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.

Subduction dynamics: From the trench to the core-mantle boundary

NASA Astrophysics Data System (ADS)

Kincaid, Chris

1995-07-01

Subduction occurs along convergent plate boundaries where one of the colliding lithospheric plates descends into the mantle. Subduction zones are recognized where plates converge at ˜2-15 cm/yr, although well developed trenches and volcanic arcs (e.g. the line of active volcanoes lying parallel to most ocean trenches, such as the Aleutian Islands in the North Pacific) occur when convergence rates are higher, 4-10 cm/yr. This report is meant to provide a brief review on the general topic of subduction dynamics. A recent spin on subduction studies is the growing realization that the need to understand this global Earth process may be argued not only on purely scientific grounds, but also in terms of societal relevance. While subducting slabs of oceanic lithosphere clearly provide the dominant driving force for mantle dynamics and plate tectonics, over half of the Earth's present 40,000 km of subduction zones are associated with continental margins where a large and rapidly increasing percentage of the Earth's population resides. Subductioninduced hazards along active continental margins include those associated with volcanic hazards (Blong, 1984; Tilling, 1989) such as lava flows, pyroclastic flows and ash fallout and tectonic processes, such as faulting, tsunamis and earthquakes. With regards to earthquake hazards, all of the great (magnitude >9) earthquakes in recorded history have occurred at subduction zones, with 50% of all energy released since 1900 being in four events (1964-Alaska; 1960-Chile; 1957- Aleutians; 1952-Kamchatka). Subduction zone hazards have significant impact on long time scales, such as contributions to global climate change (Robock, 1991; Simarski, 1992; Johnson, 1993; Bluth et al., 1993) and short time scales such as airline safety (Casadevall, 1992). Moreover, accretionary wedges are important in terms of resource potential and trenches have occasionally been suggested as nuclear waste disposal sites.

Introduction to the structures and processes of subduction zones

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhao, Zi-Fu

2017-09-01

Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction processes and products from post-subduction processes and products. Nevertheless, available results indicate that our definition and understanding of subduction zone processes and products can be advanced by the convergence of observations and interpretations from geochemical, geological, geophysical and geodynamic studies of both oceanic and continental subduction zones. Therefore, insights into subduction zones can be provided by intergration of different approaches from different targets in the near future.

Introduction to the structures and processes of subduction zones

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhao, Zi-Fu

2017-09-01

Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction processes and products from post-subduction processes and products. Nevertheless, available results indicate that our definition and understanding of subduction zone processes and products can be advanced by the convergence of observations and interpretations from geochemical, geological, geophysical and geodynamic studies of both oceanic and continental subduction zones. Therefore, insights into subduction zones can be provided by integration of different approaches from different targets in the near future.

Nonvolcanic Deep Tremors in the Transform Plate Bounding San Andreas Fault Zone

NASA Astrophysics Data System (ADS)

Nadeau, R. M.; Dolenc, D.

2004-12-01

Recently, deep ( ˜ 20 to 40 km) nonvolcanic tremor activity has been observed on convergent plate boundaries in Japan and in the Cascadia region of North America (Obara, 2002; Rodgers and Dragert, 2003; Szeliga et al., 2004). Because of the abundance of available fluids from subduction processes in these convergent zones, fluids are believed to play an important role in the generation of the tremor activity. The transient rates of tremor activity in these regions are also observed to correlate either with the occurrence of larger earthquakes (Obara, 2002) or with geodetically determined transient creep events that release large amounts of strain energy deep beneath the locked Cascadia megathrust (M.M. Miller et al., 2002; Rodgers and Dragert, 2003; Szeliga et al., 2004). These associations suggest that nonvolcanic tremor activity may participate in a fundamental mode of deep moment release and in the triggering of large subduction zone events (Rodgers and Dragert, 2003). We report the discovery of deep ( ˜ 20 to 45 km) nonvolcanic tremor activity on the transform plate bounding San Andreas Fault (SAF) in central California where, in contrast to subduction zones, long-term deformation directions are horizontal and fluid availability from subduction zone processes is absent. The source region of the SAF tremors lies beneath the epicentral region of the great 1857 magnitude (M) ˜ 8, Fort Tejon earthquake whose rupture zone is currently locked (Sieh, 1978). Activity rate transients of the tremors occurring since early 2001 also correlate with seismicity rate variations above the tremor source region.

The geological and petrological studies of the subduction boundaries and suggestion for the geological future work in Japan　- How to avoid ultra-mega-earthquakes -

NASA Astrophysics Data System (ADS)

Ishii, T.

2015-12-01

The Pacific plate is surrounded by circum-Pacific active margin, along which volcanic and seismic activities are very high. Ultra-Mega-Earthquakes (=UMEs, M>9.0) are occasionally observed along the margin, where sedimentary rocks of subducting slaves contact with the accreted sedimentary rocks of subducted slaves. But, those UME have never been occured along western Pacific islandarc-trench system including Izu-Ogasawara (=Bonin)-Mariana-Yap-Palau-Philippine-Tonga-Kermadec Trenches. I assume that the geological and petrological characteristics of the subduction boundaries are very important to understand those different seismic activities. Along the above mentioned trench inner wall, especially in the southern Mariana, mantle peridotites are widely distributed. Subducting slave contacts directly with the olivine dominant mantle peridotites of subducted slave, serpentinite layer can be deposited easily under hydrous oceanic sub-bottom environment and very slippery subduction boundaries are left along the subduction zone.On the other hand, those geological evidences give us some ideas on how to avoid UMEs in the Japanese Islands along Japan Trench and Nankai Trough in future. We will be able to change artificially from normal subduction boundaries with asperity zone into slippery subduction boundaries with serpentine layer, by means of serpentine mud injection toward the subduction boundaries interior by combining the following improved drilling technologies A and B. (A) Deep Sea Drilling Vessel CHIKYU has a drilling ability to reach subduction boundary with asperity zone in the Nankai Trough. (B) Advanced drilling technology in the shale gas industry is tremendous, that is, after one vertical deep drilling, horizontal drilling towards several direction are performed, then shale gas is collected by hydraulic fracturing method. I hope that, after several generations, our posterity will be able to avoid UMEs by continuous serpentine mud injection.

Forearc deformation and great subduction earthquakes: implications for cascadia offshore earthquake potential.

PubMed

McCaffrey, R; Goldfinger, C

1995-02-10

The maximum size of thrust earthquakes at the world's subduction zones appears to be limited by anelastic deformation of the overriding plate. Anelastic strain in weak forearcs and roughness of the plate interface produced by faults cutting the forearc may limit the size of thrust earthquakes by inhibiting the buildup of elastic strain energy or slip propagation or both. Recently discovered active strike-slip faults in the submarine forearc of the Cascadia subduction zone show that the upper plate there deforms rapidly in response to arc-parallel shear. Thus, Cascadia, as a result of its weak, deforming upper plate, may be the type of subduction zone at which great (moment magnitude approximately 9) thrust earthquakes do not occur.

Diverse continental subduction scenarios along the Arabia-Eurasia collision zone

NASA Astrophysics Data System (ADS)

Kaban, M. K.; Petrunin, A.; El Khrepy, S.; Al-Arifi, N. S.

2017-12-01

The Arabia-Eurasia continental collision zone is one of the largest and most active on the Earth. It has been discussed already long ago that the convergence of these plates implies subduction of the lithosphere. However, scenarios of this process are still debatable. Even direction of the present-day continental subduction is not clear. Previously, principal conclusions about structure of the upper mantle in this region were chiefly based on seismic tomography results. However, seismic velocities not always provide a complete image of the deep interiors since they are chiefly affected by temperature variations and less - by composition. Here we construct a 3D model of the mantle down to 700 km, which is based on a joint inversion of seismic tomography, residual (crust free) gravity field and residual topography (Kaban et al., 2016). Several cross-sections across the collision zone demonstrate principal variations of the continental subduction scenarios from northwest to southeast. In the southeastern part we observe subduction of the Eurasian plate under the West Great Caucasus, Pontic mountains and further under the northwestern part of the Arabian plate. However, the situation is changed when we move to the East Great Caucasus and Zagros, where clear double-sided subduction is observed. The Arabian plate is subducting under the Zagros, while the Eurasian plate - under the Caucasus merging in the transition zone. This situation persists further to the southeast, where we observe the subduction of the South Caspian block under Alborz accompanied by the counteracting penetration of the Arabian plate from the south. More to the southeast, the subduction of the Arabian plate is stagnated, while the subduction of the Eurasian plate can be traced down to the bottom of the transition zone under the northeastern flank of the Arabian plate. In the southern rim of the collision zone under Makran, we don't find any evidence for the present day subduction; remnants of the formerly subducted slabs are located below 200 km. Kaban, M. K., S. El Khrepy, N. Al-Arifi, M. Tesauro, and W. Stolk (2016), Three dimensional density model of the upper mantle in the Middle East: Interaction of diverse tectonic processes, J. Geophys. Res. Solid Earth, 121.

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.

Effects of geodynamic setting on the redox state of fluids released by subducted mantle lithosphere

NASA Astrophysics Data System (ADS)

Evans, K. A.; Reddy, S. M.; Tomkins, A. G.; Crossley, R. J.; Frost, B. R.

2017-05-01

Magnetite breakdown during subduction of serpentinised ultramafic rocks may produce oxidised fluids that oxidise the deep Earth and/or the sub-arc mantle, either via direct transport of ferric iron, or via redox reactions between ferric iron and other elements, such as sulfur. However, so far, there is no consensus on the oxidation state of fluids released during subduction of ultramafic rocks, or the factors that control this oxidation state. Subducted samples from a magma-poor rifted margin and a supra-subduction zone geodynamic setting were compared to examine evidence of changes in opaque phase assemblage and ferric iron content as a consequence of subduction, and as a function of geodynamic setting. Thermodynamic calculations in the system Fe-Ni-O-H-S and Fe-Ni-O-S at the pressures and temperatures of interest were used to constrain oxygen activities and fluid compositions. Samples from New Caledonia, which exemplify supra-subduction zone mantle, contain awaruite (FeNi3) and equilibrated with hydrogen-bearing fluids at oxygen activity less than the FMQ (fayalite-magnetite-quartz) buffer. In contrast, samples from the Zermatt Saas Zone ophiolite, Western Alps, which are thought to represent mantle from a subducted magma-poor rifted margin, contain magnetite plus sulfur-rich phases such as pyrite (FeS2), and are inferred to have equilibrated with hydrogen-poor fluids at oxygen activity greater than FMQ. This major difference is independent of differences in subduction pressure-temperature conditions, variation in peridotite protolith composition, or the nature of adjacent units. We propose that the Zermatt Saas Zone samples would have undergone more complete serpentinisation prior to subduction than the supra-subduction zone (SSZ) New Caledonian samples. This difference explains the different fluid compositions, because incompletely serpentinised rocks containing olivine and brucite retain or evolve awaruite-bearing assemblages that buffer fluid compositions to high hydrogen activity (aH2). Ultramafic rocks are associated with two distinctly different fluid compositions during pre-subduction and subduction serpentinisation. Initially, while olivine is in equilibrium with infiltrating fluid, mineral assemblages that include awaruite in the rocks buffer fluids to H2-bearing, low aO2 compositions. Deserpentinisation of incompletely serpentinised rocks in which awaruite is present also produces H2-bearing fluids. Once awaruite is exhausted, H2-poor, high aO2 fluids co-exist with awaruite-absent assemblages, and deserpentinisation of such rocks would produce H2O-rich fluids. Thus, deserpentinisation of ultramafic rocks could produce either hydrogen-bearing fluids that could infiltrate and reduce the sub-arc mantle, or more oxidised fluids, which could transfer redox budget to other geochemical reservoirs such as the sub-arc mantle. Therefore, the redox contribution of subducted ultramafic rocks to the deep Earth and sub-arc mantle depends on the extent of protolith serpentinisation. Pre-subduction settings that promote extensive serpentinisation by oxidised fluids at high fluid:rock ratios in open systems, such as slow and ultraslow spreading ridges, transform faults, oceanic core complexes, and exhumed mantle at rifted continental margins, may produce more oxidised fluids than those associated with less pervasive serpentinisation and fluids that may be rock-buffered to a reduced state.

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.

Geophysical signature of hydration-dehydration processes in active subduction zones

NASA Astrophysics Data System (ADS)

Reynard, Bruno

2013-04-01

Seismological and magneto-telluric tomographies are potential tools for imaging fluid circulation when combined with petrophysical models. Recent measurements of the physical properties of serpentine allow refining hydration of the mantle and fluid circulation in the mantle wedge from geophysical data. In the slab lithospheric mantle, serpentinization caused by bending at the trench is limited to a few kilometers below the oceanic crust (<5 km). Double Wadati-Benioff zones, 20-30 km below the crust, are explained by deformation of dry peridotites, not by serpentine dehydration. It reduces the required amount of water stored in solid phases in the slab (Reynard et al., 2010). In the cold (<700°C) fore-arc mantle wedge above the subducting slab, serpentinization is caused by the release of large amounts of hydrous fluids in the cold mantle above the dehydrating subducted plate. Low seismic velocities in the wedge give a time-integrated estimate of hydration and serpentinization. Serpentinization reaches 50-100% in hot subduction, while it is below 10% in cold subduction (Bezacier et al., 2010; Reynard, 2012). Electromagnetic profiles of the mantle wedge reveal high electrical-conductivity bodies. In hot areas of the mantle wedge (> 700°C), water released by dehydration of the slab induces melting of the mantle under volcanic arcs, explaining the observed high conductivities. In the cold melt-free wedge (< 700°C), high conductivities in electromagnetic profiles provide "instantaneous" images of fluid circulation because the measured electrical conductivity of serpentine is below 0.1 mS/m (Reynard et al., 2011). A small fraction (ca. 1% in volume) of connective high-salinity fluids accounts for the highest observed conductivities. Low-salinity fluids (≤ 0.1 m) released by slab dehydration evolve towards high-salinity (≥ 1 m) fluids during progressive serpentinization in the wedge. These fluids can mix with arc magmas at depths and account for high-chlorine melt inclusions in arc lavas. High electrical conductivities up to 1 S/m in the hydrated wedge of the hot subductions (Ryukyu, Kyushu, Cascadia) reflect high fluid concentration, while low to moderate (<0.01 S/m) conductivities in the cold subductions (N-E Japan, Bolivia) reflect low fluid flow. This is consistent with the seismic observations of extensive shallow serpentinization in hot subduction zones, while serpentinization is sluggish in cold subduction zones. Bezacier, L., et al. 2010. Elasticity of antigorite, seismic detection of serpentinites, and anisotropy in subduction zones. Earth and Planetary Science Letters, 289, 198-208. Reynard, B., 2012. Serpentine in active subduction zones. Lithos, http://dx.doi.org/10.1016/j.lithos.2012.10.012. Reynard, B., Mibe, K. & Van de Moortele, B., 2011. Electrical conductivity of the serpentinised mantle and fluid flow in subduction zones. Earth and Planetary Science Letters, 307, 387-394. Reynard, B., Nakajima, J. & Kawakatsu, H., 2010. Earthquakes and plastic deformation of anhydrous slab mantle in double Wadati-Benioff zones. Geophysical Research Letters, 37, L24309.

Investigating the origins of rhythmic major-element zoning in HP/LT garnets from worldwide subduction mélanges

NASA Astrophysics Data System (ADS)

Viete, D. R.; Hacker, B. R.; Seward, G.; Allen, M. B.

2016-12-01

Rhythmic major-element zoning has been documented in garnets from high pressure/low temperature (HP/LT) lenses within a number of worldwide subduction mélanges (e.g. California, Chinese Tianshan, Cuba, Greek Cyclades, Guatemala, Japan, Venezuela). The origin of these features has implications for the nature of subduction-zone processes. Conditions of rhythmic zoning acquirement in HP/LT garnets of California and Venezuela were investigated by use of Raman and FTIR microspectroscopy, and thermodynamic modelling of phase equilibria. Quartz-in-garnet Raman barometry reveals varying P—on the order of 100­-300 MPa, over radial distances of 10s of µm—in association with the high-Mn (and low-Mg) bands that define the fine-scale rhythmic zoning. Results from FTIR microspectroscopy demonstrate association between the high-Mn bands and locally depressed (structural) OH and elevated (molecular) H2O concentrations. The microspectroscopy results suggest changes in P and fluid activity attended development of the cryptic rhythmic zoning. Perple_X modelling of phase equilibria shows that, for specific rock chemistry and subduction P-T conditions, garnet modal abundance is extremely sensitive to changes in P (e.g. 10-20 vol.% growth/dissolution for ΔP = 200 MPa). Rhythmic major-element zoning may reflect P- and/or fluid-driven cycles of garnet stability-instability and/or varying reaction progress/kinetics during subduction. Steep compositional gradients that define the rhythmic major-element zoning limit time scales at subduction T, requiring that such individual stability-instability and/or accelerated reaction cycles were extremely brief. Seismic cycles or porosity waves represent ephemeral phenomena capable of accounting for development of rhythmic major-element zoning in HP/LT garnet, during subduction, as a result of fluctuations in both P and fluids. Metamorphic rocks may well carry detailed records of the catastrophism that punctuates longer-term tectonometamorphic processes.

Simulation of active tectonic processes for a convecting mantle with moving continents

USGS Publications Warehouse

Trubitsyn, V.; Kaban, M.; Mooney, W.; Reigber, C.; Schwintzer, P.

2006-01-01

Numerical models are presented that simulate several active tectonic processes. These models include a continent that is thermally and mechanically coupled with viscous mantle flow. The assumption of rigid continents allows use of solid body equations to describe the continents' motion and to calculate their velocities. The starting point is a quasi-steady state model of mantle convection with temperature/ pressure-dependent viscosity. After placing a continent on top of the mantle, the convection pattern changes. The mantle flow subsequently passes through several stages, eventually resembling the mantle structure under present-day continents: (a) Extension tectonics and marginal basins form on boundary of a continent approaching to subduction zone, roll back of subduction takes place in front of moving continent; (b) The continent reaches the subduction zone, the extension regime at the continental edge is replaced by strong compression. The roll back of the subduction zone still continues after closure of the marginal basin and the continent moves towards the upwelling. As a result the ocean becomes non-symmetric and (c) The continent overrides the upwelling and subduction in its classical form stops. The third stage appears only in the upper mantle model with localized upwellings. ?? 2006 The Authors Journal compilation ?? 2006 RAS.

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.

New Insights on the Geologic Framework of Alaska and Potential Targets of Opportunity for Future Research

NASA Astrophysics Data System (ADS)

Ridgway, K.; Trop, J. M.; Finzel, E.; Brennan, P. R.; Gilbert, H. J.; Flesch, L. M.

2015-12-01

Studies the past decade have fundamentally changed our perspective on the Mesozoic and Cenozoic tectonic configuration of Alaska. New concepts include: 1) A link exists between Mesozoic collisional zones, Cenozoic strike-slip fault systems, and active deformation that is related to lithospheric heterogeneities that remain over geologic timescales. The location of the active Denali fault and high topography, for example, is within a Mesozoic collisional zone. Rheological differences between juxtaposed crustal blocks and crustal thickening in this zone have had a significant influence on deformation and exhumation in south-central Alaska. In general, the original configuration of the collisional zone appears to set the boundary conditions for long-term and active deformation. 2) Subduction of a spreading ridge has significantly modified the convergent margin of southern Alaska. Paleocene-Eocene ridge subduction resulted in surface uplift, unconformity development and changes in deposystems in the forearc region, and magmatism that extended from the paleotrench to the retroarc region. 3) Oligocene to Recent shallow subduction of an oceanic plateau has markedly reconfigured the upper plate of the southern Alaska convergent margin. This ongoing process has prompted growth of some of the largest mountain ranges on Earth, exhumation of the forearc and backarc regions above the subducted slab, development of a regional gap in arc magmatism above the subducted slab as well as slab-edge magmatism, and displacement on the Denali fault system. In the light of these new tectonic concepts for Alaska, we will discuss targets of opportunity for future integrated geologic and geophysical studies. These targets include regional strike-slip fault systems, the newly recognized Bering plate, and the role of spreading ridge and oceanic plateau subduction on the location and pace of exhumation, sedimentary basin development, and magmatism in the upper plate.

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.

Active submarine eruption of boninite in the northeastern Lau Basin

NASA Astrophysics Data System (ADS)

Resing, Joseph A.; Rubin, Kenneth H.; Embley, Robert W.; Lupton, John E.; Baker, Edward T.; Dziak, Robert P.; Baumberger, Tamara; Lilley, Marvin D.; Huber, Julie A.; Shank, Timothy M.; Butterfield, David A.; Clague, David A.; Keller, Nicole S.; Merle, Susan G.; Buck, Nathaniel J.; Michael, Peter J.; Soule, Adam; Caress, David W.; Walker, Sharon L.; Davis, Richard; Cowen, James P.; Reysenbach, Anna-Louise; Thomas, Hans

2011-11-01

Subduction of oceanic crust and the formation of volcanic arcs above the subduction zone are important components in Earth's geological and geochemical cycles. Subduction consumes and recycles material from the oceanic plates, releasing fluids and gases that enhance magmatic activity, feed hydrothermal systems, generate ore deposits and nurture chemosynthetic biological communities. Among the first lavas to erupt at the surface from a nascent subduction zone are a type classified as boninites. These lavas contain information about the early stages of subduction, yet because most subduction systems on Earth are old and well-established, boninite lavas have previously only been observed in the ancient geological record. Here we observe and sample an active boninite eruption occurring at 1,200m depth at the West Mata submarine volcano in the northeast Lau Basin, southwest Pacific Ocean. We find that large volumes of H2O, CO2 and sulphur are emitted, which we suggest are derived from the subducting slab. These volatiles drive explosive eruptions that fragment rocks and generate abundant incandescent magma-skinned bubbles and pillow lavas. The eruption has been ongoing for at least 2.5 years and we conclude that this boninite eruption is a multi-year, low-mass-transfer-rate eruption. Thus the Lau Basin may provide an important site for the long-term study of submarine volcanic eruptions related to the early stages of subduction.

Tear geometry at active STEPs: an analogue model approach

NASA Astrophysics Data System (ADS)

Broerse, Taco; Sokoutis, Dimitrios; Willingshofer, Ernst; Govers, Rob

2017-04-01

At the lateral end of a subduction zone, tearing of lithosphere is the result of subduction of oceanic lithosphere while adjacent buoyant continental lithosphere stays at the surface. The location of lithospheric tearing is called a Subduction-Transform-Edge-Propagator (STEP), which continuously extends the plate boundary between overriding plate and continental lithosphere. One of our areas of interest is the southern Caribbean where Atlantic lithosphere subducts below the Caribbean plate. Mantle tomography suggests a clear southern edge of the Lesser Antilles slab, which makes the boundary between the Caribbean and South America a clear STEP candidate. At the surface, the San Sebastián/El Pilar fault zone forms the plate boundary between the Caribbean and South America and the active STEP is located near Trinidad. For the deeper part of the damage/shear zone, some information is available from a recent 3D gravity study: significant lateral variability in densities of the lithospheric mantle to the south of the STEP fault zone. The low-density zone may result from higher sub-crustal temperatures, such as would arise from an asthenospheric window resulting from detachment. Interpreted in this way, the mantle part of the damage zone may be 200-250 km wide. So, while the location of the plate boundary at the surface is relatively well resolved, little is known about the deeper continuation of the active STEP in the mantle lithosphere. We study the evolution of the tearing process at a STEP using analogue models. In our models we use silicone putty (lithosphere) and glucose (asthenosphere). Solely gravitational forces resulting from density differences between oceanic lithosphere and asthenosphere drive our model. Lithospheric tearing commences after subduction has initiated. The geometry of the tear varies with the rheology of the lithosphere and asthenosphere, particularly Newtonian versus power-law. We investigate the dependence on model parameters of the width of the tearing zone and the depth at which tearing occurs.

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.

Aleutian Array of Arrays (A-cubed) to probe a broad spectrum of fault slip under the Aleutian Islands

NASA Astrophysics Data System (ADS)

Ghosh, A.; LI, B.

2016-12-01

Alaska-Aleutian subduction zone is one of the most seismically active subduction zones in this planet. It is characterized by remarkable along-strike variations in seismic behavior, more than 50 active volcanoes, and presents a unique opportunity to serve as a natural laboratory to study subduction zone processes including fault dynamics. Yet details of the seismicity pattern, spatiotemporal distribution of slow earthquakes, nature of interaction between slow and fast earthquakes and their implication on the tectonic behavior remain unknown. We use a hybrid seismic network approach and install 3 mini seismic arrays and 5 stand-alone stations to simultaneously image subduction fault and nearby volcanic system (Makushin). The arrays and stations are strategically located in the Unalaska Island, where prolific tremor activity is detected and located by a solo pilot array in summer 2012. The hybrid network is operational between summer 2015 and 2016 in continuous mode. One of the three arrays starts in summer 2014 and provides additional data covering a longer time span. The pilot array in the Akutan Island recorded continuous seismic data for 2 months. An automatic beam-backprojection analysis detects almost daily tremor activity, with an average of more than an hour per day. We imaged two active sources separated by a tremor gap. The western source, right under the Unalaska Island shows the most prolific activity with a hint of steady migration. In addition, we are able to identify more than 10 families of low frequency earthquakes (LFEs) in this area. They are located within the tremor source area as imaged by the bean-backprojection technique. Application of a match filter technique reveals that intervals between LFE activities are shorter during tremor activity and longer during quiet time period. We expect to present new results from freshly obtained data. The experiment A-cubed is illuminating subduction zone processes under Unalaska Island in unprecedented detail.

Subduction initiation and Obduction: insights from analog models

NASA Astrophysics Data System (ADS)

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

2013-12-01

Subduction initiation and obduction are two poorly constrained geodynamic processes which are interrelated in a number of natural settings. Subduction initiation can be viewed as the result of a regional-scale change in plate convergence partitioning between the set of existing subduction (and collision or obduction) zones worldwide. Intraoceanic subduction initiation may also ultimately lead to obduction of dense oceanic "ophiolites" atop light continental plates. A classic example is the short-lived Peri-Arabic obduction, which took place along thousands of km almost synchronously (within ~5-10 myr), from Turkey to Oman, while the subduction zone beneath Eurasia became temporarily jammed. We herein present analog models designed to study both processes and more specifically (1) subduction initiation through the partitioning of deformation between two convergent zones (a preexisting and a potential one) and, as a consequence, (2) the possible development of obduction, which has so far never been modeled. These models explore the mechanisms of subduction initiation and obduction and test various triggering hypotheses (i.e., plate acceleration, slab crossing the 660 km discontinuity, ridge subduction; Agard et al., 2007). The experimental setup comprises an upper mantle modelled as a low-viscosity transparent Newtonian glucose syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by pushing on a piston at one end of the model with plate tectonics like velocities (1-10 cm/yr) onto (i) a continental margin, (ii) a weakness zone with variable resistance and dip (W), (iii) an oceanic plate - with or without a spreading ridge, (iv) a subduction zone (S) dipping away from the piston and (v) an upper active continental margin, below which the oceanic plate is being subducted at the start of the experiment (as for the Oman case). Several configurations were tested over thirty-five parametric experiments. 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. Measurements of displacements and internal deformation allow for a very precise and reproducible tracking of deformation. Experiments consistently demonstrate that subduction initiation chiefly depends on how the overall shortening (or convergence) is partitionned between the weakness zone (W) and the preexisting subduction zone (S). Part of the deformation is transfered to W as soon as the increased coupling across S results in 5-10% of the convergence being transfered to the upper plate. Whether obduction develops further depends on the effective strength of W. Results (1) constrain the range of physical conditions required for subduction initiation and 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. [Agard P., Jolivet L., Vrielynck B., Burov E. & Monié P., 2007. Plate acceleration : the obduction trigger? Earth and Planetary Science Letters, 258, 428-441.

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.

Helium isotopic and chemical composition of gases from volcanic-hydrothermal systems in the Philippines

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

Giggenbach, W.F.; Poreda, R.J.

1993-10-01

Relative {sup 3}He, {sup 4}He, Ne, Ar, N{sub 2} and CO{sub 2} contents of gases collected from high temperature geothermal systems associated with active subduction zones in the Philippines, (Cagua, Alto Peak, Mahagnao, Mambucal and Mount Apo) correspond to {sup 3}He/{sup 4}He ratios of 6.9 to 7.6 R{sub A}, CO{sub 2}/{sup 3}He ratios of 10 to 50 {times} 10{sup 9} and N{sub 2}/Ar ratios of 200 to 500. The data suggest that He is essentially of upper mantle origin and that CO{sub 2} and N{sub 2} are predominantly derived from subducted sediments. Highest CO{sub 2}/He ratios are observed for systemsmore » along the centerline between major zones of active subduction to the E and W of the Philippine islands. The high values may tentatively be explained in terms of derivation from an especially CO{sub 2}-rich magmatic environment created by the supply of volatiles from two subduction systems. The low {sup 3}He/{sup 4}He ratios of 0.4 and 0.6 R{sub A} for two springs on the island of Palawan indicate that the thermal waters there are the result of only residual geothermal activity associated with an, at present, inactive subduction zone.« less

Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria

NASA Astrophysics Data System (ADS)

Maffione, Marco; van Hinsbergen, Douwe J. J.; de Gelder, Giovanni I. N. O.; van der Goes, Freek C.; Morris, Antony

2017-05-01

Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Suprasubduction zone ophiolites (i.e., emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers) were generated during this subduction event and are today distributed in the eastern Mediterranean region along three E-W trending ophiolitic belts. Several models have been proposed to explain the formation of these ophiolites and the evolution of the associated intra-Neo-Tethyan subduction zone. Here we present new paleospreading directions from six Upper Cretaceous ophiolites of Turkey, Cyprus, and Syria, calculated by using new and published paleomagnetic data from sheeted dyke complexes. Our results show that NNE-SSW subduction zones were formed within the Neo-Tethys during the Late Cretaceous, which we propose were part of a major step-shaped subduction system composed of NNE-SSW and WNW-ESE segments. We infer that this subduction system developed within old (Triassic?) lithosphere, along fracture zones and perpendicular weakness zones, since the Neo-Tethyan spreading ridge formed during Gondwana fragmentation would have already been subducted at the Pontides subduction zone by the Late Cretaceous. Our new results provide an alternative kinematic model of Cretaceous Neo-Tethyan subduction initiation and call for future research on the mechanisms of subduction inception within old (and cold) lithosphere and the formation of metamorphic soles below suprasubduction zone ophiolites in the absence of nearby spreading ridges.

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.

Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip.

PubMed

Rogers, Garry; Dragert, Herb

2003-06-20

We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.

Activity of Small Repeating Earthquakes along Izu-Bonin and Ryukyu Trenches

NASA Astrophysics Data System (ADS)

Hibino, K.; Matsuzawa, T.; Uchida, N.; Nakamura, W.; Matsushima, T.

2014-12-01

There are several subduction systems near the Japanese islands. The 2011 Mw9.0 Tohoku-oki megathrust earthquake occurred at the NE Japan (Tohoku) subduction zone. We have revealed a complementary relation between the slip areas for huge earthquakes and small repeating earthquakes (REs) in Tohoku. Investigations of REs in these subduction zones and the comparison with Tohoku area are important for revealing generation mechanism of megathrust earthquakes. Our target areas are Izu-Bonin and Ryukyu subduction zones, which appear to generate no large interplate earthquake. To investigate coupling of plate boundary in these regions, we estimated spatial distribution of slip rate by using REs. We use seismograms from the High Sensitivity Seismograph Network (Hi-net), Full Range Seismograph Network of Japan (F-net), and permanent seismic stations of Japan Meteorological Agency (JMA), Tohoku University, University of Tokyo, and Kagoshima University from 8 May 2003 (Izu-Bonin) and 14 July 2005 (Ryukyu) to 31 December 2012 to detect REs along the two trenches, by using similarity of seismograms. We mainly follow the procedure adopted in Uchida and Matsuzawa (2013) that studied REs in Tohoku area to compare our results with the REs in Tohoku. We find that the RE distribution along the Ryukyu trench shows two bands parallel to the trench axis. This feature is similar to the pattern in Tohoku where relatively large earthquakes occur between the bands. Along the Izu-Bonin trench, on the other hand, we find much fewer REs than in Tohoku or Ryukyu subduction zones and only one along-trench RE band, which corresponds to the area where the subducting Pacific plate contacts with the crust of the Philippine Sea plate. We also estimate average slip rate and coupling coefficient by using an empirical relationship between seismic moment and slip for REs (Nadeau and Johnson, 1998) and relative plate motion model. As a result, we find interplate slip rate in the deeper band is higher than shallower one along the Ryukyu trench suggesting larger locking along the shallower band. This feature is also similar to the pattern in the NE Japan. Our results indicate that the Ryukyu subduction zone is very similar to the NE Japan subduction zone, while the Izu-Bonin subduction zone appears to be different from the other two zones according to the RE analyses.

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.

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...

On the initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, Sierd; Wortel, Rinus; Vlaar, N. J.

1989-03-01

Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin. Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas. Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.

Shear deformation in the northeastern margin of the Izu collision zone, central Japan, inferred from GPS observations

NASA Astrophysics Data System (ADS)

Doke, R.; Harada, M.; Miyaoka, K.; Satomura, M.

2016-12-01

The Izu collision zone, which is characterized by the collision between the Izu-Bonin arc (Izu Peninsula) and the Honshu arc (the main island of Japan), is located in the northernmost part of the Philippine Sea (PHS) plate. Particularly in the northeastern margin of the zone, numerous large earthquakes have occurred. To clarify the convergent tectonics of the zone related to the occurrence of these earthquakes, in this study, we performed Global Positioning System (GPS) observations and analysis around the Izu collision zone. Based on the results of mapping the steady state of the GPS velocity and strain rate fields, we verified that there has been wide shear deformation in the northeastern part of the Izu collision zone, which agrees with the maximum shear directions in the left-lateral slip of the active faults in the study area. Based on the relative motion between the western Izu Peninsula and the eastern subducting forearc, the shear zone can be considered as a transition zone affected by both collision and subduction. The Higashi-Izu Monogenic Volcano Group, which is located in the southern part of the shear deformation zone, may have formed as a result of the steady motion of the subducting PHS plate and the collision of the Izu Peninsula with the Honshu arc. The seismic activities in the Tanzawa Mountains, which is located in the northern part of the shear deformation zone, and the eastern part of the Izu Peninsula may be related to the shear deformation zone, because the temporal patterns of the seismic activity in both areas are correlated.

Do submesoscale frontal processes ventilate the oxygen minimum zone off Peru?

NASA Astrophysics Data System (ADS)

Thomsen, S.; Kanzow, T.; Colas, F.; Echevin, V.; Krahmann, G.; Engel, A.

2016-08-01

The Peruvian upwelling system encompasses the most intense and shallowest oxygen minimum zone (OMZ) in the ocean. This system shows pronounced submesoscale activity like filaments and fronts. We carried out glider-based observations off Peru during austral summer 2013 to investigate whether submesoscale frontal processes ventilate the Peruvian OMZ. We present observational evidence for the subduction of highly oxygenated surface water in a submesoscale cold filament. The subduction event ventilates the oxycline but does not reach OMZ core waters. In a regional submesoscale-permitting model we study the pathways of newly upwelled water. About 50% of upwelled virtual floats are subducted below the mixed layer within 5 days emphasizing a hitherto unrecognized importance of subduction for the ventilation of the Peruvian oxycline.

Propagation of back-arc extension in the arc of the southern New Hebrides Subduction Zone (South West Pacific) and possible relation to subduction initiation.

NASA Astrophysics Data System (ADS)

Fabre, M.; Patriat, M.; Collot, J.; Danyushevsky, L. V.; Meffre, S.; Falloon, T.; Rouillard, P.; Pelletier, B.; Roach, M. J.; Fournier, M.

2015-12-01

Geophysical data acquired during three expeditions of the R/V Southern Surveyor allows us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone where it bends 90° eastward along the Hunter Ridge. As shown by GPS measurements and earthquake slip vectors systematically orthogonal to the trench, this 90° bend does not mark a transition from subduction to strike slip as usually observed at subduction termination. Here the convergence direction remains continuously orthogonal to the trench notwithstanding its bend. Multibeam bathymetric data acquired in the North Fiji Basin reveals active deformation and fragmentation of the upper plate. It shows the southward propagation of a N-S back-arc spreading ridge into the pre-existing volcanic arc, and the connection of the southern end of the spreading axis with an oblique active rift in the active arc. Ultimately the active arc lithosphere is sheared as spreading progressively supersedes rifting. Consequently to such incursion of back-arc basin extension into the arc, peeled off and drifted pieces of arc crust are progressively isolated into the back-arc basin. Another consequence is that the New Hebrides arc is split in two distinct microplates, which move independently relative to the lower plate, and thereby define two different subduction systems. We suggest arc fragmentation could be a consequence of the incipient collision of the Loyalty Ridge with the New Hebrides Arc. We further speculate that this kinematic change could have resulted, less than two million year ago, in the initiation of a new subduction orthogonal to the New Hebrides Subduction possibly along the paleo STEP fault. In this geodynamic setting, with an oceanic lithosphere subducting beneath a sheared volcanic arc, a particularly wide range of primitive subduction-related magmas have been produced including adakites, island arc tholeiites, back-arc basin basalts, and medium-K subduction-related lavas.

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.

Subduction zone seismicity and the thermo-mechanical evolution of downgoing lithosphere

NASA Astrophysics Data System (ADS)

Wortel, M. J. R.; Vlaar, N. J.

1988-09-01

In this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km, with emphasis on the role of temperature and rheology in controlling the deformation of, and the seismic energy release in downgoing lithosphere. This is done in two steps. After a brief review of earlier developments, we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth-dependent critical value T cr. Second, the variation of seismic energy release (frequency of events, magnitude) with depth is addressed by inferring a rheological evolution from the slab's thermal evolution and by combining this with models for the system of forces acting on the subducting lithosphere. It is found that considerable stress concentration occurs in a reheating slab in the depth range of 400 to 650 700 km: the slab weakens, but the stress level strongly increases. On the basis of this stress concentration a model is formulated for earthquake generation within subducting slabs. The model predicts a maximum depth of seismic activity in the depth range of 635 to 760 km and, for deep earthquake zones, a relative maximum in seismic energy release near the maximum depth of earthquakes. From our modelling it follows that, whereas such a maximum is indeed likely to develop in deep earthquake zones, zones with a maximum depth around 300 km (such as the Aleutians) are expected to exhibit a smooth decay in seismic energy release with depth. This is in excellent agreement with observational data. In conclusion, the incoroporation of both depth-dependent forces and depth-dependent rheology provides new insight into the generation of intermediate and deep earthquakes and into the variation of seismic activity with depth. Our results imply that no barrier to slab penetration at a depth of 650 700 km is required to explain the maximum depth of seismic activity and the pattern of seismic energy release in deep earthquake zones.

Trading Time with Space - Development of subduction zone parameter database for a maximum magnitude correlation assessment

NASA Astrophysics Data System (ADS)

Schaefer, Andreas; Wenzel, Friedemann

2017-04-01

Subduction zones are generally the sources of the earthquakes with the highest magnitudes. Not only in Japan or Chile, but also in Pakistan, the Solomon Islands or for the Lesser Antilles, subduction zones pose a significant hazard for the people. To understand the behavior of subduction zones, especially to identify their capabilities to produce maximum magnitude earthquakes, various physical models have been developed leading to a large number of various datasets, e.g. from geodesy, geomagnetics, structural geology, etc. There have been various studies to utilize this data for the compilation of a subduction zone parameters database, but mostly concentrating on only the major zones. Here, we compile the largest dataset of subduction zone parameters both in parameter diversity but also in the number of considered subduction zones. In total, more than 70 individual sources have been assessed and the aforementioned parametric data have been combined with seismological data and many more sources have been compiled leading to more than 60 individual parameters. Not all parameters have been resolved for each zone, since the data completeness depends on the data availability and quality for each source. In addition, the 3D down-dip geometry of a majority of the subduction zones has been resolved using historical earthquake hypocenter data and centroid moment tensors where available and additionally compared and verified with results from previous studies. With such a database, a statistical study has been undertaken to identify not only correlations between those parameters to estimate a parametric driven way to identify potentials for maximum possible magnitudes, but also to identify similarities between the sources themselves. This identification of similarities leads to a classification system for subduction zones. Here, it could be expected if two sources share enough common characteristics, other characteristics of interest may be similar as well. This concept technically trades time with space, considering subduction zones where we have likely not observed the maximum possible event yet. However, by identifying sources of the same class, the not-yet observed temporal behavior can be replaced by spatial similarity among different subduction zones. This database aims to enhance the research and understanding of subduction zones and to quantify their potential in producing mega earthquakes considering potential strong motion impact on nearby cities and their tsunami potential.

Structure and Deformation of the Hikurangi-Kermadec Subduction Zone - Transitions Revealed by Seismic Wide-angle Data

NASA Astrophysics Data System (ADS)

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

2008-12-01

The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate of continental character in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 deg S. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper mantle of both plates are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, whereas the data from MANGO 2 likely show thermal activity from the adjacent arc volcanism.

Ups and downs in western Crete (Hellenic subduction zone)

PubMed Central

Tiberti, Mara Monica; Basili, Roberto; Vannoli, Paola

2014-01-01

Studies of past sea-level markers are commonly used to unveil the tectonic history and seismic behavior of subduction zones. We present new evidence on vertical motions of the Hellenic subduction zone as resulting from a suite of Late Pleistocene - Holocene shorelines in western Crete (Greece). Shoreline ages obtained by AMS radiocarbon dating of seashells, together with the reappraisal of shoreline ages from previous works, testify a long-term uplift rate of 2.5–2.7 mm/y. This average value, however, includes periods in which the vertical motions vary significantly: 2.6–3.2 mm/y subsidence rate from 42 ka to 23 ka, followed by ~7.7 mm/y sustained uplift rate from 23 ka to present. The last ~5 ky shows a relatively slower uplift rate of 3.0–3.3 mm/y, yet slightly higher than the long-term average. A preliminary tectonic model attempts at explaining these up and down motions by across-strike partitioning of fault activity in the subduction zone. PMID:25022313

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.

Estimation of Peak Ground Acceleration (PGA) for Peninsular Malaysia using geospatial approach

NASA Astrophysics Data System (ADS)

Nouri Manafizad, Amir; Pradhan, Biswajeet; Abdullahi, Saleh

2016-06-01

Among the various types of natural disasters, earthquake is considered as one of the most destructive events which impose a great amount of human fatalities and economic losses. Visualization of earthquake events and estimation of peak ground motions provides a strong tool for scientists and authorities to predict and mitigate the aftereffects of earthquakes. In addition it is useful for some businesses like insurance companies to evaluate the amount of investing risk. Although Peninsular Malaysian is situated in the stable part of Sunda plate, it is seismically influenced by very active earthquake sources of Sumatra's fault and subduction zones. This study modelled the seismic zones and estimates maximum credible earthquake (MCE) based on classified data for period 1900 to 2014. The deterministic approach was implemented for the analysis. Attenuation equations were used for two zones. Results show that, the PGA produced from subduction zone is from 2-64 (gal) and from the fault zone varies from 1-191(gal). In addition, the PGA generated from fault zone is more critical than subduction zone for selected seismic model.

Difference of the seismic structure between the Hyuga-nada and the Nankai seismogenic segments

NASA Astrophysics Data System (ADS)

Yamamoto, Y.; Obana, K.; Takahashi, T.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.

2010-12-01

In the Nankai Trough, three major seismogenic zones of megathrust earthquake exist (Tokai, Tonankai and Nankai earthquake regions). The Hyuga-nada region was distinguished from these seismogenic zones because of the lack of megathrust earthquake. In the Hyuga-nada region, interplate earthquakes of M~7 occur repeatedly at intervals of about 20 years whereas no megathrust (M > 8) earthquakes had been recognized up to now. However, recent studies show the possibility of simultaneous rupture of the Tokai, Tonankai, Nankai and Hyuga-nada segments was also pointed out [e.g., Hori et al., 2009 AOGS]. To understand the possibility of seismic linkage of Nankai and Hyuga-nada segments, Japan Agency for Marine-Earth Science and Technology has been carried out a wide-angle active source survey and local seismic observation in the western end of the Nankai seismogenic zone, as a part of Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. Nakanishi et al [2009, AGU] showed that subducting Philippine Sea Plate can be divided into three zones and there is the zone of the thin oceanic crust of the subducting Philippine Sea Plate between Nankai segment and Kyushu-Palau Ridge segment by analyzing of the active source survey. Deep structure of the subducting slab is also important to consider the possibility of the seismic linkage and the location of the boundary among three zones described above. To obtain the deep seismic image, we performed a three-dimensional seismic tomography using the local seismic data recorded on 158 ocean bottom seismographs and 105 land seismic stations. From these data, we could detect 1141 earthquakes in the Hyuga-nada region. From the result of hypocenter relocation, microseismicity near the trough axis is active on the western part of the ‘thin oceanic crust’, whereas inactive on the eastern part. Besides, velocity structure of the uppermost part of the subducting slab mantle shows spatial heterogeneities. In the thin oceanic crust zone, high velocity slab mantle is imaged from near the trough to coastline. On the other hands, there is low velocity zone in the slab mantle near the trough axis in the Kyusyu-Palau Ridge segment. This low velocity zone may be related to the location of the eastern end of subducted Kyusyu-Palau Ridge.

Cenozoic lithospheric deformation in Northeast Asia and the rapidly-aging Pacific Plate

NASA Astrophysics Data System (ADS)

Yang, Ting; Moresi, Louis; Zhao, Dapeng; Sandiford, Dan; Whittaker, Joanne

2018-06-01

Northeast Asia underwent widespread rifting and magmatic events during the Cenozoic. The geodynamic origins of these tectonic events are often linked to Pacific plate subduction beneath Northeast Asia. However, the Japan Sea did not open until the late Oligocene, tens of millions of years after Pacific Plate subduction initiation in the Paleocene. Moreover, it is still not clear why the Baikal Rift Zone extension rate increased significantly after the late Miocene, while the Japan Sea opening ceased at the same time. Geodynamic models suggest these enigmatic events are related to the rapidly-aging Pacific Plate at the trench after Izanagi-Pacific spreading ridge subduction. Subduction of the young Pacific Plate delayed the Japan Sea opening during the Eocene while advection of the old Pacific Plate towards the trench increases seafloor age rapidly, allowing the Japan Sea to open after the early Miocene. The Japan Sea opening promotes fast trench retreat and slab stagnation, with subduction-induced wedge zone convection gradually increasing its extent during this process. The active rifting center associated with wedge zone convection upwelling also shifts inland-ward during slab stagnation, preventing further Japan Sea spreading while promoting the Baikal Rift Zone extension. Our geodynamic model provides a good explanation for the temporal-spatial patterns of the Cenozoic tectonic and magmatic events in Northeast Asia.

Mechanical decoupling along a subduction boundary fault: the case of the Tindari-Alfeo Fault System, Calabrian Arc (central Mediterranean Sea)

NASA Astrophysics Data System (ADS)

Maesano, F. E.; Tiberti, M. M.; Basili, R.

2017-12-01

In recent years an increasing number of studies have been focused in understanding the lateral terminations of subduction zones. In the Mediterranean region, this topic is of particular interest for the presence of a "land-locked" system of subduction zones interrupted by continental collision and back-arc opening. We present a 3D reconstruction of the area surrounding the Tindari-Alfeo Fault System (TAFS) based on a dense set of deep seismic reflection profiles. This fault system represents a major NNW-SSE trending subduction-transform edge propagator (STEP) that controls the deformation zone bounding the Calabrian subduction zone (central Mediterranean Sea) to the southwest. This 3D model allowed us to characterize the mechanical and kinematic evolution of the TAFS during the Plio-Quaternary. Our study highlights the presence of a mechanical decoupling between the deformation observed in the lower plate, constituted by the Ionian oceanic crust entering the subduction zone, and the upper plate, where a thick accretionary wedge has formed. The lower plate hosts the master faults of the TAFS, whereas the upper plate is affected by secondary deformation (bending-moment faulting, localized subsidence, stepovers, and restraining/releasing bends). The analysis of the syn-tectonic sedimentary basins related to the activity of the TAFS at depth allow us to constrain the propagation rate of the deformation and of the vertical component of the slip-rate. Our findings provide a comprehensive framework of the structural setting that can be expected along a STEP boundary where contractional and transtensional features coexist at close distance from one another.

Seismic structure off the Kii Peninsula, Japan, deduced from passive- and active-source seismographic data

NASA Astrophysics Data System (ADS)

Yamamoto, Yojiro; Takahashi, Tsutomu; Kaiho, Yuka; Obana, Koichiro; Nakanishi, Ayako; Kodaira, Shuichi; Kaneda, Yoshiyuki

2017-03-01

We conduct seismic tomography to model subsurface seismicity between 2010 and 2012 and structural heterogeneity off the Kii Peninsula, southwestern Japan, and to investigate their relationships with segmentation of the Nankai and Tonankai seismogenic zones of the Nankai Trough. In order to constrain both the shallow and deep structure of the offshore seismogenic segments, we use both active- and passive-source data recorded by both ocean-bottom seismometers and land seismic stations. The relocated microearthquakes indicate a lack of seismic activity in the Tonankai seismogenic segment off Kumano, whereas there was active intraslab seismicity in the Kii Channel area of the Nankai seismogenic segment. Based on comparisons among the distribution of seismicity, age, and spreading rate of the subducting Philippine Sea plate, and the slip-deficit distribution, we conclude that seismicity in the subducting slab under the Kii Channel region nucleated from structures in the Philippine Sea slab that pre-date subduction and that fluids released by dehydration are related to decreased interplate coupling of these intraslab earthquakes. Our velocity model clearly shows the areal extent of two key structures reported in previous 2-D active-source surveys: a high-velocity zone beneath Cape Shionomisaki and a subducted seamount off Cape Muroto, both of which are roughly circular and of 15-20 km radius. The epicenters of the 1944 Tonankai and 1946 Nankai earthquakes are near the edge of the high-velocity body beneath Cape Shionomisaki, suggesting that this anomalous structure is related to the nucleation of these two earthquakes. We identify several other high- and low-velocity zones immediately above the plate boundary in the Tonankai and Nankai seismogenic segments. In comparison with the slip-deficit model, some of the low-velocity zones appear to correspond to an area of strong coupling. Our observations suggest that, unlike the Japan Trench subduction zone, in our study area there is not a simple correspondence between areas of large coseismic slip or strong interplate coupling and areas of high velocity in the overriding plate.

Propagation of back-arc extension into the arc lithosphere in the southern New Hebrides volcanic arc

NASA Astrophysics Data System (ADS)

Patriat, M.; Collot, J.; Danyushevsky, L.; Fabre, M.; Meffre, S.; Falloon, T.; Rouillard, P.; Pelletier, B.; Roach, M.; Fournier, M.

2015-09-01

New geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 90° bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back-arc domain, the active deformation is characterized by propagation of the back-arc spreading ridge into the Hunter volcanic arc. The N-S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike-slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction-related magmas has been produced including adakites, island arc tholeiites, back-arc basin basalts, and medium-K subduction-related lavas.

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.

3D geodynamic models for the development of opposing continental subduction zones: The Hindu Kush-Pamir example

NASA Astrophysics Data System (ADS)

Liao, Jie; Gerya, Taras; Thielmann, Marcel; Webb, A. Alexander G.; Kufner, Sofia-Katerina; Yin, An

2017-12-01

The development of opposing continental subduction zones remains scantly explored in three dimensions. The Hindu Kush-Pamir orogenic system at the western end of the Himalayan orogen provides a rare example of continental collision linked to two opposing intra-continental subduction zones. The subducted plates feature a peculiar 3D geometry consisting of two distinct lithospheric fragments with different polarities, subduction angles and slab-curvatures beneath the Hindu Kush and Pamir, respectively. Using 3D geodynamic modeling, we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the Hindu Kush-Pamir orogenic system. Our geodynamic model reproduces the major tectonic elements observed: (1) the deeper subduction depth, the steeper dip angle and the southward offset of the Hindu Kush subduction zone relative to the Pamir naturally occur if convergence direction of the subducting Indian plate and dip-direction of the Hindu Kush subduction zone match. (2) The formation of the highly asymmetrically curved Pamir region and the south-dipping subduction is promoted by the initial geometry of the indenting Indian lithosphere together with the existence of a major strike-slip fault on the eastern margin of the Pamir region. (3) Subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components, and that (4) the subduction of mainly lower crust then facilitates that conditions for intermediate-depth seismicity can be reached. (5) The secondary tectonic features modeled here such as strike-slip-fault growth, north-northwest striking extension zone, and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region. (6) Model results are further compared to the potentially similar orogenic system, i.e., the Alpine orogen, in terms of the curved Western Alpine arc and the two opposing subducted slabs beneath the Alps and the Dinarides.

Diffuse Extension of the Southern Mariana Margin: Implications for Subduction Zone Infancy and Plate Tectonics

NASA Astrophysics Data System (ADS)

Martinez, F.; Stern, R. J.; Kelley, K. A.; Ohara, Y.; Sleeper, J. D.; Ribeiro, J. M.; Brounce, M. N.

2017-12-01

Opening of the southern Mariana margin takes place in contrasting modes: Extension normal to the trench forms crust that is passively accreted to a rigid Philippine Sea plate and forms along focused and broad accretion axes. Extension also occurs parallel to the trench and has split apart an Eocene-Miocene forearc terrain accreting new crust diffusely over a 150-200 km wide zone forming a pervasive volcano-tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates that the forearc extension is active over this broad area and basement samples date young although waning volcanic activity. Diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that, in a near-trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid-ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting.

Numerical Modelling of Subduction Zones: a New Beginning

NASA Astrophysics Data System (ADS)

Ficini, Eleonora; Dal Zilio, Luca; Doglioni, Carlo; Gerya, Taras V.

2016-04-01

Subduction zones are one of the most studied although still controversial geodynamic process. Is it a passive or an active mechanism in the frame of plate tectonics? How subduction initiates? What controls the differences among the slabs and related orogens and accretionary wedges? The geometry and kinematics at plate boundaries point to a "westerly" polarized flow of plates, which implies a relative opposed flow of the underlying Earth's mantle, being the decoupling located at about 100-200 km depth in the low-velocity zone or LVZ (Doglioni and Panza, 2015 and references therein). This flow is the simplest explanation for determining the asymmetric pattern of subduction zones; in fact "westerly" directed slabs are steeper and deeper with respect to the "easterly or northeasterly" directed ones, that are less steep and shallower, and two end members of orogens associated to the downgoing slabs can be distinguished in terms of topography, type of rocks, magmatism, backarc spreading or not, foredeep subsidence rate, etc.. The classic asymmetry comparing the western Pacific slabs and orogens (low topography and backarc spreading in the upper plate) and the eastern Pacific subduction zones (high topography and deep rocks involved in the upper plate) cannot be ascribed to the age of the subducting lithosphere. In fact, the same asymmetry can be recognized all over the world regardless the type and age of the subducting lithosphere, being rather controlled by the geographic polarity of the subduction. All plate boundaries move "west". Present numerical modelling set of subduction zones is based on the idea that a subducting slab is primarily controlled by its negative buoyancy. However, there are several counterarguments against this assumption, which is not able to explain the global asymmetric aforementioned signatures. Moreover, petrological reconstructions of the lithospheric and underlying mantle composition, point for a much smaller negative buoyancy than predicted, if any (e.g., Doglioni et al., 2007; Afonso et al., 2008). Therefore we attempt to generate a different model setup in which are included both a decoupling at the lithosphere base and the "westward" drift of the lithosphere that implies a relative "eastward" mantle flow. The method used for this task is an implementation of I2VIS code, a 2D thermomechanical code incorporating both a characteristics based marker-in-cell method and conservative finite-difference (FD) schemes (Gerya and Yuen, 2003). The implementation involves both the integration of the LVZ and the application of an incoming and outgoing mantle flow through the lateral boundaries of the rectangular box (that represent the basic setup of the models). This new insight in numerical modelling of subduction zones could help to have a more accurate comprehension on what is actually influencing subduction zones dynamics in order to successively explain what are the causes of this fundamental process and what are its implications on plate tectonics dynamics.

On the initiation of subduction

NASA Technical Reports Server (NTRS)

Mueller, Steve; Phillips, Roger J.

1991-01-01

Estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation. Three environments proposed as preferential sites of incipient subduction are investigated: passive continental margins, transform faults/fracture zones, and extinct ridges. None of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses. Subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins. The attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates. This is the only tectonic force sufficient to trigger the nucleation of a new subduction zone. The ubiquitous distribution of transform faults and fracture zones, combined with the common proximity of these features to mature subduction complexes, suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere.

Deep Structure of Northern Apennines Subduction Orogen (Italy) as Revealed by a Joint Interpretation of Passive and Active Seismic Data

NASA Astrophysics Data System (ADS)

Piana Agostinetti, Nicola; Faccenna, Claudio

2018-05-01

The Apennines is a well-studied orogeny formed by the accretion of continental slivers during the subduction of the Adriatic plate, but its deep structure is still a topic of controversy. Here we illuminated the deep structure of the Northern Apennines belt by combining results from the analysis of active seismic (CROP03) and receiver function data. The result from combining these two approaches provides a new robust view of the structure of the deep crust/upper mantle, from the back-arc region to the Adriatic subduction zone. Our analysis confirms the shallow Moho depth beneath the back-arc region and defines the top of the downgoing plate, showing that the two plates separate at depth about 40 km closer to the trench than reported in previous reconstructions. This spatial relationship has profound implications for the geometry of the shallow subduction zone and of the mantle wedge, by the amount of crustal material consumed at trench.

Variations in fluid transport and seismogenic properties in the Lesser Antilles subduction zone: constraints from joint active-source and local earthquake tomography

NASA Astrophysics Data System (ADS)

Paulatto, M.; Laigle, M.; Charvis, P.; Galve, A.

2015-12-01

The degree of coupling and the seismogenic properties of the plate interface at subduction zones are affected by the abundance of slab fluids and subducted sediments. High fluid input can cause high pore-fluid pressures in the subduction channel and decrease coupling leading to aseismic behaviour. Constraining fluid input and transfer is therefore important for understanding plate coupling and large earthquake hazard, particularly in places where geodetic and seismological constraints are scarce. We use P-wave traveltimes from several active source seismic experiments and P- and S-wave traveltimes from shallow and intermediate depth (< 150 km) local earthquakes recorded on a vast amphibious array of OBSs and land stations to recover the Vp and Vp/Vs structure of the central Lesser Antilles subduction zone. Our model extends between Martinique and Antigua from the prism to the arc and from the surface to a depth of 160 km. We find low Vp and high Vp/Vs ratio (> 1.80) on the top of the slab, at depths of up to 100 km. We interpret this high Vp/Vs ratio anomaly as evidence of elevated fluid content either as free fluids or as bound fluids in hydrated minerals (e.g. serpentinite). The strength and depth extent of the anomaly varies strongly from south to north along the subduction zone and correlates with variations in forearc morphology and with sediment input constrained by multi-channel seismic reflection profiles. The anomaly is stronger and extends to greater depth in the south, offshore Martinique, where sediment input is elevated due to the vicinity of the Orinoco delta. The gently dipping forearc slope observed in this region may be the result of weak coupling of the plate interface. A high Vp/Vs ratio is also observed in the forearc likely indicating a fractured and water-saturated overriding plate. On the other hand the anomaly is weaker and shallower offshore Guadeloupe, where sediment input is low due to subduction of the Barracuda ridge. Here a strong plate coupling is likely responsible for uplifting the inner forearc and formation of the Karukera spur. We infer that variations in plate coupling modulated by slab fluid transport and release are a major factor in determining the distribution of seismic slip in the Lesser Antilles subduction zone.

Drainage Asperities on Subduction Megathrusts

NASA Astrophysics Data System (ADS)

Sibson, R. H.

2012-12-01

Geophysical observations coupled with force-balance analyses suggest that the seismogenic shear zone interface of subduction megathrusts is generally fluid-overpressured to near-lithostatic values (λv = Pf/σv > 0.9) below the forearc hanging-wall, strongly modulating the profile of frictional shear resistance. Fluid sources include the accretionary prism at shallow levels and, with increasing depth, metamorphic dehydration of material entrained within the subduction shear zone together with progressive metamorphism of oceanic crust in the downgoing slab. Solution transfer in fine-grained material contained within the deeper subduction shear zone (150 < T < 350°C) likely contributes to hydrothermal sealing of fractures. A dramatic difference may therefore exist between low prefailure permeability surrounding the megathrust and high postfailure fracture permeability along the rupture zone and adjacent areas of aftershock activity. Observed postseismic changes in the velocity structure of the fore-arc hanging-wall led Husen and Kissling (2001) to propose massive fluid loss across the subduction interface following the 1995 Antofagasta, Chile, Mw8.0 megathrust rupture. Such trans-megathrust discharges represent a variant of 'fault-valve' action in which the subduction interface itself acts as a seal trapping overpressured fluids derived from metamorphic dehydration beneath. In low-permeability assemblages the maximum sustainable overpressure is limited by the activation or reactivation of brittle faults and fractures under the prevailing stress state. Highest overpressures tend to occur at low differential stress in compressional stress regimes. Loci for fluid discharge are likely determined by stress heterogeneities along the megathrust (e.g. the hangingwall of the rupture at its downdip termination). Discharge sites may be defined by swarm aftershocks defining activated fault-fracture meshes. However, fluid loss across a subduction interface will be enhanced when the stress-state in the forearc hanging-wall switches from compressional reverse-slip faulting before failure to extensional normal-slip faulting postfailure, as occurred during the 2011 Mw9.0 Tohoku megathrust rupture. Mean stress and fault-normal stress then change from being greater than vertical stress prefailure, to less than vertical stress postfailure. Postfailure reductions in overpressure are expected from a combination of poroelastic effects and fluid loss through fault-fracture networks, enhancing vertical permeability. Mineralised fault-fracture meshes in exhumed fore-arc assemblages (e.g. the Alaska-Juneau Au-quartz vein swarm) testify to the episodic discharge of substantial volumes of hydrothermal fluid (< tens of km3). Localized drainage from the subduction interface shear zone increases frictional strength significantly, giving rise to a postfailure strength asperities. Anticipated strength increases from such fluid discharge depends on the magnitude of the drop in overpressure but are potentially large (< hundreds of MPa). Time to the subsequent failure is then governed by reaccumulation of fluid overpressure as well as shear stress along the subduction interface.

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.

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.

Fore- and Back-Arc Structures Along the Hikurangi-Kermadec Subduction Zone

NASA Astrophysics Data System (ADS)

Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

2009-04-01

The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate is continental in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 degrees South. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper arc mantle are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. Here, arc volcanism is relatively active, with many large volcanoes directly on the ridge. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by the activity of the arc. The arc crust of the northern MANGO 3 becomes significantly thinner in the backarc region due to extension, and much reduced volcanism behind the ridge. The structures on MANGO 2, on the other hand, cover strong and densely spaced thermal activity from the adjacent arc volcanism, possibly linked to a recent, fluid-rich passage of the Hikurangi Plateau.

Trench curvature and deformation of the subducting lithosphere

NASA Astrophysics Data System (ADS)

Schettino, Antonio; Tassi, Luca

2012-01-01

The subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation. The downdip component of strain is associated with external forces that are applied to the slab during its sinking, namely the gravitational force and the mantle resistance to penetration. Here, we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone, independently from the long-term physical processes that have determined the actual curvature of the subduction zone. In particular, we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters, such as trench curvature, dip function and subduction velocity. We also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths, and may include large extensional lateral deformation at intermediate depths, whereas a state of lateral mechanical equilibrium can only represent a localized exception. Our formulation overcomes the flaws of the classic 'ping-pong ball' model for the bending of the lithosphere at subduction zones, which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs. A study of the geometry and seismicity of eight modern subduction zones is performed, to assess the validity of the theoretical relationship between trench curvature, slab dip function, and lateral strain rate. The strain pattern within the eight present-day slabs, which is reconstructed through an analysis of Harvard CMT solutions, shows that tectonic plates cannot be considered as flexible-inextensible spherical caps, whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms of deviations from the mechanical equilibrium.

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.

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.

Three-dimensional structure and seismicity beneath the Central Vanuatu subduction zone

NASA Astrophysics Data System (ADS)

Foix, Oceane; Crawford, Wayne; Pelletier, Bernard; Regnier, Marc; Garaebiti, Esline; Koulakov, Ivan

2017-04-01

The 1400-km long Vanuatu subduction zone results from subduction of the oceanic Australian plate (OAP) beneath the North-Fijian microplate (NFM). Seismic and volcanic activity are both high, and several morphologic features enter into subduction, affecting seismicity and probably plate coupling. The Entrecasteaux Ridge, West-Torres plateau, and Bougainville seamount currently enter into subduction below the large forearc islands of Santo and Malekula. This collision coincides with a strongly decreased local convergence velocity rate - 35 mm/yr compared to 120-160 mm/yr to the north and south - and significant uplift on the overriding plate, indicating a high degree of deformation. The close proximity of large uplifted forearc islands to the trench provides excellent coverage of the megathrust seismogenic zone for a seismological study. We used 10 months of seismological data collected using the 30-instrument land and sea ARC-VANUATU seismology network to construct a 3D velocity model — using the LOTOS joint location/model inversion software — and locate 11655 earthquakes using the NonLinLoc software suite. The 3-D model reveals low P and S velocities in the first tens of kilometers beneath both islands, probably due to water infiltration in the heavily faulted upper plate. The model also suggests the presence of a subducted seamount beneath south Santo. The earthquake locations reveal a complex interaction of faults and stress zones related to high and highly variable deformation. Both brittle deformation and the seismogenic zone depth limits vary along-slab and earthquake clusters are identified beneath central and south Santo, at about 10-30 km of depth, and southwest of Malekula island between 10-20 km depth.

Trench curvature initiation: Upper plate strain pattern and volcanism Insights from the Lesser Antilles arc, St Barthélemy Island, FWI.

NASA Astrophysics Data System (ADS)

Philippon, M. M.; Legendre, L.; Münch, P.; Léticée, J. L.; Lebrun, J. F.; Maincent, G.; Mazabraud, Y.

2017-12-01

Upper plate deformation pattern reflect the mechanical behavior of subduction zones. In this study, we focus on the consequence of the entrance of a buoyant plateau within the Caribbean subduction zone during Eocene by studying the oldest cropping out rocks of the Lesser Antilles volcanic arc. Based on novel geochronological ages and available bio-stratigraphic data we show that St Barthélemy Island was built during three successive volcanic events over the Mid- Eocene to Oligo-Miocene time span. We show that magmatism is mainly Oligocene, not Eocene. Moreover, we demonstrate that tholeitic and calc-alkaline magmatism co-existed all along the arc activity. And ultimately we evidence a westward migration of the volcanism at the island scale. Furthermore, We demonstrate that during 21 Ma, the built of theses volcanoes, the stress regime evolves from pure to radial extension with a sub-horizontal σ3 showing N30° mean trend. To conclude, our novel results invalidate the chronological, geochemical and spatial evolution of the island arc magmatism formerly proposed in the early eighties. Indeed, arc magmatism in St Barthélemy was mainly related to the West-dipping Lesser Antilles subduction zone and not to the South-dipping Greater Antilles subduction and upper plate deformation evolution observed at local scale reflects large scale mechanical behavior of the Lesser Antilles subduction zone. A two steps restoration of the regional deformation shows that the switch from pure parallel to the trench extension to radial extension within the Caribbean upper plate reflects trench curvature that followed the entrance of the Bahamas bank in the Greater Antilles subduction zone and its collision.

Identifying tectonic parameters that influence tsunamigenesis

NASA Astrophysics Data System (ADS)

van Zelst, Iris; Brizzi, Silvia; van Dinther, Ylona; Heuret, Arnauld; Funiciello, Francesca

2017-04-01

The role of tectonics in tsunami generation is at present poorly understood. However, the fact that some regions produce more tsunamis than others indicates that tectonics could influence tsunamigenesis. Here, we complement a global earthquake database that contains geometrical, mechanical, and seismicity parameters of subduction zones with tsunami data. We statistically analyse the database to identify the tectonic parameters that affect tsunamigenesis. The Pearson's product-moment correlation coefficients reveal high positive correlations of 0.65 between, amongst others, the maximum water height of tsunamis and the seismic coupling in a subduction zone. However, these correlations are mainly caused by outliers. The Spearman's rank correlation coefficient results in more robust correlations of 0.60 between the number of tsunamis in a subduction zone and subduction velocity (positive correlation) and the sediment thickness at the trench (negative correlation). Interestingly, there is a positive correlation between the latter and tsunami magnitude. In an effort towards multivariate statistics, a binary decision tree analysis is conducted with one variable. However, this shows that the amount of data is too scarce. To complement this limited amount of data and to assess physical causality of the tectonic parameters with regard to tsunamigenesis, we conduct a numerical study of the most promising parameters using a geodynamic seismic cycle model. We show that an increase in sediment thickness on the subducting plate results in a shift in seismic activity from outerrise normal faults to splay faults. We also show that the splay fault is the preferred rupture path for a strongly velocity strengthening friction regime in the shallow part of the subduction zone, which increases the tsunamigenic potential. A larger updip limit of the seismogenic zone results in larger vertical surface displacement.

Noble gases recycled into the mantle through cold subduction zones

NASA Astrophysics Data System (ADS)

Smye, Andrew J.; Jackson, Colin R. M.; Konrad-Schmolke, Matthias; Hesse, Marc A.; Parman, Steve W.; Shuster, David L.; Ballentine, Chris J.

2017-08-01

Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole - a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5-35% and 60-80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior.

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.

Reducing risk where tectonic plates collide—U.S. Geological Survey subduction zone science plan

USGS Publications Warehouse

Gomberg, Joan S.; Ludwig, Kristin A.; Bekins, Barbara; Brocher, Thomas M.; Brock, John C.; Brothers, Daniel; Chaytor, Jason D.; Frankel, Arthur; Geist, Eric L.; Haney, Matt; Hickman, Stephen H.; Leith, William S.; Roeloffs, Evelyn A.; Schulz, William H.; Sisson, Thomas W.; Wallace, Kristi; Watt, Janet; Wein, Anne M.

2017-06-19

The U.S. Geological Survey (USGS) serves the Nation by providing reliable scientific information and tools to build resilience in communities exposed to subduction zone earthquakes, tsunamis, landslides, and volcanic eruptions. Improving the application of USGS science to successfully reduce risk from these events relies on whole community efforts, with continuing partnerships among scientists and stakeholders, including researchers from universities, other government labs and private industry, land-use planners, engineers, policy-makers, emergency managers and responders, business owners, insurance providers, the media, and the general public.Motivated by recent technological advances and increased awareness of our growing vulnerability to subduction-zone hazards, the USGS is uniquely positioned to take a major step forward in the science it conducts and products it provides, building on its tradition of using long-term monitoring and research to develop effective products for hazard mitigation. This science plan provides a blueprint both for prioritizing USGS science activities and for delineating USGS interests and potential participation in subduction zone science supported by its partners.The activities in this plan address many USGS stakeholder needs:High-fidelity tools and user-tailored information that facilitate increasingly more targeted, neighborhood-scale decisions to mitigate risks more cost-effectively and ensure post-event operability. Such tools may include maps, tables, and simulated earthquake ground-motion records conveying shaking intensity and frequency. These facilitate the prioritization of retrofitting of vulnerable infrastructure;Information to guide local land-use and response planning to minimize development in likely hazardous zones (for example, databases, maps, and scenario documents to guide evacuation route planning in communities near volcanoes, along coastlines vulnerable to tsunamis, and built on landslide-prone terrain);New tools to assess the potential for cascading hazards, such as landslides, tsunamis, coastal changes, and flooding caused by earthquakes or volcanic eruptions;Geospatial models of permanent, widespread land- and sea-level changes that may occur in the immediate aftermath of great (M ≥8.0) subduction zone earthquakes;Strong partnerships between scientists and public safety providers for effective decision making during periods of elevated hazard and risk;Accurate forecasts of far-reaching hazards (for example, ash clouds, tsunamis) to avert catastrophes and unnecessary disruptions in air and sea transportation;Aftershock forecasts to guide decisions about when and where to re-enter, repair, or rebuild buildings and infrastructure, for all types of subduction zone earthquakes.

Spatial Gravity Analysis of the Cascadia Subduction Zone using Satellite Data

NASA Astrophysics Data System (ADS)

Hanatan, A.; Hartantyo, E.; Niasari, S. W.

2018-04-01

Cascadia Subduction Zone is a subduction zone elongated about 1000 km length. The remnants of Farallon plate subduct the North American plate and form this subduction area. One of Farallon plate remnants, i.e. Juan de Fuca plate, subducts dominantly the North American plate. We focused on the observation of three states, i.e. Oregon, Idaho, and Wyoming. This research aims to determine the direction, the shape, and the initial coordinates of subduction in our study area. We obtained free air corrected gravity data from TOPEX. Then we visualized data to get contour map and found that Cascadia Subduction Zone has direction from west to east that can be proofed by increasing of gravity anomaly. The gravity anomaly ranges from -140 mGals until 320 mGals. We applied upward continuation and got the result that the subduction is elongated from north to south. Initial coordinate detail of subduction shown by SVD result. The subduction starts from coordinate 46.811° Northern Hemisphere and Longitude of 123.436° into 41.260° Northern Hemisphere and longitude of -123.204°. This coordinate appropriate with the result of magnetotelluric research that shows a high resistivity. We can conclude that from gravity satellite data, we can visualize the contour map then take several steps to get details information of subduction.

Effect of Sediments on Rupture Dynamics of Shallow Subduction Zone Earthquakes and Tsunami Generation

NASA Astrophysics Data System (ADS)

Ma, S.

2011-12-01

Low-velocity fault zones have long been recognized for crustal earthquakes by using fault-zone trapped waves and geodetic observations on land. However, the most pronounced low-velocity fault zones are probably in the subduction zones where sediments on the seafloor are being continuously subducted. In this study I focus on shallow subduction zone earthquakes; these earthquakes pose a serious threat to human society in their ability in generating large tsunamis. Numerous observations indicate that these earthquakes have unusually long rupture durations, low rupture velocities, and/or small stress drops near the trench. However, the underlying physics is unclear. I will use dynamic rupture simulations with a finite-element method to investigate the dynamic stress evolution on faults induced by both sediments and free surface, and its relations with rupture velocity and slip. I will also explore the effect of off-fault yielding of sediments on the rupture characteristics and seafloor deformation. As shown in Ma and Beroza (2008), the more compliant hanging wall combined with free surface greatly increases the strength drop and slip near the trench. Sediments in the subduction zone likely have a significant role in the rupture dynamics of shallow subduction zone earthquakes and tsunami generation.

Amphibious Shear Velocity Structure of the Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Janiszewski, H. A.; Gaherty, J. B.; Abers, G. A.; Gao, H.

2017-12-01

The amphibious Cascadia Initiative crosses the coastline of the Cascadia subduction zone (CSZ) deploying seismometers from the Juan de Fuca ridge offshore to beyond the volcanic arc onshore. This allows unprecedented seismic imaging of the CSZ, enabling examination of both the evolution of the Juan de Fuca plate prior to and during subduction as well as the along strike variability of the subduction system. Here we present new results from an amphibious shear velocity model for the crust and upper mantle across the Cascadia subduction zone. The primary data used in this inversion are surface-wave phase velocities derived from ambient-noise Rayleigh-wave data in the 10 - 20 s period band, and teleseismic earthquake Rayleigh wave phase velocities in the 20 - 160 s period band. Phase velocity maps from these data reflect major tectonic structures including the transition from oceanic to continental lithosphere, Juan de Fuca lithosphere that is faster than observations in the Pacific for oceanic crust of its age, slow velocities associated with the accretionary prism, the front of the fast subducting slab, and the Cascades volcanic arc which is associated with slower velocities in the south than in the north. Crustal structures are constrained by receiver functions in the offshore forearc and onshore regions, and by active source constraints on the Juan de Fuca plate prior to subduction. The shear-wave velocities are interpreted in their relationships to temperature, presence of melt or hydrous alteration, and compositional variation of the CSZ.

Controls on continental strain partitioning above an oblique subduction zone, Northern Andes

NASA Astrophysics Data System (ADS)

Schütt, Jorina M.; Whipp, David M., Jr.

2016-04-01

Strain partitioning is a common process at obliquely convergent plate margins dividing oblique convergence into margin-normal slip on the plate-bounding fault and horizontal shearing on a strike-slip system parallel to the subduction margin. In subduction zones, strain partitioning in the upper continental plate is mainly controlled by the shear forces acting on the plate interface and the strength of the continental crust. The plate interface forces are influenced by the subducting plate dip angle and the obliquity angle between the normal to the plate margin and the convergence velocity vector, and the crustal strength of the continent is strongly affected by the presence or absence of a volcanic arc, with the presence of the volcanic arcs being common at steep subduction zones. Along the ˜7000 km western margin of South America the convergence obliquity, subduction dip angles and presence of a volcanic arc all vary, but strain partitioning is only observed along parts of it. This raises the questions, to what extent do subduction zone characteristics control strain partitioning in the overriding continental plate, and which factors have the largest influence? We address these questions using lithospheric-scale 3D numerical geodynamic experiments to investigate the influence of subduction dip angle, convergence obliquity, and weaknesses in the crust owing to the volcanic arc on strain partitioning behavior. We base the model design on the Northern Volcanic Zone of the Andes (5° N - 2° S), characterized by steep subduction (˜ 35°), a convergence obliquity between 31° -45° and extensive arc volcanism, and where strain partitioning is observed. The numerical modelling software (DOUAR) solves the Stokes flow and heat transfer equations for a viscous-plastic creeping flow to calculate velocity fields, thermal evolution, rock uplift and strain rates in a 1600 km x 1600 km box with depth 160 km. Subduction geometry and material properties are based on a simplified, generic subduction zone similar to the northern Andes. The upper surface is initially defined to resemble the Andes, but is free to deform during the experiments. We consider two main model designs, one with and one without a volcanic arc (weak continental zone). A relatively high angle of convergence obliquity is predicted to favor strain partitioning, but preliminary model results show no strain partitioning for a uniform continental crustal strength with a friction angle of Φ = 15° . However, strain partitioning does occur when including a weak zone in the continental crust resulting from arc volcanic activity with Φ = 5° . This results in margin-parallel northeastward translation of a continental sliver at 3.2 cm/year. The presence of the sliver agrees well with observations of a continental sliver identified by GPS measurements in the Northern Volcanic Zone with a translation velocity of about 1 cm/year, though the GPS-derived velocity may not be representative of the long-term rate of translation depending on whether the observation period includes one or more seismic cycles. Regardless, the observed behavior is consistent with the observed earthquake focal mechanisms and GPS measurements, suggesting significant northeastward transport of Andean crust along the margin of the northern Andes.

H2O and CO2 devolatilization in subduction zones: implications for the global water and carbon cycles (Invited)

NASA Astrophysics Data System (ADS)

van Keken, P. E.; Hacker, B. R.; Syracuse, E. M.; Abers, G. A.

2010-12-01

Subduction of sediments and altered oceanic crust functions as a major carbon sink. Upon subduction the carbon may be released by progressive metamorphic reactions, which can be strongly enhanced by free fluids. Quantification of the CO2 release from subducting slabs is important to determine the provenance of CO2 that is released by the volcanic arc and to constrain the flux of carbon to the deeper mantle. In recent work we used a global set of high resolution thermal models of subduction zones to predict the flux of H2O from the subducting slab (van Keken, Hacker, Syracuse, Abers, Subduction factory 4: Depth-dependent flux of H2O from subducting slabs worldwide, J. Geophys. Res., under review) which provides a new estimate of the dehydration efficiency of the global subducting system. It was found that mineralogically bound water can pass efficiently through old and fast subduction zones (such as in the western Pacific) but that warm subduction zones (such as Cascadia) see nearly complete dehydration of the subducting slab. The top of the slab is sufficiently hot in all subduction zones that the upper crust dehydrates significantly. The degree and depth of dehydration is highly diverse and strongly depends on (p,T) and bulk rock composition. On average about one third of subducted H2O reaches 240 km depth, carried principally and roughly equally in the gabbro and peridotite sections. The present-day global flux of H2O to the deep mantle translates to an addition of about one ocean mass over the age of the Earth. We extend the slab devolatilization work to carbon by providing an update to Gorman et al. (Geochem. Geophys. Geosyst, 2006), who quantified the effects of free fluids on CO2 release. The thermal conditions were based on three end-member subduction zones with linear interpolation to provide a global CO2 flux. We use the new high resolution and global set of models to provide higher resolution predictions for the provenance and pathways of CO2 release to the mantle wedge and a more robust prediction of the global CO2 flux in subduction.

Wedge geometry, frictional properties and interseismic coupling of the Java megathrust

NASA Astrophysics Data System (ADS)

Koulali, Achraf; McClusky, Simon; Cummins, Phil; Tregoning, Paul

2018-06-01

The mechanical interaction between rocks at fault zones is a key element for understanding how earthquakes nucleate and propagate. Therefore, estimating frictional properties along fault planes allows us to infer the degree of elastic strain accumulation throughout the seismic cycle. The Java subduction zone is an active plate boundary where high seismic activity has long been documented. However, very little is known about the seismogenic processes of the megathrust, especially its shallowest portion where onshore geodetic networks are insensitive to recover the pattern of elastic strain. Here, we use the geometry of the offshore accretionary prism to infer frictional properties along the Java subduction zone, using Coulomb critical taper theory. We show that large portions of the inner wedge in the eastern part of the Java subduction megathrust are in a critical state, where the wedge is on the verge of failure everywhere. We identify four clusters with an internal coefficient of friction μint of ∼ 0.8 and hydrostatic pore pressure within the wedge. The average effective coefficient of friction ranges between 0.3 and 0.4, reflecting a strong décollement. Our results also show that the aftershock sequence of the 1994 Mw 7.9 earthquake halted adjacent to a critical segment of the wedge, suggesting that critical taper wedge areas in the eastern Java subduction interface may behave as a permanent barrier to large earthquake rupture. In contrast, in western Java topographic slope and slab dip profiles suggest that the wedge is mechanically stable, i.e deformation is restricted to sliding along the décollement, and likely to coincide with a seismogenic portion of the megathrust. We discuss the seismic hazard implications and highlight the importance of considering the segmentation of the Java subduction zone when assessing the seismic hazard of this region.

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.

Subduction in the Southern Caribbean

NASA Astrophysics Data System (ADS)

Levander, A.; Schmitz, M.; Bezada, M.; Masy, J.; Niu, F.; Pindell, J.

2012-04-01

The southern Caribbean is bounded at either end by subduction zones: In the east at the Lesser Antilles subduction zone the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west under the Southern Caribbean Deformed Belt accretionary prism, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system. A variety of seismic probes identify where the two plates tear as they begin to subduct (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Masy et al, 2009). The El Pilar system forms at the southeastern corner of the Antilles subduction zone by the Atlantic tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. In northwestern South America the Caribbean plate tears, its southernmost element subducting at shallow angles under northernmost Colombia and then rapidly descending to transition zone depths under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab produces the Merida Andes, the Perija, and the Santa Marta ranges. The southern edge of the nonsubducting Caribbean plate underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009). We infer that the underthrust Caribbean plate supports the coastal mountains, and controls continuing deformation.

Implications for metal and volatile cycles from the pH of subduction zone fluids

NASA Astrophysics Data System (ADS)

Galvez, Matthieu E.; Connolly, James A. D.; Manning, Craig E.

2016-11-01

The chemistry of aqueous fluids controls the transport and exchange—the cycles—of metals and volatile elements on Earth. Subduction zones, where oceanic plates sink into the Earth’s interior, are the most important geodynamic setting for this fluid-mediated chemical exchange. Characterizing the ionic speciation and pH of fluids equilibrated with rocks at subduction zone conditions has long been a major challenge in Earth science. Here we report thermodynamic predictions of fluid-rock equilibria that tie together models of the thermal structure, mineralogy and fluid speciation of subduction zones. We find that the pH of fluids in subducted crustal lithologies is confined to a mildly alkaline range, modulated by rock volatile and chlorine contents. Cold subduction typical of the Phanerozoic eon favours the preservation of oxidized carbon in subducting slabs. In contrast, the pH of mantle wedge fluids is very sensitive to minor variations in rock composition. These variations may be caused by intramantle differentiation, or by infiltration of fluids enriched in alkali components extracted from the subducted crust. The sensitivity of pH to soluble elements in low abundance in the host rocks, such as carbon, alkali metals and halogens, illustrates a feedback between the chemistry of the Earth’s atmosphere-ocean system and the speciation of subduction zone fluids via the composition of the seawater-altered oceanic lithosphere. Our findings provide a perspective on the controlling reactions that have coupled metal and volatile cycles in subduction zones for more than 3 billion years7.

Neogene collision and deformation of convergent margins along the backbone of the Americas

USGS Publications Warehouse

von Huene, Roland E.; Ranero, C.R.

2009-01-01

Along Pacific convergent margins of the Americas, high-standing relief on the subducting oceanic plate "collides" with continental slopes and subducts. Features common to many collisions are uplift of the continental margin, accelerated seafloor erosion, accelerated basal subduction erosion, a flat slab, and a lack of active volcanism. Each collision along America's margins has exceptions to a single explanation. Subduction of an ???600 km segment of the Yakutat terrane is associated with >5000-m-high coastal mountains. The terrane may currently be adding its unsubducted mass to the continent by a seaward jump of the deformation front and could be a model for docking of terranes in the past. Cocos Ridge subduction is associated with >3000-m-high mountains, but its shallow subduction zone is not followed by a flat slab. The entry point of the Nazca and Juan Fernandez Ridges into the subduction zone has migrated southward along the South American margin and the adjacent coast without unusually high mountains. The Nazca Ridge and Juan Fernandez Ridges are not actively spreading but the Chile Rise collision is a triple junction. These collisions form barriers to trench sediment transport and separate accreting from eroding segments of the frontal prism. They also occur at the separation of a flat slab from a steeply dipping one. At a smaller scale, the subduction of seamounts and lesser ridges causes temporary surface uplift as long as they remain attached to the subducting plate. Off Costa Rica, these features remain attached beneath the continental shelf. They illustrate, at a small scale, the processes of collision. ?? 2009 The Geological Society of America. All rights reserved.

Deformation and stress change associated with plate interaction at subduction zones: a kinematic modelling

NASA Astrophysics Data System (ADS)

Zhao, Shaorong; Takemoto, Shuzo

2000-08-01

The interseismic deformation associated with plate coupling at a subduction zone is commonly simulated by the steady-slip model in which a reverse dip-slip is imposed on the down-dip extension of the locked plate interface, or by the backslip model in which a normal slip is imposed on the locked plate interface. It is found that these two models, although totally different in principle, produce similar patterns for the vertical deformation at a subduction zone. This suggests that it is almost impossible to distinguish between these two models by analysing only the interseismic vertical deformation observed at a subduction zone. The steady-slip model cannot correctly predict the horizontal deformation associated with plate coupling at a subduction zone, a fact that is proved by both the numerical modelling in this study and the GPS (Global Positioning System) observations near the Nankai trough, southwest Japan. It is therefore inadequate to simulate the effect of the plate coupling at a subduction zone by the steady-slip model. It is also revealed that the unphysical assumption inherent in the backslip model of imposing a normal slip on the locked plate interface makes it impossible to predict correctly the horizontal motion of the subducted plate and the stress change within the overthrust zone associated with the plate coupling during interseismic stages. If the analysis made in this work is proved to be correct, some of the previous studies on interpreting the interseismic deformation observed at several subduction zones based on these two models might need substantial revision. On the basis of the investigations on plate interaction at subduction zones made using the finite element method and the kinematic/mechanical conditions of the plate coupling implied by the present plate tectonics, a synthesized model is proposed to simulate the kinematic effect of the plate interaction during interseismic stages. A numerical analysis shows that the proposed model, designed to simulate the motion of a subducted slab, can correctly produce the deformation and the main pattern of stress concentration associated with plate coupling at a subduction zone. The validity of the synthesized model is examined and partially verified by analysing the horizontal deformation observed by GPS near the Nankai trough, southwest Japan.

Multidisciplinary Observations of Subduction (MOOS) Experiment in South-Central Alaska

NASA Astrophysics Data System (ADS)

Christensen, D.; Abers, G.; Freymueller, J.

2008-12-01

Seismic and geodetic data are being collected in the Kenai Peninsula and surrounding area of south central Alaska as part of the PASSCAL experiment MOOS. A total of 34 broadband seismic stations were deployed between the summers of 2007 and 2008. Seventeen of these stations continue to operate for an additional year and are scheduled to be removed in the summer of 2009. Numerous GPS campaign sites have and will be visited during the same time period. The MOOS seismic deployment provides coverage across the interplate coupled zone and adjacent transition zone in the shallow parts of the Alaskan subduction zone. It is a southern extension of an earlier broadband deployment BEAAR (Broadband Experiment Across the Alaska Range) to the north. When integrated with the previous BEAAR experiment, these data will allow high-resolution broadband imaging along a 600 km long transect over the Alaska subduction zone, at 10-15 km station spacing. The MOOS deployment allows us to test several hypotheses relating to the postulated subduction of the Yakutat Block and the nature of the coupled zone which ruptured in the great 1964 earthquake. The seismic and geodetic stations cover an area that includes part of the 1964 main asperity and the adjacent, less coupled, region to the southwest. Data gathered from this experiment will shed light on the nature of this boundary from both a geodetic and seismic (or earth structure) perspective. Shallow seismicity recorded by this network greatly improves the catalog of events in this area and helps to delineate active features in the subduction complex. Preliminary results from this project will be presented.

Seismicity of the Earth 1900-2013, seismotectonics of South America (Nazca Plate Region)

USGS Publications Warehouse

Hayes, Gavin P.; Smoczyk, Gregory M.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio

2015-01-01

The South American arc extends over 7,000 kilometers (km), from the Chilean margin triple junction offshore of southern Chile, to its intersection with the Panama fracture zone, offshore of the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their descent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate, the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 millimeters/year (mm/yr) in the south, to approximately 65 mm/yr in the north. Although the rate of subduction varies little along the entire arc, there are complex changes in the geologic processes along the subduction zone that dramatically influence volcanic activity, crustal deformation, earthquake generation and occurrence all along the western edge of South America.

P and S wave attenuation tomography of the Japan subduction zone

NASA Astrophysics Data System (ADS)

Wang, Zewei; Zhao, Dapeng; Liu, Xin; Chen, Chuanxu; Li, Xibing

2017-04-01

We determine the first high-resolution P and S wave attenuation (Q) tomography beneath the entire Japan Islands using a large number of high-quality t∗ data collected from P and S wave velocity spectra of 4222 local shallow and intermediate-depth earthquakes. The suboceanic earthquakes used in this study are relocated precisely using sP depth phases. Significant landward dipping high-Q zones are revealed clearly, which reflect the subducting Pacific slab beneath Hokkaido and Tohoku, and the subducting Philippine Sea (PHS) slab beneath SW Japan. Prominent low-Q zones are visible in the crust and mantle wedge beneath the active arc volcanoes in Hokkaido, Tohoku, and Kyushu, which reflect source zones of arc magmatism caused by fluids from the slab dehydration and corner flow in the mantle wedge. Our results also show that nonvolcanic low-frequency earthquakes (LFEs) in SW Japan mainly occur in the transition zone between a narrow low-Q belt and its adjacent high-Q zones right above the flat segment of the PHS slab. This feature suggests that the nonvolcanic LFEs are caused by not only fluid-affected slab interface but also specific conditions such as high pore pressure which is influenced by the overriding plate.

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.

Subduction and dehydration of slow-spread oceanic lithosphere

NASA Astrophysics Data System (ADS)

Paulatto, M.; Laigle, M.; Galve, A.; Charvis, P.

2016-12-01

Water transported by subducting slabs affects the dynamics of subduction zones and is a major gateway in the global geochemical water cycle. During subduction much of the water stored in the slab is released via pore fluid escape and through metamorphic reactions that depend on the thermal regime. The most notable are eclogitization of hydrated basalt and gabbro and breakdown of serpentinite. Most constraints to date have been obtained at Pacific subduction zones, and have contributed to a model of slab dehydration applicable to normal fast-spread oceanic lithosphere with a mafic crust. Slow-spread crust however, is heterogeneous in thickness and composition and has a different water distribution than fast-spread crust. We use P-wave traveltimes from several active source seismic experiments and P- and S-wave traveltimes from shallow and intermediate depth (< 160 km) local earthquakes recorded on a vast amphibious array of OBSs and land seismometers to recover the 3D Vp and Vp/Vs structure of the central Lesser Antilles subduction zone from the surface to 160 km depth. This slab was formed by slow accretion at the Mid-Atlantic ridge and represents the global slow accretion rate end-member. We image the dipping low-Vp layer at the top of the slab corresponding to the hydrated slab crust penetrating to about 100 km depth. High Vp/Vs ratio on the slab top and in the forearc crust is interpreted as evidence of elevated fluid content either as free fluids or as bound water in hydrated minerals. A local minimum in Vp is observed on the slab top at 50 km depth, and forms an elongated trench-parallel anomaly. This anomaly is interrupted at the projection of the Marathon fracture zone. We suggest that this is the result of lateral variations in slab crust composition from normal mafic oceanic crust to tectonized oceanic crust consisting to a large extent of serpentinized peridotite near the fracture zone. Slab regions with normal mafic oceanic crust likely undergo eclogitization, resulting in voluminous water release over a narrow depth range. Serpentinized ultramafic crust, in contrast, may release water at a more constant rate. We infer that subduction of slow-spread lithosphere may result in heterogeneous water transport and release at subduction zones with implications for seismicity, magma generation and the geochemical budget.

Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up

PubMed Central

Kelemen, Peter B.; Manning, Craig E.

2015-01-01

Carbon fluxes in subduction zones can be better constrained by including new estimates of carbon concentration in subducting mantle peridotites, consideration of carbonate solubility in aqueous fluid along subduction geotherms, and diapirism of carbon-bearing metasediments. Whereas previous studies concluded that about half the subducting carbon is returned to the convecting mantle, we find that relatively little carbon may be recycled. If so, input from subduction zones into the overlying plate is larger than output from arc volcanoes plus diffuse venting, and substantial quantities of carbon are stored in the mantle lithosphere and crust. Also, if the subduction zone carbon cycle is nearly closed on time scales of 5–10 Ma, then the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing. Such an increase is consistent with inferences from noble gas data. Carbon in diamonds, which may have been recycled into the convecting mantle, is a small fraction of the global carbon inventory. PMID:26048906

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.

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology.

PubMed

Keenan, Timothy E; Encarnación, John; Buchwaldt, Robert; Fernandez, Dan; Mattinson, James; Rasoazanamparany, Christine; Luetkemeyer, P Benjamin

2016-11-22

Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence.

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

PubMed Central

Keenan, Timothy E.; Encarnación, John; Buchwaldt, Robert; Fernandez, Dan; Mattinson, James; Rasoazanamparany, Christine; Luetkemeyer, P. Benjamin

2016-01-01

Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence. PMID:27821756

Upper Pleistocene uplifted shorelines as tracers of (local rather than global) subduction dynamics

NASA Astrophysics Data System (ADS)

Henry, Hadrien; Regard, Vincent; Pedoja, Kevin; Husson, Laurent; Martinod, Joseph; Witt, Cesar; Heuret, Arnauld

2014-08-01

Past studies have shown that high coastal uplift rates are restricted to active areas, especially in a subduction context. The origin of coastal uplift in subduction zones, however, has not yet been globally investigated. Quaternary shorelines correlated to the last interglacial maximum (MIS 5e) were defined as a global tectonic benchmark (Pedoja et al., 2011). In order to investigate the relationships between the vertical motion and the subduction dynamic parameters, we cross-linked this coastal uplift database with the “geodynamical” databases from Heuret (2005), Conrad and Husson (2009) and Müller et al. (2008). Our statistical study shows that: (1) the most intuitive parameters one can think responsible for coastal uplift (e.g., subduction obliquity, trench motion, oceanic crust age, interplate friction and force, convergence variation, dynamic topography, overriding and subducted plate velocity) are not related with the uplift (and its magnitude); (2) the only intuitive parameter is the distance to the trench which shows in specific areas a decrease from the trench up to a distance of ˜300 km; (3) the slab dip (especially the deep slab dip), the position along the trench and the overriding plate tectonic regime are correlated with the coastal uplift, probably reflecting transient changes in subduction parameters. Finally we conclude that the first order parameter explaining coastal uplift is small-scale heterogeneities of the subducting plate, as for instance subducting aseismic ridges. The influence of large-scale geodynamic setting of subduction zones is secondary.

Subduction and Plate Edge Tectonics in the Southern Caribbean

NASA Astrophysics Data System (ADS)

Levander, A.; Schmitz, M.; Niu, F.; Bezada, M. J.; Miller, M. S.; Masy, J.; Ave Lallemant, H. G.; Pindell, J. L.

2012-12-01

The southern Caribbean plate boundary consists of a subduction zone at at either end connected by a strike-slip fault system: In the east at the Lesser Antilles subduction zone, the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west in the Colombia basin, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system that has been cut off at the Bocono fault, the southeastern boundary of the Maracaibo block. A variety of seismic probes identify where the two plates tear as they begin to subduct (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Growdon et al., 2009; Huang et al., 2010; Masy et al., 2011). The El Pilar system forms at the southeastern corner of the Antilles subduction zone with the Atlantic plate tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. In northwestern South America the Caribbean plate very likely also tears, as its southernmost element subducts at shallow angles under northernmost Colombia and the northern, nonsubducting part underthrusts the continental edge. The subducting segment rapidly descends to transition zone depths under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab produces the Merida Andes, the Perija, and the Santa Marta ranges. The nonsubducting part of the Caribbean plate underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009), where the plate edge supports the coastal mountains, and controls continuing deformation.

Multivariate statistical analysis to investigate the subduction zone parameters favoring the occurrence of giant megathrust earthquakes

NASA Astrophysics Data System (ADS)

Brizzi, S.; Sandri, L.; Funiciello, F.; Corbi, F.; Piromallo, C.; Heuret, A.

2018-03-01

The observed maximum magnitude of subduction megathrust earthquakes is highly variable worldwide. One key question is which conditions, if any, favor the occurrence of giant earthquakes (Mw ≥ 8.5). Here we carry out a multivariate statistical study in order to investigate the factors affecting the maximum magnitude of subduction megathrust earthquakes. We find that the trench-parallel extent of subduction zones and the thickness of trench sediments provide the largest discriminating capability between subduction zones that have experienced giant earthquakes and those having significantly lower maximum magnitude. Monte Carlo simulations show that the observed spatial distribution of giant earthquakes cannot be explained by pure chance to a statistically significant level. We suggest that the combination of a long subduction zone with thick trench sediments likely promotes a great lateral rupture propagation, characteristic of almost all giant earthquakes.

Oceanic crust in the mid-mantle beneath Central-West Pacific subduction zones: Evidence from S-to-P converted waveforms

NASA Astrophysics Data System (ADS)

He, X.

2015-12-01

The fate of subducted slabs is enigmatic, yet intriguing. We analyze seismic arrivals at ~20-50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analyzing techniques of 4th root vespagram and beam-form analysis to constrain the slowness and back azimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the back azimuth deviates slightly from the great-circle direction. Along with calculation of one-dimensional synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S-to-P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ~700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

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.

Visualizing Three-dimensional Slab Geometries with ShowEarthModel

NASA Astrophysics Data System (ADS)

Chang, B.; Jadamec, M. A.; Fischer, K. M.; Kreylos, O.; Yikilmaz, M. B.

2017-12-01

Seismic data that characterize the morphology of modern subducted slabs on Earth suggest that a two-dimensional paradigm is no longer adequate to describe the subduction process. Here we demonstrate the effect of data exploration of three-dimensional (3D) global slab geometries with the open source program ShowEarthModel. ShowEarthModel was designed specifically to support data exploration, by focusing on interactivity and real-time response using the Vrui toolkit. Sixteen movies are presented that explore the 3D complexity of modern subduction zones on Earth. The first movie provides a guided tour through the Earth's major subduction zones, comparing the global slab geometry data sets of Gudmundsson and Sambridge (1998), Syracuse and Abers (2006), and Hayes et al. (2012). Fifteen regional movies explore the individual subduction zones and regions intersecting slabs, using the Hayes et al. (2012) slab geometry models where available and the Engdahl and Villasenor (2002) global earthquake data set. Viewing the subduction zones in this way provides an improved conceptualization of the 3D morphology within a given subduction zone as well as the 3D spatial relations between the intersecting slabs. This approach provides a powerful tool for rendering earth properties and broadening capabilities in both Earth Science research and education by allowing for whole earth visualization. The 3D characterization of global slab geometries is placed in the context of 3D slab-driven mantle flow and observations of shear wave splitting in subduction zones. These visualizations contribute to the paradigm shift from a 2D to 3D subduction framework by facilitating the conceptualization of the modern subduction system on Earth in 3D space.

A non extensive statistical physics analysis of the Hellenic subduction zone seismicity

NASA Astrophysics Data System (ADS)

Vallianatos, F.; Papadakis, G.; Michas, G.; Sammonds, P.

2012-04-01

The Hellenic subduction zone is the most seismically active region in Europe [Becker & Meier, 2010]. The spatial and temporal distribution of seismicity as well as the analysis of the magnitude distribution of earthquakes concerning the Hellenic subduction zone, has been studied using the concept of Non-Extensive Statistical Physics (NESP) [Tsallis, 1988 ; Tsallis, 2009]. Non-Extensive Statistical Physics, which is a generalization of Boltzmann-Gibbs statistical physics, seems a suitable framework for studying complex systems (Vallianatos, 2011). Using this concept, Abe & Suzuki (2003;2005) investigated the spatial and temporal properties of the seismicity in California and Japan and recently Darooneh & Dadashinia (2008) in Iran. Furthermore, Telesca (2011) calculated the thermodynamic parameter q of the magnitude distribution of earthquakes of the southern California earthquake catalogue. Using the external seismic zones of 36 seismic sources of shallow earthquakes in the Aegean and the surrounding area [Papazachos, 1990], we formed a dataset concerning the seismicity of shallow earthquakes (focal depth ≤ 60km) of the subduction zone, which is based on the instrumental data of the Geodynamic Institute of the National Observatory of Athens (http://www.gein.noa.gr/, period 1990-2011). The catalogue consists of 12800 seismic events which correspond to 15 polygons of the aforementioned external seismic zones. These polygons define the subduction zone, as they are associated with the compressional stress field which characterizes a subducting regime. For each event, moment magnitude was calculated from ML according to the suggestions of Papazachos et al. (1997). The cumulative distribution functions of the inter-event times and the inter-event distances as well as the magnitude distribution for each seismic zone have been estimated, presenting a variation in the q-triplet along the Hellenic subduction zone. The models used, fit rather well to the observed distributions, implying the complexity of the spatiotemporal properties of seismicity and the usefulness of NESP in investigating such phenomena, exhibiting scale-free nature and long range memory effects. Acknowledgments. This work was supported in part by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled "Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non extensive statistical physics - Application to the geodynamic system of the Hellenic Arc. SEISMO FEAR HELLARC". GM and GP wish to acknowledge the partial support of the Greek State Scholarships Foundation (ΙΚΥ).

Evolution of a Subduction Zone

NASA Astrophysics Data System (ADS)

Noack, Lena; Van Hoolst, Tim; Dehant, Veronique

2014-05-01

The purpose of this study is to understand how Earth's surface might have evolved with time and to examine in a more general way the initiation and continuance of subduction zones and the possible formation of continents on an Earth-like planet. Plate tectonics and continents seem to influence the likelihood of a planet to harbour life, and both are strongly influenced by the planetary interior (e.g. mantle temperature and rheology) and surface conditions (e.g. stabilizing effect of continents, atmospheric temperature), but may also depend on the biosphere. Employing the Fortran convection code CHIC (developed at the Royal Observatory of Belgium), we simulate a subduction zone with a pre-defined weak zone (between oceanic and continental crust) and a fixed plate velocity for the subducting oceanic plate (Quinquis et al. in preparation). In our study we first investigate the main factors that influence the subduction process. We simulate the subduction of an oceanic plate beneath a continental plate (Noack et al., 2013). The crust is separated into an upper crust and a lower crust. We apply mixed Newtonian/non-Newtonian rheology and vary the parameters that are most likely to influence the subduction of the ocanic plate, as for example density of the crust/mantle, surface temperature, plate velocity and subduction angle. The second part of our study concentrates on the long-term evolution of a subduction zone. Even though we model only the upper mantle (until a depth of 670km), the subducted crust is allowed to flow into the lower mantle, where it is no longer subject to our investigation. This way we can model the subduction zone over long time spans, for which we assume a continuous inflow of the oceanic plate into the investigated domain. We include variations in mantle temperatures (via secular cooling and decay of radioactive heat sources) and dehydration of silicates (leading to stiffening of the material). We investigate how the mantle environment influences the subduction of the oceanic crust in terms of subduction velocity and subduction angle over time. We develop scaling laws combining the subduction velocity and angle depending on the mantle environment (and thus time). These laws can then be applied to continental growth simulations with 1D parameterized models (Höning et al., in press) or 2D/3D subduction zone simulations at specific geological times (using the correct subduction zone setting). References: Quinquis, M. et al. (in preparation). A comparison of thermo-mechanical subduction models. In preparation for G3. Noack, L., Van Hoolst, T., Dehant, V., and Breuer, D. (2013). Relevance of continents for habitability and self-consistent formation of continents on early Earth. XIII International Workshop on Modelling of Mantle and Lithosphere Dynamics, Hønefoss, Norway, 31. Aug. - 5. Sept. 2013. Höning, D., Hansen-Goos, H., Airo, A., and Spohn, T. (in press). Biotic vs. abiotic Earth: A model for mantle hydration and continental coverage. Planetary and Space Science.

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.

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.

Possibility of existence of serpentinized material at the Izu-Bonin subduction plate boundary around 31N using Q structure by FDM-simulation

NASA Astrophysics Data System (ADS)

Kamimura, A.; Kasahara, J.

2003-12-01

At the Izu-Bonin subduction zone (IBSZ), there is a chain of serpentine seamounts at the forearc slope of trench axis, and few large earthquakes occurred at shallow depth (<100km) in spite of many large ones at greater depth (>400km). To elucidate these characteristics we carried out a seismic refraction-reflection study at the forearc slope of the IBSZ around 31N using 22 OBSs and chemical explosives and airguns as seismic sources in 1998. As the results of forward and travel-time inversion modeling of the study, P-wave velocity structures were obtained along E-W and N-S survey lines which is perpendicular to and parallel to the trench axis, respectively (Kamimura et al., 2002). The result of E-W line (transect a summit of serpentine seamount) suggests presence of a low velocity zone just above the subducting Pacific plate, and this zone connects to the Torishima Serpentine Forearc Seamount. The interpretation of the result was: dehydration of hydrated oceanic crust supplies water to the mantle wedge, and peridotites of the mantle wedge were serpentinized. The serpentinized peridotites have moved between the oceanic slab and the overriding island arc crust and were diapiring into the serpentine seamount. The serpentine on the plate boundary might act as a lubricant and decrease seismic activity along the subduction zone, and this can explain the characteristics of seismicity of IBSZ. In order to evaluate Q structures of the above low velocity zone on the subducting slab, we calculated synthetic waveforms using FDM (Finite Difference Method) with elastodynamic formulation (E3D code, developed by Dr. Shawn Larsen) and the P-wave velocity 2D structure of Kamimura et al. (2002). The E3D uses staggered grid, and 2nd order and 4th order approximation in time and space, respectively. Grid spacing of the calculation is 30 m in x and z, and 1.5 msec in time. Five-Hz and 0-phase Ricker wavelet_@pressure source was used. Several structure models are used for comparison. One model has no low-Q zone, another one has low-Q zone only just below the serpentine seamount. Other models have low-Q zones just below the serpentine seamount and above the subducting slab, horizontal width of the low-Q zone are different one another. Comparing synthetic waveforms and observed data, we can conclude that there must be a low-Q zone just below the serpentine seamount and on the subducting oceanic slab. The low-Q zone on the slab has ca. 80 km wide east to west and connects to the serpentine seamount. It is very important to understand where serpentinites of the seamounts came from to explain the characteristics of seismicity at the IBSZ. In this presentation we are going to explain an interpretation that serpentine moved through the plate boundary and reached just below the serpentine seamount, using an existence of the low-Q zone. Kamimura, A., Kasahara, J., Masanao S., Hino, R., Shiobara, H., Fujie, G., Kanazawa, T., 2002. Crustal structure study at the Izu-Bonin subduction zone around 31° N: implications of serpentinized materials along the subduction plate boundary, Physics of the Earth and Planetary Interiors, 132, 105-129.

Diapir versus along-channel ascent of crustal material during plate convergence: constrained by the thermal structure of subduction zones

NASA Astrophysics Data System (ADS)

Liu, M. Q.; Li, Z. H.

2017-12-01

Crustal rocks can be subducted to mantle depths, interact with the mantle wedge, and then exhume to the crustal depth again, which is generally considered as the mechanism for the formation of ultrahigh-pressure metamorphic rocks in nature. The crustal rocks undergo dehydration and melting at subarc depths, giving rise to fluids that metasomatize and weaken the overlying mantle wedge. There are generally two ways for the material ascent from subarc depths: one is along subduction channel; the other is through the mantle wedge by diapir. In order to study the conditions and dynamics of these contrasting material ascent modes, systematic petrological-thermo-mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates, ages of the subducting oceanic plate, as well as the plate convergence rates. The model results suggest that the thermal structures of subduction zones control the thermal condition and fluid/melt activity at the slab-mantle interface in subcontinental subduction channels, which further strongly affect the material transportation and ascent mode. Thick overriding continental plate and low-angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge, where the along-channel exhumation occurs exclusively to result in the exhumation of HP-UHP metamorphic rocks. In contrast, thin overriding lithosphere and steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels, which lead to significant hydration and metasomatism, melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge-derived melts by diapir through the mantle wedge. This may corresponds to the origination of continental arc volcanism from mafic to ultramafic metasomatites in the bottom of the mantle wedge. In addition, the plate convergence rate can also affect the material ascent mode, e.g., diapiric extrusion versus along-channel exhumation, by changing the amount of supracrustal rocks carried into the subduction channels, which further regulate the fluid/melt activity and thermo-rheological properties.

A comparison of seismicity in world's subduction zones: Implication by the difference of b-values

NASA Astrophysics Data System (ADS)

Nishikawa, T.; Ide, S.

2013-12-01

Since the pioneering study of Uyeda and Kanamori (1979), it has been thought that world's subduction zones can be classified into two types: Chile and Mariana types. Ruff and Kanamori (1980) suggested that the maximum earthquake size within each subduction zone correlates with convergence rate and age of subducting lithosphere. Subduction zones with younger lithosphere and larger convergence rates are associated with great earthquakes (Chile), while subduction zones with older lithosphere and smaller convergence rates have low seismicity (Mariana). However, these correlations are obscured after the 2004 Sumatra earthquake and the 2009 Tohoku earthquake. Furthermore, McCaffrey (2008) pointed out that the history of observation is much shorter than the recurrence times of very large earthquakes, suggesting a possibility that any subduction zone may produce earthquakes larger than magnitude 9. In the present study, we compare world's subduction zones in terms of b-values in the Gutenberg-Richer relation. We divided world's subduction zones into 146 regions, each of which is bordered by a trench section of about 500 km and extends for 200 km from the trench section in the direction of relative plate motion. In each region, earthquakes equal to or larger than M4.5 occurring during 1988-2009 were extracted from ISC catalog. We find a positive correlation between b-values and ages of subducting lithosphere, which is one of the two important variables discussed in Ruff and Kanamori (1980). Subduction zones with younger lithosphere are associated with high b-values and vice versa, while we cannot find a correlation between b-values and convergence rates. We used the ages determined by Müller et al. (2008) and convergence rate calculated using PB2002 (Bird, 2003) for convergence rate. We also found a negative correlation between b-values and the estimates of seismic coupling, which is defined as the ratio of the observed seismic moment release rate to the rate calculated from plate tectonic velocities (Scholz and Campos, 2012). Lithosphere age also has a weak negative correlation with the degree of seismic coupling. Based on differences in b-values for the types of faulting, Schorlemmer et al. (2005) suggested that b-value depends inversely on differential stress. This idea, taken together with correlations in the present study, suggests a model where the buoyancy of subducting slabs which depends on the lithosphere age determines stress state and the b-value in each sunbduction zone. The stress state also controls the seismic coupling. This model is basically consistent with the idea of Ruff and Kanamori (1980). Subduction zones with younger and lighter lithosphere are in a compressive stress state and associate with high coupling and small b-values (Chile), while those with older and heavier lithosphere are in a tensional stress state and correlate with low coupling and large b-values (Mariana). Subduction zones such as Nicaragua and El Salvador where b-values are much higher than the expectation from the above correlations may be explained by considering the fact that local tectonics affects the seismic coupling (LaFemina et al., 2009; Scholz and Campos, 2012).

Structural control of the upper plate on the down-dip segmentation of subduction dynamics

NASA Astrophysics Data System (ADS)

Shi, Q.; Barbot, S.; Karato, S. I.; Shibazaki, B.; Matsuzawa, T.; Tapponnier, P.

2017-12-01

The geodetic and seismic discoveries of slow earthquakes in subduction zones have provided the observational evidence for the existence of the transition between megathrust earthquakes and the creeping behaviors. However, the mechanics behind slow earthquakes, and the period differential motion between the subducting slab and the overlying plate below the seismogenic zone, remain controversial. In Nankai subduction zone, the very-low-frequency earthquakes (VLFE), megathrust earthquakes, long-term slow earthquakes (duration of months or years) and the episodic tremor and slip zone (ETS) are located within the accretionary prism, the continental upper crust, the continental lower crust and the upmost mantle of the overriding plate, respectively. We use the rate-and-state friction law to simulate the periodic occurrence of VLFEs, megathrust earthquakes and the tremors in the ETS zone because of relatively high rock strength within these depth ranges. However, it is not feasible to use frictional instabilities to explain the long-term slow earthquakes in the lower crust where the ductile rock physics plays a significant role in the large-scale deformation. Here, our numerical simulations show that slow earthquakes at the depth of the lower crust may be the results of plastic instabilities in a finite volume of ductile material accompanying by the grain-size evolution. As the thickness of the fault zone increases with depth, deformation becomes distributed and the dynamic equilibrium of grain size, as a competition between thermally activated grain growth and damage-related grain size reduction, results in cycles of strain acceleration and strain deficit. In addition, we took into account the elevated pore pressure in the accretinary prism which is associated with small stress drop and low-frequency content of VLFEs and may contribute to the occurrence of tsunamigenic earthquakes. Hence, in our numerical simulations for the plate boundary system in Nankai, the down-sip segmentation of the subduction dynamic is attributed to the upper plate structure that vary with depth. The high pore pressure, grain-size evolution and alternation of the rock physics may explain the existence and the periodicity of different slow earthquakes from shallow to deep regions in the subduction zone.

Imaging the deep structures of the convergent plates along the Ecuadorian subduction zone through receiver function analysis

NASA Astrophysics Data System (ADS)

Galve, A.; Charvis, P.; Regnier, M. M.; Font, Y.; Nocquet, J. M.; Segovia, M.

2017-12-01

The Ecuadorian subduction zone was affected by several large M>7.5 earthquakes. While we have low resolution on the 1942, 1958 earthquakes rupture zones extension, the 2016 Pedernales earthquake, that occurs at the same location than the 1942 earthquake, give strong constraints on the deep limit of the seismogenic zone. This downdip limit is caused by the onset of plasticity at a critical temperature (> 350-450 °C for crustal materials, or serpentinized mantle wedge, and eventually > 700 °C for dry mantle). However we still don't know exactly where is the upper plate Moho and therefore what controls the downdip limit of Ecuadorian large earthquakes seismogenic zone. For several years Géoazur and IG-EPN have maintained permanent and temporary networks (ADN and JUAN projects) along the margin to register the subduction zone seismological activity. Although Ecuador is not a good place to perform receiver function due to its position with respect to the worldwide teleseismic sources, the very long time deployment compensate this issue. We performed a frequency dependent receiver function analysis to derive (1) the thickness of the downgoing plate, (2) the interplate depth and (3) the upper plate Moho. These constraints give the frame to interpretation on the seismogenic zone of the 2016 Pedernales earthquake.

Seismicity of the Earth 1900-2012 Philippine Sea plate and vicinity

USGS Publications Warehouse

Smoczyk, Gregory M.; Hayes, Gavin P.; Hamburger, Michael W.; Benz, Harley M.; Villaseñor, Antonio; Furlong, Kevin P.

2013-01-01

The complex tectonics surrounding the Philippine Islands are dominated by the interactions of the Pacific, Sunda, and Eurasia plates with the Philippine Sea plate (PSP). The latter is unique because it is almost exclusively surrounded by zones of plate convergence. At its eastern and southeastern edges, the Pacific plate is subducted beneath the PSP at the Izu-Bonin, Mariana, and Yap trenches. Here, the subduction zone exhibits high rates of seismic activity to depths of over 600 km, though no great earthquakes (M>8.0) have been observed, likely because of weak coupling along the plate interface. In the northeast, the PSP subducts beneath Japan and the eastern margin of the Eurasia plate at the Nankai and Ryukyu trenches, extending westward to Taiwan. The Nankai portion of this subduction zone has hosted some of the largest earthquakes along the margins of the PSP, including a pair of Mw8.1 megathrust events in 1944 and 1946. Along its western margin, the convergence of the PSP and the Sunda plate is responsible for a broad and active plate boundary system extending along both sides of the Philippine Islands chain. The region is characterized by opposite-facing subduction systems on the east and west sides of the islands, and the archipelago is cut by a major transform structure: the Philippine Fault. Subduction of the Philippine Sea plate occurs at the eastern margin of the islands along the Philippine Trench and its northern extension, the East Luzon Trough. On the west side of Luzon, the Sunda Plate subducts eastward along a series of trenches, including the Manila Trench in the north, the smaller Negros Trench in the central Philippines, and the Sulu and Cotabato trenches in the south. Twentieth and early twentyfirst century seismic activity along the boundaries of the Philippine Sea plate has produced seven great (M>8.0) earthquakes and 250 large (M>7) events. Among the most destructive events were the 1923 Kanto, the 1948 Fukui, and the 1995 Kobe, Japan, earthquakes; the 1935 and the 1999 Chi-Chi, Taiwan, earthquakes; and the 1976 M7.6 Moro Gulf and 1990 M7.6 Luzon, Philippines, earthquakes.

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).

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.

Subduction on Venus and Implications for Volatile Cycling, Early Earth and Exoplanets

NASA Astrophysics Data System (ADS)

Smrekar, S. E.; Davaille, A.; Mueller, N. T.; Dyar, M. D.; Helbert, J.; Barnes, H.

2017-12-01

Plate tectonics plays a key role in long-term climate evolution by cycling volatiles between the interior, surface and atmosphere. Subduction is a critical process. It is the first step in transitioning between a stagnant and a mobile lid, a means for conveying volatiles into the mantle, and a mechanism for creating felsic crust. Laboratory experiments using realistic rheology illuminate the deformation produced by plume-induced subduction (Davaille abstract). Characteristics include internal rifting and volcanism, external rift branches, with a partial arc of subduction creating a trench on the margins of the plume head, and an exterior flexural bulge with small strain extension perpendicular to the trench. These characteristics, along with a consistent gravity signature, occur at the two largest coronae (quasi-circular volcano-tectonic features) on Venus (Davaille et al. Nature Geos. 2017). This interpretation resolves a long-standing debate about the dual plume and subduction characteristics of these features. Numerous coronae also show signs of plume-induced subduction. At Astkhik Planum, subduction appears to have migrated beyond the margins of Selu Corona to create a 1600 km-long, linear subduction zone, along Vaidilute Rupes. The fractures that define Selu Corona merge with the trench to the north and a rift zone to the east, consistent with plume-induced subduction migrating outward from the corona. The lithosphere and crust are much thinner here than in other potential subduction zones. Subduction appears to have generated massive volcanism which could explain the 400 m elevation of the plateau. Within the plateau there are low-viscosity flow sets nearly 1000 km that may be associated with near infrared low emissivity in VIRTIS data. Unusual lava compositions might be indicative of recycling of CO2 or other volatiles into the lithosphere. Little evidence exists to illustrate how plate tectonics initiated on Earth, but Venus' high surface temperature makes it a good analog of Earth's Archean. There is increasing evidence that Venus is a dynamic planet with possible active and/or recent volcanism and subduction. Studying these processes on Venus provides a window into both early Earth and offers constraints on the conditions needed to initiate plate tectonics on exoplanets.

The Hikurangi Plateau: Tectonic Ricochet and Accretion

NASA Astrophysics Data System (ADS)

Willis, David; Moresi, Louis; Betts, Peter; Whittaker, Joanne

2015-04-01

80 million years between interactions with different subduction systems provided time for the Hikurangi Plateau and Pacific Ocean lithosphere to cool, densify and strengthen. Neogene subduction of the Hikurangi Plateau occurring orthogonal to its Cretaceous predecessor, provides a unique opportunity to explore how changes to the physical properties of oceanic lithosphere affect subduction dynamics. We used Underworld to build mechanically consistent collision models to understand the dynamics of the two Hikurangi collisions. The Hikurangi Plateau is a ~112 Ma, 15km thick oceanic plateau that has been entrained by subduction zones immediately preceding the final break-up of Eastern Gondwana and currently within the active Hikurangi Margin. We explore why attempted subduction of the plateau has resulted in vastly different dynamics on two separate occasions. Slab break-off occured during the collision with Gondwana, currently there is apparent subduction of the plateau underneath New Zealand. At ~100Ma the young, hot Hikurangi Plateau, positively buoyant with respect to the underlying mantle, impacted a Gondwana Margin under rapid extension after the subduction of an mid-ocean ridge 10-15Ma earlier. Modelling of plateaus within young oceanic crust indicates that subduction of the thickened crust was unlikely to occur. Frontal accretion of the plateau and accompanying slab break-off is expected to have occured rapidly after its arrival. The weak, young slab was susceptible to lateral propagation of the ~1500 km window opened by the collision, and break-off would have progressed along the subduction zone inhibiting the "step-back" of the trench seen in older plates. Slab break-off coincided with a world-wide reorganisation of plate velocites, and orogenic collapse along the Gondwana margin characterised by rapid extension and thinning of the over-riding continental plate from ~60 to 30km. Following extension, Zealandia migrated to the NW until the Miocene allowing the oceanic crust time to densify and strengthen. At ~23Ma, the inception of the Hikurangi Subduction Zone drove the scissor rotation of the Australian and Pacific Plates creating displacement along the Alpine Fault. The Hikurangi Plateau was once again drawn into the subduction system, this time with subduction occurring orthogonal to the Cretaceous suture. The northern margin of the plateau has begun to subduct, but towards the southern terminus, the trench appears to be pinned. The result of the locked subduction zone is the asymmetric roll-back of the Hikurangi-Kermadec-Tonga subduction system around the point where the trench transitions from roll-back to shortening. The oceanic Pacific lithosphere is now signficantly negatively buoyant while the thickened lithosphere of the plateau maintains a slight positive buoyancy. The oceanic crust provides sufficient slab pull to drive subduction of the northern plateau, aided by the thin ~500km width of the plateaus subducting front. The increased strength profile of the older subducting lithosphere allows buoyancy forces to be transmitted to the over-riding plate, allowing continued convergence and hindering slab-breakoff.

Incorporating Cutting Edge Scientific Results from the Margins-Geoprisms Program into the Undergraduate Curriculum: The Subduction Factory

NASA Astrophysics Data System (ADS)

Penniston-Dorland, S.; Stern, R. J.; Edwards, B. R.; Kincaid, C. R.

2014-12-01

The NSF-MARGINS Program funded a decade of research on continental margin processes. The NSF-GeoPRISMS Mini-lesson Project, funded by NSF-TUES, is designed to integrate fundamental results from the MARGINS program into open-source college-level curriculum. Three Subduction Factory (SubFac) mini-lessons were developed as part of this project. These include hands-on examinations of data sets representing 3 key components of the subduction zone system: 1) Heat transfer in the subducted slab; 2) Metamorphic processes happening at the plate interface; and 3) Typical magmatic products of arc systems above subduction zones. Module 1: "Slab Temperatures Control Melting in Subduction Zones, What Controls Slab Temperature?" allows students to work in groups using beads rolling down slopes as an analog for the mathematics of heat flow. Using this hands-on, exploration-based approach, students develop an intuition for the mathematics of heatflow and learn about heat conduction and advection in the subduction zone environment. Module 2: "Subduction zone metamorphism" introduces students to the metamorphic rocks that form as the subducted slab descends and the mineral reactions that characterize subduction-related metamorphism. This module includes a suite of metamorphic rocks available for instructors to use in a lab, and exercises in which students compare pressure-temperature estimates obtained from metamorphic rocks to predictions from thermal models. Module 3: "Central American Arc Volcanoes, Petrology and Geochemistry" introduces students to basic concepts in igneous petrology using the Central American volcanic arc, a MARGINS Subduction Factory focus site, as an example. The module relates data from two different volcanoes - basaltic Cerro Negro (Nicaragua) and andesitic Ilopango (El Salvador) including hand sample observations and major element geochemistry - to explore processes of mantle and crustal melting and differentiation in arc volcanism.

Spatio-temporal Variations in Slow Earthquakes along the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Ide, S.; Maury, J.; Cruz-Atienza, V. M.; Kostoglodov, V.

2017-12-01

Slow earthquakes in Mexico have been investigated independently in different areas. Here, we review differences in tremor behavior and slow slip events along the entire subduction zone to improve our understanding of its segmentation. Some similarities are observed between the Guerrero and Oaxaca areas. By combining our improved tremor detection capabilities with previous results, we suggest that there is no gap in tremor between Guerrero and Oaxaca. However some differences between Michoacan and Guerrero are seen (e.g., SSE magnitude, tremor zone width, tremor rate), suggesting that these two areas behave differently. Tremor initiation shows clear tidal sensitivity along the entire subduction zone. Tremor in Guerrero is sensitive to small tidal normal stress as well as shear stress suggesting the subduction plane may include local variations in dip. Estimation of the energy rate shows similar values along the subduction zone interface. The scaled tremor energy estimates are similar to those calculated in Nankai and Cascadia, suggesting a common mechanism. Along-strike differences in slow deformation may be related to variations in the subduction interface that yield different geometrical and temperature profiles.

Spatiotemporal Variations in Slow Earthquakes Along the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Maury, J.; Ide, S.; Cruz-Atienza, V. M.; Kostoglodov, V.

2018-02-01

Slow earthquakes in Mexico have been investigated independently in different areas. Here we review differences in tremor behavior and slow slip events along the entire subduction zone to improve our understanding of its segmentation. Some similarities are observed between the Guerrero and Oaxaca areas. By combining our improved tremor detection capabilities with previous results, we suggest that there is no gap in tremor between Guerrero and Oaxaca. However, some differences between Michoacan and Guerrero are seen (e.g., SSE magnitude, tremor zone width, and tremor rate), suggesting that these two areas behave differently. Tremor initiation shows clear tidal sensitivity along the entire subduction zone. Tremor in Guerrero is sensitive to small tidal normal stress as well as shear stress, suggesting that the subduction plane may include local variations in dip. Estimation of the energy rate shows similar values along the subduction zone interface. The scaled tremor energy estimates are similar to those calculated in Nankai and Cascadia, suggesting a common mechanism. Along-strike differences in slow deformation may be related to variations in the subduction interface that yield different geometrical and temperature profiles.

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.

A quantitative analysis of global intermediate and deep seismicity

NASA Astrophysics Data System (ADS)

Ruscic, Marija; Becker, Dirk; Le Pourhiet, Laetitita; Agard, Philippe; Meier, Thomas

2017-04-01

The seismic activity in subduction zones around the world shows a large spatial variabilty with some regions exhibiting strong seismic activity down to depths of almost 700km while in other places seismicity terminates at depths of about 200 or 300 km. Also the decay of the number of seismic events or of the seismic moment with depth is more pronounced in some regions than in others. The same is true for the variability of the ratio of large to small events (the b-value of the Gutenberg-Richter relation) that is varying with depth. These observations are often linked to parameters of the downgoing plate like age or subduction velocity. In this study we investigate a subset of subduction zones utilizing the revised ISC catalogue of intermediate and deep seismicity to determine statistical parameters well suited to describe properties of intermediate deep and deep events. The seismicity is separated into three depth intervals from 50-175km, 175-400km and >400km based on the depth at which the plate contact decouples, the observed nearly exponential decay of the event rate with depth and the supposed depth of phase transition at 410 km depth where also an increase of the event number with depth is observed. For estimation of the b-value and the exponential decay with depth, a restriction of the investigated time interval to the period after 1997 produced significantly better results indicating a globally homogeneous magnitude scale with the magnitude of completeness of about Mw 5. On a global scale the b-value decreases with depth from values of about 1 at 50-175km to values of slightly below 0.8 for events below 400km. Also, there is a slight increase of the b-value with the age of the subducting plate. These changes in the b-value with depth and with age may indicate a varying fragmentation of the slab. With respect to the ratio of the seismic moment between deeper and shallower parts of the subduction zones a dependence on the age is apparent with older slabs exhibiting higher ratios indicating stronger hydration of older slabs and consequently stronger seismic activity at depth in older and thicker slabs. Furthermore, older slabs show the tendency to larger b-values. This indicates stronger fragmentation of older slabs favoring smaller events. Between 50 km and 300 km depth, seismicity in subduction zones decays nearly exponentially with depth. However, the majority of subduction zones show between about 60 km and 100 km lower seismic activity than expected by an exponential decay. This observation correlates well with findings from petrological studies that rocks are rarely scraped off from the downgoing plate at these depths indicating low seismic coupling and low stresses at the plate interface in a depth range below the seismogenic zone and above 100 km depth were dehydration reactions become virulent. Interestingly, the percentage of this deficit becomes larger with plate age for event frequency (reduced number of events), but decreases for moment release (events have larger magnitudes). It is observed that the forearc high is located above the plate interface with reduced seismic coupling. The forearc high is thus an indication of upward directed return flow along the seismically decoupled plate interface. In addition, it is found that the topography of the forearc high is larger above shallow dipping slabs. A correlation of the depth dependent seismic behavior with the subduction or trench velocity is not observed for the investigated subduction zones. Plate age seems to be the dominating factor for properties of intermediate deep and deep seismicity.

Obduction: Why, how and where. Clues from analog models

NASA Astrophysics Data System (ADS)

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

2014-05-01

Obduction is an odd geodynamic process characterized by the emplacement of dense oceanic “ophiolites” atop light continental plates in convergent settings. We herein present analog models specifically designed to explore the conditions (i.e., sharp increase of plate velocities - herein coined as ‘acceleration’, slab interaction with the 660 km discontinuity, ridge subduction) under which obduction may develop as a result of subduction initiation. The experimental setup comprises an upper mantle modeled as a low-viscosity transparent Newtonian glucose syrup filling a rigid Plexiglas tank and high-viscosity silicone plates. Convergence is simulated by pushing a piston with plate tectonics like velocities (1-10 cm/yr) onto a model comprising a continental margin, a weakness zone with variable resistance and dip (W), an oceanic plate (with or without a spreading ridge), a preexisting subduction zone (S) dipping away from the piston and an upper active continental margin, below which the oceanic plate is being subducted at the start of the model (as for the Neotethyan natural example). Several configurations were tested over thirty-five parametric models, with special emphasis on comparing different types of weakness zone and the degree of mechanical coupling across them. Measurements of displacements and internal deformation allow for a precise and reproducible tracking of deformation. Models consistently demonstrate that once conditions to initiate subduction are reached, obduction may develop further depending on the effective strength of W. Results (1) constrain the range of physical conditions required for obduction to develop/nucleate and (2) underline the key role of such perturbations for triggering obduction, particularly plate ‘acceleration’. They provide an explanation to the short-lived Peri-Arabic obduction, which took place along thousands of km almost synchronously (within ∼50-10 Myr), from Turkey to Oman, while the subduction zone beneath Eurasia became temporarily jammed. They also demonstrate that the emplacement of dense, oceanic material on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but results from large-scale, normal (oceanic then continental) subduction processes.

Scaly fabrics and veins of the Mugi and Makimine mélanges in the Shimanto belt, SW Japan

NASA Astrophysics Data System (ADS)

Ramirez, G. E.; Fisher, D. M.; Yamaguchi, A.; Kimura, G.

2016-12-01

Two regionally extensive ancient subduction fault zones provide a microstructural record of the plate boundary deformation associated with underthrusting. These rocks exhibit many of the characteristics associated with exposed ancient subduction fault zones worldwide, including: (1) σ1 is near orthogonal to the deformation fabric (2) there are microstructurally pervasive quartz and calcite filled veins concentrated in coarser blocks and along extensional jogs on slip surfaces, (3) evidence for local diffusion of silica sourced from web-like arrays of slip surfaces (i.e., scaly fabrics), and (4) evidence for cycles of cracking and sealing that record cyclic variations in stress. We present new backscatter SEM observations of scaly fabrics from two ancient subduction-related shear zones from the Shimanto Belt in Japan that exemplify these characteristics and represent the full temperature range of the seismogenic zone: 1) the Mugi mélange (lower ( 130-150 °C) and upper ( 170-200 °C) sections) and 2) Makimine mélange (peak temperatures of 340 °C). The Mugi mélange is an underplated duplex consisting of two horses separated by an OOST. The upper section is bounded at the top by a pseudotachylite-bearing paleodécollement. The Makimine mélange was underplated at the downdip limit of the seismogenic zone. The scaly fabrics associated with these shear zones display significantly different microstructural characteristics. A slip surface from along the upper Mugi is characterized by broader ( 20-30 μm), zones of quartz-poor, anastomosing shear zones composed of fine-grained (0.5-2 μm in length) phyllosilicates. The Makimine mélange exhibits thinner (10-20 μm), anastomosing shear zones with coarser (1-4 μm in length) phyllosilicate grains that are more strongly oriented into parallelism with slip surfaces. Quartz veins are pervasively developed in more competent blocks and are oriented at near perpendicular angles to the slip surfaces. Microstructural analyses of ancient subduction-related faults show differences with temperature that highlight the importance of establishing the geochemical processes and activation energies that contribute to slip, fracturing, and healing of rocks that underthrust the subduction interface.

Transfer of subduction fluids into the deforming mantle wedge during nascent subduction: Evidence from trace elements and boron isotopes (Semail ophiolite, Oman)

NASA Astrophysics Data System (ADS)

Prigent, C.; Guillot, S.; Agard, P.; Lemarchand, D.; Soret, M.; Ulrich, M.

2018-02-01

The basal part of the Semail ophiolitic mantle was (de)formed at relatively low temperature (LT) directly above the plate interface during "nascent subduction" (the prelude to ophiolite obduction). This subduction-related LT deformation was associated with progressive strain localization and cooling, resulting in the formation of porphyroclastic to ultramylonitic shear zones prior to serpentinization. Using petrological and geochemical analyses (trace elements and B isotopes), we show that these basal peridotites interacted with hydrous fluids percolating by porous flow during mylonitic deformation (from ∼850 down to 650 °C). This process resulted in 1) high-T amphibole crystallization, 2) striking enrichments of minerals in fluid mobile elements (FME; particularly B, Li and Cs with concentrations up to 400 times those of the depleted mantle) and 3) peridotites with an elevated δ11B of up to +25‰. These features indicate that the metasomatic hydrous fluids are most likely derived from the dehydration of subducting crustal amphibolitic materials (i.e., the present-day high-T sole). The rapid decrease in metasomatized peridotite δ11B with increasing distance to the contact with the HT sole (to depleted mantle isotopic values in <1 km) suggests an intense interaction between peridotites and rapid migrating fluids (∼1-25 m.y-1), erasing the initial high-δ11B subduction fluid signature within a short distance. The increase of peridotite δ11B with increasing deformation furthermore indicates that the flow of subduction fluids was progressively channelized in actively deforming shear zones parallel to the contact. Taken together, these results also suggest that the migration of subduction fluids/melts by porous flow through the subsolidus mantle wedge (i.e., above the plate interface at sub-arc depths) is unlikely to be an effective mechanism to transport slab-derived elements to the locus of partial melting in subduction zones.

Influence of the Amlia fracture zone on the evolution of the Aleutian Terrace forearc basin, central Aleutian subduction zone

USGS Publications Warehouse

Ryan, Holly F.; Draut, Amy E.; Keranen, Katie M.; Scholl, David W.

2012-01-01

During Pliocene to Quaternary time, the central Aleutian forearc basin evolved in response to a combination of tectonic and climatic factors. Initially, along-trench transport of sediment and accretion of a frontal prism created the accommodation space to allow forearc basin deposition. Transport of sufficient sediment to overtop the bathymetrically high Amlia fracture zone and reach the central Aleutian arc began with glaciation of continental Alaska in the Pliocene. As the obliquely subducting Amlia fracture zone swept along the central Aleutian arc, it further affected the structural evolution of the forearc basins. The subduction of the Amlia fracture zone resulted in basin inversion and loss of accommodation space east of the migrating fracture zone. Conversely, west of Amlia fracture zone, accommodation space increased arcward of a large outer-arc high that formed, in part, by a thickening of arc basement. This difference in deformation is interpreted to be the result of a variation in interplate coupling across the Amlia fracture zone that was facilitated by increasing subduction obliquity, a change in orientation of the subducting Amlia fracture zone, and late Quaternary intensification of glaciation. The change in coupling is manifested by a possible tear in the subducting slab along the Amlia fracture zone. Differences in coupling across the Amlia fracture zone have important implications for the location of maximum slip during future great earthquakes. In addition, shaking during a great earthquake could trigger large mass failures of the summit platform, as evidenced by the presence of thick mass transport deposits of primarily Quaternary age that are found in the forearc basin west of the Amlia fracture zone.

The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West

NASA Astrophysics Data System (ADS)

Levander, A.; Bezada, M. J.; Niu, F.; Schmitz, M.

2015-12-01

The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta block, the Perija Range, and the Merida Andes (Kellogg and Bonini, 1982). The steep descent of the CAR under Maracaibo implies that the CAR plate is torn somewhere between the Merida Andes and the Caribbean Sea, where it forms the ocean floor. An upcoming broadband seismic experiment will examine the CAR flat slab and the suspected slab tear in detail.

Historical seismicity

USGS Publications Warehouse

Dengler, L.

1992-01-01

The North Coast region of California in the vicinity of Cape Mendocino is one of the state's most seismically active areas, accounting for 25 percent of seismic energy release in California during the last 50 years. the region is located in a geologically dynamic are surrounding the Mendocino triple junction where three of the Earth's tectonic plates join together ( see preceding article by Sam Clarke). In the historic past the North Coast has been affected by earthquakes occurring on the San Andreas fault system to the south, the Mendocino fault to the southwest, and intraplate earthquakes within both the Gorda and North American plates. More than sixty of these earthquakes have caused damage since the mid-1800's. Recent studies indicate that California's North Coast is also at risk with respect to very large earthquakes (magnitude >8) originating along the Cascadia subduction zone. Although the subduction zone has not generated great earthquakes in historic time, paleoseismic evidence suggests that such earthquakes have been generated by the subduction zone in the recent prehistoric past. 

Understanding Seismotectonic Aspects of Central and South American Subduction Zones

NASA Astrophysics Data System (ADS)

Vargas-Jiménez, Carlos A.; Monsalve-Jaramillo, Hugo; Huérfano, Victor

2004-10-01

The Circum-Pacific, and particularly the Central and South America, subduction zones are complex structures that are subject to frequent, large-magnitude earthquakes, volcanic activity, tsunamis, and geological hazards. Among these natural hazards, earthquakes produce the most significant social and economic impacts in Latin America, and the subduction zones therefore demand constant vigilance and intensive study. The American continent has witnessed serveral earthquakes that rank among the most destrive in the world. Earthquakes such as the ones that occurred in Colombia-Ecuador [Mw = 8.9, 1906], Chile [Mw = 9.6, 1960; Mw = 8.9, 1995], Mexico [Mw = 9.6, 1985], and Peru [Mw = 8.0, 2001], as well as a number of destuctive events related to crustal fault systems and volcanic eruptions [e.g., Soufrière, El Ruiz, Galeras, ect.], have produced significant human and economic loss.The latent seismic hazards in the Caribbean, and Central and South America demand from the regional Earth sciences community accurate models to explain the mechanisms of these natural phenomena.

Aftereffects of Subduction-Zone Earthquakes: Potential Tsunami Hazards along the Japan Sea Coast.

PubMed

Minoura, Koji; Sugawara, Daisuke; Yamanoi, Tohru; Yamada, Tsutomu

2015-10-01

The 2011 Tohoku-Oki Earthquake is a typical subduction-zone earthquake and is the 4th largest earthquake after the beginning of instrumental observation of earthquakes in the 19th century. In fact, the 2011 Tohoku-Oki Earthquake displaced the northeast Japan island arc horizontally and vertically. The displacement largely changed the tectonic situation of the arc from compressive to tensile. The 9th century in Japan was a period of natural hazards caused by frequent large-scale earthquakes. The aseismic tsunamis that inflicted damage on the Japan Sea coast in the 11th century were related to the occurrence of massive earthquakes that represented the final stage of a period of high seismic activity. Anti-compressive tectonics triggered by the subduction-zone earthquakes induced gravitational instability, which resulted in the generation of tsunamis caused by slope failing at the arc-back-arc boundary. The crustal displacement after the 2011 earthquake infers an increased risk of unexpected local tsunami flooding in the Japan Sea coastal areas.

Diapir versus along-channel ascent of crustal material during plate convergence: Constrained by the thermal structure of subduction zones

NASA Astrophysics Data System (ADS)

Liu, Ming-Qi; Li, Zhong-Hai; Yang, Shao-Hua

2017-09-01

Subduction channel processes are crucial for understanding the material and energy exchange between the Earth's crust and mantle. Crustal rocks can be subducted to mantle depths, interact with the mantle wedge, and then exhume to the crustal depth again, which is generally considered as the mechanism for the formation of ultrahigh-pressure metamorphic rocks in nature. In addition, the crustal rocks generally undergo dehydration and melting at subarc depths, giving rise to fluids that metasomatize and weaken the overlying mantle wedge. There are generally two ways for the material ascent from subarc depths: one is along subduction channels; the other is through the mantle wedge by diapir. In order to study the conditions and dynamics of these contrasting material ascent modes, systematic petrological-thermo-mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates, ages of the subducting oceanic plate, as well as the plate convergence rates. The model results suggest that the thermal structures of subduction zones control the thermal condition and fluid/melt activity at the slab-mantle interface in subcontinental subduction channels, which further strongly affect the material transportation and ascent mode. The thick overriding continental plate and the low-angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channels with strong overriding mantle wedge, where the along-channel exhumation occurs exclusively to result in the exhumation of HP-UHP metamorphic rocks. In contrast, the thin overriding lithosphere and the steep subduction style induced by old subducting oceanic plate are the favorable conditions for hot subduction channels, which lead to significant hydration and metasomatism, melting and weakening of the overriding mantle wedge and thus cause the ascent of mantle wedge-derived melts by diapir through the mantle wedge. This may correspond to the origination of continental arc volcanism from mafic to ultramafic metasomatites in the bottom of the mantle wedge. In addition, the plate convergence rate can also affect the material ascent mode, e.g., diapiric extrusion versus along-channel exhumation, by changing the amount of supracrustal rocks carried into the subduction channels, which further regulate the fluid/melt activity and thermo-rheological properties.

Dynamic topography in subduction zones: insights from laboratory models

NASA Astrophysics Data System (ADS)

Bajolet, Flora; Faccenna, Claudio; Funiciello, Francesca

2014-05-01

The topography in subduction zones can exhibit very complex patterns due to the variety of forces operating this setting. If we can deduce the theoretical isostatic value from density structure of the lithosphere, the effect of flexural bending and the dynamic component of topography are difficult to quantify. In this work, we attempt to measure and analyze the topography of the overriding plate during subduction compared to a pure shortening setting. We use analog models where the lithospheres are modeled by thin-sheet layers of silicone putty lying on low-viscosity syrup (asthenosphere). The model is shorten by a piston pushing an oceanic plate while a continental plate including a weak zone to localize the deformation is fixed. In one type of experiments, the oceanic plate bends and subducts underneath the continental one; in a second type the two plates are in contact without any trench, and thus simply shorten. The topography evolution is monitored with a laser-scanner. In the shortening model, the elevation increases progressively, especially in the weak zone, and is consistent with expected isostatic values. In the subduction model, the topography is characterized, from the piston to the back-wall, by a low elevation of the dense oceanic plate, a flexural bulge, the trench forming a deep depression, the highly elevated weak zone, and the continental upper plate of intermediate elevation. The topography of the upper plate is consistent with isostatic values for very early stages, but exhibits lower elevations than expected for later stages. For a same amount of shortening of the continental plate, the thickening is the same and the plate should have the same elevation in both types of models. However, comparing the topography at 20, 29 and 39% of shortening, we found that the weak zone is 0.4 to 0.6 mm lower when there is an active subduction. Theses values correspond to 2.6 to 4 km in nature. Although theses values are high, there are of the same order as dynamic topography and could represent the dynamic effect of the slab sinking into the asthenosphere and lowering the elevation of the upper plate.

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.

The northern Lesser Antilles oblique subduction zone: new insight about the upper plate deformation, 3D slab geometry and interplate coupling.

NASA Astrophysics Data System (ADS)

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

2017-12-01

In subduction zones, the 3D geometry of the plate interface is thought to be a key parameter for the control of margin tectonic deformation, interplate coupling and seismogenic behavior. In the northern Caribbean subduction, precisely between the Virgin Islands and northern Lesser Antilles, these subjects remain controversial or unresolved. During the ANTITHESIS cruises (2013-2016), we recorded wide-angle seismic, multichannel reflection seismic and bathymetric data along this zone in order to constrain the nature and the geometry of the subducting and upper plate. This experiment results in the following conclusions: 1) The Anegada Passage is a 450-km long structure accross the forearc related to the extension due to the collision with the Bahamas platform. 2) More recently, the tectonic partitioning due to the plate convergence obliquity re-activated the Anegada Passage in the left-lateral strike-slip system. The partitioning also generated the left-lateral strike-slip Bunce Fault, separating the accretionary prism from the forearc. 3) Offshore of the Virgin Islands margin, the subducting plate shows normal faults parallel to the ancient spreading center that correspond to the primary fabric of the oceanic crust. In contrast, offshore of Barbuda Island, the oceanic crust fabric is unresolved (fracture zone?, exhumed mantle? ). 4) In the direction of the plate convergence vector, the slab deepening angle decreases northward. It results in a shallower slab beneath the Virgin Islands Platform compared to the St Martin-Barbuda forearc. In the past, the collision of the Bahamas platform likely changed the geodynamic settings of the northeastern corner of the Caribbean subduction zone and we present a revised geodynamic history of the region. Currently, various features are likely to control the 3D geometry of the slab: the margin convexity, the convergence obliquity, the heterogeneity of the primary fabric of the oceanic crust and the Bahamas docking. We suggest that the slab deepening angle lower beneath the Virgin Islands segment than beneath the St Martin-Barbuda segment possibly generates a northward increasing interplate coupling. As a result, it possibly favors an increase in the seismic activity and the tectonic partitioning beneath the Virgin Islands margin contrary to the St Martin-Barbuda segment.

Mapping seismic azimuthal anisotropy of the Japan subduction zone

NASA Astrophysics Data System (ADS)

Zhao, D.; Liu, X.

2016-12-01

We present 3-D images of azimuthal anisotropy tomography of the crust and upper mantle of the Japan subduction zone, which are determined using a large number of high-quality P- and S-wave arrival-time data of local earthquakes and teleseismic events recorded by the dense seismic networks on the Japan Islands. A tomographic method for P-wave velocity azimuthal anisotropy is modified and extended to invert S-wave travel times for 3-D S-wave velocity azimuthal anisotropy. A joint inversion of the P and S wave data is conducted to constrain the 3-D azimuthal anisotropy of the Japan subduction zone. Main findings of this work are summarized as follows. (1) The high-velocity subducting Pacific and Philippine Sea (PHS) slabs exhibit trench-parallel fast-velocity directions (FVDs), which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals formed at the mid-ocean ridge as well as shape-preferred orientation such as normal faults produced at the outer-rise area near the trench axis. (2) Significant trench-normal FVDs are revealed in the mantle wedge, which reflects corner flow in the mantle wedge due to the active subduction and dehydration of the oceanic plates. (3) Obvious toroidal FVDs and low-velocity anomalies exist in and around a window (hole) in the aseismic PHS slab beneath Southwest Japan, which may reflect a toroidal mantle flow pattern resulting from hot and wet mantle upwelling caused by the joint effects of deep dehydration of the Pacific slab and the convective circulation process in the mantle wedge above the Pacific slab. (4) Significant low-velocity anomalies with trench-normal FVDs exist in the mantle below the Pacific slab beneath Northeast Japan, which may reflect a subducting oceanic asthenosphere affected by hot mantle upwelling from the deeper mantle. ReferencesLiu, X., D. Zhao (2016) Seismic velocity azimuthal anisotropy of the Japan subduction zone: Constraints from P and S wave traveltimes. J. Geophys. Res. 121, doi:10.1002/2016JB013116. Zhao, D., S. Yu, X. Liu (2016) Seismic anisotropy tomography: New insight into subduction dynamics. Gondwana Res. 33, 24-43.

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.

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.

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°).

Subduction Related Crustal and Mantle Deformations and Their Implications for Plate Dynamics

NASA Astrophysics Data System (ADS)

Okeler, Ahmet

Ocean-continent convergence and subsequent continental collision are responsible for continental growth, mountain building, and severe tectonic events including volcanic eruptions and earthquake activity. They are also key driving forces behind the extensive thermal and compositional heterogeneities at crustal and mantle depths. Active subduction along the Calabrian Arc in southern Italy and the Hellenic Arc are examples of such collisional tectonics. The first part of this thesis examines the subduction related deformations within the crust beneath the southern Apennines. By modeling regional surface wave recordings of the largest temporary deployment in the southern Apennines, a lower-crustal/upper-mantle low-velocity volume extending down to 50 km beneath the mountain chain is identified. The magnitude (˜ 0.4 km/s slower) and anisotropic nature (˜ 10%) of the anomaly suggest the presence of hot and partially molten emplacement that may extend into the upper-crust towards Mt. Vulture, a once active volcano. Since the Apulian basement units are deformed during the compressional and consequent extensional events, our observations favor the "thick-skin" tectonic growth model for the region. In the deeper mantle, active processes are thermodynamically imprinted on the depth and strength of the phase transitions. This thesis examines more than 15000 SS precursors and provides the present-day reflectivity structure and topography associated with these phase transitions. Through case studies I present ample evidence for both slab penetration into the lower mantle (beneath the Hellenic Arc, Kurile Island and South America) and slab stagnation at the bottom of the Mantle Transition Zone (beneath the Tyrrhenian Sea and eastern China). Key findings include (1) thermal anomalies (˜ 200 K) at the base of the MTZ, which represent the deep source for Cenozoic European Rift Zone, Mount Etna and Mount Cameroon volcanism, (2) significant depressions (by 20-40 km) at the bottom of the Mantle Transition Zone beneath subducting slabs, (3) a strong 520-km reflector near subducting slabs, (4) a weak and elevated (15-25 km) 410-km reflector within active deformation zones, (5) strong lower mantle reflectors (˜ 900 km) while slabs penetrate into the lower mantle, and (6) consistency between the topography of a 300-km reflector and an exothermic phase transformation.

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.

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.

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.

Deformation cycles of subduction earthquakes in a viscoelastic Earth.

PubMed

Wang, Kelin; Hu, Yan; He, Jiangheng

2012-04-18

Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised.

Slow Earthquakes in the Alaska-Aleutian Subduction Zone Detected by Multiple Mini Seismic Arrays

NASA Astrophysics Data System (ADS)

LI, B.; Ghosh, A.; Thurber, C. H.; Lanza, F.

2017-12-01

The Alaska-Aleutian subduction zone is one of the most seismically and volcanically active plate boundaries on earth. Compared to other subduction zones, the slow earthquakes, such as tectonic tremors (TTs) and low frequency earthquakes (LFEs), are relatively poorly studied due to the limited data availability and difficult logistics. The analysis of two-months of continuous data from a mini array deployed in 2012 shows abundant tremor and LFE activities under Unalaska Island that is heterogeneously distributed [Li & Ghosh, 2017]. To better study slow earthquakes and understand their physical characteristics in the study region, we deployed a hybrid array of arrays, consisting of three well-designed mini seismic arrays and five stand alone stations, in the Unalaska Island in 2014. They were operational for between one and two years. Using the beam back-projection method [Ghosh et al., 2009, 2012], we detect continuous tremor activities for over a year when all three arrays are running. The sources of tremors are located south of the Unalaska and Akutan Islands, at the eastern and down-dip edge of the rupture zone of the 1957 Mw 8.6 earthquake, and they are clustered in several patches, with a gap between the two major clusters. Tremors show multiple migration patterns with propagation in both along-strike and dip directions and a wide range of velocities. We also identify tens of LFE families and use them as templates to search for repeating LFE events with the matched-filter method. Hundreds to thousands of LFEs for each family are detected and their activities are spatiotemporally consistent with tremor activities. The array techniques are revealing a near-continuous tremor activity in this area with remarkable spatiotemporal details. It helps us to better recognize the physical properties of the transition zone, provides new insights into the slow earthquake activities in this area, and explores their relation with the local earthquakes and the potential slow slip events.

Limits on great earthquake size at subduction zones

NASA Astrophysics Data System (ADS)

McCaffrey, R.

2012-12-01

Subduction zones are where the world's greatest earthquakes occur due to the large fault area available to slip. Yet some subduction zones are thought to be immune from these massive events, where quake size is limited by some physical processes or properties. Accordingly, the size of the 2011 Tohoku-oki Mw 9.0 earthquake caught some in the earthquake research community by surprise. The expectations of these massive quakes have been driven in the past by reliance on our short, incomplete history of earthquakes and causal relationships derived from it. The logic applied is that if a great earthquake has not happened in the past, that we know of, one cannot happen in the future. Using the ~100-year global earthquake seismological history, and in some cases extended with geologic observations, relationships between maximum earthquake sizes and other properties of subduction zones are suggested, leading to the notion that some subduction zones, like the Japan Trench, would never produce a magnitude ~9 event. Empirical correlations of earthquake behavior with other subduction parameters can give false positive results when the data are incomplete or incorrect, of small numbers and numerous attributes are examined. Given multi-century return times of the greatest earthquakes, ignorance of those return times and our relatively limited temporal observation span (in most places), I suggest that we cannot yet rule out great earthquakes at any subduction zones. Alternatively, using the length of a subduction zone that is available for slip as the predominant factor in determining maximum earthquake size, we cannot rule out that any subduction zone of a few hundred kilometers or more in length may be capable of producing a magnitude 9 or larger earthquake. Based on this method, the expected maximum size for the Japan Trench was 9.0 (McCaffrey, Geology, p. 263, 2008). The same approach indicates that a M > 9 off Java, with twice the population density as Honshu and much lower building standards, is possible. The Java Trench, and others that are considered of the low-coupling type (i.e., Hikurangi, Marianas, Tonga, Kermadec), require increased awareness of the possibility for a great earthquake and tsunami.

Thermal structure and geodynamics of subduction zones

NASA Astrophysics Data System (ADS)

Wada, Ikuko

The thermal structure of subduction zones depends on the age-controlled thermal state of the subducting slab and mantle wedge flow. Observations indicate that the shallow part of the forearc mantle wedge is stagnant and the slab-mantle interface is weakened. In this dissertation, the role of the interface strength in controlling mantle wedge flow, thermal structure, and a wide range of subduction zone processes is investigated through two-dimensional finite-element modelling and a global synthesis of geological and geophysical observations. The model reveals that the strong temperature-dependence of the mantle strength always results in full slab-mantle decoupling along the weakened part of the interface and hence complete stagnation of the overlying mantle. The interface immediately downdip of the zone of decoupling is fully coupled, and the overlying mantle is driven to flow at a rate compatible with the subduction rate. The sharpness of the transition from decoupling to coupling depends on the rheology assumed and increases with the nonlinearity of the flow system. This bimodal behaviour of the wedge flow gives rise to a strong thermal contrast between the cold stagnant and hot flowing parts of the mantle wedge. The maximum depth of decoupling (MDD) thus dictates the thermal regime of the forearc. Observed surface heat flow patterns and petrologically and geochemically estimated mantle wedge temperatures beneath the volcanic arc require an MDD of 70--80 km in most, if not all, subduction zones regardless of their thermal regime of the slab. The common MDD of 70--80 km explains the observed systematic variations of the petrologic, seismological, and volcanic processes with the thermal state of the slab and thus explains the rich diversity of subduction zones in a unified fashion. Models for warm-slab subduction zones such as Cascadia and Nankai predict shallow dehydration of the slab beneath the cold stagnant part of the mantle wedge, which provides ample fluid for mantle wedge serpentinization in the forearc but little fluid for melt generation beneath the arc. In contrast, models for colder-slab subduction zones such as NE Japan and Kamchatka predict deeper dehydration, which provides greater fluid supply for melt generation beneath the arc and allows deeper occurrence of intraslab earthquakes but less fluid for forearc mantle wedge serpentinization. The common MDD also explains the intriguing uniform configuration of subduction zones, that is, the volcanic arc always tends to be situated where the slab is at about 100 km depth. The sudden onset of mantle wedge flow downdip of the common MDD overshadows the thermal effect of the slab, and the resultant thermal field and slab dehydration control the location of the volcanic arc. The recognition of the fundamental importance of the MDD has important implications to the study of geodynamics and earthquake hazard in subduction zones.

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.

Petrofabrics of high-pressure rocks exhumed at the slab-mantle interface from the "point of no return" in a subduction zone (Sivrihisar, Turkey)

NASA Astrophysics Data System (ADS)

Whitney, Donna L.; Teyssier, Christian; Seaton, Nicholas C. A.; Fornash, Katherine F.

2014-12-01

The highest pressure recorded by metamorphic rocks exhumed from oceanic subduction zones is 2.5 GPa, corresponding to the maximum decoupling depth (MDD) (80 ± 10 km) identified in active subduction zones; beyond the MDD (the "point of no return") exhumation is unlikely. The Sivrihisar massif (Turkey) is a coherent terrane of lawsonite eclogite and blueschist facies rocks in which assemblages and fabrics record P-T-fluid-deformation conditions during exhumation from 80 to 45 km. Crystallographic fabrics and other features of high-pressure metasedimentary and metabasaltic rocks record transitions during exhumation. In quartzite, microstructures and crystallographic fabrics record deformation in the dislocation creep regime, including dynamic recrystallization during decompression, and a transition from prism slip to activation of rhomb and basal slip that may be related to a decrease in water fugacity during decompression ( 2.5 to 1.5 GPa). Phengite, lawsonite, and omphacite or glaucophane in quartzite and metabasalt remained stable during deformation, and omphacite developed an L-type crystallographic fabric. In marble, aragonite developed columnar textures with strong crystallographic fabrics that persisted during partial to complete dynamic recrystallization that was likely achieved in the stability field of aragonite (P > 1.2 GPa). Results of kinematic vorticity analysis based on lawsonite shape fabrics are consistent with shear criteria in quartzite and metabasalt and indicate a large component of coaxial deformation in the exhuming channel beneath a simple shear dominated interface. This large coaxial component may have multiplied the exhuming power of the subduction channel and forced deeply subducted rocks to flow back from the point of no return.

Subduction obliquity as a prime indicator for geotherm in subduction zone

NASA Astrophysics Data System (ADS)

Plunder, Alexis; Thieulot, Cédric; van Hinsbergen, Douwe

2016-04-01

The geotherm of a subduction zone is thought to vary as a function of subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction can strongly vary due to changes in the angle between the trench and the plate convergence vector, namely the subduction obliquity. This phenomenon is observed all around the Pacific (i.e., Marianna, South America, Aleutian…). However due to observed differences in subducting lithosphere age or lateral convergence rate in nature, the quantification of temperature variation due to obliquity is not obvious. In order to investigate this effect, 3D generic numerical models were carried out using the finite element code ELEFANT. We designed a simplified setup to avoid interaction with other parameters. An ocean/ocean subduction setting was chosen and the domain is represented by a 800 × 300 × 200 km Cartesian box. The trench geometry is prescribed by means of a simple arc-tangent function. Velocity of the subducting lithosphere is prescribed using the analytical solution for corner flow and only the energy conservation equation is solved in the domain. Results are analysed after steady state is reached. First results show that the effect of the trench curvature on the geotherm with respect to the convergence direction is not negligible. A small obliquity yields isotherms which are very slightly deflected upwards where the obliquity is maximum. With an angle of ˜30°, the isotherms are deflected upwards of about 10 kilometres. Strong obliquity (i.e., angles from 60° to almost 90°) reveal extreme effects of the position of the isotherms. Further model will include other parameter as the dip of the slab and convergence rate to highlight their relative influence on the geotherm of subduction zone.

Subduction processes related to the Sea of Okhotsk

NASA Astrophysics Data System (ADS)

Zabarinskaya, Ludmila P.; Sergeyeva, Nataliya

2017-04-01

It is obviously important to study a role of subduction processes in tectonic activity within the continental margins. They are marked by earthquakes, volcanic eruptions, tsunami and other natural disasters hazardous to the people,plants and animals that inhabit such regions. The northwest part of the Sea of Okhotsk including the northern part of Sakhalin Island and the Deryugin Basin is the area of the recent intensive tectonic movements. The geological and geophysical data have made it possible to construct the geodynamic model of a deep structure of a lithosphere for this region. This geodynamic model has confirmed the existence of the ophiolite complex in the region under consideration. It located between the North Sakhalin sedimentary basin and the Deryugin basin. The Deryugin basin was formed on the side of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous-Paleogene. The North Sakhalin Basin with oil and gas resources was formed on the side of back-arc basin at that time. Approximately in the Miocene period the subduction process, apparently, has stopped. The remains of the subduction zone in the form of ophiolite complex have been identified according to geological and geophysical data. On a surface the subduction zone is shown as deep faults stretched along Sakhalin.

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

McNally, K.C.; Minster, J.B.

Revised estimates of seismic slip rates along the Middle America Trench are lower on the average than plate convergence rates but match them locally (for example, Oaxaca). Along the Cocos-North American plate boundary this can be explained by nonuniformities in slip at points of aseismic ridge or fracture zone subduction. For at least 81 yr (and possibly several hundred years), no major (M/sub s/> or =7.5) shallow earthquake is known to have occurred near the Orozco Fracture Zone and Tehuantepec Ridge areas. Compared with the average recurrence periods for large earthquakes (33 +- 8 yr since 1898 and 35 +-more » 24 yr between 1542 and 1979), this suggests that either a large (M> or =8.4) event may be anticipated at such locations, or that these are points of aseismic subduction. Large coastal terraces and evidence suggesting tectonic uplift are found onshore near the Orozco Fracture zone. The larger discrepancy between plate convergence and seismic slip rates along the Cocos-Carribbean plate boundary is more likely due to decoupling and downbending of the subducted plate. We used the limited statistical evidence available to characterize both spatial and temporal deficiencies in recent seismic slip. The observations appear consistent with a possible forthcoming episode of more intense seismic activity. Based on a series of comparisons with carefully delineated aftershock zones, we conclude that the zones of anomalous seismic activity can be indentified by a systematic, automated analysis of the worldwide earthquake catalog (m/sub b/> or =4).« less

Subduction and Plate Edge Tectonics in the Southern Caribbean

NASA Astrophysics Data System (ADS)

Levander, A.; Schmitz, M.; Niu, F.; Bezada, M. J.; Miller, M. S.; Masy, J.; Ave Lallemant, H. G.; Pindell, J. L.; Bolivar Working Group

2013-05-01

The southern Caribbean plate boundary consists of a subduction zone at at either end of a complex strike-slip fault system: In the east at the Lesser Antilles subduction zone, the Atlantic part of the South American plate subducts beneath the Caribbean. In the north and west in the Colombia basin, the Caribbean subducts under South America. In a manner of speaking, the two plates subduct beneath each other. Finite-frequency teleseismic P-wave tomography confirms this, imaging the Atlantic and the Caribbean plates subducting steeply in opposite directions to transition zone depths under northern South America (Bezada et al, 2010). The two subduction zones are connected by the El Pilar-San Sebastian strike-slip fault system, a San Andreas scale system that has been cut off at the Bocono fault, the southeastern boundary fault of the Maracaibo block. A variety of seismic probes identify subduction features at either end of the system (Niu et al, 2007; Clark et al., 2008; Miller et al. 2009; Growdon et al., 2009; Huang et al., 2010; Masy et al, 2011). The El Pilar system forms at the southeastern corner of the Antilles subduction zone with the Atlantic plate tearing from South America. The deforming plate edges control mountain building and basin formation at the eastern end of the strike-slip system. Tearing the Atlantic plate from the rest of South America appears to cause further lithospheric instability continentward. In northwestern South America the Caribbean plate very likely also tears, as its southernmost element subducts at shallow angles under northernmost Colombia but then rapidly descends to the transition zone under Lake Maracaibo (Bezada et al., 2010). We believe that the flat slab controls the tectonics of the Neogene Merida Andes, Perija, and Santa Marta ranges. The nonsubducting part of the Caribbean plate also underthrusts northern Venezuela to about the width of the coastal mountains (Miller et al., 2009). We infer that the edge of the underthrust Caribbean plate supports the elevations of the coastal mountains and controls continuing deformation.

Linking giant earthquakes with the subduction of oceanic fracture zones

NASA Astrophysics Data System (ADS)

Landgrebe, T. C.; Müller, R. D.; EathByte Group

2011-12-01

Giant subduction earthquakes are known to occur in areas not previously identified as prone to high seismic risk. This highlights the need to better identify subduction zone segments potentially dominated by relatively long (up to 1000 years and more) recurrence times of giant earthquakes. Global digital data sets represent a promising source of information for a multi-dimensional earthquake hazard analysis. We combine the NGDC global Significant Earthquakes database with a global strain rate map, gridded ages of the ocean floor, and a recently produced digital data set for oceanic fracture zones, major aseismic ridges and volcanic chains to investigate the association of earthquakes as a function of magnitude with age of the downgoing slab and convergence rates. We use a so-called Top-N recommendation method, a technology originally developed to search, sort, classify, and filter very large and often statistically skewed data sets on the internet, to analyse the association of subduction earthquakes sorted by magnitude with key parameters. The Top-N analysis is used to progressively assess how strongly particular "tectonic niche" locations (e.g. locations along subduction zones intersected with aseismic ridges or volcanic chains) are associated with sets of earthquakes in sorted order in a given magnitude range. As the total number N of sorted earthquakes is increased, by progressively including smaller-magnitude events, the so-called recall is computed, defined as the number of Top-N earthquakes associated with particular target areas divided by N. The resultant statistical measure represents an intuitive description of the effectiveness of a given set of parameters to account for the location of significant earthquakes on record. We use this method to show that the occurrence of great (magnitude ≥ 8) earthquakes on overriding plate segments is strongly biased towards intersections of oceanic fracture zones with subduction zones. These intersection regions are linked with 8 of the largest 10, 18 of the largest 25, about half of the largest 100 subduction earthquakes, as well as with the 2011 Tohoku-Oki earthquake. Subduction zone intersections with volcanic chains are not found to be associated with a significantly elevated risk for great earthquakes globally. This difference likely arises from subducting fracture zone ridges leading to stronger seismic coupling than subducting volcanic chains.

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.

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.

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.

Oxygen isotopes in garnet and accessory minerals to constrain fluids in subducted crust

NASA Astrophysics Data System (ADS)

Rubatto, Daniela; Gauthiez-Putallaz, Laure; Regis, Daniele; Rosa Scicchitano, Maria; Vho, Alice; Williams, Morgan

2017-04-01

Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. Garnet and U-Pb accessory minerals constitute some of the most robust and ubiquitous minerals in subducted crust and can preserve multiple growth zones that track the metamorphic evolution of the sample they are hosted in. Microbeam investigation of the chemical (major and trace elements) and isotopic composition (oxygen and U-Pb) of garnet and accessory minerals is used to track significant fluid-rock interaction at different stages of the subduction system. This approach requires consideration of the diffusivity of oxygen isotopes particularly in garnet, which has been investigated experimentally. The nature of the protolith and ocean floor alteration is preserved in relict accessory phases within eclogites that have been fully modified at HP conditions (e.g. Monviso and Dora Maira units in the Western Alps). Minerals in the lawsonite-blueschists of the Tavsanli zone in Turkey record pervasive fluid exchange between mafic and sedimentary blocks at the early stage of subduction. High pressure shear zones and lithological boundaries show evidence of intense fluid metasomatism at depth along discontinuities in Monviso and Corsica. In the UHP oceanic crust of the Zermatt-Saas Zone, garnet oxygen isotopes and tourmaline boron isotopes indicate multistage fluid infiltration during prograde metamorphism. Localized exchanges of aqueous fluids are also observed in the subducted continental crust of the Sesia-Lanzo Zone. In most cases analyses of distinct mineral zones enable identification of multiple pulses of fluids during the rock evolution.

Escape tectonics and the extrusion of Alaska: Past, present, and future

USGS Publications Warehouse

Redfield, T.F.; Scholl, D. W.; Fitzgerald, P.G.; Beck, M.E.

2007-01-01

The North Pacific Rim is a tectonically active plate boundary zone parts of which may be characterized as a laterally moving orogenic stream. Crustal blocks are transported along large-magnitude strike-slip faults in western Canada and central Alaska toward the Aleutian-Bering Sea subduction zones. Throughout much of the Cenozoic, at and west of its Alaskan nexus, the North Pacific Rim orogenic Stream (NPRS) has undergone tectonic escape. During transport, relatively rigid blocks acquired paleomagnetic rotations and fault-juxtaposed boundaries while flowing differentially through the system, from their original point of accretion and entrainment toward the free face defined by the Aleutian-Bering Sea subduction zones. Built upon classical terrane tectonics, the NPRS model provides a new framework with which to view the mobilistic nature of the western North American plate boundary zone. ?? 2007 The Geological Society of America.

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.

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

Hochstein, M.P.; Sudarman, Sayogi

There are at least 30 high temperatures systems (with inferred reservoir temperatures > 200 C) along the active Sumatra Arc that transfer heat from crustal intrusions to the surface. These systems, together with eleven active volcanoes, five degassing volcanoes and one caldera volcano (Lake Toba), are controlled by the Sumatra Fault Zone, an active mega shear zone that follows the median axis of the arc. At least half of the active and degassing volcanoes are associated with volcanic geothermal reservoirs containing magmatic gases and acid fluids. Large, low temperature resources exist in the Tertiary sedimentary basins of east Sumatra (back-arcmore » region), where anomalously higher thermal gradients (up to 8 C/100 m) have been measured. Volcanic activity was not continuous during the Cenozoic; subduction and arc volcanism probably decreased after the Eocene as a result of a clockwise rotation of Sumatra. In the Late Miocene, subduction started again, and andesitic volcanism reached a new peak of intensity in the Pliocene and has been continuous ever since. Rhyolitic volcanism, which has produced voluminous ignimbrite flows, began later (Pliocene/Pleistocene). All known rhyolitic centers associated with ignimbrite flows appear to lie along the Sumatra Fault Zone.« less

Cascadia Initiative Reveals Accumulation of Buoyant Material Beneath the Subducting Juan de Fuca Plate

NASA Astrophysics Data System (ADS)

Hawley, W. B.; Allen, R. M.; Richards, M. A.

2015-12-01

The Cascadia Initiative is a four-year (2011-2015) amphibious seismic deployment that covers the Juan de Fuca plate and the Cascadia Subduction Zone. It is comprised of 70 broadband ocean-bottom seismometers that occupy 120 sites in total, as well as 27 land-based stations. This array offers a unique opportunity to study the 3D structure of a subduction zone in unprecedented detail. We present the results of an inversion using teleseismic body waves recorded by the Cascadia Initiative, EarthScope, and other regional and temporary networks in the Pacific Northwest. A low-velocity feature is visible beneath the subducting slab at shallow depths. Previous studies report ponding of low-viscosity, buoyant material at the top of the asthenosphere, unable to rise through the impermeable lithospheric lid. We show that as the lithospheric lid descends into the mantle, this material is not advected with it; rather, due to its own weakness and buoyancy, it accumulates at the subduction zone. Such material could be partly responsible for the rapid uplift and volcanism in the Coast Range of California, in the wake of the northward migration of the Mendocino Triple Junction. This newly observed feature may play an important role in the structure of subduction zones, but understanding the extent of that role on a global scale will require amphibious seismic deployments in other subduction zones.

Links between fluid circulation, temperature, and metamorphism in subducting slabs

USGS Publications Warehouse

Spinelli, G.A.; Wang, K.

2009-01-01

The location and timing of metamorphic reactions in subducting lithosph??re are influenced by thermal effects of fluid circulation in the ocean crust aquifer. Fluid circulation in subducting crust extracts heat from the Nankai subduction zone, causing the crust to pass through cooler metamorphic faci??s than if no fluid circulation occurs. This fluid circulation shifts the basalt-to-eclogite transition and the associated slab dehydration 14 km deeper (35 km farther landward) than would be predicted with no fluid flow. For most subduction zones, hydrothermal cooling of the subducting slab will delay eclogitization relative to estimates made without considering fluid circulation. Copyright 2009 by the American Geophysical Union.

Plume type ophiolites in Japan, East Russia and Mongolia: Peculiarity of the Late Jurassic examples

NASA Astrophysics Data System (ADS)

Ishiwatari, Akira; Ichiyama, Yuji; Ganbat, Erdenesaikhan

2013-04-01

Dilek and Furnes (2011; GSAB) provided a new comprehensive classification of ophiolites. In addition to the mid-ocean ridge (MOR) and supra-subduction zone (SSZ) types that are known for decades, they introduced rift-zone (passive margin) type, volcanic arc (active margin) type, and plume type. The last type is thought to be originated in oceanic large igneous provinces (LIPs; oceanic plateaus), and is preserved in the subduction-accretion complexes in the Pacific margins. The LIP-origin greenstones occur in the Middle Paleozoic (Devonian) accretionary complex (AC) in central Mongolia (Ganbat et al. 2012; AGU abst.). The Late Paleozoic and Mesozoic plume-type ophiolites are abundant in Japan. They are Carboniferous greenstones covered by thick limestone in the Akiyoshi belt (Permian AC, SW Japan; Tatsumi et al., 2000; Geology), Permian greenstones in the Mino-Tamba belt (Jurassic AC, SW Japan; Ichiyama et al. 2008; Lithos), and Late Jurassic-Early Cretaceous greenstone in the Sorachi (Hokkaido; Ichiyama et al, 2012; Geology) and Mikabu (SW Japan; this study) belts. The LIP origin of these greenstones is indicated by abundance of picrite (partly komatiite and meimechite), geochemical features resembling HIMU basalts (e.g. high Nb/Y and Zr/Y) and Mg-rich (up to Fo93) picritic olivines following the "mantle array", suggesting very high (>1600oC) temperature of the source mantle plume. The Sorachi-Mikabu greenstones are characterized by the shorter time interval between magmatism and accretion than the previous ones, and are coeval with the meimechite lavas and Alaskan-type ultramafic intrusions in the Jurassic AC in Sikhote-Alin Mountains of Primorye (E. Russia), that suggest a superplume activity in the subduction zone (Ishiwatari and Ichiyama, 2004; IGR). The Mikabu greenstones extend for 800 km along the Pacific coast of SW Japan, and are characterized by the fragmented "olistostrome" occurrence of the basalts, gabbros and ultramafic cumulate rocks (but no mantle peridotite), suggesting tectonism in a sediment-starved subduction zone or a transform fault zone that transected the thick oceanic LIP crust. The Sorachi greenstones are associated with depleted mantle peridotite, and are covered by the thick Cretaceous turbidite formation (Yezo Group), and Takashima et al. (2002; JAES) concluded the marginal basin origin for the "Sorachi ophiolite". We know that some oceanic LIPs were developed into marginal basins (e.g. Caribbean basin). The Late Jurassic-Early Cretaceous greenstone belts of Japan and eastern Russia may represent relics of a 2000 km-size superplume activity that hit the subduction zone and the adjacent ocean floor in NW Pacific.

Deep Sources: New constraints on the tectonic origin of the Klyuchevskoy Group upper mantle anomaly

NASA Astrophysics Data System (ADS)

Bourke, J. R.; Nikulin, A.; Levin, V. L.

2017-12-01

Volcanoes of the Klyuchevskoy Group (KG) form one of the most active volcanic clusters on the planet, yet its position relative to the subducting Pacific Plate seems to be in violation of the understood principles of the flux-induced arc volcanism. Positioned at 170km above the accepted subduction contact, the KG is seemingly outside the maximum fluid flux release zone of 100km, as observed across global subduction zone environments. Past geophysical studies indicate presence of a planar seismic anomaly 110km below the KG, and it has been noted that the KG lavas exhibit anomalous geochemical signatures, possibly associated with two separate melt generation regions. This interpretation was largely based on receiver function analysis of seismic data recorded by 3 stations of the Partnership in International Research and Education (PIRE) network, done prior to this data becoming publically available. We present results of receiver function and a teleseismic, regional, and local source shear wave splitting study, focused on datasets obtained by the full PIRE network of 12 stations, as well as a hybrid summation of all stations. We present our findings in the form of depth migrated receiver function images convolved with a three-dimensional model of the subduction zone and shear-wave splitting measurements. Our results vastly increase the resolution of the previously identified upper mantle anomaly, further constraining its geometry both vertically and laterally. We complement our observations with a forward modeling effort aimed at assessing the geological nature of the anomaly. Specifically, we test three scenarios that were previously invoked to explain the presence of the low-velocity anomaly in the upper mantle below the KG: a 3D flow of mantle material around the corner of the subducting Pacific Plate, a sinking paleoslab left behind as a result of subduction rollback, and a plume of sediments from the subducting plate. We show that presence of remnant paleoslab material remains a likely geodynamic scenario that explains both the observed geophysical anomaly and its impact on volcanic activity of the KG.

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.

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.

Earthquakes, fluid pressures and rapid subduction zone metamorphism

NASA Astrophysics Data System (ADS)

Viete, D. R.

2013-12-01

High-pressure/low-temperature (HP/LT) metamorphism is commonly incomplete, meaning that large tracts of rock can remain metastable at blueschist- and eclogite-facies conditions for timescales up to millions of years [1]. When HP/LT metamorphism does take place, it can occur over extremely short durations (<<1 Myr) [1-2]. HP/LT metamorphism must be associated with processes that allow large volumes of rock to remain unaffected over long periods of time, but then suddenly undergo localized metamorphism. Existing models for HP/LT metamorphism have focussed on the role of fluids in providing heat for metamorphism [2] or catalyzing metamorphic reactions [1]. Earthquakes in subduction zone settings can occur to depths of 100s of km. Metamorphic dehydration and the associated development of elevated pore pressures in HP/LT metamorphic rocks has been identified as a cause of earthquake activity at such great depths [3-4]. The process of fracturing/faulting significantly increases rock permeability, causing channelized fluid flow and dissipation of pore pressures [3-4]. Thus, deep subduction zone earthquakes are thought to reflect an evolution in fluid pressure, involving: (1) an initial increase in pore pressure by heating-related dehydration of subduction zone rocks, and (2) rapid relief of pore pressures by faulting and channelized flow. Models for earthquakes at depth in subduction zones have focussed on the in situ effects of dehydration and then sudden escape of fluids from the rock mass following fracturing [3-4]. On the other hand, existing models for rapid and incomplete metamorphism in subduction zones have focussed only on the effects of heating and/or hydration with the arrival of external fluids [1-2]. Significant changes in pressure over very short timescales should result in rapid mineral growth and/or disequilibrium texture development in response to overstepping of mineral reaction boundaries. The repeated process of dehydration-pore pressure development-earthquake-pore pressure relief could conceivably produce a record of episodic HP/LT metamorphism driven by rapid pressure pulses. A new hypothesis is presented for the origins of HP/LT metamorphism: that HP/LT metamorphism is driven by effective pressure pulses caused by localized, earthquake-related modifications to fluid pressures in the subducted slab. In other words, HP/LT metamorphism marks abrupt changes in stress state within the subducted slab, driven by earthquake rupture and fluid flow, and involving a rapid return toward lithostatic pressure from effective pressures well below lithostatic. References: 1. Bjørnerud, MG, Austrheim, H & Lund, MG, 2002. Processes leading to eclogitization (densification) of subducted and tectonically buried crust. Journal of Geophysical Research 107, 2252. 2. Camacho, A, Lee, JKW, Hensen, BJ & Braun, J, 2005. Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids. Nature 435, 1191-1196. 3. Green, HW & Houston, H, 1995. The mechanics of deep earthquakes. Annual Reviews of Earth and Planetary Sciences 23, 169-213. 4. Hacker, BR, Peacock, SM, Abers, GA & Holloway, SD, 2003. Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?. Journal of Geophysical Research 108, 2030.

Tectono-sedimentary features in the Yap subduction zone, Western Pacific: constraints from latest integrated geophysical survey

NASA Astrophysics Data System (ADS)

Dong, D.; Zhang, G.; Bai, Y.; Fan, J.; Zhang, Z.

2017-12-01

The Yap subduction zone, western Pacific, is a typical structure related to the ridge subduction, but comparative shortage of the geophysical data makes the structural details unknown in this area. In this study, we present the latest and high-quality multi-beam swath bathymetry and multi-channel seismic data acquired synchronously in the year 2015 across the Yap subduction zone. Multichannel seismic and multi-beam data are mainly applied to investigate the topography of major tectonic units and stratigraphic structure in the Yap subduction zone and discuss the tectonic characteristics controlled by ridge subduction. It suggests that, Parece Vela Basin, as the regional sedimentary center, developed sedimentary layers nearly 800 meters thick. On the contrast, the horizontal sedimentary layers were not obviously identified in the Yap trench, where subduction erosion occurred. Caroline ridge changed the tectonic characteristics of subduction zone, and influenced magmatism of the Yap arc because of the special topography. The seismic profile clearly reveals landslide deposits at the upper slope break of the forearc, north of the Yap Island, which was identified as the fault notch denoting a lithological boundary in previous work. Detailed topography and geological structure of horst and graben in the north of Yap are depicted, and topographic high of Caroline ridge is supposed to bring greater bending and tension and the subsequent horst and graben belt. Multichannel seismic evidence has been provided for interpreting the expansion of Sorol Trough and its inferred age. A modified model for the Yap subduction zone evolution is proposed, incorporating three major tectonic events: proto-Yap Arc rupture in the Oligocene, collision of the Caroline Ridge and the Yap Trench in the Late Oligocene or Middle Miocene, and onset of the Sorol Trough rifting in the Late Miocene. Acknowledge: This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11030102), the National Natural Science Foundation of China (No. 41476042, 41506055 )

Heterogeneous structure of the incoming plate in the Japan Trench

NASA Astrophysics Data System (ADS)

Nakamura, Y.; Fujie, G.; Yamaguchi, A.; Kodaira, S.; Miura, S.

2017-12-01

We have conducted seismic surveys in around the Japan Trench subduction zone, northeastern Japan, to investigate the structural features of the incoming Pacific plate and the frontal prism. Thickness of the hemiplegic sediments on the deposited on the incoming Pacific plate shows the variation along trench axis between 200 and 600 ms two-way travel time (TWT). This is remarkably thinner than other subduction zones with megathrust earthquakes like Sumatra subduction zone. Off Miyagi, central part of the Japan Trench which is the main ruptured region of 2011 Tohoku earthquake, has 200 - 300 ms TWT of the incoming sediments thickness. Off Iwate, northern part of the Japan Trench, has thicker incoming sediments 500 ms TWT, and Off Fukushima, southern part of the Japan Trench, has 300 - 400 ms TWT. We found at least three areas with anomalously thin sediments; Area I: 38N 145N, Area II: 39.5N 144.5E, Area III: 39N 144.5N. At the Area I, located on the outer rise off Miyagi, the receiver function analysis using Ocean Bottom Seismograph data revealed the existence of PS conversion surfaces below the interpreted basement on the seismic sections. This implies that the interface between sediments and the igneous basement is located below the interpreted basement reflections. Previous studies suggested the existence of the petit spots in this Area I. Area II shows apparently very thin sediments near the trench axis on seismic profiles, where the petit spot volcanism was observed. Shallow sediment sampling conducted in this area indicates no major surface erosion. These observations suggest that the petit spot volcanism, like sill intrusion, masked the original deeper basement reflections and caused the apparent thin sediments on seismic profiles. Area III also has thin sediments and rough basement topography, which has possibly been caused by another petit spot activity. Petit spot area with apparent very thin sediments in the trench axis (Area II) is located next to the northern edge of the large slip zone of the 2011 Tohoku earthquake. The volcanic activities like petit spots on the incoming plate introduce heterogeneous input into the subduction zone, which could be important factors to control the megathrust seismo- and tsunamigenesis in the subduction zone.

A detailed map of the 660-kilometer discontinuity beneath the izu-bonin subduction zone.

PubMed

Wicks, C W; Richards, M A

1993-09-10

Dynamical processes in the Earth's mantle, such as cold downwelling at subduction zones, cause deformations of the solid-state phase change that produces a seismic discontinuity near a depth of 660 kilometers. Observations of short-period, shear-to-compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the Izu-Bonin subduction zone. The discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab, with a deformation profile consistent with the expected thermal signature of the slab, the experimentally determined Clapeyron slope of the phase transition, and the regional tectonic history.

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.

Topographic and sedimentary features in the Yap subduction zone and their implications for the Caroline Ridge subduction

NASA Astrophysics Data System (ADS)

Dong, Dongdong; Zhang, Zhengyi; Bai, Yongliang; Fan, Jianke; Zhang, Guangxu

2018-01-01

The Yap subduction zone in the western Pacific presents some unique features compared to normal intra-oceanic subduction zones such as the subduction of an oceanic plateau. However, due to the relative paucity of geophysical data, the detailed structure remains unknown in this area. In this study, we present the latest high-quality swath bathymetry and multi-channel seismic data acquired synchronously in 2015 across the Yap subduction zone. The topographic and sedimentary features are intensively investigated and a modified evolutionary model of the Yap subduction zone is proposed. The two-stage evolution of the Parece Vela Basin (PVB) produced fabrics that are N-S trending and NW-SE trending. Our seismic data clearly reveal landslide deposits at the upper slope break of the forearc, to the north of the Yap Island, which was identified as the fault notch denoting a lithological boundary in previous work. The swath bathymetry and seismic profile reveal detailed horst and graben structures, including a crescent-shaped fault zone near the contact between the Yap Trench and the Caroline Ridge. A simple geometric model is proposed to explain the structure formation, indicating that the higher topography of the Caroline Ridge resulted in enhanced bending-related extension. A seismic angular unconformity (named R1) is identified in the Sorol Trough, marking the onset of rifting in the trough. Based on the sequence thickness and deposition rate by Deep Sea Drilling Project (DSDP), it is deduced that the Sorol Trough formed at 10 Ma or even earlier. A modified model for the Yap subduction zone evolution is proposed, incorporating three major tectonic events: the proto-Yap Arc rupture in the Oligocene, the collision of the Caroline Ridge and the Yap Trench in the late Oligocene or middle Miocene, and the onset of the Sorol Trough rifting in the late Miocene.

Subduction, Extension, and a Mantle Plume in the Pacific Northwest

NASA Astrophysics Data System (ADS)

Hawley, W. B.; Allen, R. M.; Richards, M. A.

2016-12-01

Subduction zones are some of the most important systems that control the dynamics and evolution of the earth. The Cascadia Subduction Zone offers a unique natural laboratory for understanding the subduction process, and how subduction interacts with other large-scale geodynamical phenomena. The small size of the Juan de Fuca (JdF) plate and the proximity of the system to the Yellowstone Hotspot and the extensional Basin and Range province allow for detailed study of the effects these important systems have on each other. We present both a P-wave and an S-wave tomographic model of the Pacific Northwestern United States using regional seismic arrays, including the amphibious Cascadia Initiative. These models share important features, such as the Yellowstone plume, the subducting JdF slab, a gap in the subducting slab, and a low-velocity feature beneath the shallowest portions of the slab. But subtle differences in these features between the models—the size of the gap in the subducting JdF slab and the shape of the Yellowstone plume shaft above the transition zone, for example—provide physical insight into the interpretation of these models. The physics that we infer from our seismic tomography and other studies of the region will refine our understanding of subduction zones worldwide, and will help to identify targets for future amphibious seismic array studies. The discovery of a pronounced low-velocity feature beneath the JdF slab as it subducts beneath the coastal Pacific Northwest is, thus far, the most surprising result from our imaging work, and implies a heretofore unanticipated regime of dynamical interaction between the sublithospheric oceanic asthenosphere and the subduction process. Such discoveries are made possible, and rendered interpretable, by ever-increasing resolution that the Cascadia Initiative affords seismic tomography models.

A new view into the Cascadia subduction zone and volcanic arc: Implications for earthquake hazards along the Washington margin

USGS Publications Warehouse

Parsons, T.; Trehu, A.M.; Luetgert, J.H.; Miller, K.; Kilbride, F.; Wells, R.E.; Fisher, M.A.; Flueh, E.; ten Brink, Uri S.; Christensen, N.I.

1998-01-01

In light of suggestions that the Cascadia subduction margin may pose a significant seismic hazard for the highly populated Pacific Northwest region of the United States, the U.S. Geological Survey (USGS), the Research Center for Marine Geosciences (GEOMAR), and university collaborators collected and interpreted a 530-km-long wide-angle onshore-offshore seismic transect across the subduction zone and volcanic arc to study the major structures that contribute to seismogenic deformation. We observed (1) an increase in the dip of the Juan de Fuca slab from 2°–7° to 12° where it encounters a 20-km-thick block of the Siletz terrane or other accreted oceanic crust, (2) a distinct transition from Siletz crust into Cascade arc crust that coincides with the Mount St. Helens seismic zone, supporting the idea that the mafic Siletz block focuses seismic deformation at its edges, and (3) a crustal root (35–45 km deep) beneath the Cascade Range, with thinner crust (30–35 km) east of the volcanic arc beneath the Columbia Plateau flood basalt province. From the measured crustal structure and subduction geometry, we identify two zones that may concentrate future seismic activity: (1) a broad (because of the shallow dip), possibly locked part of the interplate contact that extends from ∼25 km depth beneath the coastline to perhaps as far west as the deformation front ∼120 km offshore and (2) a crustal zone at the eastern boundary between the Siletz terrane and the Cascade Range.

Structure of the Cascadia Subduction Zone Imaged Using Surface Wave Tomography

NASA Astrophysics Data System (ADS)

Schaeffer, A. J.; Audet, P.

2017-12-01

Studies of the complete structure of the Cascadia subduction zone from the ridge to the arc have historically been limited by the lack of offshore ocean bottom seismograph (OBS) infrastructure. On land, numerous dense seismic deployments have illuminated detailed structures and dynamics associated with the interaction between the subducting oceanic plate and the overriding continental plate, including cycling of fluids, serpentinization of the overlying forearc mantle wedge, and the location of the upper surface of the Juan de Fuca plate as it subducts beneath the Pacific Northwest. In the last half-decade, the Cascadia Initiative (CI), along with Neptune (ONC) and several other OBS initiatives, have instrumented both the continental shelf and abyssal plains off shore of the Cascadia subduction zone, facilitating the construction of a complete picture of the subduction zone from ridge to trench and volcanic arc. In this study, we present a preliminary azimuthally anisotropic surface-wave phase-velocity based model of the complete system, capturing both the young, unaltered Juan de Fuca plate from the ridge, to its alteration as it enters the subduction zone, in addition to the overlying continent. This model is constructed from a combination of ambient noise cross-correlations and teleseismic two station interferometry, and combines together concurrently running offshore OBS and onshore stations. We furthermore perform a number of representative 1D depth inversions for shear velocity to categorize the pristine oceanic, subducted oceanic, and continental crust and lithospheric structure. In the future the dispersion dataset will be jointly inverted with receiver functions to constrain a 3D shear-velocity model of the complete region.

Temporal Activity Modulation of Deep Very Low Frequency Earthquakes in Shikoku, Southwest Japan

NASA Astrophysics Data System (ADS)

Baba, Satoru; Takeo, Akiko; Obara, Kazushige; Kato, Aitaro; Maeda, Takuto; Matsuzawa, Takanori

2018-01-01

We investigated long-term changes in the activity of deep very low frequency earthquakes (VLFEs) in western Shikoku, southwest part of the Nankai subduction zone in Japan for 13 years by the matched-filter technique. VLFE activity is expected to be a proxy of interplate slips. In the Bungo channel, where long-term slow slip events (SSEs) occurred frequently, the cumulative number of detected VLFEs increased rapidly in 2010 and 2014, which were modulated by long-term SSEs. In the neighboring inland region near the Bungo channel, the cumulative number increased steeply every 6 months. This stepwise change was accompanied by episodic tremors and slips. Deep VLFE activity in western Shikoku has been low since the latter half of 2014. This decade-scale quiescence may be attributed to the change in interplate coupling strength in the Nankai subduction zone.

Teleseismic P and S wave attenuation constraints on temperature and melt of the upper mantle in the Alaska Subduction Zone.

NASA Astrophysics Data System (ADS)

Soto Castaneda, R. A.; Abers, G. A.; Eilon, Z.; Christensen, D. H.

2017-12-01

Recent broadband deployments in Alaska provide an excellent opportunity to advance our understanding of the Alaska-Aleutians subduction system, with implications for subduction processes worldwide. Seismic attenuation, measured from teleseismic body waves, provides a strong constraint on thermal structure as well as an indirect indication of ground shaking expected from large intermediate-depth earthquakes. We measure P and S wave attenuation from pairwise amplitude and phase spectral ratios for teleseisms recorded at 204 Transportable Array, Alaska Regional, and Alaska Volcano Observatory, SALMON (Southern Alaska Lithosphere & Mantle Observation Network) and WVLF (Wrangell Volcanics & subducting Lithosphere Fate) stations in central Alaska. The spectral ratios are inverted in a least squares sense for differential t* (path-averaged attenuation operator) and travel time anomalies at every station. Our preliminary results indicate a zone of low attenuation across the forearc and strong attenuation beneath arc and backarc in the Cook Inlet-Kenai region where the Aleutian-Yakutat slab subducts, similar to other subduction zones. This attenuation differential is observed in both the volcanic Cook Inlet segment and amagmatic Denali segments of the Aleutian subduction zone. By comparison, preliminary results for the Wrangell-St. Elias region past the eastern edge of the Aleutian slab show strong attenuation beneath the Wrangell Volcanic Field, as well as much further south than in the Cook Inlet-Kenai region. This pattern of attenuation seems to indicate a short slab fragment in the east of the subduction zone, though the picture is complex. Results also suggest the slab may focus or transmit energy with minimal attenuation, adding to the complexity. To image the critical transition between the Alaska-Aleutian slab and the region to its east, we plan to incorporate new broadband data from the WVLF array, an ongoing deployment of 37 PASSCAL instruments installed in 2016. These stations have 10-20 km spacing, spanning the edge of the subducting slab, and so will provide a zone of increased resolution in the region where slab behavior is poorly understood. We will discuss these data in the context of enigmatic Wrangell volcanism and its relationship to the eastern end of the Alaska-Aleutian Wadati-Benioff zone.

Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters. An Overview of the Activities of SFB 574.

NASA Astrophysics Data System (ADS)

Reston, T. J.

2005-12-01

The special research program SFB 574 at the University of Kiel investigates the role of fluid and volatile recycling in subduction zones along the Central American convergent margin (Guatemala to Panama) through integrated geophysical, geological, volcanological, geochemical, petrological and oceanographic studies. The work is carried out by over 50 scientists within 12 focussed scientific projects, evenly distributed between the tectonics of the subduction zone, the dewatering through the forearc, and the transfer of fluids from the slab to the atmosphere through the arc. During Phase I (2001-2004), we concentrated on a segment of the erosive subduction zone system onshore and offshore Costa Rica and Nicaragua, one of the focus areas for the MARGIN initiatives SubFac and SEIZE. Along this margin, the dip of subduction, the nature of the incoming plate, and magmatic compositions along the volcanic arc are all known to change significantly. In addition to work carried out during cruises and fieldwork from the 1990s, in the past 4 years we have collected new data during a total 10 months of shiptime on the research vessels SONNE and METEOR, and during 20 man-months of fieldwork, mainly in Costa Rica and Nicaragua. In Phase II (2004-2008) we will finish work off Central America, and start working in an accretionary segment of the Chile margin between 32 and 38S. In this presentation I outline some of the main results concentrating on the effect of variable input and on the output at the arc. Key effects include the influence of the Galapagos hotspot on the incoming section (and on the output at the arc), the thickness of the volcanic crust and the effects of mantle serpentinization.

Crustal Seismicity and 3-D Velocity Structure in the Principal Cordillera of Central Chile (33- 34.5 S, 69.5-71 W): Implications on Andean Geodynamic and Seismic Hazard

NASA Astrophysics Data System (ADS)

Pardo, M.; Monfret, T.; Vera, E.; Yañez, G.; Eisenberg, A.

2007-12-01

Based on data from a dense local temporary seismological network, crustal seismicity is characterized, and a 3- D body wave velocity structure is obtained by tomographic inversion down to the subducted slab. In the framework of Fondecyt 1050758, GeoAzur-IRD and ACT-18 projects, 35 broadband and short period instruments, were deployed in the studied zone for 135 days recording in continuous mode. At this zone the Andean active volcanism reappears after a gap of volcanic activity since late Miocene occurring north of 33 S due to the Central Chile flat slab subduction zone. Crustal seismicity in the depth range 0-30 km is well correlated with known geological faults that become now important in the assessment of the regional seismic hazard. This seismicity also clusters around the giant porphyry cooper deposits in the region (Rio Blanco, El Teniente), and are neither related to mine-blasts nor induced by mining activity. Moreover, the local 3-D velocity structure shows that the zone surrounding each deposit is characterized by high Vp/Vs greater than 1.8, which may indicate fluid phases located in the weakest and more fractured zone of the crust. The body wave velocity pattern shown at depth by the local tomography indicates channels of high Vp/Vs connecting the subducted slab with the surface at places where active volcanism is present, suggesting upward migration of hydrous or melted rocks. This pattern agrees with the one observed with a previous regional tomography that includes this zone, while this Vp/Vs pattern tends to be horizontal at the flat slab zone. At depths of 20-25 km, a layer of high Vp/Vs is observed beneath the Andes Cordillera that could be associated to changes in the rheological properties between the upper and lower crust, or to accumulation of magma. The average stress tensor, derived from focal mechanisms, indicate that the Andean zone is under compression in the plate convergence direction.

Tectonic evolution of the Troodos Ophiolite within the Tethyan Framework

NASA Astrophysics Data System (ADS)

Dilek, Yildirim; Thy, Peter; Moores, Eldridge M.; Ramsden, Todd W.

1990-08-01

A new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the Troodos ophiolite, a well-preserved remnant of Neotethyan oceanic crust. Grabens and normal faults within the sheeted dike complex and the extrusive sequence of the Troodos ophiolite resemble those of oceanic spreading centers. Diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along- and across-strike variation in volcanic and tectonic activity during development of oceanic crust. Coupled with the existence of the Arakapas transform fault to the south, these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault. The arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue, however, for generation of part of the ophiolite within a subduction zone environment. Regional reconstructions suggest that the Mesozoic Neotethys may have evolved as a marginal basin both to the Afro-Arabian continent and the Paleotethyan ocean over an active or recently active south dipping subduction zone. The Troodos ophiolite and other eastern Mediterranean ophiolites, whose magma compositions were affected by the subducted Paleotethyan slab, may have formed along east-west trending spreading centers separated by north-south trending transform faults within this marginal basin. A rapid change in relative plate motion in late Cretaceous time between Eurasia and Afro-Arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the Neotethyan realm and in initiation of north dipping subduction zone(s) beneath the Troodos and other ophiolites in the region. The apparent forearc setting of the Troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation.

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

Shear heating and metamorphism in subduction zones, 1. Thermal models

NASA Astrophysics Data System (ADS)

Kohn, M. J.; Castro, A. E.; Spear, F. S.

2017-12-01

Popular thermal-mechanical models of modern subduction systems are 100-500 °C colder at c. 50 km depth than pressure-temperature (P-T) conditions determined from exhumed metamorphic rocks. This discrepancy has been ascribed by some to profound bias in the rock record, i.e. metamorphic rocks reflect only anomalously warm subduction, not normal subduction. Accurately inferring subduction zone thermal structure, whether from models or rocks, is crucial for predicting depths of seismicity, fluid release, and sub-arc melting conditions. Here, we show that adding realistic shear stresses to thermal models implies P-T conditions quantitatively consistent with those recorded by exhumed metamorphic rocks, suggesting that metamorphic rock P-T conditions are not anomalously warm. Heat flow measurements from subduction zone fore-arcs typically indicate effective coefficients of friction (µ) ranging from 0.025 to 0.1. We included these coefficients of friction in analytical models of subduction zone interface temperatures. Using global averages of subducting plate age (50 Ma), subduction velocity (6 cm/yr), and subducting plate geometry (central Chile), temperatures at 50 km depth (1.5 GPa) increase by c. 200 °C for µ=0.025 to 700 °C for µ=0.1. However, at high temperatures, thermal softening will reduce frictional heating, and temperatures will not increase as much with depth. Including initial weakening of materials ranging from wet quartz (c. 300 °C) to diabase (c. 600 °C) in the analytical models produces concave-upward P-T distributions on P-T diagrams, with temperatures c. 100 to 500 °C higher than models with no shear heating. The absolute P-T conditions and concave-upward shape of the shear-heating + thermal softening models almost perfectly matches the distribution of P-T conditions derived from a compilation of exhumed metamorphic rocks. Numerical models of modern subduction zones that include shear heating also overlap metamorphic data. Thus, excepting the very hottest examples, exhumed metamorphic rocks represent the products of normal, not anomalous, subduction. Consequently numerous geochemical, petrologic, and geophysical interpretations that have been founded on models that lack shear heating must be re-evaluated.

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.

Crustal growth in subduction zones

NASA Astrophysics Data System (ADS)

Vogt, Katharina; Castro, Antonio; Gerya, Taras

2015-04-01

There is a broad interest in understanding the physical principles leading to arc magmatisim at active continental margins and different mechanisms have been proposed to account for the composition and evolution of the continental crust. It is widely accepted that water released from the subducting plate lowers the melting temperature of the overlying mantle allowing for "flux melting" of the hydrated mantle. However, relamination of subducted crustal material to the base of the continental crust has been recently suggested to account for the growth and composition of the continental crust. We use petrological-thermo-mechanical models of active subduction zones to demonstrate that subduction of crustal material to sublithospheric depth may result in the formation of a tectonic rock mélange composed of basalt, sediment and hydrated /serpentinized mantle. This rock mélange may evolve into a partially molten diapir at asthenospheric depth and rise through the mantle because of its intrinsic buoyancy prior to emplacement at crustal levels (relamination). This process can be episodic and long-lived, forming successive diapirs that represent multiple magma pulses. Recent laboratory experiments of Castro et al. (2013) have demonstrated that reactions between these crustal components (i.e. basalt and sediment) produce andesitic melt typical for rocks of the continental crust. However, melt derived from a composite diapir will inherit the geochemical characteristics of its source and show distinct temporal variations of radiogenic isotopes based on the proportions of basalt and sediment in the source (Vogt et al., 2013). Hence, partial melting of a composite diapir is expected to produce melt with a constant major element composition, but substantial changes in terms of radiogenic isotopes. However, crustal growth at active continental margins may also involve accretionary processes by which new material is added to the continental crust. Oceanic plateaus and other crustal units may collide with continental margins to form collisional orogens and accreted terranes in places where oceanic lithosphere is recycled back into the mantle. We use thermomechanical-petrological models of an oceanic-continental subduction zone to analyse the dynamics of terrane accretion and its implications to arc magmatisim. It is shown that terrane accretion may result in the rapid growth of continental crust, which is in accordance with geological data on some major segments of the continental crust. Direct consequences of terrane accretion may include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes and partial melting (Vogt and Gerya., 2014), forming complex suture zones of accreted and partially molten units. Castro, A., Vogt, K., Gerya, T., 2013. Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model. Gondwana Research, 23, 1554-1566. Vogt, K., Castro, A., Gerya, T., 2013. Numerical modeling of geochemical variations caused by crustal relamination. Geochemistry, Geophysics, Geosystems, 14, 470-487. Vogt, K., Gerya, T., 2014. From oceanic plateaus to allochthonous terranes: Numerical Modelling. Gondwana Research, 25, 494-508

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.

Interaction of the subduction process and forearc tectonics: An example from the active N - Chilean margin

NASA Astrophysics Data System (ADS)

Victor, P.; Sobiesiak, M.

2005-12-01

Convergent plate boundaries at continental margins belong to the tectonically most active areas on earth and are endangered by devastating earthquakes and tsunamis. The north Chilean margin is a high strain continental margin driven by fast plate convergence rate. The greatest amount of strain is accommodated along the subduction interface. Nevertheless there is extensive crustal deformation obvious by surface ruptures along reactivated segments of large fault systems and vertical surface motions reflecting the interaction between subducting and overriding plates. The historical seismicity record indicates that great earthquakes affect the Chilean Forearc with recurrence intervals of about 112+/- 21 y . The last great event in northern Chile occurred in 1995 near Antofagasta. The Mw= 8.0 event ruptured the subduction interface 180 km along strike with an average slip of about 5m in the depth interval between 10-50 km. From careful evaluation of the aftershock sequence by examining the different catagories of aftershock focal mechanisms we can define three segments of the seismogenic zone affected by the Antofagasta main shock. The non-ruptured northern segment beneath Mejillones Peninsula is seperated by a broad transition zone from the central segment which hosts the earthquakes' rupture plane. The southern fault plane boundary is identified by linear alignment of all apparent aftershock mechanisms. Along this southern boundary the strike slip mechanisms are exclusively left lateral whereas the strike slip mechanisms along the northern transition zone are right lateral. The orientations of summed moment tensors calculated from aftershock fault plane solutions on the northern segment and in the northern transition zone differ from the orientations exhibited by moment tensors on the central segment. This might indicate a rotational component in the coseismic movement of the ruptured segment relative to the non-ruptured segment. The observed segmentation of the downgoing plate correlates well with changes in the coseismic surface displacement field and coseismic rotations derived from GPS data (Allmendinger et al. in press). We can localize a transition zone at Mejillones peninsula (23,5°S) striking approximately N 80°E dominated by clockwise vertical axis rotations also marked by rotations of the summed moment tensors on the downgoing plate. The calculated strain tensor for this transition zone does not correspond with long term surface deformation, implying that coseismic as well as early postseismic effects on the subduction interface do not contribute to long term deformation of crustal fault zones. The Antofagasta earthquake took place just south of the large 1877 gap which extends from southern Peru to Mejillones Peninsula, being the surface expression of a barrier seperating the Antofagasta fault plane from the expected future fault plane. From our studies of the Antofagasta subduction zone and the surface displacement field we hope to find evidences for interface-crust-surface interactions which can be extrapolated also to the 1877 gap.

Seismic Wave Velocity in the Subducted Oceanic Crust from Autocorrelation of Tectonic Tremor Signals

NASA Astrophysics Data System (ADS)

Ducellier, A.; Creager, K.

2017-12-01

Hydration and dehydration of minerals in subduction zones play a key role in the geodynamic processes that generate seismicity and that allow tectonic plates to subduct. Detecting the presence of water in the subducted plate is thus crucial to better understand the seismogenesis and the consequent seismic hazard. A landward dipping, low velocity layer has been detected in most subduction zones. In Cascadia, this low velocity zone is characterized by a low S-wave velocity and a very high Poisson's ratio, which has been interpreted as high pore-fluid pressure in the upper half part of the subducted oceanic crust. Most previous studies were based on seismic reflection imaging, receiver function analysis, or body wave tomography, with seismic sources located far from the low velocity zone. In contrast, the sources of the tectonic tremors generated during Episodic Tremor and Slip (ETS) events are located on the plate boundary. As the sources of the tremors are much closer to the low velocity zone, seismic waves recorded during ETS events should illuminate the area with greater precision. Most methods to detect and locate tectonic tremors and low-frequency earthquakes are based on the cross correlation of seismic signals; either signals at the same station for different events, or the same event at different stations. We use the autocorrelation of the seismic signal recorded by eight arrays of stations, located in the Olympic Peninsula, Washington. Each tremor, assumed to be on the plate boundary, generates a direct wave and reflected and converted waves from both the strong shear-wave velocity contrast in the mid-oceanic crust, and from the Moho of the subducted oceanic crust. The time lag between the arrivals of these different waves at a seismic station corresponds to a peak of amplitude on the autocorrelation signals. Using the time lags observed for different locations of the tremor source, we intend to invert for the seismic wave velocity of the subducted oceanic crust under the arrays. Identifying zones with lower S-wave velocity and a high Poisson's ratio will then help detecting the presence of water in the subducted oceanic crust. Our ultimate goal is contributing to a better understanding of the mechanism of ETS and subduction zone processes.

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.

An Offshore Geophysical Network in the Pacific Northwest for Earthquake and Tsunami Early Warning and Hazard Research

NASA Astrophysics Data System (ADS)

Wilcock, W. S. D.; Schmidt, D. A.; Vidale, J. E.; Harrington, M.; Bodin, P.; Cram, G.; Delaney, J. R.; Gonzalez, F. I.; Kelley, D. S.; LeVeque, R. J.; Manalang, D.; McGuire, C.; Roland, E. C.; Tilley, J.; Vogl, C. J.; Stoermer, M.

2016-12-01

The Cascadia subduction zone hosts catastrophic earthquakes every few hundred years. On land, there are extensive geophysical networks available to monitor the subduction zone, but since the locked portion of the plate boundary lies mostly offshore, these networks are ideally complemented by seafloor observations. Such considerations helped motivate the development of scientific cabled observatories that cross the subduction zone at two sites off Vancouver Island and one off central Oregon, but these have a limited spatial footprint along the strike of the subduction zone. The Pacific Northwest Seismic Network is leading a collaborative effort to implement an earthquake early warning system in the Washington and Oregon using data streams from land networks as well as the few existing offshore instruments. For subduction zone earthquakes that initiate offshore, this system will provide a warning. However, the availability of real time offshore instrumentation along the entire subduction zone would improve its reliability and accuracy, add up to 15 s to the warning time, and ensure an early warning for coastal communities near the epicenter. Furthermore, real-time networks of seafloor pressure sensors above the subduction zone would enable monitoring and contribute to accurate predictions of the incoming tsunami. There is also strong scientific motivation for offshore monitoring. We lack a complete knowledge of the plate convergence rate and direction. Measurements of steady deformation and observations of transient processes such as fluid pulsing, microseismic cycles, tremor and slow-slip are necessary for assessing the dimensions of the locked zone and its along-strike segmentation. Long-term monitoring will also provide baseline observations that can be used to detect and evaluate changes in the subduction environment. There are significant engineering challenges to be solved to ensure the system is sufficiently reliable and maintainable. It must provide continuous monitoring over its operational life in the harsh ocean environment and at least parts of the system must continue to operate following a megathrust event. These requirements for robustness must be balanced with the desire for a flexible design that can accommodate new scientific instrumentation over the life of the project.

The thermochemical, two-phase dynamics of subduction zones: results from new, fully coupled models

NASA Astrophysics Data System (ADS)

Rees Jones, D. W.; Katz, R. F.; May, D.; Tian, M.; Rudge, J. F.

2017-12-01

Subduction zones are responsible for most of Earth's subaerial volcanism. However, previous geodynamic modelling of subduction zones has largely neglected magmatism. We previously showed that magmatism has a significant thermal impact, by advecting sensible heat into the lithosphere beneath arc volcanos [1]. Inclusion of this effect helps reconcile subduction zone models with petrological and heat flow observations. Many important questions remain, including how magma-mantle dynamics of subduction zones affects the position of arc volcanos and the character of their lavas. In this presentation, we employ a fully coupled, thermochemical, two-phase flow theory to investigate the dynamics of subduction zones. We present the first results from our new software (SubFUSc), which solves the coupled equations governing conservation of mass, momentum, energy and chemical species. The presence and migration of partial melts affect permeability and mantle viscosity (both directly and through their thermal impact); these, in turn, feed back on the magma-mantle flow. Thus our fully coupled modelling improves upon previous two-phase models that decoupled the governing equations and fixed the thermal structure [2]. To capture phase change, we use a novel, simplified model of the mantle melting in the presence of volatile species. As in the natural system, volatiles are associated with low-degree melting at temperatures beneath the anhydrous solidus; dehydration reactions in the slab supply volatiles into the wedge, triggering silicic melting. We simulate the migration of melts under buoyancy forces and dynamic pressure gradients. We thereby demonstrate the dynamical controls on the pattern of subduction-zone volcanism (particularly its location, magnitude, and chemical composition). We build on our previous study of the thermal consequences of magma genesis and segregation. We address the question of what controls the location of arc volcanoes themselves [3]. [1] Rees Jones, D. W., Katz, R. F., Tian, M and Rudge, J. F. (2017). Thermal impact of magmatism in subduction zones. arxiv.org/abs/1701.02550 [2] Wilson, C. R., Spiegelman, M., van Keken, P. E., & Hacker, B. R. (2014). EPSL, doi:10.1016/j.epsl.2014.05.052 [3] England, P. C., Katz, Richard F. (2010). Nature, doi:10.1038/nature09417

Carbon dioxide released from subduction zones by fluid-mediated reactions

NASA Astrophysics Data System (ADS)

Ague, Jay J.; Nicolescu, Stefan

2014-05-01

The balance between the subduction of carbonate mineral-bearing rocks into Earth's mantle and the return of CO2 to the atmosphere by volcanic and metamorphic degassing is critical to the carbon cycle. Carbon is thought to be released from subducted rocks mostly by simple devolatilization reactions. However, these reactions will also retain large amounts of carbon within the subducting slab and have difficulty in accounting for the mass of CO2 emitted from volcanic arcs. Carbon release may therefore occur via fluid-induced dissolution of calcium carbonate. Here we use carbonate δ18O and δ13C systematics, combined with analyses of rock and fluid inclusion mineralogy and geochemistry, to investigate the alteration of the exhumed Eocene Cycladic subduction complex on the Syros and Tinos islands, Greece. We find that in marble rocks adjacent to two fluid conduits that were active during subduction, the abundance of calcium carbonate drastically decreases approaching the conduits, whereas silicate minerals increase. Up to 60-90% of the CO2 was released from the rocks--far greater than expected via simple devolatilization reactions. The δ18O of the carbonate minerals is 5-10 lighter than is typical for metamorphosed carbonate rocks, implying that isotopically light oxygen was transported by fluid infiltration from the surroundings. We suggest that fluid-mediated carbonate mineral removal, accompanied by silicate mineral precipitation, provides a mechanism for the release of enormous amounts of CO2 from subduction zones.

Probing the Detailed Seismic Velocity Structure of Subduction Zones Using Advanced Seismic Tomography Methods

NASA Astrophysics Data System (ADS)

Zhang, H.; Thurber, C. H.

2005-12-01

Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the ability of double-difference tomography [Zhang and Thurber, 2003] to resolve the fine-scale structure near the source region and the favorable seismicity distribution inside many subducting slabs, it is now possible to characterize the fine details of the velocity structure and earthquake locations inside the slab, as shown in the study of the Japan subduction zone [Zhang et al., 2004]. We further develop the double-difference tomography method in two aspects: the first improvement is to use an adaptive inversion mesh rather than a regular inversion grid and the second improvement is to determine a reliable Vp/Vs structure using various strategies rather than directly from Vp and Vs [see our abstract ``Strategies to solve for a better Vp/Vs model using P and S arrival time'' at Session T29]. The adaptive mesh seismic tomography method is based on tetrahedral diagrams and can automatically adjust the inversion mesh according to the ray distribution so that the inversion mesh nodes are denser where there are more rays and vice versa [Zhang and Thurber, 2005]. As a result, the number of inversion mesh nodes is greatly reduced compared to a regular inversion grid with comparable spatial resolution, and the tomographic system is more stable and better conditioned. This improvement is quite valuable for characterizing the fine structure of the subduction zone considering the highly uneven distribution of earthquakes within and around the subducting slab. The second improvement, to determine a reliable Vp/Vs model, lies in jointly inverting Vp, Vs, and Vp/Vs using P, S, and S-P times in a manner similar to double-difference tomography. Obtaining a reliable Vp/Vs model of the subduction zone is more helpful for understanding its mechanical and petrologic properties. Our applications of the original version of double-difference tomography to several subduction zones beneath northern Honshu, Japan, the Wellington region, New Zealand, and Alaska, United States, have shown evident velocity variations within and around the subducting slab, which likely is evidence of dehydration reactions of various hydrous minerals that are hypothesized to be responsible for intermediate depth earthquakes. We will show the new velocity models for these subduction zones by applying our advanced tomographic methods.

Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block

PubMed Central

Chan, Lung Sang; Gao, Jian-Feng

2017-01-01

The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab. PMID:28182640

GPS Monitoring of Subduction Zone Deformation in Costa Rica

NASA Technical Reports Server (NTRS)

Lundgren, Paul

1997-01-01

The subduction of the Cocos plate beneath Costa Rica is among the highest convergence rates in the world. The high subduction rate and nearness of the Nicoya Peninsula, Costa Rica to the Middle America Trench (MAT) provide a unique opportunity to map variations in interseismic strain of the crust above the seismogenic zone in response to variations in seismic coupling.

Molybdenum isotope systematics in subduction zones

NASA Astrophysics Data System (ADS)

König, Stephan; Wille, Martin; Voegelin, Andrea; Schoenberg, Ronny

2016-08-01

This study presents Mo isotope data for arc lavas from different subduction zones that range between δ 98 / 95 Mo = - 0.72 and + 0.07 ‰. Heaviest isotope values are observed for the most slab fluid dominated samples. Isotopically lighter signatures are related to increasing relevance of terrigenous sediment subduction and sediment melt components. Our observation complements previous conclusions that an isotopically heavy Mo fluid flux likely mirrors selective incorporation of isotopically light Mo in secondary minerals within the subducting slab. Analogue to this interpretation, low δ 98 / 95 Mo flux that coincides with terrigenous sediment subduction and sediment melting cannot be simply related to a recycled input signature. Instead, breakdown of the controlling secondary minerals during sediment melting may release the light component and lead to decreasing δ 98 / 95 Mo influx into subarc mantle sources. The natural range between slab dehydration and hydrous sediment melting may thus cause a large spread of δ 98 / 95 Mo in global subduction zone magmas.

A geophysical potential field study to image the Makran subduction zone in SE of Iran

NASA Astrophysics Data System (ADS)

Abedi, Maysam; Bahroudi, Abbas

2016-10-01

The Makran subduction wedge as one of the largest subduction complexes has been forming due to the Arabian oceanic lithosphere subducting beneath the Lut and the Afghan rigid block microplates. To better visualize the subducting oceanic crust in this region, a geophysical model of magnetic susceptibility from an airborne magnetic survey (line spacing about 7.5 km) over the Makran zone located at southeast of Iran is created to image various structural units in Iran plate. The constructed geophysical model from the 3D inverse modeling of the airborne magnetic data indicates a thin subducting slab to the north of the Makran structural zone. It is demonstrated that the thickness of sedimentary units varies approximately at an interval of 7.5-11 km from north to south of this zone in the Iranian plate, meanwhile the curie depth is also estimated approximately < 26 km. It is also shown the Jazmurian depression zone adjacent to the north of the Makran indicates high intensity magnetic anomalies due to being underlain by an ophiolite oceanic basement, while such intensity reduces over the Makran. The directional derivatives of the magnetic field data have subtle changes in the Makran, but strongly increase in the Jazmurian by enhancing and separating different structural boundaries in this region. In addition, the density variations of the subsurface geological layers were determined by 3D inversion of the ground-based gravity data over the whole study area, where the constructed density model was in good agreement with the magnetic one. According to the outputs of the magnetic susceptibility and the density contrast, the Arabian plate subducts to the north under the Eurasia with a very low dip angle in the Makran structural zone.

Experimental study of boron geochemistry: implications for fluid processes in subduction zones

NASA Astrophysics Data System (ADS)

You, C. F.; Spivack, A. J.; Gieskes, J. M.; Rosenbauer, R.; Bischoff, J. L.

1995-06-01

A comprehensive experimental study, utilizing an autoclave hydrothermal apparatus with a 10B isotopic tracer, has been conducted to monitor the geochemical behavior of sediment B during early subduction zone processes. The partition coefficient of exchangeable B ( K D) was determined over a temperature range of 25-350°C, at 800 bars and a water/rock ratio of 3-1.5 w/w. These K D are shown to be a complex function of temperature, pH, and possibly mineralogy. At low temperatures, K D is significantly high at ˜4 in contrast to the value of essentially zero at temperatures higher than ˜100°C. A K D of zero represents no B adsorption, implying efficient mobilization of exchangeable B at shallow depths during sediment subduction. Our experimental results demonstrate high mobilization of bulk B in sediments (both exchangeable and lattice bound) at elevated temperatures (200-350°C), in good agreement with previous observations of B in metasediments indicating progressive depletion during metamorphism. In addition, this study emphasizes the importance of a possible water/rock ratio dependence of B mobilization. In other words, the degree of sedimentary B mobilization in subduction zones strongly depends on the local thermal structure and porosity distribution. In low geothermal gradient areas, large amounts of porewater are expelled before significant B mobilization has occurred, so that some sedimentary B will survive and get into the deeper parts of the subduction zone. Our results imply that efficient mobilization of B from the subducted slab must occur and that arc magmatism recycles most of the remaining subducted B back to surface reservoirs. A reconsideration of the B budget in subduction zones provides critical information with respect to B sources and sinks in the ocean.

High-resolution numerical modeling of tectonic underplating in circum-Pacific subduction zones: toward a better understanding of deformation in the episodic tremor and slip region?

NASA Astrophysics Data System (ADS)

Menant, A.; Angiboust, S.; Gerya, T.; Lacassin, R.; Simoes, M.; Grandin, R.

2017-12-01

Study of now-exhumed ancient subduction systems have evidenced km-scale tectonic units of marine sediments and oceanic crust that have been tectonically underplated (i.e. basally accreted) from the downgoing plate to the overriding plate at more than 30-km depth. Such huge mass transfers must have a major impact, both in term of long-term topographic variations and seismic/aseismic deformation in subduction zones. However, the quantification of such responses to the underplating process remains poorly constrained. Using high-resolution visco-elasto-plastic thermo-mechanical models, we present with unprecedented details the dynamics of formation and destruction of underplated complexes in subductions zones. Initial conditions in our experiments are defined in order to fit different subduction systems of the circum-Pacific region where underplating process is strongly suspected (e.g. the Cascadia, SW-Japan, New Zealand, and Chilean subduction zones). It appears that whatever the subduction system considered, underplating of sediments and oceanic crust always occur episodically forming a coherent nappe stacking at depths comprised between 10 and 50 km. At higher depth, a tectonic mélange with a serpentinized mantle wedge matrix developed along the plates interface. The size of these underplated complexes changes according to the subduction system considered. For instance, a 15-km thick nappe stacking is obtained for the N-Chilean subduction zone after a series of underplating events. Such an episodic event lasts 4-5 Myrs and can be responsible of a 2-km high uplift in the forearc region. Subsequent basal erosion of these underplated complexes results in their only partial preservation at crustal and mantle depth, suggesting that, after exhumation, only a tiny section of the overall underplated material can be observed nowadays in ancient subduction systems. Finally, tectonic underplating in our numerical models is systematically associated with (1) an increasing thickness of the high-strained subduction channel and (2) an accumulation of fluid-rich materials that serve as an environment for episodic tremor and slip events assisted by tectonic shearing and fluid release and percolation.

Tearing, segmentation, and backstepping of subduction in the Aegean: New insights from seismicity

NASA Astrophysics Data System (ADS)

Bocchini, G. M.; Brüstle, A.; Becker, D.; Meier, T.; van Keken, P. E.; Ruscic, M.; Papadopoulos, G. A.; Rische, M.; Friederich, W.

2018-06-01

This study revisits subduction processes at the Hellenic Subduction Zone (HSZ) including tearing, segmentation, and backstepping, by refining the geometry of the Nubian slab down to 150-180 km depth using well-located hypocentres from global and local seismicity catalogues. At the western termination of the HSZ, the Kefalonia Transform Fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it, respectively. A discontinuity is suggested to exist between the two slabs at shallow depths. The Kefalonia Transform Fault is interpreted as an active Subduction-Transform-Edge-Propagator-fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it. A model reconstructing the evolution of the subduction system in the area of Peloponnese since 34 Ma, involving the backstepping of the subduction to the back-side of Adria, provides seismological evidence that supports the single-slab model for the HSZ and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere. In the area of Rhodes, earthquake hypocentres indicate the presence of a NW dipping subducting slab that rules out the presence of a NE-SW striking Subduction-Transform-Edge-Propagator-fault in the Pliny-Strabo trenches region. Earthquake hypocentres also allow refining the slab tear beneath southwestern Anatolia down to 150-180 km depth. Furthermore, the distribution of microseismicity shows a first-order slab segmentation in the region between Crete and Karpathos, with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east. Thermal models indicate the presence of a colder slab beneath the southeastern Aegean that leads to deepening of the intermediate-depth seismicity. Slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity along the interplate seismogenic zone.

Varying Structure and Physical Properties of the Lithosphere Subducting Beneath Indonesia, Consequences on the Subduction

NASA Astrophysics Data System (ADS)

Jacob, J.; Dyment, J.

2013-12-01

We make inferences on the structure, age and physical properties of the subducting northern Wharton Basin lithosphere by (1) modeling the structure and age of the lithosphere subducted under the Sumatra trench through three-plate reconstructions involving Australia, Antarctica, and India, and (2) superimposing the resulting fracture zones and magnetic isochrons to the geometry of the subducting plate as imaged by seismic tomography. The model of Pesicek et al. (2010) was digitized and smoothed in order to get a realistic topography of the subducting plate. The fracture zone and magnetic isochron geometry was draped on this topography assuming a N18°E direction of subduction. This model provides an effective means to study the effect of varying physical properties of the subducting lithosphere on the subduction along the Sumatra trench. 1) The age of the oceanic lithosphere determines its thickness and buoyancy, then its ability to comply with or resist subduction. We define the "subductability" of the lithosphere as the extra weight applied on the asthenosphere by the part of the bulk lithospheric density exceeding the asthenospheric density. A negative subductability means that the bulk lithospheric density is lower than the asthenospheric density, i.e. the plate will resist subduction, which is the case for lithosphere less than ~23 Ma. The area off Sumatra corresponds to oceanic lithosphere formed between 80 and 38 Ma, with a lower subductability than other areas along the Sunda Trench. 2) The spreading rate at which the oceanic lithosphere was formed has implications of the structure and composition of the oceanic crust, and therefore on its rheology. In a subduction zone, the contact between the subducting and overriding plates is often considered to be the top of the oceanic crust and the overlying sediments. The roughness of this interface and the rheology of its constitutive material are essential parameters constraining the slip of the down going plate in the seismogenic zone, and therefore the characteristics of the resulting earthquakes. Indeed the rough topography of a slow crust may offer more asperities, and therefore a more irregular slip, than the smooth topography of a fast crust. Conversely, the weak rheology of serpentines present in a slow crust would favor a regular slip, unlike the brittle magmatic rocks of the fast crust and the underlying dry olivine mantle. 3) Local features, including fracture zones and seamounts, may affect the seismic segmentation of the subduction zone. Many seamounts have been mapped in the Wharton Basin between 10°S and 15°S., their age decreasing from 136 Ma to the East to 47 Ma to the West, with anomalously younger ages in Christmas Island. Similar seamounts belonging to the same province may have existed further north and subducted in the Sunda Trench from southern Sumatra to Java and eastward. Conversely, the Roo Rise, a larger plateau located south of Eastern Java, may have more difficulty to enter the subduction, as suggested by the geometry of the Sunda Trench in this area, diverting from the regular arc by a maximum of 60 km. References Pesicek, J.D., C.H. Thurber, S. Widiyantoro, H. Zhang, H.R. DeShon, and E.R. Engdahl (2010), Sharpening the tomographic image of the subducting slab below Sumatra, the Andaman Islands and Burma, Geophys. J. Int., 182, 433-453.

Frictional behavior of carbonate-rich sediments in subduction zones

NASA Astrophysics Data System (ADS)

Rabinowitz, H. S.; Savage, H. M.; Carpenter, B. M.; Collettini, C.

2016-12-01

Deformation in rocks and sediments is controlled by multiple mechanisms, each governed by its own pressure- (P), temperature- (T), and slip velocity- (v) dependent kinetics. Frictional behavior depends on which of these mechanisms are dominant, and, thus, varies with P, T, and v. Carbonates are a useful material with which to interrogate the PTv controls on friction due to the fact that a wide range of mechanisms can be easily accessed in the lab at geologically relevant conditions. In addition, carbonate-rich layers make up a significant component of subducting sediments around the world and may impact the frictional behavior of shallow subduction zones. In order to investigate the effect of carbonate subduction and the evolution of friction at subduction zone conditions, we conducted deformation experiments on input sediments for two subduction zones, the Hikurangi trench, New Zealand (ODP Site 1124) and the Peru trench (DSDP Site 321), which have carbonate/clay contents of 40/60 wt% and 80/20 wt%, respectively. Samples were saturated with distilled water mixed with 35g/l sea salt and deformed at room temperature. Experiments were conducted at σeff = 1-100 MPa and T = 20-100 °C with sliding velocities of 1-300 μm/s and hold times of 1-1000 s. We test the changes in velocity dependence and healing over these PT conditions to elucidate the frictional behavior of carbonates in subduction zone settings. The mechanical results are complemented by microstructural analysis. In lower stress experiments, there is no obvious shear localization; however, by 25 MPa, pervasive boundary-parallel shears become dominant, particularly in the Peru samples. Optical observations of these shear zones under cross-polarized light show evidence of plastic deformation (CPO development) while SEM-EDS observations indicate phase segregation in the boundary shears. Degree of microstructural localization appears to correspond with the trends observed in velocity-dependence. Our preliminary results indicate that carbonate/clay compositions could have a significant impact on the frictional behavior of subducting sediments.

Subduction of aseismic ridges beneath the Caribbean Plate: Implications for the tectonics and seismic potential of the northeastern Caribbean

NASA Astrophysics Data System (ADS)

McCann, William R.; Sykes, Lynn R.

1984-06-01

Normal seafloor entering the Puerto Rico and northern Lesser Antillean trenches in the northeastern Caribbean is interrupted by a series of aseismic ridges on the North and South American plates. These topographic features lie close to the expected trend of fracture zones created about 80-110 m.y. ago when this seafloor was formed at the Mid-Atlantic Ridge. The northernmost of the ridges that interact with the Lesser Antillean subduction zone, the Barracuda Ridge, intersects the arc in a region of high seismic activity. Some of this seismicity including a large shock in 1974, occurs within the overthrust plate and may be related to the deformation of the Caribbean plate as it overrides the ridge. A large bathymetric high, the Main Ridge, is oriented obliquely to the Puerto Rico trench and intersects the subduction zone north of the Virgin Islands in another cluster of seismic activity along the inner wall of the trench. Data from a seismic network in the northeastern Caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity. Magnetic anomalies, bathymetric trends, and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the Main and Barracuda ridges are parts of a formerly continuous aseismic ridge, a segment of which has recently been overridden by the Caribbean plate. Reconstruction of mid-Miocene to Recent plate motions also suggest that at least two aseismic ridges, and possibly fragments of the Bahama Platform, have interacted with the subduction zone in the northeastern Caribbean. The introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long. These ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes. They also leave a geologic imprint on segments of the arc with which they have interacted. A 50-km landward jump of the locus of island arc volcanism occurred in Late Miocene time along the northern half of the Lesser Antilles. We postulate that the subduction of a segment of seafloor of anomolously thick crust, being more buoyant than adjacent seafloor, resulted in a marked shoaling in the dip of the descending slab and, therefore, a shift of the locus of volcanism. In the region near western Puerto Rico and eastern Hispanolia, Plio-Pleistocene interaction with a similar feature, in this case a part of the Bahama Platform, about 3-4 m.y. ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone. That segment of the Bahama Platform interferred with the subduction process and was subsequently sutured onto the Caribbean plate when the boundary jumped about 60 km to the northeast. The maximum size of historic shallow earthquakes along the Lesser Antillean arc varies from about 7.0-7.5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8.0-8.5 along the northern part of the arc where the dip is shallow. The interaction of anomalous seafloor, as along the northern Lesser Antilles, can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones. Major seismic gaps and regions of high seismic potential currently exist along the northern Lesser Antilles and to the north of Puerto Rico. Both gaps are bounded by anomalous features on the downgoing plate. The intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes. A great shock in 1787 may have ruptured an existing seismic gap north of Puerto Rico between 65° and 67°W. Thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes.

Has El Salvador Fault Zone produced M ≥ 7.0 earthquakes? The 1719 El Salvador earthquake

NASA Astrophysics Data System (ADS)

Canora, C.; Martínez-Díaz, J.; Álvarez-Gómez, J.; Villamor, P.; Ínsua-Arévalo, J.; Alonso-Henar, J.; Capote, R.

2013-05-01

Historically, large earthquakes, Mw ≥ 7.0, in the Εl Salvador area have been attributed to activity in the Cocos-Caribbean subduction zone. Τhis is correct for most of the earthquakes of magnitude greater than 6.5. However, recent paleoseismic evidence points to the existence of large earthquakes associated with rupture of the Εl Salvador Fault Ζone, an Ε-W oriented strike slip fault system that extends for 150 km through central Εl Salvador. Τo calibrate our results from paleoseismic studies, we have analyzed the historical seismicity of the area. In particular, we suggest that the 1719 earthquake can be associated with paleoseismic activity evidenced in the Εl Salvador Fault Ζone. Α reinterpreted isoseismal map for this event suggests that the damage reported could have been a consequence of the rupture of Εl Salvador Fault Ζone, rather than rupture of the subduction zone. Τhe isoseismal is not different to other upper crustal earthquakes in similar tectonovolcanic environments. We thus challenge the traditional assumption that only the subduction zone is capable of generating earthquakes of magnitude greater than 7.0 in this region. Τhis result has broad implications for future risk management in the region. Τhe potential occurrence of strong ground motion, significantly higher and closer to the Salvadorian populations that those assumed to date, must be considered in seismic hazard assessment studies in this area.

Processes in continental collision zones: Preface

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Zhang, Lifei; McClelland, William C.; Cuthbert, Simon

2012-04-01

Formation and exhumation of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in continental subduction zones are the two fundamental geodynamic aspects of collisional orogensis. This volume is based on the Session 08c titled "Geochemical processes in continental collision zones" at Goldschmidt 2010 in Knoxville, USA. It focuses on micro- to macro-scale processes that are temporally and spatially linked to different depths of crustal subduction/exhumation and associated mineralogical changes. They are a key to understanding a wide spectrum of phenomena, involving HP/UHP metamorphism and syn-/post-collisional magmatism. Papers in this volume report progresses in petrological, geochronological and geochemical studies of UHP metamorphic rocks and their derivatives in China, with tectonic settings varying from arc-continent collision to continent-continent collision. Microbeam in-situ analyses of metamorphic and magmatic minerals are successfully utilized to solve various problems in the study of continental deep subduction and UHP metamorphism. In addition to their geochronological applications to dating of HP to UHP metamorphic events during continental collision, microbeam techniques have also served as an efficient means to recognize different generations of mineral growth during continental subduction-zone metamorphism. Furthermore, metamorphic dehydration and partial melting of UHP metamorphic rocks during subduction and exhumation are highlighted with respect to their effects on fluid action and element mobilization. These have provided new insights into chemical geodynamics in continental subduction zones.

­­New Finite-Frequency Teleseismic P-wave Tomography of the Anatolian Sub-continent and the Fate of the Subducted Cyprean Slab

NASA Astrophysics Data System (ADS)

Portner, D. E.; Biryol, C. B.; Delph, J. R.; Beck, S. L.; Zandt, G.; Özacar, A.; Sandvol, E. A.; Turkelli, N.

2016-12-01

The eastern Mediterranean region is characterized by active subduction of Tethyan lithosphere beneath the Anatolian sub-continent at the Aegean and Cyprean trenches. The subduction system is historically characterized by slab roll-back, detachment, and slab settling in the mantle transition zone. Prior mantle tomography studies reveal segmentation of the subducted Tethyan lithosphere, which is thought to have a strong control on surface volcanism and uplift across Anatolia. However, tomographic resolution, particularly in central Anatolia, has been limited, thus making detailed delineations of the subducted slab segments difficult. To improve resolution, we combine two years of seismic data from the recent Continental Dynamics - Central Anatolia Tectonics (CD-CAT) seismic deployment and Turkey's national seismic network ( 33,000 residuals) to 33,000 travel time residuals from Biryol et al. (2011, GJI) in a new finite-frequency teleseismic P-wave tomographic inversion. Our new images reveal with detail a complicated geometry of fast velocity anomalies associated with subducted Tethyan lithosphere. At shallow depths, slow velocities separate the fast anomalies connected to the Aegean and Cyprean trenches. The fast anomaly connected to the Cyprean trench has an arcuate shape in map view, following the trace of the Central Taurus Mountains. This anomaly is separated from a high-amplitude block to the north that appears to dip sub-vertically throughout the upper mantle (200-660 km depth). Other blocks of fast material that may represent subducted Tethyan lithosphere appear down-dip of the vertical block. Additionally, our images indicate that some of the fast velocity anomalies previously seen to flatten in the mantle transition zone may continue into the lower mantle. Thus, our new images provide a more detailed picture of the fate of the Cyprean slab and suggest that some of the fast anomalies associated with the slab continue into the lower mantle, bringing to question the traditional view of a slab graveyard in the mantle transition zone in this region.

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

USGS Publications Warehouse

Sorensen, Sorena S.; Grossman, J.N.

1989-01-01

The abundance, P-T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600??C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high-T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of "average" island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. "Exotic" LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by "subduction-zone metasomatism," although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments. ?? 1989.

Improved High Resolution Models of Subduction Dynamics: Use of transversely isotropic viscosity with a free-surface

NASA Astrophysics Data System (ADS)

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

2017-12-01

Dynamic topography, or uncompensated topography, is controlled by internal dynamics, and provide constraints on the buoyancy structure and rheological parameters in the mantle. Compared with other surface manifestations such as the geoid, dynamic topography is very sensitive to shallower and more regional mantle structure. For example, the significant dynamic topography above the subduction zone potentially provides a rich mine for inferring the rheological and mechanical properties such as plate coupling, flow, and lateral viscosity variations, all critical in plate tectonics. However, employing subduction zone topography in the inversion study requires that we have a better understanding of the topography from forward models, especially the influence of the viscosity formulation, numerical resolution, and other factors. One common approach to formulating a fault between the subducted slab and the overriding plates in viscous flow models assumes a thin weak zone. However, due to the large lateral variation in viscosity, topography from free-slip numerical models typically has artificially large magnitude as well as high-frequency undulations over subduction zone, which adds to the difficulty in making comparisons between model results and observations. In this study, we formulate a weak zone with the transversely isotropic viscosity (TI) where the tangential viscosity is much smaller than the viscosity in the normal direction. Similar with isotropic weak zone models, TI models effectively decouple subducted slabs from the overriding plates. However, we find that the topography in TI models is largely reduced compared with that in weak zone models assuming an isotropic viscosity. Moreover, the artificial `tooth paste' squeezing effect observed in isotropic weak zone models vanishes in TI models, although the difference becomes less significant when the dip angle is small. We also implement a free-surface condition in our numerical models, which has a smoothing effect on the topography. With the improved model configuration, we can use the adjoint inversion method in a high-resolution model and employ topography in addition to other observables such as the plate motion to infer critical mechanical and rheological parameters in the subduction zone.

Megathrust Slip and the Care and Feeding of the Subduction Channel Through which the Seismogenic Zone Runs

NASA Astrophysics Data System (ADS)

Scholl, D. W.; Kirby, S. H.; Keranen, K. M.; Wells, R. E.; Blakely, R. J.; Michael, F.; von Huene, R.

2007-12-01

HABITATS OF GREAT OFFSHORE EARTHQUAKES: High-magnitude earthquakes (Mw = or >8.5) and trans- oceanic tsunamis commonly nucleate along subduction zones (SZ) bordered by laterally continuous, sediment- flooded trenches. Examples include: south-central Chile (1960 Mw=9.5), eastern Alaska (1964 Mw=9.2), Sumatra (2004, Mw=9.1), Cascadia (historic 1700 Mw=9.0), Colombia (1906 Mw=8.8), Sumatra (historic 1883, Mw=8.8), west-central Aleutian (1965 Mw=8.7), central Aleutian (1986, Mw=8.7), Sumatra (2005 Mw=8.6), and Nankai (historic 1707, Mw=8.5). In thickness, sediment entering these SZ ranges from 2 to 3 km and the column is axially continuous for more than 800 km. The depositional pile is typically the clastic beds of a trench-axis turbidite wedge and underlying fan and abyssal plain deposits that accrued seaward of the trench axis. Great rupture events also occur at subduction zones receiving little sediment, for example the Kamchatka (1952, Mw=9.0) and the north Chile SZs (historic 1868 Mw=8.9). Both SZs are areas of rapid upper plate thinning, subsidence, and truncation effected by subduction erosion. WORKINGS OF THE SUBDUCTION CHANNEL (SC): Beneath the submerged forearc, the SC functions to transport subducted ocean floor sediment and tectonically eroded forearc debris toward and into the mantle. The SC is the lowest structural unit containing upper plate crustal material. It hosts the seismogenic zone, which probably runs along the SC's upper boundary commonly referred to as the interplate decollement. A thick, laterally continuous SC structurally smoothes or simplifies the surface of the interplate decollement and sets up conditions for lengthy, high moment-release ruptures. Maximum slip is commonly concentrated beneath the thinned crust underlying forearc basins. These structures, in positive feed-back, are likely deepened co- seismically by high-slip-rate enhanced basal subduction erosion. The detached material lowers the effective stress on the decollement and further evens this interface. The channel also works tectonically to underplate the base of the inner margin and induce uplift and co-seismic activation of high-angle reverse faults. CONSEQUENCES OF WHAT IS FED SUBDUCTION ZONES: Ridges and high relief entering the SZ can act to arrest lateral rupturing. Supplying sedimentary and erosional debris to the subduction channel appears to act differently and favors the continuation of rupture, rapid slip beneath crustally thinned areas that can be translated upward at forearc splay faults to generate trans-oceanic tsunamis, and nearshore reverse-fault can spawn near- field tsunamis. The potential for great earthquake nucleation along thickly sediment SZs must be set high. Similarly, seismogenic risk for highly erosional SZ little perturbed by subducting relief must also be set high. Margins undergoing rapid tectonic erosion produce regional tsunamis but perhaps not trans-oceanic waves of great destructiveness.

The Hellenic Subduction Zone: A tomographic image and its geodynamic implications

NASA Astrophysics Data System (ADS)

Spakman, W.; Wortel, M. J. R.; Vlaar, N. J.

1988-01-01

New tomographic images of the Hellenic subduction zone demonstrate slab penetration in the Aegean Upper Mantle to depths of at least 600 km. Beneath Greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern Aegean. Schematically we derive minimum time estimates for the duration of the Hellenic subduction zone that range from 26 to 40 Ma. This is considerably longer than earlier estimates which vary between 5 and about 13 Ma.

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).

Evolution of passive continental margins and initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, S. A. P. L.; Wortel, M. J. R.; Vlaar, N. J.

1982-05-01

Although the initiation of subduction is a key element in plate tectonic schemes for evolution of lithospheric plates, the underlying mechanisms are not well understood. Plate rupture is an important aspect of the process of creating a new subduction zone, as stresses of the order of kilobars are required to fracture oceanic lithosphere1. Therefore initiation of subduction could take place preferentially at pre-existing weakness zones or in regions where the lithosphere is prestressed. As such, transform faults2,3 and passive margins4,5 where the lithosphere is downflexed under the influence of sediment loading have been suggested. From a model study of passive margin evolution we found that ageing of passive margins alone does not make them more suitable sites for initiation of subduction. However, extensive sediment loading on young lithosphere might be an effective mechanism for closure of small ocean basins.

Carbonation by fluid-rock interactions at high-pressure conditions: Implications for carbon cycling in subduction zones

NASA Astrophysics Data System (ADS)

Piccoli, Francesca; Vitale Brovarone, Alberto; Beyssac, Olivier; Martinez, Isabelle; Ague, Jay J.; Chaduteau, Carine

2016-07-01

Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon, regulating its fluxes between shallow and deep reservoirs. Recent estimates predict that almost all subducted carbon is transferred into the crust and lithospheric mantle during subduction metamorphism via decarbonation and dissolution reactions at high-pressure conditions. Here we report the occurrence of eclogite-facies marbles associated with metasomatic systems in Alpine Corsica (France). The occurrence of these marbles along major fluid-conduits as well as textural, geochemical and isotopic data indicating fluid-mineral reactions are compelling evidence for the precipitation of these carbonate-rich assemblages from carbonic fluids during metamorphism. The discovery of metasomatic marbles brings new insights into the fate of carbonic fluids formed in subducting slabs. We infer that rock carbonation can occur at high-pressure conditions by either vein-injection or chemical replacement mechanisms. This indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but can interact with slab and mantle-forming rocks. Rock-carbonation by fluid-rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Furthermore, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

Small-scale Forearc Structure from Residual Bathymetry and Vertical Gravity Gradients at the Cocos-North America Subduction Zone offshore Mexico

NASA Astrophysics Data System (ADS)

Garcia, E. S. M.; Ito, Y.

2017-12-01

The subduction of topographic relief on the incoming plate at subduction zones causes deformation of the plate interface as well as the overriding plate. Whether the resulting geometric irregularities play any role in inhibiting or inducing seismic rupture is a topic of relevance for megathrust earthquake source studies. A method to discern the small-scale structure at subduction zone forearcs was recently developed by Bassett and Watts (2015). Their technique constructs an ensemble average of the trench-perpendicular topography, and the removal of this regional tectonic signal reveals the short-wavelength residual bathymetric anomalies. Using examples from selected areas at the Tonga, Mariana, and Japan subduction zones, they were able to link residual bathymetric anomalies to the subduction of seamount chains, given the similarities in wavelength and amplitude to the morphology of seamounts that have yet to subduct. We focus here on an analysis of forearc structures found in the Mexico segment of the Middle America subduction zone, and their potential mechanical interaction with areas on the plate interface that have been previously identified as source regions for earthquake ruptures and aseismic events. We identified several prominent residual bathymetric anomalies off the Guerrero and Oaxaca coastlines, mainly in the shallow portion of the plate interface and between 15 and 50 kilometers away from the trench axis. The residual amplitude of these bathymetric anomalies is typically in the hundreds of meters. Some of the residual bathymetric anomalies offshore Oaxaca are found landward of seamount chains on the incoming Cocos Plate, suggesting that these anomalies are associated with the prior subduction of seamounts at the margin. We also separated the residual and regional components of satellite-based vertical gravity gradient data using a directional median filter to isolate the possible gravity signals from the seamount edifices.

The great Lisbon earthquake and tsunami of 1755: lessons from the recent Sumatra earthquakes and possible link to Plato's Atlantis

NASA Astrophysics Data System (ADS)

Gutscher, M.-A.

2006-05-01

Great earthquakes and tsunami can have a tremendous societal impact. The Lisbon earthquake and tsunami of 1755 caused tens of thousands of deaths in Portugal, Spain and NW Morocco. Felt as far as Hamburg and the Azores islands, its magnitude is estimated to be 8.5 9. However, because of the complex tectonics in Southern Iberia, the fault that produced the earthquake has not yet been clearly identified. Recently acquired data from the Gulf of Cadiz area (tomography, seismic profiles, high-resolution bathymetry, sampled active mud volcanoes) provide strong evidence for an active east dipping subduction zone beneath Gibraltar. Eleven out of 12 of the strongest earthquakes (M>8.5) of the past 100 years occurred along subduction zone megathrusts (including the December 2004 and March 2005 Sumatra earthquakes). Thus, it appears likely that the 1755 earthquake and tsunami were generated in a similar fashion, along the shallow east-dipping subduction fault plane. This implies that the Cadiz subduction zone is locked (like the Cascadia and Nankai/Japan subduction zones), with great earthquakes occurring over long return periods. Indeed, the regional paleoseismic record (contained in deep-water turbidites and shallow lagoon deposits) suggests great earthquakes off South West Iberia every 1500 2000 years. Tsunami deposits indicate an earlier great earthquake struck SW Iberia around 200 BC, as noted by Roman records from Cadiz. A written record of even older events may also exist. According to Plato's dialogues The Critias and The Timaeus, Atlantis was destroyed by ‘strong earthquakes and floods … in a single day and night’ at a date given as 11,600 BP. A 1 m thick turbidite deposit, containing coarse grained sediments from underwater avalanches, has been dated at 12,000 BP and may correspond to the destructive earthquake and tsunami described by Plato. The effects on a paleo-island (Spartel) in the straits of Gibraltar would have been devastating, if inhabited, and may have formed the basis for the Atlantis legend.

Large Earthquake Potential in the Southeast Caribbean

NASA Astrophysics Data System (ADS)

Mencin, D.; Mora-Paez, H.; Bilham, R. G.; Lafemina, P.; Mattioli, G. S.; Molnar, P. H.; Audemard, F. A.; Perez, O. J.

2015-12-01

The axis of rotation describing relative motion of the Caribbean plate with respect to South America lies in Canada near Hudson's Bay, such that the Caribbean plate moves nearly due east relative to South America [DeMets et al. 2010]. The plate motion is absorbed largely by pure strike slip motion along the El Pilar Fault in northeastern Venezuela, but in northwestern Venezuela and northeastern Colombia, the relative motion is distributed over a wide zone that extends from offshore to the northeasterly trending Mérida Andes, with the resolved component of convergence between the Caribbean and South American plates estimated at ~10 mm/yr. Recent densification of GPS networks through COLOVEN and COCONet including access to private GPS data maintained by Colombia and Venezuela allowed the development of a new GPS velocity field. The velocity field, processed with JPL's GOA 6.2, JPL non-fiducial final orbit and clock products and VMF tropospheric products, includes over 120 continuous and campaign stations. This new velocity field along with enhanced seismic reflection profiles, and earthquake location analysis strongly suggest the existence of an active oblique subduction zone. We have also been able to use broadband data from Venezuela to search slow-slip events as an indicator of an active subduction zone. There are caveats to this hypothesis, however, including the absence of volcanism that is typically concurrent with active subduction zones and a weak historical record of great earthquakes. A single tsunami deposit dated at 1500 years before present has been identified on the southeast Yucatan peninsula. Our simulations indicate its probable origin is within our study area. We present a new GPS-derived velocity field, which has been used to improve a regional block model [based on Mora and LaFemina, 2009-2012] and discuss the earthquake and tsunami hazards implied by this model. Based on the new geodetic constraints and our updated block model, if part of the region slipped 2.5 m (500 yrs x 5 mm/yr) in a single 200 km x 200 km rupture, the moment-magnitude of the event would exceed Mw = 8.3. We hypothesize that an active subduction zone exists and supports great earthquake events with a strong possibility of destructive tsunamis, which makes this region the one with the largest seismic hazard in the circum-Caribbean.

Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai

NASA Astrophysics Data System (ADS)

Saffer, Demian M.

2003-05-01

At subduction zones, pore pressure affects fault strength, deformation style, structural development, and potentially the updip limit of seismogenic faulting behavior through its control on effective stress and consolidation state. Despite its importance for a wide range of subduction zone processes, few detailed measurements or estimates of pore pressure at subduction zones exist. In this paper, I combine logging-while-drilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones, to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction. At Costa Rica, my results suggest that the lower portion of the underthrust section remains nearly undrained, whereas the upper portion is partially drained. An inferred minimum in effective stress developed within the section ˜1.5 km landward of the trench is consistent with core and seismic observations of faulting, and illustrates the important effects of heterogeneous drainage on structural development. Inferred pore pressures at the Nankai and northern Barbados subduction zones indicate nearly undrained conditions throughout the studied intervals, and are consistent with existing direct measurements and consolidation test results. Slower dewatering at Nankai and Barbados than at Costa Rica can be attributed to higher permeability and larger compressibility of near-surface sediments underthrust at Costa Rica. Results for the three margins indicate that the pore pressure ratio (λ) in poorly drained underthrust sediments should increase systematically with distance landward of the trench, and may vary with depth.

Plate tectonic controls on atmospheric CO2 levels since the Triassic.

PubMed

Van Der Meer, Douwe G; Zeebe, Richard E; van Hinsbergen, Douwe J J; Sluijs, Appy; Spakman, Wim; Torsvik, Trond H

2014-03-25

Climate trends on timescales of 10s to 100s of millions of years are controlled by changes in solar luminosity, continent distribution, and atmosphere composition. Plate tectonics affect geography, but also atmosphere composition through volcanic degassing of CO2 at subduction zones and midocean ridges. So far, such degassing estimates were based on reconstructions of ocean floor production for the last 150 My and indirectly, through sea level inversion before 150 My. Here we quantitatively estimate CO2 degassing by reconstructing lithosphere subduction evolution, using recent advances in combining global plate reconstructions and present-day structure of the mantle. First, we estimate that since the Triassic (250-200 My) until the present, the total paleosubduction-zone length reached up to ∼200% of the present-day value. Comparing our subduction-zone lengths with previously reconstructed ocean-crust production rates over the past 140 My suggests average global subduction rates have been constant, ∼6 cm/y: Higher ocean-crust production is associated with longer total subduction length. We compute a strontium isotope record based on subduction-zone length, which agrees well with geological records supporting the validity of our approach: The total subduction-zone length is proportional to the summed arc and ridge volcanic CO2 production and thereby to global volcanic degassing at plate boundaries. We therefore use our degassing curve as input for the GEOCARBSULF model to estimate atmospheric CO2 levels since the Triassic. Our calculated CO2 levels for the mid Mesozoic differ from previous modeling results and are more consistent with available proxy data.

Plate tectonic controls on atmospheric CO2 levels since the Triassic

PubMed Central

Van Der Meer, Douwe G.; Zeebe, Richard E.; van Hinsbergen, Douwe J. J.; Sluijs, Appy; Spakman, Wim; Torsvik, Trond H.

2014-01-01

Climate trends on timescales of 10s to 100s of millions of years are controlled by changes in solar luminosity, continent distribution, and atmosphere composition. Plate tectonics affect geography, but also atmosphere composition through volcanic degassing of CO2 at subduction zones and midocean ridges. So far, such degassing estimates were based on reconstructions of ocean floor production for the last 150 My and indirectly, through sea level inversion before 150 My. Here we quantitatively estimate CO2 degassing by reconstructing lithosphere subduction evolution, using recent advances in combining global plate reconstructions and present-day structure of the mantle. First, we estimate that since the Triassic (250–200 My) until the present, the total paleosubduction-zone length reached up to ∼200% of the present-day value. Comparing our subduction-zone lengths with previously reconstructed ocean-crust production rates over the past 140 My suggests average global subduction rates have been constant, ∼6 cm/y: Higher ocean-crust production is associated with longer total subduction length. We compute a strontium isotope record based on subduction-zone length, which agrees well with geological records supporting the validity of our approach: The total subduction-zone length is proportional to the summed arc and ridge volcanic CO2 production and thereby to global volcanic degassing at plate boundaries. We therefore use our degassing curve as input for the GEOCARBSULF model to estimate atmospheric CO2 levels since the Triassic. Our calculated CO2 levels for the mid Mesozoic differ from previous modeling results and are more consistent with available proxy data. PMID:24616495

Frictional behavior of carbonate-rich incoming sediment in the Hikurangi subduction zone

NASA Astrophysics Data System (ADS)

Rabinowitz, H. S.; Savage, H. M.; Carpenter, B.; Ikari, M.; Collettini, C.

2017-12-01

In recent years, the traditional view of the seismogenic zone has been challenged by observations of a range of seismic behaviors both above and below the depths previously considered capable of nucleating earthquakes. The Hikurangi trench is one of the few subduction zones where this transitional seismic behavior has been observed at the shallowest portions of the subduction zone, providing an opportunity to investigate the mechanical controls on seismic behavior through measurements of directly sampled sediment. To this end, an IODP cruise (March-May, 2018; Exp. 375) will recover sample from the faults that participate in this shallow seismic behavior. In order to obtain preliminary frictional characterization of the sedimentary inputs to the Hikurangi Trench, we conducted deformation experiments on samples from an ocean drill core through the incoming sediments (ODP Site 1124). The sedimentary package subducting at Hikurangi contains carbonate-rich lithologies, which have been shown to be more frictionally unstable (velocity-weakening, high healing rates) than the clays that comprise the majority of the sedimentary inputs to global subduction zones. Such frictional properties could promote seismic behavior in the shallower reaches of the subduction zone. We focus on a section of ODP Site 1124 which has a carbonate content of 40 wt% to investigate the effect of this lithology. Samples were saturated with distilled water mixed with 35 g/l sea salt. Velocity-stepping and slide-hold-slide tests were performed in multiple biaxial and triaxial deformation apparatus to investigate a range of pressures, temperatures and velocities relevant to the shallow subduction zone (σeff = 1-150 MPa, sliding velocities of 1.7 nm/s-300 μm/s, hold times of 1-1000 s, and T = 20-100 ºC). We observe transitions from velocity-strengthening to velocity-weakening behavior over these conditions which could contribute to shallow seismic behavior in the Hikurangi trench.

Self-Sustained Mode-3 Tear Controls Dynamics of Narrow Retreating Subduction Zones

NASA Astrophysics Data System (ADS)

Munch, J.; Gerya, T.; Ueda, K.

2017-12-01

The Caribbean oroclinal basin exhibits several narrow retreating slabs in an oceanic domain. The slabs show a curved shape associated to a bent topography (trench). We propose that the curvature of the topography depends on slab retreat mechanisms following mode-3 tearing at the edges of the slab (out of the plane fracture propagation). While first-order characteristics have been principally reproduced in self-sustained subduction initiation models (Gerya et al., 2015, Nature, 527, 221-225), the relevant observations have not been quantified and the exact mechanism is not understood. In this work, we study the long-term 3D evolution of narrowing oceanic subduction zones during retreat, and investigate the link between mode-3 tear and orocline formation. Numerical experiments are carried out with a thermo-mechanical 3D finite-difference code. To allow the observation of developing topography, the precise location of the internal surface and its evolution by material diffusion is tracked. Retreating subduction is facilitated via a strong age contrast between a young lithosphere window enclosed by shear zones and the surrounding lithosphere. By varying the length and thickness of the shear zones and location of the age transition, the influence of these parameters on the tearing process and the development of topography is assessed. Experiments trigger subduction initiation and slab retreat via fracture zone collapse and spontaneous paired mode-3 tear propagation within the oceanic plate interior. Narrow retreating subducting slabs form as a natural result of the spontaneous paired tearing process. A curved trench forms along with slab retreat. Topography evolution and tearing trajectory appear to be dependent on the initial shear zones and young window dimensions. We also note a strong narrowing of the slab during the retreat (several tens of kilometers over 800 km of retreat). Overall, results indicate that narrowing of retreating slabs is a self-consistent consequence of tear propagation dynamics. This plate tearing mechanism may control dynamics of other narrow retreating subduction zones worldwide.

Thermal state of the Explorer segment of the Cascadia subduction zone: Implications for seismic and tsunami hazards

NASA Astrophysics Data System (ADS)

Gao, Dawei; Wang, Kelin; Davis, Earl E.; Jiang, Yan; Insua, Tania L.; He, Jiangheng

2017-04-01

The Explorer segment of northernmost Cascadia is an end-member "warm" subduction zone with very young incoming plate and slow-convergence rate. Understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance. Available geodetic observations indicate that the subduction megathrust of the Explorer segment is currently locked to some degree, but the downdip extent of the fault area that is potentially seismogenic is not known. Here we construct finite-element models to estimate the thermally allowed megathrust seismogenic zone, using available knowledge of regional plate kinematics, structural data, and heat flow observations as constraints. Despite ambiguities in plate interface geometry constrained by hypocenter locations of low-frequency earthquakes beneath Vancouver Island, the thermal models suggest a potential rupture zone of ˜60 km downdip width located fully offshore. Using dislocation modeling, we further illustrate that a rupture zone of this size, even with a conservative assumption of ˜100 km strike length, can cause significant tsunami-genic deformation. Future seismic and tsunami hazard assessment in northern Cascadia must take the Explorer segment into account.

Rheological separation of the megathrust seismogenic zone and episodic tremor and slip

NASA Astrophysics Data System (ADS)

Gao, Xiang; Wang, Kelin

2017-03-01

Episodic tremor and accompanying slow slip, together called ETS, is most often observed in subduction zones of young and warm subducting slabs. ETS should help us to understand the mechanics of subduction megathrusts, but its mechanism is still unclear. It is commonly assumed that ETS represents a transition from seismic to aseismic behaviour of the megathrust with increasing depth, but this assumption is in contradiction with an observed spatial separation between the seismogenic zone and the ETS zone. Here we propose a unifying model for the necessary geological condition of ETS that explains the relationship between the two zones. By developing numerical thermal models, we examine the governing role of thermo-petrologically controlled fault zone rheology (frictional versus viscous shear). High temperatures in the warm-slab environment cause the megathrust seismogenic zone to terminate before reaching the depth of the intersection of the continental Mohorovičić discontinuity (Moho) and the subduction interface, called the mantle wedge corner. High pore-fluid pressures around the mantle wedge corner give rise to an isolated friction zone responsible for ETS. Separating the two zones is a segment of semi-frictional or viscous behaviour. The new model reconciles a wide range of seemingly disparate observations and defines a conceptual framework for the study of slip behaviour and the seismogenesis of major faults.

Subduction-zone magnetic anomalies and implications for hydrated forearc mantle

USGS Publications Warehouse

Blakely, R.J.; Brocher, T.M.; Wells, R.E.

2005-01-01

Continental mantle in subduction zones is hydrated by release of water from the underlying oceanic plate. Magnetite is a significant byproduct of mantle hydration, and forearc mantle, cooled by subduction, should contribute to long-wavelength magnetic anomalies above subduction zones. We test this hypothesis with a quantitative model of the Cascadia convergent margin, based on gravity and aeromagnetic anomalies and constrained by seismic velocities, and find that hydrated mantle explains an important disparity in potential-field anomalies of Cascadia. A comparison with aeromagnetic data, thermal models, and earthquakes of Cascadia, Japan, and southern Alaska suggests that magnetic mantle may be common in forearc settings and thus magnetic anomalies may be useful in mapping hydrated mantle in convergent margins worldwide. ?? 2005 Geological Society of America.

Building a risk-targeted regional seismic hazard model for South-East Asia

NASA Astrophysics Data System (ADS)

Woessner, J.; Nyst, M.; Seyhan, E.

2015-12-01

The last decade has tragically shown the social and economic vulnerability of countries in South-East Asia to earthquake hazard and risk. While many disaster mitigation programs and initiatives to improve societal earthquake resilience are under way with the focus on saving lives and livelihoods, the risk management sector is challenged to develop appropriate models to cope with the economic consequences and impact on the insurance business. We present the source model and ground motions model components suitable for a South-East Asia earthquake risk model covering Indonesia, Malaysia, the Philippines and Indochine countries. The source model builds upon refined modelling approaches to characterize 1) seismic activity from geologic and geodetic data on crustal faults and 2) along the interface of subduction zones and within the slabs and 3) earthquakes not occurring on mapped fault structures. We elaborate on building a self-consistent rate model for the hazardous crustal fault systems (e.g. Sumatra fault zone, Philippine fault zone) as well as the subduction zones, showcase some characteristics and sensitivities due to existing uncertainties in the rate and hazard space using a well selected suite of ground motion prediction equations. Finally, we analyze the source model by quantifying the contribution by source type (e.g., subduction zone, crustal fault) to typical risk metrics (e.g.,return period losses, average annual loss) and reviewing their relative impact on various lines of businesses.

Lithospheric Shear Stresses Over And Around Africa

NASA Astrophysics Data System (ADS)

Greff-Lefftz, M.; Jean, B.; Vicente De Gouveia, S.

2017-12-01

We use a simple model for mantle dynamics combining contributions of subducted lithosphere, domes at the bottom of the mantle and upwelling plumes. A dominant feature of plate tectonics is the quasi permanence of a girdle of subductions around the Pacific ocean (or its ancestor), which creates large-wavelength positive topography anomaly within the ring they form. The superimposition of the resultant extension with the one induced by the dome leads to a permanent extensional regime over Africa and the future Indian ocean which creates faults with azimuth directions depending on the direction of the most active part of the ring of subductions. We thus obtain fractures with NW-SE azimuth during the period 275-165 Ma parallel to the strike of the subduction zone of the West South American active margin, which appears to be very active during this period. Between 155-95 Ma, subduction became more active along the Eastern Australian coast involving a change in the direction of the faults toward an E-W direction, in agreement with the observed fault systems between Africa and India, Antartica and Australia. During the Mesozoic and the Cenozoic, we correlate the permanent extensional regime over Africa and Indian ocean with the observed rift systems.Finally we emphasize the role of three primary hotspots as local additional contributors to the stress field imposed by our proposed subduction-doming system, which help in the opening of Indian and South Atlantic oceans.

Tidal modulation of slow slip events in the Nankai trough subduction zone detected by borehole strainmeters

NASA Astrophysics Data System (ADS)

Kikuchi, J.; Ide, S.; Matsumoto, N.

2016-12-01

Slow slip events (SSEs) often occur in the Nankai subduction zone, Japan, within a band-like zone extended from the center of Honshu to western Shikoku. SSEs are believed as shear slip on the plate interface, where the frictional property changes from velocity weakening to strengthening in the dip direction. Therefore the dynamics of SSEs may give some hints on the depth dependent friction and plate subduction. The tidal modulation of SSEs has been identified by statistical analysis using strain data of Plate Boundary Observatory, in the Cascadia subduction zone [Hawthorne & Rubin, 2010]. Here, we perform similar statistical analyses using strain data recorded at borehole stations maintained by National Institute of Advanced Industrial Science and Technology, in western Japan. The correlation between the oscillation in SSEs and tidal stress was confirmed statistically. In Nankai subduction zone, it is known that SSEs are accompanied with high activity of deep tectonic tremors [Hirose & Obara, 2006]. These tremors have been known to be sensitive to tidal stress [Nakata et al., 2008]. Therefore, the tidal modulation of SSEs is another representation of tidal modulation of tremors. To clarify the relation between SSEs and tremors, we investigate whether strain changes corresponding to SSEs can be explained only by tremors activity. For an SSE occurred in Aug. 2010 in Bungo channel, we assume that the seismic moment of the SSE is 1.6 × 1018 Nm (Mw 6.1) based on the inversion of GNSS data [Nishimura et al., 2013], and that this moment is released by 715 tremors that occur during this SSE [Idehara et al., 2014]. In this case, each tremor is assigned with seismic moment of 2.2 × 1015 Nm (Mw 4.2). Then the strain change at the observation station by these tremors is calculated using the Okada [1992] method, assuming a half space and focal mechanism consistent with the regional plate motion. The calculated strain is qualitatively similar with the observed strain, suggesting that tremors almost directly represent SSE, as suggested by previous studies [e.g., Hirose & Obara, 2006]. However, the correspondence is not always apparent. For example, a similar analysis in the eastern Kii peninsula yields significant difference between observation and calculation.

Building a Subduction Zone Observatory

USGS Publications Warehouse

Gomberg, Joan S.; Bodin, Paul; Bourgeois, Jody; Cashman, Susan; Cowan, Darrel; Creager, Kenneth C.; Crowell, Brendan; Duvall, Alison; Frankel, Arthur; González, Frank I.; Houston, Heidi; Johnson, Paul; Kelsey, Harvey; Miller, Una; Roland, Emily C.; Schmidt, David; Staisch, Lydia; Vidale, John; Wilcock, William; Wirth, Erin

2016-01-01

Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

Thermal impact of magmatism in subduction zones

NASA Astrophysics Data System (ADS)

Rees Jones, David W.; Katz, Richard F.; Tian, Meng; Rudge, John F.

2018-01-01

Magmatism in subduction zones builds continental crust and causes most of Earth's subaerial volcanism. The production rate and composition of magmas are controlled by the thermal structure of subduction zones. A range of geochemical and heat flow evidence has recently converged to indicate that subduction zones are hotter at lithospheric depths beneath the arc than predicted by canonical thermomechanical models, which neglect magmatism. We show that this discrepancy can be resolved by consideration of the heat transported by magma. In our one- and two-dimensional numerical models and scaling analysis, magmatic transport of sensible and latent heat locally alters the thermal structure of canonical models by ∼300 K, increasing predicted surface heat flow and mid-lithospheric temperatures to observed values. We find the advection of sensible heat to be larger than the deposition of latent heat. Based on these results we conclude that thermal transport by magma migration affects the chemistry and the location of arc volcanoes.

Metamorphic records of multiple seismic cycles during subduction

PubMed Central

Hacker, Bradley R.; Seward, Gareth G. E.; Kelley, Chris S.

2018-01-01

Large earthquakes occur in rocks undergoing high-pressure/low-temperature metamorphism during subduction. Rhythmic major-element zoning in garnet is a common product of such metamorphism, and one that must record a fundamental subduction process. We argue that rhythmic major-element zoning in subduction zone garnets from the Franciscan Complex, California, developed in response to growth-dissolution cycles driven by pressure pulses. Using electron probe microanalysis and novel techniques in Raman and synchrotron Fourier transform infrared microspectroscopy, we demonstrate that at least four such pressure pulses, of magnitude 100–350 MPa, occurred over less than 300,000 years. These pressure magnitude and time scale constraints are most consistent with the garnet zoning having resulted from periodic overpressure development-dissipation cycles, related to pore-fluid pressure fluctuations linked to earthquake cycles. This study demonstrates that some metamorphic reactions can track individual earthquake cycles and thereby opens new avenues to the study of seismicity. PMID:29568800

Investigation of complex slow slip behavior along the Hikurangi subduction zone with earthquake simulator RSQSim

NASA Astrophysics Data System (ADS)

Colella, H.; Ellis, S. M.; Williams, C. A.

2015-12-01

The Hikurangi subduction zone (New Zealand) is one of many subudction zones that exhibit slow slip behavior. Geodetic observations along the Hikurangi subduction zone are unusual in that not only does the subduction zone exhibit periodic slow slip events at "typical" subduction-zone depths of 25-50 km along the southern part of the margin, but also much shallower depths of 8-15 km along the northern part of the margin. Furthermore, there is evidence for interplay between slow slip events at these different depth ranges (between the deep and shallow events) along the central part of the margin, and some of the slow slip behavior is observed along regions of the interface that were previously considered locked, which raises questions about the slip behavior of this region. This study employs the earthquake simulator, RSQSim, to explore variations in the effective normal stress (i.e., stress after the addition of pore fluid pressures) and the frictional instability necessary to generate the complex slow slip events observed along the Hikurangi margin. Preliminary results suggest that to generate slow slip events with similar recurrence intervals to those observed the effective normal stress (MPa) is 3x higher in the south than the north, 6-9MPa versus 2-3MPa, respectively. Results also suggest that, at a minimum, that some overlap along the central margin must exist between the slow slip sections in the north and south to reproduce the types of slip events observed along the Hikurangi subduction zone. To further validate the results from the simulations, Okada solutions for surface displacements will be compared to geodetic solution to more accurately constrain the areas in which slip behavior varies and the cause(s) for the variation(s).

Initiation of Subduction Zones: A Consequence of Lateral Compositional Buoyancy Contrast Within the Lithosphere

NASA Astrophysics Data System (ADS)

Niu, Y.; O'Hara, M. J.; Pearce, J. A.

2001-12-01

Subduction of oceanic lithosphere into deep mantle is one of the key aspects of plate tectonics. Pull by the subducting-slab due to its negative buoyancy is widely accepted as the major driving force for plate motion and plate tectonics. Hence, there would be no plate tectonics if there were no subduction zones. Yet how a subduction zone initiates remains poorly known. Here we show that lateral compositional (vs. thermal) buoyancy contrast within the lithosphere creates the favored and necessary condition for the initiation of a subduction zone by (1) comparing the compositional and density differences between normal oceanic lithosphere (NOL) represented by abyssal peridotites (AP) and subarc lithosphere (SAL) represented by forearc peridotites (FP), and (2) simple physical analysis. As the gravitational attraction is the principal driving force of the subducting slab, it would be optimal if one part of the lithosphere experiences a greater gravitational attraction than its adjacent neighbor prior to or during the initiation of a subduction. This requires the pre-existence of a density contrast within the lithosphere. If the lithosphere is thermally uniform as is often the case, then the density contrast must result from a compositional contrast. This hypothesis can be tested by examining the lithospheric materials on both sides of a subduction zone. Subduction of a dense NOL beneath a buoyant continental lithosphere is straightforward, but intra-oceanic subduction such as in the western Pacific requires a scrutiny. Our data show that FP of Mariana and Tonga - two of the most important intra-oceanic subduction zones on Earth - are compositionally more depleted than AP: Cr#-sp (mean+/- 1σ ) = 0.584+/-0.084(FP) vs. 0.307+/-0.134(AP); Mg#-ol = 0.915+/-0.006(FP) vs. 0.898+/-0.082(AP); Mg#-opx = 0.917+/-0.006(FP) vs. 0.908+/-0.006(AP); Mg#-cpx = 0.929+/-0.021(FP) vs. 0.917+/-0.011(AP). As a result, SAL is > 0.7% less dense than NOL. This density contrast due to compositional difference is equivalent to Δ T = ~230° C, which is similar to or greater than the postulated thermal buoyancy contrast between a hot mantle plume and its surroundings. While the depleted nature of FP has been interpreted to result from subducting-slab dehydration induced high extents of mantle wedge melting, evidence indicates that the depletion of these FP predates the inception of the subduction, thus these FP are not residues of present-day arc magmatism. Hence, the compositional buoyancy contrast already existed within the lithosphere before the inception of the subduction in the western Pacific. Much of the Mariana SAL may be fragments of old continental lithosphere, whereas the Tonga/Fiji plateau and Kamchatka lithosphere may be remnants of buoyant, hence unsubductable oceanic plateaus (mantle plume head materials) for the Louisville and Hawaiian hotspots respectively. Passive continental margins, where the largest compositional buoyancy contrast exists within the lithosphere, are the loci of future subduction zones. Geometrical analysis shows that the compositional buoyancy contrast within the lithosphere under compression (e.g., ridge push) induces transtensional planes. The weakest plane in the vicinity of the compositional buoyancy contrast develops into a reverse fault. The dense NOL (the foot-wall) tends to sink into the hot and less dense asthenosphere. Calculations show that this tendency to sink reduces both the normal stress to, and shear resistance along, the fault plane, thus easing the sinking and favoring the initiation of a subduction zone. This concept also explains other observations and makes testable predictions on important geodynamic problems.

Strength of the Subduction Plate Interface beneath the Seismogenic Zone: A Microstructural Investigation of Deformation Mechanisms within a Phyllosilicate- and Amphibole-rich Shear Zone

NASA Astrophysics Data System (ADS)

Seyler, C.; Kirkpatrick, J. D.; Šilerová, D.

2017-12-01

Localization of strain at plate boundaries requires rheological weakening of the lithosphere. The rheology of the subduction plate interface is dictated by the dominant grain-scale deformation mechanisms. However, little is known about the deformation mechanisms within phases commonly found in subduction zones, such as phyllosilicates and amphiboles. We investigate the Leech River Shear Zone on Vancouver Island, British Columbia to explore deformation processes downdip of the seismogenic zone and evaluate the bulk rheology of the plate interface. This shear zone juxtaposes a metamorphosed accretionary prism against a metabasaltic oceanic plateau, representing a paleo-plate interface from the ancient Cascadia subduction zone. Preliminary geothermometry results record a prograde deformation temperature of 573.6±11.2 ˚C in the overriding accretionary wedge, and the hornblende-chlorite-epidote-plagioclase mineral assemblage suggests upper greenschist to lower amphibolite facies metamorphism of the downgoing oceanic crust. Detailed mapping of the plate interface documents a 200 m wide mylonitic shear zone developed across the lithologic contact. Asymmetric shear fabrics, isoclinal folding, boudinage, and a steeply plunging, penetrative stretching lineation are consistent with sinistral-oblique subduction. Numerous discordant quartz veins are variably sheared into sigmoidal shapes as well as isoclinally folded and boudinaged, indicating cyclical synkinematic fracture and vein formation. At the grain-scale, interconnected, anastomosing layers of muscovite, chlorite, and graphite in the accretionary prism rocks likely deformed through kinking and dislocation glide. Framework minerals such as quartz and feldspar deformed by dislocation creep. In the metabasalt, hornblende and chlorite form a continuous S—C fabric in which asymmetric hornblende porphyroclasts deformed by rigid grain rotation and dissolution-precipitation creep. The strength of the subduction plate interface beneath the seismogenic zone was therefore controlled by multiple syn-kinematic mechanisms, with overall strength dominated by the rheology of phyllosilicates and amphibole, generating very low viscosities at the plate interface and enhancing strain localization.

Trench-parallel variations in Pacific and Indo-Australian crustal velocity structure due to Louisville Ridge seamount subduction

NASA Astrophysics Data System (ADS)

Stratford, W. R.; Knight, T. P.; Peirce, C.; Watts, A. B.; Grevemeyer, I.; Paulatto, M.; Bassett, D.; Hunter, J.; Kalnins, L. M.

2012-12-01

Variations in trench and forearc morphology, and lithospheric velocity structure are observed where the Louisville Ridge seamount chain subducts at the Tonga-Kermadec Trench. Subduction of these seamounts has affected arc and back-arc processes along the trench for the last 5 Myr. High subduction rates (80 mm/yr in the north, 55 mm/yr in the south), a fast southwards migrating collision zone (~180 km/myr), and the obliquity of the subducting plate and the seamount chain to the trench, make this an ideal location to study the effects of seamount subduction on lithospheric structure. The "before and after" subduction regions have been targeted by several large-scale geophysical projects in recent years; the most recent being the R/V Sonne cruise SO215 in 2011. The crust and upper mantle velocity structure observed in profiles along strike of the seamount chain and perpendicular to the trench from this study, are compared to a similar profile from SO195, recorded ~100 km to the north. The affects of the passage of the seamounts through the subduction system are indicated by velocity anomalies in the crust and mantle of the overriding plate. Preliminary results indicate that in the present collision zone, mantle velocities (Pn) are reduced by ~5%. Around 100 km to the north, where seamounts are inferred to have subducted ~1 Myr ago, a reduction of 7% in mantle P-wave velocity is observed. The width of the trench slope and elevation of the forearc also vary along strike. At the collision zone a >100 km wide collapse region of kilometre-scale block faults comprise the trench slope, while the forearc is elevated. The elevated forearc has a 5 km think upper crust with a Vp of 2.5-5.5 km/s and the collapse zone also has upper crustal velocities as low as 2.5 km/s. To the east in the Pacific Plate, lower P-wave velocities are also observed and attributed to serpentinization due to deep fracturing in the outer trench high. Large bending faults permeate the crust and the Osbourn Seamount, currently on the verge of subduction, is fractured stepwise down into the trench. Pn velocities in the hinge zone of the Pacific Plate are as low as 7.3 km/s indicating that fracturing and serpentinization may also extend to sub-crustal depths. Finally, trench-parallel variations in subduction zone velocity structure are used to infer the degree to which seamount subduction has altered the physical state of the Pacific and Indo-Australian plates both pre- and post subduction.

Subduction Zones: Facts, Ideas, and Speculations.

ERIC Educational Resources Information Center

Uyeda, Seiya

1979-01-01

Recent research studies of both classifications of ocean margins (active or of Pacific type and passive or of Atlantic variety) have yielded a considerable amount of new information leading to some new theories. These theories regarding different kinds of tectonic activity are discussed. (BT)

Seismicity, Deformation, and Metamorphism in the Western Hellenic Subduction Zone: New Constraints From Tomography

NASA Astrophysics Data System (ADS)

Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

2018-04-01

The Western Hellenic Subduction Zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches depths of 100 km to 190 km, while the northern continental portion rarely exhibits deep earthquakes. Our study investigates how this oceanic-continental transition affects fluid release and related seismicity along strike. We present results from local earthquake tomography and double-difference relocation in conjunction with published images based on scattered teleseismic waves. Our tomographic images recover both subducting oceanic and continental crusts as low-velocity layers on top of high-velocity mantle. Although the northern and southern trenches are offset along the Kephalonia Transform Fault, continental and oceanic subducting crusts appear to align at depth. This suggests a smooth transition between slab retreat in the south and slab convergence in the north. Relocated hypocenters outline a single-planed Wadati-Benioff Zone with significant along-strike variability in the south. Seismicity terminates abruptly north of the Kephalonia Transform Fault, likely reflecting the transition from oceanic to continental subducted crust. Near 90 km depth, the low-velocity signature of the subducting crust fades out and the Wadati-Benioff Zone thins and steepens, marking the outline of the basalt-eclogite transition. Subarc melting of the mantle is only observed in the southernmost sector of the oceanic subduction, below the volcanic part of the arc. Beneath the nonvolcanic part, the overriding crust appears to have undergone large-scale silica enrichment. This enrichment is observed as an anomalously low Vp/Vs ratio and requires massive transport of dehydration-derived fluids updip through the subducting crust.

Numerical modeling of fluid migration in subduction zones

NASA Astrophysics Data System (ADS)

Walter, M. J.; Quinteros, J.; Sobolev, S. V.

2015-12-01

It is well known that fluids play a crucial role in subduction evolution. For example, mechanical weakening along tectonic interfaces, due to high fluid pressure, may enable oceanic subduction. Hence, the fluid content seems to be a critical parameter for subduction initiation. Studies have also shown a correlation between the location of slab dehydration and intermediate seismic activity. Furthermore, expelled fluids from the subduction slab affect the melting temperature, consequently, contributing to partial melting in the wedge above the down-going plate and extensive volcanism. In summary, fluids have a great impact on tectonic processes and therefore should be incorporated into geodynamic numerical models. Here we use existing approaches to couple and solve fluid flow equations in the SLIM-3D thermo-mechanical code. SLIM-3D is a three-dimensional thermo-mechanical code capable of simulating lithospheric deformation with elasto-visco-plastic rheology. It has been successfully applied to model geodynamic processes at different tectonic settings, including subduction zones. However, although SLIM-3D already includes many features, fluid migration has not been incorporated into the model yet. To this end, we coupled solid and fluid flow assuming that fluids flow through a porous and deformable solid. Thereby, we introduce a two-phase flow into the model, in which the Stokes flow is coupled with the Darcy law for fluid flow. Ultimately, the evolution of porosity is governed by a compaction pressure and the advection of the porous solid. We show the details of our implementation of the fluid flow into the existing thermo-mechanical finite element code and present first results of benchmarks and experiments. We are especially interested in the coupling of subduction processes and the evolution of the magmatic arc. Thereby, we focus on the key factors controlling magma emplacement and its influence on subduction processes.

Volatile transfer and recycling at convergent margins: Mass-balance and insights from high-P/T metamorphic rocks

NASA Astrophysics Data System (ADS)

Bebout, Gray E.

The efficiency with which volatiles are deeply subducted is governed by devolatilization histories and the geometries and mechanisms of fluid transport deep in subduction zones. Metamorphism along the forearc slab-mantle interface may prevent the deep subduction of many volatile components (e.g., H2O, Cs, B, N, perhaps As, Sb, and U) and result in their transport in fluids toward shallower reservoirs. The release, by devolatilization, and transport of such components toward the seafloor or into the forearc mantle wedge, could in part explain the imbalances between the estimated amounts of subducted volatiles and the amounts returned to Earth's surface. The proportion of the initially subducted volatile component that is retained in rocks subducted to depths greater than those beneath magmatic arcs (>100 km) is largely unknown, complicating assessments of deep mantle volatile budgets. Isotopic and trace element data and volatile contents for the Catalina Schist, the Franciscan Complex, and eclogite-facies complexes in the Alps (and elsewhere) provide insight into the nature and magnitude of fluid production and transport deep in subduction zones and into the possible effects of metamorphism on the compositions of subducting rocks. Compatibilities of the compositions of the subduction-related rocks and fluids with the isotopic and trace element compositions of various mantle-derived materials (igneous rocks, xenoliths, serpentinite seamounts) indicate the potential to trace the recycling of rock and fluid reservoirs chemically and isotopically fractionated during subduction-zone metamorphism.

Scaly fabrics and veins of tectonic mélanges in the Shimanto Belt, SW Japan

NASA Astrophysics Data System (ADS)

Ramirez, G. E.; Fisher, D. M.; Smye, A.; Hashimoto, Y.; Yamaguchi, A.

2017-12-01

Mélanges in ancient subduction fault zones provide a microstructural record of the plate boundary deformation associated with underthrusting. These rocks exhibit many of the characteristics associated with exposed ancient subduction fault zones worldwide, including: 1) σ1 is near orthogonal to the deformation fabric, 2) microstructurally pervasive quartz and calcite filled veins concentrated in coarser blocks and along extensional jogs on slip surfaces, 3) evidence for local diffusion of silica sourced from web-like arrays of slip surfaces (i.e., scaly fabrics), and 4) repeated cracking and sealing that record cyclic variations in stress. We present XRD, XRF, and EPMA observations of scaly fabrics from five ancient subduction-related shear zones (Yokonami, Mugi, Kure, Okitsu, and Makimine mélanges) from the Shimanto Belt in Japan that exemplify these characteristics and represent the full temperature range of the seismogenic zone ( 150-340 °C). The scaly fabrics associated with these shear zones display significantly different microstructural and geochemical characteristics. Individual slip surfaces in the scaly fabrics of Mugi mélange, underplated at the updip limit of the seismogenic zone, are characterized by broader (50-300 µm) anastomosing shear zones while the Makimine mélange, underplated at the downdip limit of the seismogenic zone, exhibits thinner (10-20 µm) anastomosing shear zones. XRD analyses also imply geochemical differences such as a decrease in albite concentration and an increase in illite concentration with increasing temperature/depth of underthrusting. Scaly fabrics are sites of silica redistribution in which silica is depleted on the slip surfaces and precipitated as mostly quartz in crack-seal veins. The time to seal, or heal, fractures is mainly temperature-dependent but can also be significantly quickened by fluid salinity, degree of fluid-rock interactions, and geochemical reactions (i.e. incongruent pressure solution). Microstructural and geochemical characteristics that show differences with temperature/depth of underthrusting highlight the importance of establishing the geochemical processes and activation energies that contribute to slip, fracturing, and healing of rocks that underthrust the subduction interface.

Empirical relationships between instrumental ground motions and observed intensities for two great Chilean subduction zone earthquakes

NASA Astrophysics Data System (ADS)

Cilia, M. G.; Baker, L. M.

2015-12-01

We determine empirical relationships between instrumental peak ground motions and observed intensities for two great Chilean subduction earthquakes: the 2010 Mw8.8 Maule earthquake and the 2014 Mw8.2 Iquique earthquake. Both occurred immediately offshore on the primary plate boundary interface between the Nazca and South America plates. They are among the largest earthquakes to be instrumentally recorded; the 2010 Maule event is the second largest earthquake to produce strong motion recordings. Ground motion to intensity conversion equations (GMICEs) are used to reconstruct the distribution of shaking for historical earthquakes by using intensities estimated from contemporary accounts. Most great (M>8) earthquakes, like these, occur within subduction zones, yet few GMICEs exist for subduction earthquakes. It is unclear whether GMICEs developed for active crustal regions, such as California, can be scaled up to the large M of subduction zone events, or if new data sets must be analyzed to develop separate subduction GMICEs. To address this question, we pair instrumental peak ground motions, both acceleration (PGA) and velocity (PGV), with intensities derived from onsite surveys of earthquake damage made in the weeks after the events and internet-derived felt reports. We fit a linear predictive equation between the geometric mean of the maximum PGA or PGV of the two horizontal components and intensity, using linear least squares. We use a weighting scheme to express the uncertainty of the pairings based on a station's proximity to the nearest intensity observation. The intensity data derived from the onsite surveys is a complete, high-quality investigation of the earthquake damage. We perform the computations using both the survey data and community decimal intensities (CDI) calculated from felt reports volunteered by citizens (USGS "Did You Feel It", DYFI) and compare the results. We compare the GMICEs we developed to the most widely used GMICEs from California and central US earthquakes, and global earthquakes. Existing GMICEs consistently over-predict intensity for these two subduction events. This may be a regional difference, or a magnitude-dependent effect. Currently, however, there is not enough data from these great subduction earthquakes to prefer one interpretation over the other.

Overview of Recent Coastal Tectonic Deformation in the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Ramírez-Herrera, M. Teresa; Kostoglodov, Vladimir; Urrutia-Fucugauchi, Jaime

2011-08-01

Holocene and Pleistocene tectonic deformation of the coast in the Mexico subudction margin is recorded by geomorphic and stratigraphic markers. We document the spatial and temporal variability of active deformation on the coastal Mexican subduction margin. Pleistocene uplift rates are estimated using wave-cut platforms at ca. 0.7-0.9 m/ka on the Jalisco block coast, Rivera-North America tectonic plate boundary. We examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the Holocene up to ca. 3 ± 0.5 m/ka. In contrast, steady rates of uplift (ca. 0.5-1.0 m/ka) in the Pleistocene and Holocene characterize the Michoacan coastal sector, south of El Gordo graben and north of the Orozco Fracture Zone (OFZ), incorporated within the Cocos-North America plate boundary. Significantly higher rates of surface uplift (ca. 7 m/ka) across the OFZ subduction may reflect the roughness of subducting plate. Absence of preserved marine terraces on the coastal sector across El Gordo graben likely reflects slow uplift or coastal subsidence. Stratigraphic markers and their radiocarbon ages show late Holocene (ca. last 6 ka bp) coastal subsidence on the Guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence. Temporal and spatial variability in recent deformation rates on the Mexican Pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction, such as crustal faults, subduction erosion and underplating of subducted materials under the southern Mexico continental margin.

Radial and Azimuthal Anisotropy Tomography of the NE Japan Subduction Zone: Implications for the Pacific Slab and Mantle Wedge Dynamics

NASA Astrophysics Data System (ADS)

Ishise, Motoko; Kawakatsu, Hitoshi; Morishige, Manabu; Shiomi, Katsuhiko

2018-05-01

We investigate slab and mantle structure of the NE Japan subduction zone from P wave azimuthal and radial anisotropy using travel time tomography. Trench normal E-W-trending azimuthal anisotropy (AA) and radial anisotropy (RA) with VPV > VPH are found in the mantle wedge, which supports the existence of small-scale convection in the mantle wedge with flow-induced LPO of mantle minerals. In the subducting Pacific slab, trench parallel N-S-trending AA and RA with VPH > VPV are obtained. Considering the effect of dip of the subducting slab on apparent anisotropy, we suggest that both characteristics can be explained by the presence of laminar structure, in addition to AA frozen-in in the subducting plate prior to subduction.

Resolution testing and limitations of geodetic and tsunami datasets for finite fault inversions along subduction zones

NASA Astrophysics Data System (ADS)

Williamson, A.; Newman, A. V.

2017-12-01

Finite fault inversions utilizing multiple datasets have become commonplace for large earthquakes pending data availability. The mixture of geodetic datasets such as Global Navigational Satellite Systems (GNSS) and InSAR, seismic waveforms, and when applicable, tsunami waveforms from Deep-Ocean Assessment and Reporting of Tsunami (DART) gauges, provide slightly different observations that when incorporated together lead to a more robust model of fault slip distribution. The merging of different datasets is of particular importance along subduction zones where direct observations of seafloor deformation over the rupture area are extremely limited. Instead, instrumentation measures related ground motion from tens to hundreds of kilometers away. The distance from the event and dataset type can lead to a variable degree of resolution, affecting the ability to accurately model the spatial distribution of slip. This study analyzes the spatial resolution attained individually from geodetic and tsunami datasets as well as in a combined dataset. We constrain the importance of distance between estimated parameters and observed data and how that varies between land-based and open ocean datasets. Analysis focuses on accurately scaled subduction zone synthetic models as well as analysis of the relationship between slip and data in recent large subduction zone earthquakes. This study shows that seafloor deformation sensitive datasets, like open-ocean tsunami waveforms or seafloor geodetic instrumentation, can provide unique offshore resolution for understanding most large and particularly tsunamigenic megathrust earthquake activity. In most environments, we simply lack the capability to resolve static displacements using land-based geodetic observations.

Identifying tectonic parameters that affect tsunamigenesis

NASA Astrophysics Data System (ADS)

van Zelst, I.; Brizzi, S.; Heuret, A.; Funiciello, F.; van Dinther, Y.

2016-12-01

The role of tectonics in tsunami generation is at present poorly understood. However, the fact thatsome regions produce more tsunamis than others indicates that tectonics could influencetsunamigenesis. Here, we complement a global earthquake database that contains geometrical,mechanical, and seismicity parameters of subduction zones with tsunami data. We statisticallyanalyse the database to identify the tectonic parameters that affect tsunamigenesis. The Pearson'sproduct-moment correlation coefficients reveal high positive correlations of 0.65 between,amongst others, the maximum water height of tsunamis and the seismic coupling in a subductionzone. However, these correlations are mainly caused by outliers. The Spearman's rank correlationcoefficient results in statistically significant correlations of 0.60 between the number of tsunamisin a subduction zone and subduction velocity (positive correlation) and the sediment thickness atthe trench (negative correlation). Interestingly, there is a positive correlation between the latter andtsunami magnitude. These bivariate statistical methods are extended to a binary decision tree(BDT) and multivariate analysis. Using the BDT, the tectonic parameters that distinguish betweensubduction zones with tsunamigenic and non-tsunamigenic earthquakes are identified. To assessphysical causality of the tectonic parameters with regard to tsunamigenesis, we complement ouranalysis by a numerical study of the most promising parameters using a geodynamic seismic cyclemodel. We show that the inclusion of sediments on the subducting plate results in an increase insplay fault activity, which could lead to larger vertical seafloor displacements due to their steeperdips and hence a larger tsunamigenic potential. We also show that the splay fault is the preferredrupture path for a strongly velocity strengthening friction regime in the shallow part of thesubduction zone, which again increases the tsunamigenic potential.

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.

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.

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.

Asymmetric Subductions in an Asymmetric Earth: Geodynamics and Numerical Modeling

NASA Astrophysics Data System (ADS)

Dal Zilio, L.; Ficini, E.; Doglioni, C.; Gerya, T.

2016-12-01

The driving mechanism of plate tectonics is still controversial. Moreover, mantle kinematics is still poorly constrained due to the limited information available on its composition, thermal state, and physical parameters. The net rotation of the lithosphere, or so-called W-ward drift, however, indicates a decoupling of the plates relative to the underlying asthenosphere at about 100-200 km depth in the Low-Velocity Zone and a relative "E-ward" mantle counterflow. This mantle flow can account for a number of tectonic asymmetries on subduction dynamics such as steep versus shallow slab dip, diverging versus converging subduction hinge, low versus high topography of mountain belts, etc. This asymmetry is generally interpreted to reflect the age-dependent negative buoyancy of the subducting lithosphere. However, slab dip is insensitive to the age of the lithosphere. Here we investigate the role of mantle flow in controlling subduction dynamics using a high-resolution rheologically consistent two-dimensional numerical modeling. Results show the evolution of a subducting oceanic plate beneath a continent: when the subducting plate is dipping in opposite direction with respect to the mantle flow, the slab is sub-vertically deflected by the mantle flow, thus leading the coeval development of a back-arc basin. In contrast, agreement between mantle flow and dipping of the subducting slab relieves shallow dipping subduction zone, which in turn controls the development of a pronounced topography. Moreover, this study confirms that the age of the subducting oceanic lithosphere (i.e. its negative buoyancy) has a second order effect on the dip angle of the slab and, more generally, on subduction dynamics. Our numerical experiments show strong similarities to the observed evolution of subduction zone worldwide and demonstrate that the possibility of a horizontal mantle flow is universally valid.

An Evaluation of Proposed Mechanisms of Slab Flattening in Central Mexico

NASA Astrophysics Data System (ADS)

Skinner, Steven M.; Clayton, Robert W.

2011-08-01

Central Mexico is the site of an enigmatic zone of flat subduction. The general geometry of the subducting slab has been known for some time and is characterized by a horizontal zone bounded on either side by two moderately dipping sections. We systematically evaluate proposed hypotheses for shallow subduction in Mexico based on the spatial and temporal evidence, and we find no simple or obvious explanation for the shallow subduction in Mexico. We are unable to locate an oceanic lithosphere impactor, or the conjugate of an impactor, that is most often called upon to explain shallow subduction zones as in South America, Japan, and Laramide deformation in the US. The only bathymetric feature that is of the right age and in the correct position on the conjugate plate is a set of unnamed seamounts that are too small to have a significant effect on the buoyancy of the slab. The only candidate that we cannot dismiss is a change in the dynamics of subduction through a change in wedge viscosity, possibly caused by water brought in by the slab.

Highly oxidising fluids generated during serpentinite breakdown in subduction zones.

PubMed

Debret, B; Sverjensky, D A

2017-09-04

Subduction zones facilitate chemical exchanges between Earth's deep interior and volcanism that affects habitability of the surface environment. Lavas erupted at subduction zones are oxidized and release volatile species. These features may reflect a modification of the oxidation state of the sub-arc mantle by hydrous, oxidizing sulfate and/or carbonate-bearing fluids derived from subducting slabs. But the reason that the fluids are oxidizing has been unclear. Here we use theoretical chemical mass transfer calculations to predict the redox state of fluids generated during serpentinite dehydration. Specifically, the breakdown of antigorite to olivine, enstatite, and chlorite generates fluids with high oxygen fugacities, close to the hematite-magnetite buffer, that can contain significant amounts of sulfate. The migration of these fluids from the slab to the mantle wedge could therefore provide the oxidized source for the genesis of primary arc magmas that release gases to the atmosphere during volcanism. Our results also show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs.

Preliminary deformation model for National Seismic Hazard map of Indonesia

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

Meilano, Irwan; Gunawan, Endra; Sarsito, Dina

Preliminary deformation model for the Indonesia’s National Seismic Hazard (NSH) map is constructed as the block rotation and strain accumulation function at the elastic half-space. Deformation due to rigid body motion is estimated by rotating six tectonic blocks in Indonesia. The interseismic deformation due to subduction is estimated by assuming coupling on subduction interface while deformation at active fault is calculated by assuming each of the fault‘s segment slips beneath a locking depth or in combination with creeping in a shallower part. This research shows that rigid body motion dominates the deformation pattern with magnitude more than 15 mm/year, except inmore » the narrow area near subduction zones and active faults where significant deformation reach to 25 mm/year.« less

Metamorphic density controls on early-stage subduction dynamics

NASA Astrophysics Data System (ADS)

Duesterhoeft, Erik; Oberhänsli, Roland; Bousquet, Romain

2013-04-01

Subduction is primarily driven by the densification of the downgoing oceanic slab, due to dynamic P-T-fields in subduction zones. It is crucial to unravel slab densification induced by metamorphic reactions to understand the influence on plate dynamics. By analyzing the density and metamorphic structure of subduction zones, we may gain knowledge about the driving, metamorphic processes in a subduction zone like the eclogitization (i.e., the transformation of a MORB to an eclogite), the breakdown of hydrous minerals and the release of fluid or the generation of partial melts. We have therefore developed a 2D subduction zone model down to 250 km that is based on thermodynamic equilibrium assemblage computations. Our model computes the "metamorphic density" of rocks as a function of pressure, temperature and chemical composition using the Theriak-Domino software package at different time stages. We have used this model to investigate how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within subduction systems. These processes are commonly neglected by other approaches (e.g., gravitational or thermomechanical in nature) reproducing the density distribution within this tectonic setting. The process of eclogitization is assumed as being important to subduction dynamics, based on the very high density (3.6 g/cm3) of eclogitic rocks. The eclogitization in a MORB-type crust is possible only if the rock reaches the garnet phase stability field. This process is primarily temperature driven. Our model demonstrates that the initiation of eclogitization of the slab is not the only significant process that makes the descending slab denser and is responsible for the slab pull force. Indeed, our results show that the densification of the downgoing lithospheric mantle (due to an increase of pressure) starts in the early subduction stage and makes a significant contribution to the slab pull, where eclogitization does not occur. Thus, the lithospheric mantle acts as additional ballast below the sinking slab shortly after the initiation of subduction. Our calculation shows that the dogma of eclogitized basaltic, oceanic crust as the driving force of slab pull is overestimated during the early stage of subduction. These results improve our understanding of the force budget for slab pull during the intial and early stage of subduction. Therefore, the complex metamorphic structure of a slab and mantle wedge has an important impact on the development and dynamics of subduction zones. Further Reading: Duesterhoeft, Oberhänsli & Bousquet (2013), submitted to Earth and Planetary Science Letters

A benchmark for subduction zone modeling

NASA Astrophysics Data System (ADS)

van Keken, P.; King, S.; Peacock, S.

2003-04-01

Our understanding of subduction zones hinges critically on the ability to discern its thermal structure and dynamics. Computational modeling has become an essential complementary approach to observational and experimental studies. The accurate modeling of subduction zones is challenging due to the unique geometry, complicated rheological description and influence of fluid and melt formation. The complicated physics causes problems for the accurate numerical solution of the governing equations. As a consequence it is essential for the subduction zone community to be able to evaluate the ability and limitations of various modeling approaches. The participants of a workshop on the modeling of subduction zones, held at the University of Michigan at Ann Arbor, MI, USA in 2002, formulated a number of case studies to be developed into a benchmark similar to previous mantle convection benchmarks (Blankenbach et al., 1989; Busse et al., 1991; Van Keken et al., 1997). Our initial benchmark focuses on the dynamics of the mantle wedge and investigates three different rheologies: constant viscosity, diffusion creep, and dislocation creep. In addition we investigate the ability of codes to accurate model dynamic pressure and advection dominated flows. Proceedings of the workshop and the formulation of the benchmark are available at www.geo.lsa.umich.edu/~keken/subduction02.html We strongly encourage interested research groups to participate in this benchmark. At Nice 2003 we will provide an update and first set of benchmark results. Interested researchers are encouraged to contact one of the authors for further details.

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.

Update on GPS-Acoustics Measurements on the Continental Slope of the Cascadia Subduction Zone

NASA Astrophysics Data System (ADS)

Chadwell, C. D.

2017-12-01

Land-based GPS measurements suggest the megathrust is locked offshore along the Cascadia Subduction Zone. However, land-based data alone lack geometric resolution to constrain the how the slip is distributed. GPS-Acoustic measurements can provide these constraints, but using traditional GPS-Acoustic approaches employing a ship is costly. Wave Gliders, a wave- and solar-powered, remotely-piloted sea surface platform, provide a low cost method for collecting GPS-A data. We have adapted GPS-Acoustic technology to the Wave Glider and in 2016 began annual measurements at three sites in the Cascadia Subduction Zone (CSZ). Here, we review positioning results collected during summer 2017 at two sites on the continental slope of the Cascadia Subduction Zone: One site is approximately 45 NM offshore central Oregon and the other approximately 50 NM offshore central Washington State. A third site is approximately 90 NM offshore central Oregon on the incoming Juan de Fuca plate. We will report on initial results of the GPS-A data collection and operational experiences of the missions in 2016 and 2017. Wave Glider based GPS-A measurement have the potential to significantly increase the number and frequency of measurements of strain accumulation in Cascadia Subduction Zone and elsewhere.

Slab temperature controls on the Tonga double seismic zone and slab mantle dehydration

PubMed Central

Wei, S. Shawn; Wiens, Douglas A.; van Keken, Peter E.; Cai, Chen

2017-01-01

Double seismic zones are two-layered distributions of intermediate-depth earthquakes that provide insight into the thermomechanical state of subducting slabs. We present new precise hypocenters of intermediate-depth earthquakes in the Tonga subduction zone obtained using data from local island–based, ocean-bottom, and global seismographs. The results show a downdip compressional upper plane and a downdip tensional lower plane with a separation of about 30 km. The double seismic zone in Tonga extends to a depth of about 300 km, deeper than in any other subduction system. This is due to the lower slab temperatures resulting from faster subduction, as indicated by a global trend toward deeper double seismic zones in colder slabs. In addition, a line of high seismicity in the upper plane is observed at a depth of 160 to 280 km, which shallows southward as the convergence rate decreases. Thermal modeling shows that the earthquakes in this “seismic belt” occur at various pressures but at a nearly constant temperature, highlighting the important role of temperature in triggering intermediate-depth earthquakes. This seismic belt may correspond to regions where the subducting mantle first reaches a temperature of ~500°C, implying that metamorphic dehydration of mantle minerals in the slab provides water to enhance faulting. PMID:28097220

Aqueous Silicate Polymers: An Alternative to `Supercritical' Fluids as Transport Agents in Subduction Zones

NASA Astrophysics Data System (ADS)

Mannig, C. E.

2005-12-01

The chemistry of subduction-zone fluids is complicated by melt-vapor miscibility and the existence of critical end-points in rock-H2O systems. It is commonly assumed that fluids in subduction zones attain properties intermediate in composition between hydrous silicate liquid and H2O, and that such fluids possess enhanced material transport capabilities. However, the relevance of supercritical, intermediate fluids to subduction zones presents four problems. (1) Albite-H2O is typically used as an analogue system, but the favorable position of its critical curve is not representative; critical curves for polymineralic subduction-zone lithologies lie at substantially higher P. (2) Even if albite-H2O is relevant, jadeite may interfere because of its different solubility and the positive clapeyron slope of its solidus, which points to liquid-structure changes that could cause reappearance of the liquid+vapor field. (3) Critical curves are features of very H2O-rich compositions; low-porosity, H2O-poor natural systems will coexist with intermediate fluids only over a narrow PT interval. (4) Intermediate fluids are expected only over short length scales because their migration will likely result in compositional shifts via reaction and mineral precipitation in the mantle wedge. Although supercritical, intermediate fluids are probably relatively unimportant in subduction zones, they reflect a chemical process that may hold the key to understanding high- P mass transfer. Miscibility in melt-vapor systems is a consequence of polymerization of dissolved components, primarily Si ± Al, Na and Ca. This behavior yields, e.g., aqueous Si-Si, Si-Al, Si-Na-Al, and Si-Ca oxide dimers and other multimers of varying stoichiometry (silicate polymers), even in subcritical, dilute, H2O-rich vapor. Silicate polymers in subcritical aqueous solutions have been inferred from high- P mineral-solubility experiments. The abundance of these species at high P shows that the chemistry of aqueous fluids in subduction-zones differs fundamentally from the more familiar ionic solutions of the upper crust. This has important consequences for minor element transport. Measurements of Fe, phosphorous and Ti solubility reveal that dissolved concentrations rise with increased aqueous albite content at fixed P and T, with maximum enhancements exceeding 10X at melt saturation. Subcritical silicate polymerization thus permits transport of low solubility components via their substitution into sites on aqueous multimers constructed of "polymer formers" such as Na, Al, and Si, even in dilute solutions. The partitioning of elements between the bulk fluid, the polymer network, and the rock matrix likely controls the overall compositional evolution of subduction-zone fluids. Because they form over a wider PT and bulk X range, subcritical silicate polymers in dilute solutions are likely responsible for more mass transfer in subduction zones than intermediate, supercritical fluids.

Tomography reveals buoyant asthenosphere accumulating beneath the Juan de Fuca plate

NASA Astrophysics Data System (ADS)

Hawley, William B.; Allen, Richard M.; Richards, Mark A.

2016-09-01

The boundary between Earth’s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth, perhaps indicating a thin, weak layer that may strongly influence plate motion dynamics. The behavior of such a layer at subduction zones remains unexplored. We present a tomographic model, derived from on- and offshore seismic experiments, that reveals a strong low-velocity feature beneath the subducting Juan de Fuca slab along the entire Cascadia subduction zone. Through simple geodynamic arguments, we propose that this low-velocity feature is the accumulation of material from a thin, weak, buoyant layer present beneath the entire oceanic lithosphere. The presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics.

Horizontal mantle flow controls subduction dynamics.

PubMed

Ficini, E; Dal Zilio, L; Doglioni, C; Gerya, T V

2017-08-08

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age -the main control on buoyancy- exhibits little correlation with most of the present-day subduction velocities and slab dips. "West"-directed subduction zones are on average steeper (~65°) than "East"-directed (~27°). Also, a "westerly"-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an "easterly"-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth.

Storage of fluids and melts at subduction zones detectable by seismic tomography

NASA Astrophysics Data System (ADS)

Luehr, B. G.; Koulakov, I.; Rabbel, W.; Brotopuspito, K. S.; Surono, S.

2015-12-01

During the last decades investigations at active continental margins discovered the link between the subduction of fluid saturated oceanic plates and the process of ascent of these fluids and partial melts forming a magmatic system that leads to volcanism at the earth surface. For this purpose the geophysical structure of the mantle and crustal range above the down going slap has been imaged. Information is required about the slap, the ascent paths, as well as the reservoires of fluids and partial melts in the mantle and the crust up to the volcanoes at the surface. Statistically the distance between the volcanoes of volcanic arcs down to their Wadati Benioff zone results of approximately 100 kilometers in mean value. Surprisingly, this depth range shows pronounced seismicity at most of all subduction zones. Additionally, mineralogical laboratory investigations have shown that dehydration of the diving plate has a maximum at temperature and pressure conditions we find at around 100 km depth. The ascent of the fluids and the appearance of partial melts as well as the distribution of these materials in the crust can be resolved by seismic tomographic methods using records of local natural seismicity. With these methods these areas are corresponding to lowered seismic velocities, high Vp/Vs ratios, as well as increased attenuation of seismic shear waves. The anomalies and their time dependence are controlled by the fluids. The seismic velocity anomalies detected so far are within a range of a few per cent to more than 30% reduction. But, to explore plate boundaries large and complex amphibious experiments are required, in which active and passive seismic investigations should be combined to achieve best results. The seismic station distribution should cover an area from before the trench up to far behind the volcanic chain, to provide under favorable conditions information down to 150 km depth. Findings of different subduction zones will be compared and discussed.

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.

Resolution Study of Marine CSEM Imaging of Subduction Zones

NASA Astrophysics Data System (ADS)

Gustafson, C.; Key, K.

2016-12-01

Marine controlled source electromagnetic (CSEM) data allow us to image seafloor electrical resistivity from which we can constrain the porosity and fluid content of the subsurface. In subduction zones, CSEM data can be used to constrain geologic structure, hydrogeology and fluid-tectonic processes. The scales of features we are interested in recovering with CSEM data range from large-scale features such as the incoming tectonic plate and subducting slab, to the narrow dipping plate boundary interface where slip occurs, to thin faults that cut the overriding forearc crust and shallow fluid seeps and mounds on the seafloor. Thus electrical structure is expected to vary on scales ranging from scales of meters to tens of kilometers. CSEM data collected by Scripps at the Middle America Trench in 2010 is the first and to-date the only application of the method for studying a subduction zone. The results from this pioneering data set highlight the types of new discoveries that are possible with CSEM data, such as imaging conductive bending faults and a water-rich channel of subducting sediments. In this work we explore the magnitude and scale of 2D resistivity structures that can be resolved with CSEM data through a suite of synthetic inversion studies. We build resistivity models that are representative of various known and hypothesized subduction zone plate boundary structures. We generate synthetic noisy data for these models and invert them using the freely available MARE2DEM inversion code. We compare the recovered models to the original models in order to determine which resistivity structures may be successfully identified using CSEM. We explore the potential effects of receiver spacing, frequency bandwidth and system noise levels on the ability of CSEM to recover these different subduction zone structures.

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.

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.

Permian to recent volcanism in northern sumatra, indonesia: a preliminary study of its distribution, chemistry, and peculiarities

NASA Astrophysics Data System (ADS)

Rock, N. M. S.; Syah, H. H.; Davis, A. E.; Hutchison, D.; Styles, M. T.; Lena, Rahayu

1982-06-01

Sumatra has been a ‘volcanic arc’, above an NE-dipping subduction zone, since at least the Late Permian. The principal volcanic episodes in Sumatra N of the Equator have been in the Late Permian, Late Mesozoic, Palaeogene, Miocene and Quaternary. Late Permian volcanic rocks, of limited extent, are altered porphyritic basic lavas interstratified with limestones and phyllites. Late Mesozoic volcanic rocks, widely distributed along and W of the major transcurrent. Sumatra Fault System (SFS), which axially bisects Sumatra, include ophiolite-related spilites, andesites and basalts. Possible Palaeogene volcanic rocks include an altered basalt pile with associated dyke-swarm in the extreme NW, intruded by an Early Miocene (19 my) dioritic stock; and variable pyroxene rich basic lavas and agglomerates ranging from alkali basaltic to absarokitic in the extreme SW. Miocene volcanic rocks, widely distributed (especially W of the SFS), and cropping out extensively along the W coast, include calc-alkaline to high-K calc-alkaline basalts, andesites and dacites. Quaternary volcanoes (3 active, 14 dormant or extinct) are irregularly distributed both along and across the arc; thus they lie fore-arc of the SFS near the Equator but well back-arc farther north. The largest concentration of centres, around Lake Toba, includes the >2000 km3 Pleistocene rhyolitic Toba Tuffs. Quaternary volcanics are mainly calc-alkaline andesites, dacites and rhyolites with few basalts; they seem less variable, but on the whole more acid, than the Tertiary. The Quaternary volcanism is anomalous in relation to both southern Sumatra and adjacent Java/Bali: in southern Sumatra, volcanoes are regularly spaced along and successively less active away from the SFS, but neither rule holds in northern Sumatra. Depths to the subduction zone below major calc-alkaline volcanoes in Java/Bali are 160-210 km, but little over 100 km in northern Sumatra, which also lacks the regular K2O-depth correlations seen in Java. These anomalies may arise because Sumatra — being underlain by continental crust — is more akin to destructive continental margins than typical island-arcs such as E Java or Bali, and because the Sumatran subduction zone has a peculiar structure due to the oblique approach of the subducting plate. A further anomaly — an E-W belt of small centres along the back-arc coast — may relate to an incipient S-dipping subduction zone N of Sumatra and not the main NE-dipping zone to its W. Correlation of the Tertiary volcanism with the present tectonic regime is hazardous, but the extensive W coastal volcanism (which includes rather alkaline lavas) is particularly anomalous in relation to the shallow depth (<100 km) of the present subduction zone. The various outcrops may owe their present locations to extensive fault movements (especially along the SFS), to the peculiar structure of the fore-arc (suggested by equally anomalous Sn- and W-bearing granitic batholiths also along the W coast), or they may not be subduction-related at all.

Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth

NASA Technical Reports Server (NTRS)

Rapp, R. P.

1994-01-01

Subduction zones are presently the dominant sites on Earth for recycling and mass transfer between the crust and mantle; they feed hydrated basaltic oceanic crust into the upper mantle, where dehydration reactions release aqueous fluids and/or hydrous melts. The loci for fluid and/or melt generation will be determined by the intersection of dehydration reaction boundaries of primary hydrous minerals within the subducted lithosphere with slab geotherms. For metabasalt of the oceanic crust, amphibole is the dominant hydrous mineral. The dehydration melting solidus, vapor-absent melting phase relationships; and amphibole-out phase boundary for a number of natural metabasalts have been determined experimentally, and the pressure-temperature conditions of each of these appear to be dependent on bulk composition. Whether or not the dehydration of amphibole is a fluid-generating or partial melting reaction depends on a number of factors specific to a given subduction zone, such as age and thickness of the subducting oceanic lithosphere, the rate of convergence, and the maturity of the subduction zone. In general, subduction of young, hot oceanic lithosphere will result in partial melting of metabasalt of the oceanic crust within the garnet stability field; these melts are characteristically high-Al2O3 trondhjemites, tonalites and dacites. The presence of residual garnet during partial melting imparts a distinctive trace element signature (e.g., high La/Yb, high Sr/Y and Cr/Y combined with low Cr and Y contents relative to demonstrably mantle-derived arc magmas). Water in eclogitized, subducted basalt of the oceanic crust is therefore strongly partitioned into melts generated below about 3.5 GPa in 'hot' subduction zones. Although phase equilibria experiments relevant to 'cold' subduction of hydrated natural basalts are underway in a number of high-pressure laboratories, little is known with respect to the stability of more exotic hydrous minerals (e.g., ellenbergite) and the potential for oceanic crust (including metasediments) to transport water deeper into the mantle.

Slab geometry of the South American margin from joint inversion of body waves and surface waves

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Ward, K. M.; Porter, R. C.; Portner, D. E.; Lynner, C.; Beck, S. L.; Zandt, G.

2016-12-01

The western margin of South America is a long subduction zone with a complex, highly three -dimensional geometry. The first order structure of the slab has previously been inferred from seismicity patterns and locations of volcanoes, but confirmation of the slab geometry by seismic imaging for the entire margin has been limited by either shallow, lithospheric scale models or broader, upper mantle images, often defined on a limited spatial footprint. Here, we present new teleseismic tomographic SV seismic models of the upper mantle from 10°S to 40°S along the South American subduction zone with resolution to a depth of 1000 km as inferred from checkerboard tests. In regions near the Peru Bolivia border (12°S to 18°S) and near central Chile and western Argentina (29.5°S to 33°S) we jointly invert the multi-band direct S and SKS relative delay times with Rayleigh wave phase velocities from ambient noise and teleseismic surface wave tomography. This self-consistent model provides information from the upper crust to below the mantle transition zone along the western margin in these two regions. This consistency allows tracing the slab from the South American coastline to the sub-transition zone upper mantle. From this model we image several features, but most notable is a significant eastward step near the southern edge of the margin (24°-30° S). West of this step, a large high shear velocity body is imaged in the base of and below the transition zone. We suggest this may be a stagnant slab, which is descending into the lower mantle now that it is no longer attached to the surface. This suggests a new component to the subduction history of western South America when an older slab lead the convergence before anchoring in the transition zone, breaking off from the surface, and being overtaken by the modern, actively subducting slab now located further east.

Seismicity pattern: an indicator of source region of volcanism at convergent plate margins

NASA Astrophysics Data System (ADS)

Špičák, Aleš; Hanuš, Václav; Vaněk, Jiří

2004-04-01

The results of detailed investigation into the geometry of distribution of earthquakes around and below the volcanoes Korovin, Cleveland, Makushin, Yake-Dake, Oshima, Lewotobi, Fuego, Sangay, Nisyros and Montagne Pelée at convergent plate margins are presented. The ISC hypocentral determinations for the period 1964-1999, based on data of global seismic network and relocated by Engdahl, van der Hilst and Buland, have been used. The aim of this study has been to contribute to the solution of the problem of location of source regions of primary magma for calc-alkaline volcanoes spatially and genetically related to the process of subduction. Several specific features of seismicity pattern were revealed in this context. (i) A clear occurrence of the intermediate-depth aseismic gap (IDAG) in the Wadati-Benioff zone (WBZ) below all investigated active volcanoes. We interpret this part of the subducted slab, which does not contain any teleseismically recorded earthquake with magnitude greater than 4.0, as a partially melted domain of oceanic lithosphere and as a possible source of primary magma for calc-alkaline volcanoes. (ii) A set of earthquakes in the shape of a seismically active column (SAC) seems to exists in the continental wedge below volcanoes Korovin, Makushin and Sangay. The seismically active columns probably reach from the Earth surface down to the aseismic gap in the Wadati-Benioff zone. This points to the possibility that the upper mantle overlying the subducted slab does not contain large melted domains, displays an intense fracturing and is not likely to represent the site of magma generation. (iii) In the continental wedge below the volcanoes Cleveland, Fuego, Nisyros, Yake-Dake, Oshima and Lewotobi, shallow seismicity occurs down to the depth of 50 km. The domain without any earthquakes between the shallow seismically active column and the aseismic gap in the Wadati-Benioff zone in the depth range of 50-100 km does not exclude the melting of the mantle also above the slab. (iv) Any earthquake does not exist in the lithospheric wedge below the volcano Montagne Pelée. The source of primary magma could be located in the subducted slab as well as in the overlying mantle wedge. (v) Frequent aftershock sequences accompanying stronger earthquakes in the seismically active columns indicate high fracturing of the wedge below active volcanoes. (vi) The elongated shape of clusters of epicentres of earthquakes of seismically active columns, as well as stable parameters of the available fault plane solutions, seem to reflect the existence of dominant deeply rooted fracture zones below volcanoes. These facts also favour the location of primary magma in the subducting slab rather than in the overlying wedge. We suppose that melts advancing from the slab toward the Earth surface may trigger the observed earthquakes in the continental wedge that is critically pre-stressed by the process of subduction. However, for definitive conclusions it will be necessary to explain the occurrence of earthquake clusters below some volcanoes and the lack of seismicity below others, taking into account the uncertainty of focal depth determination from global seismological data in some regions.

The origin of alkaline magmas in an intraplate setting near a subduction zone: the Ngatutura Basalts, North Island, New Zealand

NASA Astrophysics Data System (ADS)

Briggs, R. M.; Utting, A. J.; Gibson, I. L.

1990-01-01

The Ngatutura Basalts are one of a series of Pliocene-Quaternary alkalic basalt volcanic fields in North Island, New Zealand. They are situated in an intraplate tectonic setting behind the currently active Taupo Volcanic Zone, and 300 km above the subducting slab. The volcanic field consists of 16 small-volume monogenetic volcanic centres composed mainly of eroded scoria cones and lava flows, that occupy an extensional tectonic environment characterized by NE-striking block faulting. In some cases the faults have controlled the localization of volcanic vents. The lavas have restricted compositions, ranging from hawaiites to nepheline hawaiites, and are characterized by enriched LILE, LREE, and HFS elements, with particularly high Nb and Ta, low Ba/Nb, and high Zr/Y and Ce N/Yb N ratios. Nepheline hawaiites are slightly more differentiated than hawaiites and have higher Ce N/Yb N ratios. Petrogenetic modelling suggests that the range in composition was mainly controlled by fractional crystallization of olivine, clinopyroxene, and minor plagioclase and titanomagnetite, which is consistent with the modal phenocryst abundances. Fractionation is explained by side-wall crystallization and flowage differentiation during rapid ascent, rather than gravitative settling in a magma chamber. Ngatutura magmas were probably derived from an enriched garnet lherzolite source within the low-velocity mantle. The process of source enrichment is speculative but our preferred model calls on metasomatizing fluids in the low-velocity zone. There is no geochemical evidence for any influence of the subducted slab on their composition, even though they overlie the Pacific plate subduction zone. This implies that the extent of subduction-related contamination in the mantle wedge is not pervasive, but is confined to a limited region overlying the subducted slab. Also, the "deep mantle plume" responsible for alkalic magmatism must have originated above the slab, because it seems unlikely that such a plume could have occurred at a deeper level and penetrate the slab without some evidence. This therefore limits the depth of origin of these "deep mantle plumes" to less than 300 km.

Backarc spreading and mantle wedge flow beneath the Japan Sea: insight from Rayleigh-wave anisotropic tomography

NASA Astrophysics Data System (ADS)

Liu, Xin; Zhao, Dapeng

2016-10-01

We present the first high-resolution Rayleigh-wave phase-velocity azimuthal anisotropy tomography of the Japan subduction zone at periods of 20-150 s, which is determined using a large number of high-quality amplitude and phase data of teleseismic fundamental-mode Rayleigh waves. The obtained 2-D anisotropic phase-velocity models are then inverted for a 3-D shear-wave velocity azimuthal anisotropy tomography down to a depth of ˜300 km beneath Japan. The subducting Pacific slab is imaged as a dipping high-velocity zone with trench-parallel fast-velocity directions (FVDs) which may indicate the anisotropy arising from the normal faults produced at the outer-rise area near the Japan trench axis, overprinting the slab fossil fabric, whereas the mantle wedge generally exhibits lower velocities with trench-normal FVDs which reflect subduction-driven corner flow and anisotropy. Depth variations of azimuthal anisotropy are revealed in the big mantle wedge beneath the Japan Sea, which may reflect past deformations in the Eurasian lithosphere related to backarc spreading during 21 to 15 Ma and complex current convection in the asthenosphere induced by active subductions of both the Pacific and Philippine Sea plates.

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.

Remarkably Consistent Thermal State of the south Central Chile Subduction Zone from 36°S to 45°S

NASA Astrophysics Data System (ADS)

Rotman, H.; Spinelli, G. A.

2013-12-01

Delineating the rupture areas of large subduction zone earthquakes is necessary for understanding the controls on seismic and aseismic slip on faults. For the largest recorded earthquake, an event in south central Chile in 1960 with moment magnitude 9.5, the rupture area is only loosely defined due to limitations in the global seismic network at the time. The rupture extends ~900 km along strike on the margin. Coastal deformation is consistent with either a constant rupture width of ~200 km along the entire length, or a much narrower width (~115 km) for the southern half of the rupture. A southward narrowing of the seismogenic zone has been hypothesized to result from warming of the subduction zone to the south, where the subducting plate is younger. Here, we present results of thermal models at 36°S, 38°S, 43°S, and 45°S to examine potential along-strike changes the thermal state of the margin. We find that temperatures in the subduction zone are strongly affected by both fluid circulation in the high permeability upper oceanic crust and frictional heating on the plate boundary fault. Hydrothermal circulation preferentially cools transects with young subducting lithosphere; frictional heating preferentially warms transects with older subducting lithosphere. The combined effects of frictional heating and hydrothermal circulation increase decollement temperatures in the 36°S and 38°S transects by up to ~155°C, and decrease temperatures in the 45°S transect by up to ~150°C. In our preferred models, decollement temperatures 200 km landward of the trench in all four transects are ~350-400°C. This is consistent with a constant ~200 km wide seismogenic zone for the 1960 Mw 9.5 rupture, with decreasing slip magnitude in the southern half of the rupture.

Variability of High Resolution Vp/Vs and Seismic Velocity Structure Along the Nicaragua/Costa Rica Segment of the Middle America Subduction Zone

NASA Astrophysics Data System (ADS)

Moore-Driskell, M. M.; DeShon, H. R.

2012-12-01

Previous studies of subduction zone earthquakes have shown that fault conditions control earthquake rupture and behavior. There are many potential properties that may vary along the subduction margin that could cause fault zone variability, including plate age, temperature, and/or geometry, convergence rate, state of hydration, overriding geology, subducting sediment packages, or subducting seamounts/ridges. The Nicaragua/Costa Rica segment of the Middle America subduction zone is highly variable along strike and down dip. We use this margin to examine how these variable conditions affect earthquake behavior by determining local ratios of compressional to shear wave velocities (Vp/Vs) and detailed seismic velocity structure. Vp/Vs is one of the best tools available to reliably define fault conditions because it is directly related to the Poisson's ratio of the fault material, and it is sensitive to the presence of fluids and changing permeability. Thus with well-resolved near source Vp/Vs measurements we can infer composition and/or high fluid pressures. Here, we use a technique developed by Lin and Shearer (2007) to determine local Vp/Vs in small areas (~2 x 2 x 2 km) with high seismicity. Within the seismogenic zone, we find the margin to be highly variable along strike in Vp/Vs and seismic velocity. These changes correlate to documented variability in incoming plate properties. Increased Vp/Vs is associated with intraplate earthquakes along Nicaragua and northern Costa Rica. We compare our results with other geophysical studies including new high-resolution images of seismic velocity structure, an extensive catalog of high quality relocated events, apparent stress calculations, coupling, and SSE/NVT occurrence. A better understanding of the connection between fault properties and earthquake behavior gives insight into the role of fluids in seismogenesis, the spectrum of earthquake rupture, and possible hazard at subduction zones.

Characterizing Mega-Earthquake Related Tsunami on Subduction Zones without Large Historical Events

NASA Astrophysics Data System (ADS)

Williams, C. R.; Lee, R.; Astill, S.; Farahani, R.; Wilson, P. S.; Mohammed, F.

2014-12-01

Due to recent large tsunami events (e.g., Chile 2010 and Japan 2011), the insurance industry is very aware of the importance of managing its exposure to tsunami risk. There are currently few tools available to help establish policies for managing and pricing tsunami risk globally. As a starting point and to help address this issue, Risk Management Solutions Inc. (RMS) is developing a global suite of tsunami inundation footprints. This dataset will include both representations of historical events as well as a series of M9 scenarios on subductions zones that have not historical generated mega earthquakes. The latter set is included to address concerns about the completeness of the historical record for mega earthquakes. This concern stems from the fact that the Tohoku Japan earthquake was considerably larger than had been observed in the historical record. Characterizing the source and rupture pattern for the subduction zones without historical events is a poorly constrained process. In many case, the subduction zones can be segmented based on changes in the characteristics of the subducting slab or major ridge systems. For this project, the unit sources from the NOAA propagation database are utilized to leverage the basin wide modeling included in this dataset. The length of the rupture is characterized based on subduction zone segmentation and the slip per unit source can be determined based on the event magnitude (i.e., M9) and moment balancing. As these events have not occurred historically, there is little to constrain the slip distribution. Sensitivity tests on the potential rupture pattern have been undertaken comparing uniform slip to higher shallow slip and tapered slip models. Subduction zones examined include the Makran Trench, the Lesser Antilles and the Hikurangi Trench. The ultimate goal is to create a series of tsunami footprints to help insurers understand their exposures at risk to tsunami inundation around the world.

Slab-plume interaction beneath the Pacific Northwest

NASA Astrophysics Data System (ADS)

Obrebski, Mathias; Allen, Richard M.; Xue, Mei; Hung, Shu-Huei

2010-07-01

The Pacific Northwest has undergone complex plate reorganization and intense tectono-volcanic activity to the east during the Cenozoic (last 65 Ma). Here we show new high-resolution tomographic images obtained using shear and compressional data from the ongoing USArray deployment that demonstrate first that there is a continuous, whole-mantle plume beneath the Yellowstone Snake River Plain (YSRP) and second, that the subducting Juan de Fuca (JdF) slab is fragmented and even absent beneath Oregon. The analysis of the geometry of our tomographic models suggests that the arrival and emplacement of the large Yellowstone plume had a substantial impact on the nearby Cascadia subduction zone, promoting the tearing and weakening of the JdF slab. This interpretation also explains several intriguing geophysical properties of the Cascadia trench that contrast with most other subduction zones, such as the absence of deep seismicity and the trench-normal fast direction of mantle anisotropy. The DNA velocity models are available for download and slicing at http://dna.berkeley.edu.

3D receiver function Kirchhoff depth migration image of Cascadia subduction slab weak zone

NASA Astrophysics Data System (ADS)

Cheng, C.; Allen, R. M.; Bodin, T.; Tauzin, B.

2016-12-01

We have developed a highly computational efficient algorithm of applying 3D Kirchhoff depth migration to telesismic receiver function data. Combine primary PS arrival with later multiple arrivals we are able to reveal a better knowledge about the earth discontinuity structure (transmission and reflection). This method is highly useful compare with traditional CCP method when dipping structure is met during the imaging process, such as subduction slab. We apply our method to the reginal Cascadia subduction zone receiver function data and get a high resolution 3D migration image, for both primary and multiples. The image showed us a clear slab weak zone (slab hole) in the upper plate boundary under Northern California and the whole Oregon. Compare with previous 2D receiver function image from 2D array(CAFE and CASC93), the position of the weak zone shows interesting conherency. This weak zone is also conherent with local seismicity missing and heat rising, which lead us to think about and compare with the ocean plate stucture and the hydralic fluid process during the formation and migration of the subduction slab.

Regional P wave velocity structure of the Northern Cascadia Subduction Zone

USGS Publications Warehouse

Ramachandran, K.; Hyndman, R.D.; Brocher, T.M.

2006-01-01

This paper presents the first regional three-dimensional, P wave velocity model for the Northern Cascadia Subduction. Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The velocity model images the structure of the subducting Juan de Fuca plate, megathrust, and the fore-arc crust and upper mantle. Beneath southern Vancouver Island the megathrust above the Juan de Fuca plate is characterized by a broad zone (25-35 km depth) having relatively low velocities of 6.4-6.6 km/s. This relative low velocity zone coincides with the location of most of the episodic tremors recently mapped beneath Vancouver Island, and its low velocity may also partially reflect the presence of trapped fluids and sheared lower crustal rocks. The rocks of the Olympic Subduction Complex are inferred to deform aseismically as evidenced by the lack of earthquakes withi the low-velocity rocks. The fore-arc upper mantle beneath the Strait of Georgia and Puget Sound is characterized by velocities of 7.2-7.6 km/s. Such low velocities represent regional serpentinization of the upper fore-arc mantle and provide evidence for slab dewatering and densification. Tertiary sedimentary basins in the Strait of Georgia and Puget Lowland imaged by the velocity model lie above the inferred region of slab dewatering and densification and may therefore partly result from a higher rate of slab sinking. In contrast, sedimentary basins in the Strait of Juan de Fuca lie in a synclinal depression in the Crescent Terrane. The correlation of in-slab earthquake hypocenters M>4 with P wave velocities greater than 7.8 km/s at the hypocenters suggests that they originate near the oceanic Moho of the subducting Juan de Fuca plate. Copyright 2006 by the American Geophysical Union.

Beach ridges as paleoseismic indicators of abrupt coastal subsidence during subduction zone earthquakes, and implications for Alaska-Aleutian subduction zone paleoseismology, southeast coast of the Kenai Peninsula, Alaska

USGS Publications Warehouse

Kelsey, Harvey M.; Witter, Robert C.; Engelhart, Simon E.; Briggs, Richard; Nelson, Alan R.; Haeussler, Peter J.; Corbett, D. Reide

2015-01-01

The Kenai section of the eastern Alaska-Aleutian subduction zone straddles two areas of high slip in the 1964 great Alaska earthquake and is the least studied of the three megathrust segments (Kodiak, Kenai, Prince William Sound) that ruptured in 1964. Investigation of two coastal sites in the eastern part of the Kenai segment, on the southeast coast of the Kenai Peninsula, identified evidence for two subduction zone earthquakes that predate the 1964 earthquake. Both coastal sites provide paleoseismic data through inferred coseismic subsidence of wetlands and associated subsidence-induced erosion of beach ridges. At Verdant Cove, paleo-beach ridges record the paleoseismic history; whereas at Quicksand Cove, buried soils in drowned coastal wetlands are the primary indicators of paleoearthquake occurrence and age. The timing of submergence and death of trees mark the oldest earthquake at Verdant Cove that is consistent with the age of a well documented ∼900-year-ago subduction zone earthquake that ruptured the Prince William Sound segment of the megathrust to the east and the Kodiak segment to the west. Soils buried within the last 400–450 years mark the penultimate earthquake on the southeast coast of the Kenai Peninsula. The penultimate earthquake probably occurred before AD 1840 from its absence in Russian historical accounts. The penultimate subduction zone earthquake on the Kenai segment did not rupture in conjunction with the Prince William Sound to the northeast. Therefore the Kenai segment, which is presently creeping, can rupture independently of the adjacent Prince William Sound segment that is presently locked.

1D minimum p-velocity model of the Kamchatka subducting zone

NASA Astrophysics Data System (ADS)

Nizkous, I.; Sanina, I.; Gontovaya, L.

2003-04-01

Kamchatka peninsula is a very active seismic zone. The old Pacific plate subducts below the North American Plate and this causes high seismic and volcanic activity in this region. The extensive Kamchatka Regional Seismic Network (KRSN) has operated since 1962 and registers around 600 earthquakes per year. This provides a large number of high quality seismic data. In this work we are investigate P-velocity structure of the Kamchatka peninsula and subducting zone in Western Pacific. This region is well studied, but we would like to try a little bit different approach. We would like to present 1D minimum P-velocity model of the Kamchatka region created using VELEST program [3]. Data set based on 84 well-located earthquakes (IP, EP, IS and ES phases) recorded by KRSN in 1998 and in 1999. As the initial model Kuzin's model have been taken [1]. But in our calculations we split model into 17 layers instead of initial 5. Maximal investigated depth is 120 km. Using VELEST simultaneous mode we solve coupled hypocenter-velocity model problem for local earthquakes. In this case it is very important to utilize well locatable events for the sake of minimizing a priori added uncertainties. And this is major point of the approach. We apply this idea and the result is looks like the result obtained by A. Gorbatov et. al. [2] Using this 1D minimum model we redefine earthquakes hypocenter parameters and recalculate p-wave travel time residuals. This work is the first step in 3D modeling of the Kamchatka subducting zone. References: 1. I.P Kuzin. 'Focal zone and upper mantle structure of the East Kamchatka region', Moscow, Nauka, 1974. 2. A. Gorbatov, J. Domingues, G.Suarez, V.kostoglodov, D.Zhao, and E. Gordeev, 'Tomographic imaging of the P-wave velocity structure beneath the Kamchatka peninsula', Geophys. J. Int, 1999, 137, 269-279. 3. Kissling, E., W.L. Ellsworth, D. Eberhart-Phillips, and U. Kradolfer: Initial reference models in local earthquake tomography, J. Geophys. Res., 99, 19635-19646, 1994.

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.

Seismicity of the Earth 1900‒2013 Mediterranean Sea and vicinity

USGS Publications Warehouse

Herman, Matthew W.; Hayes, Gavin P.; Smoczyk, Gregory M.; Turner, Rebecca; Turner, Bethan; Jenkins, Jennifer; Davies, Sian; Parker, Amy; Sinclair, Allison; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio

2015-09-08

The Mediterranean region is seismically active due to the convergence of the Africa Plate with the Eurasia plate. Present day Africa-Eurasia motion ranges from ~4 millimeters per year (mm/yr) in a northwest-southeast direction in the western Mediterranean to ~10 mm/yr (north-south) in the eastern Mediterranean. The Africa-Eurasia plate boundary is complex, and includes extensional and translational zones in addition to the dominant convergent regimes characterized by subduction and continental collision. This convergence began at approximately 50 million years ago and was associated with the closure of the Tethys Sea; the Mediterranean Sea is all that remains of the Tethys. The highest rates of seismicity in the Mediterranean region are found along the Hellenic subduction zone of southern Greece and the North Anatolian Fault Zone of northwestern Turkey, but significant rates of current seismicity and large historical earthquakes have occurred throughout the region spanning the Mediterranean Sea.

A model for the termination of the Ryukyu subduction zone against Taiwan: A junction of collision, subduction/separation, and subduction boundaries

USGS Publications Warehouse

Wu, F.T.; Liang, W.-T.; Lee, J.-C.; Benz, H.; Villasenor, A.

2009-01-01

The NW moving Philippine Sea plate (PSP) collides with the Eurasian plate (EUP) in the vicinity of Taiwan, and at the same time, it subducts toward the north along SW Ryukyu. The Ryukyu subduction zone terminates against eastern Taiwan. While the Ryukyu Trench is a linear bathym??trie low about 100 km east of Taiwan, closer to Taiwan, it cannot be clearly identified bathymetrically owing to the deformation related to the collision, making the location of the intersection of the Ryukyu with Taiwan difficult to decipher. We propose a model for this complex of boundaries on the basis of seismicity and 3-D velocity structures. In this model the intersection is placed at the latitude of about 23.7??N, placing the northern part of the Coastal Range on EUP. As PSP gets deeper along the subduction zone it collides with EUP on the Taiwan side only where they are in direct contact. Thus, the Eurasian plate on the Taiwan side is being pushed and compressed by the NW moving Philippine Sea plate, at increasing depth toward the north. Offshore of northeastern Taiwan the wedge-shaped EUP on top of the Ryukyu subducting plate is connected to the EUP on the Ryukyu side and coupled to the NW moving PSP by friction at the plate interface. The two sides of the EUP above the western end of the subduction zone are not subjected to the same forces, and a difference in motions can be expected. The deformation of Taiwan as revealed by continuous GPS measurements, geodetic movement along the east coast of Taiwan, and the formation of the Hoping Basin can be understood in terms of the proposed model. Copyright 2009 by the American Geophysical Union.

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.

Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents

NASA Astrophysics Data System (ADS)

Hacker, Bradley R.; Abers, Geoffrey A.; Peacock, Simon M.

2003-01-01

We present a new compilation of physical properties of minerals relevant to subduction zones and new phase diagrams for mid-ocean ridge basalt, lherzolite, depleted lherzolite, harzburgite, and serpentinite. We use these data to calculate H2O content, density and seismic wave speeds of subduction zone rocks. These calculations provide a new basis for evaluating the subduction factory, including (1) the presence of hydrous phases and the distribution of H2O within a subduction zone; (2) the densification of the subducting slab and resultant effects on measured gravity and slab shape; and (3) the variations in seismic wave speeds resulting from thermal and metamorphic processes at depth. In considering specific examples, we find that for ocean basins worldwide the lower oceanic crust is partially hydrated (<1.3 wt % H2O), and the uppermost mantle ranges from unhydrated to ˜20% serpentinized (˜2.4 wt % H2O). Anhydrous eclogite cannot be distinguished from harzburgite on the basis of wave speeds, but its ˜6% greater density may render it detectable through gravity measurements. Subducted hydrous crust in cold slabs can persist to several gigapascals at seismic velocities that are several percent slower than the surrounding mantle. Seismic velocities and VP/VS ratios indicate that mantle wedges locally reach 60-80% hydration.

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.

Plate tectonics on the Earth triggered by plume-induced subduction initiation.

PubMed

Gerya, T V; Stern, R J; Baes, M; Sobolev, S V; Whattam, S A

2015-11-12

Scientific theories of how subduction and plate tectonics began on Earth--and what the tectonic structure of Earth was before this--remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

Shallow velocity structure of the Alaska Peninsula subduction zone and implications for controls on seismic behavior

NASA Astrophysics Data System (ADS)

Li, J.; Shillington, D. J.; Becel, A.; Nedimovic, M. R.; Kuehn, H.; Webb, S. C.; Abers, G. A.; Keranen, K. M.; Saffer, D. M.

2014-12-01

Downdip and along-strike variations in the seismic behavior of subduction zone megathrust faults are thought to be strongly controlled by changes in the material properties along the plate boundary. Roughness and hydration of the incoming plate, fluid pressure and lithology in the subducting sediment channel are likely to control the distribution of shallower rupture. Here, we focus on the subduction zone offshore of the Alaska Peninsula. In 2011, the ALEUT program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data across the apparently freely sliding Shumagin Gap, the locked Semidi segment that last ruptured in 1938 M8.2 earthquake, and the locked western Kodiak asperity, which ruptured in the 1964 M9.2 earthquake. Seismic reflection data from the ALEUT cruise reveal significant variability in the thickness of sediment on the incoming plate and entering the trench, and the roughness and degree of hydration of the incoming plate. Oceanic crust entering the trench in the Shumagin gap is rugged with extensive faults and only a thin layer of sediment (<0.5 km thick). Farther east in the Semidi segment, the subducting plate has a smoother surface with thicker sediments (~1 km thick) and less faulting/hydration. To better constrain the properties of the accretionary prism and shallow part of the plate boundary, we are undertaking travel time tomography using reflection/refraction phases in OBS and MCS data, and constraints on the interface geometry from MCS images to estimate the detailed shallow velocity structure, with particular focus on properties within the shallow subduction channel. We observe refractions and reflections in OBS data from the shallow part of the subduction zone in both the Shumagin Gap and Semidi segment, including reflections off the top and base of what appears to be a layer of subducting sediment, which can be used for this work. We plan to present initial models of the shallow part of the subduction zone from both segments and discuss comparisons between the two.

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.

Seismic attenuation structure beneath Nazca Plate subduction zone in southern Peru

NASA Astrophysics Data System (ADS)

Jang, H.; Kim, Y.; Clayton, R. W.

2017-12-01

We estimate seismic attenuation in terms of quality factors, QP and QS using P and S phases, respectively, beneath Nazca Plate subduction zone between 10°S and 18.5°S latitude in southern Peru. We first relocate 298 earthquakes with magnitude ranges of 4.0-6.5 and depth ranges of 20-280 km. We measure t*, which is an integrated attenuation through the seismic raypath between the regional earthquakes and stations. The measured t* are inverted to construct three-dimensional attenuation structures of southern Peru. Checkerboard test results for both QP and QS structures ensure good resolution in the slab-dip transition zone between flat and normal slab subduction down to a depth of 200 km. Both QP and QS results show higher attenuation continued down to a depth of 50 km beneath volcanic arc and also beneath the Quimsachata volcano, the northernmost young volcano, located far east of the main volcanic front. We also observe high attenuation in mantle wedge especially beneath the normal subduction region in both QP and QS (100-130 in QP and 100-125 in QS) and slightly higher QP and QS beneath the flat-subduction and slab-dip transition regions. We plan to relate measured attenuation in the mantle wedge to material properties such as viscosity to understand the subduction zone dynamics.

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.

Tomography reveals buoyant asthenosphere accumulating beneath the Juan de Fuca plate.

PubMed

Hawley, William B; Allen, Richard M; Richards, Mark A

2016-09-23

The boundary between Earth's strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth, perhaps indicating a thin, weak layer that may strongly influence plate motion dynamics. The behavior of such a layer at subduction zones remains unexplored. We present a tomographic model, derived from on- and offshore seismic experiments, that reveals a strong low-velocity feature beneath the subducting Juan de Fuca slab along the entire Cascadia subduction zone. Through simple geodynamic arguments, we propose that this low-velocity feature is the accumulation of material from a thin, weak, buoyant layer present beneath the entire oceanic lithosphere. The presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics. Copyright © 2016, American Association for the Advancement of Science.

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.

Lateral Variations of Interplate Coupling along the Mexican Subduction Interface: Relationships with Long-Term Morphology and Fault Zone Mechanical Properties

NASA Astrophysics Data System (ADS)

Rousset, Baptiste; Lasserre, Cécile; Cubas, Nadaya; Graham, Shannon; Radiguet, Mathilde; DeMets, Charles; Socquet, Anne; Campillo, Michel; Kostoglodov, Vladimir; Cabral-Cano, Enrique; Cotte, Nathalie; Walpersdorf, Andrea

2016-10-01

Although patterns of interseismic strain accumulation above subduction zones are now routinely characterised using geodetic measurements, their physical origin, persistency through time, and relationships to seismic hazard and long-term deformation are still debated. Here, we use GPS and morphological observations from southern Mexico to explore potential mechanical links between variations in inter-SSE (in between slow slip events) coupling along the Mexico subduction zone and the long-term topography of the coastal regions from Guerrero to Oaxaca. Inter-SSE coupling solutions for two different geometries of the subduction interface are derived from an inversion of continuous GPS time series corrected from slow slip events. They reveal strong along-strike variations in the shallow coupling (i.e. at depths down to 25 km), with high-coupling zones (coupling >0.7) alternating with low-coupling zones (coupling <0.3). Coupling below the continent is typically strong (>0.7) and transitions to uncoupled, steady slip at a relatively uniform ˜ 175-km inland from the trench. Along-strike variations in the coast-to-trench distances are strongly correlated with the GPS-derived forearc coupling variations. To explore a mechanical explanation for this correlation, we apply Coulomb wedge theory, constrained by local topographic, bathymetric, and subducting-slab slopes. Critical state areas, i.e. areas where the inner subduction wedge deforms, are spatially correlated with transitions at shallow depth between uncoupled and coupled areas of the subduction interface. Two end-member models are considered to explain the correlation between coast-to-trench distances and along-strike variations in the inter-SSE coupling. The first postulates that the inter-SSE elastic strain is partitioned between slip along the subduction interface and homogeneous plastic permanent deformation of the upper plate. In the second, permanent plastic deformation is postulated to depend on frictional transitions along the subduction plate interface. Based on the location and friction values of the critical state areas identified by our Coulomb wedge analysis, we parameterise frictional transitions in plastic-static models of deformation over several seismic cycles. This predicts strong shear dissipation above frictional transitions on the subduction interface. The comparison of modelled surface displacements over a critical zone at a frictional transition and over a stable area with no internal wedge deformation shows differences of long-term uplift consistent with the observed along-strike variations in the coast-to-trench distances. Our work favours a model in which frictional asperities partly control short-term inter-SSE coupling as measured by geodesy and in which those asperities persist through time.

Tectono-metamorphic evolution of high-P/T and low-P/T metamorphic rocks in the Tia Complex, southern New England Fold Belt, eastern Australia: Insights from K-Ar chronology

NASA Astrophysics Data System (ADS)

Fukui, Shiro; Tsujimori, Tatsuki; Watanabe, Teruo; Itaya, Tetsumaru

2012-10-01

The Tia Complex in the southern New England Fold Belt is a poly-metamorphosed Late Paleozoic accretionary complex. It consists mainly of high-P/low-T type pumpellyite-actinolite facies (rare blueschist facies) schists, phyllite and serpentinite (T = 300 °C and P = 5 kbar), and low-P/high-T type amphibolite facies schist and gneiss (T = 600 °C and P < 5 kbar) associated with granodioritic plutons (Tia granodiorite). White mica and biotite K-Ar ages distinguish Carboniferous subduction zone metamorphism and Permian granitic intrusions, respectively. The systematic K-Ar age mapping along a N-S traverse of the Tia Complex exhibits a gradual change. The white mica ages become younger from the lowest-grade zone (339 Ma) to the highest-grade zone (259 Ma). In contrast, Si content of muscovite changes drastically only in the highest-grade zone. The regional changes of white mica K-Ar ages and chemical compositions of micas indicate argon depletion from precursor high-P/low-T type phengitic white mica during the thermal overprinting and recrystallization by granitoids intrusions. Our new K-Ar ages and available geological data postulate a model of the eastward rollback of a subduction zone in Early Permian. The eastward shift of a subduction zone system and subsequent magmatic activities of high-Mg andesite and adakite might explain formation of S-type granitoids (Hillgrove suite) and coeval low-P/high-T type metamorphism in the Tia Complex.

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.

Deformation of the Japanese Islands and seismic coupling: an interpretation based on GSI permanent GPS observations

NASA Astrophysics Data System (ADS)

Le Pichon, Xavier; Mazzotti, Stéphane; Henry, Pierre; Hashimoto, Manabu

1998-08-01

The entire area of the Japanese Islands has been covered by the permanent GPS observation network of the Geographical Survey Institute since 1994. In this paper we use a solution for the vectors of motion during 1995 for a selection of 116 stations to discuss the origin of the observed deformation field. We refer the displacement field to Eurasia using the VLBI-determined motion of Kashima and demonstrate that other choices such as the Okhotsk or North American plates for north Japan are not compatible with the data. 1 yr GPS velocities are much higher than geological constraints would allow because these short-term measurements include transient elastic deformation. However, the good qualitative agreement between the observed geodetic deformation tensors and those inferred from active faults and earthquakes suggests that the Quaternary permanent deformation is essentially the result of the transfer of part of the subduction-induced elastic deformation into permanent plastic deformation. We then compute the elastic deformation of the Japanese Islands caused by interseismic loading of the Pacific and Philippine subduction planes. The geometry of the coupled zone and its downward extension are determined from the distribution of earthquakes for the Pacific slab. For the Philippine slab we use the geometry proposed by Hyndman et al. (1995). These elastic models account for most of the observed velocity field if the subduction movement of the Philippine Sea Plate is 100 per cent locked and if that of the Pacific Plate is 75-85 per cent locked. We note that the boundaries of the areas where significant elastic deformation is predicted (more than 10 mm yr-1 of motion with respect to Eurasia) coincide with the main zones of permanent deformation: the Eastern Japan Sea deformation zone for the Pacific subduction elastic deformation field and the Setouchi/MTL deformation zone for the Nankai field. Each zone probably accommodates 10-15 mm yr-1 of motion in the long term (convergence in the Eastern Japan Sea; strike-slip in the Setouchi/MTL zone). To account for this deformation, the effect of elastic loading from the trench must be combined with 5-10 mm yr-1 of motion of the Amur Plate with respect to Eurasia. Because loading during the subduction earthquake cycle causes an increase in stress in the Eastern Japan Sea and Setouchi/MTL deformation zones, the probability of earthquake occurrence in these zones may be higher near the end of the cycle.

Simulation of tsunamis from great earthquakes on the cascadia subduction zone.

PubMed

Ng, M K; Leblond, P H; Murty, T S

1990-11-30

Large earthquakes occur episodically in the Cascadia subduction zone. A numerical model has been used to simulate and assess the hazards of a tsunami generated by a hypothetical earthquake of magnitude 8.5 associated with rupture of the northern sections of the subduction zone. Wave amplitudes on the outer coast are closely related to the magnitude of sea-bottom displacement (5.0 meters). Some amplification, up to a factor of 3, may occur in some coastal embayments. Wave amplitudes in the protected waters of Puget Sound and the Strait of Georgia are predicted to be only about one fifth of those estmated on the outer coast.

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.

Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite

NASA Astrophysics Data System (ADS)

Walowski, K. J.; Wallace, P. J.; Hauri, E. H.; Wada, I.; Clynne, M. A.

2015-05-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water--subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate--is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab--hydrated mantle peridotite in the slab interior--compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Slab melting beneath the Cascades Arc driven by dehydration of altered oceanic peridotite

USGS Publications Warehouse

Walowski, Kristina J; Wallace, Paul J.; Hauri, E.H.; Wada, I.; Clynne, Michael A.

2015-01-01

Water is returned to Earth’s interior at subduction zones. However, the processes and pathways by which water leaves the subducting plate and causes melting beneath volcanic arcs are complex; the source of the water—subducting sediment, altered oceanic crust, or hydrated mantle in the downgoing plate—is debated; and the role of slab temperature is unclear. Here we analyse the hydrogen-isotope and trace-element signature of melt inclusions in ash samples from the Cascade Arc, where young, hot lithosphere subducts. Comparing these data with published analyses, we find that fluids in the Cascade magmas are sourced from deeper parts of the subducting slab—hydrated mantle peridotite in the slab interior—compared with fluids in magmas from the Marianas Arc, where older, colder lithosphere subducts. We use geodynamic modelling to show that, in the hotter subduction zone, the upper crust of the subducting slab rapidly dehydrates at shallow depths. With continued subduction, fluids released from the deeper plate interior migrate into the dehydrated parts, causing those to melt. These melts in turn migrate into the overlying mantle wedge, where they trigger further melting. Our results provide a physical model to explain melting of the subducted plate and mass transfer from the slab to the mantle beneath arcs where relatively young oceanic lithosphere is subducted.

Coupled deformation and dehydration processes in smectite-rich sediments constrained by laboratory experiments

NASA Astrophysics Data System (ADS)

Huepers, Andre; Kopf, Achim J.

2013-04-01

Subduction zones play a central role in the geological activity of the earth which is expressed as devastating events such as earthquakes, tsunamis and explosive volcanism. Many processes that lead to such catastrophic behavior are driven by fluids, which in turn affect the rock mechanical behavior. The kinetic reaction of hydrous smectite to illite is widely accepted as a fluid source in subduction zone forearcs that also affects the mechanical state of subduction zone sediments. The released fluids are characterized by low-chlorinity and high volatile content. Also, previous workers demonstrated in uniaxial deformation tests that smectite partially dehydrates with increasing effective stress. To shed light on this process we performed uniaxial deformation experiments on smectite-rich samples from the Nankai and Costa Rica subduction zones. Experiments were conducted at temperatures of up to 100°C under constant rate of strain and effective stresses of up to ~100MPa. Fluids expelled during the experiments were analyzed for major and minor element content. The fluids are characterized by fluid-freshening and increasing volatile content that starts at ~1.3MPa effective stress. During the course of the experiments the smectite interlayer water content decreases from 27 wt-% to 20 wt-%. The released interlayer water comprises up to 17% of the total fluid volume released from the consolidating sediment. The onset of fluid freshening is characterized by a change in deformation behavior of the samples. The porosity decrease with increasing effective stress is smaller at effective stresses greater 1.3MPa. We propose that dehydration of the low permeable smectite leads to excess pore pressures in the sample, which causes a load transfer from the solid phase to the pore fluid.

Depth-varying azimuthal anisotropy in the Tohoku subduction channel

NASA Astrophysics Data System (ADS)

Liu, Xin; Zhao, Dapeng

2017-09-01

We determine a detailed 3-D model of azimuthal anisotropy tomography of the Tohoku subduction zone from the Japan Trench outer-rise to the back-arc near the Japan Sea coast, using a large number of high-quality P and S wave arrival-time data of local earthquakes recorded by the dense seismic network on the Japan Islands. Depth-varying seismic azimuthal anisotropy is revealed in the Tohoku subduction channel. The shallow portion of the Tohoku megathrust zone (<30 km depth) generally exhibits trench-normal fast-velocity directions (FVDs) except for the source area of the 2011 Tohoku-oki earthquake (Mw 9.0) where the FVD is nearly trench-parallel, whereas the deeper portion of the megathrust zone (at depths of ∼30-50 km) mainly exhibits trench-parallel FVDs. Trench-normal FVDs are revealed in the mantle wedge beneath the volcanic front and the back-arc. The Pacific plate mainly exhibits trench-parallel FVDs, except for the top portion of the subducting Pacific slab where visible trench-normal FVDs are revealed. A qualitative tectonic model is proposed to interpret such anisotropic features, suggesting transposition of earlier fabrics in the oceanic lithosphere into subduction-induced new structures in the subduction channel.

Tsunami Size Distributions at Far-Field Locations from Aggregated Earthquake Sources

NASA Astrophysics Data System (ADS)

Geist, E. L.; Parsons, T.

2015-12-01

The distribution of tsunami amplitudes at far-field tide gauge stations is explained by aggregating the probability of tsunamis derived from individual subduction zones and scaled by their seismic moment. The observed tsunami amplitude distributions of both continental (e.g., San Francisco) and island (e.g., Hilo) stations distant from subduction zones are examined. Although the observed probability distributions nominally follow a Pareto (power-law) distribution, there are significant deviations. Some stations exhibit varying degrees of tapering of the distribution at high amplitudes and, in the case of the Hilo station, there is a prominent break in slope on log-log probability plots. There are also differences in the slopes of the observed distributions among stations that can be significant. To explain these differences we first estimate seismic moment distributions of observed earthquakes for major subduction zones. Second, regression models are developed that relate the tsunami amplitude at a station to seismic moment at a subduction zone, correcting for epicentral distance. The seismic moment distribution is then transformed to a site-specific tsunami amplitude distribution using the regression model. Finally, a mixture distribution is developed, aggregating the transformed tsunami distributions from all relevant subduction zones. This mixture distribution is compared to the observed distribution to assess the performance of the method described above. This method allows us to estimate the largest tsunami that can be expected in a given time period at a station.

Tracing halogen and B cycling in subduction zones based on obducted, subducted and forearc serpentinites of the Dominican Republic.

PubMed

Pagé, Lilianne; Hattori, Keiko

2017-12-19

Serpentinites are important reservoirs of fluid-mobile elements in subduction zones, contributing to volatiles in arc magmas and their transport into the Earth's mantle. This paper reports halogen (F, Cl, Br, I) and B abundances of serpentinites from the Dominican Republic, including obducted and subducted abyssal serpentinites and forearc mantle serpentinites. Abyssal serpentinite compositions indicate the incorporation of these elements from seawater and sediments during serpentinization on the seafloor and at slab bending. During their subduction and subsequent lizardite-antigorite transition, F and B are retained in serpentinites, whilst Cl, Br and I are expelled. Forearc mantle serpentinite compositions suggest their hydration by fluids released from subducting altered oceanic crust and abyssal serpentinites, with only minor sediment contribution. This finding is consistent with the minimal subduction of sediments in the Dominican Republic. Forearc mantle serpentinites have F/Cl and B/Cl ratios similar to arc magmas, suggesting the importance of serpentinite dehydration in the generation of arc magmatism in the mantle wedge.

Constraints on Subduction Zone Coupling along the Philippine and Manila Trenches based on GPS and Seismological Data

NASA Astrophysics Data System (ADS)

Hamburger, M. W.; Johnson, K. M.; Nowicki, M. A. E.; Bacolcol, T. C.; Solidum, R., Jr.; Galgana, G.; Hsu, Y. J.; Yu, S. B.; Rau, R. J.; McCaffrey, R.

2014-12-01

We present results of two techniques to estimate the degree of coupling along the two major subduction zone boundaries that bound the Philippine Mobile Belt, the Philippine Trench and the Manila Trench. Convergence along these plate margins accommodates about 100 mm/yr of oblique plate motion between the Philippine Sea and Sundaland plates. The coupling estimates are based on a recently acquired set of geodetic data from a dense nationwide network of continuous and campaign GPS sites in the Philippines. First, we use a kinematic, elastic block model (tdefnode; McCaffrey, 2009) that combines existing fault geometries, GPS velocities and focal mechanism solutions to solve for block rotations, fault coupling, and intra-block deformation. Secondly, we use a plate-block kinematic model described in Johnson (2013) to simultaneously estimate long-term fault slip rates, block motions and interseismic coupling on block-bounding faults. The best-fit model represents the Philippine Mobile Belt by 14 independently moving rigid tectonic blocks, separated by active faults and subduction zones. The model predicts rapid convergence along the Manila Trench, decreasing progressively southwards, from > 100 mm/yr in the north to less than 20 mm/yr in the south at the Mindoro Island collision zone. Persistent areas of high coupling, interpreted to be asperities, are observed along the Manila Trench slab interface, in central Luzon (16-18°N) and near its southern and northern terminations. Along the Philippine Trench, we observe ~50 mm/yr of oblique convergence, with high coupling observed at its central and southern segments. We identify the range of allowable coupling distributions and corresponding moment accumulation rates on the two subduction zones by conducting a suite of inversions in which the total moment accumulation rate on a selected fault is fixed. In these constrained moment inversions we test the range of possible solutions that meet criteria for minimum, best-fit, and maximum coupling that still fit the data, based on reduced chi-squared calculations. In spite of the variable coupling, the total potential moment accumulation rate along each of the two subduction zones is estimated to range from 3.98 x 1019 to 2.24 x 1020 N-m yr-1, equivalent to a magnitude Mw 8.4 to 8.9 earthquake per 100 years.

Defining the worst case scenario for the Makran Subduction Zone: the 1008 AD tsunami

NASA Astrophysics Data System (ADS)

Hoffmann, Goesta

2016-04-01

The Makran Subduction Zone is located within the Arabian Sea (Northern Indian Ocean) and marks the boundary between the Arabian and the Eurasian plate. The sinistral strike-slip Sonne fault separates the subduction zone in an eastern and western segment. The convergence rate is about 40 mm/yr and slightly faster in the east than in the west. The seismicity is low in general and the few documented seismic events are concentrated in the eastern segment. No seismic activity is known from the western segment in historic times. The hazard potential is enigmatic as the only documented and recorded tsunamigenic earthquake (MW 8.1) within the subduction zone occurred in Nov 1945. However, thermal modelling suggests a wide potential seismogenic zone, apparently capable of generating very significant (>MW 8.5) tsunamigenic earthquakes. Furthermore, submarine slumping is another tsunami trigger which has to be taken into account. We used the modelling results as a hypothesis and mapped extreme wave event deposits along the coastline of Oman, bordering the Arabian Sea. We were able to document extensive boulder fields along rocky parts of the coastline. These boulders are decorated with marine sessile organism such as e.g oysters or barnacles testifying for an intertidal setting of the boulder prior to dislocation. The organism remains were used for radiocarbon dating assuming that the death of the organism was related to the relocation of the boulder. Storm-induced boulder movement is possible as the coastline is subject to infrequent tropical cyclone impact. However, boulder movement was not observed during the strongest storm on record in 2007. The dating exercise revealed a cluster of dates around 1000 AD, coinciding with a potential earthquake event known from a historic Persian text dating to the year 1008 AD. Archaeological evidence, mainly pottery artefacts found along the sea shore near the capital area Muscat/Oman also indicate a catastrophic event which may be correlated to the 1008 AD earthquake and tsunami inundation. The boulder deposits as well as the archaeological remains testify for a maximum tsunami runup of 15m, exceeding by far the inundation as observed in 1945. We define this as the worst case scenario for the Makran Subduction Zone. However, the return period is rather large (>500 years).

Advancing Understanding of Earthquakes by Drilling an Eroding Convergent Margin

NASA Astrophysics Data System (ADS)

von Huene, R.; Vannucchi, P.; Ranero, C. R.

2010-12-01

A program of IODP with great societal relevance is sampling and instrumenting the seismogenic zone. The zone generates great earthquakes that trigger tsunamis, and submarine slides thereby endangering coastal communities containing over sixty percent of the earth’s population. To asses and mitigate this endangerment it is urgent to advance understanding of fault dynamics that allows more timely anticipation of hazardous seismicity. Seismogenesis on accreting and eroding convergent plate boundaries apparently differ because of dissimilar materials along the interplate fault. As the history of instrumentally recorded earthquakes expands the difference becomes clearer. The more homogeneous clay, silt and sand subducted at accreting margins is associated with great earthquakes (M 9) whereas the fragmented upper plate rock that can dominate subducted material along an eroding margin plate interface is associated with many tsunamigenic earthquakes (Bilek, 2010). Few areas have been identified where the seismogenic zone can be reached with scientific drilling. In IODP accreting margins are studied on the NanTroSeize drill transect off Japan where the ultimate drilling of the seismogenic interface may occur by the end of IODP. The eroding Costa Rica margin will be studied in CRISP where a drill program will begin in 2011. The Costa Rican geophysical site survey will be complete with acquisition and processing of 3D seismic data in 2011 but the entire drilling will not be accomplished in IODP. It is appropriate that the accreting margin study be accomplished soon considering the indications of a pending great earthquake that will affect a country that has devoted enormous resources to IODP. However, understanding the erosional end-member is scientifically as important to an understanding of fault mechanics. Transoceanic tsunamis affect the entire Pacific rim where most subduction zones are eroding margins. The Costa Rican subduction zone is less complex operationally and perhaps geologically than the Nankai margin. The developing Central American countries do not have the resources to contribute to IODP but this should not deter acquiring the scientific insights proposed in CRISP considering the broader scientific benefits. Such benefits include the first sampling and instrumentation of an actively eroding plate interface and drilling near or into an earthquake asperity. Drilling an eroding margin should significantly advance understanding of subduction zone fault mechanisms and help improve assessment of future hazardous earthquakes and tsunamis.

Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)

USGS Publications Warehouse

Sorensen, Sorena S.; Grossman, Jeffrey N.

1993-01-01

Data from the Gee Point and Catalina mélanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.

Evidence for Complex P-T-t Histories in Subduction Zone Rocks: A Case Study from Syros, Greece

NASA Astrophysics Data System (ADS)

Gorce, J. S.; Kendall, J.; Caddick, M. J.; Baxter, E. F.

2017-12-01

Numerical models predict that material can move freely at the interface between the subducting slab and the overlying mantle wedge (mélange zone) independent of the motion of the subducting slab (i.e. Cloos 1982, Gerya et al. 2002). This is possible because the mélange zone consists of rigid blocks of metagabbroic and metabasic material suspended in a strongly sheared matrix of serpentinite, talc, and chlorite. The implication of this is that blocks of subducted material exposed in outcrops at the earth's surface could experience complex Pressure-Temperature-time (P-T-t) paths due to the cycling and recycling of subducted material within the mélange zone. Such behavior can affect the expulsion and retention of fluid during metamorphism and thus affect elemental cycles, geodynamics, mineral phase equilibra and mass transport of materials in the mélange zone depending on the physical properties and location of the blocks. The island of Syros, Greece preserves rocks that experienced blueschist-eclogite grade metamorphism during the subduction of the Pindos Oceanic Unit and thus provides a natural laboratory for investigating the evolution of subducted lithologies. Complex compositional zoning in a garnet-bearing quartz mica schist indicates that garnet crystals grew in two distinct stages. The presence of distinct cores and rims is interpreted as the result of a complex P-T-t history. Through the use of thermodynamic modeling, we calculate that the core of the garnet equilibrated at 485oC and 22.5 kbars. The edge of the first growth zone is predicted to stop growing at approximately 530oC and 20.5 kbars. We calculate that the rim began to grow at 21.7 kbars and 560oC and that the end of garnet growth occurred at approximately 16 kbars and 500oC. Sm/Nd garnet geochronology was used to date the cores of the garnets at 47 ± 3 Ma, with preliminary results suggesting that the rims grew at a significantly younger age. These data support the hypothesis that the cycling and recycling of material in the mélange zone is responsible for the two distinct phases of metamorphism recorded in the garnet.

The energy release in earthquakes, and subduction zone seismicity and stress in slabs. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Vassiliou, M. S.

1983-01-01

Energy release in earthquakes is discussed. Dynamic energy from source time function, a simplified procedure for modeling deep focus events, static energy estimates, near source energy studies, and energy and magnitude are addressed. Subduction zone seismicity and stress in slabs are also discussed.

Phanerozoic tectonic evolution of the Circum-North Pacific

USGS Publications Warehouse

Nokleberg, Warren J.; Parfenov, Leonid M.; Monger, James W.H.; Norton, Ian O.; Khanchuk, Alexander I.; Stone, David B.; Scotese, Christopher R.; Scholl, David W.; Fujita, Kazuya

2000-01-01

The Phanerozoic tectonic evolution of the Circum-North Pacific is recorded mainly in the orogenic collages of the Circum-North Pacific mountain belts that separate the North Pacific from the eastern part of the North Asian Craton and the western part of the North American Craton. These collages consist of tectonostratigraphic terranes that are composed of fragments of igneous arcs, accretionary-wedge and subduction-zone complexes, passive continental margins, and cratons; they are overlapped by continental-margin-arc and sedimentary-basin assemblages. The geologic history of the terranes and overlap assemblages is highly complex because of postaccretionary dismemberment and translation during strike-slip faulting that occurred subparallel to continental margins.We analyze the complex tectonics of this region by the following steps. (1) We assign tectonic environments for the orogenic collages from regional compilation and synthesis of stratigraphic and faunal data. The types of tectonic environments include cratonal, passive continental margin, metamorphosed continental margin, continental-margin arc, island arc, oceanic crust, seamount, ophiolite, accretionary wedge, subduction zone, turbidite basin, and metamorphic. (2) We make correlations between terranes. (3) We group coeval terranes into a single tectonic origin, for example, a single island arc or subduction zone. (4) We group igneous-arc and subduction- zone terranes, which are interpreted as being tectonically linked, into coeval, curvilinear arc/subduction-zone complexes. (5) We interpret the original positions of terranes, using geologic, faunal, and paleomagnetic data. (6) We construct the paths of tectonic migration. Six processes overlapping in time were responsible for most of the complexities of the collage of terranes and overlap assemblages around the Circum-North Pacific, as follows. (1) During the Late Proterozoic, Late Devonian, and Early Carboniferous, major periods of rifting occurred along the ancestral margins of present-day Northeast Asia and northwestern North America. The rifting resulted in the fragmentation of each continent and the formation of cratonal and passive continental-margin terranes that eventually migrated and accreted to other sites along the evolving margins of the original or adjacent continents. (2) From about the Late Triassic through the mid-Cretaceous, a succession of island arcs and tectonically paired subduction zones formed near the continental margins. (3) From about mainly the mid-Cretaceous through the present, a succession of igneous arcs and tectonically paired subduction zones formed along the continental margins. (4) From about the Jurassic to the present, oblique convergence and rotations caused orogenparallel sinistral and then dextral displacements within the upper-plate margins of cratons that have become Northeast Asia and North America. The oblique convergences and rotations resulted in the fragmentation, displacement, and duplication of formerly more nearly continuous arcs, subduction zones, and passive continental margins. These fragments were subsequently accreted along the expanding continental margins. (5) From the Early Jurassic through Tertiary, movement of the upper continental plates toward subduction zones resulted in strong plate coupling and accretion of the former island arcs and subduction zones to the continental margins. Accretions were accompanied and followed by crustal thickening, anatexis, metamorphism, and uplift. The accretions resulted in substantial growth of the North Asian and North American Continents. (6) During the middle and late Cenozoic, oblique to orthogonal convergence of the Pacifi c plate with present-day Alaska and Northeast Asia resulted in formation of the modern-day ring of volcanoes around the Circum-North Pacific. Oblique convergence between the Pacific plate and Alaska also resulted in major dextral-slip faulting in interior and southern Alaska and along the western p

Double seismic zone for deep earthquakes in the izu-bonin subduction zone.

PubMed

Iidaka, T; Furukawa, Y

1994-02-25

A double seismic zone for deep earthquakes was found in the Izu-Bonin region. An analysis of SP-converted phases confirms that the deep seismic zone consists of two layers separated by approximately 20 kilometers. Numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 degrees to 550 degrees C, which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high-pressure phase in the subducting slab.

Fundamental structure model of island arcs and subducted plates in and around Japan

NASA Astrophysics Data System (ADS)

Iwasaki, T.; Sato, H.; Ishiyama, T.; Shinohara, M.; Hashima, A.

2015-12-01

The eastern margin of the Asian continent is a well-known subduction zone, where the Pacific (PAC) and Philippine Sea (PHS) plates are being subducted. In this region, several island arcs (Kuril, Northeast Japan, Southwest Japan, Izu-Bonin and Ryukyu arcs) meet one another to form a very complicated tectonic environment. At 2014, we started to construct fundamental structure models for island arcs and subducted plates in and around Japan. Our research is composed of 6 items of (1) topography, (2) plate geometry, (3) fault models, (4) the Moho and brittle-ductile transition zone, (5) the lithosphere-asthenosphere boundary, and (6) petrological/rheological models. Such information is basic but inevitably important in qualitative understanding not only for short-term crustal activities in the subduction zone (particularly caused by megathrust earthquakes) but also for long-term cumulative deformation of the arcs as a result of strong plate-arc/arc-arc interactions. This paper is the first presentation of our research, mainly presenting the results of items (1) and (2). The area of our modelling is 12o-54o N and 118o-164o E to cover almost the entire part of Japanese Islands together with Kuril, Ryukyu and Izu-Bonin trenches. The topography model was constructed from the 500-m mesh data provided from GSJ, JODC, GINA and Alaska University. Plate geometry models are being constructed through the two steps. In the first step, we modelled very smooth plate boundaries of the Pacific and Philippine Sea plates in our whole model area using 42,000 earthquake data from JMA, USGS and ISC. For 7,800 cross sections taken with several directions to the trench axes, 2D plate boundaries were defined by fitting to the earthquake distribution (the Wadati-Benioff zone), from which we obtained equi-depth points of the plate boundary. These equi-depth points were then approximated by spline interpolation technique to eliminate shorter wave length undulation (<50-100 km). The obtained models represent the plate geometry with longer wave lengths (>75-150 km), but provide a rather clear undulation of the PHS plate under the SW Japan arc. In the second step, finer scale plate configuration is being constrained especially in the vicinity of Japan by recent results from seismic tomography, RF analysis and active source experiment.

When Boundary Layers Collide: Plumes v. Subduction Zones

NASA Astrophysics Data System (ADS)

Moresi, L. N.; Betts, P. G.; Miller, M. S.; Willis, D.; O'Driscoll, L.

2014-12-01

Many subduction zones retreat while hotspots remain sufficiently stable in the mantle to provide an approximate reference frame. As a consequence, the mantle can be thought of as an unusual convecting system which self-organises to promote frequent collisions of downgoing material with upwellings. We present three 3D numerical models of subduction where buoyant material from a plume head and an associated ocean-island chain or plateau produce flat slab subduction and deformation of the over-riding plate. We observe transient instabilities of the convergent margin including: contorted trench geometry; trench migration parallel with the plate margin; folding of the subducting slab and orocline development at the convergent margin; and transfer of the plateau to the overriding plate. The presence of plume material beneath the oceanic plateau causes flat subduction above the plume, resulting in a "bowed" shaped subducting slab. In the absence of a plateau at the surface, the slab can remain uncoupled from the over-riding plate during very shallow subduction and hence there is very little shortening at the surface or advance of the plate boundary. In plateau-only models, plateau accretion at the edge of the overriding plate results in trench migration around the edge of the plateau before subduction re-establishes directly behind the trailing edge of the plateau. The plateau shortens during accretion and some plateau material subducts. In a plateau-plus-plume model, accretion is associated with rapid trench advance as the flat slab drives the plateau into the margin. This indentation stops once a new convergent boundary forms close to the original trench location. A slab window formed beneath the accreted plateau allows plume material to flow from beneath the subducting plate to the underside of the overriding plate. In all of these models the subduction zone maintains a relatively stable configuration away from the buoyancy anomalies within the downgoing plate. The models provide a dynamic context for plateau and plume accretion in accretionary orogenic systems.

Constraining the hydration of the subducting Nazca plate beneath Northern Chile using subduction zone guided waves

NASA Astrophysics Data System (ADS)

Garth, Tom; Rietbrock, Andreas

2017-09-01

Guided wave dispersion is observed from earthquakes at 180-280 km depth recorded at stations in the fore-arc of Northern Chile, where the 44 Ma Nazca plate subducts beneath South America. Characteristic P-wave dispersion is observed at several stations in the Chilean fore-arc with high frequency energy (>5 Hz) arriving up to 3 s after low frequency (<2 Hz) arrivals. This dispersion has been attributed to low velocity structure within the subducting Nazca plate which acts as a waveguide, retaining and delaying high frequency energy. Full waveform modelling shows that the single LVL proposed by previous studies does not produce the first motion dispersion observed at multiple stations, or the extended P-wave coda observed in arrivals from intermediate depth events within the Nazca plate. These signals can however be accurately accounted for if dipping low velocity fault zones are included within the subducting lithospheric mantle. A grid search over possible LVL and faults zone parameters (width, velocity contrast and separation distance) was carried out to constrain the best fitting model parameters. Our results imply that fault zone structures of 0.5-1.0 km thickness, and 5-10 km spacing, consistent with observations at the outer rise are present within the subducted slab at intermediate depths. We propose that these low velocity fault zone structures represent the hydrated structure within the lithospheric mantle. They may be formed initially by normal faults at the outer rise, which act as a pathway for fluids to penetrate the deeper slab due to the bending and unbending stresses within the subducting plate. Our observations suggest that the lithospheric mantle is 5-15% serpentinised, and therefore may transport approximately 13-42 Tg/Myr of water per meter of arc. The guided wave observations also suggest that a thin LVL (∼1 km thick) interpreted as un-eclogitised subducted oceanic crust persists to depths of at least 220 km. Comparison of the inferred seismic velocities with those predicted for various MORB assemblages suggest that this thin LVL may be accounted for by low velocity lawsonite-bearing assemblages, suggesting that some mineral-bound water within the oceanic crust may be transported well beyond the volcanic arc. While older subducting slabs may carry more water per metre of arc, approximately one third of the oceanic material subducted globally is of a similar age to the Nazca plate. This suggests that subducting oceanic lithosphere of this age has a significant role to play in the global water cycle.

The influence of regional extensional tectonic stress on the eruptive behaviour of subduction-zone volcanoes

NASA Astrophysics Data System (ADS)

Tost, M.; Cronin, S. J.

2015-12-01

Regional tectonic stress is considered a trigger mechanism for explosive volcanic activity, but the related mechanisms at depth are not well understood. The unique geological setting of Ruapehu, New Zealand, allows investigation on the effect of enhanced regional extensional crustal tension on the eruptive behaviour of subduction-zone volcanoes. The composite cone is located at the southwestern terminus of the Taupo Volcanic Zone, one of the most active silicic magma systems on Earth, which extends through the central part of New Zealand's North Island. Rhyolitic caldera eruptions are limited to its central part where crustal extension is highest, whereas lower extension and additional dextral shear dominate in the southwestern and northeastern segments characterized by andesitic volcanism. South of Ruapehu, the intra-arc rift zone traverses into a compressional geological setting with updoming marine sequences dissected by reverse and normal faults. The current eruptive behaviour of Ruapehu is dominated by small-scaled vulcanian eruptions, but our studies indicate that subplinian to plinian eruptions have frequently occurred since ≥340 ka and were usually preceded by major rhyolitic caldera unrest in the Taupo Volcanic Zone. Pre-existing structures related to the NNW-SSE trending subduction-zone setting are thought to extend at depth and create preferred pathways for the silicic magma bodies, which may facilitate the development of large (>100 km3) dyke-like upper-crustal storage systems prior to major caldera activity. This may cause enhanced extensional stress throughout the entire intra-arc setting, including the Ruapehu area. During periods of caldera dormancy, the thick crust underlying the volcano and the enhanced dextral share rate likely impede ascent of larger andesitic magma bodies, and storage of andesitic melts dominantly occurs within small-scaled magma bodies at middle- to lower-crustal levels. During episodes of major caldera unrest, ascent and storage of voluminous rhyolitic magma bodies at upper crustal levels may cause the extensional stress to supercede the dextral shear rate in the Ruapehu area, facilitating ascent of larger andesitic magma bodies at depth, and changing the volcano's eruptive behaviour from dominantly vulcanian to violently subplinian/plinian.

Shear wave reflectivity imaging of the Nazca-South America subduction zone: Stagnant slab in the mantle transition zone?

NASA Astrophysics Data System (ADS)

Contenti, Sean; Gu, Yu Jeffrey; Ökeler, Ahmet; Sacchi, Mauricio D.

2012-01-01

In this study we utilize over 5000 SS waveforms to investigate the high-resolution mantle reflectivity structure down to 1200 km beneath the South American convergent margin. Our results indicate that the dynamics of the Nazca subduction are more complex than previously suggested. The 410- and 660-km seismic discontinuities beneath the Pacific Ocean and Amazonian Shield exhibit limited lateral depth variations, but their depths vary substantially in the vicinity of the subducting Nazca plate. The reflection amplitude of the 410-km discontinuity is greatly diminished in a ˜1300-km wide region in the back-arc of the subducting plate, which is likely associated with a compositional heterogeneity on top of the upper mantle transition zone. The underlying 660-km discontinuity is strongly depressed, showing localized depth and amplitude variations both within and to the east of the Wadati-Benioff zone. The width of this anomalous zone (˜1000 km) far exceeds that of the high-velocity slab structure and suggesting significant slab deformation within the transition zone. The shape of the 660-km discontinuity and the presence of lower mantle reflectivity imply both stagnation and penetration are possible as the descending Nazca slab impinges upon the base of the upper mantle.

Rupture process of large earthquakes in the northern Mexico subduction zone

NASA Astrophysics Data System (ADS)

Ruff, Larry J.; Miller, Angus D.

1994-03-01

The Cocos plate subducts beneath North America at the Mexico trench. The northernmost segment of this trench, between the Orozco and Rivera fracture zones, has ruptured in a sequence of five large earthquakes from 1973 to 1985; the Jan. 30, 1973 Colima event ( M s 7.5) at the northern end of the segment near Rivera fracture zone; the Mar. 14, 1979 Petatlan event ( M s 7.6) at the southern end of the segment on the Orozco fracture zone; the Oct. 25, 1981 Playa Azul event ( M s 7.3) in the middle of the Michoacan “gap”; the Sept. 19, 1985 Michoacan mainshock ( M s 8.1); and the Sept. 21, 1985 Michoacan aftershock ( M s 7.6) that reruptured part of the Petatlan zone. Body wave inversion for the rupture process of these earthquakes finds the best: earthquake depth; focal mechanism; overall source time function; and seismic moment, for each earthquake. In addition, we have determined spatial concentrations of seismic moment release for the Colima earthquake, and the Michoacan mainshock and aftershock. These spatial concentrations of slip are interpreted as asperities; and the resultant asperity distribution for Mexico is compared to other subduction zones. The body wave inversion technique also determines the Moment Tensor Rate Functions; but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes. An appendix describes the Moment Tensor Rate Functions methodology in detail. The systematic bias between global and regional determinations of epicentral locations in Mexico must be resolved to enable plotting of asperities with aftershocks and geographic features. We have spatially “shifted” all of our results to regional determinations of epicenters. The best point source depths for the five earthquakes are all above 30 km, consistent with the idea that the down-dip edge of the seismogenic plate interface in Mexico is shallow compared to other subduction zones. Consideration of uncertainties in the focal mechanisms allows us to state that all five earthquakes occurred on fault planes with the same strike (N65°W to N70°W) and dip (15±3°), except for the smaller Playa Azul event at the down-dip edge which has a steeper dip angle of 20 to 25°. However, the Petatlan earthquake does “prefer” a fault plane that is rotated to a more east-west orientation—one explanation may be that this earthquake is located near the crest of the subducting Orozco fracture zone. The slip vectors of all five earthquakes are similar and generally consistent with the NUVEL-predicted Cocos-North America convergence direction of N33°E for this segment. The most important deviation is the more northerly slip direction for the Petatlan earthquake. Also, the slip vectors from the Harvard CMT solutions for large and small events in this segment prefer an overall convergence direction of about N20°E to N25°E. All five earthquakes share a common feature in the rupture process: each earthquake has a small initial precursory arrival followed by a large pulse of moment release with a distinct onset. The delay time varies from 4 s for the Playa Azul event to 8 s for the Colima event. While there is some evidence of spatial concentration of moment release for each event, our overall asperity distribution for the northern Mexico segment consists of one clear asperity, in the epicentral region of the 1973 Colima earthquake, and then a scattering of diffuse and overlapping regions of high moment release for the remainder of the segment. This character is directly displayed in the overlapping of rupture zones between the 1979 Petatlan event and the 1985 Michoacan aftershock. This character of the asperity distribution is in contrast to the widely spaced distinct asperities in the northern Japan-Kuriles Islands subduction zone, but is somewhat similar to the asperity distributions found in the central Peru and Santa Cruz Islands subduction zones. Subduction of the Orozco fracture zone may strongly affect the seismogenic character as the overlapping rupture zones are located on the crest of the subducted fracture zone. There is also a distinct change in the physiography of the upper plate that coincides with the subducting fracture zone, and the Guerrero seismic gap to the south of the Petatlan earthquake is in the “wake” of the Orozco fracture zone. At the northern end, the Rivera fracture zone in the subducting plate and the Colima graben in the upper plate coincide with the northernmost extent of the Colima rupture 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

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.

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.

Along-strike variations in seismic structure of the locked-sliding transition on the plate boundary beneath the southern part of Kii Peninsula, southwestern Japan

NASA Astrophysics Data System (ADS)

Kurashimo, E.; Iidaka, T.; Iwasaki, T.; Saiga, A.; Umeyama, E.; Tsumura, N.; Sakai, S.; Hirata, N.

2013-12-01

The Nankai trough region, where the Philippine Sea Plate (PHS) subducts beneath the SW Japan arc, is a well-known seismogenic zone of interplate earthquakes. A narrow zone of nonvolcanic tremor has been found in the SW Japan fore-arc, along strike of the arc (Obara, 2002). The epicentral distribution of tremor corresponds to the locked-sliding transition estimated from thermal and deformation models (Hyndman et al., 1995). The spatial distribution of the tremor is not homogeneous in a narrow belt but is spatially clustered. Obara [2002] suggested fluids as a source for tremor because of the long duration and the mobility of the tremor activity. The behavior of fluids at the plate interface is a key factor in understanding fault slip processes. Seismic reflection characteristics and seismic velocity variations can provide important information on the fluid-related heterogeneity of structure around plate interface. However, little is known about the deeper part of the plate boundary, especially the transition zone on the subducting plate. To reveal the seismic structure of the transition zone, we conducted passive and active seismic experiments in the southern part of Kii Peninsula, SW Japan. Sixty 3-component portable seismographs were installed on a 60-km-long line (SM-line) nearly perpendicular to the direction of the subduction of the PHS with approximately 1 km spacing. To improve accuracy of hypocenter locations, we additionally deployed six 3-component seismic stations around the survey line. Waveforms were continuously recorded during a five-month period from December, 2009. In October of 2010, a deep seismic profiling was also conducted. 290 seismometers were deployed on the SM-line with about 200 m spacing, on which five explosives shots were fired as controlled seismic sources. Arrival times of local earthquakes and explosive shots were used in a joint inversion for earthquake locations and 3-D Vp and Vp/Vs structures, using the iterative damped least-squares algorithm, simul2000 (Thurber and Eberhart-Phillips, 1999). To obtain the detailed structure image of the transition zone on the subducting plate, the explosive shot data recorded on the SM-line were processed using the seismic reflection technique. Seismic reflection image shows the lateral variation of the reflectivity along the top of the PHS. A clear reflection band is present where the clustered tremors occurred. The depth section of Vp/Vs structure shows the lateral variation of the Vp/Vs values along the top of the PHS. Clustered tremors are located in and around the high Vp/Vs zone. These results suggest the occurrence of the tremors may be associated with fluids dehydrated from the subducted oceanic lithosphere.

Late Cretaceous-Early Eocene Climate Change Linked to Tectonic Eevolution of Neo-Tethyan Subduction Systems

NASA Astrophysics Data System (ADS)

Jagoutz, O. E.; Royden, L.; Macdonald, F. A.

2015-12-01

In this presentation we demonstrate that the two tectonic events in the late Cretaceous-Early Tertiary triggered the two distinct cooling events that followed the Cretaceous Thermal Maximum (CTM). During much of the Cretaceous time, the northern Neo Tethyan ocean was dominated by two east-west striking subduction system. Subduction underneath Eurasia formed a continental arc on the southern margin of Eurasia and intra oceanic subduction in the equatorial region of the Neo Tethys formed and intra oceanic arc. Beginning at ~85-90 Ma the western part of the TTSS collided southward with the Afro-Arabian continental margin, terminating subduction. This resulted in southward obduction of the peri-Arabian ophiolite belt, which extends for ~4000 km along strike and includes the Cypus, Semail and Zagros ophiolites. At the same time also the eastern part of the TTS collided northwards wit Eurasia. After this collisional event, only the central part of the subduction system remained active until it collided with the northern margin of the Indian continent at ~50-55 Ma. The collision of the arc with the Indian margin, over a length of ~3000 km, also resulted in the obduction of arc material and ophiolitic rocks. Remnants of these rocks are preserved today as the Kohistan-Ladakh arc and ophiolites of the Indus-Tsangpo suture zone of the Himalayas. Both of these collision events occurred in the equatorial region, near or within the ITCZ, where chemical weathering rates are high and are contemporaneous with the onset of the global cooling events that mark the end of the CTM and the EECO. The tectonic collision events resulted in a shut down of subduction zone magmatism, a major CO2 source and emplacement of highly weatherable basaltic rocks within the ITCZ (CO2 sink). In order to explore the effect of the events in the TTSS on atmospheric CO2, we model the potential contribution of subduction zone volcanism (source) and ophiolite obduction (sink) to the global atmospheric CO2 budget. Our results show that the global ocean bottom water temperature are highly correlated with CO2 variation modeled due to the arc-continent collisions along the TTSS. Our results show that global climate in the Late Cretaceous to Early Eocene have likely been strongly changed due to the tectonic evolution of the Neo-Tethys.

Geochemistry of subduction zone serpentinites: A review

NASA Astrophysics Data System (ADS)

Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

2013-09-01

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zone geodynamics. Their presence and role in subduction environments are recognized through geophysical, geochemical and field observations of modern and ancient subduction zones and large amounts of geochemical database of serpentinites have been created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical data of abyssal, mantle wedge and exhumed serpentinites after subduction. The aim was to better understand the geochemical evolution of these rocks during their subduction as well as their impact in the global geochemical cycle. When studying serpentinites, it is essential to determine their protoliths and their geological history before serpentinization. The geochemical data of serpentinites shows little mobility of compatible and rare earth elements (REE) at the scale of hand-specimen during their serpentinization. Thus, REE abundance can be used to identify the protolith for serpentinites, as well as magmatic processes such as melt/rock interactions before serpentinization. In the case of subducted serpentinites, the interpretation of trace element data is difficult due to the enrichments of light REE, independent of the nature of the protolith. We propose that enrichments are probably not related to serpentinization itself, but mostly due to (sedimentary-derived) fluid/rock interactions within the subduction channel after the serpentinization. It is also possible that the enrichment reflects the geochemical signature of the mantle protolith itself which could derive from the less refractory continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous analyses have been carried out, notably using in situ approaches, to better constrain the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) incorporated in serpentine phases. The abundance of these elements provides information related to the fluid/rock interactions during serpentinization and the behavior of FME, from their incorporation to their gradual release during subduction. Serpentinites are considered as a reservoir of the FME in subduction zones and their role, notably on arc magma composition, is underestimated presently in the global geochemical cycle.

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

PubMed Central

Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J. -Michael

2016-01-01

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

The upper-mantle transition zone beneath the Chile-Argentina flat subduction zone

NASA Astrophysics Data System (ADS)

Bagdo, Paula; Bonatto, Luciana; Badi, Gabriela; Piromallo, Claudia

2016-04-01

The main objective of the present work is the study of the upper mantle structure of the western margin of South America (between 26°S and 36°S) within an area known as the Chile-Argentina flat subduction zone. For this purpose, we use teleseismic records from temporary broad band seismic stations that resulted from different seismic experiments carried out in South America. This area is characterized by on-going orogenic processes and complex subduction history that have profoundly affected the underlying mantle structure. The detection and characterization of the upper mantle seismic discontinuities are useful to understand subduction processes and the dynamics of mantle convection; this is due to the fact that they mark changes in mantle composition or phase changes in mantle minerals that respond differently to the disturbances caused by mantle convection. The discontinuities at a depth of 410 km and 660 km, generally associated to phase changes in olivine, vary in width and depth as a result of compositional and temperature anomalies. As a consequence, these discontinuities are an essential tool to study the thermal and compositional structure of the mantle. Here, we analyze the upper-mantle transition zone discontinuities at a depth of 410 km and 660 km as seen from Pds seismic phases beneath the Argentina-Chile flat subduction.

Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.

PubMed

Nagaya, Takayoshi; Walker, Andrew M; Wookey, James; Wallis, Simon R; Ishii, Kazuhiko; Kendall, J-Michael

2016-07-20

It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

Triggering of destructive earthquakes in El Salvador

NASA Astrophysics Data System (ADS)

Martínez-Díaz, José J.; Álvarez-Gómez, José A.; Benito, Belén; Hernández, Douglas

2004-01-01

We investigate the existence of a mechanism of static stress triggering driven by the interaction of normal faults in the Middle American subduction zone and strike-slip faults in the El Salvador volcanic arc. The local geology points to a large strike-slip fault zone, the El Salvador fault zone, as the source of several destructive earthquakes in El Salvador along the volcanic arc. We modeled the Coulomb failure stress (CFS) change produced by the June 1982 and January 2001 subduction events on planes parallel to the El Salvador fault zone. The results have broad implications for future risk management in the region, as they suggest a causative relationship between the position of the normal-slip events in the subduction zone and the strike-slip events in the volcanic arc. After the February 2001 event, an important area of the El Salvador fault zone was loaded with a positive change in Coulomb failure stress (>0.15 MPa). This scenario must be considered in the seismic hazard assessment studies that will be carried out in this area.

A review about the mechanisms associated with active deformation, regional uplift and subsidence in southern South America

NASA Astrophysics Data System (ADS)

Folguera, Andrés; Gianni, Guido; Sagripanti, Lucía; Rojas Vera, Emilio; Novara, Iván; Colavitto, Bruno; Alvarez, Orlando; Orts, Darío; Tobal, Jonathan; Giménez, Mario; Introcaso, Antonio; Ruiz, Francisco; Martínez, Patricia; Ramos, Victor A.

2015-12-01

A broad range of processes acted simultaneously during the Quaternary producing relief in the Andes and adjacent foreland, from the Chilean coast, where the Pacific Ocean floor is being subducted beneath South American, to the Brazilian and the Argentinean Atlantic platform area. This picture shows to be complex and responds to a variety of processes. The Geoid exemplifies this spectrum of uplift mechanisms, since it reflects an important change at 35°S along the Andes and the foreland that could be indicating the presence of dynamic forces modeling the topography with varying intensity through the subduction margin. On the other hand, mountains uplifted in the Atlantic margin, along a vast sector of the Brazilian Atlantic coast and inland regions seem to be created at the area where the passive margin has been hyper-extended and consequently mechanically debilitated and the forearc region shifts eastwardly at a similar rate than the westward advancing continent. Therefore the forearc at the Arica latitudes can be considered as relatively stationary and dynamically sustained by a perpendicular-to-the-margin asthenospheric flow that inhibits trench roll back, determining a highly active orogenic setting at the eastern Andes in the Subandean region. To the south, the Pampean flat subduction zone creates particular conditions for deformation and rapid propagation of the orogenic front producing a high-amplitude orogen. In the southern Central and Patagonian Andes, mountain (orogenic) building processes are attenuated, becoming dominant other mechanisms of exhumation such as the i) impact of mantle plumes originated in the 660 km mantle transition, ii) the ice-masse retreat from the Andes after the Pleistocene producing an isostatic rebound, iii) the dynamic topography associated with the opening of an asthenospheric window during the subduction of the Chile ridge and slab tearing processes, iv) the subduction of oceanic swells linked to transform zones and v) the accretion of oceanic materials beneath the forearc region. Additionally and after last geodetic studies, vi) exhumation due to co- and post-seismic lithospheric stretching associated with large earthquakes along the subduction zone, also shows to be a factor associated with regional uplift that needs to be further considered as an additional mechanism from the Chilean coast to the western retroarc area. Finally, this revision constitutes a general picture about the different mechanisms of uplift and active deformation along the Southern Andes, in which orogenic processes become dominant north of 35°S, while south of these latitudes dynamic forces seem to predominate all over the Patagonian platform.

Faulting induced by precipitation of water at grain boundaries in hot subducting oceanic crust.

PubMed

Zhang, Junfeng; Green, Harry W; Bozhilov, Krassimir; Jin, Zhenmin

2004-04-08

Dehydration embrittlement has been proposed to explain both intermediate- and deep-focus earthquakes in subduction zones. Because such earthquakes primarily occur at shallow depths or within the core of the subducting plate, dehydration at relatively low temperatures has been emphasized. However, recent careful relocation of subduction-zone earthquakes shows that at depths of 100-250 km, earthquakes continue in the uppermost part of the slab (probably the former oceanic crust that has been converted to eclogite) where temperatures are higher. Here we show that at such pressures and temperatures, eclogite lacking hydrous phases but with significant hydroxyl incorporated as defects in pyroxene and garnet develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt. This new faulting mechanism satisfactorily explains high-temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone (400-700 km depth). Of potential importance for all proposed high-pressure earthquake mechanisms is the very small amount of fluid required to trigger this instability.

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.

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.

Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

NASA Astrophysics Data System (ADS)

Molli, Giancarlo; Menegon, Luca; Malasoma, Alessandro

2017-04-01

The switching in deformation mode (from distributed to localized) and mechanism (viscous versus frictional) represent a relevant issue in the frame of processes of crustal deformation in turn connected with the concept of the brittle-"ductile" transition and seismogenesis. On the other hand the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as having a fundamental role in the localization of deformation and shear zone development, thus representing a case in which switching deformation mode and mechanisms interact and relate to each other. This contribution analyses an example of a crystal plastic shear zone localized by brittle precursor formed within a host granitic-mylonite during deformation in subduction-related environment. The studied sample come from the external Corsican continental crust units involved in alpine age subduction and characterized by a low grade blueschist facies peak assemblages. The blueschist facies host rock is cut by a thin (< 1 cm thick) brittle-viscous shear zone that preserves domains with a cataclastic microstructure overprinted by mylonitic deformation. Blue amphibole is stable in the shear zone foliation, which therefore formed under HP/LT metamorphic conditions in a subduction environment. Quartz microstructure in the damage zone flanking the brittle-viscous shear zone shows evidence of both microcracking and dislocation glide, with limited recrystallization localized in intracrystalline bands. In the mylonite portion of the shear zone, quartz forms polycrystalline ribbons of dynamically recrystallized grains with a crossed-girdle c-axis CPO. Extrapolation of laboratory-derived flow laws indicates strain rate of ca. 3.5 * 10-12 s-1 during viscous flow in the shear zone. The studied structures, possibly formed by transient instability related to episodic stress/strain rate variations, may be considered as a small scale example of fault behaviour associated with a cycle of interseismic creep with coseismic rupture and then a fossil example of stick-slip strain accommodation in subduction environment of continental crust.

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.

Putting the slab back: First steps of creating a synthetic seismic section of subducted lithosphere

NASA Astrophysics Data System (ADS)

Zertani, S.; John, T.; Tilmann, F. J.; Leiss, B.; Labrousse, L.; Andersen, T. B.

2016-12-01

Imaging subducted lithosphere is a difficult task which is usually tackled with geophysical methods. To date, the most promising method is receiver function imaging (RF), which concentrates on first order conversions from p- to s-waves at boundaries (e.g. lithological and structural) with contrasting seismic velocities. The resolution is high for the upper parts of the subducting material. However, in greater depths (40-80 km) the visualization of the subducted slab becomes increasingly blurry, until the slab cannot be distinguished from Earth's mantle anymore, rendering a visualization impossible. This blurry zone is thought to occur due to advancing eclogitization of the subducting slab. However, it is not well understood how micro- to macro-scale structures related to progressive eclogitization affect RF signals. The island of Holsnoy in the Bergen Arcs of western Norway represents a partially eclogitized formerly subducted block of lower crust and serves as an analogue to the aforementioned blurry zone in RF images. This eclogitization can be observed in static fluid induced eclogitization patches or fingers, but is mainly present in localized shear zones of variable sizes (mm to 100s of meters). We mapped the area to gain a better understanding of the geometries of such shear zones, which could possibly function as seismic reflectors. Further, we calculated seismic velocities from thermodynamic modelling on the basis of XRF whole rock analysis and compared these results to velocities calculated from a combination of thin section information, EMPA and physical mineral properties (Voigt-Reuss-Hill averaging). Both methods yield consistent results for p- and s-wave velocities of eclogites and granulites from Holsnoy. In combination with X-ray measurements to identify the microtextures of the characteristic samples to incorporate seismic anisotropy caused by e.g. foliation or lineation, these seismic velocities are used as an input for seismic models to reconstruct the progressive eclogitization of a subducting slab as seen in many RF-images (i.e. blurry zone).

Plate tectonics, damage and inheritance.

PubMed

Bercovici, David; Ricard, Yanick

2014-04-24

The initiation of plate tectonics on Earth is a critical event in our planet's history. The time lag between the first proto-subduction (about 4 billion years ago) and global tectonics (approximately 3 billion years ago) suggests that plates and plate boundaries became widespread over a period of 1 billion years. The reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics. Here we suggest that when sufficient lithospheric damage (which promotes shear localization and long-lived weak zones) combines with transient mantle flow and migrating proto-subduction, it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone. We simulate this process using a grain evolution and damage mechanism with a composite rheology (which is compatible with field and laboratory observations of polycrystalline rocks), coupled to an idealized model of pressure-driven lithospheric flow in which a low-pressure zone is equivalent to the suction of convective downwellings. In the simplest case, for Earth-like conditions, a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate, with passive spreading and strike-slip margins that persist and localize further, even though flow is driven only by subduction. But for hotter surface conditions, such as those on Venus, accumulation and inheritance of damage is negligible; hence only subduction zones survive and plate tectonics does not spread, which corresponds to observations. After plates have developed, continued changes in driving forces, combined with inherited damage and weak zones, promote increased tectonic complexity, such as oblique subduction, strike-slip boundaries that are subparallel to plate motion, and spalling of minor plates.

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

Ramdhan, Mohamad; Nugraha, Andri Dian

Toba area has complex tectonic setting attracting many earth scientists to study and understand tectonic and geological process or setting. The area is affected by oblique subduction zone, Renun Sumatran fault sub segment and some volcanoes that are near it. The earthquake catalogue provided by BMKG from April, 2009 to December, 2011 must be relocated firstly to get the precise hypocenter. We used catalogue data of P and S phase or P phase only and double-difference method to relocate the earthquakes. The results show hypocenter position enhancement that can be interpreted tectonically. The earthquakes after relocation relating to the Sumatranmore » fault, subduction zone, volcanoes and seismic activities beneath Toba caldera can be mapped clearly. The relocated hypocenters in this study are very important to provide information for seismic hazard assessment and disaster mitigation study.« less

The Calabrian Arc: three-dimensional modelling of the subduction interface.

PubMed

Maesano, Francesco E; Tiberti, Mara M; Basili, Roberto

2017-08-21

The Calabrian Arc is a one-of-a-kind subduction zone, featuring one of the shortest slab segments (<150 km), one of the thickest accretionary wedges, and one of the oldest oceanic crust in the world. Despite a convergence rate of up to 5 mm/y and well-known intraslab seismicity below 40 km, its shallow interface shows little signs of seismic activity. Nonetheless, it has been attributed as generating historical large earthquakes and tsunamis. To gain insights into this subduction zone, we first made a geological reconstruction of the shallower slab interface (<20 km) and its overlying accretionary wedge by interpreting a grid of 54 seismic reflection lines (8,658 km) with 438 intersections within an area of 10 5  km 2 . Then, we constrained a deeper portion of the slab surface (40-350 km) using the seismicity distribution. Finally, we interpolated the two parts to obtain a seamless 3D surface highlighting geometric details of the subduction interface, its lateral terminations and down-dip curvature, and a slab tear at 70-100 km depth. Our 3D slab model of the Calabrian Arc will contribute to understanding of the geodynamics of a cornerstone in the Mediterranean tectonic puzzle and estimates of seismic and tsunami hazards in the region.

Seismic imaging along a 600 km transect of the Alaska Subduction zone (Invited)

NASA Astrophysics Data System (ADS)

Calkins, J. A.; Abers, G. A.; Freymueller, J. T.; Rondenay, S.; Christensen, D. H.

2010-12-01

We present earthquake locations, scattered wavefield migration images, and phase velocity maps from preliminary analysis of combined seismic data from the Broadband Experiment Across the Alaska Range (BEAAR) and Multidisciplinary Observations of Onshore Subduction (MOOS) projects. Together, these PASSCAL broadband arrays sampled a 500+ km transect across a portion of the subduction zone characterized by the Yakutat terrane/Pacific plate boundary in the downgoing plate, and the Denali volcanic gap in the overriding plate. These are the first results from the MOOS experiment, a 34-station array that was deployed from 2006-2008 to fill in the gap between the TACT offshore refraction profile (south and east of the coastline of the Kenai Peninsula), and the BEAAR array (spanning the Alaska Range between Talkeetna and Fairbanks). 2-D images of the upper 150 km of the subduction zone were produced by migrating forward- and back-scattered arrivals in the coda of P waves from large teleseismic earthquakes, highlighting S-velocity perturbations from a smoothly-varying background model. The migration images reveal a shallowly north-dipping low velocity zone that is contiguous near 20 km depth on its updip end with previously obtained images of the subducting plate offshore. The low velocity zone steepens further to the north, and terminates near 120 km beneath the Alaska Range. We interpret this low velocity zone to be the crust of the downgoing plate, and the reduced seismic velocities to be indicative of hydrated gabbroic compositions. Earthquakes located using the temporary arrays and nearby stations of the Alaska Regional Seismic Network correlate spatially with the inferred subducting crust. Cross-sections taken along nearly orthogonal strike lines through the MOOS array reveal that both the dip angle and the thickness of the subducting low velocity zone change abruptly across a roughly NNW-SSE striking line drawn through the eastern Kenai Peninsula, coincident with a distinct change in locking at the subduction interface as revealed by previous geodetic studies. On the west end of the Kenai Peninsula, where seismically imaged downgoing crust appears oceanic, the geodetic signal mainly reflects postseismic deformation from the 1964 earthquake as evinced by southeast trending displacement vectors (with respect to fixed North America). While postseismic relaxation continues east of the boundary, NNW-directed elastic deformation due to locking at the plate boundary dominates the geodetic signal, and imaging reveals thickened Yakutat crust is subducting. The collocation of sharp changes in both deep structure and surface deformation suggest that the nature of the plate interface changes drastically across the western edge of the Yakutat block and that variations in downgoing plate structure control the strain field in the overriding plate.

Zinc isotope systematics of subduction-zone magmas

NASA Astrophysics Data System (ADS)

Huang, J.; Zhang, X. C.; Huang, F.; Yu, H.

2016-12-01

Subduction-zone magmas are generated by partial melting of mantle wedge triggered by addition of fluids derived from subducted hydrothermally altered oceanic lithosphere. Source of the fluids may be sediment, altered oceanic crust and serpentinized peridotite/serpentinite. Knowledge of the exact fluid source can facilitate our better understanding of the mechanism of fluid flux, element cycling and crust-mantle interaction in subduction zones. Zinc isotopes have the potential to place a constraint on this issue, because (1) Zn has an intermediate mobility during fluid-rock interaction and is enriched in subduction-zone fluids (e.g., Li et al., 2013); (2) sediment, altered oceanic crust and serpentinite have distinct Zn isotopic compositions (Pons et al., 2011); and (3) the mantle has a homogeneous Zn isotope composition (δ66Zn = 0.28 ± 0.05‰, Chen et al., 2013). Thus, the Zn isotopic composition of subduction-zone magmas reflects the characteristics of slab-derived fluids of different sources. Here, high-precision Zn isotope analyses were conducted on igneous rocks from arcs of Central America, Kamchatka, South Lesser Antilles, and Aleutian. One rhyolite with 75.1 wt.% SiO2 and 0.2 wt.% FeOT displays the heaviest δ66Zn value of 0.394‰ (relative to JMC Lyon) that probably results from the crystallization of Fe-Ti oxides during the late-stage differentiation. The rest of rocks have Zn isotopic compositions (0.161 to 0.339‰) similar to or lighter than those of the mantle. In an individual arc, the δ66Zn values of rocks show broad negative correlations with Ba/Th and 87Sr/86Sr ratios, suggesting that the slab-derived fluids should have lighter δ66Zn as well as higher Ba/Th and 87Sr/86Sr ratios relative to the mantle. These features are in accordance with those of serpentinites. Thus, addition of serpentinite-derived 66Zn-depleted fluids into the mantle wedge can explain the declined δ66Zn of subduction-zone magmas. ReferenceChen et al. (2013) EPSL 369-370:34-42; Li et al. (2013) GCA 120:326-362; Pons et al. (2011) PNAS 108:17639-17643.

Water, oceanic fracture zones and the lubrication of subducting plate boundaries—insights from seismicity

NASA Astrophysics Data System (ADS)

Schlaphorst, David; Kendall, J.-Michael; Collier, Jenny S.; Verdon, James P.; Blundy, Jon; Baptie, Brian; Latchman, Joan L.; Massin, Frederic; Bouin, Marie-Paule

2016-03-01

We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg-Richter law. This power law describes the earthquake-magnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its along-strike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov-Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north-south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value `bull's-eyes' along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the related seismicity. Our results suggest serpentinization around mid-ocean ridge transform faults, which go on to become fracture zones on the incoming plate, plays a significant role in the delivery of water into the mantle at subduction zones.

Geophysical and geochemical constraints on the geodynamic origin of the Vrancea Seismogenic Zone Romania

NASA Astrophysics Data System (ADS)

Fillerup, Melvin A.

The Vrancea Seismogenic Zone (VSZ) of Romania is a steeply NW-dipping volume (30 x 70 x 200 km) of intermediate-depth seismicity in the upper mantle beneath the bend zone of the Eastern Carpathians. The majority of tectonic models lean heavily on subduction processes to explain the Vrancea mantle seismicity and the presence of a Miocene age calc-alkaline volcanic arc in the East Carpathian hinterland. However, recent deep seismic reflection data collected over the Eastern Carpathian bend zone image an orogen lacking (1) a crustal root and (2) dipping crustal-scale fabrics routinely imaged in modern and ancient subduction zones. The DRACULA I and DACIA-PLAN deep seismic reflection profiles show that the East Carpathian orogen is supported by crust only 30-33 km thick while the Focsani basin (foreland) and Transylvanian basin (hinterland) crust is 42 km and 46 km thick respectively. Here the VSZ is interpreted as the former Eastern Carpathian orogenic root which was removed as a result of continental lithospheric delamination and is seismically foundering beneath the East Carpathian bend zone. Because large volumes of calc-alkaline volcanism are typically associated with subduction settings existing geochemical analyses from the Calimani, Gurghiu, and Harghita Mountains (CGH) have been reinterpreted in light of the seismic data which does not advocate the subduction of oceanic lithosphere. CGH rocks exhibit a compositional range from basalt to rhyolite, many with high-Mg# (Mg/Mg+Fe > 0.60), high-Sr (>1000 ppm), and elevated delta-O18 values (6-8.7 /) typical of arc lavas, and are consistent with mixing of mantle-derived melts with a crustal component. The 143Nd/144Nd (0.5123-0.5129) and 87Sr/86Sr (0.7040-0.7103) ratios similarly suggest mixing of mantle and crustal end members to obtain the observed isotopic compositions. A new geochemical model is presented whereby delamination initiates a geodynamic process like subduction but with the distinct absence of subducted oceanic lithosphere to produce the CGH lavas. The origin of the VSZ presented here suggests that the delamination of continental lithosphere is a process capable of producing mantle earthquakes and calc-alkaline volcanism without subduction tectonics.

Assembly of the Lhasa and Qiangtang terranes in central Tibet by divergent double subduction

NASA Astrophysics Data System (ADS)

Zhu, Di-Cheng; Li, Shi-Min; Cawood, Peter A.; Wang, Qing; Zhao, Zhi-Dan; Liu, Sheng-Ao; Wang, Li-Quan

2016-02-01

Integration of lithostratigraphic, magmatic, and metamorphic data from the Lhasa-Qiangtang collision zone in central Tibet (including the Bangong suture zone and adjacent regions of the Lhasa and Qiangtang terranes) indicates assembly through divergent double sided subduction. This collision zone is characterized by the absence of Early Cretaceous high-grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca. 120-110 Ma. Two Jurassic-Cretaceous magmatic arcs are identified from the Caima-Duobuza-Rongma-Kangqiong-Amdo magmatic belt in the western Qiangtang Terrane and from the Along Tso-Yanhu-Daguo-Baingoin-Daru Tso magmatic belt in the northern Lhasa Terrane. These two magmatic arcs reflect northward and southward subduction of the Bangong Ocean lithosphere, respectively. Available multidisciplinary data reconcile that the Bangong Ocean may have closed during the Late Jurassic-Early Cretaceous (most likely ca. 140-130 Ma) through arc-arc "soft" collision rather than continent-continent "hard" collision. Subduction zone retreat associated with convergence beneath the Lhasa Terrane may have driven its rifting and separation from the northern margin of Gondwana leading to its accretion within Asia.

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism.

PubMed

Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Matsukage, Kyoko N; Ono, Shigeaki

2012-11-13

Subduction-zone magmatism is triggered by the addition of H(2)O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry.

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

PubMed Central

Kawamoto, Tatsuhiko; Kanzaki, Masami; Mibe, Kenji; Ono, Shigeaki

2012-01-01

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry. PMID:23112158

Slab anisotropy from subduction zone guided waves in Taiwan

NASA Astrophysics Data System (ADS)

Chen, K. H.; Tseng, Y. L.; Hu, J. C.

2014-12-01

Frozen-in anisotropic structure in the oceanic lithosphere and faulting/hydration in the upper layer of the slab are expected to play an important role in anisotropic signature of the subducted slab. Over the past several decades, despite the advances in characterizing anisotropy using shear wave splitting method and its developments, the character of slab anisotropy remains poorly understood. In this study we investigate the slab anisotropy using subduction zone guided waves characterized by long path length in the slab. In the southernmost Ryukyu subduction zone, seismic waves from events deeper than 100 km offshore northern Taiwan reveal wave guide behavior: (1) a low-frequency (< 1 Hz) first arrival recognized on vertical and radial components but not transverse component (2) large, sustained high-frequency (3-10 Hz) signal in P and S wave trains. The depth dependent high-frequency content (3-10Hz) confirms the association with a waveguide effect in the subducting slab rather than localized site amplification effects. Using the selected subduction zone guided wave events, we further analyzed the shear wave splitting for intermediate-depth earthquakes in different frequency bands, to provide the statistically meaningful shear wave splitting parameters. We determine shear wave splitting parameters from the 34 PSP guided events that are deeper than 100 km with ray path traveling along the subducted slab. From shear wave splitting analysis, the slab and crust effects reveal consistent polarization pattern of fast directions of EN-WS and delay time of 0.13 - 0.27 sec. This implies that slab anisotropy is stronger than the crust effect (<0.1 s) but weaker than the mantle wedge and sub-slab mantle effect (0.3-1.3 s) in Taiwan.

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.

Geochemistry of continental subduction-zone fluids

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Hermann, Joerg

2014-12-01

The composition of continental subduction-zone fluids varies dramatically from dilute aqueous solutions at subsolidus conditions to hydrous silicate melts at supersolidus conditions, with variable concentrations of fluid-mobile incompatible trace elements. At ultrahigh-pressure (UHP) metamorphic conditions, supercritical fluids may occur with variable compositions. The water component of these fluids primarily derives from structural hydroxyl and molecular water in hydrous and nominally anhydrous minerals at UHP conditions. While the breakdown of hydrous minerals is the predominant water source for fluid activity in the subduction factory, water released from nominally anhydrous minerals provides an additional water source. These different sources of water may accumulate to induce partial melting of UHP metamorphic rocks on and above their wet solidii. Silica is the dominant solute in the deep fluids, followed by aluminum and alkalis. Trace element abundances are low in metamorphic fluids at subsolidus conditions, but become significantly elevated in anatectic melts at supersolidus conditions. The compositions of dissolved and residual minerals are a function of pressure-temperature and whole-rock composition, which exert a strong control on the trace element signature of liberated fluids. The trace element patterns of migmatic leucosomes in UHP rocks and multiphase solid inclusions in UHP minerals exhibit strong enrichment of large ion lithophile elements (LILE) and moderate enrichment of light rare earth elements (LREE) but depletion of high field strength elements (HFSE) and heavy rare earth elements (HREE), demonstrating their crystallization from anatectic melts of crustal protoliths. Interaction of the anatectic melts with the mantle wedge peridotite leads to modal metasomatism with the generation of new mineral phases as well as cryptic metasomatism that is only manifested by the enrichment of fluid-mobile incompatible trace elements in orogenic peridotites. Partial melting of the metasomatic mantle domains gives rise to a variety of mafic igneous rocks in collisional orogens and their adjacent active continental margins. The study of such metasomatic processes and products is of great importance to understanding of the mass transfer at the slab-mantle interface in subduction channels. Therefore, the property and behavior of subduction-zone fluids are a key for understanding of the crust-mantle interaction at convergent plate margins.

Seismotectonics of the central segment of the Indonesian Arc

NASA Astrophysics Data System (ADS)

Eva, C.; Cattaneo, M.; Merlanti, F.

1988-01-01

In this paper, a revision of seismicity affecting the central segment of the Indonesian island arc ranging between 110° and 126° E is presented. Using the areal and in-depth distribution of seismic activity, strain release maps and focal mechanisms, lateral changes in the Wadati-Benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries. The seismicity pattern indicates that the Sumbawa-Flores-Wetar sector shows different forms of behaviour with respect to the adjacent sectors. These include driving mechanism, inclination and continuity of the subducting slab and subduction features. This area therefore seems to be isolated from the Sunda and Banda arcs by two principal boundaries, these having a nearly N-S trend in the Bali region and with a nearly E-W trend in the region ranging between Wetar-Northern Timor and Tanibar. The first boundary, characterized by an absolute minimum of seismic activity at all ranges of depth, has been interpreted in terms of subduction of the Roo Rise aseismic bathymetric ridge. For the second boundary, dividing a northwardly steeply-dipping slab from an E-W subducting slab dipping with an angle of 30 ° -40 °, a tear in the upper part (depth less than 300 km) and a hinge fault system in the deepest part of the lithosphere, have been proposed. From the analysis of focal mechanisms of shallow earthquakes, it was inferred that the central part of the Indonesian Arc is subject to a vortex-shaped stress field centred on the Savu Basin. In this model, the compressive axes appear to rotate counterclockwise (from SW to NNE) in the Sumba-Sumbawa-Western Flores region and clockwise (from W to NNW) in the Timor-Eastern Flores zone. To interpret these features, on the basis of seismological evidence, a lateral discontinuity in the arc-trench system close to Sumba, a collision between Sumba and Sumbawa and a rotation towards the north-northeast of Sumba have been suggested. The proposed structural discontinuity, trending NW-SE, may represent a major transcurrent fault zone through which the Australian continental lithosphere comes into contact with the Indian oceanic lithosphere.

Temperature Models for the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Manea, V. C.; Kostoglodov, V.; Currie, C.; Manea, M.; Wang, K.

2002-12-01

It is well known that the temperature is one of the major factors which controls the seismogenic zone. The Mexican subduction zone is characterized by a very shallow flat subducting interplate in its central part (Acapulco, Oaxaca), and deeper subduction slabs northern (Jalisco) and southern (Chiapas). It has been proposed that the seismogenic zone is controlled, among other factors, by a temperature. Therefore, we have developed four two-dimensional steady state thermal models for Jalisco, Guerrero, Oaxaca and Chiapas. The updip limit of the seismogenic zone is taken between 100 §C and 150 §C, while the downdip limit is thought to be at 350 §C because of the transition from stick-slip to stable-sliding. The shape of the subducting plate is inferred from gravity and seismicity. The convergence velocity between oceanic and continental lithospheric plates is taken as the following: 5 cm/yr for Jalisco profile, 5.5 for Guerrero profile, 5.8 for Oaxaca profile, and 7.8 for Chiapas profile. The age of the subducting plates, since they are young, and provides the primary control on the forearc thermal structure, are as the following: 11 My for Jalisco profile, 14.5 My for Guerrero profile, 15 My for Oaxaca profile, and 28 My for Chiapas profile. We also introduced in the models a small quantity of frictional heating (pore pressure ratio 0.98). The value of 0.98 for pore pressure ratio was obtained for the Guerrero profile, in order to fit the intersection between the 350 §C isotherm and the subducting plate at 200 Km from trench. The value of 200 km coupling zone from trench is inferred from GPS data for the steady interseismic period and also for the last slow aseismic slip that occurred in Guerrero in 2002. We have used this value of pore pressure ratio (0.98) for all the other profiles. For the others three profiles we obtained the following coupling extents: Jalisco - 100 km, Oaxaca - 170 km and Chiapas - 125 km (from the trench). Independent constrains of the thermal models come from the surface thermal measurements (offshore - Prol-Ledesma et al (1989) and onshore - Ziagos et al. (1985)). Unfortunately these measurements are very sparse, and present an important dispersion and have large uncertainties. In our models, all profiles show a decrease in heat flow from the trench towards the continent, which is characteristic for subduction zones. We also have included a mantle wedge flow current in order to keep a constant depth to the lithosphere-asthenosphere boundary. This mantle wedge convection provides an increase in heat flow near the volcanic arc which is consistent with the surface observations. Our models indicate that the seismogenic zone in Mexico comprised between 100 §C and 350 §C is in good agreement with the coseismic rupture width inferred from the megathrust earthquake aftershocks and seismic models of rupture. These thermal models will be used to calculate the thermal stresses induced by the subducting plate.

Megathrust and accretionary wedge properties and behaviour in the Makran subduction zone

NASA Astrophysics Data System (ADS)

Penney, Camilla; Tavakoli, Farokh; Saadat, Abdolreza; Nankali, Hamid Reza; Sedighi, Morteza; Khorrami, Fateme; Sobouti, Farhad; Rafi, Zahid; Copley, Alex; Jackson, James; Priestley, Keith

2017-06-01

We study the Makran subduction zone, along the southern coasts of Iran and Pakistan, to gain insights into the kinematics and dynamics of accretionary prism deformation. By combining techniques from seismology, geodesy and geomorphology, we are able to put constraints on the shape of the subduction interface and the style of strain across the prism. We also address the long-standing tectonic problem of how the right-lateral shear taken up by strike-slip faulting in the Sistan Suture Zone in eastern Iran is accommodated at the zone's southern end. We find that the subduction interface in the western Makran may be locked, accumulating elastic strain, and move in megathrust earthquakes. Such earthquakes, and associated tsunamis, present a significant hazard to populations around the Arabian Sea. The time-dependent strain within the accretionary prism, resulting from the megathrust earthquake cycle, may play an important role in the deformation of the Makran region. By considering the kinematics of the 2013 Balochistan and Minab earthquakes, we infer that the local gravitational and far-field compressive forces in the Makran accretionary prism are in balance. This force balance allows us to calculate the mean shear stress and effective coefficient of friction on the Makran megathrust, which we find to be 5-35 MPa and 0.01-0.03, respectively. These values are similar to those found in other subduction zones, showing that the abnormally high sediment thickness in the offshore Makran does not significantly reduce the shear stress on the megathrust.

Hunting for shallow slow-slip events at Cascadia

NASA Astrophysics Data System (ADS)

Tan, Y. J.; Bletery, Q.; Fan, W.; Janiszewski, H. A.; Lynch, E.; McCormack, K. A.; Phillips, N. J.; Rousset, B.; Seyler, C.; French, M. E.; Gaherty, J. B.; Regalla, C.

2017-12-01

The discovery of slow earthquakes at subduction zones is one of the major breakthroughs of Earth science in the last two decades. Slow earthquakes involve a wide spectrum of fault slip behaviors and seismic radiation patterns, such as tremor, low-frequency earthquakes, and slow-slip events. The last of these are particularly interesting due to their large moment releases accompanied by minimal ground shaking. Slow-slip events have been reported at various subduction zones ; most of these slow-slip events are located down-dip of the megathrust seismogenic zone, while a few up-dip cases have recently been observed at Nankai and New Zealand. Up-dip slow-slip events illuminate the structure of faulting environments and rupture mechanisms of tsunami earthquakes. Their possible presence and location at a particular subduction zone can help assess earthquake and tsunami hazard for that region. However, their typical location distant from the coast requires the development of techniques using offshore instrumentation. Here, we investigate the absolute pressure gauges (APG) of the Cascadia Initiative, a four year amphibious seismic experiment, to search for possible shallow up-dip slow-slip events in the Cascadia subduction zone. These instruments are collocated with ocean bottom seismometers (OBS) and located close to buoys and onshore GPS stations, offering the opportunity to investigate the utility of multiple datasets. Ultimately, we aim to develop a protocol to analyze APG data for offshore shallow slow-slip event detections and quantify uncertainties, with direct applications to understanding the up-dip subduction interface system in Cascadia.

Fractal analysis of the spatial distribution of earthquakes along the Hellenic Subduction Zone

NASA Astrophysics Data System (ADS)

Papadakis, Giorgos; Vallianatos, Filippos; Sammonds, Peter

2014-05-01

The Hellenic Subduction Zone (HSZ) is the most seismically active region in Europe. Many destructive earthquakes have taken place along the HSZ in the past. The evolution of such active regions is expressed through seismicity and is characterized by complex phenomenology. The understanding of the tectonic evolution process and the physical state of subducting regimes is crucial in earthquake prediction. In recent years, there is a growing interest concerning an approach to seismicity based on the science of complex systems (Papadakis et al., 2013; Vallianatos et al., 2012). In this study we calculate the fractal dimension of the spatial distribution of earthquakes along the HSZ and we aim to understand the significance of the obtained values to the tectonic and geodynamic evolution of this area. We use the external seismic sources provided by Papaioannou and Papazachos (2000) to create a dataset regarding the subduction zone. According to the aforementioned authors, we define five seismic zones. Then, we structure an earthquake dataset which is based on the updated and extended earthquake catalogue for Greece and the adjacent areas by Makropoulos et al. (2012), covering the period 1976-2009. The fractal dimension of the spatial distribution of earthquakes is calculated for each seismic zone and for the HSZ as a unified system using the box-counting method (Turcotte, 1997; Robertson et al., 1995; Caneva and Smirnov, 2004). Moreover, the variation of the fractal dimension is demonstrated in different time windows. These spatiotemporal variations could be used as an additional index to inform us about the physical state of each seismic zone. As a precursor in earthquake forecasting, the use of the fractal dimension appears to be a very interesting future work. Acknowledgements Giorgos Papadakis wish to acknowledge the Greek State Scholarships Foundation (IKY). References Caneva, A., Smirnov, V., 2004. Using the fractal dimension of earthquake distributions and the slope of the recurrence curve to forecast earthquakes in Colombia. Earth Sci. Res. J., 8, 3-9. Makropoulos, K., Kaviris, G., Kouskouna, V., 2012. An updated and extended earthquake catalogue for Greece and adjacent areas since 1900. Nat. Hazards Earth Syst. Sci., 12, 1425-1430. Papadakis, G., Vallianatos, F., Sammonds, P., 2013. Evidence of non extensive statistical physics behavior of the Hellenic Subduction Zone seismicity. Tectonophysics, 608, 1037-1048. Papaioannou, C.A., Papazachos, B.C., 2000. Time-independent and time-dependent seismic hazard in Greece based on seismogenic sources. Bull. Seismol. Soc. Am., 90, 22-33. Robertson, M.C., Sammis, C.G., Sahimi, M., Martin, A.J., 1995. Fractal analysis of three-dimensional spatial distributions of earthquakes with a percolation interpretation. J. Geophys. Res., 100, 609-620. Turcotte, D.L., 1997. Fractals and chaos in geology and geophysics. Second Edition, Cambridge University Press. Vallianatos, F., Michas, G., Papadakis, G., Sammonds, P., 2012. A non-extensive statistical physics view to the spatiotemporal properties of the June 1995, Aigion earthquake (M6.2) aftershock sequence (West Corinth rift, Greece). Acta Geophys., 60, 758-768.

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.

Seismicity and structure of Nazca Plate subduction zone in southern Peru

NASA Astrophysics Data System (ADS)

Lim, H.; Kim, Y.; Clayton, R. W.

2015-12-01

We image the Nazca plate subduction zone system by detecting and (re)locating intra-slab earthquakes in southern Peru. Dense seismic arrays (PeruSE, 2013) were deployed along four lines to target geophysical characterization of the subduction system in the transition zone between flat and normal dipping segments of the Nazca plate (2-15°S). The arc volcanism is absent near the flat slab segment, and currently, the correlation between the location of the active volcanic front and corresponding slab depth is neither clear nor consistent between previously published models from seismicity. We detect 620 local earthquakes from August 2008 to February 2013 by manually picking 6559 and 4145 arrival times for P- and S-phases, respectively. We observe that the S-phase data is helpful to reduce the trade-off between origin time and depth of deeper earthquakes (>100 km). Earthquake locations are relocated to constrain the Nazca slab-mantle interface in the slab-dip transition zone using 7322 measurements of differential times of nearby earthquake pairs by waveform cross-correlation. We also employ the double-difference tomography (Zhang and Thurber, 2003) to further improve earthquake source locations and the spatial resolution of the velocity structure simultaneously. The relocated hypocenters clearly delineate the dipping Wadati-Benioff zone in the slab-dip transition zone between the shallow- (25°) to-flat dipping slab segment in the north and the normal (40°) dipping segment in the south. The intermediate-depth seismicity in the flat slab region stops at a depth of ~100 km and a horizontal distance of ~400 km from the trench. We find a significant slab-dip difference (up to 10°) between our relocated seismicity and previously published slab models along the profile region sampling the normal-dip slab at depth (>100 km).

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.

Unrevealing the History of Earthquakes and Tsunamis of the Mexican Subduction Zone

NASA Astrophysics Data System (ADS)

Ramirez-Herrera, M. T.; Castillo-Aja, M. D. R.; Cruz, S.; Corona, N.; Rangel Velarde, V.; Lagos, M.

2014-12-01

The great earthquakes and tsunamis of the last decades in Sumatra, Chile, and Japan remind us of the need for expanding the record of history of such catastrophic events. It can't be argued that even countries with extensive historical documents and tsunami sand deposits still have unsolved questions on the frequency of them, and the variables that control them along subduction zones. We present here preliminary results of a combined approach using historical archives and multiple proxies of the sedimentary record to unrevealing the history of possible great earthquakes and their tsunamis on the Mexican Subduction zone. The Mexican subduction zone extends over 1000 km long and little is known if the entire subduction zone along the Middle American Trench behaves as one enormous unit rather than in segments that rupture at different frequencies and with different strengths (as the short instrumental record shows). We searched on historical archives and earthquake databases to distinguish tsunamigenic events registered from the 16th century to now along the Jalisco-Colima and Guerrero-Oaxaca coastal stretches. The historical data referred are mostly from the 19th century on since the population on the coast was scarce before. We found 21 earthquakes with tsunamigenic potential, and of those 16 with doubtful to definitive accompanying tsunami on the Jalisco-Colima coast, and 31 tsunamigenic earthquakes on the Oaxaca-Guerrero coast. Evidence of great earthquakes and their tsunamis from the sedimentary record are scarce, perhaps due poor preservation of tsunami deposits in this tropical environment. Nevertheless, we have found evidence for a number of tsunamigenic events, both historical and prehistorical, 1932 and 1400 AD on Jalisco, and 3400 BP, 1789 AD, 1979 ad, and 1985 AD on Guerrero-Oaxaca. We continue working and a number of events are still to be dated. This work would aid in elucidating the history of earthquakes and tsunamis on the Mexican subduction zone.

Transform push, oblique subduction resistance, and intraplate stress of the Juan de Fuca plate

USGS Publications Warehouse

Wang, K.; He, J.; Davis, E.E.

1997-01-01

The Juan de Fuca plate is a small oceanic plate between the Pacific and North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress a, constrained by earthquake focal mechanisms is N-S. Off Oregon, and possibly off Washington, NW trending left-lateral faults cutting the Juan de Fuca plate indicate a a, in a NE-SW to E-W direction. The magnitude of differential stress increases from north to south; this is inferred from the plastic yielding and distribution of earthquakes throughout the Gorda deformation zone. To understand how tectonic forces determine the stress field of the Juan de Fuca plate, we have modeled the intraplate stress using both elastic and elastic-perfectly plastic plane-stress finite element models. We conclude that the right-lateral shear motion of the Pacific and North America plates is primarily responsible for the stress pattern of the Juan de Fuca plate. The most important roles are played by a compressional force normal to the Mendocino transform fault, a result of the northward push by the Pacific plate and a horizontal resistance operating against the northward, or margin-parallel, component of oblique subduction. Margin-parallel subduction resistance results in large N-S compression in the Gorda deformation zone because the force is integrated over the full length of the Cascadia subduction zone. The Mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike-normal compressive stresses. Internal failure of the Gorda deformation zone potentially places limits on the magnitude of the fault-normal stresses being transmitted and correspondingly on the magnitude of strike-parallel subduction resistance. Transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner. Copyright 1997 by the American Geophysical Union.

Geochemistry of primary-carbonate bearing K-rich igneous rocks in the Awulale Mountains, western Tianshan: Implications for carbon-recycling in subduction zone

NASA Astrophysics Data System (ADS)

Yang, Wu-Bin; Niu, He-Cai; Shan, Qiang; Chen, Hua-Yong; Hollings, Pete; Li, Ning-Bo; Yan, Shuang; Zartman, Robert E.

2014-10-01

Arc magmatism plays an important role in the recycling of subducted carbon and returning it to the surface. However, the transfer mechanisms of carbon are poorly understood. In this study, the contribution of subducted carbonate-rich sediments to the genesis of the carbonate-bearing K-rich igneous rocks from western Tianshan was investigated. Four key triggers are involved, including sediments subduction, slab decarbonation, partial melting and magma segregation. The globular carbonate ocelli show C-O isotope signatures intermediate between oceanic sediments and mantle, suggesting that the carbon of the primary carbonate ocelli was derived from recycled subducted sediments in the mantle. Decarbonation of the subducted slab is regarded as the primary agent to carbonize the mantle wedge. Geochemical features indicate that the carbonate ocelli are primary, and that the parental K- and carbon-rich mafic alkaline magma was derived from partial melting of carbonated mantle wedge veined with phlogopite. Major and trace element compositions indicate that globular carbonate ocelli hosted in the Bugula K-rich igneous rocks are calcio-carbonate and formed primarily by segregation of the differentiated CO2-rich alkaline magma after crystallization fractionation. The K-rich alkaline magma, which formed from partial melting of metasomatized (i.e., phlogopite bearing) mantle wedge in the sub-arc region, is a favorable agent to transport subducted carbon back to the Earth's surface during carbon recycling in subduction zones, because of the high CO2 solubility in alkaline mafic magma. We therefore propose a model for the petrogenesis of the carbonate-bearing K-rich igneous rocks in western Tianshan, which are significant for revealing the mechanism of carbon recycling in subduction zones.

Interseismic Coupling and Seismic Potential along the Indo-Burmese Arc and the Sagaing fault

NASA Astrophysics Data System (ADS)

Earnest, A.

2017-12-01

The Indo-burmese arc is formed by the oblique subduction of the Indian plate under the Eurasia. This region is a transition zone between the main Himalayan collision belt and the Andaman subduction zone. This obliquity causes strain partitioning which causes separation of a sliver plate, the Burma Plate. Considering the geomorphic, tectonic and geophysical signatures, IBR comprises all the structural features of an active subduction zone, whereas the present day tectonics of this region is perplexing. Ni et al. [1989] and Rao and Kalpana [2005] suggested that the subduction might have stopped in recent times or continues relatively in an aseismic fashion. This is implied by the NNE compressional stress orientations, instead of its downdip direction. The focal mechanism stress inversions show distinct stress fields above and below the 90 km depth. It is widely believed that the partitioning of Indian-Eurasia plate motion along the Indo-buremse arc and the Sagaing fault region the reason for earthquake occurrence in this region. The relative motion of 36mm/yr, between India and Eurasia, is partitioned across the Sagaing fault through a dextral movement of ˜20mm/yr and remaining velocity is accommodated at the Churachandapur-Mao fault (CMF) through dextral motion. The CMF and its surroundings are considered as seismically a low hazard region, an observation made from the absence of significant earthquakes and lack of field evidences. This made Kundu and Gahalaut [2013] to propose that the motion across the CMF happens in an aseismic manner. Recently, based on GPS studies Steckler et al. [2016] suggested that the region is still actively subducting and the presence of a locked megathrust plate boundary depicts the region as highly vulnerable for large magnitude seismic activities. Our study, based on various geodetic solutions and earthquake slip vectors, focus on interseisimic block models for the Indo-burmese arc and Sagaing fault region so as to model the crustal deformation of this area using an elastic block modelling approach. Results from our best fit model predicts the spatial distribution of interseismic coupling coefficient (φ) and the backslip component. These coefficients characterize the fault interface, which helps in estimating the seismic potential across Indo-burmese arc and the Sagaing fault region.

Fluid inclusions in jadeitite and jadeite-rich rock from serpentinite mélanges in northern Hispaniola: Trapped ambient fluids in a cold subduction channel

NASA Astrophysics Data System (ADS)

Kawamoto, Tatsuhiko; Hertwig, Andreas; Schertl, Hans-Peter; Maresch, Walter V.

2018-05-01

Freezing-point depression was measured in aqueous fluid inclusions to determine salinities in six samples of jadeitite and jadeite-rich rock from the Jagua Clara serpentinite mélange of the Rio San Juan Complex, Dominican Republic. The mélange represents a fossil subduction-zone channel from a cold, mature subduction zone with a geothermal gradient of 6 °C/km. One hundred and twenty-five determinations of salinity in primary inclusions hosted in jadeite, quartz, apatite and lawsonite range between extremes of 1.2 and 8.7, but yield a well-defined mean of 4.5 ± 1.1 wt% (±1 s.d.) NaCl equiv, slightly higher than mean seawater (3.5 wt%). In one sample, eight additional fluid inclusions in quartz aligned along grain boundaries yield slightly lower values of 2.7 ± 1.3 wt% NaCl equiv. Homogenization temperatures were also measured for 47 fluid inclusions in two samples, but primary entrapment densities are not preserved. It is significant that the suite includes two types of samples: those precipitated directly from an aqueous fluid as well as examples of metasomatic replacement of a pre-existing magmatic rock. Nevertheless, the results indicate identical salinity for both types and suggest a much stronger genetic link between the two types of jadeitite and jadeite-rich rock than has previously been assumed. Based on the results of conductivity measurements in modern subduction zones, we envision a pervasive fluid in the subduction channel that evolved from salinity levels lower than those in sea-water up to the measured values due to on-going but largely completed serpentinization in the subduction channel. The present data represent a reference marker for the subduction channel of the Rio San Juan intra-oceanic subduction zone at 30-50 km depth and after 50-60 Myr of operation.

Seismic anisotropy in the Hellenic subduction zone: Effects of slab segmentation and subslab mantle flow

NASA Astrophysics Data System (ADS)

Evangelidis, C. P.

2017-12-01

The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE-SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve a more definite image of the structure and geodynamics of this area.

Distribution of flexural deflection in the worldwide outer rise area

NASA Astrophysics Data System (ADS)

Lin, Zi-Jun; Lin, Jing-Yi; Lin, Yi-Chin; Chin, Shao-Jinn; Chen, Yen-Fu

2015-04-01

The outer rise on the fringe of a subduction system is caused by an accreted load on the flexed oceanic lithosphere. The magnitude of the deflection is usually linked to the stress state beard by the oceanic plate. In a coupled subduction zone, the stress is abundantly accumulated across the plate boundary which should affect the flexural properties of the subducted plate. Thus, the variation of the outer rise in shape may reflect the seismogenic characteristics of the subduction system. In this study, we intent to find the correlation between the flexure deflection (Wb) of the outer rise and the subduction zone properties by comparing several slab parameters and the Wb distribution. The estimation of Wb is performed based on the available bathymetry data and the statistic analysis of earthquakes is from the global ISC earthquake catalog for the period of 1900-2015. Our result shows a progressive change of Wb in space, suggesting a robust calculation. The average Wb of worldwise subduction system spreads from 348 to 682 m. No visible distinction in the ranging of Wb was observed for different subduction zones. However, in a weak coupling subduction system, the standard variation of Wb has generally larger value. Relatively large Wb generally occurs in the center of the trench system, whereas small Wb for the two ends of trench. The comparison of Wb and several slab parameters shows that the Wb may be correlated with the maximal magnitude and the number of earthquakes. Otherwise, no clear relationship with other parameters can be obtained.

Seismic and aseismic slip on the ``uncoupled'' Tonga subduction megathrust

NASA Astrophysics Data System (ADS)

Beavan, R. J.; Wang, X.; Bevis, M. G.; Kautoke, R'

2010-12-01

The Tonga subduction zone has been a type example of a weakly coupled subduction interface since soon after the birth of plate tectonics. Yet in the September 2009 double earthquake, the northern Tonga subduction interface failed in a great Mw 8 earthquake that was probably dynamically triggered by a Mw 8 extensional intraplate earthquake in the outer trench slope region of the incoming Pacific Plate. There are some discrepancies between models of the September 2009 doublet derived from seismic data and those derived from geodetic and DART tsunami data, in particular about which fault plane failed in the intraplate earthquake. In this presentation we explore how well the geodetic and tsunami data can be fit using the alternative fault plane. We also present new GPS data that show the subduction interface is continuing to slip faster than its 1996-2005 “long-term” rate, and we speculate on what this means for the mechanisms by which interplate slip is accommodated at the Tonga subduction zone.

Origin of back-arc basins and effects of western Pacific subduction systems on eastern China geology

NASA Astrophysics Data System (ADS)

Niu, Y.

2013-12-01

Assuming that subduction initiation is a consequence of lateral compositional buoyancy contrast within the lithosphere [1], and recognizing that subduction initiation within normal oceanic lithosphere is unlikely [1], we can assert that passive continental margins that are locations of the largest compositional buoyancy contrast within the lithosphere are the loci of future subduction zones [1]. We hypothesize that western Pacific back-arc basins were developed as and evolved from rifting at passive continental margins in response to initiation and continuation of subduction zones. This hypothesis can be tested by demonstrating that intra-oceanic island arcs must have basement of continental origin. The geology of the Islands of Japan supports this. The highly depleted forearc peridotites (sub-continental lithosphere material) from Tonga and Mariana offer independent lines of evidence for the hypothesis [1]. The origin and evolution of the Okinawa Trough (back-arc basin) and Ryukyu Arc/Trench systems represents the modern example of subduction initiation and back-arc basin formation along a (Chinese) continental margin. The observation why back-arc basins exit behind some subduction zones (e.g., western Pacific) but not others (e.g., in South America) depends on how the overlying plate responds to subduction, slab-rollback and trench retreat. In the western Pacific, trench retreat towards east results in the development of extension in the upper Eurasian plate and formation of back-arc basins. In the case of South America, where no back-arc basins form because trench retreat related extension is focused at the 'weakest' South Mid-Atlantic Ridge. It is thus conceptually correct that the South Atlantic is equivalent to a huge 'back-arc basin' although its origin may be different. Given the negative Clayperon slope of the Perovskite-ringwoodite phase transition at the 660 km mantle seismic discontinuity (660-D), slab penetration across the 660-D is difficult and trench retreat in the western Pacific readily result in the horizontal stagnation of the Pacific plate in the transition zone beneath eastern Asian continent [2]. Dehydration of this slab supplies water, which rises and results in 'basal hydration weakening' of the eastern China lithosphere and its thinning by converting it into weak material of asthenospheric property [3]. We note the proposal that multiple subduction zones with more water (i.e., subduction of the South China Block beneath the North China Craton, NCC; subduction of the Siberian/Mongolian block beneath the NCC) all contribute to the lithosphere thinning beneath the NCC [4]. However, 'South China-NCC' and 'Siberian/Mongolian-NCC' represent two collisional tectonics involving no trench retreat, causing no transition-zone slab stagnation, supplying no water, and thus contributing little to lithosphere thinning beneath the NCC. Furthermore, lithosphere thinning happened to the entire eastern China, not just limited to the NCC, emphasizing the effects of the western Pacific subduction system on eastern China geology. References: [1] Niu et al., 2003, Journal of Petrology, 44, 851-866. [2] Kárason & van der Hilst, R., 2000, Geophysical Monograph, 121, 277-288. [3] Niu, 2005, Geological Journal of China Universities, 11, 9-46. [4] Windley et al., 2010, American Journal of Science, 310, 1250-1293.

Slab dehydration in Cascadia and its relationship to volcanism, seismicity, and non-volcanic tremor

NASA Astrophysics Data System (ADS)

Delph, J. R.; Levander, A.; Niu, F.

2017-12-01

The characteristics of subduction beneath the Pacific Northwest (Cascadia) are variable along strike, leading to the segmentation of Cascadia into 3 general zones: Klamath, Siletzia, and Wrangelia. These zones show marked differences in tremor density, earthquake density, seismicity rates, and the locus and amount of volcanism in the subduction-related volcanic arc. To better understand what controls these variations, we have constructed a 3D shear-wave velocity model of the upper 80 km along the Cascadia margin from the joint inversion of CCP-derived receiver functions and ambient noise surface wave data using 900 temporary and permanent broadband seismic stations. With this model, we can investigate variations in the seismic structure of the downgoing oceanic lithosphere and overlying mantle wedge, the character of the crust-mantle transition beneath the volcanic arc, and local to regional variations in crustal structure. From these results, we infer the presence and distribution of fluids released from the subducting slab and how they affect the seismic structure of the overriding lithosphere. In the Klamath and Wrangelia zones, high seismicity rates in the subducting plate and high tremor density correlate with low shear velocities in the overriding plate's forearc and relatively little arc volcanism. While the cause of tremor is debated, intermediate depth earthquakes are generally thought to be due to metamorphic dehydration reactions resulting from the dewatering of the downgoing slab. Thus, the seismic characteristics of these zones combined with rather sparse arc volcanism may indicate that the slab has largely dewatered by the time it reaches sub-arc depths. Some of the water released during earthquakes (and possibly tremor) may percolate into the overriding plate, leading to slow seismic velocities in the forearc. In contrast, Siletzia shows relatively low seismicity rates and tremor density, with relatively higher shear velocities in the forearc. Siletzia also contains most of the young arc volcanoes in the Cascades, indicating that water is retained in the slab to depths where it can feed arc volcanism. Thus, the along strike variations in volcanic activity and seismic activity in Cascadia appear to be related to variations in depth of dewatering of the downgoing oceanic lithosphere.

Textures of eclogites and blueschists from Syros island, Greece: Inferences for elastic anisotropy of subducted oceanic crust

NASA Astrophysics Data System (ADS)

Keppler, Ruth; Behrmann, Jan H.; Stipp, Michael

2017-07-01

Many blueschists and eclogites are inferred to have formed from oceanic basalts in subducted slabs. Knowledge of their elastic behavior is essential for reconstructing the internal structure of subduction zones. The Cycladic blueschist unit, exposed on Syros Island (Greece), contains rocks belonging to an exhumed Tertiary subduction complex. They were possibly part of a subduction channel, a shear zone above the subducting slab in which exhumation is possible during subduction. Intense plastic deformation, forming crystallographic preferred orientations (CPO), accompanied blueschist and eclogite metamorphism. CPO of the constituent minerals in the collected samples was determined by time-of-flight neutron diffraction. Two samples are foliated fine-grained blueschists with strong CPO, rich in glaucophane, zoisite, and phengite. Two coarser-grained eclogite samples rich in omphacite and clinozoisite, or glaucophane, have weaker CPO. Vp and Vs anisotropies were computed from the orientation distribution function and single-crystal elastic constants. All samples show velocity maxima parallel to the mineral lineation, and minima normal to the foliation, providing important constraints on orientations of seismic anisotropy in subduction channels. Vp anisotropies are up to 3 times higher (6.5-12%) in the blueschists than in the eclogites (3-4%), pointing to a potentially important lithological control of elastic anisotropy in subducted oceanic crust.

Forward Analyses of Dehydration Reactions in Mafic Rocks Along the P-T Trajectories of the Subducting Slabs

NASA Astrophysics Data System (ADS)

Kuwatani, T.; Okamoto, A.; Toriumi, M.

2005-12-01

Fluids in the subduction zone play an important role in magmatism, metamorphism, and mechanical processes involving seismic activity. Additionally, recent geophysical researches found low-frequency tremors which may be related to the movement of fluid (Obara, 2002) and a zone of high Poisson_fs ratio which reflects high pore fluid pressure (Kodaira et al.,2004) in the Southwest Japan fore-arc. It is widely accepted that these fluids are supplied by the dehydration of hydrous metamorphic minerals in the subducting oceanic plate. Although many previous studies attempted to estimate the water content of the subducting oceanic crust experimentally and theoretically (e.g., Schmidt and Poli, 1998; Hacker et al., 2003), there have been no studies which quantify the continuous dehydration reactions in detail. The aim of this study is to quantify the progress of the continuous dehydration reactions of mafic rocks in the condition of greenschist facies, corresponding to low-intermediate depth (10-50km) of warm subduction zone. We use the differential thermodynamics (Spear 1993) which include mass balance to predict the continuous metamorphic reaction history of mafic rocks along the P-T trajectory of the subducting slab. With fixed bulk chemical composition the thermodynamic system is divariant, as specified in Duhem_fs theorem. In differential thermodynamics, applying a series of changes in pressure and temperature (ΔP and ΔT, respectively) from initial conditions (P0, T0, X0s, M0s), we can trace ΔXs and ΔMs, that is, the progress (history) of the metamorphic reactions along the arbitrary P-T trajectory (Thermodynamic forward modeling). According to Okamoto and Toriumi, 2001, we modeled the greenschist/ blueschist/ (epidote -) amphibolite assemblage of mafic rocks, which consist of the following phases: Amphibole ± Epidote ± Chlorite + Plagioclase + Quartz + Fluid (H2O), in the system of Na2O - CaO - MgO - FeO - Fe2O3 - Al2O3 - SiO2 - H2O. The reference compositions and modes of minerals were assumed according to the natural sample of greenschist which has MORB-like bulk composition (Hacker et al. 2003). The reference temperature and pressure were set to be 300°C, 0.3GPa. Calculations were performed along the P-T paths of the Southwest Japan (4MPa/°C) and the Cape Mendocino (the North California, 2MPa/°C) predicted by Yamasaki and Seno, 2003. As a result, the water production rates have the peak depths at the boundary between the greenschist facies and the epidote-amphibolite facies in the Southwest Japan, and at the boundary between the greenschist facies and the amphibolite facies in the Cape Mendocino, respectively. Chlorite decomposition is the main dehydration reaction. These peak depths correspond to the zone of low frequency tremors, high Poisson_fs ratio and active seismicity (30-50km) in the Southwest Japan, and active seismicity (10-20km) in the Cape Mendocino, respectively.

A tectonic model for the Tertiary evolution of strike slip faults and rift basins in SE Asia

NASA Astrophysics Data System (ADS)

Morley, C. K.

2002-04-01

Models for the Tertiary evolution of SE Asia fall into two main types: a pure escape tectonics model with no proto-South China Sea, and subduction of proto-South China Sea oceanic crust beneath Borneo. A related problem is which, if any, of the main strike-slip faults (Mae Ping, Three Pagodas and Aliao Shan-Red River (ASRR)) cross Sundaland to the NW Borneo margin to facilitate continental extrusion? Recent results investigating strike-slip faults, rift basins, and metamorphic core complexes are reviewed and a revised tectonic model for SE Asia proposed. Key points of the new model include: (1) The ASRR shear zone was mainly active in the Eocene-Oligocene in order to link with extension in the South China Sea. The ASRR was less active during the Miocene (tens of kilometres of sinistral displacement), with minor amounts of South China Sea spreading centre extension transferred to the ASRR shear zone. (2) At least three important regions of metamorphic core complex development affected Indochina from the Oligocene-Miocene (Mogok gneiss belt; Doi Inthanon and Doi Suthep; around the ASRR shear zone). Hence, Paleogene crustal thickening, buoyancy-driven crustal collapse, and lower crustal flow are important elements of the Tertiary evolution of Indochina. (3) Subduction of a proto-South China Sea oceanic crust during the Eocene-Early Miocene is necessary to explain the geological evolution of NW Borneo and must be built into any model for the region. (4) The Eocene-Oligocene collision of NE India with Burma activated extrusion tectonics along the Three Pagodas, Mae Ping, Ranong and Klong Marui faults and right lateral motion along the Sumatran subduction zone. (5) The only strike-slip fault link to the NW Borneo margin occurred along the trend of the ASRR fault system, which passes along strike into a right lateral transform system including the Baram line.

Characterizing an "uncharacteristic" ETS event in northern Cascadia

USGS Publications Warehouse

Wang, Kelin; Dragert, Herb; Kao, Honn; Roeloffs, Evelyn

2008-01-01

GPS and borehole strainmeter data allowed the detection and model characterization of a slow slip event in northern Cascadia in November 2006 accompanying a brief episode of seismic tremor. The event is much smaller in area and duration than other well-known ETS events in northern Cascadia but is strikingly similar to typical ETS events at the Nankai subduction zone. The 30-45 km depth range and the 2-3 cm slip magnitude as interpreted for this event appear to be common to most ETS events in these two subduction zones, regardless of their sizes. We infer that the Nankai-type small ETS events must be abundant at Cascadia and that ETS events at the two subduction zones are governed by a similar physical process.

Joint Local/Teleseismic Tomographic Inversion in Taiwan Using TAIGER and Other Data

NASA Astrophysics Data System (ADS)

Lee, E.; Wu, F. T.; Huang, B.; Liang, W.; Wang, C.; Rawlinson, N.; Okaya, D. A.

2008-12-01

Taiwan, one of the most active orogenic belts, is at the intersection of two subduction zones. In southern Taiwan, the South China Sea Slab (SCSS), part of Eurasian Plate (EP), subducts beneath the Luzon arc along the Manila trench. In northern Taiwan, the Philippine Sea Plate (PSP) subducts beneath the Ryukyu arc along the Ryukyu trench. The thin skinned model and lithospheric deformation model have been proposed to explain the formation of orogeny. To distinguish between these two geodynamically possible processes, imaging of the deep structures below Taiwan is necessary. In this study, explosion data, local/regional earthquakes and teleseisms are used to invert the velocity structures of Taiwan from surface to about 150 km. Temporary passive broadband (on land and at the ocean bottom), active sources array datasets of the TAIGER (TAiwan Integrated GEodynamics Research) project and permanent array datasets of the BATS (Broadband Array in Taiwan for Seismology) and CWB (Central Weather Bureau) are used in this study. FMTOMO (fast marching tomography) of Rawlinson et al. (2006) is employed to invert the 3D P-wavespeed beneath Taiwan. The derived velocity perturbations dVp (dVp= Vfinal-Vinital) are clearly related to geology and tectonics. At shallow depth (< 10km), dVp >0 under the Central Range (Pre-Tertiary metamorphic rocks) and dVp < 0 under the Foothills (Pliocene sedimentary). Below a depth about 20 km, the placement of the high and low anomalies is reversed, i.e., dVp>0 under the Foothills and dVp<0 under the Central Range; the low velocity core of the Central Ranges extend down to about 50 km, forming the mountain root. A steeply dipping high velocity zone lies under the thickening 'mountain root' in central Taiwan. In southern Taiwan, the high velocity zone dips eastward coinciding with the Benioff Zone. The geometry of the high velocity zones in the upper mantle are key to understanding the Taiwan orogeny.

Deep Seismic Reflection Images of the Sumatra Seismic and Aseismic Gaps

NASA Astrophysics Data System (ADS)

Singh, S. C.; Hananto, N. D.; Chauhan, A.; Carton, H. D.; Midenet, S.; Djajadihardja, Y.

2009-12-01

The Sumatra subduction zone is seismically most active region on the Earth, and has been the site of three great earthquakes only in the last four years. The first of the series, the 2004 Boxing Day earthquake, broke 1300 km of the plate boundary and produced the devastating tsunami around the Indian Ocean. The second great earthquake occurred three months later in March 2005, about 150 km SE of the 2004 event. The Earth waited for three years, and then broke again in September 2007 at 1300 km SE of the 2004 event producing a twin earthquake of magnitudes of 8.5 and 7.9 at an interval of 12 hours, leaving a seismic gap of about 600 km between the second and third earthquake, the Sumatra Seismic Gap. Seismological and geodetic studies suggest that this gap is fully locked and may break any time. In order to study the seismic and tsunami risk in this locked region, a deep seismic reflection survey (Tsunami Investigation Deep Evaluation Seismic -TIDES) was carried out in May 2009 using the CGGVeritas vessel Geowave Champion towing a 15 long streamer, the longest ever used during a seismic survey, to image the nature of the subducting plate and associated features, including the seismogenic zone, from seafloor down to 50 km depth. A total of 1700 km of deep seismic reflection data were acquired. Three dip lines traverse the Sumatra subduction zone; one going through the Sumatra Seismic Gap, one crossing the region that broke during the 2007 great earthquake, and one going through the aseismic zone. These three dip profiles should provide insight about the locking mechanism and help us to understand why an earthquake occurs in one zone and not in aseismic zone. A strike-line was shot in the forearc basin connecting the locked zone with broken zone profiles, which should provide insight about barriers that might have stopped propagation of 2007 earthquake rupture further northward.

Dominant seismic sources for the cities in South Sumatra

NASA Astrophysics Data System (ADS)

Sunardi, Bambang; Sakya, Andi Eka; Masturyono, Murjaya, Jaya; Rohadi, Supriyanto; Sulastri, Putra, Ade Surya

2017-07-01

Subduction zone along west of Sumatra and Sumatran fault zone are active seismic sources. Seismotectonically, South Sumatra could be affected by earthquakes triggered by these seismic sources. This paper discussed contribution of each seismic source to earthquake hazards for cities of Palembang, Prabumulih, Banyuasin, OganIlir, Ogan Komering Ilir, South Oku, Musi Rawas and Empat Lawang. These hazards are presented in form of seismic hazard curves. The study was conducted by using Probabilistic Seismic Hazard Analysis (PSHA) of 2% probability of exceedance in 50 years. Seismic sources used in analysis included megathrust zone M2 of Sumatra and South Sumatra, background seismic sources and shallow crustal seismic sources consist of Ketaun, Musi, Manna and Kumering faults. The results of the study showed that for cities relatively far from the seismic sources, subduction / megathrust seismic source with a depth ≤ 50 km greatly contributed to the seismic hazard and the other areas showed deep background seismic sources with a depth of more than 100 km dominate to seismic hazard respectively.

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.

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.

Recycling and transport of continental material through the mantle wedge above subduction zones: A Caribbean example

NASA Astrophysics Data System (ADS)

Rojas-Agramonte, Yamirka; Garcia-Casco, Antonio; Kemp, Anthony; Kröner, Alfred; Proenza, Joaquín A.; Lázaro, Concepción; Liu, Dunyi

2016-02-01

Estimates of global growth rates of continental crust critically depend upon knowledge of the rate at which crustal material is delivered back into the mantle at subduction zones and is then returned to the crust as a component of mantle-derived magma. Quantification of crustal recycling by subduction-related magmatism relies on indirect chemical and isotopic tracers and is hindered by the large range of potential melt sources (e.g., subducted oceanic crust and overlying chemical and clastic sediment, sub-arc lithospheric mantle, arc crust), whose composition may not be accurately known. There is also uncertainty about how crustal material is transferred from subducted lithosphere and mixed into the mantle source of arc magmas. We use the resilient mineral zircon to track crustal recycling in mantle-derived rocks of the Caribbean (Greater Antilles) intra-oceanic arc of Cuba, whose inception was triggered after the break-up of Pangea. Despite juvenile Sr and Nd isotope compositions, the supra-subduction zone ophiolitic and volcanic arc rocks of this Cretaceous (∼135-70 Ma) arc contain old zircons (∼200-2525 Ma) attesting to diverse crustal inputs. The Hf-O isotope systematics of these zircons suggest derivation from exposed crustal terranes in northern Central America (e.g. Mexico) and South America. Modeling of the sedimentary component in the most mafic lavas suggests a contribution of no more than 2% for the case of source contamination or less than 4% for sediment assimilation by the magma. We discuss several possibilities for the presence of inherited zircons and conclude that they were transported as detrital grains into the mantle beneath the Caribbean Plate via subduction of oceanic crust. The detrital zircons were subsequently entrained by mafic melts that were rapidly emplaced into the Caribbean volcanic arc crust and supra-subduction mantle. These findings suggest transport of continental detritus, through the mantle wedge above subduction zones, in magmas that otherwise do not show strong evidence for crustal input and imply that crustal recycling rates in some arcs may be higher than hitherto realized.

GPS measurements and finite element modeling of the earthquake cycle along the Middle America subduction zone

NASA Astrophysics Data System (ADS)

Correa Mora, Francisco

We model surface deformation recorded by GPS stations along the Pacific coasts of Mexico and Central America to estimate the magnitude of and variations in frictional locking (coupling) along the subduction interface, toward a better understanding of seismic hazard in these earthquake-prone regions. The first chapter describes my primary analysis technique, namely 3-dimensional finite element modeling to simulate subduction and bounded-variable inversions that optimize the fit to the GPS velocity field. This chapter focuses on and describes interseismic coupling of the Oaxaca segment of the Mexican subduction zone and introduces an analysis of transient slip events that occur in this region. Our results indicate that coupling is strong within the rupture zone of the 1978 Ms=7.8 Oaxaca earthquake, making this region a potential source of a future large earthquake. However, we also find evidence for significant variations in coupling on the subduction interface over distances of only tens of kilometers, decreasing toward the outer edges of the 1978 rupture zone. In the second chapter, we study in more detail some of the slow slip events that have been recorded over a broad area of southern Mexico, with emphasis on their space-time behavior. Our modeling indicates that transient deformation beneath southern Mexico is focused in two distinct slip patches mostly located downdip from seismogenic areas beneath Guerrero and Oaxaca. Contrary to conclusions reached in one previous study, we find no evidence for a spatial or temporal correlation between transient slip that occurs in these two widely separated source regions. Finally, chapter three extends the modeling techniques to new GPS data in Central America, where subduction coupling is weak or zero and the upper plate deformation is much more complex than in Mexico. Cocos-Caribbean plate convergence beneath El Salvador and Nicaragua is accompanied by subduction and trench-parallel motion of the forearc. Our GPS velocity field is best fit by a model with strongly locked faults in the volcanic arc and a weakly coupled subduction interface. In this region, seismic hazards associated with subduction are therefore low, but are high for crustal faults, in agreement with records of historic seismicity.

Modelling guided waves in the Alaskan-Aleutian subduction zone

NASA Astrophysics Data System (ADS)

Coulson, Sophie; Garth, Thomas; Reitbrock, Andreas

2016-04-01

Subduction zone guided wave arrivals from intermediate depth earthquakes (70-300 km depth) have a huge potential to tell us about the velocity structure of the subducting oceanic crust as it dehydrates at these depths. We see guided waves as the oceanic crust has a slower seismic velocity than the surrounding material, and so high frequency energy is retained and delayed in the crustal material. Lower frequency energy is not retained in this crustal waveguide and so travels at faster velocities of the surrounding material. This gives a unique observation at the surface with low frequency energy arriving before the higher frequencies. We constrain this guided wave dispersion by comparing the waveforms recorded in real subduction zones with simulated waveforms, produced using finite difference full waveform modelling techniques. This method has been used to show that hydrated minerals in the oceanic crust persist to much greater depths than accepted thermal petrological subduction zone models would suggest in Northern Japan (Garth & Rietbrock, 2014a), and South America (Garth & Rietbrock, in prep). These observations also suggest that the subducting oceanic mantle may be highly hydrated at intermediate depth by dipping normal faults (Garth & Rietbrock 2014b). We use this guided wave analysis technique to constrain the velocity structure of the down going ~45 Ma Pacific plate beneath Alaska. Dispersion analysis is primarily carried out on guided wave arrivals recorded on the Alaskan regional seismic network. Earthquake locations from global earthquake catalogues (ISC and PDE) and regional earthquake locations from the AEIC (Alaskan Earthquake Information Centre) catalogue are used to constrain the slab geometry and to identify potentially dispersive events. Dispersed arrivals are seen at stations close to the trench, with high frequency (>2 Hz) arrivals delayed by 2 - 4 seconds. This dispersion is analysed to constrain the velocity and width of the proposed waveguide. The velocity structure of this relatively young subducting plate is compared to the velocity structure resolved in the older oceanic lithosphere subducted beneath Northern Japan. We also use guided wave observations to investigate the thickness and low velocity structure of the subducting Yakutat terrain. Additionally we discuss the dependence of the inferred slab geometry on the earthquake catalogues that are used.

A silent slip event on the deeper Cascadia subduction interface.

PubMed

Dragert, G; Wang, K; James, T S

2001-05-25

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore. In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion. No seismicity was associated with this event. The sudden displacements are best explained by approximately 2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses.

Abrupt tectonics and rapid slab detachment with grain damage

PubMed Central

Bercovici, David; Schubert, Gerald; Ricard, Yanick

2015-01-01

A simple model for necking and detachment of subducting slabs is developed to include the coupling between grain-sensitive rheology and grain-size evolution with damage. Necking is triggered by thickened buoyant crust entrained into a subduction zone, in which case grain damage accelerates necking and allows for relatively rapid slab detachment, i.e., within 1 My, depending on the size of the crustal plug. Thick continental crustal plugs can cause rapid necking while smaller plugs characteristic of ocean plateaux cause slower necking; oceanic lithosphere with normal or slightly thickened crust subducts without necking. The model potentially explains how large plateaux or continental crust drawn into subduction zones can cause slab loss and rapid changes in plate motion and/or induce abrupt continental rebound. PMID:25605890

Abrupt tectonics and rapid slab detachment with grain damage.

PubMed

Bercovici, David; Schubert, Gerald; Ricard, Yanick

2015-02-03

A simple model for necking and detachment of subducting slabs is developed to include the coupling between grain-sensitive rheology and grain-size evolution with damage. Necking is triggered by thickened buoyant crust entrained into a subduction zone, in which case grain damage accelerates necking and allows for relatively rapid slab detachment, i.e., within 1 My, depending on the size of the crustal plug. Thick continental crustal plugs can cause rapid necking while smaller plugs characteristic of ocean plateaux cause slower necking; oceanic lithosphere with normal or slightly thickened crust subducts without necking. The model potentially explains how large plateaux or continental crust drawn into subduction zones can cause slab loss and rapid changes in plate motion and/or induce abrupt continental rebound.

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.

Brittle and ductile friction modeling of triggered tremor in Guerrero, Mexico

NASA Astrophysics Data System (ADS)

Zhang, Y.; Daub, E. G.; Wu, C.

2017-12-01

Low frequency earthquakes (LFEs), which make up the highest amplitude portions of non-volcanic tremor, are mostly found along subduction zones at a depth of 30-40km which is typically within the brittle-ductile transition zone. Previous studies in Guerrero, Mexico demonstrated a relationship between the bursts of LFEs and the contact states of fault interfaces, and LFEs that triggered by different mechanisms were observed along different parts of the subduction zone. To better understand the physics of fault interfaces at depth, especially the influence of contact states of these asperities, we use a brittle-ductile friction model to simulate the occurrence of LFE families from a model of frictional failure and slip. This model takes the stress state, slip rate, perturbation force, fault area, and brittle-ductile frictional contact characteristics and simulates the times and amplitudes of LFE occurrence for a single family. We examine both spontaneous and triggered tremor occurrence by including stresses due to external seismic waves, such as the 2010 Maule Earthquake, which triggered tremor and slow slip on the Guerrero section of the subduction zone. By comparing our model output with detailed observations of LFE occurrence, we can determine valuable constraints on the frictional properties of subduction zones at depth.

Subduction of the Pacific Plate Beneath the Kamchatka: Volcanism and Tectonic Earthquakes

NASA Astrophysics Data System (ADS)

Gordeev, E. I.

2008-12-01

The results of studying subduction process of the Pacific plate beneath the Kamchatka and related processes are described. The focal mechanism solutions estimated from Centroid Moment Tensor (CMT) catalog and sequence of the largest earthquakes occurred in Kamchatka were used to asses velocity of subducted slab. The boundary of contact for subducted slab is determined at a depth of 30-70 km, and is considered as a plane at azimuth 217° and with a dip angle of 25°. The rate of subduction estimated from CMT mechanisms yields V=0.9 cm/yr for southern zone (south of Shipunsky Cape), and V=1.4 cm/yr for central zone (from Shipunsky Cape to Kronotsky Cape). The largest coupled consistent earthquakes recorded from 1737 were used for analysis. The results show, that for the southern area V=6.6 - 7.1 cm yr (two couples), and for the central part V=6.6 cm yr. The estimated value of velocity for the creep part of subducted slab is about 5 to 15 per cent of the bulk velocity. The Pacific plate subducts at a rate of 8 cm yr. Series of GPS observations conducted from 1997 up to 2007 were used to estimate the rate at which Kamchatka is deformed under the effect of the subducted slab (along-slab direction). The average values of rate and velocity variations versus the average rate were estimated response to permanent GPS station PETR. It was shown that the motion at BKI (Bering) regardless KlU (Klyuchi) is uneven: variations of velocity reach up to 30 per cent (at average running window of 1 year). There are about 28 active volcanoes in Kamchatka that provide intensive volcanic activity in this region. The volcanoes produce about 16-17% of magmatic rocks erupted by all volcanoes in the Earth. Over the past 5 years, eruptions of Sheveluch, Klyuchevskoy, Bezymianny, Karymsky, and Mutnovsky volcanoes have occurred. Although many of these volcanoes are in sparsely populated areas, they lie adjacent to the heavily North Pacific air routes between North America, Europe and Asia. The Institute of Volcanology and Seismology (IVS) of the Russian Academy of Sciences (RAS) and Kamchatka Branch of Geophysical Survey (KB GS) of RAS monitor and study Kamchatka's hazardous volcanoes, to forecast and record eruptive activity, and implement public safety measures. To meet its mission, the IVS and KB GS maintains a volcano monitoring network to detect signs of volcanic unrest; conducts basic geological, geophysical, and geochemical investigations of Kamchakan volcanoes; and provides accurate and timely warnings of imminent activity to local, state and federal agencies.

Episodic vs. Continuous Accretion in the Franciscan Accretionary Prism and Direct Plate Motion Controls vs. More Local Tectonic Controls on Prism Evolution

NASA Astrophysics Data System (ADS)

Dumitru, T. A.; Ernst, W. G.; Wakabayashi, J.

2011-12-01

Subduction at the Franciscan trench began ≈170-165 Ma and continues today off Oregon-Washington. Plate motion reconstructions, high-P metamorphic rocks, and the arc magmatic record suggest that convergence and thus subduction were continuous throughout this period, although data for 170 to 120 Ma are less definitive. About 25% of modern subduction zones are actively building an accretionary prism, whereas 75% are nonaccretionary, in which subduction erosion is gradually removing the prism and/or forearc basement. These contrasting behaviors in modern subduction zones suggest that the Franciscan probably fluctuated between accretionary and nonaccretionary modes at various times and places during its 170 million year lifespan. Accumulating geochronologic data are beginning to clarify certain accretionary vs. nonaccretionary intervals. (1) The oldest Franciscan rocks are high-P mafic blocks probably metamorphosed in a subophiolitic sole during initiation of subduction. They yield garnet Lu-Hf and hornblende Ar/Ar ages from ≈169 to 147 Ma. Their combined volume is extremely small and much of the Franciscan was probably in an essentially nonaccretionary mode during this period. (2) The South Fork Mountain Schist forms the structural top of the preserved wedge in northern California and thus was apparently the first genuinely large sedimentary body to accrete. This occurred at ≈123 Ma (Ar/Ar ages), suggesting major accretion was delayed a full ≈45 million years after the initiation of subduction. The underlying Valentine Spring Fm. accreted soon thereafter. This shift into an accretionary mode was nearly synchronous with the end of the Early Cretaceous magmatic lull and the beginning of the prolonged Cretaceous intensification of magmatism in the Sierra Nevada arc. (3) The Yolla Bolly terrane has generally been assigned a latest Jurassic to earliest Cretaceous age. Detrital zircon data confirm that some latest Jurassic sandstones are present, but they may be blocks in olistotromes and the bulk of the terrane may be mid-Cretaceous trench sediments. (4) New data from the Central mélange belt are pending. (5) Detrital zircon ages suggest much of the voluminous Coastal belt was deposited in a short, rapid surge in the Middle Eocene, coincident with major extension, core complex development, volcanism, and erosion in sediment source areas in Idaho-Montana. Rapid Tyee Fm deposition in coastal Oregon occurred at virtually the same time from the same sources. (6) Exposed post-Eocene Franciscan rocks are rare. It is tempting to ascribe subduction zone tectonic events directly to changes in relative motions between the subducting and overriding lithospheric plates. However, in modern subduction zones, varying sediment supply to the trench appears to be a more important control on accretionary prism evolution and this seems to be the case in the Franciscan as well. Franciscan accretion was apparently influenced primarily by complex continental interior tectonics controlling sediment supply from the North American Cordillera (which may in part reflect plate motion changes), rather than directly by changes in the motions of tectonic plates.

Forearc collapse, plate flexure, and seismicity within the downgoing plate along the Sunda Arc west of Sumatra

NASA Astrophysics Data System (ADS)

Craig, Timothy J.; Copley, Alex

2018-02-01

Deformation within the downgoing oceanic lithosphere seawards of subduction zones is typically characterised by regimes of shallow extension and deeper compression, due to the bending of the oceanic plate as it dips into the subduction zone. However, offshore Sumatra there are shallow compressional earthquakes within the downgoing oceanic plate outboard of the region of high slip in the 2004 Aceh-Andaman earthquake, occurring at the same depth as extensional faulting further seaward from the trench. A clear separation is seen in the location of intraplate earthquakes, with extensional earthquakes occurring further seawards than compressional earthquakes at the same depth within the plate. The adjacent section of the forearc prism west of Aceh is also anomalous in its morphology, characterised by a wide prism with a steep bathymetric front and broad, gradually-sloping top. This shape is in contrast to the narrower and more smoothly-sloping prism to the south, and along other subduction zones. The anomalous near-trench intraplate earthquakes and prism morphology are likely to be the result of the geologically-rapid gravitational collapse of the forearc, which leads to induced bending within the subducting plate, and the distinctive plateau-like morphology of the forearc. Such collapse of the forearc could be caused by changes through time of the material properties of the forearc rocks, or of the thickness of the sediments entering the subduction zone.

Possible emplacement of crustal rocks into the forearc mantle of the Cascadia Subduction Zone

USGS Publications Warehouse

Calvert, A.J.; Fisher, M.A.; Ramachandran, K.; Trehu, A.M.

2003-01-01

Seismic reflection profiles shot across the Cascadia forearc show that a 5-15 km thick band of reflections, previously interpreted as a lower crustal shear zone above the subducting Juan de Fuca plate, extends into the upper mantle of the North American plate, reaching depths of at least 50 km. In the extreme western corner of the mantle wedge, these reflectors occur in rocks with P wave velocities of 6750-7000 ms-1. Elsewhere, the forearc mantle, which is probably partially serpentinized, exhibits velocities of approximately 7500 ms-1. The rocks with velocities of 6750-7000 ms-1 are anomalous with respect to the surrounding mantle, and may represent either: (1) locally high mantle serpentinization, (2) oceanic crust trapped by backstepping of the subduction zone, or (3) rocks from the lower continental crust that have been transported into the uppermost mantle by subduction erosion. The association of subparallel seismic reflectors with these anomalously low velocities favours the tectonic emplacement of crustal rocks. Copyright 2003 by the American Geophysical Union.

Water and the oxidation state of subduction zone magmas.

PubMed

Kelley, Katherine A; Cottrell, Elizabeth

2009-07-31

Mantle oxygen fugacity exerts a primary control on mass exchange between Earth's surface and interior at subduction zones, but the major factors controlling mantle oxygen fugacity (such as volatiles and phase assemblages) and how tectonic cycles drive its secular evolution are still debated. We present integrated measurements of redox-sensitive ratios of oxidized iron to total iron (Fe3+/SigmaFe), determined with Fe K-edge micro-x-ray absorption near-edge structure spectroscopy, and pre-eruptive magmatic H2O contents of a global sampling of primitive undegassed basaltic glasses and melt inclusions covering a range of plate tectonic settings. Magmatic Fe3+/SigmaFe ratios increase toward subduction zones (at ridges, 0.13 to 0.17; at back arcs, 0.15 to 0.19; and at arcs, 0.18 to 0.32) and correlate linearly with H2O content and element tracers of slab-derived fluids. These observations indicate a direct link between mass transfer from the subducted plate and oxidation of the mantle wedge.

Triggered Slow Slip and Afterslip on the Southern Hikurangi Subduction Zone Following the Kaikōura Earthquake

NASA Astrophysics Data System (ADS)

Wallace, Laura M.; Hreinsdóttir, Sigrún; Ellis, Susan; Hamling, Ian; D'Anastasio, Elisabetta; Denys, Paul

2018-05-01

The 2016 MW7.8 Kaikōura earthquake ruptured a complex sequence of strike-slip and reverse faults in New Zealand's northeastern South Island. In the months following the earthquake, time-dependent inversions of Global Positioning System and interferometric synthetic aperture radar data reveal up to 0.5 m of afterslip on the subduction interface beneath the northern South Island underlying the crustal faults that ruptured in the earthquake. This is clear evidence that the far southern end of the Hikurangi subduction zone accommodates plate motion. The MW7.8 earthquake also triggered widespread slow slip over much of the subduction zone beneath the North Island. The triggered slow slip included immediate triggering of shallow (<15 km), short (2-3 weeks) slow slip events along much of the east coast, and deep (>30 km), long-term (>1 year) slow slip beneath the southern North Island. The southern Hikurangi slow slip was likely triggered by large (0.5-1.0 MPa) static Coulomb stress increases.

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.

Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones

USGS Publications Warehouse

Brown, Justin R.; Beroza, Gregory C.; Ide, Satoshi; Ohta, Kazuaki; Shelly, David R.; Schwartz, Susan Y.; Rabbel, Wolfgang; Thorwart, M.; Kao, Honn

2009-01-01

Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low-frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low-frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity.

Beginning the Modern Regime of Subduction Tectonics in Neoproterozoic time: Inferences from Ophiolites of the Arabian-Nubian Shield

NASA Astrophysics Data System (ADS)

Stern, R.

2003-04-01

It is now clear that the motive force for plate tectonics is provided by the sinking of dense lithosphere in subduction zones. Correspondingly, the modern tectonic regime is more aptly called ``subduction tectonics" than plate tectonics, which only describes the way Earth's thermal boundary layer adjusts to subduction. The absence of subduction tectonics on Mars and Venus implies that special circumstances are required for subduction to occur on a silicate planet. This begs the question: When did Earth's oceanic lithosphere cool sufficiently for subduction to began? This must be inferred from indirect lines of evidence; the focus here is on the temporal distribution of ophiolites. Well-preserved ophiolites with ``supra-subduction zone" (SSZ) affinities are increasingly regarded as forming when subduction initiates as a result of lithospheric collapse (± a nudge to get it started), and the formation of ophiolitic lithosphere in evolving forearcs favors their emplacement and preservation. The question now is what percentage of ophiolites with ``supra-subduction zone" (SSZ) chemical signatures formed in forearcs during subduction initiation events? Most of the large, well-preserved ophiolites (e.g., Oman, Cyprus, California, Newfoundland) may have this origin. If so, the distribution in space and time of such ophiolites can be used to identify ``subduction initiation" events, which are important events in the evolution of plate tectonics. Such events first occurred at the end of the Archean (˜2.5Ga) and again in the Paleoproterozoic (˜1.8 Ga), but ophiolites become uncommon after this. Well-preserved ophiolites become abundant in Neoproterozoic time, at about 800±50 Ma. Ophiolites of this age are common and well-preserved in the Arabian-Nubian Shield (ANS) of Egypt, Sudan, Ethiopia, Eritrea, and Saudi Arabia. ANS ophiolites mostly contain spinels with high Cr#, indicating SSZ affinities. Limited trace element data on pillowed lavas supports this interpretation. Boninites are unusual melts of harzburgite that result from asthenospheric upwelling interactng with slab-derived water. This environment is only common during subduction initiation events. Boninites associated with ophiolites have been reported from Egypt, Ethiopia and Eritrea, but most of the geochemical studies of ANS ophiolitic basalts are based on studies that are a decade or more old. The abundance of ANS ophiolites implies an episode of subduction initiation occurred in Neoproterozoic time.

GPS-derived coupling estimates for the Central America subduction zone and volcanic arc faults: El Salvador, Honduras and Nicaragua

NASA Astrophysics Data System (ADS)

Correa-Mora, F.; DeMets, C.; Alvarado, D.; Turner, H. L.; Mattioli, G.; Hernandez, D.; Pullinger, C.; Rodriguez, M.; Tenorio, C.

2009-12-01

We invert GPS velocities from 32 sites in El Salvador, Honduras and Nicaragua to estimate the rate of long-term forearc motion and distributions of interseismic coupling across the Middle America subduction zone offshore from these countries and faults in the Salvadoran and Nicaraguan volcanic arcs. A 3-D finite element model is used to approximate the geometries of the subduction interface and strike-slip faults in the volcanic arc and determine the elastic response to coupling across these faults. The GPS velocities are best fit by a model in which the forearc moves 14-16 mmyr-1 and has coupling of 85-100 per cent across faults in the volcanic arc, in agreement with the high level of historic and recent earthquake activity in the volcanic arc. Our velocity inversion indicates that coupling across the potentially seismogenic areas of the subduction interface is remarkably weak, averaging no more than 3 per cent of the plate convergence rate and with only two poorly resolved patches where coupling might be higher along the 550-km-long segment we modelled. Our geodetic evidence for weak subduction coupling disagrees with a seismically derived coupling estimate of 60 +/- 10 per cent from a published analysis of earthquake damage back to 1690, but agrees with three other seismologic studies that infer weak subduction coupling from 20th century earthquakes. Most large historical earthquakes offshore from El Salvador and western Nicaragua may therefore have been intraslab normal faulting events similar to the Mw 7.3 1982 and Mw 7.7 2001 earthquakes offshore from El Salvador. Alternatively, the degree of coupling might vary with time. The evidence for weak coupling indirectly supports a recently published hypothesis that much of the Middle American forearc is escaping to the west or northwest away from the Cocos Ridge collision zone in Costa Rica. Such a hypothesis is particularly attractive for El Salvador, where there is little or no convergence obliquity to drive the observed trench-parallel forearc motion.

3D velocity imaging of Hikurangi subduction beneath the Wellington region, New Zealand

NASA Astrophysics Data System (ADS)

Wech, A.; Henrys, S. A.; Sutherland, R.; Seward, A. M.; Stern, T. A.; Sato, H.; Okaya, D. A.; Bassett, D.

2011-12-01

We present first results from the Seismic Array HiKurangi Experiment (SAHKE). This joint project involving institutions from New Zealand, Japan and the USA aims to investigate the subduction zone fault characteristics beneath the southernmost part of New Zealand's North Island. Situated above where the Pacific Plate is subducting beneath the Australian plate at a rate of ~42 mm/yr, the Wellington region provides a unique opportunity to investigate the frictional properties, geometry, and seismic potential of a shallow, locked megathrust fault. Here the coupled plate interface is 20-30 km deep beneath land and can be sampled with onshore-offshore data from 3 sides. An added interest to this project is that the elevated, oceanic, Hikurangi plateau has entered the subduction zone, east of Wellington, but it is still unclear how far the plateau has advanced westward into the subduction zone. SAHKE combines active and passive source data comprising 4 distinct data sets. 1) A dense temporary array of 50 seismometers with ~7 km spacing augmented 25 regional network instruments to record 49 local and 45 teleseismic earthquakes over a four month period. 2) These stations also recorded 69,000 offshore airgun shots from 17 lines crisscrossing two sides of the array. 3) An additional coast-to-coast transect of 50 stations cutting through the temporary array recorded ~2000 offshore shots on either side. 4) 1000 stations with 100m spacing along that same transect separately recorded 12 in-line, 500 kg onshore dynamite explosions. First inspection of the recent onshore shot gathers show excellent signal to noise and a band of three strong reflectors between 20 and 38 km at the western end of the profile. We combine shot and earthquake recordings to simultaneously invert ~750,000 first arrivals for velocity structure and hypocenters in the densely sampled volume. First results from 3D, Vp tomography and relocated hypocenters agree with previous studies and suggest the later weak signals are reflections from the top of the Pacific plate. Our improved velocity model provides a high-resolution geometry of the subducting plate to support interpretation of other phases identified in SAHKE shot gathers.

A real-time cabled observatory on the Cascadia subduction zone

NASA Astrophysics Data System (ADS)

Vidale, J. E.; Delaney, J. R.; Toomey, D. R.; Bodin, P.; Roland, E. C.; Wilcock, W. S. D.; Houston, H.; Schmidt, D. A.; Allen, R. M.

2015-12-01

Subduction zones are replete with mystery and rife with hazard. Along most of the Pacific Northwest margin, the traditional methods of monitoring offshore geophysical activity use onshore sensors or involve conducting infrequent oceanographic expeditions. This results in a limited capacity for detecting and monitoring subduction processes offshore. We propose that the next step in geophysical observations of Cascadia should include real-time data delivered by a seafloor cable with seismic, geodetic, and pressure-sensing instruments. Along the Cascadia subduction zone, we need to monitor deformation, earthquakes, and fluid fluxes on short time scales. High-quality long-term time series are needed to establish baseline observations and evaluate secular changes in the subduction environment. Currently we lack a basic knowledge of the plate convergence rate, direction and its variations along strike and of how convergence is accommodated across the plate boundary. We also would like to seek cycles of microseismicity, how far locking extends up-dip, and the transient processes (i.e., fluid pulsing, tremor, and slow slip) that occur near the trench. For reducing risk to society, real-time monitoring has great benefit for immediate and accurate assessment through earthquake early warning systems. Specifically, the improvement to early warning would be in assessing the location, geometry, and progression of ongoing faulting and obtaining an accurate tsunami warning, as well as simply speeding up the early warning. It would also be valuable to detect strain transients and map the locked portion of the megathrust, and detect changes in locking over the earthquake cycle. Development of the US portion of a real-time cabled seismic and geodetic observatory should build upon the Ocean Observatories Initiative's cabled array, which was recently completed and is currently delivering continuous seismic and pressure data from the seafloor. Its implementation would require substantial initial and ongoing investments from federal and state governments, private partners and the academic community but would constitute a critical resource in mitigating the hazard both through improved earthquake and tsunami warning and an enhanced scientific understanding of subduction processes in Cascadia.

An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle

NASA Astrophysics Data System (ADS)

Ribeiro, Julia M.; Lee, Cin-Ty A.

2017-12-01

The depth of slab dehydration is thought to be controlled by the thermal state of the downgoing slab: cold slabs are thought to mostly dehydrate beneath the arc front while warmer slabs should mostly dehydrate beneath the fore-arc. Cold subduction zone lavas are thus predicted to have interacted with greater extent of water-rich fluids released from the downgoing slab, and should thus display higher water content and be elevated in slab-fluid proxies (i.e., high Ba/Th, H2O/Ce, Rb/Th, etc.) compared to hot subduction zone lavas. Arc lavas, however, display similar slab-fluid signatures regardless of the thermal state of the slab, suggesting more complexity to volatile cycling in subduction zones. Here, we explore whether the serpentinized fore-arc mantle may be an important fluid reservoir in subduction zones and whether it can contribute to arc magma generation by being dragged down with the slab. Using simple mass balance and fluid dynamics calculations, we show that the dragged-down fore-arc mantle could provide enough water (∼7-78% of the total water injected at the trenches) to account for the water outfluxes released beneath the volcanic arc. Hence, we propose that the water captured by arc magmas may not all derive directly from the slab, but a significant component may be indirectly slab-derived via dehydration of dragged-down fore-arc serpentinites. Fore-arc serpentinite dehydration, if universal, could be a process that explains the similar geochemical fingerprint (i.e., in slab fluid proxies) of arc magmas.

Topographic form of the Coast Ranges of the Cascadia Margin in relation ot coastal uplift rates and plate subduction

NASA Technical Reports Server (NTRS)

Kelsey, Harvey M.; Engebretson, David C.; Mitchell, Clifton E.; Ticknor, Robert L.

1994-01-01

The Coast Ranges of the Cascadia margin are overriding the subducted Juan de Fuca/Gorda plate. We investigate the extent to which the latitudinal change in attributes related to the subduction process. These attributes include the varibale age of the subducted slab that underlies the Coast Ranges and average vertical crustal velocities of the western margin of the Coast Rnages for two markedly different time periods, the last 45 years and the last 100 kyr. These vertical crustal velocities are computed from the resurveying of highway bech marks and from the present elevation of shore platforms that have been uplifted in the late Quaternary, respectively. Topogarphy of the Coast Ranges is in part a function of the age and bouyancy of the underlying subducted plate. This is evident in the fact that the two highest topographic elements of the Coast Rnages, the Klamath Mountains and the Olympic Mountains, are underlain by youngest subducted oceanic crust. The subducted Blanco Fracture Zone in southernmost Oregon is responsible for an age discontinuity of subducted crust under the Klamath Mountains. The norhtern terminus of hte topographically higher Klamaths is offset to the north relative to the position of the underlying Blanco Fracture Zone, teh offset being in the direction of migration of the farcture zone, as dictated by relative plate motions. Vertical crustal velocities at the coast, derived from becnh mark surveys, are as much as an order of magnitude greater than vertical crustal velocities derived from uplifted shore platforms. This uplift rate discrepancy indicates that strain is accumulating on the plate margin, to be released during the next interplate earthquake. In a latitudinal sense, average Coast Rnage topography is relatively high where bench mark-derived, short-term vertical crustal velocities are highest. Becuase the shore platform vertical crustal velocities reflect longer-term, premanent uplift, we infer that a small percentage of the interseismic strain that accumulates as rapid short-term uplift is not recovered by subduction earthquakes but rather contributes to rock uplift of the Coast Ranges. The conjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduction zone subject to greater interseismic uplift rates are at approximately the same latitudes as those segments of the Coast Ranges that have higher magnitudes of rock uplift over the long term.

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.

2013-05-01

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 (<6.5 km) low-velocity layer (shear wave velocity of ~3 km/s), which is ~20-30% slower than normal oceanic crustal velocities, between the subducted slab and the overriding North American plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio (Vp/Vs) 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 11-12 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. 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 25-30 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.

Geochemistry of serpentinites in subduction zones: A review

NASA Astrophysics Data System (ADS)

Deschamps, Fabien; Godard, Marguerite; Guillot, Stéphane; Hattori, Kéiko

2013-04-01

Over the last decades, numerous studies have emphasized the role of serpentinites in the subduction zones geodynamics. Their presence and effective role in this environment is acknowledged notably by geophysical, geochemical and field observations of (paleo-) subduction zones. In this context, with the increasing amount of studies concerning serpentinites in subduction environments, a huge geochemical database was created. Here, we present a review of the geochemistry of serpentinites, based on the compilation of ~ 900 geochemical analyses of abyssal, mantle wedge and subducted serpentinites. The aim was to better understand the geochemical evolution of these rocks during their subduction history as well as their impact in the global geochemical cycle. When studying serpentinites, it is often a challenge to determine the nature of the protolith and their geological history before serpentinisation. The present-day (increasing) geochemical database for serpentinites indicates little to no mobility of incompatible elements at the scale of the hand-sample in most serpentinized peridotites. Thus, Rare Earth Elements (REE) distribution can be used to identify the initial protolith for abyssal and mantle wedge serpentinites, as well as magmatic processes such as melt/rock interactions taking place before serpentinisation. In the case of subducted serpentinites, the interpretation of trace element data is more difficult due to secondary enrichments independent of the nature of the protolith, notably in (L)REE. We propose that these enrichments reflect complex interactions probably not related to serpentinisation itself, but mostly to fluid/rock or sediment/rock interactions within the subduction channel, as well as intrinsic feature of the mantle protolith which could derive from the continental lithosphere exhumed at the ocean-continent transition. Additionally, during the last ten years, numerous studies have been carried out, notably using in situ approaches, to better constrain the geochemical budget of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, U, Th, Sr) stored in serpentinites and serpentine phases. These elements are good markers of the fluid/rock interactions taking place during serpentinisation. Today, the control of serpentinites on the behaviour of these elements, from their incorporation to their gradually release during subduction, is better understood. Serpentinites must be considered as a component of the FME budget in subduction zones and their role, notably on arc magmas composition, is undoubtedly underestimated presently in the global geochemical cycle.

Seismicity, shear failure and modes of deformation in deep subduction zones

NASA Technical Reports Server (NTRS)

Lundgren, Paul R.; Giardini, Domenico

1992-01-01

The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

Modeling Diverse Pathways to Age Progressive Volcanism in Subduction Zones.

NASA Astrophysics Data System (ADS)

Kincaid, C. R.; Szwaja, S.; Sylvia, R. T.; Druken, K. A.

2015-12-01

One of the best, and most challenging clues to unraveling mantle circulation patterns in subduction zones comes in the form of age progressive volcanic and geochemical trends. Hard fought geological data from many subduction zones, like Tonga-Lau, the Cascades and Costa-Rica/Nicaragua, reveal striking temporal patterns used in defining mantle flow directions and rates. We summarize results from laboratory subduction models showing a range in circulation and thermal-chemical transport processes. These interaction styles are capable of producing such trends, often reflecting apparent instead of actual mantle velocities. Lab experiments use a glucose working fluid to represent Earth's upper mantle and kinematically driven plates to produce a range in slab sinking and related wedge transport patterns. Kinematic forcing assumes most of the super-adiabatic temperature gradient available to drive major downwellings is in the tabular slabs. Moreover, sinking styles for fully dynamic subduction depend on many complicating factors that are only poorly understood and which can vary widely even for repeated parameter combinations. Kinematic models have the benefit of precise, repeatable control of slab motions and wedge flow responses. Results generated with these techniques show the evolution of near-surface thermal-chemical-rheological heterogeneities leads to age progressive surface expressions in a variety of ways. One set of experiments shows that rollback and back-arc extension combine to produce distinct modes of linear, age progressive melt delivery to the surface through a) erosion of the rheological boundary layer beneath the overriding plate, and deformation and redistribution of both b) mantle residuum produced from decompression melting and c) formerly active, buoyant plumes. Additional experiments consider buoyant diapirs rising in a wedge under the influence of rollback, back-arc spreading and slab-gaps. Strongly deflected diapirs, experiencing variable rise rates, also commonly surface as linear, age progressive tracks. Applying these results to systems like the Cascades and Tonga-Lau suggest there are multiple ways to produce timing trends due both to linear flows and waves of heterogeneity obliquely impacting surface plates.

The Impact of Mass Movement and Fluid Flow during Ridge Subduction inferred from Physical Properties and Zeolite Assemblage in the Upper Plate Slope of the Costa Rica Subduction Zone

NASA Astrophysics Data System (ADS)

Hamahashi, M.; Screaton, E.; Tanikawa, W.; Hashimoto, Y.; Martin, K. M.; Saito, S.; Kimura, G.

2015-12-01

The Costa Rica subduction zone offshore Osa Peninsula is known as an erosive margin with active seismicity and the subduction of the Cocos Ridge. One of the major unknowns in this margin is the nature of the unconformity at the base of the slope sediments in the upper plate and the high velocity materials below. To investigate the geologic processes across the unconformity, we examined the consolidation state and mineral assemblages of the sediments at the mid-slope Site 1380 drilled during IODP Expedition 344 by conducting microstructural observation, particle size analysis, X-ray fluorescence/diffraction analysis and resistivity measurement. The general compaction trend is controlled primarily by grain-size sorting and the physical property transition is likely caused by massive sediment removal under normal fault regime, thickness of which range between ~600-850 m determined from the composite porosity-depth curve. Across the unconformity between the late Pliocene~late Pleistocene silty clay (Unit 1) and late Pliocene~early Pleistocene clayey siltstone (Unit 2), the mineral/element components of the sediments is marked by the transitions in zeolite compositions; Unit 1 consists of laumontite and heulandite, whereas below the unconformity, Unit 2 sediments contain analcime, laumontite, and heulandite, but laumontite become less abundant at lower depth. The experienced temperature of the sediments in Unit 2 is estimated to have reached between ~86 and 122℃ as inferred from analcime burial diagenesis. This may correspond with the greater depth range prior to mass movement and normal faulting. The initial analcime burial diagenetic zone was likely cut off by the sediment removal across the unconformity, and later overprinted by high temperature fluid along the boundary forming laumontite and heulandite in the vicinity. These results illustrate that ridge subduction has substantial potential to cause mass movement, an extensional stress regime, and fluid flow from depth.

Variations in seismic velocity distribution along the Ryukyu (Nansei-Shoto) Trench subduction zone at the northwestern end of the Philippine Sea plate

NASA Astrophysics Data System (ADS)

Nishizawa, Azusa; Kaneda, Kentaro; Oikawa, Mitsuhiro; Horiuchi, Daishi; Fujioka, Yukari; Okada, Chiaki

2017-06-01

The Ryukyu (Nansei-Shoto) island arc-trench system, southwest of Japan, is formed by the subduction of the Philippine Sea (PHS) plate. Among the subduction zones surrounding the Japan Islands, the Ryukyu arc-trench system is unique in that its backarc basin, the Okinawa Trough, is the area with current extensively active rifting. The length of the trench is around 1400 km, and the geological and geophysical characteristics vary significantly along the trench axis. We conducted multichannel seismic (MCS) reflection and wide-angle seismic surveys to elucidate the along-arc variation in seismic structures from the island arc to the trench regions, shooting seven seismic lines across the arc-trench system and two along-arc lines in the island arc and the forearc areas. The obtained P-wave velocity models of the Ryukyu arc crust were found to be heterogeneous (depending on the seismic lines), but they basically consist of upper, middle, and lower crusts, indicating a typical island arc structure. Beneath the bathymetric depressions cutting the island arc—for example, the Kerama Gap and the Miyako Saddle—the MCS record shows many across-arc normal faults, which indicates the presence of an extensional regime along the island arc. In the areas from the forearc to the trench, the subduction of the characteristic seafloor features on the PHS plate affects seismic structures; the subducted bathymetric high of the Amami Plateau is detected in the northern trench: the Luzon-Okinawa fracture zone beneath the middle and southern trenches. There are low-velocity (< 4.5 km/s) wedges along the forearc areas, except for off Miyako-jima Island. The characteristic high gravity anomaly at the forearc off Miyako-jima Island is caused not by a bathymetric high of a large-scale accretionary wedge but by shallower materials with a high P-wave velocity of 4.5 km/s.[Figure not available: see fulltext.

Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise.

PubMed

Chaves, Esteban J; Schwartz, Susan Y

2016-01-01

In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise-based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [M w (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure

NASA Astrophysics Data System (ADS)

Pilet, S.; Abe, N.; Rochat, L.; Kaczmarek, M.-A.; Hirano, N.; Machida, S.; Buchs, D. M.; Baumgartner, P. O.; Müntener, O.

2016-12-01

Oceanic lithospheric mantle is generally interpreted as depleted mantle residue after mid-ocean ridge basalt extraction. Several models have suggested that metasomatic processes can refertilize portions of the lithospheric mantle before subduction. Here, we report mantle xenocrysts and xenoliths in petit-spot lavas that provide direct evidence that the lower oceanic lithosphere is affected by metasomatic processes. We find a chemical similarity between clinopyroxene observed in petit-spot mantle xenoliths and clinopyroxene from melt-metasomatized garnet or spinel peridotites, which are sampled by kimberlites and intracontinental basalts respectively. We suggest that extensional stresses in oceanic lithosphere, such as plate bending in front of subduction zones, allow low-degree melts from the seismic low-velocity zone to percolate, interact and weaken the oceanic lithospheric mantle. Thus, metasomatism is not limited to mantle upwelling zones such as mid-ocean ridges or mantle plumes, but could be initiated by tectonic processes. Since plate flexure is a global mechanism in subduction zones, a significant portion of oceanic lithospheric mantle is likely to be metasomatized. Recycling of metasomatic domains into the convecting mantle is fundamental to understanding the generation of small-scale mantle isotopic and volatile heterogeneities sampled by oceanic island and mid-ocean ridge basalts.

Gravity modelling of the Hellenic subduction zone — a regional study

NASA Astrophysics Data System (ADS)

Casten, U.; Snopek, K.

2006-05-01

The Hellenic subduction zone is clearly expressed in the arc-shaped distribution of earthquake epicenters and gravity anomalies, which connect the Peloponnesos with Crete and Anatolia. In this region, oceanic crust of the African plate collides northward with continental crust of the Aegean microplate, which itself is pushed apart to the south-west by the Anatolian plate and, at the same time, is characterised by crustal extension. The result is an overall collision rate of up to 4 cm/year and a retreating subduction process. Recent passive and active seismic studies on and around Crete gave first, but not in all details consistent, structural results useful for supporting gravity modelling. This was undertaken with the aim of presenting the first 3D density structure of the entire subduction zone. Gravity interpretation was based on a Bouguer map, newly compiled using data from land, marine and satellite sources. The anomalies range from + 170 mGal (Cretan Sea) to - 10 mGal (Mediterranean Ridge). 3D gravity modelling was done applying the modelling software IGMAS. The computed Bouguer map fits the low frequency part of the observed one, which is controlled by variations in Moho depth (less than 20 km below the Cretan Sea and extending 30 km below Crete) and the extremely thick sedimentary cover (partly up to 18 km) of the Mediterranean Ridge. The southernmost edge of the Eurasian plate, with its more triangular-shaped backstop area, was traced south off Crete. Only 50 to 100 km further to the south, the edge of the African continent was traced as well. In between these boundaries there is African oceanic crust, which has a clear arc-shaped detachment line situated at the Eurasian continental edge. The subduction arc is open towards the north, its slab separates hotter mantle material (lower density) below the updoming Moho of the Cretan Sea from colder one (higher density) in the south. Subjacent to the upper continental crust of Crete is a thickened layer of lower crust followed by the subducted oceanic crust with some mantle material as intermediate layer. The depth of the oceanic Moho below Crete is 50 km. The presence and structure of subducted or underplated sediments remains uncertain.

3-D Seismic Tomographic Inversion to Image Segmentation of the Sumatra Subduction Zone near Simeulue Island

NASA Astrophysics Data System (ADS)

Tang, G.; Barton, P. J.; Dean, S. M.; Vermeesch, P. M.; Jusuf, M. D.; Henstock, T.; Djajadihardja, Y.; McNeill, L. C.; Permana, H.

2009-12-01

Oceanic subduction along the Sunda trench to the west of Sumatra (Indonesia) shows significant along-strike variations in seismicity. For example, the great M9.3 earthquake in 2004 occurred in the forearc basin north of Simeulue island, rupturing the fault predominantly towards the northwest, while the 2005 Nias earthquake nucleated near the Banyak islands, rupturing towards the southeast (Ammon et al., 2005; Ishii et al. 2005). The gap between these two active areas indicates segmentation of the subduction zone, but the cause of the segmentation remains enigmatic. To investigate the apparent barriers to rupture, two 3-D refraction surveys were conducted in 2008, one, the topic of this study, around Simeulue island and the other to the southeast of Nias island. Seismic data were collected using ocean bottom seismometers and a 12-airgun tuned array with a total capacity of 5420 cu. in., together with high resolution bathymetry data and gravity data. 174,515 traveltimes of first refracted arrivals were picked for the study area, and 128,138 of them were inverted for a model of minimum structure required by the data using the ‘FAST’ method (Zelt et.al, 1998). Resolution tests show that the model is resolvable mostly on a scale of >40 km horizontally. The final velocity model produced has two distinct features: i. the subducted oceanic plates (represented by 6 km/s contours) seem to be discontinuous along strike; ii. the subduction dip angle appears to be steeper in the southern part of the survey area than in the north. The geometric variation in the subducted plate appears to coincide with the segment boundary approximately across the centre of Simeulue island, and may perhaps associated with the segmentation of the seismicity noted from the earthquake record. More accurate velocity models will be developed by jointly inverting traveltimes of first and later arrivals as well as normal incidence data using the tomographic inversion program JIVE-3D (Hobro et.al, 2003). Some passive earthquake data may also be available for the inversion for this area. These new results will provide insights into along-strike variations in subsurface structure and/or physical properties within the Sumatra subduction zone, which maybe related to the observed segmentation.

Fracturation Pattern in the Limestone Loyaute Islands and its Relation to the Neighbouring Vanuatu Subduction Zone (SW PAcific)

NASA Astrophysics Data System (ADS)

Bogdanov, I.; Genthon, P.; Thovert, J.; Adler, P. M.

2006-12-01

The Loyauté Islands are a series of limestone karstified islands that are currently uplifted and deformed on the elastic bulge of the Australian plate before its subduction at the Vanuatu Trench (SW Pacific). As part of the SAGE program of the New Caledonian Province des Iles, they have been extensively surveyed for geology and hydrogeology. As part of this project, a map of fracturation deduced from aerial photos, and from SPOT4 and ENVISAT satellite data has been produced and a field trip allowed to verify that the main fracture orientations were also present on the most recent terranes bordering the islands. Since their formation during the Miocene, these islands are in a tectonically stable area. Thus, they provide a unique opportunity to study their fracture distribution in relation with their recent tectonic context. We will present the results of a statistical analysis of fracture distribution both in number and in fracture length and an attempt to model the fracture orientations as resulting from the elastic deformation of the Australian lithosphere before its subduction. Three main fracture families have been determined for the three island, with very few differences if fracture number of fracture length statistic is considered. These families are N62.5, N107.5, and N152.5 for Lifou, which is the largest and central island, which are further termed as F1, F2, F3. F2 is at least 5 times more important than F1 and F3, which are 45° apart on both sides of F2. The orientation of families F1-F3 are N 65, N110, and N155 in Maré, which located less than 100 km apart from the subduction zone, and N60, N105, and N150 in Ouvéa , which is the most distant island from the subduction and is only uplifted in its NorthEastern part. The main family F2 does not correspond either to the subduction zone orientation (N150) nor to that of the Loyauté ridge (N135) on which the three islands are located. Thus, the fracture pattern of the three island cannot be explained by a 2-dimensional bulging of the Australian plate approaching the Vanuatu subduction zone. We will present two new analytical models for the elastic deformation of the Australian lithosphere. The first one takes into account the curvature of the subduction zone while the second one introduces a punctual force which account the first stages of a collision between the Loyalty ridge and this subduction zone. The directions of principal stresses deduced from these models are compared to the deformation recorded in the fracture netword of the three islands

Svecofennian orogeny in an evolving convergent margin setting

NASA Astrophysics Data System (ADS)

Korja, Annakaisa

2015-04-01

The dominant tectonic mode changes from extension to convergence at around 1.9 Ga in Fennoscandian. The lithological record suggests short lived subduction-related magmatic events followed by deformation and low-pressure high temperature metamorphism. At around 1.8 Ga the subduction systems seem to have stabilized implying continuous supply of oceanic lithosphere. The evolution of the convergent margin is recorded in the rock record and crustal architecture of the long lived Svecofennian orogeny (1.9-1.7 Ga). A closer look at the internal structure of the Svecofennian orogen reveals distinct regional differences. The northern and central parts of the Svecofennian orogen that have been formed during the initial accretionary phase - or compilation of the nucleus - have a thick three-layer crust and with thick mafic lower crust (10-30 km) and block-like internal architecture. Reflection profiles (FIRE1-3) image listric structures flattening on crustal scale décollement zones at the upper-middle crust and middle-upper crust boundaries. The crustal architecture together with large volumes of exposed granitoid rocks suggests spreading of the orogen and the development of an orogenic plateau west of the continental convergence boundary. The architecture is reminiscent of a large hot orogen. Within the western and southwestern part of the Svecofennian orogen (BABEL B, 1, 2, 3&4), which have been envisioned to have formed during continuous subduction phase, the crust is thinner (45-50 km) and it is hosting crustal blocks having one to two crustal layers. Layering is poorly developed in crustal blocks that are found S-SW of NE-dipping mantle reflections previously interpreted as paleo-subduction zones. Within these blocks, the crustal scale reflective structures dip NE (prowedge) or form pop-up wedges (uplifted plug) above the paleo-subduction zones. Crustal blocks with well-developed two-layer crust are located NE of the paleo-subduction zone. The architecture can be interpreted to image a series of abandoned accretion zones where the orogenic structure has developed from a young and cold orogen (BABEL 2,3&4) to a transitional (BABEL 1,6,B) one as the plate boundary is retreating during SW wards. The fast retreating rate of the subduction zone may not only have formed continental back-arc environment but may have restricted the thickening of the upper plate and the growth rate of the orogen. Altogether the architecture suggests a long-lived southwesterly retreating subduction system, with continental back-arc formation in its rear parts and well developed system of prowedge-retrowedge-uplifted plug close to a subduction conduit. Changes in the relative velocities of the upper and lower plate may have resulted in repetitive extensional and compressional phases of the orogeny as has been previously suggested for the southern part of the Svecofennian orogen.

The Relationships of Upper Plate Ridge-Trench-Trench and Ridge-Trench-Transform Triple Junction Evolution to Arc Lengthening, Subduction Zone initiation and Ophiolitic Forearc Obduction

NASA Astrophysics Data System (ADS)

Casey, J.; Dewey, J. F.

2013-12-01

The principal enigma of large obducted ophiolite slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plate-accretion, such as may be envisioned for the oceanic ridges, yet the volcanics commonly have arc affinity (Miyashiro) with boninites (high-temperature/low-pressure, high Mg and Si andesites), which are suggestive of a forearc origin. PT conditions under which boninites and metamorphic soles form and observations of modern forearc systems lead us to the conclusion that ophiolite formation is associated with overriding plate spreading centers that intersect the trench to form ridge-trench-trench of ridge-trench-tranform triple junctions. The spreading centers extend and lengthen the forearc parallel to the trench and by definition are in supra-subduction zone (SSZ) settings. Many ophiolites likewise have complexly-deformed associated mafic-ultramafic assemblages that suggest fracture zone/transform along their frontal edges, which in turn has led to models involving the nucleation of subduction zones on fracture zones or transpressional transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc boninites) or post-arc (classic Karig-style back arc basins that trench-parallel split arcs). Syn-arc boninites and forearc oceanic spreading centers that involve a stable ridge/trench/trench triple or a ridge-trench-transform triple junction, the ridge being between the two upper plates, are consistent with large slab ophiolite formation in an obduction-ready settting. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created within the forearc, the arc and fore-arc lengthen significantly, and a syn-arc ophiolite forearc complex is generated by this mechanism. The ophiolite ages along arc-strike; a distinctive diachronous MORB-like to boninitic to arc volcanic stratigraphy develops vertically in the forearc and eruption centers progressively migrate from the forearc back to the main arc massif with time. Dikes in the ophiolite are commonly highly oblique to the trench (as are back-arc magnetic anomalies in modern environments). Boninites and high-mg andesites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the instantaneously developed subducting and dehydrating slab. We review both modern subduction environments and ancient obducted ophiolite analogues that illustrate this tectonic model for subduction initiation and the creation and rapid divergent-convergent plate tectonic transitions to ophiolitic forearcs.

Shear-wave splitting observations of mantle anisotropy beneath Alaska

NASA Astrophysics Data System (ADS)

Bellesiles, A. K.; Christensen, D. H.; Entwistle, E.; Litherland, M.; Abers, G. A.; Song, X.

2009-12-01

Observations of seismic anisotropy were obtained from three different PASSCAL broadband experiments throughout Alaska, using shear-wave splitting from teleseismic SKS phases. The MOOS (Multidisciplinary Observations Of Subduction), BEAAR (Broadband Experiment Across the Alaska Range), and ARCTIC (Alaska Receiving Cross-Transects for the Inner Core) networks were used along with selected permanent broadband stations operated by AEIC (Alaska Earthquake Information Center) to produce seismic anisotropy results for the state of Alaska along a north south transect from the active subduction zone in the south, through continental Alaska, to the passive margin in the north. The BEAAR network is in-between the ARCTIC and MOOS networks above the subducting Pacific Plate and mantle wedge and shows a tight ~90 degree rotation of anisotropy above the 70km contour of the subducting plate. The southern stations in BEAAR yield anisotropy results that are subparallel to the Pacific Plate motion as it subducts under North America. These stations have an average fast direction of -45 degrees and 1.03 seconds of delay on average. The MOOS network in south central Alaska yielded similar results with an average fast direction of -30 degrees and delay times of .9 seconds. In the north portion of the BEAAR network the anisotropy is along strike of the subduction zone and has an average fast direction of 27 degrees with an average delay time of 1.4 seconds, although the delay times above the mantle wedge range from 1 to 2.5 seconds and are directly correlated to the length of ray path in the mantle wedge. This general trend NE/SW is seen in the ARCTIC stations to the north although the furthest north stations are oriented more NNE compared to those in BEAAR. The average fast direction for the ARCTIC network is 40 degrees with an average delay time of 1.05 seconds. These results show two distinct orientations of anisotropy in Alaska separated by the subducting Pacific Plate.

Subduction-driven recycling of continental margin lithosphere.

PubMed

Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

2014-11-13

Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones.

The blueschits from the Kopina Mt., West Sudetes, Poland - what do they tell us about accretion of the Variscides?

NASA Astrophysics Data System (ADS)

Majka, Jarosław; Mazur, Stanisław; Kośmińska, Karolina; Dudek, Krzysztof

2015-04-01

Blueschists are tracers of sutures, thus marking fossil subduction zones at convergent plate boundaries and providing important constraints on plate tectonic reconstructions. Their occurrences are scarce in the Variscan belt owing to a strong collisional overprint but just because of that each locality deserves particular attention. The Variscan blueschists must have formed during the early stage of the Variscan Orogeny and may represent a vestige of missing marginal basins fringing the Rheic Ocean at the onset of subduction. The studied rocks from the Kopina Mt. consist mainly of garnet, glaucophane, clinozoisite-epidote, chlorite-I, titanite, hematite and quartz. The original high-pressure assemblage is overprinted by later, lower pressure paragenesis, which comprises mostly Ca-amphiboles, chlorite-II, albite and K-feldspar. The latter occurs in polymineral inclusions in other phases together with albite and chlorite that are interpreted as phengite breakdown products. Garnet shows chemical compositional variation from Alm54Prp3Grs30Sps13 in the cores to Alm66Prp4Grs29Sps1 in the rims. The almandine zoning is bowl-shaped, whereas spessartine profiles show bell-shaped trends. The grossular and pyrope contents are generally constant throughout the grain. Rather gradual changes in the chemical zoning suggest a progressive, one-step garnet growth pattern. Glaucophane, although commonly well preserved, in some cases disintegrates to the albite-chlorite assemblage. The pressure-temperature (P-T) conditions were estimated using the phase equilibrium modelling in the NCKFMMnASHTO system using the PerpleX software. The compositional isopleths cross cut in the stability field of Grt+Gln+Ep+Chl+Pheng+Ttn+Hem+Q. P-T estimates indicate that the peak conditions occur at c. 14-17 kbar and 470-500°C, which corresponds to quite a low geothermal gradient in the range of 8-10°C/km. The P-T conditions estimated lie on a low temperature geotherm that is typical for a relatively cool subduction of the oceanic crust. Therefore, the origin of the studied rocks dates back to the time preceding accretion of the eastern Variscides and defines one of the key tectonic boundaries in the Bohemian Massif. A mechanism for syn-collisional emplacement and exhumation of the Kopina blueschists can be tentatively explained through activation of the double subduction system operating towards the east. First subduction commenced already in the Early Devonian and operated beneath an island arc located in proximity to the Saxothuringian margin, within the Rheic Ocean. After the mid-Devonian exhumation of the Central Sudetes allochthon, another subduction system was initiated along the eastern margin of the Rheic Ocean, beneath the Brunia microplate. Subducted oceanic crust of the Rheic Ocean (including the Kopina Mt. blueschists) reached peak metamorphic conditions in the Late Devonian, the event pronounced by a continental arc volcanism along the Brunian margin. Exhumation of the subducted oceanic crust was accommodated by the slab roll-back, which is inferred from the bimodal age and spatial distribution of the volcanic activity within the Brunian active margin. Shortly after the Kopina Mt. blueschists exhumation this eastern subduction system became probably inactive. In contrast, the western one involving the Saxothuringian margin was still operating leading to the subsequent collision with Brunia in the Early Carboniferous that produced a widespread high temperature overprint mostly wiping up the earlier metamorphic history.

Subduction of lower continental crust beneath the Pamir imaged by receiver functions from the seismological TIPAGE network

NASA Astrophysics Data System (ADS)

Schneider, F. M.; Yuan, X.; Schurr, B.; Mechie, J.; Sippl, C.; Kufner, S.; Haberland, C. A.; Minaev, V.; Oimahmadov, I.; Gadoev, M.; Abdybachaev, U.; Orunbaev, S.

2013-12-01

As the northwestern promontory of the Tibetan Plateau, the Pamir forms an outstanding part of the India-Asia convergence zone. The Pamir plateau has an average elevation of more than 4000 m surrounded by peaks exceeding 7000 m at its northern, eastern and southern borders. The Pamir is thought to consist of the same collage of continental terranes as Tibet. However, in this region the Indian-Asian continental collision presents an extreme situation since, compared to Tibet, in the Pamir a similar amount of north-south convergence has been accommodated within a much smaller distance. The Pamir hosts a zone of intermediate depth earthquakes being the seismic imprint of Earth's most spectacular active intra-continental subduction zone. We present receiver function (RF) images from the TIPAGE seismic profile giving evidence that the intermediate depth seismicity is situated within a subducted layer of lower continental crust: We observe a southerly dipping 10-15 km thick low-velocity zone (LVZ), that starts from the base of the crust and extends to a depth of more than 150 km enveloping the intermediate depth earthquakes that have been located with high precision from our local network records. In a second northwest to southeast cross section we observe that towards the western Pamir the dip direction of the LVZ bends to the southeast following the geometry of the intermediate depth seismic zone. Our observations imply that the complete arcuate intermediate depth seismic zone beneath the Pamir traces a slab of subducting Eurasian continental lower crust. These observations provide important implications for the geodynamics of continental collision: First, it shows that under extreme conditions lower crust can be brought to mantle depths despite its buoyancy, a fact that is also testified by the exhumation of ultra-high pressure metamorphic rocks. Recent results from teleseismic tomography show a signal of Asian mantle lithosphere down to 600 km depth, implying a great amount of mantle lithosphere to be involved in the subduction, which possibly transmits pull forces to the lower crust to overcome its buoyancy. Secondly, the observation that earthquakes occur within the subducted crust implies that similar to oceanic subduction, metamorphic processes within the lower continental crust can cause or enable earthquakes at depths, where the high pressure and temperature conditions would normally not allow brittle failure of rocks. For imaging of the dipping LVZ, cross sections of Q- and T-component RFs are generated using a migration technique that accounts for the inclination of the conversion layers. Furthermore we present a Moho map of the Pamir, showing crustal thickness in most places of the Pamir ranging between 65 km and 75 km, while the greatest Moho depths of around 80 km are observed at the upper end of the LVZ. The surrounding areas namely the Tajik Depression, and the Ferghana and Tarim Basins show Moho depths of around 40 to 45 km giving an estimate of the pre-collisional crustal thickness of the former Basin area that was overthrust by the Pamir.

Characterizing an "uncharacteristics" ETS event in northern Cascadia

USGS Publications Warehouse

Wang, K.; Dragert, H.; Kao, H.; Roeloffs, E.

2008-01-01

GPS and borehole strainmeter data allowed the detection and model characterization of a slow slip event in northern Cascadia in November 2006 accompanying a brief episode of seismic tremor. The event is much smaller in area and duration than other well-known ETS events in northern Cascadia but is strikingly similar to typical ETS events at the Nankai subduction zone. The 30-45 km depth range and the 2-3 cm slip magnitude as interpreted for this event appear to be common to most ETS events in these two subduction zones, regardless of their sizes. We infer that the Nankai-type small ETS events must be abundant at Cascadia and that ETS event at the two subduction zones are governed by a similar physical process. Copyright 2008 by the American Geophysical Union.

Bifurcation of the Yellowstone plume driven by subduction-induced mantle flow

NASA Astrophysics Data System (ADS)

Kincaid, C.; Druken, K. A.; Griffiths, R. W.; Stegman, D. R.

2013-05-01

The causes of volcanism in the northwestern United States over the past 20 million years are strongly contested. Three drivers have been proposed: melting associated with plate subduction; tectonic extension and magmatism resulting from rollback of a subducting slab; or the Yellowstone mantle plume. Observations of the opposing age progression of two neighbouring volcanic chains--the Snake River Plain and High Lava Plains--are often used to argue against a plume origin for the volcanism. Plumes are likely to occur near subduction zones, yet the influence of subduction on the surface expression of mantle plumes is poorly understood. Here we use experiments with a laboratory model to show that the patterns of volcanism in the northwestern United States can be explained by a plume upwelling through mantle that circulates in the wedge beneath a subduction zone. We find that the buoyant plume may be stalled, deformed and partially torn apart by mantle flow induced by the subducting plate. Using plausible model parameters, bifurcation of the plume can reproduce the primary volcanic features observed in the northwestern United States, in particular the opposite progression of two volcanic chains. Our results support the presence of the Yellowstone plume in the northwestern United States, and also highlight the power of plume-subduction interactions to modify surface geology at convergent plate margins.

Regional distribution of volcaniclastic layer and its implication for segmentation of the Nankai seismogenic zone

NASA Astrophysics Data System (ADS)

Sasaki, T.; Lim, J.; Higashi, M.; Park, J.

2010-12-01

The Nankai Trough is known as one of the best-suited convergent plate margins for studying accretionary prism growth as well as subduction zone earthquakes. Along the Nankai accretionary margin off southwest Japan, the Shikoku Basin which formed 26-15 Ma as backarc spreading in the Philippine Sea Plate is being subducted about 4 cm/year to the northwest. The Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) penetrated the Nankai accretionary prism and the incoming sedimentary section along the Ashizuri and Muroto transects, off Shikoku Island. Also, Integrated Ocean Drilling Program (IODP), which represented just one part of a multi-stage project known as the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) has been conducting drilling cruises now. IODP Expedition 322 in 2009, the coring was carried out at two drilling sites on the northern part of the Shikoku Basin in the subducting Philippine Sea plate. One of the major achievements of Expedition 322 is a discovery of late Miocene (10.2-7.6 Ma) tuffaceous and volcaniclastic sandstone layer (Underwood et al., IODP Prel. Rept. 322, 2009) that has not been previously recognized in the Nankai Trough. Based on age and volcanic sand content analysis, these volcaniclastic layers were unique to the Shikoku Basin off Kii Peninsula. The closest source of this volcanic layer was supposed to be the Izu-Bonin arc. Subducted sediments ultimately affect subduction zone geochemistry, thermal structure, and seismogenesis. High porosity of the volcaniclastic sandstone layer suggests the transportation of fluid to the subduction zone, it might affect the initiation and evolution of the decollement zone or plate boundary fault in the Nankai Trough. We interpreted single channel and multichannel seismic reflection profiles that have been acquired in the Nankai Trough margin by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) since the year of 1997. We tried to map the major seismic layers such as volcaniclastic layer, volcanic ash layer and turbidite layers which were found at drilling sites in the IODP Expedition 322 in the northern Shikoku Basin. As a result, we recognized that these prominent seismic layers are widely distributed in the northern Shikoku Basin. In this talk, we will show specific seismic layers directly connecting to the decollement at the Nankai Trough axis, and discuss its implications for subduction processes in the Nankai Trough margin.

Numerical modeling the genetic mechanism of Cenozoic intraplate Volcanoes in Northeastern China

NASA Astrophysics Data System (ADS)

Qu, Wulin; Chen, Yongshun John; Zhang, Huai; Jin, Yimin; Shi, Yaolin

2017-04-01

Changbaishan Volcano located about 1400 km west of Japan Trench is an intra continental volcano which having different origin from island arc volcanoes. A number of different mechanisms have been proposed to interpret the origin of intraplate volcanoes, such as deep mantle plumes, back-arc extension and decompressional partial melting, asthenosphere upwelling and decompressional melting, and deep stagnant slab dehydration and partial melting. The recent geophysical research reveals that the slow seismic velocity anomaly extends continuously just below 660 km depth to surface beneath Changbaishan by seismic images and three-dimensional waveform modelling [Tang et al., 2014]. The subduction-induced upwelling occurs within a gap in the stagnant subducted Pacific Plate and produces decompressional melting. Water in deep Earth can reduce viscosity and lower melting temperature and seismic velocity and has effects on many other physical properties of mantle materials. The water-storage capacity of wadsleyite and ringwoodite, which are the main phase in the mantle transition zone, is much greater than that of upper mantle and lower mantle. Geophysical evidences have shown that water content in the mantle transition zone is exactly greater than that of upper mantle and lower mantle [Karato, 2011]. Subducted slab could make mantle transition zone with high water content upward or downward across main phase change surface to release water, and lead to partial melting. We infer that the partial melting mantle and subducted slab materials propagate upwards and form the Cenozoic intraplate Volcanoes in Northeastern China. We use the open source code ASPECT [Kronbichler et al., 2012] to simulate the formation and migration of magma contributing to Changbaishan Volcano. We find that the water entrained by subducted slab from surface has only small proportion comparing to water content of mantle transition zone. Our model provide insights into dehydration melting induced by water transport out of the mantle transition zone associated with dynamic interactions between the subducted slab and surrounding mantle. References Karato, S. (2011), Water distribution across the mantle transition zone and its implications for global material circulation, EARTH PLANET SC LETT, 301(3), 413-423. Kronbichler, M., et al. (2012), High accuracy mantle convection simulation through modern numerical methods, GEOPHYS J INT, 191(1), 12-29. Tang, Y., et al. (2014), Changbaishan volcanism in northeast China linked to subduction-induced mantle upwelling, NAT GEOSCI, 7(6), 470-475.

New Insights on the Structure of the Cascadia Subduction Zone from Amphibious Seismic Data

NASA Astrophysics Data System (ADS)

Janiszewski, Helen Anne

A new onshore-offshore seismic dataset from the Cascadia subduction zone was used to characterize mantle lithosphere structure from the ridge to the volcanic arc, and plate interface structure offshore within the seismogenic zone. The Cascadia Initiative (CI) covered the Juan de Fuca plate offshore the northwest coast of the United States with an ocean bottom seismometer (OBS) array for four years; this was complemented by a simultaneous onshore seismic array. Teleseismic data recorded by this array allows the unprecedented imaging of an entire tectonic plate from its creation at the ridge through subduction initiation and back beyond the volcanic arc along the entire strike of the Cascadia subduction zone. Higher frequency active source seismic data also provides constraints on the crustal structure along the plate interface offshore. Two seismic datasets were used to image the plate interface structure along a line extending 100 km offshore central Washington. These are wide-angle reflections from ship-to-shore seismic data from the Ridge-To-Trench seismic cruise and receiver functions calculated from a densely spaced CI OBS focus array in a similar region. Active source seismic observations are consistent with reflections from the plate interface offshore indicating the presence of a P-wave velocity discontinuity. Until recently, there has been limited success in using the receiver function technique on OBS data. I avoid these traditional challenges by using OBS constructed with shielding deployed in shallow water on the continental shelf. These data have quieter horizontals and avoid water- and sediment-multiple contamination at the examined frequencies. The receiver functions are consistently modeled with a velocity structure that has a low velocity zone (LVZ) with elevated P to S-wave velocity ratios at the plate interface. A similar LVZ structure has been observed onshore and interpreted as a combination of elevated pore-fluid pressures or metasediments. This new offshore result indicates that the structure may persist updip indicating the plate interface may be weak. To focus more broadly on the entire subduction system, I calculate phase velocities from teleseismic Rayleigh waves from 20-100 s period across the entire onshore-offshore array. The shear-wave velocity model calculated from these data can provide constrains on the thermal structure of the lithosphere both prior to and during subduction of the Juan de Fuca plate. Using OBS data in this period band requires removal of tilt and compliance noise, two types of water-induced noise that affect long period data. To facilitate these corrections on large seismic arrays such as the CI, an automated quality control routine was developed for selecting noise windows for the calculation of the required transfer functions. These corrections typically involve either averaging out transient signals, which requires the assumption of stationarity of the noise over the long periods of time, or laborious hand selection of noise segments. This new method calculates transfer functions based on daily time series that exclude transient signals, but allows for the investigation of long-term variation over the course of an instrument's deployment. I interpret these new shoreline-crossing phase velocity maps in terms of the tectonics associated with the Cascadia subduction system. Major findings include that oceanic plate cooling models do not explain the velocities observed beneath the Juan de Fuca plate, that slow velocities in the forearc appear to be more prevalent in areas modeled to have experienced high slip in past Cascadia megathrust earthquakes, and along strike variations in phase velocity reflect variations in arc structure and backarc tectonics.

Trace element mobility at the slab-mantle interface: constraints from "hybrid

NASA Astrophysics Data System (ADS)

Marocchi, M.; Tropper, P.; Mair, V.; Bargossi, G. M.; Hermann, J.

2009-04-01

Subduction mélanges and hybrid rocks are considered, together with mafic rocks, metasediments and serpentinite as an important volatile-bearing portion of subducting slabs (cf. Spandler et al., 2008 and references therein; Miller et al., 2009). In particular, metasomatic rocks occurring in exhumed HP mélanges have recently attracted growing interest for two main reasons: i) metasomatic rocks forming at the interface between ultramafic and crustal rocks of subducting slabs constitute new bulk compositions which can affect the redistribution of major and trace elements and modify the composition of slab fluids moving to the mantle wedge and ii) these mineral assemblages, consisting mainly of hydrous phases can potentially store and transport water at great depth in subduction zones. Ultramafic rocks belonging to the Hochwart peridotite (Ulten Zone, central-eastern Italian Alps) preserve a series of metasomatic mineral zones generated by infiltration of hydrous fluids/melts, which occurred at the gneiss-peridotite interface (Tumiati et al., 2007; Marocchi et al., 2009). The peridotite body of Mt. Hochwart represents an almost unique occurrence where subduction-related mantle metasomatism can be studied on an outcrop scale. The ultramafic body consists of metaperidotites exposed as a hectometre-size lens along a steep gully, associated to monomineralic zones that developed at the contact between the peridotite body and the garnet-bearing gneiss country rocks. The formation of the metasomatic zones composed exclusively of hydrous phases involved extensive H2O-metasomatism as already documented for the Ulten peridotites (Scambelluri et al., 2006; Marocchi et al., 2007). Whole-rock geochemistry and trace element composition of hydrous phases (phlogopite and amphibole) in different metasomatic zones indicate mobility of many elements, including elements such as Ta, which are considered to have scarce mobility in fluids. Trace element composition of accessory minerals in the phlogopite-rich zone suggests that the trace element signature of subduction zone fluids may be fractionated in this zone. The progressive depletion in some trace elements (LREE and LILE) and enrichment in Li from the gneiss towards the peridotite suggests a strong influence of bulk composition on the trace element budget of hydrous minerals. Since these metasomatic zones can be representative of the processes occurring at the slab-mantle interface, we can infer that metasomatic reactions between slab-derived fluids and ultramafic mantle wedge will follow a specific series of reactions and create mineral zones similar to those observed in this study. Despite the mobility of many elements, in the trace element profiles for amphibole and phlogopite across the different zones, we observe a rapid decrease even of the "fluid mobile" element contents within the reaction zone. With the exception of Li, we assist to an abrupt decrease of most of trace element concentrations going towards the peridotite side contact. Thus, according to the present study, it is not likely that the "crustal trace element signature" (i.e. LILE and LREE-enriched) could be able to travel far into the mantle. Our results further favour the evidence that the primary composition of subduction zone fluids reaching the source region of arc magmas is substantially modified by metasomatic reactions occurring in the mantle wedge. Furthermore, we underline that metasomatic rocks such as those observed at Mt. Hochwart are potentially able to transport H2O and other trace elements to greater depths in subduction zones. References: Marocchi M, Hermann J, Morten L (2007)-Lithos 99: 85-104. Marocchi M, Mair V, Tropper P, Bargossi GM (2009)-Mineral Petrol, in press Miller DP, Marschall RH, Schumacher JC (2009)- Lithos 107: 53-67. Scambelluri M, Hermann J, Morten L, Rampone E (2006)- Contrib Mineral Petrol 151:372-394. Spandler CJ, Hermann J, Faure K, Mavrogenes JA, Arculus RJ (2008)- Contrib Mineral Petrol 155: 181-198. Tumiati S, Godard G, Martin S, Klőtzli U, Monticelli D (2007)- Lithos 94: 148-167.

Foreshock occurrence rates before large earthquakes worldwide

USGS Publications Warehouse

Reasenberg, P.A.

1999-01-01

Global rates of foreshock occurrence involving shallow M ??? 6 and M ??? 7 mainshocks and M ??? 5 foreshocks were measured, using earthquakes listed in the Harvard CMT catalog for the period 1978-1996. These rates are similar to rates ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span. The observed worldwide rates were compared to a generic model of earthquake clustering, which is based on patterns of small and moderate aftershocks in California, and were found to exceed the California model by a factor of approximately 2. Significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region, with the rate before thrust events higher and the rate before strike-slip events lower than the worldwide average. Among the thrust events a large majority, composed of events located in shallow subduction zones, registered a high foreshock rate, while a minority, located in continental thrust belts, measured a low rate. These differences may explain why previous surveys have revealed low foreshock rates among thrust events in California (especially southern California), while the worldwide observations suggest the opposite: California, lacking an active subduction zone in most of its territory, and including a region of mountain-building thrusts in the south, reflects the low rate apparently typical for continental thrusts, while the worldwide observations, dominated by shallow subduction zone events, are foreshock-rich.

Slab Geometry and Deformation in the Northern Nazca Subduction Zone Inferred From The Relocation and Focal mechanisms of Intermediate-Depth Earthquakes

NASA Astrophysics Data System (ADS)

Chang, Y.; Warren, L. M.; Prieto, G. A.

2015-12-01

In the northern Nazca subduction zone, the Nazca plate is subducting to the east beneath the South American Plate. At ~5.6ºN, the subducting plate has a 240-km east-west offset associated with a slab tear, called the Caldas tear, that separates the northern and southern segments. Our study seeks to better define the slab geometry and deformation in the southern segment, which has a high rate of intermediate-depth earthquakes (50-300 km) between 3.6ºN and 5.2ºN in the Cauca cluster. From Jan 2010 to Mar 2014, 228 intermediate-depth earthquakes in the Cauca cluster with local magnitude Ml 2.5-4.7 were recorded by 65 seismic stations of the Colombian National Seismic Network. We review and, if necessary, adjust the catalog P and S wave arrival picks. We use the travel times to relocate the earthquakes using a double difference relocation method. For earthquakes with Ml ≥3.8, we also use waveform modeling to compute moment tensors . The distribution of earthquake relocations shows an ~15-km-thick slab dipping to the SE. The dip angle increases from 20º at the northern edge of the cluster to 38º at the southern edge. Two concentrated groups of earthquakes extend ~40 km vertically above the general downdip trend, with a 20 km quiet gap between them at ~100 km depth. The earthquakes in the general downdip seismic zone have downdip compressional axes, while earthquakes close to the quiet gap and in the concentrated groups have an oblique component. The general decrease in slab dip angle to the north may be caused by mantle flow through the Caldas tear. The seismicity gap in the slab may be associated with an active deformation zone and the concentrated groups of earthquakes with oblique focal mechanisms could be due to a slab fold.

Tectonics and Current Plate Motions of Northern Vancouver Island and the Adjacent Mainland

NASA Astrophysics Data System (ADS)

Jiang, Y.; Leonard, L. J.; Henton, J.; Hyndman, R. D.

2016-12-01

Northern Vancouver Island comprises a complex transition zone along the western margin of the North America plate, between the subducting Juan de Fuca plate to the south and the transcurrent Queen Charlotte Fault to the north off Haida Gwaii. The tectonic history and seismic potential for this region are unclear. Here we present current plate motions for northern Vancouver Island and the adjacent mainland, determined from continuous and campaign GPS measurements processed in a consistent manner. Immediately to the north of the mid-Vancouver Island Nootka Fault Zone, the northern limit of Juan de Fuca plate subduction, GPS velocity vectors show slower Explorer plate subduction than the Juan de Fuca Plate. Off northernmost Vancouver Island, the Winona Block is possibly converging at a slow rate that decreases northward to zero. We find a constant northward margin-parallel translation of up to 5 mm/year from northern Vancouver Island extending to Alaska. The southern limit of this translation coincides with areas of high heat flow that may reflect extension and the northern limit of episodic tremor and slip (ETS) on the Cascadia megathrust. The origin of the northward translation is poorly understood. We find a mainland coastal shear zone extends as far south as northern Vancouver Island where the offshore plate boundary is likely subduction. The pattern of the observed coastal shear cannot reflect interseismic locking on a major offshore transcurrent fault. The geodetically determined mainland coastal zone velocities decrease landward from 5 to 0 mm/yr across a region where no active faults have been identified and there is very little current seismicity. In Haida Gwaii, oblique convergence is apparent in the GPS data, consistent with partitioning between margin-parallel and margin-perpendicular strain. After removing the margin parallel translation from the data, we determine an average maximum locking depth of 15 km for the Queen Charlotte transcurrent fault, consistent with seismicity and seismic structure data.

Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

USGS Publications Warehouse

Nelson, Alan R.; Shennan, Ian; Long, Antony J.

1996-01-01

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Structure and seismic activity of the Lesser Antilles subduction zone

NASA Astrophysics Data System (ADS)

Evain, M.; Galve, A.; Charvis, P.; Laigle, M.; Ruiz Fernandez, M.; Kopp, H.; Hirn, A.; Flueh, E. R.; Thales Scientific Party

2011-12-01

Several active and passive seismic experiments conducted in 2007 in the framework of the European program "Thales Was Right" and of the French ANR program "Subsismanti" provided a unique set of geophysical data highlighting the deep structure of the central part of the Lesser Antilles subduction zone, offshore Dominica and Martinique, and its seismic activity during a period of 8 months. The region is characterized by a relatively low rate of seismicity that is often attributed to the slow (2 cm/yr) subduction of the old, 90 My, Atlantic lithosphere beneath the Caribbean Plate. Based on tomographic inversion of wide-angle seismic data, the forearc can clearly be divided into an inner forearc, characterised by a high vertical velocity gradient in the igneous crust, and an outer forearc with lower crustal velocity gradient. The thick, high velocity, inner forearc is possibly the extension at depth of the Mesozoic Caribbean crust outcropping in La Désirade Island. The outer forearc, up to 70 km wide in the northern part of the study area, is getting narrower to the south and disappears offshore Martinique. Based on its seismic velocity structure with velocities higher than 6 km/s the backstop consists, at least partly, of magmatic rocks. The outer forearc is also highly deformed and faulted within the subducting trend of the Tiburon Ridge. With respect to the inner forearc velocity structure the outer forearc basement could either correspond to an accreted oceanic terrane or made of highly fractured rocks. The inner forearc is a dense, poorly deformable crustal block, tilted southward as a whole. It acts as a rigid buttress increasing the strain within both the overriding and subducting plates. This appears clearly in the current local seismicity affecting the subducting and the overriding plates that is located beneath the inner forearc. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. The main seismic activity is concentrated in the lower crust and in the mantle wedge, close to the island arc beneath the inner forearc domain. In comparison, little seismicity is observed beneath the outer forearc domain. We propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense, thick and rigid crustal block and the lighter and more flexible outer forearc. Seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate. At depth, interplate earthquakes observed between 35 and 45 km depth, deeper than the Moho of the forearc (~30 km), possibly reveal the downdip limit of the seismogenic zone. The Thales Scientific Party is composed of: Bayrakci, G., Bécel, A., Charvis, P., Diaz, J., Evain, M., Flueh, E., Gallart, J., Gailler, A., Galve, A., Hello, Y., Hirn, A., Kopp, H., Krabbenhoeft, A., Laigle, M., Lebrun, J. F., Monfret, T., Papenberg, C., Planert, L., Ruiz, M., Sapin, M., Weinzierl, W.

Slab seismicity in the Western Hellenic Subduction Zone: Constraints from tomography and double-difference relocation

NASA Astrophysics Data System (ADS)

Halpaap, Felix; Rondenay, Stéphane; Ottemöller, Lars

2016-04-01

The Western Hellenic subduction zone is characterized by a transition from oceanic to continental subduction. In the southern oceanic portion of the system, abundant seismicity reaches intermediate depths of 100-120 km, while the northern continental portion rarely exhibits deep earthquakes. Our study aims to investigate how this oceanic-continental transition affects fluid release and related seismicity along strike, by focusing on the distribution of intermediate depth earthquakes. To obtain a detailed image of the seismicity, we carry out a tomographic inversion for P- and S-velocities and double-difference earthquake relocation using a dataset of unprecedented spatial coverage in this area. Here we present results of these analyses in conjunction with high-resolution profiles from migrated receiver function images obtained from the MEDUSA experiment. We generate tomographic models by inverting data from 237 manually picked, well locatable events recorded at up to 130 stations. Stations from the permanent Greek network and the EGELADOS experiment supplement the 3-D coverage of the modeled domain, which covers a large part of mainland Greece and surrounding offshore areas. Corrections for the sphericity of the Earth and our update to the SIMULR16 package, which now allows S-inversion, help improve our previous models. Flexible gridding focusses the inversion on the domains of highest gradient around the slab, and we evaluate the resolution with checker board tests. We use the resulting velocity model to relocate earthquakes via the Double-Difference method, using a large dataset of differential traveltimes obtained by crosscorrelation of seismograms. Tens of earthquakes align along two planes forming a double seismic zone in the southern, oceanic portion of the subduction zone. With increasing subduction depth, the earthquakes appear closer to the center of the slab, outlining probable deserpentinization of the slab and concomitant eclogitization of dry crustal rocks. Against expectations, we relocate one robust deep event at ≈70 km depth in the northern, continental part of the subduction zone.

Investigating the 3-D Subduction Initiation Processes at Transform Faults and Passive Margins

NASA Astrophysics Data System (ADS)

Peng, H.; Leng, W.

2017-12-01

Studying the processes of subduction initiation is a key for understanding the Wilson cycle and improving the theory of plate tectonics. Previous studies investigated subduction initiation with geological synthesis and geodynamic modeling methods, discovering that subduction intends to initiate at the transform faults close to oceanic arcs, and that its evolutionary processes and surface volcanic expressions are controlled by plate strength. However, these studies are mainly conducted with 2-D models, which cannot deal with lateral heterogeneities of crustal thickness and strength along the plate interfaces. Here we extend the 2-D model to a 3-D parallel subduction model with high computational efficiency. With the new model, we study the dynamic controlling factors, morphology evolutionary processes and surface expressions for subduction initiation with lateral heterogeneities of material properties along transform faults and passive margins. We find that lateral lithospheric heterogeneities control the starting point of the subduction initiation along the newly formed trenches and the propagation speed for the trench formation. New subduction tends to firstly initiate at the property changing point along the transform faults or passive margins. Such finds may be applied to explain the formation process of the Izu-Bonin-Mariana (IBM) subduction zone in the western Pacific and the Scotia subduction zone at the south end of the South America. Our results enhance our understanding for the formation of new trenches and help to provide geodynamic modeling explanations for the observed remnant slabs in the upper mantle and the surface volcanic expressions.

Noble Gases Trace Earth's Subducted Water Flux

NASA Astrophysics Data System (ADS)

Smye, A.; Jackson, C.; Konrad-Schmolke, M.; Parman, S. W.; Ballentine, C. J.

2016-12-01

Volatile elements are transported from Earth's surface reservoirs back into the mantle during subduction of oceanic lithosphere [e.g. 1]. Here, we investigate the degree to which the fate of slab-bound noble gases and water are linked through the subduction process. Both water and noble gases are soluble in ring-structured minerals, such as amphibole, that are common constituents of subducted oceanic lithosphere. Heating and burial during subduction liberates noble gases and water from minerals through a combination of diffusion and dissolution. Combining a kinetic model, parameterized for noble gas fractionation in amphibole [2], with thermodynamic phase equilibria calculations, we quantify the effect of subduction dehydration on the elemental composition of slab-bound noble gases. Results show that post-arc slab water and noble gas fluxes are highly correlated. Hot subduction zones, which likely dominate over geologic history, efficiently remove noble gases and water from the down-going slab; furthermore, kinetic fractionation of noble gases is predicted to occur beneath the forearc. Conversely, hydrated portions of slab mantle in cold subduction zones transport noble gases and water to depths exceeding 200 km. Preservation of seawater-like abundances of Ar, Kr and Xe in the convecting mantle [1] implies that recycling of noble gases and water occurred during cold subduction and that the subduction efficiency of these volatile elements has increased over geological time, driven by secular cooling of the mantle. [1] Holland, G. and Ballentine, C. (2006). Nature 441, 186-191. [2] Jackson et al. (2013). Nat.Geosci. 6, 562-565.

The Rise of Oxygen in the Earth's Atmosphere Controlled by the Efficient Subduction of Organic Carbon

NASA Astrophysics Data System (ADS)

Duncan, M. S.; Dasgupta, R.

2017-12-01

Carbon cycling between the Earth's surface environment, i.e., the ocean-atmosphere system, and the Earth's interior is critical for differentiation, redox evolution, and long-term habitability of the planet. This carbon cycle is influenced heavily by the extent of carbon subduction. While the fate of carbonates during subduction has been discussed in numerous studies [e.g., 1], little is known how organic carbon is quantitatively transferred from the Earth's surface to the interior. Efficient subduction of organic carbon would remove reduced carbon from the surface environment over the long-term (≥100s Myrs) while release at subduction zone arc volcanoes would result in degassing of CO2. Here we conducted high pressure-temperature experiments to determine the carbon carrying capacity of slab derived, rhyolitic melts under graphite-saturated conditions over a range of P (1.5-3.0 GPa) and T (1100-1400 °C) at a fixed melt H2O content (2 wt.%) [2]. Based on our experimental data, we developed a thermodynamic model of CO2 dissolution in C-saturated slab melts, that allows us to quantify the extent of organic carbon mobility as a function of slab P, T, and fO2 during subduction through time. Our experimental data and thermodynamic model suggest that the subduction of graphitized organic C, and graphite/diamond formed by reduction of carbonates with depth [e.g., 3], remained efficient even in ancient, hotter subduction zones - conditions at which subduction of carbonates likely remained limited [1]. Considering the efficiency the subduction of organic C and potential conditions for ancient subduction, we suggest that the lack of remobilization in subduction zones and deep sequestration of organic C in the mantle facilitated the rise and maintenance atmospheric oxygen in the Paleoproterozoic and is causally linked to the Great Oxidation Event (GOE). Our modeling shows that episodic subduction and organic C sequestration pre-GOE may also explain occasional whiffs of atmospheric oxygen observed in the Archean [4]. [1] Dasgupta (2013) Rev. Mineral. Geochem. 75, 183-229. [2] Duncan and Dasgupta (2017) Nat. Geosci. 10, 387-392. [3] Galvez et al. (2013) Nat. Geosci. 6, 473-477. [4] Anbar et al. (2007) Sci. 317, 1903-1906.

A revised dislocation model of interseismic deformation of the Cascadia subduction zone

USGS Publications Warehouse

Wang, Kelin; Wells, Ray E.; Mazzotti, Stephane; Hyndman, Roy D.; Sagiya, Takeshi

2003-01-01

CAS3D‐2, a new three‐dimensional (3‐D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc. Horizontal deformation data, including strain rates and surface velocities from Global Positioning System (GPS) measurements, provide primary geodetic constraints, but uplift rate data from tide gauges and leveling also provide important validations for the model. A locked zone, based on the results of previous thermal models constrained by heat flow observations, is located entirely offshore beneath the continental slope. Similar to previous dislocation models, an effective zone of downdip transition from locking to full slip is used, but the slip deficit rate is assumed to decrease exponentially with downdip distance. The exponential function resolves the problem of overpredicting coastal GPS velocities and underpredicting inland velocities by previous models that used a linear downdip transition. A wide effective transition zone (ETZ) partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model. The pattern of coseismic deformation is expected to be different from that of interseismic deformation at present, 300 years after the last great subduction earthquake. The downdip transition from full rupture to no slip should take place over a much narrower zone.

The Processes Producing the Actively Uplifting Mackenzie Mountains in the Yukon and Northwest Territories, Canada

NASA Astrophysics Data System (ADS)

Rasmussen, B.; Aster, R. C.; Schutt, D.

2016-12-01

The actively uplifting and seismically active Mackenzie Mountains in the Yukon and Northwest Territories of Canada exist nearly 800 km from the Pacific plate subduction zone. As such, it is clear that traditional subduction zone orogenic mechanics are not at play. This mountain range may present a model for uplift of other ranges distant from plate boundaries, such as the Rockies or Ancestral Rockies. Due to its remote location, this region's lithospheric structure is poorly constrained. However, two hypotheses have been developed recently. The first proposes that stress from the Yakutat Indentor as it subducts under North America at the Gulf of Alaska is transferred deep inland through the upper crust, and that the lower crust and mantle lithosphere are very weak. As this weak lithosphere meets the strong Canadian Craton, lateral translation turns into uplift, forming the Mackenzies (Mazzotti and Hyndman, 2002, Geology, v. 30, no.6). Alternatively, it may be that mantle flow from the north is deflected eastward by the Yakutat slab, producing large scale mantle flow and stress which propagates through the crust to uplift the Mackzenzie Mountains without an abnormally weak lithosphere (Finzel, 2015, Geophys. Res. Lett., 42, 4350-4358). Both cases imply distinct isotropic and anisotropic structure that will be constrained through Rayleigh wave tomography. Notably, we will take advantage of the recent deployment of several Earthscope Transportable Array stations nearby, and some preliminary data from the ongoing Mackenzie Mountains Earthscope Project.

Crustal earthquake triggering by pre-historic great earthquakes on subduction zone thrusts

USGS Publications Warehouse

Sherrod, Brian; Gomberg, Joan

2014-01-01

Triggering of earthquakes on upper plate faults during and shortly after recent great (M>8.0) subduction thrust earthquakes raises concerns about earthquake triggering following Cascadia subduction zone earthquakes. Of particular regard to Cascadia was the previously noted, but only qualitatively identified, clustering of M>~6.5 crustal earthquakes in the Puget Sound region between about 1200–900 cal yr B.P. and the possibility that this was triggered by a great Cascadia thrust subduction thrust earthquake, and therefore portends future such clusters. We confirm quantitatively the extraordinary nature of the Puget Sound region crustal earthquake clustering between 1200–900 cal yr B.P., at least over the last 16,000. We conclude that this cluster was not triggered by the penultimate, and possibly full-margin, great Cascadia subduction thrust earthquake. However, we also show that the paleoseismic record for Cascadia is consistent with conclusions of our companion study of the global modern record outside Cascadia, that M>8.6 subduction thrust events have a high probability of triggering at least one or more M>~6.5 crustal earthquakes.

The global distribution of magnitude 9 earthquakes

NASA Astrophysics Data System (ADS)

McCaffrey, R.

2011-12-01

The 2011 Tohoku M9 earthquake once again caught some in the earthquake community by surprise. The expectation of these massive quakes has been driven in the past by the over-reliance on our short, incomplete history of earthquakes and causal relationships derived from it. The logic applied is that if a great earthquake has not happened in the past, that we know of, one cannot happen in the future. Using the ~100-year global earthquake history, seismologists have promoted relationships between maximum earthquake sizes and other properties of subduction zones, leading to the notion that some subduction zones, like the Japan Trench, would never produce a magnitude ~9 event. The 2004 Andaman Mw = 9.2 earthquake, that occurred where there is slow subduction of old crust and a history of only moderate-sized earthquakes, seriously undermined such ideas. Given multi-century return times of the greatest earthquakes, ignorance of those return times and our very limited observation span, I suggest that we cannot yet make such determinations. Alternatively, using the length of a subduction zone that is available for slip as the predominant factor in determining maximum earthquake size, we cannot rule out that any subduction zone of a few hundred kilometers or more in length may be capable of producing a magnitude 9 or larger earthquake. Based on this method, the expected maximum size for the Japan Trench was 9.0 (McCaffrey, Geology, p. 263, 2008). The same approach portends a M > 9 for Java, with twice the population density as Honshu and much lower building standards. The Java Trench, and others where old crust subducts (Hikurangi, Marianas, Tonga, Kermadec), require increased awareness of the possibility for a great earthquake.

SubductionGenerator: A program to build three-dimensional plate configurations

NASA Astrophysics Data System (ADS)

Jadamec, M. A.; Kreylos, O.; Billen, M. I.; Turcotte, D. L.; Knepley, M.

2016-12-01

Geologic, geochemical, and geophysical data from subduction zones indicate that a two-dimensional paradigm for plate tectonic boundaries is no longer adequate to explain the observations. Many open source software packages exist to simulate the viscous flow of the Earth, such as the dynamics of subduction. However, there are few open source programs that generate the three-dimensional model input. We present an open source software program, SubductionGenerator, that constructs the three-dimensional initial thermal structure and plate boundary structure. A 3D model mesh and tectonic configuration are constructed based on a user specified model domain, slab surface, seafloor age grid file, and shear zone surface. The initial 3D thermal structure for the plates and mantle within the model domain is then constructed using a series of libraries within the code that use a half-space cooling model, plate cooling model, and smoothing functions. The code maps the initial 3D thermal structure and the 3D plate interface onto the mesh nodes using a series of libraries including a k-d tree to increase efficiency. In this way, complicated geometries and multiple plates with variable thickness can be built onto a multi-resolution finite element mesh with a 3D thermal structure and 3D isotropic shear zones oriented at any angle with respect to the grid. SubductionGenerator is aimed at model set-ups more representative of the earth, which can be particularly challenging to construct. Examples include subduction zones where the physical attributes vary in space, such as slab dip and temperature, and overriding plate temperature and thickness. Thus, the program can been used to construct initial tectonic configurations for triple junctions and plate boundary corners.

How the gas hydrate system gives insight into subduction wedge dewatering processes in a zone of highly-oblique convergence on the southern Hikurangi margin of New Zealand

NASA Astrophysics Data System (ADS)

Crutchley, Gareth; Klaeschen, Dirk; Pecher, Ingo; Henrys, Stuart

2017-04-01

The southern end of New Zealand's Hikurangi subduction margin is characterised by highly-oblique convergence as it makes a southward transition into a right-lateral transform plate boundary at the Alpine Fault. Long-offset seismic data that cross part of the offshore portion of this transition zone give new insight into the nature of the plate boundary. We have carried out 2D pre-stack depth migrations, with an iterative reflection tomography to update the velocity field, on two seismic lines in this area to investigate fluid flow processes that have implications for the mechanical stability of the subduction interface. The results show distinct and focused fluid expulsion pathways from the subduction interface to the shallow sub-surface. For example, on one of the seismic lines there is a clear disruption of the gas hydrate system at its intersection with a splay fault - a clear indication of focused fluid release from the subduction interface. The seismic velocities derived from tomography also highlight a broad, pronounced low velocity zone beneath the deforming wedge that we interpret as a thick zone of gas-charged fluids that may have important implications for the long-term frictional stability of the plate boundary in this area. The focused flow upward toward the seafloor has the potential to result in the formation of concentrated gas hydrate deposits. Our on-going work on these data will include amplitude versus offset analysis in an attempt to better characterise the nature of the subduction interface, the fluids in that region, and also the shallower gas hydrate system.

Stratigraphic Signatures of Forearc Basin Formation Mechanisms

NASA Astrophysics Data System (ADS)

Mannu, U.; Ueda, K.; Gerya, T.; Willett, S.; Strasser, M.

2014-12-01

Forearc basins are loci of active sedimentation above the landward portion of accretionary prisms. Although these basins typically remain separated from the frontal prism by a forearc high, their evolution has a significant impact on the structure and deformation of the entire wedge. Formation of forearc basins has been proposed as a consequence of changes in wedge stability due to an increase of slab dip in subduction zones. Another hypothesis attributes this to higher hinterland sedimentation, which causes the rear of the wedge to stabilize and eventually develop a forearc basin. Basin stratigraphic architecture, revealed by high-resolution reflection seismic data and borehole data allows interpretation of structural development of the accretionary prism and associated basins with the goal of determining the underlying driving mechanism(s) of basin formation. In this study we supplement data interpretation with thermo-mechanical numerical models including high-resolution isochronal surface tracking to visualize the developing stratigraphy of basins that develop in subduction zone and wedge dynamic models. We use a dynamic 2D thermo mechanical model incorporating surface processes, strain weakening and sediment subduction. The model is a modification of I2VIS model, which is based on conservative, fully staggered finite differences and a non-diffusive marker- in-cell technique capable of modelling mantle convection. In the model different driving mechanisms for basin formation can be explored. Stratigraphic simulations obtained by isochronal surface tracking are compared to reflection pattern and stratigraphy of seismic and borehole data, respectively. Initial results from a model roughly representing the Nankai Trough Subduction Zone offshore Japan are compared to available seismic and Integrated Ocean Drilling (IODP) data. A calibrated model predicting forearc basin stratigraphy will be used to discern the underlying process of basins formation and wedge dynamics.

Numerical modelling of volatiles in the deep mantle

NASA Astrophysics Data System (ADS)

Eichheimer, Philipp; Thielmann, Marcel; Golabek, Gregor J.

2017-04-01

The transport and storage of water in the mantle significantly affects several material properties of mantle rocks and thus water plays a key role in a variety of geodynamical processes (tectonics, magmatism etc.). The processes driving transport and circulation of H2O in subduction zones remain a debated topic. Geological and seismological observations suggest different inflow mechanisms of water e.g. slab bending, thermal cracking and serpentinization (Faccenda et al., 2009; Korenaga, 2017), followed by dehydration of the slab. On Earth both shallow and steep subduction can be observed (Li et al., 2011). However most previous models (van Keken et al., 2008; Wilson et al., 2014) did not take different dip angles and subduction velocities of slabs into account. To which extent these parameters and processes influence the inflow of water still remains unclear. We present 2D numerical models simulating the influence of the various water inflow mechanisms on the mantle with changing dip angle and subduction velocity of the slab over time. The results are used to make predictions regarding the rheological behavior of the mantle wedge, dehydration regimes and volcanism at the surface. References: van Keken, P. E., et al. A community benchmark for subduction zone modeling. Phys. Earth Planet. Int. 171, 187-197 (2008). Faccenda, M., T.V. Gerya, and L. Burlini. Deep slab hydration induced by bending-related variations in tectonic pressure. Nat. Geosci. 2, 790-793 (2009). Korenaga, J. On the extent of mantle hydration caused by plate bending. Earth Planet. Sci. Lett. 457, 1-9 (2017). Wilson, C. R., et al. Fluid flow in subduction zones: The role of solid rheology and compaction pressure. Earth Planet. Sci. Lett. 401, 261-274 (2014). Li, Z. H., Z. Q. Xu, and T. V. Gerya. Flat versus steep subduction: Contrasting modes for the formation and exhumation of high- to ultrahigh-pressure rocks in continental collision zones. Earth Planet. Sci. Lett. 301, 65-77 (2011).

Subduction Top to Bottom: A Brief History of an Idea and Publication Concept

NASA Astrophysics Data System (ADS)

Bebout, G. E.; Scholl, D. W.; Kirby, S. H.

2016-12-01

INTRODUCTION: In 1991, Gray Bebout co-organized a GSA field trip to Catalina Island, CA, to examine exposures of the high P/T Catalina Schist accretionary complex. After the field trip the two of us, Gray (Lehigh), conducting research on exposed accretionary complexes, and Dave (USGS), carrying out offshore geophysical and geological studies of modern subduction zones, recognized that significant advances in subduction zone studies required a more interdisciplinary approach. To promulgate this, we agreed to convene a cross-disciplinary gathering of the then smaller communities of colleagues involved in offshore, onshore, and laboratory studies of modern subduction zones and the rock and fluid records they produce. SUBCON CONFERENCE AND PUBLICATION: It was agreed that the subduction conference (SUBCON) would be on Catalina Island to facilitate a conference field trip to the Catalina Schist. The general idea of SUBCON was discussed with our colleague Steve Kirby (USGS) who, to conceptually include the mantle, christened the conference as "Subduction Top to Bottom" (ST2B). Funding was largely provided by the USGS with supporting contributions from JOI USSAC (NSF). The conference was convened during the week of 12-17 June, 1994, at the Catalina Canyon Resort. A collection of ST2B papers was published in 1996 as AGU Geophysical Monograph v.96-known to many as "Big Purple". ST2B E-PUBLICATION: 20 years later, it seemed timely to organize a 2nd, or ST2B-2, conference. However, in recognition of the huge expansion of colleagues engaged in subduction zone science, and other multidisciplinary workshops, it was decided to convene a "virtual" conference by taking advantage of the publication speed, open-access availability, and ms-enhancing attributes of online E-pubs. GSA's Geosphere was selected as the venue of choice. Although open to all contributors, an editorial board of nearly 30 individuals was assembled to guarantee thematic coverage. Submission window is now open.

Geodynamic models of the Wilson Cycle: From rifts to mountains to rifts

NASA Astrophysics Data System (ADS)

Buiter, Susanne; Tetreault, Joya; Torsvik, Trond

2015-04-01

The Wilson Cycle theory that oceans close and reopen along the former suture is a fundamental concept in plate tectonics. The theory suggests that subduction initiates at a passive margin, closing the ocean, and that future continental extension localises at the ensuing collision zone. Each stage of the Wilson Cycle will therefore be characterised by inherited structural and thermal heterogeneities. Here we investigate the role of Wilson Cycle inheritance by considering the influence of (1) passive margin structure on continental collision and (2) collision zones on passive margin formation. Passive margins may be preferred locations for subduction initiation because inherited faults and areas of exhumed serpentinized mantle may weaken a margin enough to localise shortening. If subduction initiates at a passive margin, the shape and structure of the passive margins will affect future continental collision. Our review of present-day passive margins along the Atlantic and Indian Oceans reveals that most passive margins are located on former collision zones. Continental break-up occurs on relatively young sutures, such as Morocco-Nova Scotia, and on very old sutures, such as the Greenland-Labrador and East Antarctica-Australia systems. This implies that it is not always post-collisional collapse that initiates the extensional phase of a Wilson Cycle. We highlight the impact of collision zone inheritance on continental extension and rifted margin architecture. We show numerical experiments of one Wilson Cycle of subduction, collision, and extension. Subduction initiates at a tapered passive margin. Closure of a 60 Ma ocean leads to continental collision and slab break-off, followed by some tens of kilometres of slab eduction. Mantle flow above the sinking detached slab enhances deformation in the rift area. The resulting rift exposes not only continental crust, but also subduction-related sediments and oceanic crust remnants. Renewed subduction in the post-collision phase is enabled by lithosphere delamination and slab rollback, leading to back-arc extension in a style similar to the Tyrrhenian Sea.

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.

Revisiting the structure, age, and evolution of the Wharton Basin to better understand subduction under Indonesia

NASA Astrophysics Data System (ADS)

Jacob, Jensen; Dyment, Jérôme; Yatheesh, V.

2014-01-01

the subduction processes along the Sunda Trench requires detailed constraints on the subducting lithosphere. We build a detailed tectonic map of the Wharton Basin based on reinterpretation of satellite-derived gravity anomalies and marine magnetic anomalies. The Wharton Basin is characterized by a fossil ridge, dated 36.5 Ma, offset by N-S fracture zones. Magnetic anomalies 18 to 34 (38-84 Ma) are identified on both flanks, although a large part of the basin has been subducted. We analyze the past plate kinematic evolution of the Wharton Basin by two-plate (India-Australia) and three-plate (India-Australia-Antarctica) reconstructions. Despite the diffuse plate boundaries within the Indo-Australian plate for the last 20 Ma, we obtain finite rotation parameters that we apply to reconstruct the subducted Wharton Basin and constrain the thickness, buoyancy, and rheology of the subducting plate. The lower subductability of younger lithosphere off Sumatra has important consequences on the morphology, with a shallower trench, forearc islands, and a significant inward deviation of the subduction system. This deviation decreases in the youngest area, where the Wharton fossil spreading center enters subduction: The discontinuous magmatic crust and serpentinized upper mantle, consequences of the slow spreading rates at which this area was formed, weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism. Deeper effects include the possible creation of asthenospheric windows beneath the Andaman Sea, in relation to the long-offset fracture zones, and east of 105°E, as a result of subduction of the spreading center.

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.

Plateau subduction, intraslab seismicity and the Denali Volcanic Gap

NASA Astrophysics Data System (ADS)

Bostock, M. G.; Chuang, L. Y.; Wech, A.; Plourde, A. P.

2017-12-01

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40-58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region's unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.

Plateau subduction, intraslab seismicity, and the Denali (Alaska) volcanic gap

USGS Publications Warehouse

Chuang, Lindsay Yuling; Bostock, Michael; Wech, Aaron; Plourde, Alexandre

2018-01-01

Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40–58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region’s unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.

Migration Imaging of the Java Subduction Zones

NASA Astrophysics Data System (ADS)

Dokht, Ramin M. H.; Gu, Yu Jeffrey; Sacchi, Mauricio D.

2018-02-01

Imaging of tectonically complex regions can greatly benefit from dense network data and resolution enhancement techniques. Conventional methods in the analysis of SS precursors stack the waveforms to obtain an average discontinuity depth, but smearing due to large Fresnel zones can degrade the fine-scale topography on the discontinuity. To provide a partial solution, we introduce a depth migration algorithm based on the common scattering point method while considering nonspecular diffractions from mantle transition zone discontinuities. Our analysis indicates that, beneath the Sunda arc, the depth of the 410 km discontinuity (the 410) is elevated by 30 km and the 660 km discontinuity (the 660) is depressed by 20-40 km; the region of the strongest anticorrelation is correlated with the morphology of the subducting Indo-Australian slab. In eastern Java, a "flat" 410 coincides with a documented slab gap, showing length scales greater than 400 km laterally and 200 km vertically. This observation could be explained by the arrival of a buoyant oceanic plateau at the Java trench at approximately 8 Ma ago, which may have caused a temporary cessation of subduction and formed a tear in the subducting slab. Our results highlight contrasting depths of the 410 and 660 along the shallow-dipping slab below the Banda trench. The 660, however, becomes significantly uplifted beneath the Banda Sea, which is accompanied by enhanced reflection amplitudes. We interpret these observations as evidence for a subslab low-velocity zone, possibly related to the lower mantle upwelling beneath the subducting slab.

Structures in the transition zone of the northeast South China Sea: serpentinite dome vs mantle exhumation, or evidence of Mesozoic active subduction transferring to Cenozoic passive extension?

NASA Astrophysics Data System (ADS)

Sun, Z.; Zhou, D.

2013-12-01

Complete sedimentary sequences and weak erosion make the transition zone of the South China Sea the optimal place to study the entire evolution history of marginal sea basins, as well as the transition mechanism from active subduction to passive extension. 2D long cable seismic profiles revealed that both Baiyun and Liwan sag in the northeast South China Sea margin were lack of large controlling faults, especially in Liwan sag, syn-rift sequences waved above the basement. Dome-like uplifts(serpetinite uplifts?) or diapirs(?) came from below the basement, caused the syn-rift sequences pushed up around 36Ma(T80). Gravity inversion based on seismic reflection indicated that the dome has a lower density and a lower layer velocity than normal crust. Also around the Continent-Ocean Boundary (COB), a small segment similar to the lower crust was exposed. Between this exposed segment and the Cenozoic oceanic crust, mantle seems to be exhumed along the breakup point. Between the COB and roughly the shelf break, high velocity lower crust was discriminated in the northeast continental margin. Structures in northeast South China Sea seems having many similarities with Newfoundland-Iberia margin, by serpentinite(?) dome and exhumed mantle, although spreading rate here is intermediate. In fact, regional background suggests that there might be another interpretation: transition from Mesozoic subduction to Cenozoic extension occurred through paleo oceanic crust breakup in the northeast, which in turn retained Mesozoic subduction system beneath the northeast continental margin. Confined with magnetic anomaly, Bouguer gravity gradient anomaly, and well drilling lithological evidences, Cenozoic Baiyun sag developed upon Mesozoic fore-arc, while Cenozoic Liwan sag developed upon Mesozoic accretionary prism. The high velocity lower crust was caused by both remnant subducted slab and by Oceanic-Continent interaction due to subduction. There might also be serpentinite dome and exhumed mantle, but may be caused by extension and breakup of paleo oceanic slab, not the depth-dependent extension. IODP drillings are needed to test all these scientific conjectures.

Indo-Burmese subduction of the Bengal basin controlled by 90°E ridge collison imaged from deep seismic reflection data

NASA Astrophysics Data System (ADS)

Rangin, C.; Maurin, T.

2009-12-01

As a result of the Indo Burmese active hyper-oblique subduction, part of the Bay of Bengal is presently subducting eastward below the Burmese microplate. We have conducted two deep penetration seismic reflection surveys in the north-eastern Bay of Bengal, providing the first high resolution seismic image of the Bengal basin fill and basement. On basis of these data, we are able to trace the 90°E ridge much more northward than previously thought, i.e. up to 20°N along the Indo-Burmese plate boundary. We found out that the surface deformation, the deep structure of the subduction zone and the geometry of the plate boundary could all be strongly influenced by the impact of a prominent asperity, the 90°E ridge. These effects are variable along the margin. Between 15°N and 18°N, the ridge asperity brushes the active burmese plate boundary that strikes N10°E. At this latitude, all the structures framing the Indo-Burmese wedge have a similar N10°E trend. Deformation at the plate boundary is mainly strike slip. This is confirmed by the absence of subducted slab at depth as indicated by both seismicity and tomography. The small component of shortening along this plate boundary is probably accommodated partly by the flexure of the ridge and partly within the deformed upper plate. North of 19°N, the ridge vanishes progressively. The absence of basement topography together with the large amount of sediments provided by the Brahmaputra delta facilitates the fast westward growth of the Indo-Burmese wedge. The seismicity fits a well developed subducted slab at depth,. In the narrow transition zone between 18°N and 19°N, the 90°E ridge northern tips collides with the Burmese microplate. This collision could explain the rise of a subsuface flat and ramp system offshore Ramree and Cheduba islands, and the strong uplift of the Indo-Burmese wedge in Mount Victoria area.

Evidence for subduction-related magmatism during the Cretaceous and Cenozoic in Myanmar

NASA Astrophysics Data System (ADS)

Sevastjanova, Inga; Sagi, David Adam; Webb, Peter; Masterton, Sheona; Hill, Catherine; Davies, Clare

2017-04-01

Myanmar's complex geological history, numerous controversies around its tectonic evolution and the presence of prospective hydrocarbon basins make it a key area of interest for geologists. Understanding whether a passive or an active margin existed in the region during the Cenozoic is particularly important for the production of accurate basin models; active Cenozoic subduction would imply that hydrocarbon basins in the forearc experienced extension due to slab rollback. The geology of Myanmar was influenced by the regional tectonics associated with the Cretaceous and Cenozoic closure of the Neotethys Ocean. During this time, India travelled rapidly from Gondwana to Asia at speeds up to 20 cm/yr. To accommodate the north-eastward motion of India, the Neotethys Ocean was consumed at the subduction zone along the southern margin of Eurasia. Based on our Global Plate Model, this subduction zone can reasonably be expected to extend for the entire width of the Neotethys Ocean as far as Myanmar and Southeast Asia at their eastern extent. Moreover, a) Cretaceous volcanism onshore Myanmar, b) the middle Cenozoic arc-related extension in the Present Day eastern Andaman Sea and c) the late Cenozoic uplift of the Indo-Burman Ranges are all contemporaneous with the subduction ages predicted by the global plate motions. However, because of the geological complexity of the area, additional evidence would augment interpretations that are based on structural data. In an attempt to reduce the uncertainty in the existing interpretations, we have compiled published zircon geochronological data from detrital and igneous rocks in the region. We have used published zircon U-Pb ages and, where available, published Hf isotope data and CL images (core/rim) in order to distinguish 'juvenile' mantle-derived zircons from those of reworked crustal origin. The compilation shows that Upper Cretaceous and Cenozoic zircons, which are interpreted to have a volcanic provenance, are common across the Wuntho-Popa Arc and in the sedimentary basins onshore Myanmar (including the onshore Rakhine Basin and the Myanmar Central Basin), providing evidence for ongoing, although non-continuous, subduction in the region.

Structure and Evolution of the Forearc-Arc Crust Along the Tonga-Kermadec Subduction System from Integrated Geophysical Data

NASA Astrophysics Data System (ADS)

Funnell, M.; Peirce, C.; Robinson, A. H.; Watts, A. B.; Grevemeyer, I.

2016-12-01

Variations in tectonic forces and inputs to subduction systems generate, alter, and deform overriding crustal material. Although these processes are recorded in the crustal structure of volcanic arcs and their backarcs, the continuous nature of plate convergence superimposes subsequent episodes of crustal evolution on older features. Seismic imaging at modern subduction zones enhances our understanding of forearc development and variations in present-day deformation caused by inherited structures. In 2011 a set of multichannel and wide-angle seismic profiles imaged the forearc-arc crust and upper mantle structure along the 2700 km-long NNE-SSW trending Tonga-Kermadec subduction zone. The Tonga forearc region exhibits an 100 km-wide, 2 km high bathymetric elevation, with a 3 km-thick upper and mid-crust (Vp <6 km s-1), and a lower-crustal ridge 30 km wide comprising velocities up to 7.4 km s-1 that characterize an extinct Eocene ( 50 Ma) arc. By contrast, the active arc is <10 km wide and exhibits lower-crustal velocities below 7.0 km s-1, most likely representing intermediate compositions. This structural change suggests significant evolution, alteration, and modification of the overriding crust since the onset of subduction at this margin. Gravity anomaly modelling suggests that the extinct arc within the Tonga forearc region comprises relatively dense mafic-ultrabasic material that extends south beneath the Kermadec forearc and terminates at 32°S. The apparent southern termination of the extinct arc coincides with the partitioning of morphological features at 32°S, including a 10-km westward-step of the active arc and a 1.5 km deeper backarc to the south. We propose that tectonic partitioning about the 32°S boundary is the result of variations in the inherited crustal structure, which is divided by the presence and absence, to the north and south respectively, of the extinct volcanic arc.

Acoustic Reverse Time Migration of the Cascadia Subduction Zone Dataset

NASA Astrophysics Data System (ADS)

Jia, L.; Mallick, S.

2017-12-01

Reverse time migration (RTM) is a wave-equation based migration method, which provides more accurate images than ray-based migration methods, especially for the structures in deep areas, making it an effective tool for imaging the subduction plate boundary. In this work, we extend the work of Fortin (2015) and applied acoustic finite-element RTM on the Cascadia Subduction Zone (CSZ) dataset. The dataset was acquired by Cascadia Open-Access Seismic Transects (COAST) program, targeting the megathrust in the central Cascadia subduction zone (Figure 1). The data on a 2D seismic reflection line that crosses the Juan de Fuca/North American subduction boundary off Washington (Line 5) were pre-processed and worked through Kirchhoff prestack depth migration (PSDM). Figure 2 compares the depth image of Line 5 of the CSZ data using Kirchhoff PSDM (top) and RTM (bottom). In both images, the subducting plate is indicated with yellow arrows. Notice that the RTM image is much superior to the PSDM image by several aspects. First, the plate boundary appears to be much more continuous in the RTM image than the PSDM image. Second, the RTM image indicates the subducting plate is relatively smooth on the seaward (west) side between 0-50 km. Within the deformation front of the accretionary prism (50-80 km), the RTM image shows substantial roughness in the subducting plate. These features are not clear in the PSDM image. Third, the RTM image shows a lot of fine structures below the subducting plate which are almost absent in the PSDM image. Finally, the RTM image indicates that the plate is gently dipping within the undeformed sediment (0-50 km) and becomes steeply dipping beyond 50 km as it enters the deformation front of the accretionary prism. Although the same conclusion could be drawn from the discontinuous plate boundary imaged by PSDM, RTM results are far more convincing than the PSDM.

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.

Active-source 3-D tomography near Nias and Batu Islands, offshore central Sumatra

NASA Astrophysics Data System (ADS)

Karplus, M.; Henstock, T.; McNeill, L. C.; Vermeesch, P. M.; Hall, T. R.; Harmon, N.; Barton, P. J.

2013-12-01

Wide-angle reflection and refraction tomography constrain 3-D lithospheric P-wave velocity structure beneath the central Sumatra subduction zone from Nias Island to Siberut, offshore Indonesia at the southern boundary of the 2005 megathrust earthquake rupture. This area includes the earthquake segment boundary near the Batu Islands where the Investigator Fracture Zone is subducted beneath the Eurasian plate. We report along- and across-strike variations in structure of the downgoing slab and overriding plate. Seismic wide-angle data were collected during cruise SO198-1 in May-June 2008. Air gun shots were recorded by 47 temporary ocean bottom seismometers (OBS) deployed in a roughly 200 km by 190 km area, 10 three-component long-term OBS (with differential pressure gauge), and 52 land stations. First arrival refraction modeling using ray tracing and least squares inversion has yielded a lithospheric P-wave velocity model, best-resolved in the top 25 km. We observe velocities of ~4.5-6 km/s within the accretionary prism, which varies by several km in its depth extent. The forearc basin is underlain by high velocities of ~7-8 km/s as shallow as 8 km depth. This high velocity region is likely older forearc oceanic crust, as seen in Cascadia and near Simeulue, offshore Sumatra. The top of the subducting slab ranges in depth from ~10 km near the trench to ~20 km beneath the prism. The top of the slab dips approximately 4-4.5° towards the NE between the trench and the prism. Earthquake hypocenters show the slab dip steepens significantly NE of the forearc basin. We compare our velocity models with models derived from other regions to the north and south along-strike in the Sumatra Subduction Zone, including the 2004-2005 segment boundary at Simeulue. Multi-channel seismic reflection data show that fault structures and reflectivity change considerably along- and across-strike in the central Sumatra subduction zone. Furthermore, regional earthquake locations indicate rupture segmentation along the plate boundary. The Nias segment in the north ruptured in the 2005 M8.7 earthquake. The weakly-coupled Batu segment experiences sporadic clusters of events near the break in the forearc slope. The offshore forearc west of Siberut is characterized by almost aseismic behavior, reflecting the locked state of the plate interface, which hasn't ruptured since the 1797 M8.6-8.8 earthquake. The subducting Investigator Fracture Zone is believed to act as a barrier for propagation of slip during large ruptures. We compare our velocity model with reflection data and rupture segments to characterize differences in the lower plate, upper plate, and plate boundary properties.

Seismic Structure of Mantle Transition Zone beneath Northwest Pacific Subduction Zone and its Dynamic Implication

NASA Astrophysics Data System (ADS)

Li, J.; Guo, G.; WANG, X.; Chen, Q.

2017-12-01

The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Various and complex geometry of the Pacific subducting slab can be well traced downward from the Kuril, Japan and Izu-Bonin trench using seismicity and tomography images (Fukao and Obayashi, 2013). Due to the sparse distribution of seismic stations in the sea, investigation of the deep mantle structure beneath the broad sea regions is very limited. In this study, we applied the well- developed multiple-ScS reverberations method (Wang et al., 2017) to analyze waveforms recorded by the Chinese Regional Seismic Network, the densely distributed temporary seismic array stations installed in east Asia. A map of the topography of the upper mantle discontinuities beneath the broad oceanic regions in northwest Pacific subduction zone is imaged. We also applied the receiver function analysis to waveforms recorded by stations in northeast China and obtain the detailed topography map beneath east Asia continental regions. We then combine the two kinds of topography of upper mantle discontinuities beneath oceanic and continental regions respectively, which are obtained from totally different methods. A careful image matching and spatial correlation is made in the overlapping study regions to calibrate results with different resolution. This is the first time to show systematically a complete view of the topography of the 410-km and 660-km discontinuities beneath the east Asia "Big mantle wedge" (Zhao and Ohtani, 2009) covering the broad oceanic and continental regions in the Northwestern Pacific Subduction zone. Topography pattern of the 660 and 410 is obtained and discussed. Especially we discovered a broad depression of the 410-km discontinuity covering more than 1000 km in lateral, which seems abnormal in the cold subducting tectonic environment. Based on plate tectonic reconstruction studies and HTHP mineral experiments, we argue that the east-retreat trench motion of the subducting Pacific slab might play an important role in the observed broad depression of the 410-km discontinuity.

Deformation fabrics of blueschist facies phengite-rich, epidote-glaucophane schists from Ring Mountain, California and implications for seismic anisotropy in subduction zone

NASA Astrophysics Data System (ADS)

Jung, H.; HA, Y.; Raymond, L. A.

2016-12-01

In many subduction zones, strong seismic anisotropy is observed. A part of the seismic anisotropy can be attributed to the subducting oceanic crust, which is transformed to blueschist facies rocks under high-pressure, high-temperature conditions. Because glaucophane, epidote, and phengite constituting the glaucophane schists are very anisotropic elastically, seismic anisotropy in the oceanic crust in hot subduction zones can be attributed to the lattice preferred orientation (LPO) of these minerals. We studied deformation fabrics and seismic properties of phengite-rich, epidote-glaucophane schists from the Franciscan Complex of Ring Mountain, California. The blueschist samples are mainly composed of glaucophane, epidote, and phengite, with minor garnet, titanite, and chlorite. Some samples contain abundant phengite (up to 40 %). We determined LPOs of minerals using SEM/EBSD and calculated seismic anisotropy of minerals and whole rocks. LPOs of glaucophane have [001] axes aligned subparallel to lineation, and both (110) poles and [100] axes subnormal to foliation. Epidote [001] axes are aligned subnormal to foliation, with both (110) and (010) poles subparallel to lineation. LPOs of phengite are characterized by maxima of [001] axes subnormal to foliation, and both (110) and (010) poles and [100] axes aligned in a girdle subparallel to foliation. Phengite showed much stronger seismic anisotropy (AVP = 42%, max.AVS = 37%) than glaucophane or epidote. Glaucophane schist with abundant phengite showed much stronger seismic anisotropy (AVP = 30%, max.AVS = 23%) than epidote-glaucophane schist without phengite (AVP = 13%, max.AVS = 9%). Therefore, phengite clearly can significantly affect seismic anisotropy of whole rocks. When the subduction angle of phengite-rich blueschist facies rocks is considered for a 2-D corner flow model, the polarization direction of fast S-waves for vertically propagating S-waves changed to a nearly trench-parallel direction for the subduction angle of 45-70° and shear wave anisotropy (AVS) became stronger for vertically propagating S-waves with increasing subduction angle. Our data showed that phengite-rich blueschist, therefore, can contribute to strong trench-parallel seismic anisotropy observed in many subduction zones.

Extensive hydrothermal activity in the NE Lau basin revealed by ROV dives

NASA Astrophysics Data System (ADS)

Embley, R. W.; Resing, J. A.; Tebo, B.; Baker, E. T.; Butterfield, D. A.; Chadwick, B.; Davis, R.; de Ronde, C. E. J.; Lilley, M. D.; Lupton, J. E.; Merle, S. G.; Rubin, K. H.; Shank, T. M.; Walker, S. L.; Arculus, R. J.; Bobbitt, A. M.; Buck, N. J.; Caratori Tontini, F.; Crowhurst, P. V.; Mitchell, E.; Olson, E. J.; Ratmeyer, V.; Richards, S.; Roe, K. K.; Kenner-Chavis, P.; Martinez-Lyons, A.; Sheehan, C.; Brian, R.

2014-12-01

Dives with the QUEST 4000 ROV (Remotely Operated Vehicle) in September 2012 discovered nine hydrothermal sites in the arc and rear-arc region of the NE Lau Basin in 1150 m to 2630 m depth. These sites, originally detected by water column and seafloor surveys conducted in 2008-2011, include: (1) a paired sulfur-rich/black smoker field on the summit of a tectonically deformed magmatic arc volcano (Niua), (2) fracture-controlled black smoker venting on several small en echelon seamounts (north Matas) that lie between the magmatic arc and the backarc spreading center and (3) a magmatic degassing site on the summit of a dacite cone within a large (~12 km diameter) caldera volcano (Niuatahi). Dives at West Mata Seamount, which was undergoing strombolian volcanic activity and effusive rift-zone eruptions from 2008 to 2010, revealed a dormant volcanic phase in September 2012, with continued low-temperature diffuse venting. The high-temperature venting is likely driven by magmatic heat indicative of underlying partial melt zones and/or melt pockets distributed through the region. The occurrence of the youngest known boninite eruptions on the Mata volcanoes is consistent with subduction fluid flux melting extending into the rear-arc zone. Extension related to the transition from subduction to strike-slip motion of the northern Tonga Arc over the active Subduction-Transform Edge Propagator (STEP) fault probably contributes to the enhanced volcanism/hydrothermal activity in the NE Lau Basin. Chemosynthetic ecosystems at these sites range from mostly motile, lower diversity ecosystems at the eruptive/magmatically-degassing sites to higher diversity ecosystems with less mobile faunal components at the black-smoker systems. The wide range of fluid chemistry, water depth and geologic settings of the hydrothermal systems in this area provides an intriguing template to study the interaction of hydrothermal fluid chemistry, chemosynthetic habitats and their geologic underpinning within an arc/backarc setting.

Hyperactive hydrothermal activity in the NE Lau basin revealed by ROV dives

NASA Astrophysics Data System (ADS)

Embley, R. W.; Resing, J. A.; Tebo, B.; Baker, E. T.; Butterfield, D. A.; Chadwick, B.; Davis, R.; de Ronde, C. E.; Lilley, M. D.; Lupton, J. E.; Merle, S. G.; Rubin, K. H.; Shank, T. M.; Walker, S. L.; Arculus, R. J.; Bobbitt, A. M.; Buck, N.; Caratori Tontini, F.; Crowhurst, P. V.; Mitchell, E.; Olson, E. J.; Ratmeyer, V.; Richards, S.; Roe, K. K.; Keener, P.; Martinez Lyons, A.; Sheehan, C.; Brian, R.

2013-12-01

Dives with the QUEST 4000 ROV (Remotely Operated Vehicle) in September 2012 discovered nine hydrothermal sites in the arc and rear-arc region of the NE Lau Basin in 1150 m to 2630 m depth. These sites, originally detected by water column and seafloor surveys conducted in 2008-2011, include: (1) a paired sulfur-rich/black smoker field on the summit of a tectonically deformed magmatic arc volcano (Niua), (2) fracture-controlled black smoker venting on several small en echelon seamounts (north Matas) that lie between the magmatic arc and the backarc spreading center and (3) a magmatic degassing site on the summit of a dacite cone within a large (~12 km diameter) caldera volcano (Niuatahi). Dives at West Mata Seamount, which was undergoing strombolian volcanic activity and effusive rift-zone eruptions from 2008 to 2010, revealed a dormant volcanic phase in September 2012, with continued low-temperature diffuse venting. The high-temperature venting is likely driven by magmatic heat indicative of underlying partial melt zones and/or melt pockets distributed through the region. The occurrence of the youngest known boninite eruptions on the Mata volcanoes is consistent with subduction fluid flux melting extending into the rear-arc zone. Extension related to the transition from subduction to strike-slip motion of the northern Tonga Arc over the active Subduction-Transform Edge Propagator (STEP) fault probably contributes to the enhanced volcanism/hydrothermal activity in the NE Lau Basin. Chemosynthetic ecosystems at these sites range from mostly motile, lower diversity ecosystems at the eruptive/magmatically-degassing sites to higher diversity ecosystems with less mobile faunal components at the black-smoker systems. The wide range of fluid chemistry, water depth and geologic settings of the hydrothermal systems in this area provides an intriguing template to study the interaction of hydrothermal fluid chemistry, chemosynthetic habitats and their geologic underpinning within an arc/backarc setting.

Numerical modeling of fluid migration in subduction zones

NASA Astrophysics Data System (ADS)

Walter, Marius J.; Quinteros, Javier; Sobolev, Stephan V.

2015-04-01

It is well known that fluids play a crucial role in subduction evolution. For example, excess mechanical weakening along tectonic interfaces, due to excess fluid pressure, may enable oceanic subduction. Hence, the fluid content seems to be a critical parameter for subduction initiation. Studies have also shown a correlation between the location of slab dehydration and intermediate seismic activity. Furthermore, expelled fluids from the subduction slab affect the melting temperature, consequently, contributing to partial melting in the wedge above the downgoing plate, and resulting in chemical changes in earth interior and extensive volcanism. In summary, fluids have a great impact on tectonic processes and therefore should be incorporated into geodynamic numerical models. Here we use existing approaches to couple and solve fluid flow equations in the SLIM-3D thermo-mechanical code. SLIM-3D is a three-dimensional thermo-mechanical code capable of simulating lithospheric deformation with elasto-visco-plastic rheology. It incorporates an arbitrary Lagrangian Eulerian formulation, free surface, and changes in density and viscosity, due to endothermic and exothermic phase transitions. It has been successfully applied to model geodynamic processes at different tectonic settings, including subduction zones. However, although SLIM-3D already includes many features, fluid migration has not been incorporated into the model yet. To this end, we coupled solid and fluid flow assuming that fluids flow through a porous and deformable solid. Thereby, we introduce a two-phase flow into the model, in which the Stokes flow is coupled with the Darcy law for fluid flow. This system of equations becomes, however, nonlinear, because the rheology and permeability are depended on the porosity (fluid fraction of the matrix). Ultimately, the evolution of porosity is governed by the compaction pressure and the advection of the porous solid. We show the details of our implementation of the fluid flow into the existing thermo-mechanical finite element code and present first results of benchmarks (e.g. solitary wave) and experiments. We are especially interested in the coupling of subduction processes and the evolution of the magmatic arc. Thereby, we focus on the key factors controlling magma emplacement and its influence on subduction processes.

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

NASA Astrophysics Data System (ADS)

Tetreault, J. L.; Buiter, S. J. H.

2014-12-01

Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to accrete and others to subduct. In many modern FATs on the ocean floor, a sub-crustal layer of high seismic velocities, interpreted as ultramafic material, could serve as a detachment or delaminate during subduction.

Cenozoic tectonics of western North America controlled by evolving width of Farallon slab.

PubMed

Schellart, W P; Stegman, D R; Farrington, R J; Freeman, J; Moresi, L

2010-07-16

Subduction of oceanic lithosphere occurs through two modes: subducting plate motion and trench migration. Using a global subduction zone data set and three-dimensional numerical subduction models, we show that slab width (W) controls these modes and the partitioning of subduction between them. Subducting plate velocity scales with W(2/3), whereas trench velocity scales with 1/W. These findings explain the Cenozoic slowdown of the Farallon plate and the decrease in subduction partitioning by its decreasing slab width. The change from Sevier-Laramide orogenesis to Basin and Range extension in North America is also explained by slab width; shortening occurred during wide-slab subduction and overriding-plate-driven trench retreat, whereas extension occurred during intermediate to narrow-slab subduction and slab-driven trench retreat.

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.

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.

Experimental constraints on the serpentinization rate of fore-arc peridotites: Implications for the upwelling condition of the slab-derived fluid

NASA Astrophysics Data System (ADS)

Nakatani, T.; Nakamura, M.

2016-08-01

To constrain the water circulation in subduction zones, the hydration rates of peridotites were investigated experimentally in fore-arc mantle conditions. Experiments were conducted at 400-580°C and 1.3 and 1.8 GPa, where antigorite is expected to form as a stable serpentine phase. Crushed powders of olivine ± orthopyroxene and orthopyroxene + clinopyroxene were reacted with 15 wt % distilled water for 4-19 days. The synthesized serpentine varieties were lizardite and aluminous lizardite (Al-lizardite) in all experimental conditions except those of 1.8 GPa and 580°C in the olivine + orthopyroxene system, in which antigorite was formed. In the olivine + orthopyroxene system, the reactions were interface-controlled except for the reaction at 400°C, which was transport-controlled. The corresponding reaction rates were 7.0 × 10-12 to 1.5 × 10-11 m s-1 at 500-580°C and 7.5 × 10-16 m2 s-1 at 400°C for the interface and transport-controlled reactions, respectively. Based on a simple reaction-transport model including these hydration rates, we infer that penetration of the slab-derived fluid all the way through a water-unsaturated fore-arc mantle is allowed only when focused flow occurs with a spacing larger than 77-229 km in hot subduction zones such as Nankai and Cascadia. However, the necessary spacing is only 2.3-4.6 m in intermediate-temperature subduction zones such as Kyushu and Costa Rica. These calculations imply that fluid leakage in hot subduction zones may occur after the fore-arc mantle is totally hydrated, whereas in intermediate-temperature subduction zones, leakage through a water-unsaturated fore-arc mantle may be facilitated.

Towards Estimating the Magnitude of Earthquakes from EM Data Collected from the Subduction Zone

NASA Astrophysics Data System (ADS)

Heraud, J. A.

2016-12-01

During the past three years, magnetometers deployed in the Peruvian coast have been providing evidence that the ULF pulses received are indeed generated at the subduction or Benioff zone. Such evidence was presented at the AGU 2015 Fall meeting, showing the results of triangulation of pulses from two magnetometers located in the central area of Peru, using data collected during a two-year period. The process has been extended in time, only pulses associated with the occurrence of earthquakes and several pulse parameters have been used to estimate a function relating the magnitude of the earthquake with the value of a function generated with those parameters. The results shown, including an animated data video, are a first approximation towards the estimation of the magnitude of an earthquake about to occur, based on electromagnetic pulses that originated at the subduction zone. During the past three years, magnetometers deployed in the Peruvian coast have been providing evidence that the ULF pulses received are indeed generated at the subduction or Benioff zone. Such evidence was presented at the AGU 2015 Fall meeting, showing the results of triangulation of pulses from two magnetometers located in the central area of Peru, using data collected during a two-year period. The process has been extended in time, only pulses associated with the occurrence of earthquakes have been used and several pulse parameters have been used to estimate a function relating the magnitude of the earthquake with the value of a function generated with those parameters. The results shown, including an animated data video, are a first approximation towards the estimation of the magnitude of an earthquake about to occur, based on electromagnetic pulses that originated at the subduction zone.

Estimates of effective elastic thickness of oceanic lithosphere using model including surface and subsurface loads and effective elastic thickness of subduction zones

NASA Astrophysics Data System (ADS)

Yang, A.; Yongtao, F.

2016-12-01

The effective elastic thickness (Te) is an important parameter that characterizes the long term strength of the lithosphere, which has great significance on understanding the mechanical properties and evolution of the lithosphere. In contrast with many controversies regarding elastic thickness of continent lithosphere, the Te of oceanic lithosphere is thought to be in a simple way that is dependent on the age of the plate. However, rescent studies show that there is no simple relationship between Te and age at time of loading for both seamounts and subduction zones. As subsurface loading is very importand and has large influence in the estimate of Te for continent lithosphere, and many oceanic features such as subduction zones also have considerable subsurface loading. We introduce the method to estimate the effective elastic thickness of oceanic lithosphere using model including surface and subsurface loads by using free-air gravity anomaly and bathymetric data, together with a moving window admittance technique (MWAT). We use the multitaper spectral estimation method to calculate the power spectral density. Through tests with synthetic subduction zone like bathymetry and gravity data show that the Te can be recovered in an accurance similar to that in the continent and there is also a trade-off between spatial resolution and variance for different window sizes. We estimate Te of many subduction zones (Peru-Chile trench, Middle America trench, Caribbean trench, Kuril-Japan trench, Mariana trench, Tonga trench, Java trench, Ryukyu-Philippine trench) with an age range of 0-160 Myr to reassess the relationship between elastic thickness and the age of the lithosphere at the time of loading. The results do not show a simple relationship between Te and age.

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.

Pronounced zonation of seismic anisotropy in the Western Hellenic subduction zone and its geodynamic significance

NASA Astrophysics Data System (ADS)

Olive, Jean-Arthur; Pearce, Frederick; Rondenay, Stéphane; Behn, Mark D.

2014-04-01

Many subduction zones exhibit significant retrograde motion of their arc and trench. The observation of fast shear-wave velocities parallel to the trench in such settings has been inferred to represent trench-parallel mantle flow beneath a retreating slab. Here, we investigate this process by measuring seismic anisotropy in the shallow Aegean mantle. We carry out shear-wave splitting analysis on a dense array of seismometers across the Western Hellenic Subduction Zone, and find a pronounced zonation of anisotropy at the scale of the subduction zone. Fast SKS splitting directions subparallel to the trench-retreat direction dominate the region nearest to the trench. Fast splitting directions abruptly transition to trench-parallel above the corner of the mantle wedge, and rotate back to trench-normal over the back-arc. We argue that the trench-normal anisotropy near the trench is explained by entrainment of an asthenospheric layer beneath the shallow-dipping portion of the slab. Toward the volcanic arc this signature is overprinted by trench-parallel anisotropy in the mantle wedge, likely caused by a layer of strained serpentine immediately above the slab. Arcward steepening of the slab and horizontal divergence of mantle flow due to rollback may generate an additional component of sub-slab trench-parallel anisotropy in this region. Poloidal flow above the retreating slab is likely the dominant source of back-arc trench-normal anisotropy. We hypothesize that trench-normal anisotropy associated with significant entrainment of the asthenospheric mantle near the trench may be widespread but only observable at shallow-dipping subduction zones where stations nearest the trench do not overlie the mantle wedge.

Evolution of the Grenada and Tobago basins and the onset of the Lesser Antilles subduction zone

NASA Astrophysics Data System (ADS)

Zitter, T. A. C.; Rangin, C.

2012-04-01

The Lesser Antilles active island arc marks the eastern boundary of the Caribbean plate, where the Atlantic oceanic crust is subducted. Geodynamic history of the Grenada and Tobago basins, accepted as both the back arc and fore arc basins respectively for this convergent zone, is the key for a better understanding of the Antilles arc subduction onset. Still, recent studies propose that these two basins formed as a single paleogene depocenter. Analysis of industrial and academical seismic profiling supports this hypothesis, and shows these basins are two half-graben filled by 15 kilometers of cenozoic sediments. The seismic profiles across these basins, and particularly the Geodinos Bolivar seismic profiles, indicate that the Antilles magmatic arc develops in the midst of the previously-extended Grenada-Tobago basin from Miocene time to present. The pre-cenozoic basement of the Grenada-Tobago basin can be traced from the Aves ridge to the Tobago Island where cretaceous meta-volcanic rocks are cropping out. Therefore, this large basin extension has been initiated in early Paleocene time during stretching or subsidence of the great cretaceous Caribbean arc and long time before the onset of the lesser Antilles volcanic arc. The question arises for the mechanism responsible of this intra-plate extension. The Tobago Ridge consists of the backstop of the Barbados prism. The innermost wedge is particularly well imaged on seismic data along the Darien Ridge, where the isopach paleogene sediments are jointly deformed in latest Oligocene. This deformation is starved with the early miocene piggy-back basin. Hence, we conclude the innermost wedge in contact with the butresss is late Oligocene in age and can be considered as the onset of the subduction along the Antilles arc. This 30 Ma subduction onset is also supported by the 750 km long Atlantic slab, imaged in tomography, indicating this subduction was active with constant velocity of 2.5 km/yr. Consequently, another mechanism, than the Atlantic subduction, has to be invoked for the formation of the Grenada-Tobago depocenter prior to 30 Ma. These results are part of a cooperative research-industry programm conducted by CEREGE/EGERIE, Aix-en-Provence and GeoAzur, Nice, with Frontier Basin study group TOTAL S.A., Paris.

Dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust

NASA Astrophysics Data System (ADS)

Okazaki, Keishi; Hirth, Greg

2016-02-01

Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the mantle. In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Mohorovičić discontinuity. These observations have stimulated interest in relationships between blueschist-facies metamorphism and seismicity, particularly through dehydration reactions involving the mineral lawsonite. Here we conducted deformation experiments on lawsonite, while monitoring acoustic emissions, in a Griggs-type deformation apparatus. The temperature was increased above the thermal stability of lawsonite, while the sample was deforming, to test whether the lawsonite dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (that is, stick-slip) occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R1 and B shears), and that the fault surface develops slickensides (very smooth fault surfaces polished by frictional sliding). The unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions, regardless of the strain rate and temperature ramping rate. A thermomechanical scaling factor for the experiments is within the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers.

Detailed Image of the Subducting Plate and Upper mantle Seismic Discontinuities in the Mariana Subduction Zone

NASA Astrophysics Data System (ADS)

Tibi, R.; Wiens, D. A.; Shiobara, H.; Sugioka, H.; Yuan, X.

2006-12-01

We use P-to-S converted teleseismic phases recorded at island and ocean bottom stations in Mariana to image the subducting plate and the upper mantle seismic discontinuities in the Mariana subduction zone. The land and seafloor stations which operated from June 2003 to May 2004, were deployed within the framework of the MARGINS Subduction Factory experiment of the Mariana system. The crust in the sudducting plate is observed at about 80--90 km depth beneath the islands of Saipan, Tinian and Rota. For most of the island stations, a low velocity layer is imaged in the forearc at depth between about 20 and 60 km, with decreasing depths toward the arc. The nature of this feature is not yet clear. We found evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of ~650--715 km, and a deeper interface lies at ~740-- 770 km depth. The amplitudes of the seismic signals suggest that the shear velocity contrasts across the two features are comparable. These characteristics support the interpretation that the discontinuities are the results of the phase transformations in olivine (ringwoodite to post-spinel) and garnet (ilminite to perovskite), respectively, for the pyrolite model of mantle composition.

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'.

Fossil intermediate-depth earthquakes in subducting slabs linked to differential stress release

NASA Astrophysics Data System (ADS)

Scambelluri, Marco; Pennacchioni, Giorgio; Gilio, Mattia; Bestmann, Michel; Plümper, Oliver; Nestola, Fabrizio

2017-12-01

The cause of intermediate-depth (50-300 km) seismicity in subduction zones is uncertain. It is typically attributed either to rock embrittlement associated with fluid pressurization, or to thermal runaway instabilities. Here we document glassy pseudotachylyte fault rocks—the products of frictional melting during coseismic faulting—in the Lanzo Massif ophiolite in the Italian Western Alps. These pseudotachylytes formed at subduction-zone depths of 60-70 km in poorly hydrated to dry oceanic gabbro and mantle peridotite. This rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present-day subduction. The pseudotachylytes locally preserve high-pressure minerals that indicate an intermediate-depth seismic environment. These pseudotachylytes are important because they are hosted in a near-anhydrous lithosphere free of coeval ductile deformation, which excludes an origin by dehydration embrittlement or thermal runaway processes. Instead, our observations indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong dry metastable rocks.

Ediacaran 2,500-km-long synchronous deep continental subduction in the West Gondwana Orogen

NASA Astrophysics Data System (ADS)

Ganade de Araujo, Carlos E.; Rubatto, Daniela; Hermann, Joerg; Cordani, Umberto G.; Caby, Renaud; Basei, Miguel A. S.

2014-10-01

The deeply eroded West Gondwana Orogen is a major continental collision zone that exposes numerous occurrences of deeply subducted rocks, such as eclogites. The position of these eclogites marks the suture zone between colliding cratons, and the age of metamorphism constrains the transition from subduction-dominated tectonics to continental collision and mountain building. Here we investigate the metamorphic conditions and age of high-pressure and ultrahigh-pressure eclogites from Mali, Togo and NE-Brazil and demonstrate that continental subduction occurred within 20 million years over at least a 2,500-km-long section of the orogen during the Ediacaran. We consider this to be the earliest evidence of large-scale deep-continental subduction and consequent appearance of Himalayan-scale mountains in the geological record. The rise and subsequent erosion of such mountains in the Late Ediacaran is perfectly timed to deliver sediments and nutrients that are thought to have been necessary for the subsequent evolution of sustainable life on Earth.

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

Slab melting versus slab dehydration in subduction-zone magmatism

PubMed Central

Mibe, Kenji; Kawamoto, Tatsuhiko; Matsukage, Kyoko N.; Fei, Yingwei; Ono, Shigeaki

2011-01-01

The second critical endpoint in the basalt-H2O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones. PMID:21536910

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.

Tidal modulation of nonvolcanic tremor.

PubMed

Rubinstein, Justin L; La Rocca, Mario; Vidale, John E; Creager, Kenneth C; Wech, Aaron G

2008-01-11

Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks and differ from "normal" earthquakes in their source location and moment-duration scaling. The three most recent episodes in the Puget Sound/southern Vancouver Island portion of the Cascadia subduction zone were exceptionally well recorded. In each episode, we saw clear pulsing of tremor activity with periods of 12.4 and 24 to 25 hours, the same as the principal lunar and lunisolar tides. This indicates that the small stresses associated with the solid-earth and ocean tides influence the genesis of tremor much more effectively than they do the genesis of normal earthquakes. Because the lithostatic stresses are 10(5) times larger than those associated with the tides, we argue that tremor occurs on very weak faults.

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.

The vanadium isotope compositions of subduction zone lavas

NASA Astrophysics Data System (ADS)

Tian, S.; Huang, F.

2017-12-01

Vanadium is a redox sensitive element with multiple oxidation states, and thus it has the potential to trace redox-related processes. With the advancement of analytical method for V isotopes, we are now able to recognize V isotope fractionation for igneous rocks. Subduction zones are critical zones on the Earth for the interaction between crust and mantle where undergo complex geological processes. However, V isotope data of subduction zone lavas are still rare except those reported in [1]. To investigate the V isotope variations of subduction zones and discuss the potential to apply V to trace mantle redox state. In this contribution, we report δ51V for three subduction zone lavas from Kamchatka, Lesser Antilles, and Aleutians which are characterized by well-documented magmatic evolutionary series. 47 arc lava samples have been analyzed and the δ51V data of them range from -0.91‰ to -0.53‰ (2sd = 0.10 ‰). Among these samples, primitive arc basalts with MgO > 6 wt. % have an average δ51V of -0.80 ± 0.15‰ (2sd, n = 20), broadly consistent with δ51V data of MORB [2, 3]. Within the single arc of Kamchatka, δ51V data of primitive basalts from the arc front to the back-arc is almost constant, suggesting limited influences of mantle melting and source heterogeneity on V isotopes. δ51V data of these samples show no correlation with Ba/Nb, suggesting that fluids have little impact on V isotopes. On the other hand, δ51V data of the more involved samples with MgO < 6 wt. % are negatively correlated with MgO contents, indicating that the 50V preferentially enters crystalline minerals, which produces heavier V isotope compositions of residual melts. Distinct to the observation showing 2‰ fractionation reported in [1], the magnitude of V isotope fractionation in arc lavas is much smaller (0.38‰) in this study. Future works are needed for better understanding the V isotope fractionation mechanisms of subduction zone lavas. [1]Prytulak et al., 2017, Geochem. Persp. Let. 3, 75-84. [2]Huang et al., 2016, Goldschmidt Abstracts. 1190. [3] Prytulak et al., 2013, EPSL. 365, 177-189.

Seismicity of the Indo-Australian/Solomon Sea Plate boundary in the Southeast Papua region

NASA Astrophysics Data System (ADS)

Ripper, I. D.

1982-08-01

Seismicity and earthquake focal mechanism plots of the Southeast Papua and Woodlark Basin region for the period January 1960 to May 1979 show that: (a) the West Woodlark Basin spreading centre extends from the deep West Woodlark Basin, through Dawson Strait into Goodenough Bay, Southeast Papua; (b) a southeast seismic trend in the West Woodlark Basin is associated with a left-lateral transform fault, but a gap exists between this zone and the seismic East Woodlark Basin spreading centre; (c) Southeast Papua Seismicity divides into a shallow earthquake zone in which the earthquakes occur mainly in the northeast side of the Owen Stanley Range, and an intermediate depth southwest dipping Benioff zone which extends almost from Mt. Lamington to Goroka. The Benioff zone indicates the presence of a southwest dipping slab of Solomon Sea Plate beneath the Indo-Australian Plate in the Southeast Papua and Ramu-Markham Valley region. This subduction zone has collided with the New Britain subduction zone of the Solomon Sea Plate along the Ramu-Markham Valley. The Solomon Sea Plate is now hanging suspended in the form of an arch beneath Ramu-Markham Valley, inhibiting further subduction beneath Southeast Papua.

2D Simulations of Earthquake Cycles at a Subduction Zone Based on a Rate and State Friction Law -Effects of Pore Fluid Pressure Changes-

NASA Astrophysics Data System (ADS)

Mitsui, Y.; Hirahara, K.

2006-12-01

There have been a lot of studies that simulate large earthquakes occurring quasi-periodically at a subduction zone, based on the laboratory-derived rate-and-state friction law [eg. Kato and Hirasawa (1997), Hirose and Hirahara (2002)]. All of them assume that pore fluid pressure in the fault zone is constant. However, in the fault zone, pore fluid pressure changes suddenly, due to coseismic pore dilatation [Marone (1990)] and thermal pressurization [Mase and Smith (1987)]. If pore fluid pressure drops and effective normal stress rises, fault slip is decelerated. Inversely, if pore fluid pressure rises and effective normal stress drops, fault slip is accelerated. The effect of pore fluid may cause slow slip events and low-frequency tremor [Kodaira et al. (2004), Shelly et al. (2006)]. For a simple spring model, how pore dilatation affects slip instability was investigated [Segall and Rice (1995), Sleep (1995)]. When the rate of the slip becomes high, pore dilatation occurs and pore pressure drops, and the rate of the slip is restrained. Then the inflow of pore fluid recovers the pore pressure. We execute 2D earthquake cycle simulations at a subduction zone, taking into account such changes of pore fluid pressure following Segall and Rice (1995), in addition to the numerical scheme in Kato and Hirasawa (1997). We do not adopt hydrostatic pore pressure but excess pore pressure for initial condition, because upflow of dehydrated water seems to exist at a subduction zone. In our model, pore fluid is confined to the fault damage zone and flows along the plate interface. The smaller the flow rate is, the later pore pressure recovers. Since effective normal stress keeps larger, the fault slip is decelerated and stress drop becomes smaller. Therefore the smaller flow rate along the fault zone leads to the shorter earthquake recurrence time. Thus, not only the frictional parameters and the subduction rate but also the fault zone permeability affects the recurrence time of earthquake cycle. Further, the existence of heterogeneity in the permeability along the plate interface can bring about other slip behaviors, such as slow slip events. Our simulations indicate that, in addition to the frictional parameters, the permeability within the fault damage zone is one of essential parameters, which controls the whole earthquake cycle.

Fluid pressure and shear zone development over the locked to slow slip region in Cascadia.

PubMed

Audet, Pascal; Schaeffer, Andrew J

2018-03-01

At subduction zones, the deep seismogenic transition from a frictionally locked to steady sliding interface is thought to primarily reflect changes in rheology and fluid pressure and is generally located offshore. The development of fluid pressures within a seismic low-velocity layer (LVL) remains poorly constrained due to the scarcity of dense, continuous onshore-offshore broadband seismic arrays. We image the subducting Juan de Fuca oceanic plate in northern Cascadia using onshore-offshore teleseismic data and find that the signature of the LVL does not extend into the locked zone. Thickening of the LVL down dip where viscous creep dominates suggests that it represents the development of an increasingly thick and fluid-rich shear zone, enabled by fluid production in subducting oceanic crust. Further down dip, episodic tremor, and slip events occur in a region inferred to have locally increased fluid pressures, in agreement with electrical resistivity structure and numerical models of fault slip.

Fluid pressure and shear zone development over the locked to slow slip region in Cascadia

PubMed Central

Audet, Pascal; Schaeffer, Andrew J.

2018-01-01

At subduction zones, the deep seismogenic transition from a frictionally locked to steady sliding interface is thought to primarily reflect changes in rheology and fluid pressure and is generally located offshore. The development of fluid pressures within a seismic low-velocity layer (LVL) remains poorly constrained due to the scarcity of dense, continuous onshore-offshore broadband seismic arrays. We image the subducting Juan de Fuca oceanic plate in northern Cascadia using onshore-offshore teleseismic data and find that the signature of the LVL does not extend into the locked zone. Thickening of the LVL down dip where viscous creep dominates suggests that it represents the development of an increasingly thick and fluid-rich shear zone, enabled by fluid production in subducting oceanic crust. Further down dip, episodic tremor, and slip events occur in a region inferred to have locally increased fluid pressures, in agreement with electrical resistivity structure and numerical models of fault slip. PMID:29536046

Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation

NASA Astrophysics Data System (ADS)

Villegas-Lanza, J. C.; Chlieh, M.; Cavalié, O.; Tavera, H.; Baby, P.; Chire-Chira, J.; Nocquet, J.-M.

2016-10-01

Over 100 GPS sites measured in 2008-2013 in Peru provide new insights into the present-day crustal deformation of the 2200 km long Peruvian margin. This margin is squeezed between the eastward subduction of the oceanic Nazca Plate at the South America trench axis and the westward continental subduction of the South American Plate beneath the Eastern Cordillera and Subandean orogenic wedge. Continental active faults and GPS data reveal the rigid motion of a Peruvian Forearc Sliver that extends from the oceanic trench axis to the Western-Eastern Cordilleras boundary and moves southeastward at 4-5 mm/yr relative to a stable South America reference frame. GPS data indicate that the Subandean shortening increases southward by 2 to 4 mm/yr. In a Peruvian Sliver reference frame, the residual GPS data indicate that the interseismic coupling along the Nazca megathrust is highly heterogeneous. Coupling in northern Peru is shallow and coincides with the site of previous moderate-sized and shallow tsunami-earthquakes. Deep coupling occurs in central and southern Peru, where repeated large and great megathrust earthquakes have occurred. The strong correlation between highly coupled areas and large ruptures suggests that seismic asperities are persistent features of the megathrust. Creeping segments appear at the extremities of great ruptures and where oceanic fracture zones and ridges enter the subduction zone, suggesting that these subducting structures play a major role in the seismic segmentation of the Peruvian margin. In central Peru, we estimate a recurrence time of 305 ± 40 years to reproduce the great 1746 Mw 8.8 Lima-Callao earthquake.

Brittle deformation during Alpine basal accretion and the origin of seismicity nests above the subduction interface

NASA Astrophysics Data System (ADS)

Menant, Armel; Angiboust, Samuel; Monié, Patrick; Oncken, Onno; Guigner, Jean-Michel

2018-04-01

Geophysical observations on active subduction zones have evidenced high seismicity clusters at 20-40 km depth in the fore-arc region whose origin remains controversial. We report here field observations of pervasive pseudotachylyte networks (interpreted as evidence for paleo-seismicity) in the now-exhumed Valpelline continental unit (Dent Blanche complex, NW. Alps, Italy), a tectonic sliver accreted to the upper plate at c. 30 km depth during the Paleocene Alpine subduction. Pre-alpine granulite-facies paragneiss from the core of the Valpelline unit are crosscut by widespread, mm to cm-thick pseudotachylyte veins. Co-seismic heating and subsequent cooling led to the formation of Ti-rich garnet rims, ilmenite needles, Ca-rich plagioclase, biotite microliths and hercynite micro-crystals. 39Ar-40Ar dating yields a 51-54 Ma age range for these veins, thus suggesting that frictional melting events occurred near peak burial conditions while the Valpelline unit was already inserted inside the duplex structure. In contrast, the base of the Valpelline unit underwent synchronous ductile and brittle, seismic deformation under water-bearing conditions followed by a re-equilibration at c. 40 Ma (39Ar-40Ar on retrograded pseudotachylyte veins) during exhumation-related deformation. Calculated rheological profiles suggest that pseudotachylyte veins from the dry core of the granulite unit record upper plate micro-seismicity (Mw 2-3) formed under very high differential stresses (>500 MPa) while the sheared base of the unit underwent repeated brittle-ductile deformation at much lower differential stresses (<40 MPa) in a fluid-saturated environment. These results demonstrate that some of the seismicity clusters nested along and above the plate interface may reflect the presence of stiff tectonic slivers rheologically analogous to the Valpelline unit acting as repeatedly breaking asperities in the basal accretion region of active subduction zones.

What Controls Subduction Earthquake Size and Occurrence?

NASA Astrophysics Data System (ADS)

Ruff, L. J.

2008-12-01

There is a long history of observational studies on the size and recurrence intervals of the large underthrusting earthquakes in subduction zones. In parallel with this documentation of the variability in both recurrence times and earthquake sizes -- both within and amongst subduction zones -- there have been numerous suggestions for what controls size and occurrence. In addition to the intrinsic scientific interest in these issues, there are direct applications to hazards mitigation. In this overview presentation, I review past progress, consider current paradigms, and look toward future studies that offer some resolution of long- standing questions. Given the definition of seismic moment, earthquake size is the product of overall static stress drop, down-dip fault width, and along-strike fault length. The long-standing consensus viewpoint is that for the largest earthquakes in a subduction zone: stress-drop is constant, fault width is the down-dip extent of the seismogenic portion of the plate boundary, but that along-strike fault length can vary from one large earthquake to the next. While there may be semi-permanent segments along a subduction zone, successive large earthquakes can rupture different combinations of segments. Many investigations emphasize the role of asperities within the segments, rather than segment edges. Thus, the question of earthquake size is translated into: "What controls the along-strike segmentation, and what determines which segments will rupture in a particular earthquake cycle?" There is no consensus response to these questions. Over the years, the suggestions for segmentation control include physical features in the subducted plate, physical features in the over-lying plate, and more obscure -- and possibly ever-changing -- properties of the plate interface such as the hydrologic conditions. It seems that the full global answer requires either some unforeseen breakthrough, or the long-term hard work of falsifying all candidate hypotheses except one. This falsification process requires both concentrated multidisciplinary efforts and patience. Large earthquake recurrence intervals in the same subduction zone segment display a significant, and therefore unfortunate, variability. Over the years, many of us have devised simple models to explain this variability. Of course, there are also more complicated explanations with many additional model parameters. While there has been important observational progress as both historical and paleo-seismological studies continue to add more data pairs of fault length and recurrence intervals, there has been a frustrating lack of progress in elimination of candidate models or processes that explain recurrence time variability. Some of the simple models for recurrence times offer a probabilistic or even deterministic prediction of future recurrence times - and have been used for hazards evaluation. It is important to know if these models are correct. Since we do not have the patience to wait for a strict statistical test, we must find other ways to test these ideas. For example, some of the simple deterministic models for along-strike segment interaction make predictions for variation in tectonic stress state that can be tested during the inter-seismic period. We have seen how some observational discoveries in the past decade (e.g., the episodic creep events down-dip of the seismogenic zone) give us additional insight into the physical processes in subduction zones; perhaps multi-disciplinary studies of subduction zones will discover a new way to reliably infer large-scale shear stresses on the plate interface?

Plate coupling across the northern Manila subduction zone deduced from mantle lithosphere buoyancy

NASA Astrophysics Data System (ADS)

Lo, Chung-Liang; Doo, Wen-Bin; Kuo-Chen, Hao; Hsu, Shu-Kun

2017-12-01

The Manila subduction zone is located at the plate boundary where the Philippine Sea plate (PSP) moves northwestward toward the Eurasian plate (EU) with a high convergence rate. However, historically, no large earthquakes greater than Mw7 have been observed across the northern Manila subduction zone. The poorly understood plate interaction between these two plates in this region creates significant issues for evaluating the seismic hazard. Therefore, the variation of mantle lithospheric buoyancy is calculated to evaluate the plate coupling status across the northern Manila subduction zone, based on recently published forward gravity modeling constrained by the results of the P-wave seismic crustal structure of the TAIGER (Taiwan Integrated Geodynamic Research) project. The results indicate weak plate coupling between the PSP and EU, which could be related to the release of the overriding PSP from the descending EU's dragging force, which was deduced from the higher elevation of the Luzon arc and the fore-arc basin northward toward the Taiwan orogen. Moreover, serpentinized peridotite is present above the plate boundary and is distributed more widely and thickly closer to offshore southern Taiwan orogen. We suggest that low plate coupling may facilitate the uplifting of serpentinized mantle material up to the plate boundary.

Subducting plate geology in three great earthquake ruptures of the western Alaska margin, Kodiak to Unimak

USGS Publications Warehouse

von Huene, Roland E.; Miller, John J.; Weinrebe, Wilhelm

2012-01-01

Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.

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.

Continental crustal formation and recycling: Evidence from oceanic basalts

NASA Technical Reports Server (NTRS)

Saunders, A. D.; Tarney, J.; Norry, M. J.

1988-01-01

Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.