismic events than the STEP fault. The northern Vanuatu trench hosted a M8.0 earthquake in 2013. In 2014, at the juncture of the western terminus of the SCT and the southern South Solomon Trench, two earthquakes (M7.4 and M7.6) occurred with disparate mechanisms (dominantly thrust and strike-slip respectively), which we interpret to indicate the tearing of the Australia plate as its northern section subducts and southern section translates along the SCT. During the 2013-2014 timeframe, little seismic activity occurred along the STEP fault. However, in May 2015, three M6.8-6.9 strike-slip events occurred in rapid succession as the STEP fault ruptured east to west. These recent events share similarities with a 1993 strike-slip STEP sequence on the SCT. Analysis of the 1993 and 2015 STEP earthquake sequences provides constraints on the plate boundary geometry of this major transform fault. Preliminary research suggests that plate motion along the STEP fault is partitioned between larger east-west oriented strike-slip events and smaller north-south thrust earthquakes. Additionally, the differences in seismic activity between the subducting slabs and the STEP fault can provide insights into how stress is transferred along the plate boundary and the mechanisms by which that stress is released.

Shallow depth of seismogenic coupling in southern Mexico: implications for the maximum size of earthquakes in the subduction zone

NASA Astrophysics Data System (ADS)

Suárez, Gerardo; Sánchez, Osvaldo

1996-01-01

Studies of locally recorded microearthquakes and the centroidal depths of the largest earthquakes analyzed using teleseismic data show that the maximum depth of thrust faulting along the Mexican subduction zone is anomalously shallow. This observed maximum depth of about 25 ± 5 km is about half of that observed in most subduction zones of the world. A leveling line that crosses the rupture zone of the 19 September 1985 Michoacan event was revisited after the earthquake and it shows anomalously low deformation during the earthquake. The comparison between the observed coseismic uplift and dislocation models of the seismogenic interplate contact that extend to depths ranging from 20 to 40 km shows that the maximum depth at which seismic slip took place is about 20 km. This unusually shallow and narrow zone of seismogenic coupling apparently results in the occurrence of thrust events along the Mexican subduction zone that are smaller than would be expected for a trench where a relatively young slab subducts at a rapid rate of relative motion. A comparison with the Chilean subduction zone shows that the plate interface in Mexico is half that in Chile, not only in the down-dip extent of the seismogenic zone of plate contact, but also in the distance of the trench from the coast and in the thickness of the upper continental plate. It appears that the narrow plate contact produced by this particular plate geometry in Mexico is the controlling variable defining the size of the largest characteristic earthquakes in the Mexican subduction zone.

Geometry and kinematics of accretionary wedge faults inherited from the structure and rheology of the incoming sedimentary section; insights from 3D seismic reflection data

NASA Astrophysics Data System (ADS)

Bell, Rebecca; Orme, Haydn; Lenette, Kathryn; Jackson, Christopher; Fitch, Peter; Phillips, Thomas; Moore, Gregory

2017-04-01

Intra-wedge thrust faults represent important conduits for fluid flow in accretionary prisms, modulating pore fluid pressure, effective stress and, ultimately, the seismic hazard potential of convergent plate boundaries. Despite its importance, we know surprisingly little regarding the 3D geometry and kinematics of thrust networks in accretionary prisms, largely due to a lack of 3D seismic reflection data providing high-resolution, 3D images. To address this we here present observations from two subduction zones, the Nankai and Lesser Antilles margins, where 3D seismic and borehole data allow us to constrain the geometry and kinematics of intra-wedge fault networks and to thus shed light on the mechanisms responsible for their structural style variability. At the Muroto transect, Nankai margin we find that the style of protothrust zone deformation varies markedly along-strike over distances of only a few km. Using structural restoration and quantitative fault analysis, we reveal that in the northern part of the study area deformation occurred by buckle folding followed by faulting. Further south, intra-wedge faults nucleate above the décollement and propagate radially with no folding, resulting in variable connectivity between faults and the décollement. The seismic facies character of sediments immediately above the décollement varies along strike, with borehole data revealing that, in the north, where buckle folding dominates un-cemented Lower Shikoku Basin sediments overlie the décollement. In contrast, further south, Opal CT-cemented, and thus rigid Upper Shikoku Basin sediments overlie the décollement. We suggest these along-strike variations in diagenesis and thus rheology control the observed structural style variability. Near Barbados, at the Lesser Antilles margin, rough subducting plate relief is blanketed by up to 700 m of sediment. 3D seismic data reveal that basement relief is defined by linear normal fault blocks and volcanic ridges, and sub-circular seamounts. The youngest, most basinward thrusts in the wedge strike NW-SE; however, 17 km landward, towards the wedge core, they strike NE-SW. The orientation of the more landward faults correlates with the trend of linear basement relief, whereas thrust fault orientations close to the deformation front are perpendicular to the convergence direction. We notice that oceanic crust that has been subducted is characterised by NE-SW striking, now-inverted normal faults, with some faults extending up through the entire sedimentary section. We suggest that the NE-SW orientation of thrust faults has been inherited from linear basement ridges. In contrast, basement currently subducting beneath the deformation front is dominated by seamounts and is devoid of more linear features. Here, there are no pre-existing normal faults available for reactivation and thrust faults develop perpendicular to the convergence direction. We show that the incoming plate properties have a profound effect on the geometry of accretionary wedges; it would be difficult to elucidate this without 3D seismic data. Our insights provide new hypotheses that can be tested with numerical and laboratory models.

The 2017/09/08 Mw 8.2 Tehuantepec, Mexico Earthquake: A Large but Compact Dip-Slip Faulting Event Severing the Slab

NASA Astrophysics Data System (ADS)

Hjorleifsdottir, V.; Iglesias, A.; Suarez, G.; Santoyo, M. A.; Villafuerte, C. D.; Ji, C.; Franco-Sánchez, S. I.; Singh, S. K.; Cruz-Atienza, V. M.; Ando, R.

2017-12-01

The Mw 8.2 September 8 earthquake occurred in the middle of the "Tehuantepec Gap", a segment of the Mexican subduction zone that has no historical mentions of a large earthquake. It was, however, not the expected subduction megathrust earthquake, but rather an intraplate, normal faulting event, in the subducting oceanic Cocos plate. The earthquake rupture initiated at a depth of 50 km and propagated NW on a near-vertical plane, breaking towards the surface. Most of the slip was concentrated in the distance range 30-100 km from the hypocenter and at depth between 15 and 50 km, with maximum slip of 15m. The earthquake seems to have broken the entire lithosphere, estimated to be 35 km thick. The strike of the fault is about 20 degrees oblique to the trench but aligned with the existing fabric on the incoming oceanic plate, suggesting a structural control by preexisting intraslab fractures and activation by the extensional stress due to the slab bending and pulling. Aftershocks occurred along the fault plane during the first day after the event, with activation of other parallel structures within the subducting plate, towards the east, as well as in upper plate, in the following days. Coulomb stress modeling suggests that the stress on the plate interface above the rupture was significantly increased where shallow thrust aftershoks took place, and reduced updip of the earthquake. There are several other examples of large intraslab normal faulting earthquakes, near the downdip edge (1931 Mw 7.8 and 1999 Mw 7.5, Oaxaca) or directly below (1997 Mw 7.1, Michoacan) the coupled plate interface, along the Mexican subduction zone. The possibility of events of similar magnitude to the 2017 earthquake occurring close to the coastline, all along this part of the subduction zone, cannot be ruled out.

Geological perspectives of shallow slow earthquakes deduced from deformation in subduction mélanges

NASA Astrophysics Data System (ADS)

Ujiie, K.; Saishu, H.; Kinoshita, T.; Nishiyama, N.; Otsubo, M.; Ohta, K.; Yamashita, Y.; Ito, Y.

2017-12-01

Shallow (< 15 km depth) slow earthquakes are important to understand, as they occur along the subduction thrust where devastating tsunamis are generated. Geophysical studies have revealed that shallow slow earthquakes are not restricted to specific temperature conditions and depths but occur in regions of high fluid pressure. In the Nankai subduction zone, the shallow slow slip appears to trigger tremor and very-low-frequency-earthquake. However, the geologic perspectives for shallow slow earthquakes remain enigmatic. The Makimine mélange in the Late Cretaceous Shimanto accretionary complex of southwest Japan was formed during the subduction of young oceanic plate. Within the mélange, the quartz-filled veins and viscous shear zones are concentrated in the zones of 10 to 60 m-thick. The veins consist of shear veins showing low-angle thrust or normal faulting mechanisms and extension veins parallel or at high angle to mélange foliation. The geometrical relationship between shear and extension veins indicates that shear slip and tensile fracturing occur by small differential stress under elevated fluid pressure. The shear and extension veins typically show crack-seal textures defined by the solid inclusions bands. The time scale of each crack-seal event, which is determined from the quartz kinetics considering inclusion band spacing and vein length, is a few years. The shear slip increments estimated from the spacing of inclusions bands at dilational jogs are 0.1 mm. The viscous shear is accommodated by pressure solution creep and consistently shows low-angle thrust shear sense. These geologic features are suggested to explain seismogenic environment for shallow slow earthquakes. The shear veins and viscous shear zones showing low-angle thrust faulting mechanism could represent episodic tremor and slip, while the shear veins showing low-angle normal faulting mechanism may represent the tremor that occurred after the passage of slow slip front.

Areas of slip of recent earthquakes in the Mexican subduction zone

NASA Astrophysics Data System (ADS)

Hjorleifsdottir, V.; Sánchez-Reyes, H. S.; Singh, S.; Ji, C.; Iglesias, A.; Perez-Campos, X.

2012-12-01

The Mexican subduction zone is unusual: the width of the seismogenic zone is relatively narrow and a large portion of the co-seismic slip generally occurs below the coast, ~ 45 to 80 km from the trench. The earthquake recurrence interval is relatively short and almost the entire length of the zone has experienced a large (Mw≥7.4) earthquake in the last 100 years (Singh et al., 1981). In this study we present detailed analysis of the areas of significant slip during several recent (last 20 years) large earthquakes in the Mexican subduction zone. The most recent earthquake of 20 March 2012 (Mw7.4) occurred near the Guerrero/Oaxaca border. The slip was concentrated on the plate interface below land and the epicentral PGAs ranged between 0.2 and 0.7g. The updip portion of the plate interface had previously broken during the 25 Feb 1996 earthquake (Mw7.1), which was a slow earthquake and produced anomalously low PGAs (Iglesias et al., 2003). This indicates that in this region the area close to the trench is at least partially locked, with some earthquakes breaking the down-dip portion of the interface and others rupturing the up-dip portion. The Jalisco/Colima segment of the subduction zone seems to behave in a similar fashion. The 9 October 1995 (Mw 8.0) earthquake generated small accelerations relative to its size. The energy to moment ratio, E0/M0, is 4.2e-6 (Pérez-Campos, Singh and Beroza, 2003), a value similar to the Feb, 1996 earthquake. This value is low compared to other thrust events in the region. The earthquake also had the largest (Ms-Mw) disparity along the Mexican subduction zone, 7.4 vs 8.0. The event produced relatively large tsunami. On the contrary, the 3 June 1932 earthquake (Ms8.2, Mw8.0), that is believed to have broken the same segment of the subduction zone, appears to be "normal." Based on the available evidence, it may be concluded that the 1932 event broke a deeper patch of the plate interface relative to the 1995 event. The mode of rupture in the subduction zone between the two areas mentioned above is not known. This part of the subduction zone includes the rupture area of the 1985 Michoacán earthquake (Mw8.0) and the "Guerrero Gap" which is a section of the subduction zone that has not had a large earthquake in the last 100 years. The downdip and updip patches on the plate interface, which, generally, rupture independently may slip during one great earthquake. This possibility must be accounted for in the estimation of maximum-magnitude earthquake along the subduction zone.

Strain accumulation and rotation in western Oregon and southwestern Washington

USGS Publications Warehouse

Svarc, J.L.; Savage, J.C.; Prescott, W.H.; Murray, M.H.

2002-01-01

Velocities of 75 geodetic monuments in western Oregon and southwestern Washington extending from the coast to more than 300 km inland have been determined from GPS surveys over the interval 1992-2000. The average standard deviation in each of the horizontal velocity components is ??? 1 mm yr-1. The observed velocity field is approximated by a combination of rigid rotation (Euler vector relative to interior North America: 43. 40??N ?? 0.14??, 119.33??W ?? 0.28??, and 0.822 ?? 0.057?? Myr-1 clockwise; quoted uncertainties are standard deviations), uniform regional strain rate (??EE = -7.4 ?? 1.8, ??EN = -3.4 ?? 1.0, and ??NN = -5.0 ?? 0.8 nstrain yr-1, extension reckoned positive), and a dislocation model representing subduction of the Juan de Fuca plate beneath North America. Subduction south of 44.5??N was represented by a 40-km-wide locked thrust and subduction north of 44.5??N by a 75-km-wide locked thrust.

Deformation during terrane accretion in the Saint Elias orogen, Alaska

USGS Publications Warehouse

Bruhn, R.L.; Pavlis, T.L.; Plafker, G.; Serpa, L.

2004-01-01

The Saint Elias orogen of southern Alaska and adjacent Canada is a complex belt of mountains formed by collision and accretion of the Yakutat terrane into the transition zone from transform faulting to subduction in the northeast Pacific. The orogen is an active analog for tectonic processes that formed much of the North American Cordillera, and is also an important site to study (1) the relationships between climate and tectonics, and (2) structures that generate large- to great-magnitude earthquakes. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin of southern Alaska. Interaction between the Yakutat terrane and the North American and Pacific plates causes significant differences in the style of deformation within the terrane. Deformation in the eastern part of the terrane is caused by strike-slip faulting along the Fairweather transform fault and by reverse faulting beneath the coastal mountains, but there is little deformation immediately offshore. The central part of the orogen is marked by thrusting of the Yakutat terrane beneath the North American plate along the Chugach-Saint Elias fault and development of a wide, thin-skinned fold-and-thrust belt. Strike-slip faulting in this segment may he localized in the hanging wall of the Chugach-Saint Elias fault, or dissipated by thrust faulting beneath a north-northeast-trending belt of active deformation that cuts obliquely across the eastern end of the fold-and-thrust belt. Superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen, where the sedimentary cover of the Yakutat terrane is accreted into the upper plate of the Aleutian subduction zone. These three structural segments are separated by transverse tectonic boundaries that cut across the Yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the Saint Elias Mountains onto the coastal plain. The Malaspina fault-Pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding. Onshore, most of this boundary is buried beneath the western or "Agassiz" lobe of the Malaspina piedmont glacier. The boundary between the central fold-and-thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the Bering piedmont glacier. ?? 2004 Geological Society of America.

Tectonics and geology of spreading ridge subduction at the Chile Triple Junction: a synthesis of results from Leg 141 of the Ocean Drilling Program

USGS Publications Warehouse

Behrmann, J.H.; Lewis, S.D.; Cande, S.C.

1994-01-01

An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. This process has played a major part in the evolution of most of the continental margins that border the Pacific Ocean basin. A combination of high resolution swath bathymetric maps, seismic reflection profiles and drillhole and core data from five sites drilled during Ocean Drilling Program (ODP) Leg 141 provide important data that define the tectonic, structural and stratigraphic effects of this modern example of spreading ridge subduction. A change from subduction accretion to subduction erosion occurs along-strike of the South American forearc. This change is prominently expressed by normal faulting, forearc subsidence, oversteepening of topographic slopes and intensive sedimentary mass wasting, overprinted on older signatures of sediment accretion, overthrusting and uplift processes in the forearc. Data from drill sites north of the triple junction (Sites 859-861) show that after an important phase of forearc building in the early to late Pliocene, subduction accretion had ceased in the late Pliocene. Since that time sediment on the downgoing oceanic Nazca plate has been subducted. Site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis. Here, thick and intensely folded and faulted trench slope sediments of Pleistocene age are currently involved in the frontal deformation of the forearc. Early faults with thrust and reverse kinematics are overprinted by later normal faults. The Chile Triple Junction is also the site of apparent ophiolite emplacement into the South American forearc. Drilling at Site 862 on the Taitao Ridge revealed an offshore volcanic sequence of Plio-Pleistocene age associated with the Taitao Fracture Zone, adjacent to exposures of the Pliocene-aged Taitao ophiolite onshore. Despite the large-scale loss of material from the forearc at the triple junction, ophiolite emplacement produces a large topographic promontory in the forearc immediately after ridge subduction, and represents the first stage of forearc rebuilding. ?? 1994 Springer-Verlag.

Subduction Thermal Regime, Slab Dehydration, and Seismicity Distribution Beneath Hikurangi Based on 3-D Simulations

NASA Astrophysics Data System (ADS)

Suenaga, Nobuaki; Ji, Yingfeng; Yoshioka, Shoichi; Feng, Deshan

2018-04-01

The downdip limit of seismogenic interfaces inferred from the subduction thermal regime by thermal models has been suggested to relate to the faulting instability caused by the brittle failure regime in various plate convergent systems. However, the featured three-dimensional thermal state, especially along the horizontal (trench-parallel) direction of a subducted oceanic plate, remains poorly constrained. To robustly investigate and further map the horizontal (trench-parallel) distribution of the subduction regime and subsequently induced slab dewatering in a descending plate beneath a convergent margin, we construct a regional thermal model that incorporates an up-to-date three-dimensional slab geometry and the MORVEL plate velocity to simulate the plate subduction history in Hikurangi. Our calculations suggest an identified thrust zone featuring remarkable slab dehydration near the Taupo volcanic arc in the North Island distributed in the Kapiti, Manawatu, and Raukumara region. The calculated average subduction-associated slab dehydration of 0.09 to 0.12 wt%/km is greater than the dehydration in other portions of the descending slab and possibly contributes to an along-arc variation in the interplate pore fluid pressure. A large-scale slab dehydration (>0.05 wt%/km) and a high thermal gradient (>4 °C/km) are also identified in the Kapiti, Manawatu, and Raukumara region and are associated with frequent deep slow slip events. An intraslab dehydration that exceeds 0.2 wt%/km beneath Manawatu near the source region of tectonic tremors suggests an unknown relationship in the genesis of slow earthquakes.

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

NASA Astrophysics Data System (ADS)

Li, Duo; McGuire, Jeffrey J.; Liu, Yajing; Hardebeck, Jeanne L.

2018-03-01

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

USGS Publications Warehouse

Li, Duo; McGuire, Jeffrey J.; Liu, Yajing; Hardebeck, Jeanne L.

2018-01-01

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

Sediment Accretion During Horst and Graben Subduction associated with the Tohoku Oki M9 Earthquake, Northern Japan

NASA Astrophysics Data System (ADS)

Moore, J. C.; Chester, F. M.

2015-12-01

The stratigraphic sequence within the frontal accretionary prism of the Japan Trench, the site of large slip during the Tohoku earthquake, is unique due to horst and graben subduction. Boreholes at IODP Site C0019, penetrating the toe of the Tohoku accretionary prism, document a younger over older intraprism thrust contact with a 9 Ma age gap across the basal plate boundary fault. The anomalously young (Quaternary to Pliocene), fault-bounded sediment package is 130 m thick, of a total of 820 m of sediment above the plate boundary fault. In contrast, typical accretionary prism structure consists of stacked sediment packages on imbricate faults above the basal decollement resulting in an overall increase in age downward. Site C0019 penetrates the prism directly above a horst of the subducting Pacific oceanic crust. Here the plate-boundary fault consists of a thin, weak smectitic pelagic clay that is probably the principal slip surface of ~50 m offset in the 2011 Tohoku earthquake. The fault continues seaward deepening off the seaward edge of the horst and beneath the sediment fill of the adjacent graben, dying out at the landward base of the next incoming horst. The plate boundary fault and its splays in the graben form a narrow-taper protoprism and a small sedimentary basin of trench fill marking the seaward edge of the upper plate. The modern fault and sediment distributions within the graben are used to motivate a viable model for the presence of anomalously young sediments directly above the plate boundary fault. In this model sediments in the trench are thrust over the incoming horst by propagation of the plate boundary thrust up the landward-dipping fault of the incoming horst and along the smectitic clay layer to emplace Quaternary and Pliocene trench deposits directly on top of the incoming horst. These young deposits are in turn overlain by sediments 9 Ma or older that have been transported out of the graben along imbricate faults associated with the necessary increase in the taper of the prism above the graben. The Quaternary to Pliocene units thicken due to internal deformation accounting for the 130 m thickness now observed over the plate boundary fault at Site C0019. Conversely emplacement of very young sediment directly above a basal detachment would be unexpected in accretionary prisms subducting smoother oceanic crust.

Integrated Geophysical Characteristics of the 2015 Illapel, Chile, Earthquake

NASA Astrophysics Data System (ADS)

Herman, M. W.; Yeck, W. L.; Nealy, J. L.; Hayes, G. P.; Barnhart, W. D.; Benz, H.; Furlong, K. P.

2015-12-01

On September 16th, 2015, an Mw 8.3 earthquake (USGS moment magnitude) ruptured offshore of central Chile, 50 km west of the city of Illapel and 200 km north of Santiago. The earthquake occurred just north of where the Juan Fernandez Ridge enters the subduction zone. In this study, we integrate multiple seismic and geodetic datasets, including multiple-event earthquake relocations; moment tensors of the Illapel mainshock, aftershocks, and prior regional seismicity; finite fault models (FFMs) of the mainshock rupture; subduction zone geometry; Coulomb stress transfer calculations; and co-seismic GPS offsets and InSAR images. These datasets allow us to (a) assess the context of the Illapel earthquake sequence with respect to historical seismicity in central Chile; (b) constrain the relationship between subduction geometry and the kinematic characteristics of the earthquake sequence; and (c) understand the distribution of aftershocks with respect to the rupture zone. Double source W-phase moment tensor analysis indicates the Illapel mainshock rupture began as a smaller Mw ~7.2 thrusting event before growing into a great-sized Mw 8.3 earthquake. Relocated aftershock seismicity is concentrated around the main region of slip, and few aftershocks occur on the megathrust shallower than ~15 km, despite the FFM indicating slip near the trench. This distribution is consistent with the aftershock behavior following the 2010 Maule and 2014 Iquique earthquakes: aftershocks primarily surround the rupture zones and are largely absent from regions of greatest slip. However, in contrast to the recent 2014 Iquique and 2010 Maule events, which ruptured in regions of the Chilean subduction zone that had not had large events in over a century, this earthquake occurred in a section of the subduction zone that hosted a large earthquake as recently as 1943, as well as earlier significant events in 1880 and 1822. At this section of the subduction zone, in addition to the impinging Juan Fernandez Ridge, the slab geometry changes from steeply dipping south of the Illapel earthquake to a nearly horizontal dip adjacent to the event. Combining these various observations provides insight into the links between regional tectonics and the timing and distribution of megathrust earthquakes at this segment of the central Chilean subduction zone.

Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

USGS Publications Warehouse

Hacker, B.R.; Andersen, T.B.; Root, D.B.; Mehl, L.; Mattinson, J.M.; Wooden, J.L.

2003-01-01

The Solund-Hyllestad-Lavik area affords an excellent opportunity to understand the ultrahigh-pressure Scandian orogeny because it contains a near-complete record of ophiolite emplacement, high-pressure metamorphism and large-scale extension. In this area, the Upper Allochthon was intruded by the c. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410-400 Ma. Within 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near-ultrahigh-pressure rocks to within c. 3 km vertical distance from the low-grade Solund Conglomerate.

Paleothermal structure of the Nankai inner accretionary wedge estimated from vitrinite reflectance of cuttings

NASA Astrophysics Data System (ADS)

Fukuchi, Rina; Yamaguchi, Asuka; Yamamoto, Yuzuru; Ashi, Juichiro

2017-08-01

The paleothermal structure and tectonic evolution of an accretionary prism is basic information for understanding subduction zone seismogenesis. To evaluate the entire paleotemperature profile of the Integrated Ocean Drilling Program (IODP) Site C0002 located in the off-Kumano region of the Nankai Trough and penetrate the inner accretionary wedge down to 3058.5 m below the seafloor (mbsf), we performed a vitrinite reflectance analysis for cuttings and core samples during IODP expeditions 338 and 348: Nankai Trough seismogenic zone experiment. Although vitrinite reflectance values (Ro) tend to increase with depth, two reversals of these values suggested the existence of thrust fault zones with sufficient displacements to offset the paleothermal structure. The estimated maximum paleotemperatures are 42-70°C at 1200-1300 mbsf, 44-100°C at 1600-2400 mbsf, and 56-115°C at 2600-3000 mbsf, respectively. These temperatures roughly coincide with estimated modern temperatures; however, at a smaller scale, the reconstructed partial paleogeothermal gradient (˜60-150°C/km) recorded at the hanging- and footwall of the presumed thrust fault zone is higher than the modern geothermal gradient (˜30-40°C/km). This high paleogeothermal gradient was possibly obtained prior to subduction, reflecting the large heat flow of the young Philippine Sea Plate.

Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

USGS Publications Warehouse

Choy, G.L.; Kirby, S.H.

2004-01-01

The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (?? a = ??Es/Mo, where E s is radiated energy and Mo is seismic moment) of all globally distributed shallow (depth, ?? 1 MPa) are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower ??a (< 1 MPa) is associated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average ?? a of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average ??a with focal mechanism and tectonic regime suggests that the level of ?? a is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high ??a oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which Mc (an energy magnitude) is shown to be a far better estimate of the potential for earthquake damage than Mw. This discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate-thrust events that represented much higher total moment release. ?? 2004 RAS.

Seismicity of the Earth 1900–2010 Middle East and vicinity

USGS Publications Warehouse

Jenkins, Jennifer; Turner, Bethan; Turner, Rebecca; Hayes, Gavin P.; Davies, Sian; Dart, Richard L.; Tarr, Arthur C.; Villaseñor, Antonio; Benz, Harley M.

2013-01-01

No fewer than four major tectonic plates (Arabia, Eurasia, India, and Africa) and one smaller tectonic block (Anatolia) are responsible for seismicity and tectonics in the Middle East and surrounding region. Geologic development of the region is a consequence of a number of first-order plate tectonic processes that include subduction, large-scale transform faulting, compressional mountain building, and crustal extension. In the east, tectonics are dominated by the collision of the India plate with Eurasia, driving the uplift of the Himalaya, Karakorum, Pamir and Hindu Kush mountain ranges. Beneath the Pamir‒Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Along the western margin of the India plate, relative motions between India and Eurasia are accommodated by strike-slip, reverse, and oblique-slip faulting, resulting in the complex Sulaiman Range fold and thrust belt, and the major translational Chaman Fault in Afghanistan. Off the south coasts of Pakistan and Iran, the Makran trench is the surface expression of active subduction of the Arabia plate beneath Eurasia. Northwest of this subduction zone, collision between the two plates forms the approximately 1,500-km-long fold and thrust belts of the Zagros Mountains, which cross the whole of western Iran and extend into northeastern Iraq. Tectonics in the eastern Mediterranean region are dominated by complex interactions between the Africa, Arabia, and Eurasia plates, and the Anatolia block. Dominant structures in this region include: the Red Sea Rift, the spreading center between the Africa and Arabia plates; the Dead Sea Transform, a major strike-slip fault, also accommodating Africa-Arabia relative motions; the North Anatolia Fault, a right-lateral strike-slip structure in northern Turkey accommodating much of the translational motion of the Anatolia block westwards with respect to Eurasia and Africa; and the Cyprian Arc, a convergent boundary between the Africa plate to the south, and Anatolia Block to the north.

Subduction Initiation under Unfavorable Conditions and New Fault Formation

NASA Astrophysics Data System (ADS)

Mao, X.; Gurnis, M.; May, D.

2017-12-01

How subduction initiates with unfavorable dipping lithospheric heterogeneities is an important and rarely studied topic. We build a geodynamic model starting with a vertical weak zone for the Puysegur incipient subduction zone (PISZ). A true free surface is tracked in pTatin3D, based on the Arbitrary Lagrangian Eulerian (ALE) finite element method, and is used to follow the dynamic mantle-surface interaction and topographic evolution. A simplified surface process, based on linear topography diffusion, is implemented. Density and free water content for different phase assemblages are gained by referring to precalculated 4D (temperature, pressure, rock type and total water content) phase maps using Perplex. Darcy's law is used to migrate free water, and a linear water weakening is applied to the mantle material. A new visco-elastic formulation called Elastic Viscous Stress Splitting (EVSS) method is also included. Our predictions fit the morphology of the Puysegur Trench and Ridge and the deformation history on the overriding plate. We show a new thrust fault forms and evolves into a smooth subduction interface, and the preexisting weak zone becomes a vertical fault inboard of the thrust fault during subduction initiation, which explains the two-fault system at PISZ. Our model suggests that the PISZ may not yet be self-sustaining. We propose that the Snares Trough is caused by plate coupling differences between shallower and deeper parts, the tectonic sliver between two faults experiences strong rotation, and low density materials accumulate beneath the Snares trough. Extended models show that with favorable dipping heterogeneities, no new fault forms, and subduction initiates with smaller resisting forces.

Stress interaction between subduction earthquakes and forearc strike-slip faults: Modeling and application to the northern Caribbean plate boundary

USGS Publications Warehouse

ten Brink, Uri S.; Lin, J.

2004-01-01

Strike-slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers. We explore the interaction between thrust events on the subduction interface and strike-slip faults within the forearc region using three-dimensional models of static Coulomb stress change. Model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the Coulomb stress on strike-slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region. In contrast, subduction events with slip vectors perpendicular to the trench axis enhance the Coulomb stress on strike-slip faults farther back in the forearc, while reducing the stress adjacent to the trench. A significant contribution to Coulomb stress increase on strike-slip faults in the back region of the forearc comes from "unclamping" of the fault, i.e., reduction in normal stress due to thrust motion on the subduction interface. We argue that although Coulomb stress changes from individual subduction earthquakes are ephemeral, their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant. We use the Coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the Caribbean subduction zone. Subduction earthquakes with slip vectors nearly perpendicular to the Caribbean trench axis is dominant in the Hispaniola segment, where the strike-slip faults are more than 60 km inland from the trench. In contrast, subduction slip motion is nearly parallel to the Caribbean trench axis along the Puerto Rico segment, where the strike-slip fault is less than 15 km from the trench. This observed jump from a strike-slip fault close to the trench axis in the Puerto Rico segment to the inland faults in Hispaniola is explained by different distributions of Coulomb stress in the forearc region of the two segments, as a result of the change from the nearly trench parallel slip on the Puerto Rico subduction interface to the more perpendicular subduction slip beneath Hispaniola. The observations and modeling suggest that subduction-induced strike-slip seismic hazard to Puerto Rico may be smaller than previously assumed but the hazard to Hispaniola remains high. Copyright 2004 by the American Geophysical Union.

Two-dimensional Numerical Models of Accretionary Wedges Deformation in Response to Subduction and Obduction: Evidence from the Middle Part of the Manila Trench

NASA Astrophysics Data System (ADS)

Ma, L.; Ding, W.; Chen, L.; Gerya, T.

2016-12-01

The Manila Trench is located at the eastern boundary of the South China Sea (SCS). It was created by the subduction of the South China Sea Plate beneath the Philippine Sea Plate since the early Neogene, and also influenced by the northwestern movement of the Philippine Sea Plate. There is wide discussion whether the dual-subduction and widespread seamounts in the South China Sea would have play important roles in the 'S-shaped' geometry and the different diving angle along the Manila Trench. Multi-beam tectono-geomorphological studies on the accretionary wedges have suggested that: (1) the stress direction of the subduction along the middle part of the Manila Trench, between 17o and 18 o N, is NW55 o; (2) The Manila Trench is actually caused by obduction due to the northwestern movement of the Philippine Sea Plate. Although the NW 55 o stress direction has been supported by detailed analysis on the trend of the folds, thrust faults, extension fractures and large sea-floor canyon, its obduction-origin is purely based on regional structure. Here we use 2D numerical modeling experiments to investigate the deformation style of accretionary wedge in response to the seamounts subduction and obduction, and provide new insights into the mechanism responsible for the Luzon obduction along the Manila Trench. Our preliminary results show that: (1) the accretionary wedge is eroded faster in subduction model; (2) the velocity field direction of the slab differs in two models at the beginning of seamount subduction, which is vertical in obduction model, but oblique in subduction model; (3) both sides of the accretionary wedge deform strongly in subduction model, whereas in obduction model only the leading edge shows intensive deformation. Further modelling will focus on other parts of the Manila Trench with different slab age and subduction velocity to see their tectonic influences on the accretionary wedges.

Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface.

PubMed

Nedimović, Mladen R; Hyndman, Roy D; Ramachandran, Kumar; Spence, George D

2003-07-24

At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm x yr(-1) (ref. 1), resulting in the potential for destructive great earthquakes. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700) occurred before instrumental recording. Thermal and deformation studies indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of approximately 60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9) have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.

Seafloor Structural Geomorphic Evolution in Response to Seamount Subduction, Poverty Bay Indentation, New Zealand

NASA Astrophysics Data System (ADS)

Bodger, K. L.; Pettinga, J. R.; Barnes, P. M.

2006-12-01

More than 4000 km2 of high quality bathymetric and backscatter imaging of the Poverty Bay Indentation across the northern part of the Hikurangi subduction zone provide new insights into the relationship between seafloor morphology and active structures. The swath bathymetry extends from the edge of the continental shelf to the abyssal plain, at depths of between 100 to 3500 metres. The origin of the slope re-entrant is inferred to be related to multiple seamount impacts, and these collisions have initiated numerous large-scale gravitational collapse structures, multiple debris flow and avalanche deposits, which range in down-slope length from a few hundred metres to more than 40 km. The Poverty Bay Indentation has been simultaneously eroded by canyon systems that exhibit many of the features of incised river systems onshore. The swath images are complemented by the availability of excellent high-quality processed multi-channel seismic reflection data, single channel high-resolution 3.5 kHz seismic reflection data, as well as a limited number of core samples. Seismic reflection profiles and seafloor morphology are used to provide three morpho-structural sections. The comparison of these sections highlights the different effects of seamount subduction on the evolution of the margin and the re-entrant. The northern two sections are located to the north side of the re-entrant and reveal the role of seamount impact on the interrelationship between the structural evolution with respect to seafloor morphology. Here the development of an over-steepened margin with fault reactivation, inversion and over- printing leads to very complex structural styles of deformation and geometry in both seismic reflection profiles and seafloor morphology. There is evidence of an older, inactive thrust front buried beneath the upper and mid- slope basins. Beneath the mid-slope a subducted seamount is revealed by the presence of relief on the subduction interface and associated structural complexity in the over-riding wedge. The Poverty Bay canyon represents a structural transition zone coinciding with the re-entrant. The accretionary slope south of the re- entrant conforms more closely to the classic accretionary slope style of deformation. Backthrusts in this section propagate from a much shallower level than in the northern sections. Inversion is commonly observed in the mid slope and continental shelf basins, particularly to the south. Initial interpretations indicate that: i) seamount impact significantly influences the structural evolution, and submarine geomorphology of the inboard slope of the Hikurangi subduction zone, including the generation of large-scale gravitational collapse features; ii) the large gully systems located at the upper shelf slope boundary represent the most likely source areas for the multiple mega debris flows recognised from seafloor morphology and in seismic sections; iii) there exists a complex interaction between the evolving thrust-driven submarine ridges, ponded slope basins and the structural geometry and evolution of the near-surface fault zones (imbrication); iv) the submarine canyons may initiate complex patterns of fault zone segmentation and displacement transfer within the accretionary slope; and v) seamount subduction and subsequent instability of the margin may directly result in tsunami generation.

Potential Seismic Signatures of Megathrust Preparatory Zones

NASA Astrophysics Data System (ADS)

Parameswaran, R. M.; Maheswari, K.; Rajendran, K.

2017-12-01

The Mw 9.2, 2004 Sumatra earthquake awakened the otherwise inactive Andaman-Sumatra subduction zone (ASSZ), pushing it into an era of amplified seismicity. The subduction zone has since witnessed an array of inter- and intra-plate events along and around its trench. Several intra-plate events like the 2012 Wharton Basin earthquakes (Mw 8.6 and 8.2), are believed to be the triggered response of the plateward transmission of stresses due to the 2004 earthquake (Ishii et al., 2013). On the other hand, the Mw 7.5, 2009 33-km-deep intra-plate normal-faulting event in the northern Andaman segment is an example of outer-rise seismicity resulting from the post-seismic relaxation of the subducting slab (Andrade and Rajendran, 2011). These are aftermaths of a drastic change in the stress regime from compressional to extensional, following the 2004 megathrust event. But, pre-megathrust, aside from the inter-plate thrust mechanisms that are widely observed along the trench, how does the plate-motion-driven compression manifest in the regional seismicity? What happens to the stresses accumulating within the bending subducting slab; does it source deeper compressional events prior to a megathrust? The 2009 normal outer-rise earthquake was preceded by the 13 September 2002, Mw 6.5 Diglipur outer-rise thrust earthquake (22 km depth), both occurring at the northern terminus of the 2004-rupture, in the compressing forearc that experienced surface uplift pre-megathrust (Rajendran et al., 2003; Rajendran et al., 2007). This work, therefore, examines the slip models of such pre-event outer-rise thrust earthquakes along the stretch of the 2004 rupture zone in the ASSZ. The work is also being extended to understand the preparatory zones of other global megathrust earthquakes.

Localized fluid discharge in subduction zones: Insights from tension veins around an ancient megasplay fault (Nobeoka Thrust, SW Japan)

NASA Astrophysics Data System (ADS)

Otsubo, M.; Hardebeck, J.; Miyakawa, A.; Yamaguchi, A.; Kimura, G.

2017-12-01

Fluid-rock interactions along seismogenic faults are of great importance to understand fault mechanics. The fluid loss by the formation of mode I cracks (tension cracks) increases the fault strength and creates drainage asperities along the plate interface (Sibson, 2013, Tectonophysics). The Nobeoka Thrust, in southwestern Japan, is an on-land example of an ancient megasplay fault and provides an excellent record of deformation and fluid flow at seismogenic depths of a subduction zone (Kondo et al., 2005, Tectonics). We focus on (1) Pore fluid pressure loss, (2) Amount of fault strength recovery, and (3) Fluid circulation by the formation of mode I cracks in the post-seismic period around the fault zone of the Nobeoka Thrust. Many quartz veins that filled mode I crack at the coastal outcrops suggest a normal faulting stress regime after faulting of the Nobeoka Thrust (Otsubo et al., 2016, Island Arc). We estimated the decrease of the pore fluid pressure by the formation of the mode I cracks around the Nobeoka Thrust in the post-seismic period. When the pore fluid pressure exceeds σ3, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997, Jour. Struct. Geol.). We call the pore fluid pressure that exceeds σ3 "pore fluid over pressure". The differential stress in the post-seismic period and the driving pore fluid pressure ratio P* (P* = (Pf - σ3) / (σ1 - σ3), Pf: pore fluid pressure) are parameters to estimate the pore fluid over pressure. In the case of the Nobeoka Thrust (P* = 0.4, Otsubo et al., 2016, Island Arc), the pore fluid over pressure is up to 20 MPa (assuming tensile strength = 10 MPa). 20 MPa is equivalent to <10% of the total pore fluid pressure around the Nobeoka Thrust (depth = 10 km, density = 2.7 kg/m3). When the pore fluid pressure decreases by 4%, the normalized pore pressure ratio λ* (λ* = (Pf - Ph) / (Pl - Ph), Pl: lithostatic pressure; Ph: hydrostatic pressure) changes from 0.95 to 0.86. In the case of the Nobeoka Thrust, the fault strength can increase by up to 10 MPa (assuming frictional coefficient = 0.6). 10 MPa is almost equivalent to the stress drop values in large trench type earthquakes. Hence, we suggest that the fluid loss caused by the formation of mode I cracks in the post-seismic period may play an important role by increasing frictional strength along the megasplay fault.

The 1945 Balochistan earthquake and probabilistic tsunami hazard assessment for the Makran subduction zone

NASA Astrophysics Data System (ADS)

Höchner, Andreas; Babeyko, Andrey; Zamora, Natalia

2014-05-01

Iran and Pakistan are countries quite frequently affected by destructive earthquakes. For instance, the magnitude 6.6 Bam earthquake in 2003 in Iran with about 30'000 casualties, or the magnitude 7.6 Kashmir earthquake 2005 in Pakistan with about 80'000 casualties. Both events took place inland, but in terms of magnitude, even significantly larger events can be expected to happen offshore, at the Makran subduction zone. This small subduction zone is seismically rather quiescent, but a tsunami caused by a thrust event in 1945 (Balochistan earthquake) led to about 4000 casualties. Nowadays, the coastal regions are more densely populated and vulnerable to similar events. Additionally, some recent publications raise the question of the possiblity of rare but huge magnitude 9 events at the Makran subduction zone. We first model the historic Balochistan event and its effect in terms of coastal wave heights, and then generate various synthetic earthquake and tsunami catalogs including the possibility of large events in order to asses the tsunami hazard at the affected coastal regions. Finally, we show how an effective tsunami early warning could be achieved by the use of an array of high-precision real-time GNSS (Global Navigation Satellite System) receivers along the coast.

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

PubMed

Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

1999-09-01

Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin.

A Hydrous Seismogenic Fault Rock Indicating A Coupled Lubrication Mechanism

NASA Astrophysics Data System (ADS)

Okamoto, S.; Kimura, G.; Takizawa, S.; Yamaguchi, H.

2005-12-01

In the seismogenic subduction zone, the predominant mechanisms have been considered to be fluid induced weakening mechanisms without frictional melting because the subduction zone is fundamentally quite hydrous under low temperature conditions. However, recently geological evidence of frictional melting has been increasingly reported from several ancient accretionary prisms uplifted from seismogenic depths of subduction zones (Ikesawa et al., 2003; Austrheim and Andersen, 2004; Rowe et al., 2004; Kitamura et al., 2005) but relationship between conflicting mechanisms; e.g. thermal pressurization of fluid and frictional melting is still unclear. We found a new exposure of pseudotachylyte from a fossilized out-of-sequence thrust (OOST) , Nobeoka thrust in the accretionary complex, Kyushu, southwest Japan. Hanging-wall and foot-wall are experienced heating up to maximum temperature of about 320/deg and about 250/deg, respectively. Hanging-wall rocks of the thrust are composed of shales and sandstones deformed plastically. Foot-wall rocks are composed of shale matrix melange with sandstone and basaltic blocks deformed in a brittle fashion (Kondo et al, 2005). The psudotachylyte was found from one of the subsidiary faults in the hanging wall at about 10 m above the fault core of the Nobeoka thrust. The fault is about 1mm in width, and planer rupture surface. The fault maintains only one-time slip event because several slip surfaces and overlapped slip textures are not identified. The fault shows three deformation stages: The first is plastic deformation of phyllitic host rocks; the second is asymmetric cracking formed especially in the foot-wall of the fault. The cracks are filled by implosion breccia hosted by fine carbonate minerals; the third is frictional melting producing pseudotachylyte. Implosion breccia with cracking suggests that thermal pressurization of fluid and hydro-fracturing proceeded frictional melting.

Reducing risk where tectonic plates collide

USGS Publications Warehouse

Gomberg, Joan S.; Ludwig, Kristin A.

2017-06-19

Most of the world’s earthquakes, tsunamis, landslides, and volcanic eruptions are caused by the continuous motions of the many tectonic plates that make up the Earth’s outer shell. The most powerful of these natural hazards occur in subduction zones, where two plates collide and one is thrust beneath another. The U.S. Geological Survey’s (USGS) “Reducing Risk Where Tectonic Plates Collide—A USGS Plan to Advance Subduction Zone Science” is a blueprint for building the crucial scientific foundation needed to inform the policies and practices that can make our Nation more resilient to subduction zone-related hazards.

Slab1.0: A three-dimensional model of global subduction zone geometries

NASA Astrophysics Data System (ADS)

Hayes, Gavin P.; Wald, David J.; Johnson, Rebecca L.

2012-01-01

We describe and present a new model of global subduction zone geometries, called Slab1.0. An extension of previous efforts to constrain the two-dimensional non-planar geometry of subduction zones around the focus of large earthquakes, Slab1.0 describes the detailed, non-planar, three-dimensional geometry of approximately 85% of subduction zones worldwide. While the model focuses on the detailed form of each slab from their trenches through the seismogenic zone, where it combines data sets from active source and passive seismology, it also continues to the limits of their seismic extent in the upper-mid mantle, providing a uniform approach to the definition of the entire seismically active slab geometry. Examples are shown for two well-constrained global locations; models for many other regions are available and can be freely downloaded in several formats from our new Slab1.0 website, http://on.doi.gov/d9ARbS. We describe improvements in our two-dimensional geometry constraint inversion, including the use of `average' active source seismic data profiles in the shallow trench regions where data are otherwise lacking, derived from the interpolation between other active source seismic data along-strike in the same subduction zone. We include several analyses of the uncertainty and robustness of our three-dimensional interpolation methods. In addition, we use the filtered, subduction-related earthquake data sets compiled to build Slab1.0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far, concluding that the width of these seismogenic zones is on average 30% larger than previous studies have suggested.

Impact of great subduction earthquakes on the long-term forearc morphology, insight from mechanical modelling

NASA Astrophysics Data System (ADS)

Cubas, Nadaya

2017-04-01

The surge of great subduction earthquakes during the last fifteen years provided numerous observations requiring revisiting our understanding of large seismic events mechanics. For instance, we now have clear evidence that a significant part of the upper plate deformation is permanently acquired. The link between great earthquakes and long-term deformation offers a new perspective for the relief construction understanding. In addition, a better understanding of these relations could provide us with new constraints on earthquake mechanics. It is also of fundamental importance for seismic risk assessment. In this presentation, I will compile recent results obtained from mechanical modelling linking megathrust ruptures with upper-plate permanent deformation and discuss their impact. We will first show that, in good accordance with lab experiments, aseismic zones are characterized by frictions larger or equal to 0.1 whereas seismic asperities have dynamic frictions lower than 0.05. This difference will control the long-term upper-plate morphology. The larger values along aseismic zones allow the wedge to reach the critical state, and will lead to active thrust systems forming a relief. On the contrary, low dynamic friction along seismic asperities will place the taper in the sub-critical domain impeding any internal deformation. This will lead to the formation of forearc basins inducing negative gravity anomalies. Since aseismic zones have higher friction and larger taper, fully creeping segments will tend to develop peninsulas. On the contrary, fully locked segments with low dynamic friction and very low taper will favor subsiding coasts. The taper variation due to megathrust friction is also expressed through a correlation between coast-to-trench distance and forearc coupling (e.g., Mexican and South-American subduction zones). We will then discuss how variations of frictional properties along the megathrust can induce splay fault activation. For instance, we can reactivate normal faults at the down-dip limit of the seismogenic zone or at an increasing slip transition (e.g., Chile and Japan). Finally, we will show that the fault vergence is controlled by the frictional properties. Sudden and successive decreases of the megathrust effective friction during frontal propagation of earthquakes will lead to the formation of landward-vergent frontal thrusts in the accretionary prism. Therefore, a particular attention needs to be paid to accretionary prisms with normal faults implying large up-dip ruptures (e.g., Alaska and Japan) or with frontal landward-vergent thrust faults, markers of past seafloor coseismic ruptures leading to very large tsunamis (e.g., Cascadia and Sumatra). If the forearc long-term deformation seems in good accordance with our understanding of earthquake mechanics, recent studies have pointed to a major discrepancy between short- and long-term deformation at the coast (i.e., the Central Andes subduction zone). An analogue discrepancy has been pointed out for the Himalaya after the 2015 Mw 7.8 Gorkha earthquake. Melnick (2016) proposed that the coastal long-term deformation could be related to deep and less frequent earthquakes instead of standard subduction events. It is now of fundamental importance to understand the link between the coastal long-term record and the short-term deformation for seismic risk assessment and relief building processes understanding. It will probably constitute the next challenge for mechanical modelling.

Rupture Dynamics along Thrust Dipping Fault: Inertia Effects due to Free Surface Wave Interactions

NASA Astrophysics Data System (ADS)

Vilotte, J. P.; Scala, A.; Festa, G.

2017-12-01

We numerically investigate the dynamic interaction between free surface and up-dip, in-plane rupture propagation along thrust faults, under linear slip-weakening friction. With reference to shallow along-dip rupture propagation during large subduction earthquakes, we consider here low dip-angle fault configurations with fixed strength excess and depth-increasing initial stress. In this configuration, the rupture undergoes a break of symmetry with slip-induced normal stress perturbations triggered by the interaction with reflected waves from the free surface. We found that both body-waves - behind the crack front - and surface waves - at the crack front - can trigger inertial effects. When waves interact with the rupture before this latter reaches its asymptotic speed, the rupture can accelerate toward the asymptotic speed faster than in the unbounded symmetric case, as a result of these inertial effects. Moreover, wave interaction at the crack front also affects the slip rate generating large ground motion on the hanging wall. Imposing the same initial normal stress, frictional strength and stress drop while varying the static friction coefficient we found that the break of symmetry makes the rupture dynamics dependent on the absolute value of friction. The higher the friction the stronger the inertial effect both in terms of rupture acceleration and slip amount. When the contact condition allows the fault interface to open close to the free surface, the length of the opening zone is shown to depend on the propagation length, the initial normal stress and the static friction coefficient. These new results are shown to agree with analytical results of rupture propagation in bounded media, and open new perspectives for understanding the shallow rupture of large subduction earthquakes and tsunami sources.

Structural evidence for northeastward movement on the Chocolate Mountains Thrust, southeasternmost California

USGS Publications Warehouse

Dillon, J.T.; Haxel, G.B.; Tosdal, R.M.

1990-01-01

The Late Cretaceous Chocolate Mountains Thrust of southeastern California and southwestern Arizona places a block of Proterozoic and Mesozoic continental crust over the late Mesozoic continental margin oceanic sedimentary and volcanic rocks of the Orocopia Schist. The Chocolate Mountains Thrust is interpreted as a thrust (burial, subduction) fault rather than a low-angle normal fault. An important parameter required to understand the tectonic significance of the Chocolate Mountains and related thrusts is their sense of movement. The only sense of movement consistent with collective asymmetry of the thrust zone folds is top to the northeast. Asymmetric microstructures studied at several localities also indicate top to the northeast movement. Paleomagnetic data suggest that the original sense of thrusting, prior to Neogene vertical axis tectonic rotation related to the San Andreas fault system, was northward. Movement of the upper plate of the chocolate Mountains thrust evidently was continentward. Continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern California. -from Authors

Collision-induced rotation in an arc-continent collision: Constrained by continuous GPS observations in Mindoro, Philippines

NASA Astrophysics Data System (ADS)

Rau, R.; Hung, H.; Yang, C.; Tsai, M.; Ching, K.; Bacolcol, T.; Solidum, R.; Chang, W.

2012-12-01

The Mindoro Island, situated at the southern end of the Manila trench, is a modern arc-continent collision. Seismic activity in Mindoro concentrates mainly in the northern segment of the island as part of the Manila subduction processes; in contrast, seismicity in the middle and the southern parts of the island is rather diffuse. Although the Mindoro Island has been experiencing intense seismic activities and is a type example of arc-continent collision, the modern mode of deformation of the Mindoro collision remains unclear. We have installed eight dual-frequency continuous GPS stations in the island since May 2010. The questions we want to address by using continuous GPS observations are (1) if there are still compressions within the Mindoro collision? Have they ceased as seen by the diffuse seismicity, or are the thrust faults locked? (2) What is the mode of deformation in the Mindoro collision and what are the roles of thrust and strike-slip faults playing in the collision? (3) How does the Mindoro collision compare with the other collision, such as the Taiwan orogen? Do they share similar characteristics for the subduction-collision transition zone? For the results of the first two years GPS measurements, if we take the Sablayan site near the southern end of the Manila trench as the reference station, a large counterclockwise rotation from south to north, with horizontal velocities of 1.9-31.1 mm/yr from 165 to 277 degrees, are found in the island. The deformation of the Mindoro is similar to the pattern of the transition zone from collision to subduction in northeastern Taiwan. This result suggests that collision-induced rotation is occurring in the Mindoro Island and the Mindoro arc-continent collision is still active.

The South Sandwich "Forgotten" Subduction Zone and Tsunami Hazard in the South Atlantic

NASA Astrophysics Data System (ADS)

Okal, E. A.; Hartnady, C. J. H.; Synolakis, C. E.

2009-04-01

While no large interplate thrust earthquakes are know at the "forgotten" South Sandwich subduction zone, historical catalogues include a number of events with reported magnitudes 7 or more. A detailed seismological study of the largest event (27 June 1929; M (G&R) = 8.3) is presented. The earthquake relocates 80 km North of the Northwestern corner of the arc and its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg. respectively). The seismic moment of 1.7*10**28 dyn*cm supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. This event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations were performed using the MOST method [Titov and Synolakis, 1997]. Computed deep-water tsunami amplitudes of 30cm and 20cm were found off the coast of Brazil and along the Gulf of Guinea (Ivory Coast, Ghana) respectively. The 1929 moment was assigned to the geometries of other know earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. Tsunami hydrodynamic simulation of these scenarios revealed strong focusing of tsunami wave energy by the SAR, the SWIOR and the Agulhas Rise, in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

South Sandwich: The Forgotten Subduction Zone and Tsunami Hazard in the South Atlantic

NASA Astrophysics Data System (ADS)

Okal, E. A.; Hartnady, C. J.

2008-12-01

While no large interplate thrust earthquakes are known at the South Sandwich subduction zone, historical catalogues include a number of earthquakes with reported magnitudes of 7 or more. We present a detailed seismological study of the largest one (27 June 1929; M (G&R) = 8.3). The earthquake relocates 80 km North of the Northwestern corner of the arc. Its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg.). The seismic moment, 1.7 10**28 dyn*cm, supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. The 1929 event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations using the MOST method [Titov and Synolakis, 1997] suggest deep-water tsunami amplitudes reaching 30 cm off the coast of Brazil, where it should have had observable run-up, and 20 cm along the Gulf of Guinea (Ivory Coast, Ghana). We also simulate a number of potential sources obtained by assigning the 1929 moment to the geometries of other known earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. A common feature of these models is the strong focusing of tsunami waves by the SAR, the SWIOR, and the Agulhas Rise, resulting in amplitudes always enhanced in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

Multi-Channel Seismic Images of the Mariana Forearc: EW0202 Initial Results

NASA Astrophysics Data System (ADS)

Oakley, A. J.; Goodliffe, A. M.; Taylor, B.; Moore, G. F.; Fryer, P.

2002-12-01

During the Spring of 2002, the Mariana Subduction Factory was surveyed using multi-channel seismics (MCS) as the first major phase of a US-Japanese collaborative NSF-MARGINS funded project. The resulting geophysical transects extend from the Pacific Plate to the West Mariana remnant arc. For details of this survey, including the results from the back-arc, refer to Taylor et al. (this session). The incoming Pacific Plate and its accompanying seamounts are deformed by plate flexure, resulting in extension of the upper crust as it enters the subduction zone. The resultant trench parallel faults dominate the bathymetry and MCS data. Beneath the forearc, in the southern transects near Saipan, the subducting slab is imaged to a distance of 50-60 km arcward. In addition to ubiquitous trench parallel normal faulting, a N-S transect of the forearc clearly shows normal faults perpendicular to the trench resulting from N-S extension. On the east side of the Mariana Ridge, thick sediment packages extend into the forearc. Directly east of Saipan and Tinian, a large, deeply scouring slide mass is imaged. Several serpentine mud volcanoes (Big Blue, Turquoise and Celestial) were imaged on the Mariana Forearc. Deep horizontal reflectors (likely original forearc crust) are imaged under the flanks of some of these seamounts. A possible "throat" reflector is resolved on multiple profiles at the summit of Big Blue, the northern-most seamount in the study area. The flanks of Turquoise seamount terminate in toe thrusts that represent uplift and rotation of surrounding sediments as the volcano grows outward. These thrusts form a basal ridge around the seamount similar to that previously noted encircling Conical Seamount. Furthermore, MCS data has revealed that some forearc highs previously thought to be fault blocks are in actuality mud volcanoes.

Sandbox Simulations of the Evolution of a Subduction Wedge following Subduction Initiation

NASA Astrophysics Data System (ADS)

Brandon, M. T.; Ma, K. F.; DeWolf, W.

2012-12-01

Subduction wedges at accreting subduction zones are bounded by a landward dipping pro-shear zone (= subduction thrust) and a seaward-dipping retro-shear zone in the overriding plate. For the Cascadia subduction zone, the surface trace of the retro-shear zone corresponds to the east side of the Coast Ranges of Oregon and Washington and the Insular Mountains of Vancouver Island. This coastal high or forearc high shows clear evidence of long-term uplift and erosion along its entire length, indicating that it is an active part of the Cascadia subduction wedge. The question addressed here is what controls the location of the retro-shear zone? In the popular double-sided wedge model of Willet et al (Geology 1993), the retro-shear zone remains pinned to the S point, which is interpreted to represent where the upper-plate Moho intersects the subduction zone. For this interpretation, the relatively strong mantle is considered to operate as a flat backstop. That model, however. is somewhat artificial in that the two plates collide in a symmetric fashion with equal crustal thicknesses on both sides. Using sandbox experiments, we explore a more realistic configuration where the upper and lower plate are separated by a gentle dipping (10 degree) pro-shear zone, to simulate the initial asymmetric geometry of the subduction thrust immediately after initiation of subduction. The entire lithosphere must fail along some plane for subduction to begin and this failure plane must dip in the direction of subduction. Thus, the initial geometry of the overriding plate is better approximated as a tapered wedge than as a layer of uniform thickness, as represented in the Willett et al models. We demonstrate this model using time-lapse movies of a sand wedge above a mylar subducting plate. We use particle image velocimetry (PIV) to show the evolution of strain and structure within the overriding plate. Material accreted to the tapered end of the overriding plate drives deformation and causes the retro-shear zone to propagate rearward with time. The main conclusion is that the rearward propagation will cease only when 1) the retro shear zone reaches the S point (i.e. the mantle cutoff in the upper plate) or 2) the erosion outflux from the subduction wedge matches the accretionary influx. Given the location of the upper plate Moho at Cascadia, it seems that erosion is the control factor in pinning the retro shear zone there.

Structure and tectonic evolution of the southwestern Trinidad dome, Escambray complex, Central Cuba: Insights into deformation in an accretionary wedge

NASA Astrophysics Data System (ADS)

Despaigne-Díaz, Ana Ibis; García Casco, Antonio; Cáceres Govea, Dámaso; Wilde, Simon A.; Millán Trujillo, Guillermo

2017-10-01

The Trinidad dome, Escambray complex, Cuba, forms part of an accretionary wedge built during intra-oceanic subduction in the Caribbean from the Late Cretaceous to Cenozoic. The structure reflects syn-subduction exhumation during thickening of the wedge, followed by extension. Field mapping, metamorphic and structural analysis constrain the tectonic evolution into five stages. Three ductile deformation events (D1, D2 and D3) are related to metamorphism in a compressional setting and formation of several nappes. D1 subduction fabrics are only preserved as relict S1 foliation and rootless isoclinal folds strongly overprinted by the main S2 foliation. The S2 foliation is parallel to sheared serpentinised lenses that define tectonic contacts, suggesting thrust stacks and underthrusting at mantle depths. Thrusting caused an inverted metamorphic structure with higher-grade on top of lower-grade nappes. Exhumation started during D2 when the units were incorporated into the growing accretionary wedge along NNE-directed thrust faults and was accompanied by substantial decompression and cooling. Folding and thrusting continued during D3 and marks the transition from ductile to brittle-ductile conditions at shallower crustal levels. The D4-5 events are related to extension and contributed to the final exhumation (likely as a core complex). D4 is associated with a regional spaced S4 cleavage, late open folds, and numerous extension veins, whereas D5 is recorded by normal and strike-slip faults affecting all nappes. The P-t path shows rapid exhumation during D2 and slower rates during D3 when the units were progressively incorporated into the accretionary prism. The domal shape formed in response to tectonic denudation assisted by normal faulting and erosion at the surface during the final stages of structural development. These results support tectonic models of SW subduction of the Proto-Caribbean crust under the Caribbean plate during the latest Cretaceous and provide insights into the tectonic evolution of accretionary wedges in an intra-arc setting.

Seismicity associated with the Sumatra-Andaman Islands earthquake of 26 December 2004

USGS Publications Warehouse

Dewey, J.W.; Choy, G.; Presgrave, B.; Sipkin, S.; Tarr, A.C.; Benz, H.; Earle, P.; Wald, D.

2007-01-01

The U.S. Geological Survey/National Earthquake Information Center (USGS/ NEIC) had computed origins for 5000 earthquakes in the Sumatra-Andaman Islands region in the first 36 weeks after the Sumatra-Andaman Islands mainshock of 26 December 2004. The cataloging of earthquakes of mb (USGS) 5.1 and larger is essentially complete for the time period except for the first half-day following the 26 December mainshock, a period of about two hours following the Nias earthquake of 28 March 2005, and occasionally during the Andaman Sea swarm of 26-30 January 2005. Moderate and larger (mb ???5.5) aftershocks are absent from most of the deep interplate thrust faults of the segments of the Sumatra-Andaman Islands subduction zone on which the 26 December mainshock occurred, which probably reflects nearly complete release of elastic strain on the seismogenic interplate-thrust during the mainshock. An exceptional thrust-fault source offshore of Banda Aceh may represent a segment of the interplate thrust that was bypassed during the mainshock. The 26 December mainshock triggered a high level of aftershock activity near the axis of the Sunda trench and the leading edge of the overthrust Burma plate. Much near-trench activity is intraplate activity within the subducting plate, but some shallow-focus, near-trench, reverse-fault earthquakes may represent an unusual seismogenic release of interplate compressional stress near the tip of the overriding plate. The interplate-thrust Nias earthquake of 28 March 2005, in contrast to the 26 December aftershock sequence, was followed by many interplate-thrust aftershocks along the length of its inferred rupture zone.

The influence of tectonic inheritance on crustal extension style following failed subduction of continental crust: applications to metamorphic core complexes in Papua New Guinea

NASA Astrophysics Data System (ADS)

Biemiller, J.; Ellis, S. M.; Little, T.; Mizera, M.; Wallace, L. M.; Lavier, L.

2017-12-01

The structural, mechanical and geometric evolution of rifted continental crust depends on the lithospheric conditions in the region prior to the onset of extension. In areas where tectonic activity preceded rift initiation, structural and physical properties of the previous tectonic regime may be inherited by the rift and influence its development. Many continental rifts form and exhume metamorphic core complexes (MCCs), coherent exposures of deep crustal rocks which typically surface as arched or domed structures. MCCs are exhumed in regions where the faulted upper crust is displaced laterally from upwelling ductile material along a weak detachment fault. Some MCCs form during extensional inversion of a subduction thrust following failed subduction of continental crust, but the degree to which lithospheric conditions inherited from the preceding subduction phase control the extensional style in these systems remains unclear. For example, the Dayman Dome in Southeastern Papua New Guinea exposes prehnite-pumpellyite to greenschist facies rocks in a smooth 3 km-high dome exhumed with at least 24 km of slip along one main detachment normal fault, the Mai'iu Fault, which dips 21° at the surface. The extension driving this exhumation is associated with the cessation of northward subduction of Australian continental crust beneath the oceanic lithosphere of the Woodlark Plate. We use geodynamic models to explore the effect of pre-existing crustal structures inherited from the preceding subduction phase on the style of rifting. We show that different geometries and strengths of inherited subduction shear zones predict three distinct modes of subsequent rift development: 1) symmetric rifting by newly formed high-angle normal faults; 2) asymmetric rifting along a weak low-angle detachment fault extending from the surface to the brittle-ductile transition; and 3) extension along a rolling-hinge structure which exhumes deep crustal rocks in coherent rounded exposures. We propose the latter mode as an exhumation model for Dayman Dome and compare the model predictions to regional geophysical and geological evidence. Our models find that tectonically inherited subduction structures may strongly control subsequent extension style when the subduction thrust is weak and well-oriented for reactivation.

The Story of a Yakima Fold and How It Informs Late Neogene and Quaternary Backarc Deformation in the Cascadia Subduction Zone, Manastash Anticline, Washington, USA

NASA Astrophysics Data System (ADS)

Kelsey, Harvey M.; Ladinsky, Tyler C.; Staisch, Lydia; Sherrod, Brian L.; Blakely, Richard J.; Pratt, Thomas L.; Stephenson, William J.; Odum, Jack K.; Wan, Elmira

2017-10-01

The Yakima folds of central Washington, USA, are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line- and area-balanced cross sections, the crust has horizontally shortened by 11% (0.8-0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west-northwest striking frontal thrust faults and shorter north to northeast striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault-cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west trending Yakima folds and west striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low strain rate environment in the backarc of the Cascadia Subduction Zone.

The story of a Yakima fold and how it informs Late Neogene and Quaternary backarc deformation in the Cascadia subduction zone, Manastash anticline, Washington, USA

USGS Publications Warehouse

Kelsey, Harvey M.; Ladinsky, Tyler C.; Staisch, Lydia; Sherrod, Brian; Blakely, Richard J.; Pratt, Thomas; Stephenson, William; Odum, Jackson K.; Wan, Elmira

2017-01-01

The Yakima folds of central Washington, USA, are prominent anticlines that are the primary tectonic features of the backarc of the northern Cascadia subduction zone. What accounts for their topographic expression and how much strain do they accommodate and over what time period? We investigate Manastash anticline, a north vergent fault propagation fold typical of structures in the fold province. From retrodeformation of line- and area-balanced cross sections, the crust has horizontally shortened by 11% (0.8–0.9 km). The fold, and by inference all other folds in the fold province, formed no earlier than 15.6 Ma as they developed on a landscape that was reset to negligible relief following voluminous outpouring of Grande Ronde Basalt. Deformation is accommodated on two fault sets including west-northwest striking frontal thrust faults and shorter north to northeast striking faults. The frontal thrust fault system is active with late Quaternary scarps at the base of the range front. The fault-cored Manastash anticline terminates to the east at the Naneum anticline and fault; activity on the north trending Naneum structures predates emplacement of the Grande Ronde Basalt. The west trending Yakima folds and west striking thrust faults, the shorter north to northeast striking faults, and the Naneum fault together constitute the tectonic structures that accommodate deformation in the low strain rate environment in the backarc of the Cascadia Subduction Zone.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana

NASA Astrophysics Data System (ADS)

Skipp, Betty

1987-03-01

The Clearwater orogenic zone in central Idaho and western Montana contains at least two major northeast-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the western continental margin produced a younger northern Bitterroot lobe of the Idaho batholith relative to an older southern Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.

Using the Vertical Component of the Surface Velocity Field to Map the Locked Zone at Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Moulas, E.; Brandon, M. T.; Podladchikov, Y.; Bennett, R. A.

2014-12-01

At present, our understanding of the locked zone at Cascadia subduction zone is based on thermal modeling and elastic modeling of horizontal GPS velocities. The thermal model by Hyndman and Wang (1995) provided a first-order assessment of where the subduction thrust might be cold enough for stick-slip behavior. The alternative approach by McCaffrey et al. (2007) is to use a Green's function that relates horizontal surface velocities, as recorded by GPS, to interseismic elastic deformation. The thermal modeling approach is limited by a lack of information about the amount of frictional heating occurring on the thrust (Molnar and England, 1990). The GPS approach is limited in that the horizontal velocity component is fairly insensitive to the structure of the locked zone. The vertical velocity component is much more useful for this purpose. We are fortunate in that vertical velocities can now be measured by GPS to a precision of about 0.2 mm/a. The dislocation model predicts that vertical velocities should range up to about 20 percent of the subduction velocity, which means maximum values of ~7 mm/a. The locked zone is generally entirely offshore at Cascadia, except for the Olympic Peninsula region, where the underlying Juan De Fuca plate has an anomalously low dip. Previous thermal and GPS modeling, as well as tide gauge data and episodic tremors indicate the locked zone there extends about 50 to 75 km onland. This situation provides an opportunity to directly study the locked zone. With that objective in mind, we have constructed a full 3D geodynamic model of the Cascadia subduction zone. At present, the model provides a full representation of the interseismic elastic deformation due to variations of slip on the subduction thrust. The model has been benchmarked against the Savage (2D) and Okada (3D) analytical solutions. This model has an important advantage over traditional dislocation modeling in that we include temperature-sensitive viscosity for the upper and lower plates, and also use realistic constitutive models to represent the locked zone. Another important advantage is that the 3D model provides a full representation of the interseismic deformation, which is important for interpreting GPS data.

Geodetic Imaging of Glacio-Seismotectonic Processes in Southern Alaska

NASA Astrophysics Data System (ADS)

Sauber, J.; Bruhn, R.; Forster, R.; Hofton, M.

2008-12-01

Across southern Alaska the northwest directed motion of the Pacific plate is accompanied by migration and collision of the Yakutat terrane. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin. Over the last couple of decades the rate of ongoing deformation associated with subduction and a locked main thrust zone has been estimated by geodetic measurements. In the last five years more extensive geodetic measurements, structural and tectonic field studies, thermochronolgy, and high-resolution lidar have been acquired and analyzed as part of the STEEP project [Pavlis et al., 2006]. The nature and magnitude of accretion and translation on upper crustal faults and folds remains uncertain, however, due to complex variations in the style of tectonic deformation, pervasive and changing glaciation, and the logistical challenges of conducting field studies in formidable topography. In this study, we analyze new high-resolution lidar data to extract locations, geometry, and heights of seismogenic faults and zones of active folding across the Malaspina-Seward-Bagley region of the southern Alaska plate boundary that is hypothesized to accommodate upper crustal shortening and right-lateral slip. Airborne Topographic Mapper (ATM) lidar swath data acquired by Krabill et al. in the summer of 2005 and ICESat data (1993-present) cross a number of proposed faults and folds partially masked by glaciation, including the Malaspina thrust, Esker Creek, Chugach-St.Elias thrust, and Contact. Focal mechanisms from this region indicate mostly shallow (0-30 km) thrust and oblique strike-slip faulting. Similarly, rupture in the 1979 St. Elias earthquake (M=7.4) started as a shallow, north-dipping thrust that later changed to more steeply NE dipping with a large right-lateral strike-slip component. Additionally, we are using the morphology and dynamics of glaciers derived from L-Band SAR ice velocities and SAR images to infer the large scale sub-ice structures that form the structural framework of the Seward-Bagley Basins. The new lidar, InSAR, and STEEP results provide constraints that enable us to critically re-evaluate alternate models of the nature of tectonics and structures hidden beneath the ice originally proposed by Ford et al [2003] . Ford, A.L., R.R. Forster, and R.L. Bruhn, 2003, Ice surface velocity patterns on Seward Glacier, Alaska/Yukon, and their implications for regional tectonics in the Saint Elias Mountains, Annals of Glaciology, 36, 21-28.

Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data

NASA Astrophysics Data System (ADS)

Teixell, A.; Labaume, P.; Ayarza, P.; Espurt, N.; de Saint Blanquat, M.; Lagabrielle, Y.

2018-01-01

This paper provides a synthesis of current data and interpretations on the crustal structure of the Pyrenean-Cantabrian orogenic belt, and presents new tectonic models for representative transects. The Pyrenean orogeny lasted from Santonian ( 84 Ma) to early Miocene times ( 20 Ma), and consisted of a spatial and temporal succession of oceanic crust/exhumed mantle subduction, rift inversion and continental collision processes at the Iberia-Eurasia plate boundary. A good coverage by active-source (vertical-incidence and wide-angle reflection) and passive-source (receiver functions) seismic studies, coupled with surface data have led to a reasonable knowledge of the present-day crustal architecture of the Pyrenean-Cantabrian belt, although questions remain. Seismic imaging reveals a persistent structure, from the central Pyrenees to the central Cantabrian Mountains, consisting of a wedge of Eurasian lithosphere indented into the thicker Iberian plate, whose lower crust is detached and plunges northwards into the mantle. For the Pyrenees, a new scheme of relationships between the southern upper crustal thrust sheets and the Axial Zone is here proposed. For the Cantabrian belt, the depth reached by the N-dipping Iberian crust and the structure of the margin are also revised. The common occurrence of lherzolite bodies in the northern Pyrenees and the seismic velocity and potential field record of the Bay of Biscay indicate that the precursor of the Pyrenees was a hyperextended and strongly segmented rift system, where narrow domains of exhumed mantle separated the thinned Iberian and Eurasian continental margins since the Albian-Cenomanian. The exhumed mantle in the Pyrenean rift was largely covered by a Mesozoic sedimentary lid that had locally glided along detachments in Triassic evaporites. Continental margin collision in the Pyrenees was preceded by subduction of the exhumed mantle, accompanied by the pop-up thrust expulsion of the off-scraped sedimentary lid above. To the west, oceanic subduction of the Bay of Biscay under the North Iberian margin is supported by an upper plate thrust wedge, gravity and magnetic anomalies, and 3D inclined sub-crustal reflections. However, discrepancies remain for the location of continent-ocean transitions in the Bay of Biscay and for the extent of oceanic subduction. The plate-kinematic evolution during the Mesozoic, which involves issues as the timing and total amount of opening, as well as the role of strike-slip drift, is also under debate, discrepancies arising from first-order interpretations of the adjacent oceanic magnetic anomaly record.

Strain measurements and the potential for a great subduction earthquake off the coast of washington.

PubMed

Savage, J C; Lisowski, M

1991-04-05

Geodetic measurements of deformation in northwestern Washington indicate that strain is accumulating at a rate close to that predicted by a model of the Cascadia subduction zone in which the plate interface underlying the continental slope and outer continental shelf is currently locked but the remainder of the interface slips continuously. Presumably this locked segment will eventually rupture in a great thrust earthquake with a down-dip extent greater than 100 kilometers.

Tsunamigenic earthquake simulations using experimentally derived friction laws

NASA Astrophysics Data System (ADS)

Murphy, S.; Di Toro, G.; Romano, F.; Scala, A.; Lorito, S.; Spagnuolo, E.; Aretusini, S.; Festa, G.; Piatanesi, A.; Nielsen, S.

2018-03-01

Seismological, tsunami and geodetic observations have shown that subduction zones are complex systems where the properties of earthquake rupture vary with depth as a result of different pre-stress and frictional conditions. A wealth of earthquakes of different sizes and different source features (e.g. rupture duration) can be generated in subduction zones, including tsunami earthquakes, some of which can produce extreme tsunamigenic events. Here, we offer a geological perspective principally accounting for depth-dependent frictional conditions, while adopting a simplified distribution of on-fault tectonic pre-stress. We combine a lithology-controlled, depth-dependent experimental friction law with 2D elastodynamic rupture simulations for a Tohoku-like subduction zone cross-section. Subduction zone fault rocks are dominantly incohesive and clay-rich near the surface, transitioning to cohesive and more crystalline at depth. By randomly shifting along fault dip the location of the high shear stress regions ("asperities"), moderate to great thrust earthquakes and tsunami earthquakes are produced that are quite consistent with seismological, geodetic, and tsunami observations. As an effect of depth-dependent friction in our model, slip is confined to the high stress asperity at depth; near the surface rupture is impeded by the rock-clay transition constraining slip to the clay-rich layer. However, when the high stress asperity is located in the clay-to-crystalline rock transition, great thrust earthquakes can be generated similar to the Mw 9 Tohoku (2011) earthquake.

Triggered aseismic slip adjacent to the 6 February 2013 Mw 8.0 Santa Cruz Islands megathrust earthquake

USGS Publications Warehouse

Hayes, Gavin P.; Furlong, Kevin P.; Benz, Harley M.; Herman, Matthew W.

2014-01-01

Aseismic or slow slip events have been observed in many subduction zones, but whether they affect the occurrence of earthquakes or result from stress changes caused by nearby events is unclear. In an area lacking direct geodetic observations, inferences can be made from seismological studies of co-seismic slip, associated stress changes and the spatiotemporal nature of aftershocks. These observations indicate that the February 2013 Mw 8.0 Santa Cruz Islands earthquake may have triggered slow or aseismic slip on an adjacent section of the subduction thrust over the following hours to days. This aseismic event was equivalent to Mw 7.6, significantly larger than any earthquakes in the aftershock sequence. The aseismic slip was situated within the seismogenic portion of the subduction interface, and must have occurred to the south of the main seismic slip and most aftershocks in order to promote right-lateral faulting in the upper plate, the dominant deformation style of the aftershock sequence. This plate boundary segment can support either stable sliding (aseismic) or stick-slip (seismic) deformation in response to different driving conditions. The complete lack of aftershocks on the thrust interface implies this pair of megathrust slip episodes (seismic and aseismic) released a substantial portion of the stored strain on the northernmost section of the Vanuatu subduction zone.

New Orogenic Model for Taiwan Collision Zone Inferred From Three-dimensional P- and S-wave Velocity Structures and Seismicity

NASA Astrophysics Data System (ADS)

Nagai, S.; Hirata, N.; Sato, H.

2008-12-01

The island of Taiwan is located in the site of ongoing arc-continent collision zone between the Philippine Sea Plate (PSP) and the Eurasian Plate (EUP). Numerous geophysical and geological studies are done in and around Taiwan to develop various models to explain the tectonic processes in the Taiwan region. However, their details have not been known enough, especially under the Central Range. We suggest a new orogenic model for Taiwan orogeny, named 'Upper Crustal Stacking Model', inferred from our tomographic images using three temporary seismic networks with the Central Weather Bureau Seismic Network. These three temporary networks are the aftershock observation after the 1999 Chi-Chi Taiwan earthquake and two dense array observations across central and southern Taiwan, respectively. Tomographic images by the double-difference tomography [Zhang and Thurber, 2003] show a lateral alternate variation of high- and low-velocity, which are well correlated to surface geology and separated by east-dipping boundaries. These images have reliable high-resolution by dense arrays to be able to discuss this alternate variation. We found three high-velocity zones (> 6.0km/s). The westernmost zone corresponds to the subducting EUP. Other two zones are located beneath the Hsuehshan Range and the Eastern Central Range with trends of eastward dipping, respectively. And, we could image low-velocity zone located beneath Backbone Range between the two high-velocity zones clearly. We interpret that these east-dipping high- and low-velocity zones can be divided into two layered blocks and the subducting EUP, each of which consists of a high-velocity body under low-velocity one. Layered blocks can be interpreted as stacked thrust sheets between the subducting EUP and the Northern Luzon Arc, a part of PSP. These thrust sheets are parts of upper- and mid-crust detached from the subducting EUP. The model of continental subduction followed by buoyancy-driven exhumation can explain the existence of stacked thrust sheets. Thus we propose a new orogenic model, as referred to as the 'Upper Crustal Stacking Model'.

Interseismic deformation at the leading edge of obliquely converging Burmese plate in densely populated Bangladesh.

NASA Astrophysics Data System (ADS)

Akhter, S. H.; Steckler, M. S.; Seeber, L.; Mondal, D. R.; Goodbred, S. L., Jr.

2016-12-01

Densely populated Bangladesh sits at the juncture of 3-tectonic plates, India to the west and southwest, Eurasia to the north and non-rigid Burma platelet to the east. Moreover, the plate boundary between India and Burma passes through Bangladesh - the eastern part belongs to Burma plate while the western part belongs to Indian plate. Eastern Bangladesh, northeastern India and western Myanmar is characterized by the up to 250 km wide and 1400 km long Indo-Burma fold and thrust belt resulting from the oblique convergence of India-Burma plates. The northern extension of the Sumatra-Andaman subduction zone evolved from typical oceanic subduction in the Paleogene to the present subaerial subduction of the Ganges-Brahmaputra Delta. Subduction of the thick sedimentary pile has created the broad accretionary prism that prograding westward in Bangladesh. The deformation front runs near the low elevation Meghna estuary to the south and Sylhet marshes to the north. It is further demarcated by the westernmost buried anticlines of the fold and thrust belt, the Shahbazpur, Muladi, Kamta structures west of the Meghna River and Chatak structure in Sylhet. This position is reinforced by variations in the depth of the Holocene/Pleistocene boundary from shallow drilling. Recent GPS analysis demonstrates that the Indo-Burman subduction in deltaic Bangladesh is still active with convergence of 13 to 17 mm/y and that the décollement beneath the fold-thrust belt is locked (Steckler et. al., 2016). A megathrust earthquake occurred along Chittagong-Arakan coast in 1762 and a great earthquake in Upper Assam in 1548 brought remarkable changes in topography of these regions. A seismic gap exists between these two regions, i.e., in the Chittagong-Sylhet segment. The amount of elastic energy that has been stored in this seismic gap for at least 400 years is likely to slip >6m of the megathrust with a potential earthquake of Mw 8.2+ although it is unknown if the megathrust is seismogenic up to the deformation front.

The Role of Proto-Thrusts in Frontal Accretion and Accommodation of Plate Convergence, Hikurangi Subduction Margin, New Zealand

NASA Astrophysics Data System (ADS)

Barnes, P.; Ghisetti, F.; Ellis, S. M.; Morgan, J.

2016-12-01

Proto-thrusts are an enigmatic structural feature at the toe of many subduction accretionary wedges. They are commonly recognised in seismic reflection sections as relatively small-displacement (tens of metres) faults seaward of the primary deformation front. Although widely assumed to reflect incipient accretionary deformation and to mark the location of future thrusts, proto-thrusts have received relatively little attention. Few studies have attempted to characterise their displacement properties, evolution, and kinematic role in frontal accretion processes associated with propagation of the interface décollement. In this study, we make use of excellent quality geophysical and bathymetric imaging of the spectacular 25 km-wide Hikurangi margin proto-thrust zone (PTZ), the structure of which varies significantly along strike. From a detailed structural analysis, we provide the first substantial quantitative dataset on proto-thrust geometry, displacement profiles, fault scaling relationships, and fault population characteristics. These analyses provide new insights into the role of inferred stratigraphic inhomogeneity in proto-thrust development, and the role of proto-thrust arrays in frontal accretion. Our observations, combined with our own recently published reconstructions of the wedge, and ongoing numerical simulations, indicate a migrating wave of proto-thrust activity in association with forward-advancement of the décollement. Calculation of tectonic shortening accommodated by the active PTZ east of the present deformation front, from measurements of seismically-imaged fault displacements and estimates of sub-seismic faulting derived from power law relationships, reveal their surprisingly significant role in accommodating regional plate convergence. South of the colliding Bennett Knoll Seamount, the predominantly seaward-vergent PTZ has accommodated 3.3 km of tectonic shortening, of which 70% is at sub-seismic scale. In comparison, north of Bennett Knoll Seamount, the predominantly landward-vergent PTZ has accommodated 4 km of shortening, of which 87% is at sub-seismic scale. These data combined with estimates of stratigraphic ages and deformation duration, indicate that proto-thrusts potentially accommodate up 30-50% of the total convergence rate.

Controls on accretion of flysch and melange belts at convergent margins: evidence from the Chugach Bay thrust and Iceworm melange, Chugach accretionary wedge, Alaska

USGS Publications Warehouse

Kusky, Timothy M.; Bradley, Dwight C.; Haeussler, Peter J.; Karl, Susan M.

1997-01-01

Controls on accretion of flysch and melange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic-Cretaceous melange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed melange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm melange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite-pumpellyite facies prior to formation of the Iceworm melange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian-Maastrichtian depositional ages of the Valdez Group, and fault-related fabrics in the Iceworm melange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90-100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating melanges like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style including the fold-thrust structures that dominate the outcrop pattern in the Valdez belt. Rapid underplating and frontal accretion of the Valdez Group caused a critical taper adjustment of the accretionary wedge, including exhumation of the metamorphosed McHugh Complex, and its emplacement over the Valdez Group. The Iceworm melange formed in a zone of focused fluid flow at the boundary between the McHugh Complex and Valdez Group during this critical taper adjustment of the wedge to these changing boundary conditions.

Controls on accretion of flysch and mélange belts at convergent margins: Evidence from the Chugach Bay thrust and Iceworm mélange, Chugach accretionary wedge, Alaska

NASA Astrophysics Data System (ADS)

Kusky, Timothy M.; Bradley, Dwight C.; Haeussler, Peter J.; Karl, Sue

1997-12-01

Controls on accretion of flysch and mélange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic-Cretaceous mélange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed mélange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm mélange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite-pumpellyite facies prior to formation of the Iceworm mélange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian-Maastrichtian depositional ages of the Valdez Group, and fault-related fabrics in the Iceworm mélange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90-100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating mélanges like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style including the fold-thrust structures that dominate the outcrop pattern in the Valdez belt. Rapid underplating and frontal accretion of the Valdez Group caused a critical taper adjustment of the accretionary wedge, including exhumation of the metamorphosed McHugh Complex, and its emplacement over the Valdez Group. The Iceworm mélange formed in a zone of focused fluid flow at the boundary between the McHugh Complex and Valdez Group during this critical taper adjustment of the wedge to these changing boundary conditions.

Morphology and Role of the Investigator Fracture Zone on the Sumatra Subduction Zone Process using High-resolution Bathymetry and Seismic Data

NASA Astrophysics Data System (ADS)

Villanueva-Robles, F.; Singh, S. C.; Bradley, K. E.; Hananto, N.; Leclerc, F.; Qin, Y.; Wei, S.; Carton, H. D.; Tapponnier, P.; Sieh, K.; Permana, H.; Avianto, P.

2016-12-01

The Sumatran subduction zone is one of the most seismically active areas on Earth. Within the last decade, it has produced three great earthquakes plus one earthquake that produced a much larger tsunami than predicted from the magnitude alone. However, the physical factors that limit the lateral extent of these ruptures as well as ancient earthquakes evidenced by paleogeodesy remain poorly understood. It has been suggested that subducted bathymetric features, such as seamounts and fracture zones, may be define many segment boundaries. Offshore of Central Sumatra, the Investigator Fracture Zone (IFZ) impinges on the trench and has been subducted to great depth beneath the overriding accretionary wedge. Where it is still exposed as a bathymetric feature, this fracture zone is 2000 km long and more than 100 km wide, and is composed of four individual ridges that exhibit up to 3.7 km of original relief. In order to study the role of the IFZ on subduction processes, we simultaneously acquired multibeam bathymetry and eight 35-km-long high-resolution seismic reflection profiles across the subduction front during the 2015 MegaTera experiment. We find that subduction of the IFZ ridges significantly deforms the morphology of the overriding accretionary wedge. The steep eastern slope of subducting ridges allowed the development of a long lived frontal thrust that reaches the surface at the trench and is associated with a very large frontal anticline and a flat portion of the accretionary wedge. Extensional deformation of the forearc and transverse basin formation occurs along the trailing edge of the ridges. We suggest that the subducted IFZ defines a segment boundary between the southern limit of coseismic slip of the Mw = 8.7, 2005 Simeulue-Nias earthquake and the northern limit of coseismic slip limit of a major 1797 earthquake recorded by coral paleogeodesy. The presence of four distinct ridges and an intervening 35-km-wide area of normal oceanic crust within the 105-km-wide IFZ should cause extremely heterogeneous coupling that is reflected by frequent earthquakes along the subducted portion of IFZ, and may enhance frictional coupling along the shallowest portions of the megathrust.

A slab expression in the Gibraltar arc?

NASA Astrophysics Data System (ADS)

Nijholt, Nicolai; Govers, Rob; Wortel, Rinus

2017-04-01

The present-day geodynamic setting of the Gibraltar arc region results from several Myrs of subduction rollback in the overall (oblique) convergence of Africa and Iberia. As for most rollback settings in a convergence zone, the interaction of these two components is complex and distinctly non-stationary. Gibraltar slab rollback is considered to have stalled, or at least diminished largely in magnitude, since the late Miocene/early Pliocene, suggesting that the effect of the slab on present-day surface motions is negligible. However, GPS measurements indicate that the Gibraltar arc region has an anomalous motion with respect to both Iberia and Africa, i.e., the Gibraltar arc region does not move as part of the rigid Iberian, or the rigid African plate. A key question is whether this surface motion is an expression of the Gibraltar slab. Seismic activity in the Gibraltar region is diffuse and considerable in magnitude, making it a region of high seismic risk. Unlike the North African margin to the east, where thrust earthquakes dominate the focal mechanism tables, a complex pattern is observed with thrust, normal and strike-slip earthquakes in a region stretching between the northern Moroccan Atlas across the Gibraltar arc and Alboran Sea (with the Trans-Alboran Shear Zone) to the Betics of southern Spain. Even though no large mega-thrust earthquakes have been observed in recent history, slab rollback may not have completely ceased. However, since no activity has been observed in the accretionary wedge, probably since the Pliocene, it is likely that the subduction interface is locked. In this study, we perform a series of numerical models in which we combine the relative plate convergence, variable magnitude of friction on fault segments, regional variations in gravitational potential energy and slab pull of the Gibraltar slab. We seek to reproduce the GPS velocities and slip sense on regional faults and thereby determine whether the Gibraltar slab has an effect on surface motion. Slab shape and slab continuity to the surface, allowing slab pull to be transfered to the surface lithosphere, are key factors controlling the force balance in the region. We explore slab geometries with or without continuity at the Betics (with a slab window between the known subduction interface and a possible Betics connection) and/or continental material attached to the slab (which lowers the slab pull magnitude). Through our methodology, we are able to study which slab shape of those proposed in the literature best fits the surface data.

How to build a model illustrating sea-floor spreading and subduction

USGS Publications Warehouse

Lahr, J.C.

1999-01-01

This report describes how to build a model of the outer 300 km (180 miles) of the Earth that can be used to develop a better understanding of the principal features of plate tectonics, including sea-floor spreading, the pattern of magnetic stripes frozen into the sea floor, transform faulting, thrust faulting, subduction, and volcanism. In addition to a paper copy of this report, the materials required are a cardboard shoebox, glue, scissors, straight edge, and safety razor blade.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana ( USA).

USGS Publications Warehouse

Skipp, B.

1987-01-01

The Clearwater orogenic zone in central Idaho and W Montana contains at least 2 major NE-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the W continental margin produced a younger N Bitterroot lobe of the Idaho batholith relative to an older S Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.-Author

Preliminary Depositional and Provenance Records of Mesozoic Basin Evolution and Cenozoic Shortening in the High Andes, La Ramada Fold-Thrust Belt, Southern-Central Andes (32-33°S)

NASA Astrophysics Data System (ADS)

Mackaman-Lofland, C.; Horton, B. K.; Fuentes, F.; Constenius, K. N.; McKenzie, R.; Alvarado, P. M.

2015-12-01

The Argentinian Andes define key examples of retroarc shortening and basin evolution above a zone of active subduction. The La Ramada fold-thrust belt (RFTB) in the High Andes provides insights into the relative influence and temporal records of diverse convergent margin processes (e.g. flat-slab subduction, convergent wedge dynamics, structural inversion). The RFTB contains Mesozoic extensional basin strata deformed by later Andean shortening. New detrital zircon U-Pb analyses of Mesozoic rift sediments reveal: (1) a dominant Permo-Triassic age signature (220-280 Ma) associated with proximal sources of effective basement (Choiyoi Group) during Triassic synrift deposition; (2) upsection younging of maximum depositional ages from Late Triassic through Early Cretaceous (230 to 100 Ma) with the increasing influence of western Andean arc sources; and (3) a significant Late Cretaceous influx of Paleozoic (~350-550 Ma) and Proterozoic (~650-1300 Ma) populations during the earliest shift from back-arc post-extensional subsidence to upper-plate shortening. The Cenozoic detrital record of the Manantiales foreland basin (between the Frontal Cordillera and Precordillera) records RFTB deformation prior to flat-slab subduction. A Permo-Triassic Choiyoi age signature dominates the Miocene succession, consistent with sources in the proximal Espinacito range. Subordinate Mesozoic (~80-250 Ma) to Proterozoic (~850-1800 Ma) U-Pb populations record exhumation of the Andean magmatic arc and recycling of different structural levels in the RFTB during thrusting/inversion of Mesozoic rift basin strata and subjacent Paleozoic units. Whereas maximum depositional ages of sampled Manantiales units cluster at 18-20 Ma, the Estancia Uspallata basin (~50 km to the south) shows consistent upsection younging of Cenozoic populations attributed to proximal volcanic centers. Ongoing work will apply low-temperature thermochronology to pinpoint basin accumulation histories and thrust timing.

Neogene subduction beneath Java, Indonesia: Slab tearing and changes in magmatism

NASA Astrophysics Data System (ADS)

Cottam, Michael; Hall, Robert; Cross, Lanu; Clements, Benjamin; Spakman, Wim

2010-05-01

Java is a Neogene calc-alkaline volcanic island arc formed by the northwards subduction of the Indo-Australian Plate beneath Sundaland, the continental core of SE Asia. The island has a complex history of volcanism and displays unusual subduction characteristics. These characteristics are consistent with the subduction of a hole in the down going slab that was formed by the arrival of a buoyant oceanic plateau at the trench. Subduction beneath Java began in the Eocene. However, the position and character of the calc-alkaline arc has changed over time. An older Paleogene arc ceased activity in the Early Miocene. Volcanic activity resumed in the Late Miocene producing a younger arc to the north of the older arc, and continues to the present day. An episode of Late Miocene thrusting at about 7 Ma is observed throughout Java and appears to be linked to northward movement of the arc. Arc rocks display typical calc-alkaline characteristics and reflect melting of the mantle wedge and subducted sediments associated with high fluid fluxes. Between West Java and Bali the present arc-trench gap is unusually wide at about 300 km. Seismicity identifies subducted Indian Ocean lithosphere that dips north at about 20° between the trench and the arc and then dips more steeply at about 60-70° from 100 to 600 km depth. In East Java there is gap in seismicity between about 250 and 500 km. Seismic tomography shows that this gap is not an aseismic section of the subduction zone but a hole in the slab. East Java is also unusual in the presence of K-rich volcanoes, now inactive, to the north of the calc-alkaline volcanoes of the active arc. In contrast to the calc-alkaline volcanism of the main arc, these K-rich melts imply lower fluid fluxes and a different mantle source. We suggest that all these observations can be explained by the tearing of the subducting slab when a buoyant oceanic plateau arrived at the trench south of East Java at about 8 Ma. With the slab unable to subduct, continued convergence caused contractional deformation and thrusting in Java. The slab then broke in front of the plateau. The trench stepped back to the south by about 150 km and subduction resumed behind the plateau, causing a hole to develop in the subducting slab. As the hole passed beneath the arc, and fluid flux declined, normal calc-alkaline volcanism ceased. With the mantle wedge melt component ‘switched off' K-rich melts, produced from a deeper mantle component that remained undiluted, dominated arc volcanism. As the hole got deeper K-rich volcanism ceased. Normal, calc-alkaline, arc activity resumed when the untorn slab following the hole was subducted.

Extensional reactivation of the Chocolate Mountains subduction thrust in the Gavilan Hills of southeastern California

USGS Publications Warehouse

Oyarzabal, F.R.; Jacobson, C.E.; Haxel, G.B.

1997-01-01

The NE vergent Chocolate Mountains fault of south-eastern California has been interpreted as either a subduction thrust responsible for burial and prograde metamorphism of the ensimatic Orocopia Schist or as a normal fault involved in the exhumation of the schist. Our detailed structural analysis in the Gavilan Hills area provides new evidence to confirm the latter view. A zone of deformation is present at the top of the Orocopia Schist in which lineations are parallel to those in the upper plate of the Chocolate Mountains fault but oblique to ones at relatively deep levels in the schist. Both the Orocopia Schist and upper plate contain several generations of shear zones that show a transition from crystalloblastic through mylonitic to cataclastic textures. These structures formed during retrograde metamorphism and are considered to record the exhumation of the Orocopia Schist during early Tertiary time as a result of subduction return flow. The Gatuna fault, which places low-grade, supracrustal metasediments of the Winterhaven Formation above the gneisses of the upper plate, also seems to have been active at this time. Final unroofing of the Orocopia Schist occurred during early to middle Miocene regional extension and may have involved a second phase of movement on the Gatuna fault. Formation of the Chocolate Mountains fault during exhumation indicates that its top-to-the-NE sense of movement provides no constraint on the polarity of the Orocopia Schist subduction zone. This weakens the case for a previous model involving SW dipping subduction, while providing support for the view that the Orocopia Schist is a correlative of the Franciscan Complex.

Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

NASA Astrophysics Data System (ADS)

Duretz, T.; Gerya, T. V.

2013-08-01

Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions.

The Seismic Sequence of the 2016 Mw 7.8 Pedernales, Ecuador Sarthquake

NASA Astrophysics Data System (ADS)

Leon, S.; Fuenzalida, A.; Bie, L.; Garth, T.; Gonzalez, P. J.; Holt, J.; Rietbrock, A.; Edwards, B.; Regnier, M. M.; Pernoud, M.; Mercerat, E. D.; Perrault, M.; Font, Y.; Alvarado, A. P.; Charvis, P.; Beck, S. L.; Meltzer, A.

2016-12-01

On the 16th April 2016, a Mw 7.8 mega-thrust earthquake occurred in Northern Ecuador, close to the city of Pedernales. The event ruptured an area of 120 x 60 km and was preceded by a Mw 5.0 foreshock, located only 15 km south of the epicentre, and registered 10 minutes before the main event.A few weeks after the main event a large array of instruments was deployed by a collaborative project between the Geophysical Institute of Ecuador (IGEPN), IRIS (USA), Géoazur (France) and the University of Liverpool (UK). This dense seismic network, with more than 70 stations, includes broadband, short period and strong motion instruments and is currently recording the aftershock activity of the earthquake. It is hoped that this data set will give further insights into the structure of the subduction zone mega thrust beneath Ecuador.Using data recorded both on the permanent and the recently deployed network we located and calculated the moment tensor solutions for the foreshock event, and the large aftershocks (M > 5). We analyse the spatial distribution of the seismicity and its relation with the co-seismic slip, estimated by inverting radar satellite interferometry data, and with previous models of inter-seismic coupling (e.g. Chlieh et al., 2014). It is possible to identify two lineations in the aftershock activity located to the north and south of the rupture. Moreover, the geodetic slip model shows that the boundaries of the maximum coseismic slip coincides with the observed lineaments in the aftershocks and with the rupture area of a previous Mw 7.8 event in 1942. This suggests that the features to the north and south may impose a barrier to rupture propagation, creating different segments in the subduction zone beneath Ecuador. In addition, we model the Coulomb stress change caused by the foreshock and mainshock in order to investigate whether this could explain the aftershock distribution and potential earthquake interactions. Previous activity has presented a northward-propagating series of ruptures greater than Mw 7 spaced approximately 20 years apart. An open question is therefore whether the present event is the start of a further series of large magnitude events in northern Ecuador, and/or whether slow slip events/creep south of the rupture have partly accommodated the strain due to subduction.

Retrodeforming the Arabia-Eurasia collision zone : Age of collision and magnitude of continental subduction

NASA Astrophysics Data System (ADS)

McQuarrie, N.; van Hinsbergen, D. J. J.

2012-04-01

When did continents collide, and how is convergence partitioned after collision are first order questions that seem to defy consensus along the Alpine-Himalyan orogen. Estimates on the age of collision for Arabia and Eurasia range from late Cretaceous to Pliocene, based on a wide variety of presumed geologic responses. Both lower Miocene synorgenic strata with growth structures adjacent to the main Zagros fault and upper Oligocene to lower Miocene overlap strata over post-collisional thrusts are derived from Eurasia and require that collision was underway at least by ~25-24 Ma. However, upper plate deformation, exhumation and sedimentation are used to argue for an older, 35 Ma collision age. Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations provides precise estimates of the relative positions between the northern Arabian margin and the southern Eurasia margin. Plate circuits indicate, from NW to SE along the collision zone 490-650 km of post-25 Ma Arabia-Eurasia convergence and 810-1070 km since 35 Ma. To assess the consequences of these collision ages for the amount of Arabian continental subduction, we compile all documented shortening within the orogen. The Zagros fold-thrust belt consists of thrusted upper crust that was offscraped from subducted Arabian continental lithosphere. Balanced cross-sections give 105-180 km of Zagros shortening (including estimates from the Zagros proper, 45-90 km, and the Zagros "crush" zone, 60-90 km). Shortening within Eurasia is estimated to be 53-75 km through the Kopet Dagh and Alborz Mountains, plus 38 km across Central Iran. These estimates suggest that the orogen has shortened 200 to 300 km since the early Miocene. Both a 25 and a 35 Ma collision estimate thus requires that a considerable portion of the Arabian plate subducted without recognized accretion of its upper crust. To balance plate circuits and documented shortening requires whole-sale subduction of ~500-800 km of continental crust since 35 Ma; for a 25 Ma collision this would be between 190-450 km. The ophiolitic fragments preserved along the suture zone allow us to test the magnitude of possible continental subduction. The Oman Ophiolite preserves the geometry and distance over which ophiolites obduced over the northern margin of Arabia in the late Cretaceous. The distance from the southwestern edge of the ophiolite to the northeastern edge of the continent is 180 km, suggesting that the Arabian continental margin plus overlying ophiolites may have extended ~200 km beyond the Main Zagros fault. Assuming that 200 km of Arabian continental margin and overlying ophiolites subducted entirely, except the few remnant ophiolite slivers remaining in the suture zone, would reconstruct ~ 400-500 km of post-collisional Arabia-Eurasia convergence, consistent with a ~25 Ma collision age. As much as 500-800 km of continental subduction required by an earlier (~35 Ma) collision age seems unlikely.

The structure of the Calabrian subduction system from the fore-arc to the back-arc: new insights from wide-angle seismic data

NASA Astrophysics Data System (ADS)

Prada, M.; Sallares, V.; Ranero, C. R.; Grevemeyer, I.; Zitellini, N.

2017-12-01

The Calabrian arc is a Neogene-Quaternary arcuate orogen result from the subduction of the Ionian Lithosphere under Calabria. The SE migration of this subduction system, triggered by slab rollback, caused the opening of the Tyrrhenian back-arc basin. The large-scale lithospheric structure of the subduction system is mostly imaged by regional earthquake tomography studies. The limited resolution of these studies, however, hinders the definition of smaller-scale details on the location, nature and transition of different lithospheric domains, which are crucial to study the geodynamic evolution of the system. Here we perform travel-time tomography of offshore and onshore active-source wide-angle seismic data to define the 2D Vp structure of the entire Calabrian subduction system. The data were acquired along a 550 km-long transect that extends from the Tyrrhenian back-arc domain to the fore-arc in the Ionian Sea, across Calabria. From NW to SE, the tomographic model shows abrupt variations of the velocity structure. In the back-arc system, particularly in the Vavilov and Marsili basins, OBS sections lack PmP-like arrivals and the velocity structure shows a continuous and strong vertical velocity gradient of 1 s-1. These results strongly support the presence of a basement made of exhumed mantle rocks. Between the Vavilov and Marsili basins, a relatively thick, low-velocity block is interpreted to be of continental affinity. The transition between Marsili Basin and Calabria is marked by a steep Moho geometry that shallows from SE to NW, revealing a dramatic crustal thinning along the N Calabrian margin. The lower crust of the margin has localized Vp of 7 km/s under the submarine volcanic arc. SE Calabria, the model shows a strong horizontal velocity gradient that is interpreted as the backstop of the subduction. In the Ionian, a 3-5 km thick sedimentary wedge thickens towards the NW. The frontal part of the wedge shows sub-vertical low-velocity anomalies indicating the presence of fluid-saturated large thrusts faults.

In search of transient subduction interfaces in the Dent Blanche-Sesia Tectonic System (W. Alps)

NASA Astrophysics Data System (ADS)

Angiboust, Samuel; Glodny, Johannes; Oncken, Onno; Chopin, Christian

2014-09-01

In this paper we study the Alpine metamorphic history of a major tectonic zone which formed during Alpine orogeny, the Dent Blanche Thrust (DBT). This contact, located in the Northern Western Alps, juxtaposes some ophiolitic metasediment-rich remnants of the Liguro-Piemontese ocean (Tsaté Complex) with a composite continental, km-sized complex (Dent Blanche Tectonic System, DBTS) of Adriatic affinity thrusted over the ophiolite. In order to better understand the geodynamic meaning of the DBT region and adjacent units, we have reconstructed the pressure-temperature-time-deformation (P-T-t-d) history of these two units using modern thermobarometric tools, Rb/Sr geochronology, and field relationships. We show that the Tsaté Complex is formed by a stack of km-thick calcschists-bearing tectonic slices having experienced variable maximum burial temperatures between 360 °C and 490 °C at depths of ca. 25-40 km. Associated deformation ages span a range between 37 Ma and 41 Ma. The Arolla gneissic mylonites at the base of the DBTS experienced high-pressure (12-14 kbar), top-to-NW deformation at ca. 450 °C between 43 and 48 Ma. A first age of ca. 58 Ma has been obtained for high-pressure ductile deformation in the Valpelline shear zone, atop Arolla gneisses. Some of the primary, peak metamorphic fabrics have been reworked and later backfolded during exhumation and collisional overprint (ca. 20 km depth, 37-40 Ma) leading to the regional greenschist-facies retrogression which is particularly prominent within Tsaté metasediments. We interpret the Dent Blanche Thrust, at the base of the Arolla unit, as a fossilized subduction interface active between 43 and 48 Ma. Our geochronological results on the shear zone lining the top of the Arolla unit, together with previous P-T-t estimates on equivalent blueschist-facies shear zones cutting the Sesia unit, indicate an older tectonic activity between 58 and 65 Ma. We demonstrate here that observed younger ages towards lowermost structural levels are witness of the transient, downwards migration of the Alpine early Cenozoic blueschist-facies subduction interface. This down-stepping is interpreted to reflect the progressive underplating acting between 30 and 40 km depth in the Alpine subduction zone between late Cretaceous and late Eocene. Underplating involved first continental material derived from the stretched Adriatic margin followed by underplating of ocean-derived rocks in the Eocene. These results shed light on subduction-zone accretion processes and therefore provide a new perspective for the understanding of geophysical results imaging the plate-interface region in active subduction zones.

Seismotectonics of New Guinea: a Model for Arc Reversal Following Arc-Continent Collision

NASA Astrophysics Data System (ADS)

Cooper, Patricia; Taylor, Brian

1987-02-01

The structure and evolution of the northern New Guinea collision zone is deduced from International Seismological Center (ISC) seismicity (1964-1985), new and previously published focal mechanisms and a reexamination of pertinent geological data. A tectonic model for the New Guinea margin is derived which illustrates the sequential stages in the collision and suturing of the Bewani-Toricelli-Adelbert-Finisterre-Huon-New Britain arc to central New Guinea followed by subduction polarity reversal in the west. East of 149°E, the Solomon plate is being subducted both to the north and south; bringing the New Britain and Trobriand forearcs toward collision. West of 149°E the forearcs have collided, and together they override a fold in the doubly subducted Solomon plate lithosphere, which has an axis that is parallel to the strike of the Ramu-Markham suture and that plunges westward at an angle of 5° beneath the coast ranges of northern New Guinea. Active volcanism off the north coast of New Guinea is related to subduction of the Solomon plate beneath the Bismarck plate. Active volcanism of the Papuan peninsula and Quaternary volcanism of the New Guinea highlands are related to slow subduction of the Solomon plate beneath the Indo-Australian plate along the Trobriand Trough and the trough's former extension to the west, respectively. From 144°-148°E, seismicity and focal mechanisms reveal that convergence between the sutured Bismarck and Indo-Australian plates is accommodated by thrusting within the Finisterre and Adelbert ranges and compression of the New Guinea orogenic belt, together with basement-involved foreland folding and thrusting to the south. The Finisterre block overthrusts the New Guinea orogenic belt, whereas the Adelbert block is sutured to New Guinea and overthrusts the oceanic lithosphere of the Bismarck Sea. Along the New Guinea Trench, west of 144°E, seismicity defines a southward dipping Wadati-Benioif zone, and focal mechanisms indicate oblique subduction. Only this oldest, westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity.

Stress Rotation Across the Cascadia Megathrust Requires a Weak Subduction Plate Boundary at Seismogenic Depths

NASA Astrophysics Data System (ADS)

Li, D.; McGuire, J. J.; Liu, Y.; Hardebeck, J.

2017-12-01

Despite the great effort spent investigating subduction zones, there are very limited constraints on the stress state on the plate boundary fault at the depth of megathrust earthquakes. Here we utilize a focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. We present a high-resolution inversion for the principal stress orientations both above and below the thrust interface in the southern Cascadia Subduction zone. The distinctive stresses above and below the interface require a significant stress rotation within 10 km of the plate boundary. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our approach utilizes the continuous traction boundary conditions between layers as well as the observed principal stress orientations and the relative magnitude ratios in the crust and subducting mantle as constraints. Our results indicate that the shear stress on the plate boundary fault is likely no more than about 50 MPa at 20 km depth. Regardless of the assumed upper mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of 0 to 0.2 at seismogenic depths. The central question for the Cascadia subduction zone is why it remains seismically quiet despite the 300+ years of stress accumulation since the last megathrust earthquake. For example, we also document that no thrust earthquakes were recorded by the 2-year Cascadia Initiative expedition down to magnitude 2.0, despite the stress perturbation generated by a nearby Mw5.7 earthquake on Jan 28th, 2015, on the Mendocino Transform fault. To help answer that question, we provide a new and fundamental constraint on the absolute level of stress accumulation to date in the current seismic cycle. Our technique for evaluating the absolute level of stress in subduction zones can be applied at a number of regions around the globe as datasets improve.

West margin of North America - A synthesis of recent seismic transects

USGS Publications Warehouse

Fuis, G.S.

1998-01-01

A comparison of the deep structure along nine recent transects of the west margin of North America shows many important similarities and differences. Common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust, accreted oceanic/arc (or oceanic-like) lithosphere of Mesozoic through Cenozoic ages. Cenozoic accretionary prisms, Mesozoic accretionary prisms, backstops to the Mesozoic prisms, and undivided lower crust. Not all of these elements are present along all transects. In this study, nine transects, including four crossing subduction zones and five crossing transform faults, are plotted at the same scale and vertical exaggeration (V.E. 1:1), using the above scheme for identifying tectonic elements. The four subduction-zone transects contain actively subducting oceanic crust. Cenozoic accretionary prisms, and bodies of basaltic rocks accreted in the Cenozoic, including remnants of a large, oceanic plateau in the Oregon and Vancouver Island transects. Rocks of age and composition (Eocene basalt) similar to the oceanic plateau are currently subducting in southern Alaska, where they are doubled up on top of Pacific oceanic crust and have apparently created a giant asperity, or impediment to subduction. Most of the subduction-zone transects also contain Mesozoic accretionary prisms, and two of them, Vancouver Island and Alaska, also contain thick, technically underplated bodies of late Mesozoic/early Cenozoic oceanic lithosphere, interpreted as fragments of the extinct Kula plate. In the upper crust, most of the five transform-fault transects (all in California) reflect: (1) tectonic wedging of a Mesozoic accretionary prism into a backstop, which includes Mesozoic/early Cenozoic forearc rocks and Mesozoic ophiolitic/arc basement rocks: and (2) shuffling of the subduction margin of California by strike-slip faulting. In the lower crust, they may reflect migration of the Mendocino triple junction northward (seen in rocks east of the San Andreas fault) and cessation of Farallon-plate subduction (seen in rocks west of the San Andreas fault). In northern California, lower-crustal rocks east of the San Andreas fault have oceanic-crustal velocity and thickness and contain patches of high reflectivity. They may represent basaltic rocks magmatically underplated in the wake of the migration of the Mendocino triple junction, or they may represent stalled, subducted fragments of the Farallon/Gorda plate. The latter alternative does not fit the accepted 'slabless window' model for the migration of the triple junction. This lower-crustal layer and the Moho are offset at the San Andreas and Maacama faults. In central California, a similar lower-crustal layer is observed west of the San Andreas fault. West of the continental slope, it is Pacitic oceanic crust, but beneath the continent it may represent either Pacific oceanic crust, stalled, subducted fragments (microplates) of the Farallon plate, or basaltic rocks magmatically underplated during subduction of the Pacific/Farallon ridge or during breakup of the subducted Farallon plate. The transect in southern California is only partly representative of regional structure, as the structure here is 3-dimensional. In the upper crust, a Mesozoic prism has been thrust beneath crystalline basement rocks of the San Gabriel Mountains and Mojave Desert. In the mid-crust, a bright reflective zone is interpreted as a possible 'master' decollement that can be traced from the fold-and-thrust belt of the Los Angeles basin northward to at least the San Andreas fault. A Moho depression beneath the San Gabriel Mountains is consistent with downwelling of lithospheric mantle beneath the Transverse Ranges that appears to be driving the compression across the Transverse Ranges and Los Angeles basin. ?? 1998 Elsevier Science B.V. All rights reserved.

Tectonic stratification and seismicity of the accretionary prism of the Azerbaijani part of Greater Caucasus

NASA Astrophysics Data System (ADS)

Alizade, Akif; Kangarli, Talat; Aliyev, Fuad

2013-04-01

The Greater Caucasus has formed during last stage of the tectogenesis in a geodynamic condition of the lateral compression, peculiar to the zone pseudo-subduction interaction zone between Northern and Southern Caucasian continental microplates. Its present day structure formed as a result of horizontal movements of the different phases and sub-phases of Alpine tectogenesis (from late Cimmerian to Valakhian), and is generally regarded as zone where, along Zangi deformation, the insular arc formations of the Northern edge of South Caucasian microplate thrust under the Meso-Cenozoic substantial complex contained in the facials of marginal sea of Greater Caucasus. The last, in its turn, has been pushed beneath the North-Caucasus continental margin of the Scythian plate along Main Caucasus Thrust fault. Data collected from the territory of Azerbaijan and its' sector of the Caspian area stands for pseudo-subduction interaction of microplates which resulted in the tectonic stratification of the continental slope of Alpine formations, marginal sea and insular arc into different scale plates of south vergent combined into napping complexes. In the orogeny's present structure, tectonically stratified Alpine substantial complex of the marginal sea of Greater Caucasus bordered by Main Caucasus and Zangi thrusts, is represented by allochthonous south vergent accretionary prism in the front of first deformation with its' root buried under the southern border of Scythian plate. Allocated beneath mentioned prism, the autochthonous bedding is presented by Meso-Cenosoic complex of the Northern flank of the South-Caucasian miroplate, which is in its' turn crushed and lensed into southward shifted tectonic microplates gently overlapping the northern flank of Kura flexure along Ganykh-Ayrichay-Alyat thrust. Data of real-time GPS measurement of regional geodynamics indicates that pseudo-subduction of South Caucasian microplate under the North Caucasian microplate still continues during present stage of alpine tectogenesis. Among others, ongoing pseudo-subduction is indicated by data of regional seismicity which is irregularly distributed by depth (foci levels 2-6; 8-12; 17-22; 25-45 km). Horizontal and vertical seismic zoning is explained by Earth crust's block divisibility and tectonic stratification, within the structure of which the earthquake focuses are mainly confined to the crossing nodes of differently oriented ruptures, or to the planes of deep tectonic disruptions and lateral displacements along unstable contacts of the substantial complexes with various degree of competence. At present stage of tectogenesis, seismically most active are the structures of the northern flank of South Caucasian microplate, controlled by Ganyx-Ayrichay-Alyat deep thrust with "General Caucasus" spread in the west, and sub-meridian right-lateral strike slip zone of the Western Caspian fault in the east of Azerbaijani part of Greater Caucasus.

Links Between Earthquake Characteristics and Subducting Plate Heterogeneity in the 2016 Pedernales Ecuador Earthquake Rupture Zone

NASA Astrophysics Data System (ADS)

Bai, L.; Mori, J. J.

2016-12-01

The collision between the Indian and Eurasian plates formed the Himalayas, the largest orogenic belt on the Earth. The entire region accommodates shallow earthquakes, while intermediate-depth earthquakes are concentrated at the eastern and western Himalayan syntaxis. Here we investigate the focal depths, fault plane solutions, and source rupture process for three earthquake sequences, which are located at the western, central and eastern regions of the Himalayan orogenic belt. The Pamir-Hindu Kush region is located at the western Himalayan syntaxis and is characterized by extreme shortening of the upper crust and strong interaction of various layers of the lithosphere. Many shallow earthquakes occur on the Main Pamir Thrust at focal depths shallower than 20 km, while intermediate-deep earthquakes are mostly located below 75 km. Large intermediate-depth earthquakes occur frequently at the western Himalayan syntaxis about every 10 years on average. The 2015 Nepal earthquake is located in the central Himalayas. It is a typical megathrust earthquake that occurred on the shallow portion of the Main Himalayan Thrust (MHT). Many of the aftershocks are located above the MHT and illuminate faulting structures in the hanging wall with dip angles that are steeper than the MHT. These observations provide new constraints on the collision and uplift processes for the Himalaya orogenic belt. The Indo-Burma region is located south of the eastern Himalayan syntaxis, where the strike of the plate boundary suddenly changes from nearly east-west at the Himalayas to nearly north-south at the Burma Arc. The Burma arc subduction zone is a typical oblique plate convergence zone. The eastern boundary is the north-south striking dextral Sagaing fault, which hosts many shallow earthquakes with focal depth less than 25 km. In contrast, intermediate-depth earthquakes along the subduction zone reflect east-west trending reverse faulting.

New insight on the Great Sumatra Fault, offshore NW Sumatra, from recent marine data

NASA Astrophysics Data System (ADS)

Ghosal, D.; Singh, S. C.; Chauhan, A.; Hananto, N. D.

2009-12-01

The Sumatra subduction system is a classic example of an oblique subduction where the slip is portioned into pure thrust along the Sumatra-Andaman megathrust and strike-slip along the Great Sumatra Fault (GSF). Only in the last five years there have been three great pure thrust earthquakes along the Sumatran subduction zone. However, the 1900 km long GSF has been rather silent and is likely to produce a large earthquake in the near future, and hence it is important study the GSF in order to mitigate seismic risks. Over the last 20 years, GSF has been studied on land, but we have no information about its offshore extension NW of Sumatra. The problem is further complicated by its vicinity with the volcanic arc, which switches back and forth centering the GSF. Here we present analyses of recently acquired high-resolution bathymetry and shallow and deep reflection seismic data. We find that GSF bifurcates into two branches north of Banda Aceh, both producing 15-20 km wide deep adjacent basins. Southern basin is 1-2 km deep and has a flower structure with a push-up ridge, suggesting the presence of an active strike-slip fault. The presence of strike-slip earthquakes beneath this basin further suggests that GSF passes through this basin. The northern basin is up to 4 km deep, bounded by normal faults. The absence of recent sediments on the basin floor suggests that the basin is very young. The presence of a chain of volcanoes in the centre of the basin suggests that the volcanic arc passes through this basin. The fact that the basin is 4 km deep in the presence of volcanoes, which tend to fill in a basin and hence make them shallower, suggests that this might be the site of an onset of back-arc spreading centre. We shall examine all the new observations in the light of plate motion, local deformation and possible seismic risk.

The brittle-ductile transition in porous sedimentary rocks: geological implications for accretionary wedge aseismicity

NASA Astrophysics Data System (ADS)

Zhang, Jiaxiang; Davis, Dan M.; Wong, Teng-Fong

1993-07-01

Thrusting earthquakes in subduction zones generally occur along only part of the plate boundary, with motion along the shallowest part of the plate boundary occurring ascismically. The maximum size of subduction boundary thrust earthquakes depends strongly upon the down-dip width of the seismogenic zone. The single most uncertain factor in determining that width is the location of the up-dip limit of the zone (the seismic front), which depends upon the mechanical state of the sedimentary rocks in the plate boundary zone. In order to come to a better understanding of the seismic potential of sediments in a subduction zone, we carried out a series of triaxial experiments on Berea and Kayenta sandstones. Based on our experimental data, a brittle-ductile transition map was constructed showing that both porosity and effective pressure are important factors controlling the transition from brittle to macroscopically ductile behavior in porous rocks. In the brittle field, a sample fails by shear localization on one slip plane accompanied by strain softening and dilatancy, whereas in the ductile field, a sample deforms homogeneously with a constant yield stress or slight hardening. By comparing such a map with the estimated porosity profile of an accretionary wedge, the likely nature and rough location of the boundary between brittle and ductile behavior can be inferred. If the sediments along a plate boundary are too young and undercompacted to be capable of brittle shear localization, then their deformation is likely to be aseismic. In this way, it may be possible for even a very broad fore-arcs to produce no great earthquakes. However, great earthquakes are to be expected at margins that have large zones of plate contact along which many sediments are compacted and well lithified. Such rocks are expected to be capable of shear localization and brittle failure with the potential for stick-slip behavior.

Frontal compression along the Apennines thrust system: The Emilia 2012 example from seismicity to crustal structure

NASA Astrophysics Data System (ADS)

Chiarabba, Claudio; De Gori, Pasquale; Improta, Luigi; Lucente, Francesco Pio; Moretti, Milena; Govoni, Aladino; Di Bona, Massimo; Margheriti, Lucia; Marchetti, Alessandro; Nardi, Anna

2014-12-01

The evolution of the Apennines thrust-and-fold belt is related to heterogeneous process of subduction and continental delamination that generates extension within the mountain range and compression on the outer front of the Adria lithosphere. While normal faulting earthquakes diffusely occur along the mountain chain, the sparse and poor seismicity in the compressional front does not permit to resolve the ambiguity that still exists about which structure accommodates the few mm/yr of convergence observed by geodetic data. In this study, we illustrate the 2012 Emilia seismic sequence that is the most significant series of moderate-to-large earthquakes developed during the past decades on the compressional front of the Apennines. Accurately located aftershocks, along with P-wave and Vp/Vs tomographic models, clearly reveal the geometry of the thrust system, buried beneath the Quaternary sediments of the Po Valley. The seismic sequence ruptured two distinct adjacent thrust faults, whose different dip, steep or flat, accounts for the development of the arc-like shape of the compressional front. The first shock of May 20 (Mw 6.0) developed on the middle Ferrara thrust that has a southward dip of about 30°. The second shock of May 29 (Mw 5.8) ruptured the Mirandola thrust that we define as a steep dipping (50-60°) pre-existing (Permo-Triassic) basement normal fault inverted during compression. The overall geometry of the fault system is controlled by heterogeneity of the basement inherited from the older extension. We also observe that the rupture directivity during the two main-shocks and the aftershocks concentration correlate with low Poisson ratio volumes, probably indicating that portions of the fault have experienced intense micro-damage.

Cascadia Subduction Zone

USGS Publications Warehouse

Frankel, Arthur D.; Petersen, Mark D.

2008-01-01

The geometry and recurrence times of large earthquakes associated with the Cascadia Subduction Zone (CSZ) were discussed and debated at a March 28-29, 2006 Pacific Northwest workshop for the USGS National Seismic Hazard Maps. The CSZ is modeled from Cape Mendocino in California to Vancouver Island in British Columbia. We include the same geometry and weighting scheme as was used in the 2002 model (Frankel and others, 2002) based on thermal constraints (Fig. 1; Fluck and others, 1997 and a reexamination by Wang et al., 2003, Fig. 11, eastern edge of intermediate shading). This scheme includes four possibilities for the lower (eastern) limit of seismic rupture: the base of elastic zone (weight 0.1), the base of transition zone (weight 0.2), the midpoint of the transition zone (weight 0.2), and a model with a long north-south segment at 123.8? W in the southern and central portions of the CSZ, with a dogleg to the northwest in the northern portion of the zone (weight 0.5). The latter model was derived from the approximate average longitude of the contour of the 30 km depth of the CSZ as modeled by Fluck et al. (1997). A global study of the maximum depth of thrust earthquakes on subduction zones by Tichelaar and Ruff (1993) indicated maximum depths of about 40 km for most of the subduction zones studied, although the Mexican subduction zone had a maximum depth of about 25 km (R. LaForge, pers. comm., 2006). The recent inversion of GPS data by McCaffrey et al. (2007) shows a significant amount of coupling (a coupling factor of 0.2-0.3) as far east as 123.8? West in some portions of the CSZ. Both of these lines of evidence lend support to the model with a north-south segment at 123.8? W.

Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska

USGS Publications Warehouse

Miller, M.L.; Bradley, D.C.; Bundtzen, T.K.; McClelland, W.

2002-01-01

New geologic mapping and geochronology show that margin-parallel strike-slip faults on the western limb of the southern Alaska orocline have experienced multiple episodes of dextral motion since ~100 Ma. These faults are on the upper plate of a subduction zone ~350-450 km inboard of the paleotrench. In southwestern Alaska, dextral displacement is 134 km on the Denali fault, at least 88-94 km on the Iditarod-Nixon Fork fault, and perhaps tens of kilometers on the Dishna River fault. The strike-slip regime coincided with Late Cretaceous sedimentation and then folding in the Kuskokwim basin, and with episodes of magmatism and mineralization at ~70, ~60, and ~30 Ma. No single driving mechanism can explain all of the ~95 million-year history of strike-slip faulting. Since ~40 Ma, the observed dextral sense of strike slip has run contrary to the sense of subduction obliquity. This may be explained by northward motion of the Pacific plate driving continental margin slivers into and/or around the oroclinal bend. From 44 to 66 Ma, oroclinal rotation, perhaps involving large-scale flexural slip, may have been accompanied by westward escape of crustal blocks along strike-slip faults. However, reconstructions of this period involve unproven assumptions about the identity of the subducting plate, the position of subducting ridges, and the exact timing of oroclinal bending, thus obscuring the driving mechanisms of strike slip. Prior to 66 Ma, oblique subduction is the most plausible driving mechanism for dextral strike slip. Cumulative displacement on all faults of the western limb of the orocline is at least 400 km, about half that on the eastern limb; this discrepancy might be explained by a combination of thrusting and unrecognized strike-slip faulting.

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

USGS Publications Warehouse

Fuis, G.S.; Moore, Thomas E.; Plafker, G.; Brocher, T.M.; Fisher, M.A.; Mooney, W.D.; Nokleberg, W.J.; Page, R.A.; Beaudoin, B.C.; Christensen, N.I.; Levander, A.R.; Lutter, W.J.; Saltus, R.W.; Ruppert, N.A.

2008-01-01

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism. ?? 2008 The Geological Society of America.

Gondwana breakup via double-saloon-door rifting and seafloor spreading in a backarc basin during subduction rollback

NASA Astrophysics Data System (ADS)

Martin, A. K.

2007-12-01

A model has been developed where two arc-parallel rifts propagate in opposite directions from an initial central location during backarc seafloor spreading and subduction rollback. The resultant geometry causes pairs of terranes to simultaneously rotate clockwise and counterclockwise like the motion of double-saloon-doors about their hinges. As movement proceeds and the two terranes rotate, a gap begins to extend between them, where a third rift initiates and propagates in the opposite direction to subduction rollback. Observations from the Oligocene to Recent Western Mediterranean, the Miocene to Recent Carpathians, the Miocene to Recent Aegean and the Oligocene to Recent Caribbean point to a two-stage process. Initially, pairs of terranes comprising a pre-existing retro-arc fold thrust belt and magmatic arc rotate about poles and accrete to adjacent continents. Terrane docking reduces the width of the subduction zone, leading to a second phase during which subduction to strike-slip transitions initiate. The clockwise rotated terrane is caught up in a dextral strike-slip zone, whereas the counterclockwise rotated terrane is entrained in a sinistral strike-slip fault system. The likely driving force is a pair of rotational torques caused by slab sinking and rollback of a curved subduction hingeline. By analogy with the above model, a revised five-stage Early Jurassic to Early Cretaceous Gondwana dispersal model is proposed in which three plates always separate about a single triple rift or triple junction in the Weddell Sea area. Seven features are considered diagnostic of double-saloon-door rifting and seafloor spreading: earliest movement involves clockwise and counterclockwise rotations of the Falkland Islands Block and the Ellsworth Whitmore Terrane respectively; terranes comprise areas of a pre-existing retro-arc fold thrust belt (the Permo-Triassic Gondwanide Orogeny) attached to an accretionary wedge/magmatic arc; the Falklands Islands Block is initially attached to Southern Patagonia/West Antarctic Peninsula, while the Ellsworth Whitmore Terrane is combined with the Thurston Island Block; paleogeographies demonstrate rifting and extension in a backarc environment relative to a Pacific margin subduction zone/accretionary wedge where simultaneous crustal shortening occurs; a ridge jump towards the subduction zone from east of the Falkland Islands to the Rocas Verdes Basin evinces subduction rollback; this ridge jump combined with backarc extension isolated an area of thicker continental crust — The Falkland Islands Block; well-documented EW oriented seafloor spreading anomalies in the Weddell Sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback; the dextral strike-slip Gastre and sub-parallel faults form one boundary of the Gondwana subduction rollback, whereas the other boundary may be formed by inferred sinistral strike-slip motion between a combined Thurston Island/Ellsworth Whitmore Terrane and Marie Byrd Land/East Antarctica.

In the Footsteps of Charles Darwin: Patterns of Coastal Subsidence and Uplift Associated with Seamount Subduction, Basal Fore-arc Erosion and Seamount Accretion in Latin America

NASA Astrophysics Data System (ADS)

Fisher, D. M.; Kirby, S. H.; David, S. W.

2004-12-01

In Geological Observations on South America (1846), Charles Darwin described beds of late Cenozoic marine seashells that were uplifted to elevations as much as several hundred meters above some localities on the western coastline of South America and implied that the whole coast was uplifting at geologic time scales. We know now that such evidence is generally restricted to coastal embayments above fore-arc basins where offshore seamounts are colliding with the South American fore arc (e.g., the Juan Fernandez seamount chain, Valpariso Basin and Valpariso Bay). We suggest that the phenomena of basal fore-arc erosion and basin formation and coastal uplift are closely related to effects of seamount subduction. Marine multibeam sonar images and multichannel seismic reflection surveys by others demonstrate that seamounts, although locally cut by normal faults in the outer-rise/near-trench region, initally subduct intact and the primary interaction with the toe of the fore arc is plowing, with material eroded from the fore arc that accumulates above and on the margins of the seamount. Submarine landslides above such regions over-steepened by plowing can lead to coastal embayments far upslope of the plowing. Such plowing interaction can therefore lead to the formation of large forearc basins and coastal embayments such as those at Valpariso, Chile, or narrow corridors of subsidence in the wake of subducting seamounts in Costa Rica. It is also known that the transition between interplate thrust seismicity, representing mechanical coupling between the plates, and aseismic slip occurs at depths of typically 30-60 km and often geographically near coastlines that mark the boundary between outer fore-arc subsidence and inner fore-arc uplift. We suggest that decoupling can occur at the base of seamounts (i.e., the originally sedimented seafloor on which the seamount lavas are laid down) and that such seamounts can be accreted to the fore arc above and lead to coastal uplift. Such basal decoupling is known to occur under active volcanic islands in the open ocean in connection with rifting and gravitational spreading, such as beneath the island of Hawaii. The spatial and temporal patterns of coastal uplift and subsidence on active margins can therefore record the local history of seamount subduction. This conceptual model explains the spatial patterns of offshore subsidence and coastal uplift in Chile and Costa Rica and also has implications for patterns of seismicity along the interplate thrust boundary.

Lithologic Controls on Structure Highlight the Role of Fluids in Failure of a Franciscan Complex Accretionary Prism Thrust Fault

NASA Astrophysics Data System (ADS)

Bartram, H.; Tobin, H. J.; Goodwin, L. B.

2015-12-01

Plate-bounding subduction zone thrust systems are the source of major earthquakes and tsunamis, but their mechanics and internal structure remain poorly understood and relatively little-studied compared to faults in continental crust. Exposures in exhumed accretionary wedges present an opportunity to study seismogenic subduction thrusts in detail. In the Marin Headlands, a series of thrusts imbricates mechanically distinct lithologic units of the Mesozoic Franciscan Complex including pillow basalt, radiolarian chert, black mudstone, and turbidites. We examine variations in distribution and character of structure and vein occurrence in two exposures of the Rodeo Cove thrust, a fossil plate boundary exposed in the Marin Headlands. We observe a lithologic control on the degree and nature of fault localization. At Black Sand Beach, deformation is localized in broad fault cores of sheared black mudstone. Altered basalts, thrust over greywacke, mudstone, and chert, retain their coherence and pillow structures. Veins are only locally present. In contrast, mudstone is virtually absent from the exposure 2 km away at Rodeo Beach. At this location, deformation is concentrated in the altered basalts, which display evidence of extensive vein-rock interaction. Altered basalts exhibit a pervasive foliation, which is locally disrupted by both foliation-parallel and cross-cutting carbonate-filled veins and carbonate cemented breccia. Veins are voluminous (~50%) at this location. All the structures are cut by anastomosing brittle shear zones of foliated cataclasite or gouge. Analyses of vein chemistry will allow us to compare the sources of fluids that precipitated the common vein sets at Rodeo Beach to the locally developed veins at Black Sand Beach. These observations lead us to hypothesize that in the absence of a mechanically weak lithology, elevated pore fluid pressure is required for shear failure. If so, the vein-rich altered basalt at Rodeo Beach may record failure of an igneous basement asperity.

Earthquakes, gravity, and the origin of the Bali Basin: An example of a Nascent Continental Fold-and-Thrust Belt

NASA Astrophysics Data System (ADS)

McCaffrey, Robert; Nabelek, John

1987-01-01

We infer from the bathymetry and gravity field and from the source mechanisms and depths of the eight largest earthquakes in the Bali region that the Bali Basin is a downwarp in the crust of the Sunda Shelf produced and maintained by thrusting along the Flores back arc thrust zone. Earthquake source mechanisms and focal depths are inferred from the inversion of long-period P and SH waves for all events and short-period P waves for two of the events. Centroidal depths that give the best fit to the seismograms range from 10 to 18 km, but uncertainties in depth allow a range from 7 to 24 km. The P wave nodal planes that dip south at 13° to 35° (±7°) strike roughly parallel to the volcanic arc and are consistent with thrusting of crust of the Bali Basin beneath it. The positions of the earthquakes with respect to crustal features inferred from seismic and gravity data suggest that the earthquakes occur in the basement along the western end of the Flores thrust zone. The slip direction for the back arc thrust zone inferred from the orientation of the earthquake slip vectors indicates that the thrusting in the Bali Basin is probably part of the overall plate convergence, as it roughly coincides with the convergence direction between the Sunda arc and the Indian Ocean plate. Summation of seismic moments of earthquakes between 1960 and 1985 suggests a minimum rate of convergence across the thrust zone of 4 ± 2 mm/a. The presence of back arc thrusting suggests that some coupling between the Indian Ocean plate and the Sunda arc occurs but mechanisms such as continental collision or a shallow subduction of the Indian Ocean plate probably can be ruled out. The present tectonic setting and structure of the Bali Basin is comparable to the early forelands of the Andes or western North America in that a fold-and-thrust belt is forming on the continental side of an arc-trench system at which oceanic lithosphere is being subducted. The Bali Basin is flanked by the Tertiary Java Basin to the west and the oceanic Flores Basin to the east and thus provides an actualistic setting for the development of a fold-and-thrust belt in which structure and timing of deformation can change significantly along strike on the scale a few hundred kilometers.

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

USGS Publications Warehouse

Schlup, Micha; Steck, Albrecht; Carter, Andrew; Cosca, Michael; Epard, Jean-Luc; Hunziker, Johannes

2011-01-01

High temperatures and anatexis of the subducting upper Indian crust engendered the buoyancy-driven ductile detachment and extrusion of the High Himalayan nappe in the zone of continental collision. Late extrusion of the High Himalayan nappe started about 26 Ma ago, accompanied by ductile extensional shearing in the Zanskar shear zone in its roof between 22 and 19 Ma concomitant with thrusting along the basal Main Central Thrust to the south. The northern part of the nappe was then rapidly exhumed to shallow depth (

Potential earthquake faults offshore Southern California, from the eastern Santa Barbara Channel south to Dana Point

USGS Publications Warehouse

Fisher, M.A.; Sorlien, C.C.; Sliter, R.W.

2009-01-01

Urban areas in Southern California are at risk from major earthquakes, not only quakes generated by long-recognized onshore faults but also ones that occur along poorly understood offshore faults. We summarize recent research findings concerning these lesser known faults. Research by the U.S. Geological Survey during the past five years indicates that these faults from the eastern Santa Barbara Channel south to Dana Point pose a potential earthquake threat. Historical seismicity in this area indicates that, in general, offshore faults can unleash earthquakes having at least moderate (M 5-6) magnitude. Estimating the earthquake hazard in Southern California is complicated by strain partitioning and by inheritance of structures from early tectonic episodes. The three main episodes are Mesozoic through early Miocene subduction, early Miocene crustal extension coeval with rotation of the Western Transverse Ranges, and Pliocene and younger transpression related to plate-boundary motion along the San Andreas Fault. Additional complication in the analysis of earthquake hazards derives from the partitioning of tectonic strain into strike-slip and thrust components along separate but kinematically related faults. The eastern Santa Barbara Basin is deformed by large active reverse and thrust faults, and this area appears to be underlain regionally by the north-dipping Channel Islands thrust fault. These faults could produce moderate to strong earthquakes and destructive tsunamis. On the Malibu coast, earthquakes along offshore faults could have left-lateral-oblique focal mechanisms, and the Santa Monica Mountains thrust fault, which underlies the oblique faults, could give rise to large (M ??7) earthquakes. Offshore faults near Santa Monica Bay and the San Pedro shelf are likely to produce both strike-slip and thrust earthquakes along northwest-striking faults. In all areas, transverse structures, such as lateral ramps and tear faults, which crosscut the main faults, could segment earthquake rupture zones. ?? 2009 The Geological Society of America.

Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent zone)

NASA Astrophysics Data System (ADS)

Phinney, Eric J.; Mann, Paul; Coffin, Millard F.; Shipley, Thomas H.

2004-10-01

Possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau-arc accretion and resulting collisional orogenesis. Deep penetration, multi-channel seismic reflection (MCS) data from the northern flank of the Solomon Islands reveal the sequence stratigraphy, structural style, and age of deformation of an accretionary prism formed during late Neogene (5-0 Ma) convergence between the ˜33-km-thick crust of the Ontong Java oceanic plateau and the ˜15-km-thick Solomon island arc. Correlation of MCS data with the satellite-derived, free-air gravity field defines the tectonic boundaries and internal structure of the 800-km-long, 140-km-wide accretionary prism. We name this prism the "Malaita accretionary prism" or "MAP" after Malaita, the largest and best-studied island exposure of the accretionary prism in the Solomon Islands. MCS data, gravity data, and stratigraphic correlations to islands and ODP sites on the Ontong Java Plateau (OJP) reveal that the offshore MAP is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the Solomon the subducting Ontong Java Plateau (Pacific plate) and transferred to the Solomon arc. With the exception of an upper, sequence of Quaternary? island-derived terrigenous sediments, the deformed stratigraphy of the MAP is identical to that of the incoming Ontong Java Plateau in the North Solomon trench. We divide the MAP into four distinct, folded and thrust fault-bounded structural domains interpreted to have formed by diachronous, southeast-to-northwest, and highly oblique entry of the Ontong Java Plateau into a former trench now marked by the Kia-Kaipito-Korigole (KKK) left-lateral strike-slip fault zone along the suture between the Solomon arc and the MAP. The structural style within each of the four structural domains consists of a parallel series of three to four fault propagation folds formed by the seaward propagation of thrust faults roughly parallel to sub-horizontal layering in the upper crystalline part of the OJP. Thrust fault offsets, spacing between thrusts, and the amplitude of related fault propagation folds progressively decrease to the west in the youngest zone of active MAP accretion (Choiseul structural domain). Surficial faulting and folding in the most recently deformed, northwestern domain show active accretion of greater than 1 km of sedimentary rock and 6 km, or about 20%, of the upper crystalline part of the OJP. The eastern MAP (Malaita and Ulawa domains) underwent an earlier, similar style of partial plateau accretion. A pre-late Pliocene age of accretion (˜3.4 Ma) is constrained by an onshore and offshore major angular unconformity separating Pliocene reefal limestone and conglomerate from folded and faulted pelagic limestone of Cretaceous to Miocene age. The lower 80% of the Ontong Java Plateau crust beneath the MAP thrust decollement appears unfaulted and unfolded and is continuous with a southwestward-dipping subducted slab of presumably denser plateau material beneath most of the MAP, and is traceable to depths >200 km in the mantle beneath the Solomon Islands.

Deformation driven by subduction and microplate collision: Geodynamics of Cook Inlet basin, Alaska

USGS Publications Warehouse

Bruhn, R.L.; Haeussler, Peter J.

2006-01-01

Late Neogene and younger deformation in Cook Inlet basin is caused by dextral transpression in the plate margin of south-central Alaska. Collision and subduction of the Yakutat microplate at the northeastern end of the Aleutian subduction zone is driving the accretionary complex of the Chugach and Kenai Mountains toward the Alaska Range on the opposite side of the basin. This deformation creates belts of fault-cored anticlines that are prolific traps of hydrocarbons and are also potential sources for damaging earthquakes. The faults dip steeply, extend into the Mesozoic basement beneath the Tertiary basin fill, and form conjugate flower structures at some localities. Comparing the geometry of the natural faults and folds with analog models created in a sandbox deformation apparatus suggests that some of the faults accommodate significant dextral as well as reverse-slip motion. We develop a tectonic model in which dextral shearing and horizontal shortening of the basin is driven by microplate collision with an additional component of thrust-type strain caused by plate subduction. This model predicts temporally fluctuating stress fields that are coupled to the recurrence intervals of large-magnitude subduction zone earthquakes. The maximum principal compressive stress is oriented east-southeast to east-northeast with nearly vertical least compressive stress when the basin's lithosphere is mostly decoupled from the underlying subduction megathrust. This stress tensor is compatible with principal stresses inferred from focal mechanisms of earthquakes that occur within the crust beneath Cook Inlet basin. Locking of the megathrust between great magnitude earthquakes may cause the maximum principal compressive stress to rotate toward the northwest. Moderate dipping faults that strike north to northeast may be optimally oriented for rupture in the ambient stress field, but steeply dipping faults within the cores of some anticlines are unfavorably oriented with respect to both modeled and observed stress fields, suggesting that elevated fluid pressure may be required to trigger fault rupture. ?? 2006 Geological Society of America.

How much does geometry of seismic sources matter in tsunami modeling? A sensitivity analysis for the Calabrian subduction interface

NASA Astrophysics Data System (ADS)

Tonini, R.; Maesano, F. E.; Tiberti, M. M.; Romano, F.; Scala, A.; Lorito, S.; Volpe, M.; Basili, R.

2017-12-01

The geometry of seismogenic sources could be one of the most important factors concurring to control the generation and the propagation of earthquake-generated tsunamis and their effects on the coasts. Since the majority of potentially tsunamigenic earthquakes occur offshore, the corresponding faults are generally poorly constrained and, consequently, their geometry is often oversimplified as a planar fault. The rupture area of mega-thrust earthquakes in subduction zones, where most of the greatest tsunamis have occurred, extends for tens to hundreds of kilometers both down dip and along strike, and generally deviates from the planar geometry. Therefore, the larger the earthquake size is, the weaker the planar fault assumption become. In this work, we present a sensitivity analysis aimed to explore the effects on modeled tsunamis generated by seismic sources with different degrees of geometric complexities. We focused on the Calabrian subduction zone, located in the Mediterranean Sea, which is characterized by the convergence between the African and European plates, with rates of up to 5 mm/yr. This subduction zone has been considered to have generated some past large earthquakes and tsunamis, despite it shows only in-slab significant seismic activity below 40 km depth and no relevant seismicity in the shallower portion of the interface. Our analysis is performed by defining and modeling an exhaustive set of tsunami scenarios located in the Calabrian subduction and using different models of the subduction interface with increasing geometrical complexity, from a planar surface to a highly detailed 3D surface. The latter was obtained from the interpretation of a dense network of seismic reflection profiles coupled with the analysis of the seismicity distribution. The more relevant effects due to the inclusion of 3D complexities in the seismic source geometry are finally highlighted in terms of the resulting tsunami impact.

Structure of backarc inner rifts as a weakest zone of arc-backarc system: a case study of the Sea of Japan

NASA Astrophysics Data System (ADS)

Sato, Hiroshi; Ishiyama, Tasuya; Kato, Naoko; Abe, Susumu; Saito, Hideo; Shiraishi, Kazuya; Abe, Shiori; Iwasaki, Takaya; Inaba, Mitsuru; No, Tetsuo; Sato, Takeshi; Kodaira, Shuichi; Takeda, Tetsuya; Matsubara, Makoto; Kodaira, Chihiro

2015-04-01

A backarc inner rift is formed after a major opening of backarc basin near a volcanic front away from the spreading center of a major backarc basin. An obvious example is the inner rift along the Izu-Bonin arc. Similar inner rift zones have been developed along the Sea of Japan coast of Honshu island, Japan. NE and SW Japan arcs experienced strong shortening after the Miocene backarc rifting. The amount of shortening shows its maximum along the backarc inner rifts, forming a fold-and-thrust of thick post-rift sediments over all the structure of backarc. The rift structure has been investigated by onshore-offshore deep seismic reflection/wide-angle reflection surveys. We got continuous onshore-offshore image using ocean bottom cable and collected offshore seismic reflection data using two ships to obtain large offset data in the difficult area for towing a long streamer cable. The velocity structure beneath the rift basin was deduced by refraction tomography in the upper curst and earthquake tomography in the deeper part. It demonstrates larger P-wave velocity in upper mantle and lower crust, suggesting a large amount of mafic intrusion and thinning of upper continental crust. The deeper seismicity in the lower crust beneath the rift basin accords well to the mafic intrusive rocks. Syn-rift volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation. Continued compression produced fault-related folds in the post-rift sediments, characterized by thin-skin style of deformation. The syn-rift mafic intrusion in the crust forms convex shape and the boundary between pre-rift crust and mafic intrusive shows outward dipping surface. Due to the post rift compression, the boundary of rock units reactivated as reverse faults, commonly forming a large-scale wedge thrust and produced subsidence of rift basin under compressional stress regime. Large amount of convergence of overriding plate is accommodated along the inner rift, suggesting that it is a weakest zone in whole arc-backarc system. The convergence between young (15 Ma) Shikoku basin and SW Japan arc produced intense shortening along the inner failed rift along the Sea of Japan coast. After the onset of subduction along the Nankai trough, the fold-and-thrust belt was covered by Pliocene marine sediment. Before the 2011 off-Tohoku earthquake (M9), several damaging earthquakes occurred along the backarc fold-and-thrust belt. These represents that a weak backarc inner rift is very sensitive for the stress produce by the subduction interface.

Geochemistry and field geology of shoshonitic magmas in the Late Cretaceous foreland fold and thrust belt of southwestern Montana: Results from the North Doherty Mountain Intrusive Complex

NASA Astrophysics Data System (ADS)

Beranek, L. P.; Burton, B. R.; Ihinger, P. D.

2002-12-01

The North Doherty Mountain Intrusive Complex (NDMIC) is one of several satellite plutons related to the areally extensive Boulder batholith of southwestern Montana. The Boulder batholith comprises multiple plutons and intrusive phases, and the magmatism has long been thought to be the result of subduction due to its calc-alkaline granodioritic composition. The batholith is situated in the Helena salient, which differs from other parts of the North American Cordilleran foreland because there, magmatism spatially and temporally overlaps with deformation in the foreland fold and thrust belt. The North Doherty Mountain Intrusive Complex (NDMIC) is one of several satellite plutons related to the Boulder batholith and represents an ideal microcosm of the batholith for petrogenetic and structural studies because it exposes both mafic and felsic units and was emplaced in the limb of a major thrust related fold. We present new geologic mapping and detailed trace element geochemical analyses to show that the entire mafic-to-felsic suite of rocks in the NDMIC are cogenetic and shoshonitic in character. Shoshonites are unusual magmas that are distinguished by their high concentrations of K, Rb, Sr, Ba, Zr, and Th contents, and are thought to represent partial melting at great depths within the mantle wedge above a subducting slab. The presence of shoshonitic magma in the Cordilleran foreland fold and thrust belt provides important clues into the nature of the formation of this unusual magma type and can provide insights into our understanding of magmatism in foreland structural settings.

Foreland sedimentary record of Andean mountain building during advancing and retreating subduction

NASA Astrophysics Data System (ADS)

Horton, Brian K.

2016-04-01

As in many ocean-continent (Andean-type) convergent margins, the South American foreland has long-lived (>50-100 Myr) sedimentary records spanning not only protracted crustal shortening, but also periods of neutral to extensional stress conditions. A regional synthesis of Andean basin histories is complemented by new results from the Mesozoic Neuquén basin system and succeeding Cenozoic foreland system of west-central Argentina (34-36°S) showing (1) a Late Cretaceous shift from backarc extension to retroarc contraction and (2) an anomalous mid-Cenozoic (~40-20 Ma) phase of sustained nondeposition. New detrital zircon U-Pb geochronological results from Jurassic through Neogene clastic deposits constrain exhumation of the evolving Andean magmatic arc, retroarc thrust belt, foreland basement uplifts, and distal eastern craton. Abrupt changes in sediment provenance and distal-to-proximal depositional conditions can be reconciled with a complex Mesozoic-Cenozoic history of extension, post-extensional thermal subsidence, punctuated tectonic inversion involving thick- and thin-skinned shortening, alternating phases of erosion and rapid accumulation, and overlapping igneous activity. U-Pb age distributions define the depositional ages of several Cenozoic stratigraphic units and reveal a major late middle Eocene-earliest Miocene (~40-20 Ma) hiatus in the Malargüe foreland basin. This boundary marks an abrupt shift in depositional conditions and sediment sources, from Paleocene-middle Eocene distal fluviolacustrine deposition of sediments from far western volcanic sources (Andean magmatic arc) and subordinate eastern cratonic basement (Permian-Triassic Choiyoi igneous complex) to Miocene-Quaternary proximal fluvial and alluvial-fan deposition of sediments recycled from emerging western sources (Malargüe fold-thrust belt) of Mesozoic basin fill originally derived from basement and magmatic arc sources. Neogene eastward advance of the fold-thrust belt involved thick-skinned basement inversion with geometrically and kinematically linked thin-skinned thrust structures at shallower levels in the eastern foreland, including well-dated late Miocene growth strata. The mid-Cenozoic hiatus potentially signifies nondeposition during passage of a flexural forebulge or nondeposition during neutral to extensional conditions possibly driven by a transient retreating-slab configuration along the western margin of South America. Similar long-lived stratigraphic gaps are commonly observed in other foreland records of continental convergent margins. It is proposed that Andean orogenesis along the South American convergent margin has long been sensitive to variations in subduction dynamics throughout Mesozoic-Cenozoic time, such that shifts in relative convergence and degree of mechanical coupling along the subduction interface (i.e., transitions between advancing versus retreating modes of subduction) have governed fluctuating contractional, extensional, and neutral conditions. Unclear is whether these various modes affected the entire convergent margin simultaneously due to continental-scale changes (e.g., temporal shifts in plate convergence, absolute motion of upper plate, or mantle wedge circulation) or whether parts of the margin behaved independently due to smaller-scale fluctuations (e.g., spatial variations in the age of the subducted plate, buoyant asperities in the downgoing slab, or asthenospheric anomalies).

Local thickening of the Cascadia forearc crust and the origin of seismic reflectors in the uppermost mantle

USGS Publications Warehouse

Calvert, A.J.; Ramachandran, K.; Kao, H.; Fisher, M.A.

2006-01-01

Seismic reflection profiles from three different surveys of the Cascadia forearc are interpreted using P wave velocities and relocated hypocentres, which were both derived from the first arrival travel time inversion of wide-angle seismic data and local earthquakes. The subduction decollement, which is characterized beneath the continental shelf by a reflection of 0.5 s duration, can be traced landward into a large duplex structure in the lower forearc crust near southern Vancouver Island. Beneath Vancouver Island, the roof thrust of the duplex is revealed by a 5–12 km thick zone, identified previously as the E reflectors, and the floor thrust is defined by a short duration reflection from a − 1. We suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion. The absence of seismicity from within the E reflectors implies that they are significantly weaker than the overlying crust, and the reflectors may be a zone of active ductile shear. In contrast, seismicity in parts of the D reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex. Merging of the D and E reflectors at 42–46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km. We suggest that pervasive reflectivity in the upper mantle elsewhere beneath Puget Sound and the Strait of Georgia arises from similar shear zones.

Slab-pull and slab-push earthquakes in the Mexican, Chilean and Peruvian subduction zones

NASA Astrophysics Data System (ADS)

Lemoine, A.; Madariaga, R.; Campos, J.

2002-09-01

We studied intermediate depth earthquakes in the Chile, Peru and Mexican subduction zones, paying special attention to slab-push (down-dip compression) and slab-pull (down-dip extension) mechanisms. Although, slab-push events are relatively rare in comparison with slab-pull earthquakes, quite a few have occurred recently. In Peru, a couple slab-push events occurred in 1991 and one slab-pull together with several slab-push events occurred in 1970 near Chimbote. In Mexico, several slab-push and slab-pull events occurred near Zihuatanejo below the fault zone of the 1985 Michoacan event. In central Chile, a large M=7.1 slab-push event occurred in October 1997 that followed a series of four shallow Mw>6 thrust earthquakes on the plate interface. We used teleseismic body waveform inversion of a number of Mw>5.9 slab-push and slab-pull earthquakes in order to obtain accurate mechanisms, depths and source time functions. We used a master event method in order to get relative locations. We discussed the occurrence of the relatively rare slab-push events in the three subduction zones. Were they due to the geometry of the subduction that produces flexure inside the downgoing slab, or were they produced by stress transfer during the earthquake cycle? Stress transfer can not explain the occurence of several compressional and extensional intraplate intermediate depth earthquakes in central Chile, central Mexico and central Peru. It seemed that the heterogeneity of the stress field produced by complex slab geometry has an important influence on intraplate intermediate depth earthquakes.

Modeling the Geometry of Plate Boundary and Seismic Structure in the Southern Ryukyu Trench Subduction Zone, Japan, Using Amphibious Seismic Observations

NASA Astrophysics Data System (ADS)

Yamamoto, Y.; Takahashi, T.; Ishihara, Y.; Kaiho, Y.; Arai, R.; Obana, K.; Nakanishi, A.; Miura, S.; Kodaira, S.; Kaneda, Y.

2018-02-01

Here we present the new model, the geometry of the subducted Philippine Sea Plate interface beneath the southern Ryukyu Trench subduction zone, estimated from seismic tomography and focal mechanism estimation by using passive and active data from a temporary amphibious seismic network and permanent land stations. Using relocated low-angle thrust-type earthquakes, repeating earthquakes, and structural information, we constrained the geometry of plate boundary from the trench axis to a 60 km depth with uncertainties of less than 5 km. The estimated plate geometry model exhibited large variation, including a pronounced convex structure that may be evidence of a subducted seamount in the eastern portion of study area, whereas the western part appeared smooth. We also found that the active earthquake region near the plate boundary, defined by the distance from our plate geometry model, was clearly separated from the area dominated by short-term slow-slip events (SSEs). The oceanic crust just beneath the SSE-dominant region, the western part of the study area, showed high Vp/Vs ratios (>1.8), whereas the eastern side showed moderate or low Vp/Vs (<1.75). We interpreted this as an indication that high fluid pressures near the surface of the slab are contributing to the SSE activities. Within the toe of the mantle wedge, P and S wave velocities (<7.5 and <4.2 km/s, respectively) lower than those observed through normal mantle peridotite might suggest that some portions of the mantle may be at least 40% serpentinized.

Submarine gas seepage in a mixed contractional and shear deformation regime: Cases from the Hikurangi oblique-subduction margin

NASA Astrophysics Data System (ADS)

Plaza-Faverola, Andreia; Pecher, Ingo; Crutchley, Gareth; Barnes, Philip M.; Bünz, Stefan; Golding, Thomas; Klaeschen, Dirk; Papenberg, Cord; Bialas, Joerg

2014-02-01

Gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir, seabed ecology, and climate change. Although the relationship between hydrates, gas chimneys, and seafloor seepage is well established, the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems. In this study, we present the analysis of new three-dimensional high-resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at Omakere Ridge, on the Hikurangi subduction margin, New Zealand. The analysis reveals at high resolution, complex overprinting fault structures (i.e., protothrusts, normal faults from flexural extension, and shallow (<1 km) arrays of oblique shear structures) implicated in fluid migration within the gas hydrate stability zone in an area of 2 × 7 km. In addition to fluid migration systems sustaining seafloor seepage on both sides of a central thrust fault, the data show seismic evidence for subseafloor gas-rich fluid accumulation associated with proto-thrusts and extensional faults. In these latter systems fluid pressure dissipation through time has been favored, hindering the development of gas chimneys. We discuss the elements of the distinct fluid migration systems and the influence that a complex partitioning of stress may have on the evolution of fluid flow systems in active subduction margins.

Uncoupled vs. coupled thrust belt-foreland deformation: a model for northern Patagonia inferred from U-Th/He and apatite fission track dating

NASA Astrophysics Data System (ADS)

Savignano, Elisa; Mazzoli, Stefano; Zattin, Massimiliano; Gautheron, Cécile; Franchini, Marta

2017-04-01

The study of the Cretaceous - Cenozoic evolution of the Patagonian Andes represents a great opportunity to investigate the effects of coupling between deep lithospheric processes and near-surface deformation. Low-temperature thermochronological systems are ideally suited for detecting events involving rocks in the uppermost part of the crust because they record time and rates of cooling related to exhumation of the top few kilometers of the crust. The Patagonia region, although characterized by a general continuity of the Andean orogen along its strike, shows an appreciable internal tectonic segmentation (marked by a variable position of the magmatic arc and of the deformation front in the retroarc area) at various latitudes. This complex structural architecture has been interpreted as the result of different processes acting since the Late Cretaceous. The present-day configuration of the southern Andes is interpreted to have been controlled by alternating stages of flat- and steep-slab subduction, which produced shortening and upper plate extension episodes,, respectively. Furthermore, the deformation in this whole retroarc sector varied not only in time (i.e. with major 'cycles' of mountain building and orogenic collapse), but also in space, due to the variable transmission of horizontal compressive stress away from the orogen, that produced an irregular unroofing pattern. In this study, we have integrated field structural observations with new apatite (U-Th)/He data (AHe) and apatite fission-track (AFT) ages in the north Patagonia region (at latitudes between 40° and 44°S) in order to analyse and compare the exhumation patterns from the frontal part of the orogen and from the adjacent foreland sector, as well as to gain new insights into the timing and modes of coupling vs. uncoupling of the deformation between the northern Patagonian fold and thrust belt and its foreland. The obtained data indicate a markedly different unroofing pattern between the 'broken foreland' area (characterized by Late Cretaceous to Paleogene exhumation) and the adjacent Andean sector to the west, which is dominated by Miocene-Pliocene exhumation. Our study supports the idea that the configuration of the slab (flat vs. steep) during subduction controls the coupling vs. uncoupling of the deformation between the thrust belt and the foreland. Along the studied transect, late Miocene to Pliocene AHe ages from the frontal part of the northern Patagonian Andes correlate well with a rapid recent shortening and exhumation stage that took place in the thrust belt during steep-slab subduction and rollback. On the other hand, AHe ages obtained for the 'broken foreland' unravelled exhumation at near-surface conditions during Late Cretaceous to Paleogene times, when a prolonged phase of flat-slab subduction favoured the coupling between the thrust belt and the foreland area and associated widespread shortening able to reactivate inherited rift-related structures.

Geo-Thermochronometric Insights on the Cycladic Basement and Cycladic Blueschist Unit Contact in the Southern Cyclades, Ios Island, Greece

NASA Astrophysics Data System (ADS)

Flansburg, M. E.; Stockli, D. F.; Poulaki, E. M.; Soukis, K. I.

2017-12-01

The North Cycladic Detachment System, the West Cycladic Detachment System, and the Naxos-Paros Detachment accommodated large-scale Oligo-Miocene exhumation in the backarc of the retreating Hellenic subduction zone. While bivergent detachment faults in the northern and western Cyclades are either contained within the Cycladic Blueschist Unit (CBU) or at the CBU-Upper Unit interface, the sheared contact between the CBU and the underlying Cycladic Basement in the southern Cyclades (Ios) has been debated for over 30 years, largely due to the ambiguous coexistence of both top-to-the-N and top-to-the-S shear sense indicators and a lack of robust timing information. Reliable chronostratigraphic and thermal history constraints allow us to test whether the contact is a low-angle normal fault-possibly part of a larger detachment system-or the South Cycladic Thrust by placing absolute ages on deformation, determining older over younger relationships or vice versa, and quantifying possible differential exhumation during Cenozoic extension. Zircon U-Pb dating for the granitic Basement core of Ios gave Carboniferous-Permian age and shows that surrounding Basement metasedimentary units can be divided into two groups based on detrital zircon signatures. An older group of metasedimentary rocks have maximum depositional ages (MDAs) ranging from 450 Ma to 354 Ma and predate the intrusions, and late Permian Basement paragneisses are younger than the intrusions and likely originally deposited unconformably on the older units. Samples from the CBU in northern Ios yielded MDAs ranging from Mid-Jurassic to Late Cretaceous and appear to be repeated due to either thrusting or subduction accretion and exhibit older over younger relationships. MDA data from mapped CBU at the southern end of Ios yielded Ordovician to Permian ages, calling into question their assignment as CBU, while also revealing older over younger relationships. Zircon (U-Th)/He ages for the Basement and the CBU on Ios are 9-14 Ma and do not exhibit any differential cooling-suggesting that they were juxtaposed prior to Miocene detachment faulting and exhumed together in response to top-to-the-N detachment faulting. This is supported by the fact that both units experienced Eocene subduction metamorphism as evidenced by 60-45 Ma metamorphic zircon rims.

Temporary Seismic Installation in Eastern Bangladesh: Microseismicity and Structure of an On-Land Accretionary Prism

NASA Astrophysics Data System (ADS)

Foster, A. E.; Almeida, R. V.; Akhter, S. H.; Hubbard, J.; Bhattacharya, R.; Shing, U. M.; Hosain, A.; Bulbul, M. A.

2016-12-01

Eastern Bangladesh is underlain by a fold and thrust belt accumulating 13-17 mm/yr of plate convergence on a locked, shallowly dipping décollement (Steckler et al., 2016). We have installed a network of short-period and broadband instruments in this region to better assess microseismicity and investigate the structure of the fold belt. Stations will remain in place for 1-2 years and will complement the temporary stations installed by the University of Dhaka and the Lamont-Doherty Earth Observatory. Thirteen Lennartz 1-Hz instruments and three Trillium Compact 120-s instruments were installed in the Sylhet area in June 2016. Station spacing is between 15-30 km. Installations ranged from shallow vaults to above-ground locations on solid foundations, as required by local conditions. One focus of this deployment is to locate microseismicity. Neither large numbers nor magnitudes of earthquakes are expected in this area, but improved locations may help define active areas of the convergence as well as illuminate the interactions between the Dauki fault (responsible for uplift of the Shillong Plateau), the Sylhet anticline, and the rest of the Indo-Burman Ranges. Nine additional short-period instruments and three additional broadband instruments will be installed in the Chittagong area in September 2016. The southern-most stations are located over the Andaman subduction zone, thought to be associated with a large subduction earthquake in 1762 (Steckler et al., 1998). Thus, the full deployment will span the transition from an on-land accretionary prism to an area considered a classic subduction zone. Combined, the northern and southern parts of the network should provide good locations for events within the Tripura area (India) as well. We present initial estimates of station quality and the potential for an earthquake catalogue. We plan to obtain earthquake locations and focal mechanisms, when possible, as the data is collected. We will apply ambient noise tomography methods to investigate the structure of the fold belt. These applications and others may contribute to unanswered questions in the area, such as the location of the frontal-most deformation associated with the fold and thrust belt and the geometry of the basal décollement at depth, and allow better characterization of the risk to nearby populations.

Arc-arc Collision Structure in the Southernmost Part of the Kuril Trench Region -Results from Integrated Analyses of the 1998-2000 Hokkaido Transect Seismic Data-

NASA Astrophysics Data System (ADS)

Iwasaki, Takaya; Tsumura, Noriko; Ito, Tanio; Sato, Hiroshi; Kurashimo, Eiji; Hirata, Naoshi; Arita, Kazunori; Noda, Katsuya; Fujiwara, Akira; Abe, Susumu; Kikkuchi, Shunsuke; Suzuki, Kazuko

2015-04-01

The Hokkaido Island, located in the southernmost part of the Kuril trench region, has been under a unique tectonic environment of arc-arc collision. Due to the oblique subduction of the Pacific (PAC) plate, the Kuril forearc sliver started to collide against Northeast (NE) Japan arc from the east at the time of middle Miocene to form complicated structures in the Hidaka collision zone (HCZ), as characterized by the westward obduction of the crustal rocks of the Kuril arc (the Hidaka metamorphic belt (HMB)) along the Hidaka main thrust (HMT) and a thick foreland fold-and-thrust belt. In and around the HCZ, a series of seismic reflection/refraction experiments were undertaken from 1994 to 2000, which provided important structural features including crustal delamination in the southern HCZ and a thick fold-and-thrust belt with velocity reversals (low velocity layers) in the northern HCZ. Reprocessing/reinterpretation for these data sets, which started in 2012, is aimed to construct a more detailed collision model through new processing and interpretation techniques. A multi-disciplinary project of the 1998-2000 Hokkaido Transect, crossing the northern part of the HCZ in EW direction, collected high-quality seismic data on a 227-km seismic refraction/wide-angle reflection profile and three seismic reflection lines. Our reanalyses revealed interesting collision structure ongoing in the northern part of the HCZ. The westward obduction of the Kuril arc crust was clearly imaged along the HMT. This obduction occurs at a depth of 27-30 km, much deeper than in the southern HCZ (23-25 km). The CRS/MDRS processing to the reflection data firstly succeeded in imaging clear reflection events at a 30-45 km depth below the obducted Kuril arc crust. These events show an eastward dip, probably corresponding to the lower crust/Moho within the NE Japan arc descending down to the east under the collision zone. Gently eastward dipping structures above these events (in a depth range of 5-10 km) are interpreted to be fragments of Cretaceous subduction/arc complexes or deformation interfaces branched from the HMT. The refraction/wide-angle reflection analysis revealed a series of eastward dipping interfaces at depths of 15-30 km east of the HMT, some of which show a very large Vp contrast exceeding 0.5-1.0 km/s. The subducted NE Japan arc meets the Kuril arc 20-40 km east of the HMT at a depth of 20-30 km. The above mentioned high Vp contrasts may result from the mixture of the upper crustal (low Vp) materials of the NE Japan arc and lower crustal (high Vp) materials of the Kuril arc. Seismic reflection image in the southern HCZ reprocessed by almost the same techniques confirms a clear crustal delamination, where the upper 23-km crust is thrust up along the HMT while the lower part of the crust descends down to the subducted PAC plate. At the moment, the results in the northern HCZ do not provide positive evidence on shallow crustal delamination as found in the case of the southern HCZ, suggesting regional difference in collision style along the HMT.

Nurture Versus Nature: Accounting for the Differences Between the Taiwan and Timor active arc-continent collisions

NASA Astrophysics Data System (ADS)

Harris, R. A.

2011-12-01

The active Banda arc/continent collision of the Timor region provides many important contrasts to what is observed in Taiwan, which is mostly a function of differences in the nature of the subducting plate. One of the most important differences is the thermal state of the respective continental margins: 30 Ma China passive margin versus 160 Ma NW Australian continental margin. The subduction of the cold and strong NW Australian passive margin beneath the Banda trench provides many new constraints for resolving longstanding issues about the formative stages of collision and accretion of continental crust. Some of these issues include evidence for slab rollback and subduction erosion, deep continental subduction, emplacement or demise of forearc basement, relative amounts of uplift from crustal vs. lithospheric processes, influence of inherited structure, partitioning of strain away from the thrust front, extent of mélange development, metamorphic conditions and exhumation mechanisms, continental contamination and accretion of volcanic arcs, does the slab tear, and does subduction polarity reverse? Most of these issues link to the profound control of lower plate crustal heterogeneity, thermal state and inherited structure. The thermomechanical characteristics of subducting an old continental margin allow for extensive underthrusting of lower plate cover units beneath the forearc and emplacement and uplift of extensive nappes of forearc basement. It also promotes subduction of continental crust to deep enough levels to experience high pressure metamorphism (not found in Taiwan) and extensive contamination of the volcanic arc. Seismic tomography confirms subduction of continental lithosphere beneath the Banda Arc to at least 400 km with no evidence for slab tear. Slab rollback during this process results in massive subduction erosion and extension of the upper plate. Other differences in the nature of the subducting plates in Taiwan in Timor are differences in the lateral continuity of the continental margins. The northern Australian continental margin is highly irregular with many rift basins subducting parallel to their axes. This feature gives rise to irregularities in the uplift pattern of the collision and its continental margin parallel structural grain. Another major difference between Taiwan and Timor is the mechanical stratigraphy entering the trench. The Australian continental margin bears a carbonate rich pre and post rift sequence that is separated by a 1000 m thick, over pressured mudstone unit that acts as major detachment and promotes extensive mud diapirism. The post breakup Australian Passive Margin Sequence is incorporated into the orogenic wedge by frontal accretion and forms a classic imbricate thrust stack near the front of the Banda forearc. The pre breakup Gondwana Sequence below the detachment continues at least to depth of 30 km in the subduction channel beneath the Banda forearc upper plate and stacks up into a duplex zone that forms structural culminations throughout Timor. The upper plate of both collisions is similar in nature but is deformed in different ways due to the strong influence of the lower plate. However, both have extensive subduction erosion and demise of the forearc and systematic accretion of the arc.

Complexity in Size, Recurrence and Source of Historical Earthquakes and Tsunamis in Central Chile

NASA Astrophysics Data System (ADS)

Cisternas, M.

2013-05-01

Central Chile has a 470-year-long written earthquake history, the longest of any part of the country. Thanks to the early and continuous Spanish settlement of this part of Chile (32°- 35° S), records document destructive earthquakes and tsunamis in 1575, 1647, 1730, 1822, 1906 and 1985. This sequence has promoted the idea that central Chile's large subduction inter-plate earthquakes recur at regular intervals of about 80 years. The last of these earthquakes, in 1985, was even forecast as filling a seismic gap on the thrust boundary between the subducting Nazca Plate and the overriding South America Plate. Following this logic, the next large earthquake in metropolitan Chile will not occur until late in the 21st century. However, here I challenge this conclusion by reporting recently discovered historical evidence in Spain, Japan, Peru, and Chile. This new evidence augments the historical catalog in central Chile, strongly suggests that one of these earthquakes previously assumed to occur on the inter-plate interface in fact occurred elsewhere, and forces the conclusion that another of these earthquakes (and its accompanying tsunami) dwarfed the others. These findings complicate the task of assessing the hazard of future earthquakes in Chile's most populated region.

Forearc structure beneath southwestern British Columbia: A three-dimensional tomographic velocity model

USGS Publications Warehouse

Ramachandran, K.; Dosso, S.E.; Spence, G.D.; Hyndman, R.D.; Brocher, T.M.

2005-01-01

This paper presents a three-dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting Juan de Fuca plate beneath southwestern British Columbia and the adjoining straits of Georgia and Juan de Fuca. The velocity model was constructed through joint tomographic inversion of 50,000 first-arrival times from earthquakes and active seismic sources. Wrangellia rocks of the accreted Paleozoic and Mesozoic island arc assemblage underlying southern Vancouver Island in the Cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6.5-7.2 km/s, similar to observations at other island arc terranes. The mafic Eocene Crescent terrane, thrust landward beneath southern Vancouver Island, exhibits crustal velocities in the range of 6.0-6.7 km/s and is inferred to extend to a depth of more than 20 km. The Cenozoic Olympic Subduction Complex, an accretionary prism thrust beneath the Crescent terrane in the Olympic Peninsula, is imaged as a low-velocity wedge to depths of at least 20 km. Three zones with velocities of 7.0-7.5 km/s, inferred to be mafic and/or ultramafic units, lie above the subducting Juan de Fuca plate at depths of 25-35 km. The forearc upper mantle wedge beneath southeastern Vancouver Island and the Strait of Georgia exhibits low velocities of 7.2-7.5 km/s, inferred to correspond to ???20% serpentinization of mantle peridotites, and consistent with similar observations in other warm subduction zones. Estimated dip of the Juan de Fuca plate beneath southern Vancouver Island is ???11??, 16??, and 27?? at depths of 30, 40, and 50 km, respectively. Copyright 2005 by the American Geophysical Union.

Structure and tectonics of the Main Himalayan Thrust and associated faults from recent earthquake and seismic imaging studies using the NAMASTE array

NASA Astrophysics Data System (ADS)

Karplus, M. S.; Pant, M.; Velasco, A. A.; Nabelek, J.; Kuna, V. M.; Sapkota, S. N.; Ghosh, A.; Mendoza, M.; Adhikari, L. B.; Klemperer, S. L.

2017-12-01

The India-Eurasia collision zone presents a significant earthquake hazard, as demonstrated by the recent, devastating April 25, 2015 M=7.8 Gorkha earthquake and the following May 12, 2015 M=7.3 earthquake. Important questions remain, including distinguishing possible geometries of the Main Himalayan Thrust (MHT), the role of other regional faults, the crustal composition and role of fluids in faulting, and the details of the rupture process, including structural causes and locations of rupture segmentation both along-strike and down-dip. These recent earthquakes and their aftershocks provide a unique opportunity to learn more about this collision zone. In June 2015, funded by NSF, we deployed the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake (NAMASTE) array of 46 seismic stations distributed across eastern and central Nepal, spanning the region with most of the aftershocks. This array remained in place for 11 months from June 2015 to May 2016. We combine new results from this aftershock network in Nepal with previous geophysical and geological studies across the Himalaya to derive a new understanding of the tectonics of the Himalaya and southern Tibet in Nepal and surrounding countries. We focus on structure and composition of the Main Himalayan Thrust and compare this continent-continent subduction megathrust with megathrusts in other subduction zones.

Slip distribution and tectonic implication of the 1999 Chi-Chi, Taiwan, earthquake

USGS Publications Warehouse

Ji, C.; Helmberger, D.V.; Song, T.-R.A.; Ma, K.-F.; Wald, D.J.

2001-01-01

We report on the fault complexity of the large (Mw = 7.6) Chi-Chi earthquake obtained by inverting densely and well-distributed static measurements consisting of 119 GPS and 23 doubly integrated strong motion records. We show that the slip of the Chi-Chi earthquake was concentrated on the surface of a "wedge shaped" block. The inferred geometric complexity explains the difference between the strike of the fault plane determined by long period seismic data and surface break observations. When combined with other geophysical and geological observations, the result provides a unique snapshot of tectonic deformation taking place in the form of very large (>10m) displacements of a massive wedge-shaped crustal block which may relate to the changeover from over-thrusting to subducting motion between the Philippine Sea and the Eurasian plates.

Splay fault slip in a subduction margin, a new model of evolution

NASA Astrophysics Data System (ADS)

Conin, Marianne; Henry, Pierre; Godard, Vincent; Bourlange, Sylvain

2012-08-01

In subduction zones, major thrusts called splay faults are thought to slip coseismically during large earthquakes affecting the main plate interface. We propose an analytical condition for the activation of a splay fault based on force balance calculations and suggest thrusting along the splay fault is generally conditioned by the growth of the accretionary wedge, or by the erosion of the hanging wall. In theory, normal slip on the splay fault may occur when the décollement has a very low friction coefficient seaward. Such a low friction also implies an unstable extensional state within the outer wedge. Finite element elasto-plastic calculations with a geometry based on the Nankai Kumano section were performed and confirm that this analytical condition is a valid approximation. Furthermore, localized extension at a shallow level in the splay hanging wall is observed in models for a wide range of friction coefficients (from ∼0 to the value of internal friction coefficient of the rock, here equals to 0.4). The timing of slip established for the splay fault branch drilled on Nankai Kumano transect suggests a phase of concurrent splay and accretionary wedge growth ≈2 Ma to ≈1.5 Ma, followed by a locking of the splay ≈1.3 Ma. Active extension is observed in the hanging wall. This evolution can be explained by the activation of a deeper and weaker décollement, followed by an interruption of accretion. Activation of a splay as a normal fault, as hypothesized in the case of the Tohoku 2011 earthquake, can be achieved only if the friction coefficient on the décollement drops to near zero. We conclude that the tectonic stress state largely determines long-term variations of tightly related splay fault and outer décollement activity and thus influences where and how coseismic rupture ends, but that occurrence of normal slip on a splay fault requires coseismic friction reduction.

Dynamic Simulations for the Seismic Behavior on the Shallow Part of the Fault Plane in the Subduction Zone during Mega-Thrust Earthquakes

NASA Astrophysics Data System (ADS)

Tsuda, K.; Dorjapalam, S.; Dan, K.; Ogawa, S.; Watanabe, T.; Uratani, H.; Iwase, S.

2012-12-01

The 2011 Tohoku-Oki earthquake (M9.0) produced some distinct features such as huge slips on the order of several ten meters around the shallow part of the fault and different areas with radiating seismic waves for different periods (e.g., Lay et al., 2012). These features, also reported during the past mega-thrust earthquakes in the subduction zone such as the 2004 Sumatra earthquake (M9.2) and the 2010 Chile earthquake (M8.8), get attentions as the distinct features if the rupture of the mega-thrust earthquakes reaches to the shallow part of the fault plane. Although various kinds of observations for the seismic behavior (rupture process and ground motion characteristics etc.) on the shallow part of the fault plane during the mega-trust earthquakes have been reported, the number of analytical or numerical studies based on dynamic simulation is still limited. Wendt et al. (2009), for example, revealed that the different distribution of initial stress produces huge differences in terms of the seismic behavior and vertical displacements on the surface. In this study, we carried out the dynamic simulations in order to get a better understanding about the seismic behavior on the shallow part of the fault plane during mega-thrust earthquakes. We used the spectral element method (Ampuero, 2009) that is able to incorporate the complex fault geometry into simulation as well as to save computational resources. The simulation utilizes the slip-weakening law (Ida, 1972). In order to get a better understanding about the seismic behavior on the shallow part of the fault plane, some parameters controlling seismic behavior for dynamic faulting such as critical slip distance (Dc), initial stress conditions and friction coefficients were changed and we also put the asperity on the fault plane. These understandings are useful for the ground motion prediction for future mega-thrust earthquakes such as the earthquakes along the Nankai Trough.

Neogene-Quaternary evolution of the offshore sector of the Southern Apennines accretionary wedge, Gulf of Taranto, Italy

NASA Astrophysics Data System (ADS)

Teofilo, G.; Antoncecchi, I.; Caputo, R.

2018-07-01

Southern Apennines represent a collisional orogenic belt whose compressional regime is commonly assumed to have ceased during Middle Quaternary. On the other hand, to the south the Calabria Arc is still characterized by subduction and the principal aim of the present research is to shed some light on the space and time transition from the ceased collision to the active subduction. Accordingly, we investigated the offshore sector of the Southern Apennines accretionary wedge, corresponding to the Taranto Gulf. To gain insights into the offshore accretionary wedge, we reconstructed a 3D geological and tectonic model by interpreting a grid of 40 seismic reflection lines (1100 km, 80 intersections), within an area of ca. 104 km2, calibrated with 17 wells. The geometric and chronological constraints allow documenting a systematic Messinian-Quaternary thrust migration from internal towards external sectors of the wedge. The migrating deformational process was essentially associated with a leading-imbricate thrust system with a general NE-younging direction, where we could recognize and distinguish some major advancing phases characterized by alternating fast thrust propagation events and strain accumulation periods within the wedge. This process is well emphasized by the jump of the foredeep and piggy-back basins. The NE-wards wedge migration was also associated with a lithospheric-scale flexural folding that generated a set of normal faults striking parallel to the coeval thrusts, likely reactivating optimally oriented structures inherited from Mesozoic events. Finally, a persisting thrust activity up to the latest Quaternary and possibly up to Present in correspondence of the externalmost sector of the accretionary wedge has been documented and explained in terms of strain partitioning in the frame of a recent oblique convergence. The results of this research have possible implications for the seismic hazard assessment of the broader region which is possibly greater than previously assumed.

Seismicity of the Earth 1900-2012 Java and vicinity

USGS Publications Warehouse

Jones, Eric S.; Hayes, Gavin P.; Bernardino, Melissa; Dannemann, Fransiska K.; Furlong, Kevin P.; Benz, Harley M.; Villaseñor, Antonio

2014-01-01

The Sunda convergent margin extends for 5,600 km from the Bay of Bengal and the Andaman Sea, both located northwest of the map area, towards the island of Sumba in the southeast, and then continues eastward as the Banda arc system. This tectonically active margin is a result of the India and Australia plates converging with and subducting beneath the Sunda plate at a rate of approximately 50 to 70 mm/yr. The main physiographic feature associated with this convergent margin is the Sunda-Java Trench, which stretches for 3,000 km parallel to the Java and Sumatra land masses and terminates at 120° E. The convergence of the Indo-Australia and Sunda plates produces two active volcanic arcs: Sunda, which extends from 105 to 122° E and Banda, which extends from 122 to 128° E. The Sunda arc results solely from relatively simple oceanic plate subduction, while the Banda arc represents the transition from oceanic subduction to continental collision, where a complex, broad deforming zone is found. Based on modern activity, the Banda arc can be divided into three distinct zones: an inactive section, the Wetar Zone, bound by two active segments, the Flores Zone in the west and the Damar Zone in the east. The lack of volcanism in the Wetar Zone is attributed to the collision of Australia with the Sunda plate. The absence of gap in volcanic activity is underlain by a gap in intermediate depth seismicity, which is in contrast to nearly continuous, deep seismicity below all three sections of the arc. The Flores Zone is characterized by down-dip compression in the subducted slab at intermediate depths and late Quaternary uplift of the forearc. These unusual features, along with GPS data interpretations indicate that the Flores Zone marks the transition between subduction of oceanic crust in the west and the collision of continental crust in the east. The Java section of the Sunda arc is considered relatively aseismic historically when compared to the highly seismically active Sumatra section, despite both areas being located along the same active subduction margin. Shallow (0–20 km) events have occurred historically in the overlying Sunda plate, causing damage to local and regional communities. A recent example was the May 26, 2006 M6.3 left-lateral strike-slip event that occurred at a depth of 10 km in central Java, and caused over 5,700 fatalities. Intermediate depth (70–300 km) earthquakes frequently occur beneath Java as a result of intraplate faulting within the Australia slab. Deep (300–650 km) earthquakes occur beneath the Java Sea and the back-arc region to the north of Java. Similar to other intermediate depth events, these earthquakes are also associated with intraslab faulting. However, this subduction zone exhibits a gap in seismicity from 250 to 400 km, interpreted as the transition between extensional and compressional slab stresses. Historical examples of large intraplate events include: the 1903 M8.1 event, 1921 M7.5 event, 1977 M8.3 event, and August 2007 M7.5 event. Large thrust earthquakes close to the Java trench are typically interplate faulting events along the slab interface between the Australia and Sunda plates. These earthquakes also generally have high tsunamigenic potential due to their shallow hypocentral depths. In some cases, these events have demonstrated slow moment-release and have been defined as ‘tsunami’ earthquakes, where rupture is large in the weak crustal layers very close to the seafloor. These events are categorized by tsunamis that are significantly larger than predicted by the earthquake’s magnitude. The most notable tsunami earthquakes in the Java region occurred on June 2, 1994 (M7.8) and July 17, 2006 (M7.7). The 1994 event produced a tsunami with wave runup heights of 13 m, killing over 200 people. The 2006 event produced a tsunami of up to 15 m, and killed 730 people. Although both of these tsunami earthquakes were characterized by rupture along thrust faults, they were followed by an abundance of normal faulting aftershocks. These aftershocks are interpreted to result from extension within the subducting Australia plate, whereas the mainshocks represented interplate faulting between the Australia and Sunda plates.

Lesser Antilles Subduction Zone Investigation by a Cluster of Large Seismic Experiments in the Forearc Region

NASA Astrophysics Data System (ADS)

Last, T.

2007-12-01

Thales LAST stands for Lesser Antilles Subduction zone Team which gathers the scientific teams of a cluster of surveys and cruises that have been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project (Coord. A. Hirn). This cluster is composed by the German cruise TRAIL with the vessel F/S Merian (PI E. Flueh and H. Kopp, IFM-GEOMAR), the French cruise SISMANTILLES 2 with the IFREMER vessel N/O Atalante (PI M. Laigle, IPG Paris and JF. Lebrun, Univ. Antilles Guyane), and French cruise OBSANTILLES with the IRD vessel N/O Antea (PI P. Charvis, Geoazur, Nice, France). During these cruises and surveys, 84 Ocean Bottom 3-components Seismometers (OBS) and 20 Hydrophones (OBHs) have been brought together from several pools (Geoazur, INSU, IPGP, IFM-GEOMAR, AWI,), with up to 30 land stations (CSIC Barcelone, IPG Paris, INSU-RLBM and -LITHOSCOPE) in addition to the permanent onshore arrays of IPGP and SRU. The deployment of all these instruments has been supported principally by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT on the Antilles and Hellenic active subductions to which contribute IPGP, Geoazur, IFM-GEOMAR (Germany), ETH Zurich (Switzerland), CSIC Barcelona (Spain), Univ. Trieste (Italy) and NOA Athens (Greece). The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to southern Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the subduction mega-thrust fault plane, and which is here the main target has been localized along 3 transects to the Arc thanks to a preliminary survey in 2001, the French SISMANTILLES cuise. We will present the first results obtained during these experiments dedicated specifically to image at depth the seismic structure and activity of this region. To image faults at depth and the detailed upper-crustal structure, 3700 km of multi-beam bathymetry and multi-channel reflection seismic profiles have been collected along a grid comprising 7 strike-lines of up to 300 km long and spaced by 15 km and 12 transects of up to 150 km long and spaced by 25 km (SISMANTILLES 2). All these airgun shots dedicated to deep penetration have been recorded by the 84 OBSs and 20 OBHs deployed by the F/S Merian and N/O Atalante on the nodes of this grid of profiles. It will permit to get Vp constraints on the deep forearc region and mantle wedge by wide-angle refraction studies, as well as constraints on the updip and downdip limits of the seismogenic part of the mega-thrust fault plane. Two of these transects have been extended across the whole arc during the TRAIL survey, with up to 50 OBSs deployed along both 240 km long profiles. All these OBSs remained several months beyond the shot experiments for local earthquakes Vp and Vp/Vs tomography. They have been recovered and partly redeployed by N/O Antea during the OBSANTILLES survey. A significant number of those instruments had broadband seismometers, a notable originality in the case of the OBSs to detect low-frequency transient signals.

Shallow slip amplification and enhanced tsunami hazard unravelled by dynamic simulations of mega-thrust earthquakes

PubMed Central

Murphy, S.; Scala, A.; Herrero, A.; Lorito, S.; Festa, G.; Trasatti, E.; Tonini, R.; Romano, F.; Molinari, I.; Nielsen, S.

2016-01-01

The 2011 Tohoku earthquake produced an unexpected large amount of shallow slip greatly contributing to the ensuing tsunami. How frequent are such events? How can they be efficiently modelled for tsunami hazard? Stochastic slip models, which can be computed rapidly, are used to explore the natural slip variability; however, they generally do not deal specifically with shallow slip features. We study the systematic depth-dependence of slip along a thrust fault with a number of 2D dynamic simulations using stochastic shear stress distributions and a geometry based on the cross section of the Tohoku fault. We obtain a probability density for the slip distribution, which varies both with depth, earthquake size and whether the rupture breaks the surface. We propose a method to modify stochastic slip distributions according to this dynamically-derived probability distribution. This method may be efficiently applied to produce large numbers of heterogeneous slip distributions for probabilistic tsunami hazard analysis. Using numerous M9 earthquake scenarios, we demonstrate that incorporating the dynamically-derived probability distribution does enhance the conditional probability of exceedance of maximum estimated tsunami wave heights along the Japanese coast. This technique for integrating dynamic features in stochastic models can be extended to any subduction zone and faulting style. PMID:27725733

A low-angle normal fault and basement structures within the Enping Sag, Pearl River Mouth Basin: Insights into late Mesozoic to early Cenozoic tectonic evolution of the South China Sea area

NASA Astrophysics Data System (ADS)

Ye, Qing; Mei, Lianfu; Shi, Hesheng; Shu, Yu; Camanni, Giovanni; Wu, Jing

2018-04-01

The basement structure of the Cenozoic Enping Sag, within the Pearl River Mouth Basin on the northern margin of South China Sea, is revealed by borehole-constrained high-quality 3D seismic reflection data. Such data suggest that the Enping Sag is bounded in the north by a low-angle normal fault. We interpret this low-angle normal fault to have developed as the result of the reactivation of a pre-existing thrust fault part of a pre-Cenozoic thrust system. This is demonstrated by the selective reactivation of the pre-existing thrust and by diffuse contractional deformation recognized from the accurate analysis of basement reflections. Another significant result of this study is the finding of some residual rift basins within the basement of the Enping Sag. Both the thrust system and the residual basins are interpreted to have developed after the emplacement of continental margin arc-related granitoids (J3-K1) that define the basement within the study area. Furthermore, seismic sections show that the pre-existing residual rift basins are offset by the main thrust fault and they are both truncated by the Tg unconformity. These structural relationships, interpreted in the frame of previous studies, help us to reconstruct a six-event structural evolution model for the Enping Sag from the late Mesozoic to the early Cenozoic. In particular, we interpret the residual rift basins to have formed as the result of back-arc extension due to the slab roll-back of the Paleo-Pacific Plate subduction in the early K2. The thrust system has recorded a compressional event in the late K2 that followed the back-arc extension in the SCS area. The mechanism of this compressional event is still to be clarified, and might be related to continuous subduction of the Paleo-Pacific Plate or to the continent-continent collision between a micro-continental block and the South China margin.

Faulting and erosion in the Argentine Precordillera during changes in subduction regime: Reconciling bedrock cooling and detrital records

NASA Astrophysics Data System (ADS)

Fosdick, Julie C.; Carrapa, Barbara; Ortíz, Gustavo

2015-12-01

The Argentine Precordillera is an archetypal retroarc fold-and-thrust belt that records tectonics associated with changing subduction regimes. The interactions between exhumation and faulting in the Precordillera were investigated using apatite and zircon (U-Th-Sm)/He and apatite fission track thermochronometry from the Precordillera and adjacent geologic domains. Inverse modeling of thermal histories constrains eastward in-sequence rock cooling associated with deformation and erosion from 18 to 2 Ma across the Central Precordillera tracking thrusting during this time. The youngest AHe ages (5-2 Ma) and highest erosion rates are located in the eastern and western extremities of the Precordillera and indicate that recent denudation is concentrated at its structural boundaries. Moreover, synchronous rapid Pliocene cooling of the Frontal Cordillera, Eastern Precordillera, and Sierra del Valle Fértil was coeval with initiation of basement-involved faulting in the foreland. Detrital zircon U-Pb geochronology from the ca. 16-8.1 Ma Bermejo foreland basin strata suggests fluvial connectivity westward beyond the Frontal Cordillera to the Main Cordillera and Coast Range followed by an important shift in sediment provenance at ca. 10 Ma. At this time, we suggest that a substantial decrease in Permo-Triassic igneous sources in the Frontal Cordillera and concurrent increase in recycled zircons signatures of Paleozoic strata are best explained by uplift and erosion of the Precordillera during widening of the thrust-belt. Bedrock thermochronology and modeling indicate a 2-6 Myr lag time between faulting-related cooling in the hinterland and the detrital record of deformation in the foreland basin, suggesting that for tectonically active semi-arid settings, bedrock cooling may be more sensitive to onset of faulting. We suggest that high erosion rates in the Frontal Cordillera and Eastern Precordillera are associated with increased interplate coupling during shallowing of the subducting Nazca plate that may concentrate stress along weak structural boundaries of the Precordillera.

Earthquake hazards on the cascadia subduction zone.

PubMed

Heaton, T H; Hartzell, S H

1987-04-10

Large subduction earthquakes on the Cascadia subduction zone pose a potential seismic hazard. Very young oceanic lithosphere (10 million years old) is being subducted beneath North America at a rate of approximately 4 centimeters per year. The Cascadia subduction zone shares many characteristics with subduction zones in southern Chile, southwestern Japan, and Colombia, where comparably young oceanic lithosphere is also subducting. Very large subduction earthquakes, ranging in energy magnitude (M(w)) between 8 and 9.5, have occurred along these other subduction zones. If the Cascadia subduction zone is also storing elastic energy, a sequence of several great earthquakes (M(w) 8) or a giant earthquake (M(w) 9) would be necessary to fill this 1200-kilometer gap. The nature of strong ground motions recorded during subduction earthquakes of M(w) less than 8.2 is discussed. Strong ground motions from even larger earthquakes (M(w) up to 9.5) are estimated by simple simulations. If large subduction earthquakes occur in the Pacific Northwest, relatively strong shaking can be expected over a large region. Such earthquakes may also be accompanied by large local tsunamis.

High-velocity basal sediment package atop oceanic crust, offshore Cascadia: Impacts on plate boundary processes and fluid migration

NASA Astrophysics Data System (ADS)

Peterson, D. E.; Keranen, K. M.

2017-12-01

Differences in fluid pressure and mechanical properties at megathrust boundaries in subduction zones have been proposed to create varying seismogenic behavior. In Cascadia, where large ruptures are possible but little seismicity occurs presently, new seismic transects across the deformation front (COAST cruise; Holbrook et al., 2012) image an unusually high-wavespeed sedimentary unit directly overlying oceanic crust. Wavespeed increases before sediments reach the deformation front, and the well-laminated unit, consistently of 1 km thickness, can be traced for 50 km beneath the accretionary prism before imaging quality declines. Wavespeed is modeled via iterative prestack time migration (PSTM) imaging and increases from 3.5 km/sec on the seaward end of the profile to >5.0 km/s near the deformation front. Landward of the deformation front, wavespeed is low along seaward-dipping thrust faults in the Quaternary accretionary prism, indicative of rapid dewatering along faults. The observed wavespeed of 5.5 km/sec just above subducting crust is consistent with porosity <5% (Erickson and Jarrard, 1998), possibly reflecting enhanced consolidation, cementation, and diagenesis as the sediments encounter the deformation front. Beneath the sediment, the compressional wavespeed of uppermost oceanic crust is 3-4 km/sec, likely reduced by alteration and/or fluids, lowest within a propagator wake. The propagator wake intersects the plate boundary at an oblique angle and changes the degree of hydration of the oceanic plate as it subducts within our area. Fluid flow out of oceanic crust is likely impeded by the low-porosity basal sediment package except along the focused thrust faults. Decollements are present at the top of oceanic basement, at the top of the high-wavespeed basal unit, and within sedimentary strata at higher levels; the decollement at the top of oceanic crust is active at the toe of the deformation front. The basal sedimentary unit appears to be mechanically strong, similar to observations from offshore Sumatra, where strongly consolidated sediments at the deformation front are interpreted to facilitate megathrust rupture to the trench (Hupers et al., 2017). A uniformly strong plate interface at Cascadia may inhibit microseismicity while building stress that is released in great earthquakes.

Dynamic weakening of smectite-rich faults at intermediate to high velocities

NASA Astrophysics Data System (ADS)

Oohashi, K.; Hirose, T.; Takahashi, M.

2013-12-01

Smectite, one of the hydrous clay mineral, is ubiquitous in incoming sediments to subduction zones and is thought to weaken and stabilize subduction thrust faults. However, frictional properties of smectite alone cannot explain the nucleation and propagation of earthquake slip at the shallow plate boundary thrust which potentially causes the devastating tsunamis. Here, we investigate for the first time the effect of smectite fraction in smectite-quartz mixtures on friction at 30 μm/s to 1.3 m/s, to shed a light on the frictional response for the intermediate to high slip rates where the conventional friction experiments have not been explored. In the low slip rate of 30 μm/s, the steady-state coefficient of friction decreases non-linearly increasing smectite fraction: it drops rapidly at moderate fraction of 30-50 vol%. On the other hand, at the faster slip rates of ≥ 150 μm/s the friction lowers from 10-20 vol% fraction since drastic slip weakening appears for the mixtures of ~20 vol % smectite. Hence the fault suddenly loses the strength by adding only 20 % of smectite. The weakening seems to be associated with an excess pore pressure invoked by shear compaction and thermal pressurization during the experiments. This property weakens the fault strength and accelerates the fault slip, even if clay content is small (c.a. 15-35 %), leading to the large stress drop. In contrast, the faults rich in smectite (≥ 50 %) may cause small stress drop during the faulting owing to low friction coefficient of smectite at any slip rates. The results highlight that smectite content significantly affects frictional properties of faults and may generates the diversity in the subduction zone earthquakes. ACKNOLEDGEMENTS We thank Kyuichi Kanagawa, Masaya Suzuki, Osamu Tadai, and Hiroko Kitajima for constructive discussions and technical help. This work was supported by a JSPS Grant-in-Aid for JSPS fellows (25-04960) to KO, a JSPS Grant-in-Aid for Young Scientists (B) (20740264) to TH, and MEXT KANAME grant #21107004.

The effect of compliant prisms on subduction zone earthquakes and tsunamis

NASA Astrophysics Data System (ADS)

Lotto, Gabriel C.; Dunham, Eric M.; Jeppson, Tamara N.; Tobin, Harold J.

2017-01-01

Earthquakes generate tsunamis by coseismically deforming the seafloor, and that deformation is largely controlled by the shallow rupture process. Therefore, in order to better understand how earthquakes generate tsunamis, one must consider the material structure and frictional properties of the shallowest part of the subduction zone, where ruptures often encounter compliant sedimentary prisms. Compliant prisms have been associated with enhanced shallow slip, seafloor deformation, and tsunami heights, particularly in the context of tsunami earthquakes. To rigorously quantify the role compliant prisms play in generating tsunamis, we perform a series of numerical simulations that directly couple dynamic rupture on a dipping thrust fault to the elastodynamic response of the Earth and the acoustic response of the ocean. Gravity is included in our simulations in the context of a linearized Eulerian description of the ocean, which allows us to model tsunami generation and propagation, including dispersion and related nonhydrostatic effects. Our simulations span a three-dimensional parameter space of prism size, prism compliance, and sub-prism friction - specifically, the rate-and-state parameter b - a that determines velocity-weakening or velocity-strengthening behavior. We find that compliant prisms generally slow rupture velocity and, for larger prisms, generate tsunamis more efficiently than subduction zones without prisms. In most but not all cases, larger, more compliant prisms cause greater amounts of shallow slip and larger tsunamis. Furthermore, shallow friction is also quite important in determining overall slip; increasing sub-prism b - a enhances slip everywhere along the fault. Counterintuitively, we find that in simulations with large prisms and velocity-strengthening friction at the base of the prism, increasing prism compliance reduces rather than enhances shallow slip and tsunami wave height.

Sunda-Banda Arc Transition: Marine Multichannel Seismic Profiling

NASA Astrophysics Data System (ADS)

Lueschen, E.; Mueller, C.; Kopp, H.; Djajadihardja, Y.; Ehrhardt, A.; Engels, M.; Lutz, R.; Planert, L.; Shulgin, A.; Working Group, S.

2008-12-01

After the Indian Ocean Mw 9.3 earthquake and tsunami on December 26, 2004, intensive research activities focussed on the Sunda Arc subduction system offshore Sumatra. For this area a broad database is now available interpreted in terms of plate segmentation and outer arc high evolution. In contrast, the highly active easternmost part of this subduction system, as indicated by the south of Java Mw 7.7 earthquake and tsunami on July 17, 2006, has remained almost unexplored until recently. During RV SONNE cruise SO190 from October until December 2006 almost 5000 km of marine geophysical profiles have been acquired at the eastern Sunda Arc and the transition to the Banda Arc. The SINDBAD project (Seismic and Geoacoustic Investigations along the Sunda-Banda Arc Transition) comprises 30-fold multichannel reflection seismics with a 3-km streamer, wide-angle OBH/OBS refraction seismics for deep velocity control (see poster of Shulgin et al. in this session), swath bathymetry, sediment echosounder, gravimetric and geomagnetic measurements. We present data and interpretations of several 250-380 km long, prestack depth-migrated seismic sections, perpendicular to the deformation front, based on velocity models from focussing analysis and inversion of OBH/OBS refraction data. We focus on the variability of the lower plate and the tectonic response of the overriding plate in terms of outer arc high formation and evolution, forearc basin development, accretion and erosion processes at the base of the overriding plate. The subducting Indo-Australian Plate is characterized by three segments: i) the Roo Rise with rough topography offshore eastern Java ii) the Argo Abyssal Plain with smooth oceanic crust offshore Bali, Lombok, and Sumbawa, and iii) the Scott Plateau with continental crust colliding with the Banda island arc. The forearc responds to differences in the incoming oceanic plate with the absence of a pronounced forearc basin offshore eastern Java and with development of the 4000 m deep forearc Lombok Basin offshore Bali, Lombok, and Sumbawa. The eastern termination of the Lombok Basin is formed by Sumba Island, which shows evidence for recent uplift, probably associated with the collision of the island arc with the continental Scott Plateau. The Sumba area represents the transition from subduction to collision. Our seismic profiles image the bending of the oceanic crust seaward of the trench and associated normal faulting. Landward of the trench, they image the subducting slab beneath the outer arc high, where the former bending-related normal faults appear to be reactivated as reverse faults introducing vertical displacements in the subducting slab. The accretionary prism and the outer arc high are characterized by an ocean-verging system of imbricate thrust sheets with major thrust faults connecting seafloor and detachment. Compression results in shortening and steepening of the imbricated thrust sheets building up the outer arc high. Tilted piggy-back basins and downlaps of tilted sediments in the southern Lombok forearc basin indicate ongoing uplift of the entire outer arc high, abrupt displacements, and recent tectonic activity.

Permeability and strength structure around an ancient exhumed subduction-zone fault

NASA Astrophysics Data System (ADS)

Kato, A.; Sakaguchi, A.; Yoshida, S.; Kaneda, Y.

2003-12-01

Investigating the transporting properties of subduction zone faults is crucial for understanding shear strength and slip-stability, or instability, of subduction zone faults. Despite the influence of pore pressure on a wide range of subduction-zone fault processes, few previous studies have evaluated the permeability structure around the fault placed in a well-defined structural context. In this study, the aim is to gain the entire permeability and the shear strength structure around the ancient subduction zone fault. We have conducted a series of permeability measurements and shear failure experiments in seismogenic environments using intact rocks sampled at the outcrop of an exhumed fault zone in the Cretaceous Shimanto accretionary complex, in Shikoku, SW Japan, where a typical evidence for seismic fault rock of pseudotachylyte has been demonstrated [Ikesawa et al., 2003]. This fault zone is located at boundary between the sandstone-dominant coherent unit of the Nonokawa Formation and the Okitsu mélange. The porosity of each rock sample is less than 1 %, except for the shear zone. Cylindrical test specimens (length = 40 mm, diameter = 20 mm) were cored to an accuracy of within 0.02 mm. Most of values of permeability were evaluated at confining pressure Pc of 140 MPa and pore pressure Pp of 115 MPa simulating the depth of 5 km (suprahydrostatic pore pressure). It is found that the permeability at room temperature shows the heterogeneous structure across the fault zone. The permeability of sandstone-dominant coherent unit is the lowest (10-19 m2) across the fault zone. In contrast, high shear zone has the highest permeability (10-16 m2). Following the increase in temperature, permeability evolution has been investigated. The permeability at 250oC continuously decreases with hold time for all types of rock specimens, and the reduction rate of permeability against hold time seems to become small with hold time. It seems that the reduction rate does not significantly depend on the rock types. The specimen was loaded at a strain rate of 2*E-6 /s under the conditions (Pc, Pp, T) = (140 MPa, 105 MPa, 250oC) to conduct the shear fracture experiments. High shear zone has a minimum value in strength profile. In contrast, the largest shear strength is observed at sandstone in coherent unit. From the seismic reflection surveys in the Nankai Trough, Park et al. [2002] delineated reflections with negative polarities beneath the Nankai accretionary prism 20-60 km landward of the frontal thrust, which are located deeper than the negative polarity décollement near the frontal thrust. They interpreted that the DSRs indicate the elevated fluid pressures. The fault zone studied in this paper is consistent with the duplex-model, and corresponds to the area where the décollement near the frontal thrust stepped down. Present results show the possibility that the coherent sandstone acts as a cap rock for fluid flow, and shear zone as a conduit for the flow, which leads to the elevated pore pressures along the roof thrust.

Characteristics of the Central Costa Rican Seismogenic Zone Determined from Microseismicity

NASA Astrophysics Data System (ADS)

DeShon, H. R.; Schwartz, S. Y.; Bilek, S. L.; Dorman, L. M.; Protti, M.; Gonzalez, V.

2001-12-01

Large or great subduction zone thrust earthquakes commonly nucleate within the seismogenic zone, a region of unstable slip on or near the converging plate interface. A better understanding of the mechanical, thermal and hydrothermal processes controlling seismic behavior in these regions requires accurate earthquake locations. Using arrival time data from an onland and offshore local seismic array and advanced 3D absolute and relative earthquake location techniques, we locate interplate seismic activity northwest of the Osa Peninsula, Costa Rica. We present high resolution locations of ~600 aftershocks of the 8/20/1999 Mw=6.9 underthrusting earthquake recorded by our local network between September and December 1999. We have developed a 3D velocity model based on published refraction lines and located events within a subducting slab geometry using QUAKE3D, a finite-differences based grid-searching algorithm (Nelson & Vidale, 1990). These absolute locations are input into HYPODD, a location program that uses P and S wave arrival time differences from nearby events and solves for the best relative locations (Waldhauser & Ellsworth, 2000). The pattern of relative earthquake locations is tied to an absolute reference using the absolute positions of the best-located earthquakes in the entire population. By using these programs in parallel, we minimize location errors, retain the aftershock pattern and provide the best absolute locations within a complex subduction geometry. We use the resulting seismicity pattern to determine characteristics of the seismogenic zone including geometry and up- and down-dip limits. These are compared with thermal models of the Middle America subduction zone, structures of the upper and lower plates, and characteristics of the Nankai seismogenic zone.

Relationship between two Solomon Islands Earthquakes in 2007 (M8.1), 2010 (M7.1), and Seismic Gap along the Subduction Zone, Revealed by ALOS/PALSAR

NASA Astrophysics Data System (ADS)

Miyagi, Y.; Ozawa, T.

2010-12-01

The Solomon Islands are located in the southwest of the Pacific Ocean. The Australian, Woodlark, and Solomon Sea plates subduct toward the northeast beneath the Pacific plate. Interaction among these four plates cause complicated tectonics around the Solomon Islands, and have caused interplate earthquakes in the subduction zone (e.g. Lay and Kanamori, 1980; Xu and Schwarts, 1993). On April 1, 2007 (UTC), an M8.1 interplate earthquake occurred in the subduction zone between the Pacific Plate and the Australian Plate. This earthquake was accompanied by a large tsunami and caused considerable damage in the area. The Japan Aerospace Exploration Agency (JAXA) carried out emergency observations using the Phased Array type L-band Synthetic Aperture Rader (PALSAR) installed on Advanced Land Observing Satellite (ALOS), and detected more than 2m of maximum displacement using differential interferometric SAR (DInSAR) technique. Miyagi et al. (2009) estimated a slip distribution of the seismic fault mainly from the PALSAR/DInSAR data and suggested that most of a seismic gap was filled by the 2007 events, but a small seismic gap connecting to an Mw7.0-sized earthquake still remained. On January 3, 2010, an M7.1 earthquake occurred in the vicinity of the remnant seismic gap. ALOS/PALSAR observed epicentral area both before and after the event, and detected crustal deformation associated with the earthquake. We inferred fault model using the PALSAR/DInSAR data and concluded that the 2010 event was the supposed thrust earthquake filling the remnant seismic gap. A distribution of coulomb failure stress change in the epicentral area after the 2007 event suggested the possibility that the 2010 event was triggered by the 2007 earthquake.

Slip in Great Megathrust Earthquakes and its Relation to Crustal Structure as Revealed by Satellite Free-air Gravity

NASA Astrophysics Data System (ADS)

Wells, R. E.; Blakely, R. J.; Scholl, D.

2007-12-01

In 2003, Song and Simons and Wells et al. showed that approximately 70% of the moment released during past large, shallow subduction zone thrust earthquakes occurred beneath trench-parallel, free-air gravity lows outlining the deep-sea slope terrace and its basins. The authors suggested that the basin-centered, fore-arc gravity lows might be good predictors of high seismic slip in future earthquakes. Since 2001, ten megathrust earthquakes have occurred with magnitudes greater than Mw 7.7, including the giant, Mw 9.17 Sumatra earthquake of 2004. These earthquakes provide a robust test of the idea that seismic slip is focused beneath basin-centered gravity lows, and also the related ideas that the landward maximum gravity gradient marks the effective down-dip limit of large coseismic slip, and that intrabasin, transverse gravity highs are areas of lower slip. A compilation of seismic and geodetic slip inversions for the post-2001 earthquakes and new analyses of slip for the great Antofagasta, Jalisco, and Peru events in 1995 and 1996 indicate that more than 80% of the high-slip areas occur beneath deep-sea terrace gravity lows (DSTL), and that half of the earthquake asperities lie beneath fore-arc basins or local gravity lows. The maximum gravity gradient along the landward margin of the deep-sea terrace may mark the point where thicker overlying crust and higher temperatures on the megathrust limit the down dip extent of stick-slip behavior. Onland analogues are the mountain front of the Himalaya, which approximately marks the down-dip limit of large coseismic slip along the Main Frontal Thrust, and the front of the Taiwan Central Ranges, which coincides with the limit of slip during the 1999 Chi-Chi earthquake (Mw 7.6). In the up dip direction, coseismic slip may be partitioned onto splay faults in the wedge, as occurred in the 1964 Alaska earthquake. The observed pattern of greater slip at depth beneath fore arc basins is consistent with partitioning of slip up dip, especially if outer wedge materials deform more slowly, as suggested for parts of the 2004 Sumatra rupture. Along strike variations in fore-arc gravity also correlate with changing seismic behavior. At Cape Erimo on Hokkaido, three Mw 8+ earthquakes (1952, 1968, 2003) have occurred on either side of the gravity high that overlies the Cape, with little coseismic slip beneath the high. To the northeast, the deep-sea terrace gradually narrows, as does the rupture width of the great earthquakes, until off the central Kurile Islands, the terrace disappears and the arc gravity high occupies the fore-arc. The gravity high had been an historic seismic gap that was filled by the 2006 Kurile Island earthquake (Mw 8.3). Although the earthquake nucleated under the high, the slip occurred beneath the adjacent gravity low to the northeast. This might suggest the gravity highs are not likely sources of large seismic moment, at least in M8 earthquakes. In contrast, the main asperity associated with the 2005 Sumatra (Mw 8.7) earthquake was beneath the large gravity high of Nias Island. An alternative view is that the gravity highs are stronger asperities that only rupture in giant earthquakes. Globally, the coincidence of basin- centered coseismic slip with geologic evidence of sustained subsidence of the fore-arc suggests that subduction erosion is occurring in the seismogenic zone. Recent work off Chile, Colombia, Peru, and elsewhere shows that subduction erosion is an important process in many subduction zones.

Probabilistic tsunami hazard assessment for the Makran region with focus on maximum magnitude assumption

NASA Astrophysics Data System (ADS)

Hoechner, Andreas; Babeyko, Andrey Y.; Zamora, Natalia

2016-06-01

Despite having been rather seismically quiescent for the last decades, the Makran subduction zone is capable of hosting destructive earthquakes and tsunami. In particular, the well-known thrust event in 1945 (Balochistan earthquake) led to about 4000 casualties. Nowadays, the coastal regions are more densely populated and vulnerable to similar events. Furthermore, some recent publications discuss rare but significantly larger events at the Makran subduction zone as possible scenarios. We analyze the instrumental and historical seismicity at the subduction plate interface and generate various synthetic earthquake catalogs spanning 300 000 years with varying magnitude-frequency relations. For every event in the catalogs we compute estimated tsunami heights and present the resulting tsunami hazard along the coasts of Pakistan, Iran and Oman in the form of probabilistic tsunami hazard curves. We show how the hazard results depend on variation of the Gutenberg-Richter parameters and especially maximum magnitude assumption.

Probabilistic tsunami hazard assessment for the Makran region with focus on maximum magnitude assumption

NASA Astrophysics Data System (ADS)

Hoechner, A.; Babeyko, A. Y.; Zamora, N.

2015-09-01

Despite having been rather seismically quiescent for the last decades, the Makran subduction zone is capable of hosting destructive earthquakes and tsunami. In particular, the well-known thrust event in 1945 (Balochistan earthquake) led to about 4000 casualties. Nowadays, the coastal regions are more densely populated and vulnerable to similar events. Furthermore, some recent publications discuss rare but significantly larger events at the Makran subduction zone as possible scenarios. We analyze the instrumental and historical seismicity at the subduction plate interface and generate various synthetic earthquake catalogs spanning 300 000 years with varying magnitude-frequency relations. For every event in the catalogs we compute estimated tsunami heights and present the resulting tsunami hazard along the coasts of Pakistan, Iran and Oman in the form of probabilistic tsunami hazard curves. We show how the hazard results depend on variation of the Gutenberg-Richter parameters and especially maximum magnitude assumption.

Earthquake activity along the Himalayan orogenic belt

NASA Astrophysics Data System (ADS)

Bai, L.; Mori, J. J.

2017-12-01

The collision between the Indian and Eurasian plates formed the Himalayas, the largest orogenic belt on the Earth. The entire region accommodates shallow earthquakes, while intermediate-depth earthquakes are concentrated at the eastern and western Himalayan syntaxis. Here we investigate the focal depths, fault plane solutions, and source rupture process for three earthquake sequences, which are located at the western, central and eastern regions of the Himalayan orogenic belt. The Pamir-Hindu Kush region is located at the western Himalayan syntaxis and is characterized by extreme shortening of the upper crust and strong interaction of various layers of the lithosphere. Many shallow earthquakes occur on the Main Pamir Thrust at focal depths shallower than 20 km, while intermediate-deep earthquakes are mostly located below 75 km. Large intermediate-depth earthquakes occur frequently at the western Himalayan syntaxis about every 10 years on average. The 2015 Nepal earthquake is located in the central Himalayas. It is a typical megathrust earthquake that occurred on the shallow portion of the Main Himalayan Thrust (MHT). Many of the aftershocks are located above the MHT and illuminate faulting structures in the hanging wall with dip angles that are steeper than the MHT. These observations provide new constraints on the collision and uplift processes for the Himalaya orogenic belt. The Indo-Burma region is located south of the eastern Himalayan syntaxis, where the strike of the plate boundary suddenly changes from nearly east-west at the Himalayas to nearly north-south at the Burma Arc. The Burma arc subduction zone is a typical oblique plate convergence zone. The eastern boundary is the north-south striking dextral Sagaing fault, which hosts many shallow earthquakes with focal depth less than 25 km. In contrast, intermediate-depth earthquakes along the subduction zone reflect east-west trending reverse faulting.

Widespread afterslip and triggered slow slip events following the M7.8 Kaikoura earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Wallace, L. M.; Hreinsdottir, S.; Hamling, I. J.; D'Anastasio, E.; Bartlow, N. M.

2017-12-01

Just after midnight on 14 Nov 2016 (NZ Local time), the M7.8 Kaikoura earthquake ruptured a complex sequence of strike-slip and reverse faults over an approximately 150 km length in the northeastern South Island of New Zealand (Hamling et al., 2017, Science). In the months following the earthquake, time-dependent inversions of InSAR observations and continuous and semi-continuous GPS measurements reveal up to 0.5 m of afterslip on the subduction interface beneath the northern South Island underlying the region of large coseismic slip on crustal faults in the M7.8 earthquake. The geodetic data also require significant afterslip on a subset of the crustal faults that ruptured in the earthquake, including the Needles, Jordan Thrust, and Kekerengu faults. Our best-fitting models also suggest significant afterslip on an offshore reverse fault, in a similar position to one inferred by Clark et al. (2017, EPSL) from coseismic coastal uplift data. The M7.8 earthquake also triggered widespread slow slip occurring over much of the Hikurangi subduction zone beneath the North Island. Immediately following the earthquake, continuous GPS sites operated by GeoNet (www.geonet.org.nz) along the North Island's east coast (above the Hikurangi subduction zone) detected several to 30 mm of eastward motion over the two-week period immediately following the M7.8 event. These sites are located 350-650 km from the M7.8 earthquake. Such large eastward motion along the North Island's east coast following the earthquake is consistent with the initiation of a large slow slip event along the shallow, offshore portion of the Hikurangi subduction zone. In addition to shallow slow slip (<15 km depth) triggered offshore the east coast, we also observe deeper slow slip (>30 km depth) triggered in the Kapiti region at the southern Hikurangi margin. The Kapiti SSE was still ongoing as of August 2017, although we expect it to finish before the end of 2017. Given the large distance of the shallow east coast SSE from the M7.8 earthquake, we suggest that the shallow SSE was more likely to be triggered by dynamic stress changes, while the deeper SSEs closer to the Mw 7.8 were more likely triggered by static stress changes.

Sumatra Megathrust Earthquakes Trigger Intraplate Seismicity in the Indo-Australian Oceanic Lithosphere

NASA Astrophysics Data System (ADS)

Delescluse, M.; Chamot-Rooke, N.; Cattin, R.

2009-05-01

The present-day intraplate deformation between India and Australia started 9 Myrs ago. In the Central Indian Basin (CIB), this deformation is recorded in the thick sediments of the Bengal fan. The equatorial, dense E-W thrust fault network in this region is the result of a massive reverse reactivation of normal faults at the onset of deformation. The Wharton Basin (WB), separated from the CIB by the NinetyEast Ridge (NyR), shows a contrasting style of deformation with mainly left-lateral strike-slip seismicity. The WB finite deformation and seismicity also involve pre-existing faults, in this case the N-S paleo-transforms of the fossile Wharton spreading-ridge system. The oceanic plate seismicity after the December 2004 Aceh subduction earthquake shows strike-slip events with a clear intraplate P-axis. No thrust faults are detected. This indicates short-term reactivation of the transform faults near the trench. Spatial and temporal distribution of intraplate erthquakes, as well as their anomalous moment release suggests triggering by the Aceh megathrust earthquake, which appears to have acted as an "accelerator" for the oceanic intraplate deformation. In this study, we use Coulomb stress static variations to confirm our seismicity observations. We first assume that the reactivated transform and the neoformed thrust fault plane families are present in the oceanic lithosphere. We then compute the coseismic stresses in the vicinity of the trench from the Aceh and Nias earthquakes slip distributions. Finally, we derive the normal and shear stresses on the fault planes. The results show that the strike-slip events are all favored by the subduction earthquakes coseismic stresses. They also show that the normal fault earthquakes at oceanic bulges are supported by the modeled coseismic stresses, except offshore Myanmar. The particularly interesting result is that all the possible neoformed thrust faults perpendicular to the intraplate P-axis are inhibited by the same coseismic stresses. This suggests that the style of intraplate deformation favored near the Sumatra Trench in the short-term by subduction earthquakes is the same than the long-term style. Under the effect of northward slab pull forces, Australia tries to detach from its Indian "brake" along the WB's N-S transform faults.

Mega-thrust and Intra-slab Earthquakes Beneath Tokyo Metropolitan Area

NASA Astrophysics Data System (ADS)

Hirata, N.; Sato, H.; Koketsu, K.; Hagiwara, H.; Wu, F.; Okaya, D.; Iwasaki, T.; Kasahara, K.

2006-12-01

In central Japan the Philippine Sea plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region, where it causes mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9) which had 105,000 fatalities. The vertical proximity of this down going lithospheric plate is of concern because the greater Tokyo urban region has a population of 42 million and is the center of approximately 40% of the nation's economic activities. A M7+ earthquake in this region at present has high potential to produce devastating loss of life and property with even greater global economic repercussions. The M7+ earthquake is evaluated to occur with a probability of 70% in 30 years by the Earthquake Research Committee of Japan. In 2002, a consortium of universities and government agencies in Japan started the Special Project for Earthquake Disaster Mitigation in Urban Areas, a project to improve information needed for seismic hazards analyses of the largest urban centers. Assessment in Kanto of the seismic hazard produced by the Philippine Sea Plate (PSP) mega-thrust earthquakes requires identification of all significant faults and possible earthquake scenarios and rupture behavior, regional characterizations of PSP geometry and the overlying Honshu arc physical properties (e.g., seismic wave velocities, densities, attenuation), and local near-surface seism ic site effects. Our study addresses (1) improved regional characterization of the PSP geometry based on new deep seismic reflection profiles (Sato etal.,2005), reprocessed off-shore profiles (Kimura et al.,2005), and a dense seismic array in the Boso peninsular (Hagiwara et al., 2006) and (2) identification of asperities of the mega-thrust at the top of the PSP. We qualitatively examine the relationship between seismic reflections and asperities inferred by reflection physical properties. We also discuss the relation between deformation of PSP and intra-slab M7+ earthquakes: the PSP is subducting beneath the Hoshu arc and also colliding with the Pacific plate. The subduction and collision both contribute active seismicity in the Kanto region. We present a high resolution tomographic image to show a low velocity zone which suggests a possible internal failure of the slab; a source region of the M7+ intra-slab earthquake. Our study contributes a new assessment of the seismic hazard in the Tokyo metropolitan area. tokyo.ac.jp/daidai/index-J.html

Evidence for ˜80-75 Ma subduction jump during Anatolide-Tauride-Armenian block accretion and ˜48 Ma Arabia-Eurasia collision in Lesser Caucasus-East Anatolia

NASA Astrophysics Data System (ADS)

Rolland, Yann; Perincek, Dogan; Kaymakci, Nuretdin; Sosson, Marc; Barrier, Eric; Avagyan, Ara

2012-05-01

Orogens formed by a combination of subduction and accretion are featured by a short-lived collisional history. They preserve crustal geometries acquired prior to the collisional event. These geometries comprise obducted oceanic crust sequences that may propagate somewhat far away from the suture zone, preserved accretionary prism and subduction channel at the interplate boundary. The cessation of deformation is ascribed to rapid jump of the subduction zone at the passive margin rim of the opposite side of the accreted block. Geological investigation and 40Ar/39Ar dating on the main tectonic boundaries of the Anatolide-Tauride-Armenian (ATA) block in Eastern Turkey, Armenia and Georgia provide temporal constraints of subduction and accretion on both sides of this small continental block, and final collisional history of Eurasian and Arabian plates. On the northern side, 40Ar/39Ar ages give insights for the subduction and collage from the Middle to Upper Cretaceous (95-80 Ma). To the south, younger magmatic and metamorphic ages exhibit subduction of Neotethys and accretion of the Bitlis-Pütürge block during the Upper Cretaceous (74-71 Ma). These data are interpreted as a subduction jump from the northern to the southern boundary of the ATA continental block at 80-75 Ma. Similar back-arc type geochemistry of obducted ophiolites in the two subduction-accretion domains point to a similar intra-oceanic evolution prior to accretion, featured by slab steepening and roll-back as for the current Mediterranean domain. Final closure of Neotethys and initiation of collision with Arabian Plate occurred in the Middle-Upper Eocene as featured by the development of a Himalayan-type thrust sheet exhuming amphibolite facies rocks in its hanging-wall at c. 48 Ma.

An integrated approach to the seismic activity and structure of the central Lesser Antilles subduction megathrust seismogenic zone

NASA Astrophysics Data System (ADS)

Hirn, Alfred; Laigle, Mireille; Charvis, Philippe; Flueh, Ernst; Gallart, Josep; Kissling, Edi; Lebrun, Jean-Frederic; Nicolich, Rinaldo; Sachpazi, Maria

2010-05-01

In order to increase the understanding of plate boundaries that show currently low seismic activity, as was the Sumatra-Andaman subduction before the major earthquake in 2004, a cluster of surveys and cruises has been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project on the Lesser Antilles subduction zone of the Carribean-America plate boundary. A segment of the corresponding transform boundary just tragically ruptured in the 2010 January 12, Haïti earthquake. This cluster is composed by the German cruise TRAIL with the vessel F/S M. A. MERIAN, the French cruise SISMANTILLES II with the IFREMER vessel N/O ATALANTE), and French cruise OBSANTILLES with the IRD vessel N/O ANTEA. During these cruises and surveys, 80 OBS, Ocean Bottom Seismometers, 64 of which with 3-components seismometers and hydrophones, and 20 OBH with hydrophones have been brought together from several pools (Geoazur Nice, INSU/IPGP Paris, IfM-GEOMAR Kiel, AWI Bremerhaven), with up to 30 land stations (CSIC Barcelona, IPG Paris, INSU-RLBM and -Lithoscope, ETH Zurich). The deployment of all these instruments has been supported principally in addition by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT. The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the subduction mega-thrust fault plane, and which is here the main target has been localized along 3 transects to the Arc, thanks to a preliminary survey in 2001, the French SISMANTILLES cruise. We will present the first results obtained during these experiments dedicated specifically to image at depth the seismic structure and activity of this region. To image faults at depth and the detailed upper-crustal structure, 3700 km of multi-beam bathymetry and multi-channel reflection seismic profiles have been collected along a grid comprising 7 strike-lines of up to 300 km long and spaced by 15 km and 12 transects of up to 150 km long and spaced by 25 km (SISMANTILLES II). All these airgun shots dedicated to deep penetration have been recorded by the 80 OBSs and 20 OBHs deployed by the F/S Merian and N/O Atalante on the nodes of this grid of profiles. It will permit to get Vp constraints on the deep forearc region and mantle wedge by wide-angle refraction studies, as well as constraints on the updip and downdip limits of the seismogenic part of the mega-thrust fault plane. Two of these transects have been extended across the whole arc during the TRAIL survey, with up to 50 OBSs deployed along both 240 km long profiles. All these OBSs remained several months after the shot experiments to gather data for accurate location of local earthquakes and possibly Vp and Vp/Vs tomography. They have been recovered and partly redeployed by N/O Antea during the OBSANTILLES survey. A significant number of those instruments had broadband seismometers, a notable originality in the case of the OBSs to probe the conditions for detecting low-frequency transient signals which have been found recently in the case of the Cascadia and Central Japan subductions and associated to their seismogenic character.

Late Palaeozoic-Cenozoic assembly of the Tethyan orogen in the light of evidence from Greece and Albania

NASA Astrophysics Data System (ADS)

Robertson, A. H. F.

2012-04-01

The objective here is to use the geology and tectonics of a critical part of the Tethyan orogen, represented by Greece and Albania, to shed light on the tectonic development of Tethys on a regional, to global scale, particularly the history of convergence during Late Palaeozoic to Cenozoic time. For Carboniferous time much evidence suggests that the Korabi-Pelagonian crustal unit as exposed in Albania and Greece formed above a northward-dipping subduction zone along the Eurasia continental margin, with Palaeotethys to the south. However, there is also some evidence of southward subduction beneath Gondwana especially from southern Greece and central southern Turkey. Palaeotethys is inferred to have closed in Europe as far to the east as the longitude of Libya, while remaining open beyond this. There is still uncertainty about the Pangea A-type reconstruction that would restore all of the present units in the area to within the E Mediterranean region, versus the Pangea B-type reconstruction that would require right-lateral displacement of exotic terranes, by up to 3,500 km eastwards. In either reconstruction, fragments of the Variscan collisional orogen are likely to have been displaced eastwards (variable distances) in the Balkan region prior to Late Permian-Early Triassic time. From ~Late Permian, the Greece-Albania crustal units were located in their present relative position within Tethys as a whole. From the mid-Permian, onwards the northern margin of Gondwana was affected by crustal extension. A Mesozoic ocean (Pindos-Mirdita ocean) then rifted during Early-Middle Triassic time, culminating in final continental break-up and seafloor spreading during the Late Triassic (Carnian-Norian). Subduction-influenced volcanics of mainly Early-Middle Triassic age probably reflect the extraction of magma from sub-continental lithosphere that was enriched in subduction-related fluids and volatiles during an earlier, ?Variscan subduction event. The existence of Upper Triassic mid-ocean ridge-type igneous rocks, known locally in Albania and Greece, points to rifting of a Red Sea-type oceanic basin rather than a back-arc basin related to contemporaneous subduction. After initial, inferred slow spreading at an Upper Triassic, rifted ocean ridge and spreading during the Early Jurassic, the ocean basin underwent regional convergence. Subduction was initiated at, or near, a spreading axis perhaps adjacent to an oceanic fracture zone. The Jurassic supra-subduction zone-type ophiolites of both Greece and Albania largely relate to melting of rising asthenosphere in the presence of volatiles (water) that originated from subducting oceanic lithosphere. High-magnesian boninite-type magmas that are present in both the Albanian and Greece ophiolites and some underlying melanges reflect remelting of previously depleted oceanic upper mantle. Localised MOR-type ophiolites of Late Middle Jurassic age, mainly exposed in NE Albania, were created at a rifted spreading axis. The amphibolite-facies metamorphic sole of the ophiolites was mainly derived from oceanic crust (including within-plate type seamounts), whereas the underlying lower-grade, greenschist facies sole was mainly sourced from the rifted continental margin. The melange, dismembered thrust sheets and polymict debris flows ("olistostromes") beneath the ophiolites formed by accretion and gravity reworking of continental margin units. The in situ radiolarian chert cover of the ophiolites in northern Albania is overlain by polymict debris flows ("olistostromes"). Pelagic carbonate deposition followed during Tithonian-Berriasian time and then restoration of a regional carbonate platform during the Cretaceous. Exhumation of deeply buried parts of the over-ridden continental margin probably took place during the Early Cretaceous. Structural evidence, mainly from northern Greece (Vourinos, Pindos and Othris areas), indicates that the ophiolites, the metamorphic sole, the accretionary melange, and the underlying continental margin units were all deformed by top-to-the-northeast thrusting during Late Middle-Early Late Jurassic time. However, such kinematic evidence is not obviously replicated in Albania, where there are reports of ~southwest-directed (or variable) emplacement. Remaining Pindos-Mirdita oceanic crust subducted ~southwestwards during Late Cretaceous-Eocene time, while oceanic crust continued to form in the south-Aegean region at least locally during Late Cretaceous time. During Early Cenozoic time the Pindos-Mirdita ocean closed progressively southwards, triggering mainly southward progradation of turbidites derived from the over-riding Korabi-Pelagonian microcontinent. Smaller volumes of sediment were also derived from the Apulia (Adria) continent. The Mesohellenic Trough of Greece and its counterpart in Albania evolved from an Eocene fore-arc-type basin above subducting oceanic lithosphere to a thrust-top basin as continental crust continued to underthrust during the Oligocene after final closure of the Pindos-Mirdita ocean. Miocene and Plio-Quaternary successor flexural foredeeps developed in response to continuing regional plate convergence. The preferred tectonic alternatives are assembled into a new overall tectonic model, which in turn needs to be tested and developed in the light of future studies. Reference: Robertson, A.H.F. Tectonic development of Greece and Albania in the context of alternative reconstructions of Tethys in the Eastern Mediterranean region during Late Palaeozoic-Cenozoic time. International Geological Review, in press.

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

PubMed

Calvert, Andrew J

2004-03-11

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.

Structure and Tectonics of the Andaman Subduction Zone from Modeling of Seismological and Gravity Data

NASA Astrophysics Data System (ADS)

Nemalikanti, P. R.; Rao, N.; Hazarika, P.; Tiwari, V. M.; Mangalampally, R.; Singh, A.

2012-12-01

The 10 August 2009 Andaman earthquake of Mw 7.5 occurred to the north of the Andaman and Nicobar Islands at 14o N and 93o E which interestingly, coincides with the northern periphery of the rupture of the Sumatra-Andaman giant mega-thrust earthquake of Mw 9.1 that occurred on 26 December 2004. The event was followed by aftershocks with a peculiar vertical distribution at the same location which was earlier devoid of any significant seismicity. Waveform modeling of five of these events recorded by ISLANDS - the broadband seismic network deployed along the Andaman and Nicobar Islands, indicates that the main shock and two of its aftershocks have a normal fault mechanism with shallow focal depths within 18 km while two others have a strike-slip mechanism occurring deeper, down to 26 km. The computed Bouger gravity anomalies in this region indicate the steepest gradient of 1.5 mgal/km exactly centered over this zone of vertical seismic distribution that characterizes a region of lithospheric split or tear which is devoid of a subducting slab. This is in contrast to a clear subduction trend visible in the southern Andaman and Sunda arcs further south, as evidenced by tomographic images. Joint inversion of waveforms of these five events simultaneously, provides the best fitting P wave velocity structure of this region, given by a Moho at a depth of 30 km and a high crustal Vp/Vs ratio of 1.81. We infer an oceanic double crustal column corresponding to a thickness of about 21 km of Burmese crust including a 5 km thick sedimentary column, underlain by a thinner Indian crust which apparently has a thickness of about 9 km, a model that is also confirmed independently by gravity modeling. We interpret the mechanism of shallow normal fault earthquakes as an intra-plate relaxation phenomenon following the buckling of the overriding Burmese plate in the accretionary wedge of the fore-arc basin, in response to the 2004 mega-thrust subduction event. The deeper strike slip events correspond to an intra-plate phenomenon within the subducting Indian lithospheric plate representing left-lateral faulting across the Andaman arc, due to uneven convergence along the subduction front. Such strike-slip movements are seen all over the Indian Ocean diffuse deformation zone and represent strain accommodation in the Indian crust in response to a grosser mechanism of wrench fault tectonics of the Indo-Australian subduction beneath the Burma-Sunda plate.

The Penokean orogeny in the Lake Superior region

USGS Publications Warehouse

Schulz, K.J.; Cannon, W.F.

2007-01-01

The Penokean orogeny began at about 1880 Ma when an oceanic arc, now the Pembine-Wausau terrane, collided with the southern margin of the Archean Superior craton marking the end of a period of south-directed subduction. The docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back-arc extension both in the initial arc and adjacent craton margin to the north. A belt of volcanogenic massive sulfide deposits formed in the extending back-arc rift within the arc. Synchronous extension and subsidence of the Superior craton resulted in a broad shallow sea characterized by volcanic grabens (Menominee Group in northern Michigan). The classic Lake Superior banded iron-formations, including those in the Marquette, Gogebic, Mesabi and Gunflint Iron Ranges, formed in that sea. The newly established subduction zone caused continued arc volcanism until about 1850 Ma when a fragment of Archean crust, now the basement of the Marshfield terrane, arrived at the subduction zone. The convergence of Archean blocks of the Superior and Marshfield cratons resulted in the major contractional phase of the Penokean orogeny. Rocks of the Pembine-Wausau arc were thrust northward onto the Superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 Ma in the south (Baraga Group rocks) and 1835 Ma in the north (Rove and Virginia Formations). A thick succession of arc-derived turbidites constitutes most of the foreland basin-fill along with lesser volcanic rocks. In the southern fold and thrust belt tectonic thickening resulted in high-grade metamorphism of the sediments by 1830 Ma. At this same time, a suite of post-tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the Penokean orogeny. The Penokean orogen was strongly overprinted by younger tectonic and thermal events, some of which were previously ascribed to the Penokean. Principal among these was a period of vertical faulting in the Archean basement and overlying Paleoproterozoic strata. This deformation is now known to have post-dated the terminal Penokean plutons by at least several tens of millions of years. Evidence of the Penokean orogen is now largely confined to the Lake Superior region. Comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive Penokean orogen have been removed or overprinted by younger tectonic events. ?? 2007 Elsevier B.V. All rights reserved.

Tectonics of the IndoBurma Oblique Subduction Zone

NASA Astrophysics Data System (ADS)

Steckler, M. S.; Seeber, L.; Akhter, S. H.; Betka, P. M.; Cai, Y.; Grall, C.; Mondal, D. R.; Gahalaut, V. K.; Gaherty, J. B.; Maung Maung, P.; Ni, J.; Persaud, P.; Sandvol, E. A.; Tun, S. T.

2016-12-01

The Ganges-Brahmaputra Delta (GBD) is obliquely colliding with the IndoBurma subduction zone. Most of the 42 mm/y of arc-parallel motion is absorbed in a set of dextral to dextral-convergent faults, the Sagaing, Kabaw and Churachandpur-Mao Faults. The 13-17 mm/y of convergence with the delta has built a 250-km wide active accretionary prism. The upper part of the 19-km sediment thickness consists of a shallowing-up stack of prograding strata that has shifted the shelf edge 3-400 km since the Himalayan orogeny at 50 Ma. The upper 3-5 km sandy shelf to fluvial strata are deformed into a broad fold and thrust belt above an overpressured décollement. It forms a flat shallow roof thrust in the frontal accretionary prism. The structure of the deeper part of the accretionary prism, which must transfer the incoming sediments to the upper plate, is unknown. GPS indicates the downdip end of the megathrust locked zone is 25 km at 92.5°E. The deformation front, marked by nascent detachment folds above the shallow décollement reaches the megacity of Dhaka in the middle of the GBD. The seismogenic potential of this portion of the prism is unknown. Arc volcanism in Myanmar, 500 km east of the deformation front, is sparse. Limited geochemical data on the arc volcanics are consistent with hot slab conditions. One possibility is that the deep GBD slab and basement are metamorphosed and dewatered early in the subduction process whereby most of the fluids are transferred to the growing prism by buoyancy driven migration or accretion of fluid-rich strata. Since it is entirely subaerial this little-studied region crossing Bangladesh, India and Myanmar provides an opportunity for a detailed multidisciplinary geophysical and geological investigation. It has the potential to highlight the role of fluids in subduction zones, the tectonics of extreme accretion and their seismic hazards, and the interplay between driving and resistance forces of a subduction zone during a soft collision.

An inverted continental Moho and serpentinization of the forearc mantle.

PubMed

Bostock, M G; Hyndman, R D; Rondenay, S; Peacock, S M

2002-05-30

Volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge, resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere. Asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back-arc region, but the forearc mantle (in the corner of the wedge) is thought to be significantly cooler. Here we explore the structure of the mantle wedge in the southern Cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers. We find very low shear-wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an 'inverted' continental Moho, which reverts to normal polarity seaward of the Cascade arc. This observation provides compelling evidence for a highly hydrated and serpentinized forearc region, consistent with thermal and petrological models of the forearc mantle wedge. This serpentinized material is thought to have low strength and may therefore control the down-dip rupture limit of great thrust earthquakes, as well as the nature of large-scale flow in the mantle wedge.

Accretion in the wake of terrane collision: The Neogene accretionary wedge off Kenai Peninsula, Alaska

USGS Publications Warehouse

Fruehn, J.; von Huene, Roland E.; Fisher, M.A.

1999-01-01

Subduction accretion and repeated terrane collision shaped the Alaskan convergent margin. The Yakutat Terrane is currently colliding with the continental margin below the central Gulf of Alaska. During the Neogene the terrane's western part was subducted after which a sediment wedge accreted along the northeast Aleutian Trench. This wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the Pacific plate. Prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop. The lateral extent of the structures is constrained by high-resolution swath bathymetry and seismic lines collected along strike. Accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating. Toward the northeast the lower slope steepens, the wedge narrows, and the accreted volume decreases notwith-standing a doubling of sediments thickness in the trench. In the northeasternmost transect, near the area where the terrane's trailing edge subducts, no frontal accretion is observed and the slope is eroded. The structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion, as the trailing edge of the Yakutat Terrane is subducting.

Structure and tectonic evolution of the NE segment of the Polish-Ukrainian Carpathians during the Late Cenozoic: subsurface cross-sections and palinspastic models

NASA Astrophysics Data System (ADS)

Kuśmierek, Jan; Baran, Urszula

2016-08-01

The discrepant arrangement of the Carpathian nappes and syntectonic deposits of the Carpathian Foredeep reveals the oroclinal migration of the subduction direction of the platform margin during the Late Cenozoic. Formation of the nappes was induced by their detachment from disintegrated segments of the European Platform; the segments were shortened as a result of their vertical rotation in zones of compressional sutures. It finds expression in local occurrence of the backward vergence of folding against the generally forward vergence toward the Carpathian Foredeep. The precompressional configuration of sedimentation areas of particular nappes was reconstructed with application of the palinspastic method, on the basis of the hitherto undervalued model which emphasizes the influence of the subduction and differentiated morphology of the platform basement on the tectonic evolution of the fold and thrust belt. Superposition of the palaeogeographic representations and the present geometry of the orogen allows understanding of the impact of the magnitudes of tectonic displacements on the differentiation of the geological structure in the NE segment of the Carpathians. The differentiation has inspired different views of Polish and Ukrainian geologists on structural classification and evolution of the frontal thrusts.

Geologic processes of accretion in the Cascadia subduction zone west of Washington State

USGS Publications Warehouse

Fisher, M.A.; Flueh, E.R.; Scholl, D. W.; Parsons, T.; Wells, R.E.; Tréhu, A.; ten Brink, Uri S.; Weaver, C.S.

1999-01-01

The continental margin west of Oregon and Washington undergoes a northward transition in morphology, from a relatively narrow, steep slope west of Oregon to a broad, midslope terrace off Washington. Multichannel seismic (MCS) reflection data collected over the accretionary complex show that the morphologic transition is accompanied by significant change in accretionary style: West of Oregon the direction of thrust vergence in the wedge toe flip-flops between landward and seaward, whereas off Washington, thrust faults in the toe verge consistently landward, except near the mouth of the Columbia River where detachment folding of accreted sediment is evident. Furthermore, rocks under the broad midslope terrace west of Washington appear to be intruded by diapirs. The combination of detachment folding, diapirs, and landward-vergent thrust faults all suggest that nearly as far landward as the shelf break, coupling along the interplate decollement is, or has been, low, as suggested by other lines of evidence.

How material contrast around subduction faults may control coseismic slip and rupture dynamics: tsunami applications for the case study of Tohoku

NASA Astrophysics Data System (ADS)

Scala, Antonio; Murphy, Shane; Romano, Fabrizio; Lorito, Stefano; Festa, Gaetano; Volpe, Manuela; Piatanesi, Alessio

2017-04-01

Recent megathrust tsunamigenic events, e.g. Maule 2010 (M8.8) and Tohoku 2011 (M9.0), generated huge tsunami waves as a consequence of high slip in the shallow part of the respective subduction zone. Other events, (e.g. the recent Mentawai 2010, M7.8, or the historical Meiji 1896, M8.2), referred to as tsunami earthquakes, produced unexpectedly large tsunami waves, probably due to large slip at shallow depth over longer rupture durations compared to deeper thrust events. Subduction zone earthquakes originate and propagate along bimaterial interfaces separating materials having different elastic properties, e.g. continental and oceanic crust, a stiffer deep mantle wedge, shallow compliant accretionary prism etc. Bimaterial interfaces have been showed, through observations (seismological and laboratory) and theoretical studies, to affect the rupture: introducing a preferred rupture direction as well as asymmetric rupture velocities and shear stress redistributions. Such features are predominantly due to the break of symmetry between the two sides of the interface in turn ascribable to the complex coupling between the frictional interfacial sliding and the slip-induced normal stress perturbations. In order to examine the influence of material contrast on a fault plane on the seismic source and tsunami waves, we modelled a Tohoku-like subduction zone to perform a large number of 2D along-dip rupture dynamics simulations in the framework of linear slip weakening both for homogeneous and bimaterial fault. In this latter model, the rupture acts as the interface between the subducting oceanic crust and the overriding layers (accretionary prism, continental crust and mantle wedge), varying the position of the shear stress asperity acting as nucleation patch. Initial results reveal that ruptures in homogeneous media produce earthquakes with large slip at depth compared to the case where bi-material interface is included. However the opposite occurs for events nucleating at intermediate depths: the compliant accretionary prism favours slip up to the free surface leading to larger events compared to the homogeneous case. These preliminary findings will be further investigated considering different material contrasts between the slab and the overriding accretionary prism to mimic the slowness of the sedimentary wedge. This will contribute to assess the influence of these contrasts in more realistic environment on the seismic source features and, in turn, on the conditional probability of exceedance for maximum tsunami wave height for a M9 event. Several source parameters, such as coseismic slip, rupture duration, rupture velocity and stress conditions, derived from the numerical simulations will be compared to those inferred from real events using existing finite fault catalogues (e.g. USGS, SRCMOD, etc.).

Seismic variability of subduction thrust faults: Insights from laboratory models

NASA Astrophysics Data System (ADS)

Corbi, F.; Funiciello, F.; Faccenna, C.; Ranalli, G.; Heuret, A.

2011-06-01

Laboratory models are realized to investigate the role of interface roughness, driving rate, and pressure on friction dynamics. The setup consists of a gelatin block driven at constant velocity over sand paper. The interface roughness is quantified in terms of amplitude and wavelength of protrusions, jointly expressed by a reference roughness parameter obtained by their product. Frictional behavior shows a systematic dependence on system parameters. Both stick slip and stable sliding occur, depending on driving rate and interface roughness. Stress drop and frequency of slip episodes vary directly and inversely, respectively, with the reference roughness parameter, reflecting the fundamental role for the amplitude of protrusions. An increase in pressure tends to favor stick slip. Static friction is a steeply decreasing function of the reference roughness parameter. The velocity strengthening/weakening parameter in the state- and rate-dependent dynamic friction law becomes negative for specific values of the reference roughness parameter which are intermediate with respect to the explored range. Despite the simplifications of the adopted setup, which does not address the problem of off-fault fracturing, a comparison of the experimental results with the depth distribution of seismic energy release along subduction thrust faults leads to the hypothesis that their behavior is primarily controlled by the depth- and time-dependent distribution of protrusions. A rough subduction fault at shallow depths, unable to produce significant seismicity because of low lithostatic pressure, evolves into a moderately rough, velocity-weakening fault at intermediate depths. The magnitude of events in this range is calibrated by the interplay between surface roughness and subduction rate. At larger depths, the roughness further decreases and stable sliding becomes gradually more predominant. Thus, although interplate seismicity is ultimately controlled by tectonic parameters (velocity of the plates/trench and the thermal regime), the direct control is exercised by the resulting frictional properties of the plate interface.

Initiation of a thrust fault revealed by analog experiments

NASA Astrophysics Data System (ADS)

Dotare, Tatsuya; Yamada, Yasuhiro; Adam, Juergen; Hori, Takane; Sakaguchi, Hide

2016-08-01

To reveal in detail the process of initiation of a thrust fault, we conducted analog experiments with dry quartz sand using a high-resolution digital image correlation technique to identify minor shear-strain patterns for every 27 μm of shortening (with an absolute displacement accuracy of 0.5 μm). The experimental results identified a number of "weak shear bands" and minor uplift prior to the initiation of a thrust in cross-section view. The observations suggest that the process is closely linked to the activity of an adjacent existing thrust, and can be divided into three stages. Stage 1 is characterized by a series of abrupt and short-lived weak shear bands at the location where the thrust will subsequently be generated. The area that will eventually be the hanging wall starts to uplift before the fault forms. The shear strain along the existing thrust decreases linearly during this stage. Stage 2 is defined by the generation of the new thrust and active displacements along it, identified by the shear strain along the thrust. The location of the new thrust may be constrained by its back-thrust, generally produced at the foot of the surface slope. The activity of the existing thrust falls to zero once the new thrust is generated, although these two events are not synchronous. Stage 3 of the thrust is characterized by a constant displacement that corresponds to the shortening applied to the model. Similar minor shear bands have been reported in the toe area of the Nankai accretionary prism, SW Japan. By comparing several transects across this subduction margin, we can classify the lateral variations in the structural geometry into the same stages of deformation identified in our experiments. Our findings may also be applied to the evaluation of fracture distributions in thrust belts during unconventional hydrocarbon exploration and production.

Glacier Ice Mass Fluctuations and Fault Instability in Tectonically Active Southern Alaska

NASA Technical Reports Server (NTRS)

SauberRosenberg, Jeanne M.; Molnia, Bruce F.

2003-01-01

Across southern Alaska the northwest directed subduction of the Pacific plate is accompanied by accretion of the Yakutat terrane to continental Alaska. This has led to high tectonic strain rates and dramatic topographic relief of more than 5000 meters within 15 km of the Gulf of Alaska coast. The glaciers of this area are extensive and include large glaciers undergoing wastage (glacier retreat and thinning) and surges. The large glacier ice mass changes perturb the tectonic rate of deformation at a variety of temporal and spatial scales. We estimated surface displacements and stresses associated with ice mass fluctuations and tectonic loading by examining GPS geodetic observations and numerical model predictions. Although the glacial fluctuations perturb the tectonic stress field, especially at shallow depths, the largest contribution to ongoing crustal deformation is horizontal tectonic strain due to plate convergence. Tectonic forces are thus the primary force responsible for major earthquakes. However, for geodetic sites located < 10-20 km from major ice mass fluctuations, the changes of the solid Earth due to ice loading and unloading are an important aspect of interpreting geodetic results. The ice changes associated with Bering Glacier s most recent surge cycle are large enough to cause discernible surface displacements. Additionally, ice mass fluctuations associated with the surge cycle can modify the short-term seismicity rates in a local region. For the thrust faulting environment of the study region a large decrease in ice load may cause an increase in seismic rate in a region close to failure whereas ice loading may inhibit thrust faulting.

The Sunda-Banda Arc Transition: New Insights from Marine Multichannel Seismic Data

NASA Astrophysics Data System (ADS)

Mueller, C.; Kopp, H.; Djajadihardja, Y.; Engels, M.; Flueh, E.; Gaedicke, C.; Lueschen, E.; Lutz, R.; Planert, L.; Shulgin, A.; Soemantri, D. D.

2007-12-01

After the Indian Ocean Mw 9.3 earthquake and tsunami on December 26, 2004, intensive research activities focussed on the Sunda Arc subduction system offshore Sumatra. For this area a broad database is now available interpreted in terms of plate segmentation and outer arc high evolution. In contrast, the highly active easternmost part of this subduction system, as indicated by the south of Java Mw 7.7 earthquake and tsunami on July 17, 2006, has remained almost unexplored until recently. During RV SONNE cruise SO190 from October until December 2006 almost 5000 km of marine geophysical profiles have been acquired at the eastern Sunda Arc and the transition to the Banda Arc. The SINDBAD project (Seismic and Geoacoustic Investigations along the Sunda-Banda Arc Transition) comprises 30-fold multichannel reflection seismics with a 3-km streamer, wide-angle OBH/OBS refraction seismics for deep velocity control (see poster of Planert et al. in this session), swath bathymetry, sediment echosounder, gravimetric and geomagnetic measurements. We present data and interpretations of several 250-380 km long, prestack depth-migrated seismic sections, perpendicular to the deformation front, based on velocity models from focussing analysis and inversion of OBH/OBS refraction data. We focus on the variability of the lower plate and the tectonic response of the overriding plate in terms of outer arc high formation and evolution, forearc basin development, accretion and erosion processes at the base of the overriding plate. The subducting Indo-Australian Plate is characterized by three segments: i) the Roo Rise with rough topography offshore eastern Java ii) the Argo Abyssal Plain with smooth oceanic crust offshore Bali, Lombok, and Sumbawa, and iii) the Scott Plateau with continental crust colliding with the Banda island arc. The forearc responds to differences in the incoming oceanic plate with the absence of a pronounced forearc basin offshore eastern Java and with development of the 4000 m deep forearc Lombok Basin offshore Bali, Lombok, and Sumbawa. The eastern termination of the Lombok Basin is formed by Sumba Island, which shows evidence for recent uplift, probably associated with the collision of the island arc with the continental Scott Plateau. The Sumba area represents the transition from subduction to collision. Our seismic profiles image the bending of the oceanic crust seaward of the trench and associated normal faulting. Landward of the trench, they image the subducting slab beneath the outer arc high, where the former bending-related normal faults appear to be reactivated as reverse faults introducing vertical displacements in the subducting slab. The accretionary prism and the outer arc high are characterized by an ocean-verging system of imbricate thrust sheets with major thrust faults connecting seafloor and detachment. Compression results in shortening and steepening of the imbricated thrust sheets building up the outer arc high. Tilted piggy-back basins and downlaps of tilted sediments in the southern Lombok forearc basin indicate ongoing uplift of the entire outer arc high, abrupt displacements, and recent tectonic activity.

Transition from strike-slip faulting to oblique subduction: active tectonics at the Puysegur Margin, South New Zealand

NASA Astrophysics Data System (ADS)

Lamarche, Geoffroy; Lebrun, Jean-Frédéric

2000-01-01

South of New Zealand the Pacific-Australia (PAC-AUS) plate boundary runs along the intracontinental Alpine Fault, the Puysegur subduction front and the intraoceanic Puysegur Fault. The Puysegur Fault is located along Puysegur Ridge, which terminates at ca. 47°S against the continental Puysegur Bank in a complex zone of deformation called the Snares Zone. At Puysegur Trench, the Australian Plate subducts beneath Puysegur Bank and the Fiordland Massif. East of Fiordland and Puysegur Bank, the Moonlight Fault System (MFS) represents the Eocene strike-slip plate boundary. Interpretation of seafloor morphology and seismic reflection profiles acquired over Puysegur Bank and the Snares Zone allows study of the transition from intraoceanic strike-slip faulting along the Puysegur Ridge to oblique subduction at the Puysegur Trench and to better understand the genetic link between the Puysegur Fault and the MFS. Seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 Geodynz survey, and single beam echo soundings acquired by the NZ Royal Navy. The Snares Zone is the key transition zone from strike-slip faulting to subduction. It divides into three sectors, namely East, NW and SW sectors. A conspicuous 3600 m-deep trough (the Snares Trough) separates the NW and East sectors. The East sector is characterised by the NE termination of Puysegur Ridge into right-stepping en echelon ridges that accommodate a change of strike from the Puysegur Fault to the MFS. Between 48°S and 47°S, in the NW sector and the Snares Trough, a series of transpressional faults splay northwards from the Puysegur Fault. Between 49°50'S and 48°S, thrusts develop progressively at Puysegur Trench into a decollement. North of 48°S the Snares Trough develops between two splays of the Puysegur Fault, indicating superficial extension associated with the subsidence of Puysegur Ridge. Seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the Snares Fault, and terminate at the top of the Puysegur trench slope. Between ca. 48°S and 46°30'S, the relative plate motion appears to be distributed over the Puysegur subduction zone and the strike-slip faults located on the edge of the upper plate. Conversely, north of ca. 46°S, a lack of active strike-slip faulting along the MFS and across most of Puysegur Bank indicates that the subduction in the northern part of Puysegur Trench accounts for most of the oblique convergence. Hence, active transpression in the Snares fault zone indicates that the relative PAC-AUS plate motion is transferred from strike-slip faulting along the Puysegur Fault to subduction at Puysegur Trench. The progressive transition from thrusts at Puysegur Trench and strike-slip faulting at the Puysegur Fault to oblique subduction at Puysegur Trench suggests that the subduction interface progressively developed from a western shallow splay of the Puysegur Fault. It implies that the transfer fault links the subduction interface at depth. A tectonic sliver is identified between Puysegur Trench and the Puysegur Fault. Its northwards motion relative to the Pacific Plate implies that is might collide with Puysegur Bank.

Tensile overpressure compartments on low-angle thrust faults

NASA Astrophysics Data System (ADS)

Sibson, Richard H.

2017-08-01

Hydrothermal extension veins form by hydraulic fracturing under triaxial stress (principal compressive stresses, σ 1 > σ 2 > σ 3) when the pore-fluid pressure, P f, exceeds the least compressive stress by the rock's tensile strength. Such veins form perpendicular to σ 3, their incremental precipitation from hydrothermal fluid often reflected in `crack-seal' textures, demonstrating that the tensile overpressure state, σ 3' = ( σ 3 - P f) < 0, was repeatedly met. Systematic arrays of extension veins develop locally in both sub-metamorphic and metamorphic assemblages defining tensile overpressure compartments where at some time P f > σ 3. In compressional regimes ( σ v = σ 3), subhorizontal extension veins may develop over vertical intervals <1 km or so below low-permeability sealing horizons with tensile strengths 10 < T o < 20 MPa. This is borne out by natural vein arrays. For a low-angle thrust, the vertical interval where the tensile overpressure state obtains may continue down-dip over distances of several kilometres in some instances. The overpressure condition for hydraulic fracturing is comparable to that needed for frictional reshear of a thrust fault lying close to the maximum compression, σ 1. Under these circumstances, especially where the shear zone material has varying competence (tensile strength), affecting the failure mode, dilatant fault-fracture mesh structures may develop throughout a tabular rock volume. Evidence for the existence of fault-fracture meshes around low-angle thrusts comes from exhumed ancient structures and from active structures. In the case of megathrust ruptures along subduction interfaces, force balance analyses, lack of evidence for shear heating, and evidence of total shear stress release during earthquakes suggest the interfaces are extremely weak ( τ < 40 MPa), consistent with weakening by near-lithostatically overpressured fluids. Portions of the subduction interface, especially towards the down-dip termination of the seismogenic megathrust, are prone to episodes of slow-slip, non-volcanic tremor, low-frequency earthquakes, very-low-frequency earthquakes, etc., attributable to the activation of tabular fault-fracture meshes at low σ 3' around the thrust interface. Containment of near-lithostatic overpressures in such settings is precarious, fluid loss curtailing mesh activity.[Figure not available: see fulltext.

Tectonic evolution and crustal-scale structure of Kyrgyz Central Asian Orogenic Belt: new insights from the Darius programme

NASA Astrophysics Data System (ADS)

Rolland, Yann; Loury, Chloé; Guillot, Stéphane; Mikolaichuk, Alexander

2014-05-01

Mechanisms and history of the Late Palaeozoic accretion followed by formation of trunscurrent strike-slip faults were studied in the southern segment of the Central Asian Orogenic Belt (CAOB) within Kyrgyz South Tianshan. 1. South Tianshan Suture: ending accretion process after docking of Tarim craton This study gives insights into the crustal-scale structure and Upper Paleozoic history of this mountain belt, currently intensely reactivated by the India-Asia collision. Structural, petrological and geochronological studies were carried out within South Tianshan suture east of the Talas-Ferghana Fault (TFF). New data highlight a south-dipping structure featured by a HP metamorphic core complex comprised of c. 320 Ma continental and oceanic eclogites exhumed by top-to-North motion. A large massif (10 x 50 km) of continental HP rocks in the Atbashi Range is comprised of hectometric boudins of eclogites embedded in metapelites and gneissesMetamorphic units exhibit blueschist to eclogite facies conditions, with oceanic (MORB) rocks in the blueschist facies representing the accretionary oceanic prism being thrusted by oceanic rocks and a continental unit in the eclogite facies (510 ± 50°C and 24 ± 2 kbar). Evidence for eclogite facies both in metasediments and mafic lithologies and geological structure are in agreement with a previously thinned continental margin. Subduction of this thinned COT (Continent-Ocean Transition) probably occurred by slab pull in a south-dipping subduction zone, while another north-dipping subduction was active below Middle Tianshan. Final stacking of Middle and South Tianshan occurred at 320-310 Ma. These opposite subduction zones are still reflected in the main structures of Tianshan. Reactivation of the South-dipping structures since 30-25 Ma is ascribed to explain the current Tianshan intra-continental subduction from seismology. 2. Talas-Ferghana Fault (TFF) activity & Basin formation After this accretionary episode, the South Tianshan suture was cross-cut by the TFF, which was active in several stages from 320 Ma to present. The main events of basin formation are ascribed to the activity of the dextral TFF (Rolland et al. 2013, JAES). Ar-Ar dating undertaken on syn-kinematic minerals that feature the phases of motion of the TFF show a first stage of activation occurred at 312 ± 4 Ma, followed by a main stage of dextral motion in the Late Permian at 256 - 250 Ma, while late stages of reactivation of TFF is featured by emplacement of 195 ± 3 Ma pegmatitic dykes, formation of transtensional basins during Jurassic, dextral offsets of river valleys and ongoing seismicity. 3. Reactivation of South Tian Shan Suture Most prominent topography in Central Asia corresponds to the former South Tianshan suture which has been reactivated since about 30 Ma, the former Carboniferous thrusts are reactivated in a pop-up structure with top-north and top-south faults bounding the high mountains of Khan Tengri and Pobeda peaks (7440 m a.s.l.).

Metamorphic P-T conditions across the Chugach Metamorphic Complex (Alaska)—A record of focussed exhumation during transpression

NASA Astrophysics Data System (ADS)

Bruand, Emilie; Gasser, Deta; Stüwe, Kurt

2014-03-01

The Chugach Metamorphic Complex (CMC) is a large high-grade metamorphic complex that developed in the Eocene within the Chugach accretionary complex along the margin of Alaska where subduction is still ongoing. The CMC has a conspicuous asymmetric structure with a migmatitic zone flanked in the north and west by amphibolite facies schists and in the south by a metabasite belt. To the north and south, major, crustal-scale fault zones juxtapose the Chugach terrane against much lower-grade and less-deformed sequences belonging to different terranes. Curiously these crustal-scale structures are known to have largely strike slip motion posing the question as to the nature of the exhumation of the high-grade complex between them. However, P-T conditions which would allow an estimation of the amount of exhumation were lacking for large parts of the complex. This paper presents petrographic descriptions, biotite-garnet thermometry, RSCM thermometry, average P-T calculations and pseudosection modelling from three major across-strike transects covering the complex from west to south-east. Our results reveal that, both temperature and pressure vary substantially across the complex. More specifically, peak metamorphic conditions evolve from 4-7 kbar and ~ 550-650 °C in the northern schist zone to 5-11 kbar and ~ 650-750 °C in the migmatite zone in the south of the complex. The higher pressure estimates in the south of the complex indicate that focussed exhumation must have occurred in this area and was probably initiated by the subduction of a high topographic relief (intra-oceanic arc or ridge subduction) and the accretion of the metabasite belt in the south. Exhumation of the CMC occurred in an overall transpressive strain regime, with strike-slip deformation concentrated along the northern Border Range fault zone and thrusting and exhumation focussed within the southern migmatite zone and splay faults of the Contact fault zone. The T/P ratios in the southern migmatite zone indicate that the thermal perturbation of the migmatites is less than previously inferred. These new results, associated with the structural data and the accretion of a metabasite belt in the south of the complex, seem incompatible with the existing ridge-subduction models.

Cenozoic structural evolution, thermal history, and erosion of the Ukrainian Carpathians fold-thrust belt

NASA Astrophysics Data System (ADS)

Nakapelyukh, Mykhaylo; Bubniak, Ihor; Bubniak, Andriy; Jonckheere, Raymond; Ratschbacher, Lothar

2018-01-01

The Carpathians are part of the Alpine-Carpathian-Dinaridic orogen surrounding the Pannonian basin. Their Ukrainian part constitutes an ancient subduction-accretion complex that evolved into a foreland fold-thrust belt with a shortening history that was perpendicular to the orogenic strike. Herein, we constrain the evolution of the Ukrainian part of the Carpathian fold-thrust belt by apatite fission-track dating of sedimentary and volcanic samples and cross-section balancing and restoration. The apatite fission-track ages are uniform in the inner―southwestern part of the fold-thrust belt, implying post-shortening erosion since 12-10 Ma. The ages in the leading and trailing edges record provenance, i.e., sources in the Trans-European suture zone and the Inner Carpathians, respectively, and show that these parts of the fold-thrust were not heated to more than 100 °C. Syn-orogenic strata show sediment recycling: in the interior of the fold-thrust belt―the most thickened and most deeply eroded nappes―the apatite ages were reset, eroded, and redeposited in the syn-orogenic strata closer to the fore- and hinterland; the lag times are only a few million years. Two balanced cross sections, one constructed for this study and based on field and subsurface data, reveal an architecture characterized by nappe stacks separated by high-displacement thrusts; they record 340-390 km shortening. A kinematic forward model highlights the fold-thrust belt evolution from the pre-contractional configuration over the intermediate geometries during folding and thrusting and the post-shortening, erosional-unloading configuration at 12-10 Ma to the present-day geometry. Average shortening rates between 32-20 Ma and 20-12 Ma amounted to 13 and 21 km/Ma, respectively, implying a two-phased deformation of the Ukrainian fold-thrust belt.

Supercycles at subduction thrusts controlled by seismogenic zone downdip width

NASA Astrophysics Data System (ADS)

van Dinther, Y.; Herrendoerfer, R.; Gerya, T.; Dalguer, L. A.

2014-12-01

Supercycles in subduction zones describe a long-term cluster of megathrust earthquakes, which recur in a similar way (Sieh et al. 2008,Goldfinger et al. 2013). It consists of two complete failures of a given subduction segment in between which, after a long period of relative quiescence, partial ruptures occur. We recognize that supercycles were proposed in those subduction zones (Sieh et al. 2008,Goldfinger et al. 2013, Metois et al. 2014, Chlieh et al. 2014) for which the seismogenic zone downdip width is estimated to be larger than average (Heuret et al. 2011, Hayes et al. 2012). We show with a two-dimensional numerical model of a subduction zone that the seismogenic zone downdip width indeed has a strong influence on the long-term seismicity pattern and rupture styles. Increasing the downdip width of the seismogenic zone leads to a transition from ordinary cycles of similar sized crack-like ruptures to supercycles consisting of a range of rupture sizes and styles. Our model demonstrates how interseismic deformation accompanied by subcritical and pulse-like ruptures effectively increases the stress throughout the seismogenic zone towards a critical state at which a crack-like superevent releases most of the accumulated stresses. We propose such stress evolution along the dip of the megathrust as the simplest explanation for supercycles. This conceptual model suggests that larger than thus far observed earthquakes could occur as part of a supercycle in subduction zones with a larger than average seismogenic zone downdip width (>120-150 km).

Interaction between the Dauki and the Indo-Burman convergence boundaries from teleseismic and locally recorded earthquake data

NASA Astrophysics Data System (ADS)

Howe, M.; Moulik, P.; Seeber, L.; Kim, W.; Steckler, M. S.

2012-12-01

The Himalayan and the Burma Arcs converge onto the Indian plate from opposite sides near their syntaxial juncture and have reduced it to a sliver. Both geology and seismicity point to recent internal deformation and high seismogenic potential within this sliver. Large historical earthquakes, including the Great Indian earthquake of 1897 (Mw ~8.1), along with the recent seismicity, suggest that the cratonic blocks in the region are bounded by active faults. The most prominent is the E-W trending Dauki Fault, a deeply-rooted, north-dipping thrust fault, situated between the Shillong massif to the north and the Sylhet Basin to the south. Along the Burma Arc, the subducted seismogenic slab of the Indian plate is continuous north to the syntaxis. Yet the Naga and Tripura segments of the accretionary fold belt, respectively north and south of the easterly extrapolation of the Dauki fault, are distinct. Accretion has advanced far westward into the foredeep of the Dauki structure along the front of the Tripura segment, while it has remained stunted facing the uplifted Shillong massif along the Naga segment. Moreover, the Dauki topographic front can be traced eastwards across the Burma Arc separating the two segments. Recent earthquakes support the hypothesis that the Dauki convergence structure continues below the Burma accretionary belt. Using teleseismic and regional data from the deployment of a local network, we explore the interaction of the Dauki thrust fault with the Burma Arc subduction zone. Preliminary observations include: While seismicity is concentrated in the slab at the eastward extrapolation of the Dauki fault, shallow seismicity is diffuse and does not illuminate the Dauki fault itself. P-axes in moment-tensor solutions of earthquakes within the Indian plate tend to be directed N-S and are locally parallel to the India-Burma boundary, particularly in the slab. T-axes tend to be oriented E-W with a strong tendency to follow the slab down dip. This pattern is remarkably consistent, despite the scattered seismicity, and suggests that the stress in the Indian plate, including the subducted oceanic portion of the plate, is still primarily controlled by the Himalayan collision to the north and down-dip pull by the Burma slab. Moment tensor solutions for some of the shallow earthquakes along the fold belt are consistent with geodetic results, showing partitioning of the oblique India-Burma convergence between belt-parallel dextral faults and belt-normal shortening by thrust faults. Relocations of the events using the double-difference algorithm may provide additional constraints on the geometry of the slab. In addition to the analysis of teleseismic data, a network of six seismic stations was also installed in Bangladesh in the region surrounding Sylhet, south of the Shillong Plateau during 2007-2008. Over 200 regional and local events are detected and located by the Sylhet array. About a dozen events are large enough allowing us to determine focal depths and mechanisms that will augment the catalog of the teleseismic events, providing additional insights into the tectonics in the region.

Constraining Source Locations of Shallow Subduction Megathrust Earthquakes in 1-D and 3-D Velocity Models - A Case Study of the 2002 Mw=6.4 Osa Earthquake, Costa Rica

NASA Astrophysics Data System (ADS)

Grevemeyer, I.; Arroyo, I. G.

2015-12-01

Earthquake source locations are generally routinely constrained using a global 1-D Earth model. However, the source location might be associated with large uncertainties. This is definitively the case for earthquakes occurring at active continental margins were thin oceanic crust subducts below thick continental crust and hence large lateral changes in crustal thickness occur as a function of distance to the deep-sea trench. Here, we conducted a case study of the 2002 Mw 6.4 Osa thrust earthquake in Costa Rica that was followed by an aftershock sequence. Initial relocations indicated that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of ocean-bottom-hydrophones and land stations 80 km to the northwest were deployed. By adding readings from permanent Costa Rican stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocated this catalog using a nonlinear probabilistic approach using a 1-D and two 3-D P-wave velocity models. The 3-D model was either derived from 3-D tomography based on onshore stations and a priori model based on seismic refraction data. All epicentres occurred close to the trench axis, but depth estimates vary by several tens of kilometres. Based on the epicentres and constraints from seismic reflection data the main shock occurred 25 km from the trench and probably along the plate interface at 5-10 km depth. The source location that agreed best with the geology was based on the 3-D velocity model derived from a priori data. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interpolate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

Slow slip events in Guerrero, Mexico, and consequences on strain accumulation over the past 15 years.

NASA Astrophysics Data System (ADS)

Radiguet, M.; Cotton, F.; Cavalié, O.; Pathier, E.; Kostoglodov, V.; Vergnolle, M.; Campillo, M.; Walpersdorf, A.; Cotte, N.; Santiago, J.; Franco, S.

2012-12-01

Continuous Global Positioning System (cGPS) time series in Guerrero, Mexico, reveal the widespread existence of large Slow Slip Events (SSEs) at the boundary between the Cocos and North American plates. The existence of these SSEs asks the question of how seismic and aseismic slips complement each other in subduction zones. We examined the last three SSEs that occurred in 2001/2002, 2006 and 2009/2010, and their impact on the strain accumulation along the Guerrero subduction margin. We use continuous cGPS time series and InSAR images to evaluate the surface displacement during SSEs and inter-SSE periods. The slip distributions on the plate interface associated with each SSE, as well as the inter-SSE (short-term) coupling rates are evaluated by inverting these surface displacements. Our results reveal that the three analyzed SSEs have equivalent moment magnitudes of around 7.5 and their lateral extension is variable.The slip distributions for the three SSEs show that in the Guerrero gap area, the slow slip occurs at shallower depth (updip limit around 15-20 km) than in surrounding regions. The InSAR data provide additional information for the 2006 SSE. The joint inversion of InSAR and cGPS data confirms the lateral variation of the slip distribution along the trench, with shallower slip in the Guerrero seismic gap, west of Acapulco, and deeper slip further east. Inversion of inter-SSE displacement rates reveal that during the inter-SSE time intervals, the interplate coupling is high in the area where the slow slip subsequently occurs. Over a 12 year period, corresponding to three cycles of SSEs, our results reveal that the accumulated slip deficit in the Guerrero gap area is only ¼ of the slip deficit accumulated on both sides of the gap. Moreover, the regions of large slip deficit coincide with the rupture areas of recent large earthquakes. We conclude that the SSEs account for a major portion of the overall moment release budget in the Guerrero gap. If large subduction thrust earthquakes occur in the Guerrero gap, their recurrence time is probably increased compared to adjacent regions.

Arc-arc collision ongoing in the southernmost part of the Kuril trench region revealed from integrated analyses of the 1998-2000 Hokkaido Transect seismic data

NASA Astrophysics Data System (ADS)

Iwasaki, T.; Tsumura, N.; Ito, T.; Sato, H.; Kurashimo, E.; Hirata, N.; Arita, K.; Noda, K.; Fujiwara, A.; Abe, S.; Kikuchi, S.; Suzuki, K.

2014-12-01

The oblique subduction of the Pacific plate beneath the southernmost part of the Kuril trench is generating a unique tectonic environment in the Hokkaido Island, Japan. In this area, the Kuril forearc sliver started to collide against Northeast (NE) Japan arc from the east at the time of middle Miocene to form the Hidaka collision zone (HCZ). This collision has been acting as a responsible factor for the westward obduction of the crustal rocks of the Kuril arc (the Hidaka metamorphic belt (HMB)) along the Hidaka main thrust (HMT) and the development of the thick foreland fold-and-thrust belt. A multi-disciplinary project of the 1998-2000 Hokkaido Transect, crossing the northern part of the HCZ in EW direction, collected high-quality seismic data on a 227-km seismic refraction/wide-angle reflection profile and three seismic reflection lines. Reprocessing/reinterpretation for this data set revealed detailed collision structure ongoing in the northern part of the HCZ. The westward obduction of the Kuril arc crust was clearly imaged along the HMT. This obduction starts at a depth of 27-30 km, much deeper than in the southern HCZ (23-25 km). In the west of the HMT, we recognize the gently eastward dipping structure, representing the fragments of Cretaceous subduction/arc complexes or deformation interfaces branched from the HMT. The most important finding from our reprocessing is a series of reflection events at a 30-45 km depth below the obducted Kuril arc crust, which probably correspond to the lower crust/Moho within the NE Japan arc descending down to the east under the collision zone. The wide-angle reflection data indicate that the subducted NE Japan arc meets the Kuril arc 30-40 km east of the HMT at a depth of 30 km. This structural geometry well explained a weak but coherent seismic phase observed at far offsets (120-180 km) on the wide-angle reflection line. The obtained structure shows the complicated collision style where the upper 30-km Kuril arc crust is thrust up with significant deformation. At the moment, we cannot find out the strong evidence of crustal delamination. This is in a marked contrast with the case of the southern part of the HCZ, where the upper 23-km crust is obducted at about 20 km distance from the HMT, while the remaining lower crust is descending down to the subducted Pacific plate.

Slip distribution of the 1952 Tokachi-Oki earthquake (M 8.1) along the Kuril Trench deduced from tsunami waveform inversion

USGS Publications Warehouse

Hirata, K.; Geist, E.; Satake, K.; Tanioka, Y.; Yamaki, S.

2003-01-01

We inverted 13 tsunami waveforms recorded in Japan to estimate the slip distribution of the 1952 Tokachi-Oki earthquake (M 8.1), which occurred southeast off Hokkaido along the southern Kuril subduction zone. The previously estimated source area determined from tsunami travel times [Hatori, 1973] did not coincide with the observed aftershock distribution. Our results show that a large amount of slip occurred in the aftershock area east of Hatori's tsunami source area, suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock. We also found more than 5 m of slip along the deeper part of the seismogenic interface, just below the central part of Hatori's tsunami source area. This region, which also has the largest stress drop during the main shock, had few aftershocks. Large tsunami heights on the eastern Hokkaido coast are better explained by the heterogeneous slip model than previous uniform-slip fault models. The total seismic moment is estimated to be 1.87 ?? 1021 N m, giving a moment magnitude of Mw = 8.1. The revised tsunami source area is estimated to be 25.2 ?? 103 km2, ???3 times larger than the previous tsunami source area. Out of four large earthquakes with M ??? 7 that subsequently occurred in and around the rupture area of the 1952 event, three were at the edges of regions with relatively small amount of slip. We also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface.

One microplate - three orogens: Alps, Dinarides, Apennines and the role of the Adriatic plate

NASA Astrophysics Data System (ADS)

Ustaszewski, Kamil; Le Breton, Eline; Balling, Philipp; Handy, Mark R.; Molli, Giancarlo; Tomljenović, Bruno

2017-04-01

The motion of the Adriatic microplate with respect to the Eurasian and African plates is responsible for the Mesozoic to present tectonic evolution of the Alps, Carpathians, the Dinarides and Hellenides as well as the Apennines. The classical approach for reconstructing plate motions is to assume that tectonic plates are rigid, then apply Euler's theorem to describe their rotation on an ideally spherical Earth by stepwise restorations of magnetic anomalies and fracture zones in oceanic basins. However, this approach is inadequate for reconstructing the motion of Mediterranean microplates like Adria, which, at present, is surrounded by convergent margins and whose oceanic portions have by now been entirely subducted. Most constraints on the motion of the Adriatic microplate come either from palaeomagnetics or from shortening estimates in the Alps, i.e., its northern margin. This approach renders plate tectonic reconstructions prone to numerous errors, yielding inadmissible misfits in the Ionian Sea between southern Italy and northern Greece. At the same time, Adria's western and eastern margins in the Apennines and in the Dinarides have hitherto not been appropriately considered for improving constraints on the motion of Adria. This presentation presents new results of ongoing collaborative research that aims at improving the relative motion path for the Adriatic microplate for the Cenozoic by additionally quantifying and restoring the amount of shortening and extension in a set of geophysical-geological transects from the Tyrrhenian Sea, the Apennines and the Dinarides. Already now, our approach yields an improved motion path for the Adriatic microplate for the last 20 Ma, which minimizes misfits in previous reconstructions. The currently largest challenge in our reconstructions is to reconcile amount and age of shortening in the Dinarides fold-and-thrust belt. For one thing, we see good agreement between the cross-sectional length of subducted material (c. 135 km, estimated from p-wave tomographic models) and shortening in the external carbonate platform of the Dinarides thrust belt (c. 127 km, from balanced cross sections). However, most of the thrust belt shortening is of Palaeogene age, which is difficult to bring into agreement with the fact that most of the subduction observed in tomographic models is most likely of Neogene age. This suggests that a substantial amount of Neogene crustal shortening must have been accommodated in the internal parts of the Dinarides fold-and-thrust belt rather than along its front. More field studies are therefore badly needed to obtain a better understanding of the timing of individual faults and their role during the Neogene evolution of the NE margin of the Adriatic plate.

Trench-breaching afterslip following deeper coseismic slip of the 2012 Mw 7.6 Costa Rica earthquake constrained by near-trench pressure and land-based geodetic observations

NASA Astrophysics Data System (ADS)

Sun, Tianhaozhe; Davis, Earl E.; Wang, Kelin; Jiang, Yan

2017-12-01

Large rupture of the shallowest portion of subduction thrust faults (megathrusts), such as during the 2011 moment magnitude (Mw) 9.0 Tohoku-oki earthquake, can generate the most devastating tsunamis. However, it remains unclear whether such trench-breaching rupture is typical of other subduction earthquakes. The main difficulty in answering this question is the common lack of near-trench geodetic monitoring in subduction zones worldwide. Seafloor and sub-seafloor fluid pressure measurements at two closely located borehole observatories in the Middle America trench have provided clear evidence for the absence of trench-breaching rupture during the 2012 Mw 7.6 Costa Rica earthquake, and for the presence of substantial trench-breaching afterslip at slow rates after the rupture (Davis et al., 2015). In this study, we compare postseismic seafloor pressure change at the trench with coastal Global Navigation Satellite System (GNSS) displacements. The same temporal characteristics of the deformation at the trench and coastal sites indicate that both offshore and onshore deformation were the consequence of afterslip that occurred over a wide spatial range updip of the rupture. By determining the co- and post-seismic slip distributions and inferring the associated shear stress changes on the megathrust, we show that the mechanical behaviour varies in the dip direction. The slip behaviour of the shallow megathrust at Costa Rica is consistent with conventional conceptual models, and contrasts with the behaviour of the shallowest megathrust during the Tohoku-oki event.

Crustal magmatism and lithospheric geothermal state of western North America and their implications for a magnetic mantle

NASA Astrophysics Data System (ADS)

Wang, Jian; Li, Chun-Feng

2015-01-01

The western North American lithosphere experienced extensive magmatism and large-scale crustal deformation due to the interactions between the Farallon and North American plates. To further understand such subduction-related dynamic processes, we characterize crustal structure, magmatism and lithospheric thermal state of western North America based on various data processing and interpretation of gravimetric, magnetic and surface heat flow data. A fractal exponent of 2.5 for the 3D magnetization model is used in the Curie-point depth inversion. Curie depths are mostly small to the north of the Yellowstone-Snake River Plain hotspot track, including the Steens Mountain and McDermitt caldera that are the incipient eruption locations of the Columbia River Basalts and Yellowstone hotspot track. To the south of the Yellowstone hotspot track, larger Curie depths are found in the Great Basin. The distinct Curie depths across the Yellowstone-Snake River Plain hotspot track can be attributed to subduction-related magmatism induced by edge flow around fractured slabs. Curie depths confirm that the Great Valley ophiolite is underlain by the Sierra Nevada batholith, which can extend further west to the California Coast Range. The Curie depths, thermal lithospheric thickness and surface heat flow together define the western edge of the North American craton near the Roberts Mountains Thrust (RMT). To the east of the RMT, large Curie depths, large thermal lithospheric thickness, and low thermal gradient are found. From the differences between Curie-point and Moho depth, we argue that the uppermost mantle in the oceanic region is serpentinized. The low temperature gradients beneath the eastern Great Basin, Montana and Wyoming permit magnetic uppermost mantle, either by serpentinization/metasomatism or in-situ magnetization, which can contribute to long-wavelength and low-amplitude magnetic anomalies and thereby large Curie-point depths.

Splay fault branching from the Hikurangi subduction shear zone: Implications for slow slip and fluid flow

NASA Astrophysics Data System (ADS)

Plaza-Faverola, A.; Henrys, S.; Pecher, I.; Wallace, L.; Klaeschen, D.

2016-12-01

Prestack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure, and seismic velocities resolved to ˜14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration, and short-term geodetically determined slow slip events. The subduction interface is a shallow dipping thrust at <7 km depth near the trench and steps down to 14 km depth along an ˜18 km long ramp, beneath Porangahau Ridge. This apparent step in the décollement is associated with splay fault branching and coincides with a zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. A low-velocity zone beneath the plate interface, updip of the plate interface ramp, is interpreted as fluid-rich overpressured sediments capped with a low permeability condensed layer of chalk and interbedded mudstones. Fluid-rich sediments have been imbricated by splay faults in a region that coincides with the step down in the décollement from the top of subducting sediments to the oceanic crust and contribute to spatial variation in frictional properties of the plate interface that may promote slow slip behavior in the region. Further, transient fluid migration along splay faults at Porangahau Ridge may signify stress changes during slow slip.

Interplay of plate convergence and arc migration in the central Mediterranean (Sicily and Calabria)

NASA Astrophysics Data System (ADS)

Nijholt, Nicolai; Govers, Rob; Wortel, Rinus

2016-04-01

Key components in the current geodynamic setting of the central Mediterranean are continuous, slow Africa-Eurasia plate convergence (~5 mm/yr) and arc migration. This combination encompasses roll-back, tearing and detachment of slabs, and leads to back-arc opening and orogeny. Since ~30 Ma the Apennnines-Calabrian and Gibraltar subduction zones have shaped the western-central Mediterranean region. Lithospheric tearing near slab edges and the accompanying surface expressions (STEP faults) are key in explaining surface dynamics as observed in geologic, geophysical and geodetic data. In the central Mediterranean, both the narrow Calabrian subduction zone and the Sicily-Tyrrhenian offshore thrust front show convergence, with a transfer (shear) zone connecting the distinct SW edge of the former with the less distinct, eastern limit of the latter (similar, albeit on a smaller scale, to the situation in New Zealand with oppositely verging subduction zones and the Alpine fault as the transfer shear zone). The ~NNW-SSE oriented transfer zone (Aeolian-Sisifo-Tindari(-Ionian) fault system) shows transtensive-to-strike slip motion. Recent seismicity, geological data and GPS vectors in the central Mediterranean indicate that the region can be subdivided into several distinct domains, both on- and offshore, delineated by deformation zones and faults. However, there is discussion about the (relative) importance of some of these faults on the lithospheric scale. We focus on finding the best-fitting assembly of faults for the transfer zone connecting subduction beneath Calabria and convergence north of Sicily in the Sicily-Tyrrhenian offshore thrust front. This includes determining whether the Alfeo-Etna fault, Malta Escarpment and/or Ionian fault, which have all been suggested to represent the STEP fault of the Calabrian subduction zone, are key in describing the observed deformation patterns. We first focus on the present-day. We use geodynamic models to reproduce observed GPS velocities in the Sicily-Calabria region. In these models, we combine far-field velocity boundary conditions, GPE-related body forces, and slab pull/trench suction at the subduction contacts. The location and nature of model faults are based on geological and seismicity observations, and as these faults do not fully enclose blocks our models require both fault slip and distributed strain. We vary fault friction in the models. Extrapolating the (short term) model results to geological time scales, we are able to make a first-order assessment of the regional strain and block rotations resulting from the interplay of arc migration and plate convergence during the evolution of this complex region.

History of India-Asia Suturing in Tibet: Constraints and Questions

NASA Astrophysics Data System (ADS)

Kapp, P. A.; Ding, L.

2011-12-01

The India-Asia collision zone is widely pointed to as the type Cenozoic example of continental suturing and collision, yet there remains considerable controversy about its geological and geodynamical evolution. This in part may reflect the richness and complexity of the geological records exposed across the collision zone and how much remains to be extracted from them. Separating the formerly Andean-style continental margin of southern Asia (Gangdese arc and forearc of the Lhasa terrane) in the north, from Indian-affinity strata deformed in the Tethyan Himalayan thrust belt to the south, is the Indus-Yarlung suture zone (IYSZ). In Tibet, ophiolitic rocks along the IYSZ crystallized and were obducted in a suprasubduction zone setting during Early Cretaceous time. The ophiolitic rocks are of the appropriate age to have formed the basement upon which Gangdese forearc strata accumulated. Alternatively, they may represent remnants of an intra-oceanic subduction system that persisted in the Tethys, far from Asia, until Greater India collided with it during the latest Cretaceous to Paleocene. There has been no documentation, however, of ophiolitic or arc fragments younger than Early Cretaceous within the IYSZ. Distinguishing between these two end-member scenarios is important for interpreting detrital records of orogenesis and seismic tomographic images of the mantle. A preponderance of evidence suggests that collision between the Tethyan Himalaya and Asia initiated by 52 Ma. Initial collision led abruptly to profound and far-field changes in paleogeography and tectonism such that by 45 Ma, major shortening and potassic volcanism was ongoing in northern Tibet, plateau-like conditions were established in central Tibet, Tethyan Himalayan crust was undergoing anatexis, and Eo-Himalayan prograde metamorphism was underway. Additional constraints on the shortening history of the Tethyan Himalayan thrust belt will be key to assessing when and how much Greater Indian lithosphere was subducted northward beneath Asia during the Paleogene. Large-scale northward underthrusting of Greater Indian lithosphere (>600 km between 45 and 30 Ma), its subsequent rapid rollback to the south of the IYSZ (30 - 20 Ma), and renewed northward underthrusting (15 Ma to Recent), is inferred from north-south temporal sweeps in Cenozoic magmatism in Tibet. This history of Greater Indian lithosphere subduction may help explain major transitions in the kinematic evolution of the Himalayan-Tibetan orogen and can account for more than half of the total convergence between India and Asia since 50 Ma.

Subduction starts by stripping slabs

NASA Astrophysics Data System (ADS)

Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Prigent, Cécile; Plunder, Alexis; Yamato, Philippe; Guillot, Stéphane

2017-04-01

Metamorphic soles correspond to tectonic slices welded beneath most large-scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as having formed during the first My of intra-oceanic subduction from heat transfer from the incipient mantle wedge towards the top of the subducting plate. Our study reappraises the formation of metamorphic sole through detailed field and petrological work on three classical key sections across the Semail ophiolite (Oman and United Arab Emirates). Geothermobarometry and thermodynamic modelling show that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating and proposed by previous studies. The upper, high-temperature metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the downgoing slab to the mylonitic base of the ophiolite. Estimated peak pressure-temperature conditions through the metamorphic sole are, from top to bottom, 850˚C - 1GPa, 725°C - 0.8 GPa and 530°C - 0.5 GPa. These estimates appear constant within each unit but separated by a gap of 100 to 200˚C and 0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ophiolite ridge axis, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow to refine the tectonic-petrological model for the genesis of metamorphic soles, formed through the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles do not so much result from downward heat transfer (ironing effect) but rather from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts are the result of rheological contrasts between the sole (initially at the subducting slab) and the peridotite above as the plate interface progressively cools down. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems.

Source Mechanisms of Destructive Tsunamigenic Earthquakes occurred along the Major Subduction Zones

NASA Astrophysics Data System (ADS)

Yolsal-Çevikbilen, Seda; Taymaz, Tuncay; Ulutaş, Ergin

2016-04-01

Subduction zones, where an oceanic plate is subducted down into the mantle by tectonic forces, are potential tsunami locations. Many big, destructive and tsunamigenic earthquakes (Mw > 7.5) and high amplitude tsunami waves are observed along the major subduction zones particularly near Indonesia, Japan, Kuril and Aleutan Islands, Gulf of Alaska, Southern America. Not all earthquakes are tsunamigenic; in order to generate a tsunami, the earthquake must occur under or near the ocean, be large, and create significant vertical movements of the seafloor. It is also known that tsunamigenic earthquakes release their energy over a couple of minutes, have long source time functions and slow-smooth ruptures. In this study, we performed point-source inversions by using teleseismic long-period P- and SH- and broad-band P-waveforms recorded by the Federation of Digital Seismograph Networks (FDSN) and the Global Digital Seismograph Network (GDSN) stations. We obtained source mechanism parameters and finite-fault slip distributions of recent destructive ten earthquakes (Mw ≥ 7.5) by comparing the shapes and amplitudes of long period P- and SH-waveforms, recorded in the distance range of 30° - 90°, with synthetic waveforms. We further obtained finite-fault rupture histories of those earthquakes to determine the faulting area (fault length and width), maximum displacement, rupture duration and stress drop. We applied a new back-projection method that uses teleseismic P-waveforms to integrate the direct P-phase with reflected phases from structural discontinuities near the source, and customized it to estimate the spatio-temporal distribution of the seismic energy release of earthquakes. Inversion results exhibit that recent tsunamigenic earthquakes show dominantly thrust faulting mechanisms with small amount of strike-slip components. Their focal depths are also relatively shallow (h < 40 km). As an example, the September 16, 2015 Illapel (Chile) earthquake (Mw: 8.3; h: 26 km) reflects the major characteristics of the Peru-Chile subduction zone between the Nazca and South America Plates. The size, location, depth and focal mechanism of this earthquake are consistent with its occurrence on the megathrust interface in this region. This study is supported by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No: CAYDAG - 114Y066).

Results of a magnetotelluric traverse across western Oregon: crustal resistivity structure and the subduction of the Juan de Fuca plate

NASA Astrophysics Data System (ADS)

Waff, Harve S.; Rygh, John T.; Livelybrooks, Dean W.; Clingman, William W.

1988-02-01

As part of project EMSLAB, we have collected and analysed wideband magnetotelluric data along an east-west transect in western Oregon. Preliminary modelling of the data using one-dimensional inversions based upon rotationally-invariant earth response functions was followed by finite-element two-dimensional modelling. The models produced indicate the presence of an electrical conductor beneath the Oregon Coast Range dipping eastward at 12-18° from a depth of 23-32 km. We believe that this conductor includes the thrust surface of the subducting Juan de Fuca plate and/or adjacent water-saturated rocks. Its high conductance (about 200 S) is thought to be due to one or more of the following mechanisms: (1) sediments subducted atop and with the Juan de Fuca plate, (2) saline fluids produced by dehydration of the former, or (3) seawater contained within subducted oceanic basalts. There is a distinct possibility that the high conductivity is due primarily to the presence of subducted sediments, in contrast with the notion that the subduction of young, buoyant lithosphere retards sediment subduction at this convergent margin. The conductive layer is overlain by relatively resistive rocks presumed to be accreted oceanic lithosphere. Model-determined resistivities for the upper part of the Coast Range section are in good agreement with deep well-log data. A strong electrical contrast appears in the determinant phase pseudosection between the Coast Range and the Willamette Valley suggesting a structural boundary between the two provinces. A surficial conductor is present in the valley to depths of 1-2 km and is due to alluvial fill. Induction arrow data show the geomagnetic coast effect and a smaller effect by the Willamette Valley alluvial fill.

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

NASA Astrophysics Data System (ADS)

Angiboust, Samuel; Kirsch, Josephine; Oncken, Onno; Glodny, Johannes; Monié, Patrick; Rybacki, Erik

2015-06-01

The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30-40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist-facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near-peak metamorphic conditions (400-500°C, 1.1-1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42-48 Ma). Some of these networks exhibit mutual crosscutting relationships between mylonites, foliated cataclasites, and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40Ar-39Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics, we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (˜10-12 s-1) accompanied by fluctuation of supralithostatic and quasi-lithostatic fluid pressures (1 ≥ λ > 0.95). DBT brittle-plastic deformation switches highlight the diversity of deformation processes and fluid-rock interactions in the transition zone region of the subduction interface.

Tectonics of the Western Gulf of Oman

DOE Office of Scientific and Technical Information (OSTI.GOV)

White, R.S.; Ross, D.A.

1979-07-10

The Oman line, running northward from the Strait of Hormuz separates a continent-continent plate boundary to the northwest (Persian Gulf region) from an ocean-continent plate boundary to the southeast (Gulf of Oman region). A large basement ridge detected on multichannel seismic reflection and gravity profiles to the west of the Oman line is probably a subsurface continuation of the Musandam peninsula beneath the Strait of Hormuz. Collision and underthrusting beneath Iran of the Arabian plate on which this ridge lies has caused many of the large earthquakes that have occurred in this region. Convergence between the oceanic crust of themore » Arabian plate beneath the Gulf of Oman and the continental Eurasian plate beneath Iran to the north is accommodated by northward dipping subduction. A deformed sediment prism which forms the offshore Makran continental margin and which extends onto land in the Iranian Makran has accumulated above the descending plate. In the western part of the Gulf of Oman, continued convergence has brought the opposing continental margin of Oman into contact with the Makran continental margin. This is an example of the initial stages of a continent-continent type collision. A model of imbricate thrusting is proposed to explain the development of the fold ridges and basins on the Makran continental margin. Sediments from the subducting plate are buckled and incorporated into the edge of the Makran continental margin in deformed wedges and subsequently uplifted along major faults that penetrate the accretionary prism further to the north.« less

Tectonic significance of Kibaran structures in Central and Eastern Africa

NASA Astrophysics Data System (ADS)

Rumvegeri, B. T.

Tectonical movements of the Kibaran belt (1400-950 Ma) can be subdivided into two major deformation events, corresponding to tight, upright or recumbent folds, thrust faults, nappes and stretching lineation with a general plunging southwards. At the regional scale, the stretching lineation, associated with thrust faults and nappes is interpreted as an indication of a northwards moving direction. The shear zone with mafic-ultramafic rocks across Burundi, MW-Tanzania, SW-Uganda and NE-Zaïre is the suture zone of the Kibaran belt. Kibaran metamorphism is plurifacial and has four epizodes. The second, syn-D2, is the most important and constitutes the climax; it reached the granulite facies. The succession of tectonic, metamorphic and magmatic features suggests geotectonic evolution by subduction-collision.

Crustal thickening in Gansu-Qinghai, lithospheric mantle subduction, and oblique, strike-slip controlled growth of the Tibet plateau

NASA Astrophysics Data System (ADS)

Meyer, B.; Tapponnier, P.; Bourjot, L.; Métivier, F.; Gaudemer, Y.; Peltzer, G.; Shunmin, Guo; Zhitai, Chen

1998-10-01

Fieldwork complemented by SPOT image analysis throws light on current crustal shortening processes in the ranges of northeastern Tibet (Gansu and Qinghai provinces, China). The ongoing deformation of Late-Pleistocene bajada aprons in the forelands of the ranges involves folding, at various scales, and chiefly north-vergent, seismogenic thrusts. The most active thrusts usually break the ground many kilometres north of the range-fronts, along the northeast limbs of growing, asymmetric ramp-anticlines. Normal faulting at the apex of other growing anticlines, between the range fronts and the thrust breaks, implies slip on blind ramps connecting distinct active décollement levels that deepen southwards. The various patterns of uplift of the bajada surfaces can be used to constrain plausible links between contemporary thrusts downsection. Typically, the foreland thrusts and décollements appear to splay from master thrusts that plunge at least 15-20 km down beneath the high ranges. Plio-Quaternary anticlinal ridges rising to more than 3000 m a.s.l. expose Palaeozoic metamorphic basement in their core. In general, the geology and topography of the ranges and forelands imply that structural reliefs of the order of 5-10 km have accrued at rates of 1-2 mm yr-1 in approximately the last 5 Ma. From hill to range size, the elongated reliefs that result from such Late-Cenozoic, NE-SW shortening appear to follow a simple scaling law, with roughly constant length/width ratio, suggesting that they have grown self-similarly. The greatest mountain ranges, which are over 5.5 km high, tens of kilometres wide and hundreds of kilometres long may thus be interpreted to have formed as NW-trending ramp anticlines, at the scale of the middle-upper crust. The fairly regular, large-scale arrangement of those ranges, with parallel crests separated by piggy-back basins, the coevality of many parallel, south-dipping thrusts, and a change in the scaling ratio (from ~5 to 8) for range widths greater than ~30 km further suggests that they developed as a result of the northeastward migration of large thrust ramps above a broad décollement dipping SW at a shallow angle in the middle-lower crust. This, in turn, suggests that the 400-500 km-wide crustal wedge that forms the northeastern edge of the Tibet-Qinghai plateau shortens and thickens as a thick-skinned accretionary prism decoupled from the stronger upper mantle underneath. Such a thickening process must have been coupled with propagation of the Altyn Tagh fault towards the ENE because most thrust traces merge northwestwards with active branches of this fault, after veering clockwise. This process appears to typify the manner in which the Tibet-Qinghai highlands have expanded their surface area in the Neogene. The present topography and structure imply that, during much of that period, the Tibet plateau grew predominantly towards the northeast or east-northeast, but only marginally towards the north-northwest. This was accomplished by the rise, in fairly fast succession, of the Arka Tagh, Qiman Tagh, Mahan shan, Tanghenan Shan, and other NW-trending mountain ranges splaying southeastwards from the Altyn Tagh, isolating the Aqqik-Ayakkum Kol, Qaidam, Suhai and other catchments and basins that became incorporated into the highland mass as intermontane troughs. The tectonic cut-off of catchments and the ultimate infilling of basins by debris from the adjacent ranges, a result of tectonically forced internal drainage, have thus been essential relief-smoothing factors, yielding the outstandingly flat topography that makes Tibet a plateau. Using Late-Mesozoic and Neogene horizons as markers, the retrodeformation of sections across the West Qilian Ranges and Qaidam basin implies at least ~150 km of N30°E Neogene shortening. On a broader scale, taking erosion into account, and assuming isostatic compensation and an initial crustal thickness comparable to that of the Gobi platform (47.5+/-5 km), minimum amounts of Late-Cenozoic crustal shortening on NE sections between the Kunlun fault and the Hexi corridor are estimated to range between 100 and 200 km. In keeping with the inference of a deep crustal décollement and with the existence of Mid-Miocene to Pliocene plutonism and volcanism south of the Kunlun range, such values suggest that the lithospheric mantle of the Qaidam plunged obliquely into the asthenosphere south of that range to minimum depths of the order of 200-300 km. A minimum of ~150 km of shortening in the last ~10 Ma, consistent with the average age of the earliest volcanic-plutonic rocks just south of the Kunlun (~10.8 Ma) would imply average Late-Cenozoic rates of shortening and regional uplift in NE Tibet of at least ~15 mm yr-1 and ~0.2 mm yr-1, respectively. Such numbers are consistent with a cumulative sinistral offset and slip rate of at least ~200 km and ~2 cm yr-1, respectively, on the Altyn Tagh fault east of 88°E. The fault may have propagated more than 1000 km, to 102°E, in the last 10 Ma. Our study of ongoing tectonics in northeast Tibet is consistent with a scenario in which, while the Himalayas-Gangdese essentially `stagnated' above India's subducting mantle, much of Tibet grew by thickening of the Asian crust, as propagation of large, lithospheric, strike-slip shear zones caused the opposite edge of the plateau to migrate far into Asia. The Asian lithospheric mantle, decoupled from the crust, appears to have subducted southwards along the two Mesozoic sutures that cut Tibet north of the Gangdese, rather than to have thickened. The Bangong-Nujiang suture was probably reactivated earlier than the Jinsha-Kunlun suture, located farther north. Overall, the large-scale deformation bears a resemblance to plate tectonics at obliquely convergent margins, including slip-partioning along large strike-slip faults such as the Altyn Tagh and Kunlun faults. Simple mechanisms at the level of the lithospheric mantle are merely hidden by the broader distribution and greater complexity of strain in the crust.

Geospeedometry in the inverted metamorphic gradient of the Nestos Thrust Zone in central Rhodope (Northern Greece)

NASA Astrophysics Data System (ADS)

Cioldi, Stefania; Moulas, Evangelos; Burg, Jean-Pierre

2015-04-01

Thrust tectonics and inverted metamorphic gradients are major consequences of large and likely fast movements of crustal segments in compressional environments. The purpose of this study is to investigate the tectonic setting and the timescale of inverted metamorphic zonations related to crustal-scale thrusting. The aim is to contribute understanding the link between mechanical and thermal evolution of major thrust zones and to clarify the nature and the origin of orogenic heat. The Rhodope metamorphic complex (Northern Greece) is interpreted as a part of the Alpine-Himalaya orogenic belt and represents a collisional system with an association of both large-scale thrusting and pervasive exhumation tectonics. The Nestos Shear Zone overprints the suture boundary with a NNE-dipping pile of schists displaying inverted isograds. The inverted metamorphic zones start from chlorite-muscovite grade at the bottom and reach kyanite-sillimanite grades with migmatites in the upper structural levels. In order to reconstruct the thermo-tectonic evolution of inverted metamorphic zonation, reliable geochronological data are essential. 40Ar/39Ar geochronology with step-heating technique on white mica from micaschists provided a temporal resolution with the potential to characterize shearing. 40Ar/39Ar dating across the Nestos Shear Zone yields Late Eocene-Early Oligocene (40-30 Ma) cooling (~400-350° C) ages, which correspond to local thermo-deformation episodes linked to late and post-orogenic intrusions. U-Pb Sensitive High Resolution Ion Microprobe (SHRIMP) zircon geochronology on leucosomes from migmatitic orthogneisses were considered to estimate the age of peak metamorphic conditions, contemporaneous with anatexis. U-Pb ages of zircon rims specify regional partial melting during the Early Cretaceous (160-120 Ma). This is in disagreement with previous assertions, which argued that the formation of leucosomes in this region is Late Eocene (42-35 Ma) and implied multiple subductions and multiple metamorphic cycles during orogeny. Garnet geospeedometry considers the kinetic response of minerals and allowed estimating the absolute time-dependent thermal evolution by diffusive element profiles in garnet. Inverse-fitting numerical model considering Fractionation and Diffusion in GarnEt (FRIDGE) calculates garnet composition profiles by introducing P-T-t paths and bulk-rock composition of a specific sample. Preliminary results of Fe-Mg - Ca - Mn garnet fractionation-diffusion modelling indicate very short timescale (between 2 and 5 Ma) for peak metamorphic conditions in the Rhodope collisional system.

Tectonic evolution of the Yarlung suture zone, Lopu Range region, southern Tibet

NASA Astrophysics Data System (ADS)

Laskowski, Andrew K.; Kapp, Paul; Ding, Lin; Campbell, Clay; Liu, XiaoHui

2017-01-01

The Lopu Range, located 600 km west of Lhasa, exposes a continental high-pressure metamorphic complex beneath India-Asia (Yarlung) suture zone assemblages. Geologic mapping, 14 detrital U-Pb zircon (n = 1895 ages), 11 igneous U-Pb zircon, and nine zircon (U-Th)/He samples reveal the structure, age, provenance, and time-temperature histories of Lopu Range rocks. A hornblende-plagioclase-epidote paragneiss block in ophiolitic mélange, deposited during Middle Jurassic time, records Late Jurassic or Early Cretaceous subduction initiation followed by Early Cretaceous fore-arc extension. A depositional contact between fore-arc strata (maximum depositional age 97 ± 1 Ma) and ophiolitic mélange indicates that the ophiolites were in a suprasubduction zone position prior to Late Cretaceous time. Five Gangdese arc granitoids that intrude subduction-accretion mélange yield U-Pb ages between 49 and 37 Ma, recording Eocene southward trench migration after collision initiation. The south dipping Great Counter Thrust system cuts older suture zone structures, placing fore-arc strata on the Kailas Formation, and sedimentary-matrix mélange on fore-arc strata during early Miocene time. The north-south, range-bounding Lopukangri and Rujiao faults comprise a horst that cuts the Great Counter Thrust system, recording the early Miocene ( 16 Ma) transition from north-south contraction to orogen-parallel (E-W) extension. Five early Miocene (17-15 Ma) U-Pb ages from leucogranite dikes and plutons record crustal melting during extension onset. Seven zircon (U-Th)/He ages from the horst block record 12-6 Ma tectonic exhumation. Jurassic—Eocene Yarlung suture zone tectonics, characterized by alternating episodes of contraction and extension, can be explained by cycles of slab rollback, breakoff, and shallow underthrusting—suggesting that subduction dynamics controlled deformation.

Slip Behavior of the Queen Charlotte Plate Boundary Before and After the 2012, MW 7.8 Haida Gwaii Earthquake: Evidence From Repeating Earthquakes

NASA Astrophysics Data System (ADS)

Hayward, Tim W.; Bostock, Michael G.

2017-11-01

The Queen Charlotte plate boundary, near Haida Gwaii, B.C., includes the dextral, strike-slip, Queen Charlotte Fault (QCF) and the subduction interface between the downgoing Pacific and overriding North American plates. In this study, we present a comprehensive repeating earthquake catalog that represents an effective slip meter for both structures. The catalog comprises 712 individual earthquakes (0.3≤MW≤3.5) arranged into 224 repeating earthquake families on the basis of waveform similarity and source separation estimates from coda wave interferometry. We employ and extend existing relationships for repeating earthquake magnitudes and slips to provide cumulative slip histories for the QCF and subduction interface in six adjacent zones within the study area between 52.3°N and 53.8°N. We find evidence for creep on both faults; however, creep rates are significantly less than plate motion rates, which suggests partial locking of both faults. The QCF exhibits the highest degrees of locking south of 52.8°N, which indicates that the seismic hazard for a major strike-slip earthquake is highest in the southern part of the study area. The 28 October 2012, MW 7.8 Haida Gwaii thrust earthquake occurred in our study area and altered the slip dynamics of the plate boundary. The QCF is observed to undergo accelerated, right-lateral slip for 1-2 months following the earthquake. The subduction interface exhibits afterslip thrust motion that persists for the duration of the study period (i.e., 3 years and 2 months after the Haida Gwaii earthquake). Afterslip is greatest (5.7-8.4 cm/yr) on the periphery of the main rupture zone of the Haida Gwaii event.

Formation of forearc basins by collision between seamounts and accretionary wedges: an example from the New Hebrides subduction zone

USGS Publications Warehouse

Collot, J.-Y.; Fisher, M.A.

1989-01-01

Seabeam data reveal two deep subcircular reentrants in the lower arc slope of the New Hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin. The Malekula reentrant lies just south of the partly subducted Bougainville seamount. This proximity, as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off Japan, suggests that the Malekula reentrant formed by the collision of a seamount with the arc. An arcuate fold-thrust belt has formed across the mouth of the reentrant, forming the toe of a new accretionary wedge. The Efate reentrant may show the next stage in basin development. This reentrant lies landward of a lower-slope ridge that may have begun to form as an arcuate fold-thrust belt across the mouth of a reentrant. This belt may have grown by continued accretion at the toe of the wedge, by underplating beneath the reentrant, and by trapping of sediment shed from the island arc. These processes could result in a roughly circular forearc basin. Basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the Aleutian trenches. -Authors

Carbon Mobility at Subduction Interfaces via Deformation-Enhanced Fluid Infiltration: Evidence from the Swiss/Italian Alps

NASA Astrophysics Data System (ADS)

Jaeckel, K. P.; Bebout, G. E.; Angiboust, S.

2016-12-01

The interplay between fluid flow and deformation along subduction interfaces, and the extent to which deformation-enhanced fluid infiltration can drive decarbonation and carbonate dissolution, remain poorly understood. Recent work on HP/UHP decarbonation in W. Alps suites has indicated that, in intact volumes of metasediment, metabasalt, and ophicarbonate away from major shear zones and with few veins, carbonate is largely retained to 80-90 km depths (Cook-Kollars et al., 2014; Collins et al., 2015; Chem. Geol.). Yet uncertain is whether forearc fluid infiltration focused in intensely sheared and fractured zones could result in greater mobilization of C from subducting sections, in quantities sufficient to impact subduction zone C cycling. Lower-plate rocks at Arosa and Dent Blanche interface exposures (Bachmann et al., 2009, JGR; Angiboust et al., 2015, G3) are primarily calc-schist intercalated with meta-ultramafic and metamafic schist and contain carbonate-bearing veins of varying abundance and texture. At some localities, these sections contain blocks of carbonate, metabasalt, and upper-plate gneiss. Strongly deformed veins concordant with the foliation parallel to the thrust interface commonly contain carbonate and quartz. In highly sheared regions in the Arosa Zone, δ18O(VSMOW) values of some host-rocks and veins are shifted from +20 ± 2‰, values observed regionally for the Schistes Lustres, to values of +11 to +13‰. These shifts can be explained by interaction with externally-derived H2O-rich fluids with δ18O of +9 to +11‰. Smaller datasets for Dent Blanche localities hint at similar δ18O shifts. Most of these rocks contain little evidence of C release by decarbonation reactions. Evidence exists for local-scale dissolution of carbonate, during pressure solution, and carbonate-bearing veins reflect C mobility in fluids. Ongoing work assesses whether volumes of carbonate removed in some regions balance with those precipitated nearby in veins and pressure shadows.

Postseismic Gravity Change After the 2006-2007 Great Earthquake Doublet and Constraints on the Asthenosphere Structure in the Central Kuril Islands

NASA Technical Reports Server (NTRS)

Shin-Chan, Han; Sauber, Jeanne; Pollitz, Fred

2016-01-01

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of approximately 4 micro-Gal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by (is) approximately 6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and approximately 10(exp 18) Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.

Postseismic gravity change after the 2006-2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands.

PubMed

Han, Shin-Chan; Sauber, Jeanne; Pollitz, Fred

2016-04-16

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 M w 8.3 thrust and 2007 M w 8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and ~10 18 Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.

Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands

PubMed Central

Han, Shin-Chan; Sauber, Jeanne; Pollitz, Fred

2016-01-01

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007–2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25–35 km for the elastic thickness and ~1018 Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone. PMID:27642200

Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands

USGS Publications Warehouse

Han, Shin-Chan; Sauber, Jeanne; Pollitz, Fred

2016-01-01

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in Gravity Recovery and Climate Experiment (GRACE) but without a discernible coseismic gravity change. The gravity increase of ~4 μGal, observed consistently from various GRACE solutions around the epicentral area during 2007–2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25–35 km for the elastic thickness and ~1018 Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.

Between Sunda subduction and Himalayan collision: fertility, people and earthquakes on the Ganges-Brahmaputra Delta

NASA Astrophysics Data System (ADS)

Seeber, L.; Steckler, M. S.; Akhter, S. H.; Goodbred, S. L., Jr.; Gale, J.; McHugh, C. M.; Ferguson, E. K.; Mondal, D. R.; Paola, C.; Reitz, M. D.; Wilson, C.

2014-12-01

A foreland (Ganges) and a suture (Brahmaputra) river, which both drain the Himalaya, have coalesced to form Ganges-Brahmaputra Delta (GBD), the world's largest. The GBD progrades along the continental margin, coupled with an advancing subduction to collision transition, deforming the delta as it grows. A better understanding of this time-transgressive system is urgent now that humans are increasing their forcing of the system and exposure to environmental hazards. Among these, earthquake risk is rapidly growing as people move from rural settings into expanding cities, creating unprecedented exposure. The megathrust 1950 M8.7 earthquake in Assam occurred during the monsoon and released 10x the annual sediment load, causing progradation at the coast and a pulse of river widening that propagated downstream. The 1762 M8.8(?) along the Arakan coast extended into the shelf of the delta where coastal tsunami deposits have been identified recently. These events bracket a segment with no credible historic megathrust earthquakes, but could affect far more people. Geodetic and geologic data along this 300 km boundary facing the GBD show oblique contraction. The subaerial accretionary prism (Burma Ranges) is up to 250 km wide with a blind thrust front that reaches ½ way across the delta. The GPS convergence rate of 14 mm/y is consistent with large displacements and long interseismic times, which can account for lack of historic ruptures, but also the potential for catastrophic events. Active folds and shallow thrust earthquakes point to an additional threat from upper-plate seismicity. Much of the current seismicity is in the lower-plate and reaches as far west as Dhaka; it may pose an immediate threat. The folds, and the uplift and subsidence patterns also influence the courses of the rivers. North of the delta, the Shillong plateau is a huge basement cored anticline bounded by the north-dipping Dauki thrust fault. 7 mm/y of N-S shortening and 5 km of structural relief here are consistent with a Pleistocene age for the structure. Subsidence of its foreland has created the seasonal inland sea in Sylhet and influences avulsions of the Brahmaputra. The 1897 M8.0 earthquake caused maximum intensities on the western part of the Shillong massif, suggesting a rupture of the western Dauki fault, leaving the eastern portion as a possible gap.

Structure and Tectonics of the Saint Elias Orogen

NASA Astrophysics Data System (ADS)

Bruhn, R. L.; Pavlis, T. L.; Plafker, G.; Serpa, L.; Picornell, C.

2001-12-01

The Saint Elias orogen of western Canada and southern Alaska is a complex mountain belt formed by transform faulting and subduction between the Pacific and North American plates, and collision of the Yakutat terrane. The orogen is segmented into three regions of different structural style caused by lateral variations in transpression and processes of terrane accretion. Deformation is strain and displacement partitioned throughout the orogen; transcurrent motion is focused along discrete strike-slip faults, and shortening is distributed among reverse faults and folds with sub-horizontal axes. Plunging folds accommodate horizontal shortening and extension in the western part of the orogen. Segment boundaries extend across the Yakutat terrane where they coincide with the courses of huge piedmont glaciers that flow from the topographic backbone of the range onto the coastal plain. The eastern segment is marked by strike-slip faulting along the Fairweather transform fault and by a narrow belt of reverse faulting where the transpression ratio is 0.4:1 shortening to dextral shear. The transpression ratio is 1.7:1 in the central part of the orogen where a broad thin-skinned fold and thrust belt deforms the Yakutat terrane south of the Chugach-Saint Elias (CSE) suture. Dextral shearing is accommodated by strike-slip faulting beneath the Seward and Bagley glaciers in the hanging wall of the CSE suture, and partly by reverse faulting along a structural belt that cuts across the Yakutat terrane along the western edge of the Malaspina Glacier and links to the Pamplona fold and thrust belt offshore. Deformation along this segment boundary is probably also driven by vertical axis bending of the Yakutat microplate during collision. Subduction & accretion in the western segment of the orogen causes re-folding of previously formed structures when they are emplaced into the upper plate of the Alaska-Aleutian mega-thrust. Second phase folds plunge at moderate to steep angles and accretion is marked by only modest amounts of uplift. The structural boundary between the central and western segments of the orogen localizes the course of the Bering piedmont glacier. The structural segments coincide with subdivisions in historical seismicity, particularly ruptures of great to large magnitude earthquakes. The results of this structural study provide the requisite geological framework to design new-generation geophysical monitoring systems to study active deformation within the orogen.

The 2016 Kaikōura earthquake: Simultaneous rupture of the subduction interface and overlying faults

NASA Astrophysics Data System (ADS)

Wang, Teng; Wei, Shengji; Shi, Xuhua; Qiu, Qiang; Li, Linlin; Peng, Dongju; Weldon, Ray J.; Barbot, Sylvain

2018-01-01

The distribution of slip during an earthquake and how it propagates among faults in the subduction system play a major role in seismic and tsunami hazards, yet they are poorly understood because offshore observations are often lacking. Here we derive the slip distribution and rupture evolution during the 2016 Mw 7.9 Kaikōura (New Zealand) earthquake that reconcile the surface rupture, space geodetic measurements, seismological and tsunami waveform records. We use twelve fault segments, with eleven in the crust and one on the megathrust interface, to model the geodetic data and match the major features of the complex surface ruptures. Our modeling result indicates that a large portion of the moment is distributed on the subduction interface, making a significant contribution to the far field surface deformation and teleseismic body waves. The inclusion of local strong motion and teleseismic waveform data in the joint inversion reveals a unilateral rupture towards northeast with a relatively low averaged rupture speed of ∼1.5 km/s. The first 30 s of the rupture took place on the crustal faults with oblique slip motion and jumped between fault segments that have large differences in strike and dip. The peak moment release occurred at ∼65 s, corresponding to simultaneous rupture of both plate interface and the overlying splay faults with rake angle changes progressively from thrust to strike-slip. The slip on the Papatea fault produced more than 2 m of offshore uplift, making a major contribution to the tsunami at the Kaikōura station, while the northeastern end of the rupture can explain the main features at the Wellington station. Our inversions and simulations illuminate complex up-dip rupture behavior that should be taken into consideration in both seismic and tsunami hazard assessment. The extreme complex rupture behavior also brings new challenges to the earthquake dynamic simulations and understanding the physics of earthquakes.

Oman metamorphic sole formation reveals early subduction dynamics

NASA Astrophysics Data System (ADS)

Soret, Mathieu; Agard, Philippe; Dubacq, Benoît; Plunder, Alexis; Ildefonse, Benoît; Yamato, Philippe; Prigent, Cécile

2016-04-01

Metamorphic soles correspond to m to ~500m thick tectonic slices welded beneath most of the large-scale ophiolites. They typically show a steep inverted metamorphic structure where the pressure and temperature conditions of crystallization increase upward (from 500±100°C at 0.5±0.2 GPa to 800±100°C at 1.0±0.2 GPa), with isograds subparallel to the contact with the overlying ophiolitic peridotite. The proportion of mafic rocks in metamorphic soles also increases from the bottom (meta-sediments rich) to the top (approaching the ophiolite peridotites). These soles are interpreted as the result of heat transfer from the incipient mantle wedge toward the nascent slab (associated with large-scale fluid transfer and possible shear heating) during the first My of intra-oceanic subduction (as indicated by radiometric ages). Metamorphic soles provide therefore major constraints on early subduction dynamics (i.e., thermal structure, fluid migration and rheology along the nascent slab interface). We present a detailed structural and petrological study of the metamorphic sole from 4 major cross-sections along the Oman ophiolite. We show precise pressure-temperature estimates obtained by pseudosection modelling and EBSD measurements performed on both the garnet-bearing and garnet-free high-grade sole. Results allow quantification of the micro-scale deformation and highlight differences in pressure-temperature-deformation conditions between the 4 different locations, showing that the inverted metamorphic gradient through the sole is not continuous in all locations. Based on these new constraints, we suggest a new tectonic-petrological model for the formation of metamorphic soles below ophiolites. This model involves the stacking of several homogeneous slivers of oceanic crust leading to the present-day structure of the sole. In this view, these thrusts are the result of rheological contrasts between the sole and the peridotite as the plate interface progressively cools down. These slivers later underwent several stages of retrogression (partly mediated by ascending fluids from the slab) from amphibolite- to prehnite/pumpellite-facies conditions.

Plate convergence and deformation, North Luzon Ridge, Philippines

NASA Astrophysics Data System (ADS)

Lewis, Stephen D.; Hayes, Dennis E.

1989-10-01

Marine geophysical and earthquake seismology data indicate that the North Luzon Ridge, a volcano-capped bathymetrie ridge system that extends between Luzon and Taiwan, is presently undergoing deformation in response to the relative motion between the Asian and Philippine Sea plates. Plate motion models predict convergence along the western side of the Philippine Sea plate, from Japan in the north to Indonesia in the south, and most of this plate margin is defined by active subduction zones. However, the western boundary of the Philippine Sea plate adjacent to the North Luzon Ridge shows no evidence of an active WNW-dipping subduction zone; this is in marked contrast to the presence of both the Philippine Trench/East Luzon Trough subduction zones to the south and the Ryukyu Trench subduction zone to the north. Crustal shortening, in response to ongoing plate convergence in the North Luzon Ridge region, apparently takes place through a complex pattern of strike-slip and thrust faulting, rather than by the typical subduction of oceanic lithosphere along a discreet zone. The curvilinear bathymetrie trends within the North Luzon Ridge represent the traces of active faults. The distribution of these faults, mapped by both multichannel and single-channel seismic reflection methods and earthquake seismicity patterns and focal mechanism solutions, suggest that right-lateral, oblique-slip faulting occurs along NE-trending faults, and left-lateral, oblique-slip faulting takes place on N- and NNW-trending faults. The relative plate convergence accommodated by the deformation of the North Luzon Ridge will probably be taken up in the future by the northward-propagating East Luzon Trough subduction zone.

Source of high tsunamis along the southernmost Ryukyu trench inferred from tsunami stratigraphy

NASA Astrophysics Data System (ADS)

Ando, Masataka; Kitamura, Akihisa; Tu, Yoko; Ohashi, Yoko; Imai, Takafumi; Nakamura, Mamoru; Ikuta, Ryoya; Miyairi, Yosuke; Yokoyama, Yusuke; Shishikura, Masanobu

2018-01-01

Four paleotsunamis deposits are exposed in a trench on the coastal lowland north of the southern Ryukyu subduction zone trench. Radiocarbon ages on coral and bivalve shells show that the four deposits record tsunamis date from the last 2000 yrs., including a historical tsunami with a maximum run-up of 30 m in 1771, for an average recurrence interval of approximately 600 yrs. Ground fissures in a soil beneath the 1771 tsunami deposit may have been generated by stronger shaking than recorded by historical documents. The repeated occurrence of the paleotsunami deposits supports a tectonic source model on the plate boundary rather than a nontectonic source model, such as submarine landslides. Assuming a thrust model at the subduction zone, the seismic coupling ratio may be as low as 20%.

From Subduction to a Compressional transform system: Diffuse Deformation Processes at the Southeastern Boundary of the Caribbean Plate

NASA Astrophysics Data System (ADS)

Deville, E.; Padron, C.; Huyghe, P.; Callec, Y.; Lallemant, S.; Lebrun, J.; Mascle, A.; Mascle, G.; Noble, M.

2006-12-01

Geophysical data acquired in the southeastern Caribbean marine area (CARAMBA survey of the French O/V Atalante) provide new information about the deformation processes occurring in this subduction-to-strike-slip transitions zone. The 65 000 km2 of multibeam data and 5600 km of seismic reflection and 3.5 kHz profiles which have been collected evidence that the connection between the Barbados accretionary prism and the south Caribbean transform system is partitioned between a wide variety of recently active tectonic superficial features (complex folding, diffuse faulting, and mud volcanism), which accommodate the relative displacement between the Caribbean and the South America plates. The active deformation within the sedimentary pile is mostly aseismic (creeping) and this deformation is relatively diffuse over a large diffuse plate boundary. There is no direct fault connection between the front of the Barbados prism and the strike-slip system of northern Venezuela. The toe thrust system at the southern edge of the Barbados prism, exhibits clear en-echelon geometry. The geometry of the syntectonic deposits evidence the diachronism of the deformation processes. Notably, it is well evidenced that early folds have been sealed by the recent turbidite deposits, whereas, some of the fold and thrust structures were active recently. Within this active compressional region, extension growth faults develop on the platform and on the slope of the Orinoco delta along a WNW-ESE trending en-echelon fault system that we called the Orinoco Delta Fault Zone (ODFZ). This fault system is clearly oblique with respect to the present-day Orinoco delta slope. These faults are not simply related to a passive gravitary collapse of the sediments accumulated on the Orinoco platform. Though there a decoupling between the shallow deformation processes in the sediments and the deep deformation characterized by earthquake activity, the ODFZ is inferred to be partly controlled by deep structures associated the shearing of the lithosphere at depth (probably at the Continent-Ocean Boundary).

Possible reactivation of the Vincent-Chocolate Mountains thrust in the Gavilan Hills area, southeasternmost California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Oyarzabal, F.R.; Jacobson, C.E.; Haxel, G.B.

The Late Cretaceous-early Tertiary Orocopia Schist (OS) of southeasternmost California consists of metamorphosed continental margin sedimentary and basaltic rocks, overlain by an upper plate of continental crust along the Vincent-Chocolate Mountains fault (VCMF). Previous analysis of late folds and shear band in OS and upper plate in the Gavilan Hills and adjacent ares indicated that the direction of transport of the upper plate was northeastward. This has been considered evidence of a SW dipping subduction zone, along which an outboard continental fragment was sutured to North America. Another view is that the VCMF was formed by underplating of the OSmore » in an Andean continental margin, and that the NE-vergent late structures formed during uplift of the OS. The authors' continuing work in the Gavilan Hills confirm the NE sense of vergence but suggests a more complex structural history. The schist is characterized by refolded folds, shear bands, and two penetrative lineations. An older lineation that ranges from N10[degree]E to N30[degree]E is widespread in the area, but is more evident at low structural levels. A second lineation ranges from N40[degree]E to N70[degree]E and is strongly developed in rocks near the VCMF. The complex folding pattern, presence of mylonitic schist, relative thinness of upper-plate mylonite, and possible retrogressive character of the shear bands suggest that the VCMF in the Gavilan Hills area may have been reactivated after original thrusting. The VCMF in the Gavilan Hills is intermediate in character between the probable subduction thrust in the San Gabriel Mountains and the reactivated faults in the Orocopia Mountains and areas surrounding the Gavilan Hills.« less

New perspectives on self-similarity for shallow thrust earthquakes

NASA Astrophysics Data System (ADS)

Denolle, Marine A.; Shearer, Peter M.

2016-09-01

Scaling of dynamic rupture processes from small to large earthquakes is critical to seismic hazard assessment. Large subduction earthquakes are typically remote, and we mostly rely on teleseismic body waves to extract information on their slip rate functions. We estimate the P wave source spectra of 942 thrust earthquakes of magnitude Mw 5.5 and above by carefully removing wave propagation effects (geometrical spreading, attenuation, and free surface effects). The conventional spectral model of a single-corner frequency and high-frequency falloff rate does not explain our data, and we instead introduce a double-corner-frequency model, modified from the Haskell propagating source model, with an intermediate falloff of f-1. The first corner frequency f1 relates closely to the source duration T1, its scaling follows M0∝T13 for Mw<7.5, and changes to M0∝T12 for larger earthquakes. An elliptical rupture geometry better explains the observed scaling than circular crack models. The second time scale T2 varies more weakly with moment, M0∝T25, varies weakly with depth, and can be interpreted either as expressions of starting and stopping phases, as a pulse-like rupture, or a dynamic weakening process. Estimated stress drops and scaled energy (ratio of radiated energy over seismic moment) are both invariant with seismic moment. However, the observed earthquakes are not self-similar because their source geometry and spectral shapes vary with earthquake size. We find and map global variations of these source parameters.

Sandstone provenance and tectonic evolution of the Xiukang Mélange from Neotethyan subduction to India-Asia collision (Yarlung-Zangbo suture, south Tibet)

NASA Astrophysics Data System (ADS)

An, Wei; Hu, Xiumian; Garzanti, Eduardo

2016-04-01

The Xiukang Mélange of the Yarlung-Zangbo suture zone in south Tibet documents low efficiency of accretion along the southern active margin of Asia during Cretaceous Neotethyan subduction, followed by final development during the early Paleogene stages of the India-Asia collision. Here we investigate four transverses in the Xigaze area (Jiding, Cuola Pass, Riwuqi and Saga), inquiry the composition in each transverse, and present integrated petrologic, U-Pb detrital-zircon geochronology and Hf isotope data on sandstone blocks. In fault contact with the Yarlung-Zangbo Ophiolite to the north and the Tethyan Himalaya to the south, the Xiukang mélange can be divided into three types: serpentinite-matrix mélange composed by broken Yarlung-Zangbo Ophiolite, thrust-sheets consisting mainly chert, quartzose or limestone sheets(>100m) with little intervening marix, and mudstone-matrix mélange displaying typical blocks-in-matrix texture. While serpentinite-matrix mélange is exposed adjacent to the ophiolite, distributions of thrust-sheets and blocks in mudstone-matrix mélange show along-strike diversities. For example, Jiding transverse is dominant by chert sheets and basalt blocks with scarcely sandstone blocks, while Cuola Pass and Saga transverses expose large amounts of limestone/quartzarenite sheets in the north and volcaniclastic blocks in the south. However, turbidite sheets and volcaniclastic blocks are outcropped in the north Riwuqi transverse with quartzarenite blocks preserved in the south. Three groups of sandstone blocks/sheets with different provenance and depositional setting are distinguished by their petrographic, geochronological and isotopic fingerprints. Sheets of turbiditic quartzarenite originally sourced from the Indian continent were deposited in pre-Cretaceous time on the northernmost edge of the Indian passive margin and eventually involved into the mélange at the early stage of the India-Asia collision. Two distinct groups of volcaniclastic-sandstone blocks were derived from the central Lhasa block and Gangdese magmatic arc. One group was deposited in the trench and/or on the trench slope of the Asian margin during the early Late Cretaceous, and the other group in a syn-collisional basin just after the onset of the India-Asia collision in the Early Eocene. The largely erosional character of the Asian active margin in the Late Cretaceous is indicated by the scarcity of off-scraped trench-fill deposits and the relatively small subduction complex developed during limited episodes of accretion. The Xiukang Mélange was finally structured in the Late Paleocene/Eocene, when sandstone of both Indian and Asian origin were progressively incorporated tectonically in the suture zone of the nascent Himalayan Orogen.

Active Deformation in the Overriding Plate Associated with Temporal Changes of the Philippine Sea Plate Motion

NASA Astrophysics Data System (ADS)

Ishiyama, T.; Sato, H.; Van Horne, A.

2015-12-01

We present detailed geologic evidence linking changes over time in Philippine Sea plate (PHS) motion and intracontinental deformation in central and southwest (SW) Japan during the Pliocene and after. In the early Pliocene, subduction of the PHS plate under SW Japan restarted in a northerly direction after period of deceleration or cessation. Later, motion changed to a more westerly direction. Corresponding geological changes found in the overriding plate include unconformities in the forearc basins, changes in slip sense on faults, depocenter migration, re-organization of drainage systems and volcanism. Quaternary intraplate deformation is prominent north of the Median Tectonic Line (MTL) inactive segment, above a shallow flat slab. In contrast, less Quaternary tectonic activity is found north of the MTL active segment which lies over a steadily-slipping portion of the subducting slab that behaves as a less-deformed rigid block. Depocenters and active thrusting have migrated north/northwestward over the past 5 My above the shallow flat slab segment of the PHS. We reconstructed the Plio-Pleistocene migration history using Neogene stratigraphy and shallow seismic reflection profiles. We see shallow PHS slab contact with the lower continental crust in our deep seismic reflection profiles, which may explain its enhanced downward drag of the overriding plate and synchronous strong compression in the crust. We find evidence of more westerly PHS plate subduction since the middle Pleistocene in (1) unconformities in the Kumano forearc basin deposits in SW Japan, (2) drastic stream captures in Shikoku, and (3) concordant changes in fault slip sense from thrust to dextral slip along the MTL. Oblique subduction could have induced stronger horizontal stress in the overriding plate above the shallow flat slab which could account for the increasing geologic slip rate observed on active structures. During four repetitions of megathrust earthquake sequences since the 17th century, ca. 65 % of all intraplate M>6.5 earthquakes have been concentrated in the area above the PHS flat slab. This also suggests that mechanical interaction between the slab and the overriding plate plays an important role in intraplate seismicity over shorter timescales as well.

The Lithospheric Structure of the Solonker Suture Zone and Adjacent Areas: Crustal Structure Revealed by a High-Resolution Magnetotelluric Study

NASA Astrophysics Data System (ADS)

Ye, Gaofeng; Jin, Sheng; Wei, Wenbo; Jing, Jian'en

2017-04-01

The closure of the Paleo-Asian Ocean along the Solonker Suture Zone (SSZ) during the Late Permian and Triassic represented the final stage in the formation of the Central Asian Orogenic Belt between the Siberian Craton and the North China Craton. In order to better understand the structure and formation of this ancient subduction zone, a high-resolution magnetotelluric (MT) profile was collected with both broadband and long-period MT data. The high resolution mapping of the lithosphere achieved in this study is due to the closely spaced MT stations (2-3 km). With the 2-D resistivity model, a south-dipping conductor was detected and extends through the entire crust. The geometry of this feature provides evidence that a southward directed subduction zone formed the Solonker suture. The enhanced conductivity was interpreted to subducted sulfide-bearing graphitic sediments. The resistive body beneath the northern margin of the North China Craton indicates a thickened lithosphere caused by the southward subduction at this region, and the resistive body beneath the Solonker Suture Zone indicates the subducted oceanic lithosphere. North-dipping low resistivity features were also detected in the crust of both the North China Craton and Central Asian Orogenic Belt, and were interpreted as post-collisional thrust faults. Strong anisotropy was found beneath the suture zone, and can be explained if the high strain rate has rotated the fold axes into the dip direction.

Global Search of Triggered Tectonic Tremor

NASA Astrophysics Data System (ADS)

Peng, Z.; Aiken, C.; Chao, K.; Gonzalez-Huizar, H.; Wang, B.; Ojha, L.; Yang, H.

2013-05-01

Deep tectonic tremor has been observed at major plate-boundary faults around the Pacific Rim. While regular or ambient tremor occurs spontaneously or accompanies slow-slip events, tremor could be also triggered by large distant earthquakes and solid earth tides. Because triggered tremor occurs on the same fault patches as ambient tremor and is relatively easy to identify, a systematic global search of triggered tremor could help to identify the physical mechanisms and necessary conditions for tremor generation. Here we conduct a global search of tremor triggered by large teleseismic earthquakes. We mainly focus on major faults with significant strain accumulations where no tremor has been reported before. These includes subduction zones in Central and South America, strike-slip faults around the Caribbean plate, the Queen Charlotte-Fairweather fault system and the Denali fault in the western Canada and Alaska, the Sumatra-Java subduction zone, the Himalaya frontal thrust faults, as well as major strike-slip faults around Tibet. In each region, we first compute the predicted dynamic stresses σd from global earthquakes with magnitude>=5.0 in the past 20 years, and select events with σd > 1 kPa. Next, we download seismic data recorded by stations from local or global seismic networks, and identify triggered tremor as a high-frequency non-impulsive signal that is in phase with the large-amplitude teleseismic waves. In cases where station distributions are dense enough, we also locate tremor based on the standard envelope cross-correlation techniques. Finally, we calculate the triggering potential for the Love and Rayleigh waves with the local fault orientation and surface-wave incident angles. So far we have found several new places that are capable of generating triggered tremor. We will summarize these observations and discuss their implications on physical mechanisms of tremor and remote triggering.

Analogue modeling of rotational orogenic wedges: implications for the Neogene structural evolution of the Southern Central Andes (33°-35°S)

NASA Astrophysics Data System (ADS)

Herrera, S. S.; Farías, M.; Pinto, L.; Yagupsky, D. L.; Guzman, C.; Charrier, R.

2017-12-01

Structural evolution of the southernmost Central Andes is a major subject of debate. Overall vergence within the range and how intra-continental subduction prompts Andean orogeny are controversial topics. Between 33°-35° S, strike of the western slope main structures shifts southwards, from N-S to NNE-SSW, defining the Maipo Orocline. Likely, width of the Principal Cordillera increases southwards. Despite, a progressive southward decrease in orogenic volume has been determined for the segment. To understand such latitudinal variations, and to provide explanations for overall vergence, we carry out analogue models of contractional wedges to explore upper-crustal thrust system development with a progressive variation of the convergence vector. The model setup consisted of a fixed plate on which a mobile plate generated a velocity discontinuity. The upper-crust was simulated using low-cohesive quartz sand. The mobile plate was fixed at its northern end to a pivot, thus progressively incrementing shortening and the obliquity of convergence southwards. PIV photogrammetry recorded wedge evolution. A classical doubly-vergent wedge was formed, consisting of a steep 35° dipping, static thrust on the retro-side, an uplifted core, and an incipient forward-breaking, 25° critically tapered imbricated thrust fan on the pro-side, wider (in plan-view) where the imposed shortening reached the maximum. The resulting wedge is reminiscent of: the steep western Andean slope, in which the bordering thrust has maintained its present position during the Neogene; and the east-vergent fold-and-thrust belt of the eastern slope. The asymmetrical doubly vergence of the model suggests west-directed subduction of the South American continent beneath the orogen. The southward width increase is geometrically comparable to the natural analogue, yet we observe a flat contrast with orogenic shortening and volume estimates for the region. This can be attributed to the fact that uplift and erosion interplay, and the role of pre-Andean structures are not addressed in this approach. Rotation within the model wedge is consistent with paleomagnetic data for the 33°-35°S segment. Nevertheless, our model fails to explain curvature of the Maipo Orocline, suggesting that other lithospheric processes might control bending of the range.

Imaging megathrust zone and Yakutat/Pacific plate interface in Alaska subduction zone

NASA Astrophysics Data System (ADS)

Kim, Y.; Abers, G. A.; Li, J.; Christensen, D. H.; Calkins, J. A.

2012-12-01

We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Dense stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relation to slab seismicity, and (2) the interplate coupled zone where the great 1964 (magnitude 9.3) had greatest rupture. The joint teleseismic migration of two array datasets (MOOS, Multidisciplinary Observations of Onshore Subduction, and BEAAR, Broadband Experiment Across the Alaska Range) based on teleseismic receiver functions (RFs) using the MOOS data reveal a shallow-dipping prominent low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of these RF amplitudes shows a thin (3-6.5 km) low-velocity layer (shear wave velocity less than 3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with Vp/Vs ratio exceeding 2.0) may be due to a thick sediment input from the trench in combination of elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of 11-15 km. Both velocities and thickness of the low-velocity channel abruptly increase as the slab bends in central Alaska, which agrees with previously published RF results. Our image also includes an unusually thick low-velocity crust subducting with a ~20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment has been suggested to be due to the subduction of the Yakutat terrane. Subduction of this buoyant crust could explain the shallow dip of the thrust zone beneath southern Alaska. We also show a clear image of the Yakutat and Pacific plate subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at ~30-42 km depth in the central Kenai along the megathrust, aligns with the western end of the geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It seems plausible that this sharp change in the nature of the downgoing plate controls the slip distribution of great earthquakes on this plate interface.

A global outer-rise/outer-trench-slope (OR/OTS) earthquake study

NASA Astrophysics Data System (ADS)

Wartman, J. M.; Kita, S.; Kirby, S. H.; Choy, G. L.

2009-12-01

Using improved seismic, bathymetric, satellite gravity and other geophysical data, we investigated the seismicity patterns and focal mechanisms of earthquakes in oceanic lithosphere off the trenches of the world that are large enough to be well recorded at teleseismic distances. A number of prominent trends are apparent, some of which have been previously recognized based on more limited data [1], and some of which are largely new [2-5]: (1) The largest events and the highest seismicity rates tend to occur where Mesozoic incoming plates are subducting at high rates (e.g., those in the western Pacific and the Banda segment of Indonesia). The largest events are predominantly shallow normal faulting (SNF) earthquakes. Less common are reverse-faulting (RF) events that tend to be deeper and to be present along with SNF events where nearby seamounts, seamount chains and other volcanic features are subducting [Seno and Yamanaka, 1996]. Blooms of SNF OR/OTS events usually occur just after and seaward of great interplate thrust (IPT) earthquakes but are far less common after smaller IPT events. (2) Plates subducting at slow rates (<20 mm/a) often show sparse OR/OTS seismicity. It is unclear if such low activity is a long-term feature of these systems or is a consequence of the long return times of great IPT earthquakes (e.g., the sparse OR/OTS seismicity before the 26 December 2004 M9.2 Sumatra earthquake and many subsequent OR/OTS events). (3) OR/OTS shocks are generally sparse or absent where incoming plates are very young (<20 Ma) (e.g., Cascadia, southern Mexico, Nankai, and South Shetlands). (4) Subducting plates of intermediate age (20 to about 65 Ma) display a diversity of focal mechanisms and seismicity patterns. In the Philippines, NE Indonesia, and Melanesia, bands of reverse faulting events occur at or near the trench and SNF earthquakes are restricted to OR/OTS sites further from the trench. (5) Clustering of OR/OTS events of all types commonly occurs where seamount chains, volcanic ridges, or volcanic plateaus enter OR/OTS regions (e.g., the Louisville Ridge in Tonga, the Juan Fernandez Ridge in Chile, the Ninety East Ridge in Sumatra, and the D’Entrecastaux Ridge in Vanuatu).

Phanerozoic growth of Asia: Geodynamic processes and evolution

NASA Astrophysics Data System (ADS)

Pubellier, Manuel; Meresse, Florian

2013-08-01

Accretion processes often obscured in mountain belts can be documented with great detail in SE Asia where these have taken place during the Tertiary. The resulting configuration showing accreted continental strips and tectonised wedges is illustrated by the Tethysides jammed between the northern Laurasian cratons (Baltica and Siberia) and Gondwanian cratons (Africa, Arabia, India and Australia). Eurasia increased progressively in size due to the amalgamation of crustal and sedimentary belts. At places where the processes are documented in the recent times, they can be included within a "collision factory" which displays the opening of basins by rifting and sea floor spreading within the upper plate, until they undergo a process of shortening, both stages being subduction-controlled. In SE Asia the early stages are illustrated in the eastern Sunda arc where the subduction of the Sunda Trench is blocked in Sumba and Timor region, and flipped into the Flores Trough in less than 2 My. The incipient shortening is at present taking place in the Pliocene Damar basins. Another stage, where half of the upper plate basin has disappeared, is documented in the Celebes Sea. The examples of deformation being transferred further inland exist in the northern Celebes Sea and the Makassar Basin. The next important stage is the complete consumption of the marginal basin where both margins collide and the accretionary wedge is thrust over the margin, as illustrated in NW Borneo and Palawan. Each of these stages is responsible for a single short-lived tectonic event, the succession of several events composes an orogen which may last for over 10 My. These events predate the arrival of the conjugate margin of the large ocean, which marks the beginning of continental subduction as observed in the Himalaya-Tibet region. These examples show that the closure is generally diachronous through time as illustrated in the Philippines. We observe that the ophiolite obducted in such context is generally of back-arc origin (upper plate) rather than the relict of the vanishing large ocean which is rarely preserved. In the Philippines, once the crust is accreted the subduction zone progressively moved southward until its present position. We propose that the lithospheric mantle of the accreted block is delaminated and rolls back in a continuous manner, whereas the crust is deformed and accreted.

Cretaceous subduction in the Pyrenees: Iberian plate-kinematics in a mantle reference frame

NASA Astrophysics Data System (ADS)

Vissers, Reinoud; van Hinsbergen, Douwe; van der Meer, Douwe; Spakman, Wim

2016-04-01

During the Cretaceous, Iberia was a microplate separated from Laurasia and Gondwana by ridges and transforms, and by a convergent margin to its northeast along which the Pyrenean fold-thrust belt developed. As a microplate, Iberia underwent a well-defined but ill-understood Albian-Aptian ~ 35° counterclockwise rotation relative to Eurasia. Three competing kinematic scenarios for Iberian motion in the late Mesozoic are all compatible with the Pyrenean geological record and comprise (1) transtensional eastward motion of Iberia versus Eurasia, (2) strike-slip motion followed by orthogonal extension and (3) scissor-style opening of the Bay of Biscay coupled with subduction in the Pyrenean realm. The last scenario is the only one consistent with paleomagnetic and ocean floor anomaly constraints showing Iberia's rotation, but is criticized because the upper mantle below the Pyrenees contains no evidence for a subducted slab. Here we show that when taking absolute plate motions into account, Aptian oceanic subduction in the Pyrenees followed by Albian slab break-off should leave a slab remnant in the present-day mid-mantle below NW Africa instead of below the Pyrenees. Mantle tomography shows a positive seismic velocity anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below Reggane in Southern Algeria. Seismic tomographic imaging of the mantle structure therefore does not falsify the Pyrenean subduction hypothesis, and provides no basis to discard marine magnetic and paleomagnetic constraints on Iberia's kinematic history. Slab break-off explains the well-dated Albian-Cenomanian high-temperature metamorphism in the Pyrenees that hitherto has been interpreted as an expression of continental break-up and hyperextension. We suspect that subduction in the Pyrenees may have played a key role in driving the rapid Aptian rotation of the Iberian microplate.

Shallow seismicity patterns in the northwestern section of the Mexico Subduction Zone

NASA Astrophysics Data System (ADS)

Abbott, Elizabeth R.; Brudzinski, Michael R.

2015-11-01

This study characterizes subduction related seismicity with local deployments along the northwestern section of the Mexico Subduction Zone where 4 portions of the plate interface have ruptured in 1973, 1985, 1995, and 2003. It has been proposed that the subducted boundary between the Cocos and Rivera plates occurs beneath this region, as indicated by inland volcanic activity, a gap in tectonic tremor, and the Manzanillo Trough and Colima Graben, which are depressions thought to be associated with the splitting of the two plates after subduction. Data from 50 broadband stations that comprised the MARS seismic array, deployed from January 2006 to June 2007, were processed with the software program Antelope and its generalized source location algorithm, genloc, to detect and locate earthquakes within the network. Slab surface depth contours from the resulting catalog indicate a change in subduction trajectory between the Rivera and Cocos plates. The earthquake locations are spatially anti-correlated with tectonic tremor, supporting the idea that they represent different types of fault slip. Hypocentral patterns also reveal areas of more intense seismic activity (clusters) that appear to be associated with the 2003 and 1973 megathrust rupture regions. Seismicity concentrated inland of the 2003 rupture is consistent with slip on a shallowly dipping trajectory for the Rivera plate interface as opposed to crustal faulting in the overriding North American plate. A prominent cluster of seismicity within the suspected 1973 rupture zone appears to be a commonly active portion of the megathrust as it has been active during three previous deployments. We support these interpretations by determining focal mechanisms and detailed relocations of the largest events within the 1973 and inland 2003 clusters, which indicate primarily thrust mechanisms near the plate interface.

Rheological Properties and Heterogeneities Along the Down-Dip Extent of a Subduction Megathrust: Insights from the Condrey Mountain Schist, Northern California

NASA Astrophysics Data System (ADS)

Tewksbury-Christle, C. M.; Behr, W. M.; Helper, M. A.

2017-12-01

Episodic tremor and slow slip (ETS) is commonly observed in warm subduction zones down-dip of a locked megathrust. Proposed mechanisms for ETS involve some form of rheological heterogeneity along the subduction interface. Observations from exhumed subduction-related rocks allow us to investigate the constitutive laws that govern the interface, as well as the types and distributions of rheological heterogeneities that develop and/or persist in the tremor source region. The Late Jurassic to Early Cretaceous Condrey Mountain Schist (CMS), Klamath Mountains, northern California, provides insight into interface rheology along the down-dip extent (350-450°C, 5-8 kbar) of a subduction megathrust. The CMS consists of greenschist and blueschist facies metasediments (including graphitic mica schists), metabasalts, and metaserpentinites, all pervasively deformed under prograde metamorphic conditions with minimal retrogressive overprint. A transect of peak metamorphic temperatures determined using graphite crystallinity shows a constant, but small, inverted thermal gradient with increasing structural depth, suggesting equilibration of temperature discontinuities during underplating. Despite the lack of thermal contrasts, rheological heterogeneities are preserved in the form of km-scale cryptic thrusts that separate lithological packages deforming by different mechanisms. Graphitic mica schists exhibit pervasive cleavage-microlithon fabrics indicative of deformation by quartz dissolution-precipitation creep. Blueschist-facies oceanic crustal sequences juxtaposed against the graphitic mica schists show coeval deformation, but are deformed primarily by dislocation creep in amphibole. These observations suggest that the subduction megathrust likely transitions down-dip into a viscous (rather than frictional) interface shear zone, but that original lithological heterogeneities persist in the form of non-Newtonian vs. Newtonian viscous patches.

Linkages between orogenic plateau build-up, fold-thrust shortening, and foreland basin evolution in the Zagros (NW Iran)

NASA Astrophysics Data System (ADS)

Barber, D. E.; Stockli, D. F.

2017-12-01

The Iranian Plateau (IP) is a thickened, low-relief morphotectonic province of diffuse deformation that formed due to Arabia-Eurasia collision and may serve as a younger analogue for the Tibetan Plateau. Despite detailed geophysical characterization of the IP, its deformation history and relationship to the Zagros fold-thrust belt and its foreland basin evolution remains unresolved. Low-temperature thermochronometry and provenance data from a transect across the internal and external Zagros track growth of the IP and delineate multiphase interaction between upper- and lower-plate processes during closure of the Neotethys and Arabia-Eurasia suturing. Inversion of zircon (U-Th)/He and fission-track data from plutonic and metamorphic basement rocks in the Sanandaj-Sirjan Zone (SSZ) of the IP reveals an initial stage of low-rate exhumation from 36-25 Ma, simultaneous with the onset of tectonic subsidence and marine incursion in the Zagros foreland basin. Overlapping apatite fission-track and (U-Th)/He ages indicate sharp acceleration in SSZ exhumation rates between 20-15 Ma, coincident with rejuvenation of foreland basin subsidence and an influx of Eurasian-derived sediments into the Zagros foreland deposited above an Oligocene unconformity. The mid-Miocene marks a transition in focused exhumation from the SSZ to Arabian lower-plate. Apatite (U-Th)/He ages suggest in-sequence fold-thrust propagation from the High Zagros to simply folded belt from 10 Ma to recent, which is reflected in the foreland by a shift in provenance to dominantly recycled Arabian-derived detritus and clastic facies progradation. Integrated thermochronometric and provenance data document a two-phase outward expansion of the Iranian Plateau and Zagros fold-thrust belt, tightly coupled to distinct phases of basin evolution and provenance shifts in the Zagros foreland. We associate multiple deformation and basin episodes with protracted collisional processes, from subduction of attenuated Arabian transitional crust beneath Eurasia causing low-rate upper-plate exhumation in the late Eocene, to accelerated Miocene unroofing and basin flexure linked to increased plate coupling and eventual to suturing as buoyant Arabian continental lithosphere entered the subduction interface.

Preliminary results of local earthquake tomography around Bali, Lombok, and Sumbawa regions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nugraha, Andri Dian, E-mail: nugraha@gf.itb.ac.id; Puspito, Nanang T; Yudistira, Tedi

Bali, Sumbawa, and Lombok regions are located in active tectonic influence by Indo-Australia plate subducts beneath Sunda plate in southern part and local back-arc thrust in northern part the region. Some active volcanoes also lie from eastern part of Java, Bali, Lombok and Sumbawa regions. Previous studies have conducted subsurface seismic velocity imaging using regional and global earthquake data around the region. In this study, we used P-arrival time from local earthquake networks compiled by MCGA, Indonesia within time periods of 2009 up to 2013 to determine seismic velocity structure and simultaneously hypocenter adjustment by applying seismic tomography inversion method.more » For the tomographic inversion procedure, we started from 1-D initial velocity structure. We evaluated the resolution of tomography inversion results through checkerboard test and calculating derivative weigh sum. The preliminary results of tomography inversion show fairly clearly high seismic velocity subducting Indo-Australian and low velocity anomaly around volcano regions. The relocated hypocenters seem to cluster around the local fault system such as back-arc thrust fault in northern part of the region and around local fault in Sumbawa regions. Our local earthquake tomography results demonstrated consistent with previous studies and improved the resolution. For future works, we will determine S-wave velocity structure using S-wave arrival time to enhance our understanding of geological processes and for much better interpretation.« less

Preliminary results of local earthquake tomography around Bali, Lombok, and Sumbawa regions

NASA Astrophysics Data System (ADS)

Nugraha, Andri Dian; Kusnandar, Ridwan; Puspito, Nanang T.; Sakti, Artadi Pria; Yudistira, Tedi

2015-04-01

Bali, Sumbawa, and Lombok regions are located in active tectonic influence by Indo-Australia plate subducts beneath Sunda plate in southern part and local back-arc thrust in northern part the region. Some active volcanoes also lie from eastern part of Java, Bali, Lombok and Sumbawa regions. Previous studies have conducted subsurface seismic velocity imaging using regional and global earthquake data around the region. In this study, we used P-arrival time from local earthquake networks compiled by MCGA, Indonesia within time periods of 2009 up to 2013 to determine seismic velocity structure and simultaneously hypocenter adjustment by applying seismic tomography inversion method. For the tomographic inversion procedure, we started from 1-D initial velocity structure. We evaluated the resolution of tomography inversion results through checkerboard test and calculating derivative weigh sum. The preliminary results of tomography inversion show fairly clearly high seismic velocity subducting Indo-Australian and low velocity anomaly around volcano regions. The relocated hypocenters seem to cluster around the local fault system such as back-arc thrust fault in northern part of the region and around local fault in Sumbawa regions. Our local earthquake tomography results demonstrated consistent with previous studies and improved the resolution. For future works, we will determine S-wave velocity structure using S-wave arrival time to enhance our understanding of geological processes and for much better interpretation.

Magnitude of crustal shortening and structural framework of the easternmost Himalayan orogen, northern Indo-Burma Ranges of northeastern India

NASA Astrophysics Data System (ADS)

Haproff, P. J.; Yin, A.

2016-12-01

Along-strike variation in crustal shortening throughout the Himalayan orogen has been attributed to (1) diachronous, eastward-increasing convergence, or (2) localized controls including pre-collisional stratigraphic configuration and climate. In this study, we present new geologic maps and balanced cross-sections across the easternmost segment of the Himalayan orogen, the N-S-trending N. Indo-Burma Ranges of northeastern India. First order structures are NE-dipping, km-wide ductile thrust shear zones with mylonitic fabrics indicating top-to-the SW motion. Major structures include the Mayodia klippe and Hunli window, generated during folding of the SW-directed Tidding thrust and duplexing of Lesser Himalayan rocks (LHS) at depth. Reconstruction of two balanced cross-sections yields minimum shortening estimates of 70% (48 km) and 71% (133 km), respectively. The widths of the orogen for each transect are 21 km and 54 km, respectively. Our percent strain values are comparable to that of western Arunachal Himalaya, reflecting eastward-increasing strain due to counterclockwise rotation of India during convergence or along-strike variation in India's subduction angle. However, shortening magnitudes much less than that of the Sikkim (641 km), Bhutan (414-615 km), and western Arunachal Himalaya (515-775 km) could signal eastward increasing shortening of a unique Himalayan stratigraphic framework, evidenced by few GHC rocks, absence of Tethyan strata, and an extensive subduction mélange and forearc complex.

Tectonics of the North American Cordillera near the Fortieth Parallel

USGS Publications Warehouse

King, P.B.

1978-01-01

The North American Cordillera near the Fortieth Parallel consists of the following tectonic units: 1. (A) To the east is a reactivated cratonic area, in the Southern Rocky Mountains and Colorado Plateau, in which the supracrustal rocks (Cambrian to Cretaceous) were broadly deformed during the late Cretaceous-Paleocene Laramide orogeny, and the Precambrian basement was raised in folds of wide amplitude. 2. (B) West of it is a miogeosynclinal belt, in the eastern Great Basin, in which a thick sequence of Paleozoic carbonates and related deposits was thrust eastward along low-angle faults during the middle to late Cretaceous Sevier orogeny. The miogeosyncline is the downwarped western margin of the original North American continent, and its rocks accumulated on Precambrian basement. 3. (C) Beyond is a eugeosynclinal belt, in the western Great Basin, in which Paleozoic graywackes, cherts, and volcanics were thrust easteastward along low-angle faults during several Paleozoic orogenies - the mid-Paleozoic Antler orogeny which produced the Roberts thrust on the east, and the end-Paleozoic Sonoma orogeny which produced the Golconda thrust farther west. The Paleozoic eugeosynclinal rocks accumulated on oceanic basement. They are overlapped from the west by Triassic and Jurassic shelf deposits, which pass westward into eugeosynclinal deposits. 4. (D) A volcanic island-arc belt existed on the sites of the Sierra Nevada in Paleozoic and early Mesozoic time, which produced thick bodies of sediments and volcanics. During the mid-Mesozoic Nevadan orogeny these were steeply deformed and thrust westward over subduction zones, and were intruded by granitic rocks that rose from the upper mantle to form great batholiths. 5. (E) West of the Sierra Nevada, in the Great Valley, is a great sedimentary embankment of later Mesozoic flysch or turbidite, largely younger than the supracrustal rocks of the Sierra Nevada and the Nevadan orogeny. It was formed of the erosional products of the supracrustal and granitic rocks of the Sierra Nevada. 6. (F) This sequence is, in turn, thrust westward over the Mesozoic Franciscan terrane of the Coast Ranges, which forms the westernmost belt of the Cordillera, and which is being treated in other papers in this symposium. The net effect of the prolonged events that produced the Cordillera in this segment has been the addition of successive tectonic belts to the North American continent at the expense of the Pacific Ocean basin during Phanerozoic time. ?? 1978.

The reactivation of the SW Iberian passive margin: a brief review

NASA Astrophysics Data System (ADS)

Duarte, Joao; Rosas, Filipe; Terrinha, Pedro; Schellart, Wouter; Almeida, Pedro; Gutscher, Marc-André; Riel, Nicolas; Ribeiro, António

2016-04-01

On the morning of the 1st of November of 1755 a major earthquake struck offshore the Southwest Iberian margin. This was the strongest earthquake ever felt in Western Europe. The shake, fire and tsunami devastated Lisbon, was felt as far as Finland and had a profound impact on the thinkers of that time, in particular on the Enlightenment philosophers such as Voltaire, Rousseau and Kant. The Great Lisbon Earthquake is considered by many as the event that marks the birth of modern geosciences; and made of this region one of the most well studied areas in the world. After the 1755 earthquake, Kant and others authors wrote several treaties dealing with the causes and dynamics of earthquakes and tsunamis and were close to identify some key elements of what we now call plate tectonics. More than two hundred years later, in the year of 1969, the region was struck by another major earthquake. This was precisely during the period in which the theory of plate tectonics was being built. Geoscientists like Fukao (1973), Purdy (1975) and Mackenzie (1977) immediately focused their attention in the area. They suggested that these events were related with "transient" subduction of Africa below Iberia, along the East-West Azores-Gibraltar plate boundary. Several years later, Ribeiro (1989) suggested that instead of Africa being subducted below Iberia, it was the West Iberian passive margin that was being reactivated, a process that may, in time, lead to the formation of a new subduction zone. In the turning of the millennium, a subducting slab was imaged bellow the Gibraltar Straits, a remanent of the Western Mediterranean arc system that according to Gutscher et al. (2002) was related with ongoing subduction. Recently, it was proposed that a causal link between the Gibraltar subduction system and the reactivation of the SW Iberian margin might exist. In addition, the large-scale Africa-Eurasia convergence is inducing compressive stresses along the West Iberian margin. The margin reactivation is expressed by the presence of several active lithospheric-scale thrust faults. In this communication, we will highlight the main moments of the journey that lead to the understanding that the Southwest Iberian is in fact being reactivated. We will present some of the data and ideas that were gathered over the years, including the most recent findings. Finally, we will see that despite the numerous endeavours and the substantial improvements in our tectonic knowledge of the region there are still many enigmas waiting to be resolved. Publication supported by project FCT UID/GEO/50019/2013 - Instituto Dom Luiz

Revisiting the physical characterisitics of the subduction interplate seismogenic zones

NASA Astrophysics Data System (ADS)

Heuret, Arnauld; Lallemand, Serge; Funiciello, Francesca; Piromallo, Claudia

2010-05-01

Based on the Centennial earthquake catalog, the revised 1964-2007 EHB hypocenters catalog and the 1976-2007 CMT Harvard catalog, we have extracted the hypocenters, nodal planes and seismic moments of worldwide subduction earthquakes for the 1900-2007 period. For the 1976-2007 period, we combine the focal solutions provided by Harvard and the revised hypocenters from Engdahl et al. (1998). Older events are extracted from the Centennial catalogue (Engdahl and Villasenor, 2002) and they are used to estimate the cumulated seismic moment only. The selection criteria for the subduction earthquakes are similar to those used by Mc Caffrey (1994), i.e., we test if the focal mechanisms are consistent with 1/ shallow thrust events (depth > 70 km, positive slips, and at least one nodal plane gets dip < 45°), and, 2/ the plate interface local geometry and orientation (one nodal plane is oriented toward the volcanic arc, the azimuth of this nodal plane ranges between ± 45° with respect to the trench one, its dip ranges between ± 20° with respect to the slab one and the epicentre is located seaward of the volcanic arc). Our study concerns segments of subduction zones that fit with estimated paleoruptures associated with major events (M > 8). We assume that the seismogenic zone coincides with the distribution of 5.5 < M < 7 subduction earthquakes. We provide a map of the interplate seismogenic zones for 80% of the trench systems including dip, length, downdip and updip limits, we revisit the statistical study done by Pacheco et al. (1993) and test some empirical laws obtained for example by Ruff and Kanamori (1980) in light of a more complete, detailed, accurate and uniform description of the subduction interplate seismogenic zone. Since subduction earthquakes result from stress accumulation along the interplate and stress depends on plates kinematics, subduction zone geometry, thermal state and seismic coupling, we aim to isolate some correlations between parameters. The statistical analysis reveals that: 1- vs, the subduction velocity is the first order controlling parameter of seismogenic zone variability, both in term of geometry and seismic behaviour; 2- steep dip, large vertical extent and narrow horizontal extent of the seismogenic zone are associated to fast subductions, and cold slabs, the opposite holding for slow subductions and warm slabs; the seismogenic zone usually ends in the fore-arc mantle rather than at the upper plate Moho depth; 3- seismic rate () variability is coherent with the geometry of the seismogenic zone:  increases with the dip and with the vertical extent of the seismogenic zone, and it fits with vs and with the subducting plate thermal state; 4- mega-events occurrence determines the level of seismic energy released along the subduction interface, whatever  is; 5- to some extent, the potential size of earthquakes fits with vs and with the seismogenic zone geometry, but second order controlling parameters are more difficult to detect; 6- the plate coupling, measured through Upper Plate Strain, is one possible second order parameter: mega-events are preferentially associated to neutral subductions, i.e. moderate compressive stresses along the plate interface; high plate coupling (compressive UPS) is thought to inhibit mega-events genesis by enhancing the locking of the plate interface and preventing the rupture to extend laterally. This research was supported as part of the Eurohorcs/ESF — European Young Investigators Awards Scheme (resp. F.F.), by funds from the National Research Council of Italy and other National Funding Agencies participating in the 3rd Memorandum of Understanding, as well as from the EC Sixth Framework Programme.

Structure of the Sumatra wedge affected by the 26th December 2004 :Effects of the lower plate volcanic ridges.

NASA Astrophysics Data System (ADS)

Rangin, C.; Sibuet, J. C.; Lin, J. Y.; Le Pichon, X.

2009-04-01

Detailed swath-bathymetry, coupled with echo-sounder data were collected offshore the northern tip of Sumatra over the rupture area of the 26th December 2004 Mw=9.2 earthquake during the Sumatra aftershock cruise. 20 ocean bottom seismometers were also deployed in the northern Sumatra area., and more than 1000 events were identified during the 12 days recording period. We mapped recently active steeply dipping thrust fault zone within the western termination of the Sunda accreted wedge. Main N10°W trending out of sequence thrust fault zones with a discrete westward vergency and some component of dextral strike-slip motion were continuously mapped within the wedge, on the basis of bathymetry and low frequency sounder profiles. The interplate boundary does not appear to extend into the frontal part of the wedge but most probably merges in its central part along these major faults, the Lower and Upper Splay Faults. After relocation, the seismicity shows different pattern in each side of this Upper Splay Fault. East of this boundary, beneath the Aceh basin, the earthquake depths ranged from 30 to 60 km allow us to illustrate the subducted plate. In the western part, the aftershock distribution is strongly influenced by the N-S orientated oceanic fracture zones. Two clusters of earthquakes between 10 and 50 km in depth trending along N-S direction are observed in the lower wedge that we interpret to be reactive fracture zones. The lower wedge is interpreted as the northern prolongation below the wedge of the lower plate NS oceanic fracture zone ridges affected by NS trending left lateral strike-slip faults. This wedge outer ridge is in the process of being transferred to the upper plate. On the other hand the central ridge is interpreted as possible stacked volcanic ridge slivers already incorporated into the upper plate along the subduction buttress (the inner ridge of the wedge). We propose that the tectonic interaction of the volcanic Indian Ocean fracture ridges of the subducted plate with the leading edge of the upper Sunda plate subduction zone is an active tectonic transfer process of oceanic material to the upper plate. The proposed emergence of the interplate boundary into the middle part of the wedge along the Lower Splay Fault, could have favoured the formation of the giant Sumatra tsunami at moderate water depth. This docking and temporary stacking of these volcanic ridges before their subduction at depth, is favoured by the strong oblique convergence that prevails up to the Bengal basin into the north.

Mineralogy and fluid content of sediments entering the Costa Rica subduction zone - Results from Site U1414, IODP Expedition 344

NASA Astrophysics Data System (ADS)

Charpentier, D.; Buatier, M.; Kutterolf, S.; Straub, S. M.; Nascimento, D.; Millan, C.

2013-12-01

Subduction zones are characterized by the largest thrust earthquakes, as quantified by both rupture area and seismic moment release. Offshore Costa Rica, the oceanic Cocos Plate subducts under the Caribbean plate forming the southern end of the Middle America trench. A high convergence rate and almost complete subduction of incoming sediments make the Costa Rica convergent margin an extremely dynamic environment. The Costa Rica Seismogenesis Project (CRISP) is designed to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. Site U1414 of IODP Exp.344 was drilled to investigate the material from the incoming Cocos Plate. A key parameter of incoming plate is fluid content and release because it impacts deformation within the subduction complex. The deposition, compaction and diagenesis of sedimentary rocks control the distribution of fluids, fluid pressures and fluid flow patterns within subduction zones. We therefore decided to characterize sediment composition and quantify the different types of water at Site U1414. Mineralogical investigations were performed using optical and electronic microscope observations, X Ray Diffraction (on bulk and clay fractions), Cation Exchange Capacity measurements, carbon analyses (to determine carbonate contents), and sequenced extractions in NaOH (to quantify the biogenic opal content). Fluid characteristics were approached by thermal gravimetric analyses. The entire sedimentary sequence was recovered at Site U1414 and can be divided into three major sedimentary units. The first one is a hemipelagic silty clay to clay with a gradual increase of calcareous nannofossils. The dominant mineral is smectite associated in the clay fractions with kaolinite and zeolites. Small amounts of biogenic opal have been analyzed. Other minerals like quartz, feldspar and calcite are also present. The second unit is composed of nannofossil-rich calcareous ooze. The proportion of biosilica is variable and can attain 15 wt.%. Smectite and zeolites are present in smaller amount. The third unit is a lithified sandstone. Biosilica and smectite are absent, but zeolites are still present in this unit. Fluid content that can be released varies from about 15 wt.% to 40 wt.%. In shallow levels a significant proportion is pore water fluid, whereas in deeper levels water stored within minerals comprises a greater proportion of the total fluid budget. The presence of smectite yields to fluid release by dehydration and dehydroxylation at temperatures less than approximately 100°C and 500°C respectively. Transformation of biogenic opal to diagenetic silice goes to completion at temperatures of 50-100°C. It seems to be an importance source of fluid in the second unit, whereas in unit three it is zeolite water.

Inverse models of plate coupling and mantle rheology: Towards a direct link between large-scale mantle flow and mega thrust earthquakes

NASA Astrophysics Data System (ADS)

Gurnis, M.; Ratnaswamy, V.; Stadler, G.; Rudi, J.; Liu, X.; Ghattas, O.

2017-12-01

We are developing high-resolution inverse models for plate motions and mantle flow to recover the degree of mechanical coupling between plates and the non-linear and plastic parameters governing viscous flow within the lithosphere and mantle. We have developed adjoint versions of the Stokes equations with fully non-linear viscosity with a cost function that measures the fit with plate motions and with regional constrains on effective upper mantle viscosity (from post-glacial rebound and post seismic relaxation). In our earlier work, we demonstrate that when the temperature field is known, the strength of plate boundaries, the yield stress and strain rate exponent in the upper mantle are recoverable. As the plate boundary coupling drops below a threshold, the uncertainty of the inferred parameters increases due to insensitivity of plate motion to plate coupling. Comparing the trade-offs between inferred rheological parameters found from a Gaussian approximation of the parameter distribution and from MCMC sampling, we found that the Gaussian approximation—which is significantly cheaper to compute—is often a good approximation. We have extended our earlier method such that we can recover normal and shear stresses within the zones determining the interface between subducting and over-riding plates determined through seismic constraints (using the Slab1.0 model). We find that those subduction zones with low seismic coupling correspond with low inferred values of mechanical coupling. By fitting plate motion data in the optimization scheme, we find that Tonga and the Marianas have the lowest values of mechanical coupling while Chile and Sumatra the highest, among the subduction zones we have studies. Moreover, because of the nature of the high-resolution adjoint models, the subduction zones with the lowest coupling have back-arc extension. Globally we find that the non-linear stress-strain exponent, n, is about 3.0 +/- 0.25 (in the upper mantle and lithosphere) and a pressure-independent yield stress is 150 +/- 25 MPa. The stress in the shear zones is just tens of MPa, and in preliminary models, we find that both the shear and the normal stresses are elevated in the coupled compared to the uncoupled subduction zones.

The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge

NASA Astrophysics Data System (ADS)

Cluzel, Dominique; Jourdan, Fred; Meffre, SéBastien; Maurizot, Pierre; Lesimple, StéPhane

2012-06-01

Amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of New Caledonia (termed Peridotite Nappe) recrystallized in the high-temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the Eocene metamorphic complex. Amphibolites mostly display the geochemical features of MORB with a slight Nb depletion and thus are similar to the youngest (Late Paleocene-Eocene) BABB components of the allochthonous Poya Terrane. Thermochronological data from hornblende (40Ar/39Ar), zircon, and sphene (U-Pb) suggest that these mafic rocks recrystallized at ˜56 Ma. Using various geothermobarometers provides a rough estimate of peak recrystallization conditions of ˜0.5 GPa at ˜800-950°C. The thermal gradient inferred from the metamorphic assemblage (˜60°C km-1), geometrical relationships, and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction/obduction system and recrystallized when they subducted below young and hot oceanic lithosphere. They were detached from the down-going plate and finally thrust onto unmetamorphosed Poya Terrane basalts. This and the occurrence of slab melts at ˜53 Ma suggest that subduction inception occurred at or near to the spreading ridge of the South Loyalty Basin at ˜56 Ma.

Cascadia subducting plate fluids channelled to fore-arc mantle corner: ETS and silica deposition

USGS Publications Warehouse

Hyndman, Roy D.; McCrory, Patricia A.; Wech, Aaron; Kao, Han; Ague, Jay

2015-01-01

In this study we first summarize the constraints that on the Cascadia subduction thrust, there is a 70 km gap downdip between the megathrust seismogenic zone and the Episodic Tremor and Slip (ETS) that lies further landward; there is not a continuous transition from unstable to conditionally stable sliding. Seismic rupture occurs mainly offshore for this hot subduction zone. ETS lies onshore. We then suggest what does control the downdip position of ETS. We conclude that fluids from dehydration of the downgoing plate, focused to rise above the fore-arc mantle corner, are responsible for ETS. There is a remarkable correspondence between the position of ETS and this corner along the whole margin. Hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature, and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore-arc mantle. Laboratory data show that such fore-arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone. At the fore-arc mantle corner these fluids are released upward into the more permeable overlying fore-arc crust. An indication of this fluid flux comes from low Poisson's Ratios (and Vp/Vs) found above the corner that may be explained by a concentration of quartz which has exceptionally low Poisson's Ratio. The rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner.

Near field earthquake sources scenarios and related tsunamis on the French-Italian Riviera (Western Mediterranean

NASA Astrophysics Data System (ADS)

Larroque, Christophe; Ioualalen, Mansour; Scotti, Oona

2014-05-01

The large system of thrust faults recently evidenced at the foot of the northern Ligurian margin accommodates the inversion of this ancient passive margin since at least 5 Ma (Messinian times). At depth, these faults are certainly connected to a major northward dipping thrust that accounts for the major part of the seismicity in the northern Ligurian Sea. The deformations of the Quaternary sediments along the faults attest to a compressive tectonic regime consistent with the focal mechanisms of earthquakes. The major event in the area (the Ligurian earthquake, 1887/02/23, Mw 6.7-6.9 and the related tsunami) could result from the activation of part of the Ligurian thrust. Starting from the Ligurian earthquake source characteristics (strike: N55°E, dip: 16°N, length: 35 km, width: 17 km, co-seismic slip: 1.5 m, focal depth: 15 km, Mw 6.9), we have built an exhaustive set of earthquake scenarios involving the 80 km long Ligurian thrust. (1) Two of these earthquake scenarios ruptured respectively the eastern (offshore Imperia) and western (offshore Nice) part of the Ligurian thrust. (2) As these scenarios must scan the range of potential events in accordance with the geology, a second group of scenarios tests an 80 km long rupture of the entire Ligurian thrust with different strikes (N55°E and N70°E) and different widths of the faulting surface (17 km and 27 km) and then co-seismic slips of 2 m and 3.3 m, respectively. As the Ligurian coast is a densely populated and industrial area, the vulnerability is high. We want to stress here that we are more concerned with tsunamis triggered by local earthquakes. This is because, considering their arrival times (a few minutes), the risk prevention cannot be handled by existing tsunami warning system. For all scenarios we evaluate the tsunami coastal impact. The spatial distribution of the maximum wave height (MWH) is provided with a tentative identification of the processes that are responsible for it. The predictions suggest that the wave impact is mostly local considering the relatively moderate size of the rupture planes compared to large subduction earthquake induced tsunamis. The studied scenarios show that for such events specific localities along the French-Italian Riviera (San Remo, Cipressa, Imperia, Diano Marina, Nice) may experience very significant MWH (in the range of 3 to 10 m depending of the co-seismic slip and magnitude) related to the shallow focal depth tested for such scenarios. We may reasonably conclude that the tsunami threat is relatively significant and uniform along the Italian side of the Riviera (from Ventimiglia to Imperia) while it is more localized along the French side from Ventimiglia to Antibes with however higher local level of inundation, e.g., Nice city center, in case of a complete rupture of the Ligurian thrust faults system.

Origin of ophiolite complexes related to intra-oceanic subduction initiation: implications of IODP Expedition 352 (Izu-Bonin fore arc)

NASA Astrophysics Data System (ADS)

Robertson, Alastair; Avery, Aaron; Carvallo, Claire; Christeson, Gail; Ferré, Eric; Kurz, Walter; Kutterolf, Steffen; Morgan, Sally; Pearce, Julian; Reagan, Mark; Sager, William; Shervais, John; Whattam, Scott; International Ocean Discovery Program Expedition 352 (Izu-Bonin-Mariana Fore Arc), the Scientific Party of

2015-04-01

Ophiolites, representing oceanic crust exposed on land (by whatever means), are central to the interpretation of many orogenic belts (e.g. E Mediterranean). Based mostly on geochemical evidence, ophiolites are widely interpreted, in many but by no means all cases, as having formed within intra-oceanic settings above subduction zones (e.g. Troodos ophiolite, Cyprus). Following land geological, dredging and submersible studies, fore arcs of the SW Pacific region became recognised as likely settings of supra-subduction zone ophiolite genesis. This hypothesis was tested by recent drilling of the Izu-Bonin fore arc. Four sites were drilled, two on the outer fore arc and two on the upper trench slope. Site survey seismic data, combined with borehole data, indicate that three of the sites are located in fault-controlled sediment ponds that formed in response to dominantly down-to the-west extensional faulting (with hints of preceding top-to-the-east compressional thrusting). The sediments overlying the igneous basement, of maximum Late Eocene to Recent age, document ash and aeolian input, together with mass wasting of the fault-bounded sediment ponds. At the two more trenchward sites (U1440 and U1441), mostly tholeiitic basalts were drilled, including massive and pillowed lavas and hyaloclastite. Geochemically, these extrusives are of near mid-oceanic ridge basalt composition (fore arc basalts). Subtle chemical deviation from normal MORB can be explained by weakly fluid-influenced melting during decompression melting in the earliest stages of supra-subduction zone spreading (not as 'trapped' older MORB). The remaining two sites, c. 6 km to the west (U1439 and U1442), penetrated dominantly high-magnesian andesites, known as boninites, largely as fragmental material. Their formation implies the extraction of highly depleted magmas from previously depleted, refractory upper mantle in a supra-subduction zone setting. Following supra-subduction zone spreading, the active modern arc formed c. 200 km westwards of the trench. The new drilling evidence proves that both fore arc-type basalt and boninite formed in a fore arc setting soon after subduction initiation (c.52 Ma). Comparisons with ophiolites reveal many similarities, especially the presence of fore arc-type basalts and low calcium boninites. The relative positions of the fore arc basalts, boninites and arc basalts in the Izu Bonin and Mariana forearc (based on previous studies) can be compared with the positions of comparable units in a range of ophiolite complexes in orogenic belts including the Troodos, Oman, Greek (e.g. Vourinos), Albanian (Mirdita), Coast Range (California) and Bay of Islands (Newfoundland) ophiolites. The comparisons support the interpretation that all of the ophiolites formed during intra-oceanic subduction initiation. There are also some specific differences between the individual ophiolites suggesting that ophiolites should be interpreted individually in their regional tectonic settings.

Seismicity of the Earth 1900-2010 New Guinea and vicinity

USGS Publications Warehouse

Benz, Harley M.; Herman, Matthew; Tarr, Arthur C.; Hayes, Gavin P.; Furlong, Kevin P.; Villaseñor, Antonio; Dart, Richard L.; Rhea, Susan

2011-01-01

There have been 22 M7.5+ earthquakes recorded in the New Guinea region since 1900. The dominant earthquake mechanisms are thrust and strike slip, associated with the arc-continent collision and the relative motions between numerous local microplates. The largest earthquake in the region was a M8.2 shallow thrust fault event in the northern Papua province of Indonesia that killed 166 people in 1996. The Australia-Pacific plate boundary is over 4,000 km long on the northern margin, from the Sunda (Java) trench in the west to the Solomon Islands in the east. The eastern section is over 2,300 km long, extending west from northeast of the Australian continent and the Coral Sea until it intersects the east coast of Papua New Guinea. The boundary is dominated by the general northward subduction of the Australia plate.

Discussion on ``Dextral transpression in Late Cretaceous continental collision, Sanandaj Sirjan Zone, western Iran'' [Journal of Structural Geology, 22(8) (2000) 1125 1139

NASA Astrophysics Data System (ADS)

Numan, Nazar M. S.

2001-12-01

The NW-SE trending Alpine Zagros Thrust Belt passes from southwest Iran into northeastern Iraq. Mohajjel and Fergusson contend in their work in Iran on the Sanandaj-Sirjan Zone (with a consistent Zagros trend) that collision of the Afro-Arabian continent and the Iranian microcontinent took place in the Late Cretaceous. It seems that tectonostratigraphic evidence from the neighbouring Iraqi territories, namely the Zagros Thrust Belt in the northern part, the Foreland Belt and the Quasiplatform of the north and the Platform in the western and southern deserts (Fig. 1), chronicles the subductional history in this part of the world to a fair degree of accuracy. It rather provides for an Eocene age of the continental collision between Arabia and the Iranian microcontinent.

Late Cretaceous-Early Palaeogene tectonic development of SE Asia

NASA Astrophysics Data System (ADS)

Morley, C. K.

2012-10-01

The Late Cretaceous-Early Palaeogene history of the continental core of SE Asia (Sundaland) marks the time prior to collision of India with Asia when SE Asia, from the Tethys in the west to the Palaeo-Pacific in the east, lay in the upper plate of subduction zones. In Myanmar and Sumatra, subduction was interrupted in the Aptian-Albian by a phase of arc accretion (Woyla and Mawgyi arcs) and in Java, eastern Borneo and Western Sulawesi by collision of continental fragments rifted from northern Australia. Subsequent resumption of subduction in the Myanmar-Thailand sector explains: 1) early creation of oceanic crust in the Andaman Sea in a supra-subduction zone setting ~ 95 Ma, 2) the belt of granite plutons of Late Cretaceous-Early Palaeogene age (starting ~ 88 Ma) in western Thailand and central Myanmar, and 3) amphibolite grade metamorphism between 70 and 80 Ma seen in gneissic outcrops in western and central Thailand, and 4) accretionary prism development in the Western Belt of Myanmar, until glancing collision with the NE corner of Greater India promoted ophiolite obduction, deformation and exhumation of marine sediments in the early Palaeogene. The Ranong strike-slip fault and other less well documented faults, were episodically active during the Late Cretaceous-Palaeogene time. N to NW directed subduction of the Palaeo-Pacific ocean below Southern China, Vietnam and Borneo created a major magmatic arc, associated with rift basins, metamorphic core complexes and strike-slip deformation which continued into the Late Cretaceous. The origin and timing of termination of subduction has recently been explained by collision of a large Luconia continental fragment either during the Late Cretaceous or Palaeogene. Evidence for such a collision is absent from the South China Sea well and seismic reflection record and here collision is discounted. Instead relocation of the subducting margin further west, possibly in response of back-arc extension (which created the Proto-South China Sea) is preferred. Lying between the two subduction related arcs, the Khorat Basin is of predominantly Late Jurassic-Early Cretaceous age but stratigraphic and apatite fission track data also indicates deposition of 1-2 km of Late Cretaceous sediments. The synformal basin geometry probably arose due to the dynamic topography created by converging Tethyan and Palaeo-Pacific subduction zones. The Aptian-Albian slowing of basin subsidence and onset of evaporite deposition coincides with collision of the Mawgyi and Woyla island arcs. Extensive Palaeogene deformation and exhumation (3 + km in places) affected all margins of the Khorat Plateau. Deformation includes folds of the Phu Phan uplift, and strike-slip faults, thrusts and folds on the southern and eastern margins. South of the Khorat Plateau outcrop, and seismic reflection data from the Ton Le Sap Basin (Cambodia), and the Gulf of Thailand, indicate syn-depositional fault-controlled subsidence was important during Cretaceous deposition. The hot, thickened crust developed during the Late Cretaceous-Palaeogene events follows the weak (Indosinian), crustal-scale Inthanon and Sukhothai zones, which persistently guided the location of later structures including Cenozoic extensional, and post-rift basins, and influenced the widespread occurrence of low-angle normal faults, metamorphic core complexes, and eastern Gulf of Thailand super-deep post-rift basins.

Probing the transition between seismically coupled and decoupled segments along an ancient subduction interface

NASA Astrophysics Data System (ADS)

Angiboust, Samuel; Kirsch, Josephine; Oncken, Onno; Glodny, Johannes; Monié, Patrick; Rybacki, Erik

2015-04-01

Although of paramount importance for understanding the nature of mechanical coupling in subduction zones, the portions downdip of the locked segments of subduction interfaces remain poorly understood. These deep transition zones often are sites of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus sites of episodic tremor and slip features, recently discovered at several plate boundaries. The extensive, exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes at the original depths of 30-40 km, typical for the depth range of such transition zones. We identify the shear zone at the base of the Dent Blanche complex (Dent Blanche Thrust, DBT) as a lower blueschist-facies, fossilized subduction interface where granitic mylonites overlie a metamorphosed ophiolite. We report field observations from the DBT region where a complex, discontinuous network of meter- to tens of meters-thick foliated cataclasites is interlayered with the basal DBT mylonites. Petrological results indicate that cataclasis took place at near peak metamorphic conditions (450-500°C, c. 1.2 GPa) during subduction of the Tethyan seafloor in Eocene times (42-48 Ma). Despite some tectonic reactivation during exhumation, these networks exhibit mutual cross-cutting relationships between mylonites, foliated cataclasites and vein systems indicating multiple switching between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40Ar-39Ar age data of newly formed phengite, and strontium isotopic signatures reveal that these rocks also underwent multiple hydrofracturing events via infiltration of fluids mainly derived from the ophiolitic metasediments underneath the DBT. From the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (c. 10-12s-1) accompanied by fluctuation of near-lithostatic fluid pressures (1>λ>0.95). We interpret the triggering of brittle deformation within DBT mylonites to reflect downwards propagation of megathrust events into the transition zone. Alternatively, these foliated cataclasites could also record the deformation associated with slow transients and other episodic slip events, reported by geophysical studies for several subduction zones worldwide for this transition zone.

Seismic hazard analysis for Jayapura city, Papua

NASA Astrophysics Data System (ADS)

Robiana, R.; Cipta, A.

2015-04-01

Jayapura city had destructive earthquake which occurred on June 25, 1976 with the maximum intensity VII MMI scale. Probabilistic methods are used to determine the earthquake hazard by considering all possible earthquakes that can occur in this region. Earthquake source models using three types of source models are subduction model; comes from the New Guinea Trench subduction zone (North Papuan Thrust), fault models; derived from fault Yapen, TareraAiduna, Wamena, Memberamo, Waipago, Jayapura, and Jayawijaya, and 7 background models to accommodate unknown earthquakes. Amplification factor using geomorphological approaches are corrected by the measurement data. This data is related to rock type and depth of soft soil. Site class in Jayapura city can be grouped into classes B, C, D and E, with the amplification between 0.5 - 6. Hazard maps are presented with a 10% probability of earthquake occurrence within a period of 500 years for the dominant periods of 0.0, 0.2, and 1.0 seconds.

Talc friction in the temperature range 25°–400 °C: relevance for fault-zone weakening

USGS Publications Warehouse

Moore, Diane E.; Lockner, David A.

2008-01-01

Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks.

Late Mesozoic deformations of the Verkhoyansk-Kolyma orogenic belt, Northeast Russia

NASA Astrophysics Data System (ADS)

Fridovsky, Valery

2016-04-01

The Verkhoyansk-Kolyma orogenic belt marks the boundary between the Kolyma-Omolon superterrane (microcontinent) and the submerged eastern margin of the North Asian craton. The orogenic system is remark able for its large number of economically viable gold deposits (Natalka, Pavlik, Rodionovskoe, Drazhnoe, Bazovskoe, Badran, Malo-Tarynskoe, etc.). The Verkhoyansk - Kolyma orogenic belt is subdivided into Kular-Nera and the Polousny-Debin terranes. The Kular-Nera terrane is mainly composed of the Upper Permian, Triassic, and Lower Jurassic black shales that are metamorphosed at lower greenschist facies conditions. The Charky-Indigirka and the Chai-Yureya faults separate the Kular-Nera from the Polousny-Debin terrane that is predominantly composed of the Jurassic flyschoi dturbidites. The deformation structure of the region evolved in association with several late Mesozoic tectonic events that took place in the north-eastern part ofthe Paleo-Pacific. In Late Jurassic-Early Cretaceous several generations of fold and thrust systems were formed due to frontal accretion of the Kolyma-Omolon superterrane to the eastern margin of the North Asian craton.Thrusting and folding was accompanied by granitic magmatism, metamorphic reworking of the Late Paleozoic and the Early Mesozoic sedimentary rocks, and formation of Au-Sn-W mineralization. Three stages of deformation related to frontal accretion can be distinguished. First stage D1 has developed in the north-eastern part of the Verkhoyansk - Kolyma orogenic belt. Early tight and isoclinal folds F1 and assosiated thrusts are characteristic of D1. Major thrusts, linear concentric folds F2 and cleavage were formed during D2. The main ore-controlling structures are thrust faults forming imbricate fan systems. Frontal and oblique ramps and systems of bedding and cross thrusts forming duplexes are common. It is notable that mineralized tectonized zones commonly develop along thrusts at the contacts of rocks of contrasting competence. The superimposed structures are recognized from the early cleavage deformations. Folds F3 are often chevron type, open or tight. D1, D2 and D3 deformations are coaxial. In the Late-Neocomian-Aptian the Kolyma-Omolon superterrane started moving to the west. As a result, the thrust faults were reactivated with sinistral strike-slip motions along fault planes. At that time, granitoids of the North and Transverse belts were emplaced in the northwestern part of the Kolyma-Omolon superterrane. The strike slip faults were associated with cross open folds. The postacrettionary stage is associated with the development of the Albian-Late Cretaceous Okhotsk-Chukotka subduction zone. During this stage strike-slip faults and associated deformation structures were superimposed upon accretion-related tectonic structures of the Verkhoyansk - Kolyma orogenic belt.

Ivrea mantle wedge and arc of the Western Alps (II): Kinematic evolution of the Alps-Apennines orogenic system

NASA Astrophysics Data System (ADS)

Schmid, Stefan; Kissling, Eduard; van Hinsbergen, Douwe J. J.; Molli, Giancarlo

2017-04-01

Integration of geological and geophysical data on the deep structure of the Alps (Kissling et al. 2017) reveals that the deep-seated Ivrea mantle played a crucial role during the formation of the arc of the Western Alps. Exhumation of the mantle beneath the Ivrea Zone to shallow crustal depths during Mesozoic rifting led to the formation of a strong Ivrea mantle wedge; its strength exceeds that of surrounding mostly quartz-bearing units, and consequently allows for indentation and wedging of a quasi-rigid Ivrea mantle wedge into the Western Alps during Alpine orogeny. A first early stage (pre-35 Ma) of the West-Alpine orogenic evolution is characterized by top-NNW thrusting in sinistral transpression causing at least some 260km displacement of internal Western Alps and E-W-striking Alps farther east, together with the Adria micro-plate, towards N to NNW with respect to stable Europe. It is during the second stage (35-25 Ma) that the Ivrea mantle wedge played a crucial role by accentuating the arc. This stage is associated with top-WNW thrusting in the external zones of the central portion of the arc and lateral indentation and wedging of the Ivrea mantle slice beneath the already existing nappe pile towards WNW by some 100-150km. The final stage of arc formation (25-0 Ma) is associated with orogeny in the Apennines leading to oroclinal bending in the southernmost Western Alps that by now became parts of the Apenninic orogen, in connection with the 50° counterclockwise rotation of the Corsica-Sardinia block and the Ligurian Alps. The lithological composition of some tectonic units originating from the Alpine Tethys (Piemont-Liguria Ocean) found in the Alps and the northern Apennines has much in common. The non-metamorphic parts of the Piemont-Liguria derived units form the upper plate of the Western Alps that is devoid of Austroalpine elements, while the lower plate includes highly metamorphic units derived from the same Piemont-Liguria Ocean. This points to a lateral transition from continent-continent collision in the Central and Eastern Alps to intra-oceanic subduction in the Western Alps during Alpine orogeny, leaving large parts of the Piemont-Liguria Ocean that belong to the Adria microplate open until about 25 Ma. It is these parts that from now on formed the highest tectonic units in the Apennines, namely the Ligurides. However, internal units of the Northern Apennines previously suffered Alpine-type shortening associated with an E-dipping Alpine subduction zone. They became " backthrusted" to the NE during Apenninic orogeny commencing in the Late Oligocene. Apenninic orogeny is associated with a change in subduction polarity from Alpine E-directed subduction, previously affecting the Internal Ligurides and other parts of the Northern Apennines, towards NW-directed subduction and roll back of the Adria slab beneath Northern Apennines, pulled by the negative buoyancy of those parts of the old oceanic lithosphere of the Piemont-Liguria Ocean that remained unaffected by Alpine orogeny. Reference: Edi Kissling, Stefan M. Schmid, Tobias Diehl (2017). Ivrea mantle wedge and arc of the Western Alps (1): Geophysical evidence for the deep structure. Abstract Volume EGU 2017.

Ambient Tremor, But No Triggered Tremor at the Northern Costa Rica Subduction Zone

NASA Astrophysics Data System (ADS)

Swiecki, Z.; Schwartz, S. Y.

2010-12-01

Non-volcanic tremor (NVT) has been found to be triggered during the passage of surface waves from various teleseismic events in locations around the world including Cascadia, Southwest Japan, Taiwan, and California. In this study we examine the northern Costa Rica subduction zone for evidence of triggered tremor. The Nicoya Peninsula segment of the northern Costa Rica margin experiences both slow-slip and tremor and is thus a prime candidate for triggered tremor observations. Eleven teleseismic events with magnitudes (Mw) greater than 8 occurring between 2006 and 2010 were examined using data from both broadband and short period sensors deployed on the Nicoya Peninsula, Costa Rica. Waveforms from several large regional events were also considered. The largest teleseismic and regional events (27 February 2010 Chile, Mw 8.8 and 28 May 2009 Honduras, Mw 7.3) induced peak ground velocities (PGV) at the NIcoya stations of ~2 and 6 mm/s, respectively; larger than PGVs in other locations that have triggered tremor. Many of the earthquakes examined occurred during small episodes of background ambient tremor. In spite of this, no triggered tremor was observed during the passage of seismic waves from any event. This is significant because other studies have demonstrated that NVT is not triggered everywhere by all events above some threshold magnitude, indicating that unique conditions are required for its occurrence. The lack of triggered tremor at the Costa Rica margin can help to better quantify the requisite conditions and triggering mechanisms. An inherent difference between the Costa Rica margin and the other subduction zones where triggered tremor exists is its erosional rather than accretionary nature. Its relatively low sediment supply likely results in a drier, lower pore fluid pressure, stronger and less compliant thrust interface that is less receptive to triggering tremor from external stresses generated by teleseismic or strong local earthquakes. Another important factor is Costa Rica’s relatively cool subduction zone structure where temperatures required for the fluid generating basalt/ecloginte reaction are not reached until far below tremor producing depths.

Metamorphic sole formation reveals plate interface rheology during early subduction

NASA Astrophysics Data System (ADS)

Mathieu, S.; Agard, P.; Dubacq, B.; Plunder, A.; Prigent, C.

2015-12-01

Metamorphic soles are m to ~500m thick tectonic slices welded beneath most large ophiolites. They correspond to highly to mildly deformed portions of oceanic lithosphere metamorphosed at amphibolite to granulite facies peak conditions. Metamorphic soles are interpreted as formed ≤1-2Ma after intraoceanic subduction initiation by heat transfer from the hot, incipient mantle wegde to the underthrusting lower plate. Their early accretion and exhumation together with the future ophiolite implies at least one jump of the subduction plate interface from above to below the metamorphic sole. Metamorphic soles thus represent one of the few remnants of the very early evolution of the subduction plate interface and provide major constraints on the thermal structure and the effective rheology of the crust and mantle along the nascent slab interface.We herein present a structural and petrological detailed description of the Oman and Turkey metamorphic soles. Both soles present a steep inverted metamorphic structure, with isograds subparallel to the peridotite contact, in which the proportion of mafic rocks, pressure and temperature conditions increase upward. They comprise, as most metamorphic soles worldwide, two main units: (1) a high-grade unit adjacent to the overlying peridotite composed of granulitized to amphibolized metabasalts, with rare metasedimentary interlayers (~800±100ºC at 10±2kbar) and (2) a low-grade greenschist facies unit composed of metasedimentary rocks with rare metatuffs (~500±100ºC at 5±2kbar). We provide for the first time refined P-T peak condition estimations by means of pseudosection modelling and maximum temperature constraints for the Oman low-grade sole by RAMAN thermometry. In order to quantify micro-scale deformations trough the sole, we also present EBSD data on the Oman garnet-bearing and garnet-free high-grade sole.With these new constraints, we finally propose a new conceptual mechanical model for metamorphic sole formation. This model excludes the presence of a continuous inverted metamorphic gradient through the sole but implies the stacking of several homogeneous slivers to constitute the present structure of the sole. These successive thrusts are the result of rheological changes as the plate interface progressively cools.

Mapping Yakutat Subduction with Tectonic Tremor

NASA Astrophysics Data System (ADS)

Wech, A.

2015-12-01

Subduction of the Yakutat microplate (YAK) in south-central Alaska may be responsible for regional high topography, large slip during the 1964 earthquake, and the anomalous gap in arc volcanism, but the exact geodynamics and its relationship with the underlying Pacific Plate (PP) are not fully understood. Refraction data support distinct subducting layers, and both GPS and body wave tomography suggest the YAK extends from the Cook Inlet volcanoes in the west to the Wrangell volcanic field in the east. Earthquakes, however, are limited to normal faulting within the PP with an abrupt eastern boundary 80 km west of the inferred YAK edge, and more recent active source seismic data suggest subduction of one homogenous thickened oceanic plateau. Here, I perform a search for tectonic tremor to investigate the role of tremor and slow slip in the system. I scan continuous waveforms from 2007-2015 using all available data from permanent and campaign seismic stations in south-central Alaska. Using envelope cross-correlation, I detect and locate ~9,000 tectonic tremor epicenters, providing a map of the transition zone downdip of the 1964 earthquake. Tremor epicenters occur downdip of discrete slow slip events, and tremor rates do not correlate temporally with slow slip behavior. Depth resolution is poor, but horizontal locations are well constrained and spatially correlate with the velocity images of the YAK. Likewise, tremor extends 80 km further east than intraslab seismicity. Tremor swarms occur intermittently and manifest as ambient tremor. I interpret tremor to mark slow, semi-continuous slip occurring at the boundary between the YAK and North American plates, whose interface continues beyond the eastern edge of the PP. In this model, the YAK is welded to the underlying PP in the west, but extends past the eastern terminus of the PP. This geometry explains the correlation between tremor and the YAK, the discrepancy between deep seismicity and tremor, and the paucity of thrust events - convergence is accommodated by the YAK-North America interface, while earthquakes mark deformation within the PP. Finally, the model corroborates the eastern edge of the YAK and its role in controlling Wrangell magmatism and the gap in Aleutian arc volcanism.

Alaska earthquake source for the SAFRR tsunami scenario: Chapter B in The SAFRR (Science Application for Risk Reduction) Tsunami Scenario

USGS Publications Warehouse

Kirby, Stephen; Scholl, David; von Huene, Roland E.; Wells, Ray

2013-01-01

Tsunami modeling has shown that tsunami sources located along the Alaska Peninsula segment of the Aleutian-Alaska subduction zone have the greatest impacts on southern California shorelines by raising the highest tsunami waves for a given source seismic moment. The most probable sector for a Mw ~ 9 source within this subduction segment is between Kodiak Island and the Shumagin Islands in what we call the Semidi subduction sector; these bounds represent the southwestern limit of the 1964 Mw 9.2 Alaska earthquake rupture and the northeastern edge of the Shumagin sector that recent Global Positioning System (GPS) observations indicate is currently creeping. Geological and geophysical features in the Semidi sector that are thought to be relevant to the potential for large magnitude, long-rupture-runout interplate thrust earthquakes are remarkably similar to those in northeastern Japan, where the destructive Mw 9.1 tsunamigenic earthquake of 11 March 2011 occurred. In this report we propose and justify the selection of a tsunami source seaward of the Alaska Peninsula for use in the Tsunami Scenario that is part of the U.S. Geological Survey (USGS) Science Application for Risk Reduction (SAFRR) Project. This tsunami source should have the potential to raise damaging tsunami waves on the California coast, especially at the ports of Los Angeles and Long Beach. Accordingly, we have summarized and abstracted slip distribution from the source literature on the 2011 event, the best characterized for any subduction earthquake, and applied this synoptic slip distribution to the similar megathrust geometry of the Semidi sector. The resulting slip model has an average slip of 18.6 m and a moment magnitude of Mw = 9.1. The 2011 Tohoku earthquake was not anticipated, despite Japan having the best seismic and geodetic networks in the world and the best historical record in the world over the past 1,500 years. What was lacking was adequate paleogeologic data on prehistoric earthquakes and tsunamis, a data gap that also presently applies to the Alaska Peninsula and the Aleutian Islands. Quantitative appraisal of potential tsunami sources in Alaska requires such investigations.

An Andean-type retro-arc foreland system beneath northwest South China revealed by SINOPROBE profiling

NASA Astrophysics Data System (ADS)

Li, Jianhua; Dong, Shuwen; Cawood, Peter A.; Zhao, Guochun; Johnston, Stephen T.; Zhang, Yueqiao; Xin, Yujia

2018-05-01

In the Mesozoic, South China was situated along the convergent margin between the Asian and Pacific plates, providing an excellent laboratory to understand the interactions between deformation, sedimentation and magmatism in a retroarc environment. The crustal architecture of northwest South China is displayed along the ∼600-km-long SINOPROBE deep seismic reflection profiles and reveals from east to west: (1) highly folded and truncated reflectors in the upper crust of the Yangtze Fold Zone, which correspond to thin- and thick-skinned thrust systems, and document large-scale intraplate structural imbrication and shortening; (2) a crustal-scale flat-ramp-flat structure, termed the Main Yangtze decollement, which forms a weak, viscous layer to accommodate strain decoupling and material transport in the thin- and thick-skinned systems; and (3) nearly flat-lying reflectors in the Sichuan Basin, which support interpretation of the basin as a weakly deformed depocentre. The Yangtze Fold Zone and the Sichuan Basin represent a retro-arc foreland basin system that is >800 km away from the continental-margin magmatic arc. We suggest that tectonic processes across the arc and retro-arc systems, including arc magma flare-up, basin sedimentation, retroarc thrust propagation, lithosphere underthrusting, root foundering, and extension-related magmatism were interrelated and governed mass transfer. Age data and geological relations link the tectonic processes to evolving geodynamics of the subducting Paleo-Pacific plate.

Large-Scale Crustal-Block-Extrusion During Late Alpine Collision.

PubMed

Herwegh, Marco; Berger, Alfons; Baumberger, Roland; Wehrens, Philip; Kissling, Edi

2017-03-24

The crustal-scale geometry of the European Alps has been explained by a classical subduction-scenario comprising thrust-and-fold-related compressional wedge tectonics and isostatic rebound. However, massive blocks of crystalline basement (External Crystalline Massifs) vertically disrupt the upper-crustal wedge. In the case of the Aar massif, top basement vertically rises for >12 km and peak metamorphic temperatures increase along an orogen-perpendicular direction from 250 °C-450 °C over horizontal distances of only <15 km (Innertkirchen-Grimselpass), suggesting exhumation of midcrustal rocks with increasing uplift component along steep vertical shear zones. Here we demonstrate that delamination of European lower crust during lithosphere mantle rollback migrates northward in time. Simultaneously, the Aar massif as giant upper crustal block extrudes by buoyancy forces, while substantial volumes of lower crust accumulate underneath. Buoyancy-driven deformation generates dense networks of steep reverse faults as major structures interconnected by secondary branches with normal fault component, dissecting the entire crust up to the surface. Owing to rollback fading, the component of vertical motion reduces and is replaced by a late stage of orogenic compression as manifest by north-directed thrusting. Buoyancy-driven vertical tectonics and modest late shortening, combined with surface erosion, result in typical topographic and metamorphic gradients, which might represent general indicators for final stages of continent-continent collisions.

UHP metamorphism in the Western Mediterranean : A tale of a Tethys fragment (Edough Massif, NE Algeria) and its geodynamic consequences

NASA Astrophysics Data System (ADS)

Bruguier, Olivier; Bosch, Delphine; Caby, Renaud; Fernandez, Laure; Abdallah, Nachida; Arnaud, Nicolas; Hammor, Dalila; Laouar, Rabah; Mechati, Medhi; Monié, Patrick; Ouabadi, Aziouz; Toubal, Abder

2016-04-01

The Edough Massif of NE Algeria is part of the Maghrebides, a peri-Mediterranean Alpine belt that extends from Morocco to Tunisia. The belt resulted mainly from the eastward retreat of the Tethyan slab and from the drift of continental fragments, some of which finally collided with the north African margin. In this study we report the recent discovery of metamorphic diamonds (5-30 μm in size) included in a garnet megacryst and identified by Raman spectroscopy and the characteristic sharp band at 1332 cm-1 for crystalline diamond. The studied megacryst was taken from a weathered actinolitite horizon inserted within a major mylonite-ultramylonite band, which outcrops at the base of an allochtonous oceanic unit thrust onto the African paleomargin. The host garnet is almandine-dominant with a sharp increase in grossular component in the rim and is rich in exsolution of small acicular rutile needles. Major and trace elements show a gradual but significant zonation from core to rim characterized by a decrease in HREE, Y and Mn, typical of a prograde growth in a closed system. Trace element analyses of large prismatic rutile (up to 300 μm) indicate that the host metamorphic rock was a mafic protolith of MORB affinity and the Zr-in-rutile thermometry indicates a temperature range of 724-778°C for rutile growth. U-Pb analyses of these large rutile crystals provide an age of 32.4 ± 3.3 Ma interpreted as dating the prograde subduction stage of the mafic protolith. Minute zircons (≤ 30μm), disseminated in the garnet, display a multifaceted appearance and low Th/U ratios consistent with a metamorphic origin. The lack of HREE depletion in these zircons indicates that their metamorphic growth was not coeval with garnet. U-Pb analyses and Ti-in-zircon thermometry indicate they nucleated at 20.9 ± 2.2 Ma during near isothermal decompression related to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. We suggest that the mafic protolith originates from the subducted retreating Calabrian branch of the Tethyan slab, that broke or tore, and which fragments were dragged upward and thrusted onto the North African margin, shortly before the formation of the Edough dome. Exhumation of these UHP units is coeval with the counter clockwise rotation of the Corsica-Sardinia block, which is associated to the extensional opening of the Ligurian Sea as a result of slab rollback (Faccenna et al., 2001). The early Miocene exhumation of the UHP units, which were detached from the downgoing plate, most likely resulted simultaneously from subduction rollback that was driven by slab pull.

Active Tectonics of the Far North Pacific Observed with GPS

NASA Astrophysics Data System (ADS)

Elliott, J.; Freymueller, J. T.; Jiang, Y.; Leonard, L. J.; Hyndman, R. D.; Mazzotti, S.

2017-12-01

The idea that the tectonics of the northeastern Pacific is defined by relatively discrete deformation along the boundary between the Pacific and North American plates has given way to a more complex picture of broad plate boundary zones and distributed deformation. This is due in large part to the Plate Boundary Observatory and several focused GPS studies, which have greatly increased the density of high-quality GPS data throughout the region. We will present an updated GPS velocity field in a consistent reference frame as well as a new, integrated block model that sheds light on regional tectonics and provides improved estimates of motion along faults and their potential seismic hazard. Crustal motions in southern Alaska are strongly influenced by the collision and flat-slab subduction of the Yakutat block along the central Gulf of Alaska margin. In the area nearest to the collisional front, small blocks showing evidence of internal deformation are required. East of the front, block motions show clockwise rotation into the Canadian Cordillera while west of the front there are counterclockwise rotations that extend along the Alaska forearc, suggesting crustal extrusion. Farther from the convergent margin, the crust appears to move as rigid blocks, with uniform motion over large areas. In western Alaska, block motions show a southwesterly rotation into the Bering Sea. Arctic Alaska displays southeasterly motions that gradually transition into easterly motion in Canada. Much of the southeastern Alaska panhandle and coastal British Columbia exhibit northwesterly motions. Although the relative plate motions are mainly accommodated along major faults systems, including the Fairweather-Queen Charlotte transform system, the St. Elias fold-and-thrust belt, the Denali-Totschunda system, and the Alaska-Aleutian subduction zone, a number of other faults accommodate lesser but still significant amounts of motion in the model. These faults include the eastern Denali/Duke River system, the Castle Mountain fault, the western Denali fault, the Kaltag fault, and the Kobuk fault. Based on the expanded GPS data set, locked or partially locked sections of the Alaska subduction zone may extend as far north and east as the eastern Alaska Range.

Impact of cascadia subduction zone earthquake on the seismic evaluation criteria of bridges : technical report : SPR 770.

DOT National Transportation Integrated Search

2016-12-01

A large magnitude long duration subduction earthquake is impending in the Pacific Northwest, which lies near the : Cascadia Subduction Zone (CSZ). Great subduction zone earthquakes are the largest earthquakes in the world and are the sole source : zo...

Tracing Geophysical Indicators of Fluid-Induced Serpentinization in the Pampean Flat Slab Subduction Region of Chile

NASA Astrophysics Data System (ADS)

Bourke, J. R.; Nikulin, A.; Park, J. J.

2016-12-01

An activity gap in the Andean volcanic arc in the Pampean section of the subduction zone in Chile ( 28°-33°S) marks a section of flat-slab subduction. Past studies connected this change in geometry to the collision and subduction of the Juan Fernandez Ridge and the resulting migration of both the thrust front and magmatism eastward to the Sierras Pampeanas. The fate of fluids released from the subducting Nazca slab remains uncertain and the degree of their interaction with the basal layer of the continental lithosphere is poorly understood. We present initial results of a receiver-function investigation and forward-modeling effort at station GO03 operated by the Chilean National Seismic Network. Receiver function analysis of 75 well-recorded teleseismic earthquake events recorded at GO03 allow us to constrain the position of the subducting Nazca slab and to address the physical properties of the interplate contact zone. Critically, our analysis indicates presence of a highly-anisotropic zone of low velocities directly above the subucting Nazca slab. We point out a remarkable similarity in geophysical characteristics between the observed seismic anomaly at GO03 and a volume of proposed serpentinization in an area of sub-horizontal subduction above the Juan de Fuca slab in Cascadia. This interpretation is further supported by forward-modeling receiver functions at GO03 relying on a velocity model that incorporates a serpentinized interplate region. The newly-identified low-velocity highly-anisotropic layer may extend beyond the GO03 area and act as a mineral reservoir that captures and, possibly, transports fluids derived from the dehydrating Nazca Plate as it subducts below South America. It is likely that there is a relationship between this feature and the lack of volcanic activity in the Pampean flat slab region. Figure Caption: A) Backazimuth sweep of receiver functions recorded at station GO03 with predicted phase arrivals plotted for 55 km, 65 km, 75 km and 85 km. B) Depth-migrated receiver functions for station GO03 relying on AK-135 velocity model and local seismicity (Mw>4.5) plotted within 15km of a 100km profile centered on GO03 along the dominant direction of subduction (74°).

Direct Observations of In Situ Stress State in a 3 Kilometer Deep Borehole in the Upper Plate, Nankai Trough Subduction Zone: IODP Site C0002

NASA Astrophysics Data System (ADS)

Tobin, H. J.; Saffer, D. M.; Castillo, D. A.; Hirose, T.

2016-12-01

During IODP Expedition 348, borehole C0002F/N/P was advanced to a depth of 3058 m below the seafloor (mbsf) into the inner forearc accretionary wedge of the Nankai subduction zone (SW Japan), now the deepest scientific drilling ever into the ocean floor. The goals were to investigate the physical properties, structure, and state of stress deep within the hanging wall of a seismogenic subduction plate boundary. Mud pressure and gas monitoring, injection tests, leak-off tests (LOT), logging-while-drilling (LWD) measurements, and observations of mud losses and hole conditions provide both direct and indirect information about in situ pore pressure and stress state. The LOTs show that the minimum principal stress is consistently less than the vertical stress defined by the overburden, ruling out a thrust faulting stress state throughout the drilled section, and define a nearly linear gradient in Shmin from the seafloor to the base of the hole. Observations of mud loss and the lack of observed gas shows indicate that formation pore fluid pressure is not significantly (< 10 MPa) greater than hydrostatic. The maximum horizontal stress, estimated from borehole breakout width and pressure spikes during pack-off events, is close in magnitude to the vertical stress. Therefore the accretionary prism lies in either a normal or strike-slip faulting regime, or is transitional between the two, from 1 to 3 km depth. At 3002 mbsf we estimate that the effective stresses are: Sv' = 33 MPa; SHmax' = 25-36 MPa; and Shmin' = 18.5-21 MPa. Differential stresses are therefore low, on the order of 10-12 MPa, in the hanging wall of the subduction thrust. We conclude that (1) the inner wedge is not critically stressed in horizontal compression; (2) basal traction along the megathrust must be low in order to permit concurrent locking of the fault and low differential stresses deep within the upper plate; and (3) although low differential stresses may persist down to the plate boundary at 5000 mbsf, the maximum horizontal stress SHmax must transition to become greater than the vertical stress, either spatially below the base of the borehole, or temporally leading up to megathrust fault rupture, in order to drive slip on the megathrust.

Preliminary earthquake locations in the Kenai Peninsula recorded by the MOOS Array and their relationship to structure in the 1964 great earthquake zone

NASA Astrophysics Data System (ADS)

Li, J.; Abers, G. A.; Christensen, D. H.; Kim, Y.; Calkins, J. A.

2011-12-01

Earthquakes in subduction zones are mostly generated at the interface between the subducting and overlying plates. In 2006-2009, the MOOS (Multidisciplinary Observations Of Subduction) seismic array was deployed around the Kenai Peninsula, Alaska, consisting of 34 broadband seismometers recording for 1-3 years. This region spans the eastern end of the Aleutian megathrust that ruptured in the 1964 Mw 9.2 great earthquake, the second largest recorded earthquake, and ongoing seismicity is abundant. Here, we report an initial analysis of seismicity recorded by MOOS, in the context of preliminary imaging. There were 16,462 events detected in one year from initial STA/LTA signal detections and subsequent event associations from the MOOS Array. We manually reviewed them to eliminate distant earthquakes and noise, leaving 11,879 local earthquakes. To refine this catalog, an adaptive auto-regressive onset estimation algorithm was applied, doubling the original dataset and producing 20,659 P picks and 22,999 S picks for one month (September 2007). Inspection shows that this approach lead to almost negligible false alarms and many more events than hand picking. Within the well-sampled part of the array, roughly 200 km by 300 km, we locate 250% more earthquakes for one month than the permanent network catalog, or 10 earthquakes per day on this patch of the megathrust. Although the preliminary locations of earthquakes still show some scatter, we can see a concentration of events in a ~20-km-wide belt, part of which can be interpreted as seismogenic thrust zone. In conjunction with the seismicity study, we are imaging the plate interface with receiver functions. The main seismicity zone corresponds to the top of a low-velocity layer imaged in receiver functions, nominally attributed to the top of the downgoing plate. As we refine velocity models and apply relative relocation algorithms, we expect to improve the precision of the locations substantially. When combined with image of velocity structure from scattered wave migration, we can test whether the thrust zone is above the Yakutat terrane or between the Yakutat terrane and the subducting Pacific plate. Our refined relocations will also improve our understanding of other active faults (e.g., splay faults) and their relationship to the plate boundary.

Mesozoic to Cenozoic tectonic transition process in Zhanhua Sag, Bohai Bay Basin, East China

NASA Astrophysics Data System (ADS)

Cheng, Yanjun; Wu, Zhiping; Lu, Shunan; Li, Xu; Lin, Chengyan; Huang, Zheng; Su, Wen; Jiang, Chao; Wang, Shouye

2018-04-01

The Zhanhua sag is part of the Bohai Bay intracontinental basin system that has developed since the Mesozoic in East China. The timing of this basin system coincides with the final assembly of East Asia and the development of Western Pacific-type plate margin. Here we use 3-D seismic and core log data to investigate the evolution of this basin and discuss its broad tectonic settings. Our new structural study of Zhanhua sag suggests that there are four major tectonic transitions occurred in the Bohai Bay Basin during Mesozoic and Cenozoic: (1) The first tectonic transition was from stable Craton to thrusting during the Triassic, mainly caused by the South China Block's subduction northward beneath the North China Block, which induced the formation of the NW-striking thrust faults. (2) The second tectonic transition was mainly characterized by a change from compression to extension, which can be further divided into two-stages. At the first stage, two episodes of NW-SE shortening occurred in East Asia during Early-Middle Jurassic and Late Jurassic-earliest Cretaceous, respectively. At the second stage, the extension and left-lateral shearing took place during Early Cretaceous while compression occurred during Late Cretaceous. The NW-striking thrust faults changed to normal faults and the NNE-striking left-lateral strike-slip faults started to influence the eastern part of the basin. (3) The third transition occurred when the NW-SE extension and NNE-striking right-lateral shearing started to form during Paleogene, and the peak deformation happen around 40 Ma due to the change of the subduction direction of Pacific Plate relative to Eurasia Plate. The NE-striking normal faults are the main structure, and the pre-existing NNE-striking strike-slip faults changed from left-lateral to right-lateral. (4) The fourth transition saw the regional subsidence during Neogene, which was probably caused by the India-Asia "Hard collision" between 25 and 20 Ma.

3D Numerical modelling of topography development associated with curved subduction zones

NASA Astrophysics Data System (ADS)

Munch, Jessica; Ueda, Kosuke; Burg, Jean-Pierre; May, Dave; Gerya, Taras

2017-04-01

Curved subduction zones, also called oroclines, are geological features found in various places on Earth. They occur in diverse geodynamic settings: 1) single slab subduction in oceanic domain (e.g. Sandwich trench in the Southern Atlantic); 2) single slab subduction in continental domain, (e.g. Gibraltar-Alboran orocline in the Western Mediterranean) 3); multi-slab subduction (e.g. Caribbean orocline in the South-East of the Gulf of Mexico). These systems present various curvatures, lengths (few hundreds to thousands of km) and ages (less than 35 Ma for Gibraltar Alboran orocline, up to 100 Ma for the Caribbean). Recent studies suggested that the formation of curved subduction systems depends on slab properties (age, length, etc) and may be linked with processes such as retreating subduction and delamination. Plume induced subduction initiation has been proposed for the Caribbean. All of these processes involve deep mechanisms such as mantle and slab dynamics. However, subduction zones always generate topography (trenches, uplifts, etc), which is likely to be influenced by surface processes. Hence, surface processes may also influence the evolution of subduction zones. We focus on different kinds of subduction systems initiated by plume-lithosphere interactions (single slab subduction/multi-slab subduction) and scrutinize their surface expression. We use numerical modeling to examine large-scale subduction initiation and three-dimensional slab retreat. We perform two kinds of simulations: 1) large scale subduction initiation with the 3D-thermomechanical code I3ELVIS (Gerya and Yuen, 2007) in an oceanic domain and 2) large scale subduction initiation in oceanic domain using I3ELVIS coupled with a robust new surface processes model (SPM). One to several retreating slabs form in the absence of surface processes, when the conditions for subduction initiation are reached (c.f. Gerya et al., 2015), and ridges occur in the middle of the extensional domain opened by slab retreat. Topography associated with slab retreat is curved. Coupling I3ELVIS with SPM yields more accurate topography of the curved subduction zone. This allows balancing the relative importance of surface and deep processes in the evolution of curved subduction zones and the development of their related topography. References: Gerya, T. V., & Yuen, D. A. (2007). Robust characteristics method for modelling multiphase visco-elasto-plastic thermo-mechanical problems. Physics of the Earth and Planetary Interiors, 163(1), 83-105. Gerya, T. V., Stern, R. J., Baes, M., Sobolev, S. V., & Whattam, S. A. (2015). Plate tectonics on the Earth triggered by plume-induced subduction initiation. Nature, 527(7577), 221-225.

Mechanical and hydraulic properties of Nankai accretionary prism sediments: Effect of stress path

NASA Astrophysics Data System (ADS)

Kitajima, Hiroko; Chester, Frederick M.; Biscontin, Giovanna

2012-10-01

We have conducted triaxial deformation experiments along different loading paths on prism sediments from the Nankai Trough. Different load paths of isotropic loading, uniaxial strain loading, triaxial compression (at constant confining pressure, Pc), undrained Pc reduction, drained Pc reduction, and triaxial unloading at constant Pc, were used to understand the evolution of mechanical and hydraulic properties under complicated stress states and loading histories in accretionary subduction zones. Five deformation experiments were conducted on three sediment core samples for the Nankai prism, specifically from older accreted sediments at the forearc basin, underthrust slope sediments beneath the megasplay fault, and overthrust Upper Shikoku Basin sediments along the frontal thrust. Yield envelopes for each sample were constructed based on the stress paths of Pc-reduction using the modified Cam-clay model, and in situ stress states of the prism were constrained using the results from the other load paths and accounting for horizontal stress. Results suggest that the sediments in the vicinity of the megasplay fault and frontal thrust are highly overconsolidated, and thus likely to deform brittle rather than ductile. The porosity of sediments decreases as the yield envelope expands, while the reduction in permeability mainly depends on the effective mean stress before yield, and the differential stress after yield. An improved understanding of sediment yield strength and hydromechanical properties along different load paths is necessary to treat accurately the coupling of deformation and fluid flow in accretionary subduction zones.

Plate Motion and Crustal Deformation Estimated with Geodetic Data from the Global Positioning System

NASA Technical Reports Server (NTRS)

Argus, Donald F.; Heflin, Michael B.

1995-01-01

We use geodetic data taken over four years with the Global Positioning System (GPS) to estimate: (1) motion between six major plates and (2) motion relative to these plates of ten sites in plate boundary zones. The degree of consistency between geodetic velocities and rigid plates requires the (one-dimensional) standard errors in horizontal velocities to be approx. 2 mm/yr. Each of the 15 angular velocities describing motion between plate pairs that we estimate with GPS differs insignificantly from the corresponding angular velocity in global plate motion model NUVEL-1A, which averages motion over the past 3 m.y. The motion of the Pacific plate relative to both the Eurasian and North American plates is observed to be faster than predicted by NUVEL-1A, supporting the inference from Very Long B ase- line Interferometry (VLBI) that motion of the Pacific plate has speed up over the past few m.y. The Eurasia-North America pole of rotation is estimated to be north of NUVEL-1A, consistent with the independent hypothesis that the pole has recently migrated northward across northeast Asia to near the Lena River delta. Victoria, which lies above the main thrust at the Cascadia subduction zone, moves relative to the interior of the overriding plate at 30% of the velocity of the subducting plate, reinforcing the conclusion that the thrust there is locked beneath the continental shelf and slope.

Late Cretaceous-recent tectonic assembly of diverse crustal blocks in Central America, the Nicaraguan Rise, the Colombian Basin and northern South America as seen on a 1600-km-long, geologic and structural transect

NASA Astrophysics Data System (ADS)

Sanchez, J.; Mann, P.

2015-12-01

We have constructed a 1600-km-long transect from northern Honduras to northern Colombia that crosses northeastward-striking crustal blocks using a combination of offshore seismic data, gravity and magnetic data, well subsidence information, nearby outcrop information, and results from previous thermochronological, geochronological, geochemical and paleostress studies. The transect defines three major crustal and structural provinces: 1) Precambrian-Paleozoic, Chortis continental block whose northern edge is defined by the North America-Caribbean plate boundary. Events in this ~20-25-km-thick province include two major unconformities at the top of the Cretaceous and Eocene, associated southeast-dipping thrust faults related to collision of the Great Arc of the Caribbean (GAC) and Caribbean Large Igneous Province (CLIP) with the Chortis continental block. A third event is Eocene to recent subsidence and transtensional basins formed during the opening of the Cayman trough; 2) Late Cretaceous GAC and CLIP of oceanic arc and plateau origin, whose northern, deformed edge corresponds to the mapped Siuna belt of northern Nicaragua. This crustal province has a ~15-20-km-thick crust and is largely undeformed and extends across the Lower Nicaraguan Rise, Hess fault, to the southern limit of the Colombian basin where about 300 km of this province has been subducted beneath the accretionary wedge of the South Caribbean deformed belt of northwestern South America; and 3) Eocene to recent accretionary prism and intramontane basins on continental crust of northern South America, where Miocene accelerated exhumation and erosion of Paleogene and Cretaceous rocks reflect either shallow subduction of the CLIP or the Panama collisional event to the southwest.

Seismic investigation of the Kunlun Fault: Analysis of the INDEPTH IV 2-D active-source seismic dataset

NASA Astrophysics Data System (ADS)

Seelig, William George

The Tibetan Plateau has experienced significant crustal thickening and deformation since the continental subduction and collision of the Asian and Indian plates in the Eocene. Deformation of the northern Tibetan Plateau is largely accommodated by strike-slip faulting. The Kunlun Fault is a 1000-km long strike-slip fault near the northern boundary of the Plateau that has experienced five magnitude 7.0 or greater earthquakes in the past 100 years and represents a major rheological boundary. Active-source, 2-D seismic reflection/refraction data, collected as part of project INDEPTH IV (International Deep Profiling of Tibet and the Himalaya, phase IV) in 2007, was used to examine the structure and the dip of the Kunlun fault. The INDEPTH IV data was acquired to better understand the tectonic evolution of the northeastern Tibetan Plateau, such as the far-field deformation associated with the continent-continent collision and the potential subduction of the Asian continent beneath northern Tibet. Seismic reflection common depth point (CDP) stacks were examined to look for reflectivity patterns that may be associated with faulting. A possible reflection from the buried North Kunlun Thrust (NKT) is identified at 18-21 km underneath the East Kunlun Mountains, with an estimated apparent dip of 15°S and thrusting to the north. Minimally-processed shot gathers were also inspected for reflections off near-vertical structures such as faults and information on first-order velocity structure. Shot offset and nearest receiver number to reflection was catalogued to increase confidence of picks. Reflections off the North Kunlun (NKF) and South Kunlun Faults (SKF) were identified and analyzed for apparent dip and subsurface geometry. Fault reflection analysis found that the North Kunlun Fault had an apparent dip of approximately 68ºS to an estimated depth of 5 km, while the South Kunlun Fault dipped at approximately 78ºN to an estimated 3.5 km depth. Constraints on apparent dip and geometry of the NKF/SKF and NKT provide information valuable for seismic hazard analysis.

A new perspective on the generation of the 2016 M6.7 Kaohsiung earthquake, southwestern Taiwan

NASA Astrophysics Data System (ADS)

Wang, Zhi

2017-04-01

In order to investigate the likely generation mechanism of the 2016 M6.7 Kaohsiung earthquake, a large number of high-quality travel times from P- and S-wave source-receiver pairs are used jointly in this study to invert three-dimensional (3-D) seismic velocity (Vp, Vs) and Poisson's ratio structures at high resolution. We also calculated crack density, saturate fracture, and bulk-sound velocity from our inverted Vp, Vs, and σgodels. In this way, multi-geophysical parameter imaging revealed that the 2016 Kaohsiung earthquake occurred along a distinctive edge portion exhibiting high-to-low variations in these parameters in both horizontal and vertical directions across the hypocenter. We consider that a slow velocity and high-σ body that has high ɛ and somewhat high ζ anomalies above the hypocenter under the Coastal Plain represents fluids contained in the young fold-and-thrust belt associated with the passive Asian continental margin in southwestern Taiwan. Intriguing, a continuous low Vp and Vs zone with high Poisson's ratio, crack density and saturate fracturegnomalies across the Laonung and Chishan faults is also clearly imaged in the northwestern upper crust beneath the Coastal Plain and Western Foothills as far as the southeastern lower crust under the Central Range. We therefore propose that this southeastern extending weakened zone was mainly the result of a fluid intrusion either from the young fold-and-thrust belt the shallow crust or the subducted Eurasian continental (EC) plate in the lower crust and uppermost mantle. We suggest that fluid intrusion into the upper Oligocene to Pleistocene shallow marine and clastic shelf units of the Eurasian continental crust and/or the relatively thin uppermost part of the transitional Pleistocene-Holocene foreland due to the subduction of the EC plate along the deformation front played a key role in earthquake generation in southwestern Taiwan. Such fluid penetration would reduce Vp, and Vs while increasing Poisson's ratio and saturate fracture across the source area, leading to mechanical strength failure of the rock matrix in the relative weakened and brittle seismogenic layer and triggering the 2016 earthquake. PIC

A new perspective on the generation of the 2016 M6.4 Meilung earthquake, southwestern Taiwan

NASA Astrophysics Data System (ADS)

Wang, Z.

2017-12-01

In order to investigate the likely generation mechanism of the 2016 M6.4 Meilung earthquake, a large number of high-quality travel times from P- and S-wave source-receiver pairs are used jointly in this study to invert three-dimensional (3-D) seismic velocity (Vp, Vs) and Poisson's ratio structures at high resolution. We also calculated crack density, saturate fracture, and bulk-sound velocity from our inverted Vp, Vs, and s models. In this way, multi-geophysical parameter imaging revealed that the 2016 Meilung earthquake occurred along a distinctive edge portion exhibiting high-to-low variations in these parameters in both horizontal and vertical directions across the hypocenter. We consider that a slow velocity and high-Poisson ratio body that has high-crack density and somewhat high-saturate fracture anomalies above the hypocenter under the coastal plain represents fluids contained in the young fold-and-thrust belt relative to the passive Asian continental margin in southwestern Taiwan. Intriguing, a continuous low Vp and Vs zone with high Poisson ratio, crack density and saturate fracture anomalies across the Laonung and Chishan faults is also clearly imaged in the northwestern upper crust beneath the coastal plain and western foothills as far as the southeastern lower crust under the central range. We therefore propose that this southeastern extending weakened zone was mainly the result of a fluid intrusion either from the young fold-and-thrust belt associated with the passive Asian continental margin in the shallow crust or the subducted Eurasian continental (EC) plate in the lower crust and uppermost mantle. We suggest that fluid intrusion into the upper Oligocene to Pleistocene shallow marine and clastic shelf units of the Eurasian continental crust and/or the relatively thin uppermost part of the transitional Pleistocene-Holocene foreland due to the subduction of the EC plate along the deformation front played a key role in earthquake generation in southwestern Taiwan. Such fluid penetration would reduce Vp, and Vs while increasing Poisson ratio and saturate fracture across the source area, leading to mechanical strength failure of the rock matrix in the relative weakened and brittle seismogenic layer and triggering the 2016 earthquake.

Formation of plate boundaries: The role of mantle volatilization

NASA Astrophysics Data System (ADS)

Seno, Tetsuzo; Kirby, Stephen H.

2014-02-01

In the early Earth, convection occurred with the accumulation of thick crust over a weak boundary layer downwelling into the mantle (Davies, G.F., 1992. On the emergence of plate tectonics. Geology 20, 963-966.). This would have transitioned to stagnant-lid convection as the mantle cooled (Solomatov, V.S., Moresi, L.-N., 1997. Three regimes of mantle convection with non-Newtonian viscosity and stagnant lid convection on the terrestrial planets. Geophys. Res. Lett. 24, 1907-1910.) or back to a magma ocean as the mantle heated (Sleep, N., 2000. Evolution of the mode of convection within terrestrial planets. J. Geophys. Res. 105(E7): 17563-17578). Because plate tectonics began operating on the Earth, subduction must have been initiated, thus avoiding these shifts. Based on an analogy with the continental crust subducted beneath Hindu Kush and Burma, we propose that the lithosphere was hydrated and/or carbonated by H2O-CO2 vapors released from magmas generated in upwelling plumes and subsequently volatilized during underthrusting, resulting in lubrication of the thrust above, and subduction of the lithosphere along with the overlying thick crust. Once subduction had been initiated, serpentinized forearc mantle may have formed in a wedge-shaped body above a dehydrating slab. In relict arcs, suture zones, or rifted margins, any agent that warms and dehydrates the wedge would weaken the region surrounding it, and form various types of plate boundaries depending on the operating tectonic stress. Thus, once subduction is initiated, formation of plate boundaries might be facilitated by a major fundamental process: weakening due to the release of pressurized water from the warming serpentinized forearc mantle.

Accretionary orogens through Earth history

USGS Publications Warehouse

Cawood, Peter A.; Kroner, A.; Collins, W.J.; Kusky, T.M.; Mooney, W.D.; Windley, B.F.

2009-01-01

Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero. ?? The Geological Society of London 2009.

Origin of a crustal splay fault and its relation to the seismogenic zone and underplating at the erosional north Ecuador-south Colombia oceanic margin

NASA Astrophysics Data System (ADS)

Collot, J.-Y.; Agudelo, W.; Ribodetti, A.; Marcaillou, B.

2008-12-01

Splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone. Prestack depth migration of a multichannel seismic line across the north Ecuador-south Colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 (Mw 7.7) tsunamogenic subduction earthquake. The splay fault separates 5-6.6 km/s velocity, inner wedge basement rocks, which belong to the accreted Gorgona oceanic terrane, from 4 to 5 km/s velocity outer wedge rocks. The outer wedge is dominated by basal tectonic erosion. Despite a 3-km-thick trench fill, subduction of 2-km-high seamount prevented tectonic accretion and promotes basal tectonic erosion. The low-velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic, décollement thrust. Subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks. Conversely, duplexes are interpreted to form from and above subducting sediment, at ˜14- to 15-km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic décollement. Coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget, which comes out negative. Intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the Gorgona terrane from Cretaceous extension to likely Eocene oblique compression. The splay faults could have resulted from tectonic inversion of listric normal faults, thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis.

Moment tensor inversion of the 2016 southeast offshore Mie earthquake in the Tonankai region using a three-dimensional velocity structure model: effects of the accretionary prism and subducting oceanic plate

NASA Astrophysics Data System (ADS)

Takemura, Shunsuke; Kimura, Takeshi; Saito, Tatsuhiko; Kubo, Hisahiko; Shiomi, Katsuhiko

2018-03-01

The southeast offshore Mie earthquake occurred on April 1, 2016 near the rupture area of the 1944 Tonankai earthquake, where seismicity around the interface of the Philippine Sea plate had been very low until this earthquake. Since this earthquake occurred outside of seismic arrays, the focal mechanism and depth were not precisely constrained using a one-dimensional velocity model, as in a conventional approach. We conducted a moment tensor inversion of this earthquake by using a three-dimensional velocity structure model. Before the analysis of observed data, we investigated the effects of offshore heterogeneous structures such as the seawater, accretionary prism, and subducting oceanic plate by using synthetic seismograms in a full three-dimensional model and simpler models. The accretionary prism and subducting oceanic plate play important roles in the moment tensor inversion for offshore earthquakes in the subduction zone. Particularly, the accretionary prism, which controls the excitation and propagation of long-period surface waves around the offshore region, provides better estimations of the centroid depths and focal mechanisms of earthquakes around the Nankai subduction zone. The result of moment tensor inversion for the 2016 southeast offshore Mie earthquake revealed low-angle thrust faulting with a moment magnitude of 5.6. According to geophysical surveys in the Nankai Trough, our results suggest that the rupture of this earthquake occurred on the interface of the Philippine Sea plate, rather than on a mega-splay fault. Detailed comparisons of first-motion polarizations provided additional constraints of the rupture that occurred on the interface of the Philippine Sea plate.

Influence of increasing convergence obliquity and shallow slab geometry onto tectonic deformation and seismogenic behavior along the Northern Lesser Antilles zone

NASA Astrophysics Data System (ADS)

Laurencin, M.; Graindorge, D.; Klingelhoefer, F.; Marcaillou, B.; Evain, M.

2018-06-01

In subduction zones, the 3D geometry of the plate interface is one of the key parameters that controls margin tectonic deformation, interplate coupling and seismogenic behavior. The North American plate subducts beneath the convex Northern Lesser Antilles margin. This convergent plate boundary, with a northward increasing convergence obliquity, turns into a sinistral strike-slip limit at the northwestern end of the system. This geodynamic context suggests a complex slab geometry, which has never been imaged before. Moreover, the seismic activity and particularly the number of events with thrust focal mechanism compatible with subduction earthquakes, increases northward from the Barbuda-Anguilla segment to the Anguilla-Virgin Islands segment. One of the major questions in this area is thus to analyze the influence of the increasing convergence obliquity and the slab geometry onto tectonic deformation and seismogenic behavior of the subduction zone. Based on wide-angle and multichannel reflection seismic data acquired during the Antithesis cruises (2013-2016), we decipher the deep structure of this subduction zone. Velocity models derived from wide-angle data acquired across the Anegada Passage are consistent with the presence of a crust of oceanic affinity thickened by hotspot magmatism and probably affected by the Upper Cretaceous-Eocene arc magmatism forming the 'Great Arc of the Caribbean'. The slab is shallower beneath the Anguilla-Virgin Islands margin segment than beneath the Anguilla-Barbuda segment which is likely to be directly related to the convex geometry of the upper plate. This shallower slab is located under the forearc where earthquakes and partitioning deformations increase locally. Thus, the shallowing slab might result in local greater interplate coupling and basal friction favoring seismic activity and tectonic partitioning beneath the Virgin Islands platform.

Reconstructing Plate Boundaries in the Jurassic Neo-Tethys From the East and West Vardar Ophiolites (Greece and Serbia)

NASA Astrophysics Data System (ADS)

Maffione, Marco; van Hinsbergen, Douwe J. J.

2018-03-01

Jurassic subduction initiation in the Neo-Tethys Ocean eventually led to the collision of the Adria-Africa and Eurasia continents and the formation of an 6,000 km long Alpine orogen spanning from Iberia to Iran. Reconstructing the location and geometry of the plate boundaries of the now disappeared Neo-Tethys during the initial moments of its closure is instrumental to perform more realistic plate reconstructions of this region, of ancient ocean basins in general, and on the process of subduction initiation. Neo-Tethyan relics are preserved in an ophiolite belt distributed above the Dinaric-Hellenic fold-thrust belt. Here we provide the first quantitative constraints on the geometry of the spreading ridges and trenches active in the Jurassic Neo-Tethys using a paleomagnetically based net tectonic rotation analysis of sheeted dykes and dykes from the West and East Vardar Ophiolites of Serbia (Maljen and Ibar) and Greece (Othris, Pindos, Vourinos, and Guevgueli). Based on our results and existing geological evidence, we show that initial Middle Jurassic ( 175 Ma) closure of the western Neo-Tethys was accommodated at a N-S trending, west dipping subduction zone initiated near and parallel to the spreading ridge. The West Vardar Ophiolites formed in the forearc parallel to this new trench. Simultaneously, the East Vardar Ophiolites formed above a second N-S to NW-SE trending subduction zone located close to the European passive margin. We tentatively propose that this second subduction zone had been active since at least the Middle Triassic, simultaneously accommodating the closure of the Paleo-Tethys and the back-arc opening of Neo-Tethys.

Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino-Garberville-Covelo 30' x 60' quadrangles, northwest California

USGS Publications Warehouse

Ernst, W.G.; McLaughlin, Robert J.

2012-01-01

The Franciscan Complex is a classic subduction-zone assemblage. In northwest California, it comprises a stack of west vergent thrust sheets: westernmost Eastern Belt outliers; Central Belt mélange; Coastal Belt Yager terrane; Coastal Belt Coastal terrane; Coastal Belt King Range/False Cape terranes. We collected samples and determined P-T conditions of recrystallization for 88 medium-fine-grained metasandstones to assess their subduction-exhumation histories and assembly of the host allochthons. Feebly recrystallized Yager, Coastal, and King Range strata retain clear detrital features. Scattered neoblastic prehnite occurs in several Coastal terrane metasandstones; traces of possible pumpellyite are present in three Yager metaclastic rocks. Pumpellyite ± lawsonite ± aragonite-bearing Central Belt metasandstones are moderately deformed and reconstituted. Intensely contorted, thoroughly recrystallized Eastern Belt affinity quartzose metagraywackes contain lawsonite + jadeitic pyroxene ± aragonite ± glaucophane. We microprobed neoblastic phases in 23 rocks, documenting mineral parageneses that constrain the tectonic accretion and metamorphic P-T evolution of these sheets. Quasi-stable mineral assemblages typify Eastern Belt metasandstones, but mm-sized domains in the Central and Coastal belt rocks failed to achieve chemical equilibrium. Eastern Belt slabs rose from subduction depths approaching 25–30 km, whereas structurally lower Central Belt mélanges returned from ∼15–18 km. Coastal Belt assemblages suggest burial depths less than 5–8 km. Eastern and Central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin; K-feldspar-rich Coastal Belt rocks were stranded along the continental edge without undergoing appreciable subduction, probably during Paleogene unroofing of the older, deeply subducted units of the Franciscan Complex in east-vergent crustal wedges.

Pleistocene vertical motions of the Costa Rican outer forearc from subducting topography and a migrating fracture zone triple junction

USGS Publications Warehouse

Edwards, Joel H.; Kluesner, Jared W.; Silver, Eli A.; Bangs, Nathan L.

2018-01-01

Understanding the links between subducting slabs and upper-plate deformation is a longstanding goal in the field of tectonics. New 3D seismic sequence stratigraphy, mapped within the Costa Rica Seismogenesis Project (CRISP) seismic-reflection volume offshore southern Costa Rica, spatiotemporally constrains several Pleistocene outer forearc processes and provides clearer connections to subducting plate dynamics. Three significant shelf and/or slope erosional events at ca. 2.5–2.3 Ma, 1.95–1.78 Ma, and 1.78–1.19 Ma, each with notable differences in spatial extent, volume removed, and subsequent margin response, caused abrupt shifts in sedimentation patterns and rates. These shifts, coupled with observed deformation, suggest three primary mechanisms for Pleistocene shelf and slope vertical motions: (1) regional subaerial erosion and rapid subsidence linked to the southeastward Panama Fracture Zone triple-junction migration, with associated abrupt bathymetric variations and plate kinematic changes; (2) transient, kilometer-scale uplift and subsidence due to inferred subducting plate topography; and (3) progressive outer wedge shortening accommodated by landward- and seaward-dipping thrust faults and fold development due to the impinging Cocos Ridge. Furthermore, we find that the present-day wedge geometry (to within ∼3 km along strike) has been maintained through the Pleistocene, in contrast to modeled landward margin retreat. We also observe that deformation, i.e., extension and shortening, is decoupled from net margin subsidence. Our findings do not require basal erosion, and they suggest that the vertical motions of the Costa Rican outer forearc are not the result of a particular continuous process, but rather are a summation of plate to plate changes (e.g., passage of a fracture zone triple junction) and episodic events (e.g., subducting plate topography).

Large scale obduction of preserved oceanic crust: linking the Lesser Caucasus and NE Anatolian ophiolites and implications for the formation of the Lesser Caucasus-Pontides Arc

NASA Astrophysics Data System (ADS)

Hassig, Marc; Rolland, Yann; Sosson, Marc; Galoyan, Ghazar; Sahakyan, Lilit; Topuz, Gultelin; Farouk Çelik, Omer; Avagyan, Ara; Muller, Carla

2014-05-01

During the Mesozoic, the Southern margin of the Eurasian continent was involved in the closure of the Paleotethys and opening Neotethys Ocean. Later, from the Jurassic to the Eocene, subductions, obductions, micro-plate accretions, and finally continent-continent collision occurred between Eurasia and Arabia, and resulted in the closure of Neotethys. In the Lesser Caucasus and NE Anatolia three main domains are distinguished from South to North: (1) the South Armenian Block (SAB) and the Tauride-Anatolide Platform (TAP), Gondwanian-derived continental terranes; (2) scattered outcrops of ophiolite bodies, coming up against the Sevan-Akera and Ankara-Erzincan suture zones; and (3) the Eurasian plate, represented by the Eastern Pontides margin and the Somkheto-Karabagh Arc. The slivers of ophiolites are preserved non-metamorphic relics of the now disappeared Northern Neotethys oceanic domain overthrusting onto the continental South Armenian Block (SAB) as well as on the Tauride-Anatolide plateform from the north to the south. It is important to point out that the major part of this oceanic lithosphere disappeared by subduction under the Eurasian Margin to the north. In the Lesser Caucasus, works using geochemical whole-rock analyses, 40Ar/39Ar dating of basalts and gabbro amphiboles and paleontological dating have shown that the obducted oceanic domain originates from a back-arc setting formed throughout Middle Jurassic times. The comprehension of the geodynamic evolution of the Lesser Caucasus supports the presence of two north dipping subduction zones: (1) a subduction under the Eurasian margin and to the south by (2) an intra-oceanic subduction allowing the continental domain to subduct under the oceanic lithosphere, thus leading to ophiolite emplacement. To the West, the NE Anatolian ophiolites have been intensely studied with the aim to characterize the type of oceanic crust which they originated from. Geochemical analyses have shown similar rock types as in Armenia, Mid Ocean Ridge Basalt (MORB) to volcanic arc rocks and Intra-Plate Basalts (IPB). Lithostratigraphic comparisons have shown that the relations between the three units, well identified in the Lesser Caucasus, are similar to those found in NE Anatolia, including the emplacement of stratigraphically conform and discordant deposits. New field data has also shed light on an outcrop of low-grade metamorphic rocks of volcanic origin overthrusted by the ophiolites towards the south on the northern side of the Erzincan basin, along the North Anatolian Fault (NAF). We extend our model for the Lesser Caucasus to NE Anatolia and infer that the missing of the volcanic arc formed above the intra-plate subduction may be explained by its dragging under the obducting ophiolite with scaling by faulting and tectonic erosion. In this large scale model the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. We propose that the ophiolites of these two zones originate from the same oceanic domain and were emplaced during the same obduction event. This reconstructed ophiolitic nappe represents a preserved non-metamorphic oceanic domain over-thrusting up to 200km of continental domain along more than 500km. Distal outcrops of this exceptional object were preserved from latter collision which was concentrated along the suture zones.

Slab flattening and exhumation of the Eastern Cordillera of Colombia

NASA Astrophysics Data System (ADS)

Siravo, G.; Faccenna, C.; Fellin, M. G.; Herman, F.; Becker, T. W.; Molin, P.

2017-12-01

Mountain belt topography is shaped by processes acting at different time scales and depths, from the surface down to the crust and mantle. In particular, subduction dynamics is expected to strongly affect upper plate topography. Here, we present the case of the Eastern Cordillera (EC) in Colombia as a case history for dynamic mantle forcing from a subduction zone on the upper plate topography. The EC is an active double-vergent fold and thrust belt formed during the Cenozoic by the inversion of a Mesozoic rift, and topography there has grown up to 5000 m (Cocuy Sierra). The EC is located far ( 500 km) from the trench where the Nazca slab subducts below the South American plate. Tomography and seismicity show the presence of a flat slab subduction north of 5° N (Chiarabba et al., 2016). Slab flattening may have occurred transitionally from 10 to 5 Ma shutting down the arc volcanism (Wagner et al., 2017). We reconstruct the exhumation of the EC based on previously published and new thermochronologic data collected in the area of the Cocuy Sierra. Results of this analysis show notably fast exhumation rates since Late Miocene. We also analyze the likely contributions to topography and show that neither the present-day crustal thickness nor the cumulative shortening in the Cenozoic as extracted form balanced cross section can isostatically explain the present day topography. We conclude that fast EC exhumation and uplift are driven by mantle dynamics and likely occurred during the recent episode of slab flattening.

Tectonics of the ophiolite belt from Naga Hills and Andaman Islands, India

NASA Astrophysics Data System (ADS)

Acharyya, S. K.; Ray, K. K.; Sengupta, S.

1990-06-01

The ophiolitic rocks of Naga Hills-Andaman belt occur as rootless slices, gently dipping over the Paleogene flyschoid sediments, the presence of blue-schists in ophiolite melange indicates an involvement of the subduction process. Subduction was initiated prior to mid-Eocene as proved by the contemporaneous lower age limit of ophiolite-derived cover sediment as against the accreted ophiolites and olistostromal trench sediment. During the late Oligocene terminal collision between the Indian and Sino-Burmese blocks, basement slivers from the Sino-Burmese block, accreted ophiolites and trench sediments from the subduction zone were thrust westward as nappe and emplaced over the down-going Indian plate. The geometry of the ophiolites and the presence of a narrow negative gravity anomaly flanking their map extent, run counter to the conventional view that the Naga-Andaman belt marks the location of the suture. The root-zone of the ophiolite nappe representing the suture is marked by a partially-exposed eastern ophiolite belt of the same age and gravity-high zone, passing through central Burma-Sumatra-Java. The ophiolites of the Andaman and Naga Hills are also conventionally linked with the subduction activity, west of Andaman islands. This activity began only in late Miocene, much later than onland emplacement of the ophiolites; it further developed west of the suture in its southern part. Post-collisional northward movement of the Indian plate subparallel to the suture, also developed leaky dextral transcurrent faults close to the suture and caused Neogene-Quatemary volcanism in central Burma and elsewhere.

Parameterization of 18th January 2011 earthquake in Dalbadin Region, Southwest Pakistan

NASA Astrophysics Data System (ADS)

Shafiq-Ur-Rehman; Azeem, Tahir; Abd el-aal, Abd el-aziz Khairy; Nasir, Asma

2013-12-01

An earthquake of magnitude 7.3 Mw occurred on 18th January 2011 in Southwestern Pakistan, Baluchistan province (Dalbadin Region). The area has complex tectonics due to interaction of Indian, Eurasian and Arabian plates. Both thrust and strike slip earthquakes are dominant in this region with minor, localized normal faulting events. This earthquake under consideration (Dalbadin Earthquake) posed constraints in depth and focal parameters due to lack of data for evaluation of parameters from Pakistan, Iran or Afghanistan region. Normal faulting mechanism has been proposed by many researchers for this earthquake. In the present study the earthquake was relocated using the technique of travel time residuals. Relocated coordinates and depth were utilized to calculate the focal mechanism solution with outcome of a dominant strike slip mechanism, which is contrary to normal faulting. Relocated coordinates and resulting mechanism are more reliable than many reporting agencies as evaluation in this study is augmented by data from local seismic monitoring network of Pakistan. The tectonics in the area is governed by active subduction along the Makran Subduction Zone. This particular earthquake has strike slip mechanism due to breaking of subducting oceanic plate. This earthquake is located where oceanic lithosphere is subducting along with relative movements between Lut and Helmand blocks. Magnitude of this event i.e. Mw = 7.3, re evaluated depth and a previous study of mechanism of earthquake in same region (Shafiq et al., 2011) also supports the strike slip movement.

Subduction- and exhumation-related structures in the Cycladic Blueschists: Insights from south Evia Island (Aegean region, Greece)

NASA Astrophysics Data System (ADS)

Xypolias, P.; Iliopoulos, I.; Chatzaras, V.; Kokkalas, S.

2012-04-01

Detailed geological mapping, structural investigation and amphibole chemistry analyses in southern Evia (Aegean Sea, Greece) allow us to place new constraints on the internal structural architecture and tectonic evolution of the Cycladic Blueschists. We show that the early deformation history was related to ESE directed thrusting resulting in the stacking of the Styra and Ochi nappes, which constitute the Cycladic Blueschist unit in Evia. These early thrust movements initiated just before and proceeded at peak conditions of the Eocene high-pressure metamorphism. Subsequent constrictional deformation gave rise to E-W trending upright folding accomplished at the early exhumation stage. The main ductile-stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite. This phase was characterized by localization of ductile deformation into a series of major, tens of meters thick, ENE directed shear zones, which cut up-section in their transport direction and restack the early thrust and fold sequence, locally bringing the structurally lower Styra nappe over the higher Ochi nappe. It is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the Oligocene ENE-ward extrusion of the blueschists. Brittle-ductile NE dipping normal faulting of post-early Miocene age was probably responsible for the final exhumation of rocks.

Kinematics of the active West Andean fold-and-thrust belt (central Chile): Structure and long-term shortening rate

NASA Astrophysics Data System (ADS)

Riesner, M.; Lacassin, R.; Simoes, M.; Armijo, R.; Rauld, R.; Vargas, G.

2017-02-01

West verging thrusts, synthetic with the Nazca-South America subduction interface, have been recently discovered at the western front of the Andes. At 33°30'S, the active San Ramón fault stands as the most frontal of these west verging structures and represents a major earthquake threat for Santiago, capital city of Chile. Here we elaborate a detailed 3-D structural map and a precise cross section of the West Andean fold-and-thrust belt based on field observations, satellite imagery, and previous structural data, together with digital topography. We then reconstruct the evolution of this frontal belt using a trishear kinematic approach. Our reconstruction implies westward propagation of deformation with a total shortening of 9-15 km accumulated over the last 25 Myr. An overall long-term shortening rate of 0.1-0.5 mm/yr is deduced. The maximum value of this shortening rate compares well with the rate that may be inferred from recent trench data across the San Ramón fault and the slip associated with the past two Mw > 7 earthquakes. This suggests that the San Ramón fault is most probably the only presently active fault of the West Andean fold-and-thrust-belt and that most—if not all—the deformation is to be released seismically.

Analog modelling of obduction processes

NASA Astrophysics Data System (ADS)

Agard, P.; Zuo, X.; Funiciello, F.; Bellahsen, N.; Faccenna, C.; Savva, D.

2012-04-01

Obduction corresponds to one of plate tectonics oddities, whereby dense, oceanic rocks (ophiolites) are presumably 'thrust' on top of light, continental ones, as for the short-lived, almost synchronous Peri-Arabic obduction (which took place along thousands of km from Turkey to Oman in c. 5-10 Ma). Analog modelling experiments were performed to study the mechanisms of obduction initiation and test various triggering hypotheses (i.e., plate acceleration, slab hitting the 660 km discontinuity, ridge subduction; Agard et al., 2007). The experimental setup comprises (1) an upper mantle, modelled as a low-viscosity transparent Newtonian glucose syrup filling a rigid Plexiglas tank and (2) high-viscosity silicone plates (Rhodrosil Gomme with PDMS iron fillers to reproduce densities of continental or oceanic plates), located at the centre of the tank above the syrup to simulate the subducting and the overriding plates - and avoid friction on the sides of the tank. Convergence is simulated by pushing on a piston at one end of the model with velocities comparable to those of plate tectonics (i.e., in the range 1-10 cm/yr). The reference set-up includes, from one end to the other (~60 cm): (i) the piston, (ii) a continental margin containing a transition zone to the adjacent oceanic plate, (iii) a weakness zone with variable resistance and dip (W), (iv) an oceanic plate - with or without a spreading ridge, (v) a subduction zone (S) dipping away from the piston and (vi) an upper, active continental margin, below which the oceanic plate is being subducted at the start of the experiment (as is known to have been the case in Oman). Several configurations were tested and over thirty different parametric tests were performed. Special emphasis was placed on comparing different types of weakness zone (W) and the extent of mechanical coupling across them, particularly when plates were accelerated. Displacements, together with along-strike and across-strike internal deformation in all plates were systematically measured, allowing for a very precise and reproducible tracking of deformation. Experiments demonstrate that obduction chiefly depends on how the overall shortening (or convergence) is partitionned between the weakness zone (W) and the preexisting subduction zone (S). Conditions favorable to obduction are shown to correspond to a specific range of coupling across (S) and resistance across (W). Our results thereby (1) constrain the range of physical conditions required for obduction to develop/nucleate and (2) underline the key role of acceleration for triggering obduction (rather than ridge subduction or slab resistance to penetration at the 660 km discontinuity). They also demonstrate that the emplacement of dense, oceanic material on continental lithosphere is not a mysterious process but results from some large scale, normal subduction process that do not require exotic boundary conditions. Agard P., Jolivet L., Vrielynck B., Burov E. & Monié P., 2007. Plate acceleration : the obduction trigger? Earth and Planetary Science Letters, 258, 428-441.

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc

NASA Astrophysics Data System (ADS)

Laigle, M.; Hirn, A.; Sapin, M.; Bécel, A.; Charvis, P.; Flueh, E.; Diaz, J.; Lebrun, J.-F.; Gesret, A.; Raffaele, R.; Galvé, A.; Evain, M.; Ruiz, M.; Kopp, H.; Bayrakci, G.; Weinzierl, W.; Hello, Y.; Lépine, J.-C.; Viodé, J.-P.; Sachpazi, M.; Gallart, J.; Kissling, E.; Nicolich, R.

2013-09-01

The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10-20° under the forearc domain, and then to 60° from 70 km depth. At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: "supraslab" earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and "deep flat-thrust" earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle. None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is expected to be serpentinized by the fluids provided from the dehydrating slab beneath. This process is commonly considered as limiting the downward extent of the interplate coupling. Interpretations are not readily available either for the large crustal thickness of this shallow water marine upper plate, except when remarking its likeness to oceanic plateaus formed above hotspots. The Caribbean Oceanic Plateau of the upper plate has been formed earlier by the material advection from a mantle plume. It could then be underlain by a correspondingly modified, heterogeneous mantle, which may include pyroxenitic material among peridotites. Such heterogeneity in the mantle corner of the present subduction zone may account for the notable peculiarities in seismic structure and activity and impose regions of stick-slip behavior on the interplate among stable-gliding areas.

Tectonic setting for ophiolite obduction in Oman.

USGS Publications Warehouse

Coleman, R.G.

1981-01-01

The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the N boundary of the Arabian-African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian-African plate drifted northward about 10o. These events, combined with the opposite rotation of Eurasia and Africa, initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin, combined with the compressional forces of closure from the Eurasian plate, initiated obduction of the Tethyan oceanic crust along preexisting transform faults and still-hot oceanic crust was detached along oblique NE dipping thrust faults. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large-scale island arc volcanism did not commence until after the Tethyan sea began to close and the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics and detached fragments of the basal amphibolites, which in turn rest on autochthonous shelf carbonates of the Arabian platform. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just N of the present Jaz Murian depression in Iran.-Author

Seismicity and the nature of plate movement along the Himalayan arc, Northeast India and Arakan-Yoma: a review

NASA Astrophysics Data System (ADS)

Verma, R. K.; Kumar, G. V. R. Krishna

1987-03-01

The Himalaya together with Arakan-Yoma form a well defined seismic belt to the north and east of the Indian Peninsula. The Seismicity along this belt is attributed mostly to collision between the Indian and the Eurasian plates. However, the exact nature of activity along the major thrusts and faults is not well understood. The seismicity along the entire Himalaya and Northern Burma has been studied in detail. It has been found that besides the Main Boundary Fault and the Main Central Thrust several transverse features are also very active. Some of these behave like steeply dipping fracture zones. Along the Arakan-Yoma most of the seismicity appears to be due to subduction of the Indian lithosphere to the east. Analysis of focal mechanism solutions for the Himalaya shows that although thrust movements are predominant, normal and strike-slip faulting is taking place along some of the transverse features. In addition to thrusting, strike-slip faulting is also taking place along the Arakan-Yoma. Orientation of P-axes for all thrust solutions show a sharp change from predominantly east-west along the Burmese arc to N-S and NE-SW along the Himalaya. The direction further changes to NW-SE along the Baluchistan arc. It appears that the Indian lithosphere is under compression from practically all sides. The present day seismicity of Northeast India and Northern Burma can be explained in terms of a plate tectonics model after Nandy (1976). No simple model appears to be applicable for the entire Himalaya.

Scale independence of décollement thrusting

USGS Publications Warehouse

McBride, John H.; Pugin, Andre J.M.; Hatcher, Robert D.

2007-01-01

Orogen-scale décollements (detachment surfaces) are an enduring subject of investigation by geoscientists. Uncertainties remain as to how crustal convergence processes maintain the stresses necessary for development of low-angle fault surfaces above which huge slabs of rock are transported horizontally for tens to hundreds of kilometers. Seismic reflection profiles from the southern Appalachian crystalline core and several foreland fold-and-thrust belts provide useful comparisons with high-resolution shallow-penetration seismic reflection profiles acquired over the frontal zone of the Michigan lobe of the Wisconsinan ice sheet northwest of Chicago, Illinois. These profiles provide images of subhorizontal and overlapping dipping reflections that reveal a ramp-and-flat thrust system developed in poorly consolidated glacial till. The system is rooted in a master décollement at the top of bedrock. These 2–3 km long images contain analogs of images observed in seismic reflection profiles from orogenic belts, except that the scale of observation in the profiles in glacial materials is two orders of magnitude less. Whereas the décollement beneath the ice lobe thrust belt lies ∼70 m below thrusted anticlines having wavelengths of tens of meters driven by an advancing ice sheet, seismic images from overthrust terranes are related to lithospheric convergence that produces décollements traceable for thousands of kilometers at depths ranging from a few to over 10 km. Dual vergence or reversals in vergence (retrocharriage) that developed over abrupt changes in depth to the décollement can be observed at all scales. The strikingly similar images, despite the contrast in scale and driving mechanism, suggest a scale- and driving mechanism–independent behavior for décollement thrust systems. All these systems initially had the mechanical properties needed to produce very similar geometries with a compressional driving mechanism directed subparallel to Earth's surface. Subduction-related accretionary complexes also produce thrust systems with similar geometries in semi- to unconsolidated materials.

Active Tectonics of Himalayan Faults/Thrusts System in Northern India on the basis of recent & Paleo earthquake Studies

NASA Astrophysics Data System (ADS)

Kumar, S.; Biswal, S.; Parija, M. P.

2016-12-01

The Himalaya overrides the Indian plate along a decollement fault, referred as the Main Himalayan Thrust (MHT). The 2400 km long Himalayan mountain arc in the northern boundary of the Indian sub-continent is one of the most seismically active regions of the world. The Himalayan Frontal Thrust (HFT) is characterized by an abrupt physiographic and tectonic break between the Himalayan front and the Indo-Gangetic plain. The HFT represents the southern surface expression of the MHT on the Himalayan front. The tectonic zone between the Main Boundary Thrust (MBT) and the HFT encompasses the Himalayan Frontal Fault System (HFFS). The zone indicates late Quaternary-Holocene active deformation. Late Quaternary intramontane basin of Dehradun flanked to the south by the Mohand anticline lies between the MBT and the HFT in Garhwal Sub Himalaya. Slip rate 13-15 mm/yr has been estimated on the HFT based on uplifted strath terrace on the Himalyan front (Wesnousky et al. 2006). An out of sequence active fault, Bhauwala Thrust (BT), is observed between the HFT and the MBT. The Himalayan Frontal Fault System includes MBT, BT, HFT and PF active fault structures (Thakur, 2013). The HFFS structures were developed analogous to proto-thrusts in subduction zone, suggesting that the plate boundary is not a single structure, but series of structures across strike. Seismicity recorded by WIHG shows a concentrated belt of seismic events located in the Main Central Thrust Zone and the physiographic transition zone between the Higher and Lesser Himalaya. However, there is quiescence in the Himalayan frontal zone where surface rupture and active faults are reported. GPS measurements indicate the segment between the southern extent of microseismicity zone and the HFT is locked. The great earthquake originating in the locked segment rupture the plate boundary fault and propagate to the Himalaya front and are registered as surface rupture reactivating the fault in the HFFS.

Detailed source process of the 2007 Tocopilla earthquake.

NASA Astrophysics Data System (ADS)

Peyrat, S.; Madariaga, R.; Campos, J.; Asch, G.; Favreau, P.; Bernard, P.; Vilotte, J.

2008-05-01

We investigated the detail rupture process of the Tocopilla earthquake (Mw 7.7) of the 14 November 2007 and of the main aftershocks that occurred in the southern part of the North Chile seismic gap using strong motion data. The earthquake happen in the middle of the permanent broad band and strong motion network IPOC newly installed by GFZ and IPGP, and of a digital strong-motion network operated by the University of Chile. The Tocopilla earthquake is the last large thrust subduction earthquake that occurred since the major Iquique 1877 earthquake which produced a destructive tsunami. The Arequipa (2001) and Antofagasta (1995) earthquakes already ruptured the northern and southern parts of the gap, and the intraplate intermediate depth Tarapaca earthquake (2005) may have changed the tectonic loading of this part of the Peru-Chile subduction zone. For large earthquakes, the depth of the seismic rupture is bounded by the depth of the seismogenic zone. What controls the horizontal extent of the rupture for large earthquakes is less clear. Factors that influence the extent of the rupture include fault geometry, variations of material properties and stress heterogeneities inherited from the previous ruptures history. For subduction zones where structures are not well known, what may have stopped the rupture is not obvious. One crucial problem raised by the Tocopilla earthquake is to understand why this earthquake didn't extent further north, and at south, what is the role of the Mejillones peninsula that seems to act as a barrier. The focal mechanism was determined using teleseismic waveforms inversion and with a geodetic analysis (cf. Campos et al.; Bejarpi et al., in the same session). We studied the detailed source process using the strong motion data available. This earthquake ruptured the interplate seismic zone over more than 150 km and generated several large aftershocks, mainly located south of the rupture area. The strong-motion data show clearly two S-waves arrivals, allowing the localization of the 2 sources. The main shock started north of the segment close to Tocopilla. The rupture propagated southward. The second source was identified to start about 20 seconds later and was located 50 km south from the hypocenter. The network configuration provides a good resolution for the inverted slip distribution in the north-south direction, but a lower resolution for the east-west extent of the slip. However, this study of the source process of this earthquake shows a complex source with at least two slip asperities of different dynamical behavior.

Strength of plate coupling in the southern Ryukyu subduction zone

NASA Astrophysics Data System (ADS)

Doo, Wen-Bin; Lo, Chung-Liang; Wu, Wen-Nan; Lin, Jing-Yi; Hsu, Shu-Kun; Huang, Yin-Sheng; Wang, Hsueh-Fen

2018-01-01

Understanding the strength of a plate coupling is critical for assessing potential seismic and tsunamic hazards in subduction zones. The interaction between an overriding plate and the associated subducting plate can be used to evaluate the strength of plate coupling by examining the mantle lithospheric buoyancy. Here, we calculate the mantle lithosphere buoyancy across the northern portion of the southern Ryukyu subduction zone based on gravity modeling with the constraints from a newly derived P-wave seismic velocity model. The result indicates that the strength of the plate coupling in the study area is relatively strong, which is consistent with previous observations in the southernmost Ryukyu subduction zone. Because few large earthquakes (Mw > 7) have occurred in the southern Ryukyu subduction zone, a large amount of energy is locked and accumulated by plate coupling, that could be released in the near future.

The impact of the South-American plate motion and the Nazca Ridge subduction on the flat subduction below South Peru

NASA Astrophysics Data System (ADS)

van Hunen, Jeroen; van den Berg, Arie P.; Vlaar, Nico J.

2002-07-01

Flat subduction near Peru occurs only where the thickened crust of the Nazca Ridge subducts. Furthermore, the South-America continent shows a westward absolute plate motion. Both the overriding motion of South-America and the subduction of the Nazca Ridge have been proposed to explain the flat slab segment below South Peru. We have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms. Results suggest that the average upper mantle viscosity should be about 3.5 × 1020 Pa s or less and basaltic crust should be able to survive 600 to 800°C ambient temperature before transforming into eclogite to explain the slab geometry below Peru. The effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction.

Intraslab rupture triggering megathrust rupture coseismically in the 17 December 2016 Solomon Islands Mw 7.9 earthquake

NASA Astrophysics Data System (ADS)

Lay, Thorne; Ye, Lingling; Ammon, Charles J.; Kanamori, Hiroo

2017-02-01

The 17 December 2016 Solomon Islands earthquake (Mw 7.9) initiated 103 km deep in the subducting Solomon Sea slab near the junction of the Solomon Islands and New Britain trenches. Most aftershocks are located near the Solomon Islands plate boundary megathrust west of Bougainville, where previous large interplate thrust faulting earthquakes occurred in 1995 (Mw 7.7) and 1971 (Mw 8.0). Teleseismic body wave modeling and aftershock relocations indicate that the initial 30 s of the 2016 rupture occurred over depths of 90 to 120 km on an intraslab fault dipping 30° to the southwest, almost perpendicular to the dipping slab interface. The next 50 s of rupture took place at depths of 32 to 47 km in the deeper (Domain C) portion of the overlying megathrust fault dipping 35° to the northeast. High susceptibility to triggering in the region accounts for this compound rupture of two separate fault planes.

The 2013 Crete (Hellenic Arc) Earthquake Sequence

NASA Astrophysics Data System (ADS)

Karakostas, V. G.; Papadimitriou, E. E.; Vallianatos, F.

2014-12-01

The western Hellenic Arc is a well known place of active interplate deformation, where the convergence motion vector is perpendicular to the subduction front. On 12 October 2013 this area was hit by a strong (Mw=6.7) earthquake, occurred on a thrust fault onto the coupled part of the overriding and descending plates, with the compression axis being oriented in the direction of plate convergence. This was the first strong (M>6.0) event to have occurred onto this segment of the descending slab, which has accommodated the largest (M8.3) known earthquake in the Mediterranean area, and to be recorded by the Hellenic Unified Seismological Network (HUSN) that has been considerably improved in the last five years. The first 2-days relocated seismicity shows activation of the upper part of the descending slab, downdip of the plate interface and forming a relatively narrow aftershock area on map view. The less densely visited by aftershocks area, where the main shock is also encompassed, is considered as the high-slip area along the downdip portion of the subducting plane. Dense concentration of the intraslab aftershocks are probably due to the increase of static stress generated by the main shock. A spectacular feature of the aftershock activity concerns the lateral extension of the slipped area, which appears very sharply defined. This provides evidence on localized coupling and aseismically creeping areas, explaining the low coupling ratio in the Hellenic Arc, as it derives from comparison between relative plate motion and seismic energy release. Elucidating the issue of how far the associated large-slip zone might be extended along the plate interface during the main rupture is crucial in assessing future earthquake hazards from subduction events in the study area. This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the "THALES Program: SEISMO FEAR HELLARC" project.

Structural interpretation and physical property estimates based on COAST 2012 seismic reflection profiles offshore central Washington, Cascadia subduction zone

NASA Astrophysics Data System (ADS)

Webb, S. I.; Tobin, H. J.; Everson, E. D.; Fortin, W.; Holbrook, W. S.; Kent, G.; Keranen, K. M.

2014-12-01

The Cascadia subduction zone has a history of large magnitude earthquakes, but a near-total lack of plate interface seismicity, making the updip limit of the seismogenic zone difficult to locate. In addition, the central Cascadia accretionary prism is characterized by an extremely low wedge taper angle, landward vergent initial thrusting, and a flat midslope terrace between the inner and outer wedges, unlike most other accretionary prisms (e.g. the Nankai Trough, Japan). The Cascadia Open Access Seismic Transect (COAST) lines were shot by R/V Marcus Langseth in July of 2012 off central Washington to image this subduction zone. Two trench-parallel and nine trench-perpendicular lines were collected. In this study, we present detailed seismic interpretation of both time- and depth-migrated stacked profiles, focused on elucidating the deposition and deformation of both pre- and syn-tectonic sediment in the trench and slope. Distribution and timing of sediments and their deformation is used to unravel the evolution of the wedge through time. Initially, interpretation of the time-sections is carried out to support the building of tomographic velocity models to aid in the pre-stack depth migration (PSDM) of selected lines. In turn, we use PSDM velocity models to estimate porosity and pore pressure conditions at the base of the wedge and across the basal plate interface décollement where possible, using established velocity-porosity transforms. Interpretation in this way incorporates both accurate structural relationships and robust porosity models to document wedge development and present-day stress state, in particular regions of potential overpressure. Results shed light on the origin and evolution of the mid-slope terrace and the low taper angle for the forearc wedge. This work may shed light ultimately on the position of the potential updip limit of the seismogenic zone beneath the wedge.

Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland

USGS Publications Warehouse

Johnson, S.Y.; Blakely, R.J.; Stephenson, W.J.; Dadisman, S.V.; Fisher, M.A.

2004-01-01

Margin-parallel shortening of the Cascadia forearc is a consequence of oblique subduction of the Juan de Fuca plate beneath North America. Strike-slip, thrust, and oblique crustal faults beneath the densely populated Puget Lowland accommodate much of this north-south compression, resulting in large crustal earthquakes. To better understand this forearc deformation and improve earthquake hazard, assessment, we here use seismic reflection surveys, coastal exposures of Pleistocene strata, potential-field data, and airborne laser swath mapping to document and interpret a significant structural boundary near the City of Tacoma. This boundary is a complex structural zone characterized by two distinct segments. The northwest trending, eastern segment, extending from Tacoma to Carr Inlet, is formed by the broad (??? 11.5 km), southwest dipping (??? 11??-2??) Rosedale monocline. This monocline raises Crescent Formation basement about 2.5 km, resulting in a moderate gravity gradient. We interpret the Rosedale monocline as a fault-bend fold, forming above a deep thrust fault. Within the Rosedale monocline, inferred Quaternary strata thin northward and form a growth triangle that is 4.1 to 6.6 km wide at its base, suggesting ??? 2-3 mm/yr of slip on the underlying thrust. The western section of the >40-km-long, north dipping Tacoma fault, extending from Hood Canal to Carr Inlet, forms the western segment of the Tacoma basin margin. Structural relief on this portion of the basin margin may be several kilometers, resulting in steep gravity and aeromagnetic anomalies. Quaternary structural relief along the Tacoma fault is as much as 350-400 m, indicating a minimum slip rate of about 0.2 mm/yr. The inferred eastern section of the Tacoma fault (east of Carr Inlet) crosses the southern part of the Seattle uplift, has variable geometry along strike, and diminished structural relief. The Tacoma fault is regarded as a north dipping backthrust to the Seattle fault, so that slip on a master thrust fault at depth could result in movement on the Seattle fault, the Tacoma fault, or both.

Structural and stratigraphic constraints on tsunamigenic rupture along the frontal Sunda megathrust from MegaTera bathymetric and seismic reflection data

NASA Astrophysics Data System (ADS)

Bradley, K. E.; Qin, Y.; Villanueva-Robles, F.; Hananto, N.; Leclerc, F.; Singh, S. C.; Tapponnier, P.; Sieh, K.; Wei, S.; Carton, H. D.; Permana, H.; Avianto, P.; Nugroho, A. B.

2017-12-01

The joint EOS/IPG/LIPI 2015 MegaTera expedition collected high-resolution seismic reflection profiles and bathymetric data across the Sunda trench, updip of the Mw7.7, 2010 Mentawai tsunami-earthquake rupture patch. These data reveal rapid lateral variations in both the stratigraphic level of the frontal Sunda megathrust and the vergence of frontal ramp faults. The stratigraphic depth of the megathrust at the deformation front correlates with ramp-thrust vergence and with changes in the basal friction angle inferred by critical-taper wedge theory. Where ramp thrusts verge uniformly seaward and have an average dip of 30°, the megathrust decollement resides atop a high-amplitude reflector that marks the inferred top of pelagic sediments. Where ramp thrusts are bi-vergent (similar throw on both landward- and seaward-vergent faults) and have an average dip of 42°, the decollement is higher, within the incoming clastic sequence, above a seismically transparent unit inferred to represent distal fan muds. Where ramp thrusts are uniformly landward vergent, the decollement sits directly on top of the oceanic crust that forms the bathymetrically prominent, subducting Investigator Ridge. The two, separate regions of large tsunamigenic ground-surface uplift during the 2010 tsunami earthquake that have been inferred from joint inversions of seismic, GPS, and tsunami data (e.g. Yue et al., 2014; Satake et al., 2013) correspond to the areas of frontal bi-vergence in the MegaTera data. We propose that enhanced surface uplift and tsunamigenesis during this event occurred when rupture propagated onto areas where the decollement sits directly above the basal muds of the incoming clastic sequence. Thus we hypothesize that frontal bi-vergence may mark areas of enhanced tsunami hazard posed by small magnitude, shallow megathrust ruptures that propagate to the trench. [Yue, H. et al., 2014, Rupture process of the…, JGR 119 doi:10.1002/2014JB011082; Satake, K. et al., 2013, Tsunami Source of the…, P&AG 170, 9-10

Large-scale deformation related to the collision of the Aleutian Arc with Kamchatka

USGS Publications Warehouse

Gesit, Eric L.; Scholl, David W.

1994-01-01

The far western Aleutian Island Arc is actively colliding with Kamchatka. Westward motion of the Aleutian Arc is brought about by the tangential relative motion of the Pacific plate transferred to major, right-lateral shear zones north and south of the arc. Early geologic mapping of Cape Kamchatka (a promontory of Kamchatka along strike with the Aleutian Arc) revealed many similarities to the geology of the Aleutian Islands. Later studies support the notion that Cape Kamchatka is the farthest west Aleutian “island” and that it has been accreted to Kamchatka by the process of arc-continent collision. Deformation associated with the collision onshore Kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rocks directly west of Cape Kamchatka. The trend of the thrust faults is concave toward the collision zone, indicating a radial distribution of maximum horizontal compressive stress. Offshore, major crustal faults trend either oblique to the Kamchatka margin or parallel to major Aleutian shear zones. These offshore faults are complex, accommodating both strike-slip and thrust displacements as documented by focal mechanisms and seismic reflection data. Earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost Aleutian shear zone and to the west by an apparent aseismic front. Analysis of focal mechanisms in the region indicate that the present-day arc-continent “contact zone” is located directly east of Cape Kamchatka. In modeling the dynamics of the collision zone using thin viscous sheet theory, the rheological parameters are only partially constrained to values of n (the effective power law exponent) ≥ 3 and Ar(the Argand number) ≤ 30. These values are consistent with a forearc thermal profile of Kamchatka, previously determined from heat flow modeling. The thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence, not only of compressive stresses resulting from the west directed collision, but also of sediment-induced coupling of the subducting Pacific plate.

Interseismic coupling, seismic potential and earthquake recurrence on the southern front of the Eastern Alps (NE Italy)

NASA Astrophysics Data System (ADS)

Cheloni, Daniele; D'Agostino, Nicola; Selvaggi, Giulio

2014-05-01

The interaction of the African, Arabian, and Eurasia plates in the "greater" Mediterranean region yields to a broad range of tectonic processes including active subduction, continental collision, major continental strike-slip faults and "intra-plate" mountain building. In this puzzling region the convergence between Adria microplate and Eurasia plate is partly or entirely absorbed within the South-Eastern Alps, where the Adriatic lithosphere underthrusts beneath the mountain belt. Historical seismicity and instrumentally recorded earthquakes show thrust faulting on north-dipping low-angle faults in agreement with geological observations of active mountain building and active fold growing at the foothills of the South-Eastern Alps. In this study, we use continuous GPS observations to document the geodetic strain accumulation across the South-Eastern Alps (NE Italy). We estimate the pattern of interseismic coupling on the intra-continental collision north-dipping thrust faults that separate the Eastern Alps and the Venetian-Friulian plain using the back-slip approach and discuss the seismic potential and earthquake recurrence. Comparison between the rigid-rotation predicted motion and the shortening observed across the studied area indicates that the South-Eastern Alpine thrust front absorbs about 80% of the total convergence rate between the Adria microplate and Eurasia plate. The modelled thrust fault is currently locked from the surface to a depth of approximately 10 km. The transition zone between locked and creeping portions of the fault roughly corresponds with the belt of microseismicity parallel and to the north of the mountain front. The estimated moment deficit rate is 1.27±0.14×10^17 Nm/yr. The comparison between the estimated moment deficit and that released historically by the earthquakes suggests that to account for the moment deficit the following two factors or their combination should be considered: (1) a significant part of the observed interseismic coupling is released aseismically by folding or creeping; (2) infrequent "large" events with long return period (>1000 years) and with magnitudes larger than the value assigned to the largest historical events (Mw≡6.7).

Dynamic Passage of Topography Beneath the Southern Costa Rica Forearc seen with Seismic Stratigraphy

NASA Astrophysics Data System (ADS)

Edwards, J. H.; Kluesner, J. W.; Silver, E. A.

2014-12-01

3D seismic reflection data (CRISP) collected across the southern Costa Rica margin reveals that a thick, deforming sedimentary wedge underlies the younger slope sediments (Silver et al., this meeting). The older wedge material and younger slope sediments are separated by a high-amplitude regional unconformity. Seismic stratigraphy of the sedimentary strata overlying this regional unconformity reflects a dynamic deformation history of the margin. The younger slope sediments contain series of more localized unconformities, separating sedimentary units as thick as 1 km that reveal a dynamically changing set of inverted, overlapping basins. The geometry of these overlapping, inverted basins indicate sequential uplift events. The direction of basin thickening varies upsection, and these basins are cut by both thrust and normal faults and are deformed by folding. Structural development appears to be controlled by relief on the subducting plate interface, which induces uplift and subsidence and thereby controls the pattern of erosion and deposition. We interpret the evolution of these inverted stratigraphic packages as forming from subducting topography. Correlating these seismic-stratigraphic packages to recent drilling based on preliminary magnetostratigraphy from IODP site U1413 (Expedition 344 Scientists, 2013), allows us to date the passage of the subducting plate topography beginning ~2 Ma.

Tectonic controls on earthquake size distribution and seismicity rate: slab buoyancy and slab bending

NASA Astrophysics Data System (ADS)

Nishikawa, T.; Ide, S.

2014-12-01

There are clear variations in maximum earthquake magnitude among Earth's subduction zones. These variations have been studied extensively and attributed to differences in tectonic properties in subduction zones, such as relative plate velocity and subducting plate age [Ruff and Kanamori, 1980]. In addition to maximum earthquake magnitude, the seismicity of medium to large earthquakes also differs among subduction zones, such as the b-value (i.e., the slope of the earthquake size distribution) and the frequency of seismic events. However, the casual relationship between the seismicity of medium to large earthquakes and subduction zone tectonics has been unclear. Here we divide Earth's subduction zones into over 100 study regions following Ide [2013] and estimate b-values and the background seismicity rate—the frequency of seismic events excluding aftershocks—for subduction zones worldwide using the maximum likelihood method [Utsu, 1965; Aki, 1965] and the epidemic type aftershock sequence (ETAS) model [Ogata, 1988]. We demonstrate that the b-value varies as a function of subducting plate age and trench depth, and that the background seismicity rate is related to the degree of slab bending at the trench. Large earthquakes tend to occur relatively frequently (lower b-values) in shallower subduction zones with younger slabs, and more earthquakes occur in subduction zones with deeper trench and steeper dip angle. These results suggest that slab buoyancy, which depends on subducting plate age, controls the earthquake size distribution, and that intra-slab faults due to slab bending, which increase with the steepness of the slab dip angle, have influence on the frequency of seismic events, because they produce heterogeneity in plate coupling and efficiently inject fluid to elevate pore fluid pressure on the plate interface. This study reveals tectonic factors that control earthquake size distribution and seismicity rate, and these relationships between seismicity and tectonic properties may be useful for seismic risk assessment.

Seismic hazard analysis for Jayapura city, Papua

DOE Office of Scientific and Technical Information (OSTI.GOV)

Robiana, R., E-mail: robiana-geo104@yahoo.com; Cipta, A.

Jayapura city had destructive earthquake which occurred on June 25, 1976 with the maximum intensity VII MMI scale. Probabilistic methods are used to determine the earthquake hazard by considering all possible earthquakes that can occur in this region. Earthquake source models using three types of source models are subduction model; comes from the New Guinea Trench subduction zone (North Papuan Thrust), fault models; derived from fault Yapen, TareraAiduna, Wamena, Memberamo, Waipago, Jayapura, and Jayawijaya, and 7 background models to accommodate unknown earthquakes. Amplification factor using geomorphological approaches are corrected by the measurement data. This data is related to rock typemore » and depth of soft soil. Site class in Jayapura city can be grouped into classes B, C, D and E, with the amplification between 0.5 – 6. Hazard maps are presented with a 10% probability of earthquake occurrence within a period of 500 years for the dominant periods of 0.0, 0.2, and 1.0 seconds.« less

Great (≥Mw8.0) megathrust earthquakes and the subduction of excess sediment and bathymetrically smooth seafloor

USGS Publications Warehouse

Scholl, David W.; Kirby, Stephe H.; von Huene, Roland E.; Ryan, Holly F.; Wells, Ray E.; Geist, Eric L.

2015-01-01

However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of high-magnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation.

Activated Very Low Frequency Earthquakes By the Slow Slip Events in the Ryukyu Subduction Zone

NASA Astrophysics Data System (ADS)

Nakamura, M.; Sunagawa, N.

2014-12-01

The Ryukyu Trench (RT), where the Philippine Sea plate is subducting, has had no known thrust earthquakes with a Mw>8.0 in the last 300 years. However, the rupture source of the 1771 tsunami has been proposed as an Mw > 8.0 earthquake in the south RT. Based on the dating of tsunami boulders, it has been estimated that large tsunamis occur at intervals of 150-400 years in the south Ryukyu arc (RA) (Araoka et al., 2013), although they have not occurred for several thousand years in the central and northern Ryukyu areas (Goto et al., 2014). To address the discrepancy between recent low moment releases by earthquakes and occurrence of paleo-tsunamis in the RT, we focus on the long-term activity of the very low frequency earthquakes (VLFEs), which are good indicators of the stress release in the shallow plate interface. VLFEs have been detected along the RT (Ando et al., 2012), which occur on the plate interface or at the accretionary prism. We used broadband data from the F-net of NIED along the RT and from the IRIS network. We applied two filters to all the raw broadband seismograms: a 0.02-0.05 Hz band-pass filter and a 1 Hz high-pass filter. After identification of the low-frequency events from the band-pass-filtered seismograms, the local and teleseismic events were removed. Then we picked the arrival time of the maximum amplitude of the surface wave of the VLFEs and determined the epicenters. VLFEs occurred on the RA side within 100 km from the trench axis along the RT. Distribution of the 6670 VLFEs from 2002 to 2013 could be divided to several clusters. Principal large clusters were located at 27.1°-29.0°N, 25.5°-26.6°N, and 122.1°-122.4°E (YA). We found that the VLFEs of the YA are modulated by repeating slow slip events (SSEs) which occur beneath south RA. The activity of the VLFEs increased to two times of its ordinary rate in 15 days after the onset of the SSEs. Activation of the VLFEs could be generated by low stress change of 0.02-20 kPa increase in Coulomb failure stress. The strain in the plate interface where the VLFEs occur frequently would be released by small change in stress. Cluster of the VLFEs is complementally to the historical tsunami source area and locked area. Continuous activity of VLFEs would release the stress patchily in the plate interface and give the constraint to the maximum size of large thrust earthquakes.

Sedimentary masses and concepts about tectonic processes at underthrust ocean margins ( subduction).

USGS Publications Warehouse

Scholl, D. W.; von Huene, Roland E.; Vallier, T.L.; Howell, D.G.

1980-01-01

Tectonic processes associated with subduction of oceanic crust, but unrelated to the collision of thick crustal masses or microplates, are presumed by many geologists to significantly affect the formation and deformation of large sedimentary bodies at underthrust ocean margins. More geologists are familiar with the concept of subduction accretion than with other noncollision processes - for example, sediment subduction, subduction erosion, and subduction kneading. In our opinion, no single subduction-related tectonic process is the dominant or typical one that forges the geologic framework of modern underthrust ocean margins. It is likely, therefore, that the rock records of ancient underthrust margins are preserved in a multitude of structural and stratigraphic forms.-from Authors

Kinematics and Dynamics of the Makran Subduction Zone

NASA Astrophysics Data System (ADS)

Penney, C.; Tavakoli, F.; Sobouti, F.; Copley, A.; Priestley, K. F.; Jackson, J. A.

2016-12-01

The Makran subduction zone, along the southern coasts of Iran and Pakistan, hosts the world's largest exposed accretionary prism. In contrast to the circum-Pacific subduction zones, the Makran has not been extensively studied, with seismic data collected in the offshore region presenting only a time-integrated picture of the deformation. We investigate spatio-temporal variations in the deformation of the accretionary prism and the insights these offer into subduction zone driving forces and megathrust rheology. We combine seismology, geodesy and field observations to study the 2013 Mw 6.1 Minab earthquake, which occurred at the western end of the accretionary prism. We find that the earthquake was a left-lateral rupture on an ENE-WSW plane, approximately perpendicular to the previously mapped faults in the region. The causative fault of the Minab earthquake is one of a series of left-lateral faults in the region which accommodate a velocity field equivalent to right-lateral shear on N-S planes by rotating clockwise about vertical axes. Another recent strike-slip event within the Makran accretionary wedge was the 2013 Mw 7.7 Balochistan earthquake, which occurred on a fault optimally oriented to accommodate the regional compression by thrusting. The dominance of strike-slip faulting within the onshore prism, on faults perpendicular to the regional compression, suggests that the prism may have reached the maximum elevation which the megathrust can support, with the compressional forces which dominated in the early stages of the collision now balanced by gravitational forces. This observation allows us to estimate the mean shear stress on the megathrust interface and its effective coefficient of friction.

Tabletop Tectonics: Diverse Mountain Ranges Using Flour and Graphite

NASA Astrophysics Data System (ADS)

Davis, D. M.

2006-12-01

It has been recognized for some time that the frontal deformation zones where plates converge (foreland fold- and-thrust belts on continents and accretionary wedges at subduction zones) involve shortening over a decoupling layer, or decollement. A simple but successful way of explaining many aspects of their behavior is called the critical Coulomb wedge model, which regards these contractional wedges as analogous to the wedge-shaped mass of soil accreted in front of a bulldozer, or the wedge of snow that piles up in front of a snow plow. The shape and deformation history of the accreted wedge of soil or snow will depend upon the frictional strength of the material being plowed up and the surface over which it is being plowed. The same is true of `bulldozer' wedges consisting of many km thick piles of sediment at convergent plate margins. Using flour (or powdered milk), sandpaper, graphite, transparency sheets, and athletic field marker chalk, manipulated with sieves, brushes, pastry bags and blocks and sheets of wood, it is possible to demonstrate a wide variety of processes and tectonic styles observed at convergent plate boundaries. Model fold-and-thrust belts that behave like natural examples with a decollement that is strong (e.g., in rock without high pore fluid pressure) or weak (e.g., in a salt horizon or with elevated pore fluid pressure) can be generated simply by placing wither sandpaper or graphite beneath the flour that is pushed across the tabletop using a block of wood (the strong basement and hiterland rocks behind the fold-thrust belt). Depending upon the strength of the decollement, the cross-sectional taper of the deforming wedge will be thin or broad, the internal deformation mild or intense, and the structures either close to symmetric or strongly forward-vergent, just as at the analogous natural fold-thrust belts. Including a horizontal sheet of wood or Plexiglas in front of the pushing block allows generation of an accretionary wedge, outer-are high, and forearc basin, just as over a subduction zone. Any dark material emplaced (a pastry bag works well) atop the experiment before deformation in the form of football-field `hash marks' every 10 cm allows for easy calculation of strain distribution at any time during or after the experiment. Finally, the entire orogen can be excavated using a plastic photocopier transparency sheet. If the original set-up included occasional thin layers of red and blue field marker chalk within sedimentary column (the rest of which consists of white flour or powdered milk), excavation reveals (quite colorfully) many internal details of the fold-thrust belts that have been generated.

Recent Mega-Thrust Tsunamigenic Earthquakes and PTHA

NASA Astrophysics Data System (ADS)

Lorito, S.

2013-05-01

The occurrence of several mega-thrust tsunamigenic earthquakes in the last decade, including but not limited to the 2004 Sumatra-Andaman, the 2010 Maule, and 2011 Tohoku earthquakes, has been a dramatic reminder of the limitations in our capability of assessing earthquake and tsunami hazard and risk. However, the increasingly high-quality geophysical observational networks allowed the retrieval of most accurate than ever models of the rupture process of mega-thrust earthquakes, thus paving the way for future improved hazard assessments. Probabilistic Tsunami Hazard Analysis (PTHA) methodology, in particular, is less mature than its seismic counterpart, PSHA. Worldwide recent research efforts of the tsunami science community allowed to start filling this gap, and to define some best practices that are being progressively employed in PTHA for different regions and coasts at threat. In the first part of my talk, I will briefly review some rupture models of recent mega-thrust earthquakes, and highlight some of their surprising features that likely result in bigger error bars associated to PTHA results. More specifically, recent events of unexpected size at a given location, and with unexpected rupture process features, posed first-order open questions which prevent the definition of an heterogeneous rupture probability along a subduction zone, despite of several recent promising results on the subduction zone seismic cycle. In the second part of the talk, I will dig a bit more into a specific ongoing effort for improving PTHA methods, in particular as regards epistemic and aleatory uncertainties determination, and the computational PTHA feasibility when considering the full assumed source variability. Only logic trees are usually explicated in PTHA studies, accounting for different possible assumptions on the source zone properties and behavior. The selection of the earthquakes to be actually modelled is then in general made on a qualitative basis or remains implicit, despite different methods like event trees have been used for different applications. I will define a quite general PTHA framework, based on the mixed use of logic and event trees. I will first discuss a particular class of epistemic uncertainties, i.e. those related to the parametric fault characterization in terms of geometry, kinematics, and assessment of activity rates. A systematic classification in six justification levels of epistemic uncertainty related with the existence and behaviour of fault sources will be presented. Then, a particular branch of the logic tree is chosen in order to discuss just the aleatory variability of earthquake parameters, represented with an event tree. Even so, PTHA based on numerical scenarios is a too demanding computational task, particularly when probabilistic inundation maps are needed. For trying to reduce the computational burden without under-representing the source variability, the event tree is first constructed by taking care of densely (over-)sampling the earthquake parameter space, and then the earthquakes are filtered basing on their associated tsunami impact offshore, before calculating inundation maps. I'll describe this approach by means of a case study in the Mediterranean Sea, namely the PTHA for some locations of Eastern Sicily coasts and Southern Crete coast due to potential subduction earthquakes occurring on the Hellenic Arc.

Double-Sided Wedge Model For Retreating Subduction Zones: Applications to the Apenninic and Hellenic Subduction Zones (Invited)

NASA Astrophysics Data System (ADS)

Brandon, M. T.; Willett, S.; Rahl, J. M.; Cowan, D. S.

2009-12-01

We propose a new model for the evolution of accreting wedges at retreating subduction zones. Advance and retreat refer to the polarity of the velocity of the overriding plate with respect to subduction zone. Advance indicates a velocity toward the subduction zone (e.g., Andes) and retreat, away from the subduction zone (e.g. Apennines, Crete). The tectonic mode of a subduction zone, whether advancing or retreating, is a result of both the rollback of the subducting plate and the absolute motion of the overriding plate. The Hellenic and Apenninic wedges are both associated with retreating subduction zones. The Hellenic wedge has been active for about 100 Ma, whereas the Apenninic wedge has been active for about 30 Ma. Comparison of maximum metamorphic pressures for exhumed rocks in these wedges (25 and 30 km, respectively) with the maximum thickness of the wedges at present (30 and 35 km, respectively) indicates that each wedge has maintained a relatively steady size during its evolution. This conclusion is based on the constraint that both frictional and viscous wedges are subject to the constraint of a steady wedge taper, so that thickness and width are strongly correlated. Both wedges show clear evidence of steady accretion during their full evolution, with accretionary fluxes of about 60 and 200 km2 Ma-1. These wedges also both show steady drift of material from the front to the rear of the wedge, with horizontal shortening dominating in the front of the wedge, and horizontal extension within the back of the wedge. We propose that these wedges represent two back-to-back wedges, with a convergent wedge on the leading side (proside), and a divergent wedge on the trailing side (retroside). In this sense, the wedges are bound by two plates. The subducting plate is familiar. It creates a thrust-sense traction beneath the proside of the wedge. The second plate is an “educting” plate, which is creates a normal-sense traction beneath the retroside of the wedge. The educting plate underlies the Tyrrenhian Sea west of the Apennines and the Cretean Sea north of Crete. The stretched crust that overlies this plate represents highly thinned wedge material that has been removed or decreted from the wedge. This decretion process accounts for the mean motion within the wedge, from pro to retro side, and the pervasive thinning within the retroside. It also explains how these wedges are able to maintain a steady wedge size with time. An important prediction of this model is that different deformational styles, involving thickening and thinning, can occur within the same tectonics setting. This is in contrast the widely cited idea that tectonic thinning is a late- or post-orogenic process.

Fore-arc Deformation in the Paola Basin Segment (Offshore Western Calabria) of the Tyrrhenian-Ionian Subduction System

NASA Astrophysics Data System (ADS)

Pepe, F.; Corradino, M.; Nicolich, R.; Barreca, G.; Bertotti, G.; Ferranti, L.; Monaco, C.

2017-12-01

The 3D stratigraphic architecture and Late Neogene to Recent tectonic evolution of the Paola Basin (offshore western Calabria), a segment in the fore-arc of the Tyrrhenian-Ionian subduction system, is reconstructed by using a grid of high-penetration reflection seismics. Oligocene to Messinian deposits are interpreted all along the profile. They tend to fossilize preexisting topography and reach the largest thicknesses between (fault controlled) basement highs. Plio-Quaternary deposits are found over the entire area and display variations in thickness and tectonic style. They are thicken up to 4.5 km in the depocenter of the basin, and decrease both in the east and west termination of the lines. The Paola Basin can be partitioned into two sectors with different tectonic deformation, separated by a NNW-SSE elongated area that coincides with the basin depocenter. Tectonic features associated with strike-slip restraining and releasing bends are widely spread over the western sector of the basin. Overall, they form an approximately NS-trending and geomorphically prominent ridge separating the Paola Basin from the Marsili abyssal plain. A high-angle, NNE-trending, normal fault system develops on the south-west tip of the basin, where the faults offset the Messinian horizon of ca. 500 m. Data suggest that limited vertical slip occurs along reverse faults detected at the border and inside the sedimentary infilling of the Paola Basin, reaching thickness of more than 3.8s two way travel time. The reflection sequence pattern can be interpreted as a result of the infilling of the thrust-top basin related to a prograding system, located between a growth ramp-anticline to the west and a culmination of basement-thrust sheets to the East. We propose that the Paola Basin developed near the northern edge of the Ionian slab where tearing of the lithosphere is expected. Also, the strike-slip fault system is a kinematic consequence of obliquely convergent subduction settings, where interplate strain is partitioned into arc-parallel strike-slip zones within the fore-arc, arc or back-arc region.

Seismic signals from the slab surface within and downdip of the thrust zone: blind men and the elephant?

NASA Astrophysics Data System (ADS)

Abers, G. A.; Janiszewski, H. A.; Keranen, K. M.; Li, J.; Saffer, D. M.; Shillington, D. J.; Schindler, K.

2016-12-01

The subduction plate interface as been variably described as a narrow discontinuity, a decoupling layer, a viscous channel, or an intensely deformed mixing zone. The 1994 Subduction Conference depicted this ambiguity with the parable of the blind men and the elephant, as participants struggled to rationalize dissimilar observations with an integrated view of subduction. We illustrate here how different seismological tools reveal contradictory natures to the slab surface at 30-80 km depth, highlighting new examples from Alaska and Cascadia. At the km scale and 0.05-0.5 Hz, the teleseismic scattered wavefield that generates receiver functions shows strong consistent patterns. It indicates a uniformly layered plate interface structure with a low-velocity channel along the thrust zone where earthquakes and slow slip events occur. These channels appear homogeneous and 1-4 km thick over wide areas, with hints of strong anisotropy. By contrast, reflection seismology shows complex discontinuous reflectivity packages at the tens of m scale and 5-20 Hz that imply much greater heterogeneity, in both in normal-incidence and wide-angle reflections. To span the intervening frequency band we analyze P-S conversions from in-slab earthquakes recorded nearby, in southern Alaska. These 1-10 Hz signals arrive between P and S and have comparable amplitude, indicating sharp or complex boundaries near the slab surface at 30-50 km depth. However the signals are not uniformly observed and indicate significant heterogeneity in the causative structure. The conversion points lie within those sampled by receiver functions so the differences are not due to geographic variations, and can be analyzed jointly. Taken together, these observations suggest that the same boundary is continuous and relatively homogenous, or highly heterogeneous and laminated, depending on its sampling. They can be reconciled if the structure at km-scale is relatively simple, while it is complex and highly heterogeneous at shorter wavelengths. A complex but sharply bounded low-velocity shear zone best explains these observations, indicating that most deformation is highly localized rather than forming broadly-distributed ductile flow features even well into the aseismic region.

Evolution of the Alpine Tethys (Sava) suture zone in Fruška Gora Mountains (N Serbia): from orogenic building to tectonic omissions

NASA Astrophysics Data System (ADS)

Toljić, Marinko; Matenco, Liviu; ĐErić, Nevenka; Milivojević, Jelena; Gerzina, Nataša.; Stojadinović, Uros

2010-05-01

The Fru\\vska Gora Mountains in northern Serbia offers an unique opportunity to study the Cretaceous-Eocene evolution of the NE part of the Dinarides, which is largely covered elsewhere beneath the thick Miocene sediments of the Pannonian basin, deposited during the back-arc collapse associated with the subduction and roll-back recorded in the external Carpathians. The structural grain of the Fru\\vska Gora Mountains is the one of a large scale antiform, exposing a complex puzzle of highly deformed metamorphic rocks in its centre and Triassic-Miocene sequence of non-metamorphosed sediments, ophiolites and volcanics along its flanks. The metamorphic rocks were the target of structural investigations coupled with paleontological dating (conodonts, palynomorphs and radiolarians) in an effort to unravel the geodynamic evolution of an area thought to be located near the suture zone between the Tisza upper plate and the Adriatic lower plate, i.e. the Sava subduction zone of the Dinarides (e.g., Pamic, 2002; Schmid et al., 2008). The existence of this subduction zone was previously inferred here by local observations, such as metamorphosed Mesozoic sediments containing Middle Triassic conodonts (Đurđanović, 1971) or Early Cretaceous blue schists metamorphism (123±5 Ma, Milovanović et al., 1995). The metamorphic sequence is characterized by a Paleozoic age meta-sedimentary basement which contains palynomorphs of Upper Paleozoic - Carboniferous age and a meta-sedimentary and meta-volcanic sequence which contain a succession of contrasting metamorphosed lithologies such sandstones, black limestones, shallow water white limestones, basic volcanic sequences, deep nodular limestiones, radiolarites, meta-ophiolites and turbiditic sequences. The lower part of the sequence is contrastingly similar with the Triassic cover of the Drina-Ivanijca thrust sheet and its metamorphosed equivalent observed in the Kopaonik and Studenica series (Schefer et al., in press). This observation is supported by the newly found micro-fauna of Upper Triassic in age in the meta-sandstones associated with meta-volcanics on the SW slopes of the mountain. The upper part of the sequence display metamorphosed "flysh"-type of sequences and meta-basalts. In these deposits, slightly metamorphosed siliciclastics (lithic sandstones with volcanic-derived clasts) previously interpreted as Upper Jurassic mélange have proved to contain Upper Cretaceous palynomorphs. Among the rocks exposed in the metamorphic core of the mountains, the SW slope of Fru\\vska Gora offers the optimal locality for the study of the kinematic evolution. Here, four phases of folding have been mapped, being associated mainly with large-scale regional contraction. The first phase is characterized by isoclinal folding, with reconstructed SW vergence. The second generation of E-W oriented and coaxial folds is asymmetric and is up to metres in size, displaying a south vergence and has largely refolded the previous generation. The third event was responsible for the formation of upright folds, yet again E-W oriented, re-folding earlier structures. The first two phases of folding are associated with metamorphic conditions, while the third was apparently near the transition with the brittle domain. The relationship with a fourth folding event observed also in the non-metamorphosed clastic-carbonate rocks is rather uncertain, but is apparently associated with the present day antiformal structure of the Fuska Gora Mountains. Interestingly, the metamorphosed Triassic and Upper Cretaceous carbonatic-clastic sequence in the core of the antiform is in structural contact along the antiformal flanks with Lower-Middle Triassic and Upper Cretaceous-Paleogene sediments which display the same facies, but these are not metamorphosed. This demonstrates a large scale tectonic omission along the flanks of the Fru\\vska Gora antiform, 9-10km of rocks being removed by what we speculatively define as an extensional detachment exhuming the metamorphic core. This detachment has been subsequently folded into the present-day antiformal geometry of the Fru\\vska Gora Mountains. These findings demonstrate that the metamorphic and non-metamorphic Upper Cretaceous - Paleogene clastic-carbonate sediments belongs to the main Alpine Tethys (Sava) subduction zone of the Dinarides. The Paleozoic-Triassic metamorphic and non-metamorphic rocks belong to the distal Adriatic lower plate, or more precisely to the Jadar-Kopaonik composite thrust sheet (Schmid et al., 2008), while the layer of serpentinized peridotite found at their contact most probably belongs to the Western Vardar ophiolites obducted over the Adriatic plate during Late Jurassic - Earliest Cretaceous. The distal Jadar-Kopaonik composite unit was partly affected by strong contractional deformation and a Late Eocene greenschist facies metamorphism during the main phase of subduction and collision, similarly to what has been observed elsewhere in the Dinarides (Pamić, 2002; Schefer et al., in press). A Miocene phase of core-complex formation was responsible for the large tectonic omission observed, being probably followed by the formation of a wide open antiformal structure during the Pliocene-Quaternary inversion of the Pannonian basin.

Magmatism and the Shallowing of the Chilean Flatslab in the Central Andes

NASA Astrophysics Data System (ADS)

Kay, S. M.

2014-12-01

The magmatic history of the flatslab region between the Central and Southern Andean volcanic zones reflects shallowing of the slab, lithospheric thinning, narrowing of the asthenospheric wedge, crustal thickening and forearc removal by subduction erosion. Newly revised contours on the northern margin of the modern flatslab (Mulcahy et al. 2014) show the flattest part extends from ~28° to 33°S and is bounded by Pleistocene volcanic activity. An eastward broadening of the magmatic arc began after 18 Ma as westward drift of South America accelerated, but the most distinctive retroarc magmatism occurred after near normal subduction of the southward drifting Juan Fernandez Ridge began at ~11 Ma and ended as magmatism ceased in the Pampean ranges, ~ 700 km east of the trench at ~4.7 Ma. Recent seismic work in the retroarc area indicate a ~60 km thick crust under the Precordillera fold-thrust belt with transitions at ~20 and ~40 km that are considered to be the top of crystalline basement and an eclogitic facies transition. Chemical constraints from ~15-7 Ma magmatic rocks suggest eclogization is related to crustal thickening over the shallowing slab in accord with field relations for major thrusting in the region by ~8-7 Ma. High Ba/Th ratios in <9 Ma volcanic rocks are interpreted to reflect phengite breakdown in the mantle wedge with the fluids facilitating eclogization of the lower crust. Evidence for mantle melt contributions in the magmas up until ~7 Ma comes from more primitive isotopic values in 1088-1251 Ma amphibolite and granulite facies xenoliths (eNd = 0 to -3; 87Sr/86Sr =704-0.710) than in Miocene volcanic rocks (eNd = 0-1.7; 0.70325-0.70345; zircon eHf ~ 0). From ~8 to 3 Ma, the active volcanic arc front near 28°S and 33°S was translated ~ 40-50 km eastward in a suspected response to forearc removal by subduction erosion. Given the position of the arc and distance to the trench, the same amount of forearc was likely removed in the intervening flatslab region. Trapping of subducted forearc material in the mantle wedge could help to explain a low Vp/Vs ratios (1.65-1.72, Wagner et al. 2008 ) in the mantle wedge above the flat-slab as a low Vp could result from orthopyroxene formation by reaction of silicic material with the mantle wedge and a high Vs can be explained by the current slab being too cool to hydrate the mantle wedge.

Miocene tectonics of the Western Alboran domain: from mantle extensional exhumation to westward thrusting

NASA Astrophysics Data System (ADS)

Gueydan, F.; Frasca, G.; Brun, J. P.

2015-12-01

In the frame of the Africa-Europe convergence, the Mediterranean tectonic system presents a complex interaction between subduction rollback and upper-plate deformation during the Tertiary. The western Mediterranean is characterized by the exhumation of the largest subcontinental mantle massif worldwide (the Ronda Peridotite) and a narrow arcuate geometryacross the Gibraltar arc within the Betic-Rif belt (the internal part being called the Alboran domain), where the relationship between slab dynamics and surface tectonics is not well understood. New structural and geochronological data are used to argue for 1/ hyperstrechting of the continental lithosphere allowing extensional mantle exhumation to shallow depths, followed by 2/ lower miocene thrusting. Two Lower Miocene E-W-trending strike-slip corridors played a major role in the deformation pattern of the Alboran Domain, in which E-W dextral strike-slip faults, N60°-trending thrusts and N140°-trending normal faults developed simultaneously during dextral strike-slip simple shear. The inferred continuous westward translation of the Alboran Domain is accommodated by a major E-W-trending lateral ramp (strike-slip) and a N60°-trending frontal thrust. At lithosphere-scale, we interpret the observed deformation pattern as the upper-plate expression of a lateral slab tear and of its westward propagation since Lower Miocene. The crustal emplacement of the Ronda Peridotites occurred at the onset of this westward motion.The Miocene tectonics of the western Alboran is therefore marked by the inversion of a continental rift, triggered by shortening of the upper continental plate and accommodated by E-W dextral strike-slip corridors. During thrusting and westward displacement of the Alboran domain with respect to Iberia, the hot upper plate, which involved the previously exhumed sub-continental mantle, underwent fast cooling.

Broadband Analysis of the Energetics of Earthquakes and Tsunamis in the Sunda Forearc from 1987-2012

NASA Astrophysics Data System (ADS)

Choy, G. L.; Kirby, S. H.; Hayes, G. P.

2013-12-01

In the eighteen years before the 2004 Sumatra Mw 9.1 earthquake, the forearc off Sumatra experienced only one large (Mw > 7.0) thrust event and experienced no earthquakes that generated measurable tsunami wave heights. In the subsequent eight years, twelve large thrust earthquakes occurred of which half generated measurable tsunamis. The number of broadband earthquakes (those events with Mw > 5.5 for which broadband teleseismic waveforms have sufficient signal to compute depths, focal mechanisms, moments and radiated energies) jumped six fold after 2004. The progression of tsunami earthquakes, as well as the profuse increase in broadband activity, strongly suggests regional stress adjustments following the Sumatra 2004 megathrust earthquake. Broadband source parameters, published routinely in the Source Parameters (SOPAR) database of the USGS's NEIC (National Earthquake Information Center), have provided the most accurate depths and locations of big earthquakes since the implementation of modern digital seismographic networks. Moreover, radiated energy and seismic moment (also found in SOPAR) are related to apparent stress which is a measure of fault maturity. In mapping apparent stress as a function of depth and focal mechanism, we find that about 12% of broadband thrust earthquakes in the subduction zone are unequivocally above or below the slab interface. Apparent stresses of upper-plate events are associated with failure on mature splay faults, some of which generated measurable tsunamis. One unconventional source for local wave heights was a large intraslab earthquake. High-energy upper-plate events, which are dominant in the Aceh Basin, are associated with immature faults, which may explain why the region was bypassed by significant rupture during the 2004 Sumatra earthquake. The majority of broadband earthquakes are non-randomly concentrated under the outer-arc high. They appear to delineate the periphery of the contiguous rupture zones of large earthquakes. A not uncommon occurrence at the outer-arc high is that of a large (Mw >7.0) earthquake followed by another event, also of large magnitude, in very close spatial (<50 km) proximity within a short time (days to months). The physical separation between these events provides constraints on the nature of barriers to rupture propagation. Some of the glaring disparities in seismic damage and tsunami excitation for earthquakes with the same magnitude can be attributed to differences between rupture properties landward and seaward of the outer-arc high. Although most of the studied broadband earthquakes occurred in the wake of the Sumatra 2004 megathrust event, they illuminate tectonic features that exert a strong influence on rupture growth and extent. The application of broadband analysis to other island arcs will complement current criteria for evaluating seismic and tsunami potential

Rapid Ice Mass Loss: Does It Have an Influence on Earthquake Occurrence in Southern Alaska?

NASA Technical Reports Server (NTRS)

Sauber, Jeanne M.

2008-01-01

The glaciers of southern Alaska are extensive, and many of them have undergone gigatons of ice wastage on time scales on the order of the seismic cycle. Since the ice loss occurs directly above a shallow main thrust zone associated with subduction of the Pacific-Yakutat plate beneath continental Alaska, the region between the Malaspina and Bering Glaciers is an excellent test site for evaluating the importance of recent ice wastage on earthquake faulting potential. We demonstrate the influence of cumulative glacial mass loss following the 1899 Yakataga earthquake (M=8.1) by using a two dimensional finite element model with a simple representation of ice fluctuations to calculate the incremental stresses and change in the fault stability margin (FSM) along the main thrust zone (MTZ) and on the surface. Along the MTZ, our results indicate a decrease in FSM between 1899 and the 1979 St. Elias earthquake (M=7.4) of 0.2 - 1.2 MPa over an 80 km region between the coast and the 1979 aftershock zone; at the surface, the estimated FSM was larger but more localized to the lower reaches of glacial ablation zones. The ice-induced stresses were large enough, in theory, to promote the occurrence of shallow thrust earthquakes. To empirically test the influence of short-term ice fluctuations on fault stability, we compared the seismic rate from a reference background time period (1988-1992) against other time periods (1993-2006) with variable ice or tectonic change characteristics. We found that the frequency of small tectonic events in the Icy Bay region increased in 2002-2006 relative to the background seismic rate. We hypothesize that this was due to a significant increase in the rate of ice wastage in 2002-2006 instead of the M=7.9, 2002 Denali earthquake, located more than 100km away.

Complex submarine landsliding processes caused by subduction of large seamounts along the Middle America Trench

NASA Astrophysics Data System (ADS)

Harders, Rieka; Ranero, Cesar R.; Weinrebe, Wilhelm; von Huene, Roland

2014-05-01

Subduction of kms-tall and tens-of-km wide seamounts cause important landsliding events at subduction zones around the word. Along the Middle America Trench, previous work based on regional swath bathymetry maps (with 100 m grids) and multichannel seismic images have shown that seamount subduction produces large-scale slumping and sliding. Some of the mass wasting event may have been catastrophic and numerical modeling has indicated that they may have produced important local tsunamis. We have re-evaluated the structure of several active submarine landlide complexes caused by large seamount subduction using side scan sonar data. The comparison of the side scan sonar data to local high-resolution bathymetry grids indicates that the backscatter data has a resolution that is somewhat similar to that produced by a 10 m bathymetry grid. Although this is an arbitrary comparison, the side scan sonar data provides comparatively much higher resolution information than the previously used regional multibeam bathymetry. We have mapped the geometry and relief of the head and side walls of the complexes, the distribution of scars and the different sediment deposits to produce a new interpretation of the modes of landsliding during subduction of large seamounts. The new higher resolution information shows that landsliding processes are considerably more complex than formerly assumed. Landslides are of notably smaller dimensions that the lower resolution data had previously appear to indicate. However, significantly large events may have occur far more often than earlier interpretations had inferred representing a more common threat that previously assumed.

Fluid-rock interaction recorded in fault rocks of the Nobeoka Thrust, fossilized megasplay fault in an ancient accretionary complex

NASA Astrophysics Data System (ADS)

Hasegawa, R.; Yamaguchi, A.; Fukuchi, R.; Kitamura, Y.; Kimura, G.; Hamada, Y.; Ashi, J.; Ishikawa, T.

2017-12-01

The relationship between faulting and fluid behavior has been in debate. In this study, we clarify the fluid-rock interaction in the Nobeoka Thrust by major/trace element composition analysis using the boring core of the Nobeoka Thrust, an exhumed analogue of an ancient megasplay fault in Shimanto accretionary complex, southwest Japan. The hanging wall and the footwall of the Nobeoka Thrust show difference in lithology and metamorphic grade, and their maximum burial temperature is estimated from vitrinite reflectance analysis to be 320 330°C and 250 270°C, respectively (Kondo et al., 2005). The fault zone was formed in a fluid-rich condition, as evidenced by warm fluid migration suggested by fluid inclusion analysis (Kondo et al., 2005), implosion brecciation accompanied by carbonate precipitation followed by formation of pseudotachylyte (Okamoto et al., 2006), ankerite veins coseismically formed under reducing conditions (Yamaguchi et al., 2011), and quartz veins recording stress rotation in seismic cycles (Otsubo et al., 2016). In this study, first we analyzed the major/trace element composition across the principal slip zone (PSZ) of the Nobeoka Thrust by using fragments of borehole cores penetrated through the Nobeoka Thrust. Many elements fluctuated just above the PSZ, whereas K increase and Na, Si decrease suggesting illitization of plagioclase, as well as positive anomalies in Li and Cs were found within the PSZ. For more detail understanding, we observed polished slabs and thin sections of the PSZ. Although grain size reduction of deformed clast and weak development of foliation were observed entirely in the PSZ by macroscopic observation, remarkable development of composite planar fabric nor evidence of friction melting were absent. In this presentation, we show the result of major/trace element composition corresponding to the internal structure of PSZ, and discuss fluid-rock interaction and its impact to megasplay fault activity in subduction zones.

Tectonic Reorganization of the Western Pacific in Eocene Time: Missing Pieces in the Subduction Initiation Puzzle

NASA Astrophysics Data System (ADS)

Bloomer, S. H.; Stern, R. J.

2002-12-01

The initiation of subduction is probably the geologic process most responsible for large-scale changes in the motions and interactions of plates. To the extent that subduction drives mantle convection, the initiation of subduction also drives major changes in the convection of the mantle. The mechanisms of subduction initiation remain, however, obscure, but it is becoming increasingly clear that Eocene sequences in the western Pacific provide an outstanding opportunity to study this phenomenon. The major subduction zones of the western Pacific (Tonga, Mariana, Izu, Bonin) all first produced volcanic products in early Eocene time (55-48 Ma). The similarity of timing and of the characteristics of these margins suggests that there may be a common process involved. There is no evidence in the forearc crust of any of these convergent margins for proximity to a continental margin at the time of initiation. Current models of plate motion (particularly given recent reinterpretations of the Hawaiian hotspot bend) show no major plate reorganization that might have provided excess compressional stress across the western Pacific margins. The only mechanically viable mechanism for subduction initiation in the region appears to be spontaneous failure due to gravitational instability of cold, old oceanic lithosphere. There are a number of geologic and geophysical unknowns in assessing the viability of such spontaneous nucleation. The lithosphere becomes stronger as it ages as well as becoming denser. Failure of such crust to form a nascent subduction zone requires a crustal weakness such as a fault and a mechanism to decrease the bending strength of the plate. Paleomagnetic data and plate reconstructions for both the IBM and the Tonga-Kermedec region provide no clear answer to these issues and in fact conflict with interpretations placing large transform faults at the site of subduction nucleation. The large-scale rotations inferred from those data for the IBM conflict, or at least complicate, geologic observations around the Philippine Sea. We will review the currrent structural, mechanical, and geologic constraints on pre-subduction geometry of the western Pacific and will discuss the most essential problems to be solved if we are to constrain how subduction began in the Pacific in Eocene time.

Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole

NASA Astrophysics Data System (ADS)

Greff-Lefftz, Marianne; Besse, Jean

2012-09-01

Apparent polar wander (APW) is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large-scale upwellings in the mantle. We investigate plate motions during the last 410 million years in a reference frame where Africa is fixed. Indeed, Africa has remained a central plate from which most continents diverged since the break-up of Pangea. The exact amount of subduction is unknown prior to 120 Ma. We propose an approach, based on one hand on the study of the past subduction volcanism to locate ancient subduction activity, and on the other hand microplate motion history in the Tethyan area derived from geology and paleomagnetism. The peri-Pacific subductions seem to be a quasi-permanent feature of the Earth's history at least since the Paleozoic, with however localized interruptions. The “Tethyan” subductions have a complex history with successive collisions of continental blocs (Hercynian, Indo-Sinian, Alpine and Himalayan) and episodical rebirth of E-W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 280 Ma. Due to conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis is computed in a mantle reference frame where the Africa plate is fixed, and compared to the apparent polar wander (APW) observed by paleomagnetism since 280 Ma. We find that a major trend of both paleomagnetic and computed APW are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps (around 230 Ma, 190 Ma and 140-110 Ma). We find that cusps result from earlier major geodynamic events: the 230 Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental collision (340-300 Ma) and to renewed subduction zone West of Laurentia, whereas the 190 Ma cusp results from the Indo-Sinian collision (270-230 Ma) and the subsequent end of the Neo-Tethys ocean subduction.

Exhumation rates in the Gran Paradiso Massif (Western Alps) constrained by in situ U-Th-Pb dating of accessory phases (monazite, allanite and xenotime)

NASA Astrophysics Data System (ADS)

Manzotti, Paola; Bosse, Valérie; Pitra, Pavel; Robyr, Martin; Schiavi, Federica; Ballèvre, Michel

2018-03-01

Exhumation rates for high-pressure metamorphic rocks need to be carefully estimated to decipher tectonic processes in subduction/collision belts. In the Gran Paradiso Massif (Western Alps), the Money Unit crops out as a tectonic window below the Gran Paradiso Unit. According to previous studies, the Gran Paradiso and Money Units reached peak pressure conditions at 18 to 20 kbar, 480-520 °C and 17 to 18 kbar, 500-550 °C, respectively. This yields a maximum difference of 9 to 10 km in the subduction depth reached by these two units during the Alpine history. Thrusting of the Gran Paradiso Unit over the Money Unit led to the simultaneous development of the main foliation under the same metamorphic conditions ( 12.5 to 14.5 kbar and 530-560 °C) in both units. The thrust contact was subsequently folded and then both units were exhumed together. The relative timing of the growth and dissolution of the accessory phases was assessed by combining thermodynamic modelling with inclusion, textural and chemical (major and trace element) data from both major and accessory phases. The age of monazite constrained the high-pressure metamorphism in both the Gran Paradiso Unit and the Money Unit at 41.5 ± 0.3 and 42.0 ± 0.6 Ma, respectively. Allanite replacing monazite in the matrix has been dated at 32.7 ± 4.2 Ma. The late growth of xenotime associated with the crystallization of biotite pseudomorphs at the expense of garnet (at about 10 kbar) was dated at 32.3 ± 1.0 Ma. Our petrochronological data indicate about 10 m.y. between the peak pressure conditions and the crystallization of xenotime leading to an exhumation rate of the order of 2.2-5 mm/year. The new ages allow to better constrain the timing of the displacement of the thrust defining the lower boundary of the extruding wedge of eclogite-facies rocks.

Extent and architecture of major fault systems between northern Victoria Land and the eastern margin of the Wilkes Subglacial Basin (East Antarctica)

NASA Astrophysics Data System (ADS)

Armadillo, E.; Ferraccioli, F.; Balbi, P.; Bozzo, E.

2013-12-01

Terrane bounding and intra-terrane faults of the Ross Orogen in East Antarctica are linked to several phases of Cambrian to Ordovician age subduction and accretion along the active paleo-Pacific margin of Gondwana. Here we compile and analyse new enhanced aeromagnetic anomaly images over the Northern Victoria Land (NVL) segment of the Ross Orogen and the eastern margin of the Wilkes Subglacial Basin (WSB) that help constrain the extent and structural architecture of these fault systems and enable us re-assess their tectonic evolution. Long-wavelength magnetic lows and residual Bouguer gravity highs are modelled as several-km thick inverted sedimentary basins of early Cambrian(?) age. Tectonic inversion occurred along major thrust faults during the late stages of the Ross Orogen, forming a major high-grade pop-up structure within the central Wilson Terrane, flanked by lower grade rocks. The Prince Albert Fault System can now be recongnised as being located to the west of the Exiles Thrust fault system rather than representing its southern continuation. Relatively thin sheets of mylonitic sheared granitoids and possible ultramafic lenses are associated with the late-Ross (ca 480 Ma) Exiles Thrust fault system, while significantly larger and thicker batholiths were emplaced along the Prince Albert Fault System. Recent zircon U-Pb dating over small exposures of gabbro-diorites within the Prince Albert Mountains to the south lead us to propose that this part of the magmatic arc was emplaced during an earlier phase of subduction (~520 Ma or older?), compared to the late-Ross intrusions to the east. Whether the Prince Albert Fault System was indeed a major cryptic suture in early Cambrian times (Ferraccioli et al., 2002, GRL) remains speculative, but possible. Our aeromagnetic interpretation leads us to conclude that these inherited terrane bounding and intra-terrane fault systems of the Ross Orogen exerted a key influence on Cenozoic tectonic blocks and faults of the Transantarctic Mountains, and that the eastern margin of the WSB adjacent to NVL was also strongly controlled by a complex array of major intraplate strike-slip fault systems.

Size Constraints on Late Miocene to Pliocene Submarine Slope Failures along the Colombian Caribbean Subduction Margin as a Basis for Assessing Circum-Caribbean Impact of Future Tsunami Events

NASA Astrophysics Data System (ADS)

Leslie, S.; Mann, P.

2015-12-01

The Colombian Caribbean margin provides an ideal setting for the formation of large mass transport deposits (MTDs): 1) the Caribbean Plate is slowly subducting at rates of 20 mm/yr with infrequent large thrust earthquakes and a complete lack of subduction events in the 400-year-long historical record; 2) the margin is a broad zone of active faults including a ~50 km-wide accretionary prism and strike-slip faults landward of the prism; 3) the active margin is draped by the Magdalena delta and submarine fan fed by the Magdalena River, the 26th largest in the world; and 4) the margin is over-steepened to slopes of up to 7° from the combination of tectonic activity and rapid rates of deltaic progradation. Using seismic data we have identified three late Miocene-Pliocene MTDs, the largest of which is between 4500 and 6000 km3, comparable in size to the well-studied Storegga slide of Norway. The tsunamigenic potential of future, analog MTD events are modeled using GeoWave tsunami modeling software. The largest and youngest of these MTDs, the Santa Marta slide, is used as an analog to infer the location and input parameters for the tsunami model. The event is modeled as a translational slide ~46 km long and ~37 km wide with the center of the slide located ~57 km W/NW from the mouth of the present day Magdalena River in water depths of 1500 m. The volume for the initial failure is conservatively estimated at ~680 km3 of material. The resulting tsunami wave from such an event has an initial maximum trough amplitude of -65.8 m and a peak amplitude of 19.2 m. The impact of such a tsunami would include: 1) Kingston, Jamaica (population 938K), tsunami height 7.5 m, peak arrival at 60 min.; 2) Santo Domingo, Dominican Republic (population 965K, height 6 m, peak arrival at 80 min.); and 3) Cartagena, Colombia (population 845K, height 21 m, peak arrival at 34 min.). A number of parameters to the model are varied to analyze sensitivity of modeling results to changes in slide depth, angle of failure, slide volume, and slide density.

Half a Century of Oman Ophiolite Studies: SSZ or MOR, the Arc Disposal Problem

NASA Astrophysics Data System (ADS)

Gregory, R. T.; Gray, D.

2014-12-01

The Samail Ophiolite, one of the largest and best exposed ophiolite complexes, is a Tethyan ophiolite obducted in the Late Cretaceous onto the formerly passive Arabian platform as Arabia began its most recent >1000 km northward migration towards a Miocene collision with Eurasia. The Oman Mountains, northeastern Arabian Peninsula have yet to collide with Eurasia; present uplift and form of the mountains also date to the Miocene. In addition to the scientific scrutiny of the ophiolite complex, the geologic constraints on the timing and emplacement of the ophiolite are abundant with no consensus on the obduction mechanism or its original tectonic setting. The crustal thickness of the ophiolite is comparable to thicknesses observed for "normal" mid-ocean ridges. Largely on the basis of structural and paleomagnetic arguments, some workers have attributed its origin to Pacific-type fast spreading ridges and complex micro plate geometries. Indeed the lower pillow lava sequences and much of the gabbroic crust have isotope and geochemical signatures consistent with a MORB source. However, because of the geochemistry of the upper pillow lavas, the ophiolite is most often characterized as a supra-subduction zone (SSZ) ophiolite, i.e. it sits in the hanging wall of some large tectonic structure for part of its history. In the absence of a preserved arc, the SSZ designation has little explanatory power only being a declaration of allochthony or about chemical properties of the mantle source. That associated continental shelf and oceanic crustal sections have suffered either clockwise or counterclockwise PT time trajectories requires some type of nascent subduction and hanging wall thrust transport of the young ridge crest. The widespread Late Cretaceous obduction of Tethyan oceanic crust and mantle over thousands of kilometers strike length is a problem for SSZ models (arc, forearc, back arc etc.) because arc initiation results in thick crust on short time scales, none of which is preserved. Similarly, fast spreading models require the subduction of >106 km2 without the initiation and preservation of a magmatic arc. Some Late Cretaceous major plate boundary reorganization that resulted in the current plate motion regime with NNE convergence between Afro-Arabia and Eurasia will be part of any final solution to the Oman problem.

Earthquake Hazard and Risk in Alaska

NASA Astrophysics Data System (ADS)

Black Porto, N.; Nyst, M.

2014-12-01

Alaska is one of the most seismically active and tectonically diverse regions in the United States. To examine risk, we have updated the seismic hazard model in Alaska. The current RMS Alaska hazard model is based on the 2007 probabilistic seismic hazard maps for Alaska (Wesson et al., 2007; Boyd et al., 2007). The 2015 RMS model will update several key source parameters, including: extending the earthquake catalog, implementing a new set of crustal faults, updating the subduction zone geometry and reoccurrence rate. First, we extend the earthquake catalog to 2013; decluster the catalog, and compute new background rates. We then create a crustal fault model, based on the Alaska 2012 fault and fold database. This new model increased the number of crustal faults from ten in 2007, to 91 faults in the 2015 model. This includes the addition of: the western Denali, Cook Inlet folds near Anchorage, and thrust faults near Fairbanks. Previously the subduction zone was modeled at a uniform depth. In this update, we model the intraslab as a series of deep stepping events. We also use the best available data, such as Slab 1.0, to update the geometry of the subduction zone. The city of Anchorage represents 80% of the risk exposure in Alaska. In the 2007 model, the hazard in Alaska was dominated by the frequent rate of magnitude 7 to 8 events (Gutenberg-Richter distribution), and large magnitude 8+ events had a low reoccurrence rate (Characteristic) and therefore didn't contribute as highly to the overall risk. We will review these reoccurrence rates, and will present the results and impact to Anchorage. We will compare our hazard update to the 2007 USGS hazard map, and discuss the changes and drivers for these changes. Finally, we will examine the impact model changes have on Alaska earthquake risk. Consider risk metrics include average annual loss, an annualized expected loss level used by insurers to determine the costs of earthquake insurance (and premium levels), and the loss exceedance probability curve used by insurers to address their solvency and manage their portfolio risk. We analyze risk profile changes in areas with large population density and for structures of economic and financial importance: the Trans-Alaska pipeline, industrial facilities in Valdez, and typical residential wood buildings in Anchorage, Fairbanks and Juneau.

Low-thrust chemical orbit transfer propulsion

NASA Technical Reports Server (NTRS)

Pelouch, J. J., Jr.

1979-01-01

The need for large structures in high orbit is reported in terms of the many mission opportunities which require such structures. Mission and transportation options for large structures are presented, and it is shown that low-thrust propulsion is an enabling requirement for some missions and greatly enhancing to many others. Electric and low-thrust chemical propulsion are compared, and the need for an requirements of low-thrust chemical propulsion are discussed in terms of the interactions that are perceived to exist between the propulsion system and the large structure.

Analysis of deep seismic reflection and other data from the southern Washington Cascades. Final report, September 15, 1992--December 31, 1993

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stanley, W.D.; Johnson, S.Y.; Nuccio, V.F.

1993-12-01

This report describes results of a synthesis of geological, geological, geophysical and geochemical data from a largely volcanic rock covered region in southwestern Washington that has been identified as a underlain by thick marine sedimentary rocks. The work was funded by the Deep Source Gas projects at the Morgantown Energy Technology Center (METC). The subproject which resulted in this report is centered in the Branch of Geophysics, US Geological Survey (USGS) has involved one task focused on the application of geophysical methods to the study of phenomena associated with fossil and active subduction zones and non-subduction suture zones that maymore » have deeply emplaced sedimentary rocks. This report represents a summary synthesis of several geophysical and geological data sets. The Southern Washington Cascades Conductor (SWCC) has been examined using several types of data in addition to MT, seismic, magnetic, and gravity Specific geological mapping tasks have been completed trough funding by the Department of Energy and the USGS in the western part of the proposed basin near Morton, WA. Other regional geological studies using wells and outcrops done as part of the USGS Evolution of Sedimentary Basins programs have added information that constraint the possible nature of the SWCC rocks and their tectonic setting. Recently, evaluation of patterns of seismicity in the SWCC region has demonstrated the likelihood of several parallel and step-over strike-slip faults that may have produced the proposed basin or altered its geometry. In addition, the seismicity patterns trace the axis of key anticlinal structures and thrusts.« less

Eclogitization-induced mechanical instanility in granulite: Implications for deep seismicity in southern Tibet

NASA Astrophysics Data System (ADS)

Wang, Y.; Shi, F.; Yu, T.; Zhu, L.; Zhang, J.; Gasc, J.; Incel, S.; Schubnel, A.; Li, Z.; Liu, W.; Jin, Z.

2017-12-01

Southern Tibet is the most active orogenic region on Earth where the Indian plate thrusts under the Eurasian continent, pushing the Moho to unusual depths of 80 km. Seismicity is wide spread, reaching 100 km depth. Mechanisms of these deep earthquakes remain enigmatic. Here we examine the hypothesis of metamorphism induced mechanical instability in granulite-facies rocks, which are the dominant constituent in subducted Indian lower crust. We conducted deformation experiments on natural and nominally dry granulite in a DDIA apparatus within the stability fields of both granulite and eclogite. The system is interfaced with an acoustic emission (AE) monitoring system, allowing in-situ detection of mechanical instability along with the progress of eclogitization. We found that granulite deformed within its own stability field behaved in a ductile fashion without any AE activity. In contrast, numerous AE events were observed during deformation of metastable granulite in the eclogite field. The observed AE activities were episodic. Correlating closely to the AE burst episodes, measured differential stresses rose and fell during deformation, suggesting unstable fault slip. Microstructural observation shows that strain is highly localized around grain boundaries, which are decorated by eclogitization products. Time-resolved event location analysis showed large episodes corresponded to the growth of branches of macroscopic faults in recovered samples. It appears that ruptures originate from weakened grain boundaries, propagate through grains, and self-organize into macroscopic fault zones. No melting is required in the fault zones to facilitate brittle failure. This process may be responsible for the deep crustal seismicity in Southern Tibet and other continental-continental subduction regions.

Active tectonics of northwestern U.S. inferred from GPS-derived surface velocities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Robert McCaffrey; Robert W. King; Suzette J. Payne

2013-02-01

Surface velocities derived from GPS observations from 1993 to 2011 at several hundred sites across the deforming northwestern United States are used to further elucidate the region's active tectonics. The new velocities reveal that the clockwise rotations, relative to North America, seen in Oregon and western Washington from earlier GPS observations, continue to the east to include the Snake River Plain of Idaho and south into the Basin and Range of northern Nevada. Regional-scale rotation is attributed to gravitationally driven extension in the Basin and Range and Pacific-North America shear transferred through the Walker Lane belt aided by potentially strongmore » pinning below the Idaho Batholith. The large rotating section comprising eastern Oregon displays very low internal deformation rates despite seismological evidence for a thin crust, warm mantle, organized mantle flow, and elevated topography. The observed disparity between mantle and surface kinematics suggests that either little stress acts between them (low basal shear) or that the crust is strong relative to the mantle. The rotation of the Oregon block impinges on Washington across the Yakima fold-thrust belt where shortening occurs in a closing-fan style. Elastic fault locking at the Cascadia subduction zone is reevaluated using the GPS velocities and recently published uplift rates. The 18 year GPS and 80 year leveling data can both be matched with a common locking model suggesting that the locking has been stable over many decades. The rate of strain accumulation is consistent with hundreds of years between great subduction events.« less

Plate Margin Deformation and Active Tectonics Along the Northern Edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska and Yukon, Canada

NASA Technical Reports Server (NTRS)

Bruhn, Ronald L.; Sauber, Jeanne; Cotton, Michele M.; Pavlis, Terry L.; Burgess, Evan; Ruppert, Natalia; Forster, Richard R.

2012-01-01

The northwest directed motion of the Pacific plate is accompanied by migration and collision of the Yakutat terrane into the cusp of southern Alaska. The nature and magnitude of accretion and translation on upper crustal faults and folds is poorly constrained, however, due to pervasive glaciation. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of the transition from strike-slip motion on the Fairweather fault to plate margin deformation on the Bagley fault, which cuts through the upper plate of the collisional suture above the subduction megathrust. The Fairweather fault terminates by oblique-extensional splay faulting within a structural syntaxis, allowing rapid tectonic upwelling of rocks driven by thrust faulting and crustal contraction. Plate motion is partly transferred from the Fairweather to the Bagley fault, which extends 125 km farther west as a dextral shear zone that is partly reactivated by reverse faulting. The Bagley fault dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a narrow fault-bounded crustal sliver in the obliquely convergent plate margin. Since . 20 Ma the Bagley fault has accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern flank during terrane accretion. The fault is considered capable of generating earthquakes because it is linked to faults that generated large historic earthquakes, suitably oriented for reactivation in the contemporary stress field, and locally marked by seismicity. The fault may generate earthquakes of Mw <= 7.5.

Earthquake source parameters along the Hellenic subduction zone and numerical simulations of historical tsunamis in the Eastern Mediterranean

NASA Astrophysics Data System (ADS)

Yolsal-Çevikbilen, Seda; Taymaz, Tuncay

2012-04-01

We studied source mechanism parameters and slip distributions of earthquakes with Mw ≥ 5.0 occurred during 2000-2008 along the Hellenic subduction zone by using teleseismic P- and SH-waveform inversion methods. In addition, the major and well-known earthquake-induced Eastern Mediterranean tsunamis (e.g., 365, 1222, 1303, 1481, 1494, 1822 and 1948) were numerically simulated and several hypothetical tsunami scenarios were proposed to demonstrate the characteristics of tsunami waves, propagations and effects of coastal topography. The analogy of current plate boundaries, earthquake source mechanisms, various earthquake moment tensor catalogues and several empirical self-similarity equations, valid for global or local scales, were used to assume conceivable source parameters which constitute the initial and boundary conditions in simulations. Teleseismic inversion results showed that earthquakes along the Hellenic subduction zone can be classified into three major categories: [1] focal mechanisms of the earthquakes exhibiting E-W extension within the overriding Aegean plate; [2] earthquakes related to the African-Aegean convergence; and [3] focal mechanisms of earthquakes lying within the subducting African plate. Normal faulting mechanisms with left-lateral strike slip components were observed at the eastern part of the Hellenic subduction zone, and we suggest that they were probably concerned with the overriding Aegean plate. However, earthquakes involved in the convergence between the Aegean and the Eastern Mediterranean lithospheres indicated thrust faulting mechanisms with strike slip components, and they had shallow focal depths (h < 45 km). Deeper earthquakes mainly occurred in the subducting African plate, and they presented dominantly strike slip faulting mechanisms. Slip distributions on fault planes showed both complex and simple rupture propagations with respect to the variation of source mechanism and faulting geometry. We calculated low stress drop values (Δσ < 30 bars) for all earthquakes implying typically interplate seismic activity in the region. Further, results of numerical simulations verified that damaging historical tsunamis along the Hellenic subduction zone are able to threaten especially the coastal plains of Crete and Rhodes islands, SW Turkey, Cyprus, Levantine, and Nile Delta-Egypt regions. Thus, we tentatively recommend that special care should be considered in the evaluation of the tsunami risk assessment of the Eastern Mediterranean region for future studies.

Cretaceous-Cenozoic Geological Evolution of Tibet: Tectonic Interpretations and Outstanding Questions (Invited)

NASA Astrophysics Data System (ADS)

Kapp, P. A.; Decelles, P. G.; Ding, L.; van Hinsbergen, D. J.

2010-12-01

The India-Asia collision, although profound, is only the most recent in a series of orogenic events that has modified the architecture of the Asian lithosphere. For instance, large parts of central Tibet (Lhasa and Qiangtang terranes) underwent >50% upper-crustal shortening, and likely substantial elevation gain, between Cretaceous and Eocene time in response to Lhasa - Qiangtang continental collision and Andean-style orogenesis along the southern margin of Asia. Findings by independent groups of Gangdese-arc-age detrital zircons in 52-50 Ma Tethyan Himalaya (TH) strata indicate that TH-Asia collision was ongoing by this time. This collision timing is consistent with multiple other, albeit less direct lines of evidence and suggests that a magmatic flare-up within the Gangdese arc (culminated at 52-51 Ma) occurred during subduction of TH lithosphere. Low-temperature thermochronologic data indicate that very low erosion rates, and likely plateau-like conditions considering the shortening history, were established in large parts of central Tibet at or by 50-45 Ma. The temporal-spatial distribution of subsequent shortening and exhumation is consistent with plateau growth northward and southward from central Tibet since the Eocene. The Cenozoic magmatic record of Tibet shows intriguing temporal-spatial patterns. Between 45 Ma and 30 Ma, volcanism swept >600 km northward from the Indus-Yarlung suture (IYS) and then back southward between 30 Ma and 25 Ma. These magmatic sweeps may have been produced by underthrusting and subsequent rollback of subducting TH lithosphere. Recent stratigraphic and structural studies suggest localized extension and elevation loss along the IYS at ~25 Ma, which is explainable in a slab rollback scenario, followed within a few million years by uplift back to near-modern elevations, perhaps in response to breakoff of TH lithosphere and northward underthrusting of Indian lithosphere. This hypothesis of TH - Indian lithosphere subduction can explain how ~2000 km of India-Asia convergence was accommodated south of the IYS since ~50 Ma (with the remaining ~1000 km accommodated by shortening of Asian lithosphere). Outstanding questions include: (1) What are the explanations for major, coeval geological changes in the Lhasa terrane, Gangdese forearc, IYS, and TH at 65-63 Ma, which have led some workers to argue for initiation of India-Asia collision at this time? (2) What was the nature of the subducted TH lithosphere and its former paleogeographic and tectonic relationships to Indian cratonic lithosphere? (3) Why has only <50% of the estimated 2000 km of post-50 Ma convergence south of the Indus-Yarlung suture been documented as shortening within the Tethyan-Himalayan thrust belts? (4) Why did Asian lithosphere in Pamir and Tibet behave so differently in response to collisional orogenesis?

Investigation of Collisional Styles of the Caribbean Large Igneous Province (CLIP) vs. Normal Oceanic Crust from Seismic Reflection Profiles

NASA Astrophysics Data System (ADS)

Mataracioglu, M.; Magnani, M.; DeShon, H. R.; Cox, R. T.

2011-12-01

The Caribbean plate subducts beneath the North American and the South American plates at the Muertos Trough and the South Caribbean Deformed Belt (SCDB), respectively. During subduction, large amount of crustal material may enter the subduction zone with the subducting plate or may be incorporated into the accretionary prism. To investigate the changes in collisional style and structures associated with subduction of the Caribbean Large Igneous Province (CLIP) versus normal oceanic crust, we interpret seismic reflection profiles collected around the northern and southern margins of the Venezuelan Basin. We refine the extent of the CLIP in the central and eastern Caribbean by identifying the structural differences at the top of the acoustic basement (Horizon B") on a dataset of 150 multi-channel seismic time stack and migrated marine reflection profiles acquired in eight cruises from 1975 to 2004, as well as some selected Integrated Ocean Drilling Program (IODP) drilling data. We will also attempt to determine whether sedimentary material enters the trench and is recycled back into the mantle, and therefore characterize the northern and southern subduction zones as accretionary or erosive. Our preliminary results show that the CLIP extends spatially to most of the Venezuelan Basin, the western part of the Columbian Basin, and a part of the Beata Ridge, but that it does not extend as far south as suggested by previous interpretations. Furthermore, some portions of the CLIP at the northern and southern boundaries subduct beneath the North and the South American plates at the Muertos Trough and the SCDB, respectively. The change in nature of the subducting plate (CLIP or normal oceanic crust) causes variations in the collisional style (i.e., accretionary versus erosive) and in structures at the accretionary wedge and on the downgoing plate.

Vertical tectonics at an active continental margin

NASA Astrophysics Data System (ADS)

Houlié, N.; Stern, T. A.

2017-01-01

Direct observations of vertical movements of the earth's surface are now possible with space-based GPS networks, and have applications to resources, hazards and tectonics. Here we present data on vertical movements of the Earth's surface in New Zealand, computed from the processing of GPS data collected between 2000 and 2015 by 189 permanent GPS stations. We map the geographical variation in vertical rates and show how these variations are explicable within a tectonic framework of subduction, volcanic activity and slow slip earthquakes. Subsidence of >3 mm/yr is observed along southeastern North Island and is interpreted to be due to the locked segment of the Hikurangi subduction zone. Uplift of 1-3 mm/yr further north along the margin of the eastern North Island is interpreted as being due to the plate interface being unlocked and underplating of sediment on the subduction thrust. The Volcanic Plateau of the central North Island is being uplifted at about 1 mm/yr, which can be explained by basaltic melts being injected in the active mantle-wedge at a rate of ∼6 mm/yr. Within the Central Volcanic Region there is a 250 km2 area that subsided between 2005 and 2012 at a rate of up to 14 mm/yr. Time series from the stations located within and near the zone of subsidence show a strong link between subsidence, adjacent uplift and local earthquake swarms.

Exhumation history of the West Kunlun Mountains, northwestern Tibet: Evidence for a long-lived, rejuvenated orogen

NASA Astrophysics Data System (ADS)

Cao, Kai; Wang, Guo-Can; Bernet, Matthias; van der Beek, Peter; Zhang, Ke-Xin

2015-12-01

How and when the northwestern Tibetan Plateau originated and developed upon pre-existing crustal and topographic features is not well understood. To address this question, we present an integrated analysis of detrital zircon U-Pb and fission-track double dating of Cenozoic synorogenic sediments from the Kekeya and Sanju sections in the southwestern Tarim Basin. These data help establishing a new chronostratigraphic framework for the Sanju section and confirm a recent revision of the chronostratigraphy at Kekeya. Detrital zircon fission-track ages present prominent Triassic-Early Jurassic (∼250-170 Ma) and Early Cretaceous (∼130-100 Ma) static age peaks, and Paleocene-Early Miocene (∼60-21 Ma) to Eocene-Late Miocene (∼39-7 Ma) moving age peaks, representing source exhumation. Triassic-Early Jurassic static peak ages document unroofing of the Kunlun terrane, probably related to the subduction of Paleotethys oceanic lithosphere. In combination with the occurrence of synorogenic sediments on both flanks of the Kunlun terrane, these data suggest that an ancient West Kunlun range had emerged above sea level by Triassic-Early Jurassic times. Early Cretaceous fission-track peak ages are interpreted to document exhumation related to thrusting along the Tam Karaul fault, kinematically correlated to the Main Pamir thrust further west. Widespread Middle-Late Mesozoic crustal shortening and thickening likely enhanced the Early Mesozoic topography. Paleocene-Early Eocene fission-track peak ages are presumably partially reset. Limited regional exhumation indicates that the Early Cenozoic topographic and crustal pattern of the West Kunlun may be largely preserved from the Middle-Late Mesozoic. The Main Pamir-Tam Karaul thrust belt could be a first-order tectonic feature bounding the northwestern margin of the Middle-Late Mesozoic to Early Cenozoic Tibetan Plateau. Toward the Tarim basin, Late Oligocene-Early Miocene steady exhumation at a rate of ∼0.9 km/Myr is likely related to initial thrusting of the Tiklik fault and reactivation of the Tam Karaul thrust. Thrusting together with upper crustal shortening in the mountain front indicates basinward expansion of the West Kunlun orogen at this time. This episode of exhumation and uplift, associated with magmatism across western Tibet, is compatible with a double-sided lithospheric wedge model, primarily driven by breakoff of the Indian crustal slab. Accelerated exhumation of the mountain front at a rate of ∼1.1 km/Myr since ∼15 Ma supports active compressional deformation at the margins of the northwestern Tibetan Plateau. We thus propose that the West Kunlun Mountains are a long-lived topographic unit, dating back to Triassic-Early Jurassic times, and have experienced Middle-Late Mesozoic to Early Cenozoic rejuvenation and Late Oligocene-Miocene expansion.

The late Archean Schreiber Hemlo and White River Dayohessarah greenstone belts, Superior Province: collages of oceanic plateaus, oceanic arcs, and subduction accretion complexes

NASA Astrophysics Data System (ADS)

Polat, A.; Kerrich, R.; Wyman, D. A.

1998-04-01

The late Archean (ca. 2.80-2.68 Ga) Schreiber-Hemlo and White River-Dayohessarah greenstone belts of the Superior Province, Canada, are supracrustal lithotectonic assemblages of ultramafic to tholeiitic basalt ocean plateau sequences, and tholeiitic to calc-alkaline volcanic arc sequences, and siliciclastic turbidites, collectively intruded by arc granitoids. The belts have undergone three major phases of deformation; two probably prior to, and one during the assembly of the southern Superior Province. Imbricated lithotectonic assemblages are often disrupted by syn-accretion strike-slip faults, suggesting that strike-slip faulting was an important aspect of greenstone belt evolution. Field relations, structural characteristics, and high-precision ICP-MS trace-element data obtained for representative lithologies of the Schreiber-Hemlo and White River-Dayohessarah greenstone belts suggest that they represent collages of oceanic plateaus, juvenile oceanic island arcs, in subduction-accretion complexes. Stratigraphic relationships, structural, and geochemical data from these Archean greenstone belts are consistent with a geodynamic evolution commencing with the initiation of a subduction zone at the margins of an oceanic plateau, similar to the modern Caribbean oceanic plateau and surrounding subduction-accretion complexes. All supracrustal assemblages include both ocean plateau and island-arc geochemical characteristics. The structural and geochemical characteristics of vertically and laterally dismembered supracrustal units of the Schreiber-Hemlo and White River-Dayohessarah greenstone belts cannot be explained either by a simple tectonic juxtaposition of lithotectonic assemblages with stratified volcanic and sedimentary units, or cyclic mafic to felsic bimodal volcanism models. A combination of out-of-sequence thrusting, and orogen-parallel strike-slip faulting of accreted ocean plateaus, oceanic arcs, and trench turbidites can account for the geological and geochemical characteristics of these greenstone belts. Following accretion, all supracrustal assemblages were multiply intruded by syn- to post-tectonic high-Al, and high-La/Yb n slab-derived trondhjemite-tonalite-granodiorite (TTG) plutons. The amalgamation processes of these lithotectonic assemblages are comparable to those of Phanerozoic subduction-accretion complexes, such as the Circum-Pacific, the western North American Cordilleran, and the Altaid orogenic belts, suggesting that subduction-accretion processes significantly contributed to the growth of the continental crust in the late Archean. The absence of blueschist and eclogite facies metamorphic rocks in Archean subduction-accretion complexes may be attributed to elevated thermal gradients and shallow-angle subduction. The melting of a hotter Archean mantle at ridges and in plumes would generate relatively small, hot, and hence shallowly subducting oceanic plates, promoting high-temperature metamorphism, migmatization, and slab melting. Larger, colder, Phanerozoic plates typically subduct at a steeper angle, generating high-pressure low-temperature conditions for blueschists and eclogites in the subduction zones, and low-La/Yb n granitoids from slab dehydration, and wedge melting. Metasedimentary subprovinces in the Superior Province, such as the Quetico and English River Subprovinces, have formerly been interpreted as accretionary complexes, outboard of the greenstone belt magmatic arcs. Here the greenstone-granitoid subprovinces are interpreted as collages of subduction-accretion complexes, island arcs and oceanic plateaus amalgamated at convergent plate margins, and the neighbouring metasedimentary subprovinces as foreland basins.

Boundary conditions traps when modeling interseismic deformation at subduction zones

NASA Astrophysics Data System (ADS)

Contreras, Marcelo; Gerbault, Muriel; Tassara, Andres; Bataille, Klaus; Araya, Rodolfo

2017-04-01

In order to gain insight on the controling factors for elastic strain build-up in subduction zones, such as those triggering the Mw 8. 2010 Maule earthquake, we published a modeling study to test the influence of the subducting plate thickness, variations in the updip and downdip limit of a 100% locked interplate zone, elastic parameters, and velocity reduction at the base of the subducted slab (Contreras et al., Andean Geology 43(3), 2016). When comparing our modeled predictions with interseismic GPS observations, our results indicated little influence of the subducting plate thickness, but a necessity to reduce the velocity at the corner-base of the subducted slab below the trench region, to 10% of the far-field convergence rate. Complementary numerical models allowed us to link this velocity reduction at the base of subducting slab with a long-term high flexural stress resulting from the mechanical interaction of the slab with the underlying mantle. This study discusses that even if only a small amount of these high deviatoric stresses transfer energy towards the upper portion of the slab, it may participate in triggering large earthquakes such as the Mw8.8 Maule event. The definition of initial and boundary conditions between short-term to long-term models evidence the mechanical inconsistencies that may appear when considering pre-flexed subducting slabs and unloaded underlying asthenosphere, potentially creating mis-balanced large stress discontinuities.

Heterogeneous coupling along Makran subduction zone

NASA Astrophysics Data System (ADS)

Zarifi, Z.; Raeesi, M.

2010-12-01

The Makran subduction zone, located in the southeast of Iran and southern Pakistan, extends for almost 900 km along the Eurasian-Arabian plate boundary. The seismic activities in the eastern and western Makran exhibit very different patterns. The eastern Makran characterized by infrequent large earthquakes and low level of seismicity. The only large instrumentally recorded earthquake in the eastern Makran, the 27 Nov. 1945 (Mw=8.1) earthquake, was followed by tsunami waves with the maximum run-up height of 13 m and disastrous effects in Pakistan, India, Iran and Oman. The western Makran, however, is apparently quiescent without strong evidence on occurrence of large earthquakes in historical times, which makes it difficult to ascertain whether the slab subducts aseismically or experiences large earthquakes separated by long periods exceeding the historical records. We used seismicity and Trench Parallel Free air and Bouguer Anomalies (TPGA and TPBA) to study the variation in coupling in the slab interface. Using a 3D mechanical Finite Element (FE) model, we show how heterogeneous coupling can influence the rate of deformation in the overriding lithosphere and the state of stress in the outer rise, overriding, and subducting plates within the shortest expected cycle of earthquake. We test the results of FE model against the observed focal mechanism of earthquakes and available GPS measurements in Makran subduction zone.

The challenges of numerically simulating analogue brittle thrust wedges

NASA Astrophysics Data System (ADS)

Buiter, Susanne; Ellis, Susan

2017-04-01

Fold-and-thrust belts and accretionary wedges form when sedimentary and crustal rocks are compressed into thrusts and folds in the foreland of an orogen or at a subduction trench. For over a century, analogue models have been used to investigate the deformation characteristics of such brittle wedges. These models predict wedge shapes that agree with analytical critical taper theory and internal deformation structures that well resemble natural observations. In a series of comparison experiments for thrust wedges, called the GeoMod2004 (1,2) and GeoMod2008 (3,4) experiments, it was shown that different numerical solution methods successfully reproduce sandbox thrust wedges. However, the GeoMod2008 benchmark also pointed to the difficulties of representing frictional boundary conditions and sharp velocity discontinuities with continuum numerical methods, in addition to the well-known challenges of numerical plasticity. Here we show how details in the numerical implementation of boundary conditions can substantially impact numerical wedge deformation. We consider experiment 1 of the GeoMod2008 brittle thrust wedge benchmarks. This experiment examines a triangular thrust wedge in the stable field of critical taper theory that should remain stable, that is, without internal deformation, when sliding over a basal frictional surface. The thrust wedge is translated by lateral displacement of a rigid mobile wall. The corner between the mobile wall and the subsurface is a velocity discontinuity. Using our finite-element code SULEC, we show how different approaches to implementing boundary friction (boundary layer or contact elements) and the velocity discontinuity (various smoothing schemes) can cause the wedge to indeed translate in a stable manner or to undergo internal deformation (which is a fail). We recommend that numerical studies of sandbox setups not only report the details of their implementation of boundary conditions, but also document the modelling attempts that failed. References 1. Buiter and the GeoMod2004 Team, 2006. The numerical sandbox: comparison of model results for a shortening and an extension experiment. Geol. Soc. Lond. Spec. Publ. 253, 29-64 2. Schreurs and the GeoMod2004 Team, 2006. Analogue benchmarks of shortening and extension experiments. Geol. Soc. Lond. Spec. Publ. 253, 1-27 3. Buiter, Schreurs and the GeoMod2008 Team, 2016. Benchmarking numerical models of brittle thrust wedges, J. Struct. Geol. 92, 140-177 4. Schreurs, Buiter and the GeoMod2008 Team, 2016. Benchmarking analogue models of brittle thrust wedges, J. Struct. Geol. 92, 116-13

Shallow subsurface imaging of northern Cascadia margin using downward continued short-streamer data

NASA Astrophysics Data System (ADS)

Yelisetti, S.; Ghosal, D.; Spence, G.

2017-12-01

Since Eocene, the Juan de Fuca plate has been subducting beneath the North American plate, scrapping off sediments from the down going plate, accreting to the margin forming frontal accretionary wedge, Crescent terrane and the Pacific Rim terrane, which are separated by landward dipping Crescent thrust and Tofino faults. In 1989, the Geological Survey of Canada has acquired several multichannel seismic lines along the northern Cascadia margin to study the subduction zone processes and the formation and distribution of methane hydrates in the accreted sediment section. Seismic reflections and refractions are recorded on a 3.6 km streamer up to 14 s using 4 ms sample rate with 183 m near-offset. In this study, we present the migrated image of line 89-06 which indicate the top of the down-going plate and several landward dipping frontal thrusts. Additionally, a bottom simulating reflector (BSR) is identified over a 20 km distance at a depth of 250 m beneath the seafloor within the accretionary wedge sediments where the water depth is around 1500-2000 m. Preliminary velocity analyses corresponding to the BSR reflection using semblance method indicate high-velocity sediment with P-wave velocities of 2.0 km/s. To better constrain the velocity distribution of such shallow subsurface features, we have analyzed the refracted arrivals from the seaward part of the Tofino basin sediment section. Specifically, we have downward continued the shot and receiver gathers to the seafloor bringing the far offset refracted phases to near offset as first arrivals. Since the refractions are not well captured over the trench deposits due to large water depth ( 2500 m) and limited streamer length, the downward continued results do not show refracted arrivals very clearly at the near offset. In contrast, moving landward along the frontal slope with gradual decrease in water depth to 1300 m and less, the effect of downward continuation seems to be more prominent bringing the refracted phases to near offset more clearly as first arrivals. More detailed first arrival tomographic velocity analysis is currently underway using these downward continued datasets. The tomographic velocity model will then be used as a starting model for future full waveform inversion to obtain the high-resolution velocity and attenuation models of the accreted sediments.

Effect of blade outlet angle on radial thrust of single-blade centrifugal pump

NASA Astrophysics Data System (ADS)

Nishi, Y.; Fukutomi, J.; Fujiwara, R.

2012-11-01

Single-blade centrifugal pumps are widely used as sewage pumps. However, a large radial thrust acts on a single blade during pump operation because of the geometrical axial asymmetry of the impeller. This radial thrust causes vibrations of the pump shaft, reducing the service life of bearings and shaft seal devices. Therefore, to ensure pump reliability, it is necessary to quantitatively understand the radial thrust and clarify the behavior and generation mechanism. This study investigated the radial thrust acting on two kinds of single-blade centrifugal impellers having different blade outlet angles by experiments and computational fluid dynamics (CFD) analysis. Furthermore, the radial thrust was modeled by a combination of three components, inertia, momentum, and pressure, by applying an unsteady conservation of momentum to this impeller. As a result, the effects of the blade outlet angle on both the radial thrust and the modeled components were clarified. The total head of the impeller with a blade outlet angle of 16 degrees increases more than the impeller with a blade outlet angle of 8 degrees at a large flow rate. In this case, since the static pressure of the circumference of the impeller increases uniformly, the time-averaged value of the radial thrust of both impellers does not change at every flow rate. On the other hand, since the impeller blade loading becomes large, the fluctuation component of the radial thrust of the impeller with the blade outlet angle of 16 degrees increases. If the blade outlet angle increases, the fluctuation component of the inertia component will increase, but the time-averaged value of the inertia component is located near the origin despite changes in the flow rate. The fluctuation component of the momentum component becomes large at all flow rates. Furthermore, although the time-averaged value of the pressure component is almost constant, the fluctuation component of the pressure component becomes large at a large flow rate. In addition to the increase of the fluctuation component of this pressure component, because the fluctuation component of the inertia and momentum components becomes large (as mentioned above), the radial thrust increases at a large flow rate, as is the case for the impeller with a large blade outlet angle.

Revisiting the North Chile seismic gap segmentation using GPS-derived interseismic coupling

NASA Astrophysics Data System (ADS)

Métois, Marianne; Socquet, Anne; Vigny, Christophe; Carrizo, Daniel; Sophie, Peyrat

2013-04-01

The North Chile area did not rupture since the 1877 Mw 8.6 earthquake that produced a huge tsunami. Considering that upper plate deformation measured there by modern geodetic tools is due to some degree of locking on the subduction interface and the long elapsed time since 1877, many consider this area is a mature seismic gap where seismic hazard is high. We present a new GPS velocity field that describes in some detail the interseismic deformation between 18°S and 24°S. We invert for coupling distribution on the subduction interface using elastic modeling. Our measurements require that, at these latitudes, 10 to 12 mm/yr (i.e ~15% of the whole convergence rate) are taken up by the clockwise rigid rotation of an Andean block bounded to the East by the subandean fold-and-thrust belt. This reduces the accumulation rate on the subduction interface to ~56 mm/yr in this area. We describe coupling variations on the subduction interface both along-strike and along-dip. We find that this gap is segmented in at least two highly locked segments and two narrow low coupled intersegment zones (Iquique and Mejillones areas). This coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region. Intersegments correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates. The rupture of either the Paranal or the Loa segment alone could easily produce a Mw 8.0-8.3 rupture, and we propose that the Loa segment (from 22.5°S to 20.8°S) may be the one that ruptured in 1877.

Probabilistic tsunami hazard assessment for Makran considering recently suggested larger maximum magnitudes and sensitivity analysis for GNSS-based early warning

NASA Astrophysics Data System (ADS)

Zamora, N.; Hoechner, A.; Babeyko, A. Y.

2014-12-01

Iran and Pakistan are countries frequently affected by destructive earthquakes, as for instance, the magnitude 6.6 Bam earthquake in 2003 in Iran with about 30 000 casualties, or the magnitude 7.6 Kashmir earthquake 2005 in Pakistan with about 80'000 casualties. Both events took place inland, but in terms of magnitude, even significantly larger events can be expected to happen offshore, at the Makran subduction zone. This small subduction zone is seismically rather quiescent, nevertheless a tsunami caused by a thrust event in 1945 (Balochistan earthquake) led to about 4000 casualties. Nowadays, the coastal regions are more densely populated and vulnerable to similar events. Furthermore, some recent publications discuss the possiblity of rather rare huge magnitude 9 events at the Makran subduction zone. We analyze the seismicity at the subduction plate interface and generate various synthetic earthquake catalogs spanning 100000 years. All the events are projected onto the plate interface using scaling relations and a tsunami model is run for every scenario. The tsunami hazard along the coast is computed and presented in the form of annual probability of exceedance, probabilistic tsunami height for different time periods and other measures. We show how the hazard reacts to variation of the Gutenberg-Richter parameters and maximum magnitudes.We model the historic Balochistan event and its effect in terms of coastal wave heights. Finally, we show how an effective tsunami early warning could be achieved by using an array of high-precision real-time GNSS (Global Navigation Satellite System) receivers along the coast by applying it to the 1945 event and by performing a sensitivity analysis.

Structural styles and zircon ages of the South Tianshan accretionary complex, Atbashi Ridge, Kyrgyzstan: Insights for the anatomy of ocean plate stratigraphy and accretionary processes

NASA Astrophysics Data System (ADS)

Sang, Miao; Xiao, Wenjiao; Orozbaev, Rustam; Bakirov, Apas; Sakiev, Kadyrbek; Pak, Nikolay; Ivleva, Elena; Zhou, Kefa; Ao, Songjian; Qiao, Qingqing; Zhang, Zhixin

2018-03-01

The anatomy of an ancient accretionary complex has a significance for a better understanding of the tectonic processes of accretionary orogens and complex because of its complicated compositions and strong deformation. With a thorough structural and geochronological study of a fossil accretionary complex in the Atbashi Ridge, South Tianshan (Kyrgyzstan), we analyze the structure and architecture of ocean plate stratigraphy in the western Central Asian Orogenic Belt. The architecture of the Atbashi accretionary complex is subdivisible into four lithotectonic assemblages, some of which are mélanges with "block-in-matrix" structure: (1) North Ophiolitic Mélange; (2) High-pressure (HP)/Ultra-high-pressure (UHP) Metamorphic Assemblage; (3) Coherent & Mélange Assemblage; and (4) South Ophiolitic Mélange. Relationships between main units are tectonic contacts presented by faults. The major structures and lithostratigraphy of these units are thrust-fold nappes, thrusted duplexes, and imbricated ocean plate stratigraphy. All these rock units are complicatedly stacked in 3-D with the HP/UHP rocks being obliquely southwestward extruded. Detrital zircon ages of meta-sediments provide robust constraints on their provenance from the Ili-Central Tianshan Arc. The isotopic ages of the youngest components of the four units are Late Permian, Early-Middle Triassic, Early Carboniferous, and Early Triassic, respectively. We present a new tectonic model of the South Tianshan; a general northward subduction polarity led to final closure of the South Tianshan Ocean in the End-Permian to Late Triassic. These results help to resolve the long-standing controversy regarding the subduction polarity and the timing of the final closure of the South Tianshan Ocean. Finally, our work sheds lights on the use of ocean plate stratigraphy in the analysis of the tectonic evolution of accretionary orogens.

Fore-arc basin deformation in the Andaman-Nicobar segment of the Sumatra-Andaman subduction zone: Insight from high-resolution seismic reflection data

NASA Astrophysics Data System (ADS)

Moeremans, Raphaële E.; Singh, Satish C.

2015-08-01

The Andaman-Nicobar region is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex active back-arc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar fore-arc basin, from 8°N to 11°N, in order to better understand its structure and evolution, focusing on (1) how obliquity of convergence affects deformation in the fore arc, (2) the nature and role of the Diligent Fault (DF), and (3) the Eastern Margin Fault (EMF). Despite the obliquity of convergence, back thrusting and compression seem to dominate the Andaman-Nicobar fore-arc basin deformation. The DF is primarily a back thrust and corresponds to the Mentawai and West Andaman Fault systems farther in the south, along Sumatra. The DF is expressed in the fore-arc basin as a series of mostly landward verging folds and faults, deforming the early to late Miocene sediments. The DF seems to root from the boundary between the accretionary complex and the continental backstop, where it meets the EMF. The EMF marks the western boundary of the fore-arc basin; it is associated with subsidence and is expressed as a deep piggyback basin, containing recent Pliocene to Pleistocene sediments. The eastern edge of the fore-arc basin is the Invisible Bank (IB), which is thought to be tilted and uplifted continental crust. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea.

What role did the Hikurangi subduction zone play in the M7.8 Kaikoura earthquake?

NASA Astrophysics Data System (ADS)

Wallace, L. M.; Hamling, I. J.; Kaneko, Y.; Fry, B.; Clark, K.; Bannister, S. C.; Ellis, S. M.; Francois-Holden, C.; Hreinsdottir, S.; Mueller, C.

2017-12-01

The 2016 M7.8 Kaikoura earthquake ruptured at least a dozen faults in the northern South Island of New Zealand, within the transition from the Hikurangi subduction zone (in the North Island) to the transpressive Alpine Fault (in the central South Island). The role that the southern end of the Hikurangi subduction zone played (or did not play) in the Kaikoura earthquake remains one of the most controversial aspects of this spectacularly complex earthquake. Investigations using near-field seismological and geodetic data suggest a dominantly crustal faulting source for the event, while studies relying on teleseismic data propose that a large portion of the moment release is due to rupture of the Hikurangi subduction interface beneath the northern South Island. InSAR and GPS data also show that a large amount of afterslip (up to 0.5 m) occurred on the subduction interface beneath the crustal faults that ruptured in the M7.8 earthquake, during the months following the earthquake. Modeling of GPS velocities for the 20 year period prior to the earthquake indicate that interseismic coupling was occurring on the Hikurangi subduction interface beneath the northern South Island, in a similar location to the suggested coseismic and postseismic slip on the subduction interface. We will integrate geodetic, seismological, tsunami, and geological observations in an attempt to balance the seemingly conflicting views from local and teleseismic data regarding the role that the southern Hikurangi subduction zone played in the earthquake. We will also discuss the broader implications of the observed coseismic and postseismic deformation for understanding the kinematics of the southern termination of the Hikurangi subduction zone, and its role in the transition from subduction to strike-slip in the central New Zealand region.

Controls on intrusion of near-trench magmas of the Sanak-Baranof Belt, Alaska, during Paleogene ridge subduction, and consequences for forearc evolution

USGS Publications Warehouse

Kusky, Timothy M.; Bradley, Dwight C.; Donely, D. Thomas; Rowley, David; Haeussler, Peter J.

2003-01-01

A belt of Paleogene near-trench plutons known as the Sanak-Baranof belt intruded the southern Alaska convergent margin. A compilation of isotopic ages of these plutons shows that they range in age from 61 Ma in the west to ca. 50 Ma in the east. This migrating pulse of magmatism along the continental margin is consistent with North Pacific plate reconstructions that suggests the plutons were generated by migration of a trench-ridge-trench triple junction along the margin. On the Kenai Peninsula the regional lower greenschist metamorphic grade of the turbiditic host rocks, texture of the plutons, contact-metamorphic assemblage, and isotopic and fluid inclusion studies suggest that the plutons were emplaced at pressures of 1.5–3.0 kbars (5.2–10.5 km) into a part of the accretionary wedge with an ambient temperature of 210–300 °C. The presence of kyanite, garnet, and cordierite megacrysts in the plutons indicates that the melts were generated at a depth greater than 20 km and minimum temperature of 650 °C. These megacrysts are probably xenocrystic remnants of a restitic or contact metamorphic phase entrained by the melt during intrusion. However, it is also possible that they are primary magmatic phases crystallized from the peraluminous melt.Plutons of the Sanak-Baranof belt serve as time and strain markers separating kinematic regimes that predate and postdate ridge subduction. Pre-ridge subduction structures are interpreted to be related to the interaction between the leading oceanic plate and the Chugach terrane. These include regional thrust faults, NE-striking map-scale folds with associated axial planar foliation, type-1 mélanges, and an arrayof faults within the contact aureole indicating shortening largely accommodated by layer-parallel extension. Syn-ridge subduction features include the plutons, dikes, and ductile shear zones within contact aureoles with syn-kinematic metamorphic mineral growth and foliation development. Many of the studied plutons have sheeted margins and appear to have intruded along extensional jogs in margin-parallel strike-slip faults, whereas others form significant angles with the main faults and may have been influenced by minor faults of other orientations. Some of the plutons of the Sanak-Baranof belt have their long axes oriented parallel to faults of an orthorhombic fault set, implying that these faults may have provided a conduit for magma emplacement. This orthorhombic set of late faults is interpreted to have initially formed during the ridge subduction event, and continued to be active for a short time after passage of the triple junction. ENE-striking dextral faults of this orthorhombic fault system exhibit mutually crosscutting relationships with Eocene dikes related to ridge subduction, and mineralized strike-slip and normal faults of this system have yielded 40Ar/39Ar ages identical to near-trench intrusives related to ridge subduction. Movement on the orthorhombic fault system accommodated exhumation of deeper levels of the southern Alaska accretionary wedge, which is interpreted as a critical taper adjustment to subduction of younger oceanic lithosphere during ridge subduction. These faults therefore accommodate both deformation of the wedge and assisted emplacement of near-trench plutons. Structures that crosscut the plutons and aureoles include the orthorhombic fault set and dextral strike-slip faults, reflecting a new kinematic regime established after ridge subduction, during underthrusting of the trailing oceanic plate with new dextral-oblique convergence vectors with the overriding plate. The observation that the orthorhombic fault set both cuts and is cut by Eocene intrusives demonstrates the importance of these faults for magma emplacement in the forearc.A younger, ca. 35 Ma suite of plutons intrudes the Chugach terrane in the Prince William Sound region, and their intrusion geometry was strongly influenced by pre-existing faults developed during ridge subduction. The generation of these plutons may be related to the sudden northward migration of the triple junction at ca. 40–33 Ma, as the ridge was being subducted nearly parallel to the trench during this interval. These younger plutons are used to provide additional constraints on the structural evolution of the wedge. Late- to post-ridge subduction fabrics include a pressure solution cleavage and additional movement on the orthorhombic fault system. After triple junction migration, subduction of the trailing oceanic plate involved a significant component of dextral transpression and northward translation of the Chugach terrane. This change in kinematics is recorded by very late gouge-filled dextral faults in the late structures of the accretionary prism.

Evidence for retrograde lithospheric subduction on Venus

NASA Technical Reports Server (NTRS)

Sandwell, David T.; Schubert, Gerald

1992-01-01

Though there is no plate tectonics per se on Venus, recent Magellan radar images and topographic profiles of the planet suggest the occurrence of the plate tectonic processes of lithospheric subduction and back-arc spreading. The perimeters of several large coronae (e.g., Latona, Artemis, and Eithinoha) resemble Earth subduction zones in both their planform and topographic profile. The planform of arcuate structures in Eastern Aphrodite were compared with subduction zones of the East Indies. The venusian structures have radii of curvature that are similar to those of terrestrial subduction zones. Moreover, the topography of the venusian ridge/trench structures is highly asymmetric with a ridge on the concave side and a trough on the convex side; Earth subduction zones generally display the same asymmetry.

Primary propulsion/large space system interactions

NASA Technical Reports Server (NTRS)

Dergance, R. H.

1980-01-01

Three generic types of structural concepts and nonstructural surface densities were selected and combined to represent potential LSS applications. The design characteristics of various classes of large space systems that are impacted by primary propulsion thrust required to effect orbit transfer were identified. The effects of propulsion system thrust-to-mass ratio, thrust transients, and performance on the mass, area, and orbit transfer characteristics of large space systems were determined.

Results from CAT/SCAN, the Calabria-Apennine-Tyrrhenian/Subduction-Accretion-Collision Network

NASA Astrophysics Data System (ADS)

Steckler, M. S.; Amato, A.; Guerra, I.; Armbruster, J.; Baccheschi, P.; Diluccio, F.; Gervasi, A.; Harabaglia, P.; Kim, W.; Lerner-Lam, A.; Margheriti, L.; Seeber, L.; Tolstoy, M.; Wilson, C. K.

2005-12-01

The Calabrian Arc region is the final remnant of a Western Mediterranean microplate driven by rollback. Calabria itself is an exotic block that rifted off Sardinia and opened the Tyrrhenian Sea back-arc basin in its wake. The Calabrian Arc rapidly advanced to the southeast, with subduction ahead and extension behind, following subduction rollback of the Mesozoic seafloor. The subduction zone meanwhile collided progressively with the Apulia to form the Apennines in peninsular Italy and with the Africa to form the Maghrebides in Sicily. The Calabrian Arc is where the transition from subduction to continental collision is occurring. The collisions on either side of Calabria have restricted oceanic subduction to a narrow 200-km salient with well-defined edges and seismicity that extends to over 500 km depth. The collisions have also slowed, or possibly even halted, the rapid advance of the arc. Whether rollback of the oceanic lower plate of the Ionian Sea continues and whether the upper plate of Calabria continues to move as an independent plate are both uncertain. The Calabrian-Apennine-Tyrrhenian/Subduction-Collision-Accretion Network (CAT/SCAN) is a passive experiment to study of the Calabrian Arc and the transition to the southern Apennines. The land deployment consisted of three phases. The initial phase included an array of 39 broadband seismometers onshore, deployed in the winter of 2003/4. In September 2004, the array was reduced to 28 broadband and 8 short-period instruments. In April 2005, the array was reduced once again to 20 broadband and 2 short-period instruments. The field deployment was completed in October 2005. Offshore, 12 broadband Ocean Bottom Seismometers (OBSs) were deployed in the beginning of October 2004. Data from 4 OBSs have been recovered so far with deployment durations from a few weeks to almost one year. Fishing activity has been strongly implicated in the early recoveries, (with one instrument returned by fishermen), and is suspected for the instruments that were not recovered. The experiment is determining the structure of the Calabrian subduction and southern Apennine collision systems and the structure of the transition from oceanic subduction in Calabria to continental collision in the southern Apennines. We have delineated a strong anisotropy with a fast direction following the curved arc, but weaker anisotropy beneath the Tyrrhenian Sea. Receiver function images show variations in crustal thickness throughout the region, consistent with previous conceptual models. We also image a negative polarity interface dipping to the southwest that we interpret as the main thrust ramp in the north transitioning to the subduction interface in the south. The transition from one to the other is marked by a loss of amplitude in the Moho conversion. Local seismicity is consistent with surface structure in showing extension normal and parallel to the Calabrian forearc as well as continuing southeastward motion of Calabria relative to the southern Apennines and Maghrebides.

A possible source mechanism of the 1946 Unimak Alaska far-field tsunami, uplift of the mid-slope terrace above a splay fault zone

USGS Publications Warehouse

von Huene, Roland E.; Miller, John J.; Klaeschen, Dirk; Dartnell, Peter

2016-01-01

In 1946, megathrust seismicity along the Unimak segment of the Alaska subduction zone generated the largest ever recorded Alaska/Aleutian tsunami. The tsunami severely damaged Pacific islands and coastal areas from Alaska to Antarctica. It is the charter member of “tsunami” earthquakes that produce outsized far-field tsunamis for the recorded magnitude. Its source mechanisms were unconstrained by observations because geophysical data for the Unimak segment were sparse and of low resolution. Reprocessing of legacy geophysical data reveals a deep water, high-angle reverse or splay thrust fault zone that leads megathrust slip upward to the mid-slope terrace seafloor rather than along the plate boundary toward the trench axis. Splay fault uplift elevates the outer mid-slope terrace and its inner area subsides. Multibeam bathymetry along the splay fault zone shows recent but undated seafloor disruption. The structural configuration of the nearby Semidi segment is similar to that of the Unimak segment, portending generation of a future large tsunami directed toward the US West coast.

Deformation of the Eastern Franciscan Belt, northern California

USGS Publications Warehouse

Jayko, A.S.; Blake, M.C.

1989-01-01

The late Jurassic and Cretaceous Eastern Franciscan belt of the northern California Coast Range consists of two multiply deformed, blueschist-facies terranes; the Pickett Peak and Yolla Bolly terranes. Four deformations have been recognized in the Pickett Peak terrane, and three in the Yolla Bolly terrane. The earliest recognized penetrative fabric, D1, occurs only in the Pickett Peak terrane. The later penetrative fabrics, D2 and D3, occur in both the Yolla Bolly and Pickett Peak terranes. D1 and D2 apparently represent fabrics that formed during subduction and accretion of the terranes. Fabrics from both D1 and D2 are consistent with SW-NE movement directions with respect to their present geographic positions. D3 postdates blueschist-facies metamorphism of the terranes and may be related to emplacement of the terranes to higher structural levels. A broad regional warping, D4, is evident from the map pattern and folding of large metamorphosed thrust sheets. D4 folds may be related to deformation associated with oblique convergence along the continental margin in late Cretaceous and (or) early Tertiary time. ?? 1989.

A Possible Source Mechanism of the 1946 Unimak Alaska Far-Field Tsunami: Uplift of the Mid-Slope Terrace Above a Splay Fault Zone

NASA Astrophysics Data System (ADS)

von Huene, Roland; Miller, John J.; Klaeschen, Dirk; Dartnell, Peter

2016-12-01

In 1946, megathrust seismicity along the Unimak segment of the Alaska subduction zone generated the largest ever recorded Alaska/Aleutian tsunami. The tsunami severely damaged Pacific islands and coastal areas from Alaska to Antarctica. It is the charter member of "tsunami" earthquakes that produce outsized far-field tsunamis for the recorded magnitude. Its source mechanisms were unconstrained by observations because geophysical data for the Unimak segment were sparse and of low resolution. Reprocessing of legacy geophysical data reveals a deep water, high-angle reverse or splay thrust fault zone that leads megathrust slip upward to the mid-slope terrace seafloor rather than along the plate boundary toward the trench axis. Splay fault uplift elevates the outer mid-slope terrace and its inner area subsides. Multibeam bathymetry along the splay fault zone shows recent but undated seafloor disruption. The structural configuration of the nearby Semidi segment is similar to that of the Unimak segment, portending generation of a future large tsunami directed toward the US West coast.

Heterogeneity in Subducting Slab Influences Fluid Properties, Plate Coupling and Volcanism: Hikurangi Subduction Zone, New Zealand

NASA Astrophysics Data System (ADS)

Eberhart-Phillips, D. M.; Reyners, M.; Bannister, S. C.

2017-12-01

Seismicity distribution and 3-D models of P- and S-attenuation (1/Q) in the Hikurangi subduction zone, in the North Island of New Zealand, show large variation along-arc in the fluid properties of the subducting slab. Volcanism is also non-uniform, with extremely productive rhyolitic volcanism localized to the central Taupo Volcanic zone, and subduction without volcanism in the southern North Island. Plate coupling varies with heterogeneous slip deficit in the northern section, low slip deficit in the central section, and high slip deficit (strong coupling) in the south. Heterogeneous initial hydration and varied dehydration history both are inferred to play roles. The Hikurangi Plateau (large igneous province) has been subducted beneath New Zealand twice - firstly at ca. 105-100 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates along the Hikurangi subduction zone. It has an uneven downdip edge which has produced spatially and temporally localized stalls in subduction rate. The mantle wedge under the rhyolitic section has a very low Q feature centred at 50-125 km depth, which directly overlies a 150-km long zone of dense seismicity. This seismicity occurs below a sharp transition in the downdip extent of the Hikurangi Plateau, where difficulty subducting the buoyant plateau would have created a zone of increased faulting and hydration that spent a longer time in the outer-rise yielding zone, compared with areas to the north and south. At shallow depths this section has unusually high fracture permeability from the two episodes of bending, but it did not experience dehydration during Gondwana subduction. This central section at plate interface depths less than 50-km has low Q in the slab crust, showing that it is extremely fluid rich, and it exhibits weak plate coupling with both deep and shallow slow-slip events. In contrast in the southern section, where there is a large deficit in slip rate, the plate interface is only moderately fluid-rich, because the underlying plateau had already had an episode of Gondwana dehydration. Here the dehydrated plateau has subducted deeper, to 140-km depth, there is no volcanism, and the mantle wedge lacks low Q.

Construction of Source Model of Huge Subduction Earthquakes for Strong Ground Motion Prediction

NASA Astrophysics Data System (ADS)

Iwata, T.; Asano, K.; Kubo, H.

2013-12-01

It is a quite important issue for strong ground motion prediction to construct the source model of huge subduction earthquakes. Iwata and Asano (2012, AGU) summarized the scaling relationships of large slip area of heterogeneous slip model and total SMGA sizes on seismic moment for subduction earthquakes and found the systematic change between the ratio of SMGA to the large slip area and the seismic moment. They concluded this tendency would be caused by the difference of period range of source modeling analysis. In this paper, we try to construct the methodology of construction of the source model for strong ground motion prediction for huge subduction earthquakes. Following to the concept of the characterized source model for inland crustal earthquakes (Irikura and Miyake, 2001; 2011) and intra-slab earthquakes (Iwata and Asano, 2011), we introduce the proto-type of the source model for huge subduction earthquakes and validate the source model by strong ground motion modeling.

Oblique stepwise rise and growth of the Tibet plateau.

PubMed

Tapponnier, P; Zhiqin, X; Roger, F; Meyer, B; Arnaud, N; Wittlinger, G; Jingsui, Y

2001-11-23

Two end member models of how the high elevations in Tibet formed are (i) continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and (ii) time-dependent, localized shear between coherent lithospheric blocks. Recent studies of Cenozoic deformation, magmatism, and seismic structure lend support to the latter. Since India collided with Asia approximately 55 million years ago, the rise of the high Tibetan plateau likely occurred in three main steps, by successive growth and uplift of 300- to 500-kilometer-wide crustal thrust-wedges. The crust thickened, while the mantle, decoupled beneath gently dipping shear zones, did not. Sediment infilling, bathtub-like, of dammed intermontane basins formed flat high plains at each step. The existence of magmatic belts younging northward implies that slabs of Asian mantle subducted one after another under ranges north of the Himalayas. Subduction was oblique and accompanied by extrusion along the left lateral strike-slip faults that slice Tibet's east side. These mechanisms, akin to plate tectonics hidden by thickening crust, with slip-partitioning, account for the dominant growth of the Tibet Plateau toward the east and northeast.

Paleozoic and mesozoic evolution of East-Central California

USGS Publications Warehouse

Stevens, C.H.; Stone, P.; Dunne, G.C.; Greene, D.C.; Walker, J.D.; Swanson, B.J.

1997-01-01

East-central California, which encompasses an area located on the westernmost part of sialic North America, contains a well-preserved record of Paleozoic and Mesozoic tectonic events that reflect the evolving nature of the Cordilleran plate margin to the west. After the plate margin was formed by continental rifting in the Neoproterozoic, sediments comprising the Cordilleran miogeocline began to accumulate on the subsiding passive margin. In east-central California, sedimentation did not keep pace with subsidence, resulting in backstepping of a series of successive carbonate platforms throughout the early and middle Paleozoic. This phase of miogeoclinal development was brought to a close by the Late Devonian-Early Mississippian Antler orogeny, during the final phase of which oceanic rocks were emplaced onto the continental margin. Subsequent Late Mississippian-Pennsylvanian faulting and apparent reorientation of the carbonate platform margin are interpreted to have been associated with truncation of the continental plate on a sinistral transform fault zone. In the Early Permian, contractional deformation in east-central California led to the development of a narrow, uplifted thrust belt flanked by marine basins in which thick sequences of deep-water strata accumulated. A second episode of contractional deformation in late Early Permian to earliest Triassic time widened and further uplifted the thrust belt and produced the recently identified Inyo Crest thrust, which here is correlated with the regionally significant Last Chance thrust. In the Late Permian, about the time of the second contractional episode, extensional faulting created shallow sedimentary basins in the southern Inyo Mountains. In the El Paso Mountains to the south, deformation and plutonism record the onset of subduction and arc magmatism in late Early Permian to earliest Triassic time along this part of the margin. Tectonism had ceased in most of east-central California by middle to late Early Triassic time, and marine sediment deposited on the subsiding continental shelf overlapped the previously deformed Permian rocks. Renewed contractional deformation, probably in the Middle Triassic, is interpreted to be associated with emplacement of the Golconda allochthon onto the margin of the continent. This event, which is identified with certainty in the Sierra Nevada, also may have significantly affected rocks in the White and Inyo Mountains to the east. Subduction and arc magmatism that created most of the Sierra Nevada batholith began in the Late Triassic and lasted through the remainder of the Mesozoic. During this time, the East Sierran thrust system (ESTS) developed as a narrow zone of intense, predominantly E-vergent contractional deformation along the eastern margin of the growing batholith. Activity on the ESTS took place over an extended part of Mesozoic time, both before and after intrusion of voluminous Middle Jurassic plutons, and is interpreted to have been mechanically linked to emplacement of the batholith. Deformation on the ESTS and magmatism in the Sierra Nevada both ended prior to the close of the Cretaceous.

Origin and dynamics of depositionary subduction margins

USGS Publications Warehouse

Vannucchi, Paola; Morgan, Jason P.; Silver, Eli; Kluesner, Jared W.

2016-01-01

Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a “depositionary forearc,” a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long-term processes, and sudden extreme events reflecting the sudden influences of large-scale morphological variations in the incoming plate. Both types of processes contribute to the large-scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long-lived megathrust surfaces.

Switches in subduction polarity, slab tearing and the opening of slab gaps along the Alpine chain - a view from the bottom up

NASA Astrophysics Data System (ADS)

Handy, M. R.; Ustaszewski, K. M.; Kissling, E. H.

2013-12-01

Kinematic reconstructions of the Alpine orogen from Late Cretaceous to present time reveal that slab tearing and switches of subduction polarity are related to two slab gaps presently imaged as low-velocity anomalies at the transition of the Eastern and Central Alps, and beneath the northern Dinarides. A lithosphere-scale transfer fault at the Alps-Dinarides join (ADT) linked S-directed subduction of the oceanic part of the European plate in the Alps with N-directed subduction of the continental part of the Adriatic plate in the Dinarides in Late Cretaceous to Paleogene time. Transfer faulting in the Dinarides was initially situated along a suture zone, then jumped westward no later than 40 Ma as thrusting and subduction affected more external units of the Alps and Dinarides. Late Eocene Alpine collision led to a slowing of Adria-Europe convergence and initial rupturing of the European and Adriatic slabs in Eocene-Oligocene time, when most of the oceanic lithosphere broke off. This thermally preconditioned the lithosphere for a radical reorganization of slabs and mantle flow in the Alpine domain beginning in early Miocene time. This included the onset of Carpathian rollback subduction, as well as counterclockwise rotation and N-ward subduction of Adriatic continental lithosphere into the space beneath the Eastern Alps that was vacated by foundering and renewed tearing of the European slab in Oligocene-early Miocene time. Our plate reconstructions indicate that this tear nucleated at the tip of a subducted sliver of European continental lithosphere coinciding with the present location of the narrow slab gap between the Eastern and Central Alps. This tear then propagated horizontally to the NE along the subducted boundary of the European margin and the Carpathian embayment of the Alpine Tethyan ocean. The surface response to slab tearing included peneplainization and uplift of part of the Eastern Alps. Transfer faulting along the ADT gave way to back-arc extension and strike-slip faulting behind the retreating Carpathian orogeny no later than 23 Ma. Continued NW-motion of the Adriatic microplate in Oligocene-Miocene time opened a gap along the former ADT which filled with upwelling asthenosphere. We speculate that this thermally eroded the Miocene slab beneath the northern Dinarides, giving rise to the present slab gap there. The forces governing motion of the Adriatic microplate changed both with time and the nature of the subducting lithosphere. From 84-35 Ma, the NW-retreat of the down-going European plate facilitated the independent motion of Adria at 1-2 cm/a with respect to Europe. Adria's motion may have been driven partly by suction behind this European slab which comprised mostly old oceanic lithosphere. With the onset of Alpine collision at c. 35 Ma, the slabs became gravitationally unstable and ruptured. N-ward subduction of a fragment of Adriatic continental lithosphere beneath the Eastern Alps in Miocene time was probably initiated by push from Africa and possibly enhanced by neutral to negative buoyancy of the slab itself which included dense lower crust of the Adriatic continental margin.

Flower-strucutre deformation pattern of theTian Shan mountains as revealed by Late Quaternary geological and modern Geodesy slip rates

NASA Astrophysics Data System (ADS)

Wu, C.; Zhang, P.; Zheng, W.; Wang, H.; Zhang, Z.; Ren, Z.; Zheng, D.; Yu, J.; Wu, G.

2017-12-01

The deformation pattern and strain distribution of the Tian Shan is a hot issue.Previous studies mainly focus on the thrust-fold systems on both sides of Tian Shan, the strike-slip faults within the mountains are rarely reported. The understanding about the deformation characteristics of Tian Shan is not complete for lacking information of these strike-slip faults.Our studies show the NEE trending structures of Maidan fault and Nalati fault in the southwestern Tian Shan are all active during the Holence. These faults are characterized by sinistral strike-slip and thrust movement. The minimum average sinistral strike-slip rate of the Maidan fault is 1.07 ± 0.13 mm/yr. During the late Quaternary, the average shortening rate and sinistral strike-slip rate of the Nalati fault are 2.1 ±0.4 mm/yr and 2.56 ±0.25 mm/yr, respectively . In the interior of the Tian Shan area, two groups of strike-slip faults were developed. The NEE trending faults with sinistral strike-slipmovement, and the NWW trending faults with dextral strike-slip movement show the shape of "X"in geometrical structure. The piedmont thrust faults and the thrust strike-slip faults in the interior mountain constitute the tectonic framework of Tian Shan. Threegroups of active fault systems are the main seismogenic and geological structures, which control the current tectonic deformation pattern of Tian Shan (Figure 1). GPS observation data also showthe similar deformation characteristics with the geological results (Figures 2, 3). In addition to the crustal shortening, there is a certain strike-slip shear movement in the interior of the Tian Shan.The strike-slip rate defined by the geological and GPS data is approximately consistent with each other near the same longitude. We suggest the two groups of strike-slip faults in the interior of mountains is a set of conjugate structures. The whole Tian Shan forms a large flower-structure in a profile view. The complete tectonic deformation of the Tian Shan mountains consists ofthe shortening deformationof the N-S direction and the lateral extrusion of the E-W direction (Figure 2). The late Cenozoic deformation of the Tian Shan mountains is due to the northward subduction of Tarim Block. Although the activedeformation of the Tian Shan decrease eastward, the geological sturcutrein eastern Tian Shan is similar.

Large-scale deformation associated with ridge subduction

USGS Publications Warehouse

Geist, E.L.; Fisher, M.A.; Scholl, D. W.

1993-01-01

Continuum models are used to investigate the large-scale deformation associated with the subduction of aseismic ridges. Formulated in the horizontal plane using thin viscous sheet theory, these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction. Modelling was used to compare the Tonga arc and Louisville ridge collision with the New Hebrides arc and d'Entrecasteaux ridge collision, which have disparate arc-ridge intersection speeds but otherwise similar characteristics. Models of both systems indicate that diffuse deformation (low values of the effective stress-strain exponent n) are required to explain the observed deformation. -from Authors

A review of the geodynamic evolution of flat slab subduction in Mexico, Peru, and Chile

NASA Astrophysics Data System (ADS)

Constantin Manea, Vlad; Manea, Marina; Ferrari, Luca; Orozco, María Teresa; Wong Valenzuela, Raul; Husker, Allen Leroy; Kostoglodovc, Vlad; Ionescu, Constantin

2017-04-01

Subducting plates around the globe display a large variability in terms of slab geometry, including regions where smooth and little variation in subduction parameters is observed. While the vast majority of subduction slabs plunge into the mantle at different, but positive dip angles, the end-member case of flat-slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle. By employing a comparative assessment for the Mexican, Peruvian and Chilean flat-slab subduction zones we find a series of parameters that apparently facilitate slab flattening. Among them, trench roll-back, as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important. However, we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones. In order to unravel the origin of flat-slab subduction, it is probably necessary a numerical approach that considers also the influence of surrounding plates, and their corresponding geometries, on 3D subduction dynamics.

A review of the geodynamic evolution of flat slab subduction in Mexico, Peru, and Chile

NASA Astrophysics Data System (ADS)

Manea, V. C.; Manea, M.; Ferrari, L.; Orozco-Esquivel, T.; Valenzuela, R. W.; Husker, A.; Kostoglodov, V.

2017-01-01

Subducting plates around the globe display a large variability in terms of slab geometry, including regions where smooth and little variation in subduction parameters is observed. While the vast majority of subduction slabs plunge into the mantle at different, but positive dip angles, the end-member case of flat-slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle. By employing a comparative assessment for the Mexican, Peruvian and Chilean flat-slab subduction zones we find a series of parameters that apparently facilitate slab flattening. Among them, trench roll-back, as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important. However, we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones. In order to unravel the origin of flat-slab subduction, it is probably necessary a numerical approach that considers also the influence of surrounding plates, and their corresponding geometries, on 3D subduction dynamics.

Quantifying potential tsunami hazard in the Puysegur subduction zone, south of New Zealand

USGS Publications Warehouse

Hayes, G.P.; Furlong, K.P.

2010-01-01

Studies of subduction zone seismogenesis and tsunami potential, particularly of large subduction zones, have recently seen a resurgence after the great 2004 earthquake and tsunami offshore of Sumatra, yet these global studies have generally neglected the tsunami potential of small subduction zones such as the Puysegur subduction zone, south of New Zealand. Here, we study one such relatively small subduction zone by analysing the historical seismicity over the entire plate boundary region south of New Zealand, using these data to determine the seismic moment deficit of the subduction zone over the past ~100 yr. Our calculations indicate unreleased moment equivalent to a magnitude Mw 8.3 earthquake, suggesting this subduction zone has the potential to host a great, tsunamigenic event. We model this tsunami hazard and find that a tsunami caused by a great earthquake on the Puysegur subduction zone would pose threats to the coasts of southern and western South Island, New Zealand, Tasmania and southeastern Australia, nearly 2000 km distant. No claim to original US government works Geophysical Journal International ?? 2010 RAS.

The Tyrrhenian stage geodinamic evolution of Apenninic-Maghrebian orogen (Southern Apennines and Sicily)

NASA Astrophysics Data System (ADS)

Lentini, F.; Carbone, S.; Barreca, G.

2009-04-01

In the Central Mediterranean region the foreland domains are represented by two continental blocks, the Apulian Block to the north and the Pelagian Block to the south, respectively belonging to the Adria and to the Africa plates. They are separated since Permo-Triassic times by the oceanic crust of the Ionian Sea. The Apenninic-Maghrebian orogen is located between two oceanic crusts: the old Ionian crust, at present time subducting beneath the Calabrian Arc, and the new crust of the opening Tyrrhenian Sea. The orogenic belt is represented by a multilayer allochthonous edifice, composed of the Calabride Chain (CC) tectonically overlying the Apenninic-Maghrebian Chain (AMC), which in turn overthrust onto the Upper Miocene and Pliocene top-levels of a deep seated thrust system, originating by the deformation of the innermost carbonates of the Pelagian/Apulian blocks (External Thrust System: ETS). The AMC tectonic units derive from the orogenic transport during Oligo-Miocene times of sedimentary sequences deposited in palaeogeographical domains located between the Europe and the Afro-Adriatic plates. These units are composed of Meso-Cenozoic shallow-water carbonate successions detached from a continental type crust sector, the Panormide/Apenninic Block, recognizable by means of seismic lines shot in the Tyrrhenian offshore of Southern Apennines and Northern Sicily. The Meso-Cenozoic basinal units, that compose the AMC, can be distinguished into two main groups of sequences, originally located on oceanic crusts separated by the Panormide/Apenninic Block: the external ones (Ionides) related to an original basin belonging to branches of the Ionian Palaeobasin involved in the orogenesis, and the internal ones ascribed to the Alpine Tethys (Sicilide Units). The terrigenous deposits of the basinal sequences belonging to the Ionides are represented by Tertiary foreland/foredeep deposits, whose relationships with the substratum are occasionally preserved, although large detachments occurred with further forward transport, which generated repeated slices with an apparent increase to the original thickness. . The Alpine Tethydes are composed of sedimentary sequences, which were deposited in the Alpine Tethys, and originally were located between the European and the Panormide/Apenninic Block. They are represented by allochthonous far travelled tectonic units, resting on both the Panormide/Apenninic Platforms and the Ionides. The Calabride Chain originated by the delamination of the European margin. This roof thrust system includes nappes of Hercynian basement with remains of the original Meso-Cenozoic covers deformed during the Paleogene and sutured by the Late Oligocene-Early Burdigalian Capo d'Orlando Flysch. The geological, geophysical data and the volcanological characters permit to restore the palaeogeography and the geodynamic evolution, and allow to recognize three orogenic stages: the Eo-Alpine, originated during Cretaceous-Eocene times, evident in the western Calabria, in the Tyrrhenian basin and the Alpine Corsica; the Balearic stage (Late Oligocene-Early Miocene), in which the Corsica-Sardinia block rotated and collided with the Adria-Africa margins with thrusting of the Alpine Tethydes over Panormide/Apenninic platforms; and the Tyrrhenian stage (Middle Miocene to Present), when the onset of the Tyrrhenian back-arc basin occurred and after the closure of the interposed Palaeoionian branches the Ionides were tectonically transported onto the foreland blocks. The CROP crustal sections allow to distinguish thickness and distribution of the crusts in this area of the Mediterranean Sea, and their clear influence on geodynamic evolution of the Tyrrhenian stage. They confirm that both the foreland blocks extend below the orogenic belt, reaching the Tyrrhenian margins, with a gradual thinning and a transition to a Palaeo-Ionian slab, probably not active at present time, from which the Ionides detached and overrode the ETS. The seismogeological data indicate the presence of the Panormide/Apenninic blocks, that took part in the closure of the branches of the Palaeo-Ionian Sea interposed between the Panormide/Apenninic crust and the Pelagian/Apulian Blocks. At the present time the Panormide/Apenninic blocks are colliding with the foreland blocks. Such a collisional stage along the Tyrrhenian coast of north-western Sicily and the contemporaneous active subduction processes below the Calabrian Arc produce the NW-SE oriented South Tyrrhenian System. This system drives the transfer of the orogenic front towards areas characterized by still subducting oceanic crust of the Ionian sector. In particular it consists of predominantly NW-SE oriented right lateral faults system with antithetical NE-SW and coeval associated N-S normal faults and south-verging thrusts. All these structures are compatible with an unique cinematic framework dominated by transcurrent tectonics. Geological mapping carried out in the on-shore areas of Sicily, integrated with stratigraphical and structural analysis, permit to recognize some main structures in connection with the geodynamic evolution of the Tyrrhenian stage and allow to propose an updated structural model of this area.

Present-day chaotic formations around the Japanese trenches: Comparison to the on land examples from the Shimanto and Miura-Boso, and from the Franciscan, Mineoka and Ankara

NASA Astrophysics Data System (ADS)

Ogawa, Yujiro; Kawamura, Kiichiro; Tsunogae, Toshiaki; Mori, Ryota; Chiba, Tae; Sasaki, Tomoyuki

2010-05-01

Four different types of chaotic formations were recognized by the submersible observation around the Japanese trenches, including the Nankai and Sagami troughs, Boso triple junction, Japan trench, and Izu-Bonin arc, and each type is summarized and discussed in view of comparison to the on land examples, such as from the Franciscan, Shimanto and Miura-Boso belts in the circum Pacifc margins, and the Ankara. The submarine geologies are present actual examples to give us a critical key to understanding the formation processes and emplacement mechanisms for the so-called mélange bodies, either sedimentary, tectonic or diapiric. Some are made of alternated beds of sandstone and mudstone that show broken or block-in-matrix fashion, in most cases in muddy matrix. These are commonly developed on the trench landward slope toe of the Nankai and Sagami troughs and Boso triple junction area as well as the Japan trench slope. One type is from the landward slope, but another type is from the oceanward slopes. The former type is in places calcareous cemented, probably caused by hydraulic fracturing by high pore pressure along the thrust fault and oxidized methane-made carbonate precipitation. They are seen on the feet of the thrust-dominated slope and to be compared to the so-called sedimentary mélanges due to the gravitational sliding, which occur because of tectonically induced steep slopes. Most of such thrusts are related to large subduction type earthquakes, and await for further critical consideration on to the relation to the asperity problem. Some of large scale gravitational collapses may be related to the seamount or ridge subduction to the trench, both in case of accretionary and non-accretionary type margins, the former is for the examples from the Nankai and Sagami troughs and the Boso triple junction, latter for the Japan trench. In all cases on land and under the sea in the trench landward slope, some calcareous breccias are associated with methane-fluid supported animals within injection or diapiric intrusion. On the other hand, in the Nankai prism and the on land Miura-Boso Peninsulas, many examples of sandy matrix supported mudstone breccia are a result of liquefaction and injection of such coarse-grained clastic fragments during the earthquake shake and subsequent landsliding. Those deposits are faulted, folded and injected in various stages, some before accretionary prism incorporation, some after. Some are of sedimentary origin by gravitational process, others tectonic or diapiric, but in most cases thrust duplexes and complex folds are common. The third and fourth are mélanges including igneous, metamorphic and/or ophiolitic rock blocks. They look similar to the on land examples in the Franciscan, Mineoka (Boso, central Japan) and the Ankara, and used to be attributable to the diapiric origin, as those that have been already known as serpenitine mud volcanoes with metamorphic block at the foot of the Izu-Bonin-Mariana forearc. However, such analogue need careful consideration how the rock association would form to the final emplacement. As the fourth new type, we found an example of deep (1.5 to 2 GPa) metamorphic rock blocks of eclogitic conditions from the fault line in the schistose serpentinite (antigorite-dominated) in the middle part of the Izu arc near the Ohmachi seamount. This implies for the incorporation and exhumation of igneous and metamorphic rocks in the island arc setting, and may give an adequate analogue to the specific mélange formation of the Franciscan, Mineoka and Ankara.

Metamorphic Perspectives of Subduction Zone Volatiles Cycling

NASA Astrophysics Data System (ADS)

Bebout, G. E.

2008-12-01

Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and δ15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like δ15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites indicates retention of seafloor N signatures and, in some cases, enrichments in sedimentary N due to forearc metamorphic fluid-rock interactions (Halama et al., this session). A global estimate of C cycling, using seafloor inputs (carbonate and organic matter) and estimates of volcanic CO2 outputs, indicates ~40% return (with large uncertainty) of the subducting C in volcanic gases. This imbalance appears plausible, given the evidence for deep carbonate subduction, in UHP marbles, and the preservation of graphite in UHP metasediments, together seemingly indicating that large fractions of subducting C survive forearc-to-subarc metamorphism. Estimates of return efficiency in the Central America arc, based on data for volcanic gases, are lower and variable along strike (12-29%), quite reasonably explained by de Leeuw et al. (2007, EPSL) as resulting from incomplete decarbonation of subducting sediment and AOC, fluid flow patterns expected given sediment section thickness, and varying degrees of forearc underplating. The attempts to mass-balance C and N across individual arc-trench systems demonstrate valuable integration of information from geophysical, field, petrologic, and geochemical observations. Studies of subduction-zone metamorphic suites can yield constraints on the evolution of deeply subducting rocks and the physicochemical characteristics of fluids released in forearcs and contributing to return flux in arc volcanic gases.

LiDAR Mapping of Earthquake Uplifted Paleo-shorelines, Southern Wairarapa Coast, North Island, New Zealand

NASA Astrophysics Data System (ADS)

Valenciano, J.; Angenent, J.; Marshall, J. S.; Clark, K.; Litchfield, N. J.

2017-12-01

The Hikurangi subduction margin along the east coast of the North Island, New Zealand accommodates oblique convergence of the Pacific Plate westward beneath the Australian plate at 45 mm/yr. Pronounced forearc uplift occurs at the southern end of the margin along the Wairarapa coast, onshore of the subducting Hikurangi plateau. Along a narrow coastal lowland, a series of uplifted Holocene marine terraces and beach ridges preserve a geologic record of prehistoric coseismic uplift events. In January 2017, we participated in the Research Experience for Undergraduates (REU) program of the NSF SHIRE Project (Subduction at Hikurangi Integrated Research Experiment). We visited multiple coastal sites for reconnaissance fieldwork to select locations for future in-depth study. For the coastline between Flat Point and Te Kaukau Point, we used airborne LiDAR data provided by Land Information New Zealand (LINZ) to create ArcGIS digital terrain models for mapping and correlating uplifted paleo-shorelines. Terrace elevations derived from the LiDAR data were calibrated through the use of Real Time Kinematic (RTK) GPS surveying at one field site (Glenburn Station). Prior field mapping and radiocarbon dating results (Berryman et al., 2001; Litchfield and Clark, 2015) were used to guide our LiDAR mapping efforts. The resultant maps show between four and seven uplifted terraces and associated beach ridges along this coastal segment. At some sites, terrace mapping and lateral correlation are impeded by discontinuous exposures and the presence of landslide debris, alluvial fan deposits, and sand dunes. Tectonic uplift along the southern Hikurangi margin is generated by a complex interaction between deep megathrust slip and shallow upper-plate faulting. Each uplifted Holocene paleo-shoreline is interpreted to represent a single coseismic uplift event. Continued mapping, surveying, and age dating may help differentiate between very large margin-wide megathrust earthquakes (M8.0-9.0+) and smaller, more localized upper-plate thrust events (M7.0-8.0). Both of these earthquake types pose a significant seismic and tsunami hazard for New Zealand residents.

Glacially-derived overpressure in the northeastern Alaskan subduction zone: combined tomographic and morphometric analysis of shallow sediments on the Yakutat shelf and slope, Gulf of Alaska

NASA Astrophysics Data System (ADS)

Clary, W. A.; Worthington, L. L.; Scuderi, L. A.; Daigle, H.; Swartz, J. M.

2017-12-01

The Pamplona zone fold and thrust belt is the offshore expression of convergence and shallow subduction of the Yakutat microplate beneath North America in the northeastern Alaska subduction zone. The combination of convergent tectonics and glaciomarine sedimentary processes create patterns of deformation and deposition resulting in a shallow sedimentary sequence with varying compaction, fluid pressure, and fault activity. We propose that velocity variations observed in our tomographic analysis represent long-lived fluid overpressure due to loading by ice sheets and sediments. Regions with bathymetric and stratigraphic evidence of recent ice sheets and associated sedimentation should be collocated with evidence of overpressure (seismic low velocity zones) in the shallow sediments. Here, we compare a velocity model with shelf seismic stratigraphic facies and modern seafloor morphology. To document glacially derived morphology we use high resolution bathymetry to identify channel and gully networks on the western Yakutat shelf-slope then analyze cross-channel shape indices across the study area. We use channel shape index measurements as a proxy of recent ice-proximal sedimentation based on previously published results that proposed a close correlation. Profiles taken at many locations were fitted with a power function and assigned a shape - U-shape channels likely formed proximal to recent ice advances. Detailed velocity models were created by a combination of streamer tomography and pre-stack depth migration velocities with seismic data including: a 2008 R/V Langseth dataset from the St. Elias Erosion and Tectonics Project (STEEP); and a 2004 high-resolution R/V Ewing dataset. Velocity-porosity-permeability relationships developed using IODP Expedition 341 drilling data inform interpretation and physical properties analyses of the shallow sediments. Initial results from a 35 km profile extending SE seaward of the Bering glacier and subparallel to the Bering trough suggest a spatial relationship between the extent of U-shaped profiles and low-velocity shallow sediments. Towards the SE end of the model we observe a large overlap of U-shaped indices, and a shallow low-velocity zone in the mapped extent of the last glacial maximum suggestive of overpressure due to loading by ice sheet activity.

A Seafloor Benchmark for 3-dimensional Geodesy

NASA Astrophysics Data System (ADS)

Chadwell, C. D.; Webb, S. C.; Nooner, S. L.

2014-12-01

We have developed an inexpensive, permanent seafloor benchmark to increase the longevity of seafloor geodetic measurements. The benchmark provides a physical tie to the sea floor lasting for decades (perhaps longer) on which geodetic sensors can be repeatedly placed and removed with millimeter resolution. Global coordinates estimated with seafloor geodetic techniques will remain attached to the benchmark allowing for the interchange of sensors as they fail or become obsolete, or for the sensors to be removed and used elsewhere, all the while maintaining a coherent series of positions referenced to the benchmark. The benchmark has been designed to free fall from the sea surface with transponders attached. The transponder can be recalled via an acoustic command sent from the surface to release from the benchmark and freely float to the sea surface for recovery. The duration of the sensor attachment to the benchmark will last from a few days to a few years depending on the specific needs of the experiment. The recovered sensors are then available to be reused at other locations, or again at the same site in the future. Three pins on the sensor frame mate precisely and unambiguously with three grooves on the benchmark. To reoccupy a benchmark a Remotely Operated Vehicle (ROV) uses its manipulator arm to place the sensor pins into the benchmark grooves. In June 2014 we deployed four benchmarks offshore central Oregon. We used the ROV Jason to successfully demonstrate the removal and replacement of packages onto the benchmark. We will show the benchmark design and its operational capabilities. Presently models of megathrust slip within the Cascadia Subduction Zone (CSZ) are mostly constrained by the sub-aerial GPS vectors from the Plate Boundary Observatory, a part of Earthscope. More long-lived seafloor geodetic measures are needed to better understand the earthquake and tsunami risk associated with a large rupture of the thrust fault within the Cascadia subduction zone. Using a ROV to place and remove sensors on the benchmarks will significantly reduce the number of sensors required by the community to monitor offshore strain in subduction zones.

New SW Pacific tectonic model: Cyclical intraoceanic magmatic arc construction and near-coeval emplacement along the Australia-Pacific margin in the Cenozoic

NASA Astrophysics Data System (ADS)

Whattam, Scott A.; Malpas, John; Ali, Jason R.; Smith, Ian E. M.

2008-03-01

Various reconstructions of the SW Pacific for the Late Cretaceous and Cenozoic suggest that northeast dipping subduction began in the South Loyalty Basin (SLB) at 55-50 Ma and that subsequent closure of the SLB resulted in the diachronous emplacement of Cretaceous-Paleocene ophiolitic nappes onto the Norfolk Ridge in New Caledonia at 40-34 Ma and in Northland, New Zealand, around 24-21 Ma. A fundamental problem with these models is that they do not account for the fact that NE dipping subduction had already been established offshore Papua New Guinea by at least 65-60 Ma which resulted in the emplacement of the Papuan Ultramafic Belt (PUB) ophiolite at 59-58 Ma. A second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the Loyalty arc and Three Kings Ridge. Finally, reconstructions of the Northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the Northland allochthon is Late Cretaceous-Paleocene, when in fact it is Oligocene. A new model is presented whereby the PUB, New Caledonia, and Northland ophiolites formed and were emplaced in a cyclical fashion above an extensive NE dipping Cenozoic intraoceanic arc system which diachronously propagated (N-S) along the entire eastern margin of the Australian Plate. These "infant arc" ophiolites represent fragments of suprasubduction zone lithosphere (SSZL) generated in the earliest stages of magmatic arc formation that were emplaced shortly after (<20 m.y.) as a result of forearc-Australian Plate collision. Subduction inception was the result of subsidence of older MORB-like lithosphere generated within an extensive "back arc basin" to the east of the Norfolk Ridge during the earliest stages of SLB formation above a southwest dipping Pacific Plate. During emplacement of each ophiolite, a crustal fragment of the older lithosphere was scraped off the NE dipping slab and subsequently back-thrust beneath each ophiolite during its emplacement.

Geodetic slip solutions for the Mw=7.4 Champerico (Guatemala) subduction earthquake of November 7 2012

NASA Astrophysics Data System (ADS)

Ellis, Andria; DeMets, Charles; Briole, Pierre; Molina, Enrique; Flores, Omar; Rivera, Jeffrey; Lasserre, Cécile; Lyon-Caen, Hélène; Lord, Neal

2014-05-01

As the first large subduction thrust earthquake off the coast of western Guatemala in the past 50 years, the 7 November 2012 Mw=7.4 earthquake offers the first opportunity for a geodetic study of coseismic and postseismic behavior for a segment of the Middle America trench where frictional coupling makes a transition from weak coupling off the coast of El Salvador to strong coupling in southern Mexico. Processing of continuous GPS measurements at 19 stations in Guatemala, El Salvador, and southern Mexico, and at 7 campaign points in Guatemala defines a highly consistent pattern of coseismic offsets during the earthquake, ranging from 47±5 mm of SW movement just inland from the earthquake epicenter to a few mm at sites located in northern Guatemala. Inversions of these offsets to find their best-fitting fault-slip solution in an elastic half space give a geodetic earthquake moment ranging between 0.75 and 1.1 x 1020 Nm, slightly smaller than the seismic estimates that range between 1.2 and 1.45 x 1020 Nm. Slip inversion using a constant slip model, assuming 293° and 29° for the fault azimuth and dip angle, indicates a nearly reverse slip of 2.8 m (rake 78°) on a fault plane 42 km-long and 20 km-wide, centered at 26 km depth. A variable slip inversion indicates that slip concentrated above depths of 40 km may have extended updip to the trench and reached a maximum of only 0.8 m, less than one-sixth the maximum slip indicated by a recent slip solution (5.3 m) obtained from waveform inversion of seismological data. Detailed model comparisons will be discussed. Transient postseismic displacements have been recorded at the nearby continuous GPS sites with amplitudes reaching 20-25 mm at some stations. The duration of the phenomenon is short: using an exponential-decay model, the estimated decay time is 90 ± 10 days. This postseismic signal is consistent with afterslip along a significantly broader area (+50%) of the subduction interface than ruptured coseismically. These results will be discussed in the tectonic framework of the area.

The initial superposition of oceanic and continental units in the southern Western Alps: constraints on geometrical restoration and kinematics of the continental subduction wedge

NASA Astrophysics Data System (ADS)

Dumont, Thierry; Schwartz, Stéphane; Matthews, Steve; Malusa, Marco; Jouvent, Marine

2017-04-01

The tectonic contact separating continental and oceanic units is preserved at outcrop in many locations within the Western Alps. The contact has experienced prolonged and progressive deformation during Oligocene collision and subsequent 'extrusive' contraction which is approximately westerly-directed (Dumont et al., 2012). Despite variable metamorphic grade, this tectonic contact displays a relative consistency of tectonostratigraphic and structural characteristics. Removal of the Oligocene and younger deformation is a critical requirement to allow assessment of the kinematic evolution during the Eocene continental subduction phase. The best preserved relationships are observed near the base of the Helminthoid Flysch nappes, in the footwall of the Penninic thrust, or in the external part of the Briançonnais zone. Here, the oceanic units are composed of detached Cretaceous sediments, but they are underlain locally by an olistostrome containing basaltic clasts. Further to the east, the internal boundary of the Briançonnais zone s.l. (including the 'Prepiedmont units'), is frequently marked by breccia or megabreccia, but is strongly affected by blueschist-facies metamorphism and by approximately easterly directed backfolding and backthrusting. At one locality, there is compelling evidence that the oceanic and continental units were already tectonically stacked and metamorphosed (together) 32Ma ago. Some megabreccias of mixed continental/oceanic provenance can be interpreted as a metamorphic equivalent of the external olistostrome, products of the initial pulses of tectonic stacking. The overlying units are composed dominantly of metasediments, containing distributed ophiolitic megaboudins (Tricart & Schwartz, 2006). Further east again, the tectonic contact separates the Dora-Maira continental basement from the Mt. Viso units which are predominantly composed of oceanic lithosphere. Both the Dora-Maira and Mt. Viso units are eclogitic, but the HP peak is apparently older in the oceanic rocks (Malusà et al. 2015). Finally, further SE, the Voltri massif shows a huge volume of serpentinized mantle which locally overlies continental basement (strongly metamorphosed), and is interpreted as an exhumed remnant of the subduction channel (Federico et al., 2007). In all these localities the transport directions during initial pulses of stacking were consistently oriented generally towards the NW to N, taking into account the subsequent Oligocene and younger collision-related deformation (complex folds, thrusts, backfolds and backthrusts, and block-rotations). It is thus possible to attempt reconstructing an early stage continental subduction wedge involving these different elements from the subduction channel to the most frontal part of the accretionary complex. However, this early Alpine orogen which was active throughout the Eocene is interpreted to have propagated generally towards the NW to N, prior to subsequent pulses of more westerly directed deformation from the Oligocene onwards within the southern part of the Western Alps arc. It is therefore essential to continually improve high-resolution 3D geophysical imaging to facilitate a better understanding of the complex western termination of the Alpine orogen. References: Dumont T., Schwartz S., Guillot S., Simon-Labric S., Tricart P. & Jourdan S. (2012), Structural and sedimentary record of the Oligocene revolution in the Western Alpine arc. Jour. Geodynamics, doi:10.1016/j.jog.2011.11.006 Federico L., Crispini L., Scambelluri M. & Capponi G. (2007), Ophiolite mélange zone records exhumation in a fossil subduction channel. Geology, 35, p. 499-502 Malusà M.G., Faccenna C., Baldwin S.L., Fitzgerald P.G., Rossetti F., Balestrieri M.L., Danišík M., Ellero A., Ottria G. & Piromallo C. (2015), Contrasting styles of (U)HP rock exhumation along the Cenozoic Adria-Europe plate boundary (Western Alps, Calabria, Corsica). Geochem. Geophys. Geosyst. ,16, p. 1786-1824 Tricart P. & Schwartz S. (2006), A north-south section across the Queyras Schistes Lustrés (Piedmont zone, western Alps): Syn-collision refolding of a subduction wedge. Eclogae Geol. Helv., 99, 3, p. 429-442

Interannual variability of Indian Ocean subtropical mode water subduction rate

NASA Astrophysics Data System (ADS)

Ma, Jie; Lan, Jian

2017-06-01

The interannual variation of Indian Ocean subtropical mode water (IOSTMW) subduction rate in the Southwest Indian Ocean from 1980 to 2007 is investigated in this paper based on Simple Ocean Data Assimilation (SODA) outputs. Climatology of subduction rate exceeds 75 m/year in the IOSTMW formation area. The renewal time of permanent pycnocline water mass based on the subduction rate is calculated for each density class: 3-6 years for IOSTMW (25.8 < σ θ < 26.2 kg m-3). Subduction rate in the Southwest Indian Ocean subtropical gyre exhibits a great year-to-year variability. This interannual variations of the IOSTMW subduction rate is primarily dominated by the lateral induction term, associated with the interannual variations of strong meridional gradient of winter mixed layer depth (MLD). The slope of the mixed layer depth in the mode water is closely linked to the large variations of deep late winter MLD in the mid-latitudes and negligible variations of shallow winter MLD in lower latitudes. It is further identified that the interannual variation of late winter MLD in this area is largely controlled by the latent and sensible heat flux components. The water volume of the permanent pycnocline in the IOSTMW distribution area is also found to show a significant interannual variability, and it is well correlated with the interannual variation of subduction rate.

The fate of carbonates along a subducting slab

NASA Astrophysics Data System (ADS)

Bouilhol, P.; Debret, B.; Inglis, E.

2017-12-01

Carbon long-term cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the mobility of carbon at large scale during subduction through field, petrographic and geochemical studies on exhumed portion of the alpine slab that have recorded different metamorphic conditions during subduction. We studied serpentinite samples, metasomatic horizon between serpentinites and sediments, as well as veins hosted in serpentinites. Samples are from the Western Alps (Queyras and Zermatt) and have recorded a prograde metamorphic history from low temperature blueshist to eclogite facies P-T conditions. We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab, as well as within the serpentinites. The early stage of subduction sees carbonate precipitation from the sediment derived fluids into the serpentnites. At higher temperature, when the dehydration shift from sediment to serpentinite dominated, the carbonates are dissolved, inducing the release of CO2 rich fluids. This occurs before the eclogite facies is attained, providing strong evidence for the mobility of carbon in fluids during the early stages of subduction. These fluids are a potential metasomatic agent for the fore-arc mantle wedge, corroborating the observation of carbonate bearing veins in sub-arc mantle ultramafic rocks. In eclogite facies conditions, olivine and carbonate veins within the serpentinites witness the mobility of CO2 during serpentinite dehydration, and may provide clues about the large scale recycling of CO2 within the deep mantle, as well as secondary precipitation associated with exhumation. Trace elements, Fe and Zn isotopic composition of the different samples provides evidence for a large scale transfer of both sulfate and carbonate bearing fluids during the early stages of subduction, and could imply an overlooked role of the fore-arc in melt genesis.

Kinematics and Ophiolite obduction in the Gerania and Helicon Mountains, central Greece

NASA Astrophysics Data System (ADS)

Kaplanis, A.; Koukouvelas, I.; Xypolias, P.; Kokkalas, S.

2013-06-01

New structural, petrofabric and palaeostress data from the Beotia area (central Greece) were used to investigate the tectonic evolution of the suture zone between the External (Parnassus microplate) and Internal Hellenides (Pelagonian microplate). Petrofabric studies of ultramafic rocks were done using conventional U-stage analysis and the electron backscatter diffraction (EBSD) technique. Detailed structural analysis enabled us to distinguish three main deformation phases that took place from the Triassic to the Eocene. Triassic-Jurassic deformation is related to continental rifting and the progressive formation of an ocean basin. Ophiolites formed above a westward-dipping supra-subduction zone (SSZ) in the Early-Late Jurassic. Trench-margin collision resulted in the southeastward emplacement of the ophiolite nappe over the Pelagonian margin. There is also evidence for a north-westward thrusting of ophiolitic rocks over the Gerania and Helicon units during Berriasian time. This latter tectonic process is closely related to the deposition of "Beotian flysch" into a foreland basin. An extensional phase of deformation accompanied by shallow-water carbonate sedimentation is documented in the Upper Cretaceous. Later, during Paleocene the area was subjected to a compressional deformation phase characterised by SW-directed thrusting and folding, as well as NE-verging backthrusts and backfolds. Our proposed geotectonic model suggests the consumption of the ocean between the Parnassus and Pelagonian microplates. This model includes Late Jurassic eastward ophiolite obduction followed by Early Cretaceous west directed ophiolite thrusting.

Understanding continental megathrust earthquake potential through geological mountain building processes: an example in Nepal Himalaya

NASA Astrophysics Data System (ADS)

Zhang, Huai; Zhang, Zhen; Wang, Liangshu; Leroy, Yves; shi, Yaolin

2017-04-01

How to reconcile continent megathrust earthquake characteristics, for instances, mapping the large-great earthquake sequences into geological mountain building process, as well as partitioning the seismic-aseismic slips, is fundamental and unclear. Here, we scope these issues by focusing a typical continental collisional belt, the great Nepal Himalaya. We first prove that refined Nepal Himalaya thrusting sequences, with accurately defining of large earthquake cycle scale, provide new geodynamical hints on long-term earthquake potential in association with, either seismic-aseismic slip partition up to the interpretation of the binary interseismic coupling pattern on the Main Himalayan Thrust (MHT), or the large-great earthquake classification via seismic cycle patterns on MHT. Subsequently, sequential limit analysis is adopted to retrieve the detailed thrusting sequences of Nepal Himalaya mountain wedge. Our model results exhibit apparent thrusting concentration phenomenon with four thrusting clusters, entitled as thrusting 'families', to facilitate the development of sub-structural regions respectively. Within the hinterland thrusting family, the total aseismic shortening and the corresponding spatio-temporal release pattern are revealed by mapping projection. Whereas, in the other three families, mapping projection delivers long-term large (M<8)-great (M>8) earthquake recurrence information, including total lifespans, frequencies and large-great earthquake alternation information by identifying rupture distances along the MHT. In addition, this partition has universality in continental-continental collisional orogenic belt with identified interseismic coupling pattern, while not applicable in continental-oceanic megathrust context.

The Cimmerian accretionary wedge of Anarak, Central Iran

NASA Astrophysics Data System (ADS)

Zanchi, Andrea; Malaspina, Nadia; Zanchetta, Stefano; Berra, Fabrizio; Benciolini, Luca; Bergomi, Maria; Cavallo, Alessandro; Javadi, Hamid Reza; Kouhpeyma, Meyssam

2015-04-01

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone. Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered "ophiolites" which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the "ophiolites" within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the "ophiolites" were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290 Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Tomography and Dynamics of Western-Pacific Subduction Zones

NASA Astrophysics Data System (ADS)

Zhao, D.

2012-01-01

We review the significant recent results of multiscale seismic tomography of the Western-Pacific subduction zones and discuss their implications for seismotectonics, magmatism, and subduction dynamics, with an emphasis on the Japan Islands. Many important new findings are obtained due to technical advances in tomography, such as the handling of complex-shaped velocity discontinuities, the use of various later phases, the joint inversion of local and teleseismic data, tomographic imaging outside a seismic network, and P-wave anisotropy tomography. Prominent low-velocity (low-V) and high-attenuation (low-Q) zones are revealed in the crust and uppermost mantle beneath active arc and back-arc volcanoes and they extend to the deeper portion of the mantle wedge, indicating that the low-V/low-Q zones form the sources of arc magmatism and volcanism, and the arc magmatic system is related to deep processes such as convective circulation in the mantle wedge and dehydration reactions in the subducting slab. Seismic anisotropy seems to exist in all portions of the Northeast Japan subduction zone, including the upper and lower crust, the mantle wedge and the subducting Pacific slab. Multilayer anisotropies with different orientations may have caused the apparently weak shear-wave splitting observed so far, whereas recent results show a greater effect of crustal anisotropy than previously thought. Deep subduction of the Philippine Sea slab and deep dehydration of the Pacific slab are revealed beneath Southwest Japan. Significant structural heterogeneities are imaged in the source areas of large earthquakes in the crust, subducting slab and interplate megathrust zone, which may reflect fluids and/or magma originating from slab dehydration that affected the rupture nucleation of large earthquakes. These results suggest that large earthquakes do not strike anywhere, but in only anomalous areas that may be detected with geophysical methods. The occurrence of deep earthquakes under the Japan Sea and the East Asia margin may be related to a metastable olivine wedge in the subducting Pacific slab. The Pacific slab becomes stagnant in the mantle transition zone under East Asia, and a big mantle wedge (BMW) has formed above the stagnant slab. Convective circulations and fluid and magmatic processes in the BMW may have caused intraplate volcanism (e.g., Changbai and Wudalianchi), reactivation of the North China craton, large earthquakes, and other active tectonics in East Asia. Deep subduction and dehydration of continental plates (such as the Eurasian plate, Indian plate and Burma microplate) are also found, which have caused intraplate magmatism (e.g., Tengchong) and geothermal anomalies above the subducted continental plates. Under Kamchatka, the subducting Pacific slab shortens toward the north and terminates near the Aleutian-Kamchatka junction. The slab loss was induced by friction with the surrounding asthenosphere, as the Pacific plate rotated clockwise 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow. The stagnant slab finally collapses down to the bottom of the mantle, which may trigger upwelling of hot mantle materials from the lower mantle to the shallow mantle. Suggestions are also made for future directions of the seismological research of subduction zones.

Quaternary deformation of the Mushi thrust-related fold, northeastern margin of the Pamir

NASA Astrophysics Data System (ADS)

Li, T.; Chen, J.; Huang, D. M.; Thompson, J.; Xiao, P. W.; Yuan, D. Z.; Burbank, D. W.

2010-12-01

The Pamir salient defines the northwestern end of the Himalayan-Tibetan orogen and has overthrust the Tajik-Tarim basin to the north by ~300km along a late Cenozoic, south-dipping intracontinental subduction zone (Burtman and Molnar, 1993). The Quaternary deformation of the salient are concentrated on the outer margins: the sinistral Darvaz fault on the northwestern margin, the Trans-Alai thrust on the north margin and the northeast margin. The GPS-based plate tectonic model indicates the convergence rate is of 8-12mm/a in an N-S direction, nearly 1/4 of that between the Indian plate and the Eurasian plate (DeMets et al., 1990; Reigber et al., 2001; Yang et al., 2008). Previous studies focused on the northwestern margin and the north margin revel their spatial distribution, temporal evolution and kinematic patterns (Burtman and Molnar, 1993; Strecker et al., 1995; Arrowsmith and Strecker, 1999; Coutand et al., 2002). Deformed strata and GPS data indicate Quaternary deformations on the northeastern margin are concentrated on the PFT (the Pamir Front Thrust), the foreland thrust system generated by the latest advancing migration of the Pamir salient, whose kinematic patterns are still poor understood. Integrated by the Mushi thrust and the Mushi anticline, the Mushi thrust-related fold located at eastern end of the PFT. Simple structure, well outcrops and evident deformed terraces make it an excellent place to recognize deformation characters and kinematic patterns of the PFT. The Mushi thrust is north-vergent, roughly parallel with the anticline axis, and west part forming several subparallel fault scarps on the terrace surface and east part buried under the late-Quaternary deposits. The Mushi thrust is north-plunging, with a gentle south limb and a steep north limb. Combining field mapping data and neighboring seismic reflection profiles, following the cross-section balance principle, we can confine the Mushi thrust-related fold is a fault propagation fold evaluating from a detachment fold, the total shortening is ~0.7km, and the total uplift is ~1.5km. The shortening of the Mushi thrust-related fold is absorbed by strata folding and slipping along the thrust surface. According to the offset and the age of the terrace surface near the dam of the Kashi power station, the shortening rate of the Mushi thrust is ~0.7mm/a. On the basis of terraces deformation analysis, the Mushi anticline grows through limb rotation in late-Quaternary, and the minimum shortening rate is ~0.6mm/a. Then the total shortening rate is ~1.3mm/a. Although the growth strata cannot be found in the field work, the comfortable contacts between the Atushi formation and the Xiyu formation at both limbs indicate the growth inception of the Mushi thrust-related fold later than the base age of the Xiyu formation, which is ~1.6Ma (Chen et al., 2007). If the shortening rate is constant during growth of the thrust-related fold, the growth inception should be earlier than 0.5-0.6Ma.

Late Jurassic-Early Cretaceous continental convergence and intracontinental orogenesis in East Asia: A synthesis of the Yanshan Revolution

NASA Astrophysics Data System (ADS)

Dong, Shuwen; Zhang, Yueqiao; Zhang, Fuqin; Cui, Jianjun; Chen, Xuanhua; Zhang, Shuanhong; Miao, Laicheng; Li, Jianhua; Shi, Wei; Li, Zhenhong; Huang, Shiqi; Li, Hailong

2015-12-01

The basic tectonic framework of continental East Asia was produced by a series of nearly contemporaneous orogenic events in the late Middle Jurassic to Early Cretaceous. Commonly, the Late Mesozoic orogenic processes were characterized by continent-continent collision, large-scale thrusting, strike-slip faulting and intense crustal shortening, crustal thickening, regional anatexis and metamorphism, followed by large-scale lithospheric extension, rifting and magmatism. To better understand the geological processes, this paper reviews and synthesizes existing multi-disciplinary geologic data related to sedimentation, tectonics, magmatism, metamorphism and geochemistry, and proposes a two-stage tectono-thermal evolutionary history of East Asia during the late Middle Jurassic to Early Cretaceous (ca. 170-120 Ma). In the first stage, three orogenic belts along the continental margins were formed coevally at ca. 170-135 Ma, i.e., the north Mongol-Okhotsk orogen, the east paleo-Pacific coastal orogen, and the west Bangong-Nujiang orogen. Tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the Late Jurassic, as well as crustal anatexis that generated syn-kinematic granites, adakites and migmatites. The lithosphere of the East Asian continent was thickened, reaching a maximum during the latest Jurassic or the earliest Cretaceous. In the second stage (ca. 135-120 Ma), delamination of the thickened lithosphere resulted in a remarkable (>120 km) lithospheric thinning and the development of mantle-derived magmatism, mineralization, metamorphic core complexes and rift basins. The Middle Jurassic-Early Cretaceous subduction of oceanic plates (paleo-Pacific, meso-Tethys, and Mongol-Okhotsk) and continent-continent collision (e.g. Lhasa and Qiangtang) along the East Asian continental margins produced broad coastal and intracontinental orogens. These significant tectonic activities, marked by widespread intracontinental orogeny and continental reconstruction, are commonly termed the Yanshan Revolution (Movement) in the Chinese literature.

Various mechanisms to induce present-day shallow flat subduction and implications for the younger Earth: a numerical parameter study

NASA Astrophysics Data System (ADS)

van Hunen, Jeroen; van den Berg, Arie P.; Vlaar, Nico J.

2004-08-01

Shallow flat subduction is a relatively common feature at present-day subduction zones. Several mechanisms to explain this feature have been proposed, and can be subdivided into three groups: overthrusting of the subducting plate, subduction of a plume-generated oceanic plateau, and slab suction forces. We developed a numerical model to investigate these mechanisms and tested it through a comparison of the model results with the observations of the Peru flat slab where all three mechanisms seem to be contributing. The ratio of contributions of overthrusting continent to plateau subduction is in the range of 1:1 to 1:2, and the role of plate suction forces is likely to be significant. By applying the overthrusting continent and plateau subduction mechanisms separately, we were able to determine the most important model parameters for each of the mechanisms. Overthrusting easily results in flat subduction, and the flat slab length is primarily a function of slab age, overriding plate motion and mantle viscosity. An oceanic plateau is much less likely to cause flat subduction, and favorable conditions for flat subduction include a young slab age, long-lived plateau buoyancy after subduction, a strong mantle, and addition of slab suction forces that are large enough to further reduce the subduction dip angle, once the plateau initiates this flattening. Furthermore, we found that even though today flat subduction can be explained with the dominant model parameters within a reasonable range, for a slightly hotter, younger Earth, these flat subduction conditions are much less favorable, and so this style of subduction was probably not present in the past. This contradicts earlier predictions that flat subduction was a more wide-spread phenomenon in the early stages of plate tectonics in a younger earth.

Seismogenic Fault Geometry of 2010 Mw 7.1 Solomon Islands Earthquake

NASA Astrophysics Data System (ADS)

Kuo, Y.; Ku, C.; Taylor, F. W.; Huang, B.; Chen, Y.; Chao, W.; Huang, H.; Kuo, Y.; Wu, Y.; Suppe, J.

2010-12-01

The Solomon Islands is located in southwestern Pacific, where the Indo-Australian Plate is subducting northeastward beneath the Pacific Plate. Due to subduction of rugged seafloor topography, including seamounts, the seismic activity and tectonic behavior may be complicated. Seismicity in this region was anomalously low until 2007 when a megathrust rupture (Mw 8.1) occurred. More recently, on 3 January 2010, a Mw7.1 earthquake occurred beneath the extreme outer forearc next to the trench. It came with one foreshock (Mw 6.6, 50 minutes ahead) and two large aftershocks (Mw 6.8 and 6.0) greater than magnitude 6 within a week. It is interesting to note that these four focal mechanisms are very much similar and appear to have occurred along the interplate thrust zone between the Indo-Australian plate and Solomon Islands forearc. This Earthquake nucleated approximately 50 km to the southeast of the M8.1 Earthquake occurring in April of 2007, which is located to the other side of Rendova Island. Because a tsunami followed the 2010 earthquake, it is likely that submarine surface deformation accompanied the event. By the results of D-InSAR on ALOS and ERS, plus limited points of ground displacement from GPS and strong motion seismometers, the continuous ground displacement field is constructed and normalized. Our preliminary result shows the ground movement in the Rendova Island can reach tens of centimeters, implying shallow earthquake source consistent with the suggestion by triggering tsunami. Besides, the earthquake sequence retrieved from our local seismometer observation network allows us to further define underground fault geometry. The spatial distribution of the epicenter also concludes the seamount located in the middle divides two seismogenic asperities which generate 2007 and 2010 earthquakes respectively.

Miocene block uplift and basin formation in the Patagonian foreland: The Gastre Basin, Argentina

NASA Astrophysics Data System (ADS)

Bilmes, A.; D'Elia, L.; Franzese, J. R.; Veiga, G. D.; Hernández, M.

2013-08-01

The intraplate fault-block mountains and intermontane deposits of the Gastre Basin, which are recorded more than 550 km east of the Andean trench in central Patagonia, Argentina, are analyzed. The Gastre Basin is one of the largest Patagonian intermontane basins, limited by uplifted blocks strongly oblique to the Andean chain. It was originated by reverse faulting and inversion of pre-existing normal faults associated with a Mesozoic rift basin and defined by older crustal heterogeneities. The deformational event occurred during the middle Miocene, related to a short contractional episode (16.1-14.86 Ma), probably in response to an eastward migration of the Andean fold and thrust belt. During Pliocene to Quaternary times, neither younger fault-block uplifts nor reconfigurations of the basin occurred. Similarities between the study area and other parts of the Patagonian foreland - such as the presence of Miocene reverse or inversion tectonics, as well as the accommodation of the Miocene sedimentary successions - suggest that the Gastre Basin is part of a major late early to middle Miocene broken foreland system (i.e. the Patagonian broken foreland) that exhumed discrete fault-block mountains and generated contemporary basins along more than 950 km parallel to the Andean trench (i.e. between 40°00' and 48°00' south latitude). Based on recent studies on the southern Andean Margin, this continental-scale contractional episode may be the result of a flat-slab subduction segment. Nevertheless, such a hypothesis is very difficult to support when analyzing such a large flat subduction segment along the entire Patagonian trench. This suggests the need to consider alternative flat-slab trigger mechanisms or other factors in the generation of broken foreland systems.

Variation in forearc basin development along the Sunda Arc, Indonesia

NASA Astrophysics Data System (ADS)

van der Werff, W.

The present forearc basin configuration along the Sunda Arc initially appears to have been controlled by extension and differential subsidence of basement blocks in response to the late Eocene collision of India with Asia. The late Oligocene increase in convergence between the South-east Asian and Indian Plates associated with a new pulse of subduction, resulted in basement uplift and the formation of a regional unconformity that can be recognized along the entire Sunda Arc. From the early to late Miocene, the Sumba and Savu forearc sectors along the eastern Sunda Arc may have been characterized by forearc extension. Submarine fan deposition on the arcward side of the evolving accretionary prism represents the first phase in forearc basin deposition. These fans were subsequently covered by basin and slope sediments derived from the evolving magmatic arc. Structural response to increased late Miocene compression varied along strike of the Sunda Arc. North of Bali, Lombok and Sumbawa, the incipient collision between Australia and the western Banda Arc caused back-arc thrusting and basin inversion. Towards the south of Java, an increase in both the size of the accretionary prism and convergence rates resulted in uplift and large scale folding of the outer forearc basin strata. Along the west coast of Sumatra, increased compression resulted in uplift along the inner side of the forearc along older transcurrent faults. Uplift of West Sumatra was followed by the deposition of a westward prograding sequence of terrigenous sediments that resulted in the development of a broad shelf. Initial forearc basin subsidence relates to the age of the subducting oceanic lithosphere, on top of which the basin is situated. Along the western Sunda Arc, both fexural loading of the evolving accretionary prism, and across arc strike-slip faulting represent additional factors that result in forearc subsidence.

Review of subduction and its association with geothermal system in Sumatera-Java

NASA Astrophysics Data System (ADS)

Ladiba, A. F.; Putriyana, L.; Sibarani, B. br.; Soekarno, H.

2017-12-01

Java and Sumatera have the largest geothermal resources in Indonesia, in which mostly are spatially associated with volcanoes of subduction zones. However, those volcanoes are not distributed in a regular pattern due to the difference of subduction position. Subduction position in java is relatively more perpendicular to the trench than in Sumatera. In addition, Java has a concentration of large productive geothermal field with vapour dominated system in the western part of Java, which may be caused by the various subduction dip along the island. In order to understand the relationship between the subduction process and geothermal system in the subduction zone volcanoes, we examined several kinematic parameters of subduction that potentially relevant to the formation of geothermal system in overriding plate such as slab dip, subduction rate, and direction of subduction. Data and information regarding tectonic setting of Sumatera and Java and productive geothermal field in Sumatera and Java have been collected and evaluated. In conclusion, there are three condition that caused the geothermal fluid to be more likely being in vapour phase, which are: the subduction is in an orthogonal position, the slab dip is high, and rate of subduction is high. Although there are plenty researches of subduction zone volcanoes, only a few of them present information about its formation and implication to the geothermal system. The result of this study may be used as reference in exploration of geothermal field in mutual geologic environment.

Is the Local Seismicity in Haiti Capable of Imaging the Northern Caribbean Subduction?

NASA Astrophysics Data System (ADS)

Corbeau, J.; Clouard, V.; Rolandone, F.; Leroy, S. D.; de Lepinay, B. M.

2017-12-01

The boundary between the Caribbean (CA) and North American (NAM) plates in the Hispaniola region is the western prolongation of the NAM plate subduction evolving from a frontal subduction in the Lesser Antilles to an oblique collision against the Bahamas platform in Cuba. We analyze P-waveforms arriving at 27 broadband seismic temporary stations deployed along a 200 km-long N-S transect across Haiti, during the Trans-Haiti project. We compute teleseismic receiver functions using the ETMTRF method, and determine crustal thickness and bulk composition (Vp/Vs) using the H-k stacking method. Three distinctive crustal domains are imaged. We relate these domains to crustal terranes that have been accreted along the plate boundary during the northeastwards displacement of the CA plate. We propose a N-S crustal profile across Haiti accounting for the surface geology, shallow structural history and these new seismological constraints. Local seismicity recorded by the temporary network from April 2013 to June 2014 is used to relocate the seismicity. A total of 593 events were identified with magnitudes ranging from 1.6 to 4.5. This local seismicity, predominantly shallow (< 20 km) and situated in the southern part of Haiti along the major Enriquillo-Plantain-Garden strike-slip fault zone (EPGFZ) and offshore in Gonâve Bay, helps us to image deep active structures. Moment tensors for earthquakes with magnitudes between 3 and 4 were calculated by full waveform inversion using the ISOLA software. The analysis of the new moment tensors for the Haiti upper lithosphere indicates that normal, thrust and strike-slip faulting are equitably distributed. We found strike-slip events along the EPGFZ, near the location of the January 12th, 2010 earthquake. Most of the normal events are located in the area of Enriquillo and Azuei lakes, while the thrust events are located on both sides of the southern Peninsula of Haiti. The preliminary seismic data of our Haitian network, even noisy, tend to confirm that the North American slab in western Hispaniola is disappearing and that the scarcity of the seismic events could not be only the effect of the lack of a seismic network. Due to the geometry of the plate boundary, the deformation is predominantly strike-slip and there is no accommodation of an important part of convergence in this area.

Nature of the Coastal Range Wedge Along the Rupture Area of the 2015, Illapel Chile Earthquake Mw 8.4

NASA Astrophysics Data System (ADS)

Farías, M.; Comte, D.; Roecker, S. W.; Brandon, M. T.

2017-12-01

Wedge theory is usually applied to the pro-side of active subduction margins, where fold-and-thrust belts related to frontal accretion develop, but rarely to the entire wedge, where the retro-side is also relevant. We present a new 3D body wave tomographic image that combines data from the Chile-Illapel Aftershock Experiment (CHILLAX) with previous temporary seismic networks, with the aim of illuminating the nature of the wedge of the continental margin above the seismogenic part of the subducting slab. The downdip extent of the coupled part, called the S-point in the wedge theory, corresponds to the place where upper plate completely decouples from the subducting slab. This point is characterized by a Vp/Vs contrast at about 60 km depth that extends upward-and-eastward in a west-dipping ramp-like geometry. This ramp emerges about 180 km from the trench, near the topographic break related to the front of the Andean retro-side. The Coastal wedge domain is characterized by a monotonous east-dipping homocline with the older rocks of this region along the coast. The offshore region, corresponding to the pro-side, exhibits normal faulting and a very small frontal accretionary complex. Normal faulting in this region is related to rapid uplift of marine terraces since ca. 2 Ma, suggesting strong basal accretion and thus high friction on the thrust. In fact, the epicentral region of the 2015 Illapel Earthquake coincides with the highest elevations along the coast, i.e., the region with the highest slope of the margin. In this region, the lack of a continental forearc basin suggests an overlapping between the Andean and Coastal wedges. The western edge of the Andean wedge is also part of the homocline about 10 km east of the topographic boundary between both wedges, suggesting that the Coastal wedge has been deforming a part of the retro-side of the Andean wedge during the Miocene. The east-ward tilting of the retro-side was acquired mainly before the late Miocene, since at least the Late Cretaceous, before the proposed arrival of the Juan Fernandez ridge at this area and the consequent flat slab that characterize this part of the margin in the Present.

Early Cretaceous wedge extrusion in the Indo-Burma Range accretionary complex: implications for the Mesozoic subduction of Neotethys in SE Asia

NASA Astrophysics Data System (ADS)

Zhang, Ji'en; Xiao, Wenjiao; Windley, Brian F.; Cai, Fulong; Sein, Kyaing; Naing, Soe

2017-06-01

The Indo-Burma Range (IBR) of Myanmar, the eastern extension of the Yarlung-Tsangpo Neotethyan belt of Tibet in China, contains mélanges with serpentinite, greenschist facies basalt, chert, sericite schist, silty slate and unmetamorphosed Triassic sandstone, mudstone and siltstone interbedded with chert in the east, and farther north high-pressure blueschist and eclogite blocks in the Naga Hills mélange. Our detailed mapping of the Mindat and Magwe sections in the middle IBR revealed a major 18 km antiformal isocline in a mélange in which greenschist facies rocks in the core decrease in grade eastwards and westwards symmetrically `outwards' to lower grade sericite schist and silty slate, and at the margins to unmetamorphosed sediments, and these metamorphic rocks are structurally repeated in small-scale imbricated thrust stacks. In the Mindat section the lower western boundary of the isoclinal mélange is a thrust on which the metamorphic rocks have been transported over unmetamorphosed sediments of the Triassic Pane Chaung Group, and the upper eastern boundary is a normal fault. These relations demonstrate that the IBR metamorphic rocks were exhumed by wedge extrusion in a subduction-generated accretionary complex. Along strike to the north in the Naga Hills is a comparable isoclinal mélange in which central eclogite lenses are succeeded `outwards' by layers of glaucophane schist and glaucophanite, and to lower grade greenschist facies sericite schist and slate towards the margins. In the Natchaung area (from west to east) unmetamorphosed Triassic sediments overlie quartzites, sericite schists, actinolite schists and meta-volcanic amphibolites derived from MORB-type basalt, which are in fault contact with peridotite. Olivine in the peridotite has undulatory extinction suggesting deformation at 600-700 °C, similar to the peak temperature of the amphibolite; these relations suggest generation in a metamorphic sole. The amphibolites have U/Pb zircon ages of 119 ± 3 Ma and 115 Ma, which are close to the zircon ages of nearby calc-alkaline granite and diorite, which belong to an active continental margin arc that extends along the western side of the Shan-Thai block. The IBR accretionary complex and the active continental margin arc were generated during Early Cretaceous (115-128 Ma) subduction of the Neotethys Ocean.

A Long-term Slip Model for the San Ramón Fault, Santiago de Chile, from Tectonically Reconcilable Boundary Conditions

NASA Astrophysics Data System (ADS)

Aron, F.; Estay, N.; Cembrano, J. M.; Yanez, G. A.

2016-12-01

We constructed a 3D Boundary Elements model simulating subduction of the Nazca plate underneath South America, from 29° to 38° S, to compute long-term surface deformation and slip rates on crustal faults imbedded in the upper-plate wedge of the Andean orogen. We tested our model on the San Ramón Fault (SRF), a major E-dipping, thrust structure limiting the western front of the Main Cordillera with surface expression along the entire, 40 km long, extension of the Santiago de Chile basin. Long-lived thrusting has produced more than 2 km of differential uplift of the mountains. Given its proximity to the country's largest city, this potentially seismogenic fault —dormant during historic times— has drawn increasing public attention. We used earthquake hypocenters captured over a one-year seismic deployment, 2D resistivity profiles, and published geologic cross-sections to determine the geometry of the SRF. The base of the lithosphere and plate interface surfaces were defined based on average Andean values and the Slab1.0 model. The simulation reproduces plate convergence and mechanic decoupling of the lithospheric plates across the subduction seismic cycle using mixed boundary conditions. Relative plate motion is achieved prescribing uniform, far-field horizontal displacement over the depth extension of both the oceanic and continental lithospheric plates. Long-term deformation is carried out in two steps. First, the modeled surfaces are allowed to slip freely emulating continuous slip on the subduction megathrust; subsequently, zero displacement is prescribed on the locking zone of the megathrust down to 40 km depth, while keeping the rest of the surfaces traction free, mimicking interseismic conditions. Long-term slip rate fields obtained for the SRF range between 0.1 and 1% the plate convergence rate, with maximum values near the surface. Interestingly, at an estimated 76-77 mm/yr relative plate motion velocity, those rates agree well with what has been reported on studies at one paleoseismic trench site across the fault. These results might contribute to determining possible seismic scenarios for Santiago but perhaps more importantly, our approach could be use in estimations of long-term slip rates and surface deformation due to other crustal structures with unknown displacement history.

Tectonic processes during oblique collision: Insights from the St. Elias orogen, northern North American Cordillera

USGS Publications Warehouse

Pavlis, T.L.; Picornell, C.; Serpa, L.; Bruhn, R.L.; Plafker, G.

2004-01-01

Oblique convergence in the St. Elias orogen of southern Alaska and northwestern Canada has constructed the world's highest coastal mountain range and is the principal driver constructing all of the high topography in northern North America. The orogen originated when the Yakutat terrane was excised from the Cordilleran margin and was transported along margin-parallel strike-slip faults into the subduction-transform transition at the eastern end of the Aleutian trench. We examine the last 3 m.y. of this collision through an analysis of Euler poles for motion of the Yakutat microplate with respect to North America and the Pacific. This analysis indicates a Yakutat-Pacific pole near the present southern triple junction of the microplate and' predicts convergence to dextral-oblique convergence across the offshore Transition fault, onland structures adjacent to the Yakutat foreland, or both, with plate speeds increasing from 10 to 30 mm/yr from southeast to northwest. Reconstructions based on these poles show that NNW transport of the collided block into the NE trending subduction zone forced contraction of EW line elements as the collided block was driven into the subduction-transform transition. This suggests the collided block was constricted as it was driven into the transition. Constriction provides an explanation for observed vertical axis refolding of both earlier formed fold-thrust systems and the collisional suture at the top of the fold-thrust stack. We also suggest that this motion was partially accommodated by lateral extrusion of the western portion of the orogen toward the Aleutian trench. Important questions remain regarding which structures accommodated parts of this motion. The Transition fault may have accommodated much of the Yakutat-Pacific convergence on the basis of our analysis and previous interpretations of GPS-based geodetic data. Nonetheless, it is locally overlapped by up to 800 m of undeformed sediment, yet elsewhere shows evidence of young deformation. This contradiction could be produced if the overlapping sediments are too young to have accumulated significant deformation, or GPS motions may be deflected by transient strains or strains from poorly understood fault interactions. In either case, more data are needed to resolve the paradox. Copyright 2004 by the American Geophysical Union.

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

USGS Publications Warehouse

Liberty, Lee M.; Finn, Shaun P.; Haeussler, Peter J.; Pratt, Thomas L.; Peterson, Andrew

2013-01-01

High-resolution sparker and crustal-scale air gun seismic reflection data, coupled with repeat bathymetric surveys, document a region of repeated coseismic uplift on the portion of the Alaska subduction zone that ruptured in 1964. This area defines the western limit of Prince William Sound. Differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 Great Alaska earthquake was focused along a series of subparallel faults beneath Prince William Sound and the adjacent Gulf of Alaska shelf. Bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the seafloor as submarine terraces on the Cape Cleare bank southwest of Montague Island. Sparker seismic reflection data provide cumulative Holocene slip estimates as high as 9 mm/yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism. Crustal seismic data show that these megathrust splay faults root separately into the subduction zone décollement. Splay fault divergence from this megathrust correlates with changes in midcrustal seismic velocity and magnetic susceptibility values, best explained by duplexing of the subducted Yakutat terrane rocks above Pacific plate rocks along the trailing edge of the Yakutat terrane. Although each splay fault is capable of independent motion, we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the Holocene.

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

NASA Astrophysics Data System (ADS)

Liberty, Lee M.; Finn, Shaun P.; Haeussler, Peter J.; Pratt, Thomas L.; Peterson, Andrew

2013-10-01

sparker and crustal-scale air gun seismic reflection data, coupled with repeat bathymetric surveys, document a region of repeated coseismic uplift on the portion of the Alaska subduction zone that ruptured in 1964. This area defines the western limit of Prince William Sound. Differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 Great Alaska earthquake was focused along a series of subparallel faults beneath Prince William Sound and the adjacent Gulf of Alaska shelf. Bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the seafloor as submarine terraces on the Cape Cleare bank southwest of Montague Island. Sparker seismic reflection data provide cumulative Holocene slip estimates as high as 9 mm/yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism. Crustal seismic data show that these megathrust splay faults root separately into the subduction zone décollement. Splay fault divergence from this megathrust correlates with changes in midcrustal seismic velocity and magnetic susceptibility values, best explained by duplexing of the subducted Yakutat terrane rocks above Pacific plate rocks along the trailing edge of the Yakutat terrane. Although each splay fault is capable of independent motion, we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the Holocene.

The behavior of a convergent plate boundary - Crustal deformation in the South Kanto district, Japan

NASA Technical Reports Server (NTRS)

Scholz, C. H.; Kato, T.

1978-01-01

The northwesternmost part of the Sagami trough, a part of the Philippine Sea-Eurasian plate boundary, was ruptured during the great South Kanto earthquake in 1923. Very extensive and frequent geodetic measurements of crustal deformation have been made in the South Kanto district since the 1890's, and these constitute the most complete data set on crustal movements in the world. These data were reanalyzed and interpreted and according to our interpretation indicate the following sequence of events. The coseismic movements were due to oblique thrust and right lateral slip of about 8 m on a fault outcropping at the base of the Sagami trough. This was followed by postseismic deformation resulting from reversed afterslip of 20-60 cm that occurred at an exponentially decaying rate in time. The interseismic deformation is produced by steady subduction at a rate of about 1.8 cm/yr. During subduction the top 10-15 km of the plate boundary is apparently locked, while deeper parts slip aseismically at an irregular rate. No significant precursory deformation was observed. The recurrence time for 1923 type earthquakes is 200-300 years. The Boso and Miura peninsulas are broken into a series of fault-bound blocks that move semi-independently of the surrounding region. The subduction zone itself, where it is exposed on land, is shown to be a wide zone encompassing several faults that are active at different times.

The Naga Hills and Andaman ophiolite belt, their setting, nature and collisional emplacement history

NASA Astrophysics Data System (ADS)

Acharyya, S. K.; Ray, K. K.; Sengupta, Subhasis

The Indo-Burmese Range and the Andaman-Nicobar Island Arc, form a continuous arcuate trend along which several ophiolite occurrences have been reported. In Naga Hills (NHO) and Andaman (ANO), these ophiolites are represented by dismembered mafic and ultramafic rocks with closely associated oceanic pelagic sediments. They occur as folded thrust slices occupying the highest tectonic levels and are brought to lie over distal shelf sediments of Eocene to Oligocene age. Ophiolites are unconformably overlain by ophiolite-derived clastics of Middle to Late Eocene age. The ophiolites preserved along this belt are remnants of a continuous, narrow, one or several intra-continental ocean basin(s) of broadly comparable age, created during the Late Mesozoic rifting of the Greater India Gondwana continent. Rifting and creation of oceanic crust date between Cretaceous and Early Eocene. In the initial stages, the ocean floor had been deeper than Carbonate Compensation Depth (CCD). Subsequently it had become uneven, when oceanic crust was being added through several seamounts or seamount chains and on top of which calcareous pelagic sediments were deposited. Both tholeiitic and alkaline volcanic rocks are present in these ophiolites. In NHO, the two groups of lavas have generated from different sources in different tectonic settings. The alkalic and some tholeiitic lavas in NHO are similar to off-axis seamount basalts. Tholeiitic lavas from ANO and some NHO resemble MORB or backarc basin basalts and on the basis of certain chemical characters these are suggested to have generated in marginal basin setting. Significant volume of acid differentiates are associated in ANO which also support the marginal basin character of the basalts. The suite of rocks in ANO indicates fractionation in a shallow level magma chamber. Closure of the small ocean basin(s) and emplacement of ophiolites took place in two stages. In the initial stage, the seamount chain brought to the subduction zone collided with the Burmese block prior to Middle Eocene. Part of the ophiolites represent clipped seamounts which got accreted to the leading edge of the eastern continental block. With continued closure, this eastern block with accreted ophiolite slices was brought in juxtaposition with distal shelf sediments of the western block marking the terminal continent-continent collision. The thrust front of ophiolitic rocks apparently advanced further westward in Andaman to the south compared to the northern sector, and thus an imbricated zone and melange involving the Eocene floor sediments (Lipa Fm) has been created, whereas in the Naga Hills the floor sediments (Disang Fm) remained virtually passive. The time of terminal continental collision is represented as the regional Late Oligocene unconformity. The entire thrust stack got deformed and folded into upright geometry after being blocked. The present subduction of oceanic crust beneath the Andaman island arc appears to be a westward jump of subduction zone due to sustained post-collisional NE drive of the Indian plate.

USGS research on geohazards of the North Pacific: past, present, and future

NASA Astrophysics Data System (ADS)

McNutt, M. K.; Eichelberger, J. C.

2012-12-01

The disastrous earthquakes and tsunamis of Sumatra in 2004 and Tohoku in 2011 have driven re-examination of where and how such events occur. Particular focus is on the North Pacific. Of the top 30 earthquakes recorded instrumentally worldwide, 50% occurred along the line of subduction from the Kuril Islands to the southern Alaska mainland. This region has seen monstrous volcanic eruptions (Katmai-Novarupta, 1912), destructive tsunamis (Severo-Kurilsk, 1952), and one of Earth's largest instrumentally-recorded earthquakes (M9.2 Alaska, 1964). Only the modest populations in these frontier towns half a century ago kept losses to a minimum. Impact of any natural disaster to population, vital infrastructure, and sea and air transportation would be magnified today. While USGS had a presence in Alaska for more than a century, the great Alaska earthquake of 1964 ushered in the first understanding of the area's risks. This was the first mega-thrust earthquake properly interpreted as such, and led to re-examination of the 1960 Chilean event. All modern conceptions of mega-thrust earthquakes and tsunamis derive some heritage from USGS research following the 1964 event. The discovery of oil in the Alaska Arctic prompted building a pipeline from the north slope of Alaska to the ice-free port of Valdez. The USGS identified risks from crossing permafrost and active faults. Accurate characterization of these hazards informed innovative designs that kept the pipeline from rupturing due to ground instability or during the M7.9 Denali earthquake of 2002. As a large state with few roads, air travel is common in Alaska. The frequent ash eruptions of volcanoes in the populous Cook Inlet basin became a serious issue, highlighted by the near-crash of a large passenger jet in 1989. In response, the USGS and its partners developed and deployed efficient seismic networks on remote volcanoes and initiated regular satellite surveillance for early warning of ash eruptions. Close collaboration developed with Russian colleagues to jointly monitor volcanoes under the international air routes that traverse the region. Impacts from eruptions on the ground have been more limited than on aviation. But because it was sited before an awareness of geohazards, the Drift River Oil Terminal has been inundated by large lahars from Redoubt in 1990 and 2009, endangering large oil storage tanks. Eruption warnings from the USGS and its partners aided the terminal's crew, and no lives were lost. The Cook Inlet Regional Citizens Advisory Council recently called for replacing the facility with an underwater pipeline within five years. The USGS is now beginning an investigation of the paleo-tsunami record of the Aleutian Islands and, with partners, marine studies aimed at understanding the cause of Aleutian subduction's propensity for the most extreme of events. We are implementing a new ash transport model that will for the first time provide ash fallout forecasts. And we look forward to enhancing our partnership with Russian colleagues through sharing of data and best practices in order to mitigate disaster risk to all communities of the North Pacific.

Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system

NASA Astrophysics Data System (ADS)

Hsu, Y. J.; Yu, S. B.; Loveless, J. P.; Bacolcol, T.; Woessner, J.; Solidum, R., Jr.

2015-12-01

The Sunda plate converges obliquely with the Philippine Sea plate with a rate of ~100 mm/yr and results in the sinistral slip along the 1300 km-long Philippine fault. Using GPS data from 1998 to 2013 as well as a block modeling approach, we decompose the crustal motion into multiple rotating blocks and elastic deformation associated with fault slip at block boundaries. Our preferred model composed of 8 blocks, produces a mean residual velocity of 3.4 mm/yr at 93 GPS stations. Estimated long-term slip rates along the Manila subduction zone show a gradual southward decrease from 66 mm/yr at the northwest tip of Luzon to 60 mm/yr at the southern portion of the Manila Trench. We infer a low coupling fraction of 11% offshore northwest Luzon and a coupling fraction of 27% near the subduction of Scarborough Seamount. The accumulated strain along the Manila subduction zone at latitudes 15.5°~18.5°N could be balanced by earthquakes with composite magnitudes of Mw 8.7 and Mw 8.9 based on a recurrence interval of 500 years and 1000 years, respectively. Estimates of sinistral slip rates on the major splay faults of the Philippine fault system in central Luzon increase from east to west: sinistral slip rates are 2 mm/yr on the Dalton fault, 8 mm/yr on the Abra River fault, and 12 mm/yr on the Tubao fault. On the southern segment of the Philippine fault (Digdig fault), we infer left-lateral slip of ~20 mm/yr. The Vigan-Aggao fault in northwest Luzon exhibits significant reverse slip of up to 31 mm/yr, although deformation may be distributed across multiple offshore thrust faults. On the Northern Cordillera fault, we calculate left-lateral slip of ~7 mm/yr. Results of block modeling suggest that the majority of active faults in Luzon are fully locked to a depth of 15-20 km. Inferred moment magnitudes of inland large earthquakes in Luzon fall in the range of Mw 7.0-7.5 based on a recurrence interval of 100 years. Using the long-term plate convergence rate between the Sunda plate and Philippine Sea plate as well as seismic moment release rate, we calculate the moment budget for the entire Luzon plate boundary zone that could be balanced by earthquakes with a composite magnitude of ~Mw 9 based on recurrence intervals of 500-1000 years.

Jurassic-Paleogene intraoceanic magmatic evolution of the Ankara Mélange, north-central Anatolia, Turkey

NASA Astrophysics Data System (ADS)

Sarifakioglu, E.; Dilek, Y.; Sevin, M.

2014-02-01

Oceanic rocks in the Ankara Mélange along the Izmir-Ankara-Erzincan suture zone (IAESZ) in north-central Anatolia include locally coherent ophiolite complexes (∼ 179 Ma and ∼ 80 Ma), seamount or oceanic plateau volcanic units with pelagic and reefal limestones (96.6 ± 1.8 Ma), metamorphic rocks with ages of 256.9 ± 8.0 Ma, 187.4 ± 3.7 Ma, 158.4 ± 4.2 Ma, and 83.5 ± 1.2 Ma indicating northern Tethys during the late Paleozoic through Cretaceous, and subalkaline to alkaline volcanic and plutonic rocks of an island arc origin (∼ 67-63 Ma). All but the arc rocks occur in a shale-graywacke and/or serpentinite matrix, and are deformed by south-vergent thrust faults and folds that developed in the middle to late Eocene due to continental collisions in the region. Ophiolitic volcanic rocks have mid-ocean ridge (MORB) and island arc tholeiite (IAT) affinities showing moderate to significant large ion lithophile elements (LILE) enrichment and depletion in Nb, Hf, Ti, Y and Yb, which indicate the influence of subduction-derived fluids in their melt evolution. Seamount/oceanic plateau basalts show ocean island basalt (OIB) affinities. The arc-related volcanic rocks, lamprophyric dikes and syenodioritic plutons exhibit high-K shoshonitic to medium- to high-K calc-alkaline compositions with strong enrichment in LILE, rare earth elements (REE) and Pb, and initial ɛNd values between +1.3 and +1.7. Subalkaline arc volcanic units occur in the northern part of the mélange, whereas the younger alkaline volcanic rocks and intrusions (lamprophyre dikes and syenodioritic plutons) in the southern part. The late Permian, Early to Late Jurassic, and Late Cretaceous amphibole-epidote schist, epidote-actinolite, epidote-chlorite and epidote-glaucophane schists represent the metamorphic units formed in a subduction channel in the northern Neotethys. The Middle to Upper Triassic neritic limestones spatially associated with the seamount volcanic rocks indicate that the northern Neotethys was an open ocean with its MORB-type oceanic lithosphere by the early Triassic (or earlier). The latest Cretaceous-early Paleocene island arc volcanic, dike and plutonic rocks with subalkaline to alkaline geochemical affinities represent intraoceanic magmatism that developed on and across the subduction-accretion complex above a N-dipping, southward-rolling subducted lithospheric slab within the northern Neotethys. The Ankara Mélange thus exhibits the record of ∼ 120-130 million years of oceanic magmatism in geological history of the northern Neotethys.

Multiscale Architecture of a Subduction Complex and Insight into Large-scale Material Movement in Subduction Systems

NASA Astrophysics Data System (ADS)

Wakabayashi, J.

2014-12-01

The >1000 km by >100 km Franciscan complex of California records >100 Ma of subduction history that terminated with conversion to a transform margin. It affords an ideal natural laboratory to study the rock record of subduction-interface and related processes exhumed from 10-70 km. The Franciscan comprises coherent and block-in-matrix (mélange) units forming a nappe stack that youngs structurally downward in accretion age, indicating progressive subduction accretion. Gaps in accretion ages indicate periods of non-accretion or subduction erosion. The Franciscan comprises siliciclastic trench fill rocks, with lesser volcanic and pelagic rocks and serpentinite derived from the downgoing plate, as well as serpentinite and felsic-intermediate igneous blocks derived as detritus from the upper plate. The Franciscan records subduction, accretion, and metamorphism (including HP), spanning an extended period of subduction, rather than a single event superimposed on pre-formed stratigraphy. Melanges (serpentinite and siliciclastic matrix) with exotic blocks, that include high-grade metamorphic blocks, and felsic-intermediate igneous blocks from the upper plate, are mostly/entirely of sedimentary origin, whereas block-in-matrix rocks formed by tectonism lack exotic blocks and comprise disrupted ocean plate stratigraphy. Mélanges with exotic blocks are interbedded with coherent sandstones. Many blocks-in-melange record two HP burial events followed by surface exposure, and some record three. Paleomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow fault zones of <100's of m thickness, and <50 m in best constrained cases; these zones lack exotic blocks. Large-scale displacements, whether paleomegathrust horizons, shortening within accreted nappes, or exhumation structures, are accommodated by discrete faults or narrow shear zones, rather than by significant penetrative strain. Exhumation of Franciscan HP units, both coherent and mélange, was accommodated by significant extension of the overlying plate, and possibly extension within the subduction complex, with cross-sectional extrusion, and like subduction burial, took place at different times.

Interseismic Strain Accumulation Across Metropolitan Los Angeles: Puente Hills Thrust

NASA Astrophysics Data System (ADS)

Argus, D.; Liu, Z.; Heflin, M. B.; Moore, A. W.; Owen, S. E.; Lundgren, P.; Drake, V. G.; Rodriguez, I. I.

2012-12-01

Twelve years of observation of the Southern California Integrated GPS Network (SCIGN) are tightly constraining the distribution of shortening across metropolitan Los Angeles, providing information on strain accumulation across blind thrust faults. Synthetic Aperture Radar Interferometry (InSAR) and water well records are allowing the effects of water and oil management to be distinguished. The Mojave segment of the San Andreas fault is at a 25° angle to Pacific-North America plate motion. GPS shows that NNE-SSW shortening due to this big restraining bend is fastest not immediately south of the San Andreas fault across the San Gabriel mountains, but rather 50 km south of the fault in northern metropolitan Los Angeles. The GPS results we quote next are for a NNE profile through downtown Los Angeles. Just 2 mm/yr of shortening is being taken up across the San Gabriel mountains, 40 km wide (0.05 micro strain/yr); 4 mm/yr of shortening is being taken up between the Sierra Madre fault, at the southern front of the San Gabriel mountains, and South Central Los Angeles, also 40 km wide (0.10 micro strain/yr). We find shortening to be more evenly distributed across metropolitan Los Angeles than we found before [Argus et al. 2005], though within the 95% confidence limits. An elastic models of interseismic strain accumulation is fit to the GPS observations using the Back Slip model of Savage [1983]. Rheology differences between crystalline basement and sedimentary basin rocks are incorporated using the EDGRN/EDCMP algorithm of Wang et al. [2003]. We attempt to place the Back Slip model into the context of the Elastic Subducting Plate Model of Kanda and Simons [2010]. We find, along the NNE profile through downtown, that: (1) The deep Sierra Madre Thrust cannot be slipping faster than 2 mm/yr, and (2) The Puente Hills Thrust and nearby thrust faults (such as the upper Elysian Park Thrust) are slipping at 9 ±2 mm/yr beneath a locking depth of 12 ±5 km (95% confidence limits). Incorporating sedimentary basin rock either reduces the slip rate by 10 per cent or increases the locking rate by 20 per cent. The 9 mm/yr rate for the Puente Hills Thrust and nearby faults exceeds the cumulative 3-5 mm/yr rate estimated using paleoseismology along the Puente Hills Thrust (1.2-1.6 mm/yr, Dolan et al. 2003), upper Elysian Park Thrust (0.6-2.2 mm/yr, Oskin et al. 2000), and western Compton Thrust (1.2 mm/yr, Leon et al. 2009], though all the paleoseismic estimates are minimums. We infer that M 7 earthquakes in northern metropolitan Los Angeles may occur more frequently that previously thought.

Upper plate contraction north of the migrating Mendocino triple junction northern California: Implications for partitioning of strain

USGS Publications Warehouse

McCrory, P.A.

2000-01-01

Geologic measurement of permanent contraction across the Cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary, the component critical for the seismic hazard assessment of crustal faults. A comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the Cascadia subduction zone indicates that these thrust faults accommodate ??10 mm/yr of convergence oriented 020??-045??. Seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust. Geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust. Both types of models underestimate the seismic hazard associated with crustal faults. Subtracting the observed contraction from the plate convergence rate (40-50 mm/yr; directed 040??-055??) leaves 30-40 mm/yr of convergence to be partitioned between slip on the megathrust, contraction within the southern Juan de Fuca plate, and crustal contraction outside the subduction complex rocks. This simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the Mendocino triple junction. The San Andreas and Cascadia limbs of the Mendocino triple junction are not collinear. The eastern edge of the broad San Andreas boundary is ??85 km east of the Cascadia subduction boundary, and across this zone the Pacific plate converges directly with the North America plate. The skewed orientation of crustal structures just north of the leading edge of the Pacific plate suggests that they are deforming in a hybrid stress field resulting from both Juan de Fuca-North America motion and Pacific-North America motion. The composite convergence direction (50 mm/yr: directed 023??) is consistent with the compressive stress axis (020??) inferred from focal mechanisms of crustal earthquakes in the Humboldt region. Deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources. The slip on crustal faults north of the Mendocino triple junction may consume 4-5 mm/yr of Pacific-Humboldt convergence. The remaining 17-18 mm/yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the Cape Mendocino region or as northward translation of the continental margin, north of the triple junction.

Shear-wave Velocity Structure and Inter-Seismic Strain Accumulation in the Up-Dip Region of the Cascadia Subduction Zone: Similarities to Tohoku?

NASA Astrophysics Data System (ADS)

Collins, J. A.; McGuire, J. J.; Wei, M.

2013-12-01

The up-dip region of subduction zone thrusts is difficult to study using land-based seismic and geodetic networks, yet documenting its ability to store and release elastic strain is critical for understanding the mechanics of great subduction earthquakes and tsunami generation. The 2011 Tohoku earthquake produced extremely large slip in the shallowest portion of the subduction zone beneath a region of the fore-arc that is comprised of extremely low-velocity, unconsolidated sediments [Tsuru et al. JGR 2012]. The influence of the sediment material properties on the co-seismic slip distribution and tsunami generation can be considerable through both the effects on the dynamic wavefield during the rupture [Kozdon and Dunham, BSSA 2012] and potentially the build up of strain during the inter-seismic period. As part of the 2010-2011 SeaJade experiment [Scherwath et al, EOS 2011], we deployed 10 ocean bottom seismographs (OBS) on the continental slope offshore of Vancouver Island in the region of the NEPTUNE Canada observatory. One goal of the experiment is to measure the shear modulus of the sediments lying above the subducting plate using the seafloor compliance technique. Using seafloor acceleration measured by broadband seismometer and seafloor pressure measured by Differential Pressure Gauge (DPG), we estimate the compliance spectrum in the infra-gravity wave band (~0.002-0.04 Hz) at 9 sites following the methodology of Crawford et al. [JGR, 1991]. We calibrated DPG sensitivities using laboratory measurements and by comparing teleseismic Rayleigh arrivals recorded on the seismometer and DPG channels [Webb, pers. comm]. We correct the vertical-component seismometer data for tilt using the procedure of Crawford and Webb [BSSA, 2000], Corrections for the gravitational attraction of the surface gravity waves [Crawford et al., JGR, 1998] are important at frequencies of 0.003-0.006 Hz only. Typically, the coherences are high (>0.7) in the 0.006 to 0.03 Hz range. We invert the measured compliances in this frequency band using a genetic algorithm that solves for the S-wave velocity, P-wave velocity, and density in a layered structure. By including constraints on the Vp distribution from active-source studies, these parameters appear well constrained down to about 4 km depth from our dataset. There is a clear difference in observed compliance values between stations close to the deformation front (~10 km) and those further up the continental slope (~30-40 km) indicating a region of unconsolidated, high-porosity sediment similar to the off-Tohoku region. The low S-wave velocities and high Vp/Vs ratios in the up-dip region correspond to unconsolidated high-porosity sediments. We calculated the effect of this material property contrast on the inter-seismic strain accumulation in the up-dip region of the subduction zone using a finite element model and find that the sediments can increase the amount of inter-seismic strain accumulated in the up-dip region by >100% relative to a homogenous elastic model.

Transition from Subduction to Strike-Slip in the Southeast Caribbean: Effects on Lithospheric Structures and Overlying Basin Evolution

NASA Astrophysics Data System (ADS)

Alvarez, T.; Mann, P.; Wood, L. J.; Vargas, C. A.; Latchman, J. L.

2013-12-01

Topography, basin structures and geomorphology of the southeast Caribbean-northeast South American margin are controlled by a 200-km-long transition from westward-directed subduction of South American lithosphere beneath the Caribbean plate, to east-west strike-slip motion of the Caribbean and South American plates. Our study of structures and basins present in the transitional area integrates a tomographic study of the lithospheric structures associated with lateral variations in the subduction of the South American lithosphere and orientation of the slab beneath the Caribbean plate as well as the evolution of overlying sedimentary basins imaged with deep-penetration seismic data kindly provided by the oil industry and Trinidad & Tobago government agencies. We use an earthquake dataset containing more than 700 events recorded by the eastern Caribbean regional seismograph network to build travel-time and attenuation tomography models used to image the mantle to depths of 100 km beneath transition zone. Approximately 10,000 km of 2D seismic reflection lines which are recorded to depths > 12 seconds TWT are used to interpret basin scale structures including tectono-stratigraphic sequences and structures which deform and displace sedimentary sequences. We use the observed satellite gravity to generate a gravity model for key sections traversing the tectonic transitional zone and to determine depth to basement in basins with sedimentary fill > 12 km. Within the study area, the dip of subducted South American oceanic lithosphere imaged on tomographic images is variable from ~44 to ~24 degrees. There is a distinct low gravity, low velocity, high attenuation, northwest - southeast trending lineation located east of Trinidad which defines the location of a Mesozoic oceanic fracture zone which accommodated the opening of the Central Atlantic during the Jurassic to Middle Cretaceous. This feature is also coincident with the present-day continent-ocean boundary and acts as a lithospheric weakness during subduction. We propose that this fracture zone is a key transition point between the subduction of South American/Atlantic oceanic lithosphere; which descends into the mantle, to the northeast, and the under-thrusting of transitional to continental South American lithosphere which resists subduction to the southwest. Maps of South American basement and its overlying Cretaceous passive margin illustrates a northwesterly basement dip with a distinct change in angle of the northwest dip across the paleo-fracture zone consistent with our tomographic model. We propose that flexure of the subducting South American plate at this location exerts a critical control on the formation and evolution of the basins and the lateral distribution of Cretaceous through Pleistocene stratigraphic fill. East of the fracture zone, the overlying strata is deformed by active subduction and accretionary prism processes with a wider zone of shortening with lower overall topography, while to the west of the fracture zone there is active oblique collision with a narrower zone of shortening and greater uplift.

Change in stress with seismic cycles identified at an out of sequence thrust in an on-land accretionary complex: The Nobeoka thrust, Shimanto Belt, Kyusyu, SW Japan

NASA Astrophysics Data System (ADS)

Yamaguchi, M.; Hashimoto, Y.; Yamaguchi, A.; Kimura, G.

2011-12-01

Seismic surveys along accretionary prisms have revealed that the out-of sequence thrusts (OSTs) are commonly developed within accretionary wedges branching from seismogenic subduction plate boundaries. The OSTs are also recognized in on-land accretionary complexes as large thrust faults cutting paleo-thermal structures. The OSTs are thought to play a role in tsunami genesis at a coseismic event. Stress history on OSTs is significant to understand the OSTs' role in seismic cycles. We estimated, thus palaeostresses from micro-faults along an OST in an on-land accretionary complexes. We focused on the Nobeoka fault which is an OST in an on-land accretionary complex, the Shimato Belt, Kyusyu, SW Japan. A gap in paleothermal temperature (up to 70 degree C) is observed at the fault. The Nobeoka thrust strikes almost EW at coastline. The Cretaceous Makimine formation and Paleogene Kitagawa formation are located at the hanging wall of the fault, comprising mainly of pelitic schist. The footwall of the fault is the Paleogene Hyuga formation composed mainly of shale. A lot of micro-faults are well developed just below the thrust for a few hundred meters to the south. Those micro faults are considered to be related to the Nobeoka thurst because slip direction and sense of the micro-faults are consistent with that of the Nobeoka thrust. The micro-faults are commonly accompanied by mineral veins of quartz and ankerite. Yamaguchi et al. (2010) suggested that the differences of mineral veins are possibly related to the seismic cycle. In this study, we conducted stress inversion analysis for the micro-faults to examine the change in stress between them, which might be related to the seismic cycle. We divided the micro-fault into two as a micro-fault with quartz veins and that with ankerite veins. Slip direction from slicken fibers and slip sense by slicken steps were obtained. HIM (hough inversion method) by Yamaji et al. (2006) was used to estimate the stress. Two stress states and three stress states are identified in the results for ankerite veins and quartz veins, respectively. For ankerite veins, SE oriented and relatively higher dipping sigma3 with axial extension and SE oriented and relatively lower dipping sigma1 with axial compression are recognized. For quartz veins, SE oriented and relatively higher dipping sigma3 with axial extension, NE oriented and almost horizontal sigma1 with triaxial stress ratio, and NW oriented and lower dipping sigma1 with axial compression are observed. While NW-SE axial stress states are observed both from ankelite and quatz veins, NE oriented triaxial stress is identified only from quartz veins. The change in stress states from NW-SE axial stress to NE triaxial stress might be explained by the dynamic Coulomb wedge model suggested by Wang and Hu (2006). The model predicts that the stress within accretionary wedge can be change with seismic cycle, horizontal sigma1 with axial compression at the co-seismic slip and relatively higher dipping sigma1 with triaxial stress in inter-seimsic period.

Formation and stability of a double subduction system: a numerical study

NASA Astrophysics Data System (ADS)

Pusok, A. E.; Stegman, D. R.

2017-12-01

Examples of double subduction systems can be found in both modern (Izu-Bonin-Marianas and Ryukyu arcs, e.g. Hall [1997]) and ancient (Kohistan arc in Western Himalayas, e.g. Burg et al. [2006]) tectonic record. A double subduction system has been proposed to explain the high convergence rate observed for the India-Eurasia convergence [Aitchison et al., 2000, Jagoutz et al., 2015; Holt et al., 2017]. Rates of convergence across coupled double subduction systems can be significantly faster than across single subduction systems because of slab pull by two slabs. However, despite significant geological and geophysical observations, questions regarding double subduction remain largely unexplored. For example, it is unclear how a double subduction system forms and remains stable over millions of years. Previous numerical studies of double subduction either introduced weak zones to initiate subduction [Mishin et al., 2008] or both the subduction systems were already initiated [Jagoutz et al., 2015, Holt et al., 2017], thus assuming a priori information regarding the initial position of the two subduction zones. Moreover, the driving forces initiating a stable double subduction system remain unclear. In the context of India-Eurasia, Cande and Stegman [2011] found evidence the Reunion mantle plume head provided an ephemeral driving force on both the Indian and African plates for as long as 25 Million years, and had significant influence on plate boundaries in the region. In this study, we perform 2D and 3D numerical simulations using the code LaMEM [Kaus et al., 2016] to investigate i) subduction initiation of a secondary system in an already initiated single subduction system, and ii) the dynamics and stability of the newly formed double subduction system. We start from a single subduction setup, where subduction is already initiated (mature) and we stress the system by controlling the convergence rate of the system (i.e. imposing influx/outflux boundary conditions). Under certain conditions, a second subduction may develop and transform into a stable double subduction system. Results suggest that the fate of the incipient secondary subduction depends on internal factors (i.e. buoyancy and rheology), but also on the dynamics of the primary subduction zone and the boundary conditions (i.e. convergence rate).

A systematic investigation into b values prior to coming large earthquakes

NASA Astrophysics Data System (ADS)

Nanjo, K.; Yoshida, A.

2017-12-01

The Gutenberg-Richter law for frequency-magnitude distribution of earthquakes is now well established in seismology. The b value, the slope of the distribution, is supposed to reflect heterogeneity of seismogenic region (e.g. Mogi 1962) and development of interplate coupling in subduction zone (e.g. Nanjo et al., 2012; Tormann et al. 2015). In the laboratory as well as in the Earth's crust, the b value is known to be inversely dependent on differential stresses (Scholz 1968, 2015). In this context, the b value could serve as a stress meter to help locate asperities, the highly-stressed patches, in fault planes where large rupture energy is released (e.g. Schorlemmer & Wiemer 2005). However, it still remains uncertain whether the b values of events prior to coming large earthquakes are always low significantly. To clarify this issue, we conducted a systematic investigation into b values prior to large earthquakes in the Japanese Mainland. Since no physical definition of mainshock, foreshock, and aftershock is known, we simply investigated b values of the events with magnitudes larger than the lower-cutoff magnitude, Mc, prior to earthquakes equal to or larger than a threshold magnitude, Mth, where Mth>Mc. Schorlemmer et al. (2005) showed that the b value for different fault types differs significantly, which is supposed to reflect the feature that the fracture stress depends on fault types. Therefore, we classified fault motions into normal, strike-slip, and thrust types based on the mechanism solution of earthquakes, and computed b values of events associated with each fault motion separately. We found that the target events (M≥Mth) and the events that occurred prior to the target events both show a common systematic change in b: normal faulting events have the highest b values, thrust events the lowest and strike-slip events intermediate values. Moreover, we found that the b values for the prior events (M≥Mc) are significantly lower than the b values for the target events (M≥Mth), though their b values change somewhat depending on the choice of the parameter values to define the target events (M≥Mth) and the prior events (M≥Mc). This finding indicates that the b value could be used as an effective index for foreseeing occurrence of large earthquakes, if the parameter values are well-tuned.

The paradox of vertical σ2 in foreland fold and thrust belts

NASA Astrophysics Data System (ADS)

Tavani, Stefano

2014-05-01

Occurrence of aesthetically appealing thrust systems and associated large scale anticlines, in both active and fossil foreland fold and thrust belts, is commonly interpreted as an evidence for Andersonian compressional framework. Indeed, these structures would testify for a roughly vertical σ3. Such a correlation between thrusts occurrence and stress field orientation, however, frequently fails to explain denser observations at a smaller scale. The syn-orogenic deformation meso-structures hosted in exposed km-scale thrust-related folds, in fact, frequently and paradoxically witness for a syn-thrusting strike-slip stress configuration, with a near-vertical σ2 and a sub-horizontal σ3. This apparent widespread inconsistency between syn-orogenic meso-structures and stress field orientation is here named "the σ2 paradox". A possible explanation for such a paradox is provided by inherited extensional deformation structures commonly developed prior to thrusting, in the flexural foreland basins located ahead of fold and thrust belts. Thrust nucleation and propagation is facilitated and driven by the positive inversion of the extensional inheritances, and their subsequent linkage. This process eventually leads to the development of large reverse fault zones and can occur both in compressive and strike-slip stress configurations.

Updated Rupture Model for the M7.8 October 28, 2012, Haida Gwaii Earthquake as Constrained by GPS-Observed Displacements

NASA Astrophysics Data System (ADS)

Nykolaishen, L.; Dragert, H.; Wang, K.; James, T. S.; de Lange Boom, B.; Schmidt, M.; Sinnott, D.

2014-12-01

The M7.8 low-angle thrust earthquake off the west coast of southern Haida Gwaii on October 28, 2012, provided Canadian scientists the opportunity to study a local large thrust earthquake and has provided important information towards an improved understanding of geohazards in coastal British Columbia. Most large events along the Pacific-North America boundary in this region have involved strike-slip motion, such as the 1949 M8.1 earthquake on the Queen Charlotte Fault. In contrast along the southern portion of Haida Gwaii, the young (~8 Ma) Pacific plate crust also underthrusts North America and has been viewed as a small-scale analogy of the Cascadia Subduction Zone. Initial seismic-based rupture models for this event were improved through inclusion of GPS observed coseismic displacements, which are as large as 115 cm of horizontal motion (SSW) and 30 cm of subsidence. Additional campaign-style GPS surveys have since been repeated by the Canadian Hydrographic Service (CHS) at seven vertical reference benchmarks throughout Haida Gwaii, significantly improving the coverage of coseismic displacement observations in the region. These added offsets were typically calculated by differencing a single occupation before and after the earthquake and preliminary displacement estimates are consistent with previous GPS observations from the Geological Survey of Canada. Addition of the CHS coseismic offset estimates may allow direct inversion of the GPS data to derive a purely GPS-based rupture model. To date, cumulative postseismic displacements at six sites indicate up to 6 cm of motion, varying in azimuth between SSW and SE. Preliminary postseismic timeseries curve fitting to date has utilized a double exponential function characteristic of mantle relaxation. The current postseismic trends also suggest afterslip on the deeper plate interface beneath central Haida Gwaii along with possible induced aseismic slip on a deeper segment of the Queen Charlotte Fault located offshore of southern Moresby Island. A physical explanation involving afterslip will entail the use of logarithmic functions for curve-fitting and will allow quantitative assessment of the relative importance of the two processes.

Balanced sections and the propagation of décollement: A Jura perspective

NASA Astrophysics Data System (ADS)

Laubscher, Hans

2003-12-01

The propagation of thrusting is an important problem in tectonics that is usually approached by forward (kinematical) modeling of balanced sections. Although modeling techniques are similar in most foreland fold-thrust belts, it turns out that in the Jura, there are modeling problems that require modifications of widely used techniques. In particular, attention is called to the role of model constraints that complement the set of observational constraints in order to fully define the model. In the eastern Jura, such model constraints may be inferred from the regional geology, which shows a peculiar noncoaxial relation between thrusts and subsequent folds. This relation implies changes in the direction of translation and the mode of deformation in the course of the propagation of décollement. These changes are conjectured to be the result of a change in partial decoupling between the thin-skinned fold-thrust system (nappe) and the obliquely subducted foreland. As a particularly instructive case in point, a cross section through the Weissenstein range is discussed. A two-step forward (kinematical) model is proposed that uses both local observational constraints as well as model constraints inferred from regional data. As a first step, a fault bend fold is generated in the hanging wall of a thrust of 1500 m shortening. As a second step, this structure is transferred by flexural slip into the actual fold observed at the surface. This requires an additional 1600 m of shortening and leads to folding of the original thrust. Thereafter, the footwall is deformed so as to respect the constraint that this deformation must fit into the space defined by the folded thrust as the upper boundary and the décollement surface as the lower boundary, and that, in addition, should be confined to the area immediately below the fold. In modeling the footwall deformation a mix of balancing methods is used: fault propagation folds for the competent intervals of the stratigraphic column and area balancing for the incompetent ones. Further propagation of décollement into the foreland is made possible by the folding process, which is dominated by a sort of kinking and which is the main contribution to structural elevation and hence to producing a sort of critical taper of the moving thin-skinned wedge.

Geometry of a large-scale low-angle mid-crustal thrust (Woodroffe Thrust, Central Australia)

NASA Astrophysics Data System (ADS)

Wex, Sebastian; Mancktelow, Neil S.; Hawemann, Friedrich; Pennacchioni, Giorgio; Camacho, Alfredo

2015-04-01

Young orogens, such as the Alps, mainly expose the upper part of the continental crust and it is not possible to follow large-scale thrusts (e.g. the Glarus Thrust) to great depth in order to study their changing rheological behavior. This knowledge, however, is crucial for determining the overall kinematic and dynamic response during collision, as middle to lower crustal rocks represent the major part of the total crustal section. Information from deeper parts of the continental crust can only be obtained directly by investigating regions where these levels are now exhumed. The Musgrave Ranges in Central Australia is a very well exposed, semi-desert area, in which numerous large-scale shear zones developed during the Petermann Orogeny around 550 Ma. The most prominent structure is the ˜400 km long E-W trending Woodroffe Thrust, which placed ˜1.2 Ga granulites onto similarly-aged amphibolite and granulite facies gneisses along a generally south-dipping thrust plane with a top-to-north shear sense. Geothermobarometric calculations on the associated mylonites established that the structure developed under mid-crustal conditions (500-650°C, 0.8-1 GPa). Regional P/T variations in the direction of thrusting are small, but show trends consistent with the south-dipping orientation of the thrust plane, which predicts deeper levels and a higher metamorphic grade in the south than in the north. They imply a very low gradient of only around 3°C/km for a distance of some 30 km in the movement direction of the thrust. Combined with a geothermal gradient on the order of 20°C/km, calculated from four separate P/T estimates from the hanging wall and footwall, this regional gradient indicates that the Woodroffe Thrust was originally shallow-dipping at an average angle of only around 9°. This suggests that upper crustal brittle thrusts do not necessarily steepen into the middle to lower crust, but can define very shallow-dipping, large-scale planar features, with dimensions in the order of hundreds of kilometres. Such a geometry would require the rocks to be weak, but field observations (e.g. large volumes of syn-tectonic pseudotachylyte) argue for strong behaviour, involving alternating fast (seismic) fracturing and slow (aseismic) creep.

Flat-slab subduction, whole crustal faulting, and geohazards in Alaska: Targets for Earthscope

NASA Astrophysics Data System (ADS)

Gulick, S. P.; Pavlis, T. L.; Bruhn, R. L.; Christeson, G. L.; Freymueller, J. T.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Roeske, S.; Reece, R.; van Avendonk, H. J.; Worthington, L. L.

2010-12-01

Crustal structure and evolution illuminated by the Continental Dynamics ST. Elias Erosion and tectonics Project (STEEP) highlights some fundamental questions about active tectonics processes in Alaska including: 1) what are the controls on far field deformation and lithospheric stabilization, 2) do strike slip faults extend through the entire crust and upper mantle and how does this influence mantle flow, and 3) how does the transition from “normal” subduction of the Pacific along the Aleutians to flat slab subduction of the Yakutat Terrane beneath southeast and central Alaska to translation of the Yakutat Terrane past North American in eastern Alaska affect geohazard assessment for the north Pacific? Active and passive seismic studies and geologic fieldwork focusing on the Yakutat Terrane show that the Terrane ranges from 15-35 km thick and is underthrusting the North American plate from the St. Elias Mountains to the Alaska Range (~500 km). Deformation of the upper plate occurs within the offshore Pamplona Zone fold and thrust belt, and onshore throughout the Robinson Mountains. Deformation patterns, structural evolution, and the sedimentary products of orogenesis are fundamentally influenced by feedbacks with glacial erosion. The Yakutat megathrust extends beneath Prince William Sound such that the 1964 Mw 9.2 great earthquake epicenter was on this plate boundary and jumped to the adjacent Aleutian megathrust coseismically; this event illuminates the potential for transitional tectonic systems to enhance geohazards. The northern, southern, and eastern limits of the Yakutat microplate are strike-slip faults that, where imaged, appear to cut the entire crustal section and may allow for crustal extrusion towards the Bering Sea. Yakutat Terrane effects on mantle flow, however, have been suggested to cross these crustal features to allow for far-field deformation in the Yukon, Brooks Range, and Amerasia Basin. From the STEEP results it is clear that the Yakutat Terrane is driving a range of tectonic and surface processes perturbing the Aleutian subduction system at its eastern extent and linking this system with Laramide style subduction and plate boundary strike-slip tectonics farther east. Targeted geodetic and seismic deployments as part of Earthscope could examine all of these features and seek to address fundamental questions about tectonic interactions.

Revisiting the North Chile seismic gap segmentation using GPS-derived interseismic coupling

NASA Astrophysics Data System (ADS)

Métois, M.; Socquet, A.; Vigny, C.; Carrizo, D.; Peyrat, S.; Delorme, A.; Maureira, E.; Valderas-Bermejo, M.-C.; Ortega, I.

2013-09-01

No major earthquake occurred in North Chile since the 1877 Mw 8.6 subduction earthquake that produced a huge tsunami. However, geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming, which reveals some degree of locking on the subduction interface. This accumulation of elastic deformation is likely to be released in a future earthquake. Because of the long elapsed time since 1877 and the rapid accumulation of deformation (thought to be 6-7 cm yr-1), many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high. We present a new Global Positioning System (GPS) velocity field, acquired between 2008 and 2012, that describes in some detail the interseismic deformation between 18°S and 24°S. We invert for coupling distribution on the Nazca-South America subduction interface using elastic modelling. Our measurements require that, at these latitudes, 10 to 12 mm yr-1 (i.e. 15 per cent of the whole convergence rate) are accommodated by the clockwise rotation of an Andean block bounded to the East by the subandean fold-and-thrust belt. This reduces the accumulation rate on the subduction interface to 56 mm yr-1 in this area. Coupling variations on the subduction interface both along-strike and along-dip are described. We find that the North Chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling. This coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region. Intersegment zones (Iquique, Mejillones) correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates. The rupture of either the Paranal or the Loa segment alone could easily produce a Mw 8.0-8.3 rupture, and we propose that the Loa segment (from 22.5°S to 20.8°S) may be the one that ruptured in 1877.

Imaging Subduction, Episodic Tremor and Slip in the Pacific Northwest: Cascadia Arrays For Earthscope (CAFE)

NASA Astrophysics Data System (ADS)

Abers, G. A.; Rondenay, S.; Creager, K. C.; Malone, S. D.; Zhang, Z.; Wech, A. G.; Sweet, J. R.; Melbourne, T. I.; Hacker, B. R.

2007-12-01

Subduction delivers fluids into the Earth's mantle by transport of hydrated crust downward in subducting plates. These fluids are released at depth and may be responsible for a wide variety of phenomena including weakened thrust faults, episodic tremor and slip (ETS), intraslab earthquakes, forearc serpentinization, and arc magmatism. Cascadia is the volcanic arc associated with the youngest subducting plate, and hence a primary EarthScope target. In 2006 we launched Cascadia Arrays For Earthscope (CAFE), an EarthScope effort utilizing Flexible Array, Transportable Array, and PBO facilities, and integrating these data with complementary constraints from geodynamics and geochemistry. Seismic imaging, the emphasis of this presentation, is employed to illuminate (i) the descending oceanic plate, from where fluids are expelled by metamorphism, and (ii) the mantle wedge, where fluids migrate to produce hydrous phases such as serpentine or, beneath the volcanic arc, primary magmas, and (iii) the interface between them where ETS may be produced. The experiment traverses a section of the Cascadia system where earthquakes extend to nearly 100 km depth, thus permitting an investigation of the relationship between the release of fluids and the generation of Wadati-Benioff-zone earthquakes, and crosses regions of ETS excitation. The basic experiment has four components: (1) a 47-element broadband imaging array of Flexible Array instruments integrated with Bigfoot; (2) three small-aperture seismic arrays with 15 additional short-period instruments near known sources of ETS; (3) analysis of the PBO and PANGA GPS data sets to define the details of episodic slip events; and (4) integrative modeling. Sixty-two seismographs were deployed in July 2006; here we present a first look at the experiment and the data collected. Initial data recovery has been excellent, with approximately 12 months of continuous data recovered as of this writing, most delivered to the IRIS DMC. This time window includes an ETS episode in Jan. 2007. Given the success of this deployment, we expect to make good progress toward understanding the relationship between subduction, ETS, and fluid cycling.

Continental Evolution Involving Subduction Underplating and Synchronous Foreland Thrusting: Evidence from the Trans-Alaska Crustal Transect

NASA Astrophysics Data System (ADS)

Fuis, G. S.; Moore, T. E.; Plafker, G.; Brocher, T. M.; Fisher, M. A.; Mooney, W. D.; Nokleberg, W. J.; Page, R. A.; Beaudoin, B. C.; Christensen, N. I.; Levander, A.; Lutter, W. J.; Saltus, R. W.; Ruppert, N. A.

2010-12-01

We investigated the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980’s and early 1990’s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted to be remnants of the extinct Kula (or Resurrection) Plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by north-vergent, crustal-scale duplexes that overlie a ramp on autochthonous North Slope crust. There, Moho has been depressed to nearly 50-km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula- (or Resurrection-) Plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two widely separated regions include “flat-slab” subduction and an “orogenic-float” model. In the Neogene, the collision of the Yakutat terrane (YAK), in southern Alaska, correlates with renewed compression in northeast Alaska and northwest Canada, in a fashion somewhat similar to the tectonics in the Paleogene. The Yakutat terrane, riding atop the subducting Pacific oceanic lithosphere (POL), spans a newly interpreted tear in the POL. East of the tear, POL is interpreted to subduct steeply and alone beneath the Wrangell arc volcanoes because the overlying YAK has been left behind as tectonically underplated rocks beneath the rising St. Elias Range in the coastal region. West of the tear, the YAK and POL are interpreted to subduct together at a gentle angle (a few degrees from 0 to 400 km from the trench), and this thickened package inhibits arc volcanism.

Unraveling the polymetamorphic history of garnet-bearing metabasites: Insights from the North Motagua Mélange (Guatemala Suture Zone)

NASA Astrophysics Data System (ADS)

Bonnet, G.; Flores, K. E.; Martin, C.; Harlow, G. E.

2014-12-01

The Guatemala Suture Zone is the fault-bound region in central Guatemala that contains the present North American-Caribbean plate boundary. This major composite geotectonic unit contains a variety of ophiolites, serpentinite mélanges, and metavolcano-sedimentary sequences along with high-grade schists, gneisses, low-grade metasediments and metagranites thrusted north and south of the active Motagua fault system (MFS). The North Motagua Mélange (NMM) outcrops north of the MFS and testifies the emplacement of exhumed subduction assemblages along a collisional tectonic setting. The NMM is composed of a serpentinite-matrix mélange that contains blocks of metabasites (subgreenschist facies metabasalt, grt-blueschist, eclogite, grt-amphibolite), vein-related rocks (jadeitite, omphacitite, albitite, mica-rock), and metatrondhjemites. Our new detailed petrographic and thermobarometric study on the garnet-bearing metabasites reveals a complex polymetamorphic history with multiple tectonic events. Eclogites show a classical clockwise PT path composed of (a) prograde blueschist/eclogite facies within garnet cores, (b) eclogite facies metamorphic peak at ~1.7 GPa and 620°C, (c) post-peak blueschist facies, (d) amphibolite facies overprint, and (e) late stage greenschist facies. Two types of garnet amphibolite blocks can be found, the first consist of (a) a relict eclogite facies peak at ~1.3 GPa and 550°C only preserved within anhedral garnet cores, and (b) surrounded by a post-peak amphibolite facies. In contrast, the second type displays a prograde amphibolite facies at 0.6-1.1 GPa and 400-650°C. The eclogites metamorphic peak suggests formation in a normal subduction zone at ~60 km depth, a subsequent exhumation to the middle section of the subduction channel (~35 km), and a later metamorphic reworking at lower P and higher T before its final exhumation. The first type of garnet amphibolite shows a similar trajectory as the eclogites but at warmer conditions. In contrast, the second type of garnet amphibolite recorded a single prograde evolution along a hotter thermal gradient. These different PT paths suggest multiple metamorphic events that may be related to subduction initiation, partial exhumation and storage of HP-LT rocks, subduction of buoyant crust, final exhumation and obduction.

The Great Maule earthquake: seismicity prior to and after the main shock from amphibious seismic networks

NASA Astrophysics Data System (ADS)

Lieser, K.; Arroyo, I. G.; Grevemeyer, I.; Flueh, E. R.; Lange, D.; Tilmann, F. J.

2013-12-01

The Chilean subduction zone is among the seismically most active plate boundaries in the world and its coastal ranges suffer from a magnitude 8 or larger megathrust earthquake every 10-20 years. The Constitución-Concepción or Maule segment in central Chile between ~35.5°S and 37°S was considered to be a mature seismic gap, rupturing last in 1835 and being seismically quiet without any magnitude 4.5 or larger earthquakes reported in global catalogues. It is located to the north of the nucleation area of the 1960 magnitude 9.5 Valdivia earthquake and to the south of the 1928 magnitude 8 Talca earthquake. On 27 February 2010 this segment ruptured in a Mw=8.8 earthquake, nucleating near 36°S and affecting a 500-600 km long segment of the margin between 34°S and 38.5°S. Aftershocks occurred along a roughly 600 km long portion of the central Chilean margin, most of them offshore. Therefore, a network of 30 ocean-bottom-seismometers was deployed in the northern portion of the rupture area for a three month period, recording local offshore aftershocks between 20 September 2010 and 25 December 2010. In addition, data of a network consisting of 33 landstations of the GeoForschungsZentrum Potsdam were included into the network, providing an ideal coverage of both the rupture plane and areas affected by post-seismic slip as deduced from geodetic data. Aftershock locations are based on automatically detected P wave onsets and a 2.5D velocity model of the combined on- and offshore network. Aftershock seismicity analysis in the northern part of the survey area reveals a well resolved seismically active splay fault in the accretionary prism of the Chilean forearc. Our findings imply that in the northernmost part of the rupture zone, co-seismic slip most likely propagated along the splay fault and not the subduction thrust fault. In addition, the updip limit of aftershocks along the plate interface can be verified to about 40 km landwards from the deformation front. Prior to the Great Maule earthquake the Collaborative Research Center SFB 574 'Volatiles and Fluids in Subduction Zones' shot several wide-angle profiles and operated a network, also consisting of OBS and land stations for six months in 2008. Both projects provide a great opportunity to study the evolution of a subduction zone within the seismic cycle of a great earthquake. The most profound features are (i) a sharp reduction in intraslab seismic activity after the Maule earthquake and (ii) a sharp increase in seismic activity at the slab interface above 50 km depth, where large parts of the rupture zone were largely aseismic prior to the Maule earthquake. Further, the aftershock seismicity shows a broader depth distribution above 50 km depth.

Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains

USGS Publications Warehouse

Barnes, C.G.; Petersen, S.W.; Kistler, R.W.; Murray, R.; Kays, M.A.

1996-01-01

In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 Ma (Early Cretaceous). It occurred about 5 to l0 m.y. after the ??? 165 to 159 Ma Josephine ophiolite was thrust beneath older parts of the province during the Nevadan orogeny (thrusting from ??? 155 to 148 Ma). The magmatism also corresponds to a period of slow or no subduction. Most of the plutons crop out in the south-central Klamath Mountains in California, but one occurs in Oregon at the northern end of the province. Compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later, more voluminous tonalitic and trondhjemitic intrusions. Most plutons consist almost entirely of tonalite and trondhjemite. Poorlydefined concentric zoning is common. Tonalitic rocks are typically of the Iow-Al type but trondhjemites are generally of the high-Al type, even those that occur in the same pluton as low-Al tonalite??. The suite is characterized by low abundances of K2O, Rb, Zr, and heavy rare earth elements. Sr contents are generally moderate ( ???450 ppm) by comparison with Sr-rich arc lavas interpreted to be slab melts (up to 2000 ppm). Initial 87Sr/ 86Sr, ??18O, and ??Nd are typical of mantle-derived magmas or of crustally-derived magmas with a metabasic source. Compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source (transitional mid-ocean ridge to island arc basalt), but not by fractional crystallyzation of a basaltic parent. Melting models require a residual assemblage of clinopyroxene+garnet??plagioclase??amphibole; residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab. Such models are consistent with the metabasic part of the Josephine ophiolite as the source. Because the Josephine ophiolite was at low T during Nevadan thrusting, an external heat source was probably necessary to achieve significant degrees of melting; heat was probably extracted from mantle-derived basaltic melts, which were parental to the mafic precursors of the tonalite-trondhjemite suite. Thus, under appropriate tectonic and thermal conditions, heterogeneous mafic crustal rocks can melt to form both low- and high-Al tonalitic and trondhjemitic magmas; slab melting is not necessary.

Boosting of Nonvolcanic Tremor by Regional Earthquakes 2011-2012 in Guerrero, Mexico

NASA Astrophysics Data System (ADS)

Real, J. A.; Kostoglodov, V.; Husker, A. L.; Payero, J. S.; G-GAP Research Team

2013-05-01

Sistematic observation of nonvolcanic tremor (NVT) in Guerrero, Mexico started in 2005 after the installation of MASE broadband seismic network. Since 2008 the new "G-GAP" network of 10 seismic mini-arrays provides the data for the NVT detailed studies together with the broadband stations of the Servicio Seimologogico Nacional (SSN). Most of the NVT recorded in the central Guerrero area are of so called ambient type, which in most cases are related with the occurrence of aseismic slow slip events (SSE). While the locations of NVT are estimated relatively well, their depths are not reliable but distributed close to the subduction plate interface. The ambient NVT activity increases periodically every 3-4 months and is strongly modulated by large SSE. Another type of tremor has been observed in Guerrero during and after several large teleseismic events, such as Mw=8.8, 2010 Maule, Chile earthquake. This NVT was triggered by the surface waves when they traveled across the tremor-generating area. Large teleseismic events may also activate a noticeable post-seismic NVT activity. In subduction zones, triggering of the NVT and its post-seismic activation by the regional and local earthquakes have not yet been observed. We tried to detect the NVT triggered or boosting of post-seismic tremor activity by two recent large earthquakes that occurred in Guerrero: December 11, 2011, Mw=6.5 Zumpango, and March 20, 2012, Mw=7.4 Ometepec. The first earthquake was of the intraplate type, with normal focal mechanism, at the depth of 58 km, and the second was the shallow interplate event of the thrust type, at the depth of ~15 km. It is technically difficult to separate the NVT signal in its characteristic 1-10 Hz frequency range from the high frequency input from the regional earthquake. The Zumpango event, which is located closer to the NVT area, produced a noticeable boosting of post-seismic NVT activity to the North of its epicenter. Meanwhile the larger magnitude Ometepec earthquake apparently had no any observable influence on the NVT occurrence, furthermore some NVT activity observed before this event has not persisted after it. Further study should reveal the role of different factors on the NVT triggering and activation such as: the type of the seismic event, its magnitude, depth, and the distance from the NVT zone.

Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

USGS Publications Warehouse

von Huene, Roland E.; Scholl, D. W.

1991-01-01

At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (???43,500 km) is calculated to be 1.5 km3/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km3/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km3/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km3/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km3/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km3/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km3/yr to a total of 1.1 km3/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km3/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted materia

Spatial heterogeneity of stress and driving fluid pressure ratio inferred from mineral vein orientation along seismogenic megasplay fault (Nobeoka Thrust, Japan)

NASA Astrophysics Data System (ADS)

Otsubo, M.; Miyakawa, A.; Kawasaki, R.; Sato, K.; Yamaguchi, A.; Kimura, G.

2015-12-01

Fault zones including the damage zone and the fault core have a controlling influence on the crust's mechanical and fluid flow properties (e.g., Faulkner et al., 2010). In the Nankai subduction zone, southwest Japan, the velocity structures indicate the contrast of the pore fluid pressure between hanging wall and footwall of the megasplay fault (Tsuji et al., 2014). Nobeoka Thrust, which is an on-land example of an ancient megasplay fault, provides an excellent record of deformation and fluid flow at seismogenic depths (Kondo et al., 2005; Yamaguchi et al., 2011). Yamaguchi et al. (2011) showed that the microchemical features of syn-tectonic mineral veins along fault zones of the Nobeoka Thrust. The inversion approaches by using the mineral vein orientations can provide stress regimes and fluid driving pressure ratio (Jolly and Sanderson, 1997) at the time of fracture opening (e.g., Yamaji et al., 2010). In this study, we show (1) stress regimes in co- and post seismic period of the Nobeoka Thrust and (2) spatial heterogeneity of the fluid driving pressure ratio by using the mineral veins (extension veins) around the fault zone in the Nobeoka Thrust. We applied the inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using the mineral vein orientations. The estimated stresses are the normal faulting stress regimes of which sigma 3 axes are almost horizontal and trend NNW-SSE in both the hanging wall and the footwall. The stress regimes are the negative stress for the reverse faulting stress regime that Kawasaki et al. (2014) estimated from the minor faults in the core samples of the Nobeoka Thrust Drilling Project (Hamahashi et al., 2013). And, the orientation of the sigma 3 axes of the estimated stress regime is parallel to the slip direction of the Nobeoka Thrust (Top to SSE; Kondo et al., 2005). These facts indicate the normal faulting stress regime at the time of fracture opening is the secondary stress generated by the slip of the Nobeoka Thrust. We estimated the fluid driving pressure ratio P* at the time of fracture opening by using the Mohr circle analysis that has been carried out using the vein orientation data. The estimated P* are 0.05 and 0.15-0.40 in the hanging wall and footwall, respectively. These results indicate that there are spatial differences of pore fluid pressure in the interseismic period.

Tremor, remote triggering and earthquake cycle

NASA Astrophysics Data System (ADS)

Peng, Z.

2012-12-01

Deep tectonic tremor and episodic slow-slip events have been observed at major plate-boundary faults around the Pacific Rim. These events have much longer source durations than regular earthquakes, and are generally located near or below the seismogenic zone where regular earthquakes occur. Tremor and slow-slip events appear to be extremely stress sensitive, and could be instantaneously triggered by distant earthquakes and solid earth tides. However, many important questions remain open. For example, it is still not clear what are the necessary conditions for tremor generation, and how remote triggering could affect large earthquake cycle. Here I report a global search of tremor triggered by recent large teleseismic earthquakes. We mainly focus on major subduction zones around the Pacific Rim. These include the southwest and northeast Japan subduction zones, the Hikurangi subduction zone in New Zealand, the Cascadia subduction zone, and the major subduction zones in Central and South America. In addition, we examine major strike-slip faults around the Caribbean plate, the Queen Charlotte fault in northern Pacific Northwest Coast, and the San Andreas fault system in California. In each place, we first identify triggered tremor as a high-frequency non-impulsive signal that is in phase with the large-amplitude teleseismic waves. We also calculate the dynamic stress and check the triggering relationship with the Love and Rayleigh waves. Finally, we calculate the triggering potential with the local fault orientation and surface-wave incident angles. Our results suggest that tremor exists at many plate-boundary faults in different tectonic environments, and could be triggered by dynamic stress as low as a few kPas. In addition, we summarize recent observations of slow-slip events and earthquake swarms triggered by large distant earthquakes. Finally, we propose several mechanisms that could explain apparent clustering of large earthquakes around the world.

Anomalous Seismic Radiation in the Shallow Subduction Zone Explained by Extensive Poroplastic Deformation in the Overriding Wedge

NASA Astrophysics Data System (ADS)

Hirakawa, E. T.; Ma, S.

2012-12-01

The deficiency of high-frequency seismic radiation from shallow subduction zone earthquakes was first recognized in tsunami earthquakes (Kanamori, 1972), which produce larger tsunamis than expected from short-period (20 s) surface wave excitation. Shallow subduction zone earthquakes were also observed to have unusually low energy-to-moment ratios compared to regular subduction zone earthquakes (e.g., Newman and Okal, 1998; Venkataraman and Kanamori, 2004; Lay et al., 2012). What causes this anomalous radiation and how it relates to large tsunami generation has remained unclear. Here we show that these anomalous observations can be due to extensive poroplastic deformation in the overriding wedge, which provides a unifying interpretation. Ma (2012) showed that the pore pressure increase in the wedge due to up-dip rupture propagation significantly weakens the wedge, leading to widespread Coulomb failure in the wedge. Widespread failure gives rise to slow rupture velocity and large seafloor uplift (landward from the trench) in the case of a shallow fault dip. Here we extend this work and demonstrate that the large seafloor uplift due to the poroplastic deformation significantly dilates the fault behind the rupture front, which reduces the normal stress on the fault and increases the stress drop, slip, and rupture duration. The spectral amplitudes of the moment-rate time function is significantly less at high frequencies than those from elastic simulations. Large tsunami generation and deficiency of high-frequency radiation are thus two consistent manifestations of the same mechanism (poroplastic deformation). Although extensive poroplastic deformation in the wedge represents a significant portion of total seismic moment release, the plastic deformation is shown to act as a large energy sink, leaving less energy to be radiated and leading to low energy-to-moment ratios as observed for shallow subduction zone earthquakes.

Orogenic front propagation in the basement involved Malargüe fold and thrust belt, Neuquén Basin, (Argentina)

NASA Astrophysics Data System (ADS)

Branellec, Matthieu; Nivière, Bertrand; Callot, Jean-Paul; Ringenbach, Jean-Claude

2015-04-01

The Malargüe fold and thrust belt (MFTB) and the San Rafael Block (SRB) are located in the northern termination of the Neuquén basin in Argentina. This basin is a wide inverted intracratonic sag basin with polyphased evolution controlled at large scale by the dynamic of the Pacific subduction. By late Triassic times, narrow rift basins developed and evolved toward a sag basin from middle Jurassic to late Cretaceous. From that time on, compression at the trench resulted in various shortening pulses in the back-arc area. Here we aim to analyze the Andean system at 35°S by comparing the Miocene structuration in the MFTB and the current deformation along the oriental border or the San Rafael Block. The main structuration stage in the MFTB occurred by Miocene times (15 to 10 Ma) producing the principal uplift of the Andean Cordillera. As shown by new structural cross sections, Triassic-early Jurassic rift border faults localized the Miocene compressive tectonics. Deformation is compartmentalized and does not exhibit a classical propagation of homogeneous deformation sequence expected from the critical taper theory. Several intramontane basins in the hangingwall of the main thrusts progressively disconnected from the foreland. In addition, active tectonics has been described in the front of the MFTB attesting for the on-going compression in this area. 100 km farther to the east, The San Rafael Block, is separated from the MFTB by the Rio Grande basin. The SRB is mostly composed of Paleozoic terranes and Triassic rift-related rocks, overlain by late Miocene synorogenic deposits. The SRB is currently uplifted along its oriental border along several active faults. These faults have clear morphologic signatures in Quaternary alluvial terraces and folded Pleistocene lavas. As in the MFTB, the active deformation localization remains localized by structural inheritance. The Andean system is thus evolving as an atypical orogenic wedge partly by frontal accretion at the front of the belt and by migration and localization of strain far from the front leading to crustal block reactivation.

Miocene crustal extension following thrust tectonic in the Lower Sebtides units (internal Rif, Ceuta Peninsula, Spain): Implication for the geodynamic evolution of the Alboran domain

NASA Astrophysics Data System (ADS)

Homonnay, Emmanuelle; Corsini, Michel; Lardeaux, Jean-Marc; Romagny, Adrien; Münch, Philippe; Bosch, Delphine; Cenki-Tok, Bénédicte; Ouazzani-Touhami, Mohamed

2018-01-01

In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated. Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with: (1) a syn-metamorphic thrusting event developed under granulite facies conditions (7-10 kbar and 780-820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene. (2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400-550 °C and 1-3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene. We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position. Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20-30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension. 1, BSE image of monazite. 2, CL image of monazite showing a thin rim zonation. 3, BSE image of zircon. 4, CL image of zircon showing zonation.

Seismotectonics of the Eastern Himalayan System and Indo-Burman Convergence Zone Using Seismic Waveform Inversion

NASA Astrophysics Data System (ADS)

Kumar, A.; Mitra, S.; Suresh, G.

2014-12-01

The Eastern Himalayan System (east of 88°E) is distinct from the rest of the India-Eurasia continental collision, due to a wider zone of distributed deformation, oblique convergence across two orthogonal plate boundaries and near absence of foreland basin sedimentary strata. To understand the seismotectonics of this region we study the spatial distribution and source mechanism of earthquakes originating within Eastern Himalaya, northeast India and Indo-Burman Convergence Zone (IBCZ). We compute focal mechanism of 32 moderate-to-large earthquakes (mb >=5.4) by modeling teleseismic P- and SH-waveforms, from GDSN stations, using least-squares inversion algorithm; and 7 small-to-moderate earthquakes (3.5<= mb <5.4) by modeling local P- and S-waveforms, from the NorthEast India Telemetered Network, using non-linear grid search algorithm. We also include source mechanisms from previous studies, either computed by waveform inversion or by first motion polarity from analog data. Depth distribution of modeled earthquakes reveal that the seismogenic layer beneath northeast India is ~45km thick. From source mechanisms we observe that moderate earthquakes in northeast India are spatially clustered in five zones with distinct mechanisms: (a) thrust earthquakes within the Eastern Himalayan wedge, on north dipping low angle faults; (b) thrust earthquakes along the northern edge of Shillong Plateau, on high angle south dipping fault; (c) dextral strike-slip earthquakes along Kopili fault zone, between Shillong Plateau and Mikir Hills, extending southeast beneath Naga Fold belts; (d) dextral strike-slip earthquakes within Bengal Basin, immediately south of Shillong Plateau; and (e) deep focus (>50 km) thrust earthquakes within IBCZ. Combining with GPS geodetic observations, it is evident that the N20E convergence between India and Tibet is accommodated as elastic strain both within eastern Himalaya and regions surrounding the Shillong Plateau. We hypothesize that the strike-slip earthquakes south of the Plateau occur on re-activated continental rifts paralleling the Eocene hinge zone. Distribution of earthquake hypocenters across the IBCZ reveal active subduction of the Indian plate beneath Burma micro-plate.

Locking of the Chile subduction zone controlled by fluid pressure before the 2010 earthquake

NASA Astrophysics Data System (ADS)

Moreno, Marcos; Haberland, Christian; Oncken, Onno; Rietbrock, Andreas; Angiboust, Samuel; Heidbach, Oliver

2014-04-01

Constraints on the potential size and recurrence time of strong subduction-zone earthquakes come from the degree of locking between the down-going and overriding plates, in the period between large earthquakes. In many cases, this interseismic locking degree correlates with slip during large earthquakes or is attributed to variations in fluid content at the plate interface. Here we use geodetic and seismological data to explore the links between pore-fluid pressure and locking patterns at the subduction interface ruptured during the magnitude 8.8 Chile earthquake in 2010. High-resolution three-dimensional seismic tomography reveals variations in the ratio of seismic P- to S-wave velocities (Vp/Vs) along the length of the subduction-zone interface. High Vp/Vs domains, interpreted as zones of elevated pore-fluid pressure, correlate spatially with parts of the plate interface that are poorly locked and slip aseismically. In contrast, low Vp/Vs domains, interpreted as zones of lower pore-fluid pressure, correlate with locked parts of the plate interface, where unstable slip and earthquakes occur. Variations in pore-fluid pressure are caused by the subduction and dehydration of a hydrothermally altered oceanic fracture zone. We conclude that variations in pore-fluid pressure at the plate interface control the degree of interseismic locking and therefore the slip distribution of large earthquake ruptures.

Laboratory experiments on subduction-induced circulation in the wedge and the evolution of mantle diapirs

NASA Astrophysics Data System (ADS)

Sylvia, R. T.; Kincaid, C. R.; Behn, M. D.; Zhang, N.

2014-12-01

Circulation in subduction zones involves large-scale, forced-convection by the motion of the down-going slab and small scale, buoyant diapirs of hydrated mantle or subducted sediments. Models of subduction-diapir interaction often neglect large-scale flow patterns induced by rollback, back-arc extension and slab morphology. We present results from laboratory experiments relating these parameters to styles of 4-D wedge circulation and diapir ascent. A glucose fluid is used to represent the mantle. Subducting lithosphere is modeled with continuous rubber belts moving with prescribed velocities, capable of reproducing a large range in downdip relative rollback plate rates. Differential steepening of distinct plate segments simulates the evolution of slab gaps. Back-arc extension is produced using Mylar sheeting in contact with fluid beneath the overriding plate that moves relative to the slab rollback rate. Diapirs are introduced at the slab-wedge interface in two modes: 1) distributions of low density rigid spheres and 2) injection of low viscosity, low density fluid to the base of the wedge. Results from 30 experiments with imposed along-trench (y) distributions of buoyancy, show near-vertical ascent paths only in cases with simple downdip subduction and ratios (W*) of diapir rise velocity to downdip plate rate of W*>1. For W* = 0.2-1, diapir ascent paths are complex, with large (400 km) lateral offsets between source and surfacing locations. Rollback and back-arc extension enhance these offsets, occasionally aligning diapirs from different along-trench locations into trench-normal, age-progressive linear chains beneath the overriding plate. Diapirs from different y-locations may surface beneath the same volcanic center, despite following ascent paths of very different lengths and transit times. In cases with slab gaps, diapirs from the outside edge of the steep plate move 1000 km parallel to the trench before surfacing above the shallow dipping plate. "Dead zones" resulting from lateral and vertical shear in the wedge above the slab gap, produce slow transit times. These 4-D ascent pathways are being incorporated into numerical models on the thermal and melting evolution of diapirs. Models show subduction-induced circulation significantly alters diapir ascent beneath arcs.

Slab interactions in 3-D subduction settings: The Philippine Sea Plate region

NASA Astrophysics Data System (ADS)

Holt, Adam F.; Royden, Leigh H.; Becker, Thorsten W.; Faccenna, Claudio

2018-05-01

The importance of slab-slab interactions is manifested in the kinematics and geometry of the Philippine Sea Plate and western Pacific subduction zones, and such interactions offer a dynamic basis for the first-order observations in this complex subduction setting. The westward subduction of the Pacific Sea Plate changes, along-strike, from single slab subduction beneath Japan, to a double-subduction setting where Pacific subduction beneath the Philippine Sea Plate occurs in tandem with westward subduction of the Philippine Sea Plate beneath Eurasia. Our 3-D numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence. We find that the observed kinematics and slab geometry of the Pacific-Philippine subduction can be understood by considering an along-strike transition from single to double subduction, and is largely independent from the detailed geometry of the Philippine Sea Plate. Important first order features include the relatively shallow slab dip, retreating/stationary trenches, and rapid subduction for single slab systems (Pacific Plate subducting under Japan), and front slabs within a double slab system (Philippine Sea Plate subducting at Ryukyu). In contrast, steep to overturned slab dips, advancing trench motion, and slower subduction occurs for rear slabs in a double slab setting (Pacific subducting at the Izu-Bonin-Mariana). This happens because of a relative build-up of pressure in the asthenosphere beneath the Philippine Sea Plate, where the asthenosphere is constrained between the converging Ryukyu and Izu-Bonin-Mariana slabs. When weak back-arc regions are included, slab-slab convergence rates slow and the middle (Philippine) plate extends, which leads to reduced pressure build up and reduced slab-slab coupling. Models without back-arcs, or with back-arc viscosities that are reduced by a factor of five, produce kinematics compatible with present-day observations.

Geometry of a large-scale, low-angle, midcrustal thrust (Woodroffe Thrust, central Australia)

NASA Astrophysics Data System (ADS)

Wex, S.; Mancktelow, N. S.; Hawemann, F.; Camacho, A.; Pennacchioni, G.

2017-11-01

The Musgrave Block in central Australia exposes numerous large-scale mylonitic shear zones developed during the intracontinental Petermann Orogeny around 560-520 Ma. The most prominent structure is the crustal-scale, over 600 km long, E-W trending Woodroffe Thrust, which is broadly undulate but generally dips shallowly to moderately to the south and shows an approximately top-to-north sense of movement. The estimated metamorphic conditions of mylonitization indicate a regional variation from predominantly midcrustal (circa 520-620°C and 0.8-1.1 GPa) to lower crustal ( 650°C and 1.0-1.3 GPa) levels in the direction of thrusting, which is also reflected in the distribution of preserved deformation microstructures. This variation in metamorphic conditions is consistent with a south dipping thrust plane but is only small, implying that a ≥60 km long N-S segment of the Woodroffe Thrust was originally shallowly dipping at an average estimated angle of ≤6°. The reconstructed geometry suggests that basement-cored, thick-skinned, midcrustal thrusts can be very shallowly dipping on a scale of many tens of kilometers in the direction of movement. Such a geometry would require the rocks along the thrust to be weak, but field observations (e.g., large volumes of syntectonic pseudotachylyte) argue for a strong behavior, at least transiently. Localization on a low-angle, near-planar structure that crosscuts lithological layers requires a weak precursor, such as a seismic rupture in the middle to lower crust. If this was a single event, the intracontinental earthquake must have been large, with the rupture extending laterally over hundreds of kilometers.

Preliminary Thermo-Chronometric and Paleo-Magnetic Results from the Western Margin of The Kırşehir Block: Implications for the Timing of Continental Collisions Occurred Along Neo-Tethyan Suture Zones (Central Anatolia, Turkey)

NASA Astrophysics Data System (ADS)

Gülyüz, Erhan; Özkaptan, Murat; Langereis, Cor G.; Kaymakcı, Nuretdin

2017-04-01

Closures of Paleo- (largely Paleozoic) and Neo-Tethys (largely Mesozoic) Oceans developed between Europe, Africa and Arabia are the main driving mechanisms behind the post-Triassic tectonics, magmatism and metamorphism occurred in Anatolia. Although various scenarios have been suggested for the timing and characteristics of the subduction systems, it is largely accepted that these blocks are progressively collided and amalgamated along the northern (İzmir-Ankara-Erzincan suture zone; IAESZ) and the southern (Bitlis-Zagros suture zone; BZSZ) branches of Neo-Tethys Ocean. The geographic positions of these suture zones in Anatolia are marked by imbricated stacks of largely metamorphosed remnants of the Paleo- and Neo-Tethys Oceans. In addition to this tectonic frame, the existence of another suture zone within the northern branch of the Neo-Tethys separating the Kırşehir Block, a triangular (200km*200km*200km) continental domain represented by mainly high-pressure (HP) meta-sedimentary rocks, from the Taurides, is proposed and named as Intra-Tauride Suture Zone (ITSZ). Although traces of the Neo-Tethyan closure and continental collisions in the Central Anatolia are recorded (1) in sedimentary basins as fold and thrust belt developments (as northern Taurides fold and thrust belt along IAESZ and central Taurides fold and thrust belt along ITSZ), (2) on metamorphic rocks with Late Cretaceous to Late Paleocene peak metamorphism, and (3) on magmatic rocks with Late Cretaceous - Paleocene arc-related intrusions and post-Paleocene post-collisional magmatism, timing of these continental collisions are discussed in limited studies and furthermore they indicate a large time span (post-Paleocene to Miocene) for the collisions. This study aims to date continental collisions occurred in Central Anatolia qualitatively. In this regard, low-temperature thermo-chronometric and paleo-magnetic studies were conducted on the sedimentary units cropped-out along the western and north-western margins of the Kırşehir Block where two suture zones coincided (IAESZ & ITSZ). Although, thermo-chronometric studies have not been completely conducted, initial results consistently indicate Oligocene-Early Miocene continental uplift along the western margin of the Kırşehir Block. In keeping with thermo-chronometric results, paleo-magnetic samples (400 cores) taken systematically from upper Cretaceous to Miocene sedimentary units exposed along the IAESZ and ITSZ suggest that concentration of vertical block rotations are accumulated in Oligocene-Early Miocene time interval indicating the timing of main deformation events. Based on the paleo-magnetic and low-temperature thermo-chronometric results, we propose that continental collisions along IAESZ and ITSZ in the Central Anatolia occurred during Oligocene - Early Miocene time interval which might also correspond to the commencement of continental deposition and the base of regional unconformities exposed in the region.

Subduction at upper ocean fronts by baroclinic instability

NASA Astrophysics Data System (ADS)

Verma, Vicky; Pham, Hieu T.; Radhakrishnan, Anand; Sarkar, Sutanu

2017-11-01

Large eddy simulations of upper ocean fronts that are initially in geostrophic balance show that the linear and subsequent nonlinear evolution of baroclinic intability are effective in restratifying the front. During the growth of baroclinic instability, the front develops thin regions with enhanced vertical vorticity, i.e., vorticity filaments. Moreover, the vorticity filaments organize into submesoscale eddies. The subsequent frontal dynamics is dominated by the vorticity filaments and the submesoscale eddies. Diagnosis of the horizontal force balance reveals that the regions occupied by these coherent structures have significantly large imbalance, and are characterized by large vertical velocity. High density fluid from the heavier side of the front is subducted by the vertical velocity to the bottom of the mixed layer. The process of subduction is illustrated by Lagrangian tracking of fluid particles released at a fixed depth.

Estimates of effective elastic thickness at subduction zones

NASA Astrophysics Data System (ADS)

Yang, An; Fu, Yongtao

2018-06-01

The effective elastic thickness (Te) is an important parameter that characterizes the long-term strength of the lithosphere. Estimates of Te at subduction zones have important tectonic and geodynamic implications, providing constraints for the strength of the oceanic lithosphere at a short-term scale. We estimated Te values in several subduction zones worldwide by using models including both surface and subsurface loads from the analysis of free-air gravity anomaly and bathymetric data, together with a moving window admittance technique (MWAT). Tests with synthetic gravity and bathymetry data show that this method is a reliable way to recover Te of oceanic lithosphere. Our results show that there is a noticeable reduction in the effective elastic thickness of the subducting plate from the outer rise to the trench axis for most studied subduction zones, suggesting plate weakening at the trench-outer rise of the subduction zones. These subduction zones have Te range of 6-60 km, corresponding to a wide range of isotherms from 200 to 800 °C. Different trenches show distinct patterns. The Caribbean, Kuril-Japan, Mariana and Tonga subduction zones show predominantly high Te. By contrast, the Middle America and Java subduction zones have a much lower Te. The Peru-Chile, Aleutian and Philippine subduction zones show considerable scatter. The large variation of the isotherm for different trenches does not show clear relationship with plate weakening at the outer rise.

Foundering and Exhumation of UHP Terranes: Race Car or School Bus?

NASA Astrophysics Data System (ADS)

Kylander-Clark, A. R.; Hacker, B. R.

2008-12-01

Recent geochronologic data from the giant ultrahigh-pressure (UHP) terrane, in the Western Gneiss Region of Norway, indicate that subduction and exhumation were relatively slow (a few mm/yr), and that the terrane was exhumed to the surface as a relatively thick, coherent body. These conclusions are in stark contrast to those reached in previous studies of some of the best-studied, smaller UHP terranes and suggest that the processes that form and/or exhume small UHP terranes are fundamentally different from the processes that affect large UHP terranes. These differences may be the result of variations in the buoyancy forces of different proportions of subducted felsic crust, mafic crust, and mantle lithosphere. Initial collision occurs via the subduction of smaller portions of continental material, such as microcontinents or ribbon continents. Because the proportion of continental crust is small, the processes involved in early UHP terrane formation are dominated by the oceanic slab; subduction rates are fast because average plate densities are high, and, as a result, subduction angles are steep. Because these smaller, thinner portions of crust are weak, they deform easily and mix readily with the mantle. As the collision matures, thicker and larger portions of continental material-such as a continental margin-are subducted, and the subduction regime changes from one that was ocean dominated to one that is continent dominated. The increased buoyancy of the larger volume of continental crust resists the pull of the leading oceanic lithosphere; subduction shallows and plate rates slow. Because the downgoing continent is thick, it is strong, remains cohesive and has limited interaction with the mantle. Although the subduction regime during early orogenesis is distinct from that during late orogenesis, the degree of mountain building and crustal thickening may be similar in both stages as small volumes and fast flow rates of buoyant material give way to large volumes and slow flow rates.

How does Subduction Interface Roughness influence Megathrust Earthquakes: Insights from Natural Data and Analogue Models

NASA Astrophysics Data System (ADS)

van Rijsingen, E.; Lallemand, S.; Peyret, M.; Corbi, F.; Funiciello, F.; Arcay, D.; Heuret, A.

2017-12-01

The role of subducting oceanic features on the seismogenic behavior of subduction zones has been increasingly addressed over the past years, although their exact relationship remains unclear. Do features like seamounts, fracture zones or submarine ridges act as barriers, prohibiting ruptures to propagate, or do they initiate megathrust earthquakes instead? With this question in mind, we aim to better understand the influence of subduction interface roughness on the location of an earthquake's hypocenter, rupture area and seismic asperity. Following the work on compiling a dual-wavelength subduction interface roughness (SubRough) database, we used this roughness proxy for a global comparison with large subduction earthquakes (MW > 7.5), which occurred since 1900 (SubQuake, new catalogue). We made a quantitative comparison between the earthquake data on the landward side of the trench and the roughness proxy on the seaward side, taking into account the most appropriate direction of roughness extrapolation. Main results show that areas with low roughness at long wavelengths (i.e. 80-100 km) are more prone to host large- to mega-earthquakes. In addition to this natural data study, we perform analogue experiments, which allow us to investigate the role subducting oceanic features play over the course of multiple seismic cycles. The experimental setup consists of a gelatin wedge and an underthrusting rigid aluminum plate (i.e. the analogues of the overriding and downgoing plates, respectively). By adding scaled 3D-printed topographic features (e.g. seamounts) on the downgoing plate, we are able to accurately monitor the initiation and propagation of ruptures with respect to the subducting features. Here we show the results of our natural data study, some preliminary results of the analogue models and our first conclusions on how the subduction interface roughness may influence the seismogenic potential of an area.

U-Pb dating and emplacement history of granitoid plutons in the northern Sanandaj-Sirjan Zone, Iran

NASA Astrophysics Data System (ADS)

Mahmoudi, Shahryar; Corfu, Fernando; Masoudi, Fariborz; Mehrabi, Behzad; Mohajjel, Mohammad

2011-05-01

The Sanandaj-Sirjan Zone (SSZ), which runs parallel to the Zagros fold and thrust belt of Iran, underwent a multistage evolution starting with Neotethys initiation, its subsequent subduction below the Iranian continental crust, and eventual closure during convergence of Arabia towards central Iran. Plutonic complexes are well developed in the northern part of the SSZ and we have dated a number of them by ID-TIMS U-Pb on zircon. The new data record the following events: a Mid Jurassic period that formed the Boroujerd Plutonic Complex (169 Ma), the Astaneh Pluton (168 Ma) and the Alvand Pluton (165 Ma); Late Jurassic emplacement of the Gorveh Pluton (157-149 Ma); Mid Cretaceous (109 Ma) formation of a I-type phase in the Hasan Salary Pluton near Saqqez, followed by Early Paleocene (60 Ma) intrusion of A-type granite in the same pluton; and the youngest intrusive event recorded so far in the SSZ with the intrusion of granite in the Gosheh-Tavandasht Complex near Boroujerd at 34.9 Ma. These different events reflect specific stages of subduction-related magmatism prior to the eventual Miocene collision between the two continental blocks.

Uplift in the Fiordland region, New Zealand: implications for incipient subduction.

PubMed

House, M A; Gurnis, M; Kamp, P J J; Sutherland, R

2002-09-20

Low-temperature thermochronometry reveals regional Late Cenozoic denudation in Fiordland, New Zealand, consistent with geodynamic models showing uplift of the overriding plate during incipient subduction. The data show a northward progression of exhumation in response to northward migration of the initiation of subduction. The locus of most recent uplift coincides with a large positive Bouguer gravity anomaly within Fiordland. Thermochronometrically deduced crustal thinning, anomalous gravity, and estimates of surface uplift are all consistent with approximately 2 kilometers of dynamic support. This amount of dynamic support is in accord with geodynamic predictions, suggesting that we have dated the initiation of subduction adjacent to Fiordland.

Subduction of a buoyant plateau at the Manila Trench: Tomographic evidence and geodynamic implications

NASA Astrophysics Data System (ADS)

Fan, Jianke; Zhao, Dapeng; Dong, Dongdong

2016-02-01

We determined P-wave tomographic images by inverting a large number of arrival-time data from 2749 local earthquakes and 1462 teleseismic events, which are used to depict the three-dimensional morphology of the subducted Eurasian Plate along the northern segment of the Manila Trench. Dramatic changes in the dip angle of the subducted Eurasian Plate are revealed from the north to the south, being consistent with the partial subduction of a buoyant plateau beneath the Luzon Arc. Slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction, and the subducted lithosphere may be absent at depths greater than 250 km at ˜19°N and ˜21°N. The subducted buoyant plateau is possibly oriented toward NW-SE, and the subducted plate at ˜21°N is slightly steeper than that at ˜19°N. These results may explain why the western and eastern volcanic chains in the Luzon Arc are separated by ˜50 km at ˜18°N, whereas they converge into a single volcanic chain northward, which may be related to the oblique subduction along the Manila Trench caused by the northwestern movement of the Philippine Sea Plate. A low-velocity zone is revealed at depths of 20-200 km beneath the Manila Accretionary Prism at ˜22°N, suggesting that the subduction along the Manila Trench may stop there and the collision develops northward. The Taiwan Orogeny may originate directly from the subduction of the buoyant plateau, because the initial time of the Taiwan Orogeny is coincident with that of the buoyant plateau subduction.

Paleoseismicity and neotectonics of the Aleutian Subduction Zone—An overview

NASA Astrophysics Data System (ADS)

Carver, Gary; Plafker, George

The Aleutian subduction zone is one of the most seismically active plate boundaries and the source of several of the world's largest historic earthquakes. The structural architecture of the subduction zone varies considerably along its length. At the eastern end is a tectonically complex collision zone where the allochthonous Yakutat terrane is moving northwest into mainland Alaska. West of the collision zone a shallow-dipping subducted plate beneath a wide forearc, nearly orthogonal convergence, and a continental-type subduction regime characterizes the eastern part of the subduction zone. In the central part of the subduction zone, convergence becomes increasingly right oblique and the forearc is divided into a series of large clockwise-rotated fault-bounded blocks. Highly oblique convergence and island arc tectonics characterize the western part of the subduction zone. At the extreme western end of the arc, the relative plate motion is nearly pure strike-slip. A series of great subduction earthquakes ruptured most of the 4000-km length of the subduction zone during a period of several decades in the mid 1900s. The majority of these earthquakes broke multiple segments as defined by the large-scale structure of the overriding plate margin and patterns of historic seismicity. Several of these earthquakes generated Pacific-wide tsunamis and significant damage in the southwestern and south-central regions of Alaska. Characterization of previous subduction earthquakes is important in assessing future seismic and tsunami hazards. However, at present such information is available only for the eastern part of the subduction zone. The 1964 Alaska earthquake (M 9.2) ruptured about ˜950 km of the plate boundary that encompassed the Kodiak and Prince William Sound (PWS) segments. Within this region, nine paleosubduction earthquakes in the past ˜5000 years are recognized on the basis of geologic evidence of sudden land level change and, at some sites, coeval tsunami deposits. Carbon 14-based chronologies indicate recurrence intervals between median calibrated ages for these paleoearthquakes range from 333 to 875 years. The most recent occurred about 489 years ago and broke only the Kodiak segment. During the previous three cycles, both the Kodiak and PWS segments were involved in either multiple-segment ruptures or closely timed pairs of single segment ruptures. Evidence for the earlier paleosubduction earthquakes has been found only at sites in the PWS segment. Thus, future work on the paleoseismicity of other segments would by particular valuable in defining the seismic behavior of the subduction zone.

Strain Variation in Accretionary Prisms Across Space and Time: Insights from the Makran Subduction Zone

NASA Astrophysics Data System (ADS)

Penney, C.; Tavakoli, F.; Saadat, A.; Nankali, H. R.; Sedighi, M.; Khorrami, F.; Sobouti, F.; Rafi, Z.; Copley, A.; Jackson, J. A.; Priestley, K. F.

2017-12-01

The Makran is one of the world's least-studied subduction zones. In particular, little is known about the accumulation and accommodation of strain in the onshore part of the subduction zone, which parallels the coasts of southern Iran and Pakistan. The deformation of the Makran accretionary prism results from both its subduction zone setting and N-S right-lateral shear between central Iran and Afghanistan. North of the Makran, this shear is accommodated by a series of right-lateral faults which offset the rocks of the Sistan Suture Zone, an abandoned accretionary prism. However, these right-lateral faults are not observed south of 27°N, and no major N-S faults cut the E-W trending structures of the Makran. How this right-lateral motion is accommodated at the southern end of the Sistan Suture Zone is a long-standing tectonic question. By combining results from geomorphology, GPS, seismology and modelling we conclude that right-lateral motion is transferred across the depression north of the accretionary prism to the region of right-lateral shear at the western end of the accretionary prism. This requires the Jaz Murian depression to be bounded by normal faults, consistent with the basin geomorphology. However, GPS data show compression across the margins of the basin, and no shallow normal-faulting earthquakes have been observed in the region. We therefore suggest that the behaviour of these faults may be time-dependent and controlled by the megathrust seismic cycle, as has been suggested elsewhere (e.g. Chile). Recent strike-slip earthquakes, including the 2013 Balochistan earthquake, have clustered at the prism's lateral edges, showing the importance of spatial, as well as temporal, variations in strain. These earthquakes have reactivated thrust faults in the Makran accretionary prism, showing that the style of strain within accretionary prisms can vary on multiple timescales and allowing us to calculate the coefficient of friction on the underlying megathrust.

A large mantle water source for the northern San Andreas Fault System: A ghost of subduction past

USGS Publications Warehouse

Kirby, Stephen H.; Wang, Kelin; Brocher, Thomas M.

2014-01-01

Recent research indicates that the shallow mantle of the Cascadia subduction margin under near-coastal Pacific Northwest U.S. is cold and partially serpentinized, storing large quantities of water in this wedge-shaped region. Such a wedge probably formed to the south in California during an earlier period of subduction. We show by numerical modeling that after subduction ceased with the creation of the San Andreas Fault System (SAFS), the mantle wedge warmed, slowly releasing its water over a period of more than 25 Ma by serpentine dehydration into the crust above. This deep, long-term water source could facilitate fault slip in San Andreas System at low shear stresses by raising pore pressures in a broad region above the wedge. Moreover, the location and breadth of the water release from this model gives insights into the position and breadth of the SAFS. Such a mantle source of water also likely plays a role in the occurrence of Non-Volcanic Tremor (NVT) that has been reported along the SAFS in central California. This process of water release from mantle depths could also mobilize mantle serpentinite from the wedge above the dehydration front, permitting upward emplacement of serpentinite bodies by faulting or by diapiric ascent. Specimens of serpentinite collected from tectonically emplaced serpentinite blocks along the SAFS show mineralogical and structural evidence of high fluid pressures during ascent from depth. Serpentinite dehydration may also lead to tectonic mobility along other plate boundaries that succeed subduction, such as other continental transforms, collision zones, or along present-day subduction zones where spreading centers are subducting.

Inferring rupture characteristics using new databases for 3D slab geometry and earthquake rupture models

NASA Astrophysics Data System (ADS)

Hayes, G. P.; Plescia, S. M.; Moore, G.

2017-12-01

The U.S. Geological Survey National Earthquake Information Center has recently published a database of finite fault models for globally distributed M7.5+ earthquakes since 1990. Concurrently, we have also compiled a database of three-dimensional slab geometry models for all global subduction zones, to update and replace Slab1.0. Here, we use these two new and valuable resources to infer characteristics of earthquake rupture and propagation in subduction zones, where the vast majority of large-to-great-sized earthquakes occur. For example, we can test questions that are fairly prevalent in seismological literature. Do large ruptures preferentially occur where subduction zones are flat (e.g., Bletery et al., 2016)? Can `flatness' be mapped to understand and quantify earthquake potential? Do the ends of ruptures correlate with significant changes in slab geometry, and/or bathymetric features entering the subduction zone? Do local subduction zone geometry changes spatially correlate with areas of low slip in rupture models (e.g., Moreno et al., 2012)? Is there a correlation between average seismogenic zone dip, and/or seismogenic zone width, and earthquake size? (e.g., Hayes et al., 2012; Heuret et al., 2011). These issues are fundamental to the understanding of earthquake rupture dynamics and subduction zone seismogenesis, and yet many are poorly understood or are still debated in scientific literature. We attempt to address these questions and similar issues in this presentation, and show how these models can be used to improve our understanding of earthquake hazard in subduction zones.

Deformation and seismicity of Taiwan.

PubMed

Vita-Finzi, C

2000-10-10

14C-dated Holocene coastal uplift, conventional and satellite geodetic measurements, and coseismic and aseismic fault slip reveal the pattern of distributed deformation at Taiwan resulting from convergence between the Philippine Sea plate and Eurasia; as in other subduction orogenic settings, the locus of strain release and accumulation is strongly influenced by changes in fault geometry across strike. Uplift evidence from the islands of Lutao and Lanhsu is consistent with progressive oblique collision between the Luzon arc and the Chinese continental margin. In the Coastal Range, geodetic and seismic records show that shortening is taken up serially by discontinuous slip on imbricate faults. The geodetic data point to net extension across the Central Range, but deformed Holocene shorelines in the Hengchun Peninsula at its southern extremity suggest that the extension is a superficial effect partly caused by blind reverse faulting. The fastest shortening rates indicated by geodesy are recorded on the Longitudinal Valley fault and across the Chukou fault within the fold-and-thrust belt. In the former, the strain is dissipated mainly as aseismic reverse and strike-slip displacement. In contrast, the fold-and-thrust belt has witnessed five earthquakes with magnitudes of 6.5 or above in the 20th century, including the 1999.9.21 Chi-Chi earthquake (magnitude approximately 7.6) on a branch of the Chukou fault. The neotectonic and geodetic data for Taiwan as a whole suggest that the fold-and-thrust belt will continue to host the majority of great earthquakes on the island.

Exhumation history of the NW Indian Himalaya revealed by fission track and 40Ar/39Ar ages

USGS Publications Warehouse

Schlup, M.; Steck, A.; Carter, A.; Cosca, M.; Epard, J.-L.; Hunziker, J.

2011-01-01

New fission track and Ar/Ar geochronological data provide time constraints on the exhumation history of the Himalayan nappes in the Mandi (Beas valley) - Tso Morari transect of the NW Indian Himalaya. Results from this and previous studies suggest that the SW-directed North Himalayan nappes were emplaced by detachment from the underthrusted upper Indian crust by 55. Ma and metamorphosed by ca. 48-40. Ma. The nappe stack was subsequently exhumed to shallow upper crustal depths (<10. km) by 40-30. Ma in the Tso Morari dome (northern section of the transect) and by 30-20. Ma close to frontal thrusts in the Baralacha La region. From the Oligocene to the present, exhumation continued slowly.Metamorphism started in the High Himalayan nappe prior to the Late Oligocene. High temperatures and anatexis of the subducting upper Indian crust engendered the buoyancy-driven ductile detachment and extrusion of the High Himalayan nappe in the zone of continental collision. Late extrusion of the High Himalayan nappe started about 26. Ma ago, accompanied by ductile extensional shearing in the Zanskar shear zone in its roof between 22 and 19. Ma concomitant with thrusting along the basal Main Central Thrust to the south. The northern part of the nappe was then rapidly exhumed to shallow depth (<10. km) between 20 and 6. Ma, while its southern front reached this depth at 10-5. Ma. ?? 2010 Elsevier Ltd.

The Generation of Continents through Subduction Zone Processing of Large Igneous Provinces: A Case Study from the Central American Subduction Zone

NASA Astrophysics Data System (ADS)

Harmon, N.; Rychert, C.

2013-12-01

Billions of years ago primary mantle magmas evolved to form the continental crust, although no simple magmatic differentiation process explains the progression to average andesitic crustal compositions observed today. A multiple stage process is often invoked, involving subduction and or oceanic plumes, to explain the strong depletion observed in Archean xenoliths and as well as pervasive tonalite-trondhjemite-granodiorite and komatiite protoliths in the greenstone belts in the crust in the cratons. Studying modern day analogues of oceanic plateaus that are currently interacting with subductions zones can provide insights into continental crust formation. Here we use surface waves to image crustal isotropic and radially anisotropic shear velocity structure above the central American subduction system in Nicaragua and Costa Rica, which juxtaposes thickened ocean island plateau crust in Costa Rica with continental/normal oceanic crust in Nicaragua. We find low velocities beneath the active arc regions (3-6% slower than the surrounding region) and up to 6% radially anisotropic structures within the oceanic crust of the Caribbean Large Igneous Province beneath Costa Rica. The low velocities and radial anisotropy suggest the anomalies are due to pervasive deep crustal magma sills. The inferred sill structures correlate spatially with increased silicic outputs in northern Costa Rica, indicating that deep differentiation of primary magmas is more efficient beneath Costa Rica relative to Nicaragua. Subduction zone alteration of large igneous provinces promotes efficient, deep processing of primary basalts to continental crust. This scenario can explain the formation of continental lithosphere and crust, by both providing strongly depleted mantle lithosphere and a means for rapidly generating a silicic crustal composition.

Mantle flow influence on subduction evolution

NASA Astrophysics Data System (ADS)

Chertova, Maria V.; Spakman, Wim; Steinberger, Bernhard

2018-05-01

The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ∼3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10-50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100-200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10-50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10-50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.

Geologic constraints on the setting and dynamics of subduction initiation

NASA Astrophysics Data System (ADS)

Encarnacion, John; Keenan, Timothy

2017-04-01

Understanding where and how subduction zones have and can initiate is important because, besides being a critical step in the plate tectonic system, it can provide insight into the complex interactions of crust and mantle rheology, forces acting on the plates, strain, metamorphic reactions, and erosional and depositional processes at the surface. Insight into the possibilities of where and how subduction zones start has been provided by numerical and analog modeling. All sites for subduction initiation are potential weak zones in the lithosphere and include the continent-ocean boundary, oceanic arc-oceanic crust boundary, oceanic transform faults and fracture zones, oceanic detachment faults, and active or recently extinct oceanic ridges/spreading centers. Within the constraints of modeling, it has also been shown that the forces involved in the initiation of subduction can be largely horizontal (induced by a collision, say, or through 'ridge push') or vertical (driven by density contrasts). The latter scenario is often referred to as "spontaneous" subduction initiation, whereas the former situation may be called "forced"or "induced" subduction initiation. It is prudent, however, not to assume that "what can happen, did happen." So, the challenge for geologists is to infer from the rock record, through structural mapping, thermochronology, thermobarometry, geochemistry, paleomagnetics, and sedimentological studies, how any given subduction zone began. Even with a complete data set, it is not always possible to fully constrain the specific geologic setting or dynamics involved in the initiation of a given subduction zone. One can, however, often rule out certain scenarios, increasing the probability of others. Part of the geologic record of subduction initiation preserved at some subduction zones are so-called "metamorphic soles," which include high-temperature (T) and high-pressure (P) metamorphosed oceanic crust that was underthrust to asthenospheric mantle depths, metamorphosed, and then preserved in the hanging wall of the eventual subduction zone. These metamorphic soles may preserve important information bearing on the timing of subduction initiation, the evolving P and T conditions during subduction initiation, and, importantly, the protolith age of the initially subducted crust. The latter parameter—the age of the initially subducted oceanic crust at the time of subduction initiation—is an important constraint that has been lacking in many previous geologic studies of subduction initiation. Recent work on metamorphic soles has provided new information on subduction initiation, including the possibility of rapidly converting oceanic divergent boundaries into subduction zones.

The new Central American seismic hazard zonation: Mutual consensus based on up to day seismotectonic framework

NASA Astrophysics Data System (ADS)

Alvarado, Guillermo E.; Benito, Belén; Staller, Alejandra; Climent, Álvaro; Camacho, Eduardo; Rojas, Wilfredo; Marroquín, Griselda; Molina, Enrique; Talavera, J. Emilio; Martínez-Cuevas, Sandra; Lindholm, Conrad

2017-11-01

Central America is one of the most active seismic zones in the World, due to the interaction of five tectonic plates (North America, Caribbean, Coco, Nazca and South America), and its internal deformation, which generates almost one destructive earthquakes (5.4 ≤ Mw ≤ 8.1) every year. A new seismological zonation for Central America is proposed based on seismotectonic framework, a geological context (tectonic and geological maps), geophysical and geodetic evidence (gravimetric maps, magnetometric, GPS observations), and previous works. As a main source of data a depurated earthquake catalog was collected covering the period from 1522 to 2015. This catalog was homogenized to a moment magnitude scale (Mw). After a careful analysis of all the integrated geological and seismological information, the seismogenic zones were established into seismic areas defined by similar patterns of faulting, seismicity, and rupture mechanism. The tectonic environment has required considering seismic zones in two particular seismological regimes: a) crustal faulting (including local faults, major fracture zones of plate boundary limits, and thrust fault of deformed belts) and b) subduction, taking into account the change in the subduction angle along the trench, and the type and location of the rupture. The seismicity in the subduction zone is divided into interplate and intraplate inslab seismicity. The regional seismic zonation proposed for the whole of Central America, include local seismic zonations, avoiding discontinuities at the national boundaries, because of a consensus between the 7 countries, based on the cooperative work of specialists on Central American seismotectonics and related topics.

Geomorphic evidence for recent uplift of the Fitzcarrald Arch (Peru): A response to the Nazca Ridge subduction

NASA Astrophysics Data System (ADS)

Regard, V.; Lagnous, R.; Espurt, N.; Darrozes, J.; Baby, P.; Roddaz, M.; Calderon, Y.; Hermoza, W.

2009-06-01

The 400 000 km 2-wide Fitzcarrald Arch constitutes a wide topographic high of the Amazon Basin against the central Andes. In order to constrain its formation mechanisms and in particular to test its relationships to the Nazca ridge subduction, a quantitative geomorphology analysis of the Arch is performed using hypsometric integrals, elongation and azimuths of 7th- and 5th-order catchments. They all express a trend from high maturity to low maturity from NW towards SE. This maturity gradient coupled with the local drainage direction demonstrate that the Fitzcarrald Arch is not a 'classical' alluvial fan, since its apex is located 100 km east to the Subandean Thrust Front and the corresponding sedimentary pile is lacking. Nor is the Arch the superficial expression of an inherited transfer zone, because its geomorphic shape is radial and it does not diverge from a symmetry axis; moreover, such a reactivated structure is not found at depth on seismic profiles. In addition, our data show that underlying geomorphic control on catchment initiation and development has progressed from NW to SE, which in combination with the observation of crustal doming by Espurt et al. [Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V., Antoine, P.O., Salas-Gismondi, R., Bolaños, R., 2007. How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? Geology 35, 515-518.] suggests that this relief is caused by the eastward sliding of the buoyant Nazca ridge beneath the South American lithosphere.

Foreland structure - Beartooth Mountains, Montana and Wyoming

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clark, D.M.

1996-06-01

Analysis of public drilling records from the AMOCO Beartooth Number 1 and 1 A sidetrack boreholes (SW1/4, SE1/4, Section 19, T.8 S., R.20 E., Carbon County, Montana) continues. Several additional inferences are made about this large foreland structure, and subsequent interpretation of the structural model of the northeast corner of the Beartooth Mountain Block and structural relationship with the Big Horn Basin. The structure is described as a large recumbent to sub-horizontal, synclinal fold with the overturned upper limb out diagonally by the Beartooth Thrust or Thrust Zone and a complex thrust fault zone below the Beartooth Thrust. The singlemore » recorded dip angle and direction of the Beartooth Thrust at depth was 19 degrees to the northwest(?). The dipmeter dip angle on the Beartooth Thrust, 19 degrees, validates foreland structural theory of decreasing dip angles at a vertical depth of 8,232 feet (2,509 m), in the Precambrian crystalline basement. The northwest dip direction may be attributable to secondary structural folding. The record of northwest, southeast, and southwest dip of bedding surfaces and faults in sections of the overturned upper limb, in both boreholes, suggests possible, less intense secondary folding, after thrust fault deformation. Given the overall geometry of this large foreland structure, there is little doubt that the average direction of maximum principal stress (sigma 1) was oriented in a northeast - southwest direction.« less

Tectonic deformation of the Andes and the configuration of the subducted slab in central Peru: Results from a micro-seismic experiment

NASA Technical Reports Server (NTRS)

Suarez, G.; Gagnepain, J. J.; Cisternas, A.; Hatzfeld, D.; Molnar, P.; Ocola, L.; Roecker, S. W.; Viode, J. P.

1983-01-01

The vast majority of the microearthquakes recorded occurred to the east: on the Huaytapallana fault in the Eastern Cordillera or in the western margin of the sub-Andes. The sub-Andes appear to be the physiographic province subjected to the most intense seismic deformation. Focal depths for the crustal events here are as deep as 50 km, and the fault plane solutions, show thrust faulting on steep planes oriented roughly north-south. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a northeast-southwest trend that agrees with the fault scarp and the east dipping nodal plane of two large earthquakes that occurred on this fault in 1969. The recorded microearthquakes of intermediate depth show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with the suggestion that beneath Peru the slab first dips at an angle of 30 deg to a depth of 100 km and then flattens following a quasi-horizontal trajectory. Fault plane solutions of intermediate depth microearthquakes have horizontal T axes oriented east-west.

Silica precipitation potentially controls earthquake recurrence in seismogenic zones.

PubMed

Saishu, Hanae; Okamoto, Atsushi; Otsubo, Makoto

2017-10-17

Silica precipitation is assumed to play a significant role in post-earthquake recovery of the mechanical and hydrological properties of seismogenic zones. However, the relationship between the widespread quartz veins around seismogenic zones and earthquake recurrence is poorly understood. Here we propose a novel model of quartz vein formation associated with fluid advection from host rocks and silica precipitation in a crack, in order to quantify the timescale of crack sealing. When applied to sets of extensional quartz veins around the Nobeoka Thrust of SW Japan, an ancient seismogenic splay fault, our model indicates that a fluid pressure drop of 10-25 MPa facilitates the formation of typical extensional quartz veins over a period of 6.6 × 10 0 -5.6 × 10 1 years, and that 89%-100% of porosity is recovered within ~3 × 10 2 years. The former and latter sealing timescales correspond to the extensional stress period (~3 × 10 1 years) and the recurrence interval of megaearthquakes in the Nankai Trough (~3 × 10 2 years), respectively. We therefore suggest that silica precipitation in the accretionary wedge controls the recurrence interval of large earthquakes in subduction zones.

Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9).

PubMed

Zhao, Dapeng; Fujisawa, Moeto; Toyokuni, Genti

2017-03-15

On 30 May 2015 an isolated deep earthquake (~670 km, Mw 7.9) occurred to the west of the Bonin Islands. To clarify its causal mechanism and its relationship to the subducting Pacific slab, we determined a detailed P-wave tomography of the deep earthquake source zone using a large number of arrival-time data. Our results show that this large deep event occurred within the subducting Pacific slab which is penetrating into the lower mantle. In the Izu-Bonin region, the Pacific slab is split at ~28° north latitude, i.e., slightly north of the 2015 deep event hypocenter. In the north the slab becomes stagnant in the mantle transition zone, whereas in the south the slab is directly penetrating into the lower mantle. This deep earthquake was caused by joint effects of several factors, including the Pacific slab's fast deep subduction, slab tearing, slab thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and the ambient mantle.

Breaking the oceanic lithosphere of a subducting slab: the 2013 Khash, Iran earthquake

USGS Publications Warehouse

Barnhart, William D.; Hayes, Gavin P.; Samsonov, S.; Fielding, E.; Seidman, L.

2014-01-01

[1] Large intermediate depth, intraslab normal faulting earthquakes are a common, dangerous, but poorly understood phenomenon in subduction zones owing to a paucity of near field geophysical observations. Seismological and high quality geodetic observations of the 2013 Mw7.7 Khash, Iran earthquake reveal that at least half of the oceanic lithosphere, including the mantle and entire crust, ruptured in a single earthquake, confirming with unprecedented resolution that large earthquakes can nucleate in and rupture through the oceanic mantle. A rupture width of at least 55 km is required to explain both InSAR observations and teleseismic waveforms, with the majority of slip occurring in the oceanic mantle. Combining our well-constrained earthquake slip distributions with the causative fault orientation and geometry of the local subduction zone, we hypothesize that the Khash earthquake likely occurred as the combined result of slab bending forces and dehydration of hydrous minerals along a preexisting fault formed prior to subduction.

A Subducted Seamount Revealed: 2016, NOAA OER Deepwater Exploration of the Marianas

NASA Astrophysics Data System (ADS)

Fryer, P. B.; Kelley, C.; Pomponi, S. A.; Glickson, D.; Amon, D.

2017-12-01

The first indisputable observation of a large expanse of intact seamount exposed in the inner slope of any convergent plate margin was in June 2016. The only other potential evidence for an exposed subducted seamount was observations from a series of Nautile submersible dives in the 1980's. On these dives, brecciated boulders of Cretaceous reefal debris lay on the deepest 30 m of the inner slope of the Japan Trench near Daiichi-Kashima Seamount. Because the subducting plate within 60 to 120 km outboard of a trench is usually heavily faulted, it has been suggested that seamounts impinging on a forearc region should be heavily deformed. This is not what we observed in the inner Mariana Trench during the third leg of the NOAA ship Okeanos Explorer's expedition to the Mariana subduction region. In June 2016 we recorded 275 m of exposed reef on Dive 4 (at 20.5°N) with the NOAA "Deep Discoverer" remotely operated vehicle (D-2 ROV), starting at 5,995 m on the inner slope of the Mariana Trench. The deposits are morphologically identical to observations on Dive 16 on a summit escarpment of the Cretaceous Fryer Guyot ( 20.5°N) just east of the trench. We interpret the inner trench slope exposure to be part of a Cretaceous reef complex of a seamount partially subducted beneath the overriding plate edge. Large-scale differences in the two exposures are the prevalence of vertical debris chutes between steep ridges seen in Dive 4 versus smoother, steeper slopes on Dive 16. The reefal sequences on Dive 16 show numerous fossils including bivalves in place, and layers with rudist morphology (S. Stanley, 2017, pers. comm.) in alternating tan and white bands. Similar sequences were observed on Dive 4. Slump scars observed on Dive 4 indicate mass wasting, but there is no indication of shearing or large-scale deformation. Thus, we interpret the exposure to reveal a large section of the reef complex that is partially subducted and largely intact beneath the overriding Philippine Sea Plate edge.

Dynamic triggering of low magnitude earthquakes in the Middle American Subduction Zone

NASA Astrophysics Data System (ADS)

Escudero, C. R.; Velasco, A. A.

2010-12-01

We analyze global and Middle American Subduction Zone (MASZ) seismicity from 1998 to 2008 to quantify the transient stresses effects at teleseismic distances. We use the Bulletin of the International Seismological Centre Catalog (ISCCD) published by the Incorporated Research Institutions for Seismology (IRIS). To identify MASZ seismicity changes due to distant, large (Mw >7) earthquakes, we first identify local earthquakes that occurred before and after the mainshocks. We then group the local earthquakes within a cluster radius between 75 to 200 km. We obtain statistics based on characteristics of both mainshocks and local earthquakes clusters, such as local cluster-mainshock azimuth, mainshock focal mechanism, and local earthquakes clusters within the MASZ. Due to lateral variations of the dip along the subducted oceanic plate, we divide the Mexican subduction zone in four segments. We then apply the Paired Samples Statistical Test (PSST) to the sorted data to identify increment, decrement or either in the local seismicity associated with distant large earthquakes. We identify dynamic triggering for all MASZ segments produced by large earthquakes emerging from specific azimuths, as well as, a decrease for some cases. We find no depend of seismicity changes due to focal mainshock mechanism.

Explaining postseismic and aseismic transient deformation in subduction zones with rate and state friction modeling constrained by lab and geodetic observations

NASA Astrophysics Data System (ADS)

Liu, Y.; Dedontney, N. L.; Rice, J. R.

2007-12-01

Rate and state friction, as applied to modeling subduction earthquake sequences, routinely predicts postseismic slip. It also predicts spontaneous aseismic slip transients, at least when pore pressure p is highly elevated near and downdip from the stability transition [Liu and Rice, 2007]. Here we address how to make such postseismic and transient predictions more fully compatible with geophysical observations. For example, lab observations can determine the a, b parameters and state evolution slip L of rate and state friction as functions of lithology and temperature and, with aid of a structural and thermal model of the subduction zone, as functions of downdip distance. Geodetic observations constrain interseismic, postseismic and aseismic transient deformations, which are controlled in the modeling by the distributions of a \\barσ and b \\barσ (parameters which also partly control the seismic rupture phase), where \\barσ = σ - p. Elevated p, controlled by tectonic compression and dehydration, may be constrained by petrologic and seismic observations. The amount of deformation and downdip extent of the slipping zone associated with the spontaneous quasi- periodic transients, as thus far modeled [Liu and Rice, 2007], is generally smaller than that observed during episodes of slow slip events in northern Cascadia and SW Japan subduction zones. However, the modeling was based on lab data for granite gouge under hydrothermal conditions because data is most complete for that case. We here report modeling based on lab data on dry granite gouge [Stesky, 1975; Lockner et al., 1986], involving no or lessened chemical interaction with water and hence being a possibly closer analog to dehydrated oceanic crust, and limited data on gabbro gouge [He et al., 2007], an expected lithology. Both data sets show a much less rapid increase of a-b with temperature above the stability transition (~ 350 °C) than does wet granite gouge; a-b increases to ~ 0.08 for wet granite at 600 °C, but to only ~ 0.01 in the dry granite and gabbro cases. We find that the lessened high-T a - b does, for the same \\barσ, modestly extend the transient slip episodes further downdip, although a majority of slip is still contributed near and in the updip rate-weakening region. However, postseismic slip, for the same \\barσ, propagates much further downdip into the rate-strengthening region. To better constrain the downdip distribution of (a - b) \\barσ, and possibly a \\barσ and L, we focus on the geodetically constrained [Hutton et al., 2001] space-time distribution of postseismic slip for the 1995 Mw = 8.0 Colima-Jalisco earthquake. This is a similarly shallow dipping subduction zone with a thermal profile [Currie et al., 2001] comparable to those that have thus far been shown to exhibit aseismic transients and non-volcanic tremor [Peacock et al., 2002]. We extrapolate the modeled 2-D postseismic slip, following a thrust earthquake with a coseismic slip similar to the 1995 event, to a spatial-temporal 3-D distribution. Surface deformation due to such slips on the thrust fault in an elastic half space is calculated and compared to that observed at western Mexico GPS stations, to constrain the above depth-variable model parameters.

Longitudinal Strain in the Forearc of a Rollback-Subduction System Forced to Change Length: Structural evolution of the Crotone Basin in NE Calabria, Southern Italy

NASA Astrophysics Data System (ADS)

Reitz, M. A.; Seeber, L.

2009-12-01

Calabria is a continental fragment incorporated into a forearc overriding the WNW directed subduction system. This system rolled back toward ESE across the central Mediterranean during the Neogene to form the Tyrrhenian Basin. Riding above the megathrust, forearcs seek a dynamic equilibrium between boundary stresses (drag below and lateral containments) with body stress (gravity acting on the shape of the forearc). Changes in boundary conditions are balanced by changes in the shape. The internal deformation history of the forearc, therefore, is expected to reflect changes in subduction tectonics during the evolution of the arc. We analyzed the structure of the Crotone Basin, located in northeastern Calabria, which is located in the exposed part of the forearc closest to the deformation front and to the Apennines. The main purpose was to compare the successive phases of deformation in the basin to the known evolution of the arc. We found four distinct events from the late Tortonian to the present. A widespread unconformity correlated with the onset of rollback marks a regional foundering with multidirectional normal growth faults. Following this pervasive and deeply rooted extension, the Crotone Basin experiences a period of parallel and distal sedimentation (Ponda clay). These sediments mark a relative long period (~5ma) of remarkable tectonic quiescence, even though subduction-rollback is moving the arc rapidly (3-5cm/yr) to the ESE. In addition, the forearc is shortening by progressive collision with Apulia (the Apennines) and Africa (the Maghrebides) during this time, but our study area is still far from the oblique collisions occurring at the ends of the forearc. The Messinian Salinity Crisis (5.3-6Ma) causes major instabilities in the accretion by loading it with evaporite deposits first and then removing the water load. Landward (westward) thrusting of the accretionary complex correlates with the Messinian in the Crotone basin and elsewhere along eastern Calabria. A characteristic fluvial conglomerate that locally caps the evaporite sequence records this thrusting by a systematic fracturing of the cobbles. After a well-known mid-Pliocene basin-forming extensional event, we find evidence of a basin-wide contractional event affecting the entire Neogene sequence up to the mid-to-late Pliocene. The data show a north-south compression with vergence to the north. This arc-longitudinal shortening may correlate with mid-Pliocene N-S shortening reported in the southern Apennines. Finally, many of these shortening structures are cut or reactivated by a recent (mid-Pleistocene?) faults, that accommodate extension also directed N-S to NW-SE. Our data show a shift from radial to longitudinal tectonics in the Pliocene as the Crotone basin nears the oblique collision with Apulia. Longitudinal forearc shortening may lead to extension in the Pleistocene, as the forearc squeezes through the narrow between Africa (Sicily) and Apulia, and begins lengthening as rollback consumes progressively wider Ionian lithosphere.

Detrital record of initial basement exhumation along the Laramide deformation front, southern Rocky Mountains

NASA Astrophysics Data System (ADS)

Bush, Meredith A.; Horton, Brian K.; Murphy, Michael A.; Stockli, Daniel F.

2016-09-01

New geochronological constraints on upper crustal exhumation in the southern Rocky Mountains help delineate the latest Cretaceous-Paleogene history of drainage reorganization and landscape evolution during Laramide flat-slab subduction beneath western North America. Detrital zircon U-Pb results for the Raton basin of southern Colorado and northern New Mexico define the inception of coarse-grained siliciclastic sedimentation and a distinctive shift in provenance, from distal to proximal sources, that recorded shortening-related uplift and unroofing along the Laramide deformation front of the northern Sangre de Cristo Mountains. This Maastrichtian-early Paleocene ( 70-65 Ma) change—from distal foreland accumulation of sediment derived from the thin-skinned Cordilleran (Sevier) fold-thrust belt to coarse-grained sedimentation proximal to a Laramide basement block uplift—reflects cratonward (eastward) deformation advance and reorganization of drainage systems that supplied a large volume of Paleocene-lower Eocene sediments to the Gulf of Mexico. The timing of unroofing along the eastern deformation front is synchronous with basement-involved shortening across the interior of the Laramide province, suggesting abrupt wholesale uplift rather than a systematic inboard advance of deformation. The growth and infilling of broken foreland basins within the interior and margins of the Laramide province had a significant impact on continental-scale drainage systems, as several ponded/axial Laramide basins trapped large volumes of sediment and induced reorganization of major source-to-sink sediment pathways.