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Sample records for active intra-continental subduction

  1. Numerical modeling of deep oceanic slab dehydration: Implications for the possible origin of far field intra-continental volcanoes in northeastern China

    NASA Astrophysics Data System (ADS)

    Sheng, Jian; Liao, Jie; Gerya, Taras

    2016-03-01

    Intra-continental volcanoes have different origins compared to arc volcanoes, and several conflicting geodynamic mechanisms of intra-continental magmatism have been proposed, which require quantitative testing. In this paper, we investigate numerically possible influences of deep subducted slab dehydration processes for the development of intra-continental volcanoes distributed in northeastern China. We employ 2D thermo-mechanical numerical models to investigate the complex dynamics of long-term subduction process, including slab interaction with mantle transition zone, deep oceanic plate dehydration, trench rollback, and back-arc extension. Our experiments show systematically that water can be transported to the transition zone by stagnant slabs hydrating the overlaying deep asthenospheric mantle. Positively buoyant partially molten hydrous plumes arising from the hydrated area formed atop the slabs can propagate upwards and form partially molten mantle regions under the far field continental plate. Our numerical models thus suggest that the development of widely distributed intra-continental volcanism in northeastern China could be related to the rising of multiple relatively small hydrous plumes triggered by stagnant paleo-Pacific slab dehydration in the transition zone.

  2. Continental subduction induced tremor activity?

    NASA Astrophysics Data System (ADS)

    Tai, H. J.; Chen, K. H.; Ide, S.; Mouyen, M.; Byrne, T. B.

    2015-12-01

    Southern Central Range of Taiwan, a place where deep-seated tectonic tremors (a proxy of slow slip) and earthquake swarms are closely located in space and highly correlated in time, provides rare opportunity towards the understanding of physical mechanisms governing different style of slip. To identify tremor events, we used the identification scheme similar to Ide et al. (2015) but applied slightly different techniques: (1) Higher waveform cross-correlation coefficient (>0.6) (2) careful visual inspection for excluding local earthquakes and short-lasted event (duration < 60 s) (3) Signal to noise ratio higher than 1.2 and lower than 30 (4) No spatio-temporal clustering technique used. During the study period of 2007-2012, we identified 2320 tremor events with duration ranging from 60 s to 1550 s. They are located underneath southern Central Range, forming a NS-striking and SE-dipping pipe-like structure at a depth of 20-40 km. The up-dip extension of this tremor structure reaches an aseismic zone under the western flank of Central Range at shallow depths, where is an area characterized by high heat flow, low Vp and Vs anomaly. Such seismic gap was explained by the buoyancy induced crust detachment during continental subduction of Eurasian Plate. This detachment may open a new channel for hot and ductile material ascending to shallow depth, producing high temperatures along the way. This provides a common mechanism for down-dip tremor and up-dip shallow seismic gap along the same eastern dipping channel. In addition, the tremor events are found to be mostly occurred in high tides and exhibit higher correlation with tide data from west coast of Taiwan. This may again imply the association between tremor activity and subduction of Eurasian Plate.

  3. Alkaline series related to Early-Middle Miocene intra-continental rifting in a collision zone: An example from Polatlı, Central Anatolia, Turkey

    NASA Astrophysics Data System (ADS)

    Temel, Abidin; Yürür, Tekin; Alıcı, Pınar; Varol, Elif; Gourgaud, Alain; Bellon, Hervé; Demirbağ, Hünkar

    2010-06-01

    A large volcanic area (˜7600 km 2), the Galatean Volcanic Province (GVP), developed in northwest Central Anatolia during the Miocene along the Neo-Tethys Ocean suture zone possibly by post-collisional processes. The GVP mainly comprises 20-14 My old acid to intermediate volcanites with a geochemical signature indicating a mantle source modified by earlier (Late Cretaceous) subduction-related events. 100 km south of the GVP, near Polatlı, Ankara, basaltic rocks that cover large areas are intercalated with the Miocene deposits of the Beypazarı basin, an intra-continental subsidence zone at the southwest of the GVP. Field observations, geochemistry and K-Ar age dating of the Polatlı volcanites show that they are Early (19.9 Ma) to mid (14.1 Ma) Miocene in age, covering an area as large as 215 km 2. Variations in lava thickness and the thickness of the underlying silicified/baked zones suggest that the basaltic lavas erupted from a southern source, possibly from the Eskişehir fault zone, and flowed northwards. Most Polatlı samples have chemical compositions that indicate derivation from a mantle source with crustal contamination during ascent. They do not display any characteristic to suggest a subductional component. Although the GVP and Polatlı lavas formed close in time and space, they were derived from different mantle sources. Considering the positions of these two magmatic regions with regard to the Tethyan suture zone, we propose that the mantle beneath the GVP and near the suture zone memorised the earlier subduction while the mantle beneath Polatlı that is located about 100 km further from the suture zone remained apparently unchanged. After a significant volume of magma was consumed in the GVP, a later (˜10 My) and last activity (Güvem activity) has produced quantitatively much less basaltic rocks where this subductional signature seems to completely disappear. Considering that the western Anatolian crust is proposed to undergo extension since the

  4. Evidence for Active Subduction Beneath Gibraltar

    NASA Astrophysics Data System (ADS)

    Gutscher, M.; Malod, J. A.; Rehault, J.; Contrucci, I. M.; Klingelhoefer, F.; Victor, L. M.; Spakman, W.

    2002-12-01

    The Gibraltar arc encompasses the Betic - Rif mountain belts with outward directed thrusting, surrounding a zone of strong Neogene subsidence and crustal thinning in the Western Alboran Sea. The SISMAR marine seismic survey conducted in April 2001 acquired over 3000 km of 360-channel seismic data with a 4.5 km long streamer and 1000 km of wide-angle data recorded by ocean bottom seismometers (OBS), completely spanning the actively deforming region between the margins of Portugal and northwest Morocco. We report on results from this seismic survey which reveal a thick chaotic sedimentary mass west of Gibraltar to be an actively deforming accretionary wedge, with east dipping thrust faults disrupting the seafloor and soleing out to an east dipping decollement. New travel-time tomographic results image a continuous east dipping body with high seismic velocities (i.e. a cold slab of oceanic lithosphere) descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate depth (60 - 120 km) seismicity beneath the Gibraltar Arc and Western Alboran Sea, and merging with a region of deep focus earthquakes 600 - 660 km below Granada Spain. Together these provide compelling evidence for an active east dipping subduction zone. Slab rollback towards the west provides a plausible mechanism for extension and subsidence in the Alboran Sea, while the associated westward advance of the Gibraltar Arc drives compressional deformation in the accretionary wedge where active mud volcanoes have recently been discovered. Active subduction beneath Gibraltar should be considered as a possible candidate for the source of the destructive Lisbon great earthquake (M 8.5-9) and tsunami of 1755 which ravaged the coast of the Gulf of Cadiz.

  5. Active Subduction Beneath The Gibraltar Arc

    NASA Astrophysics Data System (ADS)

    Gutscher, M.-A.; Malod, J.; Rehault, J.-P.; Contrucci, I.; Klingelhoefer, F.; Spakman, W.; Sismar Scientific Team

    The Gibraltar region features the arcuate Betic - Rif mountain belt with outward di- rected thrusting, surrounding a zone of strong Neogene subsidence and crustal thin- ning in the Western Alboran Sea. Until now its geodynamic interpretation has re- mained controversial. The Gibraltar Arc is located at the eastern end of the Azores- Gibraltar transform, a diffuse transpressional plate boundary between the Iberian and African Plates. Attention has recently been focussed on this plate boundary, while seeking the likely source of the destructive Lisbon great earthquake (M 8.5 - 9) and tsunami of 1755. The SISMAR marine seismic survey conducted in April 2001 ac- quired over 3000 km of 360-channel seismic data with a 4.5 km long streamer and 1000 km of wide-angle data recorded by ocean bottom seismometers (OBS), com- pletely spanning the actively deforming region between the margins of Portugal and northwest Morocco. Results from this seismic survey reveal a thick chaotic sedimen- tary mass west of Gibraltar to be an actively deforming accretionary wedge, with east dipping thrust faults disrupting the seafloor and soleing out to an east dipping decolle- ment. New travel-time tomographic results image a continuous east dipping body with high seismic velocities (i.e. a cold slab of oceanic lithosphere) descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate depth (60 - 120 km) seismicity beneath the Gibraltar Arc and Western Alboran Sea, and merging with a region of deep focus earthquakes 600 - 660 km below Granada Spain. Together these provide compelling evidence for an active east dipping subduction zone. Slab rollback towards the west provides a plausible mechanism for extension and subsidence in the Alboran Sea, while the associated westward advance of the Gibraltar Arc drives com- pressional deformation in the accretionary wedge where active mud volcanoes have recently been discovered.

  6. The Cryogenian intra-continental rifting of Rodinia: Evidence from the Laurentian margin in eastern North America

    NASA Astrophysics Data System (ADS)

    McClellan, Elizabeth; Gazel, Esteban

    2014-10-01

    The geologic history of the eastern North American (Laurentian) margin encompasses two complete Wilson cycles that brought about the assembly and subsequent disaggregation of two supercontinents, Rodinia and Pangea. In the southern and central Appalachian region, basement rocks were affected by two episodes of crustal extension separated by > 100 m.y.; a Cryogenian phase spanning the interval 765-700 Ma and an Ediacaran event at ~ 565 Ma. During the Cryogenian phase, the Mesoproterozoic continental crust was intruded by numerous A-type felsic plutons and extensional mafic dikes. At ~ 760-750 Ma a bimodal volcanic sequence erupted onto the uplifted and eroded basement. This sequence, known as the Mount Rogers Formation (MRF), comprises a bimodal basalt-rhyolite lower section and an upper section of dominantly peralkaline rhyolitic sheets. Here, we provide new geochemical evidence from the well-preserved volcanic rocks of the Cryogenian lower MRF, with the goal of elucidating the process that induced the initial stage of the break-up of Rodinia and how this affected the evolution of the eastern Laurentian margin. The geochemical compositions of the Cryogenian lavas are remarkably similar to modern continental intra-plate settings (e.g., East African Rift, Yellowstone-Snake River Plain). Geochemical, geophysical and tectonic evidence suggests that the common denominator controlling the melting processes in these settings is deep mantle plume activity. Thus, evidence from the MRF suggests that the initial phase of extension of the Laurentian margin at ~ 760-750 Ma was possibly triggered by mantle plume activity. It is possible that lithospheric weakness caused by a mantle plume that impacted Rodinia triggered the regional extension and produced the intra-continental rifting that preceded the breakup of the Laurentian margin.

  7. 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

  8. Tsunamigenic potential of Mediterranean fault systems and active subduction zones

    NASA Astrophysics Data System (ADS)

    Petricca, Patrizio; Babeyko, Andrey

    2016-04-01

    Since the North East Atlantic and Mediterranean Tsunami Warning System (NEAMTWS) is under development by the European scientific community, it becomes necessary to define guidelines for the characterization of the numerous parameters must be taken into account in a fair assessment of the risk. Definition of possible tectonic sources and evaluation of their potential is one of the principal issues. In this study we systematically evaluate tsunamigenic potential of up-to-now known real fault systems and active subduction interfaces in the NEAMTWS region. The task is accomplished by means of numerical modeling of tsunami generation and propagation. We have simulated all possible uniform-slip ruptures populating fault and subduction interfaces with magnitudes ranging from 6.5 up to expected Mmax. A total of 15810 individual ruptures were processed. For each rupture, a tsunami propagation scenario was computed in linear shallow-water approximation on 1-arc minute bathymetric grid (Gebco_08) implying normal reflection boundary conditions. Maximum wave heights at coastal positions (totally - 23236 points of interest) were recorded for four hours of simulation and then classified according to currently adopted warning level thresholds. The resulting dataset allowed us to classify the sources in terms of their tsunamigenic potential as well as to estimate their minimum tsunamigenic magnitude. Our analysis shows that almost every source in the Mediterranean Sea is capable to produce local tsunami at the advisory level (i.e., wave height > 20 cm) starting from magnitude values of Mw=6.6. In respect to the watch level (wave height > 50 cm), the picture is less homogeneous: crustal sources in south-west Mediterranean as well as East-Hellenic arc need larger magnitudes (around Mw=7.0) to trigger watch levels even at the nearby coasts. In the context of the regional warning (i.e., source-to-coast distance > 100 km) faults also behave more heterogeneously in respect to the minimum

  9. High -Temperature melting in intra-continental settings - insight from numerical modelling

    NASA Astrophysics Data System (ADS)

    Gorczyk, Weronika; Hobbs, Bruce; Gonzalez, Christopher; Smithies, Hugh

    2015-04-01

    Most of intra-continental melting is associated with interaction of deep mantle plume with mantle lithosphere. Vast amount of mafic/felsic intra-plate intrusions are located along post-collisional lines, where for longer periods of time regional tectonic conditions are more likely to be (weakly) compressional to transpressional, and more rarely extensional. Arrival of the asthenosphere-derived plume would suggest initiation of extension along the weak/post-collision zone. Alternatively, in compressional regime a surprisingly large range of instabilities can develop that lead to melting of the lower crust and mantle lithosphere. Unexpected structural complexity arises which is quite sensitive to the geometry and rheological properties. This has dramatic effects on melting and devolatilisation within the lithosphere and hence in the localisation of and melt emplacement. Melts extracted in theses circumstances lead to emplacement of all variety of magmas: mafic, intermediate and felsic, from wide range of PT conditions. In order to investigate theses intra-plate sites of deformation, melt production and crustal growth in relation to pre-worked lithospheric crust we performed a series of 2D numerical experiments by using a coupled petrological - thermomechanical numerical model. The model includes, stable mineralogy, aqueous fluid transport, partial melting, melt extraction and melt emplacement in form of extrusive volcanics and intrusive plutons. As a case study we will present Musgrave Orogeny in Central Australia. The Musgrave Province developed at the nexus of the North, West and South Australian cratons and its Mesoproterozoic evolution incorporates a 100 Ma period of ultra-high temperature (UHT) metamorphism from ca. 1220 to ca. 1120 Ma. This was accompanied by high-temperature A-type granitic magmatism over an 80 Ma period, sourced in part from mantle-derived components and emplaced as a series of pulsed events that also coincide with peaks in UHT metamorphism

  10. Locked and loading megathrust linked to active subduction beneath the Indo-Burman Ranges

    NASA Astrophysics Data System (ADS)

    Steckler, Michael S.; Mondal, Dhiman Ranjan; Akhter, Syed Humayun; Seeber, Leonardo; Feng, Lujia; Gale, Jonathan; Hill, Emma M.; Howe, Michael

    2016-08-01

    The Indo-Burman mountain ranges mark the boundary between the Indian and Eurasian plates, north of the Sumatra-Andaman subduction zone. Whether subduction still occurs along this subaerial section of the plate boundary, with 46 mm yr-1 of highly oblique motion, is contentious. About 21 mm yr-1 of shear motion is taken up along the Sagaing Fault, on the eastern margin of the deformation zone. It has been suggested that the remainder of the relative motion is taken up largely or entirely by horizontal strike-slip faulting and that subduction has stopped. Here we present GPS measurements of plate motions in Bangladesh, combined with measurements from Myanmar and northeast India, taking advantage of a more than 300 km subaerial accretionary prism spanning the Indo-Burman Ranges to the Ganges-Brahmaputra Delta. They reveal 13-17 mm yr-1 of plate convergence on an active, shallowly dipping and locked megathrust fault. Most of the strike-slip motion occurs on a few steep faults, consistent with patterns of strain partitioning in subduction zones. Our results strongly suggest that subduction in this region is active, despite the highly oblique plate motion and thick sediments. We suggest that the presence of a locked megathrust plate boundary represents an underappreciated hazard in one of the most densely populated regions of the world.

  11. Active Crustal Faults in the Forearc Region, Guerrero Sector of the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Gaidzik, Krzysztof; Ramírez-Herrera, Maria Teresa; Kostoglodov, Vladimir

    2016-10-01

    This work explores the characteristics and the seismogenic potential of crustal faults on the overriding plate in an area of high seismic hazard associated with the occurrence of subduction earthquakes and shallow earthquakes of the overriding plate. We present the results of geomorphic, structural, and fault kinematic analyses conducted on the convergent margin between the Cocos plate and the forearc region of the overriding North American plate, within the Guerrero sector of the Mexican subduction zone. We aim to determine the active tectonic processes in the forearc region of the subduction zone, using the river network pattern, topography, and structural data. We suggest that in the studied forearc region, both strike-slip and normal crustal faults sub-parallel to the subduction zone show evidence of activity. The left-lateral offsets of the main stream courses of the largest river basins, GPS measurements, and obliquity of plate convergence along the Cocos subduction zone in the Guerrero sector suggest the activity of sub-latitudinal left-lateral strike-slip faults. Notably, the regional left-lateral strike-slip fault that offsets the Papagayo River near the town of La Venta named "La Venta Fault" shows evidence of recent activity, corroborated also by GPS measurements (4-5 mm/year of sinistral motion). Assuming that during a probable earthquake the whole mapped length of this fault would rupture, it would produce an event of maximum moment magnitude Mw = 7.7. Even though only a few focal mechanism solutions indicate a stress regime relevant for reactivation of these strike-slip structures, we hypothesize that these faults are active and suggest two probable explanations: (1) these faults are characterized by long recurrence period, i.e., beyond the instrumental record, or (2) they experience slow slip events and/or associated fault creep. The analysis of focal mechanism solutions of small magnitude earthquakes in the upper plate, for the period between 1995

  12. Three-dimensional finite-element modelling of coseismic Coulomb stress changes on intra-continental dip-slip faults

    NASA Astrophysics Data System (ADS)

    Bagge, Meike; Hampel, Andrea

    2016-08-01

    Investigating fault interaction plays a crucial role in seismic hazard assessment. The calculation of Coulomb stress changes allows quantifying the stress changes on so-called receiver faults in the surrounding of the fault that experienced the earthquake. A positive stress change implies that the earthquake brought the receiver fault closer to failure while a negative value indicates a delay of the next earthquake. So far, most studies focussed on stress changes for particular faults and earthquakes. Here we present a general analysis of the Coulomb stress changes on intra-continental dip-slip faults using finite-element models with normal and thrust faults arrays, respectively. Our models allow calculating coseismic ("static") stress changes on pre-defined fault planes, whose dip and position can be varied. Gravity and ongoing regional deformation (i.e. shortening or extension) are included. The results for thrust and normal faults show that synthetic receiver faults located in the hanging wall and footwall of the source fault exhibit a symmetric stress distribution, with large areas of negative and small areas of positive Coulomb stress changes. In contrast, faults positioned in along-strike prolongation of the source fault and outside of its immediate hanging wall and footwall undergo mostly positive stress changes. The stress changes are largest at the fault tip that is closer to the source fault. Our results show that the stress change distribution depends on the fault dip while the magnitude depends on the friction coefficient and the amount of coseismic slip. The Coulomb stress changes can be explained by the spatial distribution of the coseismic strain, which shows domains of horizontal extension and shortening that alternate both at the surface and with depth. Our models allow identifying the general patterns of Coulomb stress changes on dip-slip faults, which are often concealed by the peculiarity of the specific fault or earthquake in nature.

  13. Geophysical signature of hydration-dehydration processes in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-04-01

    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.

  14. Relations between earthquake activities and configuration of the subducting Pacific plate interface along the Japan Trench

    NASA Astrophysics Data System (ADS)

    Yamada, T.; Nakahigashi, K.; Kuwano, A.; Machida, Y.; Mochizuki, K.; Shinohara, M.; Kanazawa, T.; Hino, R.; Takanami, T.

    2010-12-01

    We performed passive and active seismic experiments using 189 ocean bottom seismometers in the land slope region along the Japan Trench. The accuracy of locations for more than 5000 earthquakes along the Japan Trench were drastically improved, and we deduced the configuration of the subducting Pacific plate from the distribution while considering others such as the seismic refraction data and previous studies. These provide us a window of opportunity to compare configuration of the subducting plate and seismic activity. Our plate model shows that the dip angle of the subducting plate is small from the trench to turning points whose depth of the interface is approximately 30 km although it has lateral variation along the trench axis and its pattern seems to be correspondent with seismic boundaries. Seismicity is not much in the region, however the rate has zone characters. The dip angle increase abruptly at the turning points, and they correspond to edges of the rapture zones of almost all large earthquakes in recent years. The seismicity are relatively high in adjacent areas of the asperities including the turning points and both interplate and intraplate regions. In contrast, there are few thrust-type microearthquakes in the asperities. We can also see the double seismic planes, and one of the lower planes seems to come through the trench region and connect the seismicity beneath the outer rise.

  15. Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data

    USGS Publications Warehouse

    Eberhart-Phillips, D.; Christensen, D.H.; Brocher, T.M.; Hansen, R.; Ruppert, N.A.; Haeussler, P.J.; Abers, G.A.

    2006-01-01

    In southern and central Alaska the subduction and active volcanism of the Aleutian subduction zone give way to a broad plate boundary zone with mountain building and strike-slip faulting, where the Yakutat terrane joins the subducting Pacific plate. The interplay of these tectonic elements can be best understood by considering the entire region in three dimensions. We image three-dimensional seismic velocity using abundant local earthquakes, supplemented by active source data. Crustal low-velocity correlates with basins. The Denali fault zone is a dominant feature with a change in crustal thickness across the fault. A relatively high-velocity subducted slab and a low-velocity mantle wedge are observed, and high Vp/Vs beneath the active volcanic systems, which indicates focusing of partial melt. North of Cook Inlet, the subducted Yakutat slab is characterized by a thick low-velocity, high-Vp/Vs, crust. High-velocity material above the Yakutat slab may represent a residual older slab, which inhibits vertical flow of Yakutat subduction fluids. Alternate lateral flow allows Yakutat subduction fluids to contribute to Cook Inlet volcanism and the Wrangell volcanic field. The apparent northeast edge of the subducted Yakutat slab is southwest of the Wrangell volcanics, which have adakitic composition consistent with melting of this Yakutat slab edge. In the mantle, the Yakutat slab is subducting with the Pacific plate, while at shallower depths the Yakutat slab overthrusts the shallow Pacific plate along the Transition fault. This region of crustal doubling within the shallow slab is associated with extremely strong plate coupling and the primary asperity of the Mw 9.2 great 1964 earthquake. Copyright 2006 by the American Geophysical Union.

  16. Geomorphic Indices in the Assessment of Tectonic Activity in Forearc of the Active Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Gaidzik, K.; Ramirez-Herrera, M. T.

    2015-12-01

    Rapid development of GIS techniques and constant advancement of digital elevation models significantly improved the accuracy of extraction of information on active tectonics from landscape features. Numerous attempts were made to quantitatively evaluate recent tectonic activity using GIS and DEMs, and a set of geomorphic indices (GI), however these studies focused mainly on sub-basins or small-scale areal units. In forearc regions where crustal deformation is usually large-scale and do not concentrate only along one specific fault, an assessment of the complete basin is more accurate. We present here the first attempt to implement thirteen GI in the assessment of active tectonics of a forearc region of an active convergent margin using the entire river basins. The GIs were divided into groups: BTAI - basin geomorphic indices (reflecting areal erosion vs. tectonics) and STAI - stream geomorphic indices (reflecting vertical erosion vs. tectonics). We calculated selected indices for 9 large (> 450 km2) drainage basins. Then we categorized the obtained results of each index into three classes of relative tectonic activity: 1 - high, 2 - moderate, and 3 - low. Finally we averaged these classes for each basin to determine the tectonic activity level (TAI). The analysis for the case study area, the Guerrero sector at the Mexican subduction zone, revealed high tectonic activity in this area, particularly in its central and, to a lesser degree, eastern part. This pattern agrees with and is supported by interpretation of satellite images and DEM, and field observations. The results proved that the proposed approach indeed allows identification and recognition of areas witnessing recent tectonic deformation. Moreover, our results indicated that, even though no large earthquake has been recorded in this sector for more than 100 years, the area is highly active and may represent a seismic hazard for the region.

  17. Models of Active Glacial Isostasy Roofing Warm Subduction: Case of the South Patagonian Ice Field

    NASA Technical Reports Server (NTRS)

    Klemann, Volker; Ivins, Erik R.; Martinec, Zdenek; Wolf, Detlef

    2007-01-01

    Modern geodetic techniques such as precise Global Positioning System (GPS) and high-resolution space gravity mapping (Gravity Recovery and Climate Experiment, GRACE) make it possible to measure the present-day rate of viscoelastic gravitational Earth response to present and past glacier mass changes. The Andes of Patagonia contain glacial environments of dramatic mass change. These mass load changes occur near a tectonically active boundary between the Antarctic and South American plates. The mechanical strength of the continental side of this boundary is influenced by Neogene ridge subduction and by the subduction of a youthful oceanic slab. A ridge of young volcanos parallels the Pacific coastline. Release of volatiles (such as water) at depth along this ridge creates a unique rheological environment. To assess the influence of this rheological ridge structure on the observational land uplift rate, we apply a two dimensional viscoelastic Earth model. A numerical study is presented which examines the sensitivity of the glacial loading-unloading response to the complex structure at depth related to the subducting slab, the viscous wedge between slab and continental lithosphere, and the increase of elastic thickness from oceanic to continental lithosphere. A key feature revealed by our numerical experiments is a continuum flow wherein the slab subdues the material transport toward oceanic mantle and crust. The restricted flow is sensitive to the details of slab mechanical strength and penetration into the upper mantle. The reduced viscosity within the mantle wedge, however, enhances the load-induced material transport everywhere within the asthenosphere.

  18. Active intraplate deformation as geodynamic responses to oblique shallow subduction of a flat slab: example from central and southwest Japan

    NASA Astrophysics Data System (ADS)

    Ishiyama, Tatsuya; Sato, Hiroshi

    2015-04-01

    Subduction of a flat slab has been recognized as one of the primary driving mechanism of wide intracontinental subsidence farther away from the subduction leading edge in many subduction margins. In most cases, however, quantitative and qualitative limitations on chronological constraints prevent comprehensive understanding of these geodynamic linkages. In this study, we show distinct, geologic and seismic evidence for spatial and temporal correlation between plate subduction and intercontinental deformation, mainly driven by dynamic interaction between subducting Philippine Sea (PHS) plate and overriding continental crusts of central and southwest Japan (Eurasian plate) along the Nankai-Tonankai subduction zone since Pliocene. Based on analyses of Pliocene to Pleistocene tectonic histories by use of rich dataset of Neogene stratigraphy, drainage network evolution, and shallow to deep seismic reflection profiles, depocenters of wide sedimentary basins and active thrusting have migrated northward since ca. 5 Ma to present from forearc to backarc of the southwest Japan arc. Median tectonic line, active dextral strike-slip fault as a forearc sliver along the Nankai, is located north of the upward extension of the downdip limit of the interseismic locked zone. Southwest Japan north of the MTL, underlain by the subducting slab with steady state slip (Nakanishi et al., 2002; Kodaira et al., 2004), appears tectonically less inactive than central Japan and has behaved as a less deformed rigid block. Contrastingly, Quaternary active intraplate deformation has been prominent north of the inactive MTL above a shallow flat segment of the PHS plate along the Tonankai. Deep seismic reflection profile images upward corrugated very shallow PHS slab being contact with continental lower crust beneath actively deforming area. We interpreted temporal and spatial correlation of oblique subduction of the shallow and flat, corrugated PHS slab as an essential mechanical role to enhance

  19. Deformation of the Calabrian Arc subduction complex and its relation to STEP activity at depth.

    NASA Astrophysics Data System (ADS)

    Polonia, Alina; Wortel, Rinus; Nijholt, Nicolai; Govers, Rob; Torelli, Luigi

    2015-04-01

    Propagating tear faults at the edge of subducted slabs ("Subduction transform edge propagator", STEP) are an intrinsic part of lithospheric plate dynamics. The surface expression of a STEP is generally not known yet, and is expected to vary significantly from one region to the other. We choose the Sicily -Calabria-Ionian Sea region, of which the lithosphere-upper mantle structure has the characteristics of a STEP zone, as a study area. The area has a very prominent accretionary wedge, the formation and subsequent deformation of which presumably were affected by the STEP activity at depth. In this contribution, we use seismic data on the near surface structure and deformation in combination with numerical model results to investigate the relation between deep STEP activity and near surface expression. Prominent features in the surface tectonics are the Malta escarpment (with predominantly normal faulting), the newly identified Ionian Fault and Alfeo-Etna fault system, and a distinct longitudinal division of the wedge into a western and an eastern lobe (Polonia et al., Tectonics, 2011). The two lobes are characterized by different structural style, deformation rates and basal detachment depths. Numerical model results indicate that the regional lithospheric structure, such as the orientation of the eastern passive (albeit subsequently activated) margin of Sicily relative to the Calabrian subduction zone, has a profound effect on possible fault activity along the Malta escarpment. Fault activity along the above primary fault structures may have varied in time, implying the possibility of intermittent activity. Interpreting seismicity in the context of a possible STEP, and the accompanying deformation zone at or near the surface, is not (yet) straightforward. Although direct evidence for recognizing all aspects of STEP activity is - as usual - lacking, a comparison with two well-known STEP regions, the northern part of the Tonga subduction zone and southern part of the

  20. The River Network, Active Tectonics and the Mexican Subduction Zone, Southwest Mexico

    NASA Astrophysics Data System (ADS)

    Gaidzik, K.; Ramirez-Herrera, M. T.; Kostoglodov, V.; Basili, R.

    2014-12-01

    Rivers, their profiles and network reflect the integration of multiple processes and forces that are part of the fundamental controls on the relief structure of mountain belts. The motivation of this study is to understand active tectonic processes in the forearc region of subduction zones, by distinguishing evidence of active deformation using the river network and topography. To this end, morphotectonic and structural studies have been conducted on fifteen drainage basins on the mountain front, parallel to the Mexican subduction zone, where the Cocos plate underthrusts the North American plate. The southwest - northeast Cocos plate subduction stress regime initiated ca. 20 MA. NE-SW to NNE-SSW normal faults as well as sub-latitudinal to NW-SE strike-slip faults (both dextral and sinistral) constitute the majority of mesofaults recorded in the field within the studied drainage basins. Occasionally dextral N-S strike-slip faults also occur. The stress tensor reconstruction suggests two main evolution stages of these faults: 1) the older is dominated by a NW-SE to WNW-ESE extensional regime and 2) the younger is a transcurrent regime, with NNE-SSW σ1 axis. The drainage pattern is strongly controlled by tectonic features, whereas lithology is only a subordinate factor, with only one exception (Petatlán river). Generally, major rivers flow from north to south mainly through NE-SW and NNE-SSW normal faults, and/or sub-longitudinal dextral (also locally sinistral) strike-slip faults. In the central and eastern part of the studied area, rivers also follow NW-SE structures, which are generally normal or sinistral strike-slip faults (rarely reverse). In most cases, local deflections of the river main courses are related to sub-latitudinal strike-slip faults, both dextral and sinistral. Within the current stress field related to the active Cocos subduction, both normal and strike-slip fault sets could be reactivated. Our analysis suggests that strike-slip faults, mainly

  1. Active faulting in northern Chile: ramp stacking and lateral decoupling along a subduction plate boundary?

    NASA Astrophysics Data System (ADS)

    Armijo, Rolando; Thiele, Ricardo

    1990-04-01

    Two large features parallel to the coastline of northern Chile have long been suspected to be the sites of young or active deformation: (1) The 700-km long Coastal Scarp, with average height (above sea level) of about 1000 m; (2) The Atacama Fault zone, that stretches linearly for about 1100 km at an average distance of 30-50 km from the coastline. New field observations combined with extensive analysis of aerial photographs demonstrate that both the Coastal Scarp and the Atacama Fault are zones of Quaternary and current fault activity. Little-degraded surface breaks observed in the field indicate that these fault zones have recently generated large earthquakes ( M = 7-8). Normal fault offsets observed in marine terraces in the Coastal Scarp (at Mejillones Peninsula) require tectonic extension roughly orthogonal to the compressional plate boundary. Strike-slip offsets of drainage observed along the Salar del Carmen and Cerro Moreno faults (Atacama Fault system) imply left-lateral displacements nearly parallel to the plate boundary. The left-lateral movement observed along the Atacama Fault zone may be a local consequence of E-W extension along the Coastal Scarp. But if also found everywhere along strike, left-lateral decoupling along the Atacama Fault zone would be in contradiction with the right lateral component of Nazca-South America motion predicted by models of present plate kinematics. Clockwise rotation with left-lateral slicing of the Andean orogen south of the Arica bend is one way to resolve this contradiction. The Coastal Scarp and the Atacama Fault zone are the most prominent features with clear traces of activity within the leading edge of continental South America. The great length and parallelism of these features with the subduction zone suggest that they may interact with the subduction interface at depth. We interpret the Coastal Scarp to be a west-dipping normal fault or flexure and propose that it is located over an east-dipping ramp stack at

  2. Active and long-lived permanent forearc deformation driven by the subduction seismic cycle

    NASA Astrophysics Data System (ADS)

    Aron Melo, Felipe Alejandro

    I have used geological, geophysical and engineering methods to explore mechanisms of upper plate, brittle deformation at active forearc regions. My dissertation particularly addresses the permanent deformation style experienced by the forearc following great subduction ruptures, such as the 2010 M w8.8 Maule, Chile and 2011 Mw9.0 Tohoku, Japan earthquakes. These events triggered large, shallow seismicity on upper plate normal faults above the rupture reaching Mw7.0. First I present new structural data from the Chilean Coastal Cordillera over the rupture zone of the Maule earthquake. The study area contains the Pichilemu normal fault, which produced the large crustal aftershocks of the megathrust event. Normal faults are the major neotectonic structural elements but reverse faults also exist. Crustal seismicity and GPS surface displacements show that the forearc experiences pulses of rapid coseismic extension, parallel to the heave of the megathrust, and slow interseismic, convergence-parallel shortening. These cycles, over geologic time, build the forearc structural grain, reactivating structures properly-oriented respect to the deformation field of each stage of the interplate cycle. Great subduction events may play a fundamental role in constructing the crustal architecture of extensional forearc regions. Static mechanical models of coseismic and interseismic upper plate deformation are used to explore for distinct features that could result from brittle fracturing over the two stages of the interplate cycle. I show that the semi-elliptical outline of the first-order normal faults along the Coastal Cordillera may define the location of a characteristic, long-lived megathrust segment. Finally, using data from the Global CMT catalog I analyzed the seismic behavior through time of forearc regions that have experienced great subduction ruptures >Mw7.7 worldwide. Between 61% and 83% of the cases where upper plate earthquakes exhibited periods of increased seismicity

  3. Early Cambrian granitoids of North Gondwana margin in the transition from a convergent setting to intra-continental rifting (Ossa-Morena Zone, SW Iberia)

    NASA Astrophysics Data System (ADS)

    Sánchez-García, T.; Pereira, M. F.; Bellido, F.; Chichorro, M.; Silva, J. B.; Valverde-Vaquero, P.; Pin, Ch.; Solá, A. R.

    2014-07-01

    Two distinct Cambrian magmatic pulses are recognized in the Ossa-Morena Zone (SW Iberia): an early rift-(ER) and a main rift-related event. This Cambrian magmatism is related to intra-continental rifting of North Gondwana that is thought to have culminated in the opening of the Rheic Ocean in Lower Ordovician times. New data of whole-rock geochemistry (19 samples), Sm-Nd-Sr isotopes (4 samples) and ID-TIMS U-Pb zircon geochronology (1 sample) of the Early Cambrian ER plutonic rocks of the Ossa-Morena Zone are presented in this contribution. The ER granitoids (Barreiros, Barquete, Calera, Salvatierra de los Barros and Tablada granitoid Massifs) are mostly peraluminous granites. The Sm-Nd isotopic data show moderate negative ɛNdt values ranging from -3.5 to +0.1 and TDM ages greatly in excess of emplacement ages. Most ER granitoids are crustal melts. However, a subset of samples shows a transitional anorogenic alkaline tendency, together with more primitive isotopic signatures, documenting the participation of lower crust or mantle-derived sources and suggesting a local transient advanced stage of rifting. The Barreiros granitoid is intrusive into the Ediacaran basement of the Ossa-Morena Zone (Série Negra succession) and has yielded a crystallization age of 524.7 ± 0.8 Ma consistent with other ages of ER magmatic pulse. This age: (1) constrains the age of the metamorphism developed in the Ediacaran back-arc basins before the intrusion of granites and (2) defines the time of the transition from the Ediacaran convergent setting to the Lower Cambrian intra-continental rifting in North Gondwana.

  4. Significant foreshock activities of M>7.5 earthquakes in the Kuril subduction zone

    NASA Astrophysics Data System (ADS)

    Harada, T.; Yokoi, S.; Satake, K.

    2014-12-01

    In the Kuril subduction zone, some M>7.5 earthquakes are accompanied by significant foreshock activities, providing a good opportunity to understand the characteristics of foreshocks for large interplate events such as occur along the Japan Trench and Nankai Trough etc. Some preliminary results from our examination of the foreshock sequences are as follows. Relocated foreshocks tend to migrate with time toward the trench axis. Foreshock distributions of the interplate earthquakes do not overlap with the large coseismic slips (asperities) of the mainshocks. Foreshocks of the 2007 northern Kuril outer-rise event, however, were distributed on the entire rupture area. Foreshock sequences seem to be limited in the regions where the background seismicity rates are relatively high. The foreshock activities were found in the examination of the space-time pattern of M>7 events along the northern Japan to Kuril trench since 1913 (e.g. Harada, Satake, and Ishibashi, 2011:AGU, 2012:AOGS). The large earthquakes preceded by active foreshock sequences are: the 2006 (M8.3), 2007 (M8.1) offshore Simushir earthquakes, the 1963 (M8.5), 1991 (M7.6), 1995 (M7.9) offshore Urup events, the 1978 (M7.8) offshore Iturup events, the 1969 (M8.2) offshore Shikotan event. In contrast, M>7.5 interplate earthquakes offshore Hokkaido (1952 (M8.1), 1973 (M7.8), 2003 (M8.1)) and intraslab earthquakes (1958 (M8.3), 1978 (M7.8), 1993 (M7.6), 1994 (M8.3)) had few or no foreshocks. In the examination of the active foreshocks, we relocated foreshocks by the Modified JHD method (Hurukawa, 1995), compared relocated foreshock areas with mainshock coseismic slip distributions estimated by the teleseismic body-wave inversion (Kikuchi and Kanamori, 2003), and examined the relation between active foreshock sequences and regional background seismicity. This study was supported by the MEXT's "New disaster mitigation research project on Mega thrust earthquakes around Nankai/Ryukyu subduction zones".

  5. Upper plate deformation and seismic barrier in front of Nazca subduction zone: The Chololo Fault System and active tectonics along the Coastal Cordillera, southern Peru

    NASA Astrophysics Data System (ADS)

    Audin, Laurence; Lacan, Pierre; Tavera, Hernando; Bondoux, Francis

    2008-11-01

    The South America plate boundary is one of the most active subduction zone. The recent Mw = 8.4 Arequipa 2001 earthquake ruptured the subduction plane toward the south over 400 km and stopped abruptly on the Ilo Peninsula. In this exact region, the subduction seismic crisis induced the reactivation of continental fault systems in the coastal area. We studied the main reactivated fault system that trends perpendicular to the trench by detailed mapping of fault related-geomorphic features. Also, at a longer time scale, a recurrent Quaternary transtensive tectonic activity of the CFS is expressed by offset river gullies and alluvial fans. The presence of such extensional fault systems trending orthogonal to the trench along the Coastal Cordillera in southern Peru is interpreted to reflect a strong coupling between the two plates. In this particular case, stress transfer to the upper plate, at least along the coastal fringe, appears to have induced crustal seismic events that were initiated mainly during and after the 2001 earthquake. The seafloor roughness of the subducting plate is usually thought to be a cause of segmentation along subduction zones. However, after comparing and discussing the role of inherited structures within the upper plate to the subduction zone segmentation in southern Peru, we suggest that the continental structure itself may exert some feedback control on the segmentation of the subduction zone and thus participate to define the rupture pattern of major subduction earthquakes along the southern Peru continental margin.

  6. Distribution of dehalogenation activity in subseafloor sediments of the Nankai Trough subduction zone

    PubMed Central

    Futagami, Taiki; Morono, Yuki; Terada, Takeshi; Kaksonen, Anna H.; Inagaki, Fumio

    2013-01-01

    Halogenated organic matter buried in marine subsurface sediment may serve as a source of electron acceptors for anaerobic respiration of subseafloor microbes. Detection of a diverse array of reductive dehalogenase-homologous (rdhA) genes suggests that subseafloor organohalide-respiring microbial communities may play significant ecological roles in the biogeochemical carbon and halogen cycle in the subseafloor biosphere. We report here the spatial distribution of dehalogenation activity in the Nankai Trough plate-subduction zone of the northwest Pacific off the Kii Peninsula of Japan. Incubation experiments with slurries of sediment collected at various depths and locations showed that degradation of several organohalides tested only occurred in the shallow sedimentary basin, down to 4.7 metres below the seafloor, despite detection of rdhA in the deeper sediments. We studied the phylogenetic diversity of the metabolically active microbes in positive enrichment cultures by extracting RNA, and found that Desulfuromonadales bacteria predominate. In addition, for the isolation of genes involved in the dehalogenation reaction, we performed a substrate-induced gene expression screening on DNA extracted from the enrichment cultures. Diverse DNA fragments were obtained and some of them showed best BLAST hit to known organohalide respirers such as Dehalococcoides, whereas no functionally known dehalogenation-related genes such as rdhA were found, indicating the need to improve the molecular approach to assess functional genes for organohalide respiration. PMID:23479745

  7. Large-scale Geometry of Intra-continental Strike-slip Faults: Example of the Karakorum Fault, Western Tibet

    NASA Astrophysics Data System (ADS)

    Chevalier, M. L.; Leloup, P. H.; Li, H.

    2015-12-01

    How large-scale, active strike-slip fault systems are defined can sometimes be ambiguous, especially when viewed at different timescales (geodetic vs longer term measurements). Does every kilometer of the fault system need to be visible in the morphology (offset geomorphic features, fault trace, etc) to be considered as currently active? Does every segment of the fault need to have a unique and consistent kinematics along the entire fault system (normal, strike-slip, reverse)? Does all segments need to be physically connected at the surface to be considered part of the same fault system? To illustrate our arguments against such statements, we use the example of the right-lateral strike-slip Karakorum fault, located in western Tibet, along which lively debates have been taking place in the last ~20 years. These concern its initiation age, total geologic offsets, slip-rates, and more recently, even the location and current activity of the northern half of the fault. In particular, whether the active Kongur Shan extensional system, located in the Chinese Pamir, belongs to the Karakorum fault system remains controversial. Here, we argue that both systems are connected and that they both play a significant role in accommodating deformation at the western Himalayan syntaxis, under the form of extensional displacement in the Chinese Pamir.

  8. Seismic Activity offshore Martinique and Dominique islands (Lesser Antilles subduction zone)

    NASA Astrophysics Data System (ADS)

    Ruiz Fernandez, Mario; Galve, Audrey; Monfret, Tony; Charvis, Philippe; Laigle, Mireille; Flueh, Ernst; Gallart, Josep; Hello, Yann

    2010-05-01

    In the framework of the European project Thales was Right, two seismic surveys (Sismantilles II and Obsantilles) were carried out to better constrain the lithospheric structure of the Lesser Antilles subduction zone, its seismic activity and to evaluate the associated seismic hazards. Sismantilles II experiment was conducted in January, 2007 onboard R/V Atalante (IFREMER). A total of 90 OBS belonging to Géoazur, INSU-CNRS and IFM-Geomar were deployed on a regular grid, offshore Antigua, Guadeloupe, Dominique and Martinique islands. During the active part of the survey, more than 2500 km of multichannel seismic profiles were shot along the grid lines. Then the OBS remained on the seafloor continuously recording for the seismic activity for approximately 4 months. On April 2007 Obsantilles experiment, carried out onboard R/V Antea (IRD), was focused on the recovery of those OBS and the redeployment of 28 instruments (Géoazur OBS) off Martinique and Dominica Islands for 4 additional months of continuous recording of the seismicity. This work focuses on the analysis of the seismological data recorded in the southern sector of the study area, offshore Martinique and Dominique. During the two recording periods, extending from January to the end of August 2007, more than 3300 seismic events were detected in this area. Approximately 1100 earthquakes had enough quality to be correctly located. Station corrections, obtained from multichannel seismic profiles, were introduced to each OBS to take in to account the sedimentary cover and better constrain the hypocentral determinations. Results show events located at shallower depths in the northern sector of the array, close to the Tiburon Ridge, where the seismic activity is mainly located between 20 to 40 km depth. In the southern sector, offshore Martinique, hypocenters become deeper, ranging to 60 km depth and dipping to the west. Focal solutions have also been obtained using the P wave polarities of the best azimuthally

  9. Importance of continental subductions for the growth of the Tibetan plateau

    NASA Astrophysics Data System (ADS)

    Guillot, Stephane; Replumaz, Anne; Riel, Nicolas; Hetenyi, Gyorgy

    2013-04-01

    How and when the Tibetan plateau developed has long been a puzzling question with implications for the current understanding of the behaviour of the continental lithosphere in convergent zones. Tibet initially resulted from the accretion of the Gondwana continental blocks to the southern Asian margin during the Palaeozoic and Mesozoic eras. These successive accretions have potentially favoured the creation of local landforms, particularly in southern Tibet, but no evidence exists in favour of the existence of a proto-Tibetan plateau prior to the Cenozoic. Moreover, before the India-Asia collision, the Tibetan crust had to be sufficiently cold and rigid to transfer the horizontal forces from India to northern Tibet and localize the deformation along the major strike-slip faults. However, these successive accretions associated with subductions have contaminated the Tibetan lithospheric mantle and largely explain the potassium- and sodium-rich Cenozoic magmatism. Another consequence of this contamination by fluids is the softening of the Tibetan lithosphere, which favoured intra-continental subductions. The timing and the geochemical signatures of the magmatism and the palaeo-altitudes suggest the early growth of the Tibetan plateau. By the Eocene, the southern plateau and the northern portion of Himalaya would be at an altitude of approximately 4000 meters, while the central and northern Tibetan plateau was at altitudes of approximately 2000 to 3000 meters at the Eocene-Oligocene transition. From all of these data, we propose a model of the formation of the Tibetan plateau coupled with the formation of Himalaya, which accounts for more than 2000 km of convergence accommodated by the deformation of the continental lithospheres. During the early Eocene (55-45 Ma), the continental subduction of the high-strength Indian continental lithosphere dominates, ending with the detachment of the Indian slab. Between 45 and 35 Ma, the continental collision is established

  10. Subduction & orogeny: Introduction to the special volume

    NASA Astrophysics Data System (ADS)

    Rolland, Y.; Bosch, D.; Guillot, S.; de Sigoyer, J.; Martinod, J.; Agard, P.; Yamato, P.

    2016-05-01

    Subduction processes play a major role in plate tectonics and the subsequent geological evolution of Earth. This special issue focuses on ongoing research in subduction dynamics to a large extent (oceanic subduction, continental subduction, obduction…) for both past and active subduction zones and into mountain building processes and the early evolution of orogens. It puts together various approaches combining geophysics (imaging of subduction zones), petrology/geochemistry (metamorphic analysis of HP-UHP rocks, fluid geochemistry and magmatic signal, geochronology), seismology and geodesy (present-day evolution of subduction zones, active tectonics), structural geology (structure and evolution of mountain belts), and numerical modelling to provide a full spectrum of tools that can be used to constrain the nature and evolution of subduction processes and orogeny. Studies presented in this special issue range from the long-term (orogenic cycle) to short-term (seismic cycle).

  11. Activated Very Low Frequency Earthquakes By the Slow Slip Events in the Ryukyu Subduction Zone

    NASA Astrophysics Data System (ADS)

    Nakamura, M.; Sunagawa, N.

    2014-12-01

    The Ryukyu Trench (RT), where the Philippine Sea plate is subducting, has had no known thrust earthquakes with a Mw>8.0 in the last 300 years. However, the rupture source of the 1771 tsunami has been proposed as an Mw > 8.0 earthquake in the south RT. Based on the dating of tsunami boulders, it has been estimated that large tsunamis occur at intervals of 150-400 years in the south Ryukyu arc (RA) (Araoka et al., 2013), although they have not occurred for several thousand years in the central and northern Ryukyu areas (Goto et al., 2014). To address the discrepancy between recent low moment releases by earthquakes and occurrence of paleo-tsunamis in the RT, we focus on the long-term activity of the very low frequency earthquakes (VLFEs), which are good indicators of the stress release in the shallow plate interface. VLFEs have been detected along the RT (Ando et al., 2012), which occur on the plate interface or at the accretionary prism. We used broadband data from the F-net of NIED along the RT and from the IRIS network. We applied two filters to all the raw broadband seismograms: a 0.02-0.05 Hz band-pass filter and a 1 Hz high-pass filter. After identification of the low-frequency events from the band-pass-filtered seismograms, the local and teleseismic events were removed. Then we picked the arrival time of the maximum amplitude of the surface wave of the VLFEs and determined the epicenters. VLFEs occurred on the RA side within 100 km from the trench axis along the RT. Distribution of the 6670 VLFEs from 2002 to 2013 could be divided to several clusters. Principal large clusters were located at 27.1°-29.0°N, 25.5°-26.6°N, and 122.1°-122.4°E (YA). We found that the VLFEs of the YA are modulated by repeating slow slip events (SSEs) which occur beneath south RA. The activity of the VLFEs increased to two times of its ordinary rate in 15 days after the onset of the SSEs. Activation of the VLFEs could be generated by low stress change of 0.02-20 kPa increase in

  12. Active and passive-source imaging of the Cascadia subduction zone using both onshore and offshore data

    NASA Astrophysics Data System (ADS)

    Janiszewski, H. A.; Abers, G. A.; Carton, H. D.; Webb, S. C.; Gaherty, J. B.; Trehu, A. M.

    2013-12-01

    The Cascadia subduction zone is characterized by the subduction of young lithosphere with relatively little seismicity, despite evidence of prehistoric earthquakes, and a thick incoming sediment section that feeds the plate interface. It has been suggested that the thrust zone forms a high-porosity channel of near-lithostatic pressure to 40 km depth, but stronger metasediments may also explain many observations. To test these hypotheses, we analyze new data and integrate results from both active and passive-source seismic studies of Cascadia that sample the interplate thrust zone. In June-July 2012, fifteen seismometers were deployed in Washington from the coast to 140 km inland to record airguns from the R/V Langseth along a linear trench-perpendicular profile. We also analyze broadband data from the coincident onshore CAFE (2006-08) broadband high-density array, which provided high-resolution receiver function images of the downgoing plate, and with the offshore Grays Harbor array of the Cascadia Initiative (CI). In the active-source data, arrivals are observed at up to 140 km offset from the stations, the farthest of which are likely turning waves that travel in the slab mantle. Signals from all but the farthest inland stations are dominated by strong reverberating signals at 20-90 km offset. Preliminary calculations indicate that some of these signals have apparent velocity and timing consistent with waves that reflect off the plate interface or just above it. Bounce points for these rays map a zone of high reflectivity extending ~15-20 km on either side of the coastline. Some aspects of the signals may indicate an origin on or near the plate boundary. In addition, these reflections directly underlie CAFE stations where receiver functions have been obtained on land and the CI broadband stations where receiver functions are being obtained offshore, allowing for direct comparison and integration of all three datasets. The CI stations present several challenges

  13. Using glacial morphology to constrain the impact of the Chile active spreading ridge subduction in Central Patagonia

    NASA Astrophysics Data System (ADS)

    Scalabrino, B.; Ritz, J. F.; Lagabrielle, Y.

    2009-04-01

    The Central Patagonian Cordillera is a unique laboratory to study interaction between oceanic and continental lithospheres during the subduction of an active spreading ridge beneath a continent. The subduction of the South Chile spreading Ridge, which separates the Nazca plate from the Antarctic plate, started ca. 15-14 Ma at the southern tip of Patagonia (55°S latitude). The northwards migration of the Chile Triple Junction induces the subduction of several segments especially around 46°S latitude. There, three segments subducted at ca. 6, 3 and 0.3 Ma, leading to the formation of a large asthenospheric slab-window beneath Central Patagonia. Contemporaneously, the Central Patagonia reliefs are undergoing major glacial events since at least 7 Ma. These events are evidenced to the east of the Central Patagonian morphotectonic front within perched relict surfaces. Inset in these perched glacial surfaces are found mid-Pleistocene glacial valleys, as the Lake General Carrera-Buenos Aires amphitheatre (LGCBA), which formed between 1.1 Ma and 16 ka. We used the relationships between the glacial valleys and the volcanism associated with the asthenospheric slab-window to better constraints the structural evolution of the Patagonian Cordillera related to the subduction of the Chili active spreading Ridge. The present work focused within two well-preserved perched flat surfaces named Meseta del Lago Buenos Aires and Meseta del Cerro Galera: (i) The meseta del Lago Buenos Aires defines a plateau made of interbedded units of tills and lavas dated between 12 Ma and 3 Ma. The top surface of the meseta, ˜2000 meters high is dated at 3 Ma, and is shaped by four NE-SW trending glacial lobes characterized with kettles, lineations and moraines. The glacial valleys are beheaded westwards and define perched valleys 200 to 400 meters higher than the western Cordillera. This suggests recent vertical movement along N160 extensive/transtensive corridor located between the morphotectonic

  14. Lithosphere-Mantle Interactions Associated with Flat-Slab Subduction

    NASA Astrophysics Data System (ADS)

    Gerault, M.; Becker, T. W.; Husson, L.; Humphreys, E.

    2014-12-01

    Episodes of flat-slab subduction along the western margin of the Americas may have lead to the formation of intra-continental basins and seas, as well as mountain belts and continental plateaux. Here, we explore some of the consequences of a flat slab morphology, linking dynamic topography and stress patterns in continents to slab and mantle dynamics. Using a 2-D cylindrical code, we develop general models and apply them to the North and South America plates. The results are primarily controlled by the coupling along the slab-continent interface (due to geometry and viscosity), the viscosity of the mantle wedge, and the buoyancy of the subducted lithosphere. All models predict broad subsidence, large deviatoric stresses, and horizontal compression above the tip of the flat slab and the deep slab hinge. In models where the slab lays horizontally for hundreds of kilometers, overriding plate compression focuses on both ends of the flat segment, where normal-dip subduction exerts a direct downward pull. In between, a broad low-stress region gets uplifted proportionally to the amount of coupling between the slab and the continent. Anomalously buoyant seafloor enhances this effect but is not required. The downward bending of the flat slab extremities causes its upper part to undergo extension and the lower part to compress. These results have potential for explaining the existence of relatively undeformed, uplifted regions surrounded by mountain belts, such as in the western U.S. and parts of the Andes. Adequately modeling topography and stress in the unusual setting of southwestern Mexico requires a low-viscosity subduction interface and mantle wedge. Our results are only partially controlled by the buoyancy of the subducting plate, suggesting that the viscosity and the morphology of the slab are important, and that the often-used low resolution and "Stokeslet" models may be missing substantial effects.

  15. Online Classroom Research and Analysis Activities Using MARGINS-Related Resources for the Izu-Bonin-Mariana Subduction System

    NASA Astrophysics Data System (ADS)

    Ryan, J. G.

    2007-12-01

    Students today have online access to nearly unlimited scientific information in an entirely unfiltered state. As such, they need guidance and training in identifying and assessing high-quality information resources for educational and research use. The extensive research data resources available online for the Izu-Bonin-Mariana (IBM) subduction system that have been developed with MARGINS Program and related NSF funding are an ideal venue for focused Web research exercises that can be tailored to a range of undergraduate geoscience courses. This presentation highlights student web research activities examining: a) The 2003-2005 eruptions of Anatahan Volcano in the Mariana volcanic arc. MARGINS-supported geophysical research teams were in the region when the eruption initiated, permitting a unique "event response" data collection and analysis process, with preliminary results presented online at websites linked to the MARGINS homepage, and ultimately published in a special issue of the Journal of Volcanology and Geothermal Research. In this activity, students will conduct a directed Web surf/search effort for information on and datasets from the Anatahan arc volcano, which they will use in an interpretive study of recent magmatic activity in the Mariana arc. This activity is designed as a homework exercise for use in a junior-senior level Petrology course, but could easily be taken into greater depth for the benefit of graduate-level volcanology or geochemistry offerings. b) Geochemical and mineralogical results from ODP Legs 125 and 195 focused on diapiric serpentinite mud volcanoes, which erupt cold, high pH fluids, serpentine muds, and serpentinized ultramafic clasts at a number of sites in the forearc region of the Mariana subduction zone. The focus of this activity is an examination of the trace element chemistry of the forearc serpentines and their associated upwelling porefluids as a means of understanding the roles of ionic radius, valence, and system

  16. Transition from collision to subduction in Western Greece: the Katouna-Stamna active fault system and regional kinematics

    NASA Astrophysics Data System (ADS)

    Pérouse, E.; Sébrier, M.; Braucher, R.; Chamot-Rooke, N.; Bourlès, D.; Briole, P.; Sorel, D.; Dimitrov, D.; Arsenikos, S.

    2016-06-01

    Transition from subduction to collision occurs in Western Greece and is accommodated along the downgoing plate by the Kefalonia right-lateral fault that transfers the Hellenic subduction front to the Apulian collision front. Here we present an active tectonic study of Aitolo-Akarnania (Western Greece) that highlights how such a transition is accommodated in the overriding plate. Based on new multi-scale geomorphic and tectonic observations, we performed an accurate active fault trace mapping in the region, and provide evidence for active normal and left-lateral faulting along the Katouna-Stamna Fault (KSF), a 65-km-long NNW-striking fault system connecting the Amvrakikos Gulf to the Patras Gulf. We further show that the Cenozoic Hellenide thrusts located west of the KSF are no longer active, either in field observation or in GPS data, leading us to propose that the KSF forms the northeastern boundary of a rigid Ionian Islands-Akarnania Block (IAB). Cosmic ray exposure measurements of 10Be and 36Cl were performed on a Quaternary alluvial fan offset along the KSF (~50 m left-lateral offset). A maximum abandonment age of ~12-14 ka for the alluvial fan surface can be determined, giving an estimated KSF minimum geological left-lateral slip rate of ~4 mm year-1, in agreement with high GPS slip rates (~10 mm year-1). Despite this high slip rate, the KSF is characterized by subdued morphological evidence of tectonic activity, a gypsum-breccia bedrock and a low level of seismicity, suggesting a dominantly creeping behavior for this fault. Finally, we discuss how the IAB appears to have been progressively individualized during the Pleistocene (younger than ~1.5 Ma).

  17. Upper plate responses to active spreading ridge/transform subduction: The tectonics, basin evolution, and seismicity of the Taita area, Chile Triple Junction

    SciTech Connect

    Flint, S.; Prior, D. ); Styles, P.; Murdie, R. ); Agar, S.; Turner, P. )

    1993-02-01

    Integrated field geophysical, structural and stratigraphic studies are attempting to elucidate the mechanisms and consequences of the Late Miocene-present day subduction of the Chile Ridge triple junction system. Preliminary data indicate a shallow plane of seismicity at about 15 km to 20 km depth below the Taitao peninsula. The depths correspond to the predicted depth range of subducted upper ocean crust. The calculated Bouguer anomaly map cannot be explained by the upper plate geology, suggesting that gravity is influenced by heterogeneities in the subducting oceanic plate. Seismic data imply that a subducted transform system underlying the inner Taitao Peninsula is still an active structure. A series of Middle-Late Tertiary sedimentary basins lie inboard of the triple junction. Within the Cosmelli basin, abrupt marine to continental facies transitions give clear evidence of base level changes. The amount of basinward shift of facies across sequence boundaries gets progressively greater up stratigraphy, indicating progressively greater base level changes. The lower part of the basin fill is folded and then thrusted eastward as a series of imbricates, while the overlying, greater thickness of fluvial sediments are only gently tilted westwards. We provisionally interpret this geometry to indicate that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited. This complex basin history may reflect initiation and development of triple junction subduction.

  18. Slab pull and the seismotectonics of subducting lithosphere.

    USGS Publications Warehouse

    Spence, W.

    1987-01-01

    This synthesis links many seismic and tectonic processes at subduction zones, including great subduction earthquakes, to the sinking of subducted plate. Earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force. Interactions between the forces that drive and resist plate motions cause spatially and temporally localized stress that lead to characteristic earthquake activity, providing details on how subduction occurs.-from Author

  19. Imaging the Subduction Decollement, Hikurangi Subduction Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Bannister, S.; Toulmin, S.; Henrys, S.; Reyners, M.; Barker, D.; Pecher, I.; Sutherland, R.; Uruski, C.; Maslen, G.

    2006-12-01

    Beneath the eastern coastline of North Island, New Zealand, the subducted Pacific plate dips at less than 3 degrees to the northwest and the subduction decollement is at a depth of less than 15-km. The active-source NIGHT and passive-source CNIPSE experiments carried out in 2001 image the shallow dipping decollement down to more than 6 s (twt) at which point it steepens landward, 120 km from the Hikurangi trench. This change in dip appears to be closely associated with the onset of seismogenesis in the subducted plate. Velocity inversion of CNIPSE earthquake times reveals the forearc to be a relatively low Vp (<5.5 km/s), high Vp/Vs (>1.85), high Poisson's ratio (>0.29) region overlying the 12-15 km thick subducted crust. In March-May 2005 a new industry-seismic survey, 05CM, was carried out offshore the east coast, seismically imaging the subducted plate. More than 278000 airgun shots were also recorded by temporary seismometer stations placed along the coastline, out to offsets of more than 100 km. The combined marine and offshore- onshore seismic data highlight an area of more than 400 sq.kms. with higher reflectivity on the subduction decollement up-dip of the up-dip limit of seismogenesis, which we further examine using AVO and finite- difference modelling.

  20. 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

  1. 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

  2. Seismic activity offshore Martinique and Dominica islands (Central Lesser Antilles subduction zone) from temporary onshore and offshore seismic networks

    NASA Astrophysics Data System (ADS)

    Ruiz, M.; Galve, A.; Monfret, T.; Sapin, M.; Charvis, P.; Laigle, M.; Evain, M.; Hirn, A.; Flueh, E.; Gallart, J.; Diaz, J.; Lebrun, J. F.

    2013-09-01

    This work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the Central Lesser Antilles Island Arc from Martinique to Guadeloupe islands. During the whole recording period, extending from January to the end of August 2007, more than 1300 local seismic events were detected in this area. A subset of 769 earthquakes was located precisely by using HypoEllipse. We also computed focal mechanisms using P-wave polarities of the best azimuthally constrained earthquakes. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. At depth seismicity delineates the Wadati-Benioff Zone down to 170 km depth. The main seismic activity is concentrated in the lower crust and in the mantle wedge, close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed. 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.

  3. Tectonic and magmatic controls on the location of post-subduction monogenetic volcanoes in Baja California, Mexico, revealed through spatial analysis of eruptive vents

    NASA Astrophysics Data System (ADS)

    Germa, Aurélie; Connor, Laura J.; Cañon-Tapia, Edgardo; Le Corvec, Nicolas

    2013-12-01

    Post-subduction (12.5 Ma to less than 1 Ma) monogenetic volcanism on the Baja California peninsula, Mexico, formed one of the densest intra-continental areas of eruptive vents on Earth. It includes about 900 vents within an area ˜700 km long (N-S) and 70 to 150 km wide (W-E). This study shows that post-subduction volcanic activity was distributed along this arc and that modes exist in the volcano distribution, indicating that productivity of the magma source region was not uniform along the length of the arc. Vent clustering, vent alignments, and cone elongations were measured within eight monogenetic volcanic fields located along the peninsula. Results indicate that on a regional scale, vent clustering varies from north to south with denser spatial clustering in the north on the order of 1.9 × 10-1 vents/km2 to less dense clustering in the south on the order of 7.8 × 10-2 vents/km2. San Quintin, San Carlos, Jaraguay, and Santa Clara are spatially distinct volcanic fields with higher eruptive vent densities suggesting the existence of individual melt columns that may have persisted over time. In contrast, the San Borja, Vizcaino, San Ignacio, and La Purisima vent fields show lower degrees of vent clustering and no obvious spatial gaps between fields, thus indicating an area of more distributed volcanism. Insight into the lithospheric stress field can be gained from vent alignments and vent elongation measurements. Within the fields located along the extinct, subduction-related volcanic arc, elongation patterns of cinder cones and fissure-fed spatter cones, vent clusters, and vent alignments trend NW-SE and N-S. Within the Santa Clara field, located more to the west within the forearc, elongation patterns of the same volcanic features trend NE-SW. These patterns suggest that magmatism was more focused in the forearc and in the northern part of Baja California than in its southern region. Within the extinct arc, magma ascent created volcano alignments and elongate

  4. From rifting to active spreading in the Lau Basin - Havre Trough backarc system (SW Pacific): Locking/unlocking induced by seamount chain subduction

    NASA Astrophysics Data System (ADS)

    Ruellan, E.; Delteil, J.; Wright, I.; Matsumoto, T.

    2003-05-01

    Associated with Pacific-Australia plate convergence, the Lau Basin - Havre Trough is an active back-arc basin that has been opened since ˜5.5 Ma by rifting and southward propagating oceanic spreading. Current back-arc opening rates decrease from 159 mm yr-1 in the northern Lau Basin to 15 mm y-1 in the southern Havre Trough. Major tectonic changes occur at the transition between Havre Trough rifting and full oceanic spreading of the Eastern Lau Spreading Center (ELSC), where the oblique-to-trench, westward subducting Louisville Seamount Chain (LSC) sweeps southwards along the Tonga trench. New swath bathymetry, seismic reflection data, and limited rock sampling in this area constrain a tectonic and kinematic back-arc model that incorporates the effects of LSC subduction. The ELSC, which extends southward to 24°55'S, forms a deep rift valley propagating southward through older, rifted arc basement. Present-day seismicity and fresh and fractured pillow lavas at 23°42'S are consistent with rift valley neovolcanism. Conversely, the northern Havre Trough has low seismicity and rifted volcanic basement ridges trending 25-45° oblique to the basin axis consistent with low levels of extensional tectonism and volcanism. This latter structural fabric is interpreted as an early stage of rifting that is now "locked" due to compression on the arc exerted by LSC subduction, while in the Lau Basin such effects have passed as the LSC swept along the Tonga Trench. It is proposed that the Lau-Havre back-arc opening is controlled by tectonic constraints exerted at the limits of the system by the LSC subduction, which determines the southward migration of the Tonga Arc pole of rotation and associated Lau Basin opening. A discrete three-stage back-arc opening evolution is proposed, comprising: (1) an initial phase of back-arc rifting along the whole length of the plate boundary, beginning at ˜6-5 Ma; (2) a subsequent phase, mostly present in the southern part of the back-arc domain

  5. Repeated remobilisation of submarine landslide debris on an active subduction margin interpreted from multibeam bathymetry and multichannel seismic data

    NASA Astrophysics Data System (ADS)

    Mountjoy, J. J.; Barnes, P. M.; McKean, J.; Pettinga, J. R.

    2008-12-01

    EM300 multibeam and multichannel seismic data reveal a 230 square kilometre submarine landslide complex which exhibits many of the characteristic features of equivalent terrestrial creeping earthflow complexes. Slope failures are sourced from the shelf edge/upper slope of the Poverty Bay reentrant on the active Hikurangi subduction margin of New Zealand where tectonic deformation, via major thrust faults with slip rates of c. 3-4 mm/yr, exerts a controlling influence on seafloor physiography. Individual landslides within this submarine complex are up to 14 km long over a vertical elevation drop of 700 m. Debris streams are in excess of 2 km wide with a debris thickness of 100 m. While multibeam data is limited to c. 10 m resolution, the scale of submarine landslide features allows us to resolve internal debris detail equivalent to terrestrial landslide examples using terrestrial techniques (e.g. airborne lidar). DEM derivative surface roughness techniques are employed to delineate the geomorphic expression of features including active and abandoned lateral shears, and contractional and extensional deformation of the landslide debris. From these interpretations multiple internal failures are recognised along the length of the landslide debris. Debris deformation is also imaged in high fold multichannel seismic data and correlated to the imaged surface geomorphic features, providing insight into the failure mechanics of the landslides. Failures initiate and evolve within the quasi-stable prograding sediment wedge built onto the upper slope during lowstand sealevels. Landslides within the greater complex are at different stages of development providing information on their spatial and temporal evolution headward and laterally along the transition from shelf to upper slope margin. We infer that failures are triggered and evolve in response to sealevel rise, and/or the frequent occurrence large earthquakes along the margin.

  6. Accelerating late Quaternary uplift of the New Georgia Island Group (Solomon island arc) in response to subduction of the recently active Woodlark spreading center and Coleman seamount

    NASA Astrophysics Data System (ADS)

    Mann, Paul; Taylor, Frederick W.; Lagoe, Martin B.; Quarles, Andrew; Burr, G.

    1998-10-01

    The New Georgia Island Group of the Solomon Islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc. We have used coral reef terraces, paleobathymetry of Neogene sedimentary rocks, and existing marine geophysical data to constrain patterns of regional Quaternary deformation related to subduction of the recently active Woodlark spreading center and its overlying Coleman seamount. These combined data indicate the following vertical tectonic history for the central part of the New Georgia Island Group: (1) subsidence of the forearc region (Tetepare and Rendova Islands) to water depths of ˜1500 m and deposition of marine turbidites until after 270 ka; (2) late Quaternary uplift of the forearc to sea level and erosion of an unconformity; (3) subsidence of the forearc to ˜500 m BSL and deposition of bathyal sediments; and (4) uplift of the forearc above sea level with Holocene uplift rates up to at least 7.5 mm/yr on Tetepare and 5 mm/yr on Rendova. In the northeastern part of the New Georgia Island Group, our combined data indicate a slightly different tectonic history characterized by lower-amplitude vertical motions and a more recent change from subsidence to uplift. Barrier reefs formed around New Georgia and Vangunu Islands as they subsided >300 m. By 50-100 ka, subsidence was replaced by uplift that accelerated to Holocene rates of ˜1 mm/yr on the volcanic arc compared with rates up to ˜7.5 mm/yr in the forearc area of Tetepare and Rendova. Uplift mechanisms, such as thermal effects due to subduction of spreading ridges, tectonic erosion, or underplating of deeply subducted bathymetric features, are not likely to function on the 270-ka period that these uplift events have occurred in the New Georgia Island Group. A more likely uplift mechanism for the post-270-ka accelerating uplift of the forearc and volcanic arc of the New Georgia Island Group is progressive impingement of the Coleman seamount or

  7. Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system

    NASA Astrophysics Data System (ADS)

    Rizzo, Andrea; Caracausi, Antonio; Chavagnac, Valérie; Nomikou, Paraskevi; Polymenakou, Paraskevi; Mandalakis, Manolis; Kotoulas, Georgios; Magoulas, Antonios; Castillo, Alain

    2016-04-01

    Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important ource of mantle-derived gases. These gases are fundamental for the comprehension of mantle characteristics in key sectors where subaerial volcanism is missing or strongly modified by mixing with crustal and atmospheric components. Though, the study of submarine volcanic areas remains a challenge due to their hazardousness and sea depth. Here, we report 3He/4He measurements in CO2-dominated gases discharged at 500 m below the sea level from the high-temperature (~220°C) hydrothermal system of Kolumbo submarine volcano, located 7 km northeast off Santorini Island in the central part of Hellenic Volcanic Arc (HVA). For the first time, we demonstrate that the mantle below Kolumbo and Santorini has a 3He/4He signature of at least 7.1 Ra (being Ra the 3He/4He ratio of atmospheric He equal to 1.39×10-6), 3 Ra units higher than actually known for Santorini gases-rocks. This isotopic ratio is also the highest measured across HVA and is indicative of the direct degassing of a MORB-like mantle through lithospheric faults or a possible slab tear. We finally highlight that the 3He/4He ratio has been increasing since 2010 (ΔRa=0.3), which implies an enhanced magmatic activity beneath Kolumbo.

  8. Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system.

    PubMed

    Rizzo, Andrea Luca; Caracausi, Antonio; Chavagnac, Valèrie; Nomikou, Paraskevi; Polymenakou, Paraskevi N; Mandalakis, Manolis; Kotoulas, Georgios; Magoulas, Antonios; Castillo, Alain; Lampridou, Danai

    2016-01-01

    Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important source of mantle-derived gases. These gases are of basic importance for the comprehension of mantle characteristics in areas where subaerial volcanism is missing or strongly modified by the presence of crustal/atmospheric components. Though, the study of submarine volcanism remains a challenge due to their hazardousness and sea-depth. Here, we report (3)He/(4)He measurements in CO2-dominated gases discharged at 500 m below sea level from the high-temperature (~220 °C) hydrothermal system of the Kolumbo submarine volcano (Greece), located 7 km northeast off Santorini Island in the central part of the Hellenic Volcanic Arc (HVA). We highlight that the mantle below Kolumbo and Santorini has a (3)He/(4)He signature of at least 7.0 Ra (being Ra the (3)He/(4)He ratio of atmospheric He equal to 1.39×10(-6)), 3 Ra units higher than actually known for gases-rocks from Santorini. This ratio is also the highest measured across the HVA and is indicative of the direct degassing of a Mid-Ocean-Ridge-Basalts (MORB)-like mantle through lithospheric faults. We finally highlight that the degassing of high-temperature fluids with a MORB-like (3)He/(4)He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano. PMID:27311383

  9. Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system.

    PubMed

    Rizzo, Andrea Luca; Caracausi, Antonio; Chavagnac, Valèrie; Nomikou, Paraskevi; Polymenakou, Paraskevi N; Mandalakis, Manolis; Kotoulas, Georgios; Magoulas, Antonios; Castillo, Alain; Lampridou, Danai

    2016-06-17

    Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important source of mantle-derived gases. These gases are of basic importance for the comprehension of mantle characteristics in areas where subaerial volcanism is missing or strongly modified by the presence of crustal/atmospheric components. Though, the study of submarine volcanism remains a challenge due to their hazardousness and sea-depth. Here, we report (3)He/(4)He measurements in CO2-dominated gases discharged at 500 m below sea level from the high-temperature (~220 °C) hydrothermal system of the Kolumbo submarine volcano (Greece), located 7 km northeast off Santorini Island in the central part of the Hellenic Volcanic Arc (HVA). We highlight that the mantle below Kolumbo and Santorini has a (3)He/(4)He signature of at least 7.0 Ra (being Ra the (3)He/(4)He ratio of atmospheric He equal to 1.39×10(-6)), 3 Ra units higher than actually known for gases-rocks from Santorini. This ratio is also the highest measured across the HVA and is indicative of the direct degassing of a Mid-Ocean-Ridge-Basalts (MORB)-like mantle through lithospheric faults. We finally highlight that the degassing of high-temperature fluids with a MORB-like (3)He/(4)He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano.

  10. Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system

    PubMed Central

    Rizzo, Andrea Luca; Caracausi, Antonio; Chavagnac, Valèrie; Nomikou, Paraskevi; Polymenakou, Paraskevi N.; Mandalakis, Manolis; Kotoulas, Georgios; Magoulas, Antonios; Castillo, Alain; Lampridou, Danai

    2016-01-01

    Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important source of mantle-derived gases. These gases are of basic importance for the comprehension of mantle characteristics in areas where subaerial volcanism is missing or strongly modified by the presence of crustal/atmospheric components. Though, the study of submarine volcanism remains a challenge due to their hazardousness and sea-depth. Here, we report 3He/4He measurements in CO2–dominated gases discharged at 500 m below sea level from the high-temperature (~220 °C) hydrothermal system of the Kolumbo submarine volcano (Greece), located 7 km northeast off Santorini Island in the central part of the Hellenic Volcanic Arc (HVA). We highlight that the mantle below Kolumbo and Santorini has a 3He/4He signature of at least 7.0 Ra (being Ra the 3He/4He ratio of atmospheric He equal to 1.39×10−6), 3 Ra units higher than actually known for gases-rocks from Santorini. This ratio is also the highest measured across the HVA and is indicative of the direct degassing of a Mid-Ocean-Ridge-Basalts (MORB)-like mantle through lithospheric faults. We finally highlight that the degassing of high-temperature fluids with a MORB-like 3He/4He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano. PMID:27311383

  11. Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system

    NASA Astrophysics Data System (ADS)

    Rizzo, Andrea Luca; Caracausi, Antonio; Chavagnac, Valèrie; Nomikou, Paraskevi; Polymenakou, Paraskevi N.; Mandalakis, Manolis; Kotoulas, Georgios; Magoulas, Antonios; Castillo, Alain; Lampridou, Danai

    2016-06-01

    Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important source of mantle-derived gases. These gases are of basic importance for the comprehension of mantle characteristics in areas where subaerial volcanism is missing or strongly modified by the presence of crustal/atmospheric components. Though, the study of submarine volcanism remains a challenge due to their hazardousness and sea-depth. Here, we report 3He/4He measurements in CO2–dominated gases discharged at 500 m below sea level from the high-temperature (~220 °C) hydrothermal system of the Kolumbo submarine volcano (Greece), located 7 km northeast off Santorini Island in the central part of the Hellenic Volcanic Arc (HVA). We highlight that the mantle below Kolumbo and Santorini has a 3He/4He signature of at least 7.0 Ra (being Ra the 3He/4He ratio of atmospheric He equal to 1.39×10‑6), 3 Ra units higher than actually known for gases-rocks from Santorini. This ratio is also the highest measured across the HVA and is indicative of the direct degassing of a Mid-Ocean-Ridge-Basalts (MORB)-like mantle through lithospheric faults. We finally highlight that the degassing of high-temperature fluids with a MORB-like 3He/4He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano.

  12. Elements of the Seismic Structure and Activity of the Lesser Antilles Subduction Zone (Guadeloupe and Martinique Islands) from the SISMANTILLES Seismic Survey

    NASA Astrophysics Data System (ADS)

    Laigle, M.; Roux, E.; Sapin, M.; Hirn, A.; de Voogd, B.; Charvis, P.; Hello, Y.; Murai, Y.; Nishimura, Y.; Shimamura, H.; Galve, A.; Lepine, J.; Lebrun, J.; Diaz, J.; Gallart, J.; Beauducel, F.; Viode, J.

    2005-12-01

    The Lesser Antilles is an active subduction zone, prone to future major earthquakes as it has experienced in the past with the occurrence in 1843 of a M>7.5 probably mega-thrust earthquake that destroyed Pointe-a-Pitre city on Guadeloupe Island. The SISMANTILLES project was carried out at a regional scale for a first reconnaissance of the seismic structure and activity from northern Guadeloupe to Martinique islands. The project focused more particularly on the detection, mapping and characterisation of the potentially seimogenic part of the interplate subduction fault. The french N/O Nadir vessel acquired 2500 km of deep-penetration multichannel reflection seismic (MCS) profiles. Up to 37 3-components Ocean Bottom Seismometers (OBS) were deployed offshore over several weeks together with a set of 3-components broadened-band stations on the islands (Martinique, Dominica, Guadeloupe and Antigua). These instruments recorded continuously both the MCS shots that provided wide angle reflexion and refraction (WARR) data as well as the local, regional and teleseismic earthquakes. On MCS profiles, reflections from the top of the subducting oceanic crust and decollement can be followed down to several km depth beneath the thick accretionary prism. Detailed velocity analysis provided depth structural sections that are used as an input for the forward modeling of WARR data. Thanks to these data, we can constrain on 3 transects to the arc, the part where the forearc deep crust is in contact with the subducting oceanic plate, considered as a proxy for the seismogenic part. Its location with respect to the deformation front and the volcanic arc and its downdip size appear significantly variable along the arc. The local earthquakes now reliably located in map and depth thanks to the high-quality P and S observations of the OBS network can be discussed with respect to these imaged structures. Local earthquakes P & S tomography as well as receiver functions analysis will bring more

  13. Dynamics and Preservation Potential of Subduction Complexes in Continental Sutures: A Case Study from the Sedimentary-Marix Mélange of the Indus-Yarlung Suture Zone in Southern Tibet

    NASA Astrophysics Data System (ADS)

    Metcalf, K.; Kapp, P. A.; Orme, D. A.

    2015-12-01

    Intra-continental sutures are the geologic record of ancient subduction zones. Subduction complexes are a useful record of ancient continental collisions because they preserve sediments and/or blocks from units which have since eroded and are the first point of contact during collision. The India-Asia collision is one of the most-studied collisional orogens, but how much of the original subduction complex is preserved and what we can determine about the dynamics of the ancient subduction zone along the southern margin of Asia is poorly understood. Compared to other subduction complexes around the world, the complex preserved in the Indus Yarlung Suture Zone (IYSZ) of southern Tibet is anomalous. Blueschist facies metamorphism, a prominent mineral assemblage along intra-continental suture zones, is common in the northwest Himalaya, but not found along the central segment of the IYSZ. Most of the subduction complex is greenschist facies, inconsistent with the geotherm for a subduction zone. We present a metamorphic history for the greenschist facies rocks to reconcile this contradiction. A deep forearc basin (~5-8 km) developed during the Cretaceous, requiring an accretionary subduction zone, a topographic or structural outer forearc high behind which to trap sediment, and/or basal subduction erosion. The preserved subduction complex is almost entirely tectonic sedimentary-matrix mélange with minor outcrops of overlying turbidites. We present evidence from detrital zircon U-Pb geochronology of sandstones that indicate along-strike variability in the provenance of the sedimentary-matrix mélange. For example, both lower and upper plate material are present near the town of Ngamring, while regions along-strike to the west contain little to no upper plate material. The blocks in the sedimentary-matrix mélange are well-mixed throughout kilometers of exposed width. Sandstone blocks of Tethyan affinity, which could have entered the trench only at the onset of collision

  14. Nature of Subduction Megathrust Faults at the Ryukyu Subduction Zone

    NASA Astrophysics Data System (ADS)

    Arai, R.; Kaiho, Y.; Takahashi, T.; Nakanishi, A.; Fujie, G.; Nakamura, Y.; Miura, S.; Kodaira, S.; Kaneda, Y.

    2015-12-01

    The Ryukyu subduction zone (RSZ) has been intensively examined in terms of seismic coupling along the plate boundary and tsunami potentials. On the contrary to other subduction zones nearby, such as the Nankai Trough and the Japan Trench, the RSZ has lacked clear evidence of great interplate earthquakes (M>8) for the last few hundred years and thus the overall interplate coupling is thought to be weak (Peterson and Seno, 1984). Correspondingly, geodetic observation implies that a possible coupled zone is narrow and limited to a shallow portion of the plate boundary near the trench (Ando et al., 2009). Recent seismic studies show that very low frequency earthquakes (VLFEs) are ubiquitously distributed in the forearc region of the Ryukyu arc, implying a variety of slip behaviors along the subduction faults related to fluid distribution (Nakamura and Sunagawa, 2015). However, these findings were derived from land station network and did not have sufficient resolution near the trench to determine spatial relationship of megathrust faults to the seismic activity. Since 2013 we JAMSTEC have been carrying out marine active- and passive-source seismic experiments in the Ryukyu subduction zone to reveal the fine-scale geometry and nature of subduction faults. Here we present integrated seismological evidence for megathrust fault structure and its relation to VLFEs at the southern Ryukyu Trench. Active-source seismic data consistently reveal that the plate boundary and backstop interface form a 40-km-wide frontal prism where low-velocity sedimentary rocks fill in. We find VLFEs occur around the low-velocity wedge where fluids are distributed as suggested by negative polarity in the reflection data. This forearc structure is also coincident with the source region of Yaeyama earthquake tsunami in 1771 (Nakamura, 2009), the most devastating disaster known in this region. Slow ruptures enhanced by the fluid-rich condition at the plate boundary and/or surrounding faults may be a

  15. Subduction of fracture zones

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Gerya, Taras; Manea, Marina; Zhu, Guizhi; Leeman, William

    2013-04-01

    Since Wilson proposed in 1965 the existence of a new class of faults on the ocean floor, namely transform faults, the geodynamic effects and importance of fracture zone subduction is still little studied. It is known that oceanic plates are characterized by numerous fracture zones, and some of them have the potential to transport into subduction zones large volumes of water-rich serpentinite, providing a fertile water source for magma generated in subduction-related arc volcanoes. In most previous geodynamic studies, subducting plates are considered to be homogeneous, and there is no clear indication how the subduction of a fracture zone influences the melting pattern in the mantle wedge and the slab-derived fluids distribution in the subarc mantle. Here we show that subduction of serpentinized fracture zones plays a significant role in distribution of melt and fluids in the mantle wedge above the slab. Using high-resolution tree-dimensional coupled petrological-termomechanical simulations of subduction, we show that fluids, including melts and water, vary dramatically in the region where a serpentinized fracture zone enters into subduction. Our models show that substantial hydration and partial melting tend to concentrate where fracture zones are being subducted, creating favorable conditions for partially molten hydrous plumes to develop. These results are consistent with the along-arc variability in magma source compositions and processes in several regions, as the Aleutian Arc, the Cascades, the Southern Mexican Volcanic Arc, and the Andean Southern Volcanic Zone.

  16. Fluid flow and water-rock interaction across the active Nankai Trough subduction zone forearc revealed by boron isotope geochemistry

    NASA Astrophysics Data System (ADS)

    Hüpers, Andre; Kasemann, Simone A.; Kopf, Achim J.; Meixner, Anette; Toki, Tomohiro; Shinjo, Ryuichi; Wheat, C. Geoffrey; You, Chen-Feng

    2016-11-01

    Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water-rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (δ11B) of pore fluids sampled across the subduction zone forearc from depths of up to ∼922 m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to ∼120 μM and variable δ11B values in the range of ∼+20‰ and +49‰. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for 10B. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper (<2 km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60-150 °C. At both locations the δ11B signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the décollement zone along the central portion of the Nankai margin.

  17. Deep crust and mantle structure linked to subduction of the Pacific plate at a continental margin from an active seismic source study

    NASA Astrophysics Data System (ADS)

    Stern, T. A.; Okaya, D. A.; Henrys, S. A.; Savage, M. K.; Sato, H.; Iwasaki, T.

    2013-12-01

    We present new results that bear on mantle structure in a subduction zone of both the down going and over-riding plates. Wellington region, New Zealand, sits on the Australian plate a mere ~ 15-25 km above the subducted Pacific plate. This is rare: most active continental margins have at least 30-50 km of free board above subduction zones (cf, Pacific NW, Honshu). Such a shallow setting offers unusually good conditions for making terrestrial observations of the subduction process. In 2010-11 an active source seismic experiment (SAHKE) was carried out across the Wellington region. Both active and passive seismic methods were used to establish a model of the crustal structure and upper mantle structure beneath SAHKE (Henrys et al , 2013). We recorded 12 x 500 kg dynamite shots on ~ 1000 recorders place at 100 m spacing from coast to coast. These data contain excellent returns from the subduction zone at two-way travel times of 7-15 s, but we also recorded coherent reflection energy down to 30 s two-way-travel-time on vertical recorders and 47 s on horizontal recorders. We performed a low-fold stack of these deep reflections and see two zones. The deepest reflections define a westward 17 degree-dipping zone at ~ 100 km deep that is possibly the base of the subducted Pacific plate. The westward dip on this 100 km deep reflector matches that for the top of the plate. Moreover, the total plate thickness implied is ~ 80 km, which matches that predicted for 100 my old oceanic lithosphere. The other deep reflection zone dips eastward across the shot gathers and is typically seen at two-way travel times of 18-25 s on the vertical phones. We also see the same event as a Vs arrival on horizontal phones at two-way travel times of 34-47s. When migrated these reflections define an east dipping reflector that is within the mantle of the overriding Australian plate, and the surface projection of the reflector is ~ 80 km west of SAHKE line. This finding raises the question of what

  18. Shallow structure and its formation process of an active flexure in the forearc basin of the central Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Ashi, J.; Ikehara, K.; Omura, A.; Ojima, T.; Murayama, M.

    2013-12-01

    ENE-WSW trending active faults, named Enshu fault system, are developed in the forearc basins of the eastern and central Nankai subduction zone. Three parallel faults developed in the Enshu forearc basin of the eastern Nankai have right lateral slip on the basis of dextral displacement of the canyon axis. Moreover, bathymetry data and side-scan sonar imageries indicate relative uplift of the northern region and the multichannel seismic (MCS) reflection profiles show northward dipping fault planes. In the central Nankai subuduction zone, an ENE-WSW trending step is distributed at the northern part of the Kumano forearc basin and is regarded as the western extension of the Enshu fault system. Although MCS records show deformations including an anticlinal fold beneath the bathymetric step, they have less resolution to identify deformation of basin sequence just below the seafloor. In contrast, deformation seems to reach to the seafloor on a profile by SBP mounted on a mother ship. Investigation of shallow deformation structures is significant for understanding of recent tectonic activity. We carried out deep towed SBP survey by ROV NSS (Navigable Sampling System) during Hakuho-maru KH-11-9 cruise. High resolution mapping of shallow structures was successfully conducted by a chirp SBP system of EdgeTech DW-106. ROV NSS also has capability to take a long core with a pinpoint accuracy around complex topographic region. The Kumano forearc basin is topographically divided into the northern part at a water depth of 2038 m and the other major region at a depth of 2042 m by the ENE-WSW linear step. Three deep towed SBP lines intersected this topographical step and revealed the following structures. This step is composed of 100 m wide gentle slope with an inclination of about 8 degrees. An anticlinal axis is located beneath the upper edge of this slope. Sedimentary layers continue at this slope region without any abut/termination and rapidly increase their thickness toward the

  19. Bi-directional subduction of the South Tianshan Ocean during the Late Silurian: Magmatic records from both the southern Central Tianshan Block and northern Tarim Craton

    NASA Astrophysics Data System (ADS)

    Wang, Meng; Zhang, Jinjiang; Zhang, Bo; Liu, Kai; Ge, Maohui

    2016-10-01

    The subduction polarity of the South Tianshan Ocean (STO) is a matter of debate, primarily in that the Paleozoic structures of Tianshan orogenic belt have been strongly overprinted by the Cenozoic intra-continental deformation. Indentifying the arc-related magmatic rocks may provide a convincible clue for understanding the closure process of the STO. In this study, whole-rock geochemistry, zircon U-Pb dating and Hf isotope were presented on the andesite and monzonite from the Bayanbulak area of the southern Central Tianshan Block (CTB) and on the quartz diorite from the Ouxidaban area of the northern Tarim Craton. Geochemically, all the samples are Na-rich, enriched in light rare earth elements and large ion lithophile elements (Rb, Ba, U, K and Pb), and depleted in high strength field elements (Nb, Ta and Ti), like most arc-type igneous rocks. The Bayanbulak andesite samples display high MgO, Fe2O3T, TiO2 and Mg# values, and positive εHf(t) values, indicating magma source from the wedge mantle. But the existence of xenocrystic zircons implies that continental crust material were involved during magma ascend, suggesting a continental arc setting for the Bayanbulak andesite. The Bayanbulak monzonite and the Ouxidaban quartz diorite samples display relatively higher SiO2 contents, and lower MgO, Fe2O3T and TiO2 concentrations, indicating crustal sources. But the Mg# values of the Bayanbulak monzonite and the Ouxidaban quartz diorite are 48.76-51.85 and 50.31-53.73, and the εHf(t) values are -2.5 to 8.7 and -1.7 to 4.1, indicating that their magma sources were also mixed by mantle-derived components. LA-ICP-MS zircon U-Pb dating results reveal that the Bayanbulak andesite, the Bayanbulak monzonite and the Ouxidaban quartz diorite were formed at 423, 424 Ma, and 421 Ma, respectively. The age and geochemical data indicate that both the southern CTB and northern Tarim Craton were active continental margins during the Late Silurian, favoring a bi-directional subduction

  20. Subduction of the South Chile active spreading ridge: A 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

    NASA Astrophysics Data System (ADS)

    Boutonnet, E.; Arnaud, N.; Guivel, C.; Lagabrielle, Y.; Scalabrino, B.; Espinoza, F.

    2010-01-01

    The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/ 39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4-3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

  1. Small repeating earthquake activity, interplate quasi-static slip, and interplate coupling in the Hyuga-nada, southwestern Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Yamashita, Yusuke; Shimizu, Hiroshi; Goto, Kazuhiko

    2012-04-01

    Small repeating earthquake (RE) analysis is a useful method for estimating interplate quasi-static slip, which is a good indicator of interplate coupling. We detected 170 continual-type interplate RE groups and then estimated the spatial variation in quasi-static slip in the Hyuga-nada over the past 17 years. The RE activity in this region has different characteristics compared with that in the northeast Japan subduction zone, presumably reflecting differences in the subduction properties. Our results revealed that interplate coupling spatially changes along the trench-axis and dip-direction—a phenomenon that cannot be resolved by land-based Global Positioning System (GPS) analysis. By comparing seismicity, the low-slip-rate areas correspond with the location of hypocenters and asperities for large- and moderate-sized interplate earthquakes, suggesting strong interplate coupling at these sites. These results indicate that the slip rate distribution estimated from RE activity is reliable and useful for assessing the potential of future large earthquakes.

  2. Combined Regional Gravity Model of the Andean Convergent Subduction Zone and Its Application to Crustal Density Modelling in Active Plate Margins

    NASA Astrophysics Data System (ADS)

    Hosse, M.; Pail, R.; Horwath, M.; Holzrichter, N.; Gutknecht, B. D.

    2014-11-01

    The Central Andean subduction system is one of the most active geological structures on Earth. Although there have been a few previous studies, the structure and dynamics of the system are still not well understood. In the present study, we determine a combined regional gravity model of the Andean convergent subduction region for constraining lithospheric models. After a thorough validation and cleaning of the terrestrial gravity and height databases, the method of Least Squares Collocation was applied to consistently combine terrestrial and satellite gravity data, putting much emphasis on the stochastic modelling of the individual data components. As a result, we computed the first high-resolution regional gravity model of the study region that includes GOCE satellite gravity information. The inclusion of GOCE is an essential distinction from the independent global gravity model EGM2008. Validation against EGM2008 reveals that our regional solution is very consistent in regions where terrestrial gravity data are available, but shows systematic differences in areas with terrestrial data gaps. Artefacts in the EGM2008 of up to 150 mGal could be identified. The new combined regional model benefits from the very homogeneous error characteristics and accuracy of GOCE gravity data in the long-to-medium wavelengths down to 80-100 km. Reliable density modelling became possible also in the region of Central Andes, which lacks terrestrial gravity data. Finally, density models were adapted to fit the new regional gravity field solution. The results clearly demonstrate the capabilities of GOCE to better constrain lithospheric models.

  3. An array method for detection, location and characterization of multi-scale seismic energy release associated to the deformation processes of active subduction zones

    NASA Astrophysics Data System (ADS)

    Poiata, N.; Satriano, C.; Bernard, P.; Vilotte, J.; Obara, K.

    2013-12-01

    Detection, location and characterization of the seismic energy release associated to deformation processes in active subduction zones are fundamental for understanding the dynamics of active deformation and the mechanisms of generation and rupturing of large subduction earthquakes. The statistical analysis of this seismic energy release, spanning a wide range of space and time scales, as well as phenomena, (e.g., earthquakes, seismic repeaters, low and very low-frequency earthquakes, tectonic tremors) can provide original insides to the problem. We developed a new methodology exploiting the frequency selective coherence of the wave field at dense seismic arrays and local antennas that leads to stable and reliable detection, blind source separation, and location of distributed non-stationary sources. The methodology consist of: (1) a signal processing scheme yielding a simplified representation of a seismic signal by an adaptive time-frequency characterization of its statistical properties; (2) a fully probabilistic detection and location algorithm based on back projection of stacked local cross-correlations of the simplified signals. This new approach has been developed and tested on the Shikoku region in Japan, which is an exceptional field laboratory, due to its high seismic activity comprising a wide variety of phenomena observed by the dense Hi-net seismic network operated by NIED. We evaluate the capability and potential of the proposed methodology to detect, locate and characterize the energy release associated to possibly overlapping seismic radiation from earthquakes and low-frequency tectonic tremors. As future direction we also discuss an application to the International Maule Aftershock Deployment (IMAD) in Chile.

  4. An array method for detection, location and characterization of multi-scale seismic energy release associated to the deformation processes of active subduction zones

    NASA Astrophysics Data System (ADS)

    Poiata, N.; Satriano, C.; Bernard, P.; Vilotte, J.; Obara, K.

    2011-12-01

    Detection, location and characterization of the seismic energy release associated to deformation processes in active subduction zones are fundamental for understanding the dynamics of active deformation and the mechanisms of generation and rupturing of large subduction earthquakes. The statistical analysis of this seismic energy release, spanning a wide range of space and time scales, as well as phenomena, (e.g., earthquakes, seismic repeaters, low and very low-frequency earthquakes, tectonic tremors) can provide original insides to the problem. We developed a new methodology exploiting the frequency selective coherence of the wave field at dense seismic arrays and local antennas that leads to stable and reliable detection, blind source separation, and location of distributed non-stationary sources. The methodology consist of: (1) a signal processing scheme yielding a simplified representation of a seismic signal by an adaptive time-frequency characterization of its statistical properties; (2) a fully probabilistic detection and location algorithm based on back projection of stacked local cross-correlations of the simplified signals. This new approach has been developed and tested on the Shikoku region in Japan, which is an exceptional field laboratory, due to its high seismic activity comprising a wide variety of phenomena observed by the dense Hi-net seismic network operated by NIED. We evaluate the capability and potential of the proposed methodology to detect, locate and characterize the energy release associated to possibly overlapping seismic radiation from earthquakes and low-frequency tectonic tremors. As future direction we also discuss an application to the International Maule Aftershock Deployment (IMAD) in Chile.

  5. Tomographic search for missing link between the ancient Farallon subduction and the present Cocos subduction

    NASA Astrophysics Data System (ADS)

    Gorbatov, Alexei; Fukao, Yoshio

    2005-03-01

    A striking feature of the tomographic images of the Earth's mid-mantle is the long, high-velocity belt extending in a north-south direction under the North and South American continents, which is believed to be the remnant subduction of the Farallon Plate. In the Oligocene epoch the North Farallon Plate subduction terminated off Baja California and the South Farallon Plate broke into the Cocos and Nazca plates. This important period of the Farallon subduction history is not clearly understood, due in part to the lack of high-resolution tomographic images. Our P-wave tomographic image of the mantle below Mexico indicates that the currently subducting slab of the Cocos Plate is torn apart from the already subducted slab of the ancient Farallon Plate in a region behind the slab window or slab gap off Baja California. We suggest that the southeastward advance of this slab tearing was synchronous with the counter-clockwise rotation of the Cocos Plate against the eastward to northeastward subduction of the ancient Farallon Plate. The Cocos slab torn apart from the deeper Farallon slab dips to the north to northeast with its strike oblique to the trench axis. This slab configuration delineates well with the intermediate-depth earthquake activity and the volcanic activity known as the Trans Mexican Volcanic Belt. Further to the south of the slab tearing of the subducted slab continues from the deeper Farallon part to the shallower Cocos part but with considerable distortion in the shallower part.

  6. Structural and thermal control of seismic activity and megathrust rupture dynamics in subduction zones: Lessons from the Mw 9.0, 2011 Tohoku earthquake

    NASA Astrophysics Data System (ADS)

    Satriano, Claudio; Dionicio, Viviana; Miyake, Hiroe; Uchida, Naoki; Vilotte, Jean-Pierre; Bernard, Pascal

    2014-10-01

    The 2011 Tohoku megathrust earthquake ruptured a vast region of the northeast Japan Trench subduction zone in a way that had not been enough anticipated by earthquake and tsunami risk scenarios. We analyzed the Tohoku rupture combining high-frequency back-projection analysis with low frequency kinematic inversion of the co-seismic slip. Results support the to-day well-accepted broadband characteristics of this earthquake. Most of the seismic moment is released during the first 100 s, with large co-seismic slip (up to 55 m) offshore Miyagi in a compact region on the landward side of the trench. Coherent high-frequency radiation areas and relatively low co-seismic slip are a distinctive signature of the slab-mantle interface. The broadband characteristics of the Tohoku rupture are interpreted, integrating the seismic activity and structure information on the NE Japan forearc region, as a signature of along-dip segmentation and segment interactions, that result from thermal structure, plate geometry, material composition and fracture heterogeneities along the plate boundary interface. Deep mantle corner flow and low dehydration rates along the cold subduction slab interface lead to an extended seismogenic slab-mantle interface, with strong bi-material contrast controlling larger propagation distance in the downdip preferred rupture direction. Off Miyagi, plate bending below the mantle wedge, ∼142.3°E at ∼25 km depth, is associated with the eastern limit of the deep M7-8-class thrust-earthquakes, and of the strongest coherent high-frequency generation areas. The region of the slab-crust interface between the mantle wedge limit, ∼142.7°E at ∼20 km depth, and a trenchward plate bending, ∼143.2°E at ∼15 km, acted as an effective barrier resisting for many centuries to stress-loading gradient induced by deep stable sliding and large earthquakes along the slab-mantle interface. The 2011 Tohoku earthquake, whose hypocenter is located on the east side of the

  7. Subduction in eastern Indonesia: how many slabs?

    NASA Astrophysics Data System (ADS)

    Milsom, John

    2001-08-01

    Seismicity associated with arc-continent collision in eastern Indonesia testifies to past north-directed subduction of Indian Ocean lithosphere beneath the Banda Sea. The complex patterns of deep seismicity have been cited as evidence for simultaneous south-directed subduction at the northern margin of the sea but this interpretation has not been universally accepted. Recently available recomputations of hypocentre locations have provided increased resolution of eastern Indonesian Wadati-Benioff Zones (WBZs). Shallow to intermediate depth seismic activity around the Banda Arc appears to support models involving subduction of two separate and distinct lithospheric slabs, but between 150 and 500 km the WBZ has a continuous 'shoehorn' shape. This shape confirms the presence of subducted lithosphere beneath Seram, in the north, as well as beneath Timor, in the south, is incompatible with independent subduction of two unconnected plates and implies rapid eastwards retreat of the subduction trace across a now vanished northern spur of the Indian Ocean. This 'roll-back' is unlikely to have been driven by local gravitational forces alone and may have been sustained by injection behind the Banda slab of asthenospheric material escaping from the Molucca Sea arc-arc collision.

  8. Modeling Activity of Very-Low-Frequency Earthquakes in Shallow Subduction Zone Considering Splay Faults and High Pore Pressure Zones

    NASA Astrophysics Data System (ADS)

    Shibazaki, B.; Ito, Y.; Ujiie, K.

    2010-12-01

    Recent observations reveal that very-low-frequency (VLF) earthquakes occur in the shallow subduction zones in the Nankai trough, Hyuganada, and off the coast of Tokachi, Japan (Obara and Ito, 2005; Asano et al., 2008; Obana and Kodaira, 2009). The ongoing super drilling project, Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), involves sampling the core of seismogenic faults and conducting analyses, experiments, and in-situ borehole measurements at the Nankai trough where VLF earthquakes occur. The data obtained in this project will be used to develop a model of VLF earthquakes that integrates seismological observations, laboratory experimental results, and geological observations. In the present study, first, we perform 2D quasi-dynamic modeling of VLF earthquakes in an elastic half-space on the basis of a rate- and state-dependent friction law. We set a local unstable zone in a shallow stable zone. To explain very low stress drops and short recurrence intervals of VLF earthquakes, the effective stress is assumed to be around 0.2 MPa. The results indicate that VLF earthquakes are unstable slips that occur under high pore pressure conditions. The probable causes for the high pore pressure along the faults of VLF earthquakes are the sediment compaction and dehydration that occur during smectite-to-illite transition in the shallow subduction zone. Then, we model the generation process of VLF earthquakes by considering splay faults and the occurrences of large subduction earthquakes. We set the local unstable zones with high pore pressure in the stable splay fault zones. We assume the long-term average slip velocity of the splay faults, and that the shear stress is accumulated by the delay of the fault slip from the long-term slip motion. Depending on the frictional properties of the shallow splay faults, two types of VLF earthquakes can occur. When the effective stress is low all over the splay faults, the rupture of large earthquakes propagates to the

  9. Active upper crust deformation pattern along the southern edge of the Tyrrhenian subduction zone (NE Sicily): Insights from a multidisciplinary approach

    NASA Astrophysics Data System (ADS)

    Palano, Mimmo; Schiavone, Domenico; Loddo, Mariano; Neri, Marco; Presti, Debora; Quarto, Ruggiero; Totaro, Cristina; Neri, Giancarlo

    2015-08-01

    Using a multidisciplinary dataset based on gravimetric, seismic, geodetic and geological observations, we provide an improved picture of the shallow structure and dynamics of the southern edge of the Tyrrhenian subduction zone. With a local earthquake tomography we clearly identify two main crustal domains in the upper 15 km characterized by different P-wave velocity values: a high-velocity domain comprising southeasternmost Tyrrhenian Sea, NE Sicily and Messina Straits, and a low-velocity domain comprising Mt. Etna and eastern Sicily. The transition between the two domains shows a good spatial correspondence with a wider set of faults including the Taormina Fault System (TFS) and the Aeolian-Tindari-Letojanni Fault System (ATLFS), two nearly SE-striking fault systems crossing northeastern Sicily and ending on the Ionian shoreline of Sicily according to many investigators. Within this set of faults, most of the deformation/seismicity occurs along the northern and central segments of ATLFS, compared to low activity along TFS. A lack of seismicity (both recent and historical) is observed in the southern sector of ATLFS where, however, geodetic data reveal significant deformation. Our multidisciplinary dataset including offshore observations suggests the southeastward continuation of the ATLFS into the Ionian Sea until joining with the faults cutting the Ionian accretionary wedge described in the recent literature. Our findings imply the existence of a highly segmented crustal shear zone extending from the Aeolian Islands to the Ionian Abyssal plain, that we believe plays the role of accommodating differential motion between the Southern Tyrrhenian unit and the western compressional domain of Sicily. The ATLFS, which is a main part of the inferred shear zone, behaves similarly to what often observed at the edges of retreating subduction slabs, where the overriding plate drifts with a highly non-uniform transform motion along the lateral borders.

  10. Imaging of the subducted Kyushu-Palau Ridge in the Hyuga-nada region, western Nankai Trough subduction zone

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

    We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW-SE extending down the plate boundary to around 30 km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region.

  11. Porous fluid flow enables oceanic subduction initiation on Earth

    NASA Astrophysics Data System (ADS)

    Dymkova, D.; Gerya, T.

    2013-11-01

    Although most of the presently active intra-oceanic subduction zones are relatively young and initiated during the Cenozoic, subduction initiation process remains poorly understood. Previous models of subduction initiation assumed excessive weakening of tectonic plate boundaries that does not reconcile with laboratory rock strength measurements. The weakening was assumed to be caused by fluids present along tectonic fractures; however no self-consistent solid-fluid model of subduction initiation has been developed so far. Here we present new numerical hydro-thermo-mechanical model of spontaneous intra-oceanic subduction initiation where solid rock deformation and fluid percolation are fully coupled. Based on 2-D numerical experiments, we demonstrate that although subduction fails to initiate under fluid-absent conditions, it can naturally start when porous fluid is present inside oceanic crust and along the plate boundaries. Fluid percolation is localized along spontaneously forming faults where high fluid pressure compensates lithostatic pressure, thus dramatically decreasing friction along the incipient subduction zone. Through the parametric study, we conclude that the most important parameter for subduction initiation is the solid matrix permeability. Paradoxical at first, lowering the permeability indeed favors subduction initiation by maintaining high fluid pressure and thus decreasing friction along active faults.

  12. Seismic Characterization of the Transition from Continental to Oceanic Subduction along the western Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Pearce, F. D.; Rondenay, S.; Zhang, H.; Sachpazi, M.; Charalampakis, M.; Royden, L.

    2010-12-01

    The Hellenic subduction zone is located in the east-central Mediterranean region and exhibits large variations in convergence rate along its western edge. Differences in the lithosphere entering the subduction zone are believed to drive the different rates of convergence. While seismic reflection data has shown a transition from continental to oceanic lithosphere along the foreland, no detailed images of the mantle-wedge structure have been available to test this hypothesis. Here, we use high-resolution seismic images across northern and southern Greece to investigate differences in the subducted crust along the western Hellenic subduction zone. We deployed 40 broadband seismometers from the IRIS PASSCAL pool across Greece in a northern line (NL, across Northern Greece) and southern line (SL, across Peloponnesus, Attica, and Evia), each roughly perpendicular to the trench axis. We recorded over 50 high-quality teleseismic events with good azimuthal coverage from each line. We processed them using a 2D teleseismic migration algorithm based on the Generalized Radon Transform and a 3D receiver function algorithm that includes dipping interfaces. In addition, we constructed a 3D velocity model by applying double-difference tomography to ~5000 local earthquakes. The 3D velocity model was used to construct an optimal background model for the teleseismic imaging. Migration and RF images reveal N60E dipping low-velocity layers beneath both NL and SL. From high-resolution migration images, we interpret an ~8 km thick low-velocity layer beneath SL as subducted oceanic crust and a ~20 km thick low-velocity layer beneath NL as subducted continental crust. Relocated earthquakes show that the NL subducted crust is seismically active near the foreland down to 50 km depth presumably as a result of slab flexure. Beyond this region, the subducted crust is aseismic until its signal disappears at ~70 km depth. In contrast, the SL subducted crust is marked by seismicity that extends

  13. Active and passive seismic imaging of the Precordilleran crust, fore-arc of the North-Chilean subduction zone (Central Andes)

    NASA Astrophysics Data System (ADS)

    Wenske, Ina; Hellwig, Olaf; Buske, Stefan; Wigger, Peter; Shapiro, Serge A.

    2014-05-01

    In the fore-arc of the Chilean subduction zone the prominent trench-parallel fault systems can be traced for several thousand kilometers in the north-south direction. These fault systems possibly crosscut parts or the entire crust and are expected to have a close relationship to transient processes of the subduction earthquake cycle. With the motivation to image and characterize the structural inventory and the processes that occur in the vicinity of these large-scale fault zones, we are currently performing a combined analysis of active and passive seismic data sets. The active-seismic data analysis is intended to provide images of the faults at depth and allow linking surface information to subsurface structures. The correlation of the active seismic data with the observed seismicity around these fault systems complements the image and potentially reveals the origin and the nature of the seismicity (including tremors) bound to these fault systems. In 1996, an approximately 350 km long, west-east running reflection seismic profile was acquired to image the entire crust of the Central Andean fore-arc system (North Chile; ANCORP96 seismic line). Several features such as the downgoing plate (Nazca reflector) and the Quebrada Blanca Bright Spot at mid-crustal level were clearly imaged using both standard CMP processing and Kirchhoff prestack depth migration. The latter proved to be more successful in coping with the low data coverage and varying data quality. However, the original images did not provide conclusive information on the upper crust (< 10 km depth) due to the sparse acquisition geom- etry and the insufficient removal of source-generated noise. The major goal of our current re-processing of the ANCORP96 reflection seismic data set is to provide improved images of the upper and middle crust, Thereby, resolving the shallow and perhaps steeply dipping segments of the major fault systems, which were not detected by the original processing. This is done by using

  14. 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

  15. 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.

  16. On the initiation of subduction

    SciTech Connect

    Mueller, S.; Phillips, R.J. )

    1991-01-10

    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.

  17. Subduction Initiation at Oceanic Detachment Faults and the Origin of Supra-subduction Ophiolites

    NASA Astrophysics Data System (ADS)

    Maffione, M.; Thieulot, C.; Van Hinsbergen, D. J. J.; Morris, A.; Spakman, W.; Plümper, O.

    2015-12-01

    Subduction initiation is a critical link in the plate tectonic cycle. Intra-oceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g. the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow- and ultraslow-spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2-D numerical modeling exploring the evolution of a detachment-ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far-field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of supra-subduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three-dimensional modeling of subduction initiation and how upper plate extension is associated with that.

  18. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

    USGS Publications Warehouse

    Von Huene, R.; Scholl, D. W.

    1991-01-01

    At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is

  19. 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.

  20. Active seismic and microseismic reflection imaging of the Precordilleran crust, fore-arc of the North-Chilean subduction zone (Central Andes)

    NASA Astrophysics Data System (ADS)

    Wenske, Ina; Hellwig, Olaf; Schmelzbach, Cedric; Buske, Stefan; Kummerow, Jörn; Wigger, Peter; Shapiro, Serge A.

    2013-04-01

    In the fore-arc of the Chilean subduction zone, prominent trench-parallel fault systems can be traced for more than thousand kilometers in north-south direction. These fault systems possibly crosscut parts or the entire crust and are expected to have a close relationship to transient processes of the subduction earthquake cycle. With the motivation to image and characterize the structural inventory and the processes that occur in the vicinity of these large-scale fault zones, we are currently performing a combined analysis of active and passive seismic data sets. The active-seismic data analysis is intended to provide images of the faults at depth and allow linking surface information to subsurface structures. The correlation of the active seismic data with the observed seismicity around these fault systems complements the imaging and potentially reveals the origin and the nature of the seismicity (incl. tremors) bound to these fault systems. Furthermore, reflection information extracted from passive-seismic waveform data has the potential to complement the active seismic imaging. In 1996, an approximately 350 km long west-east running reflection seismic profile was acquired to image the entire crust of the Central Andean fore-arc system (North Chile; ANCORP96 seismic line). Several features such as the downgoing plate (Nazca reflector) and the Quebrada Blanca Bright Spot at mid-crustal level were clearly imaged using both standard CMP processing and Kirchhoff prestack depth migration. The latter proved to be more successful in coping with the low data coverage and varying data quality. However, the original images were not providing conclusive information on the upper crust (< 10 km depth) due to the sparse acquisition geometry and the partly insufficient removal of source-generated noise. The major goal of our current re-processing of the ANCORP96 reflection seismic data set using adapted noise-suppression schemes and a novel prestack depth migration technique

  1. Deep mantle subduction flux

    NASA Astrophysics Data System (ADS)

    Porter, Katherine A.; White, William M.

    2009-12-01

    We assess the flux of incompatible trace elements into the deep mantle in the Aleutian, Central America, Izu-Bonin, Kurile, Lesser Antilles, Mariana, Sunda, and Tonga subduction zones. We use a simple mass balance approach in which we assume that all of the material lost from the subducting crust and sediment (the "slab") is incorporated into the magmas erupted above the subduction zone, and we use these assumptions to calculate a residual slab composition. The calculated residual slabs are enriched in incompatible elements compared to mid-ocean ridge basalts and highly enriched compared to primitive or depleted mantle. Almost all of the subducted Nb, Ta, and intermediate and heavy rare earths survive into the deep mantle, as do most of the light rare earths. On average, 73% of Th and Pb, 74% of K, 79% of U, 80% of Rb, 80% of Sr, and 82% of Ba survive into the deep mantle. Pb/Ce ratios are systematically lower, and Nb/U ratios are systematically higher, in the deep mantle flux than they are in the flux of material into the trench. Nevertheless, most residual slabs have Pb/Ce and Nb/U ratios outside the typical mantle range. Changes to U/Pb and Th/U ratios tend to be small and are not systematic. Rb/Sr ratios significantly decrease in some subduction zones but increase in others. In contrast, Sm/Nd ratios increase by small but significant amounts in most arcs. Based on these results, we attempt to predict the Sr, Nd, and Pb composition of anciently recycled material now in the mantle. We find that such material would most resemble enriched mantle II-type oceanic island basalts (OIB). None of our calculated residual slabs would evolve to Sr-Nd-Pb isotopic compositions similar to either high 238U/204Pb or enriched mantle I. The range of Sr and Pb isotope ratios in anciently recycled material is similar to that seen in modern OIB, but Nd isotopic compositions do not range to ɛNd values as low as those in some modern OIB. Neither radiogenic nor unradiogenic Pb isotope

  2. How was the Iapetus infected with subduction

    NASA Astrophysics Data System (ADS)

    Waldron, John; Schofield, David; Brendan Murphy, J.; Thomas, Chris

    2015-04-01

    The history of the Iapetus Ocean is the archetype for the "Wilson cycle". The most poorly understood part of the Wilson cycle is the transition between ocean opening and ocean closing. It is often assumed that subduction is initiated by subsidence of old, cold ocean floor at passive margins. However, in the best modern analogue, Atlantic margins formed at ~180 Ma are still passive, suggesting that some other mechanism is required to initiate subduction. In most tectonic reconstructions of the Appalachian-Caledonide orogen, the continental blocks (Laurentia, Baltica, and Amazonia - West Africa), which separated to form the Iapetus during the breakup of Rodinia, are the same three continents that subsequently collided during closure, making the Iapetus a test case for models of subduction initiation. The margin of Laurentia underwent protracted rifting from ~615 Ma to at least 550 Ma, and perhaps later. The earliest "drift" successions on the Newfoundland margin are as young as ~515 Ma. Subduction, recorded by arc volcanics preserved in the orogen, began relatively early in the history of the new ocean at ~515-505 Ma, and the earliest collisional events are recorded almost simultaneously in peri-Laurentian and peri-Gondwanan microcontinents around 490-480 Ma. However, the stable passive margin of Laurentia survived until after 470 Ma before being converted to an active margin. Closure of the ocean between Avalonia and Laurentia was complete by ~425 Ma. These relationships are difficult to reconcile with a classic Wilson cycle in which subduction is initiated by inversion of an extensional margin. It is much more likely that closure was initiated at a subduction zone migrating westward into the Iapetus, analogous to the eastward Mesozoic-Cenozoic entry of the Caribbean and Scotia plates into the Atlantic realm. This process was probably initiated at a transform boundary between the "internal" ocean formed during the breakup of Rodinia, and "external" Panthalassan

  3. Subduction Initiation in Eastern Indonesia

    NASA Astrophysics Data System (ADS)

    Hall, R.

    2014-12-01

    Subduction is often reported to be difficult to initiate, yet in the West Pacific and Eastern Indonesia there are many young subduction zones. Few theoretical or modelling studies consider such settings in which subduction commonly began close to boundaries between ocean crust and thickened crust of arc or continental origin. In Eastern Indonesia there are subduction zones at different stages of development. Some young examples such as the Banda Arc developed by propagation of an existing trench into a new area by tearing, probably along an ocean-continent boundary. This 'solves' the problem since the older subducted slab provides the driving force to drag down unsubducted ocean lithosphere. However, similar explanations cannot account for other subduction zones, such as North Sulawesi, nearby examples in which the subducted slab is not yet at 100 km depth, or troughs where subduction appears to be beginning. These examples show that subduction initiated at a point, such as a corner in an ocean basin, where there were very great differences in elevation between land and adjacent ocean floor. Depression of ocean crust by flow of arc/continent crust is associated with granitic magmatism and detachments within the upper crust. Once the oceanic corner reaches depths of c.100 km, eclogite formation may lead to slab pull that causes the new subduction zone to grow in both directions along strike; arc magmatism may or may not begin. The close relationship between subduction and extension in Eastern Indonesia links dramatic elevation of land, exhumation of deep crust, and spectacular subsidence of basins imaged by oil exploration seismic and multibeam data. Exhumed granites and high-grade metamorphic rocks at elevations up to 3 km, separated by Neogene alluvial sediments from carbonate reefs now at depths of 2 kilometres, imply vertical movements of several kilometres in a few million years. These observations raise the question of whether subduction is driving extension

  4. Estimation of past intermittent methane seep activity using radiocarbon dating of Calyptogena shells in the eastern Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Yagasaki, K.; Ashi, J.; Yokoyama, Y.; Miyairi, Y.; Kuramoto, S.

    2013-12-01

    Radioisotope carbon dating samples from the deep ocean has always been a difficult phenomenon due to the carbon offset present. This research presents a way of utilizing such method to date shell samples in order to study past fault activities. The research presented will be based on the preliminary data collected thus far. The Nankai and the Tokai regions are common areas for cold seeps, where seepage of hydrogen sulfide and methane rich fluid occurs. These various substances encourage the growth of Calyptogena colonies to flourish at these sites. Cold seeps generally occur at tectonically active continental margins and are mostly ephemeral. This suggests that the cold seep events are possibly influenced by the tectonic activity during the plate divergence. In 1997, a submersible dive by Shinkai 2000 discovered an unusually large Calyptogena colony ranging over 200 m2 off Daini Tenryu Knoll. Majority of the shells were fossilized with few live shells remaining. It is assumed that past tectonic events in the region may have caused a high flux of methane fluid or gas to be released, making it possible to support such a vast scale colony to survive until their eventual death. Previous attempt to reconstruct the cold seep activity history through amino acid racemisation dating revealed two different age grouped shells. Further data using a different method is required to prove its reliability, as acid racemization dating technique can easily be affected by seawater temperature changes and microbial activity. This consequently alters the protein structure of the sample and its overall age. As 14C radioisotope dating is not affected by temperature change, it will provide additional information to the accuracy of the acid racemisation dating of the shell. However, the possibility of contamination is likely due to the shells incorporating older carbon from the sediments during their early stages of growth. The old carbon value can be calculated by subtracting the formerly

  5. Analysis of past recurrent methane seep activity using radiocarbon dating of Calyptogena spp. shells in the eastern Nankai subduction zone, Japan

    NASA Astrophysics Data System (ADS)

    Yagasaki, Kazuhiro; Ashi, Juichiro; Yokoyama, Yusuke; Miyairi, Yosuke; Kuramoto, Shin'ichi

    2016-04-01

    Fault activity around subduction zones have been widely studied and monitored through drilling of oceanic plates, studying piston cores, use of monitoring equipment or through visual analysis using submersible vehicles. Yet the understanding of how small scale faults near shallow regions of the seabed behave in relation to cold seep vent activity is still vague, especially determining when they were active in the past. In tectonically active margins such as the Nankai and Tokai regions off Japan, dense methane hydrate reservoirs have been identified. Cold seeps releasing methane rich hydrocarbon fluids are common here, supporting a wide variety of biological species that hold a symbiotic relationship with the chemosynthetic bacteria. In 1998 a large dead Calyptogena spp. bivalve colony (over 400m2 in size) was discovered off Tokai, Japan. It is unusual for a bivalve colony this size to mostly be dead, raising questions as to what caused their death. In this study we document the radiocarbon 14C age of these bivalve shells to attempt analysing the possible methane seep bahaviour in the past. The measured 14C age ranged in three age groups of 1396±36-1448±34, 1912±31-1938±35 and 5975±34. The 14C age of shells that were alive upon collection and the dissolved inorganic carbon (DIC) in seawater show little difference (˜100 14C age) indicating that shells are not heavily affected by the dead carbon effect from cold seeps that is of biogenic or thermogenic origin, which can make the age to become considerably older than the actual age. Thus the novel calibration model used was based on the seawater DIC collected above the Calyptogena spp. colony site (1133±31), which resulted in the dead shells to be clustered around 1900 Cal AD. This proves to be interesting as the predicted epicenter of the Ansei-Tokai earthquake (M 8.4) in 1854 is extremely close to the bibalve colony site. Using geological data obtained using visual analysis and sub-seafloor structural

  6. Buoyant subduction on Venus: Implications for subduction around coronae

    NASA Astrophysics Data System (ADS)

    Burt, J. D.; Head, J. W.

    1993-03-01

    Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system was found so far, however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross-section were described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring under thrusting instead. The effect of slab buoyancy on subduction was explored and the conditions which would lead to under thrusting versus those allowing the formation of trenches and self-perpetuating subduction were defined. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, it was found that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an under thrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion, as in the case of an expanding corona, subduction angles are expected to increase. An initial stage involved estimating the changes in slab buoyancy due to slab healing and pressurization over the course of subduction. Modeling a slab, descending at a fixed angle and heated by conduction, radioactivity, and the heat released in phase changes, slab material density changes due to changing temperature, phase, and pressure were derived.

  7. Mineralogical and chemical characteristics of newer dolerite dyke around Keonjhar, Orissa: Implication for hydrothermal activity in subduction zone setting

    NASA Astrophysics Data System (ADS)

    Sengupta, Piyali; Ray, Arijit; Pramanik, Sayantani

    2014-06-01

    The newer dolerite dykes around Keonjhar within the Singbhum Granite occur in NE-SW, NW-SE and NNE-SSW trends. The mafic dykes of the present study exhibit several mineralogical changes like clouding of plagioclase feldspars, bastitisation of orthopyroxene, and development of fibrous amphibole (tremolite-actinolite) from clinopyroxene, which are all considered products of hydrothermal alterations. This alteration involves addition and subtraction of certain elements. Graphical analyses with Alteration index and elemental abundances show that elements like Rb, Ba, Th, La and K have been added during the alteration process, whereas elements like Sc, Cr, Co, Ni, Si, Al, Fe, Mg and Ca have been removed. It is observed that in spite of such chemical alteration, correlation between major and trace elements, characteristic of petrogenetic process, is still preserved. This might reflect systematic Alteration (addition or subtraction) of elements without disturbing the original element to element correlation. It has also been established by earlier workers that the evolution of newer dolerite had occurred in an arc-back arc setting which may also be true for newer dolerites of the present study. This is evident from plots of pyroxene composition and whole rock composition of newer dolerite samples in different tectonic discrimination diagrams using immobile elements. The newer dolerite dykes of the Keonjhar area may thus be considered to represent an example of hydrothermal activity on mafic rocks in an arc setting.

  8. Incipient subduction interface formation by coupling solid deformation and fluid flow

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Dymkova, Diana

    2014-05-01

    Although most of the presently active intra-oceanic subduction zones are relatively young and initiated during the Cenozoic, subduction initiation process associated with an incipient subduction interface formation remains poorly understood. We investigated incipient spontaneously initiating intra-oceanic subduction by exploring new numerical hydro-thermo-mechanical (HTM) model, in which solid rock deformation and fluid percolation are fully coupled. Based on 2-D numerical experiments, we demonstrate that subduction can naturally start in the presence of porous fluid inside oceanic crust and pre-existing fracture zones. During subduction initiation, fluid percolation is localized along a system of multiple listric propagating thrusts with coalescing nearly horizontal roots forming near the oceanic Moho of the forming subducting slab. These coalescing roots form incipient strongly hydrated subduction interface, which decouples upper and lower plates. High pressure of aqueous fluid flow, which spontaneously focuses along this interface, compensates lithostatic pressure, thus dramatically decreasing friction between the plates. Through the parametric study, we conclude that the most important parameter for the incipient subduction interface formation is the solid matrix permeability. Paradoxical at first, lowering the permeability indeed favors subduction initiation by maintaining high fluid pressure and thus decreasing friction along the active system of coalescent listric thrusts. Citation: Dymkova, D., and T. Gerya (2013), Porous fluid flow enables oceanic subduction initiation on Earth, Geophys. Res. Lett., 40, 5671-5676, doi:10.1002/2013GL057798.

  9. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

    Fluids play a critical role in subduction zones and arc magmatism. At shallow levels in subduction zones (<40 kilometers depth), expulsion of large volumes of pore waters and CH(4)-H(2)O fluids produced by diagenetic and low-grade metamorphic reactions affect the thermal and rheological evolution of the accretionary prism and provide nutrients for deep-sea biological communities. At greater depths, H(2)O and CO(2) released by metamorphic reactions in the subducting oceanic crust may alter the bulk composition in the overlying mantle wedge and trigger partial melting reactions. The location and conse-quences of fluid production in subduction zones can be constrained by consideration of phase diagrams for relevant bulk compositions in conjunction with fluid and rock pressure-temperature-time paths predicted by numerical heat-transfer models. Partial melting of subducting, amphibole-bearing oceanic crust is predicted only within several tens of million years of the initiation of subduction in young oceanic lithosphere. In cooler subduction zones, partial melting appears to occur primarily in the overlying mantle wedge as a result of fluid infiltration. PMID:17784486

  10. Fluid processes in subduction zones.

    PubMed

    Peacock, S A

    1990-04-20

    Fluids play a critical role in subduction zones and arc magmatism. At shallow levels in subduction zones (<40 kilometers depth), expulsion of large volumes of pore waters and CH(4)-H(2)O fluids produced by diagenetic and low-grade metamorphic reactions affect the thermal and rheological evolution of the accretionary prism and provide nutrients for deep-sea biological communities. At greater depths, H(2)O and CO(2) released by metamorphic reactions in the subducting oceanic crust may alter the bulk composition in the overlying mantle wedge and trigger partial melting reactions. The location and conse-quences of fluid production in subduction zones can be constrained by consideration of phase diagrams for relevant bulk compositions in conjunction with fluid and rock pressure-temperature-time paths predicted by numerical heat-transfer models. Partial melting of subducting, amphibole-bearing oceanic crust is predicted only within several tens of million years of the initiation of subduction in young oceanic lithosphere. In cooler subduction zones, partial melting appears to occur primarily in the overlying mantle wedge as a result of fluid infiltration.

  11. 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.

  12. 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

  13. Zircon from Mesoproterozoic sediments sheds light on the subduction-collision history at the eastern active continental margin of the Archaean Kalahari-Grunehogna Craton

    NASA Astrophysics Data System (ADS)

    Marschall, H.; Hawkesworth, C. J.; Leat, P. T.; Dhuime, B.; Storey, C.

    2013-12-01

    The Grunehogna Craton (East Antarctica) was a part of the Archean Kalahari Craton of southern Africa prior to Gondwana breakup. Granite from the basement of the craton has been dated by U-Pb zircon dating to 3,067 Ma with inherited grains showing ages of up to 3,433 Ma [1]. At the eastern margin of the craton, the Ahlmannryggen nunataks comprise an ~2000 m thick pile of clastic and volcanic sediments of the Ritscherflya Supergroup. These were sourced from eroding a proximal active continental arc as demonstrated through the age distribution and internal zoning of detrital zircon [2]. Detrital zircon grains from the Ritscherflya Supergroup show an age distribution with a dominant age peak at ~1,130 Ma, i.e., close to the sedimentation age. Older age peaks include those at 1370 Ma, 1725 Ma, 1880 Ma, 2050 Ma, and 2700 Ma. Palaeo- and Mesoarchaean zircon grains (2800-3445 Ma) were also discovered, corresponding to the age of the Kalahari-Grunehogna Craton basement. Most significantly we found a number of inherited Archaean cores in ~1130 Ma zircons. They demonstrate that the volcanic arc was indeed located on Archaean continental crust, rather than in Mesoproterozoic, intra-oceanic island arcs. The age spectrum of the zircons bears strong evidence for (i) derivation of the entire Ritscherflya sediment sequence from an active continental convergent margin; (ii) a cratonic provenance of part of the sediments from population peaks coinciding with major tectono-thermal events in the Kalahari Craton; (iii) at least some of the active volcanism being located on cratonic basement rather than a juvenile island arc. Detrital zircons in the ~1130 Ma age group show several distinct populations in their Hf isotopic compositions. The dominant group shows negative ɛHf values of -11.5 corresponding to a model age (TDM) of ~2700 Ma (average crustal 176Lu/177Hf = 0.015). A smaller group shows ɛHf values of +2 to +6, which may represent mantle-derived subduction-zone volcanism at

  14. Global overview of subduction seismicity

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Presti, D.; Heuret, A.; Piromallo, C.

    2013-12-01

    In the framework of the EURYI Project ';Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modelling', we propose the first global overview of subduction seismicity. Previous studies have been focused on interplate seismicity, intraslab seismicity, upper plate deformation, or relation between interplate and intraslab seismicity, but the three components of subduction seismicity have been never approached in an systematic and exhaustive study. To allow such a study, nodal planes and seismic moments of worldwide subduction-related earthquakes heve been extracted by EHB hypocenter and CMT Harvard catalogues for the period 1976 - 2007. Data were collected for centroid depths between sea level and 700 km and for magnitude Mw 5.5. For each subduction zone, a set of trench-normal transects were constructed choosing a 120km width of the cross-section on each side of a vertical plane and a spacing of 1 degree along the trench. For each of the 505 resulting transects, the whole subduction seismogenic zone was mapped as focal mechanisms projected on to a vertical plane after their faulting type classification according to the Aki-Richards convention. Transect by transect, fist the seismicity that can be considered not related to the subduction process under investigation was removed, then was selected the upper plate seismicity (i.e. earthquakes generated within the upper plate as a result of the subduction process). After deletion from the so obtained event subset of the interplate seismicity as identified in the framework of this project by Heuret et al. (2011), we can be reasonably confident that the remaining seismicity can be related to the subducting plate. Among these earthquakes we then selected the shallow (0-70 km), intermediate (70-300 km) and deep (300-660 km) depth seismicity. Following Heuret et al. (2011), the 505 transects were merged into 62 larger segments that were ideally homogeneous in terms of their

  15. Dynamic modelling of the subduction zone of central Mexico

    NASA Astrophysics Data System (ADS)

    Gardi, A.; Cocco, M.; Negredo, A. M.; Sabadini, R.; Singh, S. K.

    2000-12-01

    In central Mexico some significant normal faulting events have occurred within the subducted oceanic Cocos plate, just below or near the down-dip edge of the strongly coupled interface. These normal faulting shocks followed large shallow thrust earthquakes. In other subduction zones such events generally precede the up-dip thrust events. A vertical 2-D finite element modelling has been used to simulate the subduction of the Cocos plate beneath the North American plate when the slab is driven by an active convergence velocity or slab pull. We find that the latter mechanism plays only a minor role due to shallow subduction. The modelling results show that the stress pattern is very sensitive to the geometry of the plates. In particular, normal faulting earthquakes that follow large thrust events can be explained on the basis of the flexural response of the overriding and subducting plates to the peculiar geometry of this subduction zone, where the subducting slab becomes horizontal at about 100km from the trench. This horizontal part of the subducting plate, down-dip with respect to the main thrust zone, is under an extensional stress field. This provides an alternative explanation to the slab pull for the occurrence of normal faulting intraplate earthquakes. In order for normal faulting earthquakes to occur in the early part of the seismic cycle, it is necessary that the large up-dip thrust events have a partial stress drop. We find that for small fractional stress drop, a wide region of extension remains below the down-dip edge of the main fault plane following a large thrust earthquake. Thus, the main thrust earthquakes do not invert the polarity of the active stress field, which is compressional and extensional up-dip and down-dip, respectively, with respect to the main thrust fault. Larger fractional stress drops result in larger delays in the occurrence of normal faulting events after the main thrust events.

  16. A plan of continuous monitoring of seismic signature at the top of the subducting Philippine Sea Plate using the ACROSS active monitoring method

    NASA Astrophysics Data System (ADS)

    Kasahara, J.; Tsuruga, K.; Hasada, Y.; Yamaoka, K.; Kunitomo, T.; Watanabe, T.; Fujii, N.; Yoshida, Y.; Kumazawa, M.

    2004-12-01

    Recently, a very strong reflected PP phase from the subducting plate boundary was found beneath the aseismic forearc slope of the Japan Trench (Fujie et al., 2003, Kasahara et al., 2003). This strong PP reflection is interpreted as a reflection from the non-asperity region along the plate boundary caused by the presence of low-Vp material. Similar strong PP reflection phase was observed near the plate boundary of the subducting Philippine Sea Plate and the Eurasian Plate, in the western part of Shizuoka Prefecture along the NS seismic survey line across the central part of Japan using explosive sources (Iidaka et al., 2003, Kodaira et al., 2004). This region is match to the aseismic slip zone since 2000 observed by GPS (Ozawa et al., 2002). This region is supposed to the lower edge of future Tokai or Tonankai earthquake along the Nankai Trough. The "ACROSS" (Accurately Controlled and Routinely Operated Signal System) (Kunitomo and Kumazawa, 2004) can continuously transmit accurately-controlled seismic waves in amplitude, frequency and phase. Using continuous transmission of seismic waves from the ACROSS source located in Toki City, Japan and /or the new ACROSS source under planning in the western part of Shizuoka Prefecture and the high sensitivity seismometers along the NS traverse line, we can delineate the temporal variation of seismic wave properties of the anomalously reflected PP phase along the boundary of subducting Philippine Sea Plate. This observation can give information on variation of the physical state of frictional slip and clues for the earthquake generation. It is also very useful for the monitoring of precursory slip and/or after-slip of the future Tokai and Tonankai earthquakes (or any large earthquakes) with sources along the Philippine Sea Plate.

  17. Deep view of the Subduction-Transform Edge Propagator (STEP) fault in the Calabrian Subduction Zone

    NASA Astrophysics Data System (ADS)

    Maesano, Francesco Emanuele; Tiberti, Mara Monica; Basili, Roberto

    2016-04-01

    The Calabrian Subduction Zone plays a key role in the evolution of the central Mediterranean in the framework of the convergence between Africa and Europe. Here, the remnants of the World's oldest oceanic crust form a narrow NW-dipping slab passively subducting beneath the Calabrian Arc. Recently published high-resolution seismic profiles and bathymetric data of the western Ionian Sea highlight the presence of a NNW-SSE faulting system connected with a series of Plio-Pleistocene syn-tectonic basins. These features are correlated with the recent activity of a major NNW-SSE deformation zone confining the active subduction to the SW and interpreted as a Subduction-Transform Edge Propagator (STEP) fault. The goal of this work is to jointly reconstruct the geometry of the STEP fault and the subduction interface in its surroundings. We use multichannel seismic profiles acquired in the southwestern part of the Calabrian accretionary wedge to focus on the STEP fault geometry at depth and to analyse its relationships with shallow deformation features. The quantitative analysis and enhancement of seismic data provided an accurate image of the internal structure of the accretionary wedge at various depths, showing growth strata in the Plio-Pleistocene succession and major discontinuities in the lower crust. Our results depict a main subvertical, slightly east-dipping, lithospheric fault cutting the oceanic crust down to the Moho, and a rich set of associated secondary synthetic and antithetic faults. This picture also provides new insights on the STEP fault propagation mechanism. In addition, the tridimensional correlation of the STEP fault occurrences in various seismic profiles provides a preliminary scheme of its segmentation and highlights the relationships of this master fault with other main structural elements of the Calabrian Arc and Eastern Sicily, including some of the faults deemed to be responsible for major historical earthquakes in the area.

  18. Subduction initiation: spontaneous and induced

    NASA Astrophysics Data System (ADS)

    Stern, Robert J.

    2004-10-01

    The sinking of lithosphere at subduction zones couples Earth's exterior with its interior, spawns continental crust and powers a tectonic regime that is unique to our planet. In spite of its importance, it is unclear how subduction is initiated. Two general mechanisms are recognized: induced and spontaneous nucleation of subduction zones. Induced nucleation (INSZ) responds to continuing plate convergence following jamming of a subduction zone by buoyant crust. This results in regional compression, uplift and underthrusting that may yield a new subduction zone. Two subclasses of INSZ, transference and polarity reversal, are distinguished. Transference INSZ moves the new subduction zone outboard of the failed one. The Mussau Trench and the continuing development of a plate boundary SW of India in response to Indo-Asian collision are the best Cenozoic examples of transference INSZ processes. Polarity reversal INSZ also follows collision, but continued convergence in this case results in a new subduction zone forming behind the magmatic arc; the response of the Solomon convergent margin following collision with the Ontong Java Plateau is the best example of this mode. Spontaneous nucleation (SNSZ) results from gravitational instability of oceanic lithosphere and is required to begin the modern regime of plate tectonics. Lithospheric collapse initiates SNSZ, either at a passive margin or at a transform/fracture zone, in a fashion similar to lithospheric delamination. The theory of hypothesis predicts that seafloor spreading will occur in the location that becomes the forearc, as asthenosphere wells up to replace sunken lithosphere, and that seafloor spreading predates plate convergence. This is the origin of most boninites and ophiolites. Passive margin collapse is a corollary of the Wilson cycle but no Cenozoic examples are known; furthermore, the expected strength of the lithosphere makes this mode unlikely. Transform collapse SNSZ appears to have engendered new

  19. Subduction initiation: spontaneous and induced

    NASA Astrophysics Data System (ADS)

    2004-10-01

    The sinking of lithosphere at subduction zones couples Earth's exterior with its interior, spawns continental crust and powers a tectonic regime that is unique to our planet. In spite of its importance, it is unclear how subduction is initiated. Two general mechanisms are recognized: induced and spontaneous nucleation of subduction zones. Induced nucleation (INSZ) responds to continuing plate convergence following jamming of a subduction zone by buoyant crust. This results in regional compression, uplift and underthrusting that may yield a new subduction zone. Two subclasses of INSZ, transference and polarity reversal, are distinguished. Transference INSZ moves the new subduction zone outboard of the failed one. The Mussau Trench and the continuing development of a plate boundary SW of India in response to Indo Asian collision are the best Cenozoic examples of transference INSZ processes. Polarity reversal INSZ also follows collision, but continued convergence in this case results in a new subduction zone forming behind the magmatic arc; the response of the Solomon convergent margin following collision with the Ontong Java Plateau is the best example of this mode. Spontaneous nucleation (SNSZ) results from gravitational instability of oceanic lithosphere and is required to begin the modern regime of plate tectonics. Lithospheric collapse initiates SNSZ, either at a passive margin or at a transform/fracture zone, in a fashion similar to lithospheric delamination. The theory of hypothesis predicts that seafloor spreading will occur in the location that becomes the forearc, as asthenosphere wells up to replace sunken lithosphere, and that seafloor spreading predates plate convergence. This is the origin of most boninites and ophiolites. Passive margin collapse is a corollary of the Wilson cycle but no Cenozoic examples are known; furthermore, the expected strength of the lithosphere makes this mode unlikely. Transform collapse SNSZ appears to have engendered new

  20. Subduction of the South-Chile active spreading ridge: a 17 Ma to 3 Ma magmatic record in central Patagonia (western edge of Meseta del Lago Buenos Aires, Argentina)

    NASA Astrophysics Data System (ADS)

    Boutonnet, Emmanuelle; Arnaud, Nicolas; Guivel, Christèle; Lagabrielle, Yves; Scalabrino, Bruno; Espinoza, Felipe

    2010-05-01

    The Chile Triple Junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent. The MLBA plateau (Meseta del Lago Buenos Aires) is one of the Neogene alkali basaltic plateaus located in the back-arc region of the Andean Cordillera at the latitude of the current Chile Triple Junction. The genesis of MLBA can be related with successive opening of slabs windows beneath Patagonia: within the subducting Nazca Plate itself and between the Nazca and Antarctic plates. Detailed 40Ar/39Ar dating and geochemical analysis of bimodal magmatism from the western flank of the MLBA show major changes in the back-arc magmatism which occurred between 14.5 Ma and 12.5 Ma with the transition from calc-alkaline lavas (Cerro Plomo) to alkaline lavas (MLBA) in relation with slab window opening. In a second step, at 4- 3 Ma, alkaline felsic intrusions were emplaced in the western flank of the MLBA coevally with the MLBA basalts with which they are genetically related. These late OIB-like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting Nazca plate during the opening of the South Chile spreading ridge-related slab window. These basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones. The close association of bimodal magmatism with extensional tectonic features in the western MLBA is a strong support to the model of Patagonian collapse event proposed to have taken place between 5 and 3 Ma as a consequence of the presence of the asthenospheric window (SCR-1 segment of South Chile Ridge) below the MLBA area.

  1. Waveform through the subducted plate under the Tokyo region in Japan observed by a ultra-dense seismic network (MeSO-net) and seismic activity around mega-thrust earthquakes area

    NASA Astrophysics Data System (ADS)

    Sakai, S.; Kasahara, K.; Nanjo, K.; Nakagawa, S.; Tsuruoka, H.; Morita, Y.; Kato, A.; Iidaka, T.; Hirata, N.; Tanada, T.; Obara, K.; Sekine, S.; Kurashimo, E.

    2009-12-01

    In central Japan, the Philippine Sea plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region, where it causes mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9) which had 105,000 fatalities. A M7 or greater earthquake in this region at present has high potential to produce devastating loss of life and property with even greater global economic repercussions. The Central Disaster Management Council of Japan estimates the next great earthquake will cause 11,000 fatalities and 112 trillion yen (1 trillion US$) economic loss. This great earthquake is evaluated to occur with a probability of 70 % in 30 years by the Earthquake Research Committee of Japan. We had started the Special Project for Earthquake Disaster Mitigation in Tokyo Metropolitan area (2007-2012). Under this project, the construction of the Metropolitan Seismic Observation network (MeSO-net) that consists of about 400 observation sites was started [Kasahara et al., 2008; Nakagawa et al., 2008]. Now, we had 178 observation sites. The correlation of the wave is high because the observation point is deployed at about 2 km intervals, and the identification of the later phase is recognized easily thought artificial noise is very large. We also discuss the relation between a deformation of PSP and intra-plate M7+ earthquakes: the PSP is subducting beneath the Honshu arc and also colliding with the Pacific plate. The subduction and collision both contribute active seismicity in the Kanto region. We are going to present a high resolution tomographic image to show low velocity zone which suggests a possible internal failure of the plate; a source region of the M7+ intra-plate earthquake. Our study will contribute a new assessment of the seismic hazard at the Metropolitan area in Japan. Acknowledgement: This study was supported by the Earthquake Research Institute cooperative research program.

  2. 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

  3. The Cascadia Subduction Zone: two contrasting models of lithospheric structure

    USGS Publications Warehouse

    Romanyuk, T.V.; Blakely, R.; Mooney, W.D.

    1998-01-01

    The Pacific margin of North America is one of the most complicated regions in the world in terms of its structure and present day geodynamic regime. The aim of this work is to develop a better understanding of lithospheric structure of the Pacific Northwest, in particular the Cascadia subduction zone of Southwest Canada and Northwest USA. The goal is to compare and contrast the lithospheric density structure along two profiles across the subduction zone and to interpet the differences in terms of active processes. The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes and volcanism, goelogic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, the other crosses the margin of central Oregon.

  4. Hydro-mechanical regimes of deforming subduction interface: modeling versus observations

    NASA Astrophysics Data System (ADS)

    Zheng, L.; Gerya, T.; May, D.

    2015-12-01

    A lot of evidence indicates that fluid flows exist in the subduction interface, including seismic observation, magnetotelluric imaging, heat flow modeling, etc. Fluid percolation should strongly modify rock deformation affected by fluid-induced weakening within the subduction interface. Hence, we study the fluid-rock interaction along the subduction interface using a visco-plastic hydro-mechanical model, in which rock deformation and fluid percolation are self-consistently coupled. Based on a series of 2D numerical experiments, we found two typical hydro-mechanical regimes of deforming subduction interface: (1) coupled and (2) decoupled. In the case of the coupled regime, the tectonic movement of the subduction interface is divided into blocks; newly generated faults are distributed uniformly , say fault band; fluid activity concentrates inside the faults. In the case of the decoupled regime, the upper layer of the subduction interface stops moving while the lower layer continues moving along with the subduction slab; a primary fault is generated at the centre of the subduction interface, or namely decoupled interface. Available observations suggests that both coupled and decoupled regimes can be observed in the nature at different scales. Systematic parameter study suggests that it is mainly the magnitude of the yield strength of subducted rocks depending on their cohesion and friction coefficient, which control the transition between the coupled and decoupled subduction interface regimes.

  5. Complex rupture processes of the Solomon Islands subduction zone earthquake and subduction controlled upper mantle structure beneath Anatolia

    NASA Astrophysics Data System (ADS)

    Biryol, Cemal Berk

    This dissertation explores subduction zone-related deformation both on short time scales in the form of subduction zone earthquakes and over larger time and geographical scales in the form of subduction rollback or detachment of the subducting lithosphere. The study presented here is composed of two parts. First, we analyzed the source-rupture processes of the April 1, 2007 Solomon Islands Earthquake (Mw=8.1) using a body-wave inversion technique. Our analysis indicated that the earthquake ruptured approximately 240 km of the southeast Pacific subduction zone in two sub-events. In the second part of this study, we used shear-wave splitting analysis to investigate the effects of the subducting African lithosphere on the upper-mantle flow field beneath the Anatolian Plate in the Eastern Mediterranean region. Our shear-wave splitting results are consistent with relatively uniform southwest-directed flow towards the actively southwestward-retreating Aegean slab. Based on spatial variations in observed delay times we identified varying flow speeds beneath Anatolia and we attribute this variation to the differential retreat rates of the Aegean and the Cyprean trenches. Finally, we used teleseismic P-wave travel-time tomography to image the geometry of the subducting African lithosphere beneath the Anatolia region. Our tomograms show that the subducting African lithosphere is partitioned into at least two segments along the Cyprean and the Aegean trenches. We observed a gap between the two segments through which hot asthenosphere ascends beneath the volcanic fields of western Anatolia. Our results show that the Cyprean slab is steeper than the Aegean slab. We inferred that this steep geometry, in part, controls the flow regime of asthenosphere beneath Anatolia causing variations in flow speeds inferred from shear-wave splitting analysis.

  6. Multi-stage barites in partially melted UHP eclogite: implications for fluid/melt activities during deep continental subduction in the Sulu orogenic belt

    NASA Astrophysics Data System (ADS)

    Wang, Songjie; Wang, Lu

    2015-04-01

    Barite (BaSO4) is well-known from deep-sea sedimentary environments but has received less attention to its presence in high-grade metamorphic rocks. Recently, barite in ultrahigh pressure (UHP) eclogite has drawn increasing attention from geologists, especially in the Dabie-Sulu orogen, since it is an important indicator for high-salinity fluid events, thus aiding in further understanding HP-UHP fluid / melt evolution. However, its formation time and mechanism in UHP eclogite are still controversial, with three representative viewpoints: (1) Liu et al. (2000) found barite-anhydrite-coesite inclusions in zircon and interpreted them to have formed by UHP metamorphic fluids; (2) Zeng et al. (2007) recognized isolated barite within K-feldspar (Kfs) and Quartz (Qz) surrounded by radial cracks in omphacite, and interpreted Kfs+Qz to be reaction products of potassium-rich fluid/melt and coesite, with the barite formed by prograde metamorphic fluids; (3) Gao et al. (2012) and Chen et al. (2014) found barite-bearing Multiphase Solid (MS) inclusions within garnet and omphacite and assumed that the barite formed by phengite breakdown possibly caused by eclogite partial melting during exhumation, though no direct evidence were proposed. The controversy above is mainly due to the lack of direct formation evidence and absence of a clear link with the metamorphic evolution of UHP eclogite along the subduction-exhumation path. We report detailed petrological and micro-structural analyses revealing four types of barites clearly linked with (1) the prograde, (2) earlier stage of partial melting and (3) later stage of crystallization differentiation, as well as (4) high-grade amphibolite-facies retrogression of a deeply subducted and partially melted intergranular coesite-bearing eclogite from Yangkou Bay, Sulu Orogen. Round barite inclusions (type-I) within UHP-stage garnet and omphacite are formed by internally buffered fluids from mineral dehydration during prograde metamorphism

  7. Geodynamic models of deep subduction

    NASA Astrophysics Data System (ADS)

    Christensen, Ulrich

    2001-12-01

    Numerical and laboratory models that highlight the mechanisms leading to a complex morphology of subducted lithospheric slabs in the mantle transition zone are reviewed. An increase of intrinsic density with depth, an increase of viscosity, or phase transitions with negative Clapeyron slope have an inhibiting influence on deep subduction. The impingement of slabs on a viscosity and density interface has been studied in laboratory tanks using corn syrup. Slab interaction with equilibrium and non-equilibrium phase transitions has been modelled numerically in two dimensions. Both the laboratory and the numerical experiments can reproduce the variety of slab behaviour that is found in tomographic images of subduction zones, including cases of straight penetration into the lower mantle, flattening at the 660-km discontinuity, folding and thickening of slabs, and sinking of slabs into the lower mantle at the endpoint of a flat-lying segment. Aside from the material and phase transition properties, the tectonic conditions play an important role. In particular, the retrograde motion of the point of subduction (trench-rollback) has an influence on slab penetration into the lower mantle. A question that still needs to be clarified is the mutual interaction between plate kinematics and the subduction process through the transition zone.

  8. The subduction of young lithosphere

    NASA Astrophysics Data System (ADS)

    Sacks, I. S.

    1983-04-01

    Studies, using a variety of techniques, of the subduction beneath western South America and the southwest Honshu-Kyushu region of Japan indicate that volcano-free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere. The subduction is initiated at normal dip angles, but the plate deforms at some depth (100 km under central Peru) to travel horizontally immediately beneath continental lithosphere. The most plausible reason for this geometry is that the subducted plate is buoyant. A model is developed constrained by age of the plate, bathymetry, and heat flow. Estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts. If the high-density tranformation of crustal basalt to eclogite is retarded by low temperatures, the plate can remain buoyant to considerable depth for long periods of time. Heat flow data from western South America are consistent with this model.

  9. A discussion of numerical subduction initiation

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne; Ellis, Susan

    2016-04-01

    In nature, subduction can initiate in various ways: Shortening can localise at oceanic transform faults, extinct spreading centres, or inherited passive margin faults; or, alternatively, subduction can be triggered from existing subduction systems by along-strike trench propagation, polarity reversals, or trench jumps. Numerical studies that specifically address subduction initiation have highlighted the roles of sediment loading, rheological strength contrasts, strain softening, and continental topographic gradients, among others. Usually, however, numerical models that aim to investigate subduction dynamics prefer to bypass the subduction initiation phase and its complexities, and focus instead on the stages during which the slab is descending into the mantle. However, even in these models, subduction still needs to begin. It is disturbingly easy to define initial model geometries that do not result in subduction. The specific combination of initial model geometries and values for rheological parameters that successfully initiates subduction has even been referred to as 'the sweet spot' in model space. One cause of subduction initiation failure is when the subducting and overriding plates lock, resulting in either indentation or severe dragging downwards of the overriding plate. This may point to a difficulty in maintaining a weak subduction interface during model evolution. A second factor that may cause difficulties is that initial model geometry and stresses need to balance, as otherwise the first model stages may show spurious deformation associated with reaching equilibrium. A third requirement that may cause problems is that the surface needs to have sufficient displacement freedom to allow the overriding plate to overthrust the subducting plate. That also implies an exclusion of sharp corners in the subduction interface near the surface. It is the interplay of subduction interface geometry, interface strength and subducting plate rheology that determines

  10. Seismicity of the eastern Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bruestle, A.; Kueperkoch, L.; Rische, M.; Meier, T.; Friederich, W.; Egelados Working Group

    2012-04-01

    The Hellenic Subduction Zone (HSZ) is the seismically most active region of Europe. The African plate is subducting beneath the Aegean lithosphere with a relative velocity of 4 cm per year. A detailed picture of the microseismicity of the eastern HSZ was obtained by the recordings of the temporary networks CYCNET (September 2002 - September 2005) and EGELADOS (October 2005 - March 2007). In total, nearly 7000 earthquakes were located with a location uncertainty of less than 20 km. The SE Aegean is dominated by (1) shallow intraplate seismicity within the Aegean plate, by (2) interplate seismicity at the plate contact and by (3) intermediate deep seismicity along the subducting African slab. Strong shallow seismicity in the upper plate is observed along the Ptolemy graben south of Crete extending towards the Karpathos Basin, indicating intense recent deformation of the forearc. In contrary, low shallow seismicity around Rhodes indicates only minor seismic crustal deformation of the upper plate. An almost NS-striking zone of microseismicity has been located, running from the Karpathos basin via the Nisyros volcanic complex towards the EW striking Gökova graben. In the SE Aegean the geometry of the Wadati-Benioff-Zone (WBZ) within the subducting African plate is revealed in detail by the observed microseismicity. Between about 50 to 100 km depth a continuous band of intermediate deep seismicity describes the strongly curved geometry of the slab. From the central to the eastern margin of the HSZ, the dip direction of the WBZ changes from N to NW with a strong increase of the dip angle beneath the eastern Cretan Sea. The margin of the dipping African slab is marked by an abrupt end of the observed WBZ beneath SW Anatolia. Below 100 km depth, the WBZ of the eastern HSZ is dominated by an isolated cluster of intense intermediate deep seismicity (at 100-180 km depth) beneath the Nisyros volcanic complex. It has an extension of about 100x80 km and is build up of 3 parallel

  11. Osmium Recycling in Subduction Zones

    PubMed

    Brandon; Creaser; Shirey; Carlson

    1996-05-10

    Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge. PMID:8662577

  12. Osmium Recycling in Subduction Zones

    PubMed

    Brandon; Creaser; Shirey; Carlson

    1996-05-10

    Peridotite xenoliths from the Cascade arc in the United States and in the Japan arc have neodymium and osmium isotopic compositions that are consistent with addition of 5 to 15 percent of subducted material to the present-day depleted mantle. These observations suggest that osmium can be partitioned into oxidized and chlorine-rich slab-derived fluids or melts. These results place new constraints on the behavior of osmium (and possibly other platinum group elements) during subduction of oceanic crust by showing that osmium can be transported into the mantle wedge.

  13. Lithification facilitates frictional instability in argillaceous subduction zone sediments

    NASA Astrophysics Data System (ADS)

    Trütner, Sebastian; Hüpers, Andre; Ikari, Matt J.; Yamaguchi, Asuka; Kopf, Achim J.

    2015-12-01

    Previous work suggests that in subduction zones, the onset of large earthquake nucleation at depths > ~ 5-10 km is likely driven by a combination of factors associated with the process of lithification. At these depths, lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall-rock. To test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low-grade metamorphism, we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the Shimanto Belt in SW Japan. These meta-sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225 °C, which are representative of seismogenic depths along the active Nankai subduction megathrust in the foreland of the Shimanto Belt. We find that intact Shimanto rock samples, which preserve the influence of diagenetic and metamorphic processes, exhibit the potential for unstable slip under in-situ pressure conditions. Powdered versions of the same samples tested under the same conditions exhibit only velocity-strengthening friction, thus demonstrating that destroying the lithification state also removes the potential for unstable slip. Using advanced porosity loss to quantify the lithification process, we demonstrate that increased velocity weakening correlates with increasingly advanced lithification. In combination with documented frictionally stable behavior of subduction zone sediments from shallower depths, our results provide evidence that the sediment lithification hypothesis can explain the depth-dependent onset of large earthquake nucleation along subduction zone megathrusts.

  14. 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.

  15. 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.

  16. The Banda Arc subduction enigma

    NASA Astrophysics Data System (ADS)

    Spakman, Wim; Hall, Robert

    2010-05-01

    The spectacularly curved Banda arc comprises young oceanic crust enclosed by a volcanic inner arc, outer arc islands, and a trough parallel to the Australian continental margin. Seismicity defines a spoon-shaped lithospheric fold in the upper mantle for which there are two contrasting explanations: deformation of a single subducted slab, or two different slabs subducted from north and south. We show that the Banda arc resulted from subduction of a single slab. Based on geology and seismic tomography, we argue that the arc formed since 15 Ma by subduction of a Jurassic oceanic embayment within the Australian plate. Viewed in an Atlantic-Indian hotspot reference frame, the stationary E-W striking Java trench propagated ESE into the Banda embayment by hinge rollback. Extension of the upper plate formed oceanic crust in the present Banda Sea between stretched continental crust of Australian origin. Slab morphology depends primarily on the geometry of the continental margin enclosing the embayment. Our model explains the first order tectonic development of the Banda region and links slab deformation to absolute plate motion.

  17. 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; Gonzalez, Frank; 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.

  18. 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

  19. Dynamics of intraoceanic subduction initiation: 1. Oceanic detachment fault inversion and the formation of supra-subduction zone ophiolites

    NASA Astrophysics Data System (ADS)

    Maffione, Marco; Thieulot, Cedric; van Hinsbergen, Douwe J. J.; Morris, Antony; Plümper, Oliver; Spakman, Wim

    2015-06-01

    Subduction initiation is a critical link in the plate tectonic cycle. Intraoceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid-ocean ridges, as in e.g., the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow-spreading and ultraslow-spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2-D numerical modeling exploring the evolution of a detachment-ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far-field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three-dimensional modeling of subduction initiation and how upper plate extension is associated with that.

  20. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, Alan; Bezada, Maximiliano; Niu, Fenglin; Palomeras, Imma; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep; Schmitz, Michael; Miller, Meghan

    2016-04-01

    Subduction recycling of oceanic lithosphere, a central theme of plate tectonics, is relatively well understood. Recycling continental lithosphere is more difficult to recognize, can take a number of different forms, and appears to require an external trigger for initiation. Delamination and localized convective downwelling are two processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. We describe a related process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone. Body wave tomograms from dense broadband seismograph arrays in northeastern South America (SA) and the western Mediterranean show larger than expected volumes of positive velocity anomalies which we identify as the subducted Atlantic slab under northeastern SA, and the Alboran slab beneath the Gibraltar arc (GA). The positive anomalies lie under and are aligned with the continental margins at sublithospheric depths. The continental margins along which the subduction zones have traversed, i.e. the northeastern SA plate boundary and east of GA, have significantly thinner lithosphere than expected. The thinner than expected lithosphere extends inland as far as the edges of nearby cratons as determined from receiver function images and surface wave tomography. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle and include the lower crust, as around GA, inferred by results from active and passive seismic experiments. Viscous removal of continental margin lithosphere creates LAB topography leading

  1. Subduction of thick crust: the Alaska example (Invited)

    NASA Astrophysics Data System (ADS)

    Abers, G. A.; Kim, Y.; Christensen, D. H.

    2013-12-01

    It is a paradigm of plate tectonics that oceanic lithosphere subducts readily, while lithosphere transporting much thicker continental crust does not. Analyses of plate buoyancy have included a variety of effects, such as eclogitization, crustal compositional stratification, and plate strength, but all lead to the conclusion that crust needs to be thinner than about 15-25 km in order to subduct. A test of this conclusion is underway in southern Alaska, where the Yakutat terrane is being driven by the Pacific plate into the Alaskan margin. Its crust is 15-30 km thick, varying along strike, with a seismic velocity structure resembling an oceanic plateau; thus it spans the predicted limit in thickness of subductable crust. In the eastern thicker part, the terrane appears to be colliding and driving orogenesis in the St. Elias-Chugach ranges, although voluminous volcanism of the Wrangell Volcanic Field may be a consequence of some crust subducting. Farther west, the Yakutat terrane is 15-20 km thick and clearly subducting beneath the Prince William Sound and Kenai Peninsula. It forms the slab subducting beneath the central Alaska Range 400 km inland. The thick crust has been imaged at all depths less than 130 km, through receiver functions, travel-time tomography, and offshore by active-source imaging, with similar structure in most images. Greater than 130 km depth the imaged crust vanishes in seismic images, consistent with predicted depths of eclogitization of weakly hydrated metagabbroic crust, and the lack of a velocity contrast between eclogite and peridotite. Lithosphere including the thick Yakutat crust gives a net buoyancy close to neutral, so its subduction will depend on other factors. The high buoyancy may be responsible for the remarkably shallow dip of the plate at depths less than 50 km, producing one of the widest seismogenic thrust zones on the planet, allowing it to host the great (Mw 9.3) 1964 Gulf of Alaska earthquake. The shallow dip may also aid

  2. A Regime Diagram for Subduction

    NASA Astrophysics Data System (ADS)

    Stegman, D. R.; Farrington, R.; Capitanio, F. A.; Schellart, W. P.

    2009-12-01

    Regime diagrams and associated scaling relations have profoundly influenced our understanding of planetary dynamics. Previous regime diagrams characterized the regimes of stagnant-lid, small viscosity contrast, transitional, and no-convection for temperature-dependent (Moresi and Solomatov, 1995), and non-linear power law rheologies (Solomatov and Moresi, 1997) as well as stagnant-lid, sluggish-lid, and mobile-lid regimes once the finite strength of rock was considered (Moresi and Solomatov, 1998). Scalings derived from such models have been the cornerstone for parameterized models of thermal evolution of rocky planets and icy moons for the past decade. While such a theory can predict the tectonic state of a planetary body, it is still rather incomplete in regards to predicting tectonics. For example, the mobile-lid regime is unspecific as to how continuous lithospheric recycling should occur on a terrestrial planet. Towards this goal, Gerya et al., (2008) advanced a new regime diagram aiming to characterize when subduction would manifest itself as a one-sided or two-sided downwelling and either symmetric or asymmetric. Here, we present a regime diagram for the case of a single-sided, asymmetric type of subduction (most Earth-like type). Using a 3-D numerical model of a free subduction, we describe a total of 5 different styles of subduction that can possibly occur. Each style is distinguished by its upper mantle slab morphology resulting from the sinking kinematics. We provide movies to illustrate the different styles and their progressive time-evolution. In each regime, subduction is accommodated by a combination of plate advance and slab rollback, with associated motions of forward plate velocity and trench retreat, respectively. We demonstrate that the preferred subduction mode depends upon two essential controlling factors: 1) buoyancy of the downgoing plate and 2) strength of plate in resisting bending at the hinge. We propose that a variety of subduction

  3. 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

  4. The role of the Hikurangi Plateau in the dynamics of the Hikurangi Subduction Margin

    NASA Astrophysics Data System (ADS)

    Willis, David; Moresi, Louis; Betts, Peter; Ailleres, Laurent

    2015-04-01

    The collision of the Hikurangi Plateau with the Hikurangi Subduction Zone coincides with a change from extension to shortening in the overriding Australian Plate. The 15km thick oceanic Hikurangi Plateau is juxtaposed with Chatham Rise at the southern edge of the subduction margin, possibly extending an unknown distance beneath the Chatham Rise. We explore the influence of the Hikurangi Plateau and its size on subduction dynamics using a series of 3D visco-elastic Underworld models with free slip boundary conditions. The models rely on negative slab buoyancy to drive subduction, with the buoyant oceanic plateau altering the dynamics. The models are benchmarked against New Zealand GPS velocities, paleomagnetic rotations, and fault zones. Both the overriding and subducting plates have undergone significant rotation since 23Ma with ~90° clockwise rotation of the overriding plate relative to the subducting plate. Distinctive fault zones have also been created with the Marlborough Fault Zone facilitating escape tectonics around the Chatham Rise; the North Island Dextral Fault Belt allowing lateral movement between the fore and back-arcs; extension in Taupo Volcanic Zone assisting back-arc extension; and active N-S extension in the Chatham Rise adjacent to the Hikurangi Plateau. In a model without an oceanic plateau, the subduction zone undergoes roll-back along its entire margin. Extension occurs along the overriding plate adjacent to subduction, with shortening limited to the collision of the Chatham Rise. Rotation rates adjacent to the Chatham Rise are extremely high, and decrease rapidly northward and there is no rotation of the subducting plate. When an oceanic plateau is included in the model, shortening occurs at the southern margin of the subduction zone as subduction shallows with the entry of the bouyant plateau. Rotation starts in the subducting plate as the plateau enters the subduction zone. This rotation is attributed to a decrease in slab pull southward

  5. Driving forces: Slab subduction and mantle convection

    NASA Technical Reports Server (NTRS)

    Hager, Bradford H.

    1988-01-01

    Mantle convection is the mechanism ultimately responsible for most geological activity at Earth's surface. To zeroth order, the lithosphere is the cold outer thermal boundary layer of the convecting mantle. Subduction of cold dense lithosphere provides tha major source of negative buoyancy driving mantle convection and, hence, surface tectonics. There are, however, importnat differences between plate tectonics and the more familiar convecting systems observed in the laboratory. Most important, the temperature dependence of the effective viscosity of mantle rocks makes the thermal boundary layer mechanically strong, leading to nearly rigid plates. This strength stabilizes the cold boundary layer against small amplitude perturbations and allows it to store substantial gravitational potential energy. Paradoxically, through going faults at subduction zones make the lithosphere there locally weak, allowing rapid convergence, unlike what is observed in laboratory experiments using fluids with temperature dependent viscosities. This bimodal strength distribution of the lithosphere distinguishes plate tectonics from simple convection experiments. In addition, Earth has a buoyant, relatively weak layer (the crust) occupying the upper part of the thermal boundary layer. Phase changes lead to extra sources of heat and bouyancy. These phenomena lead to observed richness of behavior of the plate tectonic style of mantle convection.

  6. Friction and stress coupling on the subduction interfaces

    NASA Astrophysics Data System (ADS)

    Tan, E.; Lavier, L.; van Avendonk, H.

    2011-12-01

    At a subduction zone, the down-going oceanic plate slides underneath the overriding plate. The frictional resistance to the relative motion between the plates generates great earthquakes along the subduction interface, which can cause tremendous damage in the civil life and property. There is a strong incentive to understand the frictional strength of the subduction interface. One fundamental question of mechanics of subuction is the degree of coupling between the plates, which is linked to the size of earthquakes. It has been noted that the trench-parallel (along-strike) gravity variation correlates positively with the trench-parallel topography anomaly and negatively with the activity of great earthquake (Song and Simons, 2003). Regions with a negative trench-parallel gravity anomaly are more likely to have great earthquakes. The interpretation of such correlation is that strong coupling along subduction interface will drag down the for-arc region of the overriding plate, which generates the gravity and topography anomalies, and could store more strain energy to be released during a great earthquake. We developed a 2D numerical thermo-mechanical code for modeling subduction. The numerical method is based on an explicit finite element method similar to the Fast Lagrangian Analysis of Continua (FLAC) technique. The constitutive law is visco-elasti-plastic with strain weakening. The cohesion and friction angle are reduced with increasing plastic strain after yielding. To track different petrologic phases, Lagrangian particles are distributed in the domain. Basalt-eclogite, sediment-schist and peridotite-serpentinite phase changes are included in the model. Our numerical models show that the degree of coupling negatively correlates with the coefficient of friction. In the low friction case, the subduction interface has very shallow dipping angle, which helps to elastically couple the downing plate with the overriding plate. The topography and gravity anomalies of the

  7. Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone

    NASA Astrophysics Data System (ADS)

    Lefevre, L. Victoria; McNally, Karen C.

    1985-05-01

    The regional distribution of stresses associated with the subduction of the Cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms, body and surface wave analyses of large earthquakes, and seismicity distributions. Broad patterns of consistent behavior are found across the region, from the Rivera Plate boundary in the northwest to the Guatemala/El Salvador border in the southeast, and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges, the Tehuantepec Ridge and the Orozco Fracture Zone. Information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large (Ms ≥ 7.5) earthquakes have occurred historically. We identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity. The first, closest to the trench and reflecting the mechanical interaction of the converging plates, is a zone of shallow thrust earthquakes extending 100-150 km inland from the trench. The second is a zone of normal faulting, beginning at about 200 km inland from the trench, h ≥ 60 km, which extends continuously along the entire length of the descending plate throughout the region. The third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150-200 km inland from the trench. This zone extends from the Rivera plate boundary in the northwest to the Guatamala region in the southeast. At this point, the quiet region pinches out, and the thrust and normal faulting zones abut and overlap. Superimposed on this overall pattern, we find locally only minor changes in areas of aseismic ridge subduction, aside from the prominent seismic slip gaps. Furthermore, on October 25, 1981, the Playa Azul earthquake (Ms = 7.3) occurred in the midregion of the Orozco Fracture Zone. Body and surface wave analyses of this event show a simple

  8. Overriding Plate Deformation During Subduction Evolution

    NASA Astrophysics Data System (ADS)

    Davies, J. H.; Garel, F.; Davies, R.; Goes, S. D. B.

    2015-12-01

    Subduction dynamics has been widely studied in free subduction models, which document the important control of the downgoing plate. However, various models have shown how the overriding plate can influence subduction dynamics through its thermal structure, thickness and coupling. Using the code Fluidity we investigate overriding plate deformation in a 2-D thermo-mechanical model of the two-plate subduction system. We use Fluidity's adaptive mesh and free-surface formulation. The model includes a composite temperature- and stress-dependent rheology, and plates are decoupled by a weak layer, which allows for free trench motion. We focus on the evolution of the topography and state of stress in the overriding plate during the different phases of the subduction process: early stages of subduction, free-fall sinking in the upper mantle and interaction of the slab with the high-viscosity lower mantle.

  9. Back-arc Extension: Critical Analisys of Subduction-related and Non Subduction-related Driving Mechanisms

    NASA Astrophysics Data System (ADS)

    Mantovani, E.; Viti, M.; Babbucci, D.; Tamburelli, C.; Albarello, D.

    It is argued that the opening of back arc basins can hardly be explained as an effect of subduction related forces, since this kind of interpretation has not yet provided plausible explanations for several major features of such processes in the world. In particular, it is not clear why back arc extension occurs in some subduction zones and not in others, why extension ceased in zones where subduction has remained active, why the arcs associated with back arc basins are often characterized by a strongly curved shape, why arc-trench-back arc systems do not develop along the entire length of consuming borders and why no significant correlation can be recognized between any parameter of subduction processes and the occurrence of back arc extension. In addition, modelling experiments indicate that the magnitude of the tensional stress induced in the overriding plate by subduction-related forces is significantly lower than the lithospheric strength. These problems are discussed, in particular, for three subduction-related interpretations, the "slab-pull", the "corner flow" and the "sea an- chor" models, which seem to be the most quoted in literature. It is then argued that possible solutions of the above problems may be provided by the extrusion model, which postulates that back arc basins are generated by the forced separation of the arc from the overriding plate, along a sector of the consuming border. This separa- tion is generally caused by the oblique indentation of strong and buoyant structures against the accretionary belt. In this view, subduction and back arc extension are not causally linked one to the other, but rather represent simultaneous effects of the lateral migration of the arc, driven by plate convergence. It is pointed out that the conditions required for the occurrence of this kind of mechanism may be recognized in the tec- tonic contexts where back arc basins developed in the wake of arc-trench migrating systems. On the other hand, in the zones

  10. Assessing the Seismic Potential Hazard of the Makran Subduction Zone

    NASA Astrophysics Data System (ADS)

    Frohling, E.; Szeliga, W. M.; Melbourne, T. I.; Abolghasem, A.; Lodi, S. H.

    2013-12-01

    Long quiescent subduction zones like the Makran, Sunda, and Cascadia, which have long recurrence intervals for large (> Mw 8) earthquakes, often have poorly known seismic histories and are particularly vulnerable and often ill-prepared. The Makran subduction zone has not been studied extensively, but the 1945 Mw 8.1 earthquake and subsequent tsunami, as well as more recent mid magnitude, intermediate depth (50-100 km) seismicity, demonstrates the active seismic nature of the region. Recent increases in regional GPS and seismic monitoring now permit the modeling of strain accumulations and seismic potential of the Makran subduction zone. Subduction zone seismicity indicates that the eastern half of the Makran is presently more active than the western half. It has been hypothesized that the relative quiescence of the western half is due to aseismic behavior. However, based on GPS evidence, the entire subduction zone generally appears to be coupled and has been accumulating stress that could be released in another > 8.0 Mw earthquake. To assess the degree of coupling, we utilize existing GPS data to create a fault coupling model for the Makran using a preliminary 2-D fault geometry derived from ISC hypocenters. Our 2-D modeling is done using the backslip approach and defines the parameters in our coupling model; we forego the generation of a 3-D model due to the low spatial density of available GPS data. We compare the use of both NUVEL-1A plate motions and modern Arabian plate motions derived from GPS station velocities in Oman to drive subduction for our fault coupling model. To avoid non-physical inversion results, we impose second order smoothing to eliminate steep strain gradients. The fit of the modeled inter-seismic deformation vectors are assessed against the observed strain from the GPS data. Initial observations indicate that the entire subduction zone is currently locked and accumulating strain, with no identifiable gaps in the interseismic locking

  11. Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

    NASA Astrophysics Data System (ADS)

    Ratheesh Kumar, R. T.; Windley, B. F.; Rajesh, V. J.; Santosh, M.

    2013-12-01

    We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0 < Te < 3 km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te < Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

  12. A Computer-Based Subduction-Zone-Earthquake Exercise for Introductory-Geology Classes.

    ERIC Educational Resources Information Center

    Shea, James Herbert

    1991-01-01

    Describes the author's computer-based program for a subduction-zone-earthquake exercise. Instructions for conducting the activity and obtaining the program from the author are provided. Written in IBM QuickBasic. (PR)

  13. 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

  14. Mapping Yakutat Subduction with Tectonic Tremor

    NASA Astrophysics Data System (ADS)

    Wech, A.

    2015-12-01

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

  15. Tethyan subducted slabs under India

    NASA Astrophysics Data System (ADS)

    Van der Voo, Rob; Spakman, Wim; Bijwaard, Harmen

    1999-08-01

    Tomographic imaging of the mantle under Tibet, India and the adjacent Indian Ocean reveals several zones of relatively high P-wave velocities at various depths. Under the Hindu Kush region in northeastern Afghanistan and southern Tajikistan, a regional northward-dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the Indian plate. Under northern Pakistan this same slab shows a roll-over structure with the deeper portion overturned and dipping southward, as can also be seen in the distribution of earthquake hypocenters. Farther east-southeast (e.g., in the vicinity of Nepal), a well-resolved anomaly below 450 km depth is connected to the slab under the Hindu Kush, but seems to be separated from the lithosphere above 350 km. These upper-mantle anomalies are interpreted as the remnants of delaminated sub-continental lithosphere that went down when Greater India continued to converge northward with Asia after ˜45 Ma. The deeper high-velocity anomalies under the Indian sub-continent appear clearly separated from the shallower ones as well as from each other, and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the Indian plate. They occur at depths between 1000 and 2300 km and occasionally descend down to the core-mantle boundary. The anomalies form three parallel WNW-ESE striking zones. We interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the Neo-Tethys Ocean closed between India and Tibet in the Cretaceous and earliest Tertiary. The northern deep-mantle zone under northern Afghanistan, the Himalayas and the Lhasa block in southern Tibet may represent the last-subducted remnant of the Paleo-Tethys Ocean, which is thought to have closed before the Hauterivian stage of the Early Cretaceous. The middle zone continues southeastward as a rather straight high-velocity zone towards

  16. Initiation of Subduction at Relic Arcs

    NASA Astrophysics Data System (ADS)

    Gurnis, M.; Leng, W.

    2014-12-01

    Plate tectonics have been well established for tens of years, but how subduction initiates over tectonic history remains obscure. It has been proposed that passive margins may be a possible place for subduction initiation, but there is no obvious Cenozoic example of such a scenario, including along the passive margins of the Atlantic Ocean. With a computational method that follows the deformation of a visco-elasto-plastic medium, here we show that a favourable locale for subduction initiation is the juxtaposition of an old oceanic plate adjacent to a young, but relic arc. The probable enrichment of quartz in the middle to lower arc crust leads to two major factors which may have induced subduction initiation. One is the compositional density difference between the relic arc crust and the oceanic lithospheric mantle; the other is the significantly weakened lithosphere strength due to the rheology of wet quartz. With such a setup, we observe spontaneous subduction initiation within a few million years. The evidence that Izu-Bonin-Mariana and Tonga-Kermedec subduction zones both initiate adjacent to relic island arcs supports our conclusions. Our results provide an explanation for the rarity of subduction initiation at the passive margins. The continental lithosphere is typically old and cold. Consequently, the thermal effects cancel the compositional buoyancy contrast between the continental crust and the oceanic lithospheric mantle, making subduction initiation difficult at passive margins.

  17. Variability of South Pacific Tropical Water Subduction

    NASA Astrophysics Data System (ADS)

    Lu, X.; Fine, R. A.; Qu, T.

    2014-12-01

    Collection of Argo data provides an opportunity to carefully examine South Pacific Tropical Water (SPTW) subduction rate variability. SPTW is characterized by a vertical salinity maximum exceeding 36.2 psu centered at 20°S and 120°W and lying in the upper thermocline between 24.0 and 25.0 σθ. Subduction rates for SPTW for two different periods are calculated using two methods. Monthly one degree by one degree Argo data covering the South Pacific are used to calculate subduction rates from September 2005 to August 2013, also lateral induction and vertical pumping are calculated. There are two spatial subduction maxima, and the lateral induction process dominates in both maxima. Subduction rates from Argo data vary from 15 to 26 m/yr +/- 7.5% during the 8 year period. Subduction rates are shown to be positively and highly correlated with Southern Oscillation Index. Additionally, using CFC-12 data from the 1990s World Ocean Circulation Experiment, average subduction rate is calculated to be 35 +/- 16.5 m/yr. Some of the difference between Argo and tracer rates is due to a difference in the methods, and some difference may be due to decadal variability. Thus, SPTW subduction rates are shown to vary on interannual and possible decadal time scales.

  18. Three-Dimensional Numerical Models of Subduction and Subduction-Induced Mantle Flow

    NASA Astrophysics Data System (ADS)

    Freeman, J.; Stegman, D. R.; Schellart, W. P.; Moresi, L.; May, D. A.; Turnbull, R.

    2004-12-01

    The kinematics of subduction and its influence on mantle convection and plate-scale deformation have been the focus of numerous geodynamic studies [e.g. Garfunkel et al., 1986; Gurnis and Hager, 1988; Zhong and Gurnis, 1995; Christensen, 1996; Olbertz et al., 1997; Conrad and Hager, 1999; Eberle et al., 2002]. Most geodynamic models have considered only two-dimensional aspects of subduction dynamics by incorporating the assumption that subduction zones are infinite in trench-parallel extent. However, natural subduction zones are intrinsically three-dimensional, due in part to their limited lateral extent. Lateral length scales of natural subduction zones vary from only a few hundred kilometres (e.g. the Calabrian, Hellenic and Scotia slabs) to several thousand kilometres (e.g. the Aleutian, Indonesian, Northwest Pacific and South American slabs). Here, we present results from three-dimensional numerical experiments that simulate lithospheric subduction and subduction induced mantle flow for slabs with a varying lateral extent.

  19. Potential Fields Illuminate Earthquake Sources in Subduction-Margin Settings

    NASA Astrophysics Data System (ADS)

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

    2008-05-01

    sources at mantle depths, show evidence for hydrated mantle at many subduction margins of the world, including Cascadia, northeast Japan, the Aleutians, southern Mexico, and central America. All of these subduction zones have thermal characteristics believed to be conducive for intraslab earthquakes. On the other hand, the Peru, Chile, and Nankai subduction margins, also known to be susceptible to intraslab earthquakes, have only minor forearc magnetic anomalies, indicating complexities in the relationship between hydrated mantle and intraslab seismogenesis. In the shallow crust of the forearc, magnetic fields illuminate seismically active crustal faults activated by stresses induced by the subducting plate. The Seattle fault, source of a MW 7 earthquake 1100 ka, and the Nojima Fault, source of the MW 6.9 Kobe earthquake in 1995, are well-known examples. In Cascadia, where bedrock often is concealed by vegetation and young glacial deposits, a three-pronged approach has proven useful in mapping and characterizing crustal faults: (1) Laser terrain mapping (lidar) reveals scarps cutting late- Pleistocene glacial surfaces. (2) In almost every case, these lidar scarps correspond with short-wavelength, low- amplitude magnetic anomalies in high-resolution aeromagnetic surveys, which in turn facilitate geologic mapping of the faults away from scarps and help to characterize faults at depth. (3) The lidar and aeromagnetic data together provide targets for follow-on trench excavations that yield temporal information on past earthquakes needed to assess earthquake hazards.

  20. 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

  1. Influence of paired subduction zones: insight into Central Mediterranean tectonics

    NASA Astrophysics Data System (ADS)

    Miller, Meghan Samantha; Moresi, Louis; Faccenna, Claudio; Funiciello, Francesca

    2015-04-01

    The Hellenic and Calabrian slabs are subducting the last remnant of the Ionian oceanic lithosphere into the deep mantle beneath the Central Mediterranean. Seismic tomography studies have provided clear images of the present day morphology of the subducted lithosphere [1]. Tectonic studies have shown that the Calabrian slab has rolled back into its current geometry with episodes of back-arc spreading that have now ceased [2]. Conversely, GPS observations along with tectonic reconstructions show that the Hellenic slab is currently rolling back and appears to have accelerated in the past ~15 My [3], which has resulted in the only region of backarc spreading still active in the Mediterranean. Observations of seismic anisotropy from SKS splitting [4] indicate toroidal flow patterns at the edges of the subducted slabs, which lead to interpretations of mantle convection and flow. Rollback in a confined setting has allowed the two slabs to become a plate-tectonic pushmi-pullyu [5]. The evolution of each slab is necessarily dependent on the other as they are both subducting the same lithosphere in opposite directions and are sufficiently close together that their induced mantle flow patterns must interact strongly. Although this seems to be an oddity in the classical picture of plate tectonics, we note that rollback-dominated subduction is more likely to be important in the highly-confined setting of a closing ocean where the oceanic lithosphere is not always able to develop into a freely-moving plate. Under such conditions, back-to-back pairings of subducting slabs are potentially more common. To investigate this setting, we present preliminary numerical models of paired subduction zones that we have developed using Underworld. We include variations in the strength and buoyancy of the surrounding (over-riding) plates and account for the presence of continentally-derived basement in the Adriatic sea. The geodynamic models allow for exploration into the timing, mechanics

  2. Subduction erosion and accretion in the Solomon Sea region

    NASA Astrophysics Data System (ADS)

    Honza, Eiichi; Miyazaki, Teruki; Lock, Jo

    1989-03-01

    The Solomon Sea region is an area of intense tectonic activity characterized by structural complexity, a high level of seismicity and volcanism, and rapid evolution of plate boundaries. There is little accretion in the eastern New Britain Trench. Accretion gradually increases westward with thick accretion in the western New Britain Trench and in the Trobriand Subduction System. The thick accretion in the western part of the New Britain Trench may be a result of collision from the north of Finisterre-Huon block with New Guinea mainland. The present boundary of the collision is along the Ram-Markham fault. Deformation structures and present day seismicity suggest that the northern block is under compression. Accretion has occurred in the sediment filled trenches in the Solomon Sea. The scale of the accretionary wedge depends on the amount of trench-fill sediment available. It is unlikely that there is no sediment supply to the eastern part of the New Britain Trench where no accretion is observed and subduction erosion may be occurring. There are two possible mechanisms for subduction erosion of sediment; either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench; or horizontal tensional force superimposed on both the forearc and backarc regions of the arc. Seafloor spreading in both the Manus and Woodlark basins is fan-like with nearby poles in the western margins of the basins. This may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the Solomon Sea. Therefore it is possible to conclude that the consumption of sediment in the eastern New Britain Trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system. Imbricated thrust and overthrust faults in the western New Britain Trench and Trobriand Trough are not linear over long distance, but form wavy patterns in blocks with

  3. Convection scaling and subduction on Earth and super-Earths

    NASA Astrophysics Data System (ADS)

    Valencia, Diana; O'Connell, Richard J.

    2009-09-01

    Super-Earths are the smallest class of discovered extra-solar planets. Owing to their relatively small mass, some might resemble Earth and perhaps be habitable. Because of the connection to habitability through thermal evolution, we investigate the tectonic regime of massive terrestrial planets. Two independent studies [O'Neill, C., Lenardic, A., Oct 2007. Geological consequences of super-sized earths. GRL 34, L19204, Valencia, D., O'Connell, R.J., Sasselov, D. D., Nov 2007a. Inevitability of plate tectonics on super-earths. ApJL 670, L45-L48.] have reached opposing conclusions about the likelihood of plate tectonics. Here, we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics: deformation on faults, negative buoyancy and energy dissipation during subduction. We show that in general, the ratio of driving force to plate resistance increases with planetary mass. This is a consequence of increasing convective stresses, thinning plates and similar plate structure. We conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets, the convective stress increases even more allowing deformation to take place. Also, despite shorter timescales for plate cooling, rocky super-Earths achieve negative buoyancy at subduction zones. Finally, by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active-lid tectonics. In conclusion, rocky super-Earths have more favorable conditions than Earth for the subduction of plates, and hence, for sustaining plate tectonics.

  4. Flat Subduction and Dynamic Topography

    NASA Astrophysics Data System (ADS)

    Lithgow-Bertelloni, C. R.; Dávila, F. M.; Eakin, C. M.; Crameri, F.

    2014-12-01

    Mantle dynamics manifests at the surface via the horizontal motions of plates and the vertical deflections that influence topography and the non-hydrostatic geoid. The pioneering work of Mitrovica et al. (1989) and Gurnis (1990) on this dynamic topography revolutionized our understanding of sedimentary basin formation, sea level changes and continental flooding. The temporal evolution of subduction can explain the migration of basins and even the drainage reversal of the Amazon (Shephard et al., 2012; Eakin et al., 2014). Until recently, flat subduction has been seen as enhancing downward deflection of the overriding plate and increasing flooding. However, this interpretation depends crucially on the details of the morphology and density structure of the slab, which controls the loci and amplitude of the deflection. We tend to ignore morphological details in mantle dynamics because flow can smooth out short wavelength variations. We have shown instead that details matter! Using South America as a natural laboratory because of the large changes in morphology of the Nazca slab along strike, we show that downward deflection of the overriding plate and hence basin formation, do not occur over flat segments but at the leading edge, where slabs plunge back into the mantle. This is true in both Argentina and Peru. The temporal evolution from a 'normally' dipplng slab to a flat slab leads to uplift over flat segments rather than enhanced subsidence. Critical for this result is the use of a detailed morphological model of the present-day Nazca slab with a spatial resolution of 50-100 km and based on relocated seismicity and magnetotelluric results. The density structure of the slab, due to age and the presence of overthickened crust from aseismic ridge subduction is essential. Overthickened crust leads to buoyant slabs. We reproduce formation and deposition of the Acres-Solimoes basin and the evolution of the Amazon drainage basin in Peru as well as the Mar Chiquita

  5. Constraints on the Amount of deeply subducted Water from numerical Models in comparison with natural Samples

    NASA Astrophysics Data System (ADS)

    Konrad-Schmolke, M.; Halama, R.

    2014-12-01

    The subduction of hydrated slab mantle to beyond-arc depths is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. During subduction of hydrated oceanic lithosphere, dehydration reactions in the downgoing plate lead to a partitioning of water between upper and lower plate. Water retained in the slab is recycled into the mantle where it controls its rheology and thus plate tectonic velocities. Hence, quantification of the water partitioning in subduction zones is crucial for the understanding of mass transfer between the Earth's surface and the mantle. Combined thermomechanical and thermodynamic models yield quantitative constraints on the water cycle in subduction zones, but unless model results can be linked to natural observations, the reliability of such models remains speculative. We present combined thermomechanical, thermodynamic and geochemical models of active and paleo-subduction zones, whose results can be tested with independent geochemical features in natural rocks. In active subduction zones, evidence for the validity of our model comes from the agreement between modeled and observed across-arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. In the Kamchatkan subduction zone, for example, the model successfully predicts complex geochemical patterns and the spatial distribution of arc volcanoes. In paleo-subduction zones (e.g. Western Gneiss Region and Western Alps), constraints on the water budget and dehydration behavior of the subducting slab come from trace element zoning patterns in ultra-high pressure (UHP) garnets. Distinct enrichments of Cr, Ni and REE in the UHP zones of the garnets can be reconciled by our models that predict intense rehydration and trace element re-enrichment of the eclogites at UHP conditions by fluids released from the underlying slab mantle. Models of present-day subduction zones indicate the presence of 2.5-6 wt.% of water

  6. Subduction initiation at oceanic detachment faults: a mechanism to generate extensive ophiolite belts

    NASA Astrophysics Data System (ADS)

    Maffione, Marco; Thieulot, Cedric; van Hinsbergen, Douwe; Morris, Antony; Plumper, oliver; Spakman, Wim

    2015-04-01

    One of the least understood processes of plate tectonics is the nucleation of new subduction zones and the formation of ophiolites by subsequent upper plate extension. Subduction initiation within ocean basins is thought to occur along weakness zones such as transform faults, fracture zones, and mid-ocean ridges. Detachment faults, which cut across oceanic lithosphere immediately adjacent to slow-spreading mid-ocean ridges may yields ideal rheological conditions for subduction initiation due to their pervasive serpentinization. We numerically test this hypothesis by modeling the inversion of an ocean basin cut by a serpentinized detachment fault adjacent to an active spreading center. The results of our models consistently show that the serpentinized fault effectively localizes deformation, assisting subduction initiation upon compression. Subsequent reactivation of the pre-existing spreading center preserved in the forearc above the nascent subduction zone provides an efficient mechanism for the formation of supra-subduction zone ophiolites. Application of our model of subduction initiation to the ~700 km-long ophiolite belt spanning from Albania to Greece is then discussed.

  7. 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

  8. The Trobriand Subduction System in the Western Solomon Sea

    NASA Astrophysics Data System (ADS)

    Lock, J.; Davies, H. L.; Tiffin, D. L.; Murakami, F.; Kisimoto, K.

    1987-09-01

    A south-dipping Subduction system which underlies the Trobriand Trough and 149° Embayment, on the southern margin of the Solomon Sea, is active or was recently active. Oceanic basement is overlain by 2.5 s, two-way travel time (TWTT), of sediment that shows at least two stages of deformation: early thrusts (inner wall) and normal faults (outer wall), and later normal faults that have elevated the outer trench margin. Thrust anticlines and slope basins are developed on the inner wall. The floor of the Solomon Sea Basin arches upward between the Trobriand Trough and the New Britain Trench to form isolated peaks and ridges in the east (152° Peaks) and an east-west Central Ridge in the west. Structures in the subduction system, and in the Solomon Sea Basin, plunge westward towards the point of collision with the New Britain Trench.

  9. Initiation of subduction by post-collision foreland thrusting and back-thrusting

    NASA Astrophysics Data System (ADS)

    Mitchell, A. H. G.

    1984-07-01

    While postulated causes of initial subduction and trench formation include underthrusting, controls on its location and age have not been determined. Consideration of the age of subduction zones bordering five collisional orogens suggests that subduction may have been initiated by foreland thrusts and back-thrusts. Foreland thrusts develop within a continental foreland on the subducting plate mostly within 50 my of collision with an arc system; where the foreland is narrow the thrusts may intersect the continent-ocean crust boundary. Back-thrusts develop in the fore-arc or back-arc area on the overriding plate within 10 to 20 my of collision, and can result in tectonic burial of the magmatic arc; where the arc system is oceanic the back-thrusts may intersect the arc-ocean crust boundary. Possible examples of subduction initiated by foreland thrusts are the start of subduction in the late Jurassic beneath the northern Sunda Arc, and at the end-Miocene in the Negros Trench. Examples of back-thrusts which have initiated or may initiate subduction are the late Cenozoic eastward translation of Taiwan over the Philippine Sea plate, the incipient southward subduction of the Banda Sea beneath Timor, and the W-dipping back-thrust comprising the Highland Boundary Fault zone and postulated early Ordovician thrusts to the SE in Scotland. The suggested relationship of subduction to collision helps to explain the persistence of Wilson cycles in the still-active late Mesozoic to Cenozoic orogenic belts and implies that orogeny will cease only with collision between major continents.

  10. Implications of magmatic records for Neotethyan subduction beneath the Eurasian margin (Lhasa terrane, southern Tibet)

    NASA Astrophysics Data System (ADS)

    Tan, Jieqing; Aitchison, Jonathan

    2014-05-01

    Evidence for magmatism is widely developed in the Lhasa terrane of southern Tibet. Much of this is related to northward subduction of the Neotethyan Ocean prior the India-Eurasia collision. To better understand the tectono-magmatism, we systematically studied the published data for Middle Jurassic-Eocene igneous rocks in southern Tibet. Many of these rocks formed during two important intervals from ca. 110-80 Ma and ca. 65-40 Ma. On the basis of the reported rocks in this area, we considered the possibility that a Neotethyan mid-ocean ridge was subducted during the early peak episode (ca. 110-80 Ma). With this ridge subduction system, hot asthenosphere rose up through a slab window causing both oceanic slab and mantle wedge melting that resulted in peak volcanism during the Late Cretaceous. As young and hot crust at a mid-ocean ridge has a relatively low density, and thus potentially positive buoyancy, the subduction of a buoyant mid-ocean ridge may have led to a reduction in the angle of subduction. Evidence for termination of arc magmatism by the flat subducted oceanic slab is recorded by a magmatic gap ca. 80-65 Ma. Around ca. 65 Ma, the magmatic record appears again accompanied by a southward migration that represents resumption of an oceanic slab subduction at a normal subduction angle. Subsequently, magmatism lasts to ca. 36 Ma before the India-Eurasia collision and reached a peak of activity associated with a magmatic flare-up at 50 Ma. In this subduction system, some magmatic processes triggered formation of porphyry ore deposits and affected the temporal and spatial distribution of ores.

  11. Origin and dynamics of depositionary subduction margins

    NASA Astrophysics Data System (ADS)

    Vannucchi, Paola; Morgan, Jason P.; Silver, Eli A.; Kluesner, Jared W.

    2016-06-01

    Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a "depositionary forearc," a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long-term processes, and sudden extreme events reflecting the sudden influences of large-scale morphological variations in the incoming plate. Both types of processes contribute to the large-scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long-lived megathrust surfaces.

  12. Origin and dynamics of depositionary subduction margins

    USGS Publications Warehouse

    Vannucchi, Paola; Morgan, Jason P.; Silver, Eli; Kluesner, Jared

    2016-01-01

    Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a “depositionary forearc,” a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long-term processes, and sudden extreme events reflecting the sudden influences of large-scale morphological variations in the incoming plate. Both types of processes contribute to the large-scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long-lived megathrust surfaces.

  13. Seismic anisotropy above a subducting plate

    SciTech Connect

    Shih, X.R.; Meyer, R.P. ); Schneider, J.F. )

    1991-08-01

    Shear-wave splitting observed in northeastern Colombia has provided evidence of seismic anisotropy in a shear zone immediately above a subducting plate. In an upper mantle composed mainly of olivine (57%) and orthopyroxene (17%), the splitting can be interpreted by wave propagation in an anisotropic medium of orthorhombic symmetry that results from alignment of these intrinsically anisotropic minerals. The mechanism of alignment is most likely the shearing associated with the subduction, aided by fluids migrating from the subducting plate when the plate exceeds 100 km in depth.

  14. 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

  15. Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary.

    PubMed

    Vannucchi, Paola; Remitti, Francesca; Bettelli, Giuseppe

    2008-02-01

    Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth's subduction zones. In situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone. Until now, a fossil erosive subduction channel-the shear zone marking the plate boundary-has not been recognized in the field, so that seismic observations have provided the only information on plate boundary processes at erosive margins. Here we show that a fossil erosive margin is preserved in the Northern Apennines of Italy. It formed during the Tertiary transition from oceanic subduction to continental collision, and was preserved by the late deactivation and fossilization of the plate boundary. The outcropping erosive subduction channel is approximately 500 m thick. It is representative of the first 5 km of depth, with its deeper portions reaching approximately 150 degrees C. The fossil zone records several surprises. Two décollements were simultaneously active at the top and base of the subduction channel. Both deeper basal erosion and near-surface frontal erosion occurred. At shallow depths extension was a key deformation component within this erosive convergent plate boundary, and slip occurred without an observable fluid pressure cycle. At depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast (co-seismic) and slow (inter-seismic) slip. In the deepest portions of the outcropping subduction channel, extension is finally overprinted by compressional structures. In modern subduction zones the onset of seismic activity is believed to occur at approximately 150 degrees C, but in the fossil channel the onset occurred at cooler palaeo-temperatures.

  16. Evidence for a large-scale remnant of subducted lithosphere beneath Fiji.

    PubMed

    Chen, W P; Brudzinski, M R

    2001-06-29

    We combine spatial variations of P- and S-wave speeds, 1000 fault plane solutions, and 6600 well-determined hypocenters to investigate the nature of subducted lithosphere and deep earthquakes beneath the Tonga back-arc. We show that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep-dipping Wadati-Benioff zone and a subhorizontal remnant of slab that is no longer attached to the actively subducting lithosphere. The detached slab may be from a previous episode of subduction along the fossil Vitiaz trench about 5 to 8 million years ago. The juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle. Such a configuration, if common in the past, would allow the preservation of a primordial component in the lower mantle.

  17. Subducted slabs and the geoid: Constraints on mantle rheology and flow

    NASA Technical Reports Server (NTRS)

    Hager, B. H.

    1983-01-01

    The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones.

  18. Seismicity and tectonics of the subducted Cocos Plate

    NASA Astrophysics Data System (ADS)

    Burbach, George Vanness; Frohlich, Cliff; Pennington, Wayne D.; Matumoto, Tosimatu

    1984-09-01

    We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best-defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike-slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller-scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths

  19. Subduction signature in backarc mantle?

    NASA Astrophysics Data System (ADS)

    Nelson, W. R.; Snow, J. E.; Brandon, A. D.; Ohara, Y.

    2013-12-01

    Abyssal peridotites exposed during seafloor extension provide a rare glimpse into the processes occurring within the oceanic mantle. Whole rock and mineral-scale major element data from abyssal peridotites record processes intimately associated with melt-depletion and melt-rock interaction occurring just prior to exposure of the mantle at the surface. Isotopic data, however, can provide insight into the long-term evolution of the oceanic mantle. A number of studies of mantle material exposed along mid-ocean ridges have demonstrated that abyssal peridotites from Mid-Atlantic Ridge, Gakkel Ridge, and Southwest Indian Ridge commonly display a range of whole rock Os isotopic ratios (187Os/188Os = 0.118- 0.130; Brandon et al., 2000; Standish et al., 2002; Alard et al., 2005; Harvey et al., 2006; Liu et al., 2008). The range of isotopic values in each region demonstrates that the oceanic mantle does not melt uniformly over time. Instead, anciently depleted regions (187Os/188Os ≈ 0.118) are juxtaposed against relatively fertile regions (187Os/188Os ≈ 0.130) that are isotopically similar to established primitive mantle values (187Os/188Os = 0.1296; Meisel et al. 2001). Abyssal peridotites from the Godzilla Megamullion and Chaotic Terrain in the backarc Parece Vela Basin (Philippine Sea) display a range of Os isotopic values extending to similar unradiogenic values. However, some of the backarc basin abyssal peridotites record more radiogenic 187Os/188Os values (0.135-0.170) than mid-ocean ridge peridotites. Comparable radiogenic signatures are reported only in highly weathered abyssal peridotites (187Os/188Os ≤ 0.17, Standish et al., 2002) and subduction-related volcanic arc peridotites (187Os/188Os ≤ 0.16, Brandon et al., 1996; Widom et al., 2003). In both the weathered peridotites and arc peridotites, the 187Os/188Os value is negatively correlated with Os abundance: the most radiogenic value has the lowest Os abundance (< 1 ppb) making them highly susceptible to

  20. Density model of the Cascadia subduction zone

    USGS Publications Warehouse

    Romanyuk, T.V.; Mooney, W.D.; Blakely, R.J.

    2001-01-01

    The main goal of this work is to construct self-consistent density models along two profiles crossing the northern and central Cascadia subduction zone that have been comprehensively studied on the basis of geological, geophysical, etc. data.

  1. Hydration of the incoming plate in the Kuril subduction zone

    NASA Astrophysics Data System (ADS)

    Fujie, G.; Kodaira, S.; Yamashita, M.; Sato, T.; Takahashi, T.; Takahashi, N.; Noguchi, N.

    2010-12-01

    Water supplied from the subducting oceanic plate by dehydration is inferred to cause seismicity and magmatism in subduction zones. It is important, therefore, to reveal the distribution of water within the incoming plate for understanding seismic and volcanic activities in subduction zones. In 2009 and 2010, to reveal the detailed seismic structure and hydration process within the incoming plate, we conducted a wide-angle seismic survey in the Kuril subduction zone, where the old Pacific plate formed in the eastern Pacific ridge is subducting from south to north beneath the island arc of Japan. We designed a north-south 500km-long seismic experimental line to be perpendicular to the Kuril trench. The northern end of our line is located at about 30km south of the trench axis and well-developed horst and graben structure is observed around the northern end. We deployed 80 Ocean Bottom Seismometers (OBSs) at intervals of 6km and shot a large tuned airgun array towed by R/V Kairei. In addition, we obtained MCS reflection data using a 444-channel hydrophone streamer (6km long) along the same line. We modelled both P-wave and S-wave velocity structures by the traveltime inversion using refraction, reflection and PS-conversion traveltimes. Our results show that P-wave velocity beneath the well-developed horst and graben structure is about 5% lower than that in the south of outer rise. This is consistent with a previous structure study in the Chili subduction zone that shows the P-wave velocity in the vicinity of the trench axis is lower than that of normal oceanic plate. More notable feature of our results is the regional variations of Vp/Vs. The S-wave velocity, as well as P-wave velocity, gradually decreases toward the trench axis. However Vp/Vs is not uniform; Vp/Vs immediately beneath the sediments is remarkably high beneath the well-developed horst and graben structure, and Vp/Vs decreases with depth (high Vp/Vs is confined to the top of the oceanic plate). Since the

  2. Opening and closing slab windows in congested subduction zones

    NASA Astrophysics Data System (ADS)

    Moresi, Louis

    2013-04-01

    Subduction zones often try to swallow buoyant material which is embedded in the oceanic lithosphere: plume material or hotspot residues, oceanic plateaux, and fragments of continental material. This often results in the formation of a slab window and it has been shown (Mason et al, 2010; Betts et al, 2012) that this window strongly influences the subsequent evolution of the slab and the advance/retreat rate of the trench. The buoyant material typically pushes the trench into a local state of advance, and the creation of the slab window allows the rest of the trench to retreat as the mantle behind the slab flows in through the window. This situation is inherently unstable: if the buoyancy anomaly is finite in size, then the retreating trench will soon move behind the anomaly and juxtapose negatively buoyant oceanic lithosphere with active subduction. This creates the potential to close the slab window and, in doing so, transfer the buoyant material to the over-riding plate. Models show that this closure of the window initially occurs through a lateral rollback process followed by a catastrophic re-initiation of subduction behind the colliding buoyant anomaly. This rollback leaves a characteristic, tightly rolled remnant in the mantle and significant rotation in the over-riding plate and the newly-docked block. The over-riding plate is thrown into extension perpendicular to the original orientation of the trench. This same situation applies at the late-stages of a closing ocean due to the passive margin geometry and the presence of debris collected from the closing ocean floor and it seems likely that these models can also be applied to the complicated geometry of subduction in such environments. Mason, W. G.; Moresi, L.; Betts, P. G. & Miller, M. S. Three-dimensional numerical models of the influence of a buoyant oceanic plateau on subduction zones Tectonophysics, 2010, 483, 71-79 P. Betts, W. Mason, L. Moresi, The influence of mantle plumes on subduction zone

  3. Multiscale seismic imaging of the Western-Pacific subduction zone

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2011-12-01

    We used multiscale seismic tomography to determine the detailed 3-D structure of the crust and mantle under the Western-Pacific subduction zone. The subducting Pacific and Philippine Sea (PHS) slabs are imaged clearly from their entering the mantle at the oceanic trenches to their reaching the mantle transition zone and finally to the core-mantle boundary (CMB). High-resolution local tomography of Northeast Japan has imaged the shallow portion of the slab from the Japan Trench down to about 200 km depth under Japan Sea. The 3-D Vp and Vs structures of the forearc region under the Pacific Ocean are constrained by locating suboceanic events precisely with sP depth phases. Strong structural heterogeneity is revealed in the megathrust zone under the forearc region, and there is a good correlation between the heterogeneity and the distribution of large thrust earthquakes including the great 2011 Tohoku-oki earthquake (Mw 9.0). A joint inversion of local and teleseismic data imaged the subducting Pacific slab down to 670 km depth under the Japan Islands and the Japan Sea. The PHS slab is detected down to 500 km depth under SW Japan. A mantle upwelling is found under SW Japan that rises from about 400 km depth right above the Pacific slab up to the PHS slab. Regional and global tomography revealed the Pacific slab that is stagnant in the mantle transition zone under Eastern China. A big mantle wedge (BMW) has formed in the upper mantle above the stagnant slab. Convective circulations in the BMW and deep dehydration of the stagnant slab may have caused the intraplate volcanoes in NE Asia, such as the Changbai and Wudalianchi volcanoes. The active Tengchong volcanism in SW China is caused by a similar process in the BMW above the subducting Burma (or Indian) slab. Global tomography shows pieces of fast anomalies in the middle and lower mantle as well as in the D" layer above the CMB, suggesting that the stagnant slab finally collapses down to the lower mantle and CMB as a

  4. A global view of shear wave splitting and mantle flow in subduction systems

    NASA Astrophysics Data System (ADS)

    Long, Maureen; Silver, Paul; Hanna, Jenny; Wirth, Erin; Kincaid, Chris; Montesi, Laurent

    2010-05-01

    The character of the mantle flow field in subduction zone regions remains poorly understood, despite its importance for our understanding of subduction dynamics. Observations of seismic anisotropy, which manifests itself in shear wave splitting, can shed light on the geometry of mantle flow in subduction zones, but placing constraints on anisotropy in various parts of the subduction system (including the overriding plate, the mantle wedge, the subducting slab, and the sub-slab mantle) remains challenging from an observational point of view. In order to identify dynamic processes that make first-order contributions to the pattern of mantle flow in subduction zones, we analyze a global compilation of shear wave splitting measurements for a variety of ray paths, including SK(K)S and teleseismic S phases as well as local S and source-side splitting from slab earthquakes. Key challenges associated with assembling such a compilation include correctly assessing and accounting for any dependence of local S splitting parameters on frequency and correctly characterizing any contribution to SKS splitting measurements from anisotropy within the subducting slab that is unrelated to active mantle flow. We present local case studies from the Japan and Izu-Bonin-Marianas subduction zones that explore frequency-dependent splitting due to heterogeneous anisotropy in the mantle wedge and that use a variety of raypath combinations to isolate the contribution from anisotropy within the slab. Keeping these results in mind, we have compiled shear wave splitting measurements from subduction zones globally from the literature and from our own work to produce estimates of average shear wave splitting parameters - and their spatial variation - for the mantle wedge and the sub-wedge region for individual subduction segments. These estimates are then compared to other parameters that describe subduction. The sub-wedge splitting signal is relatively simple and is dominated by trench

  5. Magmatic consequences of the transition from orthogonal to oblique subduction in Panama

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Morell, Kristin D.; Hidalgo, Paulo; Fraceschi, Pastora

    2015-12-01

    The closure of the Central American Seaway is linked with tectonic and magmatic processes that have controlled the evolution of the Isthmus of Panama. We focus on the terminal stages of arc activity in the Central Panama region, and present new geochemical data from ˜9 Ma explosive silicic volcanism preserved in three syngenetic tuff beds from the Gatun. The magmatic evolution of the Gatun Formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral, with amphibole appearing only relatively late in the fractionation sequence. Our data show Gatun lavas exhibit a strong subduction signature, consistent with plate reconstruction models showing arc-normal subduction from Costa Rica to Panama pre-8.5 Ma. However, large ion lithophile elements are depleted in the Gatun Formation in comparison to other regional suites, indicative of a lower flux of subduction fluid to the Gatun Formation mantle source, which is explained by a shift toward magma generation by decompression following the collision of the arc with South America. Oblique subduction commencing ˜8.5 Ma resulted in the shutdown of normal arc activity throughout Panama. We interpret subsequent regional Quaternary adakitic volcanism as a response to this oblique subduction. The now more refractory mantle wedge required greater fluid flux in order to melt. The resultant volatile-rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites. Deep fractionation was potentially enhanced by changing stress regimes on the upper plate caused by oblique subduction.

  6. Ups and downs in western Crete (Hellenic subduction zone).

    PubMed

    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.

  7. Elasticity of Hydrous Phases in Subduction Zones- Geophysical Implications

    NASA Astrophysics Data System (ADS)

    Mookherjee, M.; Mainprice, D.

    2014-12-01

    Globally, subduction zones are region associated with earthquakes and volcanic activities, both involving risk to local populations. These processes are intimately related to the thermodynamic stability and instabilty of hydrous phases that are subducted with the down going slab. These phases sequestrate several wt % of water in their crystallographic structure and can account for significant proportion of the hydrogen budget of the upper mantle , transition zone and perhaps the top of the lower mantle. In order to quantify the degree of mantle hydration, we need to have a good understanding of the elastic properties of layered hydrous phases, the effects of temperature, and pressure and relate them to seismological observables, such as the velocity and its anisotropy. Using first principle simulations, we have investigated several layered hydrous phases, including the important minerals antigorite, talc, and chlorite. These results are complementary to the recent experimental Brillouin Scattering results at ambient conditions. Based on the full elastic constant tensor we note that these hydrous phases have significant shear wave anisotropy and often have unusual pressure dependence of the anisotropy. Together with elasticity data, thermodynamic predictions of phase stability and experimental plastic deformation studies it is apparent that these layered hydrous phases could account for the large delay times observed in certain subduction zone settings, such as Ryukyu trench. Acknowledgement- MM is supported by the US National Science Foundation grant (EAR-1250477). MM acknowledges computing resources (request # EAR130015) from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575.

  8. 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

  9. Structural expression of forearc crust uplift due to subducting asperity

    NASA Astrophysics Data System (ADS)

    Fleury, Jean-Marc; Pubellier, Manuel; de Urreiztieta, Marc

    2009-12-01

    New structural observations and mapping of reefal terraces, carried out both on the field and on satellite remote sensing data, indicate that Sumba Island is presently undergoing a large amount of extension, associated with a significant regional uplift. This crustal uplift may have been created by a major thrust emerging in the South of the island. The uplift, which partly accommodated the Australian plate-South West Banda Arc convergence, is associated with the general northeastward tilt of the island. The consequent anomalous positive topography along the southern coast of the island is being compensated by significant tectonic erosion along large-scale curvilinear normal faults in the southeastern half of the island. The most important expression of this gravitational collapse is located at the receding side of an advancing circular dome, showing striking similarities with accretionary wedges being affected by seamount subduction. The part of the forearc basin known as the Savu Basin is moderately deformed (mostly in its central part) and appears to act as a rigid buttress in the convergence between the Banda Arc and the Australian plate. As a result the convergence appears to be transferred northward within the actively-shortening back-arc domain, which goes from the north of the Flores Island to the southwest of the Timor block. The convergent plate boundary shows a transition between a stable domain (West of Sumba) and a tectonized domain (East of Sumba), the latter coinciding with the subduction of the outer Australian passive margin. The subduction of the ocean-continent boundary of the Austral-Indian plate below the Banda arc since the Lower Pliocene may have incorporated some crustal fragments in the plunging Benioff zone. Most likely, the integration of the stretched continental lithosphere in the subduction zone caused the uplift the entire forearc domain, exhibiting inherited structures of the upper plate.

  10. Accretion, subduction, and underplating along the southern Alaska continental margin

    SciTech Connect

    Plafker, G.; Ambos, E.L.; Fuis, G.S.; Mooney, W.D.; Nokleberg, W.J.; Campbell, D.L.

    1985-01-01

    In 1984-1985 the Trans Alaska Crustal Transect (TACT) program completed geologic, seismic refraction, gravity, and magnetic studies along a 350-km-long corridor that extends northward from the Gulf of Alaska coast near Cordova to the Denali fault at the Richardson Highway. From south to north, this segment of the transect traverses: 1) part of the Prince William terrance (PWT), composed of an accreted Paleocene and Eocene deep-sea fan complex, oceanic volcanic rocks, and pelagic sediments; 2) the Chugach terrane (CGT) composed of a) accreted Late Cretaceous flysch and oceanic basaltic rocks, b) accreted and subducted (.) Late Jurassic to Early Cretaceous sheared melange, and c) subducted Early (.) Jurassic or older blueschist/greenschist; and 3) Wrangellia-Peninsular terranes (WRT/PET) consisting primarily of late Paleozoic intraoceanic andesitic arc rocks with associated mafic and ultramafic plutonic rocks, an overlying distinctive Triassic sedimentary and volcanic sequence, and superposed intrusive and extrusive magmatic rocks of the Jurassic Talkeetna arc. At the southern margin of both the CGT and WRT/PET, shallow high-velocity zones characterized by positive gravity and magnetic anomalies reflect uplift of mafic and ultramafic basement along these thrusts. The Contact and Border Ranges fault systems appear to merge into a subhorizontal low-velocity zone of uncertain origin that underlies the CGT and southern WRT/PET at 5-9 km depth. A few kilometers beneath the shallow low-velocity zone in a 30-km-thick stack of eight northward-dipping layers of alternating high and low velocity, interpreted as subducted and underplated mantle and oceanic crust rocks. Distribution of earthquake hypocenters suggests that active subduction involves at least the lowest two and possibly the lower four layers.

  11. 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.

  12. Subduction Zone Redox and the Deep Earth Cycles of Sulfur and Chalcophile Elements

    NASA Astrophysics Data System (ADS)

    Canil, D.

    2013-12-01

    Subduction at convergent plate margins is a return flux to the mantle of rocks influenced by weathering, hydrothermal activity, atmospheric exchange, or bio-mineralization in the exosphere. The latter exogenic processes modify the long-term abundance and behaviour of certain elements in the deeper earth that can be traced over time in the chemistry of mantle-derived magmas. The redox budget of subduction is controlled by the flux of oxidized versus reduced forms of Fe, S, H, or C, and impacts the long-term evolution of oxygen on the planet, critical for life in the exosphere. In particular, the sulfur cycle is specifically tied to the evolution of oxygen on Earth's surface over time and critical to biogeochemical cycles on the surface. The behaviour of sulfur in the exogenic system is well-studied and fairly well understood using sedimentary records. An originally sulfidic ocean on Earth gave way with time and oxygenation to one that is sulfate dominated over the last two billion years. In contrast, far less is known of the deep earth cycle of S, and more so its history. The record of the endogenic cycle can only be monitored via what comes out of the mantle (magmas and their gases), or what goes down via subduction (hydrothermally-altered or weathered subducted lithosphere). Interest in the endogenic cycle of S is not new but several outstanding conundrums remain for sulfur in arc magmas that point to the importance of the subduction process. A hitherto ignored component of the paradox of the sulfur cycle is the sedimentary veneer that sits atop the subducted oceanic basalt crust. Compilations show only 0.12 wt% S in altered ocean basalt crust, but up to 10 times that amount in oceanic sediments, tied to their Fe content (in pyrite). These abundances may seem trivial, but the behaviour of this small amount of S in subduction is not fully appreciated and its oxidation potential in the arc mantle is enormous. The conversion of subducted sulfide to sulfate is a 8

  13. Oceanic asthenosphere subduction and its geological implications

    NASA Astrophysics Data System (ADS)

    Zhou, Q.; Liu, L.

    2014-12-01

    We investigate the evolution of oceanic asthenosphere during subduction by exploring various scenarios including plate kinematics and plausible values of asthenosphere viscosity and density. We find that the oceanic asthenosphere will always subduct with the down-going slab as long as its average viscosity value is no smaller than 1×1018 Pa s. In order to allow slabs to subduct into the deep upper mantle, a maximum oceanic asthenosphere density reduction relative to the underlying mantle should be no larger than 0.7%, assuming a 200-km-thick asthenosphere channel. We find that a significant portion of the asthenosphere buoyancy should result from its excess temperature from the long-term thermal evolution of mantle convection. Our results are in contrast to an earlier suggestion that negligible amount (<30 km thick) of asthenosphere could get subducted, which is likely due to over-simplicity of subduction geometry and model boundary conditions. The recycling of a weak and hot asthenosphere provides a novel mechanism for the formation of slow seismic anomalies within the deep mantle. This, in turn, questions the commonly believed deep mantle plume origin of intra-plate volcanism, with a typical example being the Yellowstone volcanic system. Our current results suggest that a buoyant asthenosphere can be dragged down into the lower mantle and then moves upward due to its buoyancy when the overlying slab barrier is removed. To further test our hypothesis, we construct a 4D subduction model for western North America during the Cenozoic. We use data assimilation techniques to incorporate plate kinematics and sea floor ages as boundary conditions, and seismic anomalies converted density structure as internal buoyancy source. The subduction history is calibrated through a hybrid of forward and adjoint simulations satisfying multiple observational constraints. Some preliminary results will be presented.

  14. Mid mantle seismic anisotropy around subduction zones

    NASA Astrophysics Data System (ADS)

    Faccenda, M.

    2014-02-01

    There is increasing evidence for mid mantle seismic anisotropy around subduction zones whose interpretation remains elusive. In this study I estimate the strain-induced mid mantle fabric and associated seismic anisotropy developing in 3D petrological-thermo-mechanical subduction models where the slab is either stagnating over the 660 km discontinuity or penetrating into the lower mantle. The modelling of synthetic lattice-preferred-orientation (LPO) development of wadsleyite and perovskite has been calibrated with results from deformational experiments and ab-initio atomic scale models, and the single crystal elastic tensor of the different mineral phases is scaled by local P-T conditions. The lower transition zone (ringwoodite + garnet) is assumed to be isotropic. Mid mantle fabric develops in proximity of the subducting slab where deformation and stresses are high, except at depths where upwelling or downwelling material undergoes phase transformations, yielding to LPO reset. The upper transition zone (wadsleyite + garnet) is characterized by weak transverse isotropy (2-3%) with symmetry axes oriented and fast S wave polarized dip-normal. A slightly stronger transverse isotropy develops in the lower mantle (perovskite + periclase), where the symmetry axes, the polarization of the fast S wave and the maximum Vp and dVs are parallel to the slab dip and subduction direction. For stagnating slab models this translates into negative and positive radial anisotropy in the upper transition zone and lower mantle back-arc, respectively, minimum delay times for vertically travelling shear waves and large shear wave splitting for waves propagating horizontally in the lower mantle. These results may help in reconciling the seismic anisotropy patterns observed in some subduction zones with subduction-induced deformation, such as those measured in the mid mantle between the Australian plate and the New Hebrides-Tonga-Kermadec trenches that I interpret as related to stagnating

  15. The Interdependence of Plate Coupling Processes, Subduction Rate, and Asthenospheric Pressure Drop across Subducting Slabs

    NASA Astrophysics Data System (ADS)

    Royden, L.; Holt, A.; Becker, T. W.

    2015-12-01

    One advantage of analytical models, in which analytic expressions are used for the various components of the subduction system, is the efficient exploration of parameter space and identification of the physical mechanisms controlling a wide breadth of slab kinematics. We show that, despite subtle differences in how plate interfaces and boundary conditions are implemented, results for single subduction from a 3-D semi-analytical model for subduction FAST (Royden & Husson, 2006; Jagoutz et al., 2015) and from the numerical finite-element model CitcomCU (Moresi & Gurnis, 1996, Zhong et al., 2006) are in excellent agreement when plate coupling (via shear stress on the plate interface) takes place in the FAST without the development of topographic relief at the plate boundary. Results from the two models are consistent across a variety of geometries, with fixed upper plate, fixed lower plate, and stress-free plate ends. When the analytical model is modified to include the development of topography above the subduction boundary, subduction rates are greatly increased, indicating a strong sensitivity of subduction to the mode of plate coupling. Rates of subduction also correlate strongly with the asthenospheric pressure drop across the subducting slab, which drives toroidal flow of the asthenosphere around the slab. When the lower plate is fixed, subduction is relatively slow and the pressure drop from below to above the slab is large, inhibiting subduction and slab roll-back. When the upper plate is fixed and when the plate ends are stress-free, subduction rates are approximately 50% faster and the corresponding asthenospheric pressure drop from below to above the slab is small, facilitating rapid subduction. This qualitative correlation between plate coupling processes, asthenospheric pressure drop, and rates of subduction can be extended to systems with more than one subduction zone (Holt et al., 2015 AGU Fall Abstract). Jagoutz, O., Royden, L., Holt, A. & Becker, T. W

  16. 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.

  17. Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief

    NASA Astrophysics Data System (ADS)

    Bassett, Dan; Watts, Anthony B.

    2015-05-01

    An ensemble averaging technique is used to remove the long-wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within fore arcs, many of which are attributed to the tectonic structure of the subducting plate. The residual-gravimetric expression of subducting fracture zones extends >200 km landward of the trench axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0 <17 km). Subducting seamounts are similar in wavelength, amplitude, and morphology to unsubducted seamounts. Morphology, spatial distributions, and reduced levels of seismicity are considered inconsistent with mechanical models proposing wholesale decapitation, and the association of subducting seamounts with large-earthquakes. Subducting aseismic ridges are associated with uplift and steepening of the outer fore arc, a gradual reduction in residual bathymetric expression across the inner fore arc, and a local increase in the width and elevation of the volcanic-arc/orogen. These contrasting expressions reflect the influence of margin-normal variations in rigidity on where and how the upper plate deforms, both to accommodate subducting relief and in response to stresses transmitted across the plate interface. The outer fore arc and arc have lower rigidity due to fracturing and thermal weakening, respectively. Similar associations with complex earthquakes and fault creep suggest aseismic ridge subduction may also be accommodated by the development and evolution of a broad fracture network, the geometrical strength of which may exceed the locking strength of a smooth fault.

  18. 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.

  19. From subduction to collision: results of French POP2 program on Taiwan-Philippine festoon

    SciTech Connect

    Blanchet, R.; Stephan, J.F.; Rangin, C.; Baladad, D.; Bouysse, Ph.; Chen, M.P.; Chotin, P.; Collot, J.Y.; Daniel, J.; Drouhot, J.M.; Marsset, B.; Pelletier, B.; Richard, M.; Tardy, M.

    1986-07-01

    A sea-beam, seismic, magnetic, and gravimetric survey was conducted with the R/V Jean-Charcot in three key regions off the Taiwan-Philippine festoon in the western Pacific: (1) Ryukyu active margin and its junction with Taiwan; (2) northern part of the Manila Trench and its junction with the Taiwan tectonic prism; and (3) southern termination of Manila Trench in front of Mindoro Island. Transitions between active subduction along the Manila Trench and collision of Taiwan and Mindoro, and relations between active subduction and extension in the Okinawa-Ryukyu and the northeastern Taiwan systems are particularly studied.

  20. Subduction, collision and initiation of hominin dispersal

    NASA Astrophysics Data System (ADS)

    Schattner, Uri; Lazar, Michael

    2009-09-01

    Subduction is the main driving force of plate tectonics controlling the physiography of the Earth. The northward subduction of the Sinai plate was interrupted during the Early Pleistocene when the Eratosthenes Seamount began to collide with the Cyprian arc. A series of synchronous structural deformations was triggered across the entire eastern Mediterranean, and local topography was drastically accentuation along the Levantine corridor - one of the main pathways of hominin dispersal out of Africa. However, the choice of this preferred pathway and timing of dispersal has not been resolved. Though causes for dispersal out of Africa are in debate, we show that the transition from subduction to collision in the eastern Mediterranean set the route.

  1. 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

  2. Predicting the Isotopic Composition of Subduction-Filtered Subducted Oceanic Crust and Sediment

    NASA Astrophysics Data System (ADS)

    White, W. M.

    2010-12-01

    The chemical and isotopic character of mantle plumes, which produce oceanic island volcanoes, are widely thought to reflect the presence of recycled oceanic crust and sediment. Isotopic systematics suggest the “cycle time” for this process is 1 Ga or longer, but it should be possible to use a simple mass balance approach to discern how the presently operating subduction zone filter affects the ratios of radioactive parent to radiogenic daughter isotopes. Simple uniformitarian assumptions can then be used to predict the present isotopic composition of anciently subducted lithosphere. Our underlying assumption in deciphering the subduction zone filter is that the flux of an element into the deep mantle is simply equal to the flux of element into the subduction zone less the flux of that element into subduction zone magmas. The former is readily calculated from published data. The latter can be calculated by estimating parental magma compositions, arc accretion rates, and the assumption that arc magma compositions differ from MORB only because of material derived from subducting crust and sediment. Using this approach for 8 intra-oceanic subduction zones, we find 73% of Th and Pb, 79% of U, 80% of Rb and Sr, 93% of Nd and 98% of Sm survive the subduction zone filter. The subduction zone filter systematically increases Sm/Nd ratios in all subduction zones, but the effect is small, with a weighted mean increase of 1.5%. The effect of subduction is to decrease the Sm/Nd of the mantle, but only slightly. The effect of subduction is to increase the Rb/Sr of the mantle, but the subduction zone filter does not have a systematic effect on Rb/Sr ratios: it significantly increases in Rb/Sr in 3 subduction zones and significantly decreases it in one; the weighted mean shows no significant change. The effect of the subduction zone filter on U/Pb is also not systematic. U/Pb ratios in the mantle fluxes are bimodal, with values equal to or lower than the bulk Earth value in 4

  3. 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

  4. Surface deformation and slab-mantle interaction during Banda arc subduction rollback

    NASA Astrophysics Data System (ADS)

    Spakman, Wim; Hall, Robert

    2010-08-01

    The spectacularly curved Banda arc comprises young oceanic crust enclosed by a volcanic inner arc, outer arc islands and a trough parallel to the Australian continental margin. Strong seismic activity in the upper mantle defines a folded surface, for which there are two contrasting explanations: deformation of a single slab or two separate slabs subducting from the north and south. Here we combine seismic tomography with the plate tectonic evolution of the region to infer that the Banda arc results from subduction of a single slab. Our palaeogeographic reconstruction shows that a Jurassic embayment, which consisted of dense oceanic lithosphere enclosed by continental crust, once existed within the Australian plate. Banda subduction began about 15million years ago when active Java subduction tore eastwards into the embayment. The present morphology of the subducting slab is only partially controlled by the shape of the embayment. As the Australian plate moved northward at a high speed of about 7cmyr-1, the Banda oceanic slab rolled back towards the south-southeast accompanied by active delamination separating the crust from the denser mantle. Increasing resistance of the mantle to plate motion progressively folded the slab and caused strong deformation of the crust. The Banda arc represents an outstanding example of large-scale deformation of the Earth's crust in response to coupling between the crust, slab and surrounding mantle.

  5. 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.

  6. Subduction trench migration since the Cretaceous

    NASA Astrophysics Data System (ADS)

    Williams, S.; Flament, N. E.; Müller, D.; Butterworth, N. P.

    2015-12-01

    Much of our knowledge about subduction zone processes is derived from analyzing present-day Earth. Several studies of contemporary plate motions have investigated the balance between retreating and advancing trenches and shown that subduction zone kinematics are sensitive to the choice of Absolute Plate Motion (APM) model (or "reference frame"). For past times, the absolute motions of the lithospheric plates relative to the Earth's deep interior over tens of millions of years are commonly constrained using observations from paleomagnetism and age-progressive seamount trails. In contrast, a reference frame linking surface plate motions to subducted slab remnants mapped from seismic tomography has recently been proposed. APM models derived using different methodologies, different subsets of hotspots, or differing assumptions of hotspot motion, have contrasting implications for parameters that describe the long term state of the plate-mantle system, such as the balance between advance and retreat of subduction zones, plate velocities, and net lithospheric rotation. Here we quantitatively compare the subduction zone kinematics, net lithospheric rotation and fit to hotspot trails derived the last 130 Myr for a range of alternative reference frames and a single relative plate motion model. We find that hotspot and tomographic slab-remnant reference frames yield similar results for the last 70 Myr. For the period between 130 and 70 Ma, when hotspot trails become scarce, hotspot reference frames yield a much more dispersed distribution of slab advance and retreat velocities, which is considered geodynamically less plausible. By contrast, plate motions calculated using the slab-remnant reference frame, or using a reference frame designed to minimise net rotation, yield more consistent subduction zone kinematics for times older than 70 Ma. Introducing the global minimisation of trench migration rates as a key criterion in the construction of APM models forms the foundation

  7. Tomographically-imaged subducted slabs and magmatic history of Caribbean and Pacific subduction beneath Colombia

    NASA Astrophysics Data System (ADS)

    Bernal-Olaya, R.; Mann, P.; Vargas, C. A.; Koulakov, I.

    2013-12-01

    We define the length and geometry of eastward and southeastward-subducting slabs beneath northwestern South America in Colombia using ~100,000 earthquake events recorded by the Colombian National Seismic Network from 1993 to 2012. Methods include: hypocenter relocation, compilation of focal mechanisms, and P and S wave tomographic calculations performed using LOTOS and Seisan. The margins of Colombia include four distinct subduction zones based on slab dip: 1) in northern Colombia, 12-16-km-thick oceanic crust subducts at a modern GPS rate of 20 mm/yr in a direction of 110 degrees at a shallow angle of 8 degrees; as a result of its low dip, Pliocene-Pleistocene volcanic rocks are present 400 km from the frontal thrust; magmatic arc migration to the east records 800 km of subduction since 58 Ma ago (Paleocene) with shallow subduction of the Caribbean oceanic plateau starting ~24-33 Ma (Miocene); at depths of 90-150 km, the slab exhibits a negative velocity anomaly we associate with pervasive fracturing; 2) in the central Colombia-Panama area, we define an area of 30-km-thick crust of the Panama arc colliding/subducting at a modern 30/mm in a direction of 95 degrees; the length of this slab shows subduction/collision initiated after 20 Ma (Middle Miocene); we call this feature the Panama indenter since it has produced a V-shaped indentation of the Colombian margin and responsible for widespread crustal deformation and topographic uplift in Colombia; an incipient subduction area is forming near the Panama border with intermediate earthquakes at an eastward dip of 70 degrees to depths of ~150 km; this zone is not visible on tomographic images; 3) a 250-km-wide zone of Miocene oceanic crust of the Nazca plate flanking the Panama indenter subducts at a rate of 25 mm/yr in a direction of 55 degrees and at a normal dip of 40 degrees; the length of this slab suggests subduction began at ~5 Ma; 4) the Caldas tear defines a major dip change to the south where a 35 degrees

  8. The Cascadia Subduction Zone and related subduction systems: seismic structure, intraslab earthquakes and processes, and earthquake hazards

    USGS Publications Warehouse

    Kirby, Stephen H.; Wang, Kelin; Dunlop, Susan

    2002-01-01

    The following report is the principal product of an international workshop titled “Intraslab Earthquakes in the Cascadia Subduction System: Science and Hazards” and was sponsored by the U.S. Geological Survey, the Geological Survey of Canada and the University of Victoria. This meeting was held at the University of Victoria’s Dunsmuir Lodge, Vancouver Island, British Columbia, Canada on September 18–21, 2000 and brought 46 participants from the U.S., Canada, Latin America and Japan. This gathering was organized to bring together active research investigators in the science of subduction and intraslab earthquake hazards. Special emphasis was given to “warm-slab” subduction systems, i.e., those systems involving young oceanic lithosphere subducting at moderate to slow rates, such as the Cascadia system in the U.S. and Canada, and the Nankai system in Japan. All the speakers and poster presenters provided abstracts of their presentations that were a made available in an abstract volume at the workshop. Most of the authors subsequently provided full articles or extended abstracts for this volume on the topics that they discussed at the workshop. Where updated versions were not provided, the original workshop abstracts have been included. By organizing this workshop and assembling this volume, our aim is to provide a global perspective on the science of warm-slab subduction, to thereby advance our understanding of internal slab processes and to use this understanding to improve appraisals of the hazards associated with large intraslab earthquakes in the Cascadia system. These events have been the most frequent and damaging earthquakes in western Washington State over the last century. As if to underscore this fact, just six months after this workshop was held, the magnitude 6.8 Nisqually earthquake occurred on February 28th, 2001 at a depth of about 55 km in the Juan de Fuca slab beneath the southern Puget Sound region of western Washington. The Governor

  9. Subduction and exhumation of continental crust: insights from laboratory models

    NASA Astrophysics Data System (ADS)

    Bialas, Robert W.; Funiciello, Francesca; Faccenna, Claudio

    2011-01-01

    When slivers of continental crust and sediment overlying oceanic lithosphere enter a subduction zone, they may be scraped off at shallow levels, subducted to depths of up to 100-200 km and then exhumed as high pressure (HP) and ultra-high pressure (UHP) rocks, or subducted and recycled in the mantle. To investigate the factors that influence the behaviour of subducting slivers of continental material, we use 3-D dynamically consistent laboratory models. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. A sliver of continental material, also composed of silicone putty, overlies the subducting lithosphere, separated by a syrup detachment. The density of the sliver, viscosity of the detachment, geometry of the subducting system (attached plate versus free ridge) and dimensions of the sliver are varied in 34 experiments. By varying the density of the sliver and viscosity of the detachment, we can reproduce a range of sliver behaviour, including subduction, subduction and exhumation from various depths and offscraping. Sliver subduction and exhumation requires sufficient sliver buoyancy and a detachment that is strong enough to hold the sliver during initial subduction, but weak enough to allow adequate sliver displacement or detachment for exhumation. Changes to the system geometry alter the slab dip, subduction velocity, pattern of mantle flow and amount of rollback. Shallower slab dips with more trench rollback produce a mantle flow pattern that aids exhumation. Steeper slab dips allow more buoyancy force to be directed in the up-dip direction of the plane of the plate, and aide exhumation of subducted slivers. Slower subduction can also aide exhumation, but if slab dip is too steep or subduction too slow, the sliver will subduct to only shallow levels and not exhume. Smaller slivers are most easily subducted and exhumed and influenced by the mantle flow.

  10. Geochemical constraints on possible subduction components in lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

    USGS Publications Warehouse

    Castillo, P.R.; Newhall, C.G.

    2004-01-01

    Mayon is the most active volcano along the east margin of southern Luzon, Philippines. Petrographic and major element data indicate that Mayon has produced a basaltic to andesitic lava series by fractional crystallization and magma mixing. Trace element data indicate that the parental basalts came from a heterogeneous mantle source. The unmodified composition of the mantle wedge is similar to that beneath the Indian Ocean. To this mantle was added a subduction component consisting of melt from subducted pelagic sediment and aqueous fluid dehydrated from the subducted basaltic crust. Lavas from the highly active Taal Volcano on the west margin of southern Luzon are compositionally more variable than Mayon lavas. Taal lavas also originated from a mantle wedge metasomatized by aqueous fluid dehydrated from the subducted basaltic crust and melt plus fluid derived from the subducted terrigenous sediment. More sediment is involved in the generation of Taal lavas. Lead isotopes argue against crustal contamination. Some heterogeneity of the unmodified mantle wedge and differences in whether the sediment signature is transferred into the lava source through an aqueous fluid or melt phase are needed to explain the regional compositional variation of Philippine arc lavas. ?? Oxford University Press 2004; all rights reserved.

  11. Subduction and interplate seismicity. What have we learnt with the EURYI project?

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Corbi, F.; Heuret, A.; van Dinther, Y.; Presti, D.; Sandri, L.; Marzocchi, W.; Moroni, M.; Gerya, T.; Mai, P. M.; Dalguer, L. A.; Piromallo, C.; Lallemand, S.; Conrad, C. P.; Ranalli, G.; Faccenna, C.

    2012-12-01

    Subduction zones are the site of the largest and most dangerous seismic events occurring on the Earth. The interface of converging plates is comprised of major fault zones where most of the total seismic moment is released due to the global seismicity. Despite first order similarities between convergent margins, the statistical distribution of interplate seismic activity on different subduction zones is considerably diverse: some regions are characterized by the occurrence of megathrust earthquakes while others show only minor seismic activity,with moderate-sized events. Determining the causes of this variability is challenging. While essential features characterizing the behavior of the subduction thrust faults are known, it is still difficult to merge them in a single, comprehensive picture. This is mainly related to the lack of direct observables (i.e. subduction thrust faults are not readily accessible developing in the deeper crust, in the offshore domain) and to a short (i.e. limited to the last century) instrumental seismic record. Here we present the main results of an interdisciplinary study realized in the framework of the five-years ESF (European Science Foundation) - EURYI project 'Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modeling'. This project aimed to improve the current understanding of the physics of subduction zone thrust fault earthquakes, analyzing and gathering a) a wide range of geometric, kinematic and seismological data on current subduction zones and related statistical analysis and b) results of innovative laboratory and numerical models. In particular, our results allowed us to identify interesting features characterizing interplate earthquakes, as; 1) seismogenic zone geometry; 2) seismogenic zone activity; 3) relationship between subduction thrust fault parameters and subduction parameters; 4) conditions for megaearthquake genesis; 5) where interplate earthauqkes are more

  12. Five years of EURYI project on subduction and interplate seismicity: What have we learnt?

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Corbi, F.; Heuret, A.; van Dinther, Y.; Presti, D.; Sandri, L.; Marzocchi, W.; Moroni, M.; Gerya, T.; May, M.; Dalguer, L.; Piromallo, C.; Lallemand, S.; Conrad, C. P.; Acocella, V.; Ranalli, G.; Faccenna, C.

    2012-04-01

    Subduction zones are the site of the largest and most dangerous seismic events occurring on the Earth. The interface of converging plates is comprised of major fault zones where most of the total seismic moment is released due to the global seismicity. Despite first order similarities between convergent margins, the statistical distribution of interplate seismic activity on different subduction zones is considerably diverse: some regions are characterized by the occurrence of megathrust earthquakes while others show only minor seismic activity, with moderate-sized events. Determining the causes of this variability is challenging. While essential features characterizing the behavior of the subduction thrust faults are known, it is still difficult to merge them in a single, comprehensive picture. This is mainly related to the lack of direct observables (i.e. subduction thrust faults are not readily accessible developing in the deeper crust, in the offshore domain) and to a short (i.e. limited to the last century) instrumental seismic record. Here we present the main results of an interdisciplinary study realized in the framework of the ESF (European Science Foundation) - EURYI project 'Convergent margins and seismogenesis: defining the risk of great earthquakes by using statistical data and modeling'. This project aims to improve the current understanding of the physics of subduction zone thrust fault earthquakes, analyzing and gathering a) a wide range of geometric, kinematic and seismological data on current subduction zones and related statistical analysis and b) results of innovative laboratory and numerical models. In particular, our results allow us to identify interesting features characterizing interplate earthquakes, as; 1) seismogenic zone geometry; 2) seismogenic zone activity; 3) relationship between subduction thrust fault parameters and subduction parameters; 4) conditions for megaearthquake genesis; 5) where interplate earthauqkes are more likely to

  13. Cenozoic tectonic evolution of the Bohai Bay Basin and its coupling relationship with Pacific Plate subduction

    NASA Astrophysics Data System (ADS)

    Liang, Jintong; Wang, Hongliang; Bai, Ying; Ji, Xinyuan; Duo, Xuemei

    2016-09-01

    The Bohai Bay Basin is a Mesozoic-Cenozoic rift basin in eastern China. Based mainly on a balanced-section analysis, this study compares the spatio-temporal differences of tectonic evolution in relation to strike-slip faults among different depressions within the basin. In combination with the analysis of subsidence characteristics, the study also attempts to clarify the Cenozoic tectonic evolution of the basin and its coupling relationship with the subduction of the Pacific Plate. It was found that: (1) the strike-slip faults were activated generally from south to north and from west to east during the Cenozoic; (2) there is a negative correlation between the intensity of tectonic activity in the Bohai Bay Basin and subduction rate of the Pacific Plate; and (3) the migration direction of the basin depocenters is consistent with the direction of Pacific Plate subduction.

  14. Fluid-assisted deformation of the subduction interface: Coupled and decoupled regimes from 2-D hydromechanical modeling

    NASA Astrophysics Data System (ADS)

    Zheng, Liang; May, Dave; Gerya, Taras; Bostock, Michael

    2016-08-01

    Shear deformation, accompanied with fluid activity inside the subduction interface, is related to many tectonic energy-releasing events, including regular and slow earthquakes. We have numerically examined the fluid-rock interactions inside a deforming subduction interface using state-of-the-art 2-D hydromechanical numerical models, which incorporate the rock fracturing behavior as a plastic rheology which is dependent on the pore fluid pressure. Our modeling results suggest that two typical dynamical regimes of the deforming subduction interface exist, namely, a "coupled" and a "decoupled" regime. In the coupled regime the subduction interface is subdivided into multiple rigid blocks, each separated by a narrow shear zone inclined at an angle of 15-20° with respect to the slab surface. In contrast, in the decoupled regime the subduction interface is divided into two distinct layers moving relative to each other along a pervasive slab surface-parallel shear zone. Through a systematic parameter study, we observe that the tensile strength (cohesion) of the material within the subduction interface dictates the resulting style of deformation within the interface: high cohesion (~60 MPa) results in the coupled regime, while low cohesion (~10 MPa) leads to the decoupled regime. We also demonstrate that the lithostatic pressure and inflow/outflow fluid fluxes (i.e., fluid-fluxed boundary condition) influence the location and orientation of faults. Predictions from our numerical models are supported by experimental laboratory studies, geological data, and geophysical observations from modern subduction settings.

  15. Stress orientations in subduction zones and the strength of subduction megathrust faults

    USGS Publications Warehouse

    Hardebeck, Jeanne L.

    2015-01-01

    Subduction zone megathrust faults produce most of the world’s largest earthquakes. Although the physical properties of these faults are difficult to observe directly, their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them. A global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward, consistently making a 45°-60° angle to the subduction megathrust fault. These angles indicate that the megathrust fault is not substantially weaker than its surroundings. Together with several other lines of evidence, this implies that subduction zone megathrusts are weak faults in a low-stress environment. The deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface.

  16. Earth's oldest mantle fabrics indicate Eoarchaean subduction.

    PubMed

    Kaczmarek, Mary-Alix; Reddy, Steven M; Nutman, Allen P; Friend, Clark R L; Bennett, Vickie C

    2016-01-01

    The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics. PMID:26879892

  17. Earth's oldest mantle fabrics indicate Eoarchaean subduction

    NASA Astrophysics Data System (ADS)

    Kaczmarek, Mary-Alix; Reddy, Steven M.; Nutman, Allen P.; Friend, Clark R. L.; Bennett, Vickie C.

    2016-02-01

    The extension of subduction processes into the Eoarchaean era (4.0-3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ~3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics.

  18. Subduction and volatile recycling in Earth's mantle

    NASA Technical Reports Server (NTRS)

    King, S. D.; Ita, J. J.; Staudigel, H.

    1994-01-01

    The subduction of water and other volatiles into the mantle from oceanic sediments and altered oceanic crust is the major source of volatile recycling in the mantle. Until now, the geotherms that have been used to estimate the amount of volatiles that are recycled at subduction zones have been produced using the hypothesis that the slab is rigid and undergoes no internal deformation. On the other hand, most fluid dynamical mantle flow calculations assume that the slab has no greater strength than the surrounding mantle. Both of these views are inconsistent with laboratory work on the deformation of mantle minerals at high pressures. We consider the effects of the strength of the slab using two-dimensional calculations of a slab-like thermal downwelling with an endothermic phase change. Because the rheology and composition of subducting slabs are uncertain, we consider a range of Clapeyron slopes which bound current laboratory estimates of the spinel to perovskite plus magnesiowustite phase transition and simple temperature-dependent rheologies based on an Arrhenius law diffusion mechanism. In uniform viscosity convection models, subducted material piles up above the phase change until the pile becomes gravitationally unstable and sinks into the lower mantle (the avalanche). Strong slabs moderate the 'catastrophic' effects of the instabilities seen in many constant-viscosity convection calculations; however, even in the strongest slabs we consider, there is some retardation of the slab descent due to the presence of the phase change.

  19. The Seismic Coupling of Subduction Zones Revisited

    NASA Astrophysics Data System (ADS)

    Scholz, C.; Campos, J.

    2012-04-01

    The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the results using the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20-30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos [1995]. Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation.

  20. The seismic coupling of subduction zones revisited

    NASA Astrophysics Data System (ADS)

    Scholz, Christopher H.; Campos, Jaime

    2012-05-01

    The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20-30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos (1995). Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation.

  1. Earth's oldest mantle fabrics indicate Eoarchaean subduction

    PubMed Central

    Kaczmarek, Mary-Alix; Reddy, Steven M.; Nutman, Allen P.; Friend, Clark R. L.; Bennett, Vickie C.

    2016-01-01

    The extension of subduction processes into the Eoarchaean era (4.0–3.6 Ga) is controversial. The oldest reported terrestrial olivine, from two dunite lenses within the ∼3,720 Ma Isua supracrustal belt in Greenland, record a shape-preferred orientation of olivine crystals defining a weak foliation and a well-defined lattice-preferred orientation (LPO). [001] parallel to the maximum finite elongation direction and (010) perpendicular to the foliation plane define a B-type LPO. In the modern Earth such fabrics are associated with deformation of mantle rocks in the hanging wall of subduction systems; an interpretation supported by experiments. Here we show that the presence of B-type fabrics in the studied Isua dunites is consistent with a mantle origin and a supra-subduction mantle wedge setting, the latter supported by compositional data from nearby mafic rocks. Our results provide independent microstructural data consistent with the operation of Eoarchaean subduction and indicate that microstructural analyses of ancient ultramafic rocks provide a valuable record of Archaean geodynamics. PMID:26879892

  2. 3D Numerical simulations of oblique subduction

    NASA Astrophysics Data System (ADS)

    Malatesta, C.; Gerya, T.; Scambelluri, M.; Crispini, L.; Federico, L.; Capponi, G.

    2012-04-01

    In the past 2D numerical studies (e.g. Gerya et al., 2002; Gorczyk et al., 2007; Malatesta et al., 2012) provided evidence that during intraoceanic subduction a serpentinite channel forms above the downgoing plate. This channel forms as a result of hydration of the mantle wedge by uprising slab-fluids. Rocks buried at high depths are finally exhumed within this buoyant low-viscosity medium. Convergence rate in these 2D models was described by a trench-normal component of velocity. Several present and past subduction zones worldwide are however driven by oblique convergence between the plates, where trench-normal motion of the subducting slab is coupled with trench-parallel displacement of the plates. Can the exhumation mechanism and the exhumation rates of high-pressure rocks be affected by the shear component of subduction? And how uprise of these rocks can vary along the plate margin? We tried to address these questions performing 3D numerical models that simulate an intraoceanic oblique subduction. The models are based on thermo-mechanical equations that are solved with finite differences method and marker-in-cell techniques combined with multigrid approach (Gerya, 2010). In most of the models a narrow oceanic basin (500 km-wide) surrounded by continental margins is depicted. The basin is floored by either layered or heterogeneous oceanic lithosphere with gabbro as discrete bodies in serpentinized peridotite and a basaltic layer on the top. A weak zone in the mantle is prescribed to control the location of subduction initiation and therefore the plate margins geometry. Finally, addition of a third dimension in the simulations allowed us to test the role of different plate margin geometries on oblique subduction dynamics. In particular in each model we modified the dip angle of the weak zone and its "lateral" geometry (e.g. continuous, segmented). We consider "continuous" weak zones either parallel or increasingly moving away from the continental margins

  3. 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.

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

    USGS Publications Warehouse

    Hayes, G.P.; Wald, D.J.; Johnson, R.L.

    2012-01-01

    We describe and present a new model of global subduction zone geometries, called Slab1.0. An extension of previous efforts to constrain the two-dimensional non-planar geometry of subduction zones around the focus of large earthquakes, Slab1.0 describes the detailed, non-planar, three-dimensional geometry of approximately 85% of subduction zones worldwide. While the model focuses on the detailed form of each slab from their trenches through the seismogenic zone, where it combines data sets from active source and passive seismology, it also continues to the limits of their seismic extent in the upper-mid mantle, providing a uniform approach to the definition of the entire seismically active slab geometry. Examples are shown for two well-constrained global locations; models for many other regions are available and can be freely downloaded in several formats from our new Slab1.0 website, http://on.doi.gov/ d9ARbS. We describe improvements in our two-dimensional geometry constraint inversion, including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking, derived from the interpolation between other active source seismic data along-strike in the same subduction zone. We include several analyses of the uncertainty and robustness of our three-dimensional interpolation methods. In addition, we use the filtered, subduction-related earthquake data sets compiled to build Slab1.0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far, concluding that the width of these seismogenic zones is on average 30% larger than previous studies have suggested. Copyright 2011 by the American Geophysical Union.

  5. 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.

  6. Neotectonics of a subduction/strike-slip transition: the northeastern Dominican Republic

    SciTech Connect

    Winslow, M.A.; McCann, W.R.

    1985-01-01

    The Septentrional fault system in the northeastern Dominican Republic marks the zone where the North American-Carribean plate boundary is evolving from subduction to strike-slip motion, and where terranes appear to be forming and migrating laterally in a subduction complex/forearc region. On the Island of Hispaniola, slip vectors are oblique to the strike of the Puerto Rico trench, and oblique subduction thrusts the upper plate over normal seafloor. The offshore geology and seismicity of the northern Caribbean suggest that uplift, broad crustal warping, thrusting, and strike-slip faulting (ie. collisional tectonics) should be present in the northern part of the Dominican Republic. The high topography (>1000m), high levels of seismicity, and large earthquakes support the hypothesis of contemporary deformation in Hispaniola. In this region, the subduction regime dies out toward the west, and deformation is transferred to onshore, oblique-slip faults. As this change in tectonic style has occurred in Neogene to Recent times, we are investigating the modern evolution of a plate boundary. We have already documented: (1) the presence of a strike-slip faulting in the northeastern Dominican Republic; (2) an anomalous push-up structure; and (3) a region of numerous splay faults. In conclusion, recent seismicity suggest a wide zone of deformation and variations in interplate motions near Hispaniola. This island lies at the western limit of active underthrusting and at the eastern limit of onshore faulting, i.e., at an important transition from a subduction to strike-slip regime.

  7. 3D Finite-Difference Modeling of Scattered Teleseismic Wavefields in a Subduction Zone

    NASA Astrophysics Data System (ADS)

    Morozov, I. B.; Zheng, H.

    2005-12-01

    For a teleseismic array targeting subducting crust in a zone of active subduction, scattering from the zone underlying the trench result in subhorizontally-propagating waves that could be difficult to distinguish from converted P- and S- wave backscattered from the surface. Because back-scattered modes often provide the most spectacular images of subducting slabs, it is important to understand their differences from the arrivals scattered from the trench zone. To investigate the detailed teleseismic wavefield in a subduction zone environment, we performed a full-waveform, 3-D visco-elastic finite-difference modeling of teleseismic wave propagation using a Beowulf cluster. The synthetics show strong scattering from the trench zone, dominated by the mantle and crustal P-waves propagating at 6.2-8.1.km/s and slower. These scattered waves occupy the same time and moveout intervals as the backscattered modes, and also have similar amplitudes. Although their amplitude decay characters are different, with the uncertainties in the velocity and density structure of the subduction zone, unambiguous distinguishing of these modes appears difficult. However, under minimal assumptions (in particular, without invoking slab dehydration), recent observations of receiver function amplitudes decreasing away from the trench favor the interpretation of trench-zone scattering.

  8. Mapping mantle flow during retreating subduction: Laboratory models analyzed by feature tracking

    NASA Astrophysics Data System (ADS)

    Funiciello, F.; Moroni, M.; Piromallo, C.; Faccenna, C.; Cenedese, A.; Bui, H. A.

    2006-03-01

    Three-dimensional dynamically consistent laboratory models are carried out to model the large-scale mantle circulation induced by subduction of a laterally migrating slab. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. The circulation pattern is continuously monitored and quantitatively estimated using a feature tracking image analysis technique. The effects of plate width and mantle viscosity/density on mantle circulation are systematically considered. The experiments show that rollback subduction generates a complex three-dimensional time-dependent mantle circulation pattern characterized by the presence of two distinct components: the poloidal and the toroidal circulation. The poloidal component is the answer to the viscous coupling between the slab motion and the mantle, while the toroidal one is produced by lateral slab migration. Spatial and temporal features of mantle circulation are carefully analyzed. These models show that (1) poloidal and toroidal mantle circulation are both active since the beginning of the subduction process, (2) mantle circulation is intermittent, (3) plate width affects the velocity and the dimension of subduction induced mantle circulation area, and (4) mantle flow in subduction zones cannot be correctly described by models assuming a two-dimensional steady state process. We show that the intermittent toroidal component of mantle circulation, missed in those models, plays a crucial role in modifying the geometry and the efficiency of the poloidal component.

  9. A 2-D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey

    NASA Astrophysics Data System (ADS)

    Horning, G.; Canales, J. P.; Carbotte, S. M.; Han, S.; Carton, H.; Nedimović, M. R.; Keken, P. E.

    2016-08-01

    We report results from a wide-angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two-dimensional velocity model of the crust and upper mantle is derived from a joint reflection-refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages >1 Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5-2.1 wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8 wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (<0.3 wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.

  10. Subduction zone structures and slip behavior in megathrust

    NASA Astrophysics Data System (ADS)

    Kodaira, S.; Nakanishi, A.; Nakamura, Y.

    2014-12-01

    Earthquake, tsunami and geodetic data show that co-seismic slips of a large megathrust earthquakes do not uniformly propagate along a plate boundary. For example, a clear segmentation of slip zones of magnitude-8 class megathrust earthquakes are well recognized in the Nankai Trough. Moreover, a lateral variation of the slips are revealed even in one segment. In the Japan Trench, the most characteristic slip behavior of the 2011 Tohoku-oki earthquake is an extremely large slip reaching to the trench axis, but geodetic, tsunami or teleseismic show the slip was heterogeneous near the trench. In order to examine whether those complex slip distributions are attributed by any distinct structural factor, we have been carried out active-source seismic surveys in the subduction seismogenic. In the Nankai Trough, large-scale subducted seamounts, ridges and doming structure intruded in an overriding accretion wedge are imaged. Comparing co-seismic slip distribution of the 1944 Tonankai and the 1946 Nankai earthquakes with the seismic images, we concluded that those structures are key factors to control the slip distributions. In the central part of the Japan Trench area, we fund a rough basement geometry is overprinted on the horst-and-graben structure. Those complex geometry of basement cause a strong lateral variation of the thickness of subducting pelagic/hemi-pelagic sediment. Many geological studies suggest that properties of the plate-boundary sediment attribute the large slip near the trench. We therefore plan to acquire additional high-resolution seismic data in the entire Japan Trench in order to examine a role of incoming sediment on the large slip to the trench axis. In this presentation we present an overview of the structural factors controlling slips in megathrust earthquakes, including new data acquired in the Nankai Trough and Japan Trench.

  11. Ridge Subduction Beneath the Americas: Synthesis and New Research on Anomalous Tectonism and Magmatism

    NASA Astrophysics Data System (ADS)

    Thorkelson, D. J.; Madsen, J. K.; Breitsprecher, K.; Groome, W. G.; Sluggett, C.

    2006-12-01

    The west coast of the Americas has been repeatedly affected by ridge-trench interactions from Mesozoic to Recent time. Beneath North America, subduction of the Kula-Farallon, Kula-Resurrection and Farallon- Resurrection spreading ridges resulted in anomalous and time-transgressive forearc to backarc magmatism and related tectonism from the Late Cretaceous to the Eocene. Following consumption and redistribution of the Kula and Resurrection plates, the Neogene Farallon-Pacific ridge system intersected the North American trench in two locations - western Canada and northwestern Mexico / southwestern United States - causing pronounced magmatic and tectonic effects that continue to the present. Beneath Central America, divergent subduction of the Nazca and Cocos plates led to development of a slab window, with a present location beneath Panama and a probable pre-Pliocene position beneath Columbia or Ecuador. Patagonia has been the site of localized ridge subduction from the Eocene to the Recent, with the Phoenix-Farallon ridge subducting from the Eocene to the early Miocene, and the Nazca-Antarctic ridge from the Miocene to the present. Antarctica experienced diverging Antarctic-Phoenix plate subduction from the Eocene to the Pliocene. In all cases, normal arc magmatism was interrupted or eliminated by anomalous igneous activity ranging in signature from adakitic to intraplate. Our current research involves geochemical, tectonic, and thermal modeling of slab window environments. A new geochemical analysis on the effects of Miocene to Recent subduction of the northern segment of the Farallon (Juan de Fuca)-Pacific ridge is underway. A symmetrical arc-intraplate-arc geochemical pattern is evident in a transect from the northern Cascade Arc, through the volcanic fields of British Columbia, Yukon and eastern Alaska, and into the Aleutian Arc. This pattern can be explained by Neogene displacement of the arc-metasomatized mantle wedge caused by upwelling oceanic

  12. Development of common conversion point stacking of receiver functions for detecting subducted slabs

    NASA Astrophysics Data System (ADS)

    Abe, Y.; Ohkura, T.; Hirahara, K.; Shibutani, T.

    2010-12-01

    In subduction zones, the subducting slabs are thought to convey fluid into the mantle wedge to cause arc volcanism (Hasegawa et al., 2008. Iwamori, 2007). Kawakatsu & Watada (2007) examined the Pacific slab subducting beneath northeast Japan with receiver function (RF) analysis, and revealed where the hydrated oceanic crust and the serpentinized mantle wedge exist. In the other subduction zones, it is also essential to examine subducting slabs for better understanding of water transportation and volcanic activities. In this study, we develop a new method to migrate RFs in order to examine subducting slabs with high dip angle (Abe et al., submitted to GJI) and apply this method to examination of the Philippine Sea slab (PHS). The RF technique is one of the useful methods to obtain seismic velocity discontinuities. Ps phases converted at discontinuities in a teleseismic coda can be detected by RF analysis. RFs are usually converted to depth domain assuming a 1-d velocity structure, and the geometry of discontinuities is obtained (e.g. Yamauchi et al., 2003). In subduction zones, however, subducting slabs usually dip, and we should take into account the refraction of seismic waves at dipping interfaces. Therefore, we use the multi-stage fast marching method (FMM, de Kool et al., 2006) to convert RFs into depth domain. We stack transverse RFs, since polarity of them does not change depending on their dip angles and they are better at detecting phases converted at dipping interfaces than radial RFs. We have confirmed that this method works properly with synthetic test. We apply our method to waveform data observed in Kyushu, Japan, where PHS is subducting toward WNW and the Wadati-Benioff zone dips at 30° at depths up to 80 km, and dips at 70° at depths between 80 km and 170 km. We obtain a vertical section, on which RF amplitude is projected, across central part of Kyushu perpendicular to the depth contour of the Wadati-Benioff zone. On the section, positive peaks of

  13. Seismotectonics of the southern boundary of Anatolia, Eastern Mediterranean region: subduction, collision, and arc jumping

    SciTech Connect

    Rotstein, Y.; Kafka, A.L.

    1982-09-10

    The pattern of seismicity and fault plane solutions of earthquakes are used to outline the tectonic features of the southern boundary of Anatolia in the eastern Mediterranean and southeastern Turkey. The results of this study show that this boundary is composed of two distinct parts. One, in southeastern Turkey and Syria, is a wide and complex zone of continental collision. The other, in the Levantine basin of the eastern Mediterranean, is a zone of oceanic subduction. In the region of continental collision three zones of seismicity are observed. Most of the seismic activity in this region follows the Bitlis zone and is associated with a zone of thrusting and mountain building. This appears to be the zone of most active deformation and plate consumption in the plate boundary region between Arabia and Turkey. A less active zone of seismicity to the north of the Bitlis zone is interpreted to have been more active in the past whereas another active zone of seismicity to the south is interpreted to be a zone which may be more active in the future as the main zone of plate consumption jumps to the south. In the subduction zone of the eastern Mediterranean the depth of the subducted slab and the rate of seismicity generally increease from east to west. The zone of present-day convergence between Africa and Turkey in the Levantine basin can be best outlined by the northern edge of the Mediterranean ridge. Deep seismic activity near the Gulf of Antalya is associated with a detached subducted slab north of the Anaximander Mountains that is distinctly different from the seismic trend which is associated with present-day active subduction. Most of the focal mechanisms of the earthquakes along the entire southern boundary of Anatolia indicate that N to NNW thrusting is the dominant mode of seismic deformation.

  14. Formation of the Red Hills Ultramafic Massif during Subduction Initiation along an Oceanic Transform Fault

    NASA Astrophysics Data System (ADS)

    Tikoff, B.; Stewart, E. D.; Newman, J.; Lamb, W. M.

    2015-12-01

    The Red Hills ultramafic massif in the South Island, New Zealand, is part of the Dun Mountain Ophiolite Belt (DMOB). The DMOB was created at the onset of subduction in a forearc setting in the Middle Permian, and it likely formed immediately prior to the establishment of a magmatic arc along the New Zealand and Australian portions of the Gondwanan margin. The Red Hills ultramafic massif records a two-stage history of high temperature mantle flow during subduction initiation along the Gondwanan margin. Initial deformation was homogeneous and fabrics are constrictional. Kilometer-scale deformation zones, part of the second stage of deformation, overprinted the early homogeneous fabric throughout the western portion of the massif. Timing of all high-temperature mantle deformation in the Red Hills was between 285 and 274 Ma during subduction initiation based on the earliest ages of igneous activity in adjacent volcanic rocks, and a new U-Pb zircon age of 274.55±0.43 Ma from a cross-cutting dike. We present a kinematic model to explain the occurrence of the constructional fabrics during subduction initiation, and find that the three-dimensional boundary conditions for deformation in the incipient mantle wedge must have been transtensional, with a dominant trench-parallel component of motion. Such a scenario indicates subduction likely initiated along an active oceanic transform fault. We test this model by kinematically restoring the Red Hills ultramafics to their Permian orientation, and find the consistent elongation direction of the constructional fabrics was oriented nearly parallel to the trench. Stage 2 deformation zones were variably oriented, but all accommodated normal motion. These results support a model where the incipient mantle wedge was undergoing highly oblique transtension, and the lack of evidence for contraction suggests the onset of subduction along the Permian margin of New Zealand occurred along a transform fault due to spontaneous, density driven

  15. A geodynamical perspective on the subduction of Cocos and Rivera plates beneath Mexico and Central America

    NASA Astrophysics Data System (ADS)

    Manea, V. C.; Manea, M.; Ferrari, L.

    2013-12-01

    The Middle America subduction zone (MASZ) is one of the world’ most complex convergent margins as it involves the subduction of the Rivera and Cocos young oceanic plates beneath the North American and Caribbean plates and is bounded by the Gulf of California rift and the Panama slab window. Characterized by contorted and unusual slab geometry, irregularly distributed seismicity and volcanism, exceptionally large slow slip events (SSE) and non-volcanic tremors (NVT), this subduction system represents a great natural laboratory for better understanding geodynamic processes at a fundamental level. Based on a solid observational foundation, and incorporating the latest experimental results into a coherent geodynamical framework, we shed light on the main processes controlling the subduction system evolution in this region. The tectonics, volcanism, slab geometry and segmentation along the margin are reviewed from a geodynamical perspective. We proposed and discussed a series of evolutionary scenarios for the Mexican and Central American subduction zones, providing a coherent starting base for future geodynamical modeling studies tailored to this active margin. We discuss comparatively the recently discovered SSEs and NVTs along the MASZ, and try to differentiate among the proposed mechanisms responsible for these observations. Finally we discuss the recent seismic anisotropy observations in a geodynamic context, offering an integrated view of mantle flow pattern along the entire active margin. Although the MASZ as a whole may be considered a fairly complicated region with many unusual features and sometimes controversial interpretations, its complexity and unusual characteristics can improve our knowledge about the linkage between deep and surface processes associated with subduction zone dynamics.

  16. Initiation of the Fiordland-Puysegur subduction zone, New Zealand

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The Australia-Pacific plate boundary south of New Zealand was an active ridge 45 Ma to 30 Ma, generating oceanic crust between the Resolution Rifted Margin and the Campbell Rifted Margin. Referred to as the Macquarie Ridge Complex (MRC), this boundary progressively evolved into a strike-slip boundary from 30 Ma to 20 Ma; the northern segment, the Fiordland-Puysegur subduction zone (FPSZ), has had a substantial transpressional component. Over the last 20 Myr, 600 km of highly oblique plate motion occurred at the MRC, and resulted in a maximum total convergence of 150-200 km at the FPSZ, which some simple models suggested might be near the threshold for a self-sustaining subduction. The morphology of the Puysegur Ridge shows a diagnostic change from uplift to subsidence expected for the transition of a subduction zone from being forced externally to being internally driven by the negative buoyancy of the slab. The large negative gravity anomalies over the Snares Zone, in the middle of the FPSZ, imply strong vertical forces pulling downward the lithosphere. To better understand these observations, we use a viscous flow forward model with a free surface to simulate the geodynamics of the FPSZ since 20 Ma. The forward model describes the dynamics of an incompressible, Stokes fluid. Brittle-ductile behavior of the material within the crust-asthenosphere is modeled by using a fluid viscosity defined via a composite flow law comprised from an Arrhenius and a Drucker-Prager rheology. The well-constrained relative plate motion between the Australian and Pacific plates is used to define a Dirichlet boundary condition for velocity within the lithosphere. In the mantle, we apply the hydrostatic pressure as a normal stress boundary condition. A simplified surface process model consisting of linear diffusion is applied at the free surface to simulate short-range erosion and sedimentation. Our models show that the topographic variations within the Puysegur Ridges may correspond to

  17. 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

  18. 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

  19. Subduction zone earthquakes and stress in slabs

    NASA Technical Reports Server (NTRS)

    Vassiliou, M. S.; Hager, B. H.

    1988-01-01

    Simple viscous fluid models of subducting slabs are used to explain observations of the distribution of earthquakes as a function of depth and the orientation of stress axes of deep (greater than 300 km) and intermediate (70-300 km) earthquakes. Results suggest the following features in the distribution of earthquakes with depth: (1) an exponential decrease from shallow depths down to 250 to 300 km, (2) a minimum near 250 to 300 km, and (3) a deep peak below 300 km. Many shallow subducting slabs show only the first characteristic, while deeper extending regions tend to show all three features, with the deep peak varying in position and intensity. These data, combined with the results on the stress orientations of various-depth earthquakes, are consistent with the existence of a barrier of some sort at 670-km depth and a uniform viscosity mantle above this barrier.

  20. Subduction related fluids fractionate Nb/Ta

    NASA Astrophysics Data System (ADS)

    Salters, V. J.; Bizimis, M.; Sachi-Kocher, A.; Taylor, R.; Savov, I. P.; Stern, C. R.

    2009-12-01

    Key differences between the chemical composition of terrestrial materials and those of meteorites have led to the suggestion that a `hidden’ high Nb/Ta reservoir exists in the Earth’s mantle. In order to test this hypothesis we must identify the processes that can create such a reservoir. It has been suggested that during subduction Nb is more refractory then Ta resulting in low Nb/Ta in the subducted slab, which then serves as a reservoir for the high Nb/Ta. Here we report high precision HFSE data on products of the subduction processes thought to fractionate Nb from Ta: boninites (hydrous melting), adakites (slab melting), oceanic island arc basalts and supra subduction zone peridotites. We developed a new method for the high precision determination of Nb, Ta, Zr, Hf concentrations based on a modified version of standard addition. All analyses were performed on a single collector ICPMS (ELEMENT 1), using Y and Yb as internal standards to correct for instrumental drift during the unspiked -spiked sample sequence. Concentrations are calculated using a York- type regression that accounts for all measured and propagated errors. Long-term reproducibility (multiple dissolutions and multiple spike solutions) for the standards BHVO-1, BIR-1 AGV-1 and BCR-1 are better than 0.8% (1s) for Nb/Ta and Zr/Hf ratios. The advantages of this method compared to previous methods are fast throughput, no column chemistry and low blanks. While the Zr/Hf ratios in subduction-related volcanics and ocean island basalts vary by less than a factor of two, the Nb/Ta ratio varies by a factor of four. Most of the Nb/Ta variation is observed in subduction related rocks. Samples with the highest Nb/Ta ratio (up to 19.5) are adakites from the Austral Volcanic Zone (Andes) which are thought to represent eclogitic melts from subducted oceanic crust which was most likely dehydrated. The lowest Nb/Ta (5) was found in boninites from Chichi-Jima, Bonin Island. Samples from Chichi-Jima and from the

  1. Diamond Growth in the Subduction Factory

    NASA Astrophysics Data System (ADS)

    Bureau, H.; Frost, D. J.; Bolfan-Casanova, N.; Leroy, C.; Estève, I.

    2014-12-01

    Natural diamonds are fabulous probes of the deep Earth Interior. They are the evidence of the deep storage of volatile elements, carbon at first, but also hydrogen and chlorine trapped as hydrous fluids in inclusions. The study of diamond growth processes in the lithosphere and mantle helps for our understanding of volatile elements cycling between deep reservoirs. We know now that inclusion-bearing diamonds similar to diamonds found in nature (i.e. polycrystalline, fibrous and coated diamonds) can grow in hydrous fluids or melts (Bureau et al., GCA 77, 202-214, 2012). Therefore, we propose that the best environment to promote such diamonds is the subduction factory, where highly hydrous fluids or melts are present. When oceanic plates are subducted in the lithosphere, they carry an oceanic crust soaked with seawater. While the slabs are traveling en route to the mantle, dehydration processes generate saline fluids highly concentrated in NaCl. In the present study we have experimentally shown that diamonds can grow from the saline fluids (up to 30 g/l NaCl in water) generated in subducted slabs. We have performed multi-anvil press experiments at 6-7 GPa and from 1300 to 1400°C during 6:00 hours to 30:00 hours. We observed large areas of new diamond grown in epitaxy on pure diamond seeds in salty hydrous carbonated melts, forming coated gems. The new rims are containing multi-component primary inclusions. Detailed characterizations of the diamonds and their inclusions have been performed and will be presented. These experimental results suggest that multi-component salty fluids of supercritical nature migrate with the slabs, down to the deep mantle. Such fluids may insure the first stage of the deep Earth's volatiles cycling (C, H, halogen elements) en route to the transition zone and the lower mantle. We suggest that the subduction factory may also be a diamond factory.

  2. Subducting slabs: Jellyfishes in the Earth's mantle

    NASA Astrophysics Data System (ADS)

    Loiselet, Christelle; Braun, Jean; Husson, Laurent; Le Carlier de Veslud, Christian; Thieulot, Cedric; Yamato, Philippe; Grujic, Djordje

    2010-08-01

    The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

  3. Subducting Slabs: Jellyfishes in the Earth's Mantle

    NASA Astrophysics Data System (ADS)

    Loiselet, C.; Braun, J.; Husson, L.; Le Carlier de Veslud, C.; Thieulot, C.; Yamato, P.; Grujic, D.

    2010-12-01

    The constantly improving resolution of geophysical data, seismic tomography and seismicity in particular, shows that the lithosphere does not subduct as a slab of uniform thickness but is rather thinned in the upper mantle and thickened around the transition zone between the upper and lower mantle. This observation has traditionally been interpreted as evidence for the buckling and piling of slabs at the boundary between the upper and lower mantle, where a strong contrast in viscosity may exist and cause resistance to the penetration of slabs into the lower mantle. The distribution and character of seismicity reveal, however, that slabs undergo vertical extension in the upper mantle and compression near the transition zone. In this paper, we demonstrate that during the subduction process, the shape of low viscosity slabs (1 to 100 times more viscous than the surrounding mantle) evolves toward an inverted plume shape that we coin jellyfish. Results of a 3D numerical model show that the leading tip of slabs deform toward a rounded head skirted by lateral tentacles that emerge from the sides of the jellyfish head. The head is linked to the body of the subducting slab by a thin tail. A complete parametric study reveals that subducting slabs may achieve a variety of shapes, in good agreement with the diversity of natural slab shapes evidenced by seismic tomography. Our work also suggests that the slab to mantle viscosity ratio in the Earth is most likely to be lower than 100. However, the sensitivity of slab shapes to upper and lower mantle viscosities and densities, which remain poorly constrained by independent evidence, precludes any systematic deciphering of the observations.

  4. Internal fabric of a fossil erosive subduction channel in the Northern Apennines of Italy: implication for fluid flow and seismogenesis

    NASA Astrophysics Data System (ADS)

    Vannucchi, P.; Remitti, F.; Bettelli, G.

    2006-12-01

    In the Northern Apennines of Italy, a shallow erosive subduction channel have been formed during the Tertiary transition from subduction to collision and it has been preserved by later deactivation and fossilization of the plate boundary. Here for the first time it is possible to observe the activations of a basal and roof décollement defining an erosive subduction channel containing material detached or reworked from the upper plate. The outcropping Apennine erosive subduction channel, about 500 m thick, is representative of a portion reaching about 150°C, a critical temperature recognized in most of the subduction zones as coincident with the updip limit of seismogenesis. The onset of structural features developed in the channel testify an evolution from diffusive deformation to strain localization. In particular as the material enters the subduction channel, compaction and pervasive extensional shear fractures accommodates the longitudinal compression. The geometry and kinematic of the extensional shear fractures define a polymodal fracture pattern that in 2D cut the rock in lozenge-shaped elements. We speculate that the onset of extensional features is representative of the weak nature of erosive plate boundaries. As the material is dragged down the channel shear localization is observed, but still in the extensional strain regime. Our results indicate that this second phase of deformation was characterized by high fluid pressure. The strong dynamic interaction between fluids, shear zones and state of eroded material influence fault mechanics and have maintained a locus of broad active deformation that influenced rupture propagation within the channel. Contraction develops only as result of further strengthening caused by the complete lithification of all the components in the subduction channel and suggests locked condition. Collectively these changes indicate favorable conditions for storage of elastic energy in the subduction channel that can be released

  5. Segmentation of the Lesser Antilles subduction zone

    NASA Astrophysics Data System (ADS)

    Wadge, G.; Shepherd, J. B.

    1984-12-01

    Precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the Benioff zone beneath the Lesser Antilles island arc. There are two distinct segments to the zone: one to the north of Martinique which trends NNW and one to the south of St. Lucia which trends NNE. The northern zone dips at 60-50°. The southern zone has a dip of 50-45° in the north but is vertical in the south. Pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the Benioff zone. Mid-Miocene swarms of dykes in Martinique and St. Lucia have NW and NE azimuthal modes respectively. We interpret this to mean that this kink in the subduction zone existed as long ago as the Miocene. The kink may represent the deformation of a single subducting plate or two separate North and South American plates subducting beneath the Caribbean plate with a triple junction in the region of Martinique-St. Lucia.

  6. Continental aggregation, subduction initiation, and plume generation

    NASA Astrophysics Data System (ADS)

    Heron, P. J.; Lowman, J. P.

    2013-12-01

    Several processes unfold during the supercontinent cycle, more than one of which might result in an elevation in subcontinental mantle temperatures through the generation of mantle plumes. Paleogeographic plate reconstructions have indicated that sub-continental mantle upwellings appear below large continents that are extensively ringed by subduction zones. Moreover, several numerical simulations of supercontinent formation and dispersal attribute the genesis of sub-continental plumes to the generation of subduction zones on the edges of the supercontinent, rather than resulting from continental insulation. However, the role of the location of downwellings in producing a return-flow upwelling, and on increasing sub-continental mantle temperatures, is not fully understood. In this mantle convection study, we examine the evolution of mantle dynamics after supercontinent accretion over a subduction zone (analogous to the formation of Pangea) for a range of continental coverage. We present 2D and 3D Cartesian geometry mantle convection simulations, featuring geotherm- and pressure-dependent viscosity with thermally and mechanically distinct oceanic and continental plates. Through changing the size of the continent we are able to analyze the factors involved in the generation of mantle plumes in purely thermal convection. Furthermore, we change the upper and lower mantle viscosity to determine their relation to plume formation in vigorous mantle convection simulations. Elevated sub-continental temperatures are analyzed in relation to continental coverage to further understand the influence of continental tectonics on the thermal evolution of the mantle.

  7. 3-D numerical modeling of plume-induced subduction initiation

    NASA Astrophysics Data System (ADS)

    Baes, Marzieh; Gerya, taras; Sobolev, Stephan

    2016-04-01

    Investigation of mechanisms involved in formation of a new subduction zone can help us to better understand plate tectonics. Despite numerous previous studies, it is still unclear how and where an old oceanic plate starts to subduct beneath the other plate. One of the proposed scenarios for nucleation of subduction is plume-induced subduction initiation, which was investigated in detail, using 2-D models, by Ueda et al. (2008). Recently. Gerya et al. (2015), using 3D numerical models, proposed that plume-lithosphere interaction in the Archean led to the subduction initiation and onset of plate tectonic. In this study, we aim to pursue work of Ueda et al. (2008) by incorporation of 3-D thermo-mechanical models to investigate conditions leading to oceanic subduction initiation as a result of thermal-chemical mantle plume-lithosphere interaction in the modern earth. Results of our experiments show four different deformation regimes in response to plume-lithosphere interaction, that are a) self-sustaining subduction initiation where subduction becomes self-sustained, b) freezing subduction initiation where subduction stops at shallow depths, c) slab break-off where subducting circular slab breaks off soon after formation and d) plume underplating where plume does not pass through the lithosphere but spreads beneath it (failed subduction initiation). These different regimes depend on several parameters such as plume's size, composition and temperature, lithospheric brittle/plastic strength, age of the oceanic lithosphere and presence/absence of lithospheric heterogeneities. Results show that subduction initiates and becomes self-sustained when lithosphere is older than 10 Myr and non-dimensional ratio of the plume buoyancy force and lithospheric strength above the plume is higher than 2.

  8. 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.

  9. Seismicity of the Subducted Caribbean Plate in Panama

    NASA Astrophysics Data System (ADS)

    Camacho, E.; Pacheco, J. F.; Hutton, W.

    2007-05-01

    The Panama microplate is bounded by convergent boundaries to the north (North Panama deformed belt; NPDB) and south (South Panama deformed belt; SPDB and Middle American Trench; MAT), suturing with South America along the Atrato valley to the east and the Central Costa Rica deformed belt (CCRDB) to the west. GPS measured convergence rates between the NPDB and the Caribbean plate are to the southwest at 7 mm/a. Seismicity of the portion of the NPDB along the isthmus of Panama is poorly understood, even though the largest recorded earthquake in Panama (7 September, 1882 (M7.9)) occurred on this boundary. This earthquake caused extensive damage in central Panama and produced a tsunami that killed 75 people in Comarca de San Blas. A preliminary study of seismicity in the area, using high quality seismic data recorded by local stations between 1996-2006, suggests that most events were of intermediate depth (40-70km) with a geometry indicative of a subducted plate. We also note that focal mechanisms show a complex stress field in this area possibly due to activation of pre-existing faults and internal deformation of the Caribbean plate being subducted. The existence of a well-defined Wadati-Bennioff zone below northern Panama should be included in future risk assessment evaluations.

  10. 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.

  11. Subduction zone guided waves in Northern Chile

    NASA Astrophysics Data System (ADS)

    Garth, Thomas; Rietbrock, Andreas

    2016-04-01

    Guided wave dispersion is observed in subduction zones as high frequency energy is retained and delayed by low velocity structure in the subducting slab, while lower frequency energy is able to travel at the faster velocities associated with the surrounding mantle material. As subduction zone guided waves spend longer interacting with the low velocity structure of the slab than any other seismic phase, they have a unique capability to resolve these low velocity structures. In Northern Chile, guided wave arrivals are clearly observed on two stations in the Chilean fore-arc on permanent stations of the IPOC network. High frequency (> 5 Hz) P-wave arrivals are delayed by approximately 2 seconds compared to the low frequency (< 2 Hz) P-wave arrivals. Full waveform finite difference modelling is used to test the low velocity slab structure that cause this P-wave dispersion. The synthetic waveforms produced by these models are compared to the recorded waveforms. Spectrograms are used to compare the relative arrival times of different frequencies, while the velocity spectra is used to constrain the relative amplitude of the arrivals. Constraining the waveform in these two ways means that the full waveform is also matched, and the low pass filtered observed and synthetic waveforms can be compared. A combined misfit between synthetic and observed waveforms is then calculated following Garth & Rietbrock (2014). Based on this misfit criterion we constrain the velocity model by using a grid search approach. Modelling the guided wave arrivals suggest that the observed dispersion cannot be solely accounted for by a single low velocity layer as suggested by previous guided wave studies. Including dipping low velocity normal fault structures in the synthetic model not only accounts for the observed strong P-wave coda, but also produces a clear first motion dispersion. We therefore propose that the lithospheric mantle of the subducting Nazca plate is highly hydrated at intermediate

  12. Magnesium Isotopic Composition of Subducting Marine Sediments

    NASA Astrophysics Data System (ADS)

    Hu, Y.; Teng, F. Z.; Plank, T. A.; Huang, K. J.

    2015-12-01

    Subducted marine sediments have recently been called upon to explain the heterogeneous Mg isotopic composition (δ26Mg, ‰) found in mantle wehrlites (-0.39 to +0.09 [1]) in the context of a homogeneous mantle (-0.25 ± 0.07 [2]). However, no systematic measurements of δ26Mg on marine sediments are currently available to provide direct support to this model. To characterize the Mg inputs to global subduction zones, we measured δ26Mg data for a total of 90 marine sediments collected from 12 drill sites outboard of the world's major subduction zones. These sediments span a 1.73‰ range in δ26Mg. The detritus-dominated sediments have δ26Mg (-0.59 to +0.53) comparable to those of weathered materials on continents (e.g. -0.52 to +0.92 [3]), while the calcareous oozes yield δ26Mg (as light as -1.20) more similar to the seawater value (-0.83 [4]). The negative correlation between δ26Mg and CaO/Al2O3 in these sediments indicates the primary control of mineralogy over the Mg isotopic distribution among different sediment types, as carbonates are enriched in light Mg isotopes (-5.10 to -0.40 [5]) whereas clay-rich weathering residues generally have heavier δ26Mg (e.g. up to +0.65 in saprolite [6]). In addition, chemical weathering and grain-size sorting drive sediments to a heavier δ26Mg, as indicated by the broad positive trends between δ26Mg with CIA (Chemical Index of Alteration [7]) and Al2O3/SiO2, respectively. Collectively, the arc systems sampled in this study represent ~30% of global arc length and the extrapolated global Mg flux of subducting marine sediments accounts for ~9% of the yearly Mg riverine input with a flux-weighted average δ26Mg at -0.26. Subduction of these heterogeneous sediments may not cause significant mantle heterogeneity on a global scale, but the highly variable Mg fluxes and δ26Mg of sediments delivered to different trenches are capable of producing local mantle variations. Volcanic rocks sourced from these mantle domains are thus

  13. Geomorphic Response to Flat Slab Subduction along the Eastern Foothills of the Colombian Andes

    NASA Astrophysics Data System (ADS)

    Veloza, G.; Taylor, M. H.; Gosse, J. C.; Mora, A.; Becker, T. W.

    2013-12-01

    It is thought that in northwest South America flat slab subduction plays a key role in the recent development of the eastern Colombian Andes. Here we show that the geomorphic response to flat slab subduction is presently occurring >500 km inboard of the subduction zone plate boundary. The Llanos basin located along the eastern edge of the Colombian Andes is experiencing active uplift along the seismically active Cusiana, Yopal, Paz de Ariporo and Tame thrust faults, which we refer to as the Llanos Foothills thrust system (LFTS). The LFTS is comprised of east-directed thrust faults that are listric in geometry with shallowly west-dipping decollements. Locally, actively growing north-south plunging folds are cored by blind thrust faults, and are being incised by antecedent east-flowing streams. Using a combination of field-based observations on the geometry of faulted and folded fluvial terraces, and geochronology from terrestrial cosmogenic nuclides, we show that the fluvial terraces have been uplifted, and locally, incised >200 meters at incision rates exceeding 3 mm/yr. The field observations in combination with earthquakes and geodynamic simulations can be reconciled by flat slab subduction, but it is presently unknown whether the flat slab has a Caribbean or Nazca plate affinity. Different geodynamic scenarios can be tested to understand how the leading edge of the flat slab interacts with the South American craton, and how that interaction controls upper crustal deformation.

  14. Velocities of Subducted Sediments and Continents

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

    The growing capability to measure seismic velocities in subduction zones has led to unusual observations. For example, although most minerals have VP/ VS ratios around 1.77, ratios <1.7 and >1.8 have been observed. Here we explore the velocities of subducted sediments and continental crust from trench to sub-arc depths using two methods. (1) Mineralogy was calculated as a function of P & T for a range of subducted sediment compositions using Perple_X, and rock velocities were calculated using the methodology of Hacker & Abers [2004]. Calculated slab-top temperatures have 3 distinct depth intervals with different dP/dT gradients that are determined by how coupling between the slab and mantle wedge is modeled. These three depth intervals show concomitant changes in VP and VS: velocities initially increase with depth, then decrease beyond the modeled decoupling depth where induced flow in the wedge causes rapid heating, and increase again at depth. Subducted limestones, composed chiefly of aragonite, show monotonic increases in VP/ VS from 1.63 to 1.72. Cherts show large jumps in VP/ VS from 1.55-1.65 to 1.75 associated with the quartz-coesite transition. Terrigenous sediments dominated by quartz and mica show similar, but more-subdued, transitions from ~1.67 to 1.78. Pelagic sediments dominated by mica and clinopyroxene show near-monotonic increases in VP/ VS from 1.74 to 1.80. Subducted continental crust that is too dry to transform to high-pressure minerals has a VP/ VS ratio of 1.68-1.70. (2) Velocity anisotropy calculations were made for the same P-T dependent mineralogies using the Christoffel equation and crystal preferred orientations measured via electron-backscatter diffraction for typical constituent phases. The calculated velocity anisotropies range from 5-30%. For quartz-rich rocks, the calculated velocities show a distinct depth dependence because crystal slip systems and CPOs change with temperature. In such rocks, the fast VP direction varies from slab

  15. Archean Subduction or Not? The Archean Volcanic Record Re-assessed.

    NASA Astrophysics Data System (ADS)

    Pearce, Julian; Peate, David; Smithies, Hugh

    2013-04-01

    Methods of identification of volcanic arc lavas may utilize: (1) the selective enrichment of the mantle wedge by 'subduction-mobile' elements; (2) the distinctive preconditioning of mantle along its flow path to the arc front; (3) the distinctive combination of fluid-flux and decompression melting; and (4) the effects of fluids on crystallization of the resulting magma. It should then be a simple matter uniquely to recognise volcanic arc lavas in the Geological Record and so document past subduction zones. Essentially, this is generally true in the oceans, but generally not on the continents. Even in recent, fresh lavas and with a full battery of element and isotope tools at our disposal, there can be debate over whether an arc-like geochemical signature results from active subduction, an older, inherited subduction component in the lithosphere, or crustal contamination. In the Archean, metamorphism, deformation, a different thermal regime and potential non-uniformitarian tectonic scenarios make the fingerprinting of arc lavas particularly problematic. Not least, the complicating factor of crustal contamination is likely to be much greater given the higher magma and crustal temperatures and higher magma fluxes prevailing. Here, we apply new, high-resolution immobile element fingerprinting methods, based primarily on Th-Nb fractionation, to Archean lavas. In the Pilbara, for example, where there is a volcanic record extending for over >500 m.y., we note that lavas with high Th/Nb (negative Nb anomalies) are common throughout the lava sequence. Many older formations also follow a basalt-andesite-dacite-rhyolite (BADR) sequence resembling present-day arcs. However, back-extrapolation of their compositions to their primitive magmas demonstrates that these were almost certainly crustally-contaminated plume-derived lavas. By contrast, this is not the case in the uppermst part of the sequence where even the most primitive magmas have significant Nb anomalies. The

  16. Juan de Fuca subducting plate geometry and intraslab seismicity

    NASA Astrophysics Data System (ADS)

    Medema, Guy Frederick

    The geometry of the subducting Juan de Fuca plate beneath the Olympic Peninsula of Western Washington is modeled using wide-angle P mP reflections off the slab Moho. Active-source reflection data collected from the 1998 WET SHIPS project were augmented with earthquake-source reflections to increase the spatial distribution of reflection points over the Juan de Fuca arch structure. Approximately 1100 WET SHIPS and 500 earthquake-source reflections were used in our inversion. PmP travel-times from active sources and PmP--P differential times from earthquakes sources were simultaneously inverted for slab-Moho depth, using a combination of finite-difference and 3-D ray-tracing methods. Results show a tighter arch structure than in previous models with the shallowest dipping portion (˜ 10° dip) concentrated directly beneath the Olympic Mountains. Comparison of our slab model to intraslab earthquake hypocenters reveals a southwest-northeast trending lineament of seismicity situated just beneath the subducted Moho in the slab mantle. We interpret this seismicity as the manifestation of a subducting pseudofault along which high levels of upper mantle hydration occurred prior to subduction. Most of the remaining intraslab seismicity is concentrated on the northern and southern flanks of the arch and is likely due to increased strain rates in these regions from the combined effect of slab arch and subsequent steepening of slab dip. Earthquakes in the northern region appear to occur above the slab Moho in the crust of the oceanic plate, while uncertainties in earthquake locations and 3-D velocities in the southern region prevent an unambiguous interpretation at this time. The southern patch is especially important as it contains 3 large (magnitude 6.5 to 7.1) earthquakes during that past 60 years, including the 2001, Mw 6.8, Nisqually earthquake. Earthquakes occurring between 45 and 65 km depth in these two regions also systematically produce an anomalous low

  17. Seismic Reflection Imaging of Along-Strike Changes in the Hikurangi Margin Subduction Interface, North Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Barker, D.; Sutherland, R.; Bannister, S.; Toulmin, S.; Henrys, S.; Reyners, M.; Pecher, I.; Uruski, C.; Maslen, G.

    2006-12-01

    Seismic reflection data from the subduction margin off the east coast of the North Island, New Zealand, show first-order structural features in the upper plate and subducting slab geometry that we relate to plate coupling at the subduction interface. Beneath the eastern coastline of North Island, New Zealand, the subducted Pacific plate dips at less than 3 degrees to the northwest and is at a depth of less than 15 km. This shallow geometry is optimum for detailed geophysical studies of the subduction decollement using both active-source and passive-source experiments. In March-May 2005 a new industry seismic reflection survey, 05CM, was undertaken offshore of the east coast. Although primarily intended to identify hydrocarbon plays, the survey was also designed to be capable of seismically imaging the subducted plate, using a 12-km-long streamer and 12 s record lengths. In total, over 2800 km of data were recorded along the margin. Repeated damage to the seismic streamer from shark bites meant only some profiles were collected to the original full survey specifications. Remaining data were acquired with a shorter streamer (4-6 km) and record length (8 s), still sufficient to provide a grid of high quality reflection data revealing first-order structural features of the upper plate and along-strike changes in subducting slab geometry. We suggest that these along-strike changes reflect the transition from a coupled plate interface in the south to decoupled in the north, a feature of the Hikurangi subduction system previously inferred from other geological and geophysical observations.

  18. Subduction in Central Kermadec: Crustal Structures from the Incoming Plate and the Arc- Backarc Region From Wide-Angle Seismics

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

    The central part of the 2500-km long Tonga-Kermadec Trench is characterized by the subduction of the Louisville Ridge and unusually large seismicity approximately 200-300 km to the south of this ridge subduction. From this region we show preliminary results which have been derived from the recently acquired interpretation of seismic wide-angle reflection/refraction data. The data were collected along an almost 500-km long transect carried out in April 2007 using the R/V Sonne in order to determine the upper lithospheric structures of the incoming Pacific Plate and the overriding Australian Plate across the Colville and Kermadec Ridges. This transect lies immediately north of Raoul Island, the largest of the Kermadec Islands and which is presently a highly active volcano. This study is part of the MANGO project (Marine Geoscientific Inverstigations on the Input and Output of the Kermadec Subduction Zone) which comprises a 1000-km long working area north of New Zealand's North Island. It covers the transition from subduction of the Hikurangi Plateau in the south to erosive subduction of normal Pacific oceanic crust in the centre and thence accretionary subduction further north. Overall the subduction is accompanied by northward increasing seismicity. The aim of this project is to understand the transition throughout the different regimes, the variation of the structures to explain the accompanying seismicty, and the role and evolution of the stratovolcanoes. This will be achieved by analysing the structures of the sediment, crust and upper mantle and also material transfers from its input and output through subduction zone processes.

  19. The importance of plumes to trigger subduction of a sluggish lid: examples from laboratory experiments and planets

    NASA Astrophysics Data System (ADS)

    Davaille, Anne; Smrekar, Suzanne

    2014-05-01

    The possible role of plate tectonics in creating habitable zones and the conditions required to start plate tectonics are currently hotly debated due to the discovery of many Earth-sized exoplanets. The initiation of subduction is both the gateway to plate tectonics and a key link between interior convection and lithospheric rheology. Modeling the details of plate failure and the initiation of subduction is very challenging due to the complexity of mantle rocks. We carried out experiments on convection in aqueous colloidal dispersions heated from below, and dried and cooled from above. The rheology of these fluids depends strongly on solid particle fraction fp, being Newtonian at low fp, and presenting yield stress, elasticity, and brittle properties as fp increases. Such a behaviour is analogue to the rheology of mantle rocks as temperature decreases. So if drying is sufficiently rapid in the laboratory, a skin forms on the fluid surface and may participate (or not) to the convective motions, depending on the experimental parameters. Moreover, we observed that (1) the existence of upwelling plumes help trigger subduction, the asymmetric subduction zone being localized on the rim of the plume impingement zone under the lithosphere; (2) depending on the lithospheric rheology, the nascent subduction can then either stop as the result of subducted plate necking, or continue to sink smoothly. Inspection of the geological record on Earth suggests that such a strong association between plumes and subduction may have been instrumental in the nucleation and growth of cratons, the onset of continuous plate tectonics, and present-day initiation of subduction around some large oceanic plateaus. On Venus, interpretation of geophysical data sets suggests that Quetzelpetlatl corona overlies an active plume. The narrow trough found along 2/3 of the margin of the corona has a flexural signature and is likely an example of subduction initiated at the rim of a plume. If this is

  20. 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.

  1. Effects of subduction parameters on geothermal gradients in forearcs with an application to Franciscan subduction in California

    SciTech Connect

    Dumitru, T.A. )

    1991-01-10

    Geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate. In this paper, finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters: radiogenic heat production; thermal conductivity of forearc rocks; subduction angle; subduction rate; frictional heat production; and presubduction geothermal gradients. Pressure-temperature conditions of blueschist-facies metamorphism in the Franciscan subduction complex of California are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 MPa, but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints. Speculative application of the modeling results assuming a shear stress of 4% of lithostatic pressure to plate motion reconstructions for the Franciscan forearc suggests that forearc gradients were about 8C/km around 85 Ma when the subducting slab was perhaps 145 m.y. old and the subduction rate was perhaps 95 km/m.y. Gradients increased moderately through the latest Cretaceous to middle Tertiary as subduction became slower and the subducting slab became younger, reaching about 16C/km at 28 Ma when the slab age was about 11 m.y. and the subduction rate was about 48 km/m.y. The slab age, subduction rate, and forearc gradient then remained fairly constant until 5 Ma, when subduction slowed to about 32 km/m.y. and the slab age decreased to about 8 m.y., causing gradients to rise to about 20C/km.

  2. Subduction zone tectonic studies to develop concepts for the occurrence of sediment subduction (Phase I). Final technical report

    SciTech Connect

    Hilde, T.W.C.

    1984-08-01

    The objective was to determine the fate of sediments at convergent lithospheric plate boundaries. The study focuses on the structures of the Circum-Pacific trenches and shallow portions of the associated subduction zones. Sediment distribution and the nature of sediment deformation was defined through the various stages of plate convergence to determine if the sediments are subducted or accreted. The controlling factors for sediment subduction and/or accretion were determined. 50 figs. (ACR)

  3. The Subduction Factory of Hydrocarbon Formation

    NASA Astrophysics Data System (ADS)

    Kleschev, K. A.

    2003-04-01

    THE SUBDUCTION FACTORY OF HYDROCARBON FORMATION K. A. Kleschev VNIGNI, Moscow, Russia balanyuk@sio.rssi.ru In the zones of collision of continental lithospheric plates, the subducting plates drive down great amounts of sediments formed at the former passive margins. This sedimentary layer, enclosed between two plates, contains significant quantities of hydrocarbons and occurs under the effect of strong shear deformations and special thermodynamic conditions. As the mass of the sediments passing through this zone is rather great, there are favorable conditions for occurrence of numerous hydrocarbon fields of industrial importance, including fields giants. Combined consideration of geodynamic and fluidodynamic aspects in a model of lithospheric plates collision enables to understand the influence of P T conditions and shear deformations on the mechanism of hydrocarbon generation and to look after their migration in the lithosphere up to the formation of deposits. Such model would allow one not only to describe and to explain these processes, but also to predict some features essential for the search and exploration of hydrocarbon fields in these regions and their classification. Enormous volumes of sediments accumulated over continental slopes and shelves are involved into the subduction zones in the course of overthrusting of island arcs onto continental margins. The weight of the overthrusting island arc substantially accelerates the subsiding of the continental margin. Crustal ground waters and hydrocarbons, capable to migration, begin to be squeezed out from the sedimentary material delivered to the waveguide zone under the island arc. The waters from the waveguide zone and the thermal water, released through dehydration of sediments and rocks of the former ocean crust and supplied from the deeper parts of the subduction zone, favor this process. Although the major portion of hydrocarbons is discharged and lost still in the body of the island arc itself

  4. Interplate coupling at oblique subduction zones: influence on upper plate erosion.

    NASA Astrophysics Data System (ADS)

    Malatesta, Cristina; Gerya, Taras; Crispini, Laura; Federico, Laura; Scambelluri, Marco; Capponi, Giovanni

    2014-05-01

    In active subduction zones, when the converging plates cannot slip freely past each other, "plate coupling" occurs. The moving subducting slab and therefore the coupling/decoupling relationship between plates control both short- and long-term deformation of the upper plate. Short-term deformation is dominantly elastic, occurs at human timescales and can be directly associated with earthquakes. Long-term deformation is cumulative, permanent and prevails at the geological timescale (Hoffman-Rothe et al., 2006, Springer Berlin Heidelberg). Here we used 3D numerical simulations to test oblique subduction zones and to investigate: 1) how long-term deformation and coupling relationship vary along the trench-axis; 2) how this relationship influences erosion and down-drag of upper plate material. Our models are based on thermo-mechanical equations solved with finite differences method and marker-in-cell techniques combined with a multigrid approach (Gerya, 2010, Cambridge Univ. Press). The reference model simulates an intraoceanic subduction close to the continental margin (Malatesta et al., 2013, Nature Communications, 4:2456 DOI:10.1038/ncomms3456). The oceanic crust is layered with a 5-km-thick layer of gabbro overlain by a 3-km-thick layer of basalt. The ocean floor is covered by 1-km-thick sediments. Plates move with a total velocity of 3.15 cm/yr; the oblique convergence is obtained using velocity vectors that form an angle of 45° with the initial starting point of subduction (weak zone in the lithosphere). After initiation of plate convergence, part of sediments on top of the incoming plate enters the subduction zone and is buried; another part is suddenly transferred along strike at shallow depths and along the subducting slab according to the direction of the along-trench velocity component of subduction. The lateral migration of sediment causes the evolution of the trench along its strike from sediment-poor to sediment-rich. As soon as subduction starts, where

  5. Subduction initiation adjacent to a relic island arc

    NASA Astrophysics Data System (ADS)

    Leng, W.; Gurnis, M.

    2013-12-01

    Although plate tectonics is well established, how subduction initiates over tectonic history has remained obscure. It has been proposed that passive margins may be a possible place for subduction initiation, but there is no obvious Cenozoic example of such a scenario, including along the passive margins of the Atlantic Ocean. With a computational method that follows the deformation of a visco-elasto-plastic medium, here we show that a favourable locale for subduction initiation is the juxtaposition of an old oceanic plate adjacent to a young, but relic arc. Significant density anomalies leading to subduction initiation arise from two major factors. One is the compositional difference between the relic arc crust and the oceanic lithospheric mantle; the other is the thermal difference due to the age offset between the two plates. With such a setup, we observe spontaneous subduction initiation if the oceanic crust is significantly weakened by pore fluid pressure. If the oceanic crust is relatively strong, a small amount of plate convergence is required to induce subduction. The evidence that Izu-Bonin-Mariana and Tonga-Kermedec subduction zones both initiate adjacent to a relic island arc support our conclusions. The initiation of both subduction zones at 51-52 Ma with commensurate compression on their respective overriding plates support a causal link between both subduction initiation events through a change in Pacific Plate motion. Our results provide an explanation for the rarity of subduction initiation at the passive margins. The continental lithosphere is typically old and cold. Consequently, the thermal effects cancel the compositional buoyancy contrast between the continental crust and the oceanic lithospheric mantle, making subduction initiation difficult at passive margins.

  6. Subducted slabs and the geoid - Constraints on mantle rheology and flow

    NASA Technical Reports Server (NTRS)

    Hager, B. H.

    1984-01-01

    The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones. Previously announced in STAR as N83-22874

  7. Forearc deformation and megasplay fault system of the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Hsu, S.; Yeh, Y.; Sibuet, J.; Tsai, C.; Doo, W.

    2011-12-01

    A great tsunami caused by a subduction earthquake had struck south Ryukyu islands and killed ~12000 people in 1771. Here we report the existence of a megasplay fault system along the south Ryukyu forearc. Analyses of deep multi-channel seismic reflection profiles indicate that the megasplay fault system is rising from the summit of a ~1 km high mount sitting on a ~5° landward dipping subducted plate interface. The fault system has accumulated large strain as evidenced by the active and widespread normal faults in the inner wedge. The along-trench length of the megasplay fault system is estimated to be ~450 km. The origin of this south Ryukyu megasplay fault system is linked to the subduction of elevated ridges parallel to the fracture zones. In contrast, no similar splay fault system is found in the west of 125. 5°E where the oblique subduction has produced shear zones along the south Ryukyu forearc. We infer that the megasplay fault system is responsible for the 1771 south Ryukyu tsunami. Likewise, after a quiescence of ~240 years, a near-future great earthquake and tsunami is anticipated as the extensional feature is strongly widespread over the south Ryukyu forearc.

  8. Scenarios of tsunamigenic earthquakes generated along the Hellenic subduction zone and impact along the French coastlines

    NASA Astrophysics Data System (ADS)

    Gailler, Audrey; Hébert, Hélène; Schindelé, François

    2016-04-01

    The Hellenic subduction is an active deformation zone characterized by a sustained day-to-day seismicity (magnitude < 4.5) among the strongest in Europe. The last significant earthquake along the Hellenic subduction zone detected and characterized by the French tsunami warning center (CENALT) occurred on 16th April 2015 (Mw = 6.0) along the southeastern coasts of Crete, without any tsunami risk for the French coastlines. Even if great subduction earthquakes (magnitude > 7.5) are less frequent than in Chile or Japan, the Hellenic area experienced several strong events by the past, the biggest being associated with major tsunamis (e.g., in 551, in 1303). The last known sequence dates the end of the 19th beginning of the 20th century with a seismic gap located along the South Peloponnese - West Crete segment. The legendary 365 AD great earthquake (magnitude 8 to 8.5) is assumed to have ruptured along a major inverse fault parallel to the trench in this area, generating a large tsunami observed up to the Adriatic. In this work we investigate the tsunami potential of earthquakes localized along the Hellenic subduction zone, especially the minimum magnitude required to generate a tsunami that would be able to cross from Eastern to Western Mediterranean. The impact along Corsica coastlines is discussed through the modeling of a set of tsunami scenarios (magnitude ranging from 8.0 to 8.5) established from historical events parameters.

  9. Earthquake nucleation in weak subducted carbonates

    NASA Astrophysics Data System (ADS)

    Kurzawski, Robert M.; Stipp, Michael; Niemeijer, André R.; Spiers, Christopher J.; Behrmann, Jan H.

    2016-09-01

    Ocean-floor carbonate- and clay-rich sediments form major inputs to subduction zones, especially at low-latitude convergent plate margins. Therefore, knowledge of their frictional behaviour is fundamental for understanding plate-boundary earthquakes. Here we report results of mechanical tests performed on simulated fault gouges prepared from ocean-floor carbonates and clays, cored during IODP drilling offshore Costa Rica. Clay-rich gouges show internal friction coefficients (that is, the slope of linearized shear stress versus normal stress data) of μint = 0.44 - 0.56, irrespective of temperature and pore-fluid pressure (Pf). By contrast, μint for the carbonate gouge strongly depends on temperature and pore-fluid pressure, with μint decreasing dramatically from 0.84 at room temperature and Pf = 20 MPa to 0.27 at T = 140 °C and Pf = 120 MPa. This effect provides a fundamental mechanism of shear localization and earthquake generation in subduction zones, and makes carbonates likely nucleation sites for plate-boundary earthquakes. Our results imply that rupture nucleation is prompted by a combination of temperature-controlled frictional instability and temperature- and pore-pressure-dependent weakening of calcareous fault gouges.

  10. Acceleration spectra for subduction zone earthquakes

    USGS Publications Warehouse

    Boatwright, J.; Choy, G.L.

    1989-01-01

    We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P+pP+sP, by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from Ms = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 1027 dyn cm, exhibit intermediate slopes where u(w)???w5/4 for frequencies from 0.005 to 0.05 Hz. For these earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekstrom's (1987) moment rate functions for the Ms=8.1 Michoacan earthquake of September 19, 1985, and the Ms=7.6 Michoacan aftershock of September 21, 1985. -from Authors

  11. Contamination of the Convecting Mantle in Eastern Tethyan 'Subduction Factories'

    NASA Astrophysics Data System (ADS)

    Flower, M. F.; Nguyen, T. H.

    2003-04-01

    As subduction gives way to collision at the end of a Wilson Cycle the associated magmatic activity becomes increasingly enriched in potassium and other large-ion lithophile elements. This is usually attributed to the addition of continental crust-derived material to the convecting mantle wedge. Corresponding depletions in high-field strength elements (Ti and Nb) are more commonly explained in terms of accessory phase buffering or protracted reaction of melts with mantle wallrock. It is increasingly apparent that mantle wedge magmatic sources range from 'fertile' (lherzolitic) to 'refractory' (harzburgitic) although the extent to which this corresponds to the LILE and HFSE variation is unclear. Mantle wedge mass balances clearly hold clues to enrichment-depletion histories of the convecting asthenosphere with respect to both the overriding and subducting plates. With a view to better understanding these effects we have used the MELTS algorithm to calculate hypothetical partial melt compositions as a function of source fertility and H2O content, in the pressure range, 0-1.0 GPa as a basis comparison for natural partial melts. Primitive magmas characterizing the Mariana (western Pacific) and Sunda-Banda (Indonesia) arcs, and the northeastern syntaxis of the India-Asia collision suture (Yunnan) appear to resemble calculated equilibrium melts of refractory (basalt-depleted) peridotite, variably enriched in lithophile and light rare earth elements. These comparisons lead to three observations. 1) HFSE and Fe abundances in primitive MORB, calcalkaline, and boninite magmas, and their respective high-potassium variants are consistent with those implied by phase equilibria associated with partial melting and fractionation, suggesting accessory phases, wall-rock reaction, and slab contamination are probably not important as causes of HFSE depletions. 2) Magmatic sources at convergent and colliding margins are typically refractory (basalt-depleted) compared to those yielding

  12. Viscous Dissipation and Criticality of Subducting Slabs

    NASA Astrophysics Data System (ADS)

    Riedel, Mike; Karato, Shun; Yuen, Dave

    2016-04-01

    Rheology of subducting lithosphere appears to be complicated. In the shallow part, deformation is largely accomodated by brittle failure, whereas at greater depth, at higher confining pressures, ductile creep is expected to control slab strength. The amount of viscous dissipation ΔQ during subduction at greater depth, as constrained by experimental rock mechanics, can be estimated on the basis of a simple bending moment equation [1,2] 2ɛ˙0(z) ∫ +h/2 2 M (z) = h ṡ -h/2 4μ(y,z)y dy , (1) for a complex multi-phase rheology in the mantle transition zone, including the effects of a metastable phase transition as well as the pressure, temperature, grain-size and stress dependency of the relevant creep mechanisms; μ is here the effective viscosity and ɛ˙0(z) is a (reference) strain rate. Numerical analysis shows that the maximum bending moment, Mcrit, that can be sustained by a slab is of the order of 1019 Nm per m according to Mcrit˜=σp ∗h2/4, where σp is the Peierl's stress limit of slab materials and h is the slab thickness. Near Mcrit, the amount of viscous dissipation grows strongly as a consequence of a lattice instability of mantle minerals (dislocation glide in olivine), suggesting that thermo-mechanical instabilities become prone to occur at places where a critical shear-heating rate is exceeded, see figure. This implies that the lithosphere behaves in such cases like a perfectly plastic solid [3]. Recently available detailed data related to deep seismicity [4,5] seems to provide support to our conclusion. It shows, e.g., that thermal shear instabilities, and not transformational faulting, is likely the dominating mechanism for deep-focus earthquakes at the bottom of the transition zone, in accordance with this suggested "deep criticality" model. These new findings are therefore briefly outlined and possible implications are discussed. References [1] Riedel, M. R., Karato, S., Yuen, D. A. Criticality of Subducting Slabs. University of Minnesota

  13. Viscous Dissipation and Criticality of Subducting Slabs

    NASA Astrophysics Data System (ADS)

    Riedel, Mike; Karato, Shun; Yuen, Dave

    2016-04-01

    Rheology of subducting lithosphere appears to be complicated. In the shallow part, deformation is largely accomodated by brittle failure, whereas at greater depth, at higher confining pressures, ductile creep is expected to control slab strength. The amount of viscous dissipation ΔQ during subduction at greater depth, as constrained by experimental rock mechanics, can be estimated on the basis of a simple bending moment equation [1,2] 2ɛ˙0(z) ∫ +h/2 2 M (z) = h ṡ ‑h/2 4μ(y,z)y dy , (1) for a complex multi-phase rheology in the mantle transition zone, including the effects of a metastable phase transition as well as the pressure, temperature, grain-size and stress dependency of the relevant creep mechanisms; μ is here the effective viscosity and ɛ˙0(z) is a (reference) strain rate. Numerical analysis shows that the maximum bending moment, Mcrit, that can be sustained by a slab is of the order of 1019 Nm per m according to Mcrit˜=σp ∗h2/4, where σp is the Peierl's stress limit of slab materials and h is the slab thickness. Near Mcrit, the amount of viscous dissipation grows strongly as a consequence of a lattice instability of mantle minerals (dislocation glide in olivine), suggesting that thermo-mechanical instabilities become prone to occur at places where a critical shear-heating rate is exceeded, see figure. This implies that the lithosphere behaves in such cases like a perfectly plastic solid [3]. Recently available detailed data related to deep seismicity [4,5] seems to provide support to our conclusion. It shows, e.g., that thermal shear instabilities, and not transformational faulting, is likely the dominating mechanism for deep-focus earthquakes at the bottom of the transition zone, in accordance with this suggested "deep criticality" model. These new findings are therefore briefly outlined and possible implications are discussed. References [1] Riedel, M. R., Karato, S., Yuen, D. A. Criticality of Subducting Slabs. University of Minnesota

  14. Dehydration History of Subducted Lithologies, Sifnos, Greece

    NASA Astrophysics Data System (ADS)

    Dragovic, B.; Baxter, E. F.; Caddick, M. J.

    2011-12-01

    Garnet-forming reactions in subduction zones may be linked directly to dehydration. When coupled with precise zoned garnet geochronology, this permits quantification of H2O release rates and evaluation of models for progressive metamorphism (and thus dehydration) in subduction zones. Here, an integrated geochronologic and thermodynamic analysis seeks to determine the devolatization history of subducted lithologies from Sifnos, Greece, in the Attic Cycladic Blueschist Belt. Based on major element zoning, microsampling and Sm-Nd dating of chemically zoned garnets (greater than 1cm in diameter) from various lithologies, we constrain the rates and duration of garnet growth. Combined with bulk garnet analyses on several other samples where crystals were not sufficiently large for age zoning work, a chronology of garnet growth across all lithologies was constructed. After the slow initiation of garnet growth recorded by one 5cm crystal from a quartzofeldspathic gneiss (the innermost 1cm, radially, grew from 52.7 ± 3.3Ma to 47.19 ± 0.21 Ma), relatively rapid growth prevailed from 45.92 ± 0.18 Ma to 45.63 ± 0.22 Ma for the outermost 0.9 cm of the crystal. This equates to acceleration in volumetric growth rate of two orders of magnitude. In addition, garnet ages from six intermediate to mafic garnet-bearing lithologies span 46.48 ± 0.82 Ma to 43.90 ± 1.92 Ma, at least one of which (a mafic blueschist) indicates growth spanning just hundreds of thousands of years. Analysis of the garnet-forming reaction(s) provides information on the stoichiometric ratio between garnet produced and water released. For a range of bulk rock compositions, we find that garnet:water molar production ratios vary from approximately 0.4:1 to 1.2:1, depending mostly on the portion of the P-T path over which reaction occurs, the bulk composition, and the hydrous phases from which garnet forms. Hydrous phases contributing to garnet growth and water release typically include chloritoid, lawsonite

  15. The Sulfur Cycle at Subduction Zones

    NASA Astrophysics Data System (ADS)

    de Moor, M. J.; Fischer, T. P.; Sharp, Z. D.

    2013-12-01

    We present sulfur (S) isotope data for magmatic gases emitted along the Central American (CA) Arc (oxidizing conditions ΔQFM ~+ 1.5) and at the East African Rift (reduced conditions ΔQFM ~0). The results are interpreted through mass balance calculations to characterize the S cycle through subduction zones with implications for the redox conditions of arc magmas. Voluminous gas emissions from Masaya, an open vent basaltic volcano in Nicaragua, represent >20% of the SO2 flux from the CA arc [1]. Samples from the Masaya plume have S isotope compositions of + 4.8 × 0.4 ‰ [2]. Degassing fractionation modeling and assessment of differentiation processes in this oxidized volcano suggest that this value is close to that of the source composition. High T gas samples from other CA volcanoes (Momotombo, Cerro Negro, Poas, Turrialba) range from + 3 ‰ (Cerro Negro) to + 7 ‰ (Poas; [3]). The high δ34S values are attributed to recycling of subducted oxidized sulfur (sulfate ~ + 20 ‰) through the CA arc. The δ34S values of the more reduced samples from East African Rift volcanoes, Erta Ale - 0.5 × 0.6 ‰ [3] and Oldoinyo Lengai -0.7 ‰ to + 1.2 ‰) are far lower and are probably sourced directly from ambient mantle. The subduction of oxidized material at arcs presents a likely explanation for the oxidized nature of arc magmas relative to magmas from spreading centers. We observe no distinguishable change in melt fO2 with S degassing and attribute these differences to tectonic setting. Monte Carlo modeling suggests that subducted crust (sediments, altered oceanic crust, and serpentinized lithospheric mantle) delivers ~7.7 × 2.2 x 1010 mols of S with δ34S of -1.5 × 2.3‰ per year into the subduction zone. The total S output from the arc is estimated to be 3.4 × 1.1 x 1010 mols/yr with a δ34S value similar to that of Masaya gas (+5 × 0.5 ‰). Considering δ34S values for ambient upper mantle (0 ‰ [4]) and slab-derived fluids (+14 ‰ [5]) allows calculation

  16. Effects of Two Subducting Slabs on the Temperature Distribution Along the Subduction Faults in the Kanto Region, Japan

    NASA Astrophysics Data System (ADS)

    Wada, I.; He, J.

    2015-12-01

    In this study, we investigate the thermal effects of subduction of two oceanic plates in the Kanto region of Japan, using a 3-D numerical thermal model. The Kanto region lies in the forearc of a subduction system, where the Pacific (PAC) plate and the Philippine Sea (PHS) plate subduct beneath the North American (NA) plate. In a typical subduction setting with one subducting slab, the motion of the slab drives solid-state mantle flow in the overlying mantle wedge, and the flow brings in hot mantle from the backarc towards the forearc. In the Kanto region, however, the presence of the PHS plate between the overlying NA plate and the subducting PAC plate prevents a typical mantle flow pattern. We developed a 3-D thermal model for the Kanto region to simulate the pattern of mantle wedge flow and to quantify its effect on the thermal structure. The model incorporates realistic slab geometries that were delineated from seismological studies. Mantle wedge flow between the PHS slab and the overlying NA plate is expected to be subdued due to the small space and the relatively slow subduction of the PHS slab. We simplify the model by incorporating the results of a 2-D thermal modeling for the subduction of the PHS slab as part of boundary conditions in the 3-D model to approximate the effect of the subdued mantle wedge flow and the subduction of the PHS slab. We use geophysical observations as constraints for the 3-D thermal model and estimate the temperature distributions along the subduction plate interfaces. The model predicts a particularly cold condition in the central part of the Kanto region where the PAC and PHS slabs are in contact with one another, consistent with the observed deeper extent of seismicity along the subduction faults compared to the neighboring regions.

  17. Seamount, ridge, and transform subduction in southern Central America

    NASA Astrophysics Data System (ADS)

    Morell, Kristin D.

    2016-02-01

    Understanding the factors that control subduction zone processes is a first-order goal in the study of convergent margins. In southern Central America, a growing body of research reveals strong links between the character of the subducting slab and the mechanics of important processes that include subduction erosion, fluid flow, deformation, and seismogenesis. In this paper, I evaluate the role that seamount, ridge, and transform subduction have in the development of upper plate deformation and volcanism by summarizing previous work across a >500 km long region of Central America where each of these three scenarios are present along strike. The data show that the subduction of short-wavelength bathymetry (e.g., seamounts and faults on the seafloor) produces short-wavelength deformation that persists for relatively short timescales (104-105 years), whereas the subduction of longer-wavelength bathymetry (e.g., the aseismic Cocos Ridge) results in longer-wavelength deformation that endures over a longer time scale (106 years). The timing and distribution of upper plate deformation are consistent with subhorizontal Cocos Ridge subduction driving upper plate deformation, and the increased crustal thickness (>20 km) of the subducting Cocos Ridge is likely one of the most important factors in the production of upper plate contraction and crustal thickening. The data illustrate a fundamental connection between lower plate properties and upper plate deformation and highlight the profound influence that bathymetry and crustal thickness have in the localization and kinematics of upper plate strain and volcanism in Middle America.

  18. 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.

  19. Segmentation of Makran Subduction Zone and its consequences on tsunami hazard estimations

    NASA Astrophysics Data System (ADS)

    Mokhtari, M.

    2009-04-01

    In a plate tectonic setting like that of the Makran Accretionary Complex of Oman Sea, a fairly high earthquake activity would be expected, as in many of the other major Accretionary complexes/subduction zones around the world. But this region which is located between the Zendan-Minab Fault System and Oranch Fault Zone shows relatively low seismicity in comparison with the surrounding region. Better documented tsunami events in the Makran subduction zones are 3, including two events of seismic origin, and one of unknown origin. The latest event is the major earthquake generated tsunami of 1945 in eastern Makran that ruptured approximately one-fifth the length of the subduction zone. It is important to note that, the epicenter of this event is also close to the Sonne Fault which has created segments on the Makran Subduction Zone. The crossing points between Makran Subduction Zone and these oblique fault zones can be a location for occurrence of major earthquake activities. However, more studies are required for further clarification. In contrast to the east, the plate boundary in western Makran has no clear record of historically as well as instrumental great events. The large changes in seismicity between eastern and western Makran suggest segmentation of the subduction zone. This is being supported by Kukowski et al., (2000) where they introduce a new boundary coinciding very well with the Sonne strike-slip fault. As mentioned the western part is characterized by the absence of events. East of the Sonne fault and west of long 64°E is the only region with a clustering of events within the submarine and southernmost onshore part of the Accretionary Wedge, also including the Mw 8.1 event of 1945 (Byrne et al., 1992). Most events in the wedge appear to be pure-thrust earthquakes and are interpreted as plate boundary events (Quittmeyer and Kafka, 1984; Byrne et al., 1992). The earthquake of August 12, 1963, a few tens of kilometers east of the Sonne fault, had a large

  20. Global correlation of lower mantle structure and past subduction

    NASA Astrophysics Data System (ADS)

    Domeier, Mathew; Doubrovine, Pavel V.; Torsvik, Trond H.; Spakman, Wim; Bull, Abigail L.

    2016-05-01

    Advances in global seismic tomography have increasingly motivated identification of subducted lithosphere in Earth's deep mantle, creating novel opportunities to link plate tectonics and mantle evolution. Chief among those is the quest for a robust subduction reference frame, wherein the mantle assemblage of subducted lithosphere is used to reconstruct past surface tectonics in an absolute framework anchored in the deep Earth. However, the associations heretofore drawn between lower mantle structure and past subduction have been qualitative and conflicting, so the very assumption of a correlation has yet to be quantitatively corroborated. Here we show that a significant, time-depth progressive correlation can be drawn between reconstructed subduction zones of the last 130 Myr and positive S wave velocity anomalies at 600-2300 km depth, but that further correlation between greater times and depths is not presently demonstrable. This correlation suggests that lower mantle slab sinking rates average between 1.1 and 1.9 cm yr-1.

  1. The Fate of Eclogitized Oceanic Crust During Subduction: Implications for Subduction Zone Dynamics

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Langdon, R.; Waters, D.; Raimbourg, H.; Yamato, P.; Chopin, C.

    2011-12-01

    The Monviso ophiolite is composed of two main tectonic slices: the Monviso Unit (MU) to the west, which overlies the Lago Superiore Unit (LSU). Our PT estimates show that the MU has been subducted down to 480°C-23kbar (~70km) during Alpine subduction while the LSU reached slightly deeper conditions (540°C-26kbar, i.e. ~80km). This ophiolite, which comprises large (10-20km long) ophiolite fragments therefore does not correspond to a serpentinite mélange, and may be the southern extension of the Zermatt-Saas ophiolite (Angiboust et al., 2009; 2011). The well-preserved LSU constitutes an almost continuous upper fragment of oceanic lithosphere subducted between 50 and 40 Ma and later exhumed along the subduction interface. It therefore provides a unique opportunity to study strain partitioning and deep mechanical behaviour of the subducting lithosphere. The LSU comprises (i) several hundred meters of eclogitized basaltic crust (+ minor calcschist lenses) overlying a 100-400m thick metagabbroic body and (ii) a serpentinite sole (c. 1km thick). We herein focus on eclogite-facies shear zones, which are found at the boundary between basalts and gabbros, and between gabbros and serpentinites, i.e. between material with marked rheological contrasts. Eclogite facies blocks within the shear zones display intense fracturation, fragment rotation and dispersion in the serpentinite schists which line up the shear zones. We also report the first finding of eclogite-facies breccias, constituted of rotated eclogite mylonitic clasts cemented within an eclogite-facies matrix. Local fracturation of garnet within these breccias is attested by the presence of numerous fracture networks within garnet, generally healed by a Mg-enriched composition. The shear zones also preserve clear evidence of pervasive and channelized fluid flow (of variable duration) leading to alteration of bulk rock composition, weakening of the rock and widespread crystallization of lawsonite. Our results provide

  2. Tectonic tremor and slow slip along the northwestern section of the Mexico subduction zone

    NASA Astrophysics Data System (ADS)

    Brudzinski, Michael R.; Schlanser, Kristen M.; Kelly, Nicholas J.; DeMets, Charles; Grand, Stephen P.; Márquez-Azúa, Bertha; Cabral-Cano, Enrique

    2016-11-01

    The southwestern coast of Mexico is marked by active subduction of the Rivera and Cocos plates, producing megathrust earthquakes that tend to recur every 50-100 yr. Herein, we use seismic and GPS data from this region to investigate the potential relationship between earthquakes, tectonic (non-volcanic) tremor, and transient slip along the westernmost 200 km of the Mexico subduction zone. Visual examination of seismograms and spectrograms throughout the 18-month-long MARS seismic experiment reveals clear evidence for frequent small episodes of tremor along the Rivera and Cocos subduction zones beneath the states of Jalisco, Colima, and Michoacán. Using a semi-automated process that identifies prominent energy bursts in envelope waveforms of this new data, analyst-refined relative arrival times are inverted for source locations using a 1-D velocity model. The resulting northwest-southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 Mw 8.0 Colima-Jalisco and 2003 Mw 7.2 Tecoman subduction-thrust earthquakes and just below the regions of afterslip triggered by these earthquakes. Despite the close proximity between tremor and megathrust events, there is no evidence that the time since the last great earthquake influences the spatial or temporal pattern of tremor. A well-defined gap in the tremor beneath the western Colima Graben appears to mark a separation along the subducted Rivera-Cocos plate boundary. From the position time series of 19 continuous GPS sites in western Mexico, we present the first evidence that slow slip events occur on the Rivera plate subduction interface. Unlike the widely-recorded, large-amplitude, slow slip events on the nearly horizontal Cocos plate subduction interface below southern Mexico, slow slip events below western Mexico have small amplitudes and are recorded at relatively few, mostly coastal stations. The smaller slow slip beneath western Mexico may be due to the steeper dip, causing a

  3. Flat vs. Normal subduction, Central Chile: insights from regional seismic tomography and rock type modeling

    NASA Astrophysics Data System (ADS)

    Marot, Marianne; Monfret, Tony; Gerbault, Muriel; Nolet, Guust; Ranalli, Giorgio; Pardo, Mario

    2013-04-01

    The Central Chilean subduction zone (27-35°S) is host to a multitude of unexplained phenomena, all likely linked to one another. Here, the 35 Ma oceanic Nazca plate is subducting beneath South America with a well developed, highly seismic flat slab, very well correlated with the subducting Juan Fernandez seamount Ridge (JFR) track, and also with the absence of volcanism at the surface. The upper plate, currently under compression, is composed of a series of accreted terranes of various origins and ages. Although no general consensus on the formation of this flat slab has been yet achieved, there may have been influence of overthickened oceanic crust, delayed eclogitization and consequent fluid retain within the slab, and slab suction due to the high convergence rate with the thick Rio de Plata craton. Therefore, the main questions we address are: Does the slab dehydrate along the flat subducting segment? If so, how hydrated is the slab, at what depth does slab dehydration occur, where are the fluids transported to, and where are they stored? Is magmatism still active beneath the now inactive arc? Are accreted terranes and suture zones important attributes of this subduction zone? Do they possess their own mantle entities? To answer these questions, we analyzed recorded local seismicity and performed regional 3D seismic tomography for Vp and Vs. Combining seismic tomography with 2D instantaneous thermo-mechanical modeling for the regions of flat and normal subduction, we predict rock compositions for these two regions based on published mineral and rock elastic properties. Here, we present a comparison between the normal subduction zone to the south, reflecting typical and expected features, and the flat slab region to the north, exhibiting heterogeneities. Our results agree with other studies for a dry and cold continental mantle above the flat slab. We distinguish the Cuyania terrane with overthickened crust and/or abnormal mantle beneath it. We notice that the

  4. Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Agard, Philippe; Glodny, Johannes; Omrani, Jafar; Oncken, Onno

    2016-06-01

    Pressure-temperature-time (P- T- t) trajectories of high-pressure rocks provide important constraints to understand the tectonic evolution of convergent margins. New field observations and P- T- t constraints for the evolution of the only known blueschist-facies exposure along the Zagros suture zone in Southern Iran are reported here. These blueschists, now exposed in tectonic windows under the Sanandaj-Sirjan zone (upper plate), constitute accreted fragments of the Tethyan domain during N-directed Cretaceous subduction. Three units were identified in the field: from top to bottom, the Ashin unit (mafic and felsic gneisses), the Seghin complex (mafic tuffs and ultramafics) and the Siah Kuh massif (coherent volcanic edifice). Microstructural observations, P- T estimates and Rb-Sr deformation ages indicate that the Ashin unit possibly underwent burial down to 30-35 km and 550 °C along a relatively warm P- T gradient (c. 17°/km) and was ultimately deformed between 85 and 100 Ma. The Seghin complex exhibits remarkably well-preserved HP-LT assemblages comprising lawsonite, glaucophane, aragonite, omphacite and garnet. P- T- t reconstruction indicates that this slice was subducted down to c. 50 km at temperatures of c. 500 °C along a very cold subduction gradient (c. 7°/km). Deformation in the Seghin complex stopped at around 65 Ma, close to peak metamorphic conditions. Field relationships and estimates of the P- T trajectory followed by the Siah Kuh volcanic edifice indicate that this massif was lately subducted down to 15 km depth along the same very cold gradient. This slice-stack represents a well-preserved field example (i) highlighting the existence of transient underplating processes juxtaposing pluri-kilometric tectonic slices along the subduction channel and (ii) imaging the discontinuous down-stepping of the active main subduction thrust with ongoing accretion. The Zagros blueschists also record an apparent cooling of the Zagros subduction zone between 90

  5. Evolution of the long-wavelength, subduction-driven topography of South America since 150 Ma

    NASA Astrophysics Data System (ADS)

    Flament, N. E.; Gurnis, M.; Williams, S.; Bower, D. J.; Seton, M.; Müller, D.

    2014-12-01

    Subduction to the west of South America spans 6000 km along strike and has been active for over 250 Myr. The influence of the history of subduction on the geodynamics of South America has been profound, driving mountain building and arc volcanism in the Andean Cordillera. Here, we investigate the long-wavelength changes in the topography of South America associated with subduction and plate motion and their interplay with the lithospheric deformation associated with the opening of the South Atlantic. We pay particular attention to the topographic expression of flat-lying subduction zones. We develop time-dependent geodynamic models of mantle flow and lithosphere deformation to investigate the evolution of South American dynamic and total topography since the late Jurassic (150 Ma). Our models are semi-empirical because the computational cost of fully dynamic, evolutionary models is still prohibitive. We impose the kinematics of global plate reconstructions with deforming continents in forward global mantle convection models with compositionally distinct crust and continental lithosphere embedded within the thermal lithosphere. The shallow thermal structure of subducting slabs is imposed, allowing us to investigate the evolution of dynamic topography around flat slab segments in time-dependent models. Multiple cases are used to investigate how the evolution of South American dynamic topography is influenced by mantle viscosity, the kinematics of the opening of the South Atlantic and alternative scenarios for recent and past flat-slab subduction. We predict that the migration of South America over sinking oceanic lithosphere resulted in continental tilt to the west until ~ 45 Ma, inverting to an eastward tilt thereafter. This first-order result is consistent with the reversal of the drainage of the Amazon River system. We investigate which scenarios of flat-slab subduction since the Eocene are compatible with geological constraints on the evolution of the Solimoes

  6. 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

  7. Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone

    NASA Astrophysics Data System (ADS)

    Angiboust, Samuel; Agard, Philippe; Glodny, Johannes; Omrani, Jafar; Oncken, Onno

    2016-04-01

    Pressure-Temperature-time (P-T-t) trajectories of high-pressure rocks provide important constraints to understand the tectonic evolution of convergent margins. New field observations and P-T-t constraints for the evolution of the only known blueschist-facies exposure along the Zagros suture zone in Southern Iran are reported here. These blueschists, now exposed in tectonic windows under the Sanandaj-Sirjan zone (upper plate), constitute accreted fragments of the Tethyan domain during N-vergent Cretaceous subduction. Three units were identified in the field: from top to bottom, the Ashin unit (mafic and felsic gneisses), the Seghin complex (mafic tuffs and ultramafics) and the Siah Kuh massif (coherent volcanic edifice). Microstructural observations, P-T estimates and Rb-Sr deformation ages indicate that the Ashin unit possibly underwent burial down to 30-35 km and 550°C along a relatively warm P-T gradient (c. 17°/km) and was ultimately deformed between 85 and 100 Ma. The Seghin complex exhibits remarkably well-preserved HP-LT assemblages comprising lawsonite, glaucophane, aragonite, omphacite and garnet. P-T-t reconstruction indicates that this slice was subducted down to c. 50 km at temperatures of c. 500°C along a very cold subduction gradient (c. 7°/km). Deformation in the Seghin complex stopped at around 65 Ma, close to peak metamorphic conditions. Field relationships and estimates of the P-T trajectory followed by the Siah Kuh volcanic edifice indicate that this massif was lately subducted down to 15 km depth along the same very cold gradient. This slice-stack represents a well-preserved field example (i) highlighting the existence of transient underplating processes juxtaposing pluri-kilometric tectonic slices along the subduction channel and (ii) imaging the discontinuous down-stepping of the active main subduction thrust with ongoing accretion. The Zagros blueschists also record an apparent cooling of the Zagros subduction zone between 90 and 65 Ma

  8. Arc Evolution in Response to the Subduction of Buoyant Features

    NASA Astrophysics Data System (ADS)

    Jenkins, Luke; Fourel, Loic; Goes, Saskia; Morra, Gabriele

    2015-04-01

    The subduction of buoyant features such as aseismic ridges or oceanic plateaux has been invoked to explain arc deformation, flat subduction and increase in seismic coupling. Other studies have challenged these ideas, attributing a larger role to the overriding plate. However, many open questions remain about the dynamics of the relative simple case of a single freely subducting plate. How big does a plateau need to be to change the arc shape? What is the control of plate's strength on the impact of buoyant features? How do the velocities adapt to the subduction of less dense material? In the present study, we propose a systematic approach in order to tackle these questions. We use a new 3-D coupled fluid-solid subduction model where the interaction between the slab and the isoviscous mantle is only calculated on the slab surface, significantly increasing computational efficiency. The oceanic plate rheology is visco-elasto-plastic and its top surface is free. We find that arc shape is significantly altered by the subduction of buoyant plateaux. Along the subduction plane through the plateau and depending on its size, the dip angle and the retreat velocity significantly decrease. Flat subduction is obtained in the case of large and strongly buoyant plateau/ridge. An interesting feature is that retreat velocity increases right after the plateau or ridge has finished subducting in order to catch up with the rest of the plate. The gradient in retreat velocity obtained along the trench may cause the slab to have a heterogeneous response to ridge push, eventually leading to slab advance where buoyant material is present. We apply our models to the Izu-Bonin-Marianas (IBM) trench and propose that subduction of the buoyant Caroline Island Ridge at the southern edge of the Mariana trench can explain both trench motion history and the current morphology of the IBM slab as imaged by seismic tomography.

  9. Thermal modeling of the southern Alaska subduction zone: Insight into the petrology of the subducting slab and overlying mantle wedge

    SciTech Connect

    Ponko, S.C.; Peacock, S.M.

    1995-11-10

    This report discusses a two-dimensional thermal model of the southern Alaska subduction zone. This model allows specfic predictions to be made about the pressure-temperature conditions and mineralogy of the subducting oceanic crust and the mantle wedge and assess different petrologic models for the generation of Alaskan arc magmas.

  10. Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

    NASA Astrophysics Data System (ADS)

    Shervais, John W.; Jean, Marlon M.

    2012-10-01

    Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This "subduction factory" is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones. The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source: C=[C/[[D/(D-PF)]∗[1-(PF/D)

  11. Multiscale Finite-frequency Seismic Imaging of the Southern Alaska Subduction Zone

    NASA Astrophysics Data System (ADS)

    Song, X.; Hung, S. H.; Tong, P.; Liu, Q.

    2015-12-01

    Southern Alaska is one of the most seismically active regions in north America as the Pacific plate subducts northward beneath North America plate along the Aleutian trench. In this study, we determine 3-D variations of P- and S-wave speed and Possion's ratio (Vp/Vs) perturbations of the southern Alaska subduction zone based on broadband tele-seismic data recorded by 198 seismic stations for about 2000 events with magnitudes greater than 5.5 during the period from June 2000 to December 2014. Relative arrival times of P and S phases bwtween stations are accurately measured by adapting the efficient multi-channel cross-correlation (MCCC) technique. The obtained arrival-time data are then used to tomographically image the Vp and Vs structures beneath the stations based on 3-D finite-frequency sensitivity kernels and a wavelet-based multi-scale model parameterization. Our results show strong positive velocity anomalies in the crust and upper mantle starting at a depth of about 50km and extending to northwestward down to a depth of 200 km and covering about 350 km in horizontal distance. The high velocity feature interpreted as a cold slab has a thickness of about 50km and a subducting angle of about 45o, consistent with some previous studies of southern Alaska. We also plan to further obtain high-resolution seismic imaging of southern Alaska subduction zone by utilizing the converted and coda waves of tele-seismic main phases (e.g., P and S) based on a hybrid tomographic technique combining spectral-element method (SEM) and frequency-wavenumber (FK) method. The 3D Vp and Vs models obtained from finite-frequency traveltime tomography thus can serve as a proper starting velocity model for the hybrid SEM-FK imaging to further reveal high-resolution details of the subduction zone.

  12. A complex, young subduction zone imaged by three-dimensional seismic velocity, Fiordland, New Zealand

    NASA Astrophysics Data System (ADS)

    Eberhart-Phillips, Donna; Reyners, Martin

    2001-09-01

    The Fiordland subduction zone, where subduction developed in the late Miocene, has been imaged with P and S-P arrival-time data from 311 earthquakes in a simultaneous inversion for hypocentres and 3-D VP and VP/VS models. The three-month microearthquake survey, recorded with 24 portable seismographs, provides excellent coverage, and, since earthquakes to depths of 130km are included, parts of the model are well-resolved to depths of 100km. The crustal features are generally consistent with geology. The low velocity in the upper 10km is associated with the Te Anau and Waiau basins. The Western Fiordland Orthogneiss is associated with a prominent feature from near-surface to over 40km depth, which includes the residue from the basaltic source rocks. It is defined by high VP (7.4kms-1 at 15km depth) and slightly low VP/VS, and has distinct boundaries on its southern and eastern margins. Adjacent to the deepest earthquakes, there is high-velocity Pacific mantle below 80km depth, inferred to be the mantle expression of ongoing shortening since the early Miocene. As the subducting slab moves down and northeast, it is hindered by the high-velocity body and bends to near-vertical. Bending is accommodated by distributed fracturing evidenced by high VP/VS and persistent deep earthquake activity. Buckling of the subducted plate pushes up the Western Fiordland Orthogneiss. In the transition to the Alpine fault in northern Fiordland, a prominent low-velocity crustal root is consistent with ductile thickening in combination with downwarp of the subducted plate.

  13. Variations in plate kinematics and subduction geometries: unifying explanation of Mesozoic and Cenozoic deformation in Rocky Mountains region

    SciTech Connect

    Cross, T.A.; Pilger, R.H. Jr.

    1985-05-01

    The variety of late Mesozoic through early Cenozoic tectonic elements and events in the Rocky Mountains region shows temporal and spatial correspondence with inferred variations in kinematics of plate interactions and geometries of subducted oceanic lithosphere. From this space and time correspondence and current understanding of subduction processes and responses, it is suggested that a unified explanation for the occurrence and genesis of these features. The following tectonic elements and events are regarded as genetic expressions of variations in subduction modes and geometries: (1) the history of igneous activity in the western US, (2) the contrasting styles and loci of deformation along the foreland fold and thrust belt (Sevier style) and the basement-cored uplifts (Laramide style) bordering the northern and eastern margins of the Colorado Plateau, (3) the development and maintenance of the Colorado Plateau as a relatively rigid tectonic block, (4) the timing and geometry of subsidence in the foreland basin, (5) the disjunct history of subsidence and subsequent uplift of the Colorado-Wyoming-Utah (CWU) region beyond the foreland basin, and (6) the initial stability and subsequent subsidence of the High Plains region. During normal subduction, thin-skinned crustal deformation was continuous opposite the convergent margin. During the ensuing period of low-angle subduction, the Colorado Plateau region was underpinned by subducted lithosphere, anomalous subsidence occurred in the CWU locus, and deformation was transferred to the position of greatest contrast in mechanical properties of the crust (the eastern and northern boundaries of the plateau). Decoupling of subducted lithosphere from overlying lithosphere caused uplift and erosional stripping of the CWU region, crustal flexure to the east, and sediment accumulation on the High Plains.

  14. From subduction to collision: constraining the early history of the Taiwan Mountain Belt by plate tectonic reconstructions

    NASA Astrophysics Data System (ADS)

    von Hagke, Christoph; Philippon, Mélody; Avouac, Jean-Philippe

    2014-05-01

    Understanding formation of the Taiwan orogen is important, because it is an active case-example to test geodynamic theories of mountain building processes, such as the critical wedge model, or of subduction zone reversal. Nevertheless, large uncertainties exist regarding the pre-collisional architecture of the orogen, timing of collision, as well as peak metamorphic conditions of the Cenozoic orogeny. The goal of this contribution is to re-evaluate existing models in the light of recent geophysical datasets, and constrain the evolution towards the present day plate tectonic configuration with a comprehensive reconstruction of plate movements since the Late Cretaceous. To this end, we present a revised analysis of the plate tectonic framework of Southeast Asia since the Late Cretaceous, a time when subduction polarity was still opposite to what is observed at present (westward subduction of the Pacific Plate, as opposed to eastward subduction of Eurasia at present). This is independent of the subduction zone reversal thought to occur at present in the northern part of the Taiwan orogen. We place our reconstructions within a global plate tectonic frame, and discuss (1) the consequences of subduction zone reversal for the evolving passive margin, (2) the influence of opening on the (proto-) South China Sea on the pre-collisional architecture. This yields a new model for the collisional history of Taiwan, which reconciles the pre-collisional architecture with the metamorphic conditions of the Cenozoic orogeny, and makes predictions about timing of peak-pressures, as well as the timing of collision and present subduction zone reversal.

  15. Numerical Models of Subduction Beneath Non-Uniform Overriding Plates. Implications for Subduction Velocity and Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Rodríguez-González, J.; Billen, M. I.; Negredo, A. M.

    2015-12-01

    Subduction zones show significant along-strike variations of different subduction features. In particular there is a good correlation between variations of overriding plate thickness and variations of slab dip, seismic anisotropy and subduction velocity. Previous numerical models have shown that overriding plate thermal state influences the slab dip and variations in slab dip can cause trench-parallel flow above the slab. This suggests a causal link between overriding plate structure, slab geometry and mantle flow in subduction zones. Models also show that interplate coupling is stronger for colder overriding plates, which might lead to lower subduction velocities. We implement generic numerical models to study the effect of the overriding plate structure on the evolution of slab geometry, induced mantle flow and subduction velocity. We solve the 3D, time dependent thermo-mechanical equations with a non-linear rheology to simulate buoyancy driven subduction processes. We find that along-strike variations in thermal thickness of the overriding plate cause increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate; the variation in slab geometry drives strong trench-parallel flow beneath the slab and a complex flow pattern above the slab. Induced mantle flow varies significantly with time, suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non-steady-state behavior of these systems. This new mechanism for driving trench-parallel flow provides a good explanation for seismic anisotropy observations from the Middle and South America subduction zones. Our results also show that increased interplate coupling beneath a colder portion of the overriding plate leads to a reduction of the subduction velocity in the region, leading to along-strike variations subduction velocity. These results provide a good explanation of the observed present-day variations in

  16. Tectonic evolution of the Lachlan Fold Belt, southeastern Australia: constraints from coupled numerical models of crustal deformation and surface erosion driven by subduction of the underlying mantle

    NASA Astrophysics Data System (ADS)

    Braun, Jean; Pauselli, Cristina

    2004-04-01

    We have used a coupled thermo-mechanical finite-element (FE) model of crustal deformation driven by mantle/oceanic subduction to demonstrate that the tectonic evolution of the Lachlan Fold Belt (LFB) during the Mid-Palaeozoic (Late Ordovician to Early Carboniferous) can be linked to continuous subduction along a single subduction zone. This contrasts with most models proposed to date which assume that separate subduction zones were active beneath the western, central and eastern sections of the Lachlan Orogen. We demonstrate how the existing data on the structural, volcanic and erosional evolution of the Lachlan Fold Belt can be accounted for by our model. We focus particularly on the timing of fault movement in the various sectors of the orogen. We demonstrate that the presence of the weak basal decollement on which most of the Lachlan Fold Belt is constructed effectively decouples crustal structures from those in the underlying mantle. The patterns of faulting in the upper crust appears therefore to be controlled by lateral strength contrasts inherited from previous orogenic events rather than the location of one or several subduction zones. The model also predicts that the uplift and deep exhumation of the Wagga-Omeo Metamorphic Belt (WOMB) is associated with the advection of this terrane above the subduction point and is the only tectonic event that gives us direct constraints on the location of the subduction zone. We also discuss the implications of our model for the nature of the basement underlying the present-day orogen.

  17. Deformation and faulting of subduction overriding plate caused by a subducted seamount

    NASA Astrophysics Data System (ADS)

    Ding, Min; Lin, Jian

    2016-09-01

    We conducted numerical experiments to simulate elastoplastic deformation of the overriding plate caused by a subducted seamount. Calculations revealed development of a distinct pair of fault-like shear zones, including a landward dipping forethrust fault initiated from the seamount top and a seaward dipping backthrust fault from the landward base of the seamount. Significant dome-shaped surface uplift was predicted above the thrust faults. Lesser-developed seaward dipping backthrust faults were calculated to develop under certain conditions. The overriding plate was calculated to deform in two stages: In Stage I, elastic deformation leads to the formation of fault-like shear zones. After major faults have cut through the entire plate, plastic deformation on faults dominates Stage II. On the subduction interface, compressional normal stress was calculated to increase on the landward leading flank of the seamount and decrease on the seaward trailing flank. These changes, together with associated stress singularities at seamount edges, could affect earthquake processes.

  18. 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

    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.

  19. Fluid budgets along the northern Hikurangi subduction margin, New Zealand: the effect of a subducting seamount on fluid pressure

    NASA Astrophysics Data System (ADS)

    Ellis, Susan; Fagereng, Åke; Barker, Dan; Henrys, Stuart; Saffer, Demian; Wallace, Laura; Williams, Charles; Harris, Rob

    2015-07-01

    We estimate fluid sources around a subducted seamount along the northern Hikurangi subduction margin of New Zealand, using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data. Calculated fluid sources are input into an independent fluid-flow model to explore the key controls on overpressure generation to depths of 12 km. In the thermomechanical models, sediment transport through and beneath the wedge is calculated assuming a pressure-sensitive frictional rheology. The change in porosity, pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration. Our calculations yield more precise information about source locations in time and space than previous averaged estimates for the Hikurangi margin. The volume of fluid release in the wedge is smaller than previously estimated from margin-averaged calculations (˜14 m3 yr-1 m-1), and is exceeded by fluid release from underlying (subducting) sediment (˜16 m3 yr-1 m-1). Clay dehydration contributes only a small quantity of fluid by volume (˜2 m3 yr-1 m-1 from subducted sediment), but the integrated effect is still significant landward of the seamount. Fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid-flow models, using subducted sediment permeability derived from porosity, and testing two end-members for décollement permeability. Models in which the décollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment. However, if the subduction interface becomes impermeable with depth, significant fluid overpressure develops in subducting sediment landward of the seamount. The location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration, shallow, slow slip events (SSEs) landward of the seamount are caused by anomalous fluid pressures; alternatively

  20. Strain accumulation along the Cascadia subduction zone

    USGS Publications Warehouse

    Murray, M.H.; Lisowski, M.

    2000-01-01

    We combine triangulation, trilateration, and GPS observations to determine horizontal strain rates along the Cascadia subduction zone from Cape Mendocino to the Strait of Juan de Fuca. Shear-strain rates are significantly greater than zero (95% confidence) in all forearc regions (26-167 nanoradians/yr), and are not significant in the arc and backarc regions. The deformation is primarily uniaxial contraction nearly parallel to Juan de Fuca-North America plate convergence (N55??-80??E). The strain rates are consistent with an elastic dislocation model for interseismic slip with a shallow 100-km wide locked zone and a deeper 75-km transition zone along the entire megathrust, except along the central Oregon coast where relatively lower strain rates are consistent with 30-40 km wide locked and transition zones.

  1. Viscosity of Fluids in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Audétat, Andreas; Keppler, Hans

    2004-01-01

    The viscosities of aqueous fluids with 10 to 80 weight percent dissolved silicates have been measured at 600° to 950°C and 1.0 to 2.0 gigapascals by in situ observation of falling spheres in the diamond anvil cell. The viscosities at 800°C range from 10-4 to 100.5 pascal seconds. The combination of low viscosities with a favorable wetting angle makes silicate-rich fluid an efficient agent for material transport at low-volume fractions. Our results therefore suggest that there may be a direct relationship between the position of the volcanic front and the onset of complete miscibility between water and silicate melt in the subducting slab.

  2. Viscosity of fluids in subduction zones.

    PubMed

    Audétat, Andreas; Keppler, Hans

    2004-01-23

    The viscosities of aqueous fluids with 10 to 80 weight percent dissolved silicates have been measured at 600 degrees to 950 degrees C and 1.0 to 2.0 gigapascals by in situ observation of falling spheres in the diamond anvil cell. The viscosities at 800 degrees C range from 10(-4) to 10(0.5) pascal seconds. The combination of low viscosities with a favorable wetting angle makes silicate-rich fluid an efficient agent for material transport at low-volume fractions. Our results therefore suggest that there may be a direct relationship between the position of the volcanic front and the onset of complete miscibility between water and silicate melt in the subducting slab. PMID:14739456

  3. Gravity and Flexure Modelling of Subducting Plates

    NASA Astrophysics Data System (ADS)

    Hunter, J. A.; Watts, A. B.; SO 215 Shipboard Scientific Party

    2012-04-01

    The long-term strength of the lithosphere is determined by its flexural rigidity, which is commonly expressed through the effective elastic thickness, Te. Flexure studies have revealed a dependence of Te on thermal age. In the oceans, loads formed on young (70 Ma) seafloor. In the continents, loads on young (1000 Ma) lithosphere. Recent studies have questioned the relationship of Te with age, especially at subduction zones, where oceanic and continental lithosphere are flexed downwards by up to ~6 km over horizontal distances of up to ~350 km. We have therefore used free-air gravity anomaly and topography profile data, combined with forward and inverse modelling techniques, to re-assess Te in these settings. Preliminary inverse modelling results from the Tonga-Kermadec Trench - Outer Rise system, where the Pacific plate is subducting beneath the Indo-Australian plate, show large spatial variations in Te that are unrelated to age. In contrast to the southern end of the system, where Te is determined by the depth to the 600° C and 900° C isotherms, the northern end of the system shows a reduction in strength. Results also suggest a reduction in Te trenchward of the outer rise that is coincident with a region of pervasive extensional faulting visible in swath bathymetry data. In a continental setting, the Ganges foreland basin has formed by flexure of the Indo-Australian plate in front of the migrating loads of the Himalaya. Preliminary forward modelling results, using the Himalaya as a known surface topographic load, suggest that Te is high - consistent with the great age of Indian cratonic lithosphere. However, results from inverse modelling that solves for unknown loads (vertical shear force and bending moment) show significant scatter and display trade-offs between Te and these driving loads.

  4. Lithospheric-scale effects of a subduction-driven Alboran plate: improved neotectonic modeling

    NASA Astrophysics Data System (ADS)

    Neres, Marta; Carafa, Michele; Terrinha, Pedro; Fernandes, Rui; Matias, Luis; Duarte, João; Barba, Salvatore

    2016-04-01

    The presence of a subducted slab under the Gibraltar arc is now widely accepted. However, discussion still remains on whether subduction is active and what is its influence in the lithospheric processes, in particular in the observed geodesy, deformation rates and seismicity. Aiming at bringing new insights into the discussion, we have performed a neotectonic numerical study of a segment of the Africa-Eurasia plate boundary, from the Gloria fault to the Northern Algerian margin. Specifically, we have tested the effect of including or excluding an independently driven Alboran plate, i.e. testing active subduction versus inactive subduction (2plates versus 3plates scenarios). We used the dynamic code SHELLS (Bird et al., 2008) to model the surface velocity field and the ongoing deformation, using a new up-to-date simplified tectonic map of the region, new available lithospheric data and boundary conditions determined from two alternative Africa-Eurasia angular velocities, respectively: SEGAL2013, a new pole based on stable Africa and stable Eurasia gps data (last decades); and MORVEL, a geological-scale pole (3.16 Ma). We also extensively studied the variation within the parametric space of fault friction coefficient, subduction resistance and surface velocities imposed to the Alboran plate. The final run comprised a total of 5240 experiments, and each generated model was scored against geodetic velocities, stress direction data and seismic strain rates. The preferred model corresponds to the 3plates scenario, SEGAL2013 pole and fault friction of 0.225, with scoring results: gps misfit of 0.78 mm/yr; SHmax misfit of 13.6° and correlation with seismic strain rate of 0.62, significantly better than previous models. We present predicted fault slip rates for the recognized active structures and off-faults permanent strain rates, which can be used for seismic and tsunami hazard calculations (the initial motivation for this work was contributing for calculation of

  5. Jadeitite formed during subduction: Zircon geochronology constraints from two different tectonic events on the Guatemala Suture Zone

    NASA Astrophysics Data System (ADS)

    Flores, K. E.; Martens, U.; Harlow, G. E.; Brueckner, H. K.

    2012-12-01

    Jadeitite is a rare rock type associated with high-pressure—low-temperature blocks from serpentinite matrix mélange. Evolving models of formation involve precipitation from subduction zone aqueous fluids in veins cutting the overlying mantle wedge or metasomatism of channel blocks emplaced into it [1]. Age determination of the mélanges is based on dating included rocks for "peak metamorphism" or constituent minerals, such as micas by Ar-Ar, which may only provide exhumation constraints. New SHRIMP-RG 238U-206Pb dates from metasomatic/solution-precipitate zircons in jadeitites and mica-albite rocks from Guatemala combined with other recently reported dates show (a) older mean ages that are a clear evidence for jadeitite crystallization 10-30 Ma prior to peak subduction zone metamorphism (e.g., crystallization of eclogite), and (b) a second group of younger mean ages slightly younger or similar to exhumation ages given by Ar-Ar dates from micas. These age relationships are also observed at other jadeitite localities, such as Sym-Keu ultramafic complex in the Polar Urals (Russia) and the serpentinite mélanges of the Río San Juan complex (Dominican Republic). The data argue for formation of jadeitite in the non-subducting mantle wedge during active subduction. Thus, jadeitite provides a record of fluid introduction into the mantle wedge during subduction rather than during exhumation. Reference: [1] Tsujimori & Harlow 2012, Eur. J. Mineral. 24, 371-390.

  6. Plate deformation at the transition between collision and subduction: insights from 3D thermo-mechanical laboratory experiments

    NASA Astrophysics Data System (ADS)

    Boutelier, D. A.; Cruden, A. R.; Oncken, O.

    2012-04-01

    3-D thermo-mechanical laboratory experiments of arc-continent collision investigate plate deformation at the transition between collision and subduction. Deformation in the collision area propagates into the subduction-collision transition zone via along-strike coupling of the neighboring segments of the plate boundary. The largest along-strike gradient of trench-perpendicular compression produced by a passive margin turning by 90 degrees does not generate sufficiently localized shear strain in the transition zone to cause a strike-slip system. This is because of the fast propagation of lithosphere failure in the arc area. Deformation is thus continuous along-strike, but the deformation mechanism is three-dimensional and progressive structural variations arise because the coupling between neighboring segments induces either advanced or delayed failure of the arc lithosphere and passive margin. During the initial stage of collision, the accretionary wedge is partially subducted, the interplate zone is lubricated, and shear traction drops. Thus large convergence obliquity does not produce a migrating fore-arc sliver. Instead, the fore-arc motion is due to the pressure force generated by subduction of the buoyant continental crust. It follows that convergence obliquity does not yield trench-parallel deformation of the fore-arc and its influence on the collision process is limited. However, convergence obliquity may have shaped the active margin during the stage of oceanic subduction stage, prior to collision, and inherited structures may impact the propagation mechanism.

  7. Relamination of mafic subducting crust throughout Earth's history

    NASA Astrophysics Data System (ADS)

    Maunder, Ben; van Hunen, Jeroen; Magni, Valentina; Bouilhol, Pierre

    2016-09-01

    Earth has likely cooled by several hundred degrees over its history, which has probably affected subduction dynamics and associated magmatism. Today, the process of compositional buoyancy driven upwelling, and subsequent underplating, of subducted materials (commonly referred to as "relamination") is thought to play a role in the formation of continental crust. Given that Archean continental crust formation is best explained by the involvement of mafic material, we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early Earth subduction zones, to assess if such a process might have been viable in an early Earth setting. Our numerical parametric study illustrates that the hotter, thicker-crust conditions of the early Earth favour the upward relamination of mafic subducting crust. The amount of relaminating subducting crust is observed to vary significantly, with subduction convergence rate having the strongest control on the volume of relaminated material. Indeed, removal of the entire mafic crust from the subducting slab is possible for slow subduction (∼2 cm/yr) under Archean conditions. We also observe great variability in the depth at which this separation occurs (80-120 km), with events corresponding to shallower detachment being more voluminous, and that relaminating material has to remain metastably buoyant until this separation depth, which is supported by geological, geophysical and geodynamical observations. Furthermore, this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 Myrs, similar to timescales of episodicity observed in the Archean rock record. We demonstrate that this relamination process can result in the heating of considerable quantities of mafic material (to temperatures in excess of 900 °C), which is then emplaced below the over-riding lithosphere. As such, our results have implications for Archean subduction zone magmatism, for

  8. Constraints on Subduction Zone Temperatures and Chemical Fluxes from Accessory Phase Saturation in Subducted Sediments (Invited)

    NASA Astrophysics Data System (ADS)

    Blundy, J.; Skora, S.

    2009-12-01

    A global correlation between the incompatible trace element chemistry of subducted sediments and that of magmas erupted through the overlying plate testifies to the important role that sediments play in controlling magmagenesis. We report new experimental data on the high pressure (3 GPa) and temperature (700-1300 °C) phase relations of hydrous red clay to evaluate the role of residual accessory phases in controlling the incompatible trace element chemistry of sediment-derived fluids. A particular focus is monazite, which preferentially incorporates LREE and Th, exerting a powerful control on the fluid Th/LREE ratio. Given that arc magmas appear to inherit the Th/LREE ratio of the associated subducting sediment, understanding monazite solubility and stability has the potential to provide new constraints on temperature at the slab surface, a parameter that is notoriously hard to pin down by other means. Experiments on red clay with H2O structurally bound in hydrous minerals (<2 wt%) produced negligible melt fractions for any reasonable temperatures. Experiments with 7-15 wt% added H2O produced copious melting at temperatures only 50-100°C above the solidus (c. 720 °C), e.g. with 15 wt% added H2O, the red clay is >50% molten at 800 °C. Irrespective of the amount of added H2O the residual assemblage comprised garnet and kyanite up to the liquidus (1250 °C with 7 wt% H2O). Phengite is present ≤800 °C; ilmenite and rutile persist to c. 1000 °C; quartz/coesite-out temperature decreases from 1000 °C with 7 wt% H2O to 800 °C with 15%. Monazite was stable to 950 °C in the Th- and LREE-doped starting materials that we used. The trace element contents of the residual melts in equilibrium with monazite-thorite solid solutions were used, in conjunction with published data, to develop a thermodynamic model of LREE and Th solubility in sediment-derived fluids. These models were used to calculate the stability of monazite in a red clay with natural LREE and Th levels

  9. The initiation of subduction: criticality by addition of water?

    PubMed

    Regenauer-Lieb, K; Yuen, D A; Branlund, J

    2001-10-19

    Subduction is a major process of plate tectonics; however, its initiation is not understood. We used high-resolution (less than 1 kilometer) finite-element models based on rheological data of the lithosphere to investigate the role played by water on initiating subduction. A solid-fluid thermomechanical instability is needed to drive a cold, stiff, and negatively buoyant lithosphere into the mantle. This instability can be triggered slowly by sedimentary loading over a time span of 100 million years. Our results indicate that subduction can proceed by a double feedback mechanism (thermoelastic and thermal-rheological) promoted by lubrication due to water.

  10. 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

  11. 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.

  12. Preservation of Paleoseismic and Paleogeodetic Records of mid to late Holocene Subduction Zone Earthquakes in Different Coastal Settings

    NASA Astrophysics Data System (ADS)

    Kelsey, H. M.; Horton, B.; Rubin, C. M.; Grand Pre, C.; Hawkes, A. D.; Dura, T.; Daryono, M.; Ladinsky, T.

    2009-12-01

    Dynamic variations in sea level and solid Earth properties along active subduction zones predetermine the duration and when paleoseismic and paleogeodetic records will be preserved in coastal regions. The most direct, reliable way to chronicle the history of past subduction zone earthquakes is through coastal stratigraphic sequences that preserve abrupt and gradual relative sea level changes caused by great subduction earthquake cycles. Specifically, paleoseismic timing and paleo geodetic determination of vertical displacement can be obtained through the application of litho-, bio- and chronostratigraphic analyses of selected coastal stratigraphic sequences. Such stratigraphic sequences are only preserved under a specific set of conditions wherein sea level rise, crustal loading, local crustal thickness and imposed strain accumulation and release from megathrust and upper plate faults and folds collectively conspire to provide a long-term, gradual relative sea level rise over millenia that span at least two or three subduction earthquake cycles. Given the conditions necessary to preserve stratigraphic sequences recording multiple great subduction earthquake cycles, it is not surprising that robust paleoseismic records from coastal marsh stratigraphies are rare. To illustrate the conditions under which coastal marshes preserve paleoseismic records of great subduction zone earthquakes, we present two sites with different combinations of sea level rise, crustal loading, crustal thickness and local tectonics. Although both sites preserve a paleoseismic record of subduction zone earthquakes, the length of the records and the specific time range of the records are notably different. The coastal, equatorial, island tropical setting in the Indian Ocean preserves tidal-marsh stratigraphic records of great subduction zone earthquakes in the time window 7-5 ka. In contrast, mid-latitude, North American, northeast Pacific coastal settings preserve tidal-marsh stratigraphic

  13. 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

  14. Fluid Budgets Along the Northern Hikurangi Subduction Margin, New Zealand: the Effect of a Subducting Seamount on Fluid Pressure

    NASA Astrophysics Data System (ADS)

    Ellis, S. M.; Fagereng, A.; Saffer, D. M.; Barker, D. H.; Henrys, S. A.; Williams, C. A.; Wallace, L.; Buiter, S. J.

    2013-12-01

    The northern Hikurangi subduction margin, offshore North Island, New Zealand, has a shallow transition from locked to aseismically creeping behaviour (<5-15 km) that is accompanied by well-characterized slow-slip events. This region has recently been the focus of international efforts to understand transient slip events at subduction margins. Subduction of a seamount and excess pore fluid pressures along and above the subduction interface there have been implicated in the particularly shallow region over which slow-slip events have been observed We estimate fluid sources and sinks for the northern Hikurangi subduction margin using thermo-mechanical numerical modeling constrained by wedge structure and porosities from multichannel seismic data. Sediment transport through and beneath the accretionary wedge is calculated assuming a pressure-sensitive frictional rheology. The change in porosity, pressure and temperature with rock advection is used to compute fluid compaction and dehydration sources as sediment flows through and under the accretionary wedge. Computed fluid source terms yield more precise information about source locations in time and space than previous averaged estimates for the Hikurangi margin. The magnitude of fluid release in the accretionary wedge is an order of magnitude smaller than previously predicted for the entire margin, and is smaller than calculated fluid release from pore water in subducting sediments. Clay dehydration, while producing only small quantities of fluid, occurs over a large enough area to be a significant source landward of a subducted seamount imaged on the seismic profile. Fluid source terms are used to estimate fluid pressures around a subducting seamount. Models in which the decollement acts as a fluid conduit overlying low-permeability subducting sediment generate significant fluid overpressure landward of the seamount, supporting the idea that short duration, shallow, slow slip events in this region may be related to

  15. Crustal wedge deformation in an internally-driven, numerical subduction model

    NASA Astrophysics Data System (ADS)

    van Dinther, Ylona; Morra, Gabriele; Funiciello, Francesca; Rossetti, Federico; Faccenna, Claudio

    2010-05-01

    The Earth's active convergent margins are characterized by dynamic feedback mechanisms that interact to form an intricate system in which a crustal wedge is shaped and metamorphosed at the will of two large, converging plates. This framework is accompanied by complicated processes, such as seismogenesis and the exhumation of high pressure rocks. To honor the dynamic interaction between different entities and advance on these persisting issues, we model the interaction between the subducting and overriding lithospheres, the mantle and the crustal wedge explicitly, and observe how a crustal wedge evolves in detail within a set of rigid, internally-driven boundary conditions. We model crustal wedge evolution in an intra-oceanic subduction setting by using a plane-strain implicit solid-mechanical Finite Element Model, in which the mechanical conservation equations are solved using the software package ABAQUS. The crustal wedge is modeled as a thick-skinned accretionary wedge of inter-mediate thickness with a linear visco-elastic bulk rheology. The dynamic interaction between the subducting plate, the overriding plate, and crustal wedge is implemented using a Coulomb frictional algorithm. The interaction with the mantle is incorporated using a computationally favorable mantle drag formulation that simulates induced three-dimensional mantle flow. This results in a quasi-static framework with a freely moving slab, trench, and fault, where a weaker wedge deforms in response to self-regulating, rigid boundary conditions formed by single, frictional bounding faults. The self-regulating evolution of crustal wedge architecture follows three phases; 1) initial vertical growth, 2) coeval compression and extension leading to internal corner flow, and 3) a steady-state taper with continuous corner flow. Particle trajectories show that, as shortening continues throughout the second phase, wedge material is constantly forced upward against the backstop, while extension and ocean

  16. Water cycling beneath subduction zones in 2D and 3D numerical models (Invited)

    NASA Astrophysics Data System (ADS)

    Rupke, L.; Iyer, K. H.; Hasenclever, J.; Morgan, J.

    2013-12-01

    Tracing the cycling of fluids and volatiles through subduction zones continues to be a challenging task with budgets still having large error bars attached to them. In this contribution we show how numerical models can help to integrate various geological, geophysical, and geochemical datasets and how they can be used to put better bounds on the likely amounts of water being subducted, released into the arc and back-arc melting regions, and recycled to the deeper mantle. To achieve this task we use a suite of numerical models. Bending related faulting and hydration of the incoming lithosphere is resolved using a reactive flow model that solves for crustal scale fluid flow and mantle serpentinization using reaction kinetics. Seismic tomography studies from offshore Chile and Central America are used to evaluate and constrain the effective reaction rate. These rates are then used to assess the contribution of serpentinization to the water budget at subduction zones. The pattern of hydration is controlled by the reaction kinetics and serpentinization is most intense around the 270°C isotherm. The depth of this isotherm correlates well with the dominant spacing of double seismic zones observed globally. Comparison of the results with heat flow data suggests that observed seafloor temperature gradients in the bend-fault region are too low to be caused by ';one-pass' downward water flow into the serpentinizing lithosphere, but rather suggest that bend-faults are areas of active hydrothermal circulation. This implies that serpentine-sourced vents and chemosynthetic vent communities should be found in this deep-sea environment as well. Dehydration reactions are resolved with a 2D kinematic subduction zone model that computes the temperature field and the likely locations and volumes of slab fluid release due to metamorphic dehydration reactions. Here we find that up to 1/3 of the subducted water may be transported into the deeper mantle for the coldest subduction zones

  17. 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

  18. Subduction, back-arc spreading and global mantle flow

    NASA Technical Reports Server (NTRS)

    Hager, B. H.; Oconnell, R. J.; Raefsky, A.

    1983-01-01

    It is pointed out that the subducted lithosphere associated with Benioff zones provides the only direct evidence about the flow in the earth's interior associated with plate motions. It is the primary objective of the present investigation to study the relation between the orientation of subducting lithosphere and the flow patterns (both local and global) near subduction zones. Most of the calculations conducted are based on simple flow models for radially symmetric, Newtonian viscous spheres. The investigation is concerned with the possibility that a simple model of global mantle flow could account for some features of subduction zones. It is found that such a model can account for the orientation of the seismic zones, and, in addition, also for features related to back-arc spreading and perhaps the maximum earthquake size.

  19. Stability of hydrous phases in subducting oceanic crust

    USGS Publications Warehouse

    Liu, J.; Bohlen, S.R.; Ernst, W.G.

    1996-01-01

    Experiments in the basalt-H2O system at 600-950??C and 0.8-3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T ??? 650??C; the dehydration H2O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H2O is stored in lawsonite at 600??C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7??C/km, H2O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.

  20. Geophysics. Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface.

    PubMed

    Yamashita, Y; Yakiwara, H; Asano, Y; Shimizu, H; Uchida, K; Hirano, S; Umakoshi, K; Miyamachi, H; Nakamoto, M; Fukui, M; Kamizono, M; Kanehara, H; Yamada, T; Shinohara, M; Obara, K

    2015-05-01

    Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events. PMID:25954006

  1. How do subduction processes contribute to forearc Andean uplift? Insights from numerical models

    NASA Astrophysics Data System (ADS)

    Martinod, J.; Regard, V.; Letourmy, Y.; Henry, H.; Hassani, R.; Baratchart, S.; Carretier, S.

    2016-05-01

    We present numerical models to study how changes in the process of subduction may explain the observed Quaternary uplift of the Andean forearc region. Indeed, most segments of the South American Pacific coasts between 16 and 32° S have been uplifting since the Lower Pleistocene, following a period of stability of the forearc region. Models confirm that local uplift is expected to occur above ridges, this phenomenon being predominant in central Peru where the Nazca Ridge is subducting. We investigate the effects of slab pull, interplate friction and convergence velocity on the vertical displacements of the overriding plate. We propose that the global tendency to coastal uplift is accompanying the deceleration of the Nazca-South America convergence that occurred in the Pleistocene. In contrast, forearc subsidence may accompany increasing convergence velocities, as suggested by the subsidence history of the South America active margin.

  2. Tomographic imaging of the Cascadia subduction zone: Constraints on the Juan de Fuca slab

    NASA Astrophysics Data System (ADS)

    Chen, Chuanxu; Zhao, Dapeng; Wu, Shiguo

    2015-04-01

    We used 40,343 P-wave arrival times from 1883 local earthquakes and 105,455 P-wave arrivals from 6361 teleseismic events to study the detailed structure of the Cascadia subduction zone. We conducted tomographic inversions using a starting velocity model which includes the high-velocity subducting Juan de Fuca slab as a priori information. A number of such slab-constrained inversions are conducted by changing the slab thickness and the velocity contrast between the slab and the surrounding mantle. Our optimal 3-D velocity model fits the data much better than that determined by an inversion with a 1-D homogeneous starting model. Our results show that the subducting Juan de Fuca slab has a thickness of 30-50 km and a P-wave velocity of 1-3% higher than that of the surrounding mantle. Beneath the northern and southern parts of the Cascadia, P-wave velocity is lower in the slab and along the slab interface, which may reflect a more hydrated slab and more active slab dehydration there. The lateral velocity variations may indicate different degrees of slab dehydration and forearc mantle serpentinization. The segmentation in episodic tremor and slip (ETS) is also spatially coincident with the velocity heterogeneities, indicating that the ETS occurrence and recurrence interval are controlled by fluid activity in and around the mantle wedge corner.

  3. Electromagnetic Precursors Leading to Triangulation of Future Earthquakes and Imaging of the Subduction Zone

    NASA Astrophysics Data System (ADS)

    Heraud, J. A.; Centa, V. A.; Bleier, T.

    2015-12-01

    During several sessions in past AGU meetings, reports on the progress of analysis of magnetometer data have been given, as our research moved from a one dimensional geometry, to two and finally to a three dimensional image. In the first case, we learned how to extract one coordinate, azimuth information, on the occurrence of an earthquake based on the processing of mono-polar pulses received at a single station. A two dimensional geometry was implemented through triangulation and we showed the use of this technique to find out where a future epicenter would occur. Recently, we have obtained compelling evidence that the pressure points leading to the determination of future epicenters originate at a plane, inclined with the same angle as the subduction zone, a three-dimensional position of the future hypocenter. Hence, an image of the subduction zone or interface between the Nazca plate and the continental plate in the northern area of Lima, Peru, has been obtained, corresponding to the subduction zone obtained by traditional seismic methods. Our work with magnetometers deployed along part of the Peruvian coast since 2009, has shown that it is possible to measure, with significant precision, the azimuth of electromagnetic pulses propagating from stress points in the earth's crust due to the subduction of tectonic plates, as to be able to determine precisely the origin of the pulses. The occurrence of earthquakes approximately 11 to 18 days after the appearance of the first pulses and the recognition of grouping of such pulses, has allowed us to determine accurately the direction and the timing of future seismic events. Magnetometers, donated by Quakefinder and Telefonica del Peru were then strategically installed in different locations in Peru with the purpose of achieving triangulation. During two years since 2013, about a dozen earthquakes have been associated with future seismic activity in a pre or post occurrence way. Our presentation will be based on animated

  4. Demise of Flat-slab Subduction at the end of the Laramide Orogeny (Invited)

    NASA Astrophysics Data System (ADS)

    Humphreys, E.

    2013-12-01

    absence of major change in California subduction presumably indicates that while the flat slab fell off of North America east of California, it remained at the base of California. More recently, the arrival of Yellowstone beneath S. Oregon triggered a delamination of the flat slab remaining beneath N. Oregon, drawing flood basalt activity north.

  5. Imaging continental collision and subduction in the Pamir mountain range, Central Asia, by seismic attenuation tomography

    NASA Astrophysics Data System (ADS)

    Schurr, Bernd; Haberland, Christian; Sippl, Christian; Yuan, Xiaohui; Mechie, James; Schneider, Felix; Tipage Team

    2014-05-01

    Subduction of continental crust is the mode of shortening in continental collision that is the least well understood. It is known to occur, as testified e.g., by now exhumed ultra-high-pressure rocks, despite the fact that continental crust is generally too buoyant to submerge into the mantle. Continental crust may, however, subduct in tow of a leading dense oceanic plate at the last stage of the plate tectonic Wilson cycle. Alternatively, if upper and lower crust detach, the latter, together with the underlying cold mantle lithosphere, may become negatively buoyant, enabling their descent. The Pamir mountains in Central Asia have been one of the few places on Earth, where on-going continental subduction has been postulated based on an active Wadati-Benioff zone. The Pamir is situated on an orographic node northwest of Tibet, between the Tarim and Tajik basins, where the Hindu Kush, Karakorum, western Kunlun Shan and Tien Shan ranges coalesce. It formed in the late Paleogene to Neogene, i.e. approximately during the second half of the India-Asia collision, north of the Western Himalayan Syntaxis, on the Asian (retro)continent. We use tomography of seismic attenuation to image the lithospheric-scale structure of the Pamir orogen. Attenuation tomography has been shown to be a powerful tool to study deep process-related structures particularly in oceanic subduction zones. Attenuation at this scale may be seen as a proxy for rheology and hence is very sensitive to e.g., homologous temperature and deformation. We use data from a two-year seismic deployment of the Tien Shan-Pamir Geodynamic Program (TIPAGE). The whole path attenuation parameter t* is determined by inversion of P-wave velocity spectra from 1790 earthquakes and then inverted for a 3D attenuation model (Qp) employing a recently published 3D velocity model. We find a prominent continuous crescent-shaped high-attenuation anomaly (HAA) that penetrates from upper crustal levels to depths of more than 100 km. At

  6. Subduction of Atlantic aseismic ridges and Late Cenozoic evolution of the Lesser Antilles island arc

    NASA Astrophysics Data System (ADS)

    Bouysse, P.; Westercamp, D.

    1990-04-01

    A great amount of data concerning the Lesser Antilles arc and the near Atlantic has been reviewed: nature, timing and spatial evolution of the Oligocene to Recent volcanic sequences; the location and scale of uplifted areas; facturation of the arc ridge; the structural pattern of the Central Atlantic oceanic crust; and characteristics of the subduction beneath the Caribbean plate. These data allow us to lay stress on the role of the interaction of underthrust oceanic aseismic ridges with the evolution of the overriding arc. Depending on the degree of gravimetrical compensation (buoyancy) of these features, this interaction can induce effects of the first and second orders of magnitude. With the onset of the Late Oligocene, a buoyant ridge of anomalous mid Cretaceous crust reached the front of the northern half of the Lesser Antilles arc basement. It momentarily stopped the subduction along the whole arc, bringing the arc volcanism (Older arc) to a standstill as it was underplated beneath the eastern rim of the Caribbean lithosphere, and triggering the westward tilting of this part of the arc, with locally dramatic uplifting (La Désirade). After a gap of 9-10 m.y., the volcanic activity resumed in the early Burdigalian (Recent arc), with a westward jump of the volcanic axis north of Martinique (Inner arc, corresponding to the northern part of the Recent arc). Later, the subduction of non-buoyant (gravimetrically uncompensated) ridges (Barracuda, Tiburon and St. Lucia ridges) exerted significant effects, but of a lower extent with regard to their spatial incidence and intensity, on the arc north of St. Lucia. These effects induce a sudden transverse shift of the volcanic front, of only some kilometers in length (forward and backward), centrifugal (longitudinal) migration of the eruptive centers on either side of the arc segment above the ridge, gentle uplifts, and enhance seismic and hydrothermal ( sensu lato) manifestations. An inverse approach of reasoning leads

  7. Fluid and mass transfer into the cold mantle wedge of subduction zones: budgets and seismic constraints

    NASA Astrophysics Data System (ADS)

    Abers, G. A.; Hacker, B. R.; Van Keken, P. E.; Nakajima, J.; Kita, S.

    2015-12-01

    Dehydration of subducting plates should hydrate the shallow overlying mantle wedge where mantle is cold. In the shallow mantle wedge hydrous phases, notably serpentines, chlorite, brucite and talc should be stable to form a significant reservoir for H2O. Beneath this cold nose thermal models suggest only limited slab dehydration occurs at depths less than ca. 80 km except in warm subduction zones, but fluids may flow updip from deeper within the subducting plate to hydrate the shallow mantle. We estimate the total water storage capacity in cold noses, at temperatures where hydrous phases are stable, to be roughly 2-3% the mass of the global ocean. At modern subduction flux rates its full hydration could be achieved in 50-100 Ma if all subducting water devolatilized in the upper 100 km flows into the wedge; these estimates have at least a factor of two uncertainty. To investigate the extent to which wedge hydration actually occurs we compile and generate seismic images of forearc mantle regions. The compilation includes P- and S-velocity images with good sampling below the Moho and above the downgoing slab in forearcs, from active-source imaging, local earthquake tomography and receiver functions, while avoiding areas of complex tectonics. Well-resolved images exist for Cascadia, Alaska, the Andes, Central America, North Island New Zealand, and Japan. We compare the observed velocities to those predicted from thermal-petrologic models. Among these forearcs, Cascadia stands out as having upper-mantle seismic velocities lower than overriding crust, consistent with high (>50%) hydration. Most other forearcs show Vp close to 8.0 km/s and Vp/Vs of 1.73-1.80. We compare these observations to velocities predicted from thermal-mineralogical models. Velocities are slightly slower than expected for dry peridotite and allow 10-20% hydration, but also could also be explained as relict accreted rock, or delaminated, relaminated, or offscraped crustal material mixed with mantle

  8. 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

  9. Subduction of oceanic asthenosphere: A critical appraisal in central Alaska

    NASA Astrophysics Data System (ADS)

    Song, Teh-Ru Alex; Kawakatsu, Hitoshi

    2013-04-01

    Song and Kawakatsu (2012) have shown that the sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subduction of the oceanic asthenosphere that has strong radial anisotropy. This model not only explains the non-intuitive trench-parallel splitting pattern in most of subduction zones, but also predicts the trench-normal behavior (fast polarization direction sub-parallel to the absolute plate motion of the incoming plate) observed in several shallow subduction zones. The general validity of such a scenario is crucial to fundamental understandings of the development of mantle anisotropy in sub-lithosphere or/and sub-slab conditions, the nature and formation of oceanic asthenosphere as well as the flow pattern and mass transport near subduction zones. To validate this scenario, we examine SKS splitting patterns observed across the fore-arc in central Alaska. Here the fast splitting direction varies from plate motion sub-parallel near the trench to mostly trench-parallel beyond the 100 km slab-isodepth contour, while being strongly variable in between. After taking into account the rotation of anisotropy symmetry in the oceanic asthenosphere with respect to the local plate motion obliquity and down-dip variations in the slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as the back azimuth dependent splitting pattern across the entire fore-arc. The current validation further augments the idea that, apart from anisotropy in the mantle wedge and the subducting slab, subduction of the oceanic asthenosphere is likely to be the dominant source of seismic anisotropy in central Alaska and potentially in many subduction zones. Furthermore, this result also provides alternative views to models of seismic anisotropy in the mantle wedge and on the length scale in which the 3D mantle flow may be important.

  10. The role of deep subduction in supercontinent breakup

    NASA Astrophysics Data System (ADS)

    Capitanio, Fabio; Dal Zilio, Luca; Faccenda, Manuele

    2016-04-01

    The breakup of continents is a crucial stage of the episodic aggregation and dispersal of tectonic plates. In particular, the transition from a stable supercontinent to its rifting, breakup and subsequent drifting is one of the least understood aspects of plate tectonics. Over the last decades, several works have highlighted the potential role of pre-existing weaknesses or that of raising mantle plumes in assisting the localization of strain. However, to sustain large-scale divergent regime over geological time, extensional stresses are strictly required. Here we present results from 2-D thermo-mechanical numerical experiments and we show that rifting and drifting of continents result from lithospheric subduction at convergent margins, when this extends to lower mantle depths. We quantify the drag exerted by subduction-induced mantle flow along the basal surface of continental plates, comparing models where lithospheric slabs stagnate above the upper-lower mantle boundary with those where slabs penetrate into the lower mantle. When subduction is upper mantle-confined, divergent basal tractions localize at distances comparable to the effective upper mantle thickness (~500 km), causing the breakup of a microcontinent and opening of a marginal basin. Instead, when the descending lithosphere subducts deeper, extensional stresses localize at greater distances from the trench (≥ 2900 km), are higher and are sustained over a longer time. Although relatively low, basal shear stresses integrated over large plates generate tension forces that may exceed the strength of the continental lithosphere, eventually leading to breakup and opening of an intervening distal basin. The models illustrate that the mechanism leading to the formation of back-arc basins above upper mantle-confined subduction provides a viable explanation for the opening of larger basins above deeper subduction. Examples include the Atlantic Ocean formation and the South and North American plates drifting

  11. Advancing techniques to constrain the geometry of the seismic rupture plane on subduction interfaces a priori: Higher-order functional fits

    USGS Publications Warehouse

    Hayes, G.P.; Wald, D.J.; Keranen, K.

    2009-01-01

    Ongoing developments in earthquake source inversions incorporate nonplanar fault geometries as inputs to the inversion process, improving previous approaches that relied solely on planar fault surfaces. This evolution motivates advancing the existing framework for constraining fault geometry, particularly in subduction zones where plate boundary surfaces that host highly hazardous earthquakes are clearly nonplanar. Here, we improve upon the existing framework for the constraint of the seismic rupture plane of subduction interfaces by incorporating active seismic and seafloor sediment thickness data with existing independent data sets and inverting for the most probable nonplanar subduction geometry. Constraining the rupture interface a priori with independent geological and seismological information reduces the uncertainty in the derived earthquake source inversion parameters over models that rely on simpler assumptions, such as the moment tensor inferred fault plane. Examples are shown for a number of wellconstrained global locations. We expand the coverage of previous analyses to a more uniform global data set and show that even in areas of sparse data this approach is able to accurately constrain the approximate subduction geometry, particularly when aided with the addition of data from local active seismic surveys. In addition, we show an example of the integration of many two-dimensional profiles into a threedimensional surface for the Sunda subduction zone and introduce the development of a new global threedimensional subduction interface model: Slab1.0. ?? 2009 by the American Geophysical Union.

  12. Subduction zone earthquake probably triggered submarine hydrocarbon seepage offshore Pakistan

    NASA Astrophysics Data System (ADS)

    Fischer, David; José M., Mogollón; Michael, Strasser; Thomas, Pape; Gerhard, Bohrmann; Noemi, Fekete; Volkhard, Spiess; Sabine, Kasten

    2014-05-01

    Seepage of methane-dominated hydrocarbons is heterogeneous in space and time, and trigger mechanisms of episodic seep events are not well constrained. It is generally found that free hydrocarbon gas entering the local gas hydrate stability field in marine sediments is sequestered in gas hydrates. In this manner, gas hydrates can act as a buffer for carbon transport from the sediment into the ocean. However, the efficiency of gas hydrate-bearing sediments for retaining hydrocarbons may be corrupted: Hypothesized mechanisms include critical gas/fluid pressures beneath gas hydrate-bearing sediments, implying that these are susceptible to mechanical failure and subsequent gas release. Although gas hydrates often occur in seismically active regions, e.g., subduction zones, the role of earthquakes as potential triggers of hydrocarbon transport through gas hydrate-bearing sediments has hardly been explored. Based on a recent publication (Fischer et al., 2013), we present geochemical and transport/reaction-modelling data suggesting a substantial increase in upward gas flux and hydrocarbon emission into the water column following a major earthquake that occurred near the study sites in 1945. Calculating the formation time of authigenic barite enrichments identified in two sediment cores obtained from an anticlinal structure called "Nascent Ridge", we find they formed 38-91 years before sampling, which corresponds well to the time elapsed since the earthquake (62 years). Furthermore, applying a numerical model, we show that the local sulfate/methane transition zone shifted upward by several meters due to the increased methane flux and simulated sulfate profiles very closely match measured ones in a comparable time frame of 50-70 years. We thus propose a causal relation between the earthquake and the amplified gas flux and present reflection seismic data supporting our hypothesis that co-seismic ground shaking induced mechanical fracturing of gas hydrate-bearing sediments

  13. Destruction of Luzon forearc basin from subduction to Taiwan arc-continent collision

    NASA Astrophysics Data System (ADS)

    Hirtzel, Justin; Chi, Wu-Cheng; Reed, Donald; Chen, Liwen; Liu, Char-Shine; Lundberg, Neil

    2009-12-01

    Along offshore to the east of southern Taiwan, different stages of subduction and collision occur simultaneously along strike of the convergent boundary. As a result, the evolution of the Luzon arc and its forearc basin can be studied from the younger subduction zone to the south to the collision zone to the north. Examining more than 8000 km of seismic lines, we analyzed the seismic stratigraphy of strata in a forearc basin and its successive basins in the collision zone, to study the processes related to arc collapse and forearc basin closure. The study area presents three evolutional stages: intra-oceanic subduction, initial arc-continent collision, and arc-continent collision. We divided 9 seismic sequences in the forearc basin and found older, sub-parallel basin-fill sequences (4-9) and younger, divergent sequences (1-3). Isochron maps of the sequences were used to interpret different deformation modes and their areal extends. On the arc side of the basin of the subduction and initial collision zones, we found relatively undisturbed strata, showing little arc deformation. On the trench side, the growth strata in sequences 1 through 3 are the result of recent tectonic wedging along the rear of the accretionary prism. Tectonic wedging and back-thrusts incorporate the forearc strata into the rear of the accretionary prism until they close the forearc basin at a region with a 2200 m basement relief. This relief is not caused by active deformation, as young flat forearc strata lap onto it and mark the transition from initial collision to collision where many growth strata to the north suggest abrupt increase in active arc basement deformation. The (1) deforming basement, (2) back-thrusts, and (3) other sedimentary processes affect the architecture of the successive basins in the collision zone until the arc is juxtaposed to the rear of the fold and thrust belt on land.

  14. Downgoing plate controls on overriding plate deformation in subduction zones

    NASA Astrophysics Data System (ADS)

    Garel, Fanny; Davies, Rhodri; Goes, Saskia; Davies, Huw; Kramer, Stephan; Wilson, Cian

    2014-05-01

    Although subduction zones are convergent margins, deformation in the upper plate can be extensional or compressional and tends to change through time, sometimes in repeated episodes of strong deformation, e.g, phases of back-arc extension. It is not well understood what factors control this upper plate deformation. We use the code Fluidity, which uses an adaptive mesh and a free-surface formulation, to model a two-plate subduction system in 2-D. The model includes a composite temperature- and stress-dependent rheology, and plates are decoupled by a weak layer, which allows for free trench motion. We investigate the evolution of the state of stress and topography of the overriding plate during the different phases of the subduction process: onset of subduction, free-fall sinking in the upper mantle and interaction of the slab with the transition zone, here represented by a viscosity contrast between upper and lower mantle. We focus on (i) how overriding plate deformation varies with subducting plate age; (ii) how spontaneous and episodic back-arc spreading develops for some subduction settings; (iii) the correlation between overriding plate deformation and slab interaction with the transition zone; (iv) whether these trends resemble observations on Earth.

  15. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time.

  16. Interannual variability of Indian Ocean subtropical mode water subduction rate

    NASA Astrophysics Data System (ADS)

    Ma, Jie; Lan, Jian

    2016-08-01

    The interannual variation of Indian Ocean subtropical mode water (IOSTMW) subduction rate in the Southwest Indian Ocean from 1980 to 2007 is investigated in this paper based on Simple Ocean Data Assimilation (SODA) outputs. Climatology of subduction rate exceeds 75 m/year in the IOSTMW formation area. The renewal time of permanent pycnocline water mass based on the subduction rate is calculated for each density class: 3-6 years for IOSTMW (25.8 < σ θ < 26.2 kg m-3). Subduction rate in the Southwest Indian Ocean subtropical gyre exhibits a great year-to-year variability. This interannual variations of the IOSTMW subduction rate is primarily dominated by the lateral induction term, associated with the interannual variations of strong meridional gradient of winter mixed layer depth (MLD). The slope of the mixed layer depth in the mode water is closely linked to the large variations of deep late winter MLD in the mid-latitudes and negligible variations of shallow winter MLD in lower latitudes. It is further identified that the interannual variation of late winter MLD in this area is largely controlled by the latent and sensible heat flux components. The water volume of the permanent pycnocline in the IOSTMW distribution area is also found to show a significant interannual variability, and it is well correlated with the interannual variation of subduction rate.

  17. Subduction to Continental Delamination: Insights From Laboratory Experiments

    NASA Astrophysics Data System (ADS)

    Gogus, O. H.; Corbi, F.; Faccenna, C.; Pysklywec, R. N.

    2009-05-01

    The evolution of the lithosphere through subduction-collision and delamination and its surface/crustal response (topography/deformation) is investigated in this work. We present a series of lithosphere scale two dimensional (2-D) and three dimensional (3-D) laboratory experiments to better understand such processes. In these experiments, an idealized viscously deforming crust-mantle lithosphere-mantle system is configured with silicone putty (representing lithospheric mantle and upper crust) and glucose syrup (representing the upper mantle and lower crust). The initial focus was to investigate the physical development of delamination versus continental subduction without plate convergence. Experiments show that the delamination or continental subduction is strongly dependent on the density of the crust (both crust and mantle lithosphere subducts when crust has a higher density, instead of delamination), while in the investigated range, the viscosity of the weak layer does not have much influence on the process. In all the experiments, the topography is asymmetric with subsidence above the delaminating hinge due to the dynamic vertical pulling driven by the delaminating slab, and uplift above the delaminated region due to the buoyancy of asthenosphere. Our investigation on the oceanic subduction with a convergence rate of ~ 3cm/year plate velocity suggests that subduction -collision - delamination is well defined and at the end, the delaminating crust from the lithosphere is overthrusted on top of the overriding plate. Our results provide integrated insights on the Alpine-Himalayan type orogenies, in particular the neotectonic evolution of Eastern Anatolian plateau.

  18. Patterns of seismogenesis for giant plate-boundary earthquakes in island-arc-type subduction systems

    NASA Astrophysics Data System (ADS)

    Kirby, S. H.

    2006-12-01

    The global record of giant earthquake occurrence in subduction zones during the instrumental and historical eras is woefully short; only about 16 events with magnitudes above 8.4 are reasonably well documented since 1700. We find no examples of giant (M > 8.4) interplate thrust events and/or wide-ranging tsunamis sourced in the classic island arcs with fast backarc spreading (Bonin, Marianas, Tonga-Kermadec, Vanuatu, and South Scotia). The Sumatra-Andaman Earthquake of 2004 (SAE) ruptured a sector of the INDIA-BURMA subduction boundary and evidently had no known historical antecedents, suggesting that the return time may be many centuries to millennia and consistent with low convergence rates. Moreover, the persistence of rupture to the north in the weakly volcanic Nicobar/Andaman sector gives one pause to reflect on the assumption that island arcs, especially those with active back-arc spreading such as the Marianas, do not produce great interplate- thrust earthquakes. The Andaman/Nicobar subduction segment is an unusual island arc. Only two arc volcanoes occur between the convergent plate boundary west of the Andamans and the backarc ridge/transform system to the east. Backarc spreading in the Andaman/ Nicobar segment is unusual because the NNW spreading directions are nearly parallel to the trench/deformation-front as do the INDIA-BURMA plate motions across it. This geometry suggests that arc-normal extension, trench migration and associated slab normal motions may not mechanically decouple this subduction system. The Nicobar sector of the rupture for the 2004 event is roughly 200 km wide judging from the aftershock distribution; a distribution that persists to the east under the Nicobar Islands, suggesting that the plate-boundary dip is very shallow in that latitude range. If this is correct, then the down-dip limitation on seismogenic slip set by serpentinized forearc mantle (Hyndman et al., 2003) may not control rupture width as it apparently does for many

  19. Late cenozoic subduction complex of Sicily

    USGS Publications Warehouse

    Roure, F.; Howell, D.G.; Muller, C.; Moretti, I.

    1990-01-01

    Besides remnants of Hercynian deformations in the Peloritani nappe and of pre-Oligocene Alpine structures in the Troiani nappe, most compressive structures observed in the Sicilian accretionary wedge result from the late Cenozoic (Tortonian to Present) continental subduction of the Apulia (Iblei) block, and are thus synchronous with distensive structures related to the opening of the Tyrrhenian Sea. Syntectonic deposits fill southward-migrating foredeeps in a sequential fashion, and the dating of these deposits helps to constrain the timing of deformation. Similarly, Plio-Quaternary sediments, eroded from the accreted units, rest on top of the allochthon in either compressive piggy-back depressions or extensional basins. The age and configuration of these overlap deposits constrain our reconstructions of the subsurface geometry of the underlying peri-Tyrrhenian detachment faults or S-verging thrust-faults. Post-depositional erosion, normal faulting and syntectonic filling of basins contribute to maintaining the critical taper of the prism, whose geometry is continuously altered owing to frontal accretion, underplating and isostatic uplift. ?? 1990.

  20. Carbonate dissolution and transport in H2O fluids during subduction revealed by diamond-bearing rocks from the Alps

    NASA Astrophysics Data System (ADS)

    Frezzotti, M.; Selverstone, J.; Sharp, Z. D.; Compagnoni, R.

    2011-12-01

    Here we discuss the fate of subducted carbonates and its implications for recycling of crustal carbon. Thermodynamic models predict little decarbonation along most subduction geotherms, and the mechanisms by which carbon is transferred from the subducting slab to the overlying mantle remain poorly constrained. Diamond-bearing fluid inclusions in garnet in oceanic metasedimentary rocks from Lago di Cignana (western Alps) represent the first occurrence of diamond from a low-temperature subduction complex of clearly oceanic origin (T ≤600°C; P ≥3.5 GPa). The presence of diamonds in and associated with fluid inclusions provides clear evidence of carbon transport by fluids at depths that are directly relevant to slab-mantle fluid transfer during subduction. At room temperature, the fluid inclusions contain aqueous fluid, a vapor bubble, and multiple solid daughter crystals. Daughter crystals identified by Raman spectroscopy and microprobe analysis include ubiquitous Mg-calcite/calcite and rutile, and less common diamond, quartz, paragonite, dawsonite, rhodochrosite, dypingite, and pentahydrite. Molecular CO2 is absent or in trace amounts. The aqueous liquid phase contains ≥0.2 wt%, HCO3-, CO32-, and SO42- ions. In Raman spectra, broad peaks at 773 and 1017 cm-1 point to the presence of both Si(OH)4(aq) and deprotonated monomers (e.g., SiO(OH)3-(aq), and SiO2(OH)22-(aq)), indicative of alkaline solutions. The absence of CO2 in the vapor, and the presence of carbonate daughter minerals, CO32-(aq), and HCO3-(aq) also show that the trapped fluids are alkaline at ambient conditions. High activities of aqueous carbon species reveal that carbonate dissolution is an important mechanism for mobilizing slab carbon at sub-arc depths (100-200 km) during oceanic subduction. Our results imply that the magnitude of carbon release and transport from the slab at sub-arc depths is greater than experimentally predicted on the basis of decarbonation reactions alone.

  1. Physical characteristics of subduction-type seismogenic zones revisited

    NASA Astrophysics Data System (ADS)

    Heuret, A.; Lallemand, S.; Piromallo, C.; Funiciello, F.

    2009-12-01

    Based on both the Centennial earthquake catalog, the revised 1964-2007 EHB hypocenters and the 1976-2007 CMT Harvard catalog, we have extracted the hypocenters, nodal planes and seismic moments of worldwide subduction earthquakes for the period 1900-2007. For the period 1976-2007, we use 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 for the estimate of the cumulated seismic moment only. The criteria used to select 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 (positive slips, at least one nodal plane get dip < 45° and depth > 70 km), and, 2/ the plate interface local geometry and orientation (one nodal plane is oriented toward the volcanic arc, the azimut of this nodal plane is ± 45° with respect to the trench one, its dip is ± 20° with respect to the slab one and the epicenter 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 then provide a map of the seismogenic zone for 36% of the oceanic subduction plates boundaries including dip, length, downdip and updip limits. The remnant 64% correspond to either weakly coupled oceanic subduction zones, slow subduction rates, or long recurrence period between earthquakes. We then revisit the statistical study done by Pacheco et al. (1993) and tested some empirical laws obtained for example by Kanamori (1986) in light of a more complete, more detailed, more accurate and more uniform description of the subduction interplate seismogenic zone. Since the subduction earthquakes result from stress accumulation along the interplate and that

  2. Seismic anisotropy and texture development during early stages of subduction

    NASA Astrophysics Data System (ADS)

    Di Leo, Jeanette; Walker, Andrew; Li, Zhong-Hai; Wookey, James; Ribe, Neil; Kendall, J.-Michael; Tommasi, Andréa

    2014-05-01

    Shear wave splitting measurements are frequently used to infer upper mantle flow trajectory, based on the fact that, under strain, olivine develops lattice-preferred orientation (LPO) textures in the convecting mantle. However, such inferences ought to be made carefully, since the relationship between splitting fast polarisation and olivine LPO depends on several factors, one of them being the deformation history of the volume of mantle in question. This is especially the case in regions such as subduction zones, where complex and time-dependent mantle flow occurs. Here, we present an integrated model to simulate strain-history-dependent LPO development and measure the resulting shear wave splitting in a subduction setting. We do this for a subduction model that approximates the geometry of the double-sided Molucca Sea subduction system in eastern Indonesia. We test a single-sided and a double-sided subduction case, and compare the results to shear wave splitting observations of this region. Since the subduction zone is fairly young, early textures from the slab's descent from the near-surface to the bottom of the mantle transition zone - which we simulate in our models - have not yet been overprinted by subsequent continuous flow. It further allows us to test the significance of the double-sided geometry, i.e., the need for a rear barrier to achieve trench-parallel sub-slab mantle flow. We simulate olivine LPO evolution in polycrystalline aggregates as they move and deform along pathlines extracted from a 3-D mantle flow model. Interactions between crystals are described using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. After calculating elastic properties associated with LPO textures, we estimate the resulting splitting parameters (fast direction φ, delay time δt) for synthetic SKS phases. Our models demonstrate that complex, backazimuth

  3. Automated Tremor Analysis From the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Aguiar, A. C.; Melbourne, T. I.; Scrivner, C. W.

    2006-12-01

    Reanalysis of geodetic GPS time series from the Cascadia subduction zone have revealed at least 30 resolvable slow slip events along the megathrust since 1997, ranging from northern California to southern British Columbia. Many of the smaller and more recent events are barely detectable with GPS, but stand out clearly as tremor sequences on band-pass filtered record sections of surface seismic data. With the intent of assessing whether a fixed frequency-magnitude relationship applies to subduction zone slow slip events, and assuming that tremor and GPS transients are different manifestations of the same faulting process at depth, we have implemented automated processes to identify the frequency-magnitude relationship of slow slip events that are not resolvable with GPS. Our methodology is as follows. We first consolidate daily seismic files from the northern Puget Basin of Washington State and SW British Columbia, where GPS density is highest. Seismic traces are included from the Pacific Northwest Seismic Network, the Plate Boundary Observatory borehole seismic network, the Earthscope-funded CAFÉ experiment, and locally deployed seismometers around long-baseline tiltmeters, all running at 100 sps. We then band-pass the data between 1-6 Hz and run it through several tremor-detection algorithms. The envelopes of rectified traces are then regionally averaged to provide a single metric indicative of tremor activity. We also compute spectrograms of daily files and use power in the 1-6 Hz range to assess tremor onset and duration. To validate that this automated approach works as expected, we first manually inspected time periods of slow slip events, particularly the Sept 6-30 of 2005, when GPS transients are known. Secondly, hours per week of tremor identified on at least three stations separated by 25 Km were summed manually and show clear spikes during the GPS-inferred timing, indicating that envelope averaging does accurately identify the largest known events. We

  4. Crustal earthquake triggering by modern great earthquakes on subduction zone thrusts

    NASA Astrophysics Data System (ADS)

    Gomberg, Joan; Sherrod, Brian

    2014-02-01

    Among the many questions raised by the recent abundance of great (M > 8.0) subduction thrust earthquakes is their potential to trigger damaging earthquakes on crustal faults within the overriding plate and beneath many of the world's densely populated urban centers. We take advantage of the coincident abundance of great earthquakes globally and instrumental observations since 1960 to assess this triggering potential by analyzing centroids and focal mechanisms from the centroid moment tensor catalog for events starting in 1976 and published reports about the M9.5 1960 Chile and M9.2 1964 Alaska earthquake sequences. We find clear increases in the rates of crustal earthquakes in the overriding plate within days following all subduction thrust earthquakes of M > 8.6, within about ±10° of the triggering event centroid latitude and longitude. This result is consistent with dynamic triggering of more distant increases of shallow seismicity rates at distances beyond ±10°, suggesting that dynamic triggering may be important within the near field too. Crustal earthquake rate increases may also follow smaller M > 7.5 subduction thrust events, but because activity typically occurs offshore in the immediately vicinity of the triggering rupture plane, it cannot be unambiguously attributed to sources within the overriding plate. These observations are easily explained in the context of existing earthquake scaling laws.

  5. Th isotope and U-series studies of subduction-related volcanic rocks

    NASA Astrophysics Data System (ADS)

    Gill, James B.; Williams, Ross W.

    1990-05-01

    The activities of 6-7 radionuclides measured by alpha spectrometry are reported for 35 samples of recent volcanic rocks from 24 volcanoes in 9 arcs. These include continental margins (Cascades, Alaska, Costa Rica), island arcs (the Sunda, Banda, and Sangihe arcs of Indonesia, Marianas, Japan, Aleutians, Antilles, Tonga), and a backarc basin (Lau Basin). Enrichment of 238U over 230Th is more common (over 2/5 of the samples) and greater (up to 59%) in these subduction-related volcanics than in those from other tectonic environments. 230Th /232Th ratios also extend to higher values, both in absolute numbers and relative to other isotope ratios. Enrichment of 210Po and 226Ra over 230Th is widespread and is more common and greater in island arcs than continental margins. The level of Po or Ra enrichment (50-300% in island arcs, 10-50% at continental margins) is similar to that in ocean island and ridge basalts and decreases with differentiation. The differences in Th-U systematics between subduction-related and other volcanic rocks are attributed to variations in the process of melt extraction, changes in bulk partition coefficients within the mantle wedge, or preferential addition of U from subducted lithosphere. Ra enrichments are attributed to partial melting processes which are similar to those at ocean ridges. Smaller excesses at continental margins are attributed either to slower ascent or to differences in the process of melt formation beneath continents.

  6. The 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone

    NASA Astrophysics Data System (ADS)

    Kostoglodov, Vladimir; Husker, Allen; Shapiro, Nikolai M.; Payero, Juan S.; Campillo, Michel; Cotte, Nathalie; Clayton, Robert

    2010-12-01

    The last decade featured an explosive sequence of discoveries of slow slip events (SSE) and nonvolcanic tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE is usually associated with an increased NVT activity but it is not clear yet if those events are the result of the same process or are independent expressions of a common underlying seismotectonic source. A large SSE in Central Mexico occurred in 2006 during the Meso-American Subduction Experiment (MASE) which provided continuous observations of the NVT for the years 2005-2007. GPS and abundant seismic data show that although the NVT energy increased notably during the 2006 SSE, the two phenomena were separated spatially and not completely synchronized in time. Significant NVT episodes occur during the period between SSEs, suggesting again that large slow slip events and NVT observed in the Mexican subduction zone are of different origins. The results presented here contribute to uncovering the nature of these two separate phenomena that have been indistinguishable in some other regions.

  7. Detailed Seismic Velocity Structure of the Plate Boundary, Cascadia Subduction Zone, from Prestack Waveform Inversion

    NASA Astrophysics Data System (ADS)

    Fortin, W.; Holbrook, W.; Tobin, H. J.; Keranen, K. M.; Everson, E.; Mallick, S.; Padhi, A.

    2013-12-01

    Understanding the geologic makeup of the Cascadia Subduction Zone (CSZ) has great importance for understanding seismic hazards in the coastal margin of the U.S. Pacific Northwest. The Cascadia margin is a potential earthquake and tsunami threat to the many millions who live in the area, yet details of its structure and mechanics remain poorly understood. In particular, the character of the subduction interface is elusive due to the CSZ's relatively aseismic behavior and low seismic reflectivity, making imaging difficult for passive and active source methods, respectively. In July 2012 seismic data were acquired as a part of the COAST project, spanning the important transition from the Cascadia basin, across the deformation front, and into the accretionary wedge. This modern data, coupled with sophisticated pre-stack full waveform seismic inversion methods, allows us to create highly detailed velocity models. While still computationally expensive, current computing clusters can perform these inversions with enough lateral density to yield highly detailed velocity information in both the vertical and horizontal. Here we present pre-stack full waveform inversions of a seismic line from the center of the COAST survey offshore Washington state as a cross section of the velocity structure of the CSZ. This detailed velocity model is a necessary initial step toward a detailed porosity cross section to be used to determine the role of fluids in the CSZ. Using these new data we investigate the lateral variability in reflectivity of the subducting plate boundary reflection in terms of its seismic velocity.

  8. Bayesian geodynamic inversion to constrain the rheology of the flat subduction system in southwestern Mexico

    NASA Astrophysics Data System (ADS)

    Gérault, Mélanie; Bodin, Thomas

    2016-04-01

    The flat slab in southwestern Mexico differs from others at the present-day because (1) it is associated with abundant arc volcanism, (2) it is associated with extension in the arc and a neutral state of stress in the fore-arc, (3) it generates relatively low seismic activity, (4) the continental mantle lithosphere is very thin or nonexistent, (5) it is not directly caused by the subduction of thickened oceanic crust, and (6) there is no nearby cratonic keel. In a recent study, we showed that the topography in the area is controlled by both isostatic and dynamic contributions. The Trans-Mexican Volcanic Belt is either isostatically supported or slightly buoyed up by a low-density mantle wedge. To the contrary, the forearc is pulled downward by the flat slab, resulting in about 1 km of subsidence. Using a two-dimensional instantaneous Stokes flow finite-elements model, we found a combination of slab, mantle, and subduction interface properties that can predict the observed topography, plate velocities, and stress state in the continent. However, this solution is not unique, and there are trade-offs between these properties such that several combinations can provide a similarly good fit to the data. In this work, we present a geodynamic inversion to further investigate what viscosities and densities are required in different zones of the subduction system to explain the observations collected at the surface. The inverse problem is cast in a Bayesian framework, where model parameters (e.g. the viscosity in the mantle wedge and along the subduction interface) can be reconstructed in a probabilistic sense, and where trade-offs and uncertainties can be quantitatively constrained. We use a direct parameter search approach based on a Markov chain Monte Carlo (McMC) scheme to test a large number of potential scenarios.

  9. Influence of Peruvian flat-subduction dynamics on the evolution of western Amazonia

    NASA Astrophysics Data System (ADS)

    Eakin, Caroline M.; Lithgow-Bertelloni, Carolina; Dávila, Federico M.

    2014-10-01

    Convection in the Earth's mantle is mainly driven by cold, dense subducting slabs, but relatively little is known about how 3D variations in slab morphology and buoyancy affect mantle flow or how the surface above deforms in response (i.e. dynamic topography). We investigate this problem by studying the dynamics of an active region of flat-slab subduction located in Peru in South America. Here the slab geometry is well known, based on the regional seismicity, and we have observations from the local geological record to validate our models. Of particular interest is the widespread subsidence and deposition of the Solimões Formation across western Amazonia that coincided with the development of the Peruvian flat-slab during the Mid-Late Miocene. This formation covers an extensive area from the foredeep to the Purus Arch located ∼2000 km away from the trench. Close to the Andes the preservation of several kilometers of sedimentary thicknesses can be easily accounted for by flexure. Based on an estimate of the Andean loading we predict 2.8 to 3.6 km of accommodation space that spans 100 km. The spatial and temporal history of the Solimões Formation however, particularly the thick distal foreland accumulations up to 1.2 km deep, can only be matched with the addition of a longer-wavelength dynamic source of topography. Following the transition from normal to flat subduction, we predict over 1 km of dynamic subsidence (∼1500 km wide) that propagates over 1000 km away from the trench, tracking the subduction leading edge. This is followed by a pulse of dynamic uplift over the flat segment behind it. We therefore propose that a combination of uplift, flexure and dynamic topography during slab flattening in Peru is responsible for the sedimentation history and landscape evolution of western Amazonia that eventually led to the configuration of the Amazon Drainage Basin we know today.

  10. Plume-induced subduction: from laboratory experiments to Venus large coronae

    NASA Astrophysics Data System (ADS)

    Davaille, A.; Smrekar, S. E.; Tomlinson, S. M.

    2015-12-01

    The development of new visualization techniques and the use of complex-rheology fluids open a new area for planetary geodynamic modelling, as observations of surface patterns (i.e. faults, folds, ridges, trenches) can be related to convective instabilities inside the laboratory mantle analog. The rheology of colloidal aqueous dispersions of silica nanoparticles depends strongly on the solid particle fraction, φp, deforming in the Newtonian regime at low φp, and transitioning to strain-rate weakening, plasticity, elasticity, and brittle properties as φp increases. So, as the system is dried from above, a dense skin grows on the surface, akin to a planetary lithosphere. If it is also heated from below, hot plumes develop. When a hot plume impinges under the skin, it triggers a new mode of subduction: as the upwelling plume material breaks the lithosphere and flows above the denser skin, it forces it to sink. The subduction trenches are localized along the rim of the plumes and strong roll-back is observed. Subduction always occurs along partial circles, a situation very different from the purely viscous case. This is due to the brittle character of the upper part of the experimental lithosphere: it cannot deform viscously to accommodate roll-back and sinking motions. Instead, the plate tears, as a sheet of paper would do upon intrusion. The experiments further suggest that a weaker lithosphere than that present on Earth today is required for such a convective regime. These experimental observations strongly resemble the association of large coronae with trenches that is observed on Venus. The surface deformation structures and the subsurface density variations predicted by the laboratory agree with radar image observations and subsurface density variations inferred from modeling the gravity and topography data at Artemis and Quetzelpetlatl Coronae. Evidence for geologically recent volcanism at Quetzelpetlatl suggests that subduction may be currently active on

  11. 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.

  12. 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.

  13. Slab2 - Providing updated subduction zone geometries and modeling tools to the community

    NASA Astrophysics Data System (ADS)

    Hayes, G. P.; Hearne, M. G.; Portner, D. E.; Borjas, C.; Moore, G.; Flamme, H.

    2015-12-01

    The U.S. Geological Survey database of global subduction zone geometries (Slab1.0) combines a variety of geophysical data sets (earthquake hypocenters, moment tensors, active source seismic survey images of the shallow subduction zone, bathymetry, trench locations, and sediment thickness information) to image the shape of subducting slabs in three dimensions, at approximately 85% of the world's convergent margins. The database is used extensively for a variety of purposes, from earthquake source imaging, to magnetotelluric modeling. Gaps in Slab1.0 exist where input data are sparse and/or where slabs are geometrically complex (and difficult to image with an automated approach). Slab1.0 also does not include information on the uncertainty in the modeled geometrical parameters, or the input data used to image them, and provides no means to reproduce the models it described. Currently underway, Slab2 will update and replace Slab1.0 by: (1) extending modeled slab geometries to all global subduction zones; (2) incorporating regional data sets that may describe slab geometry in finer detail than do previously used teleseismic data; (3) providing information on the uncertainties in each modeled slab surface; (4) modifying our modeling approach to a fully-three dimensional data interpolation, rather than following the 2-D to 3-D steps of Slab1.0; (5) migrating the slab modeling code base to a more universally distributable language, Python; and (6) providing the code base and input data we use to create our models, such that the community can both reproduce the slab geometries, and add their own data sets to ours to further improve upon those models in the future. In this presentation we describe our vision for Slab2, and the first results of this modeling process.

  14. 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

  15. 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

  16. 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.

  17. Influence of subduction history on South American topography

    NASA Astrophysics Data System (ADS)

    Flament, Nicolas; Gurnis, Michael; Müller, R. Dietmar; Bower, Dan J.; Husson, Laurent

    2015-11-01

    The Cenozoic evolution of South American topography is marked by episodes of large-scale uplift and subsidence not readily explained by lithospheric deformation. The drying up of the inland Pebas system, the drainage reversal of the Amazon river, the uplift of the Sierras Pampeanas and the uplift of Patagonia have all been linked to the evolution of mantle flow since the Miocene in separate studies. Here we investigate the evolution of long-wavelength South American topography as a function of subduction history in a time-dependent global geodynamic model. This model is shown to be consistent with these inferred changes, as well as with the migration of the Chaco foreland basin depocentre, that we partly attribute to the inboard migration of subduction resulting from Andean mountain building. We suggest that the history of subduction along South America has had an important influence on the evolution of the topography of the continent because time-dependent mantle flow models are consistent with the history of vertical motions as constrained by the geological record at four distant areas over a whole continent. Testing alternative subduction scenarios reveals flat slab segments are necessary to reconcile inferred Miocene shorelines with a simple model paleogeography. As recently suggested, we find that the flattening of a subduction zone results in dynamic uplift between the leading edge of the flat slab segment and the trench, and in a wave of dynamic subsidence associated with the inboard migration of the leading edge of flat subduction. For example, the flattening of the Peruvian subduction contributed to the demise of Pebas shallow-water sedimentation, while continental-scale tilting also contributed to the drainage reversal of the Amazon River. The best correlation to P-wave tomography models for the Peruvian flat slab segment is obtained for a case when the asthenosphere, here considered to be 150 km thick and 10 times less viscous than the upper mantle, is

  18. The 2014 Mw6.2 Eketahuna earthquake, Hikurangi subduction zone - normal faulting in the subducted Pacific Plate crust

    NASA Astrophysics Data System (ADS)

    Abercrombie, R. E.; Bannister, S. C.; Francois-Holden, C.; Hamling, I. J.; Ristau, J. P.

    2014-12-01

    The 2014 January 20th M6.2 Eketahuna earthquake occurred in the subducted crust of the Pacific plate at the Hikurangi subduction zone, beneath North Island, New Zealand. Moment tensor analysis together with aftershock relocations show that this event was an oblique-normal faulting intraplate event, with hypocentre depth ca.30 km, and with rupture on a northwest-dipping fault extending through the subducted crust up to the subduction megathrust at ca.18-20 km depth. More than 3500 aftershocks were subsequently recorded by the New Zealand GeoNet network, with only minor migration of the aftershocks away from the inferred mainshock rupture, and with very few aftershocks within +/- 1 km of the subduction megathrust. The megathrust in this particular region is inferred to be interseismically locked with no seismic or aseismic slip, although slow slip is occurring ca.15-30 km down-dip (Wallace et al, 2013). Similar oblique-normal faulting events have previously occurred along the Hikurangi subduction margin, including in 1985 (ML5.7) and 1990 (Mw6.2). Earlier earthquakes in 1942 (Mw6.8) and 1921 (Mw6.8) are also inferred to have occurred at a similar depth within the subducted crust. The 1990 earthquake sequence occurred ~40 km along-strike from the 2014 Eketahuna event, and involved a Mw6.2 oblique-normal faulting event in the subducted crust, which was quickly followed by a Mw6.4 event in the overlying crust, with both thrust and dextral strike-slip components, possibly responding to deeper aseismic slip. Deeper earthquakes of similar type at other subduction margins are thought to be high stress drop. We calculate the stress drops of the mainshock and larger aftershocks, using a direct wave, empirical Green's function (EGF) approach that includes measurement uncertainties and objective criteria for assessing the quality of each spectral ratio (Abercrombie, 2013). We compare the results to those for earthquakes in other tectonic regions of New Zealand, calculated using

  19. Process Based Explanations for Correlations Between the Structural and Seismic Segmentation of the Cascadia Subduction Wedge

    NASA Astrophysics Data System (ADS)

    Fuller, C. W.; Brandon, M. T.; Willett, S. D.

    2006-12-01

    between coseismic slip and basins given the universal applicability of basin forming processes. Our models also show that the segmentation with respect to the CR structural high depends on variations in the viscous rheology of the wedge and in the effective flexural rigidity of the subducting plate. Uplift of the CR is due to viscous weakening of the landward region of the wedge, and the style of uplift depends on the viscosity and distribution of ductile material within the wedge. For example, the Olympic Mountains are the highest and deepest exhumed portion of the CR partially due to the viscously weaker felsic rocks comprising the core of the Olympics compared to stronger basaltic CRT underlying much of the remaining CR. The subducting plate beneath the Olympics also has a higher effective flexural rigidity than the rest of the Olympics due to the bend in the subducting plate as the trench trend turns from N to NNW. The increased rigidity provides less flexurally created accommodation space for accreted material leading to an earlier emergence of the CR in the Olympics. The wedge is also segmented with respect to upper plate seismicity in that minor seismic activity is observed most everywhere except within the core of the Olympic Mountains where the CRT has been removed. We hypothesize that the weaker viscous strength of the wedge within the core of the Olympics allows for the deformation to accumulate ductilely and aseismically, while the greater viscous strength of segments dominated by the CRT allows for more brittle and seismogenic deformation.

  20. "Wrong-way" subduction of South Alpine (Adriatic) lithosphere beneath the Eastern Alps - a kinematic appraisal

    NASA Astrophysics Data System (ADS)

    Handy, Mark R.; Ustaszewski, Kamil; Kissling, Eduard; Schmid, Stefan M.; Rosenberg, Claudio L.

    2013-04-01

    opposing slabs beneath the Alps and Dinarides. The fragment of the Adriatic slab presently beneath the Eastern Alps may have separated from the rest of the Adriatic slab beneath the Dinarides already in late Paleogene time. Its subduction beneath the eastern part of the Alpine orogen is attributed to counter-clockwise rotation (20°) and/or to N-ward subduction of this Adriatic slab fragment beginning at c. 20 Ma. This coincided with the onset of indentation, rapid exhumation and lateral escape in the Tauern Window and Eastern Alps, and shortening in the eastern part of the Southern Alps. We attribute the lack of a tomographically imaged Adriatic slab beneath the Dinarides (Wortel & Spakman 2000) to asthenospheric upwelling and thermal erosion of the slab within a gap that opened along the ADT during late Paleogene to Miocene time. This gap resulted from progressive NW motion of the leading edge of Adria (i.e., the Adriatic Indenter) away from the foundering part of the broken Adriatic slab beneath the Dinarides. Today, this slab gap separates the Adriatic Indenter in the Alps from the actively subducting Adriatic slab beneath the western Hellenides.

  1. Kinks in subducted slabs: Petrological evidence points to additional hindrance to the exhumation of UHP rocks

    NASA Astrophysics Data System (ADS)

    John, T.; Klemd, R.; Scherer, E. E.; Rondenay, S.; Gao, J.

    2012-12-01

    hypothesize that reaction activity, slab weakening, and deformation are most intense at the depth where the low velocity layer disappears. Just after this point, the slab is denser and has regained its strength. The depth of this critical point apparently varies from one subduction zone to another, and is about 90 km in the Tianshan. Slab kinks have important implications for the descent of slabs and for determining which parts of them may eventually exhume: In addition to the negative buoyancy of mafic UHP rocks, kinks may also act to hinder the exhumation of UHP rocks , thereby explaining their paucity. [1] Yuan et al. (2000), Nature 408, 958-961; [2] Rondenay et al. (2008), Geology 36, 275-278; [3] Austrheim (1987), EPSL 81, 221-232; [4] Jolivet et al. (2005), EPSL 237, 532-547.

  2. Thermal structure and melting conditions associated with `hot' subduction: Implications from thermobarometry of Garibaldi belt basalts, northern Cascadia Subduction System

    NASA Astrophysics Data System (ADS)

    Green, N. L.

    2005-12-01

    The northern Cascadia margin of North America is the classic example of a "hot" subduction system. The downgoing Juan de Fuca plate decreases in age from ca. 10 m.y. off the central Washington coast to less than 5 m.y. off central Vancouver Island; beneath the Garibaldi volcanic belt (GVB) 250 km east of the convergent margin, inferred age of the oceanic lithosphere decreases northward from ca. 22 m.y to 13 m.y. Primitive and near-primitive mafic lavas, which primarily occur trenchward of the GVB volcanic front, range northward from high-Al olivine tholeiites, Mg-andesites and LILE- and LREE-enriched calc-alkaline basalts at Glacier Peak, through transitional basalts in the Cheakamus Valley to alkali olivine basalts and trachybasalts at Meager Mountain, Salal Glacier and Bridge River. The more northerly GVB basaltic magmas show the least evidence of slab-derived components in their source regions. Application of various olivine-melt and pyroxene-melt thermobarometers to GVB basalts indicates a general increase in magmatic temperatures from 1150-1200 C in Mount Baker and Glacier Peak basalts to 1225-1300 C in Bridge River and Salal Glacier lavas. Fe-Ti oxide thermobarometry suggests that northernmost basalts equilibrated under oxygen fugacities conditions between QFM and NNO, whereas Glacier Peak lavas equilibrated at higher oxygen fugacities (ca. 1 log unit above NNO). Estimated P and T conditions of mantle segregation suggest that GVB basalts ascended from increasingly greater depths northward along the volcanic arc. Similar variation is indicated by calculated P-T of basalt equilibrations with both Mg- and Fe-rich peridotite mineral assemblages, based on diopside and albite activity-composition relations. Estimated mantle equilibration temperatures correlate positively with some HFSE abundances (e.g., Hf), but negatively with those of fluid mobile elements (e.g., Cs and B). These relationships are considered in terms of the influence of slab thermal structure on

  3. Two decades of spatiotemporal variations in subduction zone coupling offshore Japan

    NASA Astrophysics Data System (ADS)

    Loveless, John P.; Meade, Brendan J.

    2016-02-01

    Spatial patterns of interplate coupling on global subduction zones can be used to guide seismic hazard assessment, but estimates of coupling are often constrained using a limited temporal range of geodetic data. Here we analyze ∼19 years of geodetic observations from the GEONET network to assess time-dependent variations in the spatial distribution of coupling on the subduction zones offshore Japan. We divide the position time series into five, ∼3.75-year epochs each decomposed into best-fit velocity, annual periodic signals, coseismic offsets, and postseismic effects following seven major earthquakes. Nominally interseismic velocities are interpreted in terms of a combination of tectonic block motions and earthquake cycle activity. The duration of the inferred postseismic activity covaries with the linear velocity. To address this trade-off, we assume that the nominally interseismic velocity at each station varies minimally from epoch to epoch. This approach is distinct from prior time-series analysis across the earthquake cycle in that position data are not detrended using preseismic velocity, which inherently assumes that interseismic processes are spatially stable through time, but rather the best-fit velocity at each station may vary between epochs. These velocities reveal significant consistency since 1996 in the spatial distribution of coupling on the Nankai subduction zone, with variation limited primarily to the Tokai and Bungo Channel regions, where long-term slow slip events have occurred, and persistently coupled regions coincident with areas that slipped during historic great earthquakes. On the Sagami subduction zone south of Tokyo, we also estimate relatively stable coupling through time. On the Japan-Kuril Trench, we image significant coupling variations owing to effects of the 1994 MW = 7.7 Sanriku-oki, 2003 MW = 8.2 Tokachi-oki, and 2011 MW = 9.0 Tohoku-oki earthquakes. In particular, strong coupling becomes more spatially extensive following

  4. 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

  5. Subduction of Oceanic Asthenosphere: A Critical Appraisal in Central Alaska

    NASA Astrophysics Data System (ADS)

    Song, T.; Kawakatsu, H.

    2012-12-01

    Strong radial anisotropy, or transverse isotropy, observed in the oceanic asthenosphere has invited numerous discussions concerning its origin. Near subduction zones where the slab plunges into the mantle, shear wave birefringence (or splitting) measurements attributed to the sub-slab regime often reveal fast splitting direction sub-parallel to the trench, apparently at odds with predictions from a dominant sub-slab entrained flow. However, taking into account strong radial anisotropy observed in the oceanic asthenosphere beneath ocean basins, Song and Kawakatsu [2012, GRL in press] recently concluded that sub-slab fast splitting pattern observed in most subduction zones can be a direct consequence of subducting oceanic asthenosphere, apparently manifested at several shallow subduction zones showing fast polarization direction sub-parallel to the absolute plate motion of the incoming plate. We refer to the term "subduction of oceanic asthenosphere" as a slightly different expression from slab entrainment and put emphasis on the fact that the entrained sub-slab mantle displays anisotropy property analogous to oceanic asthenosphere beneath ocean basins except the angle of symmetry axis changes with the slab dip. To further validate this scenario and distinguish it from more sophisticated hypothesis such as sub-slab trench-parallel flow, we examine complicated SKS splitting patterns observed across the fore-arc central Alaska [Christensen and Abers, 2010; Hanna and Long, 2012], where the rate of trench migration is very low regardless of the mantle reference frame. Observations of fast splitting direction vary from plate motion parallel near the trench to mostly trench-parallel beyond 100 km slab isodepth and there are strong back-azimuth variations in between these regions. After taking into account the rotation of anisotropy symmetry in the subducted oceanic asthenosphere with respect to the obliquity of plate motion and down-dip variations in slab dip, we reproduce

  6. Towards a new thermomechanical model of subduction channel

    NASA Astrophysics Data System (ADS)

    Quinteros, J.; Sobolev, S. V.

    2009-04-01

    Subduction is known to be a major process in most collision zones, and the subduction channel is one of the key elements in the subduction process. In the last years the key-role of the subduction channel was thoroughly studied by many authors. From a mechanical point of view, its properties are highly determined by the friction coefficient, which depends on many factors, and the rheology that is set up. Despite of being a few kilometers wide, and small compared to the big subduction picture, its effects can propagate far away from the trench. For example, that was demonstrated by Sobolev and Babeyko (Geology, 2005) by means of a numerical model focused at the evolution of the Central Andean Subduction Zone. The model was based on a finite-element/finite-difference explicit code called LAPEX-2D (Babeyko et al., EPSL, 2002). Here, we present a new and enhanced 2-D thermomechanical model developed to study this type of tectonic setting. The main ideas of the technical implementation are based on the work published by Popov and Sobolev (PEPI, 2008). The domain is modeled by means of the Finite Elements Method with an implicit approach. The rheology is considered to be elasto-visco-plastic and the viscosity is temperature- and stress-dependent. Diffusion, Dislocation and Peierls types of creep and Mohr-Coulomb plasticity are included. Topography evolution is naturally tracked by a Lagrangian mesh. A particle technique similar to the particle-in-cell method was used to minimize diffusion during remeshing. One of the improvements added in this work, compared to the previous models, is a non-uniform mesh, which allows two main benefits. First, the study of specified regions of interest with more detail by means of the concentration of elements (like subduction channel), and second, the capability to define more realistic and smooth interfaces between different materials without distorsions related to mesh orientation. Also, the remeshing process includes basic automatic

  7. Evolution of texture and seismic anisotropy during subduction (Invited)

    NASA Astrophysics Data System (ADS)

    Di Leo, J. F.; Walker, A.; Li, Z.; Wookey, J. M.; Ribe, N. M.; Kendall, J. M.; Tommasi, A.

    2013-12-01

    Shear wave splitting fast directions are routinely used to infer upper mantle flow direction, based on the premise that olivine develops strain-induced lattice-preferred orientation (LPO) textures in the convecting mantle. However, such inferences ought to be made with caution, since the relationship between olivine LPO and splitting fast direction depends on many factors, including the entire deformation history of the volume of mantle in question. This is especially the case in regions where complex time-dependent mantle flow is expected, such as subduction zones. Here, we present an integrated model to simulate strain-history-dependent LPO development and estimate the resulting shear wave splitting in a subduction setting. We do this for a mantle flow model that approximates the geometry of the double-sided Molucca Sea subduction system in Eastern Indonesia. We test a single-sided and a double-sided subduction case, and compare the results to recent shear wave splitting measurements of this region by Di Leo et al. (2012). As the subduction system is fairly young, early textures from the slab's descent from the near-surface to the bottom of the mantle transition zone (which we are able to simulate in our models) have not yet been overprinted by subsequent continuous steady-state flow. It also allows us to test the significance of the double-sided geometry, or, more generally, the need for a rear barrier to achieve trench-parallel sub-slab mantle flow. We simulate olivine LPO evolution caused by deformation of polycrystalline aggregates as they deform and move along pathlines extracted from a 3-D mantle flow model. Interactions between crystals are described using the visco-plastic self-consistent (VPSC) approach. Unlike previous studies, we consider the entire subduction history from subduction initiation onwards. After calculating elastic properties associated with LPO at multiple depths, we estimate the resulting splitting parameters (fast direction φ, delay

  8. Overriding plate thickness control on subducting slab curvature

    NASA Astrophysics Data System (ADS)

    Holt, A.; Buffett, B. A.; Becker, T. W.

    2014-12-01

    The curvature of subducting lithosphere controls deformation due to bending at the trench, which results in a force that dissipates gravitational potential energy and may affect seismic coupling. We use 2-D, thermo-mechanical subduction models to explore the dependence of the radius of curvature on the thickness of the subducting and overriding plates for models with both viscous and effectively plastic lithospheric rheologies. Such a plastic rheology has been shown to reproduce the bending stresses/moment computed using a kinematic strain rate description and a laboratory derived composite rheology. Laboratory and numerical models show that the bending geometry of subducting slabs with a viscous rheology is strongly dependent on slab thickness; thicker plates have a larger radius of curvature. However, the curvature of subducting plates on Earth, illuminated by the distribution of earthquake hypocenters, shows little to no dependence on the plate thickness or age. Such an observation is instead compatible with plates that have a plastic rheology. Indeed, our numerical models show that the radius of curvature of viscous plates has a stronger dependence on subducting plate thickness than in equivalent plastic models. In viscous plates, the bending moment produces a torque, which balances the torque exerted by buoyancy. However, for the plastic plate case the bending moment saturates at a maximum value and so cannot balance the gravitational torque. The saturation of bending moment means that, (a) the radius of curvature of the bending region is not constrained by this torque balance, and, (b) other forces are required to balance the gravitational torque. We explore the role that the overriding plate could play in controlling the subducting plate curvature in plastic plate models where the bending stresses have saturated. For such plates, we find that increasing the thickness of the overriding plate causes the radius of curvature to increase. The same correlation is

  9. Role of the subduction filter in mantle recycling

    NASA Astrophysics Data System (ADS)

    Kimura, J. I.; Skora, S. E.; Gill, J.; Van Keken, P. E.

    2015-12-01

    Subduction modifies the descending basaltic and sedimentary oceanic crust and generates felsic arc materials and continental crust. Studies of element mass balances in the subduction zone therefore reveal the evolution of the Earth's two major geochemical reservoirs: the continent crust and mantle. We use the Arc Basalt Simulator ver.4 (ABS4) to model the geochemical mass balance during dehydration by prograde metamorphism and melting of the slab followed by subsequent flux melting of the wedge mantle caused by the addition of slab-derived liquids. The geochemistry of high-Mg andesite or adakite formed in a hot subduction zone is akin to the present-day bulk continental crust and to the Archean (>2 Ga) Tonalite-Trondjhemite-Granodiorite composition. Therefore, the residual slab and the metasomatized mantle wedge at hot subduction zones should be the most plausible sources for materials recycled back into the deep mantle. Model calculations of isotopic growth in the residual slab and mantle formed in hot subduction zones reproduce fairly well the EM1-FOZO-HIMU isotope arrays found in ocean island basalts (OIBs) of deep mantle plume origin, although FOZO with high 3He/4He is not generated by this slab recycling process. The recycled materials are bulk igneous ocean crust for HIMU and metasomatized mantle wedge peridotite for EM1. In contrast, the EM2-FOZO array can be generated in a cold subduction zone with igneous oceanic crust for FOZO and sediment for EM2 sources. Necessary residence time are ~2 Ga to form HIMU-FOZO-EM1 and ~1 Ga to form EM2-FOZO. The subducted oceanic crust (forming HIMU) and mantle wedge peridotite (forming EM1) may have travelled in the mantle together. They then melted together in an upwelling mantle plume to form the EM1-FOZO-HIMU isotopic variations found frequently in OIBs. In contrast, the less frequent EM2-FOZO array suggests a separate source and recycling path. These recycling ages are consistent with the change in the mantle potential

  10. The 2004 Sumatra Earthquake and Tsunami: Lessons Learned in Subduction Zone Science and Emergency Management for the Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Cassidy, John F.

    2015-03-01

    The 26 December 2004, Mw 9.3 Sumatra earthquake and tsunami was a pivotal turning point in our awareness of the dangers posed by subduction zone earthquakes and tsunamis. This earthquake was the world's largest in 40 years, and it produced the world's deadliest tsunami. This earthquake ruptured a subduction zone that has many similarities to the Cascadia Subduction Zone. In this article, I summarize lessons learned from this tragedy, and make comparisons with potential rupture characteristics, slip distribution, deformation patterns, and aftershock patterns for Cascadia using theoretical modeling and interseismic observations. Both subduction zones are approximately 1,100-1,300 km in length. Both have similar convergence rates and represent oblique subduction. Slip along the subduction fault during the 26 December earthquake is estimated at 15-25 m, similar to values estimated for Cascadia. The width of the rupture, ~80-150 km estimated from modeling seismic and geodetic data, is similar to the width of the "locked and transition zone" estimated for Cascadia. Coseismic subsidence of up to 2 m along the Sumatra coast is also similar to that predicted for parts of northern Cascadia, based on paleoseismic evidence. In addition to scientific lessons learned, the 2004 tsunami provided many critical lessons for emergency management and preparedness. As a result of that tragedy, a number of preparedness initiatives are now underway to promote awareness of earthquake and tsunami hazards along the west coast of North America, and plans are underway to develop prototype tsunami and earthquake warning systems along Cascadia. Lessons learned from the great Sumatra earthquake and tsunami tragedy, both through scientific studies and through public education initiatives, will help to reduce losses during future earthquakes in Cascadia and other subduction zones of the world.

  11. 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.

  12. Subduction of the Indian Lithospheric Slab Beneath Tibet

    NASA Astrophysics Data System (ADS)

    Zhou, H.; Murphy, M. A.

    2001-12-01

    In order to characterize the dynamics of continent-continent collisions, it is essential to define its present geometry and physical state. We report the results of a seismic tomography study of the Tibet-Himalayan collision zone, using a global data set, which indicates that the Indian lithospheric slab has been subducted subhorizontally beneath nearly the entire Tibetan plateau to depths of 165-260 km. Tibetan velocity structure is low in the crust and high in mantle lithosphere at depths between 75-120 km. An asthenospheric layer overlies the subducted Indian slab at depths between 120-165 km beneath the Tibetan plateau. There is a large low-velocity anomaly north of the Indus-Yalu suture zone between 85ºE and 93ºE that extends from the crust down to at least 310 km depth beneath the plateau. This low-velocity anomaly is indicative of mantle upwelling through a weakened zone of the subducted slab. The extent to which India has subducted beneath Tibet, as revealed by these seismic images, is comparable to estimates of crustal shortening across the Himalaya. Moreover, we hypothesize that the buoyancy due to heating of the subducted Indian slab and the existence of the asthenospheric layer contribute to the elevation and flatness of the Tibetan plateau.

  13. Plume head - trench interaction: impact on subduction dynamics

    NASA Astrophysics Data System (ADS)

    Betts, P. G.; Moresi, L. N.; Mason, W. G.; Willis, D.

    2013-12-01

    The geologic record provides numerous examples where plumes and their associated buoyancy swell have disrupted convergent plate margins. These interactions have produced a variety of responses in the overriding plate including transient episodes of arc amagmatism, transient episodes of crustal shortening followed by plume-related magmatism in the overriding plate. The latter observation implies the plume must have transitioned from the subducting plate to the overriding plate. We present several 3D Underworld numerical models of plume heads of variable dimension and buoyancy interacting with a subduction trench. The models indicate that plume heads impact enormously on trench geometry. Arcuate trenches are created as the trench retreats around the edges of the plume head, whereas trench advance occurs in front of the plume resulting in transient crustal shortening in the overriding plate. Stalling of subduction when the plume head impacts the trench causes slab windowing. The size of the slab window is dependent on the size and buoyancy of the plume. The creation of the slab window provides a potential conduit for plume migration to the overriding plate. Alternatively, the plume head may be transferred to the overriding plate as subduction is re-established behind the plume. Models with "strong" slabs, characterized by high yield strengths, display different behavior. Plume-heads are entrained in the slab and are subducted without the development of a slab window.

  14. Revised Geometry for the Hikurangi Subduction Thrust, New Zealand: Application to Slow Slip Event Inversions and Hydrological Modeling on the Hikurangi Subduction Interface

    NASA Astrophysics Data System (ADS)

    Williams, C. A.; Ellis, S. M.; Marson-Pidgeon, K.; Eberhart-Phillips, D.; Wallace, L.; Beavan, R. J.; Barker, D. H.; Henrys, S. A.; Reyners, M.; Sutherland, R.

    2012-12-01

    Slow slip events (SSEs) occur along nearly the entire Hikurangi subduction margin adjacent to the North Island, New Zealand. Long duration, deep, large events occur at the southern Hikurangi margin, while short duration, shallow, smaller events occur at the northern and central Hikurangi margin. We are attempting to understand the factors controlling the distribution of these SSEs by 1) Providing a more accurate description of the interface geometry; 2) Including the effects of material heterogeneity in our geodetic inversions of slow slip; and 3) Examining the effects of pore fluid pressure on frictional fault behavior. Accurate knowledge of interface geometry is critical in both forward and inverse models of SSEs. To address this, we have combined recent seismic reflection results and trench bathymetry with seismicity results to provide a parametric representation of the Hikurangi interface. Using this merged geometric model, we then make use of a New Zealand-wide 3D seismic velocity model and the finite element code PyLith to generate Green's functions that are used to perform inversions for slow slip distributions constrained by GPS observations. The improved geometry in combination with the inclusion of material heterogeneity should provide much more accurate predictions of the slip distributions. Finally, we examine the effects of pore fluid pressure on fault frictional behavior by examining a 2D seismic reflection profile near Gisborne. This profile captures a subducting seamount, as well as a region of SSE activity downdip of the seamount. Using geometric and material property constraints provided by the seismic reflection data, we use the finite element code SUTRA to model the fluid flow for this region. We then use PyLith to model the frictional interface, using the predicted pore fluid pressures from SUTRA to modify the initial effective normal stress on the fault. By combining the fluid flow model with a frictional fault model, we hope to gain a better

  15. High-resolution imaging of the subduction thrust location, properties and geometry, from teleseismic converted waves in the Western Hellenic and Lesser Antilles subduction zones

    NASA Astrophysics Data System (ADS)

    Sachpazi, M.; Gesret, A.; Charalampakis, M.; Laigle, M.; Diaz, J.; HIRN, A.

    2012-12-01

    The « Thales was right » experiment aimed to approach by using several methods the seismic structure and activity of the two subduction zones in the European Union (Laigle et al., T02 this meeting and Hirn et al., this session). The identification in depth of the plate boundary and material properties are critical for a quantitative comparison with the distribution of earthquakes also finely resolved in theses studies. Subduction zone imaging by teleseismic tomography results in velocity anomalies varying smoothly in space, thus not resolving finely the geometry and properties of the megathrust. Images commonly presented use either Receiver Functions (RF) of teleseismic P to S conversions, or 2D GRT inversion of the scattered wavefield. We showed that with the commonly used filters, an oceanic crust cannot be resolved, whereas its thickness can be determined with our original RF multiscale analysis going to high frequency (Gesret et al., GJI, 2010). An other important issue concerns the geometry of the subducting slab. In order to constrain the depth and the dip value of the slab beneath receivers, which record signals from a wide range of backazimuths and incidence angles, we have developed a multiangle RF approach. In the Hellenic subduction zone, our multiscale analysis allowed us to resolve for the first time a standard oceanic crust at the top of the slab beneath the Eastern coast of Peloponnesus. This was in marked contrast to the significantly larger LVL thickness obtained by Suckale et al. (2009) with the 2D GRT inversion, which interpretation needed considering particular properties above the slab top or under its crust. Correspondingly to the analysis of Gesret et al. (2010) that the primary converted could broaden the LVL image, Pearce et al. (JGR, 2012) revisited the Suckale et al. (2009) data with the same approach, but using only the backscattered multiples and not the primary conversions. They found that these data were indeed consistent with an

  16. Gravity anomalies, forearc morphology and seismicity in subduction zones

    NASA Astrophysics Data System (ADS)

    Bassett, D.; Watts, A. B.; Das, S.

    2012-12-01

    We apply spectral averaging techniques to isolate and remove the long-wavelength large-amplitude trench-normal topographic and free-air gravity anomaly "high" and "low" associated with subduction zones. The residual grids generated illuminate the short-wavelength structure of the forearc. Systematic analysis of all subduction boundaries on Earth has enabled a classification of these grids with particular emphasis placed on topography and gravity anomalies observed in the region above the shallow seismogenic portion of the plate interface. The isostatic compensation of these anomalies is investigated using 3D calculations of the gravitational admittance and coherence. In the shallow region of the megathrust, typically within 100 km from the trench, isolated residual anomalies with amplitudes of up to 2.5 km and 125 mGal are generally interpreted as accreted/subducting relief in the form of seamounts and other bathymetric features. While most of these anomalies, which have radii < 50km, are correlated with areas of reduced seismicity, several in regions such as Japan and Java appear to have influenced the nucleation and/or propagation of large magnitude earthquakes. Long-wavelength (500 - >1000 km) trench-parallel forearc ridges with residual anomalies of up to 1.5 km and 150 mGal are identified in approximately one-third of the subduction zones analyzed. Despite great length along strike, these ridges are less than 100 km wide and several appear uncompensated. A high proportion of arc-normal structure and the truncation/morphological transition of trench-parallel forearc ridges is explained through the identification and tracking of pre-existing structure on the over-riding and subducting plates into the seismogenic portion of the plate boundary. Spatial correlations between regions with well-defined trench-parallel forearc ridges and the occurrence of large magnitude interplate earthquakes, in addition to the uncompensated state of these ridges, suggest links

  17. Subduction and recycling of nitrogen along the Central American margin.

    PubMed

    Fischer, Tobias P; Hilton, David R; Zimmer, Mindy M; Shaw, Alison M; Sharp, Zachary D; Walker, James A

    2002-08-16

    We report N and He isotopic and relative abundance characteristics of volatiles emitted from two segments of the Central American volcanic arc. In Guatemala, delta15N values are positive (i.e., greater than air) and N2/He ratios are high (up to 25,000). In contrast, Costa Rican N2/He ratios are low (maximum 1483) and delta15N values are negative (minimum -3.0 per mil). The results identify shallow hemipelagic sediments, subducted into the Guatemalan mantle, as the transport medium for the heavy N. Mass balance arguments indicate that the subducted N is efficiently cycled to the atmosphere by arc volcanism. Therefore, the subduction zone acts as a "barrier" to input of sedimentary N to the deeper mantle.

  18. Preliminary results from Submarine Ring of Fire 2012 - NE Lau: First explorations of hydrothermally active volcanoes across the supra-subduction zone and a return to the West Mata eruption site

    NASA Astrophysics Data System (ADS)

    Resing, J.; Embley, R. W.

    2012-12-01

    Several expeditions in the past few years have shown that the NE Lau basin has one of the densest concentrations of volcanically and hydrothermally active volcanoes on the planet. In 2008 two active submarine volcanic eruptions were discovered during a one week period and subsequent dives with the Jason remotely operated vehicle at one of the sites (West Mata) revealed an active boninite eruption taking place at 1200 m depth. Two dives at the other revealed evidence for recent eruption along the NE Lau Spreading Center. Several more expeditions in 2010-11 discovered additional evidence about the extent and types of hydrothermal activity in this area. Data from CTDO (conductivity, temperature, depth, optical) vertical casts, tow-yos, and towed camera deployments revealed more than 15 hydrothermal sites at water depths from ~800 to 2700 m that include sites from the magmatic arc, the "rear arc," and the back arc spreading centers. These sites range from high temperature black smoker sulfide-producing systems to those dominated by magmatic degassing. Dives by remotely operated vehicle (Quest 4000) in September 2012 will explore these sites and return samples for chemical, biological and geologic studies. One of the dives will be a return visit to West Mata volcano, the site of the deepest submarine eruption yet observed (in 2009). Recent multibeam data reveal large changes in West Mata's summit, suggesting that the nature of the eruption and the location of the erupting vents may have changed. In addition to the preliminary results from the science team, we will also discuss our use and experience with continuous live video transmission (through the High Definition video camera on the Quest 4000) back to shore via satellite and through the internet. Submarine Ring of Fire 2012 Science Team: Bradley Tebo, Bill Chadwick, Ed Baker, Ken Rubin, Susan Merle, Timothy Shank, Sharon Walker, Andra Bobbitt, Nathan Buck, David Butterfield, Eric Olson, John Lupton, Richard Arculus

  19. Carboniferous granites on the northern margin of Gondwana, Anatolide-Tauride Block, Turkey - Evidence for southward subduction of Paleotethys

    NASA Astrophysics Data System (ADS)

    Candan, O.; Akal, C.; Koralay, O. E.; Okay, A. I.; Oberhänsli, R.; Prelević, D.; Mertz-Kraus, R.

    2016-06-01

    Carboniferous metagranites with U-Pb zircon crystallization ages of 331-315 Ma crop out in the Afyon zone in the northern margin of the Anatolide-Tauride Block, which is commonly regarded as part of Gondwana during the Late Palaeozoic. They are peraluminous, calc-alkaline and are characterized by increase in Rb and Ba, decrease in Nb-Ta, and enrichment in Sr and high LILE/HFSE ratios compatible with a continental arc setting. The metagranites intrude a metasedimentary sequence of phyllite, metaquartzite and marble; both the Carboniferous metagranites and metasedimentary rocks are overlain unconformably by Lower Triassic metaconglomerates, metavolcanics and Upper Triassic to Cretaceous recrystallized limestones. The low-grade metamorphism and deformation occurred at the Cretaceous-Tertiary boundary. There is no evidence for Carboniferous deformation and metamorphism in the region. Carboniferous arc-type granites and previously described Carboniferous subduction-accretion complexes on the northern margin of the Anatolide-Tauride Block suggest southward subduction of Paleotethys under Gondwana during the Carboniferous. Considering the Variscan-related arc granites in Pelagonian and Sakarya zones on the active southern margin of Laurasia, a dual subduction of Paleotethys can be envisaged between Early Carboniferous and Late Permian. However, the southward subduction was short-lived and by the Late Permian the Gondwana margin became passive.

  20. Influence of Peruvian Flat-Subduction Dynamics on the Evolution of Western Amazonia

    NASA Astrophysics Data System (ADS)

    Eakin, Caroline; Lithgow-Bertelloni, Carolina; Dávila, Federico

    2014-05-01

    Subducting slabs are the primary buoyancy force in the Earth's mantle and provide a critical source of surface deformation via their coupling to the upper plate. The impact of variations in slab morphology on this process however remains to be fully investigated. This is of particular importance for episodes of "flat" or "shallow" subduction that are often associated with slabs that have thickened oceanic crust. The shape and buoyancy content of these flat-slabs is therefore quite different to "normal" subduction scenarios. The consequences for dynamics of the mantle and how the upper plate deforms in response are not well understood. One way we can constrain this better is to study dynamic topography produced by active flat-slab systems, such as in Peru in South America, and compare them against observations from the geological record. Flat-subduction beneath Peru is thought to have begun during the Mid-Late Miocene. At the same time widespread subsidence occurred across western Amazonia resulting in the deposition of the Solimões Formation from the foredeep all the way up to the Purus Arch, 2000 km from the modern trench. We investigate how both long-wavelength changes of dynamic topography from the arrival of the flat-slab, as well as shorter wavelength flexure from Andean loading interacted to shape the modern day Amazonian landscape. We calculate dynamic topography of a slab model derived from the regional seismicity to provide a realistic flat-slab geometry that best defines the leading edge of subduction and therefore constrains the loci of dynamic subsidence. We find that >1 km of dynamic subsidence (~1500 km wide) is expected over 1000 km away from the trench. In contrast our flexural calculations predict 2.8 to 3.6 km of accommodation space that spans only 100 km across. We show that thick distal foreland accumulations of the Solimões Formation (up to 1.2 km deep), are beyond the influence of the narrow flexural subsidence but are well matched by our

  1. Sensitivity of the short-to-intermediate wavelength geoid to rheologic structure in subduction zones

    NASA Astrophysics Data System (ADS)

    Hines, J. M.; Billen, M. I.

    2010-12-01

    It is well established that the long wavelength geoid and dynamic topography are responsive to the radial viscosity structure of the mantle, but recent studies indicate that lateral viscosity variations affect the geoid at shorter wavelengths. These studies, however, only consider Newtonian viscosity structures, although experimental deformation studies of mantle minerals and seismic observations of lattice preferred orientation in the upper mantle provide evidence that dislocation creep is an active deformation mechanism at mantle conditions. In addition, the lithosphere is expected to yield plastically at high stresses based on laboratory measurements of yield strength. To quantify the effects of lateral viscosity variations and realistic flow laws on the short-to-intermediate wavelength dynamic topography and geoid near subduction zones, we consider a composite viscosity that accounts both for Newtonian and stress-dependent deformation mechanisms, including plastic yielding. Regional models of instantaneous stokes flow models are computed on a variable resolution mesh using CitcomS, where the resolution ranges from 25 km away from the subduction zone to 5 km in the vicinity of the subducting slab. The slab is defined as an 80 million year old lithosphere temperature anomaly smoothed above and below by half-space cooling models, and extends 100 km into the lower mantle. The buoyancy and stress fields are expanded to spherical harmonic degree 360, corresponding to a spatial resolution of about 110 km. These fields include the effects of self-gravitation and are used to solve for the surface geoid, as well as for dynamic topography at the surface and core-mantle boundary. Results of preliminary, layered mantle viscosity models are consistent with previous geoid studies, the main conclusion being that a more positive geoid at subduction zones is the product of relative viscosity increases with depth. In layered models, increased viscous support of the down

  2. The 1977 Sumba earthquake series: Evidence for Slab pull force acting at a subduction zone

    NASA Astrophysics Data System (ADS)

    Spence, William

    1986-06-01

    The great 1977 Sumba earthquake occurred at the eastern Sunda trench, just west of the collision of Australian continental lithosphere with the island arc. The length of the aftershock zone of this normal-faulting earthquake is about 200 km. Aftershocks are concentrated 65-115 km east of the main shock epicenter, with very few aftershocks in a 50-km-long segment that spans the main shock epicenter. Relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere. There is no evidence for thrust faulting of the deeper aftershocks. These data imply that the neutral bending surface must be at least 35-40 km deep. A second aftershock zone, about 180 km northwest of the main shock, became active immediately following the main shock, but events were concentrated during days 50-52. This zone is a 70-km-long lineation that trends toward the main shock epicenter and reflects right-lateral, strike-slip faulting within the subducted oceanic plate. Seismicity exists to a depth of about 650 km in the very old plate beneath the Sunda-Banda arc, and that plate's negative buoyancy causes very large slab pull forces. Great interface thrust earthquakes are absent at the Sumba region, and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate. This decoupling permits slab pull stresses to be guided updip to the region of the Sumba main shock. Such shallow-acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire Sumba earthquake series. In this model, slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake. Postseismically, the released oceanic plate undergoes a pulse of downdip strain, returning the plate to a less extended state. The moment of this downdip plate motion could exceed the seismic moment of the main

  3. 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

  4. Two decades of spatiotemporal variations in subduction zone coupling offshore Japan

    NASA Astrophysics Data System (ADS)

    Loveless, J. P.; Meade, B. J.

    2015-12-01

    Spatial patterns of coupling on global subduction zones can be used to guide seismic hazard assessment, but estimates of coupling are often constrained using a limited temporal range of geodetic data. Here we analyze 19 years of observations from the GEONET network to assess time-dependent variations in the spatial distribution of coupling on the subduction zones offshore Japan. We divide the position time series into five, 3.75-year epochs each decomposed into best-fit velocity, periodic signals, coseismic offsets, and postseismic effects following five major earthquakes. Nominally interseismic velocities are interpreted in terms of combined tectonic block motions and earthquake cycle activity. Our derived velocity fields reveal significant consistency since 1996 in the spatial distribution of coupling on the Nankai subduction zone, with variation limited primarily to Tokai, where long-term slow slip events have occurred, and persistently coupled regions coincident with areas that slipped in historic great earthquakes, including estimates of the 1707 Hoei earthquake based on tsunami deposits. On the Sagami subduction zone south of Tokyo, we estimate relatively stable coupling through time, though the area of coupling is reduced by the occurrence of a slow-slip event in 2002. On the Japan Trench, we image significant coupling variations owing to effects of the 1994 Sanriku-oki, 2003 Tokachi-oki, and 2011 Tohoku-oki earthquakes. In particular, strong coupling becomes more spatially extensive following the 1994 event until 2011, and a decrease in coupling and occurrence of coseismic-sense slip precede the Tohoku-oki event. Despite occurrence of the 2003 Tokachi-oki earthquake, coupling offshore Hokkaido suggests ongoing seismic hazard, possibly similar to past Mw 9-class earthquakes interpreted from coastal paleoseismic records. This time-dependent analysis of interseismic deformation illuminates rich diversity in the distribution of subduction zone processes

  5. Geochemical consequences of thermomechanical plumes in subduction zones. Implications for crustal making processes

    NASA Astrophysics Data System (ADS)

    Vogt, K.; Castro, A.; Gerya, T.

    2011-12-01

    Crustal growth rates and geochemical consequences of composite plumes formed in subduction zones have been analysed using a thermo-mechanical numerical model of an oceanic-continental subduction zone. This model includes dehydration of subducted crust, aqueous fluid transport, partial melting and melt emplacement. Subduction of crustal material to sublithospheric depth results in the formation of tectonic rock melanges composed of basalts and sediments, which may trigger Rayleigh-Taylor instabilities atop the slab. Composite plumes are formed that rise through the mantle transporting subducted crustal materials (of varying composition) towards hotter zones of the mantle wedge. We have investigated the composition and the geochemical evolution of liquids derived from composite plumes by analysing the differing proportions of the endmembers in the source, i.e. basalts and sediments. Our results show that the proportions of the components are limited to short range variations over an interval of Xb(basalt/basalt+sediment) = 0.4 - 0.8 that allows for granodioritic melt production [1]. We have further calculated Sr and Nd isotopic initial ratios of the melange at any time during the simulations, based on the fraction of the components in the melange. Liquids derived from composite plumes inherit the geochemical characteristics of the parental magma and show distinct temporal variations of radiogenic isotopes. The decoupling between radiogenic isotopes and major elements is an interesting result, and may explain short range variations observed in some batholiths along the Cordillera. Batholiths formed along active continental margins display homogeneous major element composition but substanstial variation in radiogenic isotopic compositions, suggesting widely varying proportions of mantle and crustal components in their source that may be explained by melts derived from composite plumes. [1] Castro A., Gerya, T., García-Casco, A., Fernández, C., Díaz Alvarado, J

  6. Investigating Compositional Links Between Arc Magmas And The Subducted Altered Oceanic Crust

    NASA Astrophysics Data System (ADS)

    Straub, S. M.

    2015-12-01

    Mexican (dominated by recycling of eroded crust) arcs imply a similar unradiogenic Pb of the recycled AOC at 206Pb/204Pb ~18.1. Tentatively, these results suggest that the AOC subducted in the past may be more unradiogenic on average than mean composition inferred from the erupted magmas along the active mid-oceanic ridges.

  7. Carbon in, Carbon out: Reevaluating Carbon Fluxes in Subduction Zones

    NASA Astrophysics Data System (ADS)

    Manning, C. E.; Kelemen, P. B.

    2015-12-01

    Subduction zones exert a fundamental control on the deep carbon cycle. We reevaluated carbon inputs and outputs in convergent margins considering new estimates of C concentration in subducting mantle peridotites, carbonate solubility in aqueous fluids along subduction geotherms, melting and diapirism of carbon-bearing metasediments, and diffuse degassing from arcs. Our updated estimate of carbon inputs to the global subduction system, which includes estimates for C in altered peridotite, is 40-66 megatons carbon/year (MtC/y). We find that estimates of C lost from slabs (14-66 MtC/y) must take into account the high CaCO3 solubility in aqueous fluids, which contributes significant C that must be added to that derived from mineral decarbonation reactions. When taken together with hydrous silicate and carbonatite melts and metasediment diapirs, nearly all C can be scavenged from subducting lithosphere. The return of C to the atmosphere via arc-volcano degassing is only 18-43 MtC/y, but consideration deep volatile saturation of arc magmas, magma ponding in the middle and deep arc crust, and CO2 venting in forearcs can account for the remaining C lost from the slab. Thus, 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, substantial quantities of carbon are stored in the mantle lithosphere and crust and the carbon content of the mantle lithosphere + crust + ocean + atmosphere must be increasing, at least over the last 5-10 My. This is consistent with inferences from noble gas data. Recycled carbon in diamonds is a small fraction of the global carbon inventory.

  8. Evolution of the Archaean crust by delamination and shallow subduction.

    PubMed

    Foley, Stephen F; Buhre, Stephan; Jacob, Dorrit E

    2003-01-16

    The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time. PMID:12529633

  9. Does subduction-induced mantle flow drive backarc extension?

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter P.; Strak, Vincent; Duarte, João C.

    2016-05-01

    Backarc extension is a characteristic feature of many narrow subduction zones. Seismological and geochemical studies imply the occurrence of mantle flow around the narrow subducting slabs. Previous 3D models suggested that backarc extension is related to subduction-induced toroidal mantle flow. The physical viability of this mechanism, however, has never been tested using laboratory-based geodynamic models. In this work, we present dynamic laboratory models of progressive subduction in three-dimensional (3D) space that were carried out to test this mechanism. To achieve this, we have used a stereoscopic Particle Image Velocimetry (sPIV) technique to map simultaneously overriding plate deformation and 3D subduction-induced mantle flow underneath and around an overriding plate. The results show that the strain field of the overriding plate is characterized by the localization of an area of maximum extension within its interior (at 300-500 km from the trench). The position of maximum extension closely coincides (within ∼2 cm, scaling to 100 km) with that of the maximum trench-normal horizontal mantle velocity and velocity gradient measured at a scaled depth of 15-25 km below the base of the overriding plate, and the maximum horizontal gradient of the vertical mantle velocity gradient. We propound that in narrow subduction zones backarc extension in the overriding plate is mainly a consequence of the trench-normal horizontal gradients of basal drag force at the base of the overriding plate. Such shear force gradients result from a horizontal gradient in velocity in the mantle below the base of the lithosphere induced by slab rollback. Calculations based on our models indicate a tensional horizontal trench-normal deviatoric stress in the backarc region scaling to ∼28.8 MPa, while the overriding plate trench-normal stress resulting from the horizontal component of the trench suction force is about an order of magnitude smaller, scaling to ∼2.4-3.6 MPa.

  10. Did growth of high Andes slow down Nazca plate subduction?

    NASA Astrophysics Data System (ADS)

    Quinteros, J.; Sobolev, S. V.

    2010-12-01

    The convergence velocity rate of the Nazca and South-American plate and its variations during the last 100 My are quite well-known from the global plate reconstructions. The key observation is that the rate of Nazca plate subduction has decreased by about 2 times during last 20 Myr and particularly since 10 Ma. During the same time the Central Andes have grown to its present 3-4 km height. Based on the thin-shell model, coupled with mantle convection, it was suggested that slowing down of Nazca plate resulted from the additional load exerted by the Andes. However, the thin-shell model, that integrates stresses and velocities vertically and therefore has no vertical resolution, is not an optimal tool to model a subduction zone. More appropriate would be modeling it with full thermomechanical formulation and self-consistent subduction. We performed a set of experiments to estimate the influence that an orogen like the Andes could have on an ongoing subduction. We used an enhanced 2D version of the SLIM-3D code suitable to simulate the evolution of a subducting slab in a self-consistent manner (gravity driven) at vertical crossections through upper mantle, transition zone and shallower lower mantle. The model utilizes non-linear temperature- and stress-dependant visco-elasto-plastic rheology and phase transitions at 410 and 660 km depth. We started from a reference case with a similar configuration as both Nazca and South-America plates. After some Mys of slow kinematicaly imposed subduction, to develop a coherent thermo-mechanical state, subduction was totally dynamic. On the other cases, the crust was slowly thickened artificially during 10 My to generate the Andean topography. Although our first results show no substantial changes on the velocity pattern of the subduction, we, however, consider this result as preliminary. At the meeting we plan to report completed and verified modeling results and discuss other possible cases of the late Cenozoic slowing down of

  11. 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

  12. Curved orogen and syntaxes formation during subduction and collision

    NASA Astrophysics Data System (ADS)

    Bajolet, F.; Replumaz, A.; Faccenna, C.; Lainé, R.

    2012-04-01

    The sustained convergence between India and Asia with successive stages of oceanic subduction, continental subduction and continental collision has lead to the formation of the Tibetan plateau while the Himalayan orogenic front acquired an arcuate shape convex toward the South. The Indian plate is bounded by north-south strike-slip faults, which accommodate a large indentation of Asia, between two oceanic subductions, beneath Makran to the west, beneath Indonesia to the east. Two syntaxes formed at both east and west termination of the Himalayan orogenic front at the transition between Indian and Asian plates. In order to better understand this particular configuration, we performed analog experiments at the Laboratory of Experimental Tectonics of Roma TRE to simulate, at the scale of the mantle - lithosphere system, the mechanics of the indentation process. The configuration is set to drive the India indenter towards the Asian continent with a motor-controlled-piston, to simulate far field stresses necessary for indentation. In particular, we test (1) which geometry and rheological parameters favor arcuate orogen and syntaxes formation, (2) what are the consequences on the topography of both the orogenic front and the plateau, and (3) how they relate with the subduction/collision dynamics. The setup is composed of a subducting and an overriding plate made of visco-elastic silicone putty, floating on low-viscosity syrup simulating the asthenosphere. The subducting plate simulates an oceanic lithosphere followed by a continental indenter (analog for the Indian craton) flanked or not by oceans (analog for Makran and Indonesian domains), while the upper continental plate simulates the Tibetan plateau. Results show that the curvature of the orogen and syntaxes' formation are primarily controlled by the strength and gravitational potential energy of the upper plate, and the shape of the subducting plate. A relatively strong upper plate flanked by oceans leads to a

  13. Trench migration and overriding plate stress in dynamic subduction models

    NASA Astrophysics Data System (ADS)

    Holt, A. F.; Becker, T. W.; Buffett, B. A.

    2015-04-01

    On Earth, oceanic plates subduct beneath a variety of overriding plate (OP) styles, from relatively thin and negatively buoyant oceanic OPs to thick and neutrally/positively buoyant continental OPs. The inclusion of an OP in numerical models of self-consistent subduction has been shown to reduce the rate that subducting slabs roll back relative to the equivalent single plate models. We use dynamic, 2-D subduction models to investigate how the mechanical properties, namely viscosity, thickness, and density, of the OP modify the slab rollback rate and the state of stress of the OP. In addition, we examine the role of the subducting plate (SP) viscosity. Because OP deformation accommodates the difference between the slab rollback rate and the far-field OP velocity, we find that the temporal variations in the rollback rate results in temporal variations in OP stress. The slabs in our models roll back rapidly until they reach the lower mantle viscosity increase, at which point the rollback velocity decreases. Concurrent with this reduction in rollback rate is a switch from an OP dominated by extensional stresses to a compressional OP. As in single plate models, the viscosity of the SP exerts a strong control on subducting slab kinematics; weaker slabs exhibit elevated sinking velocities and rollback rates. The SP viscosity also exerts a strong control on the OP stress regime. Weak slabs, either due to reduced bulk viscosity or stress-dependent weakening, have compressional OPs, while strong slabs have dominantly extensional OPs. While varying the viscosity of the OP alone does not substantially affect the OP stress state, we find that the OP thickness and buoyancy plays a substantial role in dictating the rate of slab rollback and OP stress state. Models with thick and/or negatively buoyant OPs have reduced rollback rates, and increased slab dip angles, relative to slabs with thin and/or positively buoyant OPs. Such elevated trench rollback for models with positively

  14. Large-scale deformation associated with ridge subduction

    USGS Publications Warehouse

    Geist, E.L.; Fisher, M.A.; Scholl, D. W.

    1993-01-01

    Continuum models are used to investigate the large-scale deformation associated with the subduction of aseismic ridges. Formulated in the horizontal plane using thin viscous sheet theory, these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction. Modelling was used to compare the Tonga arc and Louisville ridge collision with the New Hebrides arc and d'Entrecasteaux ridge collision, which have disparate arc-ridge intersection speeds but otherwise similar characteristics. Models of both systems indicate that diffuse deformation (low values of the effective stress-strain exponent n) are required to explain the observed deformation. -from Authors

  15. 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

  16. Intraplate volcanism influenced by distal subduction tectonics at Jeju Island, Republic of Korea

    NASA Astrophysics Data System (ADS)

    Brenna, Marco; Cronin, Shane J.; Kereszturi, Gábor; Sohn, Young Kwan; Smith, Ian E. M.; Wijbrans, Jan

    2015-01-01

    The drivers behind the inception of, and the variable, pulsatory eruption rates at distributed intraplate volcanic fields are not well understood. Such broad areas of monogenetic volcanism cover vast areas of the world and are often heavily populated. Reliable models to unravel their behaviour require robust spatio-temporal frameworks within the fields, but an analysis of the potential proximal and distal regional volcano-tectonic processes is also needed. Jeju Island (Republic of Korea) is a volcanic field that has been extensively drilled and dated. It is also located near one of the world's best-studied tectonic plate boundaries: the subduction zone in southwestern Japan, which generates the Ryukyu and SW Japan arcs. A new set of 40Ar/39Ar ages collected from cores penetrating the entire Jeju eruptive pile, along with geochemical information, is used to construct a temporal and volumetric model for the volcano's growth. The overall pattern indicates inception of volcanism at ~1.7 Ma, with an initial 1.2 Myr of low-rate activity, followed by over an order of magnitude rise over the last 0.5 Myr. The magma flux at Jeju correlates well with increased extension rates in the arc/backarc region. In particular, we infer that the increased trenchward mantle flow, caused by the greater rollback of the Philippine Sea Plate, activated pre-existing shear weaknesses in the mantle beneath Jeju, resulting in mantle upwelling and decompression melting that caused a change in compositions and an increase in eruption rates at Jeju. Thus, the volcanic activity of an intraplate field system can be modulated by regional subduction processes occurring more than 650 km away. This model may explain the frequent observation of pulsatory behaviour seen in many monogenetic volcanic fields worldwide that lie within 1,000 km of subduction zones.

  17. Episodic Tremor and Slip in the Cascadia Subduction Zone: A Story of Discovery

    NASA Astrophysics Data System (ADS)

    Dragert, H.

    2003-12-01

    migration velocity of slip for the GPS-determined Cascadia slip events, to the depth and migration velocity of the Japanese tremors triggered the search for seismic signatures for the Cascadia slip events. An examination of seismic records from 1996 to 2002 for sites on Vancouver Is. revealed that what had previously been deemed surface noise was signal from seismic tremors that accompanied slip events. The Cascadia tremors were found to be similar in character to the Japanese deep tremors. In addition, their source region was found to coincide with, or directly overlie, the region of the subducting slab interface where transient slip occurs. The close correlation of tremors with slip coined the naming of the phenomenon as Episodic Tremor and Slip (ETS). The physical processes which give rise to this dynamic behavior on the deeper plate interface are not yet well understood. To date, only the Nankai and Cascadia subduction zones have been observed to share aspects of this behavior, suggesting that this phenomenon may be restricted to young subduction zones. The release of fluids, contact with a hydrated mantle wedge, and episodic changes in shear strength or mechanical coupling may all play a part in governing this behavior. Possible connections of ETS with the development of "E-zone" reflector bands, basal erosion, and pulsating metamorphism await further research. In the context of seismic hazard, the ETS zone may mark the down-dip limit of coseismic rupture of the next megathrust earthquake. Also, since it is conceivable for a slip event to trigger a large subduction thrust earthquake, the onset of ETS activity could identify times of higher probability for the occurrence of megathrust earthquakes.

  18. Subduction Zone Fluid Flow and Infiltrative Metasomatism in Franciscan Complex Exotic Ultramafic Blocks

    NASA Astrophysics Data System (ADS)

    King, R. L.; Kohn, M. J.; Eiler, J.

    2001-12-01

    Exotic ultramafic blocks within mud-matrix mélange of the Franciscan Complex, CA, preserve a series of metasomatic mineral zones due to the infiltration of SiO2-rich fluids during subduction. These blocks have experienced extremely large fluid fluxes during metamorphism within the Franciscan subduction channel and appear to have recorded the composition of the fluids present within the Franciscan subduction zone. Chemical constituents dissolved in this fluid include many elements expected to be mobile in high-T aqueous fluids (e.g. SiO2), but also include elements generally believed to be immobile in fluids liberated during the devolatilization of subducted material (e.g. TiO2). We examined the petrology, whole-rock geochemistry, and oxygen isotope composition of these mineral zones to place further constraints on the petrologic structure of the forearc mantle wedge. The ultramafic blocks preserve the relict peridotite mineralogy Ol + Opx + Cpx + Cr-Spl. Fluid flow forms serpentinite (Srp + Mgt + Chl +/- relict Cr-Spl) after peridotite. Additional infiltration formed Tlc (Tlc + Chl +/- relict Cr-Spl) and is the strongest evidence for fluid-mediated addition of SiO2. A fourth mineral zone, outside the scope of the models presented by Manning [1995, 1997], was created by metasomatism of Tlc-bearing rocks, and is composed of Tr + barroisite + Chl + Czo + Ttn + Ap + Zrc +/- relict Cr-Spl. The amphiboles appear to buffer SiO2 at higher activities, stabilizing this diverse group of minerals. Whole-rock geochemical changes occurring during metasomatism include wholesale removal of approximately 6-8 wt% of CaO by through-going fluids during serpentinization. During the production of Tlc from Srp, minor amounts of Al2O3 were added to the rock in addition to the requisite SiO2. At the final stage of metasomatism preserved, Tr-rich rocks show increases in SiO2, TiO2, Al2O3, P2O5, and especially CaO. Separates of Srp from synmetamorphic Srp + Cal veins give Srp δ 18O of 8

  19. Investigations on the fate of subducted carbonates

    NASA Astrophysics Data System (ADS)

    Cerantola, Valerio; McCammon, Catherine; Dubrovinsky, Leonid

    2013-04-01

    Carbon storage in the deep Earth is currently the subject of ongoing debate. There is much evidence for the presence of carbon cycling through the Earth's interior, such as the occurrence of diamonds from the upper and lower parts of the mantle, carbonate inclusions in diamonds and mantle xenoliths, the existence of carbonatite magmas, the presence of CO2 in volcanic eruptions, etc. Now that carbonate-bearing subducting slabs are believed to pass through the transition zone and enter the lower mantle, interest has been focused on which carbon-bearing phases are the most stable at pressures and temperatures down to the core-mantle boundary. The solubility of carbon in the dominant mantle phases is low; hence carbon is likely stored in accessory phases, e.g., carbonates, diamonds/graphite, methane, carbides. Its distribution is a function of pressure, temperature, bulk composition and oxygen fugacity. At highly reducing conditions, the crystalline form of carbon is graphite or diamond, depending on P and T. At more oxidizing conditions carbonates are favored, due to the reaction between elemental carbon and oxygen to form (CO3)2- groups, which bond to other cations such as Ca2+, Mg2+, Fe2+ and Na+ depending on the original bulk assemblage. Recent discoveries favor the presence of Fe-bearing carbonates at ultra high pressures and temperatures, suggesting the presence of carbonate solid solutions in the deep Earth. Several questions remain unsolved, however: 1) Which phase compositions are stable at pressures and temperatures of the lower mantle? 2) What is the effect of Ca on the redox stability of iron-bearing carbonates at high pressures and high temperatures? 3) Do spin transitions occur at lower mantle conditions in iron-containing (Mg,Ca)CO3 phases? Our interest is currently focused on the study of carbonates at lower mantle conditions. We achieve high pressure and temperature conditions using externally heated or laser heated diamond anvil cells, which enable us

  20. Three-dimensional dynamic laboratory models of subduction with an overriding plate and variable interplate rheology

    NASA Astrophysics Data System (ADS)

    Duarte, João C.; Schellart, Wouter P.; Cruden, Alexander R.

    2013-10-01

    Subduction zones are complex 3-D features in which one tectonic plate sinks underneath another into the deep mantle. During subduction the overriding plate (OP) remains in physical contact with the subducting plate and stresses generated at the subduction zone interface and by mantle flow force the OP to deform. We present results of 3-D dynamic laboratory models of subduction that include an OP. We introduce new interplate materials comprising homogeneous mixtures of petrolatum and paraffin oil to achieve progressive subduction. The rheology of these mixtures is characterized by measurements using a strain rate controlled rheometer. The results show that the strength of the mixture increases with petrolatum content, which can be used as a proxy for the degree of mechanical coupling along the subduction interface. Results of subduction experiments are presented with different degrees of mechanical coupling and the influence this has on the dynamics and kinematics of subduction. The modelling results show that variations in the degree of mechanical coupling between the plates have a major impact on subduction velocities, slab geometry and the rate of OP deformation. In all experiments the OP is displaced following trench migration and experiences overall extension localized in the plate interior. This suggests that OP deformation is driven primarily by the toroidal component of subduction-related mantle return flow. The subduction rate is always very slow in experiments with medium mechanical coupling, and subduction stops prematurely in experiments with very high coupling. This implies that the shear forces along the plate interface in natural subduction zone systems must be relatively low and do not vary significantly. Otherwise a higher variability in natural subduction velocities should be observed for mature, non-perturbed subduction zones. The required low shear force is likely controlled by the rheology of highly hydrated sedimentary and basaltic rocks.

  1. Reconciling evidence for Tethyan intra-oceanic subduction and a two-stage collision between India and Eurasia

    NASA Astrophysics Data System (ADS)

    Gibbons, Ana D.; Zahirovic, Sabin; Dietmar Müller, R.; Whittaker, Joanne M.; Yatheesh, Vadakkeyakath

    2014-05-01

    We present a plate tectonic model for the India-Eurasia collision that includes a time-dependent network of evolving plate boundaries with synthetic plates constructed for now-subducted Tethyan ocean floor, including back-arc basins that formed on the southern Eurasian margin. Southern Eurasia and Southeast Asia are riddled with dismembered oceanic arcs indicating long-lived intra-oceanic subduction. This intra-oceanic subduction may have extended further west into the India-Eurasia convergence zone in the NeoTethys, which was consumed during Greater India's northward trajectory towards Eurasia from the Early Cretaceous. Fragments of obducted oceanic crust within the Himalayan Yarlung-Tsangpo Suture Zone, between India and Eurasia, cluster around two age groups, the Late Jurassic and mid Cretaceous (Barremian-Aptian). The adakitic, boninitic and MORB-affinities of the various ophiolites along strike suggest that there was at least one generation of intra-oceanic subduction, whose plate boundary configuration remains uncertain, though it is best preserved in the Kohistan-Ladakh Arc. Paleomagnetic and magmatic characterisation studies from the ophiolites suggest that the intra-oceanic arc was as far south as the equator during the Early Cretaceous before subduction resumed further north beneath the southern Eurasian margin (Lhasa terrane) to consume the back-arc basin. During ~80-65 Ma, a hiatus in subduction-related magmatism along the southern Lhasa terrane may indicate the approach of the back-arc spreading centre towards the active Andean-style margin. We incorporate these observations into a regional, self-consistent plate tectonic model for the dispersal of East Gondwana, simultaneously considering geophysical data and seafloor spreading histories from abyssal plains offshore West Australia and East Antarctica, including Jurassic seafloor age data from offshore NW Australia that limits northern Greater India to a maximum of ~1000 km. This Greater India collided

  2. Reconstructions of subducted ocean floor along the Andes: a framework for assessing Magmatic and Ore Deposit History

    NASA Astrophysics Data System (ADS)

    Sdrolias, M.; Müller, R.

    2006-05-01

    The South American-Antarctic margin has been characterised by numerous episodes of volcanic arc activity and ore deposit formation throughout much of the Mesozoic and Cenozoic. Although its Cenozoic subduction history is relatively well known, placing the Mesozoic arc-related volcanics and the emplacement of ore bodies in their plate tectonic context remains poorly constrained. We use a merged moving hotspot (Late Cretaceous- present) and palaeomagnetic /fixed hotspot (Early Cretaceous) reference frame, coupled with reconstructed spreading histories of the Pacific, Phoenix and Farallon plates to understand the convergence history of the South American and Antarctic margins. We compute the age-area distribution of oceanic lithosphere through time, including subducting oceanic lithosphere and estimate convergence rates along the margin. Additionally, we map the location and migration of spreading ridges along the margin and relate this to processes on the overriding plate. The South American-Antarctic margin in the late Jurassic-early Cretaceous was dominated by rapid convergence, the subduction of relatively young oceanic lithosphere (< 35 m.y. old) and extensive arc volcanism on the overriding plate. Additionally, our reconstructed position of the Farallon-Phoenix ridge during this period corresponds with the emplacement of several ore bodies in southern South America, similar to formation of Miocene to recent ore deposits in the northern Andes due to aseismic ridge subduction. A change in absolute motion of the Pacific plate after ~120 Ma, led to a significant decrease in the convergence rate and the southward migration of the Farallon-Phoenix ridge and this may have contributed to the cessation of back- arc spreading in the "Rocas Verdes" in southern South America. The speed of subduction increased again along the South American-Antarctic margin at ~105 Ma after another change in tectonic regime. Newly created crust from the Farallon-Phoenix ridge continued to be

  3. Teaching about Subduction Zone Magmagenesis using MARGINS Subduction Factory Focus Site Geochemical Compilations and ABS3 (Invited)

    NASA Astrophysics Data System (ADS)

    Stern, R. J.; Jordan, E.; Raye, U.; Carr, M. J.; Feigenson, M.; Gill, J. B.; Hacker, B. R.; Kimura, J.; Lehnert, K. A.; Tamura, Y.; van Keken, P. E.

    2010-12-01

    Processes and inputs involved in generating arc magmas are reasonably well known but quantitative modeling is often overlooked when teaching about subduction zone magmagenesis. In order to appreciate these complexities, students need to be able to explore subduction zone magmagenetic processes using trace element compositions of igneous rocks. The MARGINS Subduction Factory experiment selected two endmember convergent margins, the Izu-Bonin-Mariana (IBM) arc, which subducts old, cold, dense seafloor, and the Central American (CentAm) arc, which subducts young, hot, buoyant seafloor. We have compiled high-quality trace element and isotopic data for young, fresh lavas from along the magmatic fronts of these endmember arcs, using the EarthChem database. Comparing data for primitive magmas from the two arc systems allows for first-order distinctions, including the greater relative abundances of fluid-mobile elements (e.g., K, Sr, U) in IBM lavas and greater relative abundances of elements requiring sediment melting (e.g., Th, LREE, Zr) in CentAm lavas. These differences can be explored quantitatively using the Arc Basalt Simulator version 3 (ABS3). ABS3 is a free Excel-based spreadsheet forward model that allows the user to control compositions of subducted sediment and altered oceanic crust in tandem with realistic thermal models to predict metamorphic conditions in the subducted slab, using simplified results from Perple_X, and to understand when sediment- and slab-melting is likely (See J.-I. Kimura et al, this meeting "V15 The Subduction Filter" session for more information about ABS3). Prograde metamorphism along with experimentally-determined partition coefficients are used to predict hydrous fluid compositions; experimental results along with mineral-melt distribution coefficients are used to predict slab melt compositions. Hydrous fluid or melt is allowed to rise into and metasomatize overlying mantle, and the modified fluid allowed to trigger mantle melting

  4. Magma-derived CO2 emissions in the Tengchong volcanic field, SE Tibet: Implications for deep carbon cycle at intra-continent subduction zone

    NASA Astrophysics Data System (ADS)

    Zhang, Maoliang; Guo, Zhengfu; Sano, Yuji; Zhang, Lihong; Sun, Yutao; Cheng, Zhihui; Yang, Tsanyao Frank

    2016-09-01

    Active volcanoes at oceanic subduction zone have long been regard as important pathways for deep carbon degassed from Earth's interior, whereas those at continental subduction zone remain poorly constrained. Large-scale active volcanoes, together with significant modern hydrothermal activities, are widely distributed in the Tengchong volcanic field (TVF) on convergent boundary between the Indian and Eurasian plates. They provide an important opportunity for studying deep carbon cycle at the ongoing intra-continent subduction zone. Soil microseepage survey based on accumulation chamber method reveals an average soil CO2 flux of ca. 280 g m-2 d-1 in wet season for the Rehai geothermal park (RGP). Combined with average soil CO2 flux in dry season (ca. 875 g m-2 d-1), total soil CO2 output of the RGP and adjacent region (ca. 3 km2) would be about 6.30 × 105 t a-1. Additionally, we conclude that total flux of outgassing CO2 from the TVF would range in (4.48-7.05) × 106 t a-1, if CO2 fluxes from hot springs and soil in literature are taken into account. Both hot spring and soil gases from the TVF exhibit enrichment in CO2 (>85%) and remarkable contribution from mantle components, as indicated by their elevated 3He/4He ratios (1.85-5.30 RA) and δ13C-CO2 values (-9.00‰ to -2.07‰). He-C isotope coupling model suggests involvement of recycled organic metasediments and limestones from subducted Indian continental lithosphere in formation of the enriched mantle wedge (EMW), which has been recognized as source region of the TVF parental magmas. Contamination by crustal limestone is the first-order control on variations in He-CO2 systematics of volatiles released by the EMW-derived melts. Depleted mantle and recycled crustal materials from subducted Indian continental lithosphere contribute about 45-85% of the total carbon inventory, while the rest carbon (about 15-55%) is accounted by limestones in continental crust. As indicated by origin and evolution of the TVF

  5. Variation of b and p values from aftershocks sequences along the Mexican subduction zone and their relation to plate characteristics

    NASA Astrophysics Data System (ADS)

    Ávila-Barrientos, L.; Zúñiga, F. R.; Rodríguez-Pérez, Q.; Guzmán-Speziale, M.

    2015-11-01

    Aftershock sequences along the Mexican subduction margin (between coordinates 110ºW and 91ºW) were analyzed by means of the p value from the Omori-Utsu relation and the b value from the Gutenberg-Richter relation. We focused on recent medium to large (Mw > 5.6) events considered susceptible of generating aftershock sequences suitable for analysis. The main goal was to try to find a possible correlation between aftershock parameters and plate characteristics, such as displacement rate, age and segmentation. The subduction regime of Mexico is one of the most active regions of the world with a high frequency of occurrence of medium to large events and plate characteristics change along the subduction margin. Previous studies have observed differences in seismic source characteristics at the subduction regime, which may indicate a difference in rheology and possible segmentation. The results of the analysis of the aftershock sequences indicate a slight tendency for p values to decrease from west to east with increasing of plate age although a statistical significance is undermined by the small number of aftershocks in the sequences, a particular feature distinctive of the region as compared to other world subduction regimes. The b values show an opposite, increasing trend towards the east even though the statistical significance is not enough to warrant the validation of such a trend. A linear regression between both parameters provides additional support for the inverse relation. Moreover, we calculated the seismic coupling coefficient, showing a direct relation with the p and b values. While we cannot undoubtedly confirm the hypothesis that aftershock generation depends on certain tectonic characteristics (age, thickness, temperature), our results do not reject it thus encouraging further study into this question.

  6. Degree of serpentinization in the forearc mantle wedge of Kyushu subduction zone: quantitative evaluations from seismic velocity

    NASA Astrophysics Data System (ADS)

    Xia, Shaohong; Sun, Jinlong; Huang, Haibo

    2015-09-01

    Serpentinization is an important phenomenon for understanding the water cycle and geodynamics of subduction zones in the upper mantle. In this study, we evaluate quantitatively the degree of serpentinization using the seismic velocity. The results show that serpentinization mainly occurs in the forearc mantle wedge along the subducted oceanic crust, and the degree of serpentinization in the forearc mantle wedge of Kyushu is strongly heterogeneous and varies from 0 to 12 %, containing about 0-1.8 % water contents. In general, the degree of serpentinization gradually decreases with depth from 40 to 80 km and the largest degree usually occur in about 40-50 km depth. Localized high anomalies of serpentinization are revealed in the northern and southern portions of Kyushu, respectively. We suggest that in the northern portion of the forearc mantle wedge, the water contents are relatively large, which might result from the abundant fractures and cracks with more fluids in the subducting slab because of the subduction of Kyushu-Palau ridge and the sudden change in its subduction angle of Philippine Sea lithosphere. But the high degree of serpentinization in the southern portion is closely associated with the active left-lateral shear zone revealed by global positioning system site velocities and earthquake focal mechanisms. In addition, the present results also display that the low degree of serpentinization in the central domain of the forearc mantle wedge is consistent with the location of anomalous arc volcano. The distribution of water contents is closely associated with the degree of serpentinization in the forearc mantle wedge.

  7. Accretion, underplating and exhumation along a subduction interface: From subduction initiation to continental subduction (Tavşanlı zone, W. Turkey)

    NASA Astrophysics Data System (ADS)

    Plunder, Alexis; Agard, Philippe; Chopin, Christian; Pourteau, Amaury; Okay, Aral I.

    2015-06-01

    We herein reappraise the pressure-temperature (PT) evolution of the high-pressure and low-temperature (HP-LT) Tavşanlı zone (western Turkey) in order to (i) better characterize rock units exhumed along a cooling subduction interface, from birth to steady state and (ii) constrain exhumation and detachment dynamics, as well as mechanical coupling between plates. Based on PT estimates and field observations three oceanic complexes are recognized between the HP-LT continental margin and the obducted ophiolite, with PT estimates ranging from incipient metamorphism to blueschist-facies conditions. PT conditions for the continental unit are reappraised to 24 kbar and ~ 500 °C on the basis of pseudosection modelling and Raman spectroscopy on carbonaceous material. A tentative reconstruction of the subduction zone evolution is proposed using available radiometric and palaeogeographic data and recent thermomechanical modelling. Both PT conditions and field observations point out to the slicing of km-sized units at different preferred depths along the subduction interface, thus providing constraints on the dynamics of accretion and underplating. In particular, the comparison of PT estimates for the Tavşanlı zone and for other broadly similar fossil subduction settings (i.e., Oman, Corsica, New Caledonia, Franciscan, Schistes Lustrés) suggests that units are detached preferentially from the slab at specific depths of 30-40 km (i.e., downdip of the seismogenic zone) and ~ 80 km. We propose that these depths are controlled by major changes in mechanical coupling along the plate interface, whereas exhumation through time would rather be controlled by large-scale geodynamic boundary conditions.

  8. Crustal Structure of the Caribbean-South American Diffuse Plate Boundary: Subduction Zone Migration and Polarity Reversal Along BOLIVAR Profile 64W

    NASA Astrophysics Data System (ADS)

    Clark, S. A.; Levander, A.; Magnani, M.; Zelt, C. A.; Sawyer, D. S.; Ave Lallemant, H. G.

    2005-12-01

    The BOLIVAR (Broadband Ocean-Land Investigation of Venezuela and the Antilles arc Region) project is an NSF funded, collaborative seismic experiment in the southeast Caribbean region. The purpose of the project is to understand the diffuse plate boundary created by the oblique collision between the Caribbean and South American plates. Profile 64W of the BOLIVAR experiment, a 450 km-long, N-S transect onshore and offshore Venezuela located at ~64°W longitude, images the deep crustal structures formed by this collision. The active source components of profile 64W include 300 km of MCS reflection data, 33 coincident OBSs, and 344 land seismic stations which recorded 7500 offshore airgun shots and 2 explosive land shots. Results from the reflection and refraction seismic data along 64W show complex crustal structure across the entire span of the diffuse plate boundary. The onshore portion of 64W crosses the fold and thrust belt of the Serrania del Interior, which formed at ~16 Ma by collision of the Caribbean forearc with the northern South American passive margin. Underlying the Serrania del Interior is a south-vergent, remnant Lesser Antillean subduction zone. As this Lesser Antilles subduction impinged on continental crust, it caused a polarity reversal and jump offshore to the north. Convergence was initially localized in the closure and inversion of the Grenada Basin. However, subduction could not develop because of the ~20-km-thick crust of the Aves Ridge; instead, north-vergent subduction initiated further to the north, where ~12-km-thick Caribbean oceanic crust of the Venezuela Basin began to subduct beneath the Aves Ridge in the Pliocene (~4 Ma) and appears to continue subducting today. Between the remnant subduction zone and the modern one, the El Pilar and Coche dextral strike-slip faults accommodate most of the transform motion of the plate boundary. From the Serrania del Interior to the Aves Ridge, ~260 km of accreted orogenic float comprises the diffuse

  9. Subduction Zone Diversity and Nature of the Plate Contact

    NASA Astrophysics Data System (ADS)

    Defranco, R.; Govers, R.; Wortel, R.

    2008-12-01

    We recently showed that the overall dynamics of subduction and initial collision depends on whether the plate contact is a fault or a channel. Here, we combine results of our numerical experiments with a re-analysis of published observations. Overall, our synthesis connects seismic moment release with back-arc deformation and tectonic processes at the margin. It leads us to identify four classes of subduction zones. The first two classes results directly from our numerical experiments. In class 1, subduction zones are characterized by a plate contact that is largely fault-like with an accretionary margin. In class 2, the plate contacts are largely channel-type and have an erosive margin. Class 3, where the plate contact is entirely channel-like, consists of accretionary margins with a high sediment supply. Subduction zones of class 4, mostly characterized by an erosive convergent margin (northern Chili, Peru, Honshu and Kuril), are more complicated. They can be explained by incorporating regional observations.

  10. GPS constraints on interplate locking within the Makran subduction zone

    NASA Astrophysics Data System (ADS)

    Frohling, E.; Szeliga, W.

    2016-04-01

    The Makran subduction zone is one of the last convergent margins to be investigated using space-based geodesy. While there is a lack of historical and modern instrumentation in the region, a sparse sampling of continuous and campaign measurements over the past decade has allowed us to make the first estimates of convergence rates. We combine GPS measurements from 20 stations located in Iran, Pakistan and Oman along with hypocentral locations from the International Seismological Centre to create a preliminary 3-D estimate of the geometry of the megathrust, along with a preliminary fault-coupling model for the Makran subduction zone. Using a convergence rate which is strongly constrained by measurements from the incoming Arabia plate along with the backslip method of Savage, we find the Makran subduction zone appears to be locked to a depth of at least 38 km and accumulating strain.We also find evidence for a segmentation of plate coupling, with a 300 km long section of reduced plate coupling. The range of acceptable locking depths from our modelling and the 900 km along-strike length for the megathrust, makes the Makran subduction zone capable of earthquakes up to Mw = 8.8. In addition, we find evidence for slow-slip-like transient deformation events on two GPS stations. These observations are suggestive of transient deformation events observed in Cascadia, Japan and elsewhere.

  11. 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

  12. Slip to the Trench for Great Subduction Earthquakes

    NASA Astrophysics Data System (ADS)

    Mori, J. J.

    2015-12-01

    The 2011 Tohoku-oki earthquake had the largest ever recorded fault slip (40 to 60 meters) on the shallow portion of the subduction zone close to the trench. This large displacement was largely unexpected for this region and was mainly responsible for the very large and damaging tsunami along the northeast coast of Honshu. We investigate the possibility of large slip to the trench in great earthquakes for other subduction zones around the world. Since the trench region is generally far offshore, it is often difficult to resolve the amount of slip from onshore geodetic and strong-motion data. We use a variety of observations, including slip distribution models, aftershock locations, local coastal deformation, and tsunami heights to determine which events likely had large amounts of slip close to the trench. Tsunami earthquakes, such as 1992 Nicaragua and 2006 Java likely had large shallow slip. Some typical subduction earthquakes, such as 1968 Tokachi-oki and 2003 Tokachi-oki (located in regions north of the source area of the 2011 Tohoku-oki earthquake) likely did not.We will discuss possible factors that influence the slip distribution on the shallow area of subduction megathrusts. Using results from the Japan Trench Fast Drilling Project (JFAST) which sampled the fault in the region of large slip, we can begin to understand the conditions of very large fault slip, such as the characteristic fault zone material and level of friction on the fault.

  13. Tensor-guided fitting of subduction slab depths

    USGS Publications Warehouse

    Bazargani, Farhad; Hayes, Gavin P.

    2013-01-01

    Geophysical measurements are often acquired at scattered locations in space. Therefore, interpolating or fitting the sparsely sampled data as a uniform function of space (a procedure commonly known as gridding) is a ubiquitous problem in geophysics. Most gridding methods require a model of spatial correlation for data. This spatial correlation model can often be inferred from some sort of secondary information, which may also be sparsely sampled in space. In this paper, we present a new method to model the geometry of a subducting slab in which we use a data‐fitting approach to address the problem. Earthquakes and active‐source seismic surveys provide estimates of depths of subducting slabs but only at scattered locations. In addition to estimates of depths from earthquake locations, focal mechanisms of subduction zone earthquakes also provide estimates of the strikes of the subducting slab on which they occur. We use these spatially sparse strike samples and the Earth’s curved surface geometry to infer a model for spatial correlation that guides a blended neighbor interpolation of slab depths. We then modify the interpolation method to account for the uncertainties associated with the depth estimates.

  14. The Effect of Subduction Events on Rising Mantle Currents

    NASA Astrophysics Data System (ADS)

    Moeller, A.; Stein, C.; Hansen, U.

    2007-12-01

    Mantle convection systems with periodic events of subduction exhibit two different kinds of upwelling currents: plumes and thermals. Plumes are strong upwelling currents with a well-defined head and tail structure. Thermals having only little buoyancy do not develop this typical structure and are strongly influenced by the large- scale convection. In vigorous convection thermals are driven towards the locations of the plumes. In this study we numerically investigate the structure and evolution of the rising mantle currents in a 2D Rayleigh- Benard set-up. Periodic events of the subduction occur due to the application of a temperature- and stress- dependent rheology. Model calculations in a Cartesian geometry with large aspect ratios are run with variations in the Rayleigh number, the viscosity contrast (measure of the temperature dependence) and the yield stress (measure of the stress dependence). Our study indicates that a system with periodic subduction events is characterised by three phases. In the first phase the cold thick lid at the surface has no influence on the upwelling currents. Strong, stationary plumes drive the convection. In the second phase the subducted cold material suppresses upwellings. Only small transient currents rise in front of the cold slab. In the third phase the cold material at the bottom is heated up allowing for the formation of strong upwelling currents again.

  15. Rheological and geodynamic controls on the mechanisms of subduction, HP/UHP exhumation and PT conditions within crustal rocks during continental collision: insights from numerical models

    NASA Astrophysics Data System (ADS)

    Burov, Evgueni

    2014-05-01

    ) continental subduction occurs in very specific cases of cold strong lithospheres while in general this process takes less than 5 Myr. During the active subduction phase (simple shear accommodation of convergence) we do not detect significant deviations (+-20%) of total pressure in the subduction channel from lithostatic values , that can be rather lower than the lithostatic pressure, while intra-channel temperatures vary in quite large limits. Hence, large volumes of HP/UHP metamorphic rocks generated and brought to the surface during subduction phase would not record significant deviations from the lithostatic pressure conditions. At the same time, strong non-lithostatic pressures (extensional and compressional) are predicted for some internal parts of the colliding plates that, however, are not prone to yield "extractable" metamorphic material. The experiments also show that high non-lithostatic pressures develop in the former subduction channel at its lock-up , during and after the transition from subduction to pure shear collision or folding, while the metamorphic material generated at this stage appears to be blocked at depth and does not return to the surface (at least if the channel is not unlocked due some external conditions). We suggest that most continental orogenic belts could have started their formation from continental subduction. This process has been generally limited in time while pressures recorded in the HP and UHP material generated at this stage can be largely treated in terms of the lithostatic approximation. In case of subduction of continental terrains embedded in the oceanic lithosphere, it can be shown that their exhumation, resulting in formation of metaphoric belts and domes, may initiate series of slab roll-back and exhumation events associated with remarkably complex and spatially variable P-T-t paths.

  16. 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.

  17. Tectonic and petrologic evolution of the Western Mediterranean: the double polarity subduction model

    NASA Astrophysics Data System (ADS)

    Melchiorre, Massimiliano; Vergés, Jaume; Fernàndez, Manel; Torné, Montserrat; Casciello, Emilio

    2016-04-01

    The geochemical composition of the mantle beneath the Mediterranean area is extremely heterogeneous. This feature results in volcanic products whose geochemical features in some cases do not correspond to the geodynamic environment in which they are sampled and that is observed at present day. The subduction-related models that have been developed during the last decades to explain the evolution of the Western Mediterranean are mainly based on geologic and seismologic evidences, as well as petrography and age of exhumation of the metamorphic units that compose the inner parts of the different arcs. Except few cases, most of these models are poorly constrained from a petrologic point of view. Usually the volcanic activity that affected the Mediterranean area since Oligocene has been only used as a corollary, and not as a key constrain. This choice is strictly related to the great geochemical variability of the volcanic products erupted in the Western Mediterranean, due to events of long-term recycling affecting the mantle beneath the Mediterranean since the Variscan Orogeny, together with depletion episodes due to partial melting. We consider an evolutionary scenario for the Western Mediterranean based on a double polarity subduction model according to which two opposite slabs separated by a transform fault of the original Jurassic rift operated beneath the Western and Central Mediterranean. Our aim has been to reconstruct the evolution of the Western Mediterranean since the Oligocene considering the volcanic activity that affected this area since ~30 Ma and supporting the double polarity subduction model with the petrology of the erupted rocks.

  18. Subduction-related large tsunamis in the Mediterranean Sea: a review

    NASA Astrophysics Data System (ADS)

    Papadopoulos, G.

    2013-05-01

    A variety of historical documentary sources combined with evidence from geological signatures, geomorphological imprints, observations from selected coastal archaeological sites, as well as instrumental records, clearly indicate that seismic and non-seismic (e.g. volcanism, landslides) tsunami sources can be found in all the areas of the Mediterranean Sea region. Local, regional and basin-wide tsunamis have been documented. However, historically the largest ones were generated along active zones of lithospheric subduction. In the Hellenic Arc and Trench, which is the most active in the region, large tsunamis were produced by M8-8.3 earthquakes documented to rupture the western and eastern segments of the arc in AD 365 and 1303, respectively. In pre-historic times, the subduction-related huge Late Bronge Age (17th century BC) Plinian-type eruption in Thera (Santorini) volcano has been geologically and archaeologically documented in several coastal zones of the eastern Mediterranean basin. In the Calabrian Arc, it is under question the generation mechanisms of the large, destructive tsunamis of AD 1693, 1783 and 1908. In the Cyprean Arc, a very strong tsunami was documented to hit the southern coast of Cyprus after a possibly subduction-related earthquake in AD 1222. Understanding of such large Mediterranean tsunamis on the basis of combined historical documentary sources, on-shore and off-shore geological signatures and inputs from numerical tsunami simulations is investigated. The issue of tsunami repeat times calculation from geological and statistical methods is examined, while consequences for hazard assessment are discussed.

  19. Three-dimensional Thermal Model of the Mexican Subduction Zone

    NASA Astrophysics Data System (ADS)

    Rosas, J. C.; Pimentel, F. D. C.; Currie, C. A.; He, J.; Harris, R. N.

    2015-12-01

    Along the Mexican section of the Middle America Trench (MAT), the Cocos plate subducts beneath the North American plate. The most important feature of this subduction zone is the flat-slab section below central Mexico, extending approximately 250 km landward from the trench at a depth of 50 km. Further west, the dip changes to 45-50º. This particular geometry has several unique consequences, such as a volcanic arc that is not aligned with the trench and very shallow slab seismicity. For the mantle wedge, the abrupt change in slab geometry could lead to a three-dimensional (3D) mantle wedge flow that departs from the classical 2D subduction-driven corner flow. Evidence of 3D flow in the region comes from seismic anisotropy studies, which show that olivine fast-direction axes have a component that is parallel to the MAT. In other subduction zones, such as Costa Rica-Nicaragua and Japan, 3D flow has been observed to increase temperatures by >50º C relative to corner flow models.For this study, we have created the first 3D finite-element model of the Mexican subduction zone in order to analyze its thermal structure. Our objective is to assess the effects of 3D mantle flow and hydrothermal circulation (HC) in the subducting slab. In this region, low surface heat flow values near the trench indicate that HC may remove heat from the oceanic plate. Our model incorporates the effect of HC through conductivity proxies in the subducting crust and a 2D oceanic geotherm that includes the age variations of the Cocos plate along the MAT. For an isoviscous mantle, our model shows that the slab dip variations induce a flow that departs from 2D corner flow near the transition between the flat-slab and normal-dipping sections. The mantle flows in eastward direction toward the flat slab, and its orientation is consistent with seismic anisotropy studies. The maximum along-margin flow rate is nearly 2 cm/yr, which is >30% of the convergence rate. Temperatures at the location of this

  20. Subduction zone guided waves: 3D modelling and attenuation effects

    NASA Astrophysics Data System (ADS)

    Garth, T.; Rietbrock, A.

    2013-12-01

    Waveform modelling is an important tool for understanding complex seismic structures such as subduction zone waveguides. These structures are often simplified to 2D structures for modelling purposes to reduce computational costs. In the case of subduction zone waveguide affects, 2D models have shown that dispersed arrivals are caused by a low velocity waveguide, inferred to be subducted oceanic crust and/or hydrated outer rise normal faults. However, due to the 2D modelling limitations the inferred seismic properties such as velocity contrast and waveguide thickness are still debated. Here we test these limitations with full 3D waveform modelling. For waveguide effects to be observable the waveform must be accurately modelled to relatively high frequencies (> 2 Hz). This requires a small grid spacing due to the high seismic velocities present in subduction zones. A large area must be modelled as well due to the long propagation distances (400 - 600 km) of waves interacting with subduction zone waveguides. The combination of the large model area and small grid spacing required means that these simulations require a large amount of computational resources, only available at high performance computational centres like the UK National super computer HECTOR (used in this study). To minimize the cost of modelling for such a large area, the width of the model area perpendicular to the subduction trench (the y-direction) is made as small as possible. This reduces the overall volume of the 3D model domain. Therefore the wave field is simulated in a model ';corridor' of the subduction zone velocity structure. This introduces new potential sources of error particularly from grazing wave side reflections in the y-direction. Various dampening methods are explored to reduce these grazing side reflections, including perfectly matched layers (PML) and more traditional exponential dampening layers. Defining a corridor model allows waveguide affects to be modelled up to at least 2

  1. Along-strike Variations of Subduction Parameters at the Chilean Plate Boundary

    NASA Astrophysics Data System (ADS)

    Hoffmann-Rothe, A.; Kukowski, N.; Oncken, O.

    2004-12-01

    Newly compiled data of the geometric, kinematic and mechanic properties and their variations along-strike the oblique Chilean subduction margin between 20° S and 46° S are used to weigh their competing influence on forearc deformation. Special emphasis lies on the formation of margin-parallel strike-slip systems. Among the parameters considered are the convergence rate and obliquity, the ocean floor age, the dip of the down-going and the slope of the overriding plate, the geodetic and seismic coupling depth, the interplate seismicity, the depth of the trench-fill and the mass transfer mode at the subduction front. Commonly discussed control factors for forearc deformation can be attributed to three major elements of a subduction system, namely (1) the plate kinematic boundary conditions, (2) the plate coupling properties that govern the effectiveness of force transmission from the subducting plate to the overriding plate, and (3) the upper plate heterogeneities affecting its rheology (e.g. elasticity, shear strength) or resistance to block motion (buttressing). An example is given for each of these elements: (1) Oblique convergence is a pre-requisite for the activation of margin-parallel strike-slip systems, but apparently not a sufficient condition. For example, strike-slip motion can presently be observed along the Liquiñe-Ofqui Fault Zone in southern Chile, while neither the Atacama Fault Zone nor the Precordilleran Fault System in northern Chile accommodate significant amounts of margin-parallel slip since the Pliocene. This difference can not be explained by variations of convergence rate or obliquity as the plate kinematic framework is almost constant along the Chilean trench. (2) The plate coupling force is a function of the frictionally coupled area on the plate interface and of the shear friction that needs to be overcome. Along the Chilean margin various factors affect coupling in opposing manner: The slab-dip is shallower in southern Chile compared to

  2. Multiple Collision and Subduction Structure of the Izu-Bonin Arc: Tectonics of the Arc-arc Collision in Central Japan

    NASA Astrophysics Data System (ADS)

    Arai, R.; Iwasaki, T.; Sato, H.; Abe, S.; Hirata, N.

    2011-12-01

    The Izu collision zone in central Japan provides a great research field for studying crustal evolution of island arcs associated with collision and subduction processes. Since the middle Miocene, an intraoceanic volcanic arc named the Izu-Bonin arc has been colliding from south with the Honshu arc along with the subduction of the Philippine Sea plate. Intensive geological studies in the last few decades revealed that several crustal blocks derived from the Izu-Bonin arc, such as Koma Mountains, Misaka Mountains, Tanzawa Mountains and Izu Peninsula, were accreted onto the Honshu arc in the course of the multiple collision (e.g. Amano, 1991). In order to understand the whole crustal structure dominated by the active arc-arc collision, we carried out several seismic experiments using controlled sources (Sato et al., 2005, 2006; Arai et al., 2009). Structural models obtained by reflection and refraction/wide-angle reflection analyses show extremely complex collision styles characterized by obduction in the northernmost part (Misaka) and crustal stacking in the middle part (Tanzawa). Delamination structure is recognized at a mid-crustal level for these two blocks. On the other hand, a subduction style is dominant in the southern part (Izu). These differences may be attributed to the along-arc structural variation of the Izu-Bonin arc (Kodaira et al., 2007). It is also indicated that some portions of the Izu-Bonin middle crust were accreted at the bottom of the Honshu crust. Consideration of mass balance, however, suggests that the most Izu-Bonin crust has been subducting deep into the mantle without being accreted. So we can say that the dynamic process of this intraoceanic volcanic arc is essentially controlled by subduction rather than collision/accretion. A northwestward dipping reflector was found at depths of 25-35 km beneath the accreted crustal blocks of Misaka and Tanzawa, which is interpreted to be the upper surface of the subducting lower crust of the Izu

  3. The effect of subducting slabs in global shear wave tomography

    NASA Astrophysics Data System (ADS)

    Lu, Chang; Grand, Stephen P.

    2016-05-01

    Subducting slabs create strong short wavelength seismic anomalies in the upper mantle where much of Earth's seismicity is located. As such, they have the potential to bias longer wavelength seismic tomography models. To evaluate the effect of subducting slabs in global tomography, we performed a series of inversions using a global synthetic shear wave traveltime data set for a theoretical slab model based on predicted thermal anomalies within slabs. The spectral element method was applied to predict the traveltime anomalies produced by the 3-D slab model for paths corresponding to our current data used in actual tomography models. Inversion tests have been conducted first using the raw traveltime anomalies to check how well the slabs can be imaged in global tomography without the effect of earthquake mislocation. Our results indicate that most of the slabs can be identified in the inversion result but with smoothed and reduced amplitude. The recovery of the total mass anomaly in slab regions is about 88 per cent. We then performed another inversion test to investigate the effect of mislocation caused by subducting slabs. We found that source mislocation largely removes slab signal and significantly degrades the imaging of subducting slabs-potentially reducing the recovery of mass anomalies in slab regions to only 41 per cent. We tested two source relocation procedures-an iterative relocation inversion and joint relocation inversion. Both methods partially recover the true source locations and improve the inversion results, but the joint inversion method worked significantly better than the iterative method. In all of our inversion tests, the amplitudes of artefact structures in the lower mantle caused by the incorrect imaging of slabs (up to ˜0.5 per cent S velocity anomalies) are comparable to some large-scale lower-mantle heterogeneities seen in global tomography studies. Based on our inversion tests, we suggest including a-priori subducting slabs in the

  4. Can slabs melt beneath forearcs in hot subduction zones?

    NASA Astrophysics Data System (ADS)

    Ribeiro, J.; Maury, R.; Gregoire, M.

    2015-12-01

    At subduction zones, thermal modeling predict that the shallow part of the downgoing oceanic crust (< 80 - 100 km depth to the slab) is usually too cold to cross the water-rich solidus and melts beneath the forearc. Yet, the occasional occurrence of adakites, commonly considered as slab melts, in the forearc region challenges our understanding of the shallow subduction processes. Adakites are unusual felsic rocks commonly associated with asthenospheric slab window opening or fast subduction of young (< 25 Ma) oceanic plate that enable slab melting at shallow depths; but their genesis has remained controversial. Here, we present a new approach that provides new constraints on adakite petrogenesis in hot subduction zones (the Philippines) and above an asthenospheric window (Baja California, Mexico). We use amphibole compositions to estimate the magma storage depths and the composition of the parental melts to test the hypothesis that adakites are pristine slab melts. We find that adakites from Baja California and Philippines formed by two distinct petrogenetic scenarios. In Baja California, hydrous mantle melts mixed/mingled with high-pressure (HP) adakite-type, slab melts within a lower crustal (~30 km depth) magma storage region before stalling into the upper arc crust (~7-15 km depth). In contrast, in the Philippines, primitive mantle melts stalled and crystallized within lower and upper crustal magma storage regions to produce silica-rich melts with an adakitic signature. Thereby, slab melting is not required to produce an adakitic geochemical fingerprint in hot subduction zones. However, our results also suggest that the downgoing crust potentially melted beneath Baja California.

  5. Plate Interface Rheology, Mechanical Coupling and Accretion during Subduction Infancy

    NASA Astrophysics Data System (ADS)

    Agard, P.; Yamato, P.; Mathieu, S.; Prigent, C.; Guillot, S.; Plunder, A.; Dubacq, B.; Monie, P.; Chauvet, A.

    2015-12-01

    Understanding subduction rheology in both space and time has been a challenge since the advent of plate tectonics. We herein focus on "subduction infancy", that is the first ~1-5 My immediately following subduction nucleation, when a newly born slab penetrates into the upper plate mantle and heats up. The only remnants of this critical yet elusive geodynamic step are thin metamorphic soles, commonly found beneath pristine, 100-1000 km long portions of oceanic lithosphere emplaced on continents (i.e., ophiolites). Through the (i) worldwide compilation of pressure-temperature conditions of metamorphic sole formation augmented by pseudosection thermodynamic modeling, (ii) calculations of the viscosity of materials along the plate interface and (iii) generic numerical thermal models, we provide a conceptual model of dynamic plate interface processes during subduction infancy (and initiation s.l.). We show in particular how major rheological switches across the subduction interface control slab penetration, and the formation of metamorphic soles. Due to the downward progression of hydration and weakening of the mantle wedge with cooling, the lower plate (basalt, sediment) and the upper plate (mantle wedge) rheologies equalize and switch over a restricted temperature-time-depth interval (e.g., at ~800°C and ~1 GPa, during 0.1-2 My, for high-temperature metamorphic sole formation). These switches result in episodes of maximum interplate mechanical coupling, thereby slicing the top of the slab and welding pieces (basalt, sediment) to the base of the mantle wedge. Similar mechanical processes likely apply for the later, deeper accretion and exhumation of high-temperature oceanic eclogites in serpentinite mélanges, or for the accretion of larger tectonic slices. This model provides constraints on the effective rheologies of the crust and mantle and general understanding, at both rock and plate scale, for accretion processes and early slab dynamics.

  6. Perspectives on the Dynamics of Subduction and Trench Rollback: From the Birth of Subduction to Global Plate Motions (Invited)

    NASA Astrophysics Data System (ADS)

    Gurnis, M.; Leng, W.; Alisic, L.; Stadler, G.

    2013-12-01

    We will provide a brief overview of two classes of dynamic models of subduction zones and address issues associated with the forces driving plate tectonics and initiating new subduction zones. A common thread between the models is the origin of the intense back arc spreading and rapid roll back associated with some ocean-ocean subduction zones. We will look at the dynamics driving global plate motions and then look at the time-dependence of trench rollback regionally. Plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries, but observationally constrained high-resolution models of global mantle flow have remained a computational challenge. We capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions, with individual plate margins resolved down to a scale of 1 kilometer. We find that cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high-viscosity slabs, altering the velocity of oceanic plates. Back-arc extension and slab rollback are emergent consequences of slab descent in the upper mantle. We will show that most back arc extension follows subduction initiation and show how this arises in dynamic models.

  7. Fore-arc deformation at the transition between collision and subduction: results from first 3D thermo-mechanical laboratory experiments

    NASA Astrophysics Data System (ADS)

    Boutelier, D. A.; Oncken, O.; Ustaszewski, K. M.; Cruden, A. R.

    2011-12-01

    3-D thermo-mechanical laboratory experiments of arc-continent collision investigate the deformation of the fore-arc at the transition between collision and subduction. The deformation of the plates in the collision area propagates into the subduction-collision transition zone via along-strike coupling of the neighboring segments of the plate boundary. The largest along-strike gradient of trench-perpendicular compression produced by a passive margin turning by 90 degrees does not generate sufficiently localized shear strain in the transition zone to cause a strike-slip system because of the fast propagation of arc lithosphere failure. Deformation is thus continuous along-strike, but the deformation mechanism is three-dimensional and progressive structural variations arise because the coupling between neighboring segment induces either advanced or delayed failure of the arc lithosphere and passive margin. During the initial stage of collision, the accretionary wedge is partially subducted, the interplate zone is lubricated, and shear traction drops. Thus large convergence obliquity does not produce a migrating fore-arc sliver. Instead, the fore-arc motion is due to the pressure force generated by subduction of the buoyant continental crust. It follows that convergence obliquity does not yield trench-parallel deformation of the fore-arc and its influence on the collision process is limited. However, convergence obliquity may have shaped the active margin during the stage of oceanic subduction stage, prior to collision, and inherited structures may impact the propagation mechanism.

  8. Deformation and topography above the lateral transition from continental to oceanic subduction in three-dimensional laboratory models: what can we learn on the Hellenic subduction?

    NASA Astrophysics Data System (ADS)

    Guillaume, B.; Funiciello, F.; Faccenna, C.; Husson, L.; Royden, L. H.

    2012-04-01

    We use three-dimensional dynamically self-consistent laboratory models to analyze relationships between surface evolution and deep dynamics at convergent margins. Our models are setup with a viscous plate of silicone (lithosphere) subducting under negative buoyancy in a viscous layer of glucose syrup (upper mantle). We focus on the subduction of a laterally heterogeneous lithosphere characterized by an abrupt transition of density using negatively and positively buoyant silicone to reproduce oceanic and continental subduction, respectively. We quantify and establish relationships between the subduction dynamics and resulting slab geometry, trench kinematics and pattern of horizontal/vertical deformation for both the overriding plate and the upper mantle. Assuming that our modeling results can be representative of the natural behavior of subduction zones, we compare them to the Neogene to Quaternary evolution of the Hellenic subduction zone. We more particularly focus on the deformation and topography of the Hellenic upper plate, which may have been influenced by the difference in subduction dynamics north and south of the Kephalonia Transform Zone, with a slowly subducting Adriatic continental lithosphere in the north and a rapidly subducting Ionian oceanic lithosphere in the south.

  9. Overriding plate deformation and variability of fore-arc deformation during subduction: Insight from geodynamic models and application to the Calabria subduction zone

    NASA Astrophysics Data System (ADS)

    Chen, Zhihao; Schellart, Wouter P.; Duarte, João. C.

    2015-10-01

    In nature, subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far-field boundary conditions that influence the style of subduction and overriding plate deformation. Here we present dynamic laboratory models of progressive subduction in three-dimensional space, in which the far-field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP-Fixed, OP-Fixed, and SP-OP-Fixed. We investigate their impact on the kinematics and dynamics of subduction, particularly focusing on overriding plate deformation. The results indicate that the variation in far-field boundary conditions has an influence on the slab geometry, subduction partitioning, and trench migration partitioning. Our models also indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of back-arc basins (e.g., Tyrrhenian Sea, Aegean Sea, and Scotia Sea) is generally expected to occur at a comparable location (250-700 km from the trench), irrespective of the boundary condition. In addition, our models indicate that the style of fore-arc deformation (shortening or extension) is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP-OP-Fixed setup is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature, and accretionary configuration. Furthermore, the model can explain back-arc and fore-arc extension at the Calabria subduction zone since the latest middle Miocene as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting induced strong trench suction, driving fore-arc extension, and

  10. Thermochronological response to rifting and subduction in the Corsica-Sardinia block

    NASA Astrophysics Data System (ADS)

    Malusà, Marco Giovanni; Danišík, Martin; Kuhlemann, Joachim

    2014-05-01

    Variscan Corsica (N-S in Paleogene coordinates). Modelled time-temperature paths show that this age trend is consistent with an erosional pulse migrating northward during the Paleogene, which led to the re-exposure of the Mesozoic planation surfaces previously buried by Paleogene detrital sequences. The northward migration of erosional pulses mirrors the coeval northward trajectory of Adria relative to Europe as inferred by magnetic anomalies. Preservation of the low-T fingerprint acquired during Tethyan rifting indicates that no European continental subduction took place south of Corsica since the Mesozoic, and suggests that the post-Jurassic Adria-Europe convergence along the Sardinia transect was possibly accommodated on the Adriatic side of the subduction system, consistent with the onset of Cenozoic orogenic magmatism. The inferred tectonic reconstruction for the Paleocene - early Eocene time frame thus includes a northwestward (Apenninic) subduction that was active along the Sardinia transect, and an eastward (Alpine) subduction that was still active along the Corsica transect and choked in middle-late Eocene times, when Adria started moving towards the NNE (Malusà et al., 2011). The northward translation of the Adriatic slab beneath Sardinia and Corsica is mirrored by the coeval migration of exhumation pulses at the surface, until the slab reached the remnants of the Alpine wedge of Corsica in Oligocene times shortly before the onset of slab rollback. Danišík, M., Kuhlemann, J., Dunkl, I., Székely, B., Frisch, W., 2007. Burial and exhumation of Corsica (France) in the light of fission track data. Tectonics 26(TC1001). Malusà, M.G., Faccenna, C., Garzanti, E., Polino, R., 2011. Divergence in subduction zones and exhumation of high-pressure rocks (Eocene Western Alps). Earth Planet. Sci. Lett. 310, 21-32.