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Sample records for subducted westward beneath

  1. Subducting characteristic of the Pacific slab beneath northeast China

    Science.gov (United States)

    Jiang, G.; Zhang, G.; Xu, Y.

    2012-12-01

    The volcanoes locating in northeast China are very active. Some researchers consider that the origin of volcanoes is closely related to the subducting western Pacific plate and the upwelling asthenosphere. The thickness and the existing range of the subducted plate are not clear as far although the seismic tomography results obviously show that the Pacific plate exists below the volcano region. Therefore, in this study, we adopted the method combining the teleseismic tomography with travel time forward modeling to further study the velocity structure beneath northeast China, especially the precise model of subducted Pacific plate. Our results show that (1) the average thickness and velocity perturbation of slab is 85 km and 1%, respectively, and the slab has not been thickened compared with the previous result of the Japan Sea; (2) the Pacific plate subducted into the mantle transition zone with a shallow dip angle, and changed horizontally when it touched the bottom of mantle transition zone, and extended westward to Longitude 127°E and then stops over there; (3) the horizontal slab locates right below the volcano region. These above features help people understand the origin of intraplate volcanoes and the geodynamical process better. (a) Tomographic result along 43°N. Red and blue colors represent the high and low velocity anomalies, respectively, and the scale is shown at the right-bottom; The profile line is shown in (b); The black triangles represent the volcanoes locating near the profile; The black solid and dashed lines show the depths of upper and lower boundaries of Pacific plate, respectively. The red dots represent the deep earthquakes around the profile. (b) Location of profile AA' along 43°N. Black triangles denote volcanoes; White squares represent the stations; Blue contours denote the depth of upper boundary of Pacific plate; Black and red dots represent the deep epicenters.

  2. Nonuniform subduction of the Indian crust beneath the Himalayas.

    Science.gov (United States)

    Guo, Xiaoyu; Li, Wenhui; Gao, Rui; Xu, Xiao; Li, Hongqiang; Huang, Xingfu; Ye, Zhuo; Lu, Zhanwu; Klemperer, Simon L

    2017-10-02

    Himalayan tectonic activity is triggered by downward penetration of the Indian plate beneath the Asian plate. The subsurface geometry of this interaction has not been fully investigated. This study presents novel constraints on this geometry provided by two newly obtained, deep seismic reflection profiles. The profiles cover 100- and 60-km transects across the Yarlung-Zangbo suture of the Himalaya-Tibet orogen at c. 88°E. Both profiles show a crustal-scale outline of the subducting Indian crust. This outline clearly shows Indian understhrusting southern Tibet, but only to a limited degree. When combined with a third seismic reflection profile of the western Himalayas, the new profiles reveal progressive, eastward steepening and shortening in the horizontal advance of the subducting Indian crust.

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

    Science.gov (United States)

    Garth, Tom; Rietbrock, Andreas

    2017-09-01

    Guided wave dispersion is observed from earthquakes at 180-280 km depth recorded at stations in the fore-arc of Northern Chile, where the 44 Ma Nazca plate subducts beneath South America. Characteristic P-wave dispersion is observed at several stations in the Chilean fore-arc with high frequency energy (>5 Hz) arriving up to 3 s after low frequency (first motion dispersion observed at multiple stations, or the extended P-wave coda observed in arrivals from intermediate depth events within the Nazca plate. These signals can however be accurately accounted for if dipping low velocity fault zones are included within the subducting lithospheric mantle. A grid search over possible LVL and faults zone parameters (width, velocity contrast and separation distance) was carried out to constrain the best fitting model parameters. Our results imply that fault zone structures of 0.5-1.0 km thickness, and 5-10 km spacing, consistent with observations at the outer rise are present within the subducted slab at intermediate depths. We propose that these low velocity fault zone structures represent the hydrated structure within the lithospheric mantle. They may be formed initially by normal faults at the outer rise, which act as a pathway for fluids to penetrate the deeper slab due to the bending and unbending stresses within the subducting plate. Our observations suggest that the lithospheric mantle is 5-15% serpentinised, and therefore may transport approximately 13-42 Tg/Myr of water per meter of arc. The guided wave observations also suggest that a thin LVL (∼1 km thick) interpreted as un-eclogitised subducted oceanic crust persists to depths of at least 220 km. Comparison of the inferred seismic velocities with those predicted for various MORB assemblages suggest that this thin LVL may be accounted for by low velocity lawsonite-bearing assemblages, suggesting that some mineral-bound water within the oceanic crust may be transported well beyond the volcanic arc. While older

  4. Louisville seamount subduction and its implication on mantle flow beneath the central Tonga-Kermadec arc.

    Science.gov (United States)

    Timm, Christian; Bassett, Daniel; Graham, Ian J; Leybourne, Matthew I; de Ronde, Cornel E J; Woodhead, Jon; Layton-Matthews, Daniel; Watts, Anthony B

    2013-01-01

    Subduction of intraplate seamounts beneath a geochemically depleted mantle wedge provides a seldom opportunity to trace element recycling and mantle flow in subduction zones. Here we present trace element and Sr, Nd and Pb isotopic compositions of lavas from the central Tonga-Kermadec arc, west of the contemporary Louisville-Tonga trench intersection, to provide new insights into the effects of Louisville seamount subduction. Elevated (206)Pb/(204)Pb, (208)Pb/(204)Pb, (86)Sr/(87)Sr in lavas from the central Tonga-Kermadec arc front are consistent with localized input of subducted alkaline Louisville material (lavas and volcaniclastics) into sub-arc partial melts. Furthermore, absolute Pacific Plate motion models indicate an anticlockwise rotation in the subducted Louisville seamount chain that, combined with estimates of the timing of fluid release from the subducting slab, suggests primarily trench-normal mantle flow beneath the central Tonga-Kermadec arc system.

  5. Slab detachment of subducted Indo-Australian plate beneath Sunda ...

    Indian Academy of Sciences (India)

    2007). We investigate the northward subduction of the. Indo-Australian plate along the eastern Sunda arc right from northwestern Sumatra, along Java to. Keywords. Slab detachment; subduction zone; Sunda arc; Indo-Australian slab; trench migration. J. Earth Syst. Sci. 120, No. 2, April 2011, pp. 193–204 c Indian Academy ...

  6. Structure and evolution of subducted lithosphere beneath the Sunda arc, Indonesia

    NARCIS (Netherlands)

    Widiyantoro, Sri; Hilst, R.D. van der

    1996-01-01

    Tomographic imaging reveals seismic anomalies beneath the Sunda island arc, Indonesia, that suggest that the lithospheric slab penetrates to a depth of at least 1500 kilometers. The Sunda slab forms the eastern end of a deep anomaly associated with the past subduction of the plate underlying the

  7. Tectonic implications of tomographic images of subducted lithosphere beneath northwestern South America

    NARCIS (Netherlands)

    Hilst, R.D. van der; Mann, P.

    1994-01-01

    We used seismic tomography to investigate the complex structure of the upper mantle below northwestern South America. Images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted Caribbean-Pacific lithosphere beneath the study area.

  8. Age of the subducting Pacific slab beneath East Asia and its geodynamic implications

    Science.gov (United States)

    Liu, Xin; Zhao, Dapeng; Li, Sanzhong; Wei, Wei

    2017-04-01

    We study the age of the subducting Pacific slab beneath East Asia using a high-resolution model of P-wave tomography and paleo-age data of ancient seafloor. Our results show that the lithosphere age of the subducting slab becomes younger from the Japan Trench (∼130 Ma) to the slab's western edge (∼90 Ma) beneath East China, and the flat (stagnant) slab in the mantle transition zone (MTZ) is the subducted Pacific plate rather than the proposed Izanagi plate which should have already collapsed into the lower mantle. The flat Pacific slab has been in the MTZ for no more than ∼10-20 million years, considerably less than the age of the big mantle wedge beneath East Asia (>110 million years). Hence, the present flat Pacific slab in the MTZ has contributed to the Cenozoic destruction of the East Asian continental lithosphere with extensive intraplate volcanism and back-arc spreading, whereas the destruction of the North China Craton during the Early Cretaceous (∼140-110 Ma) was caused by the subduction of the Izanagi (or the Paleo-Pacific) plate.

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

    Science.gov (United States)

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

    2017-04-01

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

  10. A Dynamical Context for Small-scale Heterogeneity Throughout the Mantle Beneath Subduction

    Science.gov (United States)

    Frost, D. A.; Rost, S.; Garnero, E.

    2014-12-01

    Subduction zones are a source for mantle heterogeneity within the convection system and there is mounting evidence that seismic signatures can be used to track slabs down from the surface throughout the mantle. Seismic studies of the mantle beneath Central America demonstrate that subducted slab material reaches the Core-Mantle Boundary (CMB). The lowermost mantle beneath this convergent margin shows strong seismic evidence for heterogeneity. Tomographic models characterise subduction zones to be underlain by increased seismic velocities over 100s-1000s km laterally, in association with D'´ discontinuities 100-300 km above the CMB, consistent with phase transitions in the Bridgmanite system. Recent analyses have found evidence for isolated Ultra Low Velocity Zones in addition to prevalent fine-scale heterogeneity, on the order of 1-10 km, scattering high frequency waves. These techniques indicate thermal and/or chemical anomalies within the mantle on a range of scales. Numerical geodynamical simulations suggest small-scale mechanical mixing of initially coherent compositionally anomalous subducted material separating into entities of various sizes consistent with the range of heterogeneity sizes observed in the lower mantle.Investigating seismic scattering, the re-radiation of a wavefront due to interaction with a sharply contrasting volumetric anomaly, is an effective method for studying small-scale elastic heterogeneities in the Earth's mantle. Studies commonly record structure with scale lengths of about 10 km. Here we analyse scattered energy related to PKPPKP — PKP•PKP (the • indicates the scattering location along the raypath) — sampling a large volume of the mantle beneath Central America. We reveal the character of heterogeneity in various frequency bands within the whole mantle using both broadband and short-period data. These observations will be placed in context with other studies in this region illustrating the large-scale background structure

  11. Preliminary results of characteristic seismic anisotropy beneath Sunda-Banda subduction-collision zone

    Energy Technology Data Exchange (ETDEWEB)

    Wiyono, Samsul H., E-mail: samsul.wiyono@bmkg.go.id [Study Program of Earth Sciences, Faculty of Earth Sciences and Technology, Institute of Technology Bandung, Bandung 40132 (Indonesia); Indonesia’s Agency for Meteorology Climatology and Geophysics, Jakarta 10610 (Indonesia); Nugraha, Andri Dian, E-mail: nugraha@gf.itb.ac.id [Indonesia’s Agency for Meteorology Climatology and Geophysics, Jakarta 10610 (Indonesia); Global Geophysics Research Group, Faculty of Mining and Petroleum Engineering, Institute of Technology Bandung, Bandung 40132, Indonesia, Phone: +62-22 2534137 (Indonesia)

    2015-04-24

    Determining of seismic anisotropy allowed us for understanding the deformation processes that occured in the past and present. In this study, we performed shear wave splitting to characterize seismic anisotropy beneath Sunda-Banda subduction-collision zone. For about 1,610 XKS waveforms from INATEWS-BMKG networks have been analyzed. From its measurements showed that fast polarization direction is consistent with trench-perpendicular orientation but several stations presented different orientation. We also compared between fast polarization direction with absolute plate motion in the no net rotation and hotspot frame. Its result showed that both absolute plate motion frame had strong correlation with fast polarization direction. Strong correlation between the fast polarization direction and the absolute plate motion can be interpreted as the possibility of dominant anisotropy is in the asthenosphere.

  12. Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier.

    Science.gov (United States)

    McGary, R Shane; Evans, Rob L; Wannamaker, Philip E; Elsenbeck, Jimmy; Rondenay, Stéphane

    2014-07-17

    Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature. The two mechanisms are not mutually exclusive. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid-melt phase. By incorporating constraints from a collocated seismic study into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

  13. Post-Eocene Subduction Dynamics and Mantle Flow beneath Western U.S.

    Science.gov (United States)

    Liu, L.; Zhou, Q.; Leonard, T.

    2015-12-01

    Both surface geology and mantle seismic images suggest a complex late Cenozoic history of mantle dynamics over western U.S. We try to understand this history by simulating the Farallon subduction since 40 Ma. Forward subduction models assimilating time dependent seafloor ages, plate kinematics and evolving plate boundaries suggest that the present-day 3D distribution of fast seismic anomalies below western U.S. mostly represent late Cenozoic slabs, which experienced multiple phases of segmentation during subduction because of their young age and small mechanical strength (Liu & Stegman, 2011). A major slab segmentation event occurred around mid-Miocene, with the resulting slab tear and induced asthenosphere upwelling correlating with the Steens-Columbia River flood basalts (SCRB) eruption both in space and in time (Liu & Stegman, 2012). This suggests that a mantle plume is not required for the formation of the SCRB. Segmentation of the Farallon slab generates rapid toroidal flows around the newly formed slab edges beneath the Cascadia arc. These mantle flows may affect both the pattern and composition of arc volcanism through transportation of oceanic asthenosphere material into the mantle wedge. Based on the forward model, we further test the influence of slow seismic anomalies on mantle dynamics. On the one hand, we explicitly input a deep hot anomaly to represent the putative Yellowstone plume. On the other hand, we develop a hybrid scheme that combines the adjoint inverse method with the high-resolution forward simulation approach, so that the present-day mantle seismic structure is entirely consistent with the convection model. Our preliminary results suggest that a hot plume could actively rise up only when it is several hundreds of kilometers away from the slabs, as is the case prior to 20 Ma. Subsequently, the plume is dominated by the surrounding slabs, resulting in an overall downwelling mantle flow. This suggests that a plume might have contributed to

  14. Investigation of mantle kinematics beneath the Hellenic-subduction zone with teleseismic direct shear waves

    Science.gov (United States)

    Confal, Judith M.; Eken, Tuna; Tilmann, Frederik; Yolsal-Çevikbilen, Seda; Çubuk-Sabuncu, Yeşim; Saygin, Erdinc; Taymaz, Tuncay

    2016-12-01

    The subduction and roll-back of the African plate beneath the Eurasian plate along the arcuate Hellenic trench is the dominant geodynamic process in the Aegean and western Anatolia. Mantle flow and lithospheric kinematics in this region can potentially be understood better by mapping seismic anisotropy. This study uses direct shear-wave splitting measurements based on the Reference Station Technique in the southern Aegean Sea to reveal seismic anisotropy in the mantle. The technique overcomes possible contamination from source-side anisotropy on direct S-wave signals recorded at a station pair by maximizing the correlation between the seismic traces at reference and target stations after correcting the reference stations for known receiver-side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. We obtained splitting parameters at 35 stations with good-quality S-wave signals extracted from 81 teleseismic events. Employing direct S-waves enabled more stable and reliable splitting measurements than previously possible, based on sparse SKS data at temporary stations, with one to five events for local SKS studies, compared with an average of 12 events for each station in this study. The fast polarization directions mostly show NNE-SSW orientation with splitting time delays between 1.15 s and 1.62 s. Two stations in the west close to the Hellenic Trench and one in the east show N-S oriented fast polarizations. In the back-arc region three stations exhibit NE-SW orientation. The overall fast polarization variations tend to be similar to those obtained from previous SKS splitting studies in the region but indicate a more consistent pattern, most likely due to the usage of a larger number of individual observations in direct S-wave derived splitting measurements. Splitting analysis on direct shear waves typically resulted in larger split time delays compared to previous studies, possibly because S-waves travel along a longer path

  15. Variations of Converted Ps Phase Amplitude Along the Subducting Philippine Sea Slab Beneath Shikoku, Southwest Japan

    Science.gov (United States)

    Shiomi, K.

    2016-12-01

    The Nankai region southwest Japan is one of the famous regions where low-frequency tremors (LFTs) are very active. Since the LFTs are distributed along the down-dip limit of the recurrent megathrust source regions, the LFT activities might be strongly related to stress changes in the source regions along the Philippine Sea slab. To understand the mechanisms of LFTs, knowledge about the structural difference inside and outside of the LFT active zone is important. In this study, we investigate variation of converted P-to-S (Ps) phase amplitude from receiver functions (RFs) along the subducting oceanic Moho. Teleseismograms recorded at the NIED Hi-net and F-net seismic stations are used. Since converted phase amplitude depends on its ray parameter, we select limited earthquakes with ray parameter range from 0.050 to 0.077, and apply amplitude correction coefficients. We read Ps amplitudes of RFs with reference to the previous studies [e.g. Shiomi et al. (2008; 2015)]. Since the selected events are not uniformly distributed in back azimuth (BAZ), we calculate an average and its standard deviation for each 5-degree bin. Then, we fit a simple function constructed with sin(BAZ), sin(2*BAZ) and bias component with the least square fitting algorithm. The bias components, named `standard amplitude' by Shiomi and Park (2009), gradually decay as the oceanic Moho becomes deep. This feature reflects a phase transition from amphibolite to eclogite with water release in the oceanic crust. At almost all stations, sin(BAZ) component is dominate. This component corresponds to the contribution from the dipping interface mainly, and the estimated plunge azimuth (N305±10°E) is consistent with the previous models. On the other hand, only one station, located at the northern edge of the active LFT zone, shows clear 4-lobed backazimuthal distribution. The oceanic Moho is detected at 45km depth beneath this station, and temperature is estimated 600 700°C. This temperature and pressure

  16. Estimation of seismic velocity in the subducting crust of the Pacific slab beneath Hokkaido, northern Japan by using guided waves

    Science.gov (United States)

    Shiina, T.; Nakajima, J.; Toyokuni, G.; Kita, S.; Matsuzawa, T.

    2014-12-01

    A subducting crust contains a large amount of water as a form of hydrous minerals (e.g., Hacker et al., 2003), and the crust plays important roles for water transportation and seismogenesis in subduction zones at intermediate depths (e.g., Kirby et al., 1996; Iwamori, 2007). Therefore, the investigation of seismic structure in the crust is important to understand ongoing physical processes with subduction of oceanic lithosphere. A guided wave which propagates in the subducting crust is recorded in seismograms at Hokkaido, northern Japan (Shiina et al., 2014). Here, we estimated P- and S-wave velocity in the crust with guided waves, and obtained P-wave velocity of 6.6-7.3 km/s and S-wave velocity of 3.6-4.2 km/s at depths of 50-90 km. Moreover, Vp/Vs ratio in the crust is calculated to be 1.80-1.85 in that depth range. The obtained P-wave velocity about 6.6km/s at depths of 50-70 km is consistent with those estimated in Tohoku, northeast Japan (Shiina et al., 2013), and this the P-wave velocity is lower than those expected from models of subducting crustal compositions, such as metamorphosed MORB model (Hacker et al., 2003). In contrast, at greater depths (>80 km), the P-wave velocity marks higher velocity than the case of NE Japan and the velocity is roughly comparable to those of the MORB model. The obtained S-wave velocity distribution also shows characteristics similar to P waves. This regional variation may be caused by a small variation in thermal regime of the Pacific slab beneath the two regions as a result of the normal subduction in Tohoku and oblique subduction in Hokkaido. In addition, the effect of seismic anisotropy in the subducting crust would not be ruled out because rays used in the analysis in Hokkaido propagate mostly in the trench-parallel direction, while those in Tohoku are sufficiently criss-crossed.

  17. Reconstructing Farallon plate subduction beneath North America back to the Late Cretaceous.

    Science.gov (United States)

    Liu, Lijun; Spasojevic, Sonja; Gurnis, Michael

    2008-11-07

    Using an inverse mantle convection model that assimilates seismic structure and plate motions, we reconstruct Farallon plate subduction back to 100 million years ago. Models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy, requiring that the Farallon slab was flat lying in the Late Cretaceous, consistent with geological reconstructions. The simulation predicts that an extensive zone of shallow-dipping subduction extended beyond the flat-lying slab farther east and north by up to 1000 kilometers. The limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten. The results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized.

  18. Are diamond-bearing Cretaceous kimberlites related to shallow-angle subduction beneath western North America?

    Science.gov (United States)

    Currie, C. A.; Beaumont, C.

    2009-05-01

    The origin of deep-seated magmatism (in particular, kimberlites and lamproites) within continental plate interiors remains enigmatic in the context of plate tectonic theory. One hypothesis proposes a relationship between kimberlite occurrence and lithospheric subduction, such that a subducting plate releases fluids below a continental craton, triggering melting of the deep lithosphere and magmatism (Sharp, 1974; McCandless, 1999). This study provides a quantitative evaluation of this hypothesis, focusing on the Late Cretaceous- Eocene (105-50 Ma) kimberlites and lamproites of western North America. These magmas were emplaced along a corridor of Archean and Proterozoic lithosphere, 1000-1500 km inboard of the plate margin separating the subducting Farallon Plate and continental North America Plate. Kimberlite-lamproite magmatism coincides with tectonic events, including the Laramide orogeny, shut-down of the Sierra Nevada arc, and eastward migration of volcanism, that are commonly attributed to a change in Farallon Plate geometry to a shallow-angle trajectory (1999; Sharp, W.E., Earth Planet. Sci. Lett., v.21, pp.351-354, 1974.

  19. Modeling of slow slip events and their interaction with large earthquakes along the subduction interfaces beneath Guerrero and Oaxaca, Mexico

    Science.gov (United States)

    Shibazaki, B.; Cotton, F.; Matsuzawa, T.

    2013-12-01

    Recent high-resolution geodetic observations have revealed the occurrence of slow slip events (SSEs) along the Mexican subduction zone. In the Guerrero gap, large SSEs of around Mw 7.5 repeat every 3-4 years (Lowry et al., 2001; Kostoglodov et al., 2003; Radiguet et al., 2012). The 2006 Guerrero slow slip was analyzed in detail (Radiguet et al., 2011): the average velocity of propagation was 0.8 km/day, and the maximum slip velocity was 1.0E-8 m/s. On the other hand, in the Oaxaca region, SSEs of Mw 7.0-7.3 repeat every 1-2 years and last for 3 months (Brudzinski et al., 2007; Correa-Mora et al., 2008). These SSEs in the Mexican subduction zone are categorized as long-term (long-duration) SSEs; however, their recurrence interval is relatively short. It is important to investigate how SSEs in Mexico can be reproduced using a theoretical model and determine the difference in friction law parameters when compared to SSEs in other subduction zones. An Mw 7.4 subduction earthquake occurred beneath the Oaxaca-Guerrero border on March 20, 2012. The 2012 SSE coincided with this thrust earthquake (Graham et al., 2012). SSEs in Mexico can trigger large earthquakes because their magnitudes are close to that of earthquakes. The interaction between SSEs and large earthquakes is an important problem, which needs to be investigated. We model SSEs and large earthquakes along the subduction interfaces beneath Guerrero and Oaxaca. To reproduce SSEs, we use a rate- and state-dependent friction law with a small cut-off velocity for the evolution effect based on the model proposed by Shibazaki and Shimamoto (2007). We also consider the 3D plate interface, which dips at a very shallow angle at a horizontal distance of 50-150 km from the trench. We set the unstable zone from a depth of 10 to 20 km. By referring to analytical results, we set a Guerrero SSE zone, which extends to the shallow Guerrero gap. Because the maximum slip velocity is around 1.0E-8 m/s, we set the cut-off velocity

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

    Science.gov (United States)

    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. Imaging subducted high velocity slabs beneath the sea of Okhotsk using depth phases

    Science.gov (United States)

    Bai, K.; Li, D.; Helmberger, D. V.; Sun, D.; Wei, S.

    2014-12-01

    A recent study of a shallow Kuril subduction zone event displays significant waveform multi-pathing for paths propagating down the slab towards Europe(Zhan,Zhongwen 2014). Relatively fast structures (5%) are invoked to simulate such observations requiring numerical methods to capture such proportional distortions. Here, we present results from the reverse direction that is the effects on depth phases of deep events propagating up the slab. In particular the Mw6.7 Sea of Okhotsk deep earthquake occurred at a depth of 640 km is believed to be near the bottom of the slab structure and produced an abundance of depth phases. Differential travel time sP-P analysis shows a systematic decrease of up to 5 seconds from Europe to Australia and then to Pacific which is indicative of a dipping high velocity layer above the source region. Multiple simulations using WKM(An upgraded variation of the traditional WKBJ method) and finite difference methods were conducted in an effort to assess the effects of sharp structure on the whole wave-field. Results obtained from analytical methods, by the WKM code become questionable compared against the finite difference method due to its inability to handle the diffraction phases which become crucial in complex structures. In this example, seismicity clustered within a 45 degree dipping benioff zone at shallow depth but became blurred beyond 400 km. Finite difference simulations showed that a slab shapped structure that follows the benioff zone at shallow depth and steepens beyond 400 km produces a model that can account for the sP-P differential travel times of our 5s for oceanic paths.

  2. Mantle enrichment by volatiles as the Nazca plate subducts beneath the Payenia backarc of the Southern Volcanic Zone, Argentina

    DEFF Research Database (Denmark)

    Brandt, Frederik Ejvang

    , minerals, fluid and melt inclusions from the Payenia backarc province of the Andean Southern Volcanic Zone. Major emphasis has been on olivine hosted melt inclusions. The study gives evidence for the role of fluids in the metasomatism of the backarc mantle, and outlines the trend of the variation...... of the metasomatism in Payenia, which is also characterized by a variation in oxidation state and other geochemical parameters of the melt inclusions, and is moreover related to mantle lithological variations. The mantle metasomatism by melts of subducted crust and fluid-borne enrichment is quantitatively modelled......, the origin of Chlorine is explained via slab-derived fluids, and the contrast between backarc and frontal arc magmas is discussed. These results add to the understanding of the origin of the complexities in the mantle wedge under arc-backarc in a subduction zone which has transition to flat slab conditions...

  3. Formation of mantle "lone plumes" in the global downwelling zone - A multiscale modelling of subduction-controlled plume generation beneath the South China Sea

    Science.gov (United States)

    Zhang, Nan; Li, Zheng-Xiang

    2018-01-01

    It has been established that almost all known mantle plumes since the Mesozoic formed above the two lower mantle large low shear velocity provinces (LLSVPs). The Hainan plume is one of the rare exceptions in that instead of rising above the LLSVPs, it is located within the broad global mantle downwelling zone, therefore classified as a "lone plume". Here, we use the Hainan plume example to investigate the feasibility of such lone plumes being generated by subducting slabs in the mantle downwelling zone using 3D geodynamic modelling. Our geodynamic model has a high-resolution regional domain embedded in a relatively low resolution global domain, which is set up in an adaptive-mesh-refined, 3D mantle convection code ASPECT (Advanced Solver for Problems in Earth's ConvecTion). We use a recently published plate motion model to define the top mechanical boundary condition. Our modelling results suggest that cold slabs under the present-day Eurasia, formed from the Mesozoic subduction and closure of the Tethys oceans, have prevented deep mantle hot materials from moving to the South China Sea from regions north or west of the South China Sea. From the east side, the Western Pacific subduction systems started to promote the formation of a lower-mantle thermal-chemical pile in the vicinity of the future South China Sea region since 70 Ma ago. As the top of this lower-mantle thermal-chemical pile rises, it first moved to the west, and finally rested beneath the South China Sea. The presence of a thermochemical layer (possible the D″ layer) in the model helps stabilizing the plume root. Our modelling is the first implementation of multi-scale mesh in the regional model. It has been proved to be an effective way of modelling regional dynamics within a global plate motion and mantle dynamics background.

  4. Crustal structure and configuration of the subducting Philippine Sea plate beneath the Pacific coast industrial zone in Japan inferred from receiver function analysis

    Science.gov (United States)

    Igarashi, T.; Iidaka, T.; Sakai, S.; Hirata, N.

    2012-12-01

    We apply receiver function (RF) analyses to estimate the crustal structure and configuration of the subducting Philippine Sea (PHS) plate beneath the Pacific coast industrial zone stretching from Tokyo to Fukuoka in Japan. Destructive earthquakes often occurred at the plate interface of the PHS plate, and seismic activities increase after the 2011 Tohoku earthquake (Mw9.0) around the Tokyo metropolitan area. Investigation on the crustal structure is the key to understanding the stress concentration and strain accumulation process, and information on configuration of the subducting plate is important to mitigate future earthquake disasters. In this study, we searched for the best-correlated velocity structure model between an observed receiver function at each station and synthetic ones by using a grid search method. Synthetic RFs were calculated from many assumed one-dimensional velocity structures that consist of four layers with positive velocity steps. Observed receiver functions were stacked without considering back azimuth or epicentral distance. We further constructed the vertical cross-sections of depth-converted RF images transformed the lapse time of time series to depth by using the estimated structure models. Telemetric seismographic network data covered on the Japanese Islands including the Metropolitan Seismic Observation network, which constructed under the Special Project for Earthquake Disaster Mitigation in the Tokyo Metropolitan area and maintained by Special Project for Reducing Vulnerability for Urban Mega Earthquake Disasters, are used. We selected events with magnitudes greater or equal to 5.0 and epicentral distance between 30 and 90 degrees based on USGS catalogues. As a result, we clarify spatial distributions of the crustal S-wave velocities. Estimated average one-dimensional S-wave velocity structure is approximately equal to the JMA2011 structural model although the velocity from the ground surface to 5 km in depth is slow. In particular

  5. Early Jurassic calc-alkaline magmatism in northeast China: Magmatic response to subduction of the Paleo-Pacific Plate beneath the Eurasian continent

    Science.gov (United States)

    Wang, Feng; Xu, Yi-Gang; Xu, Wen-Liang; Yang, Lei; Wu, Wei; Sun, Chen-Yang

    2017-08-01

    The subduction of the Paleo-Pacific Plate played an important role in the regional evolution of the eastern margin of the Eurasian continent, but the timing and extent of this event remain ambiguous. To address these issues, we examine the geochronology and geochemistry of Early Jurassic intrusive rocks in eastern Jilin Province, NE China. The Early Jurassic gabbro-diorites, diorites, granodiorites, and monzogranites are found to have been emplaced at 183-185 Ma and are characterized by enrichment in large ion lithophile elements and depletion in high field strength elements, similar to calc-alkaline arc-type igneous rocks. The Early Jurassic gabbroic and dioritic rocks have εHf(t) values of +2.1 to +10.1 and Hf single-stage (TDM1) model ages of 430-774 Ma, whereas the monzogranites have εHf(t) values of +6.7 to +8.9 and Hf single-stage (TDM1) ages of 597-718 Ma. The gabbro-diorites, diorites, and granodiorites described in this study are genetically linked and they represent the products of the fractional crystallization of a common mafic magma that was in turn derived from the partial melting of a mantle source that was metasomatized by subduction-related fluids. In contrast, the Early Jurassic monzogranites were generated by partial melting of a depleted lower crustal block that was probably accreted during the Neoproterozoic. More importantly, the Early Jurassic calc-alkaline igneous rocks in the east part of NE China form a NE-trending belt that is oriented perpendicular to the direction of Paleo-Pacific Plate movement at that time. West of this belt, contemporaneous bimodal igneous rocks occur in the Lesser Xing'an-Zhangguangcai Ranges. This magmatic configuration is best explained by continental arc magmatism along the continental margin and extensional magmatism in a back-arc setting, in each case triggered by the initial subduction of the Paleo-Pacific Plate beneath Eurasia in the Early Jurassic.

  6. Subduction Controls of Hf and Nd Isotopes in Lavas of the Aleutian Island Arc

    Energy Technology Data Exchange (ETDEWEB)

    Yogodzinski, Gene; Vervoort, Jeffery; Brown, Shaun Tyler; Gerseny, Megan

    2010-08-29

    trench east of the Amlia Fracture Zone, which is being subducting beneath the arc at Seguam Island. Mixing trends between mantle wedge and sediment end members become flatter in Hf-Nd isotope space at locations further west along the arc, indicating that the sediment end member in the west has either higher Nd/Hf or is more radiogenic in Hf compared to Nd. This pattern is interpreted to reflect an increase in pelagic clay relative to the terrigenous subducted sedimentary component westward along the arc. Results of this study imply that Hf does not behave as a conservative element in the Aleutian subduction system, as has been proposed for some other arcs.

  7. Comments on “Arc magmatism and subduction history beneath the Zagros Mountains, Iran: A new report of adakites and geodynamic consequences” by J. Omrani, P. Agard, H. Whitechurch, M. Bennoit, G. Prouteau, L. Jolivet

    Science.gov (United States)

    Aftabi, Alijan; Atapour, Habibeh

    2009-12-01

    Based on the imprecise geochemical data for 62 samples from Qom, Anar and Baft regions in central Iranian magmatic arc Omrani et al. (Omrani, J., Agard, P., Witechurch, H., Benoit, M., Prouteau, G., Jolivet, L., 2008. Arc magmatism and subduction history beneath the Zagsros Mountains, Iran: A new report of adakites and geodynamic consequences. Lithos 106, 380-398.), suggested that all studied magmatic rocks display the geochemical affinity of subduction-related calc-alkalic rock suites. Here, we demonstrate that the incorrect altered and variable geochemical data (e.g., Al 2O 3, Sr, Y, Ni, Cr, SiO 2, Na 2O, La/Yb and Th/Ce), show that most of the samples actually display calc-alkaline, shoshonitic and calc-alkalic-adakitic affinities. Furthermore, as a result of alteration, rock samples of similar age (e.g., Qom) indicate both adakitic and non-adakitic compositional signatures, which is misleading. On the basis of more than 400 previously published geochemical analyses, we suggest that, after eliminating the false geochemical signatures, the calc-alkaline and adakitic affinities of the central Iranian magmatic arc are due to flat subduction and might be related to a second phase of Miocene- Pliocene porphyry copper mineralization, which is a considerable exploration target and thus merits further investigation.

  8. Horizontal subduction zones, convergence velocity and the building of the Andes

    CERN Document Server

    Martinod, Joseph; Roperch, Pierrick; Guillaume, Benjamin; Espurt, Nicolas; 10.1016/j.epsl.2010.09.010

    2010-01-01

    We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and...

  9. New evidence about the subduction of the Copiapó ridge beneath South America, and its connection with the Chilean-Pampean flat slab, tracked by satellite GOCE and EGM2008 models

    Science.gov (United States)

    Álvarez, Orlando; Gimenez, Mario; Folguera, Andres; Spagnotto, Silvana; Bustos, Emilce; Baez, Walter; Braitenberg, Carla

    2015-11-01

    Satellite-only gravity measurements and those integrated with terrestrial observations provide global gravity field models of unprecedented precision and spatial resolution, allowing the analysis of the lithospheric structure. We used the model EGM2008 (Earth Gravitational Model) to calculate the gravity anomaly and the vertical gravity gradient in the South Central Andes region, correcting these quantities by the topographic effect. Both quantities show a spatial relationship between the projected subduction of the Copiapó aseismic ridge (located at about 27°-30° S), its potential deformational effects in the overriding plate, and the Ojos del Salado-San Buenaventura volcanic lineament. This volcanic lineament constitutes a projection of the volcanic arc toward the retroarc zone, whose origin and development were not clearly understood. The analysis of the gravity anomalies, at the extrapolated zone of the Copiapó ridge beneath the continent, shows a change in the general NNE-trend of the Andean structures to an ENE-direction coincident with the area of the Ojos del Salado-San Buenaventura volcanic lineament. This anomalous pattern over the upper plate is interpreted to be linked with the subduction of the Copiapó ridge. We explore the relation between deformational effects and volcanism at the northern Chilean-Pampean flat slab and the collision of the Copiapó ridge, on the basis of the Moho geometry and elastic thicknesses calculated from the new satellite GOCE data. Neotectonic deformations interpreted in previous works associated with volcanic eruptions along the Ojos del Salado-San Buenaventura volcanic lineament is interpreted as caused by crustal doming, imprinted by the subduction of the Copiapó ridge, evidenced by crustal thickening at the sites of ridge inception along the trench. Finally, we propose that the Copiapó ridge could have controlled the northern edge of the Chilean-Pampean flat slab, due to higher buoyancy, similarly to the control

  10. An Alpine-style Ordovician collision complex in the Sierra de Pie de Palo, Argentina: Record of subduction of Cuyania beneath the Famatina arc

    Science.gov (United States)

    van Staal, C. R.; Vujovich, G. I.; Currie, K. L.; Naipauer, M.

    2011-03-01

    The Caucete Group and structurally overlying Pie de Palo Complex in western Argentina are characterised by two generations of west-verging folds and thrust-related shear zones, which formed under amphibolite facies conditions. The Caucete Group is separated from the Pie de Palo Complex by the Pirquitas thrust. These structures are interpreted to have formed as a result of a progressive deformation, generated during Middle Ordovician, underthrusting of the Laurentian-derived Cuyania microcontinent beneath the active Famatina margin. Geometrical relationships are most simply explained if the Pie de Palo Complex was basement to the Caucete Group prior to Ordovician orogenesis. We propose that this basement-cover relationship was established during Cambrian rifting of the Cuyania microcontinent from Laurentia. The Pirquitas fault may have been initiated during this extension prior to its long-lived remobilization as a thrust. We cannot rule out the possibility that the Pie de Palo Complex was exotic with respect to the Caucete Group, but for this to be possible we have to introduce an extra generation of structures, for which no evidence is preserved. The deformation was characterised by early strain localization followed by a more homogeneously distributed non-coaxial flow during F 2. Thermal softening probably dominated over fabric softening during this stage.

  11. Complex Subduction Imaged by Diffractional Tomography of USArray Receiver Functions

    Science.gov (United States)

    Zhou, Y.

    2016-12-01

    Subduction of a large oceanic plate beneath a continental plate is a complex process. In the Western United States, fragmentation of the Farallon slab has been reported in recent tomographic models. In this study, we measure finite-frequency travel times of P410s and P660s receiver functions recorded at USArray Transportable Array (TA) stations for teleseismic events occurred between 2015 and 2011. We calculate the finite-frequency sensitivities of receiver functions to depth perturbations of the 410-km and 660-km discontinuities to obtain high resolution mantle transition zone models based on diffractional tomography. The high-resolution discontinuity models reveal several interesting anomalies associated with complex subduction of the Farallon plate. In particular, we observe a linear feature in both the 410-km and 660-km discontinuity models. This mantle transition zone anomaly is roughly located in the western Snake River Plain and aligns with a major slab gap imaged in an earlier finite-frequency S-wave velocity model. We show that non-stationary upwellings generated by eastward propagation of a slab tearing event, together with a westward motion of the North American plate at a rate of about 1 to 1.5 centimeters per year (comparable to the half spreading rate of the Mid-Atlantic Ridge) in the past 16 million years can explain the age-progressive Snake River Plain / Yellowstone volcanic track. The slab to the west of the anomaly shows a near vertical subduction, it is heavily fragmented and the 410-km and 660-km discontinuity topography indicates that the southern fragment north of the Mendocino triple junction has subducted down to the mantle transition zone.

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

    Science.gov (United States)

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

    2015-12-01

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

  13. Effect of Aseismic Ridge Subduction on Volcanism in the NE Lesser Antilles Arc

    Science.gov (United States)

    Stinton, A. J.; Hatfield, R. G.; McCanta, M. C.

    2014-12-01

    The interaction of aseismic ridges or buoyant crust in subduction zones can affect the volcanism occurring on the overriding plate. Here we describe the affect of the subduction of an aseismic ridge on volcanism in the NE Lesser Antilles. The Lesser Antilles island arc is a result of westward subduction of the North American plate beneath the Caribbean plate and stretches 800 km from Saba in the north to Grenada in the south. From Guadeloupe northwards, the arc bifurcates into an eastern, inactive arc (known as the Limestone Caribees) and a western, active arc (Volcanic Caribees). It has been suggested that this bifurcation is the result of the subduction of buoyant crust in the form of at least two aseismic ridges, in the North American plate. In 2012, IODP Expedition 340 recovered 130 m of core from site U1396, located 55 km SW of Montserrat, Lesser Antilles. The core contains a detailed record of volcanism, in the form of more than 180 tephra layers, that stretches back nearly 4.5 Ma. This is the longest and most complete record of volcanism for the NE Lesser Antilles and provides insight into the evolution and development of the island arc in this region. A variety of techniques are being applied to the tephra layers from U1396 to determine their age, chemistry, components and origin. Here we present preliminary results from paeleomagnetic age determinations for each tephra layer to show how the subduction of aseismic ridges on the North American plate has affected the rate of volcanism and development of the island arc in the NE Lesser Antilles over the last 4.5 Ma.

  14. Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela

    Science.gov (United States)

    Masy, Jeniffer; Niu, Fenglin; Levander, Alan; Schmitz, Michael

    2015-01-01

    We have combined surface wave tomography with Ps and Sp receiver-function images based on common-conversion-point (CCP) stacking to study the upper mantle velocity structure, particularly the lithosphere-asthenosphere boundary (LAB), beneath eastern and central Venezuela. Rayleigh phase velocities in the frequency range of 0.01-0.05 Hz (20-100 s in period) were measured using the two-plane-wave method and finite-frequency kernels, and then inverted on a 0.5° × 0.5° grid. The phase velocity dispersion data at grid points were inverted for 1D shear velocity profiles using initial crust-mantle velocity models constructed from previous studies. The 3D velocity model and receiver-function images were interpreted jointly to determine the depth of the LAB and other upper mantle features. The tomographic images revealed two high velocity anomalies extending to more than ∼200 km depth. One corresponds to the top of the subducting Atlantic plate beneath the Serrania del Interior. The other anomaly is a highly localized feature beneath the Maturin Basin. The LAB depth varies significantly in the study region: It is located at ∼110 km depth beneath the Guayana Shield, and reaches ∼130 km at the northern edge of the Maturin Basin, which might be related to the downward flexural bending due to thrust loading of the Caribbean plate and pull from the subducting Atlantic plate. Immediately to the west, the lithosphere is thin (∼50-60 km) along the NE-SW trending Espino Graben from the Cariaco basin to the Orinoco River at the northern edge of the craton. The LAB in this region is the top of a pronounced low velocity zone. Westward, the lithosphere deepens to ∼80 km depth beneath the Barinas Apure Basin, and to ∼90 km beneath the Neogene Merida Andes and Maracaibo block. Both upper mantle velocity structure and lithosphere thickness correlate well with surface geology and are consistent with northern South American tectonics.

  15. Major dust storms and westward traveling waves on Mars

    Science.gov (United States)

    Wang, Huiqun

    2017-04-01

    Westward traveling waves are observed during major dust storm periods in northern fall and winter. The close correlation in timing makes westward traveling wave one of the signature responses of the Martian atmosphere to major dust storms. Westward traveling waves are dominated by zonal wave number m = 1 in the middle atmosphere and are typically characterized by long wave period. They are associated with significant temperature perturbations near the edge of the north polar vortex. Their wind signals extend to the low latitudes and the southern hemisphere. Their eddy momentum and heat fluxes exhibit complex patterns on a global scale in the middle atmosphere.

  16. Mantle constraints on the plate tectonic evolution of the Tonga-Kermadec-Hikurangi subduction zone and the South Fiji Basin region

    NARCIS (Netherlands)

    Schellart, W. P.; Spakman, W.

    The Tonga-Kermadec-Hikurangi subduction zone is a major plate boundary in the Southwest Pacific region, where the Pacific plate subducts westward underneath the Australian plate. Considerable controversy exists regarding the Cenozoic evolution of this subduction zone, its connection with the

  17. Mantle constraints on the plate tectonic evolution of the Tonga-Kermadec-Hikurangi subduction zone and the South Fiji Basin region

    NARCIS (Netherlands)

    Schellart, W.P.; Spakman, W.

    2012-01-01

    The Tonga–Kermadec–Hikurangi subduction zone is a major plate boundary in the Southwest Pacific region, where the Pacific plate subducts westward underneath the Australian plate. Considerable controversy exists regarding the Cenozoic evolution of this subduction zone, its connection with

  18. Subduction dynamics: Constraints from gravity field observations

    Science.gov (United States)

    Mcadoo, D. C.

    1985-01-01

    Satellite systems do the best job of resolving the long wavelength components of the Earth's gravity field. Over the oceans, satellite-borne radar altimeters such as SEASAT provide the best resolution observations of the intermediate wavelength components. Satellite observations of gravity contributed to the understanding of the dynamics of subduction. Large, long wavelength geoidal highs generally occur over subduction zones. These highs are attributed to the superposition of two effects of subduction: (1) the positive mass anomalies of subducting slabs themselves; and (2) the surface deformations such as the trenches convectively inducted by these slabs as they sink into the mantle. Models of this subduction process suggest that the mantle behaves as a nonNewtonian fluid, its effective viscosity increases significantly with depth, and that large positive mass anomalies may occur beneath the seismically defined Benioff zones.

  19. GPS Monitoring of Subduction Zone Deformation in Costa Rica

    Science.gov (United States)

    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.

  20. The nature of subslab slow velocity anomalies beneath South America

    Science.gov (United States)

    Portner, Daniel Evan; Beck, Susan; Zandt, George; Scire, Alissa

    2017-05-01

    Slow seismic velocity anomalies are commonly imaged beneath subducting slabs in tomographic studies, yet a unifying explanation for their distribution has not been agreed upon. In South America two such anomalies have been imaged associated with subduction of the Nazca Ridge in Peru and the Juan Fernández Ridge in Chile. Here we present new seismic images of the subslab slow velocity anomaly beneath Chile, which give a unique view of the nature of such anomalies. Slow seismic velocities within a large hole in the subducted Nazca slab connect with a subslab slow anomaly that appears correlated with the extent of the subducted Juan Fernández Ridge. The hole in the slab may allow the subslab material to rise into the mantle wedge, revealing the positive buoyancy of the slow material. We propose a new model for subslab slow velocity anomalies beneath the Nazca slab related to the entrainment of hot spot material.

  1. Transient slab flattening beneath Colombia

    Science.gov (United States)

    Wagner, L. S.; Jaramillo, J. S.; Ramírez-Hoyos, L. F.; Monsalve, G.; Cardona, A.; Becker, T. W.

    2017-07-01

    Subduction of the Nazca and Caribbean Plates beneath northwestern Colombia is seen in two distinct Wadati Benioff Zones, one associated with a flat slab to the north and one associated with normal subduction south of 5.5°N. The normal subduction region is characterized by an active arc, whereas the flat slab region has no known Holocene volcanism. We analyze volcanic patterns over the past 14 Ma to show that in the mid-Miocene a continuous arc extended up to 7°N, indicating normal subduction of the Nazca Plate all along Colombia's Pacific margin. However, by 6 Ma, we find a complete cessation of this arc north of 3°N, indicating the presence of a far more laterally extensive flat slab than at present. Volcanism did not resume between 3°N and 6°N until after 4 Ma, consistent with lateral tearing and resteepening of the southern portion of the Colombian flat slab at that time.

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

    Science.gov (United States)

    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

  3. Geophysical evidence for a transform margin offshore Western Algeria: a witness of a subduction-transform edge propagator?

    Science.gov (United States)

    Badji, Rabia; Charvis, Philippe; Bracene, Rabah; Galve, Audrey; Badsi, Madjid; Ribodetti, Alessandra; Benaissa, Zahia; Klingelhoefer, Frauke; Medaouri, Mourad; Beslier, Marie-Odile

    2015-02-01

    For the first time, a deep seismic data set acquired in the frame of the Algerian-French SPIRAL program provides new insights regarding the origin of the westernmost Algerian margin and basin. We performed a tomographic inversion of traveltimes along a 100-km-long wide-angle seismic profile shot over 40 ocean bottom seismometers offshore Mostaganem (Northwestern Algeria). The resulting velocity model and multichannel seismic reflection profiles show a thin (3-4 km thick) oceanic crust. The narrow ocean-continent transition (less than 10 km wide) is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with Miocene to Quaternary sediments. This fault system, as well as the faults organized in a negative-flower structure on the continent side, marks a major strike-slip fault system. The extremely sharp variation of the Moho depth (up to 45 ± 3°) beneath the continental border underscores the absence of continental extension in this area. All these features support the hypothesis that this part of the margin from Oran to Tenes, trending N65-N70°E, is a fossil subduction-transform edge propagator fault, vestige of the propagation of the edge of the Gibraltar subduction zone during the westward migration of the Alborán domain.

  4. IMF BY dependence of the extent of substorm westward electrojet

    Indian Academy of Sciences (India)

    BY-component of the IMF influences the extent of the substorm westward electrojet. This finding implies that the IMF may have a role in controlling the longitudinal extent of substorm occurrence. 1. Introduction. The substorm phenomenon was ..... of the GEOTAIL Comprehensive Plasma Instru- ment (CPI) experiment, Prof.

  5. IMF BY dependence of the extent of substorm westward electrojet

    Indian Academy of Sciences (India)

    Based on an initial study of the magnetograms, an arbitrary local time of 2030 MLT is fixed to define the early manifestation of the substorm westward electrojet. Using this criterion 12 substorms are identified and the possible causes examined. Many of these events are observed to be associated with a moderate to intense ...

  6. Strike up Student Interest through Song: Technology and Westward Expansion

    Science.gov (United States)

    Steele, Meg

    2014-01-01

    Sheet music, song lyrics, and audio recordings may not be the first primary sources that come to mind when considering ways to teach about changes brought about by technology during westward expansion, but these sources engage students in thought provoking ways. In this article the author presents a 1917 photograph of Mountain Chief, of the Piegan…

  7. Reaction-induced rheological weakening enables oceanic plate subduction

    OpenAIRE

    Hirauchi, Ken-ichi; Fukushima, Kumi; Kido, Masanori; Muto, Jun; Okamoto, Atsushi

    2016-01-01

    Earth is the only terrestrial planet in our solar system where an oceanic plate subducts beneath an overriding plate. Although the initiation of plate subduction requires extremely weak boundaries between strong plates, the way in which oceanic mantle rheologically weakens remains unknown. Here we show that shear-enhanced hydration reactions contribute to the generation and maintenance of weak mantle shear zones at mid-lithospheric depths. High-pressure friction experiments on peridotite goug...

  8. Why does substorm-associated auroral surge travel westward?

    Science.gov (United States)

    Ebihara, Y.; Tanaka, T.

    2018-01-01

    A substorm is a long-standing unsolved issue in solar-terrestrial physics. One of the big challenges is to explain reasonably the evolution of the morphological structure of the aurora associated with the substorm. The sudden appearance of a bright aurora and an auroral surge traveling westward (westward traveling surge, WTS) are noticeable features of the aurora during the substorm expansion phase. By using a global magnetohydrodynamics (MHD) simulation, we obtained the following results regarding the WTS. When the interplanetary magnetic field turns southward, a persistent dynamo appears in the cusp/mantle region, driving the two-cell magnetospheric convection. Then, the substorm growth phase begins. When magnetic reconnection takes place in the magnetotail, plasma is accelerated earthward in the plasma sheet, and accelerated toward the equatorial plane in the lobe. The second dynamo appears in the near-Earth region, which is closely associated with the generation of the field-aligned current (FAC) on the nightside. When the FAC reaches the ionosphere, the aurora becomes bright, and the onset of the expansion phase begins. In the ionosphere, the conductivity is intensified in the bright aurora due to the precipitation of accelerated electrons. The conductivity gradient gives rise to the overflow of the Hall current, which acts as the third dynamo. The overflow results in the accumulation of space charge, which causes a divergent electric field. The divergent electric field generates a thin, structured upward FAC adjacent to the bright aurora. The opposite process takes place on the opposite side of the bright aurora. In short, the upward FAC increases (appearance of aurora) at the leading edge of the surge, and decreases (disappearance of aurora) at the trailing edge of the surge. By repeating these processes, the surge seems to travel westward.

  9. Thermal impact of magmatism in subduction zones

    Science.gov (United States)

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

    2018-01-01

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

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

    Science.gov (United States)

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

    2015-01-01

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

  11. Detecting slab structure beneath the Mediterranean

    Science.gov (United States)

    Miller, Meghan S.; Sun, Daoyuan; Piana Agostinetti, Nicola

    2013-04-01

    The presence of subducted slabs in the Mediterranean has been well documented with seismic tomography, however, these images, which are produced by smoothed, damped inversions, underestimate the sharpness of the structures. The position and extent of the slabs and the presence possible tears or gaps in the subducted lithosphere are still debated, yet the shape and location these structures are important for kinematic reconstructions and evolution of the entire subduction zone system. Extensive distribution of broadband seismic instrumentation in the Mediterranean (Italian National Seismic Network in Italy and the NSF-PICASSO project in Spain and Morocco) has allowed us to use alternative methodologies to detect the position of the slabs and slab tears beneath the Central and Western Mediterranean. Using S receiver functions we are able to identify S-to-p conversions from the bottom of the subducted slab and a lack of these signals where there are gaps or tears in the slab. We also analyze broadband waveforms for changes in P wave coda from deep (> 300 km depth) local earthquakes. The waveform records for stations in southern Italy and around the Betic-Rif show large amplitude, high frequency (f > 5 Hz) late arrivals with long coda after relatively low-frequency onset. High frequency arrivals are the strongest from events whose raypaths travel within the slab to the stations where they are recorded allowing for mapping of where the subducted material is located within the upper mantle. These two methods, along with inferring the slab position from fast P-wave velocity perturbations in tomography and intermediate depth seismicity, provide additional geophysical evidence to aid in interpretation of the complex, segmented slab structure beneath the Mediterranean.

  12. Introduction to the structures and processes of subduction zones

    Science.gov (United States)

    Zheng, Yong-Fei; Zhao, Zi-Fu

    2017-09-01

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

  13. Surface deformation resulting from subduction and slab detachment

    NARCIS (Netherlands)

    Buiter, S.J.H.

    2000-01-01

    Convergence of lithospheric plates is accommodated at active margins by one plate moving beneath the other into the Earth's mantle. Changes in this subduction process may cause variations in the topography of the Earth's surface near a convergent plate margin. The focus of this thesis lies on

  14. Continental crust beneath southeast Iceland

    Science.gov (United States)

    Torsvik, Trond H.; Amundsen, Hans E. F.; Trønnes, Reidar G.; Doubrovine, Pavel V.; Gaina, Carmen; Kusznir, Nick J.; Steinberger, Bernhard; Corfu, Fernando; Ashwal, Lewis D.; Griffin, William L.; Werner, Stephanie C.; Jamtveit, Bjørn

    2015-01-01

    The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from the nearby continental margins. A restricted area in southeast Iceland—and especially the Öræfajökull volcano—is characterized by a unique enriched-mantle component (EM2-like) with elevated 87Sr/86Sr and 207Pb/204Pb. Here, we demonstrate through modeling of Sr–Nd–Pb abundances and isotope ratios that the primitive Öræfajökull melts could have assimilated 2–6% of underlying continental crust before differentiating to more evolved melts. From inversion of gravity anomaly data (crustal thickness), analysis of regional magnetic data, and plate reconstructions, we propose that continental crust beneath southeast Iceland is part of ∼350-km-long and 70-km-wide extension of the Jan Mayen Microcontinent (JMM). The extended JMM was marginal to East Greenland but detached in the Early Eocene (between 52 and 47 Mya); by the Oligocene (27 Mya), all parts of the JMM permanently became part of the Eurasian plate following a westward ridge jump in the direction of the Iceland plume. PMID:25825769

  15. Control of high oceanic features and subduction channel on earthquake ruptures along the Chile-Peru subduction zone

    Science.gov (United States)

    Contreras-Reyes, Eduardo; Carrizo, Daniel

    2011-05-01

    We discuss the earthquake rupture behavior along the Chile-Peru subduction zone in terms of the buoyancy of the subducting high oceanic features (HOF's), and the effect of the interplay between HOF and subduction channel thickness on the degree of interplate coupling. We show a strong relation between subduction of HOF's and earthquake rupture segments along the Chile-Peru margin, elucidating how these subducting features play a key role in seismic segmentation. Within this context, the extra increase of normal stress at the subduction interface is strongly controlled by the buoyancy of HOF's which is likely caused by crustal thickening and mantle serpentinization beneath hotspot ridges and fracture zones, respectively. Buoyancy of HOF's provide an increase in normal stress estimated to be as high as 10-50 MPa. This significant increase of normal stress will enhance seismic coupling across the subduction interface and hence will affect the seismicity. In particular, several large earthquakes (Mw ≥ 7.5) have occurred in regions characterized by subduction of HOF's including fracture zones (e.g., Nazca, Challenger and Mocha), hotspot ridges (e.g., Nazca, Iquique, and Juan Fernández) and the active Nazca-Antarctic spreading center. For instance, the giant 1960 earthquake (Mw = 9.5) is coincident with the linear projections of the Mocha Fracture Zone and the buoyant Chile Rise, while the active seismic gap of north Chile spatially correlates with the subduction of the Iquique Ridge. Further comparison of rupture characteristics of large underthrusting earthquakes and the locations of subducting features provide evidence that HOF's control earthquake rupture acting as both asperities and barriers. This dual behavior can be partially controlled by the subduction channel thickness. A thick subduction channel smooths the degree of coupling caused by the subducted HOF which allows lateral earthquake rupture propagation. This may explain why the 1960 rupture propagates

  16. Subducting Plate Breakup by Plume-Lithosphere Interaction

    Science.gov (United States)

    Koptev, A.; Gerya, T.; Jolivet, L.; Leroy, S. D.

    2016-12-01

    We use a 3D high-resolution thermo-mechanical modeling to investigate the impact of active mantle plume on a subducting lithospheric plate. Initial model setup consists of an overriding continental lithosphere and subducting lithospheric plate including oceanic and continental lithosphere. A mantle plume thermal anomaly has been initially seeded at the bottom of the model box underneath the continental segment of subducting plate. Mantle plume impingement on lithospheric bottom leads to thinning of continental lithosphere and decompressional melting of both lithospheric and sublithospheric mantle along stretched trench-parallel zone. Further continental breakup is followed by opening of an oceanic basin separating a newly formed microcontinent from the main subducting continent. Despite continuous push applied at the boundary of subducting plate, plume-induced oceanic basin opens during several Myrs reaching several hundred kilometers wide. Cooling of the mantle plume and beginning of collision between the separated microcontinent and the overriding continental plate lead to gradual closure of newly formed oceanic basin that gets further involved into subduction and collision. The final stage sees continental subduction of main body of subducting plate and simultaneous tectonic exhumation of the upper crust of the subducted microcontinent. This scenario involving a plume-induced rifting of a microcontinent away from main body of subducted plate can be compared to the Mesozoic-Cenozoic development of the African plate characterized by the consecutive separation of the Apulian microcontinent and Arabian plate (in the Jurassic and the Neogene, respectively) during subduction of Neo-Tethys oceanic lithosphere beneath the Eurasian margin.

  17. Subduction and slab tearing dynamics constrained by thermal anomalies in the Anatolia-Aegean region

    Science.gov (United States)

    Roche, Vincent; Guillou-Frottier, Laurent; Jolivet, Laurent; Loiselet, Christelle; Bouchot, Vincent

    2015-04-01

    Most previous geodynamic studies treat subduction zones with backward migration (rollback), slab tearing or slab breakoff by numerical or laboratory experiments and by integrating seismicity, tomography data and geochemical studies. Here we investigate these processes in the Aegean-Anatolian domain and particularly the western side of Turkey (western Anatolia) by incorporating thermal regime of the crust, and in particular the geothermal fields as anomalies that could reflect the thermal state of Aegean subduction zone at depth. This domain is characterized by 1) extensional crustal deformation which progressively localized during the Aegean slab retreat from late Eocene to Present, enabling the development of a hot backarc domain; this extension accelerated between 15 and 8 Ma coeval with a fast rotation of the Hellenides and 2) since the latest Miocene, extension is coupled with the development of the North Anatolian Fault that accommodates the westward escape of the Anatolian block. Both the acceleration of extension in the Middle Miocene and the recent escape of Anatolia have been proposed to result from several slab tearing events, the first one being located below western Turkey and the Eastern Aegean Sea, a second one below eastern Turkey and a last one below the Corinth Rift (Faccenna et al., 2006; Jolivet et al., 2013). The distribution of magmatism and mineral resources has been suggested to be largely controlled by these retreat and tearing events (Menant et al., submitted). The development of a widespread active geothermal province in western Anatolia is unlikely to simply result from the Quaternary magmatism whose volcanism part has a too limited extent. Conversely, the long wavelength east-west variation of surface heat flow density could reflect deep thermal processes in the lower crust and/or deeper, and we thus look for possible connections with larger-scale mantle dynamics. We use the distribution of thermal anomalies at different scales and the 3

  18. Plume-induced subduction

    Science.gov (United States)

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

    2016-12-01

    Dominant present-day subduction initiation mechanisms require acting plate forces and/or pre-existing zones of lithospheric weakness, which are themselves consequences of plate tectonics. In contrast, recently discovered plume-induced subduction initiation could have started the first subduction zone without pre-existing plate tectonics. Here, we investigate this new mechanism with high-resolution 3D numerical thermomechanical modeling experiments. We show that typical plume-induced subduction dynamics is subdivided into five different stages: (1) oceanic plateau formation by arrival of a mantle plume head; (2) formation of an incipient trench and a descending nearly-circular slab at the plateau margins; (3) tearing of the circular slab; (4) formation of several self-sustained retreating subduction zones and (5) cooling and spreading of the new lithosphere formed between the retreating subduction zones. At the final stage of plume-induced subduction initiation, a mosaic of independently moving, growing and cooling small oceanic plates heading toward individual retreating subduction zones forms. The plates are separated by spreading centers, triple junctions and transform faults and thus the newly formed multi-slab subduction system operates as an embryonic plate tectonic cell. We demonstrate that three key physical factors combine to trigger self-sustained plume-induced subduction: (1) old negatively buoyant oceanic lithosphere; (2) intense weakening of the lithosphere by plume-derived magmas; and (3) lubrication of the forming subduction interface by hydrated oceanic crust. We furthermore discuss that plume-induced subduction, which is rare at present day conditions, may have been common in the Precambrian time and likely started global plate tectonics on Earth.

  19. Shear wave anisotropy in northwestern South America and its link to the Caribbean and Nazca subduction geodynamics

    National Research Council Canada - National Science Library

    Idárraga‐García, J; Kendall, J.‐M; Vargas, C. A

    2016-01-01

    ...‐related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs...

  20. Seismic Anisotropy due to Crust and Uppermost Mantle Deformation Beneath Southern Peru and Bolivia: Constraints from Receiver Functions

    Science.gov (United States)

    Bar, N.; Long, M. D.; Wagner, L. S.; Beck, S. L.; Tavera, H.

    2016-12-01

    Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during subduction and orogenesis in continental subduction systems remains poorly understood. Observations of seismic anisotropy can provide important constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the complex slab morphology and the along-strike transition from flat to normally dipping subduction. In particular, understanding the structure and deformation of the crust and mantle will yield insight into the relationship between the flat slab and the overriding continental lithosphere. In this study we constrain seismic anisotropy within and above the subducting slab (including the mantle wedge and the overriding plate) beneath southern Peru and Bolivia using transverse component receiver functions. Because anisotropic receiver function analysis can constrain the depth distribution of anisotropy, this analysis is complementary to previous studies of shear wave splitting in this region. We examine data from two dense lines of seismometers from the PULSE and CAUGHT deployments in Peru and Bolivia, each anchored by a long-running permanent station. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. Beneath Peru, our investigation of anisotropic structure along the flat slab will help test the recently suggested hypothesis of a slab tear; beneath Bolivia, we aim to characterize the pattern of flow in the mantle wedge as well as the nature of deformation in the lower crust of the overriding plate.

  1. The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle

    NARCIS (Netherlands)

    Schellart, W. P.

    2007-01-01

    A geodynamic model exists, the westward lithospheric drift model, in which the variety of overriding plate deformation, trench migration and slab dip angles is explained by the polarity of subduction zones. The model predicts overriding plate extension, a fixed trench and a steep slab dip for

  2. Cascadia subduction tremor muted by crustal faults

    Science.gov (United States)

    Wells, Ray; Blakely, Richard J.; Wech, Aaron G.; McCrory, Patricia A.; Michael, Andrew

    2017-01-01

    Deep, episodic slow slip on the Cascadia subduction megathrust of western North America is accompanied by low-frequency tremor in a zone of high fluid pressure between 30 and 40 km depth. Tremor density (tremor epicenters per square kilometer) varies along strike, and lower tremor density statistically correlates with upper plate faults that accommodate northward motion and rotation of forearc blocks. Upper plate earthquakes occur to 35 km depth beneath the faults. We suggest that the faults extend to the overpressured megathrust, where they provide fracture pathways for fluid escape into the upper plate. This locally reduces megathrust fluid pressure and tremor occurrence beneath the faults. Damping of tremor and related slow slip caused by fluid escape could affect fault properties of the megathrust, possibly influencing the behavior of great earthquakes.

  3. Subduction of a buoyant plateau at the Manila Trench: Tomographic evidence and geodynamic implications

    Science.gov (United States)

    Fan, Jianke; Zhao, Dapeng; Dong, Dongdong

    2016-02-01

    We determined P-wave tomographic images by inverting a large number of arrival-time data from 2749 local earthquakes and 1462 teleseismic events, which are used to depict the three-dimensional morphology of the subducted Eurasian Plate along the northern segment of the Manila Trench. Dramatic changes in the dip angle of the subducted Eurasian Plate are revealed from the north to the south, being consistent with the partial subduction of a buoyant plateau beneath the Luzon Arc. Slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction, and the subducted lithosphere may be absent at depths greater than 250 km at ˜19°N and ˜21°N. The subducted buoyant plateau is possibly oriented toward NW-SE, and the subducted plate at ˜21°N is slightly steeper than that at ˜19°N. These results may explain why the western and eastern volcanic chains in the Luzon Arc are separated by ˜50 km at ˜18°N, whereas they converge into a single volcanic chain northward, which may be related to the oblique subduction along the Manila Trench caused by the northwestern movement of the Philippine Sea Plate. A low-velocity zone is revealed at depths of 20-200 km beneath the Manila Accretionary Prism at ˜22°N, suggesting that the subduction along the Manila Trench may stop there and the collision develops northward. The Taiwan Orogeny may originate directly from the subduction of the buoyant plateau, because the initial time of the Taiwan Orogeny is coincident with that of the buoyant plateau subduction.

  4. Slab melting and magma formation beneath the southern Cascade arc

    Science.gov (United States)

    Walowski, Kristina J.; Wallace, Paul J.; Clynne, Michael A.; Rasmussen, D.J.; Weis, D.

    2016-01-01

    The processes that drive magma formation beneath the Cascade arc and other warm-slab subduction zones have been debated because young oceanic crust is predicted to largely dehydrate beneath the forearc during subduction. In addition, geochemical variability along strike in the Cascades has led to contrasting interpretations about the role of volatiles in magma generation. Here, we focus on the Lassen segment of the Cascade arc, where previous work has demonstrated across-arc geochemical variations related to subduction enrichment, and H-isotope data suggest that H2O in basaltic magmas is derived from the final breakdown of chlorite in the mantle portion of the slab. We use naturally glassy, olivine-hosted melt inclusions (MI) from the tephra deposits of eight primitive (MgO>7 wt%) basaltic cinder cones to quantify the pre-eruptive volatile contents of mantle-derived melts in this region. The melt inclusions have B concentrations and isotope ratios that are similar to mid-ocean ridge basalt (MORB), suggesting extensive dehydration of the downgoing plate prior to reaching sub-arc depths and little input of slab-derived B into the mantle wedge. However, correlations of volatile and trace element ratios (H2O/Ce, Cl/Nb, Sr/Nd) in the melt inclusions demonstrate that geochemical variability is the result of variable addition of a hydrous subduction component to the mantle wedge. Furthermore, correlations between subduction component tracers and radiogenic isotope ratios show that the subduction component has less radiogenic Sr and Pb than the Lassen sub-arc mantle, which can be explained by melting of subducted Gorda MORB beneath the arc. Agreement between pMELTS melting models and melt inclusion volatile, major, and trace element data suggests that hydrous slab melt addition to the mantle wedge can produce the range in primitive compositions erupted in the Lassen region. Our results provide further evidence that chlorite-derived fluids from the mantle portion of the

  5. Mitigating and monitoring flight crew fatigue on a westward ultra-long-range flight

    National Research Council Canada - National Science Library

    Signal, T Leigh; Mulrine, Hannah M; van den Berg, Margo J; Smith, Alexander A T; Gander, Philippa H; Serfontein, Wynand

    2014-01-01

    This study examined the uptake and effectiveness of fatigue mitigation guidance material including sleep recommendations for a trip with a westward ultra-long-range flight and return long-range flight...

  6. Mantle transition zone discontinuities beneath the Tien Shan

    Science.gov (United States)

    Yu, Youqiang; Zhao, Dapeng; Lei, Jianshe

    2017-10-01

    To better understand geodynamic processes of intracontinental mountain building, we conduct a systematic investigation of the mantle transition zone (MTZ) beneath the Tien Shan and its surrounding areas using a receiver function method under non-plane wave front assumption. The resulting apparent depths of the 410 km (d410) and 660 km (d660) discontinuities and the MTZ thickness display significant lateral variations. Both the central Tien Shan and the Pamir Plateau are characterized by a thick MTZ, which can be well explained by the existence of lithospheric segments resulted from possible break-off of the subducted slab or lithosphere delamination. A thin MTZ and an obviously depressed d410, which may be induced by asthenosphere upwelling associated with the dropping lithospheric segment, are revealed beneath the Kazakh Shield. Seismic evidence is obtained for the potential existence of lower mantle upwelling beneath the Tarim Basin based on the observed thin MTZ and relatively significant uplift of d660. The subduction of the Kazakh Shield and Tarim lithosphere driven by the India-Eurasia collision possibly plays an essential role in the formation and evolution of the Tien Shan orogenic belt, and the lower mantle upwelling revealed beneath the Tarim Basin may promote the uplift of the Tien Shan by softening the upper mantle.

  7. Investigating Late Cenozoic Mantle Dynamics beneath Yellowstone

    Science.gov (United States)

    Zhou, Q.; Liu, L.

    2015-12-01

    Recent tomography models (Sigloch, 2011; Schmandt & Lin, 2014) reveal unprecedented details of the mantle structure beneath the United States (U.S.). Prominent slow seismic anomalies below Yellowstone, traditionally interpreted as due to a mantle plume, are restricted to depths either shallower than 200 km or between 500 and 1000 km, but a continuation to greater depth is missing. Compared to fast seismic anomalies, which are usually interpreted as slabs or delaminated lithosphere, origin of deep slow seismic anomalies, especially those in the vicinity of subduction zones, is more enigmatic. As a consequence, both the dynamics and evolution of these slow anomalies remain poorly understood. To investigate the origin and evolution of the Yellowstone slow anomaly during the past 20 Myr, we construct a 4D inverse mantle convection model with a hybrid data assimilation scheme. On the one hand, we use the adjoint method to recover the past evolution of mantle seismic structures beyond the subduction zones. On the other hand, we use a high-resolution forward model to simulate the subduction of the oceanic (i.e., Farallon) plate. During the adjoint iterations, features from these two approaches are blended together at a depth of ~200 km below the subduction zone. In practice, we convert fast and slow seismic anomalies to effective positive and negative density heterogeneities. Our preliminary results indicate that at 20 Ma, the present-day shallow slow anomalies beneath the western U.S. were located inside the oceanic asthenosphere, which subsequently entered the mantle wedge, through the segmented Farallon slab. The eastward encroachment of the slow anomaly largely followed the Yellowstone hotspot track migration. The present deep mantle Yellowstone slow anomaly originated at shallower depths (i.e. transition zone), and was then translated down to the lower mantle accompanying the sinking fast anomalies. The temporal evolution of the slow anomalies suggests that the deep

  8. 3D geodynamic models for the development of opposing continental subduction zones: The Hindu Kush-Pamir example

    Science.gov (United States)

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

    2017-12-01

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

  9. Convective Removal of Continental Margin Lithosphere at the Edges of Subducting Oceanic Plates

    Science.gov (United States)

    Levander, A.; Bezada, M. J.; Palomeras, I.; Masy, J.; Humphreys, E.; Niu, F.

    2013-12-01

    Although oceanic lithosphere is continuously recycled to the deeper mantle by subduction, the rates and manner in which different types of continental lithospheric mantle are recycled is unclear. Cratonic mantle can be chemically reworked and essentially decratonized, although the frequency of decratonization is unclear. Lithospheric mantle under or adjacent to orogenic belts can be lost to the deeper mantle by convective downwellings and delamination phenomena. Here we describe how subduction related processes at the edges of oceanic plates adjacent to passive continental margins removes the mantle lithosphere from beneath the margin and from the continental interior. This appears to be a widespread means of recycling non-cratonic continental mantle. Lithospheric removal requires the edge of a subducting oceanic plate to be at a relatively high angle to an adjacent passive continental margin. From Rayleigh wave and body wave tomography, and receiver function images from the BOLIVAR and PICASSO experiments, we infer large-scale removal of continental margin lithospheric mantle from beneath 1) the northern South American plate margin due to Atlantic subduction, and 2) the Iberian and North African margins due to Alboran plate subduction. In both cases lithospheric mantle appears to have been removed several hundred kilometers inland from the subduction zones. This type of ';plate-edge' tectonics either accompanies or pre-conditions continental margins for orogenic activity by thinning and weakening the lithosphere. These processes show the importance of relatively small convective structures, i.e. small subducting plates, in formation of orogenic belts.

  10. Kinematics of subduction and plate convergence under Taiwan and its geomorphic, geodetic and seismic expressions

    Science.gov (United States)

    Suppe, J.; Carena, S.; Kanda, R. V.; Wu, Y.; Huang, H.; Wu, J. E.

    2013-12-01

    Deciphering the kinematics of ongoing subduction and rapid plate convergence under Taiwan is neither trivial nor straightforward. A 3D synthesis of diverse constraints is required, for example tomography, geodesy, tectonic geomorphology, stress inversion, and Philippine Sea plate motions. Eurasian-Philippine Sea plate convergence is ~90mm/y in a mildly oblique 300° azimuth relative to the ~NS nearly vertically subducting Eurasian mantle lithosphere which extends to ~500km depth. If all the current plate convergence were consumed in subduction of Eurasian mantle, the subduction flexural hinge would migrate westward at ~80mm/y, which is fast relative to the ~30mm/y long-term slip rate on the Taiwan main detachment that represents the Eurasian subduction interface under the Taiwan Central Mountains. If this fast simple subduction were occurring, subduction would too quickly outrun the mountain belt in conflict with data. Instead we estimate that subduction of Eurasian lithosphere is proceeding at ~50mm/y with the remaining ~40mm/y convergence at a lithospheric level consumed by secondary subduction above and to the east of the main plate interface. This secondary subduction is largely transient deformation that is most obvious under the Coastal Range, which represents the deforming western margin of the Philippine Sea plate during the last ~1-1.5 Ma. The thrust faults of the Coastal Range function as subduction faults with the long-term net motion of their footwalls moving largely down relative to their only slowly uplifting hanging walls, with a net secondary subduction of ~40-50km in the last ~1-1.5Ma as estimated from seismic tomography and other data. In addition we find evidence for ongoing subduction of the eastern Central Mountains of Taiwan. The crest of the mountains coincides with the western edge of the migrating plate flexure, a band of extensional geodetic strain coincides with the flexure, and an extensional stress state in the upper 5-10km coincides

  11. Structures of Active Blind Thrusts Beneath Tokyo Metropolitan Area

    Science.gov (United States)

    Ishiyama, T.; Sato, H.; Kato, N.; Nakayama, T.; Iwasaki, T.; Abe, S.

    2011-12-01

    We show structural models of active blind thrust faults beneath Tokyo metropolitan area, based on actively deforming landforms, Quaternary stratigraphy, and deep to shallow high-resolution seismic reflection data tied with these stratigraphic constraints, resolving seismic hazards from otherwise elusive active structures beneath highly urbanized areas. At the leading edge of the subducting Philippine Sea plate beneath the Kanto region, most significant active structures are recognized as folding and/or faulting of late Pleistocene and Holocene fluvial and marine deposits. Newly obtained seismic reflection profile and reprocessed sections indicate that these active structures are surface manifestations of emergent splay thrust faults extending from a subduction megathrust that generated the A.D.1923 Kanto earthquake (M7.9). Much slower rates of slip along these structures (~ 4.2 mm/yr) than slip deficits (~ 30 mm/yr) indicate that slip partitioning might have occurred between the subduction megathrust and splay faults. In contrast to these megathrust-related structures, steeply dipping blind thrusts are distributed beneath the Kanto plain underlain by several thousand meters thick Neogene forearc basin and shallow marine to terrestrial sediments (Kazusa and Shimousa Group). Deep seismic reflection profiles corroborate that these blind thrusts are reactivated normal faults originally formed due to early to middle Miocene extensional tectonics. While rates of slip along these structures are commonly slow (~0.1 mm/yr) based on offsets of late Pleistocene terrace deposits, their proximity to the metropolitan area urges more intense efforts to identify their potential seismic hazards including locations, sizes, rates of slip, and geometries of blind thrusts.

  12. Slab remnants beneath the Baja California peninsula : Seismic constraints and tectonic implications

    NARCIS (Netherlands)

    Paulssen, Hanneke; de Vos, Denise

    2017-01-01

    The formation of the Gulf of California has been related to the cessation of subduction of the Guadalupe and Magdalena microplates. Various studies have identified features that point to the presence of a slab remnant beneath the Baja California peninsula, but its depth range and lateral extent

  13. The Geodynamics of Continental Lithosphere Entering a Subduction Zone

    Science.gov (United States)

    Steedman, C. E.; Kaus, B. J.; Becker, T. W.; Okaya, D.; Wu, F. T.

    2006-12-01

    As deformation patterns resulting from subduction of a passive continental margin are insufficiently understood, here we perform 2-D numerical simulations to explore the effects of continental lithosphere entering a subduction zone. The model setup consists of a subduction zone in which the oceanic part of a passive continental margin initially subducts beneath an oceanic plate. A particle-based 2-D visco-elasto-plastic thermo-mechanical finite element code is employed to study the dynamics of the system. A novel new feature of the code is that the resolution of the model can be significantly increased in selected parts of the domain, which allows for self-consistent modelling of mantle-lithosphere interaction. In the present study this feature is employed to study how crustal scale deformation around the subduction zone is influenced by surface processes and by flow in the upper mantle. Using systematic 2-D numerical simulations, we explore the parameters that are dominant in controlling near- surface structures, both with regards to changes in topography over time, and subsurface features such as Moho undulations. The main parameters that have been varied are: the lithospheric density structure; the strength of the lower crust; the amounts of erosion; imposed pushing versus density-driven (slab-pull and ridge- push) convergence; the upper boundary condition (free surface versus free slip); rheology (non-Newtonian versus Newtonian, viscous, visco-elasto-plastic); and finally the effect of an imposed slab breakoff. In all cases we track surface uplift, subduction evolution and rock exhumation history. The results can be compared to evidence from areas such as Taiwan where continental subduction or convergence is thought to be happening. Preliminary results indicate that a low viscosity lower crust may contribute to crustal uplift.

  14. Subduction-driven recycling of continental margin lithosphere.

    Science.gov (United States)

    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.

  15. Deep electrical resistivity structure of Costa Rican Subduction Zone

    Science.gov (United States)

    Worzewski, T.; Jegen, M.; Brasse, H.; Taylor, W.

    2009-04-01

    The water content and its distribution play an important role in the subduction process. Water is released from the subducting slab in a series of metamorphic reactions and the hydration of the mantle wedge may trigger the onset of melting, weakening and changes in the dynamics and thermal structure of subduction zones. However, the amount of water carried into the subduction zone and its distribution are not well constrained by existing data and are subject of vigorous current research in SFB574 (Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters). We will show numerical modeling studies which are used to determine the resolution and sensitivity of the MT response to fluids in the crust and subducting slab under the special condition of a coastal setting. In 2007-2008 we conducted a long-period magnetotelluric investigations in northwestern Costa Rica on- and offshore, where the Cocos Plate subducts beneath the Carribean plate. Eleven marine magnetotelluric Stations newly developed and constructed by IFM-GEOMAR and University of Kiel were deployed on the 200 km long marine extension of the profile for several months. We will present the data and its processing, as well as our attempts to eliminate motion induced noise observed on some stations on the cliffy shelf due to tidal waves hitting the shelf and trench parallel- and perpendicular currents. The marine profile was extended landwards by the Free University of Berlin over length of 160 kilometers with further 18 stations. We present preliminary modeling results of land data, which revealed interesting features, inter alia a possible image of fluid release from the downgoing slab in the forearc, as well as ongoing modeling of the combined on- and offshore data sets.

  16. On the initiation of subduction

    Science.gov (United States)

    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.

  17. Great earthquakes hazard in slow subduction zones

    Science.gov (United States)

    Marcaillou, B.; Gutscher, M.; Westbrook, G. K.

    2008-12-01

    Research on the Sumatra-Andaman earthquake of 2004 has challenged two popular paradigms; that the strongest subduction earthquakes strike in regions of rapid plate convergence and that rupture occurs primarily along the contact between the basement of the overriding plate and the downgoing plate. Subduction zones presenting similar structural and geodynamic characteristics (slow convergence and thick wedges of accreted sediment) may be capable of generating great megathrust earthquakes (M>8.5) despite an absence of thrust type earthquakes over the past 40 years. Existing deep seismic sounding data and hypocenters are used to constrain the geometry of several key slow subduction zones (Antilles, Hellenic, Sumatra). This geometry forms the basis for numerical modelling of fore-arc thermal structure, which is applied to calculate the estimated width of the seismogenic portion of the subduction fault plane. The margins with the thickest accretionary wedges are commonly found to have the widest (predicted) seismogenic zone. Furthermore, for these margins there exists a substantial (20-60 km wide) region above the up-dip limit for which the contribution to tsunami generation is poorly understood. As the rigidity (mu) of these high-porosity sediments is low, co-seismic slip here can be expected to be slow. Accordingly, the contribution to seismic moment will be low, but the contribution to tsunami generation may be very high. Indeed, recent seismological data from Nankai indicate very low frequency shallow-thrust earthquakes beneath this portion of the accretionary wedge, long-considered to be "aseismic". We propose that thick accumulations of sediment on the downgoing plate and the presence of a thick accretionary wedge can increase the maximum size of the potential rupture fault plane in two ways; 1) by thermally insulating the downgoing plate and thereby increasing the total downdip length of the fault which can rupture seismically and 2) by "smoothing out" the

  18. Subduction of the oceanic Hikurangi Plateau and its impact on the Kermadec arc.

    Science.gov (United States)

    Timm, Christian; Davy, Bryan; Haase, Karsten; Hoernle, Kaj A; Graham, Ian J; de Ronde, Cornel E J; Woodhead, Jon; Bassett, Dan; Hauff, Folkmar; Mortimer, Nick; Seebeck, Hannu C; Wysoczanski, Richard J; Caratori-Tontini, Fabio; Gamble, John A

    2014-09-17

    Large igneous province subduction is a rare process on Earth. A modern example is the subduction of the oceanic Hikurangi Plateau beneath the southern Kermadec arc, offshore New Zealand. This segment of the arc has the largest total lava volume erupted and the highest volcano density of the entire Kermadec arc. Here we show that Kermadec arc lavas south of ~32°S have elevated Pb and Sr and low Nd isotope ratios, which argues, together with increasing seafloor depth, forearc retreat and crustal thinning, for initial Hikurangi Plateau-Kermadec arc collision ~250 km north of its present position. The combined data set indicates that a much larger portion of the Hikurangi Plateau (the missing Ontong Java Nui piece) than previously believed has already been subducted. Oblique plate convergence caused southward migration of the thickened and buoyant oceanic plateau crust, creating a buoyant 'Hikurangi' mélange beneath the Moho that interacts with ascending arc melts.

  19. Extending Alaska's plate boundary: tectonic tremor generated by Yakutat subduction

    Science.gov (United States)

    Wech, Aaron G.

    2016-01-01

    The tectonics of the eastern end of the Alaska-Aleutian subduction zone are complicated by the inclusion of the Yakutat microplate, which is colliding into and subducting beneath continental North America at near-Pacific-plate rates. The interaction among these plates at depth is not well understood, and further east, even less is known about the plate boundary or the source of Wrangell volcanism. The drop-off in Wadati-Benioff zone (WBZ) seismicity could signal the end of the plate boundary, the start of aseismic subduction, or a tear in the downgoing plate. Further compounding the issue is the possible presence of the Wrangell slab, which is faintly outlined by an anemic, eastward-dipping WBZ beneath the Wrangell volcanoes. In this study, I performed a search for tectonic tremor to map slow, plate-boundary slip in south-central Alaska. I identified ∼11,000 tremor epicenters, which continue 85 km east of the inferred Pacific plate edge marked by WBZ seismicity. The tremor zone coincides with the edges of the downgoing Yakutat terrane, and tremors transition from periodic to continuous behavior as they near the aseismic Wrangell slab. I interpret tremor to mark slow, semicontinuous slip occurring at the interface between the Yakutat and North America plates. The slow slip region lengthens the megathrust interface beyond the WBZ and may provide evidence for a connection between the Yakutat slab and the aseismic Wrangell slab.

  20. A Broad Depressed 410-km Discontinuity beneath Northeast Asia

    Science.gov (United States)

    Li, J.; Guo, G.; WANG, X.

    2016-12-01

    The topography of the upper mantle discontinuities is important for good understanding of the thermal structure, composition of the mantle, and scales of mantle circulation as well. We applied both receiver function analysis and multiple-ScS reverberations to seismic waveforms recorded by stations beneath land and ocean, respectively. We obtained a complete image of the upper mantle discontinuities beneath northeast Asia, covering from the Okhotsk Sea, far east Russia, Japan Sea and northeast China. Results with different resolutions from different methods are compared in detail, and the comparison shows that long-period ScS reverberation signals is effective in extracting the robust features of the upper mantle discontinuities. Through the integrated depth undulation map covering both sea and land, we detected an obvious depression of the 410-km discontinuity with value 8-25 km, anticorrelated with a wide range of depressed 660-km discontinuity. The depression of the 660 can be explained by the temperature anomaly associated to the subducting Pacific slab. The landward extension of the depressed 410, however, is of large scale with a lateral range of at least 800-1000 km. Mechanism invoking chemical heterogeneity in the mantle transition zone was explored to explain the observation. We speculate that the broadly depressed 410 beneath west Japan Sea, part of Okhotsk Sea, and northeast China might be caused by high water content at the top of the mantle transition zone. The significant trench rollback motion of the subducting Pacific slab from the Miocene might explain the widespread distribution of the depression of the 410. The west edge of observed depressed 410-km discontinuity might pin the initial location where the Pacific subducting slab had been furthest before the occurrence of trench retreating.

  1. Geodynamic Evolution of Subduction to Collision to Escape in Central Anatolia From Surface to Mantle - Results From the CD-CAT Project

    Science.gov (United States)

    Darin, Michael

    2017-04-01

    inherited lithosphere-scale structure that has accommodated contrasting magnitudes and styles of deformation to the east and west since Arabia collision. The coincidence of a similarly NNE-oriented lower plate boundary (Africa COB) or STEP fault between the Cyprus and Arabia slabs may have amplified the role of the CAFZ in controlling differential upper plate deformation. These findings support the following tectonic scenario: the first stage involved late Eocene to early Miocene horizontal subduction of the Afro-Arabia slab from central Anatolia to the Zagros, culminating in the final suturing of the Taurides and Pontides in Anatolia. The second stage occurred during the Miocene and involved the segmentation of the downgoing slab at the longitude of the CAFZ to form the Arabia slab in the east and the Cyprus slab in the west. North of Arabia, early Miocene rollback and foundering of the Arabia slab resulted in widespread volcanism, slab delamination beneath the eastern Taurides and eventual break-off and rapid sinking into the lower mantle starting at 15-10 Ma. North of Cyprus, initial rollback, steepening and breakup of the Cyprus slab are recorded by early Miocene upper plate extension and exhumation, followed by middle Miocene voluminous CAVP magmatism and uplift of the southern Taurides margin. The final stage involved a transition from diffuse to localized strain along transcurrent structures that have facilitated the westward escape of Anatolia since the latest Miocene-Pliocene.

  2. Mapping seismic azimuthal anisotropy of the Japan subduction zone

    Science.gov (United States)

    Zhao, D.; Liu, X.

    2016-12-01

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

  3. Beneath our Feet

    Science.gov (United States)

    Vernon, Ron

    2001-01-01

    How much do you know about the rocks beneath your feet--the basic materials of our planet? Their variety and beauty is astounding, especially when viewed through the microscope, where crystals of different minerals and in various arrangements are revealed in exquisite detail. While surface rocks seem so solid, they flow deep inside the earth, and, in this spectacular book, you'll see what rocks that have flowed actually look like. Eminent geologist Ron Vernon reveals the incredible beauty and variety of earth rocks, both under and on the surface. Through breathtaking photographs he captures rocks formed by heating and melting inside the earth, rocks ejected from volcanoes, rocks formed by erosion on the earth's surface, and extraterrestrial rocks that have crashed into our planet. These stunning images are accompanied by clear, nontechnical explanations of the processes underlying the creation of rocks and minerals. No other current book so cogently explains the nature and origin of rocks for nonscientists. Beneath Our Feet: The Rocks of Planet Earth is destined to be a must-have purchase for a wide-ranging audience from professional scientists to amateur rock collectors--indeed anyone interested in natural history.

  4. Subduction initiation and Obduction: insights from analog models

    Science.gov (United States)

    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

  5. Seismic anisotropy and mantle flow below subducting slabs

    Science.gov (United States)

    Walpole, Jack; Wookey, James; Kendall, J.-Michael; Masters, T.-Guy

    2017-05-01

    Subduction is integral to mantle convection and plate tectonics, yet the role of the subslab mantle in this process is poorly understood. Some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle, although the geodynamical feasibility of this has been questioned. Here, we use the source-side shear wave splitting technique to probe anisotropy beneath subducting slabs, enabling us to test petrofabric models and constrain the geometry of mantle fow. Our global dataset contains 6369 high quality measurements - spanning ∼ 40 , 000 km of subduction zone trenches - over the complete range of available source depths (4 to 687 km) - and a large range of angles in the slab reference frame. We find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow-symmetry-axis pointing normal to the plane of the slab. This appears incompatible with purely trench-parallel flow models. On the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction. Trench parallel measurements are most commonly associated with shallow events (source depth < 50 km) - suggesting a separate region of anisotropy in the lithospheric slab. This may correspond to the shape preferred orientation of cracks, fractures, and faults opened by slab bending. Meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite.

  6. Fluid Release and the Deformation of Subducting Crust

    Science.gov (United States)

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

    2014-05-01

    It is known that slab dehydration is crucial in subduction dynamics and for the formation of arc-magmatism. Previous studies of this process have constrained this intake and subsequent release of fluids into the mantle wedge by considering the stability hydrous phases within the slab. Other, more dynamical effects of this hydration state and partial melting have also been suggested, such as the possibility of "cold plumes", crustal delamination, and subduction channel return flow. These processes have been inferred to play a role in the generation of continental crust over time through accumulation and melting beneath the overriding plate. Water content and melt fraction have a strong control on the rheology of the system. Therefore we investigate the effect of these parameters on the dynamics of a subducting slab, with the aim to establish the physical bounds on the delamination process. To do this we use a coupled geodynamical-petrological model that tracks dehydration and melting reactions in order to factor in the rheological effect of metamorphism and magmatism on slab and mantle wedge dynamics. We focus primarily on the strength of the subducting crust and the possibility of delamination. We then extend this investigation by considering whether early earth crust formation could have been the result of such a processes by looking at a hypothetical Archean setting.

  7. Reaction-induced rheological weakening enables oceanic plate subduction.

    Science.gov (United States)

    Hirauchi, Ken-Ichi; Fukushima, Kumi; Kido, Masanori; Muto, Jun; Okamoto, Atsushi

    2016-08-26

    Earth is the only terrestrial planet in our solar system where an oceanic plate subducts beneath an overriding plate. Although the initiation of plate subduction requires extremely weak boundaries between strong plates, the way in which oceanic mantle rheologically weakens remains unknown. Here we show that shear-enhanced hydration reactions contribute to the generation and maintenance of weak mantle shear zones at mid-lithospheric depths. High-pressure friction experiments on peridotite gouge reveal that in the presence of hydrothermal water, increasing strain and reactions lead to an order-of-magnitude reduction in strength. The rate of deformation is controlled by pressure-solution-accommodated frictional sliding on weak hydrous phyllosilicate (talc), providing a mechanism for the 'cutoff' of the high peak strength at the brittle-plastic transition. Our findings suggest that infiltration of seawater into transform faults with long lengths and low slip rates is an important controlling factor on the initiation of plate tectonics on terrestrial planets.

  8. Seismic imaging of a mid-lithospheric discontinuity beneath Ontong Java Plateau

    Science.gov (United States)

    Tharimena, Saikiran; Rychert, Catherine A.; Harmon, Nicholas

    2016-09-01

    Ontong Java Plateau (OJP) is a huge, completely submerged volcanic edifice that is hypothesized to have formed during large plume melting events ∼90 and 120 My ago. It is currently resisting subduction into the North Solomon trench. The size and buoyancy of the plateau along with its history of plume melting and current interaction with a subduction zone are all similar to the characteristics and hypothesized mechanisms of continent formation. However, the plateau is remote, and enigmatic, and its proto-continent potential is debated. We use SS precursors to image seismic discontinuity structure beneath Ontong Java Plateau. We image a velocity increase with depth at 28 ± 4 km consistent with the Moho. In addition, we image velocity decreases at 80 ± 5 km and 282 ± 7 km depth. Discontinuities at 60-100 km depth are frequently observed both beneath the oceans and the continents. However, the discontinuity at 282 km is anomalous in comparison to surrounding oceanic regions; in the context of previous results it may suggest a thick viscous root beneath OJP. If such a root exists, then the discontinuity at 80 km bears some similarity to the mid-lithospheric discontinuities (MLDs) observed beneath continents. One possibility is that plume melting events, similar to that which formed OJP, may cause discontinuities in the MLD depth range. Plume-plate interaction could be a mechanism for MLD formation in some continents in the Archean prior to the onset of subduction.

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

    Science.gov (United States)

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

    1993-01-01

    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.

  10. Observational evidence for westward propagation of temperature inversions in the southeastern Arabian Sea

    Digital Repository Service at National Institute of Oceanography (India)

    Shankar, D.; Gopalakrishna, V.V.; Shenoi, S.S.C.; Durand, F.; Shetye, S.R.; Rajan, C.K.; Johnson, Z.; Araligidad, N.; Michael, G.S.

    in April, when the Tropical Convergence Zone moves over the SEAS and the warm pool engulfs the region. Ocean dynamics and air- sea fluxes are together responsible for the formation and westward propagation of the inversions. 1. Introduction Prior... and samples of surface water were collected with each deployment for subsequent salinity analysis. The XBT data anal- ysed by Thadathil and Gosh [1992] were sufficient only for proving the existence of inversions. In this paper, we use XBT data from the ARMEX...

  11. Subduction of fracture zones controls mantle melting and geochemical signature above slabs.

    Science.gov (United States)

    Manea, Vlad C; Leeman, William P; Gerya, Taras; Manea, Marina; Zhu, Guizhi

    2014-10-24

    For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

  12. Westward advance of the deformation front and evolution of submarine canyons offshore of southwestern Taiwan

    Science.gov (United States)

    Han, Wei-Chung; Liu, Char-Shine; Chi, Wu-Cheng; Chen, Liwen; Lin, Che-Chuan; Chen, Song-Chuen

    2017-11-01

    This study analyzes both 2D and 3D seismic images around the Palm Ridge area offshore of southwestern Taiwan to understand how the deformation front shifted westward and how tectonic activities interact with submarine canyon paths in the transition area between the active and passive margins. Palm Ridge is a submarine ridge that developed on the passive China continental margin by down-dip erosion of several tributaries of Penghu Canyon; it extends eastward across the deformation front into the submarine Taiwan accretionary wedge. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of the westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area.

  13. Complex seismic anisotropy and mantle dynamics beneath Turkey

    Science.gov (United States)

    Lemnifi, Awad A.; Elshaafi, Abdelsalam; Karaoğlu, Özgür; Salah, Mohamed K.; Aouad, Nassib; Reed, Cory A.; Yu, Youqiang

    2017-12-01

    Seismic anisotropy is an unambiguous property of the deep Earth that is often detected through shear wave splitting (SWS) and anisotropic receiver function (RF) techniques, which are then used to infer the lithospheric and asthenospheric deformational structure. The Anatolian plate and its associated Mediterranean, Eurasian, and Arabian plate boundaries represent the consequences of a variety of convergent and transform tectonic regimes; these boundaries are thus well-suited for studying seismic anisotropy related to subduction, orogenic, and strike-slip processes. We apply a joint SWS and RF analysis to identify the magnitude and orientation of deformation associated with lithosphere-asthenosphere coupling beneath the Anatolian plate system as well as intra-plate fossil fabrics resulting from ancient and ongoing collision. SWS analysis reveals the existence of complex anisotropic fabrics beneath the Anatolian region, where the upper-layer fast orientations are either parallel to strike-slip faults or orthogonal to reverse faults. Strongly oriented NE-SW lower-layer fast orientations suggest that they originate from slab-modulated flow in the mantle wedge overlying the northward-subducting African plate. The results of the RF analysis show that the fast orientations are spatially variable but are generally consistent with crustal fabrics developed mostly through intensive faulting and are possibly associated with sub-vertical lower crustal shear zones.

  14. Geologic signature of early Tertiary ridge subduction in Alaska

    Science.gov (United States)

    Bradley, Dwight C.; Kusky, Timothy M.; Haeussler, Peter J.; Goldfarb, Richard J.; Miller, Marti L.; Dumoulin, Julie A.; Nelson, Steven W.; Karl, Susan M.

    2003-01-01

    A mid-Paleocene to early Eocene encounter between an oceanic spreading center and a subduction zone produced a wide range of geologic features in Alaska. The most striking effects are seen in the accretionary prism (Chugach–Prince William terrane), where 61 to 50 Ma near-trench granitic to gabbroic plutons were intruded into accreted trench sediments that had been deposited only a few million years earlier. This short time interval also saw the genesis of ophiolites, some of which contain syngenetic massive sulfide deposits; the rapid burial of these ophiolites beneath trench turbidites, followed immediately by obduction; anomalous high-T, low-P, near-trench metamorphism; intense ductile deformation; motion on transverse strike-slip and normal faults; gold mineralization; and uplift of the accretionary prism above sea level. The magmatic arc experienced a brief flare-up followed by quiescence. In the Alaskan interior, 100 to 600 km landward of the paleotrench, several Paleocene to Eocene sedimentary basins underwent episodes of extensional subsidence, accompanied by bimodal volcanism. Even as far as 1000 km inboard of the paleotrench, the ancestral Brooks Range and its foreland basin experienced a pulse of uplift that followed about 40 million years of quiescence.All of these events - but most especially those in the accretionary prism - can be attributed with varying degrees of confidence to the subduction of an oceanic spreading center. In this model, the ophiolites and allied ore deposits were produced at the soon-to-be subducted ridge. Near-trench magmatism, metamorphism, deformation, and gold mineralization took place in the accretionary prism above a slab window, where hot asthenosphere welled up into the gap between the two subducted, but still diverging, plates. Deformation took place as the critically tapered accretionary prism adjusted its shape to changes in the bathymetry of the incoming plate, changes in the convergence direction before and after

  15. Boron cycling in subduction zones

    OpenAIRE

    Palmer, Martin R.

    2017-01-01

    Subduction zones are geologically dramatic features, with much of the drama being driven by the movement of water. The “light and lively” nature of boron, coupled with its wide variations in isotopic composition shown by the different geo-players in this drama, make it an ideal tracer for the role and movement of water during subduction. The utility of boron ranges from monitoring how the fluids that are expelled from the accretionary prism influence seawater chemistry, to the subduction of c...

  16. Insight into the subducted Indian slab and origin of the Tengchong volcano in SE Tibet from receiver function analysis

    Science.gov (United States)

    Xu, Mijian; Huang, Hui; Huang, Zhouchuan; Wang, Pan; Wang, Liangshu; Xu, Mingjie; Mi, Ning; Li, Hua; Yu, Dayong; Yuan, Xiaohui

    2018-01-01

    The subduction of the Indian Plate beneath SE Tibet and its related volcanism in Tengchong are important geologic processes that accompany the evolution of the Tibetan Plateau. However, it is still not clear whether the subduction and volcanism are confined to the upper mantle or if they extend deep into the mantle transition zone (MTZ). Here, we imaged MTZ structures by using receiver function methods with the waveforms recorded by more than 300 temporary stations in SE Tibet. The results show significant depressions of both the 410-km and 660-km discontinuities and a thickened MTZ (260-280 km) beneath SE Tibet. The depression of the 660-km discontinuity (by 10-30 km) and the thickened MTZ correlate well with high P-wave velocity anomalies in the MTZ, indicating the presence of a subducted Indian slab within the MTZ. Significant depression of the 410-km discontinuity (by 10-20 km) beneath the Tengchong volcano indicates that the volcano originates from the MTZ and is closely related to the subducted Indian slab. Our results confirm the deep subduction of the Indian plate and the deep origin of the Tengchong volcano. However, it remains unknown whether a slab gap exists and contributes to the Tengchong volcano.

  17. Seismicity and the subduction process

    Science.gov (United States)

    Ruff, L.; Kanamori, H.

    1980-01-01

    There is considerable variation between subduction zones in the largest characteristic earthquake within each zone. Assuming that coupling between downgoing and upper plates is directly related to characteristic earthquake size, tests for correlations between variation in coupling and other physical features of subduction zones are conducted: the lateral extent and penetration depth of Benioff zones, age of subducting lithosphere, convergence rate, and back-arc spreading. Using linear multivariate regression, coupling is correlated with two variables: convergence rate and lithosphere age. Secondary correlations within the data set are penetration depth versus lithosphere age, and lateral extent versus convergence rate. Taken together, the observed correlations suggest a simple qualitative model where convergence rate and lithosphere age determine the horizontal and sinking rates, respectively, of slabs: these parameters influence the seismic coupling in the subduction zone. In the limit of a fast sinking rate and slow convergence rate, back-arc spreading occurs and thereby appears to be a passive process.

  18. Metallogeny of subduction zones

    Directory of Open Access Journals (Sweden)

    Sorokhtin N. O.

    2017-03-01

    Full Text Available The paper deals with the multistage mechanism of the Earth's crust enrichment in ore elements in underthrust zones. The processes of metamorphism and the formation of hydrothermal solutions at pulling of the watered oceanic lithospheric plate into the subduction zone have been described. Some physical and chemical transformation regularities of structural-material complexes in these areas and mechanisms of the formation of ore deposits have been discussed. Spatio-temporal patterns of the localization of a number of endogenetic and exogenetic deposits have been described using metallogeny of the Ural and the Verkhoyansk-Kolyma Fold Belts as an example. It has been shown that in nature there are several effective mechanisms of the enrichment of the crust in ore minerals. One of them is the process of pulling into subduction zone of metalliferous sediments and ferromanganese crusts as well as seabed nodules, their metamorphic transformation, partial melting and transition of ore components into magmatic melts and mineralized fluids. In the future this leads to the release of ore material by magmas and hydrothermal solutions into the folded formations of island-arc and Andean types and the formation of igneous, metasomatic and hydrothermal deposits. Another, yet no less powerful natural mechanism of a conveyor enrichment of the crust in ore elements is the process of destruction and sedimentation of mineral deposits formed in the folded areas as well as the formation of placers and their transfer to the marginal parts of the continent. Later, during the collision of active and passive margins of two lithospheric plates, such as the collision of the Kolyma Massif with the eastern part of the Siberian craton in the middle of the Mesozoic there was a thrusting of a younger lithospheric plate over a more ancient one. As a result, the sedimentary sequences of the passive margin of the Siberian plate were submerged and partially melted by the basic magmas

  19. Recycling Revisited: Where did all the Subducted Sediments go?

    Science.gov (United States)

    Hofmann, A. W.; Chauvel, C.; Lewin, E.; Kelemen, P. B.; Hacker, B. R.

    2016-12-01

    Several lines of reasoning have revived the idea [1] that subduction has recycled continent-derived sediments into the mantle on a massive scale. For example, well-known peaks in zircon ages have been reinterpreted as reflecting variable rates of crust destruction via erosion and sediment subduction [2]. In addition, assessment of the trace element budgets of subducted sediments and arc volcanics, as well as geological and geophysical studies of accretionary wedges have led to estimates that about one mass of present-day continental crust has been returned to the mantle [3]. If these ideas are correct, then recycled sedimentary components should be present in MORB and OIB sources. As previously established, Nb/U and 87Sr/86Sr are negatively correlated in all EM2-type OIBs, clearly indicating continental/sedimentary input. However, the MORB source reservoir, being depleted in incompatible elements, is particularly susceptible to "pollution" by subducted sediments. Chauvel et al. [4] modeled the Hf-Nd isotopic array of MORBs+OIBs and concluded that it requires the addition of up to 6 % subducted sediment. We revisit this issue and show that global MORBs show no decrease in Nb/U with increasing 87Sr/86Sr, ruling out extensive addition of recycled sediment into global MORB sources. Instead, the Hf-Nd array can be obtained by recycled alkali basalts derived from subducted seamounts and ocean islands, rather than sediments. Moreover, mantle plumes with clearly identifiable sediment input contribute less than 20% of the total plume flux. We conclude that most of the subducted sediment flux is not returned to the convecting mantle. Instead, its most plausible fate is to be underplated beneath existing continental crust via "relamination" [5]. These results imply that continental recycling is subordinate and the growth of the continental crust has been largely irreversible. [1] Armstrong, 1968, Rev. Geophys. 6, 175. [2] Hawkesworth et al., 2009, Science 323, 49. [3] Porter

  20. Three-dimensional Thermal Model of the Mexican Subduction Zone

    Science.gov (United States)

    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

  1. Inner Core Rotation from Geomagnetic Westward Drift and a Stationary Spherical Vortex in Earth's Core

    Science.gov (United States)

    Voorhies, C. V.

    1999-01-01

    The idea that geomagnetic westward drift indicates convective leveling of the planetary momentum gradient within Earth's core is pursued in search of a differentially rotating mean state, upon which various oscillations and secular effects might be superimposed. The desired state conforms to roughly spherical boundary conditions, minimizes dissipative interference with convective cooling in the bulk of the core, yet may aide core cooling by depositing heat in the uppermost core and lower mantle. The variational calculus of stationary dissipation applied to a spherical vortex within the core yields an interesting differential rotation profile akin to spherical Couette flow bounded by thin Hartmann layers. Four boundary conditions are required. To concentrate shear induced dissipation near the core-mantle boundary, these are taken to be: (i) no-slip at the core-mantle interface; (ii) geomagnetically estimated bulk westward flow at the base of the core-mantle boundary layer; (iii) no-slip at the inner-outer core interface; and, to describe magnetic locking of the inner core to the deep outer core, (iv) hydrodynamically stress-free at the inner-outer core boundary. By boldly assuming the axial core angular momentum anomaly to be zero, the super-rotation of the inner core is calculated to be at most 1.5 degrees per year.

  2. Fluid accumulation along the Costa Rica subduction thrust and development of the seismogenic zone

    Science.gov (United States)

    Bangs, Nathan L.; McIntosh, Kirk D.; Silver, Eli A.; Kluesner, Jared W.; Ranero, César R.

    2015-01-01

    In 2011 we acquired an 11 × 55 km, 3-D seismic reflection volume across the Costa Rica margin, NW of the Osa Peninsula, to accurately image the subduction thrust in 3-D, to examine fault zone properties, and to infer the hydrogeology that controls fluid accumulation along the thrust. Following processing to remove water column multiples, noise, and acquisition artifacts, we constructed a 3-D seismic velocity model for Kirchhoff prestack depth migration imaging. Images of the plate boundary thrust show high-reflection amplitudes underneath the middle to lower slope that we attribute to fluid-rich, poorly drained portions of the subduction thrust. At 5 km subseafloor, beneath the upper slope, the plate interface abruptly becomes weakly reflective, which we interpret as a transition to a well-drained subduction thrust. Mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude. There is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a "plumbing system." Faults commonly have fault plane reflections and are presumably fluid charged. The faults and layered fabric form three compartmentalized hydrogeologic zones: (1) a shallow NE dipping zone beneath the slope, (2) a steeply SW dipping zone beneath the shelf slope break, and (3) a NE dipping zone beneath the shelf. The more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure, elevates effective stress, and creates greater potential for unstable coseismic slip.

  3. The subduction zone flow field from seismic anisotropy: a global view.

    Science.gov (United States)

    Long, Maureen D; Silver, Paul G

    2008-01-18

    Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity |V(t)|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |V(t)|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity V(c); their relative influence is governed by the relative magnitude of |V(t)| and V(c).

  4. A discussion of numerical subduction initiation

    Science.gov (United States)

    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

  5. Major disruption of D'' beneath Alaska: D'' Beneath Alaska

    Energy Technology Data Exchange (ETDEWEB)

    Sun, Daoyuan [Laboratory of Seismology and Physics of Earth' s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei Anhui China; National Geophysics Observatory at Mengcheng, Anhui China; Helmberger, Don [Seismological Laboratory, California Institute of Technology, Caltech, Pasadena California USA; Miller, Meghan S. [Department of Earth Sciences, University of Southern California, Los Angeles California USA; Jackson, Jennifer M. [Seismological Laboratory, California Institute of Technology, Caltech, Pasadena California USA

    2016-05-01

    D'' represents one of the most dramatic thermal and compositional layers within our planet. In particular, global tomographic models display relatively fast patches at the base of the mantle along the circum-Pacific which are generally attributed to slab debris. Such distinct patches interact with the bridgmanite (Br) to post-bridgmanite (PBr) phase boundary to generate particularly strong heterogeneity at their edges. Most seismic observations for the D'' come from the lower mantle S wave triplication (Scd). Here we exploit the USArray waveform data to examine one of these sharp transitions in structure beneath Alaska. From west to east beneath Alaska, we observed three different characteristics in D'': (1) the western region with a strong Scd, requiring a sharp δVs = 2.5% increase; (2) the middle region with no clear Scd phases, indicating a lack of D'' (or thin Br-PBr layer); and (3) the eastern region with strong Scd phase, requiring a gradient increase in δVs. To explain such strong lateral variation in the velocity structure, chemical variations must be involved. We suggest that the western region represents relatively normal mantle. In contrast, the eastern region is influenced by a relic slab that has subducted down to the lowermost mantle. In the middle region, we infer an upwelling structure that disrupts the Br-PBr phase boundary. Such an interpretation is based upon a distinct pattern of travel time delays, waveform distortions, and amplitude patterns that reveal a circular-shaped anomaly about 5° across which can be modeled synthetically as a plume-like structure rising about 400 km high with a shear velocity reduction of ~5%, similar to geodynamic modeling predictions of upwellings.

  6. Crust and subduction zone structure of Southwestern Mexico

    Science.gov (United States)

    Suhardja, Sandy Kurniawan; Grand, Stephen P.; Wilson, David; Guzman-Speziale, Marco; Gomez-Gonzalez, Juan Martin; Dominguez-Reyes, Tonatiuh; Ni, James

    2015-02-01

    Southwestern Mexico is a region of complex active tectonics with subduction of the young Rivera and Cocos plates to the south and widespread magmatism and rifting in the continental interior. Here we use receiver function analysis on data recorded by a 50 station temporary deployment of seismometers known as the MARS (MApping the Rivera Subduction zone) array to investigate crustal structure as well as the nature of the subduction interface near the coast. The array was deployed in the Mexican states of Jalisco, Colima, and Michoacan. Crustal thickness varies from 20 km near the coast to 42 km in the continental interior. The Rivera plate has steeper dip than the Cocos plate and is also deeper along the coast than previous estimates have shown. Inland, there is not a correlation between the thickness of the crust and topography indicating that the high topography in northern Jalisco and Michoacan is likely supported by buoyant mantle. High crustal Vp/Vs ratios (greater than 1.82) are found beneath the trenchward edge of magmatism including below the Central Jalisco Volcanic Lineament and the Michoacan-Guanajuato Volcanic Field implying a new arc is forming closer to the trench than the Trans Mexican Volcanic Belt. Elsewhere in the region, crustal Vp/Vs ratios are normal. The subducting Rivera and Cocos plates are marked by a dipping shear wave low-velocity layer. We estimate the thickness of the low-velocity layer to be 3 to 4 km with an unusually high Vp/Vs ratio of 2.0 to 2.1 and a drop in S velocity of 25%. We postulate that the low-velocity zone is the upper oceanic crust with high pore pressures. The low-velocity zone ends from 45 to 50 km depth and likely marks the basalt to eclogite transition.

  7. Tomographic Inversion for Shear Velocity Beneath the North American Plate

    Science.gov (United States)

    Grand, Stephen P.

    1987-12-01

    A tomographic back projection scheme has been applied to S and SS travel times to invert for shear velocity below the North American plate. The data range in distance from 8° to 80°, and a total of 3923 arrival times were used. First arrivals were measured directly off the seismograms, while the arrival times of later arrivals were found by a waveform correlation technique using synthetic seismograms. The starting model was laterally heterogeneous in the upper 400 km to account for the first-order differences in ray paths already known. The model was divided into blocks with horizontal dimensions of 500 km by 500 km and varying vertical thicknesses. Good resolution was obtained for structure from just below the crust to about 1700 km depth in the mantle. In the upper mantle a high-velocity root was found directly beneath the Canadian shield to about 400 km depth with the Superior province having the highest velocity and deepest root. The east coast of the United States was found to have intermediate velocities from 100 to 350 km depth and the western United States the slowest velocities at these depths. Below 400 km depth the most significant structure found is a slab-shaped high-velocity anomaly from the eastern Carribean to the northern United States. Beneath the Carribean this anomaly is almost vertical and extends from about 700 km to 1700 km depth. Further to the north, the anomaly dips to the east with high velocities at 700 km depth in the central United States and high velocities below 1100 km depth beneath the east coast. The anomaly is about 1% in magnitude. This lower-mantle anomaly may be associated with past subduction of the Farallon plate beneath North America.

  8. Review of Westward on the High-Hilled Plains. The Later Prehistory of the West Midlands

    Directory of Open Access Journals (Sweden)

    Carmen Martin Ramos

    2017-09-01

    Full Text Available 'Westward on the High-Hilled Plains. The Later Prehistory of the West Midlands' is the second volume of the project 'The Making of the West Midlands'. This is an ambitious six-volume series which puts together the proceedings originally presented at the seminars held, between June 2002 and June 2003, under the title the 'West Midlands Regional Research Framework for Archaeology'. This second book of the series, edited by Derek Hurst, complements the assessment and research agenda for later prehistory (Hurst 2011 more recently published in 'The Archaeology of West Midlands: a framework for research' (Watt 2011 and focuses on the Late Prehistory of the West Midland region, from the Middle Bronze age to the Late Iron Age.

  9. Wind-driven coastal upwelling and westward circulation in the Yucatan shelf

    Science.gov (United States)

    Ruiz-Castillo, Eugenio; Gomez-Valdes, Jose; Sheinbaum, Julio; Rioja-Nieto, Rodolfo

    2016-04-01

    The wind-driven circulation and wind-induced coastal upwelling in a large shelf sea with a zonally oriented coast are examined. The Yucatan shelf is located to the north of the Yucatan peninsula in the eastern Gulf of Mexico. This area is a tropical shallow body of water with a smooth sloping bottom and is one of the largest shelves in the world. This study describes the wind-driven circulation and wind-induced coastal upwelling in the Yucatan shelf, which is forced by easterly winds throughout the year. Data obtained from hydrographic surveys, acoustic current profilers and environmental satellites are used in the analysis. Hydrographic data was analyzed and geostrophic currents were calculated in each survey. In addition an analytical model was applied to reproduce the currents. The results of a general circulation model were used with an empirical orthogonal function analysis to study the variability of the currents. The study area is divided in two regions: from the 40 m to the 200 m isobaths (outer shelf) and from the coast to the 40 m isobath (inner shelf). At the outer shelf, observations revealed upwelling events throughout the year, and a westward current with velocities of approximately 0.2 m s-1 was calculated from the numerical model output and hydrographic data. In addition, the theory developed by Pedlosky (2007) for a stratified fluid along a sloping bottom adequately explains the current's primary characteristics. The momentum of the current comes from the wind, and the stratification is an important factor in its dynamics. At the inner shelf, observations and numerical model output show a wind-driven westward current with maximum velocities of 0.20 m s-1. The momentum balance in this region is between local acceleration and friction. A cold-water band is developed during the period of maximum upwelling.

  10. Heterogeneous Structure and Seismicity beneath the Tokyo Metropolitan Area

    Science.gov (United States)

    Nakagawa, S.; Kato, A.; Sakai, S.; Nanjo, K.; Panayotopoulos, Y.; Kurashimo, E.; Obara, K.; Kasahara, K.; Aketagawa, T.; Kimura, H.; Hirata, N.

    2010-12-01

    Beneath the Tokyo metropolitan area, the Philippine Sea Plate (PSP) subducts and causes damaged mega-thrust earthquakes. Sato et al. (2005) revealed the geometry of upper surface of PSP, and Hagiwara et al. (2006) estimated the velocity structure beneath Boso peninsula. However, these results are not sufficient for the assessment of the entire picture of the seismic hazards beneath the Tokyo metropolitan area including those due to an intra-slab M7+ earthquake. So, we launched the Special Project for Earthquake Disaster Mitigation in the Tokyo Metropolitan area (Hirata et al., 2009). Proving the more detailed geometry and physical properties (e.g. velocities, densities, attenuation) and stress field within PSP is very important to attain this issue. The core item of this project is a dense seismic array called Metropolitan Seismic Observation network (MeSO-net) for making observations in the metropolitan area (Sakai and Hirata, 2009; Kasahara et al., 2009). We deployed the 249 seismic stations with a spacing of 5 km. Some parts of stations construct 5 linear arrays at interval of 2 km such as Tsukuba-Fujisawa (TF) array, etc. The TF array runs from northeast to southwest through the center of Tokyo. In this study, we applied the tomography method to image the heterogeneous structure under the Tokyo metropolitan area. We selected events from the Japan Meteorological Agency (JMA) unified earthquake list. All data of MeSO-net were edited into event data by the selected JMA unified earthquake list. We picked the P and S wave arrival times. The total number of stations and events are 421 and 1,256, respectively. Then, we applied the double-difference tomography method (Zhang and Thurber, 2003) to this dataset and estimated the fine-scale velocity structure. The grid nodes locate 10 km interval in parallel with the array, 20 km interval in perpendicular to the array; and on depth direction, 5 km interval to a depth of less than 50 km and 10 km interval at a depth of more

  11. Processes and consequences of deep subduction

    NARCIS (Netherlands)

    Rubie, David C.; Hilst, R.D. van der

    2001-01-01

    Subduction of slabs of oceanic lithosphere into the deep mantle involves a wide range of geophysical and geochemical processes and is of major importance for the physical and chemical evolution of the Earth. For example, subduction and subduction-related volcanism are major processes through

  12. Subduction and Slab Advance at Orogen Syntaxes: Predicting Exhumation Rates and Thermochronometric Ages with Numerical Modeling

    Science.gov (United States)

    Nettesheim, Matthias; Ehlers, Todd A.; Whipp, David M.

    2017-04-01

    The change in plate boundary orientation and subducting plate geometry along orogen syntaxes may have major control on the subduction and exhumation dynamics at these locations. Previous work documents that the curvature of subducting plates in 3D at orogen syntaxes forces a buckling and flexural stiffening of the downgoing plate. The geometry of this stiffened plate region, also called indenter, can be observed in various subduction zones around the world (e.g. St. Elias Range, Alaska; Cascadia, USA; Andean syntaxis, South America). The development of a subducting, flexurally stiffened indenter beneath orogen syntaxes influences deformation in the overriding plate and can lead to accelerated and focused rock uplift above its apex. Moreover, the style of deformation in the overriding plate is influenced by the amount of trench or slab advance, which is the amount of overall shortening not accommodated by underthrusting. While many subduction zones exhibit little to no slab advance, the Nazca-South America subduction and especially the early stages of the India-Eurasia collision provide end-member examples. Here, we use a transient, lithospheric-scale, thermomechanical 3D model of an orogen syntaxis to investigate the effects of subducting a flexurally stiffened plate geometry and slab advance on upper plate deformation. A visco-plastic upper-plate rheology is used, along with a buckled, rigid subducting plate. The free surface of the thermomechanical model is coupled to a landscape evolution model that accounts for erosion by fluvial and hillslope processes. The cooling histories of exhumed rocks are used to predict the evolution of low-temperature thermochronometer ages on the surface. With a constant overall shortening for all simulations, the magnitude of slab advance is varied stepwise from no advance, with all shortening accommodated by underthrusting, to full slab advance, i.e. no motion on the megathrust. We show that in models where most shortening is

  13. Deep vs. shallow expressions of continental cratons: Can cratonic roots be destroyed by subduction?

    Science.gov (United States)

    Perry-Houts, J.; Calo, M.; Eddy, C. L.; Guerri, M.; Holt, A.; Hopper, E.; Tesoniero, A.; Romanowicz, B. A.; Becker, T. W.; Wagner, L. S.

    2013-12-01

    Cratons are parts of continents that have remained tectonically quiescent over billion-year timescales. Although cratonic lithosphere has the stabilizing properties of chemical buoyancy and high viscosity, it can still be destroyed. The best known example of a missing cratonic root is beneath the eastern North China Craton (NCC). Despite strong evidence for the past existence of a craton in northern China, high heat flow, Mesozoic basin formation, extensive seismicity, and the lack of a fast seismic root imply that the deep cratonic lithosphere is missing. The mechanism for the lithospheric root loss is a source of much debate. Many mechanisms have been proposed, among them: shearing of the lithospheric root by asthenospheric flow induced by the Indo-Eurasian collision; ponding of the Pacific slab in the transition zone acting as a source of fluids that enable hydrous weakening; and thermal erosion due to the corner-flow upwelling of hot, deep material. It is generally agreed that the influence of subduction is key, both from the temporal coincidence of subduction with increased tectonomagmatic activity on the craton and from the spatial correlation of lithospheric loss adjacent to the Pacific trench. We investigate how cratons extend to depth through comparison between seismic signatures of the cratonic lithosphere in the upper mantle and surficial evidence of cratonic boundaries. We examine global and regional tomography, as well as receiver-function constraints on lithospheric thickness in the NCC. We define craton boundaries at the surface through analyses on crust and lithospheric mantle ages and kimberlite locations. We aim to identify regions where the fast cratonic root has been lost or altered beneath Archean and Proterozoic crust and in particular place constraints on the extent of the remaining cratonic root beneath North China. Given the common emphasis on the role of subduction as a driving force for the root loss beneath the eastern NCC, we focus on

  14. Potential methane reservoirs beneath Antarctica

    NARCIS (Netherlands)

    Wadham, J.L.; Arndt, S.|info:eu-repo/dai/nl/304835706; Tulaczyk, S.; Stibal, M.; Tranter, M.; Telling, J.; Lis, G.P.; Lawson, E.; Ridgwell, A.; Dubnick, A.; Sharp, M.J.; Anesio, A.M.; Butler, C.E.H.

    2012-01-01

    Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been

  15. Crustal structure of the Carpathian orogen from receiver function analysis: how craton subduction and active delamination affect the crust

    Science.gov (United States)

    Petrescu, Laura; Tataru, Dragos; Grecu, Bogdan

    2017-04-01

    The Carpathian arc is an uncommon curved collisional system, involving the subduction of the Eastern European craton and the Proterozoic Moesian platform beneath younger European microplates. The Cenozoic collision led to the closure of the Tethys Oceanic basin, portions of which are actively breaking off or delaminating beneath the orogen, generating deep mantle earthquakes. Neogene volcanism, possibly related to subduction slab roll-back, also formed a band of presently extinct volcanoes in the back-arc region. The Carpathian embayment is thus an ideal laboratory to investigate crustal processes related to subduction of cratonic material, multiple plate junctions and active delamination. To better understand how the crustal structure changes from the Eastern European cratonic foreland, across the curved subduction zone, to the younger European microplates, we analyse teleseismic earthquakes recorded at broadband seismic stations located across eastern and southern Carpathians, in Romania and Moldova. We processed data from permanent seismic networks (The Romanian National Seismic Network) as well as data from temporary deployments such as CALIXTO (Carpathian Arc Lithosphere X-Tomography) and SCP (South Carpathian Project). Using extended multi-taper spectral division, we compute and analyse radial and transverse receiver functions. Energy on the transverse component may be an indicator of crustal anisotropy or the existence of intracrustal dipping interfaces. Using phase-weighted H-k stacking of receiver functions, we estimate the crustal thickness and the bulk crustal Poisson's ratio as well as the seismic sharpness of the Moho discontinuity. Furthermore, we invert receiver functions to obtain the S-wave velocity structure of the crust and upper mantle beneath individual stations, which provide concurrent information on the Moho nature. Our results provide a better understanding of crustal structure across complex collisional systems involving the subduction of

  16. Building a Subduction Zone Observatory

    Science.gov (United States)

    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.

  17. Imaging Canary Island hotspot material beneath the lithosphere of Morocco and southern Spain

    Science.gov (United States)

    Miller, Meghan S.; O'Driscoll, Leland J.; Butcher, Amber J.; Thomas, Christine

    2015-12-01

    The westernmost Mediterranean has developed into its present day tectonic configuration as a result of complex interactions between late stage subduction of the Neo-Tethys Ocean, continental collision of Africa and Eurasia, and the Canary Island mantle plume. This study utilizes S receiver functions (SRFs) from over 360 broadband seismic stations to seismically image the lithosphere and uppermost mantle from southern Spain through Morocco and the Canary Islands. The lithospheric thickness ranges from ∼65 km beneath the Atlas Mountains and the active volcanic islands to over ∼210 km beneath the cratonic lithosphere in southern Morocco. The common conversion point (CCP) volume of the SRFs indicates that thinned lithosphere extends from beneath the Canary Islands offshore southwestern Morocco, to beneath the continental lithosphere of the Atlas Mountains, and then thickens abruptly at the West African craton. Beneath thin lithosphere between the Canary hot spot and southern Spain, including below the Atlas Mountains and the Alboran Sea, there are distinct pockets of low velocity material, as inferred from high amplitude positive, sub-lithospheric conversions in the SRFs. These regions of low seismic velocity at the base of the lithosphere extend beneath the areas of Pliocene-Quaternary magmatism, which has been linked to a Canary hotspot source via geochemical signatures. However, we find that this volume of low velocity material is discontinuous along strike and occurs only in areas of recent volcanism and where asthenospheric mantle flow is identified with shear wave splitting analyses. We propose that the low velocity structure beneath the lithosphere is material flowing sub-horizontally northeastwards beneath Morocco from the tilted Canary Island plume, and the small, localized volcanoes are the result of small-scale upwellings from this material.

  18. Laboratory models of the thermal evolution of the mantle during rollback subduction.

    Science.gov (United States)

    Kincaid, C; Griffiths, R W

    2003-09-04

    The subduction of oceanic lithosphere plays a key role in plate tectonics, the thermal evolution of the mantle and recycling processes between Earth's interior and surface. Information on mantle flow, thermal conditions and chemical transport in subduction zones come from the geochemistry of arc volcanoes, seismic images and geodynamic models. The majority of this work considers subduction as a two-dimensional process, assuming limited variability in the direction parallel to the trench. In contrast, observationally based models increasingly appeal to three-dimensional flow associated with trench migration and the sinking of oceanic plates with a translational component of motion (rollback). Here we report results from laboratory experiments that reveal fundamental differences in three-dimensional mantle circulation and temperature structure in response to subduction with and without a rollback component. Without rollback motion, flow in the mantle wedge is sluggish, there is no mass flux around the plate and plate edges heat up faster than plate centres. In contrast, during rollback subduction flow is driven around and beneath the sinking plate, velocities increase within the mantle wedge and are focused towards the centre of the plate, and the surface of the plate heats more along the centreline.

  19. Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.

    Science.gov (United States)

    Zhao, Zi-Fu; Dai, Li-Qun; Zheng, Yong-Fei

    2013-12-04

    Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.

  20. Thermal buoyancy on Venus - Underthrusting vs subduction

    Science.gov (United States)

    Burt, Jeffrey D.; Head, James W.

    1992-01-01

    The thermal and buoyancy consequences of the subduction endmember are modeled in an attempt to evaluate the conditions distinguishing underthrusting and subduction. Thermal changes in slabs subducting into the Venusian mantle with a range of initial geotherms are used to predict density changes and, thus, slab buoyancy. Based on a model for subduction-induced mantle flow, it is then argued that the angle of the slab dip helps differentiate between underthrusting and subduction. Mantle flow applies torques to the slab which, in combination with torques due to slab buoyancy, act to change the angle of slab dip.

  1. Shear wave splitting measurements and interpretation beneath Acapulco-Tampico transect in Mexico

    Science.gov (United States)

    Stubailo, I.; Davis, P.

    2007-12-01

    We have examined shear wave splitting in teleseismic shear waves from 100 broadband stations installed from Acapulco to Tampico in Mexico over a period of 1.5 years (2005-2007). The instruments were part of the MASE (Middle America Subduction Experiment) which has the objective to build a geodynamical model of the subduction process beneath the Middle America Trench. The stations had a 5-6 km spacing and provided a unique data set which allows examination of the variation in splitting in high detail. Tomographic and receiver function studies in this area (done by MASE colleagues) show the presence of a flat slab under the western part of the array, and a steeply dipping slab beneath its center. According to geochronological data, the onset of flat slab subduction took place ~15-20 Ma, after the Cocos plate broke off the Farallon plate. We observe large splitting delay times with, on average, a fast direction in the northeast-southwest direction, but with considerable variation along the network. We compare the splitting results with the three dimensional structure inferred from the geochemistry and seismic analyses.

  2. Pervasive upper mantle melting beneath the western US

    Science.gov (United States)

    Hier-Majumder, Saswata; Tauzin, Benoit

    2017-04-01

    We report from converted seismic waves, a pervasive seismically anomalous layer above the transition zone beneath the western US. The layer, characterized by an average shear wave speed reduction of 1.6%, spans over an area of ∼ 1.8 ×106 km2 with thicknesses varying between 25 and 70 km. The location of the layer correlates with the present location of a segment of the Farallon plate. This spatial correlation and the sharp seismic signal atop of the layer indicate that the layer is caused by compositional heterogeneity. Analysis of the seismic signature reveals that the compositional heterogeneity can be ascribed to a small volume of partial melt (0.5 ± 0.2 vol% on average). This article presents the first high resolution map of the melt present within the layer. Despite spatial variations in temperature, the calculated melt volume fraction correlates strongly with the amplitude of P-S conversion throughout the region. Comparing the values of temperature calculated from the seismic signal with available petrological constraints, we infer that melting in the layer is caused by release of volatiles from the subducted Farallon slab. This partially molten zone beneath the western US can sequester at least 1.2 ×1017 kg of volatiles, and can act as a large regional reservoir of volatile species such as H or C.

  3. Subduction of the Daiichi Kashima Seamount in the Japan Trench

    Science.gov (United States)

    Lallemand, S.; Culotta, R.; Von Huene, R.

    1989-01-01

    In 1984-1985, the Kaiko consortium collected Seabeam, single-channel seismic and submersible sampling data in the vicinity of the Daiichi-Kashima seamount and the southern Japan trench. We performed a prestack migration of a Shell multichannel seismic profile, that crosses this area, and examined it in the light of this unusually diverse Kaiko dataset. Unlike the frontal structure of the northern Japan trench, where mass-wasting appears to be the dominant tectonic process, the margin in front of the Daiichi-Kashima shows indentation, imbrication, uplift and erosion. Emplacement of the front one-third of the seamount beneath the margin front occurs without accretion. We conclude that the Daiichi-Kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin. ?? 1989.

  4. Geometry and seismic properties of the subducting Cocos plate in central Mexico

    Science.gov (United States)

    Kim, Y.; Clayton, R. W.; Jackson, J. M.

    2010-06-01

    The geometry and properties of the interface of the Cocos plate beneath central Mexico are determined from the receiver functions (RFs) utilizing data from the Meso America Subduction Experiment (MASE). The RF image shows that the subducting oceanic crust is shallowly dipping to the north at 15° for 80 km from Acapulco and then horizontally underplates the continental crust for approximately 200 km to the Trans-Mexican Volcanic Belt (TMVB). The crustal image also shows that there is no continental root associated with the TMVB. The migrated image of the RFs shows that the slab is steeply dipping into the mantle at about 75° beneath the TMVB. Both the continental and oceanic Moho are clearly seen in both images, and modeling of the RF conversion amplitudes and timings of the underplated features reveals a thin low-velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab. By inverting RF amplitudes of the converted phases and their time separations, we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting Cocos plate and its thickness. High Poisson's and Vp/Vs ratios due to anomalously low S wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure. The evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates. This may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust.

  5. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Shear Wave Splitting from PULSE

    Science.gov (United States)

    Eakin, C. M.; Long, M. D.; Beck, S. L.; Wagner, L. S.; Tavera, H.

    2013-12-01

    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for deep and teleseismic events, making use of a wide variety of available phases that sample the upper mantle directly beneath the stations (such as SKS, SKKS, PKS, sSKS, SKiKS, ScS and local/direct S). We analyze the variability of our results with respect to initial polarizations and ray paths, as well as spatial variability between stations as the underlying slab morphology changes. Preliminary results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA). Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. We carefully evaluate the different possible source locations within the subduction zone for this seismic anisotropy and observe increasing evidence for distinct anisotropy within the slab as well as the sub-slab mantle.

  6. The idol beneath the altar.

    Science.gov (United States)

    Clemens, Norman A

    2014-03-01

    Drawing on the imagery of a Mayan idol hidden beneath the altar of a Catholic mission church imposed on a Mayan city by Spanish conquerors, the author discusses the role of deeply rooted core beliefs that are not always evident on the surface-and the observation that, in clinical practice, things are not always as they seem. Psychotherapists may unconsciously be seen as invading cultural enemies.

  7. Mantle structure beneath the Alboran Sea from shear wave splitting

    Science.gov (United States)

    Alpert, L. A.; Becker, T. W.; Miller, M. S.; Allam, A. A.

    2011-12-01

    New seismological investigations in the Alboran domain of the western Mediterranean, as part of the PICASSO experiment, support geodynamic models which constrain the mantle structure beneath the Alboran Sea. We evaluate global circulation models in the context of seismic anistropy as inferred from SKS/SKKS splitting observations. Using instantaneous velocity fields from 3-D flow models with variable mantle density based on several tomography and seismicity based models, we calculate the predicted anisotropy, fast polarization direction (FPD), and delay times in order to explain the complex tectonic and geologic history of the Alboran Sea region. Slab rollback, delamination, and convective removal processes have been invoked to explain the synorogenic extension in the Alboran and recently published splitting measurements show north-east trending FPD across the Iberian margin with a rotation to the southeast that follows the curve of the Gibraltar arc, suggested by the authors as supporting west-directed slab rollback. Our new measurements from 39 stations substantiate the measurements in southern Spain, but we find a striking, nearly 90 degree rotation in azimuth and reduced delay times across the High Atlas Mountains in northern Morocco. These splitting patterns define three distinct regions we attempt to predict with our geodynamic models. Here, we test several differently-oriented subduction, slab break-off, and delamination scenarios. Our preliminary results show that density models which include a curved, northeast trending slab predict the east-northeast oriented measurements along the Iberian margin. Imposing a drip structure beneath the Alboran Sea also predicts these orientations. In order to predict the rotation of the FPD we find in Morocco, however, most models require a stiff keel beneath the African craton.

  8. Horizontal mantle flow controls subduction dynamics.

    Science.gov (United States)

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

    2017-08-08

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

  9. P-wave tomography of subduction zones around the central Philippines and its geodynamic implications

    Science.gov (United States)

    Fan, Jianke; Zhao, Dapeng; Dong, Dongdong; Zhang, Guangxu

    2017-09-01

    High-resolution tomographic images are obtained by inverting a large number of arrival-time data of local earthquakes and teleseismic events to depict the 3-D crustal and upper mantle structure beneath the central Philippines. Our tomographic results show that the subducted South China Sea slab beneath the southern segment of the Manila Trench steepens and tears, resulting in migration of the locus of active volcanism in the Macolod Corridor, due to the collision between the Palawan microcontinental block and the Philippine Mobile Belt. The subduction of the Philippine Sea Plate along the Philippine Trench started at 10-12°N or south of 12°N, the central part of the trench, from at least ∼10 Ma estimated from our tomographic images. Our results reveal clearly a high-velocity anomaly in and around the mantle transition zone, which is interpreted as the subducted Proto South China Sea slab that sinks deeper southeastward, being well consistent with geological results that the age of collision between the Palawan microcontinental block and the Philippine Mobile Belt becomes younger from the south to the north. This collision zone can be divided into northern and southern segments, demarcated by the salient point of the collision zone, which is probably the boundary between the South China Sea slab and the Proto South China Sea slab, and may be ascribed to the complete consumption of the two slabs.

  10. On the current-voltage relationship in auroral breakups and westwards-travelling surges

    Directory of Open Access Journals (Sweden)

    A. Olsson

    1996-12-01

    Full Text Available Auroral precipitating electrons pass through an acceleration region before entering the atmosphere. Regardless of what produces it, a parallel electric field is assumed to cause the acceleration. It is well known that from kinetic theory an expression for the corresponding upward field-aligned current can be calculated, which under certain assumptions can be linearized to j∥=KV. The K constant, referred to as the Lyons-Evans-Lundin constant, depends on the source density and thermal energy of the magnetospheric electrons; it is an important parameter in magnetosphere-ionosphere coupling models. However, the K parameter is still rather unknown, and values are found in a wide range of 10–8–10–10 S m–2. In this study, we investigated how the type of auroral structure affects the K values. We look at onset and westwards-travelling surge (WTS events and make comparisons with earlier results from observations of more stable auroral arcs. A new analysis technique for studying those magnetospheric parameters using ground-based measurements is introduced. Electron density measurements are taken with the EISCAT radar, and through an inversion technique the flux-energy spectra are calculated. Source densities, thermal energies and potential drops are estimated from fittings of accelerated Maxwellian distributions. With this radar technique we have the possibility to study the changes of the mentioned parameters during the development of onsets and the passage of surges over EISCAT. The study indicates that the linearization of the full Knight formulation holds even for the very high potential drops and thermal temperatures found in the dynamic onset and WTS events. The values of K are found to be very low, around 10–11 S m–2 in onset cases as well as WTS events. The results may establish a new technique where ionospheric measurements are used for studying the ionosphere-magnetosphere coupling processes.

  11. The lithosphere-asthenosphere boundary beneath the South Island of New Zealand

    Science.gov (United States)

    Hua, Junlin; Fischer, Karen M.; Savage, Martha K.

    2018-02-01

    Lithosphere-asthenosphere boundary (LAB) properties beneath the South Island of New Zealand have been imaged by Sp receiver function common-conversion point stacking. In this transpressional boundary between the Australian and Pacific plates, dextral offset on the Alpine fault and convergence have occurred for the past 20 My, with the Alpine fault now bounded by Australian plate subduction to the south and Pacific plate subduction to the north. Using data from onland seismometers, especially the 29 broadband stations of the New Zealand permanent seismic network (GeoNet), we obtained 24,971 individual receiver functions by extended-time multi-taper deconvolution, and mapped them to three-dimensional space using a Fresnel zone approximation. Pervasive strong positive Sp phases are observed in the LAB depth range indicated by surface wave tomography. These phases are interpreted as conversions from a velocity decrease across the LAB. In the central South Island, the LAB is observed to be deeper and broader to the northwest of the Alpine fault. The deeper LAB to the northwest of the Alpine fault is consistent with models in which oceanic lithosphere attached to the Australian plate was partially subducted, or models in which the Pacific lithosphere has been underthrust northwest past the Alpine fault. Further north, a zone of thin lithosphere with a strong and vertically localized LAB velocity gradient occurs to the northwest of the fault, juxtaposed against a region of anomalously weak LAB conversions to the southeast of the fault. This structure could be explained by lithospheric blocks with contrasting LAB properties that meet beneath the Alpine fault, or by the effects of Pacific plate subduction. The observed variations in LAB properties indicate strong modification of the LAB by the interplay of convergence and strike-slip deformation along and across this transpressional plate boundary.

  12. Velocity structure of the mantle transition zone beneath the southeastern margin of the Tibetan Plateau

    Science.gov (United States)

    Li, Guohui; Bai, Ling; Zhou, Yuanze; Wang, Xiaoran; Cui, Qinghui

    2017-11-01

    P-wave triplications related to the 410 km discontinuity (the 410) were clearly observed from the vertical component seismograms of three intermediate-depth earthquakes that occurred in the Indo-Burma Subduction Zone (IBSZ) and were recorded by the Chinese Digital Seismic Network (CDSN). By matching the observed P-wave triplications with synthetics through a grid search, we obtained the best-fit models for four azimuthal profiles (I-IV from north to south) to constrain the P-wave velocity structure near the 410 beneath the southeastern margin of the Tibetan Plateau (TP). A ubiquitous low-velocity layer (LVL) resides atop the mantle transition zone (MTZ). The LVL is 25 to 40 km thick, with a P-wave velocity decrement ranging from approximately - 5.3% to - 3.6% related to the standard Earth model IASP91. An abrupt transition in the velocity decrement of the LVL was observed between profiles II and III. We postulate that the mantle structure beneath the southeastern margin of the TP is primarily controlled by the southeastern extrusion of the TP to the north combined with the eastward subduction of the Indian plate to the south, but not affected by the Emeishan mantle plume. We attribute the LVL to the partial melting induced by water and/or other volatiles released from the subducted Indian plate and the stagnant Pacific plate, but not from the upwelling or the remnants of the Emeishan mantle plume. A high-velocity anomaly ranging from approximately 1.0% to 1.5% was also detected at a depth of 542 to 600 km, providing additional evidence for the remnants of the subducted Pacific plate within the MTZ.

  13. Tomographic Imaging of the Peru Subduction Zone beneath the Altiplano and Implications for Andean Tectonics

    Science.gov (United States)

    Davis, P. M.; Foote, E. J.; Stubailo, I.; Phillips, K. E.; Clayton, R. W.; Skinner, S.; Audin, L.; Tavera, H.; Dominguez Ramirez, L. A.; Lukac, M. L.

    2010-12-01

    This work describes preliminary tomography results from the Peru Seismic Experiment (PERUSE) a 100 station broadband seismic network installed in Peru. The network consists a linear array of broadband seismic stations that was installed mid-2008 that runs from the Peruvian coast near Mollendo to Lake Titicaca. A second line was added in late 2009 between Lake Titicaca and Cusco. Teleseismic and local earthquake travel time residuals are being combined in the tomographic inversions. The crust under the Andes is found to be 70-80 km thick decreasing to 30 km near the coast. The morphology of the Moho is consistent with the receiver function images (Phillips et al., 2010; this meeting) and also gravity. Ray tracing through the heterogeneous structure is used to locate earthquakes. However the rapid spatial variation in crustal thickness, possibly some of the most rapid in the world, generates shadow zones when using conventional ray tracing for the tomography. We use asymptotic ray theory that approximates effects from finite frequency kernels to model diffracted waves in these regions. The observation of thickened crust suggests that models that attribute the recent acceleration of the Altiplano uplift to crustal delamination are less likely than those that attribute it to crustal compression.

  14. Deformation Patterns and Subduction Behavior of Continental Lithosphere Entering a Trench

    Science.gov (United States)

    Steedman, C. E.; Kaus, B. J.; Becker, T. W.; Okaya, D.

    2007-05-01

    We perform 2-D numerical simulations of continental lithosphere entering a subduction zone, to better understand deformation patterns resulting from subduction of a continental margin. The model consists of a subduction zone in which an attached slab drives subduction of a passive continental margin beneath an oceanic plate. A particle-based 2-D visco-elasto-plastic thermo-mechanical finite element code is employed to study the dynamics of the system. A novel feature of the code is that the resolution of the model can be significantly increased in selected parts of the domain, which allows for self-consistent modelling of mantle-lithosphere interaction. In the present study we employ this feature to study how lithospheric-scale deformation around and within the subduction zone is influenced by surface processes such as erosion, and by flow in the upper mantle. Using systematic 2-D numerical simulations, we explore the parameters that are dominant in controlling near- surface structures, both with regards to changes in topography and trench location, and subsurface features such as Moho undulations. The main parameters that have been varied are: the lithospheric density structure; the lithospheric age and temperature structure; the strength of the lower crust; the presence of a weak zone at the plate interface; the amounts of erosion; the upper boundary condition (free surface versus free slip); rheology (non-Newtonian versus Newtonian, viscous, visco-elasto-plastic); and finally the effect of an imposed slab breakoff. In all cases we track surface uplift, subduction evolution and rock exhumation history. We find that the strength of the overriding plate influences surface uplift and the shape of subsurface deformation, and that the density and thermal structure of the subducting plate affects trench motion. Denser slab roll back, and younger, lighter slabs advance, while neither slab rheology nor the presence of erosion greatly affect trench location. For all cases

  15. Lower slab boundary in the Japan subduction zone

    Science.gov (United States)

    Tonegawa, Takashi; Hirahara, Kazuro; Shibutani, Takuo; Fujii, Naoyuki

    2006-07-01

    We have successfully detected the lower boundary of a subducting slab. The successive imaging of the lower slab boundary beneath northeastern (NE) Japan is attained by receiver function (RF) depth conversion analysis using a recent 3D tomographic velocity model. We use waveforms from 249 teleseismic events collected by Hi-net and J-array short-period stations in NE Japan. RFs are calculated through frequency domain division of radial components by vertical ones with a water level of 0.001 and a 1.0 Hz low-pass Gaussian filter. Assuming that all later phases in the radial RFs are due to Ps phases converted at discontinuities beneath stations, we calculate depth-converted RFs, mapped onto the cross-section with the CCP (common conversion point) stacking. In a cross section, the slab surface and the oceanic Moho can be imaged down to 120 km depth. For the greater depths, the RF amplitudes corresponding to them cannot be seen, because, in the oceanic crust, basalt would be completely metamorphosed to eclogite below this depth. The lower boundary of the Pacific slab can also be traced down to 200 km depth or more. It is parallel to the slab surface and the oceanic Moho, and the thickness between the slab surface and the lower boundary is ˜ 80 km. Finally, we estimate a top-to-bottom slab velocity model that explains the RFs observed at broadband stations with the synthetic RFs. This model exhibits a 13% velocity reduction downwards the lower slab boundary, which would relatively sharp for the base of the thermal boundary layer. Therefore, this sharp discontinuity is presumably considered to be the subducting G (Gutenberg) discontinuity that is formed by the change of the amount of H 2O (water), meaning that the G discontinuity is the chemical boundary at the bottom of the oceanic lithosphere. The G discontinuity depth is controlled by the potential temperature of the asthenospheric mantle beneath the mid-ocean ridge, and hence the observed thickness of 80 km, i.e. the

  16. Small subsidence of the 660-km discontinuity beneath Japan probed by ScS reverberations

    Science.gov (United States)

    Kato, Mamoru; Misawa, Mika; Kawakatsu, Hitoshi

    We investigate layering structure in the mantle beneath Japan using ScS reverberation waveforms of two recent large deep events in the northwest Pacific. We estimate regional variation of the elastic and anelastic structure of the mantle as well as properties of the major velocity discontinuities by modeling broadband seismograms recorded at two dense networks, J-Array and FREESIA. The 660-km discontinuity is the deepest in the region where the stagnant subducting slab in the transition zone is tomographically imaged, but the subsidence is of ∼10 km, much smaller than previous estimates with SS precursors. No significant elevation is detected for the 410-km discontinuity.

  17. On the current-voltage relationship in auroral breakups and westwards-travelling surges

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    A. Olsson

    Full Text Available Auroral precipitating electrons pass through an acceleration region before entering the atmosphere. Regardless of what produces it, a parallel electric field is assumed to cause the acceleration. It is well known that from kinetic theory an expression for the corresponding upward field-aligned current can be calculated, which under certain assumptions can be linearized to j=KV. The K constant, referred to as the Lyons-Evans-Lundin constant, depends on the source density and thermal energy of the magnetospheric electrons; it is an important parameter in magnetosphere-ionosphere coupling models. However, the K parameter is still rather unknown, and values are found in a wide range of 10–8–10–10 S m–2. In this study, we investigated how the type of auroral structure affects the K values. We look at onset and westwards-travelling surge (WTS events and make comparisons with earlier results from observations of more stable auroral arcs. A new analysis technique for studying those magnetospheric parameters using ground-based measurements is introduced. Electron density measurements are taken with the EISCAT radar, and through an inversion technique the flux-energy spectra are calculated. Source densities, thermal energies and potential drops are estimated from fittings of accelerated Maxwellian distributions. With this radar technique we have the possibility to study the changes of the mentioned parameters during the development of onsets and the passage of surges over EISCAT. The study indicates that the linearization of the full Knight formulation holds even for the very high potential drops and thermal temperatures found in the dynamic onset and WTS events. The values of K are found to be very low, around 10–11 S m–2 in onset cases as well as WTS events. The results may establish a new technique where ionospheric

  18. Depth dependence of westward-propagating North Atlantic features diagnosed from altimetry and a numerical 1/6° model

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    R. Tailleux

    2008-03-01

    Full Text Available A 1/6° numerical simulation is used to investigate the vertical structure of westward propagation between 1993 and 2000 in the North Atlantic ocean. The realism of the simulated westward propagating signals, interpreted principally as the signature of first-mode baroclinic Rossby waves (RW, is first assessed by comparing the simulated amplitude and zonal phase speeds of Sea Level Anomalies (SLA against TOPEX/Poseidon-ERS satellite altimeter data. Then, the (unobserved subsurface signature of RW phase speeds is investigated from model outputs by means of the Radon Transform which was specifically adapted to focus on first-mode baroclinic RW. The analysis is performed on observed and simulated SLA and along 9 simulated isopycnal displacements spanning the 0–3250 m depth range. Simulated RW phase speeds agree well with their observed counterparts at the surface, although with a slight slow bias. Below the surface, the simulated phase speeds exhibit a systematic deceleration with increasing depth, by a factor that appears to vary geographically. Thus, while the reduction factor is about 15–18% on average at 3250 m over the region considered, it appears to be much weaker (about 5–8% in the eddy-active Azores Current, where westward propagating structures might be more coherent in the vertical. In the context of linear theories, these results question the often-made normal mode assumption of many WKB-based theories that the phase speed is independent of depth. Alternatively, these results could also suggest that the vertical structure of westward propagating signals may significantly depend on their degree of nonlinearity, with the degree of vertical coherence possibly increasing with the degree of nonlinearity.

  19. Modeling Diverse Pathways to Age Progressive Volcanism in Subduction Zones.

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    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

  20. A westward propagating slab tear model for Late Triassic Qinling Orogenic Belt geodynamic evolution: Insights from the petrogenesis of the Caoping and Shahewan intrusions, central China

    Science.gov (United States)

    Hu, Fangyang; Liu, Shuwen; Zhang, Wanyi; Deng, Zhengbin; Chen, Xu

    2016-10-01

    temperature (TZr = 770-817 °C, comparing with Caoping intrusion of TZr = 727-773 °C), together with the magma mixing and isothermal decompression processes. Integrated with previous regional data, the SQB shows that mantle-derived magmatic activations initiated at 234 Ma near Wudang in the east, and westward systematically decreased to 222-219 Ma near Caoping and to 212-208 Ma near Yangba. We attribute this temporal-spatial distribution of mantle-derived magmatism within the SQB to the progressive tear of the subducted oceanic slab. Combined with sedimentary studies on the foreland basins, paleomagnetic studies as well as numerical and seismic tomography models of slab break-off, we propose that the 225-205 Ma magmatism in the SQB resulted from a westward slab tear following the collision between the NCC and YZC.

  1. Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma

    Science.gov (United States)

    Hollingsworth, James; Jackson, James; Walker, Richard; Nazari, Hamid

    2008-10-01

    We examine active deformation of the region surrounding theeastern South Caspian Sea using a combination of seismic, geological,geodetic, and geomorphological data. Global positioning system(GPS) velocities indicate a westward component of motion ofthe South Caspian basin, relative to Eurasia and central Iran.We identify major zones of shear (the Ashkabad and Shahrud faultzones) that accommodate this westward extrusion on either sideof the South Caspian block. Estimates of total strike-slip motioncould, at present-day slip rates derived from GPS observations,be achieved in ~10 m.y. Therefore, the northwest extrusion ofthe South Caspian region, which is accommodated by subductionbeneath the North Caspian region (stable Eurasia) along theApsheron-Balkan sill, may also date from that time. This suggeststhat the onset of subduction may be older than previous estimates(ranging from 1.8 to 5.5 Ma). Our results are summarized ina new kinematic model that significantly clarifies the activetectonics of this complicated region.

  2. Conjecture with water and rheological control for subducting slab in the mantle transition zone

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    Fumiko Tajima

    2015-01-01

    Full Text Available Seismic observations have shown structural variation near the base of the mantle transition zone (MTZ where subducted cold slabs, as visualized with high seismic speed anomalies (HSSAs, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the “660 km” discontinuity depths and low viscosity layers (LVLs beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a separation of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20–30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ∼660 km. The variation of the discontinuity depths and highly localized low seismic speed anomaly (LSSA zones observed from seismic P waveforms in a relatively high frequency band (∼1 Hz support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen (H+ diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not

  3. Origin and dynamics of depositionary subduction margins

    Science.gov (United States)

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

    2016-01-01

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

  4. Origin and dynamics of depositionary subduction margins

    Science.gov (United States)

    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.

  5. Electrical conductivity imaging in the western Pacific subduction zone

    Science.gov (United States)

    Utada, Hisashi; Baba, Kiyoshi; Shimizu, Hisayoshi

    2010-05-01

    Oceanic plate subduction is an important process for the dynamics and evolution of the Earth's interior, as it is regarded as a typical downward flow of the mantle convection that transports materials from the near surface to the deep mantle. Recent seismological study showed evidence suggesting the transportation of a certain amount of water by subduction of old oceanic plate such as the Pacific plate down to 150-200 km depth into the back arc mantle. However it is not well clarified how deep into the mantle the water can be transported. The electromagnetic induction method to image electrical conductivity distribution is a possible tool to answer this question as it is known to be sensitive to the presence of water. Here we show recent result of observational study from the western Pacific subduction zone to examine the electrical conductivity distribution in the upper mantle and in the mantle transition zone (MTZ), which will provide implications how water distributes in the mantle. We take two kinds of approach for imaging the mantle conductivity, (a) semi-global and (b) regional induction approaches. Result may be summarized as follows: (a) Long (5-30 years) time series records from 8 submarine cables and 13 geomagnetic observatories in the north Pacific region were analyzed and long period magnetotelluric (MT) and geomagnetic deep sounding (GDS) responses were estimated in the period range from 1.7 to 35 days. These frequency dependent response functions were inverted to 3-dimensional conductivity distribution in the depth range between 350 and 850 km. Three major features are suggested in the MTZ depth such as, (1) a high conductivity anomaly beneath the Philippine Sea, (2) a high conductivity anomaly beneath the Hawaiian Islands, and (3) a low conductivity anomaly beneath and in the vicinity of northern Japan. (b) A three-year long deployment of ocean bottom electro-magnetometers (OBEM's) was conducted in the Philippine Sea and west Pacific Ocean from 2005

  6. The Mesozoic accretionary complex in Northeast China: Evidence for the accretion history of Paleo-Pacific subduction

    Science.gov (United States)

    Zhou, Jian-Bo; Li, Long

    2017-09-01

    The Mesozoic accretionary complex in Northeast China, which mainly consists of the Jilin-Heilongjiang high-pressure (HP) metamorphic belt and the Nadanhada accretionary complex, are the key area to understand the Paleo-Pacific subduction-accretion. The Jilin-Heilongjiang HP belt is a HP metamorphic zone between the Jiamusi and Songliao blocks and consists of the Heilongjiang blueschist belt and the Zhangguangcai Complex. Previously published and our new geochronological data indicate that the collision between the Jiamusi and Songliao blocks along the Jilin-Heilongjiang HP belt occurred between 210 and 180 Ma, suggesting that the Jilin-Heilongjiang HP belt is an important unit for characterizing the geodynamic switch from the north-south closure of the Central Asian Orogenic Belt to the onset of westward accretion related to subduction of Paleo-Pacific plate in the Latest Triassic to Early Jurassic. Early Permian igneous rocks with arc affinity in the eastern margin of the Jiamusi Block are more likely related to the Mongol-Okhotsk subduction rather than the Paleo-Pacific subduction or the collision between the Jiamusi and Khanka blocks as previously considered. The Nadanhada accretionary complex lies to the east of the Jiamusi Block, and is composed of the Yuejinshan and Raohe complexes. Compilation of published geochronological data indicate that the Yuejinshan Complex was probably formed between 210 Ma and 180 Ma, similar to ages for the Jilin-Heilongjiang HP belt along the western margin of the Jiamusi-Khanka Block. The Raohe Complex was formed later in the Late Jurassic to Early Cretaceous (170-137 Ma), likely more related to the subduction-accretion of Paleo-Pacific plate. The final accretion in the target area took place in the Early Cretaceous (137-130 Ma).

  7. The ADN project : an integrated seismic monitoring of the northern Ecuadorian subduction

    Science.gov (United States)

    Nocquet, Jean-Mathieu; Yepes, Hugo; Vallee, Martin; Mothes, Patricia; Regnier, Marc; Segovia, Monica; Font, Yvonne; Vaca, Sandro; Bethoux, Nicole; Ramos, Cristina

    2010-05-01

    The subduction of the Nazca plate beneath South America has caused one of the largest megathrust earthquake sequence during the XXth century with three M>7.7 earthquakes that followed the great 1906 (Mw = 8.8) event. Better understanding the processes leading to the occurrence of large subduction earthquakes requires to monitor the ground motion over a large range of frequencies. We present a new network (ADN) developed under a collaboration between the IRD-GeoAzur (Nice, France) and the IG-EPN (Quito, Ecuador). Each station of the ADN network includes a GPS recording at 5 Hz, an accelerometer and a broadband seismometer. CGPS data will quantify the secular deformation induced by elastic locking along the subduction interface, enabling a detailed modelling of the coupling distribution. CGPS will be used to monitor any transient deformation induced by Episodic Slip Event along the subduction, together with broadband seismometers that can detect any tremors or seismic signatures that may accompany them. In case of any significant earthquake, 5 Hz GPS and accelerometer will provide near field data for earthquake source detailed study. Finally, the broadband seismometers will be used for study of the microseismicity and structure of the subduction zone. The network includes 9 stations, operating since 2008 and covering the coastal area from latitude 1.5°S to the Colombian border. In this poster, we will present preliminary assessment of the data, first hypocenters location, magnitude and focal mechanism determination, as well as results about an episodic slip event detected in winter 2008.

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

    Science.gov (United States)

    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.

  9. Westward moving dynamic substorm features observed with the IMAGE magnetometer network and other ground-based instruments

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    H. Lühr

    1998-04-01

    Full Text Available We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic field measurements, there were auroral observations and plasma flow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward field-aligned currents associated with the moving auroral patches. A detailed analysis of the electrodynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with field-aligned current filaments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma flow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction effects in the ionosphere may play a role for the orientation of the plasma flows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.Keywords. Ionosphere (Auroral ionosphere; Electric fields and currents; Ionosphere-magnetosphere interactions.

  10. Westward moving dynamic substorm features observed with the IMAGE magnetometer network and other ground-based instruments

    Directory of Open Access Journals (Sweden)

    H. Lühr

    Full Text Available We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic field measurements, there were auroral observations and plasma flow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward field-aligned currents associated with the moving auroral patches. A detailed analysis of the electrodynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with field-aligned current filaments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma flow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction effects in the ionosphere may play a role for the orientation of the plasma flows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.

    Keywords. Ionosphere (Auroral ionosphere; Electric fields and currents; Ionosphere-magnetosphere interactions.

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

    Science.gov (United States)

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

    2017-12-01

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

  12. Dynamics of intraoceanic subduction initiation: 2D thermomechanical modeling

    Science.gov (United States)

    Zhou, X.; Gerya, T.; LI, Z.; Stern, R. J.

    2016-12-01

    Intraoceanic subduction initiation occurs in previous weak zones which could be transform faults or old fracture zones, and concurrents with the change of plate motions. It is an important process to understand the beginning of plate tectonics. However, the dynamic process during (after) subduction initiation remain obscure. The process of suducting slabs move from down to downdip is also not revealed clearly. In order to obtain better understanding of the transitional process of subducting slab motion, we use finite difference and marker-in-cell methods to establish a series of self-sustainable subduction initiation models and explore many visco-plastic parameters to qualify the dynamical process of subduction initiation. The following parameters are systematic tested: (1) the age of the subducting slab; (2) friction coefficient of the mantle material; (3) the mantle potential temperature; (4) the age of the overriding slab. We find out the critical age of the oceanic lithosphere which can produce subduction initiation. And the age of subducting slab plays important roles during subduction initiation. The young subducting slab induces fast trench retreat and then trench begin to advance. For the old subducting slab, it induces relative slower trench retreat and then stop moving. The age of overriding slabs impacts coupling with the subducting slab. The friction coefficient of lithosphere also impacts the backarc spreading and subduction velocity. Stronger subducted plate gives lower subduction velocity and faster trench retreat velocity. The mantle potential temperature changes the critical age of subducted slabs.

  13. Seismic tomography reveals magma chamber location beneath Uturuncu volcano (Bolivia)

    Science.gov (United States)

    Kukarina, Ekaterina; West, Michael; Koulakov, Ivan

    2014-05-01

    Uturuncu volcano belongs to the Altiplano-Puna Volcanic Complex in the central Andes, the product of an ignimbrite ''flare-up''. The region has been the site of large-scale silicic magmatism since 10 Ma, producing 10 major eruptive calderas and edifices, some of which are multiple-eruption resurgent complexes as large as the Yellowstone or Long Valley caldera. Satellite measurements show that the hill has been rising more than half an inch a year for almost 20 years, suggesting that the Uturuncu volcano, which has erupted last time more than 300,000 years ago, is steadily inflating, which makes it fertile ground for study. In 2009 an international multidisciplinary team formed a project called PLUTONS to study Uturuncu. Under this project a 100 km wide seismic network was set around the volcano by seismologists from University of Alaska Fairbanks. Local seismicity is well distributed and provides constraints on the shallow crust. Ray paths from earthquakes in the subducting slab complement this with steep ray paths that sample the deeper crust. Together the shallow and deep earthquakes provide strong 3D coverage of Uturuncu and the surrounding region. To study the deformation source beneath the volcano we performed simultaneous tomographic inversion for the Vp and Vs anomalies and source locations, using the non-linear passive source tomographic code, LOTOS. We estimated both P and S wave velocity structures beneath the entire Uturuncu volcano by using arrival times of P and S waves from more than 600 events registered by 33 stations. To show the reliability of the results, we performed a number of different tests, including checkerboard synthetic tests and tests with odd/even data. Obtained Vp/Vs ratio distribution shows increased values beneath the south Uturuncu, at a depth of about 15 km. We suggest the high ratio anomaly is caused by partial melt, presented in expanding magma chamber, responsible for the volcano inflation. The resulting Vp, Vs and the ratio

  14. Lower Crustal and Moho Reflections Beneath Mount St. Helens

    Science.gov (United States)

    Levander, A.; Kiser, E.; Zelt, C. A.; Creager, K.; Ulberg, C. W.; Schmandt, B.; Hansen, S. M.; Abers, G. A.

    2016-12-01

    The multi-disciplinary project iMUSH (imaging Magma Under St. Helens) was designed to illuminate the magmatic system beneath Mount St Helens (MSH) from the subducting Juan de Fuca slab to the surface using seismic, magnetotelluric, and petrologic data. The iMUSH active source experiment consisted of 23 large shots and 6000 seismograph locations. Included in the active-source seismic experiment were 2 dense linear profiles striking NW-SE and NE-SW, each with over 1000 receivers ( 150 m spacing) and 8 shots. Using the 1D average velocity model around MSH determined from travel-time analysis (Kiser et al., 2016, Geology), we have common-midpoint stacked STA/LTA envelope functions from all of the data along the NW-SE profile. A number of bright reflection events in the CMP section show remarkably good correspondence with abrupt velocity changes that were imaged in the 2D travel-time analysis in the mid to lower crust and at the Moho: Reflections appear at 20-25 km depth at the tops of two lower crustal high velocity (Vp > 7.5 km/s) bodies. One of these high velocity bodies is directly beneath MSH. The other is 40 km SE of MSH, under the Indian Heaven volcanic field, a basaltic field last active 9 ka. We have interpreted the high velocity bodies as cumulates from Quaternary or Tertiary volcanism. Separating the two high Vp bodies is a lower velocity column (Vp ≤ 6.5 km/s) dipping to the SE from the midcrust to the Moho. In the CMP section, the Moho reflection is bright under the region of low velocity and dims beneath both of the high velocity lower crustal bodies. Seismicity associated with the 1980 eruption extended from the summit to 20 km depth, stopping just above the bright reflection at the top of the MSH high Vp body. Deep long period events under MSH, often associated with motion of magmatic fluids, cluster at 20-30 km depth along the southeastern edge of the same reflection. This leads us to suggest that lower crustal magmas migrate along the southeastern

  15. Slow slip rate and excitation efficiency of deep low-frequency tremors beneath southwest Japan

    Science.gov (United States)

    Daiku, Kumiko; Hiramatsu, Yoshihiro; Matsuzawa, Takanori; Mizukami, Tomoyuki

    2018-01-01

    We estimated the long-term average slip rate on the plate interface across the Nankai subduction zone during 2002-2013 using deep low-frequency tremors as a proxy for short-term slow slip events based on empirical relations between the seismic moment of short-term slow slip events and tremor activities. The slip rate in each region is likely to compensate for differences between the convergence rate and the slip deficit rate of the subducting Philippine Sea plate estimated geodetically, although the uncertainty is large. This implies that the strain because of the subduction of the plate is partially stored as the slip deficit and partially released by slow slip events during the interseismic period. The excitation efficiency of the tremors for the slow slip events differs among regions: it is high in the northern Kii region. Some events in the western Shikoku region show a somewhat large value. Antigorite serpentinite of two types exists in the mantle wedge beneath southwest Japan. Slips with more effective excitation of tremors presumably occur in high-temperature conditions in the antigorite + olivine stability field. Other slip events with low excitation efficiency are distributed in the antigorite + brucite stability field. Considering the formation reactions of these minerals and their characteristic structures, events with high excitation efficiency can be correlated with a high pore fluid pressure condition. This result suggests that variation in pore fluid pressure on the plate interface affects the magnitude of tremors excited by slow slip events.

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

    Science.gov (United States)

    Zuo, Xuran; Chan, Lung Sang; Gao, Jian-Feng

    2017-01-01

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

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

    Science.gov (United States)

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

    2016-12-01

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

  18. P-wave tomography of Northeast Asia: Constraints on the western Pacific plate subduction and mantle dynamics

    Science.gov (United States)

    Ma, Jincheng; Tian, You; Liu, Cai; Zhao, Dapeng; Feng, Xuan; Zhu, Hongxiang

    2018-01-01

    A high-resolution model of 3-D P-wave velocity structure beneath Northeast Asia and adjacent regions is determined by using 244,180 arrival times of 14,163 local and regional earthquakes and 319,857 relative travel-time residuals of 9988 teleseismic events recorded at ∼2100 seismic stations in the study region. Our tomographic results reveal the subducting Pacific slab clearly as a prominent high-velocity anomaly from the Japan Trench to the North-South Gravity lineament (NSGL) in East China. The NSGL is roughly coincident with the western edge of the stagnant Pacific slab in the mantle transition zone (MTZ). The subducting Pacific slab has partly sunk into the lower mantle beneath Northeast China, but under the Sino-Korean Craton the slab lies horizontally in the MTZ. The NSGL, as an important tectonic line in Mainland China, is marked by sharp differences in the surface topography, gravity anomaly, crustal and lithospheric thickness and mantle seismic velocity from the east to the west. These features of the NSGL and large-scale hot and wet upwelling in the big mantle wedge (BMW) in the east of the NSGL are all related to the subduction processes of the Western Pacific plate. The Changbai intraplate volcanic group is underlain by a striking low-velocity anomaly from the upper MTZ and the BMW up to the surface, and deep earthquakes (410-650 km depths) occur actively in the subducting Pacific slab to the east of the Changbai volcano. We propose that the Changbai volcanic group is caused by upwelling of hot and wet asthenospheric materials and active convection in the BMW. The formation of other volcanic groups in the east of the NSGL is also associated with the subduction-driven corner flow in the BMW.

  19. A subduction zone reference frame based on slab geometry and subduction partitioning of plate motion and trench migration

    NARCIS (Netherlands)

    Schellart, W. P.

    2011-01-01

    The geometry of subducted slabs that interact with the transition zone depends critically on the partitioning of the subduction velocity (v S⊥) at the surface into its subducting plate motion component (vSP⊥) and trench migration component (vT⊥). Geodynamic models of progressive subduction

  20. The earthquake cycle in subduction zones

    Science.gov (United States)

    Melosh, H. J.; Fleitout, L.

    1982-01-01

    A simplified model of a subduction zone is presented, which incorporates the mechanical asymmetry induced by the subducted slab to anchor the subducting plate during post-seismic rebound and thus throw most of the coseismic stream release into the overthrust plate. The model predicts that the trench moves with respect to the deep mantle toward the subducting plate at a velocity equal to one-half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, and although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back-arc region.

  1. Seismic coupling and uncoupling at subduction zones

    Science.gov (United States)

    Ruff, L.; Kanamori, H.

    1983-01-01

    Some of the correlations concerning the properties of subduction zones are reviewed. A quantitative global comparison of many subduction zones reveals that the largest earthquakes occur in zones with young lithosphere and fast convergence rates. Maximum earthquake size is directly related to the asperity distribution on the fault plane. This observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. Plate age and rate can control asperity distribution directly through the horizontal compressive stress associated with the vertical and horizontal velocities of subducting slabs. The basalt to eclogite phase change in the down-going oceanic crust may be largely responsible for the uncoupling of subduction zones below a depth of about 40 km.

  2. A numerical reference model for themomechanical subduction

    DEFF Research Database (Denmark)

    Quinquis, Matthieu; Chemia, Zurab; Tosi, Nicola

    2010-01-01

    response to systematic variations in input parameters, numerical studies often start from a 'reference' subduction model. However, the reference model often varies between different numerical studies, making it difficult to compare results directly. We aim therefore to define a numerical reference model......, and initial temperature distribution. We will show results of the evolution and dynamics of the subduction reference model using different numerical codes: a finite element code, SULEC, and two finite difference codes, YACC and FDcon....

  3. Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles

    Science.gov (United States)

    Paulatto, Michele; Laigle, Mireille; Galve, Audrey; Charvis, Philippe; Sapin, Martine; Bayrakci, Gaye; Evain, Mikael; Kopp, Heidrun

    2017-01-01

    Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60–100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120–160 km depth suggests that the slab’s mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics. PMID:28691714

  4. Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles

    Science.gov (United States)

    Paulatto, Michele; Laigle, Mireille; Galve, Audrey; Charvis, Philippe; Sapin, Martine; Bayrakci, Gaye; Evain, Mikael; Kopp, Heidrun

    2017-07-01

    Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60-100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120-160 km depth suggests that the slab's mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics.

  5. Receiver function imaging of upper mantle complexity beneath the Pacific Northwest, United States

    Science.gov (United States)

    Eagar, Kevin C.; Fouch, Matthew J.; James, David E.

    2010-08-01

    Small-scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions. Subduction of the Juan de Fuca plate and other tectonic processes dominating the Pacific Northwest, United States in the Cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3-D temperature variations. We address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km. We utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations, primarily those of the Earthscope/USArray Transportable Array. We find the average depths of the discontinuities to be 412 km and 658 km, respectively, with no obvious 520 km discontinuity detected. The peak-to-peak range is greater on the '410' than the '660', suggesting the possibility of more significant regional dynamic processes at upper mantle depths. Our results are not consistent with a mantle plume below central Oregon in the High Lava Plains region. Our observation of a thinner transition zone beneath the western Snake River Plain region, however, is consistent with a regional increase in mantle temperatures, perhaps due to either asthenospheric flow from beneath and around the southern edge of the Juan de Fuca plate, or to vertical flow in the form of regional mantle upwelling related to the Snake River Plain / Yellowstone hotspot track. Further, our results are not consistent with a simple subducting Juan de Fuca slab morphology, but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies. We find evidence for a thickened and therefore cooler mantle transition zone beneath the Wallowa / Idaho Batholith region, consistent with tomographic models which suggest

  6. On the lifetime and extent of an auroral westward flow channel (AWFC observed during a magnetospheric substorm

    Directory of Open Access Journals (Sweden)

    M. L. Parkinson

    2003-04-01

    Full Text Available A -190-nT negative bay in the geomagnetic X component measured at Macquarie Island ( -65° L showed that an ionospheric substorm occurred during 09:58 to 11:10 UT on 27 February 2000. Signatures of an auroral westward flow channel (AWFC were observed nearly simultaneously in the backscatter power, LOS Doppler velocity, and Doppler spectral width measured using the Tasman International Geospace Environment Radar (TIGER, a Southern Hemisphere HF SuperDARN radar. Many of the characteristics of the AWFC were similar to those occurring during a polarisation jet (PJ, or subauroral ion drift (SAID event, and suggest that it may have been a pre-cursor to a fully developed, intense westward flow channel satisfying all of the criteria defining a PJ/SAID. A beam-swinging analysis showed that the westward drifts (poleward electric field associated with the flow channel were very structured in time and space, but the smoothed velocities grew to ~ 800 ms-1 (47 mVm-1 during the 22-min substorm onset interval 09:56 to 10:18 UT. Maximum west-ward drifts of >1.3 km s-1 (>77 mVm-1 occurred during a ~ 5-min velocity spike, peaking at 10:40 UT during the expansion phase. The drifts decayed rapidly to ~ 300 ms-1 (18 mVm-1 during the 6-min recovery phase interval, 11:04 to 11:10 UT. Overall, the AWFC had a lifetime of 74 min, and was located near -65° L in the evening sector west of the Harang discontinuity. The large westward drifts were confined to a geographic zonal channel of longitudinal ex-tent >20° (>1.3 h magnetic local time, and latitudinal width ~2° L. Using a half-width of ~ 100 km in latitude, the peak electric potential was >7.7 kV. However, a transient velocity of >3.1 km s-1 with potential >18.4 kV was observed further poleward at the end of the recovery phase. Auroral oval boundaries determined using DMSP measurements suggest the main flow channel overlapped the equatorward boundary of the diffuse auroral oval. During the ~ 2-h interval following

  7. On the lifetime and extent of an auroral westward flow channel (AWFC observed during a magnetospheric substorm

    Directory of Open Access Journals (Sweden)

    M. L. Parkinson

    Full Text Available A -190-nT negative bay in the geomagnetic X component measured at Macquarie Island ( -65° L showed that an ionospheric substorm occurred during 09:58 to 11:10 UT on 27 February 2000. Signatures of an auroral westward flow channel (AWFC were observed nearly simultaneously in the backscatter power, LOS Doppler velocity, and Doppler spectral width measured using the Tasman International Geospace Environment Radar (TIGER, a Southern Hemisphere HF SuperDARN radar. Many of the characteristics of the AWFC were similar to those occurring during a polarisation jet (PJ, or subauroral ion drift (SAID event, and suggest that it may have been a pre-cursor to a fully developed, intense westward flow channel satisfying all of the criteria defining a PJ/SAID. A beam-swinging analysis showed that the westward drifts (poleward electric field associated with the flow channel were very structured in time and space, but the smoothed velocities grew to ~ 800 ms-1 (47 mVm-1 during the 22-min substorm onset interval 09:56 to 10:18 UT. Maximum west-ward drifts of >1.3 km s-1 (>77 mVm-1 occurred during a ~ 5-min velocity spike, peaking at 10:40 UT during the expansion phase. The drifts decayed rapidly to ~ 300 ms-1 (18 mVm-1 during the 6-min recovery phase interval, 11:04 to 11:10 UT. Overall, the AWFC had a lifetime of 74 min, and was located near -65° L in the evening sector west of the Harang discontinuity. The large westward drifts were confined to a geographic zonal channel of longitudinal ex-tent >20° (>1.3 h magnetic local time, and latitudinal width ~2° L. Using a half-width of ~ 100 km in latitude, the peak electric potential was >7.7 kV. However, a transient velocity of >3.1 km s-1 with potential >18.4 kV was observed further poleward at the end of the recovery phase. Auroral oval boundaries determined

  8. Thermal implications of the cessation of subduction in the Sierra Nevada and Baja- California arcs

    Science.gov (United States)

    Erkan, K.; Blackwell, D. D.

    2006-12-01

    The thermal regime in the extinct Sierra Nevada arc has undergone substantial transformation as a result of the cessation of subduction in the last 30 My. The dynamic mechanism of cooling in the arc has been replaced by re-equilibration of the lithosphere toward continental averages. Preliminary 1D thermal models reveal that the effect of asthenospheric heating at the bottom do not lead to credible changes in the lithospheric temperatures for a 30 My years period in terms of surface manifestations. In the Great Valley, neither the topography nor the surface heat flow show considerable variations before and after the cessation of subduction. On the other hand, Sierra Nevada has experienced substantial uplift in the post subduction California. In the Sierra Nevada, the surface heat flow lags deep temperature response but the topography responds directly to temperatures at depth. As the end of subduction migrated north with the Mendocino triple junction, the dynamic equilibrium between the cold Sierra Nevada and Basin and Range has been upset and the high temperatures of Basin and Range started to invade the Sierra Nevada lithosphere. Our 2D thermal model reveals that conductive heating of the Sierran lithosphere by the hot Basin and Range lithosphere could be the dominant source for the tilted uplift in the Sierra Nevada. The heating from the bottom is likely not very effective in Sierra Nevada as no uplift is observed in the neighboring Great Valley region. The thermal uplift due to Basin and Range heating is substantial at the east edge of the Sierra Nevada and decreases rapidly toward the west. The uplift as far as 100 km toward west could be accommodated by flexural bending of the cold Sierra Nevada lithosphere as the eastern edge thermally expanded. A similar thermo-tectonic scenario could explain the westward tilted Baja-California peninsular ranges which were part of the same tectonic setting of the Sierra Nevada during subduction. We also investigated the

  9. Sulphide-sulphate stability and melting in subducted sediment and its role in arc mantle redox and chalcophile cycling in space and time

    Science.gov (United States)

    Canil, Dante; Fellows, Steven A.

    2017-07-01

    The redox budget during subduction is tied to the evolution of oxygen and biogeochemical cycles on Earth's surface over time. The sulphide-sulphate couple in subducted crust has significant potential for redox and control on extraction of chalcophile metals from the arc mantle. We derive oxygen buffers for sulphide-sulphate stability ('SSO buffers') using mineral assemblages in subducted crust within the eclogite facies, and examine their disposition relative to the fO2 in the arc mantle along various P-T trajectories for subduction. The fO2 required for sulphide stability in subducted crust passing beneath an arc is shifted by variations in the bulk Ca/(Ca + Mg + Fe) of the subducting crust alone. Hotter slabs and more Fe-rich sediments stabilize sulphide and favour chalcophile sequestration deep into the mantle, whereas colder slabs and calcic sediment will stabilize anhydrite, in some cases at depths of melt generation in the arc mantle (earth history. Oxidation of arc mantle and the proliferation of porphyry Cu deposits may be latter-day advents in earth history partly due to the rise of planktic calcifiers in the oceans in only the past 250 million years.

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

    Science.gov (United States)

    Chaves, Esteban J; Schwartz, Susan Y

    2016-01-01

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

  11. Preliminary result of P-wave speed tomography beneath North Sumatera region

    Energy Technology Data Exchange (ETDEWEB)

    Jatnika, Jajat [Earth Science Study Program, Institute of Technology Bandung (Indonesia); Indonesian Meteorological, Climatological and Geophysical Agency (MCGA), Jakarta (Indonesia); Nugraha, Andri Dian, E-mail: nugraha@gf.itb.ac.id [Global Geophysical Research Group, Faculty of Mining and Petroleum Engineering, Insitute of Technology Bandung (Indonesia); Wandono [Indonesian Meteorological, Climatological and Geophysical Agency (MCGA), Jakarta (Indonesia)

    2015-04-24

    The structure of P-wave speed beneath the North Sumatra region was determined using P-wave arrival times compiled by MCGA from time periods of January 2009 to December 2012 combining with PASSCAL data for February to May 1995. In total, there are 2,246 local earthquake events with 10,666 P-wave phases from 63 stations seismic around the study area. Ray tracing to estimate travel time from source to receiver in this study by applying pseudo-bending method while the damped LSQR method was used for the tomographic inversion. Based on assessment of ray coverage, earthquakes and stations distribution, horizontal grid nodes was set up of 30×30 km2 for inside the study area and 80×80 km2 for outside the study area. The tomographic inversion results show low Vp anomaly beneath Toba caldera complex region and around the Sumatra Fault Zones (SFZ). These features are consistent with previous study. The low Vp anomaly beneath Toba caldera complex are observed around Mt. Pusuk Bukit at depths of 5 km down to 100 km. The interpretation is these anomalies may be associated with ascending hot materials from subduction processes at depths of 80 km down to 100 km. The obtained Vp structure from local tomography will give valuable information to enhance understanding of tectonic and volcanic in this study area.

  12. Tomography of the upper mantle beneath the African/Iberian collision zone

    Science.gov (United States)

    Bonnin, Mickael; Nolet, Guust; Thomas, Christine; Villaseñor, Antonio; Gallart, Josep; Levander, Alan

    2013-04-01

    In this study we take advantage of the dense broadband-station networks available in western Mediterranean region (IberArray, PICASSO and MOROCCO-MUENSTER networks) to develop a high-resolution 3D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone. This model is based on teleseismic arrival times recorded between 2008 and 2012 for which cross-correlation delays are measured with a new technique in different frequency bands centered between 0.03 and 1.0 Hz, and interpreted using multiple frequency tomography. Such a tomography is required to scrutinize the nature and extent of the thermal anomalies inferred beneath Northern Africa, especially in the Atlas ranges region and associated to sparse volcanic activities. Tomography is notably needed to help in determining the hypothetical connection between those hot anomalies and the Canary Island hotspot as proposed by geochemistry studies. It also provides new insights on the geometry of the subducting slab previously inferred from tomography, GPS measurements or shear-wave splitting patterns beneath the Alboran Sea and the Betic ranges and is indispensable for deciphering the complex geodynamic history of the Western Mediterranean region. We shall present the overall statistics of the delays, their geographical distribution, as well as the first inversion results.

  13. Kinematics of subduction and subduction-induced flow in the upper mantle

    NARCIS (Netherlands)

    Schellart, W. P.

    2004-01-01

    Results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle. The experiments model a dense highviscosity plate (subducting lithosphere) overlying a less dense low-viscosity layer (upper mantle). The overriding lithosphere is not

  14. Plume-subduction interaction forms large auriferous provinces.

    Science.gov (United States)

    Tassara, Santiago; González-Jiménez, José M; Reich, Martin; Schilling, Manuel E; Morata, Diego; Begg, Graham; Saunders, Edward; Griffin, William L; O'Reilly, Suzanne Y; Grégoire, Michel; Barra, Fernando; Corgne, Alexandre

    2017-10-10

    Gold enrichment at the crustal or mantle source has been proposed as a key ingredient in the production of giant gold deposits and districts. However, the lithospheric-scale processes controlling gold endowment in a given metallogenic province remain unclear. Here we provide the first direct evidence of native gold in the mantle beneath the Deseado Massif in Patagonia that links an enriched mantle source to the occurrence of a large auriferous province in the overlying crust. A precursor stage of mantle refertilisation by plume-derived melts generated a gold-rich mantle source during the Early Jurassic. The interplay of this enriched mantle domain and subduction-related fluids released during the Middle-Late Jurassic resulted in optimal conditions to produce the ore-forming magmas that generated the gold deposits. Our study highlights that refertilisation of the subcontinental lithospheric mantle is a key factor in forming large metallogenic provinces in the Earth's crust, thus providing an alternative view to current crust-related enrichment models.The lithospheric controls on giant gold deposits remain unclear. Here, the authors show evidence for native gold in the mantle from the Deseado Massif in Patagonia demonstrating that refertilisation of the lithospheric mantle is key in forming metallogenic provinces.

  15. 3-D imaging of two episodes of Hikurangi Plateau subduction in the southern South Island of New Zealand

    Science.gov (United States)

    Eberhart-Phillips, D. M.; Reyners, M.; Upton, P.; Gubbins, D.

    2016-12-01

    The Hikurangi Plateau (originally part of the Ontong Java large igneous province) has been subducted beneath New Zealand twice - firstly at ca. 108-105 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. We have investigated the southern limit of this subducted plateau by supplementing the sparse GeoNet permanent seismograph network in the southeastern South Island with a forty-station broadband portable seismograph network. We have then used local earthquake tomography to determine detailed 3-D Vp and Vp/Vs structure in the southern South Island. We track a region of Vp 8.5 km/s (which has previously been associated with an eclogite layer at the base of the Hikurangi Plateau from studies in the North Island) across most of the southwestern South Island. Its southeastern edge runs diagonally from near Christchurch to northern Fiordland. It dips both to the northwest and the southwest, and impacts the subducted Australian plate in northern Fiordland, where it currently bends the subducted Australian slab to vertical. The plateau and its leading oceanic crust are distinguished by low Vp/Vs, consistent with extensive dehydration of the thick (ca. 35 km), buoyant plateau during ca. 500 km of flat subduction at the Gondwana margin. The plateau is also revealed by dipping zones of relocated earthquakes. The backstop of Gondwana subduction appears to be the ophiolitic Maitai terrane, which extends through the crust and forms the trenchward boundary of the ca. 60-65 km-thick Median Batholith. We image the low Vp crustal root associated with orogeny at the Gondwana margin in the southeastern South Island, as well as the crustal root resulting from the current convergent episode in the western South Island. The shapes of both crustal roots are controlled by the Hikurangi Plateau.

  16. Intrinsic and Extrinsic Factors in Subduction Dynamics

    Science.gov (United States)

    Billen, Magali; Arredondo, Katrina

    2014-05-01

    Since the realization that tectonic plates sink into the mantle, in a process we now call subduction, our understanding of this process has improved dramatically through the combined application of observations, theory and modeling. During that time independent research groups focusing on different aspects of subduction have identified factors with a significant impact on subduction, such as three-dimensionality, slab rollback, rheology of the slab and mantle and magnitude of phase changes. However, as each group makes progress we often wonder how these different factors interact as we all strive to understand the real world subduction system. These factors can be divided in two groups: intrinsic factors, including the age of the slab, its thermal structure, composition, and rheology, and extrinsic factors including others forces on plates, overall mantle flow, structure of the overriding plate, rheology of the mantle and phase changes. In addition, while modeling has been a powerful tool for understanding subduction, all models make important (but often necessary) approximations, such as using two dimensions, imposed boundary conditions, and approximations of the conservation equations and material properties. Here we present results of a study in which the "training wheels" are systematically removed from 2D models of subduction to build a more realistic model of subduction and to better understand how combined effects of intrinsic and extrinsic factors contribute to the dynamics. We find that a change from the Boussinesq to the extended Boussinesq form of the conservation equations has a dramatic effect on slab evolution in particular when phase changes are included. Allowing for free (dynamically-driven) subduction and trench motion is numerically challenging, but also an important factor that allows for more direct comparison to observations of plate kinematics. Finally, compositional layering of the slab and compositionally-controlled phase changes also have

  17. Deep structure and historical earthquakes in the Calabrian subduction zone (Southern Italy): preliminary results from multi-channel seismic reflection profiles

    Science.gov (United States)

    Gallais, F.; Gutscher, M.; Torelli, L.; Polonia, A.; Riminucci, F.

    2009-12-01

    The Calabrian subduction zone is located in the complex Central Mediterranean area. This subduction is characterized by the presence of deep earthquakes under the Tyrrhenian Sea down to 500 km depth. The Tethyan remnant Ionian slab descends towards the NW at a dip of about 70° and is associated with an active volcanic arc (the Aeolian Islands). Recently reported GPS and seismicity studies suggest that the subduction of the Ionian lithosphere beneath the Calabrian Arc may be locally still active, though at very slow rates (thrust earthquakes, characteristic of active subduction zone, suggests that if subduction is active, the fault plane may be locked since the instrumental period. To seek evidence of continuous tectonic activity of the Calabrian system, we present preliminary results from reprocessed 96-channels seismic reflection profiles (French Archimede cruise, 1997) offshore Sicily. This analysis permits to recognize a well-defined stratigraphy in the Ionian Abyssal Plain, this stratigraphy becomes difficult to follow under the deformed Calabrian Prism. But the joint interpretation with the reprocessed PM01 profile (French PRISMED cruise, 1994) helps constrain this interpretation and to image some characteristic structures of an accretionary wedge (fore/back-thrusts, basal decollement...). This study also include interpretation of a more recent Italian seismic cruise (Calamare, 2008) and CROP profiles. This work will help to prepare a future cruise proposal (CIRCEE, to be submitted in January 2010) to study the Calabrian subduction with OBS, MCS seismic, heat-flow measurements and sediment coring. The goals are : 1/ to image the deep structure of this subduction zone, 2/ to characterize its thermal state to deduce a geometry of the seismogenic part of the plate interface and add new constraints on seismic risk linked with the Calabrian subduction.

  18. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods

    Science.gov (United States)

    Wu, Jonny; Suppe, John; Lu, Renqi; Kanda, Ravi

    2016-06-01

    We reconstructed Philippine Sea and East Asian plate tectonics since 52 Ma from 28 slabs mapped in 3-D from global tomography, with a subducted area of ~25% of present-day global oceanic lithosphere. Slab constraints include subducted parts of existing Pacific, Indian, and Philippine Sea oceans, plus wholly subducted proto-South China Sea and newly discovered "East Asian Sea." Mapped slabs were unfolded and restored to the Earth surface using three methodologies and input to globally consistent plate reconstructions. Important constraints include the following: (1) the Ryukyu slab is ~1000 km N-S, too short to account for ~20° Philippine Sea northward motion from paleolatitudes; (2) the Marianas-Pacific subduction zone was at its present location (±200 km) since 48 ± 10 Ma based on a >1000 km deep slab wall; (3) the 8000 × 2500 km East Asian Sea existed between the Pacific and Indian Oceans at 52 Ma based on lower mantle flat slabs; (4) the Caroline back-arc basin moved with the Pacific, based on the overlapping, coeval Caroline hot spot track. These new constraints allow two classes of Philippine Sea plate models, which we compared to paleomagnetic and geologic data. Our preferred model involves Philippine Sea nucleation above the Manus plume (0°/150°E) near the Pacific-East Asian Sea plate boundary. Large Philippine Sea westward motion and post-40 Ma maximum 80° clockwise rotation accompanied late Eocene-Oligocene collision with the Caroline/Pacific plate. The Philippine Sea moved northward post-25 Ma over the northern East Asian Sea, forming a northern Philippine Sea arc that collided with the SW Japan-Ryukyu margin in the Miocene (~20-14 Ma).

  19. Seismic evidence of the lithosphere-asthenosphere boundary beneath Izu-Bonin area

    Science.gov (United States)

    Cui, H.; Gao, Y.; Zhou, Y.

    2016-12-01

    The lithosphere-asthenosphere boundary (LAB), separating the rigid lithosphere and the ductile asthenosphere layers, is the seismic discontinuity with the negative velocity contrast of the Earth's interior [Fischer et al., 2010]. The LAB has been also termed the Gutenberg (G) discontinuity that defines the top of the low velocity zone in the upper mantle [Gutenberg, 1959; Revenaugh and Jordan, 1991]. The seismic velocity, viscosity, resistivity and other physical parameters change rapidly with the depths across the boundary [Eaton et al., 2009]. Seismic detections on the LAB in subduction zone regions are of great help to understand the interactions between the lithosphere and asthenosphere layers and the geodynamic processes related with the slab subductions. In this study, the vertical broadband waveforms are collected from three deep earthquake events occurring from 2000 to 2014 with the focal depths of 400 600 km beneath the Izu-Bonin area. The waveform data is processed with the linear slant stack method [Zang and Zhou, 2002] to obtain the vespagrams in the relative travel-time to slowness domain and the stacked waveforms. The sP precursors reflected on the LAB (sLABP), which have the negative polarities with the amplitude ratios of 0.17 0.21 relative to the sP phases, are successfully extracted. Based on the one-dimensional modified velocity model (IASP91-IB), we obtain the distributions for six reflected points of the sLABP phases near the source region. Our results reveal that the LAB depths range between 58 and 65 km beneath the Izu-Bonin Arc, with the average depth of 62 km and the small topography of 7 km. Compared with the results of the tectonic stable areas in Philippine Sea [Kawakatsu et al., 2009; Kumar and Kawakatsu, 2011], the oceanic lithosphere beneath the Izu-Bonin Arc shows the obvious thinning phenomena. We infer that the lithospheric thinning is closely related with the partial melting, which is caused by the volatiles continuously released

  20. Pressure generation beneath a new thermoplastic cast.

    Science.gov (United States)

    Mohler, L R; Pedowitz, R A; Byrne, T P; Gershuni, D H

    1996-01-01

    A new cast immobilizer that is heat-shrunk to conform to an injured extremity was examined. The purpose of these studies was to compare pressures beneath the thermoplastic cast with those beneath fiberglass casts on a laboratory model and on the forearms of human volunteers. Pressures measured beneath fiberglass casts on metal cylinders averaged 36 mm Hg. Thermoplastic casts on the smaller cylinder that allowed 42% shrinkage produced a mean pressure of 25 mm Hg; those placed on the larger cylinder that allowed 17% shrinkage produced a mean pressure of 39 mm Hg. Pressures measured on the forearms of healthy volunteers averaged 22 mm Hg beneath fiberglass casts and 31 mm Hg beneath the thermoplastic casts. These pressures were considerably less than pressures that have been shown to occlude the microcirculation of the skin. Acute compartment syndromes result from swelling within a limited space and remain a serious concern clinically when swelling is anticipated under any type of constraining cast. The results of these studies indicate that the new cast should not produce a greater risk of circulatory compromise to the limb than previously used fiberglass materials.

  1. Gas Hydrate Formation Amid Submarine Canyon Incision: Investigations From New Zealand's Hikurangi Subduction Margin

    Science.gov (United States)

    Crutchley, G. J.; Kroeger, K. F.; Pecher, I. A.; Mountjoy, J. J.; Gorman, A. R.

    2017-12-01

    We investigate gas hydrate system dynamics beneath a submarine canyon on New Zealand's Hikurangi subduction margin using seismic reflection data and petroleum systems modeling. High seismic velocities just above the base of gas hydrate stability (BGHS) indicate that concentrated gas hydrates exist beneath the canyon. Two-dimensional gas hydrate formation modeling shows how the process of canyon incision at this location alters the distribution and concentration of gas hydrate. The key modeling result is that free gas is trapped beneath the gas hydrate layer and then "captured" into a concentrated gas hydrate deposit as a result of a downward-shift in the BGHS driven by canyon incision. Our study thus provides new insight into the functioning of this process. From our data, we also conceptualize two other models to describe how canyons could significantly change gas hydrate distribution and concentration. One scenario is related to deflection of fluid flow pathways from over-pressured regions at the BGHS toward the canyon, and the other is based on relationships between simultaneous seafloor uplift and canyon incision. The relationships and processes described are of global relevance because of considerations of gas hydrate as an energy resource and the influence of both submarine canyons and gas hydrate systems on seafloor biodiversity.

  2. Evidence for retrograde lithospheric subduction on Venus

    Science.gov (United States)

    Sandwell, David T.; Schubert, Gerald

    1992-01-01

    Annular moats and outer rises around large Venus coronas such as Artemis, Latona, and Eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones. On earth, trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab; this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronas on Venus. Accordingly, it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large Venus coronas while compensating back-arc extension is occurring in the expanding coronas interiors. Similar processes may be taking place at other deep arcuate trenches or chasmata on Venus such as those in the Dali-Diana chasmata area of aestern Aphrodite Terra.

  3. Seismic Structure of the Subducted Cocos Plate

    Science.gov (United States)

    Clayton, R. W.; Davis, P. M.; Perez-Campos, X.

    2007-05-01

    The Meso-American Subduction Experiment (MASE) was designed to determine the critical parameters to necessary to simulate the subduction process in Central Mexico . A preliminary analysis of the data shows a 200km section of the slab that is subhorizontal and to within the resolution of the receiver functions it underplates the continental crust with no intervening asthenosphere. This is an interesting situation because the short-term (GPS) and long-term (geologic) strain measurements show almost no compressive strain in this region. This would imply that the crust is decoupled from the subducting slab. Near the coast, the receiver functions show that the slab cuts through the crust at an approximately a 15-degree angle, and under the Trans-Mexican Volcanic Belt the slab becomes detached from the crust, but its geometry at depth is not yet determined from the receiver functions, but a well-developed mantle wedge is apparent from the attenuation of regional earthquakes.

  4. Subduction zones seen by GOCE gravity gradients

    DEFF Research Database (Denmark)

    Švarc, Mario; Herceg, Matija; Cammarano, Fabio

    In this study, the GOCE (Gravity field and steady state Ocean Circulation Explorer) gradiometry data were used to study geologic structures and mass variations within the lithosphere in areas of known subduction zones. The advantage of gravity gradiometry over other gravity methods...... is that gradients are extremely sensitive to localized density contrasts within regional geological settings, which makes it ideally suited for detecting subduction zones. Second order gravity gradients of disturbing potential were extracted from global geopotential model, the fifth release GOCE model ‘EGM_TIM_RL05......’. In order to remove the signal which mainly corresponds to the gravity signal of the lower mantle, long wavelength part of the gravity signal was removed up to degree and order 60. Because the areas with notable topography differences coincide with subduction zones, topography correction was also performed...

  5. Impact of Mantle Wind on Subducting Plate Geometry and Interplate Pressure: Insights From Physical Modelling.

    Science.gov (United States)

    Boutelier, D.; Cruden, A. R.

    2005-12-01

    New physical models of subduction investigate the impact of large-scale mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The experiments comprise two lithospheric plates made of highly filled silicone polymer resting on a model asthenosphere of low viscosity transparent silicone polymer. Subduction is driven by a piston that pushes the subducting plate at constant rate, a slab-pull force due to the relative density of the slab, and a basal drag force exerted by flow in the model asthenosphere. Large-scale mantle flow is imposed by a second piston moving at constant rate in a tunnel at the bottom of the experiment tank. Passive markers in the mantle track the evolution of flow during the experiment. Slab structure is recorded by side pictures of the experiment while horizontal deformation is studied via passive marker grids on top of both plates. The initial mantle flow direction beneath the overriding plate can be sub-horizontal or sub-vertical. In both cases, as the slab penetrates the mantle, the mantle flow pattern changes to accommodate the subducting high viscosity lithosphere. As the slab continues to descend, the imposed flow produces either over- or under-pressure on the lower surface of the slab depending on the initial mantle flow pattern (sub-horizontal or sub-vertical respectively). Over-pressure imposed on the slab lower surface promotes shallow dip subduction while under-pressure tends to steepen the slab. These effects resemble those observed in previous experiments when the overriding plate moves horizontally with respect to a static asthenosphere. Our experiments also demonstrate that a strong vertical drag force (due to relatively fast downward mantle flow) exerted on the slab results in a decrease in strain rate in both the downgoing and overriding plates, suggesting a decrease in interplate pressure. Furthermore, with an increase in drag force deformation in the downgoing plate can switch

  6. The Role of Subducting Ridges in the Formation of Flat Slabs: Insights from the Peruvian Flat Slab

    Science.gov (United States)

    Knezevic Antonijevic, Sanja; Wagner, Lara; Kumar, Abhash; Beck, Susan; Long, Maureen; Zandt, George; Eakin, Caroline M.

    2015-04-01

    Flattening of the subducting plate is often used to explain various geological features removed far from the subducting margins, including basement-cored uplifts, the cessation of arc volcanism, ignimbrite flare-ups, and the formation of high plateaus and ore deposits [Humphreys et al., 2003; Gutscher et al., 2000; Rosenbaum et al., 2005, Kay and Mpodozis, 2001]. Today, flat slab subduction is observed in central Chile and Peru, representing the modern analogues to the immense paleo-flat slab that subducted beneath the North American continent during the Laramide orogeny (80-55 Ma) [English et al., 2003]. However, how flat slabs form and what controls their inboard and along-strike extent is still poorly understood. To better understand modern and paleo-flat slabs, we focus on the Peruvian flat slab, where the Nazca plate starts to bend at ~90 km depth and travels horizontally for several hundred kilometers beneath the South American plate. Earlier studies propose a correlation between the flat slab and the subducting Nazca Ridge that has been migrating to the south over the past 11 ~Ma [Hampel et al., 2004, Gutscher et al., 2003]. Combining 3D shear wave velocity structure and Rayleigh wave phase anisotropy between ~10° and 18° S, we find that the flat slab has the greatest inboard extent along the track of the subducting Nazca Ridge. North of the ridge track, where the flat slab was initially formed, the flat slab starts to sag, tear and re-initiate steep slab subduction, allowing inflow of warm asthenosphere. Based on our new constraints on the geometry of the subducted plate, we find that the subduction of buoyant oceanic features with overthickened oceanic crust plays a vital role in the formation of flat slabs. We further develop a model of temporal evolution of the Peruvian flab slab that forms as a result of the combined effects of the subducting ridge, trench retreat, and suction forces. Once the buoyant ridge subducts to ~90 km depth, it will fail to

  7. The Subduction of Continental Crust, the Variscan Evolution of the Bohemian Massif, and the Origin of PO Granitoids

    Science.gov (United States)

    Brueckner, H. K.

    2012-04-01

    Slices of continental crust subducted into the mantle during collisional orogeny may either undergo metamorphism and exhumation towards the surface as coherent slab-like or domal high pressure/ultrahigh pressure (HP/UHP) terranes or, if stalled or delayed in the mantle, melt and return towards the surface as magmas, or undergo a combination of exhumation and melting. Some exhumed HP/UHP terranes contain synorogenic granitoid bodies demonstrating melting does occur during exhumation. Therefore, crust that remains trapped in the mantle will also melt when temperatures reach the appropriate solidi through adiabatic decompression and/or conductive heating and/or radioactive decay. Subducted terranes with hydrous phases will undergo hydrate-breakdown melting and could melt during subduction, when stalled in the mantle or during exhumation. Terranes lacking hydrous phases probably require melting by adiabatic decompression as heated crust becomes ductile and rises as diapirs through the mantle wedge. The generated magmas will intrude through the overlying mantle wedge and into the overlying continental crust to form late orogenic and post orogenic (PO) granitoids depending on the time required to reach solidus temperatures. Geochemical characteristics will depend on P-T conditions, the age/chemistry/mineralogy of the subducted terrane (especially the presence or lack of hydrous phases), and the degree of melt interaction (i.e. the traverse length) with the mantle wedge. Melts that significantly traverse the wedge will acquire the hybrid mantle/crust nature of many PO granitoids. Melts generated by adiabatic decompression close to or within the continental crust will retain ancient crustal signatures. The Variscan evolution of the Bohemian Massif involved two episodes of subduction of continental crust: (1), the southward (present coordinates) subduction of Saxo-Thuringia beneath Bohemia (aka Tapla-Barrandia) along an east-west suture at 400-370 Ma followed by, (2), the

  8. Subduction trench migration since the Cretaceous

    Science.gov (United States)

    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

  9. Seismicity, topography, and free-air gravity of the Aleutian-Alaska subduction zone

    Science.gov (United States)

    Wells, R. E.; Blakely, R. J.; Scholl, D. W.; Ryan, H. F.

    2011-12-01

    The Aleutian-Alaska subduction zone, extending 3400 km from the Queen Charlotte Fault to Kamchatka, has been the source of six great megathrust earthquakes in the 20th Century. Four earthquakes have ruptured the 2000-km-long Aleutian segment, where the Cenozoic Aleutian arc overlies the subducting Pacific plate. These include the 1946 M 8.6 earthquake off Unimak Is., the 1957 M 8.6 and 1986 M 8.0 earthquakes off the Andreanoff Is., and the 1965 M 8.7 Rat Is. earthquake. The source regions of these earthquakes inferred from waveform inversions underlie the well-defined Aleutian deep-sea terrace. The deep-sea terrace is about 4 km deep and is underlain by Eocene arc framework rocks, which extend nearly to the trench. It is bounded on its seaward and landward margins by strong topographic and fee-air gravity gradients. The main asperities (areas of largest slip) for the great earthquakes and nearly all of the Aleutian thrust CMT solutions lie beneath the Aleutian terrace, between the maximum gradients. Similar deep-sea terraces are characteristic of non-accretionary convergent margins globally (75% of subduction zones), and, where sampled by drilling (e.g., Japan, Peru, Tonga, Central America), are undergoing sustained subsidence. Sustained subsidence requires removal of arc crust beneath the terrace by basal subduction erosion (BSE). BSE is in part linked to the seismic cycle, as it occurs in the same location as the megathrust earthquakes. Along the eastern 1400 km of the Alaskan subduction zone, the Pacific plate subducts beneath the North American continent. The boundary between the Aleutian segment and the continent is well defined in free-air gravity, and the distinctive deep-sea terrace observed along the Aleutian segment is absent. Instead, the Alaskan margin consists of exhumed, underplated accretionary complexes forming outer arc gravity highs. Superimposed on them are broad topographic highs and lows forming forearc basins (Shumagin, Stevenson) and islands

  10. Metamorphic zirconology of continental subduction zones

    Science.gov (United States)

    Chen, Ren-Xu; Zheng, Yong-Fei

    2017-09-01

    Zircon is widely used to date geological events and trace geochemical sources in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks of continental subduction zones. However, protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid-state transformation, metasomatic alteration and dissolution reprecipitation; new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks (metamorphic zircon) or peritectic reactions above the wet solidus (peritectic zircon). As a consequence, there are different origins of zircon domains in high-grade metamorphic rocks from collisional orogens. Thus, determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing. We advocate an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotope compositions. Only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology, enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction-zone conditions. The metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in HP to UHP metamorphic rocks of collisional orogens. These different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions. While the protolith nature of metamorphosed rocks dictates water availability, the P-T conditions of subduction zones dictate the property of subduction-zone fluids. The fluids of different properties may be produced at different positions of subducting and exhuming crustal slices, and they may physically and chemically mix with each other in continental

  11. A new tomographic image on the Philippine Sea Slab beneath Tokyo - Implication to seismic hazard in the Tokyo metropolitan region -

    Science.gov (United States)

    Hirata, N.; Sakai, S.; Nakagawa, S.; Ishikawa, M.; Sato, H.; Kasahara, K.; Kimura, H.; Honda, R.

    2012-12-01

    In central Japan, the Philippine Sea plate (PSP) subducts beneath the Tokyo metropolitan region. Devastating M8-class earthquakes occurred on the upper surface of the Philippine Sea plate (SPS), examples of which are the Genroku earthquake of 1703 (magnitude M=8.0) and the Kanto earthquake of 1923 (M=7.9), which had 105,000 fatalities. A M7 or greater (M7+) earthquake in this region at present has high potential to produce devastating loss of life and property with even greater global economic repercussions although it is smaller than the megathrust type M8-class earthquakes. This great earthquake is evaluated to occur with a probability of 70 % in 30 years by the Earthquake Research Committee of Japan. The M7+ earthquakes may occur either on the upper surface or intra slab of PSP. The Central Disaster Management Council of Japan estimates the next great M7+ earthquake will cause 11,000 fatalities and 112 trillion yen (1 trillion US$) economic loss at worst case if it occur beneath northern Tokyo bay with M7.3. However, the estimate is based on a source fault model by conventional studies about the PSP geometry. To evaluate seismic hazard due to the great quake we need to clarify the geometry of PSP and also the Pacific palate (PAP) that subducs beneath PSP. We identify those plates with use of seismic tomography and available deep seismic reflection profiling and borehole data in southern Kanto area. We deployed about 300 seismic stations in the greater Tokyo urban region under the Special Project for Earthquake Disaster Mitigation in Tokyo Metropolitan Area. We obtain clear P- and S- wave velocity (Vp and Vs) tomograms which show a clear image of PSP and PAP. A depth to the top of PSP, 20 to 30 kilometer beneath northern part of Tokyo bay, is about 10 km shallower than previous estimates based on the distribution of seismicity (Ishida, 1992). This shallower plate geometry changes estimations of strong ground motion for seismic hazards analysis within the Tokyo

  12. Complex structure of the lithospheric slab beneath the Banda arc, eastern Indonesia depicted by a seismic tomographic model

    Directory of Open Access Journals (Sweden)

    Sri Widiyantoro

    2011-10-01

    Full Text Available Seismic tomography with a non-linear approach has been successfully applied to image the P-wave velocity structure beneath the Banda arc in detail. Nearly one million compressional phases including the surfacereflected depth phases pP and pwP from events within the Indonesian region have been used. The depth phases have been incorporated in order to improve the sampling of the uppermantle structure, particularly below the Banda Sea in the back-arc regions. For the model parameterization, we have combined a highresolution regional inversion with a low-resolution global inversion to allow detailed images of slab structures within the study region and to minimize the mapping of distant aspherical mantle structure into the volume under study. In this paper, we focus our discussion on the upper mantle and transition zone structure beneath the curved Banda arc. The tomographic images confirm previous observations of the twisting of the slab in the upper mantle, forming a spoon-shaped structure beneath the Banda arc. A slab lying flat on the 660 km discontinuity beneath the Banda Sea is also well imaged. Further interpretations of the resulting tomograms and seismicity data support the scenario of the Banda arc subduction rollback.

  13. Variation of Seismic Velocity Structure around the Mantle Transition Zone and Conjecture of Deep Water Transport by Subducted Slabs

    Science.gov (United States)

    Tajima, F. C.; Stahler, S. C.; Ohtani, E.; Yoshida, M.; Sigloch, K.

    2011-12-01

    Seismic tomography models published in the past two decades determined common long-wavelength features of subducting plates as high velocity anomalies and upwelling plumes as low velocity anomalies, and have led to a new class of high-resolution three-dimensional (3D) modeling of global mantle convection with a link to tomography models [e.g., Becker and Boschi, 2002; Ritsema et al., 2007; Schuberth et al., 2009a,b]. However, even such high resolution numerical models do not account for the variation associated with different behaviors of subducting plates as they enter the mantle transition zone (MTZ), i.e., some flatten to form stagnant slabs with a large lateral extent and others descend further into the lower mantle. There are conventional interpretations applied for the cause of variation of the subducted slab behaviors, i.e., temperature difference due to different plate age, different geochemical compositions, different water content and subsequent possible reduction of viscosity etc., which could be taken as non-unique and somewhat equivocal. These parameters and conditions have been tested in two-dimensional numerical simulations, while the water content in the MTZ or the mechanisms of hydration and dehydration through subduction process are still in the realm of conjecture. Recent models of seismic P- and SH-wave velocities derived for the mantle structure beneath northeast China [Wang and Niu, 2010; Ye et al., 2011] using reflectivity synthetics with data from the dense Chinese networks of broadband seismic instruments, show a broader 660 km discontinuity (by about 30 to 70 km) and slower shear velocities above the MTZ than a global standard model iasp91 (Kennett and Engdahl, 1991). These features were interpreted with a mixture of different chemical properties which show delayed phase transformation, and effects of water above the flattened slab. Nonetheless, the SH-wave model has a structure similar to model TNA above the MTZ, which was derived for the

  14. Influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge

    NARCIS (Netherlands)

    Schellart, Wouter P.

    2005-01-01

    Geological observations indicate that along two active continental margins (East Asia and Mediterranean) major phases of overriding plate extension, resulting from subduction hinge-retreat, occurred synchronously with a reduction in subducting plate velocity. In this paper, results of fluid

  15. Detailed crustal thickness variations beneath the Illinois Basin area: Implications for crustal evolution of the midcontinent

    Science.gov (United States)

    Yang, Xiaotao; Pavlis, Gary L.; Hamburger, Michael W.; Marshak, Stephen; Gilbert, Hersh; Rupp, John; Larson, Timothy H.; Chen, Chen; Carpenter, N. Seth

    2017-08-01

    We present high-resolution imaging results of crustal and upper mantle velocity discontinuities across the Illinois Basin area using both common conversion point stacking and plane wave migration methods applied to P wave receiver functions from the EarthScope Ozark, Illinois, Indiana, and Kentucky experiment. The images reveal unusually thick crust (up to 62 km) throughout the central and southeastern Illinois Basin area. A significant Moho gradient underlies the NW trending Ste. Genevieve Fault Zone, which delineates the boundary between the Illinois Basin and Ozark Dome. Relatively thinner crust (Proterozoic convergent margin tectonics around 1.55-1.35 Ga; (3) by Late Precambrian magmatic underplating at the base of older crust, associated with the creation of the Eastern Granite-Rhyolite Province around 1.3 Ga; and (4) through crustal "relamination" during an episode of Proterozoic flat-slab subduction beneath the Illinois Basin, possibly associated with the Grenville Orogeny.

  16. Flat-slab subduction, whole crustal faulting, and geohazards in Alaska: Targets for Earthscope

    Science.gov (United States)

    Gulick, S. P.; Pavlis, T. L.; Bruhn, R. L.; Christeson, G. L.; Freymueller, J. T.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Roeske, S.; Reece, R.; van Avendonk, H. J.; Worthington, L. L.

    2010-12-01

    Crustal structure and evolution illuminated by the Continental Dynamics ST. Elias Erosion and tectonics Project (STEEP) highlights some fundamental questions about active tectonics processes in Alaska including: 1) what are the controls on far field deformation and lithospheric stabilization, 2) do strike slip faults extend through the entire crust and upper mantle and how does this influence mantle flow, and 3) how does the transition from “normal” subduction of the Pacific along the Aleutians to flat slab subduction of the Yakutat Terrane beneath southeast and central Alaska to translation of the Yakutat Terrane past North American in eastern Alaska affect geohazard assessment for the north Pacific? Active and passive seismic studies and geologic fieldwork focusing on the Yakutat Terrane show that the Terrane ranges from 15-35 km thick and is underthrusting the North American plate from the St. Elias Mountains to the Alaska Range (~500 km). Deformation of the upper plate occurs within the offshore Pamplona Zone fold and thrust belt, and onshore throughout the Robinson Mountains. Deformation patterns, structural evolution, and the sedimentary products of orogenesis are fundamentally influenced by feedbacks with glacial erosion. The Yakutat megathrust extends beneath Prince William Sound such that the 1964 Mw 9.2 great earthquake epicenter was on this plate boundary and jumped to the adjacent Aleutian megathrust coseismically; this event illuminates the potential for transitional tectonic systems to enhance geohazards. The northern, southern, and eastern limits of the Yakutat microplate are strike-slip faults that, where imaged, appear to cut the entire crustal section and may allow for crustal extrusion towards the Bering Sea. Yakutat Terrane effects on mantle flow, however, have been suggested to cross these crustal features to allow for far-field deformation in the Yukon, Brooks Range, and Amerasia Basin. From the STEEP results it is clear that the Yakutat

  17. Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

    Science.gov (United States)

    Stanley, Dal; Villaseñor, Antonio; Benz, Harley

    1999-01-01

    The Cascadia subduction zone is extremely complex in the western Washington region, involving local deformation of the subducting Juan de Fuca plate and complicated block structures in the crust. It has been postulated that the Cascadia subduction zone could be the source for a large thrust earthquake, possibly as large as M9.0. Large intraplate earthquakes from within the subducting Juan de Fuca plate beneath the Puget Sound region have accounted for most of the energy release in this century and future such large earthquakes are expected. Added to these possible hazards is clear evidence for strong crustal deformation events in the Puget Sound region near faults such as the Seattle fault, which passes through the southern Seattle metropolitan area. In order to understand the nature of these individual earthquake sources and their possible interrelationship, we have conducted an extensive seismotectonic study of the region. We have employed P-wave velocity models developed using local earthquake tomography as a key tool in this research. Other information utilized includes geological, paleoseismic, gravity, magnetic, magnetotelluric, deformation, seismicity, focal mechanism and geodetic data. Neotectonic concepts were tested and augmented through use of anelastic (creep) deformation models based on thin-plate, finite-element techniques developed by Peter Bird, UCLA. These programs model anelastic strain rate, stress, and velocity fields for given rheological parameters, variable crust and lithosphere thicknesses, heat flow, and elevation. Known faults in western Washington and the main Cascadia subduction thrust were incorporated in the modeling process. Significant results from the velocity models include delineation of a previously studied arch in the subducting Juan de Fuca plate. The axis of the arch is oriented in the direction of current subduction and asymmetrically deformed due to the effects of a northern buttress mapped in the velocity models. This

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

    Science.gov (United States)

    Hardebeck, Jeanne L

    2015-09-11

    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 an angle of 45° to 60° with respect 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. Copyright © 2015, American Association for the Advancement of Science.

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

    Science.gov (United States)

    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.

  20. Subduction and volatile recycling in Earth's mantle

    Science.gov (United States)

    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.

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

    Science.gov (United States)

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

    2016-02-16

    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.

  2. Earthquake nucleation in weak subducted carbonates

    NARCIS (Netherlands)

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

    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

  3. Magmatic underplating beneath the Rajmahal Traps: Gravity ...

    Indian Academy of Sciences (India)

    R. Narasimhan (Krishtel eMaging) 1461 1996 Oct 15 13:05:22

    The early Cretaceous thermal perturbation beneath the eastern continental margin of the Indian shield resulted in the eruption of the Rajmahal Traps. To understand the impact of the magmatic process that originated in the deep mantle on the lower crustal level of the eastern Indian shield and adjoining Bengal basin the ...

  4. Evidence for strong lateral seismic velocity variation in the lower crust and upper mantle beneath the California margin

    Science.gov (United States)

    Lai, Voon; Graves, Robert; Wei, Shengji; Helmberger, Don

    2017-01-01

    Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across Southern California where long period (30–50 seconds) SH waves arrive up to 15 seconds earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust-mantle interface beneath the California margin. To model these observations, we propose a fast seismic layer, with thickness growing westward from the San Andreas along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to match the observed timing of SH waveforms better than existing 3D models. The presence of a strong upper mantle buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and may explain the observed asymmetric strain rate across the boundary.

  5. Global correlations between maximum magnitudes of subduction zone interface thrust earthquakes and physical parameters of subduction zones

    NARCIS (Netherlands)

    Schellart, W. P.; Rawlinson, N.

    2013-01-01

    The maximum earthquake magnitude recorded for subduction zone plate boundaries varies considerably on Earth, with some subduction zone segments producing giant subduction zone thrust earthquakes (e.g. Chile, Alaska, Sumatra-Andaman, Japan) and others producing relatively small earthquakes (e.g.

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

    NARCIS (Netherlands)

    Maffione, Marco; Thieulot, Cedric|info:eu-repo/dai/nl/270177493; van Hinsbergen, Douwe J.J.|info:eu-repo/dai/nl/269263624; Morris, Antony; Plümper, Oliver|info:eu-repo/dai/nl/37155960X; Spakman, Wim|info:eu-repo/dai/nl/074103164

    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

  7. Slab remnants beneath the Baja California peninsula: Seismic constraints and tectonic implications

    Science.gov (United States)

    Paulssen, Hanneke; de Vos, Denise

    2017-11-01

    The formation of the Gulf of California has been related to the cessation of subduction of the Guadalupe and Magdalena microplates. Various studies have identified features that point to the presence of a slab remnant beneath the Baja California peninsula, but its depth range and lateral extent remained unclear. In this study we used surface wave phase velocity and receiver function data of NARS-Baja stations around the Gulf of California to better constrain the location of the slab. For stations in central and southern Baja California the shear velocity models show an upper mantle high-velocity layer with its top in the depth range from 115 to 135km and a thickness varying between roughly 40 and 60km. These high-velocity anomalies are interpreted as subducted slab remnants. In contrast, the models for the northern peninsula show no slab signature. This change directly correlates with the variation in relative motion between the Baja California peninsula and the Pacific plate as measured by GPS data. It is inferred that the stalled slab fragments beneath the peninsula produce strong coupling between Baja California and the Pacific plate. The shear velocity models for stations on the Mexican mainland show a layer of higher velocities above a low-velocity upper mantle. In the North, the low-velocity mantle, starting at a depth of 40km, is associated with upwelling as suggested by previous studies. Further south, the transition from higher to lower mantle velocities occurs around 80km depth, which is a typical value for the lithosphere-asthenosphere boundary. Furthermore, the models show strong crustal thinning towards the gulf, both from the peninsula as well as from the Mexican mainland.

  8. Topography of the 410 km and 660 km discontinuities beneath the Japan Sea and adjacent regions by analysis of multiple-ScS waves

    Science.gov (United States)

    Wang, Xin; Li, Juan; Chen, Qi-Fu

    2017-02-01

    The northwest Pacific subduction region is an ideal location to study the interaction between the subducting slab and upper mantle discontinuities. Due to the sparse distribution of seismic stations in the sea, previous studies mostly focus on mantle transition zone (MTZ) structures beneath continents or island arcs, leaving the vast area of the Japan Sea and Okhotsk Sea untouched. In this study, we analyzed multiple-ScS reverberation waves, and a common-reflection-point stacking technique was applied to enhance consistent signals beneath reflection points. A topographic image of the 410 km and 660 km discontinuities is obtained beneath the Japan Sea and adjacent regions. One-dimensional and 3-D velocity models are adapted to obtain the "apparent" and "true" depth. We observe a systematic pattern of depression ( 10-20 km) and elevation ( 5-10 km) of the 660, with the topography being roughly consistent with the shift of the olivine-phase transition boundary caused by the subducting Pacific plate. The behavior of the 410 is more complex. It is generally 5-15 km shallower at the location where the slab penetrates and deepened by 5-10 km oceanward of the slab where a low-velocity anomaly is observed in tomography images. Moreover, we observe a wide distribution of depressed 410 beneath the southern Okhotsk Sea and western Japan Sea. The hydrous wadsleyite boundary caused by the high water content at the top of the MTZ could explain the depression. The long-history trench rollback motion of Pacific slab might be responsible for the widely distributed depression of the 410 ranging upward and landward from the slab.

  9. Dispersion of the invasive common carp Cyprinus carpio in southern South America: changes and expectations, westward and southward.

    Science.gov (United States)

    Crichigno, S; Cordero, P; Blasetti, G; Cussac, V

    2016-07-01

    Common carp Cyprinus carpio possess multiple traits that contribute to their success as an invasive species. They have been introduced across the globe, and abundant populations can have numerous negative effects. Although ecological niche-based modelling techniques have been used to predict the potential range of C. carpio invasion in U.S.A., occurrence and abundance patterns have not yet been considered on a regional scale. In the present review new locations are documented, the status of the southernmost population has been studied and the probability of new lakes and reservoirs being colonized by C. carpio has been obtained and related to environmental conditions. The new localities for C. carpio have expanded its distribution westward, into the Andean Region, and present results from the South American southernmost population have shown a well-established population. Analysis of presence data provided two principal results: (1) the probability of a site being with C. carpio can be inferred using environmental variables and (2) the probability of a site being with C. carpio is a useful tool for the prediction of future invasions. Selective fishing on the Negro basin could constitute a potential mitigation measure, decreasing the abundance of the species and thus reducing the species' potential for southward expansion. These results reinforce the idea that artisanal fisheries, food production and conservation interests should be taken into account by local government management agencies in any discussion regarding the southern distribution of C. carpio in the near future. © 2016 The Fisheries Society of the British Isles.

  10. Seismic attenuation structure associated with episodic tremors and slip zones, southwestern Japan, in the Nankai subduction zone

    Science.gov (United States)

    Kita, S.; Matsubara, M.

    2016-12-01

    We imaged the seismic attenuation structure (frequency- independent Qp) beneath southwestern Japan using t* estimated by the S coda wave spectral ratio method to the waveform data from the Kiban nationwide seismic network. The seismic attenuation (Qp-1) structure was clearly imaged for the region beneath Shikoku, the Kii peninsula, and eastern Kyushu at depths down to 50 km. At depths of 5 to 35 km, the seismic attenuation structure changes at the Median Tectonic Line and other geological boundaries beneath Shikoku and the southwestern Kii peninsula. High-Qp zones within the lower crust of the overlying plate were found just above the slip regions at the centers and deeper parts of the long-term slow-slip events (SSEs) beneath the Bungo and Kii channels and central Shikoku. Beneath central Shikoku, within the overlying plate, a high-Qp zone bounded by low-Qp zones was located from the top of the overlying plate to the plate interface of the subducting plate. The high-Qp zone and low-Qp zones correspond to high-Vp and low-Vp zones of previous study by Matsubara et al. [2009], respectively. The boundaries of the high-Qp zone and the low-Qp zones were located at the segment boundaries of tremors. The heterogeneity of the seismic attenuation and velocity structures also appeared to correspond to the characteristics of geography (uplifting map by Ohmori [1990]) beneath Shikoku. The results indicated that the locations of the long- and short-term SSEs could be limited by the inhomogeneous distribution of the materials and/or condition of the overlying plate, which is considered to be formed due to geological and geographical process. The heterogeneity of materials and/or condition within the forearc crust possibly made an effect on inhomogeneous rheological strength distribution on the interface.

  11. The lithosphere structure beneath the central Mediterranean from S receiver functions

    Science.gov (United States)

    Bianchi, Irene; Miller, Meghan; Piana Agostinetti, Nicola; O'Driscoll, Leland

    2017-04-01

    The last piece of Mesozoic oceanic lithosphere from the Neo-Tethys Ocean is being consumed beneath Eurasia in the Central Mediterranean area, squeezed by the continuing convergence of Africa with the Eurasian plate. Convergence between Africa and Eurasia has been ongoing since the Cenozoic, forming a series of arcuate shaped subduction zones, and producing the current complex plate boundary that strikes across the Mediterranean area. Moreover, geophysical imaging depicts a variable lithospheric structure related to remnants of both oceanic and continental lithosphere within this convergent margin, which contributes to its complexity. In fact, the subduction/collision of blocks with different rheologies and thicknesses (e.g. continental or oceanic) has resulted in complex setting that includes accretionary wedges, orogenesis, and formation of an intricate back-arc/fore-arc/trench system. In order to shed light on these tectonic structures, we provide observations and interpretations of the lithospheric structure of the central Mediterranean via S receiver functions analysis. Teleseismic observations recorded at permanent and temporary seismic stations have been employed to produce images of the lithospheric discontinuities with tens of kilometers lateral resolution. We illustrate the feasibility of the lithosphere-asthenosphere boundary detection on a regional scale, and detect the occurrence of deeper seismic discontinuities due both to positive and negative seismic velocity jumps.

  12. Interplay between deformation, fluid release and migration across a nascent subduction interface: evidence from Oman-UAE and implications for warm subduction zones

    Science.gov (United States)

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

    2017-04-01

    Frozen-in subduction plate interfaces preserving the first 1-2 My of the subduction history are found beneath ophiolites. These contacts are a key target to study the inception of mantle wedge metasomatism and the mechanical coupling between the upper plate and the top part of the sinking slab shortly after subduction initiation. Combining structural field and EBSD data, detailed petrology, thermodynamic modelling and geochemistry on both sides, i.e. the base of the mantle wedge (Oman-UAE basal peridotites) and the underlying accreted crustal fragments from the subducting slab (metamorphic soles), this study documents the continuous evolution of the plate contact from 1 GPa 900-750°C to 0.6 GPa 750-600°C, with emphasis on strain localization and feedbacks between deformation and fluid migration. In the mantle wedge, the (de)formation of proto-ultramylonitic peridotites is coeval with mantle metasomatism by focused hydrous fluid migration. Peridotite metasomatism results in the precipitation of new minerals (clinopyroxene, amphibole and spinel ± olivine and orthopyroxene) and their enrichment in FMEs (particularly B, Li and Cs, with concentrations up to 40 times that of the PM). Boron concentrations and isotopes (δ11B of metasomatized peridotites up to +25‰) suggest that these fluids with a "subduction signature" are probably sourced from the dehydrating amphibolitic metamorphic sole. Concomitantly, deformation in the lower plate results in the stepwise formation, detachment and accretion to the mylonitic s.l. mantle of successive slices of HT metabasalts from the downgoing slab, equilibrated at amphibolite/granulite conditions (900-750°C). Two major stages may be outlined: - between 900 and 750°C, the garnet-clinopyroxene-amphibole bearing sinking crust (with melting < 6 vol%) gets juxtaposed and mechanically coupled to the mantle, leading to the transfer of subduction fluids and metasomatism (possibly into the arc zone ultimately). Deformation is

  13. Signature of slab fragmentation beneath Anatolia from full-waveform tomography

    Science.gov (United States)

    Govers, Rob; Fichtner, Andreas

    2016-09-01

    When oceanic basins close after a long period of convergence and subduction, continental collision and mountain building is a common consequence. Slab segmentation is expected to have been relatively common just prior to closure of other oceans in the geological past, and may explain some of the complexity that geologists have documented in the Tibetan plateau also. We focus on the eastern Mediterranean basin, which is the last remainder of a once hemispherical neo-Tethys ocean that has nearly disappeared due to convergence of the India and Africa/Arabia plates with the Eurasia plate. We present new results of full-waveform tomography that allow us to image both the crust and upper mantle in great detail. We show that a major discontinuity exists between western Anatolia lithosphere and the region to the east of it. Also, the correlation of geological features and the crustal velocities is substantially stronger in the west than in the east. We interpret these observations as the imprint in the overriding plate of fragmentation of the neo-Tethys slab below it. This north-dipping slab may have fragmented following the Eocene (about 35 million years ago) arrival of a continental promontory (Central Anatolian Core Complex) at the subduction contact. From the Eocene through the Miocene, slab roll-back ensued in the Aegean and west Anatolia, while the Cyprus-Bitlis slab subducted horizontally beneath central and east Anatolia. Following collision of Arabia (about 16 million years ago), the Cyprus-Bitlis slab steepened, exposing the crust of central and east Anatolia to high temperature, and resulting in the velocity structure that we image today. Slab fragmentation thus was a major driver of the evolution of the overriding plate as collision unfolded.

  14. New tomographic images of P- , S- wave velocity and Q on the Philippine Sea Slab beneath Tokyo: Implication to seismotectonics and seismic hazard in the Tokyo metropolitan region

    Science.gov (United States)

    Hirata, Naoshi; Sakai, Shin'ichi; Nakagawa, Shigeki; Panayotopoulos, Yannis; Ishikawa, Masahiro; Sato, Hiroshi; Kasahara, Keiji; Kimura, Hisanor; Honda, Ryou

    2013-04-01

    The Central Disaster Management Council of Japan estimates the next great M7+ earthquake in the Tokyo metropolitan region will cause 11,000 fatalities and 112 trillion yen (1 trillion US) economic loss at worst case if it occur beneath northern Tokyo bay with M7.3. However, the estimate is based on a source fault model by conventional studies about the PSP geometry. To evaluate seismic hazard due to the great quake we need to clarify the geometry of PSP and also the Pacific palate (PAP) that subducs beneath PSP. We identify those plates with use of seismic tomography and available deep seismic reflection profiling and borehole data in southern Kanto area. We deployed about 300 seismic stations in the greater Tokyo urban region under the Special Project for Earthquake Disaster Mitigation in Tokyo Metropolitan Area. We obtain clear P- and S- wave velocity (Vp and Vs) and Q tomograms which show a clear image of PSP and PAP. A depth to the top of PSP, 20 to 30 kilometer beneath northern part of Tokyo bay, is about 10 km shallower than previous estimates based on the distribution of seismicity (Ishida, 1992). This shallower plate geometry changes estimations of strong ground motion for seismic hazards analysis within the Tokyo region. Based on elastic wave velocities of rocks and minerals, we interpreted the tomographic images as petrologic images. Tomographic images revealed the presence of two stepwise velocity increase of the top layer of the subducting PSP slab. Rock velocity data reveals that subducting PSP crust transforms from blueschists to amphibolites at depth of 30km and amphibolites to eclogites at depth of 50km, which suggest that dehydration reactions occurs in subducting crust of basaltic compositions during prograde metamorphism and water is released from the subducting PSP crust. Tomograms show evidence for a low-velocity zone (LVZ) beneath the area just north of Tokyo bay. A Q tomogram show a low Q zone in PSP slab. We interpret the LVZ as a

  15. The temporal evolution of a subducting plate in the lower mantle

    Science.gov (United States)

    Loiselet, C.; Grujic, D.; Braun, J.; Fullsack, P.; Thieulot, C.; Yamato, P.

    2009-04-01

    It is now widely accepted that some subducting slabs may cross the lower/upper mantle boundary to ground below the 660 km discontinuity. Indeed, geophysical data underline long and narrow traces of fast materials, associated with subducting slabs, from the upper mantle transition zone to mid-mantle depths that are visible beneath North and South America and southern Asia (Li et al, 2008). Furthermore, seismic tomography data (Van der Hilst et al., 1997; Karason and van der Hilst, 2000, 2001) show a large variety of slab geometries and of mantle flow patterns around subducting plate boundaries (e.g. the slab geometry in the lower mantle in the Tonga subduction zone). However, seismic tomography does not elucidate the temporal evolution of the slab behaviour and geometry during its descent through the upper and lower mantle. In this work, we therefore propose to study the deformation of a thin plate (slab) falling in a viscous fluid (mantle) by means of both analogue and numerical modelling. The combination of both analogue and numerical experiments provides important insights into the shape and attitude evolution of subducting slabs. Models bring information into the controls exerted by the rheology of the slab and the mantle and other physical parameters such as the density contrast between the slab and the surrounding mantle, on the rate at which this deformation takes place. We show that in function of a viscosity ratios between the plate and the surrounding fluid, the plate will acquire a characteristic shape. For the isoviscous case, the plate shape tends toward a bubble with long tails: a "jellyfish" form. The time necessary for the plate to acquire this shape is a function of the viscosity and density contrast between the slab and the mantle. To complete our approach, we have developed a semi-analytical model based on the solution of the Hadamar-Rybinski equations for the problem of a dense, yet isoviscous and thus deforming sphere. This model helps to better

  16. A wave equation migration method for receiver function imaging: 2. Application to the Japan subduction zone

    Science.gov (United States)

    Chen, Ling; Wen, Lianxing; Zheng, Tianyu

    2005-11-01

    The newly developed wave equation poststack depth migration method for receiver function imaging is applied to study the subsurface structures of the Japan subduction zone using the Fundamental Research on Earthquakes and Earth's Interior Anomalies (FREESIA) broadband data. Three profiles are chosen in the subsurface imaging, two in northeast (NE) Japan to study the subducting Pacific plate and one in southwest (SW) Japan to study the Philippine Sea plate. The descending Pacific plate in NE Japan is well imaged within a depth range of 50-150 km. The slab image exhibits a little more steeply dipping angle (˜32°) in the south than in the north (˜27°), although the general characteristics between the two profiles in NE Japan are similar. The imaged Philippine Sea plate in eastern SW Japan, in contrast, exhibits a much shallower subduction angle (˜19°) and is only identifiable at the uppermost depths of no more than 60 km. Synthetic tests indicate that the top 150 km of the migrated images of the Pacific plate is well resolved by our seismic data, but the resolution of deep part of the slab images becomes poor due to the limited data coverage. Synthetic tests also suggest that the breakdown of the Philippine Sea plate at shallow depths reflects the real structural features of the subduction zone, rather than caused by insufficient coverage of data. Comparative studies on both synthetics and real data images show the possibility of retrieval of fine-scale structures from high-frequency contributions if high-frequency noise can be effectively suppressed and a small bin size can be used in future studies. The derived slab geometry and image feature also appear to have relatively weak dependence on overlying velocity structure. The observed seismicity in the region confirms the geometries inferred from the migrated images for both subducting plates. Moreover, the deep extent of the Pacific plate image and the shallow breakdown of the Philippine Sea plate image are

  17. Paleotethyan subduction process revealed from Triassic blueschists in the Lancang tectonic belt of Southwest China

    Science.gov (United States)

    Fan, Weiming; Wang, Yuejun; Zhang, Yanhua; Zhang, Yuzhi; Jourdan, Fred; Zi, Jianwei; Liu, Huichuan

    2015-11-01

    The subduction of the Paleotethyan Ocean and subsequent continental collision along the Lancang tectonic belt of the southeastern Paleotethyan belt is a major tectonic event in Southwest China, but the event of the subduction preceding the final collision is still not well-constrained. The mafic blueschists exposed in the Lancang accretionary complex provide crucial records of the Paleotethyan subduction process. In this paper, we present a set of new petrologic, geochronological and geochemical data for the Suyi mafic blueschists in the Lancang metamorphic zone. The mineral assemblage of these blueschists consists of zoned sodic amphibole (25-30%), albite (15-20%), epidote (25-30%), phengite (5-10%), chlorite (~ 5-10%), and minor amounts of actinolite, apatite, sphene, zircon, ilmenite, quartz and secondary limonite. This suggests a prograde metamorphism from ~ 0.5 to ~ 0.9 GPa and retrograde metamorphic overprinting (back to ~ 0.6 GPa) within the temperature range of 300-450 °C. The Suyi blueschists give a zircon U-Pb age of 260 ± 4 Ma and glaucophane minerals formed during prograde metamorphism yield a 40Ar/39Ar plateau age of 242 ± 5 Ma (MSWD = 0.77; P = 0.54). The blueschists have geochemical compositions of subalkaline basalt and show typical OIB-type REE and multi-elemental patterns and εNd(t) values ranging from + 3.35 to + 4.85. Based on available data, it is inferred that the protolith formed at 260 Ma and originated from a basaltic seamount. The basaltic rocks subducted down to 30-35 km depths beneath the Lincang arc to form the epidote blueschists at ~ 242 Ma. The blueschists were subsequently transported to shallower crustal levels in response to the continuous underthrust of the subducted slab and the continent-continent collision in the middle-late Triassic. These results provide a systematic constraint on the tectonic evolution and temporal framework of the southeastern Paleotethyan belt in Southwest China.

  18. Along-strike structure of the Costa Rican convergent margin from seismic a refraction/reflection survey: Evidence for underplating beneath the inner forearc

    Science.gov (United States)

    St. Clair, J.; Holbrook, W. S.; Van Avendonk, H. J. A.; Lizarralde, D.

    2016-02-01

    The convergent margin offshore Costa Rica shows evidence of subsidence due to subduction erosion along the outer forearc and relatively high rates of uplift (˜3-6 mm/yr) along the coast. Recently erupted arc lavas exhibit a low 10Be signal, suggesting that although nearly the entire package of incoming sediments enters the subduction zone, very little of that material is carried directly with the downgoing Cocos plate to the magma generating depths of the mantle wedge. One mechanism that would explain both the low 10Be and the coastal uplift is the underplating of sediments, tectonically eroded material, and seamounts beneath the inner forearc. We present results of a 320 km long, trench-parallel seismic reflection and refraction study of the Costa Rican forearc. The primary observations are (1) margin perpendicular faulting of the basement, (2) thickening of the Cocos plate to the northwest, and (3) two weak bands of reflections in the multichannel seismic (MCS) reflection image with travel times similar to the top of the subducting Cocos plate. The modeled depths to these reflections are consistent with an ˜40 km long, 1-3 km thick region of underplated material ˜15 km beneath some of the highest observed coastal uplift rates in Costa Rica.

  19. Structure beneath the Alboran from geodynamic flow models and seismic anisotropy

    Science.gov (United States)

    Alpert, Lisa A.; Miller, Meghan S.; Becker, Thorsten W.; Allam, Amir A.

    2013-08-01

    Upper mantle heterogeneity beneath the Alboran Sea (western Mediterranean) as inferred from seismology has been associated with a range of subduction and lithospheric delamination scenarios. However, better constraints on the deep dynamics of the region are needed to determine the cause and consequence of complex surface tectonics. Here, we use an improved set of shear wave splitting observations and a suite of mantle flow models to test a range of suggested structures. We find that the observed seismic anisotropy is best reproduced by mantle flow models that include a continuous, deeply extending slab beneath the Alboran which elongates along the Iberian margin from Granada to Gibraltar and curves southward toward the High Atlas. Other models with detached slabs, slabs with spatial gaps, or drip-like features produce results inconsistent with the splitting observations. SW-directed shear flow, when combined with sublithospheric deflection in response to a dense sinker, generates NNW-splitting orientations most similar to the patterns observed along Gibraltar. Slab viscosities of ˜250 times that of the upper mantle are preferred because they provide a balance between the poloidal flow induced by any sinker and toroidal flow induced by stiff slabs. The best match to anisotropy across the Atlas is a model with a stiff continental keel in northwestern Africa which deflects flow northward. Our results show that quantitative predictions of seismic anisotropy are useful in distinguishing the spatial and depth extent of regional density structures which may otherwise be ambiguous.

  20. Capturing the flow beneath water waves.

    Science.gov (United States)

    Nachbin, A; Ribeiro-Junior, R

    2018-01-28

    Recently, the authors presented two numerical studies for capturing the flow structure beneath water waves (Nachbin and Ribeiro-Junior 2014 Disc. Cont. Dyn. Syst. A 34 , 3135-3153 (doi:10.3934/dcds.2014.34.3135); Ribeiro-Junior et al. 2017 J. Fluid Mech. 812 , 792-814 (doi:10.1017/jfm.2016.820)). Closed orbits for irrotational waves with an opposing current and stagnation points for rotational waves were some of the issues addressed. This paper summarizes the numerical strategies adopted for capturing the flow beneath irrotational and rotational water waves. It also presents new preliminary results for particle trajectories, due to irrotational waves, in the presence of a bottom topography.This article is part of the theme issue 'Nonlinear water waves'. © 2017 The Author(s).

  1. The Ocean Boundary Layer beneath Hurricane Frances

    Science.gov (United States)

    Dasaro, E. A.; Sanford, T. B.; Terrill, E.; Price, J.

    2006-12-01

    The upper ocean beneath the peak winds of Hurricane Frances (57 m/s) was measured using several varieties of air-deployed floats as part of CBLAST. A multilayer structure was observed as the boundary layer deepened from 20m to 120m in about 12 hours. Bubbles generated by breaking waves create a 10m thick surface layer with a density anomaly, due to the bubbles, of about 1 kg/m3. This acts to lubricate the near surface layer. A turbulent boundary layer extends beneath this to about 40 m depth. This is characterized by large turbulent eddies spanning the boundary layer. A stratified boundary layer grows beneath this reaching 120m depth. This is characterized by a gradient Richardson number of 1/4, which is maintained by strong inertial currents generated by the hurricane, and smaller turbulent eddies driven by the shear instead of the wind and waves. There is little evidence of mixing beneath this layer. Heat budgets reveal the boundary layer to be nearly one dimensional through much of the deepening, with horizontal and vertical heat advection becoming important only after the storm had passed. Turbulent kinetic energy measurements support the idea of reduced surface drag at high wind speeds. The PWP model correctly predicts the degree of mixed layer deepening if the surface drag is reduced at high wind speed. Overall, the greatest uncertainty in understanding the ocean boundary layer at these extreme wind speeds is a characterization of the near- surface processes which govern the air-sea fluxes and surface wave properties.

  2. Lithospheric structure beneath the northeastern Tibetan Plateau and the western Sino-Korea Craton revealed by Rayleigh wave tomography

    Science.gov (United States)

    Li, Yonghua; Pan, Jiatie; Wu, Qingju; Ding, Zhifeng

    2017-08-01

    A new 3-D shear wave velocity model of the northeastern (NE) Tibetan Plateau and western Sino-Korea Craton is presented. The model is based on Rayleigh waves recorded at 650 portable stations deployed in the region. Interstation phase and group velocity dispersions for more than 18 000 paths were estimated using the two-station method and then inverted to produce phase velocity maps for 10-80 s period and group velocity maps for 10-60 s period. Local 1-D shear wave velocity profiles for each 0.5° × 0.5°grid node were obtained by inverting Rayleigh wave dispersions obtained in this study together with previously published Rayleigh wave group velocities between 60 and 145 s and then assembled into a 3-D shear wave velocity model. The images obtained reveal an obvious mid-crustal low-velocity zone (LVZ) and low-velocity anomaly in the upper mantle beneath the Songpan-Ganzi terrane. The mid-crustal LVZ can be explained by the presence of partial melting, most likely from asthenospheric upwelling. The existence of a relatively weak mid-crustal LVZ beneath the Qilian orogeny is also confirmed. This LVZ is likely caused by the presence of deep crustal fluids. The Yinchuan-Hetao graben is characterized by relatively low velocities that extend to at least 200 km below the Earth's surface, strongly contrasting with the seismically fast lithosphere of the Ordos and Alax blocks. The model presented here shows the presence of a relatively thick and high-velocity lithosphere beneath the Ordos and NE Alxa blocks, as well as evidence of thinning of the lithosphere beneath the southwestern Alxa and Qilian blocks, indicating that the Alxa block is not subducting beneath NE Tibet.

  3. Lithospheric Delamination or Relict Slab Beneath the Former North American Cratonic Margin in Idaho and Oregon? New Constraints From Seismic Tomography.

    Science.gov (United States)

    Stanciu, A. C.; Russo, R. M.; Mocanu, V. I.; VanDecar, J. C.; Hongsresawat, S.; Bremner, P. M.; Torpey, M. E.; Panning, M. P.

    2016-12-01

    We present a new high-resolution P-wave velocity model of the upper mantle beneath the former passive margin of the North American craton in Oregon and Idaho. We identify high velocity anomalies in the central part of the model and low velocity anomalies to the northwest and southeast. Our results derive from an integrated data set of teleseismic P waves recorded at 145 broadband stations, 85 deployed between 2011 and 2013 as part of the IDOR Passive experiment, and 60 USArray-TA stations. We determined 15,000 travel-times using multi-channel cross-correlation (VanDecar and Crosson, 1990). Phanerozoic tectonic events that affected upper mantle seismic structure here include subduction of Farallon and Juan de Fuca lithosphere, accretion of Blue Mountains terranes, Sevier and Laramide orogenies, Idaho batholith formation, Yellowstone and Columbia River volcanism, and Basin and Range extension. Our results indicate a high P-wave velocity anomaly located beneath the Idaho Batholith in west-central Idaho traceable down to 150-200 km depth. A similar anomaly identified by Schmandt and Humphrey (2011) beneath Washington and Montana was interpreted as a slab remnant from the accretion of Siletzia to North America. Alternatively, the fast Vp anomalies are delaminated North American craton lithosphere. Thickened lithosphere may have formed during Farallon subduction, terrane collision and accretion. Crust as much as 55 km thick present during Late Cretaceous (Foster et al., 2001; Gaschnig et al., 2011) is potentially indicative of lithospheric thickening leading to delamination. To the southeast, upper mantle low velocity anomalies occur beneath the Western Snake River Plain. We associate these low velocities with high temperatures generated by the Yellowstone mantle plume system. We observe a low velocity anomaly beneath the Wallowa Mountains starting at 150-200 km extending to depths below the resolution of our model.

  4. Subduction zone earthquakes and stress in slabs

    Science.gov (United States)

    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.

  5. The hidden simplicity of subduction megathrust earthquakes

    Science.gov (United States)

    Meier, M.-A.; Ampuero, J. P.; Heaton, T. H.

    2017-09-01

    The largest observed earthquakes occur on subduction interfaces and frequently cause widespread damage and loss of life. Understanding the rupture behavior of megathrust events is crucial for earthquake rupture physics, as well as for earthquake early-warning systems. However, the large variability in behavior between individual events seemingly defies a description with a simple unifying model. Here we use three source time function (STF) data sets for subduction zone earthquakes, with moment magnitude Mw ≥ 7, and show that such large ruptures share a typical universal behavior. The median STF is scalable between events with different sizes, grows linearly, and is nearly triangular. The deviations from the median behavior are multiplicative and Gaussian—that is, they are proportionally larger for larger events. Our observations suggest that earthquake magnitudes cannot be predicted from the characteristics of rupture onsets.

  6. Subduction related fluids fractionate Nb/Ta

    Science.gov (United States)

    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

  7. Diverse melanges of an ancient subduction complex

    Energy Technology Data Exchange (ETDEWEB)

    Lash, G.G.

    1987-07-01

    Three lithologically and structurally diverse melanges occur within an early Paleozoic (Early-Middle Ordovician) subduction complex in the central Appalachian orogen. Type I melange, characterized by horizons of variably deformed sandstone and scaly mudstone that alternate with coherent sandstone-rich sequences, is interpreted to reflect accretion-related deformation of water-saturated trench deposits. Type II melange, composed of exotic radiolaria-bearing mudstone clasts in a scaly mudstone matrix, can be explained by remobilization and mixing of inner-trench slope sediments. Type III melange is a poorly sorted polymict assemblage of native lithology clasts in a scaly mudstone matrix. Evidence of forceful injection of matrix mud into clasts and inferred discordant contacts between melange and surrounding bedded deposits suggest that the type III melange formed from mud diapirism. The close association of these melanges points out the diversity of tectonic and sedimentary processes previously documented from modern convergent margins that may be reflected in older subduction complexes.

  8. Bent-shaped plumes and horizontal channel flow beneath the 660 km discontinuity

    Science.gov (United States)

    Tosi, Nicola; Yuen, David A.

    2011-12-01

    values based on lower mantle mineral physics. If an increase of the thermal conductivity with depth is also included, our model can predict both long channel flows beneath the 660 km discontinuity and also values of the radial velocity of local blobs sinking in the lower mantle of around 1 cm/yr, in good agreement with those inferred for the sinking rate of cold remnants of the subducted lithosphere.

  9. Uplift of the Colorado Plateau due to lithosphere attenuation during Laramide low-angle subduction

    Science.gov (United States)

    Spencer, J.E.

    1996-01-01

    The Colorado Plateau is blanketed by Phanerozoic marine and nonmarine strata as young as Cretaceous that are now exposed at elevations of about 2 km. Crustal thickening due to magmatism and horizontal crustal shortening was far less than necessary to cause this uplift, which is commonly attributed to the consequences of mantle lithosphere thinning and heating. The Colorado Plateau and the midcontinent region around Iowa consist of Precambrian bedrock overlain by a similar amount of Paleozoic platformal strata, and thus both regions once had similar lithospheric buoyancy. Mesozoic sedimentation increased the crustal thickness and lithospheric buoyancy of the Colorado Plateau relative to the midcontinent region. Backstripping calculations yield elevation without these sediments and lead to a calculated elevation difference between the two areas of about 1200 m, which represents unexplained plateau uplift. Review of constraints on uplift timing finds little support for a late Cenozoic uplift age and allows early to middle Cenozoic uplift, which is consistent with uplift mechanisms related to low-angle subduction that ended in the middle Cenozoic. Finite element heat flow calculations of low-angle subduction and lithosphere attenuation, using a range of initial lithosphere thicknesses and degree of attenuation, indicate that required uplift can result from tectonic removal of about 120 km of mantle lithosphere from an initially 200-km-thick lithosphere. This allows for partial preservation of North American mantle lithosphere with its distinctive isotopic signature in some late Cenozoic volcanic rocks and is consistent with normal Pn velocities in the uppermost mantle beneath the plateau.

  10. One and two-dimensional Triplicated waveform modeling beneath northeast Asia

    Science.gov (United States)

    Lai, Y.; Chen, L.

    2016-12-01

    Northeast Asia has long been investigated by geoscientists for its critical importance where the Pacific plate is subducting beneath the Eurasian continent accompanying by complicated mantle processes. In our study, taking advantages of frequent occurrence of deep earthquakes(>300km) in the subduction zone and dense seismic arrays in mainland China and Japan, we conducted both 1-Dimensional (1-D) and 2-Dimensional (2-D) triplicated forward waveform modeling on several selected deep events in the Japan-kuril and Izu areas. Triplicated waveform or Triplication which consists of refracted and reflected wave caused by the 410km/660km discontinuity, is very sensitive to the mantle transition zone velocity structure compared with the most commonly adopted methods such as receiver functions and seismic tomography. From 1-D modeling, we obtained the 1-D P and SH wave velocity structure model in the research area, which has an average P wave velocity 1% with maximum 2.1% and an average SH wave velocity 2% with maximum 3.9% higher than the global Iasp91 model in the mantle transition zone, consistent with the tomographic image of a fast stagnant slab in the region. In addition, we found slightly low velocities at approximately 560 km depth. However, the distribution and properties of such a low velocity feature cannot be well constrained based purely on the simple 1-D waveform modeling. In our 2-D simulations, we constructed a series of models and adopted the SEM method to investigate the sensitivity of triplicated waveforms on the slab structure in the mantle transition zone. In particular, we tested a model with a low velocity metastable olivine wedge (mow) added in the slab in the case that seismic waves travel roughly along the trending direction of the steep slab beneath the Izu Arc. Our preliminary result shows that the synthetic seismograms with the mow model display waveform variations distinctly different from the observations, suggesting that a metastable olivine

  11. Volcanism and Subduction: The Kamchatka Region

    Science.gov (United States)

    Eichelberger, John; Gordeev, Evgenii; Izbekov, Pavel; Kasahara, Minoru; Lees, Jonathan

    The Kamchatka Peninsula and contiguous North Pacific Rim is among the most active regions in the world. Kamchatka itself contains 29 active volcanoes, 4 now in a state of semi-continuous eruption, and I has experienced 14 magnitude 7 or greater earthquakes since accurate recording began in 1962. At its heart is the uniquely acute subduction cusp where the Kamchatka and Aleutian Arcs and Emperor Seamount Chain meet. Volcanism and Subduction covers coupled magmatism and tectonics in this spectacular region, where the torn North Pacific slab dives into hot mantle. Senior Russian and American authors grapple with the dynamics of the cusp with perspectives from the west and east of it, respectively, while careful tephrostratigraphy yields a remarkably precise record of behavior of storied volcanoes such as Kliuchevskoi and Shiveluch. Towards the south, Japanese researchers elucidate subduction earthquake processes with unprecedented geodetic resolution. Looking eastward, new insights on caldera formation, monitoring, and magma ascent are presented for the Aleutians. This is one of the first books of its kind printed in the English language. Students and scientists beginning research in the region will find in this book a useful context and introduction to the region's scientific leaders. Others who wish to apply lessons learned in the North Pacific to their areas of interest will find the volume a valuable reference.

  12. Regional differences in subduction ground motions

    CERN Document Server

    Beauval, Céline; Abrahamson, N; Theodulidis, N; Delavaud, E; Rodriguez, L; Scherbaum, F; Haendel, A

    2012-01-01

    A few ground-motion prediction models have been published in the last years, for predicting ground motions produced by interface and intraslab earthquakes. When one must carry out a probabilistic seismic hazard analysis in a region including a subduction zone, GMPEs must be selected to feed a logic tree. In the present study, the aim is to identify which models provide the best fit to the dataset M6+, global or local models. The subduction regions considered are Japan, Taiwan, Central and South America, and Greece. Most of the data comes from the database built to develop the new BCHydro subduction global GMPE (Abrahamson et al., submitted). We show that this model is among best-fitting models in all cases, followed closely by Zhao et al. (2006), whereas the local Lin and Lee (2008) is well predicting the data in Taiwan and also in Greece. The Scherbaum et al. (2009) LLH method prove to be efficient in providing one number quantifying the overall fit, but additional analysis on the between-event and within-ev...

  13. From transpressional to transtensional tectonics in Northern Central America controlled by Cocos - Caribbean subduction coupling change

    Science.gov (United States)

    Alonso-Henar, Jorge; Alvarez-Gomez, José Antonio; Jesús Martinez-Diaz, José

    2017-04-01

    The Central American Volcanic Arc (CAVA) is located at the western margin of the Caribbean plate, over the Chortís Block, spanning from Guatemala to Costa Rica. The CAVA is associated to the subduction of the Cocos plate under the Caribbean plate at the Middle America Trench. Our study is focused in the Salvadorian CAVA segment, which is tectonically characterized by the presence of the El Salvador Fault Zone (ESFZ), part of the western boundary of a major block forming the Caribbean plate (the Chortis Block). The structural evolution of the western boundary of the Chortis Block, particularly in the CAVA crossing El Salvador remains unknown. We have done a kinematic analysis from seismic and fault slip data and combined our results with a review of regional previous studies. This approach allowed us to constrain the tectonic evolution and the forces that control the deformation in northern Central America. Along the active volcanic arc we identified active transtensional deformation. On the other hand, we have identified two deformation phases in the back arc region: A first one of transpressional wrenching close to simple shearing (Miocene); and a second one characterized by almost E-W extension. Our results reveal a change from transpressional to transtensional shearing coeval with a migration of the volcanism towards the trench in Late Miocene times. This strain change could be related with a coupled to decoupled transition on the Cocos - Caribbean subduction interface, which could be related to a slab roll-back of the Cocos Plate beneath the Chortis Block. The combination of different degrees of coupling on the subduction interface, together with a constant relative eastward drift of the Caribbean Plate, control the deformation style along the western boundary of the Chortis Block.

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

    Science.gov (United States)

    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.

  15. First results of high-resolution modeling of Cenozoic subduction orogeny in Andes

    Science.gov (United States)

    Liu, S.; Sobolev, S. V.; Babeyko, A. Y.; Krueger, F.; Quinteros, J.; Popov, A.

    2016-12-01

    The Andean Orogeny is the result of the upper-plate crustal shortening during the Cenozoic Nazca plate subduction beneath South America plate. With up to 300 km shortening, the Earth's second highest Altiplano-Puna Plateau was formed with a pronounced N-S oriented deformation diversity. Furthermore, the tectonic shortening in the Southern Andes was much less intensive and started much later. The mechanism of the shortening and the nature of N-S variation of its magnitude remain controversial. The previous studies of the Central Andes suggested that they might be related to the N-S variation in the strength of the lithosphere, friction coupling at slab interface, and are probably influenced by the interaction of the climate and tectonic systems. However, the exact nature of the strength variation was not explored due to the lack of high numerical resolution and 3D numerical models at that time. Here we will employ large-scale subduction models with a high resolution to reveal and quantify the factors controlling the strength of lithospheric structures and their effect on the magnitude of tectonic shortening in the South America plate between 18°-35°S. These high-resolution models are performed by using the highly scalable parallel 3D code LaMEM (Lithosphere and Mantle Evolution Model). This code is based on finite difference staggered grid approach and employs massive linear and non-linear solvers within the PETSc library to complete high-performance MPI-based parallelization in geodynamic modeling. Currently, in addition to benchmark-models we are developing high-resolution (Paleozoic-Cenozoic sediments above the uppermost crust in the Subandean Ranges. Future work will be focused on the origin of different styles of deformation and topography evolution in Altiplano-Puna Plateau and Central-Southern Andes through 3D modeling of large-scale interaction of subducting and overriding plates.

  16. The Hainan Lone Plume Prompted By Encircling Subduction Zones around the South China Sea

    Science.gov (United States)

    Zhang, N.; Li, Z. X.

    2016-12-01

    The present of the late Mesozoic Hainan plume originated from the lower mantle of northern South China Sea has been documented by both seismic tomographic and geochemical-petrological work. The Hainan plume is one of the rare mantle plumes that are located away from the two large low shear velocity provinces (LLSVPs) in the lower mantle. Instead, it is within the broad global mantle downwelling zone, thus classified as a "lone plume". It had previously been proposed that this plume could have been triggered by subducting slabs into the lower mantle in the regions surrounding the South China Sea, a mechanism similar to what had been proposed for the formation of the LLSVPs. Here, we investigate the feasibility of such a plume-generation mechanism use a geodynamic modelling. Our geodynamic model has a high resolution regional domain embedded in a relatively low resolution global domain, which is set up in an adoptive-mesh-refined, 3D mantle convection code ASPECT. The top mechanic boundary condition of the global domain uses the latest plate motion reconstruction. In a series of experiments, we explore the effects of various important mantle parameters on mantle plume generation. The results so far suggest that the Indo-Australian cold slab acted like a cold wall from the southwest side in the present-day South China Sea mantle domain since 80 Ma ago. Fossil slabs from much older Tethyan subduction systems plays a moderate role in blocking the deep mantle hot materials from escaping to the north. The Western Pacific subduction systems started to promote the initiation of Hainan plume some 50 Ma ago from near the core-mantle boundary (CMB). As the plume head rises, it first moved to the west, and finally to beneath the South China Sea. Our model results are not sensitive to whether there is a chemical layer (possible D" layer) near the CMB.

  17. Crustal Structure beneath Mexico from Receiver Functions

    Science.gov (United States)

    Espindola, V.; Quintanar, L.; Espindola, J.

    2011-12-01

    The Servicio Sismológico Nacional (SSN) is Mexico's official organism in charge of the observation of seismicity in the country. Operated by the Universidad Nacional Autonoma de Mexico, it counts with 32 broadband stations deployed throughout the country. The coverage includes most of the geologic provinces of the territory, which vary widely in their geologic characteristics. The availability of records from teleseisms at those stations makes feasible to obtain sound and homogeneous estimates of the structure of the crust in the Mexican territory through the analysis of receiver functions (RF). In this work we present the results of the analysis of RF obtained from events registered from 1998 to 2009 in the 32 stations of the SSN. The RF technique, which uses converted phases at major velocity discontinuities, is a well established technique to infer the velocity contrasts and thickness of the underlying crust. Using this method we were able to infer the depth of the Moho, a major intracrustal discontinuity and in some cases the depth to the base of the subducting plate. We present maps of crustal thickness in Mexico, which varies between about 29 km in the Yucatan peninsula to more than 40 km in central Mexico. Poisson's coefficient varies between 0.19 and 0.30. The position of the descending slab shows a large variation in the subduction angle (from about 6° in the SE margin of the Pacific coast to about 60° in the NW ) as has been found from other techniques.

  18. Teleseismic P wave tomography of South Island, New Zealand upper mantle: Evidence of subduction of Pacific lithosphere since 45 Ma

    Science.gov (United States)

    Zietlow, Daniel W.; Molnar, Peter H.; Sheehan, Anne F.

    2016-06-01

    A P wave speed tomogram produced from teleseismic travel time measurements made on and offshore the South Island of New Zealand shows a nearly vertical zone with wave speeds that are 4.5% higher than the background average reaching to depths of approximately 450 km under the northwestern region of the island. This structure is consistent with oblique west-southwest subduction of Pacific lithosphere since about 45 Ma, when subduction beneath the region began. The high-speed zone reaches about 200-300 km below the depths of the deepest intermediate-depth earthquakes (subcrustal to ~200 km) and therefore suggests that ~200-300 km of slab below them is required to produce sufficient weight to induce the intermediate-depth seismicity. In the southwestern South Island, high P wave speeds indicate subduction of the Australian plate at the Puysegur Trench to approximately 200 km depth. A band with speeds ~2-3.5% lower than the background average is found along the east coast of the South Island to depths of ~150-200 km and underlies Miocene or younger volcanism; these low speeds are consistent with thinned lithosphere. A core of high speeds under the Southern Alps associated with a convergent margin and mountain building imaged in previous investigations is not well resolved in this study. This could suggest that such high speeds are limited in both width and depth and not resolvable by our data.

  19. Mantle Subduction and Uplift of Intracontinental Mountains: A Case Study from the Chinese Tianshan Mountains within Eurasia.

    Science.gov (United States)

    Li, Jinyi; Zhang, Jin; Zhao, Xixi; Jiang, Mei; Li, Yaping; Zhu, Zhixin; Feng, Qianwen; Wang, Lijia; Sun, Guihua; Liu, Jianfeng; Yang, Tiannan

    2016-06-29

    The driving mechanism that is responsible for the uplift of intracontinental mountains has puzzled geologists for decades. This study addresses this issue by using receiver function images across the Chinese Tianshan Mountains and available data from both deep seismic profiles and surface structural deformation. The near-surface structural deformation shows that the Tianshan crust experienced strong shortening during the Cenozoic. The receiver function image across the Tianshan Mountains reveals that the lithosphere of the Junggar Basin to the north became uncoupled along the Moho, and the mantle below the Moho subducted southwards beneath the northern part of the Tianshan Mountains, thereby thickening the overlying crust. Similar deep structures, however, are not observed under the Tarim Basin and the adjacent southern Tianshan Mountains. This difference in the deep structures correlates with geomorphological features in the region. Thus, a new model of mantle subduction, herein termed M-type subduction, is proposed for the mountain-building processes in intracontinental compressional settings. The available geomorphological, geological and seismic data in the literatures show that this model is probably suitable for other high, linear mountains within the continent.

  20. Coherent tilt signals observed in the Shumagin seismic gap Detection of time-dependent subduction at depth?

    Science.gov (United States)

    Beavan, J.; Bilham, R.; Hurst, K.

    1984-01-01

    Repeated surveys of short level lines in the Shumagin Islands, Alaska, reveal coherent tilt signals associated with subduction of the Pacific plate beneath the North American plate in the Shumagin seismic gap. Ten years of steady tilt down toward the trench is interrupted during 1978-1980 by a rapid episode of reverse tilt. The 'normal' tilt represents surface deformation as subduction occurs, with the plate boundary locked to at least 60 km depth. Using all available tilt, sea level, and seismic data, the tilt reversal is interpreted as due to an episodic reverse slip of about 80 cm magnitude on the plate boundary between about 70 km and 20 km depth, downdip from the main seismogenic zone, which remains locked. This event causes an increase of stress on the locked main thrust zone. It is speculated that such events may be a regular process at subduction zones, that great plate boundary earthquakes may be more common during their occurrence, and that their onset may be detectable early enough to give warning of an increase in probability for the occurrence of a great earthquake.

  1. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Implications for the Dynamics of Flat-Slabs

    Science.gov (United States)

    Eakin, Caroline; Long, Maureen; Beck, Susan; Wagner, Lara; Tavera, Hernando

    2014-05-01

    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for both teleseismic events (such as SKS, SKKS, PKS, sSKS) that sample the upper mantle column beneath the stations as well as direct S from local events that constrain anisotropy in the upper portion of the subduction zone. We analyze the variability of our results with respect to initial polarizations, ray paths, and frequency content as well as spatial variability between stations as the underlying slab morphology changes. Teleseismic results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA) that suggests a trench-perpendicular fast direction in the lowest layer in the sub-slab mantle. Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. Local S results indicate the presence of weak (delay times typically less than 0.5 seconds) and heterogeneous supra

  2. Subduction of oceanic plate irregularities and seismicity distribution along the Mexican Subduction Zone

    Science.gov (United States)

    Manea, Marina; Constantin Manea, Vlad; Gerya, Taras; Wong, Raul-Valenzuela; Radulian, Mircea

    2017-04-01

    It is known that oceanic plates morphology is not a simple one, but rather complicated by a series of irregularities as seamounts, fracture zones and mid-ocean ridges. These features present on the oceanic floor form part of the fabric of oceanic crust, and once formed they move together with the oceanic plates until eventually enter a subduction zone. Offshore Mexico the oceanic Cocos plate seafloor is littered with relatively small but numerous seamounts and seamount chains, and also large fracture zones. In this study we investigate the relationship between these oceanic irregularities located in the vicinity of the trench in Mexico and the distribution of subduction seismicity, including the rupture history of large subduction zone earthquakes. Since the interseismic locking degree is influenced by the rheological properties of crustal and mantle rocks, any variations along strike will result in significant changes in seismic behavior due to a change in frictional stability. Our preliminary study shows a direct relationship between the presence of seamounts chains on the incoming oceanic plate and the subduction seismicity distribution. We also found a clear relationship between the subduction of the Tehuantepec fracture zone (TFZ) and the low seismic activity in the region where this fracture zone intersects the trench. This region is also long term conspicuously quiet and considered a seismic gap where no significant large earthquake has occurred in more than 100 years. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations specifically tailored for the subduction of the Cocos plate in the region of TFZ we show that the weakened serpentinized fracture zone is partially scraped out in the forearc region because of its low strength and positive buoyancy. The presence of serpentinite in the fore arc apparently lowers the degree of interseismic locking, producing a seismic gap in southern Mexico.

  3. What's happening inside the subduction factory?

    Science.gov (United States)

    Penniston-Dorland, S. C.; Bebout, G. E.; Gorman, J. K.; Piccoli, P. M.; Walker, R. J.

    2012-12-01

    Much research has focused on the inputs and outputs of the 'subduction factory,' however a variety of metamorphic processes occur within the subducting slab and at its interface with the mantle wedge that contribute to creating the mixed signals observed in arc magmas. Subduction-related metamorphic rocks from the Catalina Schist represent a range of metamorphic grades and provide a natural laboratory to investigate these processes. Hybrid rock types such as reaction zones or 'rinds' between mafic (crustal) and ultramafic (mantle) rocks have attracted recent interest since they have a different bulk chemistry and mineralogy compared to the original inputs to the subduction factory. Here we explore the mineralogical and geochemical differences between the metamorphic rocks, their reaction zones, and endmember subduction input lithologies over a range of metamorphic grades including lawsonite albite, lawsonite blueschist, and amphibolite facies (with peak T ranging from ~ 275 to ~ 750°C and peak P ranging up to ~1.1 GPa). The results shed light on chemical changes occurring within the subduction zone and the processes happening inside the 'subduction factory', including mass transfer of elements by both fluid infiltration and mechanical mixing. Elements commonly enriched in arc magmatic rocks, such as the LILE (e.g. Ba, K), are enriched in metamafic rocks at all metamorphic grades relative to likely MORB protoliths. These enrichments are interpreted as the product of metamorphic fluid infiltration. Many major- and trace-element concentrations in reaction rinds fall between those of metamafic blocks and surrounding ultramafic-rich mélange matrix (including TiO2, MgO, FeO, Al2O3, Zr, Ni and Cr). Spatial distributions of these elements within the rinds suggest that the intermediate concentrations may be due to mechanical mixing of crustal and mantle materials. Rind concentrations of the highly siderophile elements (HSE: including Os, Ir, Ru, Pt, Pd, Re) as well as

  4. Velocities of Subducted Sediments and Continents

    Science.gov (United States)

    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 1.8 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-normal at

  5. Extensive decarbonation of continuously hydrated subducting slabs

    Science.gov (United States)

    Arzilli, Fabio; Burton, Mike; La Spina, Giuseppe; Macpherson, Colin G.

    2017-04-01

    CO2 release from subducting slabs is a key element of Earth's carbon cycle, consigning slab carbon either to mantle burial or recycling to the surface through arc volcanism, however, what controls subducted carbon's fate is poorly understood. Fluids mobilized by devolatilization of subducting slabs play a fundamental role in the melting of mantle wedges and in global geochemical cycles [1]. The effect of such fluids on decarbonation in subducting lithologies has been investigated recently [2-5], but several thermodynamic models [2-3], and experimental studies [6] suggest that carbon-bearing phases are stable at sub-arc depths (80-140 km; 2.6-4.5 GPa), implying that this carbon can be carried to mantle depths of >140 km. This is inconsistent with observations of voluminous CO2 release from arc volcanoes [7-10], located above slabs that are at 2.6-4.5 GPa pressure. The aim of this study is to re-evaluate the role of metamorphic decarbonation, showing if decarbonation reactions could be feasible at sub-arc depths combined with a continuous hydration scenario. We used the PerpleX software combined with a custom-designed algorithm to simulate a pervasive fluid infiltration characterized by "continuous hydration" combined with a distillation model, in which is possible to remove CO2 when decarbonation occurs, to obtain an open-system scenario. This is performed by repeatedly flushing the sediment with pure H2O at 0.5, 1.0 or 5 wt.% until no further decarbonation occurs. Here we show that continuous hydrated of sediment veneers on subducting slabs by H2O released from oceanic crust and serpentinised mantle lithosphere [11-13], produces extensive slab decarbonation over a narrow, sub-arc pressure range, even for low temperature subduction pathways. This explains the location of CO2-rich volcanism, quantitatively links the sedimentary composition of slab material to the degree of decarbonation and greatly increases estimates for the magnitude of carbon flux through the arc

  6. Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust

    Science.gov (United States)

    Kim, Daeyeong; Katayama, Ikuo; Michibayashi, Katsuyoshi; Tsujimori, Tatsuki

    2013-09-01

    Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (1 0 0)[0 0 1] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (0 0 1)[0 1 0], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AVP) and S-wave (AVS) seismic anisotropies of glaucophane (AVP = 20.4%, AVS = 11.5%) and epidote (AVP = 9.0%, AVS = 8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [0 0 1] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest VS polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AVP = 9.6%, AVS = 19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [0 0 1] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AVS of lawsonite blueschist (5.6-9.2%) is weak compared with that of epidote blueschist (8.4-11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu

  7. Diapiric flow at subduction zones: a recipe for rapid transport.

    Science.gov (United States)

    Hall, P S; Kincaid, C

    2001-06-29

    Recent geochemical studies of uranium-thorium series disequilibrium in rocks from subduction zones require magmas to be transported through the mantle from just above the subducting slab to the surface in as little as approximately 30,000 years. We present a series of laboratory experiments that investigate the characteristic time scales and flow patterns of the diapiric upwelling model of subduction zone magmatism. Results indicate that the interaction between buoyantly upwelling diapirs and subduction-induced flow in the mantle creates a network of low-density, low-viscosity conduits through which buoyant flow is rapid, yielding transport times commensurate with those indicated by uranium-thorium studies.

  8. Analysis of groundwater flow beneath ice sheets

    Energy Technology Data Exchange (ETDEWEB)

    Boulton, G. S.; Zatsepin, S.; Maillot, B. [Univ. of Edinburgh (United Kingdom). Dept. of Geology and Geophysics

    2001-03-01

    The large-scale pattern of subglacial groundwater flow beneath European ice sheets was analysed in a previous report. It was based on a two-dimensional flowline model. In this report, the analysis is extended to three dimensions by exploring the interactions between groundwater and tunnel flow. A theory is developed which suggests that the large-scale geometry of the hydraulic system beneath an ice sheet is a coupled, self-organising system. In this system the pressure distribution along tunnels is a function of discharge derived from basal meltwater delivered to tunnels by groundwater flow, and the pressure along tunnels itself sets the base pressure which determines the geometry of catchments and flow towards the tunnel. The large-scale geometry of tunnel distribution is a product of the pattern of basal meltwater production and the transmissive properties of the bed. The tunnel discharge from the ice margin of the glacier, its seasonal fluctuation and the sedimentary characteristics of eskers are largely determined by the discharge of surface meltwater which penetrates to the bed in the terminal zone. The theory explains many of the characteristics of esker systems and can account for tunnel valleys. It is concluded that the large-scale hydraulic regime beneath ice sheets is largely a consequence of groundwater/tunnel flow interactions and that it is essential similar to non-glacial hydraulic regimes. Experimental data from an Icelandic glacier, which demonstrates measured relationships between subglacial tunnel flow and groundwater flow during the transition from summer to winter seasons for a modern glacier, and which support the general conclusions of the theory is summarised in an appendix.

  9. Mapping crustal thinning beneath the Eastern Pyrenees

    Science.gov (United States)

    Diaz, Jordi; Chevrot, Sebastien; Verges, Jaume; Sylvander, Matthieu; Ruiz, Mario; Antonio-Vigil, Amanda

    2017-04-01

    The eastern termination of the Pyrenees is a complex region marked by two large tectonic events, the building of the Pyrenees during the Alpine orogeny and the Neogene extensional processes associated to the rotation of the Sardinia-Corsica block and the opening of the Valencia Trough. This complex tectonic history has left major imprints in crustal structures. Previous studies based on gravity data and active seismic profiles have documented a crustal thinning from 40-45 km about 80 km to the west of the Mediterranean coastline to less than 25 km beneath the eastern termination of the Pyrenees. To progress in the knowledge of the geometry of this transition, two passive seismic profiles have been acquired from mid 2015 to late 2016 within the OROGEN project, an academic-industrial collaboration between CNRS-Total-BRGM and CSIC. Up to 38 broad-band stations were deployed along two orthogonal lines, with an interstation spacing close to 10 km. First results of receiver function migration on the E-W profile suggest a smooth Moho thinning smoothly from 40 km beneath the western termination of the line to 23 km close to the coastline. The NNE-SSW profile shows a clearly defined Moho beneath Iberia, slightly deepening from 32 to 35 km northwards, a 28-30 km thick crust underneath the North Pyrenean Front Thrust and a complex geometry in the Axial Zone. Data from natural events located in the Gulf of Roses and near the intersecting point of the profiles have been recorded along the lines, hence allowing to produce wide angle reflection/refraction profiles providing additional constraints on the geometry of the crust/mantle boundary in the Eastern Pyrenees. These new results will be integrated with the available geophysical and geologic information for a more accurate geodynamical interpretation of the results. (Additional founding by the MISTERIOS project, CGL2013-48601-C2-1-R)

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

    Science.gov (United States)

    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. Ablative subduction - A two-sided alternative to the conventional subduction model

    Science.gov (United States)

    Tao, Winston C.; O'Connell, Richard J.

    1992-01-01

    The plausibility of a two-sided fluid-based model of lithospheric subduction that is based upon current views of lithospheric structure is examined. In this model the viscous lower lithosphere flows downward, and the brittle upper lithosphere deforms in passive response. This process is potentially double-sided, since it is found that even a buoyant plate can be dragged downward by a dense descending neighbor. Thus an apparent overriding plate may be worn away by a process of viscous ablation, with the rate of ablation a function of plate buoyancy. This process, called 'ablative subduction,' makes it possible to simply interpret observations concerning slab profiles, interplate seismicity, back arc tectonics, and complex processes such as double subduction and subduction polarity reversal. When experiments modeling the evolution of simple fluid 'slabs' are performed, slab profile is found to be strongly influenced by ablation in the overriding plate. When ablation is weak, as when a buoyant continent borders the trench, deformable slabs adopt shallow Andean-style profiles.

  12. The melting of subducted banded iron formations

    Science.gov (United States)

    Kang, Nathan; Schmidt, Max W.

    2017-10-01

    Banded iron formations (BIF) were common shelf and ocean basin sediments 3.5-1.8 Ga ago. To understand the fate of these dense rocks upon subduction, the melting relations of carbonated BIF were determined in Fe-Ca-(Mg)-Si-C-O2 at 950-1400 °C, 6 and 10 GPa, oxidizing (fO2 = hematite-magnetite, HM) and moderately reducing (fO2 ∼CO2-graphite/diamond, CCO) conditions. Solidus temperatures under oxidizing conditions are 950-1025 °C with H2O, and 1050-1150 °C anhydrous, but 250-175 °C higher at graphite saturation (values at 6-10 GPa). The combination of Fe3+ and carbonate leads to a strong melting depression. Solidus curves are steep with 17-20 °C/GPa. Near-solidus melts are ferro-carbonatites with ∼22 wt.% FeOtot, ∼48 wt% CO2 and 1-5 wt.% SiO2 at fO2 ∼ HM and ∼49 wt.% FeOtot, ∼20 wt% CO2 and 19-25 wt.% SiO2 at fO2 ∼ CCO . At elevated subduction geotherms, as likely for the Archean, C-bearing BIF could melt out all carbonate around 6 GPa. Fe-rich carbonatites would rise but stagnate gravitationally near the slab/mantle interface until they react with the mantle through Fe-Mg exchange and partial reduction. The latter would precipitate diamond and yield Fe- and C-rich mantle domains, yet, Fe-Mg is expected to diffusively re-equilibrate over Ga time scales. We propose that the oldest subduction derived diamonds stem from BIF derived melts.

  13. P-T and structural constraints of lawsonite and epidote blueschists from Liberty Creek and Seldovia: Tectonic implications for early stages of subduction along the southern Alaska convergent margin

    Science.gov (United States)

    López-Carmona, Alicia; Kusky, Timothy M.; Santosh, M.; Abati, Jacobo

    2011-01-01

    The southern Alaska convergent margin contains several small belts of sedimentary and volcanic rocks metamorphosed to blueschist facies, located along the Border Ranges fault on the contact between the Wrangellia and Chugach terranes. These belts are significant in that they are the most inboard, and thus probably contain the oldest record of Triassic-Jurassic northward-directed subduction beneath Wrangellia. The Liberty Creek HP-LT schist belt is the oldest and the innermost section of the Chugach terrane. Within this belt lawsonite blueschists contains an initial high-pressure assemblage formed by lawsonite + phengite + chlorite + sphene + albite ± apatite ± carbonates and quartz. Epidote blueschists are composed of sodic, sodic-calcic and calcic amphiboles + epidote + phengite + chlorite + albite + sphene ± carbonates and quartz. P-T pseudosections computed from four representative samples constrain maximum pressures at 16 kbar and 250-280 °C for the Lawsonite-bearing blueschists, and 15 kbar and 400-500 °C for the epidote-bearing blueschists, suggesting a initial subduction stage of 50-55 km depth. The growth of late albite porphyroblasts in all samples suggests a dramatic decompression from ca. 9 kbar to 5 kbar. The Liberty Creek schists can be correlated with the Seldovia blueschist belt on the Kenai Peninsula. Metamorphism in both terranes took place in the Early Jurassic (191-192 Ma), recording an early stage of subduction beneath Wrangellia. In the nearby terranes of the same margin, the age of metamorphism records an early stage of subduction at 230 Ma. Based on this difference in age, a maximum of 40 Ma were necessary to subduct the protoliths of the Seldovia and Liberty Creek blueschists to depths of circa 50-55 km, suggesting a minimum vertical component of subduction of 1.2-1.5 cm/year.

  14. Subduction Zone Dewatering at the Southern End of New Zealand's Hikurangi Margin - Insights from 2D Seismic Tomography

    Science.gov (United States)

    Crutchley, G. J.; Klaeschen, D.

    2016-12-01

    The southern end of New Zealand's Hikurangi subduction margin is characterised by highly-oblique convergence as it makes a southward transition into a right-lateral transform plate boundary. Long-offset seismic data that cross part of the offshore portion of this transition zone give new insight into the nature of the margin. We have carried out two-dimensional pre-stack depth migrations with an iterative reflection tomography to update the velocity field on two seismic lines in this area. The depth-migrated sections show much-improved imaging of faulting within the wedge, and the seismic velocities themselves give clues about the distribution of gas and/or overpressured regions at the plate boundary and within the overlying wedge. A fascinating observation is a major splay fault that has been (or continues to be) a preferred dewatering pathway through the wedge, evidenced by a thermal anomaly that has left its mark on the overlying gas hydrate layer. Another interesting observation is a thick and laterally extensive low velocity zone beneath the subduction interface, which might have important implications for the long-term mechanical stability of the interface. Our on-going work on these data is focused on amplitude versus offset analysis in an attempt to better understand the nature of the subduction interface and also the shallower gas hydrate system. This study is an example of how distinct disturbances of the gas hydrate system can provide insight into subduction zone fluid flow processes that are important for understanding wedge stability and ultimately earthquake hazard.

  15. Multichannel Seismic Imaging of the Rivera Plate Subduction at the Seismogenic Jalisco Block Area (Western Mexican Margin)

    Science.gov (United States)

    Bartolome, R.; Gorriz, E.; Danobeitia, J.; Barba, D. C., Sr.; Martí, D.; L Cameselle, A.; Nuñez-Cornu, F. J.; Bandy, W. L.; Mortera, C.; Nunez, D.; Alonso, J. L.; Castellon, A.; Prada, M.

    2016-12-01

    During the TSUJAL marine geophysical survey, conducted in February and March 2014 Spanish, Mexican and British scientists and technicians explored the western margin of Mexico, considered one of the most active seismic zones in America. This work aims to characterize the internal structure of the subduction zone of the Rivera plate beneath the North American plate in the offshore part of the Jalisco Block, to link the geodynamic and the recent tectonic deformation occurring there with the possible generation of tsunamis and earthquakes. For this purpose, it has been carried out acquisition, processing and geological interpretation of a multichannel seismic reflection profile running perpendicular to the margin. Crustal images show an oceanic domain, dominated by subduction-accretion along the lower slope of the margin with a subparallel sediment thickness of up to 1.6 s two way travel time (approx. 2 km) in the Middle American Trench. Further, from these data the region appears to be prone to giant earthquake production. The top of the oceanic crust (intraplate reflector) is very well imaged. It is almost continuous along the profile with a gentle dip (<10°); however, it is disrupted by normal faulting resulting from the bending of the plate during subduction. The continental crust presents a well-developed accretionary prism consisting of highly deformed sediments with prominent slumping towards the trench that may be the result of past tsunamis. Also, a Bottom Simulating Reflector (BSR) is identified in the first half a second (twtt) of the section. High amplitude reflections at around 7-8 s twtt clearly image a discontinuous Moho, defining a very gentle dipping subduction plane.

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

    Science.gov (United States)

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

    2017-09-01

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

  17. Peru Subduction Experiment (PERUSE) Preliminary results of Gravity measurements, Earthquake locations and Regional Seismicity in Southern Peru

    Science.gov (United States)

    Foote, E. J.; Davis, P. M.; Guy, R.; Lukac, M. L.; Feng, H. S.; Clayton, R. W.; Phillips, K. E.; Skinner, S.; Audin, L.; Tavera, H.; Aguilar, V.

    2009-12-01

    The Peru Subduction Experiment (PERUSE) is a collaborative project developed by UCLA, Caltech, French L’Institut de Recherche pour le Développement (IRD) and Instituto Geofisico del Peru (IGP) to improve geophysical models of the Andean Orogenic Belt and to image the subduction process in Southern Peru. One area of particular interest is where the Nazca Plate transitions from a normally subducting slab at an angle of about 30 degrees to a shallow subducting slab beneath the South American Plate. The PERUSE project, which started in the summer of 2008, consists of a linear array of 50 broadband seismic stations that are evenly spaced about 6 kilometers apart. They are aligned perpendicular to the coast of Peru, from Mollendo to Juliaca. Caltech will deploy 50 more stations by the end of 2009. Their line will run perpendicular to the current line, from Juliaca to Cusco. By the end of 2010, a third linear array will be installed north of and perpendicular to Caltech’s line in the Altiplano. Preliminary results from gravity measurements indicate that the crustal root of the Andes dips approximately 20 degrees on both sides of the range, and extends to a depth of approximately 70km. This also agrees well with the receiver function results, which show that the crust thickens from the coast of Mollendo through the Altiplano to Juliaca to a depth about 70km (Phillips et al, Fall AGU 2009). Teleseismic studies also indicate that the crustal thickness varies laterally below southern Peru. We are developing a heterogeneous model from the topographic and gravity data, teleseismic events, and the receiver function results to accurately locate earthquakes in the area of interest and to provide a better crustal model of the region.

  18. Dynamics of interplate domain in subduction zones: influence of rheological parameters and subducting plate age

    Directory of Open Access Journals (Sweden)

    D. Arcay

    2012-12-01

    Full Text Available The properties of the subduction interplate domain are likely to affect not only the seismogenic potential of the subduction area but also the overall subduction process, as it influences its viability. Numerical simulations are performed to model the long-term equilibrium state of the subduction interplate when the diving lithosphere interacts with both the overriding plate and the surrounding convective mantle. The thermomechanical model combines a non-Newtonian viscous rheology and a pseudo-brittle rheology. Rock strength here depends on depth, temperature and stress, for both oceanic crust and mantle rocks. I study the evolution through time of, on one hand, the brittle-ductile transition (BDT depth, zBDT, and, on the other hand, of the kinematic decoupling depth, zdec, simulated along the subduction interplate. The results show that both a high friction and a low ductile strength at the asthenospheric wedge tip shallow zBDT. The influence of the weak material activation energy is of second order but not negligible. zBDT becomes dependent on the ductile strength increase with depth (activation volume if the BDT occurs at the interplate decoupling depth. Regarding the interplate decoupling depth, it is shallowed (1 significantly if mantle viscosity at asthenospheric wedge tip is low, (2 if the difference in mantle and interplate activation energy is weak, and (3 if the activation volume is increased. Very low friction coefficients and/or low asthenospheric viscosities promote zBDT = zdec. I then present how the subducting lithosphere age affects the brittle-ductile transition depth and the kinematic decoupling depth in this model. Simulations show that a rheological model in which the respective activation energies of mantle and interplate material are too close hinders the mechanical decoupling at the down-dip extent of the interplate

  19. Three-dimensional magnetotelluric imaging of Cascadia subduction zone from an amphibious array

    Science.gov (United States)

    Yang, B.; Egbert, G. D.; Key, K.; Bedrosian, P.; Livelybrooks, D.; Schultz, A.

    2016-12-01

    We present results from three-dimensional inversion of an amphibious magnetotelluric (MT) array consisting of 71 offshore and 75 onshore sites in the central part of Cascadia, to image down-dip and along strike variations of electrical conductivity, and constrain the 3D distribution of fluids and melt in the subduction zone. A larger scale array consisting of EarthScope transportable-array data and several 2D legacy profiles (e.g. EMSLAB, CAFE-MT, SWORMT) which covers WA, OR, northern CA and northern NV has been inverted separately, to provide a broader view of the subduction zone. Inverting these datasets including seafloor data, and involving strong coast effects presents many challenges, especially for the nominal TE mode impedances which have very anomalous phases in both land and seafloor sites. We find that including realistic bathymetry and conductive seafloor sediments significantly stabilizes the inversion, and that a two stage inversion strategy, first emphasizing fit to the more challenging TE data, improved overall data fits. We have also constrained the geometry of the (assumed resistive) subducting plates by extracting morphological parameters (e.g. upper boundary and thickness) from seismological models (McCrory et al 2012, Schmandt and Humphreys 2010). These constraints improve recovery and resolution of subduction related conductivity features. With the strategies mentioned above, we improved overall data fits, resulting in a model which reveals (for the first time) a conductive oceanic asthenosphere, extending under the North America plate. The most striking model features are conductive zones along the plate interface, including a continuous stripe of high conductivity just inboard of the coast, extending from the northern limits of our model in Washington state, to north-central Oregon. High conductivities also occur in patches near the tip of the mantle wedge, at depths appropriate for eclogitization, and at greater depth beneath the arc, in

  20. Constraining the Rate of Water-Releasing Metamorphic Reactions in Subduction Zones

    Science.gov (United States)

    Mehl, L. Y.; Barkman, J. E.; Baxter, E. F.

    2006-12-01

    Metamorphic reactions in subduction zones have implications for the timing and depth over which water is released from subducting lithologies. Water release is related to the densification of the slab, the fluxing of arc magmatism, and seismogenesis. Garnet-forming reactions frequently involve the release of water. In order to quantify the rate at which water is produced during the P-T-t interval represented by garnet growth, we seek to constrain the rate and duration of garnet formation. Garnets in blueschists were collected from Sifnos, Greece. This island is in an accretionary wedge setting where the Apulian microplate subducted beneath the Eurasia plate from late Cretaceous to Eocene times. Peak metamorphic conditions were certainly less than 600 C and 2.0 GPa [1]. Blueschists from this subduction zone are preserved in the northern part of Sifnos; we collected most of our samples from Vroulidia Bay. This locality was chosen because large garnets (5-10mm) are preserved in many lithologies. Also, the peak temperatures determined by previous workers indicates that the garnet has remained closed to diffusive re-equilibration of Nd permitting the use of Sm/Nd geochronology to constrain prograde reaction history. Samples taken from Vroulidia Bay are fresh, with compositions that likely include mafic oceanic protoliths. Common assemblages include glaucophane, epidote, garnet, zoisite, quartz, phengite, partially replaced titanite, and rutile, as well as patches of carbonate and rare chlorite. The garnets sampled are dominantly almandine but also include a grossular component. Possible garnet-forming reactions in the Sifnos rocks may include: Chlorite + quartz = Almandine + H2O where the consumption of Fe-rich chlorite forms the almandine and releases water. The break down of Ca-rich clinozoisite, lawsonite, and/or titanite could contribute to the grossular component. Using the Sm/Nd isochron method, microsampled cores and rims of garnets may be directly dated. The

  1. Large-scale trench-normal mantle flow beneath central South America

    Science.gov (United States)

    Reiss, M. C.; Rümpker, G.; Wölbern, I.

    2018-01-01

    results do not provide any evidence for a significant contribution of trench-parallel mantle flow beneath the subducting slab.

  2. Continental basalts record the crust-mantle interaction in oceanic subduction channel: A geochemical case study from eastern China

    Science.gov (United States)

    Xu, Zheng; Zheng, Yong-Fei

    2017-09-01

    the crustal metasomatism through melt-peridotite reaction at the slab-mantle interface in oceanic subduction channels. Continental basalts of Mesozoic to Cenozoic ages from eastern China are used as a case example to illustrate the above petrogenetic mechanism. Subduction of the paleo-Pacific oceanic slab beneath the eastern edge of Eurasian continent in the Early Mesozoic would have transferred the crustal signatures into the mantle sources of these basalts. This process would be associated with rollback of the subducting slab at that time, whereas the partial melting of metasomatites takes place mainly in the Late Mesozoic to Cenozoic to produce the continental basalts. Therefore, OIB-like continental basalts are also the product of subduction-zone magmatism though they occur in intraplate settings.

  3. Noble gases recycled into the mantle through cold subduction zones

    Science.gov (United States)

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

    2017-08-01

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

  4. The subduction dichotomy of strong plates and weak slabs

    Science.gov (United States)

    Petersen, Robert I.; Stegman, Dave R.; Tackley, Paul J.

    2017-03-01

    A key element of plate tectonics on Earth is that the lithosphere is subducting into the mantle. Subduction results from forces that bend and pull the lithosphere into the interior of the Earth. Once subducted, lithospheric slabs are further modified by dynamic forces in the mantle, and their sinking is inhibited by the increase in viscosity of the lower mantle. These forces are resisted by the material strength of the lithosphere. Using geodynamic models, we investigate several subduction models, wherein we control material strength by setting a maximum viscosity for the surface plates and the subducted slabs independently. We find that models characterized by a dichotomy of lithosphere strengths produce a spectrum of results that are comparable to interpretations of observations of subduction on Earth. These models have strong lithospheric plates at the surface, which promotes Earth-like single-sided subduction. At the same time, these models have weakened lithospheric subducted slabs which can more easily bend to either lie flat or fold into a slab pile atop the lower mantle, reproducing the spectrum of slab morphologies that have been interpreted from images of seismic tomography.

  5. Crustal structure beneath two seismic stations in the Sunda-Banda arc transition zone derived from receiver function analysis

    Energy Technology Data Exchange (ETDEWEB)

    Syuhada, E-mail: hadda9@gmail.com [Graduate Research on Earthquake and Active Tectonics (GREAT), Bandung Institute of Technology, Jalan Ganesha 10, Bandung 40132 (Indonesia); Research Centre for Physics - Indonesian Institute of Sciences (LIPI), Kompleks Puspiptek Serpong, Tangsel 15314, Banten Indonesia (Indonesia); Hananto, Nugroho D.; Handayani, Lina [Research Centre for Geotechnology - Indonesian Institute of Sciences (LIPI), Jl. Sangkuriang (Kompleks LIPI) Bandung 40135 (Indonesia); Puspito, Nanang T; Yudistira, Tedi [Faculty of Mining and Petroleum Engineering ITB, Jalan Ganesha 10, Bandung 40132 (Indonesia); Anggono, Titi [Research Centre for Physics - Indonesian Institute of Sciences (LIPI), Kompleks Puspiptek Serpong, Tangsel 15314, Banten Indonesia (Indonesia)

    2015-04-24

    We analyzed receiver functions to estimate the crustal thickness and velocity structure beneath two stations of Geofon (GE) network in the Sunda-Banda arc transition zone. The stations are located in two different tectonic regimes: Sumbawa Island (station PLAI) and Timor Island (station SOEI) representing the oceanic and continental characters, respectively. We analyzed teleseismic events of 80 earthquakes to calculate the receiver functions using the time-domain iterative deconvolution technique. We employed 2D grid search (H-κ) algorithm based on the Moho interaction phases to estimate crustal thickness and Vp/Vs ratio. We also derived the S-wave velocity variation with depth beneath both stations by inverting the receiver functions. We obtained that beneath station PLAI the crustal thickness is about 27.8 km with Vp/Vs ratio 2.01. As station SOEI is covered by very thick low-velocity sediment causing unstable solution for the inversion, we modified the initial velocity model by adding the sediment thickness estimated using high frequency content of receiver functions in H-κ stacking process. We obtained the crustal thickness is about 37 km with VP/Vs ratio 2.2 beneath station SOEI. We suggest that the high Vp/Vs in station PLAI may indicate the presence of fluid ascending from the subducted plate to the volcanic arc, whereas the high Vp/Vs in station SOEI could be due to the presence of sediment and rich mafic composition in the upper crust and possibly related to the serpentinization process in the lower crust. We also suggest that the difference in velocity models and crustal thicknesses between stations PLAI and SOEI are consistent with their contrasting tectonic environments.

  6. The Run-Up of Subduction Zones

    Science.gov (United States)

    Riquelme, S.; Bravo, F. J.; Fuentes, M.; Matias, M.; Medina, M.

    2016-12-01

    Large earthquakes in subduction zones are liable to produce tsunamis that can cause destruction and fatalities. The Run-up is a geophysical parameter that quantifies damage and if critical facilities or population are exposed to. Here we use the coupling for certain subduction regions measured by different techniques (Potency and GPS observations) to define areas where large earthquakes can occur. Taking the slab 1.0 from the United States Geological Survey (USGS), we can define the geometry of the area including its tsunamigenic potential. By using stochastic earthquakes sources for each area with its maximum tsunamigenic potential, we calculate the numerical and analytical run-up for each case. Then, we perform a statistical analysis and calculate the envelope for both methods. Furthermore, we build an index of risk using: the closest slope to the shore in a piecewise linear approach (last slopecriteria) and the outputsfrom tsunami modeling. Results show that there are areas prone to produce higher run-up than others based on the size of the earthquake, geometrical constraints of the source, tectonic setting and the coast last slope. Based on these results, there are zones that have low risk index which can define escape routes or secure coastal areas for tsunami early warning, urban and planning purposes when detailed data is available.

  7. Neotectonic fault detection and lithosphere structure beneath SW of High Atlas (Morocco)

    Science.gov (United States)

    Timoulali, Youssef; Radi, Said; Azguet, Roumaissae; Bachaoui, Mostapha

    2016-08-01

    subduction zone marking the limit between SW of High Atlas and the western Meseta. A second high velocity body, dipping north beneath the Hercynian Tichka Massif is detected. This positive velocity anomaly can be interpreted as an old subduction zone marking the limit between Meseta Domain and West African Craton. The occurrence of tholeiitic and alkaline magmatism respectively in the Essaouira basin zone and in the Hercynian Ticka Massif zone leads us to conclude that remains of subduction exist in these zones. The negative lithospheric anomalies are interpreted as heat or hot asthenospheric material upwelling from depth crossing the main crustal structures in SW High Atlas. The negative lithospheric anomalies are interpreted as hot asthenospheric material or heat coming from depth to replace the detached crust. Based on our analysis of local P wave velocities and profiles, the crustal thickness beneath SW of High Atlas region varies from 30 km to 45 km in central and western part.

  8. Viscous Dissipation and Criticality of Subducting Slabs

    Science.gov (United States)

    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

  9. Mapping subduction interface coupling using magnetotellurics: Hikurangi margin, New Zealand

    Science.gov (United States)

    Heise, W.; Caldwell, T. G.; Bannister, S.; Bertrand, E. A.; Ogawa, Y.; Bennie, S. L.; Ichihara, H.

    2017-09-01

    The observation of slow-slip, seismic tremor, and low-frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction (megathrust) earthquakes. However, the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood. Using magnetotelluric data, we show in situ that an electrically resistive patch on the Hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data. This resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone. Together, the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the Hikurangi margin may be controlled by the interface fluid and sediment content: the resistive patch marking a fluid- and sediment-starved area with an increased density of small, seismogenic-asperities, and therefore a greater likelihood of subduction earthquake nucleation.

  10. Gravity and Magnetic Anomaly Interpretations and 2.5D Cross-Section Models over the Border Ranges Fault System and Aleutian Subduction Zone, Alaska

    Science.gov (United States)

    Mankhemthong, N.; Doser, D. I.; Baker, M. R.; Kaip, G.; Jones, S.; Eslick, B. E.; Budhathoki, P.

    2011-12-01

    Quaternary glacial covers and lack of dense geophysical data on the Kenai Peninsula cause a location and geometry of the Border Ranges fault system (BRFS) within a recent forearc-accretionary boundary of Aleutian subduction zone in southern Alaska are unclear. Using new ~1,300 gravity collections within the Anchorage and Kenai Peninsula regions complied with prior 1997 gravity and aeromagnetic data help us better imaging these fault and the subduction structures. Cook Inlet forearc basin is corresponded by deep gravity anomaly lows; basin boundaries are characterized by a strong gravity gradient, where are considered to be traces of Border Ranges fault system on the east and Castle Mountain and Bruin Bay fault system on the west and northwest of the forearc basin respectively. Gravity anomaly highs over accreted rocks generally increase southeastward to the Aleutian trench, but show a gravity depression over the Kenai Mountains region. The lineament between gravity high and low in the same terrenes over the Kenai Peninsula is may be another evidence to determine the Southern Edge of the Yakutat Microplate (SEY) as inferred by Eberhart-Phillips et al. (2006). Our 2.5-D models illustrate the main fault of the BRFS dips steeply toward the west with a downslip displacement. Gravity and Magnetic anomaly highs, on the east of the BRFS, probably present a slice of the ultramafic complex emplaced by faults along the boundary of the forearc basin and accretionary wedge terranes. Another magnetic high beneath the basin in the southern forearc basin support a serpentiznied body inferred by Saltus et al. (2001), with a decreasing size toward the north. Regional density-gravity models show the Pacific subducting slab beneath the foreacre-arc teranes with a gentle and flatted dip where the subducting plate is located in north of SEY and dips more steeply where it is located on the south of SEY. The gravity depression over the accreted terrene can be explained by a density low

  11. Seismic structure of the subducting seamounts on the trench axis: Erimo Seamount and Daiichi-Kashima Seamount, northern and southern ends of the Japan Trench

    Science.gov (United States)

    Nishizawa, A.; Kaneda, K.; Watanabe, N.; Oikawa, M.

    2009-03-01

    We present detailed P-wave velocity models of subducting seamounts from two wide-angle seismic experiments across the Erimo Seamount and Daiichi-Kashima Seamount, northern and southern ends of the Japan Trench. Common characteristics of the velocity models of the seamounts are that the maximum crustal thicknesses of the seamounts are 12-17 km thicker than a typical oceanic crust and that Pn velocities beneath the seamounts are approximately 7.7 km/s, i.e., slower then those of the neighboring area. These features are very similar to the crustal models for the seamounts produced by the Cretaceous off-ridge volcanism on the Pacific Basin.

  12. Bubble streams rising beneath an inclined surface

    Science.gov (United States)

    Bird, James; Brasz, Frederik; Kim, Dayoung; Menesses, Mark; Belden, Jesse

    2017-11-01

    Bubbles released beneath a submerged inclined surface can tumble along the wall as they rise, dragging the surrounding fluid with them. This effect has recently regained attention as a method to mitigate biofouling in marine environment, such as a ship hull. It appears that the efficacy of this approach may be related to the velocity of the rising bubbles and the extent that they spread laterally as they rise. Yet, it is unclear how bubble stream rise velocity and lateral migration depend on bubble size, flow rate, and inclination angle. Here we perform systematic experiments to quantify these relationships for both individual bubble trajectories and ensemble average statistics. Research supported by the Office of Naval Research under Grant Number award N00014-16-1-3000.

  13. Geothermal structure of the eastern Black Sea basin and the eastern Pontides orogenic belt: Implications for subduction polarity of Tethys oceanic lithosphere

    Directory of Open Access Journals (Sweden)

    Nafiz Maden

    2013-07-01

    Full Text Available The numerical results of thermal modeling studies indicate that the lithosphere is cold and strong beneath the Black Sea basin. The thermal lithospheric thickness increases southward from the eastern Pontides orogenic belt (49.4 km to Black Sea basin (152.2 km. The Moho temperature increases from 367 °C in the trench to 978 °C in the arc region. The heat flow values for the Moho surface change between 16.4 mW m−2 in the Black Sea basin and 56.9 mW m−2 in the eastern Pontides orogenic belt. Along the southern Black Sea coast, the trench region has a relatively low geothermal potential with respect to the arc and back-arc region. The numerical studies support the existence of southward subduction beneath the Pontides during the late Mesozoic–Cenozoic.

  14. Shear wave anisotropy beneath the Tibetan Plateau

    Science.gov (United States)

    McNamara, Daniel E.; Owens, Thomas J.; Silver, Paul G.; Wu, Frances T.

    1994-07-01

    Eleven broadband digital seismic stations were deployed across the central Tibetan Plateau in the first extensive passive-source experiment attempted within the Tibetan Plateau. One year of recording resulted in 186 event-station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array. Measurements of the fast polarization direction (phi) and delay time (delta-t) for SKS and direct S arrivals reveal systematic variations along the north-south oriented array. In the north central region of the plateau, very large delay times are observed at three stations, the largest of which is BUDO with delta-t = 2.4 s. However, at TUNL, which is off the northern edge of the plateau and 110 km from BUDO, and at sites in the south central plateau, delta-t decreases by nearly a factor of 3. We also observe a systematic rotation of phi from about 45 deg (NE) to 90 deg (E-W) from south to north along the array. A previously identified zone of inefficient Sn propagation correlates well with our region of large delta-t observations. The large delay times suggest that a relatively high number of anisotropic crystals are preferentially alligned within the mantle-lid, beneath the north central portion of the Tibetan Plateau. In most cases, fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the Tibetan Plateau.

  15. Thermochronological response to rifting and subduction in the Corsica-Sardinia block

    Science.gov (United States)

    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.

  16. A detailed look beneath the surface: Evidence of a surface reconstruction beneath a capping layer

    Energy Technology Data Exchange (ETDEWEB)

    Krull, D., E-mail: dominique.krull@tu-dortmund.de [Lehrstuhl für Experimentelle Physik I and DELTA, Dortmund University of Technology, Otto-Hahn-Str. 4, 44221 Dortmund (Germany); Tesch, M.F. [University of Applied Sciences Münster, Stegerwaldstr. 39, 48565 Steinfurt (Germany); Schönbohm, F.; Lühr, T.; Keutner, C.; Berges, U. [Lehrstuhl für Experimentelle Physik I and DELTA, Dortmund University of Technology, Otto-Hahn-Str. 4, 44221 Dortmund (Germany); Mertins, H.-Ch. [University of Applied Sciences Münster, Stegerwaldstr. 39, 48565 Steinfurt (Germany); Westphal, C. [Lehrstuhl für Experimentelle Physik I and DELTA, Dortmund University of Technology, Otto-Hahn-Str. 4, 44221 Dortmund (Germany)

    2016-03-30

    Highlights: • Demonstration of a detailed look into internal interface structures. • Close-up view to an internal surface beneath a capping layer. • Resolving of inter-diffusion layers. • Imaging of phase-transition (local layers). • Demonstration of achieving magnetic information of an interlayer. - Abstract: Many physical effects are strongly depending on the composition of the interfaces between separating layers. Hence, the knowledge of the interfacial characteristics such as structure, chemical bonds, or magnetic properties of the corresponding materials is essential for an understanding and optimization of these effects. This study reports on a combined magnetic and structural analysis using X-ray photoelectron diffraction (XPD) and transverse magneto-optical Kerr effect (T-MOKE). The information depth of these methods is demonstrated by investigating the uppermost GaAs(001) layer beneath a Fe-film and the interfacial regimes of Fe/GaAs(001) beneath an MgO capping layer. Iron was prepared on a clean GaAs(001) surface and a GaAs(001)-(4 × 2)-reconstructed surface. Beneath the Fe-film, the (4 × 2)-reconstruction is not lifted, which is clearly shown by the diffraction pattern of the GaAs(4 × 2)-Fe surface. It is shown that Fe inter-diffusion, resulting in an amorphous interface, is almost prevented by the Ga-rich reconstruction. The magneto-optical measurements with T-MOKE clearly demonstrated the Fe-interlayer in a ferromagnetic state. We find no evidence for magnetic properties neither within the signal of the GaAs-substrate nor the MgO-film.

  17. A detailed look beneath the surface: Evidence of a surface reconstruction beneath a capping layer

    Science.gov (United States)

    Krull, D.; Tesch, M. F.; Schönbohm, F.; Lühr, T.; Keutner, C.; Berges, U.; Mertins, H.-Ch.; Westphal, C.

    2016-03-01

    Many physical effects are strongly depending on the composition of the interfaces between separating layers. Hence, the knowledge of the interfacial characteristics such as structure, chemical bonds, or magnetic properties of the corresponding materials is essential for an understanding and optimization of these effects. This study reports on a combined magnetic and structural analysis using X-ray photoelectron diffraction (XPD) and transverse magneto-optical Kerr effect (T-MOKE). The information depth of these methods is demonstrated by investigating the uppermost GaAs(001) layer beneath a Fe-film and the interfacial regimes of Fe/GaAs(001) beneath an MgO capping layer. Iron was prepared on a clean GaAs(001) surface and a GaAs(001)-(4 × 2)-reconstructed surface. Beneath the Fe-film, the (4 × 2)-reconstruction is not lifted, which is clearly shown by the diffraction pattern of the GaAs(4 × 2)-Fe surface. It is shown that Fe inter-diffusion, resulting in an amorphous interface, is almost prevented by the Ga-rich reconstruction. The magneto-optical measurements with T-MOKE clearly demonstrated the Fe-interlayer in a ferromagnetic state. We find no evidence for magnetic properties neither within the signal of the GaAs-substrate nor the MgO-film.

  18. Magnetolluric Imaging of the Lithosphere beneath the Tibetan Plateau: A Synoptic View

    Science.gov (United States)

    Dong, H.; Wei, W.; Jin, S.; Ye, G.; Jing, J.; Zhang, L.; Jones, A. G.; Xie, C.; Yin, Y.

    2016-12-01

    To better understand the collision and convergence process between India and Asia continent, magnetotelluric (MT) data have been used to image crustal electrical resistivity and to estimate the rheological structure in the south, east and north margin of Tibetan Plateau in the last decades. However, most of these MT data were collected as isolated profiles and insufficient to convey a synoptic view of the Plateau. Recently, the MT data coverage is greatly improved from the 1 by 1 degree SINOPROBE MT array data from 2011-2013. In this study, a three-dimensional (3D) geo-electrical model of the crust and uppermost mantle is derived from the combined broadband and long period array dataset. The resulting model features broadly resistive structure in the upper crust, with the exception of small scale conductors lining along the Yarlung-Zangbo Suture (YZS) and Bangong-Nujiang Suture (BNS). The lower crust consists of widely spread high conductive structures beneath Lhasa and Qiangtang Block except for the southeast part of Tibetan Plateau. Instead of the previous proposed homogenous high conductive layer, these lower crustal conductors are generally N-S directed and perpendicular to the above Cenozoic and Mesozoic sutures. While the lower crustal structures probably indicate partial melts induced by radiogenic heat from the thickened crust, the conductor along the suture zones in the upper crust might imply pathways for deep fluids into the shear zones of YZS and BNS. One of the most prominent feature revealed is the upper mantle conductive sturcture in the central part of Tibet Plateau (85-92°E) accompanied by two resistive regions in the southeast and southwest part of the Plateau. While the southwest resistive zone is broadly consistent with the "flat subduction" region of Indian lithosphere revealed by seismic receiver function study, the conductive middle part of Tibetan Plateau might represent the upper mantle melts induced by the water carried by the steeper

  19. Helium as a tracer for fluids released from Juan de Fuca lithosphere beneath the Cascadia forearc

    Science.gov (United States)

    McCrory, Patricia A.; Constantz, James E.; Hunt, Andrew G.; Blair, James Luke

    2016-01-01

    The ratio between helium isotopes (3He/4He) provides an excellent geochemical tracer for investigating the sources of fluids sampled at the Earth's surface. 3He/4He values observed in 25 mineral springs and wells above the Cascadia forearc document a significant component of mantle-derived helium above Juan de Fuca lithosphere, as well as variability in 3He enrichment across the forearc. Sample sites arcward of the forearc mantle corner (FMC) generally yield significantly higher ratios (1.2-4.0 RA) than those seaward of the corner (0.03-0.7 RA). The highest ratios in the Cascadia forearc coincide with slab depths (40-45 km) where metamorphic dehydration of young oceanic lithosphere is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab depths (25-30 km) beneath sites seaward of the corner.Tremor (considered a marker for high fluid pressure) and high RA values in the forearc are spatially correlated. The Cascadia tremor band is centered on its FMC, and we tentatively postulate that hydrated forearc mantle beneath Cascadia deflects a significant portion of slab-derived fluids updip along the subduction interface, to vent in the vicinity of its corner. Furthermore, high RA values within the tremor band just arcward of the FMC, suggest that the innermost mantle wedge is relatively permeable.Conceptual models require: (1) a deep fluid source as a medium to transport primordial 3He; (2) conduits through the lithosphere which serve to speed fluid ascent to the surface before significant dilution from radiogenic 4He can occur; and (3) near lithostatic fluid pressure to keep conduits open. Our spatial correlation between high RA values and tectonic tremor provides independent evidence that tremor is associated with deep fluids, and it further suggests that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through ductile upper mantle and lower crust.

  20. Depositionary Margins: The Destruction and Renovation of Subduction Forearcs

    Science.gov (United States)

    Vannucchi, P.; Morgan, J. P.; Silver, E. A.; Kluesner, J.

    2016-12-01

    A depositionary margin is 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 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.

  1. Subducted oceanic relief locks the shallow megathrust in central Ecuador

    Science.gov (United States)

    Collot, Jean-Yves; Sanclemente, Eddy; Nocquet, Jean-Mathieu; Leprêtre, Angélique; Ribodetti, Alessandra; Jarrin, Paul; Chlieh, Mohamed; Graindorge, David; Charvis, Philippe

    2017-05-01

    Whether subducted oceanic reliefs such as seamounts promote seismic rupture or aseismic slip remains controversial. Here we use swath bathymetry, prestack depth-migrated multichannel seismic reflection lines, and wide-angle seismic data collected across the central Ecuador subduction segment to reveal a broad 55 km × 50 km, 1.5-2.0 km high, low height-to-width ratio, multipeaked, sediment-bare, shallow subducted oceanic relief. Owing to La Plata Island and the coastline being located, respectively, 35 km and 50-60 km from the trench, GPS measurements allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, 80 km × 55 km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its highly jagged topography, the absence of a subduction channel, and a stiff erosive oceanic margin are found to be long-term geological characteristics associated with the shallow locking of the megathrust. Although the size and level of locking observed at the subducted relief scale could produce an Mw >7+ event, no large earthquakes are known to have happened for several centuries. On the contrary, frequent slow slip events have been recorded since 2010 within the locked patch, and regular seismic swarms have occurred in this area during the last 40 years. These transient processes, together with the rough subducted oceanic topography, suggest that interplate friction might actually be heterogeneous within the locked patch. Additionally, we find that the subducted relief undergoes internal shearing and produces a permanent flexural bulge of the margin, which uplifted La Plata Island.

  2. IODP Expedition 334: An Investigation of the Sedimentary Record, Fluid Flow and State of Stress on Top of the Seismogenic Zone of an Erosive Subduction Margin

    Directory of Open Access Journals (Sweden)

    Paola Vannucchi

    2013-03-01

    Full Text Available The Costa Rica Seismogenesis Project (CRISP is an experiment to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. Integrated Ocean Drililng Program (IODP Expedition 334 by R/V JOIDES Resolution is the first step toward deep drilling through the aseismic and seismicplate boundary at the Costa Rica subduction zone offshore the Osa Peninsula where the Cocos Ridge is subducting beneath the Caribbean plate. Drilling operations included logging while drilling (LWD at two slope sites (Sites U1378 and U1379 and coring at three slope sites (Sites U1378–1380and at one site on the Cocos plate (Site U1381. For the first time the lithology, stratigraphy, and age of the slope and incoming sediments as well as the petrology of the subducting Cocos Ridge have been characterized at this margin.The slope sites recorded a high sediment accumulation rate of 160–1035m m.y.-1 possibly caused by on-land uplift triggered by the subduction of the Cocos Ridge. The geochemical data as well as the in situ temperature data obtained at the slope sites suggest that fluids are transported from greater depths. The geochemical profiles at Site U1381 reflect diffusional communication of a fluid with seawater-likechemistry and the igneous basement of the Cocos plate (Solomon et al., 2011; Vannucchi et al., 2012a. The present-day in situ stress orientation determined by borehole breakouts at Site U1378 in the middle slope and Site U1379 in the upper slope shows a marked change in stress state within ~12 km along the CRISP transect; that maycorrespond to a change from compression (middle slope to extension (upper slope.

  3. How the gas hydrate system gives insight into subduction wedge dewatering processes in a zone of highly-oblique convergence on the southern Hikurangi margin of New Zealand

    Science.gov (United States)

    Crutchley, Gareth; Klaeschen, Dirk; Pecher, Ingo; Henrys, Stuart

    2017-04-01

    The southern end of New Zealand's Hikurangi subduction margin is characterised by highly-oblique convergence as it makes a southward transition into a right-lateral transform plate boundary at the Alpine Fault. Long-offset seismic data that cross part of the offshore portion of this transition zone give new insight into the nature of the plate boundary. We have carried out 2D pre-stack depth migrations, with an iterative reflection tomography to update the velocity field, on two seismic lines in this area to investigate fluid flow processes that have implications for the mechanical stability of the subduction interface. The results show distinct and focused fluid expulsion pathways from the subduction interface to the shallow sub-surface. For example, on one of the seismic lines there is a clear disruption of the gas hydrate system at its intersection with a splay fault - a clear indication of focused fluid release from the subduction interface. The seismic velocities derived from tomography also highlight a broad, pronounced low velocity zone beneath the deforming wedge that we interpret as a thick zone of gas-charged fluids that may have important implications for the long-term frictional stability of the plate boundary in this area. The focused flow upward toward the seafloor has the potential to result in the formation of concentrated gas hydrate deposits. Our on-going work on these data will include amplitude versus offset analysis in an attempt to better characterise the nature of the subduction interface, the fluids in that region, and also the shallower gas hydrate system.

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

    Science.gov (United States)

    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.

  5. Deformation cycles of subduction earthquakes in a viscoelastic Earth.

    Science.gov (United States)

    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.

  6. A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc

    NARCIS (Netherlands)

    Arculus, Richard J.; Ishizuka, Osamu; Bogus, Kara A.; Gurnis, Michael; Hickey-Vargas, Rosemary; Aljahdali, Mohammed H.; Bandini-Maeder, Alexandre N.; Barth, Andrew P.; Brandl, Philipp A.; Drab, Laureen; Do Monte Guerra, Rodrigo; Hamada, Morihisa; Jiang, Fuqing; Kanayama, Kyoko; Kender, Sev; Kusano, Yuki; Li, He; Loudin, Lorne C.; Maffione, Marco; Marsaglia, Kathleen M.; McCarthy, Anders; Meffre, Sebastién; Morris, Antony; Neuhaus, Martin; Savov, Ivan P.; Sena, Clara; Tepley, Frank J.; Van Der Land, Cees; Yogodzinski, Gene M.; Zhang, Zhaohui

    2015-01-01

    The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence

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

    NARCIS (Netherlands)

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

    2013-01-01

    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 flowforce the

  8. Tomography of the subducting Cocos plate in central Mexico: Images of a truncated slab

    Science.gov (United States)

    Husker, A. L.; Davis, P. M.

    2007-12-01

    The location of the subducting slab beneath Mexico City and its relation to the Trans-Mexican Volcanic Belt (TMVB) has been unknown because of the absence of deep seismicity that could be used to define the Wadati-Benioff zone. We used data from a temporary seismic network to locate the slab using seismic tomography. A break is seen in the Cocos plate under the TMVB. The break is seen with both P-wave and S-wave tomography and in a constrained tomographic inversion that finds parameters for a simple slab temperature model. The data used are 172 teleseismic earthquakes recorded by the Middle American Subduction Experiment (MASE). MASE was made up of 100 broadband seismometers spaced every 5 km running from Acapulco north through Mexico City almost to the Gulf Coast. In order to determine arrival time differences, Dt, across the array, waveforms were cross correlated. When Dt is plotted with respect to the latitude of the seismometer at which it was recorded, a Dt minimum (early arrivals) is seen near the TMVB. This minimum is shifted northward for back azimuths from the south, and southward for back azimuths from the north. The shift in the Dt minimum is indicative of a fast structure at depth. If there were no break in the slab, the localized minimum would not be seen. Tomography reveals an approximately 50-80 km thick slab diving into the mantle at about 75° to approximately 550 km depth and 375 km inland from Acapulco. We speculate the absence of deep earthquakes is due to low stresses in a young plate that has been truncated at depth.

  9. The upliftment in the Mejillones peninsula during 1995-2015 with two subduction earthquakes

    Science.gov (United States)

    Narayan Shrivastava, Mahesh; Gonzalez, Gabriel; Moreno, Marcos

    2017-04-01

    The Mejillones peninsula in northern Chile show the significant Quaternary surface uplift, which contrast with the surrounding coastal areas. The continuous GPS sites in this peninsula have detected significant upliftment during propagation of subduction earthquakes. The Mejillones peninsula region shows the low locking degree [Métois et al., 2013 & Bejar et al., 2014] and acts as a seismic barrier for the last two megathrust earthquakes such as 1995, Mw 8.1 Antofagasta earthquake and the 2007, Mw 7.7 Tocopilla earthquake in the south and north of peninsula. However, significant afterslip has taken place beneath the Mejillones peninsula in both earthquakes. We describe the vertical deformation in the Mejillones peninsula with the megathrust architecture during 1995-2015 with megathrust earthquakes and locking degree. We simulate the vertical displacement with the coseismic slip and 3.3 years afterslip of Antofagasta, 1995 by [Chlieh et al., 2004] and coseismic slip and 3.3 years of afterslip of Tocopilla earthquake 2007 modeled in this present study. With both earthquakes due to coseismic slip and afterslip in 3.3 years, Mejillones peninsula has exhibited 105 cm cumulative upliftment at the JRGN GPS site. The modeled new locking model by additional dataset of continuous and survey GPS from 2012-2015, it shows the low locking degree in the Mejillones peninsula region confirming the previous locking models. Our interseismic model and observation suggest that JRGN Continuous GPS site shows subsidence rate 2.0 mm/year after the Tocopilla earthquake 2007. But in contrast the continuous GPS sites UCNF, PB05 and VLZL show the significant upliftment. By considering the modeled locking degree same during 1995 to 2015, we conclude that Mejillones peninsula would have remained upliftment 98 cm. The vertical tectonic rate fluctuates at various timescale in the subduction zones, the study of elevated shorelines may provide an efficient tool to develop our understanding of long

  10. Seismic Investigation of Sediments Beneath Antarctic Ice Streams

    Science.gov (United States)

    Luthra, Tarun

    Fault zones are areas of localized deformation that accommodate strain in the Earth's crust accumulated over time due to tectonic motion or stress transfer from adjacent areas. Faults are traditionally considered to accommodate this strain by either constant slow movement (creep), or by rapid catastrophic failure events (earthquakes). The behavior of faults which produce earthquakes has been extensively studied, including characterization of the time and slip predictability of earthquakes, frequency-magnitude distributions, aftershock decay patterns, dynamic triggering, and frictional processes. Chance observations in global positioning system (GPS) data from the Vancouver Island area in 2001 revealed a new kind of fault slip that had not been considered before, slowslip events. Since those early observations, slow-slip events have been observed at most major subduction interfaces and even in glacial systems. In this dissertation, I strive to answer some of the fundamental questions about slow-slip systems. Little is known about the dynamics of these systems and how they operate. Scattered laboratory observations have provided clues, but this study is the first systematic examination of slow-slip earthquakes and their frictional behavior in the laboratory. I examine questions such as what controls how a fault zone will fail and what the velocity and normal stress sensitives are, then connect those mechanisms to observations from a natural slow-slip system beneath Whillans Ice Stream in western Antarctica. In chapter 1, I demonstrate how to modify the stiffness of the testing machine to create slow-slip events in the laboratory inartificial granular material. I also present a method to automatically calculate the stiffness of each slip event in a given experiment. Chapter 2 extends this work into a synthetic fault gouge material and carefully examines the sensitivity of the system to the stiffness of the testing apparatus. Chapter 3 introduces the new parameter of

  11. Mohorovicic discontinuity depth analysis beneath North Patagonian Massif

    Science.gov (United States)

    Gómez Dacal, M. L.; Tocho, C.; Aragón, E.

    2013-05-01

    The North Patagonian Massif is a 100000 km2, sub-rectangular plateau that stands out 500 to 700 m higher in altitude than the surrounding topography. The creation of this plateau took place during the Oligocene through a sudden uplift without noticeable internal deformation. This quite different mechanical response between the massif and the surrounding back arc, the short time in which this process took place and a regional negative Bouguer anomaly in the massif area, raise the question about the isostatic compensation state of the previously mentioned massif. In the present work, a comparison between different results about the depth of the Mohorovicic discontinuity beneath the North Patagonian Massif and a later analysis is made. It has the objective to analyze the crustal thickness in the area to contribute in the determination of the isostatic balance and the better understanding of the Cenozoic evolution of the mentioned area. The comparison is made between four models; two of these were created with seismic information (Feng et al., 2006 and Bassin et al., 2000), another model with gravity information (Barzaghi et al., 2011) and the last one with a combination of both techniques (Tassara y Etchaurren, 2011). The latter was the result of the adaptation to the work area of a three-dimensional density model made with some additional information, mainly seismic, that constrain the surfaces. The work of restriction and adaptation of this model, the later analysis and comparison with the other three models and the combination of both seismic models to cover the lack of resolution in some areas, is presented here. According the different models, the crustal thickness of the study zone would be between 36 and 45 Km. and thicker than the surrounding areas. These results talk us about a crust thicker than normal and that could behave as a rigid and independent block. Moreover, it can be observed that there are noticeable differences between gravimetric and seismic

  12. The Ionian Abyssal Plain - closure of a remnant Mesozoic oceanic domain: subbottom structures, deep deformation and the Calabrian subduction zone

    Science.gov (United States)

    Gallais, F.; Gutscher, M.; Graindorge, D.; Klaeschen, D.

    2010-12-01

    The Ionian Abyssal Plain (IAP), located in the Central Mediterranean area is a deep triangular shaped basin, surrounded by the Calabrian subduction zone to the NW, the Mediterranean Ridge to the NE and the Medina Ridges to the South. Available heat flow measurements show very low values under the Ionian Abyssal plain, suggesing a very old age of 180-200 Ma for the basin. The Bouguer gravity anomaly map shows only a strong positive anomaly in this region and the depth of the Moho is around 16 to 18 km corresponding to high refraction velocities of 8.1-8.2 km/s. The Ionian basin is interpreted as one of the oldest basins in the Mediterranean area, and is thought to represent a remnant part of the Mesozoic Tethyan ocean. Due to the complex relative motions of microplates and blocks, currently, the oceanic lithosphere of the Ionian basin is being simultaneously consumed by subduction to the NE beneath the Hellenic system and to the NW beneath Calabria. We present the most relevant lines of the Archimede multi-channel seismic cruise (1997, R/V Le Nadir) crossing the Ionian Abyssal Plain and the Calabrian subduction zone. Interpretation of this seismic dataset is based on correlation with published seismic data and with ESP results. Beneath the IAP, we identify a thick sedimentary cover (> 5km) from the Jurassic to the Plio-Quaternary in age, which overlies the oceanic basement. The Pre-Messinian sequences are affected by a set of NE/SW striking compressional faults with some syn-tectonic basins NW of these faults. These features are interpreted as a re-activated set of normal faults, possibly formed during rifting and/or subsequent accretion of oceanic crust. The orientation of the subbottom structures and the thickness of the Messinian deposits in the south-eastern part of the IAP may be linked with the presence of these faults and their activity through time. On the Calabrian side of the IAP, the Post-Messinian sequences are accreted to the Calabrian wedge. The weak

  13. Mechanical behaviour of the Oman metamorphic sole: rheology of amphibolites at lower crustal conditions during subduction initiation

    Science.gov (United States)

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

    2017-04-01

    Amphibolites are commonly found in the middle to lower continental crust and along oceanic transform faults and detachments. Amphibolites are also the main component of metamorphic soles beneath highly strained peridotites at the base of large-scale ophiolites as exemplified in Oman. Metamorphic soles are crustal slivers stripped from the slab during early subduction and underplated below the upper plate (future ophiolite) mantle when the subduction interface is still young and warm (i.e. during the first million years -My- of intra-oceanic subduction). Understanding the rheological behaviour of amphibolitic rocks is therefore of major interest to model and quantify deformation and strain localisation in varied geodynamical environments. This contribution focuses on the deformation mechanisms of amphibole through a microstructural and petrological study of garnet-bearing and garnet-free clinopyroxene-bearing amphibolites, using EBSD analysis. The first aim is to test the influence of progres- sive changes in PT conditions during deformation and of the appearance/disappearance of anhydrous minerals (plagioclase, clinopyroxene and garnet) on the mechanical behaviour of mafic amphibolites. The second aim is to track deformation mechanisms during early subduction, through the study of these metamorphosed oceanic rocks, commonly 10-100 m thick, which range from high- to low-grade away from the contact with the peridotites (i.e. from 800 ± 100˚C - 0.9 ± 0.2 GPa to 500 ± 100˚C - 0.5 ± 0.1 GPa) and are essentially mafic at the top). Our study points out the existence of two major steps of deformation in the high-temperature amphibolite slices of the metamorphic soles during the early subdduction dynamics. These two steps witness important mechanical coupling and progressive strain localization at plate interface under cooling and hydrated conditions after subduction initiation. During the accretion of the first slice of metamorphic sole at 850 ± 50˚C (the garnet

  14. Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction

    Science.gov (United States)

    Erkan, Kamil; Blackwell, David

    2009-02-01

    the cessation of the subduction. We suggest that the significant lateral heat transfer from the Basin and Range in the Sierra Nevada (and from the Gulf of California spreading center in the Peninsular block since ˜5 Ma) may be the main driving mechanism of the postsubduction volcanism/magmatism along the extinct volcanic arc and the recent tilted uplift of the Sierra Nevada block. The low lithospheric temperatures in Sierra Nevada region may also explain the observation of the high seismic velocities in the mantle beneath the southern Sierra Nevada where the downwelling of the mantle lithosphere proposed.

  15. Turbulence beneath finite amplitude water waves

    Energy Technology Data Exchange (ETDEWEB)

    Beya, J.F. [Universidad de Valparaiso, Escuela de Ingenieria Civil Oceanica, Facultad de Ingenieria, Valparaiso (Chile); The University of New South Wales, Water Research Laboratory, School of Civil and Environmental Engineering, Sydney, NSW (Australia); Peirson, W.L. [The University of New South Wales, Water Research Laboratory, School of Civil and Environmental Engineering, Sydney, NSW (Australia); Banner, M.L. [The University of New South Wales, School of Mathematics and Statistics, Sydney, NSW (Australia)

    2012-05-15

    Babanin and Haus (J Phys Oceanogr 39:2675-2679, 2009) recently presented evidence of near-surface turbulence generated below steep non-breaking deep-water waves. They proposed a threshold wave parameter a {sup 2}{omega}/{nu} = 3,000 for the spontaneous occurrence of turbulence beneath surface waves. This is in contrast to conventional understanding that irrotational wave theories provide a good approximation of non-wind-forced wave behaviour as validated by classical experiments. Many laboratory wave experiments were carried out in the early 1960s (e.g. Wiegel 1964). In those experiments, no evidence of turbulence was reported, and steep waves behaved as predicted by the high-order irrotational wave theories within the accuracy of the theories and experimental techniques at the time. This contribution describes flow visualisation experiments for steep non-breaking waves using conventional dye techniques in the wave boundary layer extending above the wave trough level. The measurements showed no evidence of turbulent mixing up to a value of a {sup 2}{omega}/{nu} = 7,000 at which breaking commenced in these experiments. These present findings are in accord with the conventional understandings of wave behaviour. (orig.)

  16. Channelization of plumes beneath ice shelves

    KAUST Repository

    Dallaston, M. C.

    2015-11-11

    © 2015 Cambridge University Press. We study a simplified model of ice-ocean interaction beneath a floating ice shelf, and investigate the possibility for channels to form in the ice shelf base due to spatial variations in conditions at the grounding line. The model combines an extensional thin-film description of viscous ice flow in the shelf, with melting at its base driven by a turbulent ocean plume. Small transverse perturbations to the one-dimensional steady state are considered, driven either by ice thickness or subglacial discharge variations across the grounding line. Either forcing leads to the growth of channels downstream, with melting driven by locally enhanced ocean velocities, and thus heat transfer. Narrow channels are smoothed out due to turbulent mixing in the ocean plume, leading to a preferred wavelength for channel growth. In the absence of perturbations at the grounding line, linear stability analysis suggests that the one-dimensional state is stable to initial perturbations, chiefly due to the background ice advection.

  17. Atmospheric Waves and Dynamics Beneath Jupiters Clouds from Radio Wavelength Observations

    Science.gov (United States)

    Cosentino, Richard G.; Butler, Bryan; Sault, Bob; Morales-Juberias, Raul; Simon, Amy; De Pater, Imke

    2017-01-01

    We observed Jupiter at wavelengths near 2 cm with the Karl G. Jansky Very Large Array (VLA) in February 2015. These frequencies are mostly sensitive to variations in ammonia abundance and probe between approx. 0.5- 2.0 bars of pressure in Jupiters atmosphere; within and below the visible cloud deck which has its base near 0.7 bars. The resultant observed data were projected into a cylindrical map of the planet with spatial resolution of approx. 1500 km at the equator. We have examined the data for atmospheric waves and observed a prominent bright belt of radio hotspot features near 10 N, likely connected to the same equatorial wave associated with the 5-m hotspots. We conducted a passive tracer power spectral wave analysis for the entire map and latitude regions corresponding to eastward and westward jets and compare our results to previous studies. The power spectra analysis revealed that the atmosphere sampled in our observation (excluding the NEB region) is in a 2-D turbulent regime and its dynamics are predominately governed by the shallow water equations. The Great Red Spot (GRS) is also very prominent and has a noticeable meridional asymmetry and we compare it, and nearby storms, with optical images. We find that the meridional radio profile has a global north-south hemisphere distinction and find correlations of it to optical intensity banding and to shear zones of the zonal wind profile over select regions of latitude. Amateur optical images taken before and after our observation complemented the radio wave- length map to investigate dynamics of the equatorial region in Jupiters atmosphere. We find that two radio hotspots at 2 cm are well correlated with optical plumes in the NEB, additionally revealing they are not the same 5 m hotspot features correlated with optical dark patches between adjacent plumes. This analysis exploits the VLAs upgraded sensitivity and explores the opportunities now possible when studying gas giants, especially atmospheric

  18. Subduction-stage P-T path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction

    Science.gov (United States)

    Aoya, M.; Uehara, S.; Wallis, S. R.; Enami, M.

    2003-12-01

    The Sambagawa belt in SW Japan is a subduction-type high-P/T metamorphic belt. Subduction-stage P-T paths of its constituent rocks are important because they directly constrain physical conditions of the EarthOs interior at the time exhumation of high-P/T metamorphic rocks became feasible. Although a few examples of subduction-stage P-T paths for the Sambagawa rocks have been recognized, these are limited to relatively low-pressure regions (~10 kbar). To augment these data the subduction-stage P-T path of the Kotsu glaucophane (Gln) eclogite is derived. The tectonic significance of the derived and previously determined P-T paths is further examined using a new thermal model. By using compositions of matrix minerals and rims of porphyroblastic garnet (Grt), the peak-T conditions of the Kotsu Gln eclogite have been estimated as ~20 kbar/ 600° C. However, the dP/dT of the P-T path leading to the peak-T conditions is unknown. Petrological studies focusing on inclusion minerals in Grt show: (1) albite is absent as inclusions within Grt; (2) acmite (Acm) component of cpx decreased during growth of Grt; (3) Tschermakite (Ts) component of amphibole decreased and Gln component increased during growth of Grt; and (4) Grt-Cpx thermometry shows a temperature increase during growth of Grt. Along with mineral textures observed in the matrix, the Gln-formation reaction can be determined as: 4Acm + 2Ts + 2quartz + H2O (R) 2Gln + 2epidote + hematite. P-T curve of this reaction always has a large positive dP/dT (>7.1 kbar/100 ?C) with the Gln stability field on the high-P/T side. To cross this reaction curve into the Gln stability field during a rise in temperature, the Kotsu eclogite must trace a very steep subduction-type P-T path. Compilation of previously obtained subduction-stage P-T paths for the Sambagawa rocks along with the P-T path of the Kotsu Gln eclogite shows that the series of subduction-stage P-T paths are not distributed on a straight line starting from the origin

  19. Seismic evidence for a possible deep crustal hot zone beneath Southwest Washington

    Science.gov (United States)

    Flinders, Ashton; Shen, Yang

    2017-01-01

    Crustal pathways connecting deep sources of melt and the active volcanoes they supply are poorly understood. Beneath Mounts St. Helens, Adams, and Rainier these pathways connect subduction-induced ascending melts to shallow magma reservoirs. Petrogenetic modeling predicts that when these melts are emplaced as a succession of sills into the lower crust they generate deep crustal hot zones. While these zones are increasingly recognized as a primary site for silicic differentiation at a range of volcanic settings globally, imaging them remains challenging. Near Mount Rainier, ascending melt has previously been imaged ~28 km northwest of the volcano, while to the south, the volcano lies on the margin of a broad conductive region in the deep crust. Using 3D full-waveform tomography, we reveal an expansive low-velocity zone, which we interpret as a possible hot zone, linking ascending melts and shallow reservoirs. This hot zone may supply evolved magmas to Mounts St. Helens and Adams, and possibly Rainier, and could contain approximately twice the melt volume as the total eruptive products of all three volcanoes combined. Hot zones like this may be the primary reservoirs for arc volcanism, influencing compositional variations and spatial-segmentation along the entire 1100 km-long Cascades Arc.

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

    Science.gov (United States)

    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.

  1. Full-waveform seismic tomography of the Vrancea, Romania, subduction region

    Science.gov (United States)

    Baron, Julie; Morelli, Andrea

    2017-12-01

    The Vrancea region is one of the few locations of deep seismicity in Europe. Seismic tomography has been able to map lithospheric downwelling, but has not been able yet to clearly discriminate between competing geodynamic interpretations of the geological and geophysical evidence available. We study the seismic structure of the Vrancea subduction zone, using adjoint-based, full-waveform tomography to map the 3D vP and vS structure in detail. We use the database that was built during the CALIXTO (Carpathian Arc Lithosphere X-Tomography) temporary experiment, restricted to the broadband sensors and local intermediate-depth events. We fit waveforms with a cross-correlation misfit criterion in separate time windows around the expected P and S arrivals, and perform 17 iterations of vP and vS model updates (altogether, requiring about 16 million CPU hours) before reaching stable convergence. Among other features, our resulting model shows a nearly vertical, high-velocity body, that overlaps with the distribution of seismicity in its northeastern part. In its southwestern part, a slab appears to dip less steeply to the NW, and is suggestive of ongoing - or recently concluded - subduction geodynamic processes. Joint inversion for vP and vS allow us to address the vP/vS ratio distribution, that marks high vP/vS in the crust beneath the Focsani sedimentary basin - possibly due to high fluid pressure - and a low vP/vS edge along the lower plane of the subducting lithosphere, that in other similar environment has been attributed to dehydration of serpentine in the slab. In spite of the restricted amount of data available, and limitations on the usable frequency pass-band, full-waveform inversion reveals its potential to improve the general quality of imaging with respect to other tomographic techniques - although at a sensible cost in terms of computing resources. Our study also shows that re-analysis of legacy data sets with up-to-date techniques may bring new, useful

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

    Science.gov (United States)

    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.

  3. Impact-driven subduction on the Hadean Earth

    Science.gov (United States)

    O'Neill, C.; Marchi, S.; Zhang, S.; Bottke, W.

    2017-10-01

    Impact cratering was a dominant geologic process in the early Solar System that probably played an active role in the crustal evolution of the young terrestrial planets. The Earth's interior during the Hadean, 4.56 to 4 billion years ago, may have been too hot to sustain plate tectonics. However, whether large impacts could have triggered tectonism on the early Earth remains unclear. Here we conduct global-scale tectonic simulations of the evolution of the Earth through the Hadean eon under variable impact fluxes. Our simulations show that the thermal anomalies produced by large impacts induce mantle upwellings that are capable of driving transient subduction events. Furthermore, we find that moderate-sized impacts can act as subduction triggers by causing localized lithospheric thinning and mantle upwelling, and modulate tectonic activity. In contrast to contemporary subduction, the simulated localized subduction events are relatively short-lived (less than 10 Myr) with relatively thin, weak plates. We suggest that resurgence in subduction activity induced by an increased impact flux between 4.1 and 4.0 billion years ago may explain the coincident increase in palaeointensity of the magnetic field. We further suggest that transient impact-driven subduction reconciles evidence from Hadean zircons for tectonic activity with other lines of evidence consistent with an Earth that was largely tectonically stagnant from the Hadean into the Archaean.

  4. POTENTIAL OF TSUNAMI GENERATION ALONG THE COLOMBIA/ECUADOR SUBDUCTION MARGIN AND THE DOLORES-GUAYAQUIL MEGA-THRUST

    Directory of Open Access Journals (Sweden)

    George Pararas-Carayannis

    2012-01-01

    Full Text Available The Colombia/Ecuador subduction zone is a region where high seismic stress is presently accumulating. Statistical probability studies and GPS measurements of crustal deformation indicate that the region has an increased potential to generate in the near future a major or great tsunamigenic earthquake similar to the 1979 or 1906. Although most of the major earthquakes along this margin usually generate local tsunamis, the recurrence of a great mega-thrust, inter-plate earthquake, similar in magnitude and rupture to the 1906 event (Mw=8.8, rupture 600 km., can generate a tsunami with destructive near and far-field impacts. To understand the potential for such destructive tsunami generation in this region, the present study examines and evaluates: a the controlling inter-plate coupling mechanisms of the tectonic regime of the margin – including lithospheric structure deformation, sea-floor relief and the subduction or accretion of highly folded, hydrated sediments along the seismogenic zone of southern Colombia/North Ecuador; b the seismo-dynamics and role in tsunami generation as affected by the Carnegie Ridge’s oblique subduction beneath the South American continent; and c the seismotectonic extensional processes in the vicinity of the Gulf of Guayaquil-Tumbes Basin and how the northwestward movement of the North Andes block away from the South American continent along the Dolores Guayaquil mega-thrust and the resulting strain rotation may cause sudden detachment, décollement and deformation, with the potential for local tsunami generation that may affect the Gulf of Guayaquil and other coastal areas along southern Ecuador.

  5. Imaging of the Deep Structure and Extension of the North Anatolian Fault Zone by Magnetotelluric Method Beneath the Marmara Sea

    Science.gov (United States)

    Kaya, T.; Ogawa, Y.; Kasaya, T.; Tank, B.; Honkura, Y.; Tuncer, M. K.; Oshiman, N.; Matsushima, M.

    2011-12-01

    Relative motions of the Arabian and African plates with respect to stable Eurasian plate resulted in westward movement of the Anatolian block and produced two main fault zones in Turkey. The most active one, the North Anatolian Fault Zone (NAFZ), hosted destructive earthquakes during the history causing not only damage in the buildings but also thousands of causalities. The migration of large earthquakes along the NAFZ from east to west in 20th century, occurrence of the last earthquakes (1999 Izmit and Duzce) by the side of Marmara Sea, and owning the fault segment which has not ruptured since 1766 made the Marmara Sea a potential location for the next large rupture on the NAFZ. Seismic, geodetic and other studies showed complexity of the structure suggesting various estimates about the extension of the NAFZ through the Marmara Sea. In this study, we benefit from the high depth resolution of the Magnetotelluric (MT) method to resolve the electrical resistivity structure beneath the Marmara Sea and disclose its relation with the geologic structure. In order to investigate extension of the NAFZ beneath the Marmara Sea we deployed long period ocean bottom electromagnetic data at 16 sites which form 4 profiles perpendicular to the possible traces of the NAFZ. Variation of the geoelectric strike from east to west demonstrates different oriented faults in the Marmara Sea. The highly conductive anomaly in electrical resistivity models extends from crustal depths to the lithosphere and merges with the melted mantle material. This conductive anomaly is surrounded by relatively resistive anomalies which imply continuation of the fault structure from land to the Marmara Sea. Our results clear the location of the highly conductive and resistive anomalies that has crucial implications in two aspects; conductive anomaly may trigger the micro-seismic activity and resistive anomalies may refer to the asperity zones where stress accumulation result in large earth quakes.

  6. Structure of the mantle lithosphere beneath the Siberian kimberlite pipes reconstructed by monomineral thermobarometry

    Science.gov (United States)

    Ashchepkov, I. V.

    2009-04-01

    columns (10-13) is reconstructed by the stepped TPX trends formed at first by the combinations of subduction and superplume events coinciding with the Re/Os ages (Spetsius, 2007), overprinted by the reactions with the plume and other percolating melts The Fe# increase near the 60 kbar refer to the last superplume events the previous leave similar rhythmic Fe- dunite horizons at 11-12 levels. The comparison of the compositions of minerals and reconstruction of mantle roots for several phases for Yubileinay, Udachnaya and Nyurbinskaya pipes allow to reveal the evolution of the magmatic sources and their interaction with the mantle lithosphere. Reconstruction of the mantle columns beneath 60 pipes allow to make the transsects of the kimberlite fields and the 3D model of the mantle beneath the dense kimberlite clusters with many close located diatrems Mesozoic mantle columns beneath the Anabar, Olenek, Aldan show the HT -Fe# alteration in 60-40 kbar due to interaction with the PT superplume, but relic and LT and low Fe# associations occurs to 60 kbar also. RBRF 05-05-74718, 06-05-65021, 06-05-64416.

  7. Multiscale Architecture of a Subduction Complex and Insight into Large-scale Material Movement in Subduction Systems

    Science.gov (United States)

    Wakabayashi, J.

    2014-12-01

    The >1000 km by >100 km Franciscan complex of California records >100 Ma of subduction history that terminated with conversion to a transform margin. It affords an ideal natural laboratory to study the rock record of subduction-interface and related processes exhumed from 10-70 km. The Franciscan comprises coherent and block-in-matrix (mélange) units forming a nappe stack that youngs structurally downward in accretion age, indicating progressive subduction accretion. Gaps in accretion ages indicate periods of non-accretion or subduction erosion. The Franciscan comprises siliciclastic trench fill rocks, with lesser volcanic and pelagic rocks and serpentinite derived from the downgoing plate, as well as serpentinite and felsic-intermediate igneous blocks derived as detritus from the upper plate. The Franciscan records subduction, accretion, and metamorphism (including HP), spanning an extended period of subduction, rather than a single event superimposed on pre-formed stratigraphy. Melanges (serpentinite and siliciclastic matrix) with exotic blocks, that include high-grade metamorphic blocks, and felsic-intermediate igneous blocks from the upper plate, are mostly/entirely of sedimentary origin, whereas block-in-matrix rocks formed by tectonism lack exotic blocks and comprise disrupted ocean plate stratigraphy. Mélanges with exotic blocks are interbedded with coherent sandstones. Many blocks-in-melange record two HP burial events followed by surface exposure, and some record three. Paleomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow fault zones of structures, are accommodated by discrete faults or narrow shear zones, rather than by significant penetrative strain. Exhumation of Franciscan HP units, both coherent and mélange, was accommodated by significant extension of the overlying plate, and possibly extension within the subduction complex, with cross-sectional extrusion, and like subduction burial, took place at

  8. Late Neogene oroclinal bending in the central Taurides: A record of terminal eastward subduction in southern Turkey?

    Science.gov (United States)

    Koç, Ayten; van Hinsbergen, Douwe J. J.; Kaymakci, Nuretdin; Langereis, Cornelis G.

    2016-01-01

    The Tauride fold-thrusts belt formed during ∼S-N convergence between Africa and Eurasia since Cretaceous time. The western end of the central Taurides strike NW-SE, highly obliquely to the overall convergence direction, and connect to the NE-SW Beydağları-Lycian Nappe flank of the western Taurides, forming the so-called 'Isparta Angle'. In Neogene time, the western and central Taurides and the inner part of the Isparta Angle became overlain by Neogene sedimentary basins including Manavgat, Köprüçay and Aksu, characterized by marine clastics and carbonates. The eastern limb of the Isparta Angle experienced multidirectional Miocene to Present extension, whereas E-W shortening affected the marine sedimentary basins in the heart of the Isparta Angle. To quantitatively reconstruct the Neogene kinematic evolution of the Taurides, towards restoring the subduction system accommodating Africa-Eurasia convergence, we paleomagnetically assess if and when vertical axis rotations affected the Manavgat, Köprüçay, and Aksu basins in Early Miocene to Pliocene times. We show that the northern Köprüçay Basin rotated ∼20-30° clockwise, the Manavgat Basin underwent ∼25-35° counterclockwise rotation, and the Aksu Basin underwent no rotation since the Early-Middle Miocene. It was previously shown that the Beydağları region underwent a post-Middle Miocene ∼20° counterclockwise rotation. These results show that the prominent oroclinal salient geometry of the western Taurides thus acquired, at least in part, since Miocene times, that the Köprüçay Basin rotated relative to the Aksu Basin along the Aksu thrust, and that the Beydağları platform rotated relative to the Aksu Basin along the Bucak thrust, which must have both been active until Late Neogene time. This synchronous E-W shortening in the heart of the Isparta Angle, and multidirectional extension in its eastern limb may be explained by relative westward retreat of an eastward dipping subducting Antalya

  9. Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska

    Science.gov (United States)

    Ferguson, Kelly M; Armstrong, Phillip A; Arkle Jeanette C,; Haeussler, Peter J.

    2014-01-01

    Megathrust splay fault systems in accretionary prisms have been identified as conduits for long-term plate motion and significant coseismic slip during subduction earthquakes. These fault systems are important because of their role in generating tsunamis, but rarely are emergent above sea level where their long-term (million year) history can be studied. We present 32 apatite (U-Th)/He (AHe) and 27 apatite fission-track (AFT) ages from rocks along an emergent megathrust splay fault system in the Prince William Sound region of Alaska above the shallowly subducting Yakutat microplate. The data show focused exhumation along the Patton Bay megathrust splay fault system since 3–2 Ma. Most AHe ages are younger than 5 Ma; some are as young as 1.1 Ma. AHe ages are youngest at the southwest end of Montague Island, where maximum fault displacement occurred on the Hanning Bay and Patton Bay faults and the highest shoreline uplift occurred during the 1964 earthquake. AFT ages range from ca. 20 to 5 Ma. Age changes across the Montague Strait fault, north of Montague Island, suggest that this fault may be a major structural boundary that acts as backstop to deformation and may be the westward mechanical continuation of the Bagley fault system backstop in the Saint Elias orogen. The regional pattern of ages and corresponding cooling and exhumation rates indicate that the Montague and Hinchinbrook Island splay faults, though separated by only a few kilometers, accommodate kilometer-scale exhumation above a shallowly subducting plate at million year time scales. This long-term pattern of exhumation also reflects short-term seismogenic uplift patterns formed during the 1964 earthquake. The increase in rock uplift and exhumation rate ca. 3–2 Ma is coincident with increased glacial erosion that, in combination with the fault-bounded, narrow width of the islands, has limited topographic development. Increased exhumation starting ca. 3–2 Ma is interpreted to be due to rock uplift

  10. Imaging voids beneath bridge bent using electrical resistivity tomography.

    Science.gov (United States)

    2014-02-01

    Five electrical resistivity tomography (ERT) profiles and borehole control were acquired beneath two bridges on the bank of the : Gasconade River in order to determine extension of the underground water-filled openings in rock encountered during a dr...

  11. Electrical conductivity beneath the volcanoes of the NW Argentinian Puna

    National Research Council Canada - National Science Library

    Lezaeta, Pamela; Brasse, Heinrich

    2001-01-01

    ...., in the eastern Puna and backarc zone. The 2‐D conductivity models show a conductive zone beneath the eastern Puna shoshonitic volcanoes and nearby Tuzgle volcano, which reaches from the upper crust to the upper mantle...

  12. 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)

    Science.gov (United States)

    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.

  13. Origin of magmas in subduction zones: a review of experimental studies.

    Science.gov (United States)

    Kushiro, Ikuo

    2007-02-01

    Studies of the origin of magmas in subduction zones, particularly in the Japanese island arc, have been significantly advanced by petrological, geochemical, geophysical and experimental studies during last 50 years. Kuno's original model(1)) for magma generation in the Japanese island arc, that tholeiite magmas are formed at relatively shallow levels in the mantle on the Pacific Ocean side whereas alkali basalt magmas are formed in deeper levels on the Japan Sea side, stimulated subsequent studies, particularly high-pressure experimental studies in which the author participated. Recent seismic tomographic studies of regions beneath the Japanese island arc demonstrate that seismic low-velocity zones where primary magmas are formed have finger-like shapes and rise obliquely from the Japan Sea side toward the Pacific Ocean side. Based on experimental studies, it is suggested that the compositions of primary magmas depend mainly on the H2O content and degree of melting in the melting zones, and that primary tholeiite magmas are formed by 10-25% melting of the source mantle containing less than 0.2 wt.% H2O. High-alumina basalt and alkali basalt magmas are formed by smaller degrees of melting of similar mantle, whereas primary boninite magmas are formed by more than 20% melting of the source mantle with more than 0.2 wt.% H2O, and finally, high-magnesia andesite magmas are formed by smaller degrees of melting of similar mantle.

  14. Geomorphologic Indices for Transition from Subduction to Arc-Continent Collision in Sumba Island, Indonesia

    Science.gov (United States)

    Authemayou, C.; Delcaillau, B.; Brocard, G. Y.; Molliex, S.; Nexer, M.; Pedoja, K.

    2014-12-01

    The Sumba Island lies in a key area to study the eastern Indonesia geodynamics. It is located in the Sunda-Banda fore-arc in the area of transition from subduction of the Indian oceanic lithosphere (W) beneath the Sunda-Banda arc to arc-continent collision between the Australian continental margin and the Sunda-Banda arc (E). East of the Sumba Island, the western boundary of the Savu basin originated during Middle Miocene by the southeastward slab retreat below the Sunda-Banda arc (Rigg and Hall, 2001). Previous studies has detected a global uplift of the island accommodating the Australian plate - South West Banda Arc convergence (Fleury et al., 2009). This uplift is associated with northeastward tilting and gravitational collapse to the South. Analyses of various geomorphic markers (perched low relief landscapes, rockyshore platform, coral reef terraces, drainages) and of morphometric indices allowed us to localise new structures (faults and folds), to determine the chronology of their activation and to better constraint the uplift history of Sumba Island. These results aim to understand the evolution of the deformation in Sumba Island in regard with its geodynamic context from Middle Miocene to present-day.

  15. Deep-crustal magma reservoirs beneath the Nicaraguan volcanic arc, revealed by 2-D and semi 3-D inversion of magnetotelluric data

    Science.gov (United States)

    Brasse, Heinrich; Schäfer, Anja; Díaz, Daniel; Alvarado, Guillermo E.; Muñoz, Angélica; Mütschard, Lutz

    2015-11-01

    A long-period magnetotelluric (MT) experiment was conducted in early 2009 in western Nicaragua to study the electrical resistivity and thus fluid/melt distribution at the Central American continental margin where the Cocos plate subducts beneath the Caribbean plate. Strike analysis yields a preference direction perpendicular to the profile, with moderate deviation from two-dimensionality, however. Two-dimensional modeling maps the sediments of the Nicaraguan Depression and a high-conductivity zone in the mid-crust, slightly offset from the arc. Further conductors are modeled in the backarc. However, these features are probably artifacts when a 2-D program is applied to data which show moderate 3-D characteristics. 3-D inversion clarifies the situation, and the major remaining conductive structure is now quasi directly beneath the volcanic chain and interpreted as a deep-seated magma deposit. Conductivity in the backarc is also relatively high and may either be caused by still existing partial melts beneath the Paleocene to Miocene volcanic arcs or by related metallic deposits in the aureoles of hydrothermal alteration.

  16. Westward extension of the Levantine Basin to the Eratosthenes Seamount and the Cyprus Arc - no evidence for strike-slip motion

    Science.gov (United States)

    Klimke, Jennifer; Ehrhardt, Axel

    2013-04-01

    The Eastern Mediterranean represents a complex pattern of micro plates. A side by side distribution of diverse tectonic situations like collision, subduction, obduction and shear makes this area a very interesting spot on earth. Whereas subduction of Neo-Tethys oceanic crust is still ongoing at the Hellenic Arc, a collision occurred eastward when the Eratosthenes Seamount (ESM) entered the Cyprus Arc. If subduction is still active further east towards the Syrian coast remains unclear. The collision related deformation of the ESM and the adjacent Levantine Basin will be discussed in this paper. We present a new set of 2D multichannel seismic data, acquired in 2010 with the RV Maria S Merian, which is a dense line grid with NW-SE and NE-SW trending profiles crossing the ESM and the western part of the Levantine Basin south of Cyprus. We show first results of the profiles that were processed up to Pre-Stack Depth Migration. Based on the dense line grid with distances of not more than 5 nautical miles, we picked the key horizons in the Levantine Basin and generated reliable 3D-grids of the horizons. With this dense line grid, it was possible to trace the western extension of the Levantine Basin sometimes also referred to as Baltim Hecataeus Line (BHL), which is a fault lineament of Mesozoic age separating the Levantine Basin from the ESM. This extension is observed on every NW-SE and NE-SW trending profile and we were able to trace it even further north and south of the ESM. The BHL is believed to be reactivated as a linear sinistral transform fault that compensates the northward motion of the African-Arabian plate with respect to the blocked ESM. With our data we can show that the western extension of the Levantine Basin does not coincide with a sinistral transform fault and that it is rather a normal fault with a meandering NNE-SSW trending strike.

  17. Subduction zone decoupling/retreat modeling explains south Tibet (Xigaze) and other supra-subduction zone ophiolites and their UHP mineral phases

    Science.gov (United States)

    Butler, Jared P.; Beaumont, Christopher

    2017-04-01

    The plate tectonic setting in which proto-ophiolite 'oceanic' lithosphere is created remains controversial with a number of environments suggested. Recent opinions tend to coalesce around supra-subduction zone (SSZ) forearc extension, with a popular conceptual model in which the proto-ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction. This mechanism is favored in intra-oceanic settings where the subducting lithosphere is old and the upper plate is young and thin. We investigate an alternative mechanism; namely, decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin, followed by subduction zone (trench) retreat and creation of a forearc oceanic rift basin, containing proto-ophiolite lithosphere, between the continental margin and the retreating subduction zone. A template of 2D numerical model experiments examines the trade-off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere. Three tectonic styles are observed: 1) C, continuous subduction without forearc decoupling; 2) R, forearc decoupling followed by rapid subduction zone retreat; 3) B, breakoff of subducting lithosphere followed by re-initiation of subduction and in some cases, forearc decoupling (B-R). In one case (BA-B-R; where BA denotes backarc) subduction zone retreat follows backarc rifting. Subduction zone decoupling is analyzed using frictional-plastic yield theory and the Stefan solution for the separation of plates containing a viscous fluid. The numerical model results are used to explain the formation of Xigaze group ophiolites, southern Tibet, which formed in the Lhasa terrane forearc, likely following earlier subduction and not necessarily during subduction initiation. Either there was normal coupled subduction before subduction zone decoupling, or precursor slab breakoff, subduction re-initiation and then decoupling

  18. Subduction processes related to the Sea of Okhotsk

    Science.gov (United States)

    Zabarinskaya, Ludmila P.; Sergeyeva, Nataliya

    2017-04-01

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

  19. Highly oxidising fluids generated during serpentinite breakdown in subduction zones.

    Science.gov (United States)

    Debret, B; Sverjensky, D A

    2017-09-04

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

  20. Fluid flux and melting reactions in subduction zones

    Science.gov (United States)

    Bouilhol, Pierre; Magni, Valentina; van Hunen, Jeroen; Kaislaniemi, Lars

    2014-05-01

    Understanding the metamorphic reactions that occurs within the slab is a must to constrain subduction zone processes. Slab dehydration reactions ultimately permit the mantle wedge to melt, by lowering its solidus, thus forming arcs above descending slabs. Alternatively the slab crust may cross its solidus in warm hydrated slabs. Moreover, slab dehydration allows chemical fractionation to occur between residual phases and transferred fluid phase, giving arc magmas part of their typical subduction zone chemical characteristics. To better comprehend such complex thermo-chemical open system, we are using a numerical model that reproduces the thermo-mechanical behaviour of a subducting slab and computes the thermodynamic equilibrium paragenesis at each P-T-X conditions of the system. Hence we generate a "paragenetic map" of a subduction system, allowing us to track the fate of water during dehydration and subsequent re-hydration or melting reactions. Here we highlight the role of dehydration and re-hydration reactions occurring in the slab's igneous crust and mantle and the mantle wedge for different slab configuration hence presenting the evolution of a subduction paragenetic map for different regimes. We intend to show the key roles of a) antigorite and chlorite breakdown in the hydrated part of the slab mantle, b) amphibole and lawsonite in the slab crust, and c) the role of amphibole and chlorite in the mantle wedge. Our results show the crucial role of dehydration and re-hydration reactions on slab and mantle wedge melting potential.

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

    Science.gov (United States)

    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.

  2. Nonlinear Responses of High-rise Buildings in Seattle for Simulated Ground Motions From Giant Cascadia Subduction Earthquakes (Mw 9.2)

    Science.gov (United States)

    Yang, J.; Heaton, T. H.

    2008-12-01

    With the exception of the 2003 Tokachi-oki earthquake, strong ground recordings from large subduction earthquakes (Mw > 8.0) are meager. Furthermore there are no strong motion recordings of giant earthquakes. However, there is a growing set of high-quality broadband teleseismic recordings of large and giant earthquakes. In this poster, we use recordings from the 2003 Tokachi-oki (Mw 8.3) earthquake as empirical Green's functions to simulate the rock and soil ground motions from a scenario Mw 9.2 subduction earthquake on Cascadia subduction zone in the frequency band of interest to flexible and large- scale buildings (0.075 to 1 Hz). The effect of amplification by the Seattle basin is considered by using a basin response Green's function which is derived from deconvolving the teleseismic waves recorded at rock sites from soil sites at the SHIP02 experiment. These strong ground motions are used to excite simulation of the fully nonlinear seismic responses of 20-story and 6-story steel moment-frame buildings designed according to both the U.S. 1994 UBC and also the Japanese building code published in 1987. We consider several realizations of the hypothetical subduction earthquake; the down-dip limit of rupture is of particular importance to the simulated ground motions in Seattle. If slip is assumed to be limited to offshore regions, then the building simulations indicate that the building responses are mostly in the linear range. However, our simulation shows that buildings with brittle welds would collapse for rupture models where rupture extends beneath the Olympic Mountains. The ground motions all have very long durations (more than 4 minutes), and our building simulations should be considered as a minimum estimate since we have used a very simple model of degradation of the structure.

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

    Science.gov (United States)

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

    2014-12-01

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

  4. Prominent reflector beneath around the segmentation boundary between Tonankai-Nankai earthquake area

    Science.gov (United States)

    Nakanishi, A.; Shimomura, N.; Fujie, G.; Kodaira, S.; Obana, K.; Takahashi, T.; Yamamoto, Y.; Yamashita, M.; Takahashi, N.; Kaneda, Y.; Mochizuki, K.; Kato, A.; Iidaka, T.; Kurashimo, E.; Shinohara, M.; Takeda, T.; Shiomi, K.

    2013-12-01

    In the Nankai Trough subduction seismogenic zone, the Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. In most cases, first break of such large events of Nankai Trough usually begins from southwest off the Kii Peninsula so far. The idea of split Philippine Sea plate between the Kii Peninsula and the Shikoku Island, which explains seismicity, tectonic background, receiver function image and historical plate motion, was previously suggested. Moreover, between the Kii Peninsula and the Shikoku Island, there is a gap of deep low-frequency events observed in the belt-like zone along the strike of the subducting Philippine Sea plate. In 2010 and 2011, we conducted the large-scale high-resolution wide-angle and reflection (MCS) seismic study, and long-term observation from off Shikoku and Kii Peninsula. Marine active source seismic data have been acquired along grid two-dimensional profiles having the total length of ~800km/year. A three-dimensional seismic tomography using active and passive seismic data observed both land and ocean bottom stations have been also performed. From those data, we found a possible prominent reflector imaged in the offshore side in the Kii channel at the depth of ~18km. The velocity just beneath the reflector cannot be determined due to the lack of ray paths. Based of the amplitude information, we interpret the reflector as the forearc Moho based on the velocity gap (from ~6.4km/s to ~7.4km/s). However, the reflector is shallower than the forearc Moho of other area along the Nankai Trough. Similar reflectors are recognized along other seismic profiles around the Kii channel. In this presentation, we will show the result of structure analysis to understand the peculiar structure including the prominent reflector around the Kii channel. Relation between the structure and the existence of the segmentation of the Nankai megathrust earthquake or seismic gap of the deep low-frequency events will be also

  5. Receiver Function Imaging of Upper Mantle Discontinuities Beneath the Oregon High Lava Plains and Surrounding Regions

    Science.gov (United States)

    Eagar, K. C.; Fouch, M. J.

    2007-12-01

    The Pacific Northwestern United States has experienced wide-spread tectonomagmatism from the late Cenozoic to present-day, the reasons for which are likely linked to complex interactions between subduction zone processes near the edge of a Precambrian craton, possible hotspot effects, and mantle flow within the asthenosphere. The goal of this study is to examine upper mantle seismic velocity discontinuities beneath this region to investigate the connection between these geodynamic processes. We present results from analysis of Ps receiver functions using stations from the High Lava Plains temporary broadband seismic experiment, the USArray Transportable Array (TA), and other regional broadband stations for a total of over 108 stations in the region. We compute receiver functions at individual stations using an iterative deconvolution approach, and generate common conversion point (CCP) stacks using moveout corrections defined by the Tectonic North America (TNA) shear wave velocity model. Results from our imaging show that conversions from the 410 km discontinuity have narrow, high amplitude peaks across most of the region, with local complexity in peaks for some areas. Conversely, most conversions near the 660 km discontinuity exhibit peaks that are broad and less well-defined. Estimates in transition zone thickness show local variability, with notable areas of thickening in northeastern Oregon into Idaho and thinning under the coastal regions. The complexity in upper mantle discontinuity structure in this region is likely due to a combination of strong variations in isotropic and anisotropic seismic wavespeeds. The local complexities of the 410 km discontinuity correlate well with seismic wavespeed anomalies imaged by regional P-wave tomography. The source of the variations in the 660 km discontinuity are less apparent, but we note that seismic anisotropy is strong and regionally homogenous across much of the region, perhaps producing backazimuthal variations in

  6. Sr-Nd-Hf-Pb isotopic evidence for modification of the Devonian lithospheric mantle beneath the Chinese Altai

    Science.gov (United States)

    Yu, Yang; Sun, Min; Huang, Xiao-Long; Zhao, Guochun; Li, Pengfei; Long, Xiaoping; Cai, Keda; Xia, Xiaoping

    2017-07-01

    , and are thought to have formed from a magmas generated by the partial melting of the depleted lithospheric mantle metasomatised by fluids derived from subducted sediments. The melting and dehydration of subducted oceanic crust and sediments indicate the Devonian subduction of a hot slab beneath the Chinese Altai, which could be related to a ridge subduction model previously proposed. The involvement of hydrous melts and fluids promoted the melting of the lithospheric mantle which was responsible for intensive felsic magmatism with abrupt increase of juvenile component and high temperature metamorphism in the region.

  7. Os and HSE of the hot upper mantle beneath southern Tibet: Indian mantle affinity?

    Science.gov (United States)

    Zhao, Z.; Dale, C. W.; Pearson, D. G.; Niu, Y.; Zhu, D.; Mo, X.

    2011-12-01

    The subduction of the Indian plate (including cratonic continental crust and/or upper mantle) beneath southern Tibet is widely accepted from both geological and geophysical studies. Mantle-derived xenoliths from this region provide a means of directly investigating the mantle underlying the southern part of the plateau. Studies of xenoliths hosted in the Sailipu ultrapotassic volcanic rocks, erupted at ~17 Ma, have indicated that the subcontinental mantle of southern Tibetan Plateau is hot and strongly influenced by metasomatism (Zhao et al., 2008a, b; Liu et al., 2011). Here we report comprehensive EPMA and LA-ICP-MS major and trace element data for the Sailipu xenoliths and also whole rock Os isotope and HSE data in order to constrain the depletion history of the mantle and to identify the presence of any potential Indian cratonic mantle. The xenoliths, ranging in size from 0.5cm to 1.5cm in diameter, are mostly peridotites. The calculated temperatures are 1010-1230°C at the given pressures of ~1.6-2.0 GPa (n=47). These P-T conditions are similar to rift-related upper mantle regimes (e.g. Kenya), indicating the influence of regional extension beneath southern Tibet in the Miocene. A series of compositional discriminations for minerals (Cpx, Opx, Ol, and Phl), e.g. Fo ~91), with a clear metasomatic signature We obtained Os isotope data and abundances of highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) on a set of six olivine-dominated peridotite samples from Sailipu volcanics, less than 1 cm in dimension. They allow us to further constrain the nature and state of the upper mantle beneath the southern Tibet. Sailipu samples display low total HSE abundances (Os+Ir+Ru+Pt+Pd+Re) ranging from 8.7 to 25 ppb, with nearly constant Os, Ir , and Ru, but rather varied Pt (2-13), Pd (0.4-5.2), and Re (0.01-0.5). Chondrite-normalised Pd/Ir ratios range from 0.2 to 2.4 reflecting significant metasomatism of some samples. The xenoliths exhibit 187Os/188Os

  8. Trench dynamics: Effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems

    Science.gov (United States)

    Yoshida, Masaki

    2017-07-01

    Understanding the mechanisms of trench migration (retreat or advance) is crucial to characterizing the driving forces of Earth's tectonics plates, the origins of subducting slab morphologies in the deep mantle, and identifying the characteristics of subduction zones systems, which are among the fundamental issues of solid Earth science. A series of numerical simulations of mantle convection, focusing on plate subduction in a three-dimensional (3-D) regional spherical shell coordinate system, was performed to examine subduction zone characteristics, including geodynamic relationships among trench migration, back-arc stress, and slab morphology. The results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub-horizontal slab because its front edge (its 'toe') is subject to resistance from the highly viscous lower mantle. As the sub-horizontal slab starts to penetrate into the lower mantle from its 'heel,' the toe of the slab is drawn into the lower mantle. The results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self-consistent phenomenon in trench migration. The reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub-horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab. In fact, a 'mantle suction force,' which is generated in the mantle wedge to fill space left by the retreating subducting plate, is enhanced by the subsequent trench retreat. Even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410 km (as inferred from independent seismic tomographic, geodynamic, geochemical, and mineral physics), reaches the base of the overriding plate, and the overriding plate is slightly thinned, lithospheric stress tends to be

  9. The transition zone below the Chile-Argentina flat subduction region

    Science.gov (United States)

    Bonatto, Luciana; Piromallo, Claudia; Badi, Gabriela

    2017-04-01

    We study the fine structure of the upper mantle (below 200 km depth) beneath the western margin of South America, within an area known as the Chile-Argentina flat subduction zone (between 26°S and 36°S). Unlike what happens in most subduction zones, in this region the Nazca Plate subducts with an angle close to the horizontal -initially dips underneath the continent and flattens at a depth of approximately 100 km, remaining almost horizontal for about 300 km before descending more steeply into the mantle. Moreover, the flat slab follows the path of the subducting Juan Fernández Ridge, a hot spot seamount chain on the Nazca Plate. The complex tectonic setting makes this region an excellent laboratory to explore and quantify the relative contributions of thermal and compositional heterogeneities to the mantle discontinuity structure. In this study we combine data available from four past temporary experiments: 18 seismic stations from CHARGE; 43 from SIEMBRA, 12 from ESP and 30 from PUDEL. The research tools are the Pds phases (the direct P wave converted to an S wave while passing through a seismic discontinuity at depth d). These signals arrive in the coda of the P-phase in the radial component and are expected to be coherent with the waveform of the first arrival for conversion at discontinuities thinner than one half of the P-wavelength. In order to extract these converted phases by means of waveform similarity, we use the receiver function (RF) technique, i.e. the deconvolution of the vertical from the radial component in the frequency domain. The Pds phases are then detected on stacked RF (globally and by common conversion point) in the relative time-slowness domain. Since the incidence angle of converted phases is larger than the incidence angle of the P phase, they are expected with negative slowness. This permits to separate them from the multiples, which are instead expected with positive slowness. We measure amplitudes and arrival times for the

  10. Mapping fluids to subduction megathrust locking and slip behavior

    Science.gov (United States)

    Saffer, Demian M.

    2017-09-01

    In subduction zones, high fluid content and pore pressure are thought to promote aseismic creep, whereas well-drained conditions are thought to promote locking and failure in earthquakes. However, observations directly linking fluid content and seismic coupling remain elusive. Heise et al. (2017) use a magnetotelluric survey to image the electrical resistivity structure of the northern Hikurangi subduction thrust to 30 km depth, as an indicator of interconnected fluid content. The authors document a clear correlation between high resistivity and a distinct geodetically locked patch and between conductive areas and weak coupling. Their study, together with other recent geophysical investigations, provides new evidence for the role of fluids in governing subduction thrust locking.

  11. A Silent Slip Event on the Deeper Cascadia Subduction Interface

    Science.gov (United States)

    Dragert, Herb; Wang, Kelin; James, Thomas S.

    2001-05-01

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

  12. A silent slip event on the deeper Cascadia subduction interface.

    Science.gov (United States)

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

    2001-05-25

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

  13. Friction and stress coupling on the subduction interfaces

    Science.gov (United States)

    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

  14. An auroral westward flow channel (AWFC and its relationship to field-aligned current, ring current, and plasmapause location determined using multiple spacecraft observations

    Directory of Open Access Journals (Sweden)

    M. L. Parkinson

    2007-02-01

    Full Text Available An auroral westward flow channel (AWFC is a latitudinally narrow channel of unstable F-region plasma with intense westward drift in the dusk-to-midnight sector ionosphere. AWFCs tend to overlap the equatorward edge of the auroral oval, and their life cycle is often synchronised to that of substorms: they commence close to substorm expansion phase onset, intensify during the expansion phase, and then decay during the recovery phase. Here we define for the first time the relationship between an AWFC, large-scale field-aligned current (FAC, the ring current, and plasmapause location. The Tasman International Geospace Environment Radar (TIGER, a Southern Hemisphere HF SuperDARN radar, observed a jet-like AWFC during ~08:35 to 13:28 UT on 7 April 2001. The initiation of the AWFC was preceded by a band of equatorward expanding ionospheric scatter (BEES which conveyed an intense poleward electric field through the inner plasma sheet. Unlike previous AWFCs, this event was not associated with a distinct substorm surge; rather it occurred during an interval of persistent, moderate magnetic activity characterised by AL~−200 nT. The four Cluster spacecraft had perigees within the dusk sector plasmasphere, and their trajectories were magnetically conjugate to the radar observations. The Waves of High frequency and Sounder for Probing Electron density by Relaxation (WHISPER instruments on board Cluster were used to identify the plasmapause location. The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE EUV experiment also provided global-scale observations of the plasmapause. The Cluster fluxgate magnetometers (FGM provided successive measurements specifying the relative location of the ring current and filamentary plasma sheet current. An analysis of Iridium spacecraft magnetometer measurements provided estimates of large-scale ionospheric FAC in relation to the AWFC evolution. Peak flows in the AWFC were located close to the peak of a Region 2

  15. Splay fault branching along the Nankai subduction zone.

    Science.gov (United States)

    Park, Jin-Oh; Tsuru, Tetsuro; Kodaira, Shuichi; Cummins, Phil R; Kaneda, Yoshiyuki

    2002-08-16

    Seismic reflection profiles reveal steeply landward-dipping splay faults in the rupture area of the magnitude (M) 8.1 Tonankai earthquake in the Nankai subduction zone. These splay faults branch upward from the plate-boundary interface (that is, the subduction zone) at a depth of approximately 10 kilometers, approximately 50 to 55 kilometers landward of the trough axis, breaking through the upper crustal plate. Slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion.

  16. Seismological studies of the plumbing system beneath the Klyuchevskoy volcanic group in Kamchataka, Russia

    Science.gov (United States)

    Koulakov, I.; Shapiro, N.; Abkadyirov, I.; Gordeev, E.; Jakovlev, A.; Kugaenko, Y.; Droznin, D.; Droznina, S.; Senyukov, S.; Novgorodova, A.; Kulakov, R.; Deev, E.

    2016-12-01

    The Klyuchevskoy volcanic group (KVG) is one of largest and most active clusters of subduction-related volcanoes in the World. Within an area of 70 km size, there are 13 closely located stratovolcanoes, of which Klyuchevskoy, Bezymianny, and Tolbachik produced numerous strong eruptions during recent decades. The Klyuchevskoy and Tolbachik recent eruptions produced lavas of basaltic to basaltic-andesitic composition while Bezimyanny is an explosive andesitic volcano. We present a seismological study of the KVG plumbing system. The first part of our study is the tomographic imaging based on data of permanent and temporary seismic stations operated in vicinity of the KVG. Beneath the Klyuchevskoy volcano at depths between 28 and 35 km, we observe an anomaly with very high Vp/Vs ratio collocated with a large cluster of long-period volcanic earthquakes. We propose that this feature represents the top of the mantle magmatic reservoir feeding the KVG. A narrow seismicity cluster beneath the Klyuchevskoy volcano marks a straight vertical conduit bringing basic magma from the deep reservoir at 30 km to the surface. For the Bezymianny volcano, we observe a middle-crust reservoir at 15 km depth, where the light andesitic magma is fractioned and separated. For the Tolbachik volcano, linearly clustered seismicity and seismic anomalies indicate two distinct pathways of magma ascending. In summary, the new seismic model has revealed three different types of plumbing systems explaining the variability of eruption styles in Klyuchevskoy, Bezymyanny and Tolbachik volcanoes. In the second part we study the occurrence of long-period (LP) seismic events occurring within KVG during two years preceding the large eruption of Tolbachik started at the end of 2012. We find two distinct groups of LP sources: events that occurred just below the edifices of the active volcanoes, and events at depths of 30 km in the vicinity of a deep magmatic reservoir. The time histories of these deep and

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

    Science.gov (United States)

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

    2018-01-01

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

  18. Influence of trench width on subduction hinge retreat rates in 3-D models of slab rollback

    NARCIS (Netherlands)

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

    Subduction of tectonic plates limited in lateral extent and with a free-trailing tail, i.e., "free subduction,'' is modeled in a three-dimensional (3-D) geometry. The models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate

  19. Review of subduction and its association with geothermal system in Sumatera-Java

    Science.gov (United States)

    Ladiba, A. F.; Putriyana, L.; Sibarani, B. br.; Soekarno, H.

    2017-12-01

    Java and Sumatera have the largest geothermal resources in Indonesia, in which mostly are spatially associated with volcanoes of subduction zones. However, those volcanoes are not distributed in a regular pattern due to the difference of subduction position. Subduction position in java is relatively more perpendicular to the trench than in Sumatera. In addition, Java has a concentration of large productive geothermal field with vapour dominated system in the western part of Java, which may be caused by the various subduction dip along the island. In order to understand the relationship between the subduction process and geothermal system in the subduction zone volcanoes, we examined several kinematic parameters of subduction that potentially relevant to the formation of geothermal system in overriding plate such as slab dip, subduction rate, and direction of subduction. Data and information regarding tectonic setting of Sumatera and Java and productive geothermal field in Sumatera and Java have been collected and evaluated. In conclusion, there are three condition that caused the geothermal fluid to be more likely being in vapour phase, which are: the subduction is in an orthogonal position, the slab dip is high, and rate of subduction is high. Although there are plenty researches of subduction zone volcanoes, only a few of them present information about its formation and implication to the geothermal system. The result of this study may be used as reference in exploration of geothermal field in mutual geologic environment.

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

    Science.gov (United States)

    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

  1. The lithospheric structure beneath southeast Tibet revealed by P and S receiver functions

    Science.gov (United States)

    Yang, Haiyan; Peng, Hengchu; Hu, Jiafu

    2017-05-01

    Yunnan is located at the margin of southeast Tibet, where dramatic tectonic activities occur. In this study, we calculated the P and S receiver functions by the deconvolution of three-component seismic data from 48 permanent broad-band stations deployed in Yunnan region. In order to improve signal-noise ratios of the receiver functions, we move-out corrected receiver functions to a reference epicentral distance of 67°, and then stacked them to one trace for each station. Finally, the stacked traces were converted to the depth domain to obtain the crustal and lithospheric thicknesses beneath each station. In southwestern Yunnan (at the west side of the Jinshajiang-Red River Fault), the crustal thicknesses from the P-wave receiver functions (PRFs) and from the S-wave receiver functions (SRFs) are in the ranges of 30-36 km, and of 33-39 km, respectively. But in northwestern Yunnan, the crustal thicknesses from PRFs and SRFs are from 66 to 69 km and from 63 to 66 km, respectively. Our results show that the crustal thicknesses in Yunnan from PRFs and SRFs are consistent, with a maximum deviation of 3 km; and increase gradually from ∼30 km in the south to ∼68 km in the northwest. Besides, the lithospheric thickness from PRFs is also similar to that from SRFs, with the largest difference of 15-20 km in southeastern Yunnan. At the west side of the Jinshajiang-Red River Fault in western Yunnan, it is only about 80-100 km, and increases to 140-150 km regionally in northern and southeastern Yunnan. The thinned lithosphere extends eastward from western Yunnan to eastern Yunnan. We attribute the thinned lithosphere to the upwelling of hot upper mantle materials associated with the eastward subduction of the Indian plate.

  2. Anelastic Mantle Structure beneath the Northern Philippine Sea from Phase Pair Method

    Science.gov (United States)

    Shito, A.; Shibutani, T.

    2001-12-01

    Anelasticity of the mantle provides important constraints on its dynamics, in complement to elasticity, however, studies of lateral variation in attenuation are few, especially at short periods. In this study, we determine the body wave attenuation structure of the mantle beneath the northern part of the Philippine Sea. Elastic tomography studies [e.g. van der Hilst et al., 1991; Fukao et al., 1992] show a stagnant Pacific lithosphere just above the 660 km discontinuity in this region. This stagnation was caused by the trench retreat due to the back arc spreading during the past 17 - 30 Ma [Seno et al., 1993; van der Hilst, 1995; Shito and Shibutani, 2001]. Anelastic properties of the mantle and lithosphere may play important roles in the interaction of the slab and mantle of this area. To study the attenuation structure, we examine the difference of the observed attenuation between P and S waves. The S-P phase pair method measures δ t* using the differential spectral decay between S and P waves arriving at the same station, assuming a constant Qα }/Q{β over the frequency band of 0.5 to 1.25 Hz. We use 20 earthquakes in the Izu-Bonin slab which were recorded at 43 broad-band stations of the J-array and FREESIA networks in Japan. About 700 phase pairs are used to invert for the 2-D attenuation structure. The resultant preliminary Q model shows the local variations of attenuation in the subduction zone. The slab is imaged as a low attenuation area (Qα > 1000), while Qα values in the range of 100 - 350 are found in the mantle wedge.

  3. Formation and stability of a double subduction system: a numerical study

    Science.gov (United States)

    Pusok, Adina E.; Stegman, Dave

    2017-04-01

    Examples of double subduction systems can be found in both modern (Izu-Bonin-Marianas and Ryukyu arcs, e.g. Hall [1997]) and ancient (Kohistan arc in Western Himalayas, e.g. Burg [2006], Burg et al. [2006]) tectonic record. A double subduction system has also been proposed to explain the high convergence rate observed for the India-Eurasia convergence [Jagoutz et al., 2015; Holt et al., 2016, 2017]. Rates of convergence across coupled double subduction systems can be significantly faster than across single subduction systems because of slab pull by two slabs. However, despite significant geological and geophysical observations, our understanding about this process is limited, and questions regarding double subduction remain largely unexplored in terms of physical factors controlling its initiation, duration and dynamics. Subduction initiation (of a single system) in itself has been a popular and challenging topic in the research community for the last few years, and various mechanisms (i.e., collapse at a passive margin or transform fault [Gerya et al., 2008; Stern, 2004], driven by compression [Hall et al., 2003; Toth and Gurnis, 1998], due to shear heating under compression [Thielmann and Kaus, 2012] or plume induced initiation [Gerya et al., 2015]) have been proposed. However, initiation of a secondary subduction, and formation of a stable double subduction system has not been studied before. Previous studies of double subduction either introduced weak zones to initiate subduction [Mishin et al., 2008] or both the subduction systems were already initiated [Jagoutz et al., 2015], thus assuming a priori information regarding the initial position of the two subduction zones. In this study, we perform 2D and 3D numerical simulations to investigate i) subduction initiation of a secondary system in an already initiated single subduction system, and ii) the dynamics and stability of the newly formed double subduction system. For this, we employ the code LaMEM [Kaus et

  4. Spatial distribution of random velocity inhomogeneities in the western part of Nankai subduction zone

    Science.gov (United States)

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

    2011-12-01

    In the Nankai trough, there are three seismogenic zones of megathrust earthquakes (Tokai, Tonankai and Nankai earthquakes). Lithospheric structures in and around these seismogenic zones are important for the studies on mutual interactions and synchronization of their fault ruptures. Recent studies on seismic wave scattering at high frequencies (>1Hz) make it possible to estimate 3D distributions of random inhomogeneities (or scattering coefficient) in the lithosphere, and clarified that random inhomogeneity is one of the important medium properties related to microseismicity and damaged structure near the fault zone [Asano & Hasegawa, 2004; Takahashi et al. 2009]. This study estimates the spatial distribution of the power spectral density function (PSDF) of random inhomogeneities the western part of Nankai subduction zone, and examines the relations with crustal velocity structure and seismic activity. Seismic waveform data used in this study are those recorded at seismic stations of Hi-net & F-net operated by NIED, and 160 ocean bottom seismographs (OBSs) deployed at Hyuga-nada region from Dec. 2008 to Jan. 2009. This OBS observation was conducted by JAMSTEC as a part of "Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes" funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. Spatial distribution of random inhomogeneities is estimated by the inversion analysis of the peak delay time of small earthquakes [Takahashi et al. 2009], where the peak delay time is defined as the time lag from the S-wave onset to its maximal amplitude arrival. We assumed the von Karman type functional form for the PSDF. Peak delay times are measured from root mean squared envelopes at 4-8Hz, 8-16Hz and 16-32Hz. Inversion result can be summarized as follows. Random inhomogeneities beneath the Quaternary volcanoes are characterized by strong inhomogeneities at small spatial scale (~ a few hundreds meter) and weak spectral gradient

  5. Phase change in subducted lithosphere, impulse, and quantizing Earth surface deformations

    Science.gov (United States)

    Bowin, C. O.; Yi, W.; Rosson, R. D.; Bolmer, S. T.

    2015-09-01

    The new paradigm of plate tectonics began in 1960 with Harry H. Hess's 1960 realization that new ocean floor was being created today and is not everywhere of Precambrian age as previously thought. In the following decades an unprecedented coming together of bathymetric, topographic, magnetic, gravity, seismicity, seismic profiling data occurred, all supporting and building upon the concept of plate tectonics. Most investigators accepted the premise that there was no net torque amongst the plates. Bowin (2010) demonstrated that plates accelerated and decelerated at rates 10-8 times smaller than plate velocities, and that globally angular momentum is conserved by plate tectonic motions, but few appeared to note its existence. Here we first summarize how we separate where different mass sources may lie within the Earth and how we can estimate their mass. The Earth's greatest mass anomalies arise from topography of the boundary between the metallic nickel-iron core and the silicate mantle that dominate the Earth's spherical harmonic degree 2 and 3 potential field coefficients, and overwhelm all other internal mass anomalies. The mass anomalies due to phase changes in olivine and pyroxene in subducted lithosphere are hidden within the spherical harmonic degree 4-10 packet, and are an order of magnitude smaller than those from the core-mantle boundary. Then we explore the geometry of the Emperor and Hawaiian seamount chains and the 60° bend between them that aids in documenting the slow acceleration during both the Pacific Plate's northward motion that formed the Emperor seamount chain and its westward motion that formed the Hawaiian seamount chain, but it decelerated at the time of the bend (46 Myr). Although the 60° change in direction of the Pacific Plate at of the bend, there appears to have been nary a pause in a passive spreading history for the North Atlantic Plate, for example. This, too, supports phase change being the single driver for plate tectonics and

  6. Alakit and Daldyn kimberlite fields, Siberia, Russia: Two types of mantle sub-terranes beneath central Yakutia?

    Directory of Open Access Journals (Sweden)

    I.V. Ashchepkov

    2017-07-01

    Full Text Available Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle. Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields. In the Alakit field, Cr-diopsides are alkaline, and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions, while in the Daldyn field harzburgitic pyropes are frequent. The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions, while in the Daldyn field they show varying compositions and often continuous Pressure–Temperature (P–T ranges with increasing Fe# with decreasing pressures. In Alakit, Cr-pargasites to richterites were found in all pipes, while in Daldyn, pargasites are rare Dalnyaya and Zarnitsa pipes. Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements (LREE and stronger REE-slopes, and enrichment in light Rare Earth Elements (LREE, sometimes Th-U, and small troughs in Nb-Ta-Zr. In the Daldyn field, the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances, while those from sheared and refertilized peridotites have smooth patterns. Garnets from Alakit show HREE minima, but those from Daldyn often have a trough at Y and high U and Pb. PTXfO2 diagrams from both regions show similarities, suggesting similar layering and structures. The degree of metasomatism is often higher for pipes which show dispersion in P–Fe# trends for garnets. In the mantle beneath Udachnaya and Aykhal, pipes show 6–7 linear arrays of P–Fe# in the lower part of the mantle section at 7.5–3.0 GPa, probably reflecting primary subduction horizons. Beneath the Sytykanskaya pipe, there are several horizons with opposite inclinations which reflect metasomatic processes. The high dispersion of the P–Fe# trend indicating widespread metasomatism is associated with decreased

  7. Evolution of passive continental margins and initiation of subduction zones

    NARCIS (Netherlands)

    Cloetingh, Sierd

    1982-01-01

    The initiation of subduction is a key element in plate tectonic schemes for the evolution of the Earth's lithosphere. Nevertheless, up to present, the underlying mechanism has not been very well understood (e.g. Dickinson and Seely, 1979; Hager, 1980; Kanamori, 1980). The insight into the initiation

  8. Evolution of passive continental margins and initiation of subduction zones

    NARCIS (Netherlands)

    Cloetingh, S.A.P.L.

    1982-01-01

    The initiation of subduction is a key element in plate tectonic schemes for the evolution of the Earth's lithosphere. Nevertheless, up to present, the underlying mechanism has not been very well understood (e.g. Dickinson and Seely, 1979; Hager, 1980; Kanamori, 1980). The insight into the

  9. The course of water in Archean subduction systems

    Science.gov (United States)

    Bouilhol, P.; Magni, V.; Van Hunen, J.; Kaislaniemi, L.

    2012-12-01

    The andesitic nature of the bulk continental crust, as well as its characteristic trace element ratios, have a close resemblance to the differentiated crust of volcanic arcs, thus leading to models for formation of continental crust in subduction zone settings. If the modern processes leading to continental crust formation at convergent margins are well constrained, the extrapolation to early Earth conditions is hazardous, because the composition of Earth's early crust can be achieved through several processes. We study the different scenarios that may have operated during early Earth subduction to form differentiated crust. Each scenario (e.g. arc crust melting, slab melting, mantle melting followed by differentiation…) has a common denominator that is the fate of water, because it reflects slab devolatilization and controls the melting process and the stability of minerals such as garnet, amphibole and plagioclase, which are major players in the final melt composition. To this end, we present thermomechanical numerical models that incorporate internally consistent thermodynamic data in order to simulate slab dehydration. Our goal is to track the fate of subducted water in an Archean style subduction regime to better comprehend its modus operandi.

  10. Slab melting versus slab dehydration in subduction-zone magmatism.

    Science.gov (United States)

    Mibe, Kenji; Kawamoto, Tatsuhiko; Matsukage, Kyoko N; Fei, Yingwei; Ono, Shigeaki

    2011-05-17

    The second critical endpoint in the basalt-H(2)O 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.

  11. Some consequences of the subduction of young slabs

    NARCIS (Netherlands)

    England, P.; Wortel, R.

    The negative buoyancy force exerted by a subducting oceanic slab depends on its descent velocity, and strongly on its age. For lithosphere close to thermal equilibrium, this force dominates by a large margin the resisting forces arising from friction on the plate boundary and compositional buoyancy.

  12. Tensor-guided fitting of subduction slab depths

    Science.gov (United States)

    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.

  13. Hafnium at subduction zones: isotopic budget of input and output fluxes; L'hafnium dans les zones de subduction: bilan isotopique des flux entrant et sortant

    Energy Technology Data Exchange (ETDEWEB)

    Marini, J.Ch

    2004-05-15

    Subduction zones are the primary regions of mass exchanges between continental crust and mantle of Earth through sediment subduction toward the earth's mantle and by supply of mantellic magmas to volcanic arcs. We analyze these mass exchanges using Hafnium and Neodymium isotopes. At the Izu-Mariana subduction zone, subducting sediments have Hf and Nd isotopes equivalent to Pacific seawater. Altered oceanic crust has Hf and Nd isotopic compositions equivalent to the isotopic budget of unaltered Pacific oceanic crust. At Luzon and Java subduction zones, arc lavas present Hf isotopic ratios highly radiogenic in comparison to their Nd isotopic ratios. Such compositions of the Luzon and Java arc lavas are controlled by a contamination of their sources by the subducted oceanic sediments. (author)

  14. Atmospheric waves and dynamics beneath Jupiter's clouds from radio wavelength observations

    Science.gov (United States)

    Cosentino, Richard G.; Butler, Bryan; Sault, Bob; Morales-Juberías, Raúl; Simon, Amy; de Pater, Imke

    2017-08-01

    We observed Jupiter at wavelengths near 2 cm with the Karl G. Jansky Very Large Array (VLA) in February 2015. These frequencies are mostly sensitive to variations in ammonia abundance and probe between ∼ 0.5 - 2.0 bars of pressure in Jupiter's atmosphere; within and below the visible cloud deck which has its base near 0.7 bars. The resultant observed data were projected into a cylindrical map of the planet with spatial resolution of ∼1500 km at the equator. We have examined the data for atmospheric waves and observed a prominent bright belt of radio hotspot features near 10°N, likely connected to the same equatorial wave associated with the 5-μm hotspots. We conducted a passive tracer power spectral wave analysis for the entire map and latitude regions corresponding to eastward and westward jets and compare our results to previous studies. The power spectra analysis revealed that the atmosphere sampled in our observation (excluding the NEB region) is in a 2-D turbulent regime and its dynamics are predominately governed by the shallow water equations. The Great Red Spot (GRS) is also very prominent and has a noticeable meridional asymmetry and we compare it, and nearby storms, with optical images. We find that the meridional radio profile has a global north-south hemisphere distinction and find correlations of it to optical intensity banding and to shear zones of the zonal wind profile over select regions of latitude. Amateur optical images taken before and after our observation complemented the radio wavelength map to investigate dynamics of the equatorial region in Jupiter's atmosphere. We find that two radio hotspots at 2 cm are well correlated with optical plumes in the NEB, additionally revealing they are not the same 5 μm hotspot features correlated with optical dark patches between adjacent plumes. This analysis exploits the VLA's upgraded sensitivity and explores the opportunities now possible when studying gas giants, especially atmospheric dynamics

  15. Subduction and vertical coastal motions in the eastern Mediterranean

    Science.gov (United States)

    Howell, Andy; Jackson, James; Copley, Alex; McKenzie, Dan; Nissen, Ed

    2017-10-01

    Convergence in the eastern Mediterranean of oceanic Nubia with Anatolia and the Aegean is complex and poorly understood. Large volumes of sediment obscure the shallow structure of the subduction zone, and since much of the convergence is accommodated aseismically, there are limited earthquake data to constrain its kinematics. We present new source models for recent earthquakes, combining these with field observations, published GPS velocities and reflection-seismic data to investigate faulting in three areas: the Florence Rise, SW Turkey and the Pliny and Strabo Trenches. The depths and locations of earthquakes reveal the geometry of the subducting Nubian plate NE of the Florence Rise, a bathymetric high that is probably formed by deformation of sediment at the surface projection of the Anatolia-Nubia subduction interface. In SW Turkey, the presence of a strike-slip shear zone has often been inferred despite an absence of strike-slip earthquakes. We show that the GPS-derived strain-rate field is consistent with extension on the orthogonal systems of normal faults observed in the region and that strike-slip faulting is not required to explain observed GPS velocities. Further SW, the Pliny and Strabo Trenches are also often interpreted as strike-slip shear zones, but almost all nearby earthquakes have either reverse-faulting or normal-faulting focal mechanisms. Oblique convergence across the trenches may be accommodated either by a partitioned system of strike-slip and reverse faults or by oblique slip on the Aegean-Nubia subduction interface. The observed late-Quaternary vertical motions of coastlines close to the subduction zone are influenced by the interplay between: (1) thickening of the material overriding the subduction interface associated with convergence, which promotes coastal uplift; and (2) subsidence due to extension and associated crustal thinning. Long-wavelength gravity data suggest that some of the observed topographic contrasts in the eastern

  16. Frictional behavior of carbonate-rich sediments in subduction zones

    Science.gov (United States)

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

    2016-12-01

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

  17. Composition, melting and evolution of the upper mantle beneath the Jurassic Pindos ocean inferred by ophiolitic ultramafic rocks in East Othris, Greece

    Science.gov (United States)

    Magganas, Andreas; Koutsovitis, Petros

    2015-07-01

    Large ultramafic bodies of the East Othris ophiolite in Central Greece consist of serpentinites, of harzburgite precursors, as well as serpentinized lherzolites, which have been intruded by thin dykes of olivine-rich and olivine-poor pyroxenites. They represent parts of partially altered upper mantle wedge rocks in a Mid-Late Jurassic intraoceanic subduction setting of the Pindos microocean, a western strand of the Tethyan oceanic realm. Serpentinization and rodingitization occurred during their exhumation toward the fore-arc oceanic region and accretionary prism through a subduction channel. Petrography and geochemistry show that protoliths of most serpentinites and serpentinized peridotites are harzburgites, while few are more fertile lherzolites. Petrogenetic modeling reveals that the former harzburgites correspond to highly depleted residual mantle peridotites, which formed after moderate degrees (~13-20 %) of hydrous partial melting, whereas lherzolites, being closely related to the ophiolitic mantle peridotites of West Othris, resulted after lower partial melting degrees (~7-10 %). Mineral chemistry and geochemical data from pyroxenites imply that they have been derived after crystallization of a subduction-related IAT hydrous magma that formed after moderate partial melting degrees (~14-19 %), quite similar to those that produced the harzburgites. Melting processes for the East Othris mantle peridotites occurred in the spinel-stability field, at estimated equilibrium temperatures ranging between 900 and 1,050 °C and pressures between 1.4 and 1.7 GPa, in a rather highly oxidized environment. It is estimated that the primary IAT magma, formed under relatively high temperatures with liquidus temperature at ~1,260 °C and mantle potential temperature at ~1,372 °C. Cooling rates of the shallow mantle beneath the Pindos oceanic basin, from its Mid-Triassic rift/drift phase and the subsequently developed Mid-Late Triassic short-lived intraoceanic subduction, to

  18. Wind-induced subduction at the South Atlantic subtropical front

    Science.gov (United States)

    Calil, Paulo H. R.

    2017-10-01

    The South Atlantic Subtropical Front, associated with the eastward-flowing South Atlantic Current, separates the colder, nutrient-rich waters of the subpolar gyre from the warmer, nutrient-poor waters of the subtropical gyre. Perturbations to the quasi-geostrophic, eastward flow generate meanders and filaments which induce cross-frontal exchange of water properties. Down-front winds transport denser waters from the South over warm waters from the North, inducing convective instability and subduction. Such processes occur over spatial scales of the order of 1 km and thus require high horizontal spatial resolution. In this modeling study, a high-resolution (4 km) regional grid is embedded in a basin-wide configuration (12 km) of the South Atlantic Ocean in order to test the importance of submesoscale processes in water mass subduction along the subtropical front. Stronger and more numerous eddies obtained in the high-resolution run yield more intense zonal jets along the frontal zone. Such stronger jets are more susceptible to instabilities, frontogenesis, and the generation of submesoscale meanders and filaments with O(1) Rossby number. As a consequence, vertical velocities larger than 100 md 1 are obtained in the high-resolution run, one order of magnitude larger than in the low-resolution run. Wind-driven subduction occurs along the frontal region, associated with negative Ertel potential vorticity in the surface layer. Such processes are not observed in the low-resolution run. A passive tracer experiment shows that waters with density characteristics similar to subtropical mode waters are preferentially subducted along the frontal region. The wind-driven buoyancy flux is shown to be much larger than thermal or haline fluxes during the wintertime, which highlights the importance of the frictional component in extracting PV from the surface ocean and inducing subduction, a process that has been overlooked in subtropical mode water formation in the region.

  19. The dynamical control of subduction parameters on surface topography

    Science.gov (United States)

    Crameri, F.; Lithgow-Bertelloni, C. R.; Tackley, P. J.

    2017-04-01

    The long-wavelength surface deflection of Earth's outermost rocky shell is mainly controlled by large-scale dynamic processes like isostasy or mantle flow. The largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces. Distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates. Having a mantle convection model with a free surface that is able to reproduce both realistic single-sided subduction and long-wavelength surface topography self-consistently, we are now able to better investigate this interaction. We separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters. Results are diagnosed by splitting the topographic signal into isostatic and residual components, and by considering various physical aspects like viscous dissipation during plate bending. Performing several systematic suites of experiments, we are then able to quantify the topographic impact of the buoyancy, rheology, and geometry of the subduction-zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws. We identify slab dip and, slightly less importantly, slab buoyancy as the major agents controlling surface topography at subduction zones on Earth. Only the island-arc high and the back-arc depression extent are mainly controlled by plate strength. Overall, his modeling study sets the basis to better constrain deep-seated mantle structures and their physical properties via the observed surface topography on present-day Earth and back through time.

  20. Project Skippy explores the lithosphere and mantle beneath Australia

    NARCIS (Netherlands)

    Hilst, R.D. van der; Kennett, Brian; Christie, Doug; Grant, John

    1994-01-01

    A new project is probing the seismic structure of the lithosphere and mantle beneath Australia. The Skippy Project, named after the bush kangaroo, exploits Australia's regional seismicity and makes use of recent advances in digital recording technology to collect three-component broadband

  1. Deep structure beneath Lake Ontario: Crustal-scale Grenville subdivisions

    Science.gov (United States)

    Forsyth, D. A.; Milkereit, B.; Zelt, Colin A.; White, D. J.; Easton, R. M.; Hutchinson, Deborah R.

    1994-01-01

    Lake Ontario marine seismic data reveal major Grenville crustal subdivisions beneath central and southern Lake Ontario separated by interpreted shear zones that extend to the lower crust. A shear zone bounded transition between the Elzevir and Frontenac terranes exposed north of Lake Ontario is linked to a seismically defined shear zone beneath central Lake Ontario by prominent aeromagnetic and gravity anomalies, easterly dipping wide-angle reflections, and fractures in Paleozoic strata. We suggest the central Lake Ontario zone represents crustal-scale deformation along an Elzevir–Frontenac boundary zone that extends from outcrop to the south shore of Lake Ontario.Seismic images from Lake Ontario and the exposed western Central Metasedimentary Belt are dominated by crustal-scale shear zones and reflection geometries featuring arcuate reflections truncated at their bases by apparent east-dipping linear reflections. The images show that zones analogous to the interpreted Grenville Front Tectonic Zone are also present within the Central Metasedimentary Belt and support models of northwest-directed crustal shortening for Grenvillian deep crustal deformation beneath most of southeastern Ontario.A Precambrian basement high, the Iroquoian high, is defined by a thinning of generally horizontal Paleozoic strata over a crestal area above the basement shear zone beneath central Lake Ontario. The Iroquoian high helps explain the peninsular extension into Lake Ontario forming Prince Edward County, the occurrence of Precambrian inlier outcrops in Prince Edward County, and Paleozoic fractures forming the Clarendon–Linden structure in New York.

  2. Living and Working Beneath the Sea – Next Approach

    Directory of Open Access Journals (Sweden)

    Rowiński Lech

    2017-04-01

    Full Text Available The idea of living beneath the sea is very new if compared with millennia of shipping activity. In fact, ocean surface was considered mainly as medium suitable for transport of persons and goods as well as aggression and robbery. More practical attempts to live “on” the water surface are limited to well protected internal waters.

  3. Simulation of Wave-Plus-Current Scour beneath Submarine Pipelines

    DEFF Research Database (Denmark)

    Eltard-Larsen, Bjarke; Fuhrman, David R.; Sumer, B. Mutlu

    2016-01-01

    A fully coupled hydrodynamic and morphologic numerical model was utilized for the simulation of wave-plus-current scour beneath submarine pipelines. The model was based on incompressible Reynolds-averaged Navier–Stokes equations, coupled with k-ω turbulence closure, with additional bed and suspen...

  4. Deep long-period earthquakes beneath Washington and Oregon volcanoes

    Science.gov (United States)

    Nichols, M.L.; Malone, S.D.; Moran, S.C.; Thelen, W.A.; Vidale, J.E.

    2011-01-01

    Deep long-period (DLP) earthquakes are an enigmatic type of seismicity occurring near or beneath volcanoes. They are commonly associated with the presence of magma, and found in some cases to correlate with eruptive activity. To more thoroughly understand and characterize DLP occurrence near volcanoes in Washington and Oregon, we systematically searched the Pacific Northwest Seismic Network (PNSN) triggered earthquake catalog for DLPs occurring between 1980 (when PNSN began collecting digital data) and October 2009. Through our analysis we identified 60 DLPs beneath six Cascade volcanic centers. No DLPs were associated with volcanic activity, including the 1980-1986 and 2004-2008 eruptions at Mount St. Helens. More than half of the events occurred near Mount Baker, where the background flux of magmatic gases is greatest among Washington and Oregon volcanoes. The six volcanoes with DLPs (counts in parentheses) are Mount Baker (31), Glacier Peak (9), Mount Rainier (9), Mount St. Helens (9), Three Sisters (1), and Crater Lake (1). No DLPs were identified beneath Mount Adams, Mount Hood, Mount Jefferson, or Newberry Volcano, although (except at Hood) that may be due in part to poorer network coverage. In cases where the DLPs do not occur directly beneath the volcanic edifice, the locations coincide with large structural faults that extend into the deep crust. Our observations suggest the occurrence of DLPs in these areas could represent fluid and/or magma transport along pre-existing tectonic structures in the middle crust. ?? 2010 Elsevier B.V.

  5. Stuck Schools Revisited: Beneath the Averages. K-12 Policy

    Science.gov (United States)

    Ushomirsky, Natasha

    2011-01-01

    "Stuck Schools Revisited: Beneath the Averages" shows why a national focus on turning around the lowest performing schools, while needed, is not enough to raise achievement and close gaps. The report analyzes student achievement data from Maryland and Indiana, which reflect the outcomes seen in other states. The results confirm a…

  6. A high resolution 3D velocity model beneath the Tokyo Metropolitan area by MeSO-net

    Science.gov (United States)

    Nakagawa, S.; Sakai, S.; Honda, R.; Kimura, H.; Hirata, N.

    2015-12-01

    Beneath the Tokyo metropolitan area, the Philippine Sea Plate (PSP) subducts and causes devastating mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9). An M7 or greater (M7+) earthquake in this area at present has high potential to produce devastating serious loss of life and property with even greater global economic repercussions. The Central Disaster Management Council of Japan estimates that an M7+ earthquake will cause 23,000 fatalities and 95 trillion yen (about 1 trillion US$) economic loss. We have launched the Special Project for Reducing Vulnerability for Urban Mega Earthquake Disasters in collaboration with scientists, engineers, and social-scientists in nationwide institutions since 2012. We analyze data from the dense seismic array called Metropolitan Seismic Observation network (MeSO-net), which has 296 seismic stations with spacing of 5 km (Sakai and Hirata, 2009; Kasahara et al., 2009). We applied the double-difference tomography method (Zhang and Thurber, 2003) and estimated the velocity structure and the upper boundary of PSP (Nakagawa et al., 2010). The 2011 Tohoku-oki earthquake (M9.0) has activated seismicity also in Kanto region, providing better coverage of ray paths for tomographic analysis. We obtain much higher resolution velocity models from whole dataset observed by MeSO-net between 2008 and 2015. A detailed image of tomograms shows that PSP contacts Pacific plate at a depth of 50 km beneath northern Tokyo bay. A variation of velocity along the oceanic crust suggests dehydration reaction to produce seismicity in a slab, which may related to the M7+ earthquake. Acknowledgement: This study was supported by the Special Project for Reducing Vulnerability for Urban Mega Earthquake Disasters of MEXT, Japan and the Earthquake Research Institute cooperative research program.

  7. 40Ar/39Ar geochronology of subaerial lava flows of Barren Island volcano and the deep crust beneath the Andaman Island Arc, Burma Microplate

    Science.gov (United States)

    Ray, Jyotiranjan S.; Pande, Kanchan; Bhutani, Rajneesh

    2015-06-01

    Little was known about the nature and origin of the deep crust beneath the Andaman Island Arc in spite of the fact that it formed part of the highly active Indonesian volcanic arc system, one of the important continental crust forming regions in Southeast Asia. This arc, formed as a result of subduction of the Indian Plate beneath the Burma Microplate (a sliver of the Eurasian Plate), contains only one active subaerial magmatic center, Barren Island volcano, whose evolutional timeline had remained uncertain. In this work, we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island, by incremental heating 40Ar/39Ar method, in an attempt to understand the evolutionary histories of the volcano and its basement. Based on concordant plateau and isochron ages, we establish that the oldest subaerial lava flows of the volcano are 1.58 ± 0.04 (2σ) Ma, and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 ± 3 (2σ) Ma. Mineralogy (anorthite + Cr-rich diopside + minor olivine) and isotopic compositions (87Sr/86Sr 7.0) of xenoliths not only indicate their derivation from a lower (oceanic) crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the Andaman accretionary prism. We speculate that the basements of the forearc and volcanic arc of the Andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the Eurasian Plate.

  8. Thin lithosphere-asthenosphere boundary beneath Eastern Indian craton

    Science.gov (United States)

    Shalivahan; Bhattacharya, Bimalendu B.; Rao, N. V. Chalapathi; Maurya, V. P.

    2014-02-01

    The lithosphere-asthenosphere boundary (LAB) separates the hard and rigid outer layer of the earth (lithosphere) and the weaker, hotter, and deeper part of the upper mantle (asthenosphere) and plays a pivotal role in plate tectonics. However, its definitive detection, especially beneath the cratons, is proving elusive. One of the geophysical tools used to map the LAB beneath the cratons is through magnetotelluric (MT) observations. The resistivity at boundary falls in the range of 5-25 Ω-m and can be explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Here, we report thickness of the LAB in one of the oldest dated ancient cratons of India-Eastern Indian Craton (EIC) of ~ 3.3 Gyr, from MT studies. The two prominent phase-sensitive strike directions, one each for crust and mantle, and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath EIC. Our results show that the LAB beneath the EIC is at 95 km. The region is interesting as the electrical properties of the crust and mantle and the Moho depth are similar to those of the Slave Craton, Canada (~ 4.0 Gyr) but the depth of the LAB beneath the EIC is half that of the Slave craton. As cratonic signatures, depicted by ultrapotassic rocks from Gondwana coal fields close to EIC, are preserved at least till early Cretaceous (117 Ma) it is likely that Himalayan orogeny could have played a major role in delamination of the lithospheric roots of the EIC in addition to attendant seismicity.

  9. Three-dimensional shallow velocity structure beneath Taal Volcano, Philippines

    Science.gov (United States)

    You, Shuei-Huei; Konstantinou, Konstantinos I.; Gung, Yuancheng; Lin, Cheng-Horng

    2017-11-01

    Based on its numerous historical explosive eruptions and high potential hazards to nearby population of millions, Taal Volcano is one of the most dangerous "Decade Volcanoes" in the world. To provide better investigation on local seismicity and seismic structure beneath Taal Volcano, we deployed a temporary seismic network consisting of eight stations from March 2008 to March 2010. In the preliminary data processing stage, three periods showing linear time-drifting of internal clock were clearly identified from noise-derived empirical Green's functions. The time-drifting errors were corrected prior to further data analyses. By using VELEST, 2274 local earthquakes were manually picked and located. Two major earthquake groups are noticed, with one lying beneath the western shore of Taal Lake showing a linear feature, and the other spreading around the eastern flank of Taal Volcano Island at shallower depths. We performed seismic tomography to image the 3D structure beneath Taal Volcano using the LOTOS algorithm. Some interesting features are revealed from the tomographic results, including a solidified magma conduit below the northwestern corner of Taal Volcano Island, indicated by high Vp, Vs, and low Vp/Vs ratio, and a large potential hydrothermal reservoir beneath the center of Taal Volcano Island, suggested by low Vs and high Vp/Vs ratio. Furthermore, combining earthquake distributions and tomographic images, we suggest potential existence of a hydrothermal reservoir beneath the southwestern corner of Taal Lake, and a fluid conduit extending to the northwest. These seismic features have never been proposed in previous studies, implying that new hydrothermal activity might be formed in places away from the historical craters on Taal Volcano Island.

  10. Modelling the Crust beneath the Kashmir valley in Northwestern Himalaya

    Science.gov (United States)

    Mir, R. R.; Parvez, I. A.; Gaur, V. K.; A.; Chandra, R.; Romshoo, S. A.

    2015-12-01

    We investigate the crustal structure beneath five broadband seismic stations in the NW-SE trendingoval shaped Kashmir valley sandwiched between the Zanskar and the Pir Panjal ranges of thenorthwestern Himalaya. Three of these sites were located along the southwestern edge of the valley andthe other two adjoined the southeastern. Receiver Functions (RFs) at these sites were calculated usingthe iterative time domain deconvolution method and jointly inverted with surface wave dispersiondata to estimate the shear wave velocity structure beneath each station. To further test the results ofinversion, we applied forward modelling by dividing the crust beneath each station into 4-6homogeneous, isotropic layers. Moho depths were separately calculated at different piercing pointsfrom the inversion of only a few stacked receiver functions of high quality around each piercing point.These uncertainties were further reduced to ±2 km by trial forward modelling as Moho depths werevaried over a range of ±6 km in steps of 2 km and the synthetic receiver functions matched with theinverted ones. The final values were also found to be close to those independently estimated using theH-K stacks. The Moho depths on the eastern edge of the valley and at piercing points in itssouthwestern half are close to 55 km, but increase to about 58 km on the eastern edge, suggesting thathere, as in the central and Nepal Himalaya, the Indian plate dips northeastwards beneath the Himalaya.We also calculated the Vp/Vs ratio beneath these 5 stations which were found to lie between 1.7 and1.76, yielding a Poisson's ratio of ~0.25 which is characteristic of a felsic composition.

  11. Long term (since the late palaeogene) tectono-sedimentary evolution of the Lesser Antilles fore-arc at Marie-Galante Basin: a clue for geodynamical behavior at the subduction interfac

    Science.gov (United States)

    Jean-Frederic, L.; DeMin, L.; Garrigou, J.; Münch, P.; Léticée, J. L.; Cornée, J. J.

    2015-12-01

    Oblique subduction of late cretaceous lithosphere of the Atlantic ocean beneath the thick (25km) crust of the Caribbean plate results in widespread deformation and vertical motions in the Lesser Antilles fore-arc. The present-day deformation includes a major transtensive left lateral fault system along the arc and several forearc transverse basins accommodating lengthening of the fore-arc northward. These deformations result from plate motion partitioning under increasing subduction obliquity from the Marie-Galante Basin (MGB) latitude (central Lesser Antilles) northward. Vertical motions in the fore-arc at a regional scale were interpreted as resulting from the effect of subducting ridges and reliefs. The present day uplift of the fore-arc islands acting since the late(?) Pleistocene is believed to attest for long wavelength bending of the plate under strongly coupled plate interface. Recent GPS data suggests a mostly uncoupled plate interface. To decipher between the models and to understand the long-term evolution of the Lesser Antilles forearc since the Late Palaeogene, we interpret high-resolution bathymetric and seismic data from the MGB, together with the onland geology of shallow water carbonate platforms. The tectonic pattern reveals both inherited and late Neogene structures (re)activated under multidirectional extensive tectonic. The sismo-stratigraphic interpretation of sedimentary deposit displays long-term drowning and flexing of the upper plate similar to that occurring under intensive tectonic erosion at the subduction interface. Several short term period of second order uplift can correlate with sweeping of subducting ridges or transient events at the plate interface. The evolution of the Lesser Antilles fore-arc since the Late Palaeogene is interpreted within the regional geodynamical evolution of the plate boundary following its last major reorganization: collision of the Bahamas Bank and inception of the Greater Antilles strike-slip fault zone.

  12. Subduction Mode Selection During Slab and Mantle Transition Zone Interaction: Numerical Modeling

    Science.gov (United States)

    Shi, Yanan; Wei, Dongping; Li, Zhong-Hai; Liu, Ming-Qi; Liu, Mengxue

    2017-12-01

    Global seismic tomography of the subduction zones shows that the subducting slabs could either stagnate around the 660-km discontinuity, or penetrate into the lower mantle. The stagnating slabs also have various morphologies. These are directly related to the interaction between the subducting slabs and the mantle transition zone (MTZ), the dynamics of which are still debated. Using a 2-D thermo-mechanical model, we systematically investigated the modes of subduction in the mantle transition zone and explored the key constraints of various subduction styles. Four basic subduction modes are obtained in the numerical experiments, including one with slab penetrating through the 660-km discontinuity and three other modes with slab stagnating in the MTZ (i.e. folding, lying and rolling-back). The numerical models indicate that the age of subducting oceanic plate, the thickness of overriding continental lithosphere and the convergence velocity play crucial roles in the dynamics of subducting slab and MTZ interaction. In general, the young subducting slab favors the penetration or folding mode, whereas the old subducting slab tends to result in lying or rolling-back mode, although other parameters can also affect. Our models also show a strong correlation between the subduction mode selection and dip angle of the slab tip when reaching the 660-km phase boundary.

  13. Reevaluating plate driving forces from 3-D models of subduction

    Science.gov (United States)

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

    2004-12-01

    Subducting lithospheric slabs mechanically attached to tectonic plates provide the main driving force for surface plate motion. Numerical models historically simulate slab dynamics as a 2-D process and further simplify the problem into either a density driven model (no heat transfer) or a corner-flow problem (thermal convection) [Christensen, 2001; Enns et al., (in revision); van Keken, 2003]. Recent 3-D global models of density driven flow incorporating a history of plate motion (Conrad and Lithgow-Bertelloni, 2002) have succussfully ruled out slab "suction" (basal shear traction induced by downward flow of the slabs) as a major driving force, but exact partitioning of the remaining forces acting on the slab remain unconstrained. A survey of trenches around the world reveals that over half of the slabs presently subducted in the upper mantle have a discontinuous edge (either a slab tip on a young slab, or the side edge of a slab with finite width) around which mantle can flow: prime examples being slabs in the Mediterranean and Carribean. However, even slabs with a wide lateral extent (and where a 2-D approximation may seem appropriate), show signs of having 3-D complexity. For example, on the surface Tonga appears relatively symmetric, but when the history of subduction is considered, the slab has a twisted, 3-D structure due to significant eastward retreat of just the northern part of an originally N-S oriented trench edge. Similarly the widest slabs, South American and Kamchatka, show seismic anisotropy attributed to trench parallel mantle flow (Russo and Silver, 1994; Peyton, et al., 2001, respectively), while the Aleutian trench has oblique subduction varying in magnitude from west to east, and medium width Central American slab likely has a slab window allowing 3-D flow (Johnston and Thorkelson, 1997). Recent laboratory experiments of subduction have demonstrated the full complexity of flow occuring in 3-D geometry (Kincaid and Griffiths, 2003; Schellart

  14. From subduction to arc-continent collision: Geodynamic modeling of strain partitioning and mountain building in the Indonesia Archipelago

    Science.gov (United States)

    Yang, Y.; Liu, M.; Harris, R.

    2006-12-01

    The arc-continental collision in the eastern Indonesia archipelago began 3-5 million years ago. This process has been continuously building one of the youngest orogenic belts on Earth. Both GPS measurement and geological observation indicated westward weakening of crustal shortening in this region, correlating with the transition from continent-arc collision in the Timor trough to oceanic slab-arc convergence in the Java trench. We have developed a finite element model to investigate how the continent-ocean floor transition on the subducting Australian plate and rheologic heterogeneity of the Sunda region have controlled strain partitioning and mountain building in the overriding plate. To simulate the progressive oblique collision between the Indo- Australian plate and Southeast Asian plate, we constructed the model in 3D with nonlinear viscous rheology that also includes fault zones and rheological heterogeneity in both horizontal and vertical direction. Topographic loading is considered, and tectonic loading is calculated from the velocity boundary condition based on either Nuvel-1A or GPS measurements. Our model showed direct link between plate coupling and crustal shortening in the upper plate. When the coupling is weak, deformation concentrates near the trench. As coupling increases, more plate convergence is accommodated by crustal shorting in the upper plate. The initiation of the continent-arc collision in the eastern Timor could explain the cease of oceanic floor spreading in the south Banda basin and the development of back arc thrust. Slipping of the back arc thrusts played a critical role in uplift and tilting of the upper plate in the double-vergent Timor islands. Within a reasonable range of rheological parameters, our model is able to predict surface velocity, uplift rate, and stress states consistent with the GPS data, geological observation and earthquake mechanism solutions. Results of this model provide some useful insights into the evolution of

  15. 3D Numerical modelling of topography development associated with curved subduction zones

    Science.gov (United States)

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

    2017-04-01

    Curved subduction zones, also called oroclines, are geological features found in various places on Earth. They occur in diverse geodynamic settings: 1) single slab subduction in oceanic domain (e.g. Sandwich trench in the Southern Atlantic); 2) single slab subduction in continental domain, (e.g. Gibraltar-Alboran orocline in the Western Mediterranean) 3); multi-slab subduction (e.g. Caribbean orocline in the South-East of the Gulf of Mexico). These systems present various curvatures, lengths (few hundreds to thousands of km) and ages (less than 35 Ma for Gibraltar Alboran orocline, up to 100 Ma for the Caribbean). Recent studies suggested that the formation of curved subduction systems depends on slab properties (age, length, etc) and may be linked with processes such as retreating subduction and delamination. Plume induced subduction initiation has been proposed for the Caribbean. All of these processes involve deep mechanisms such as mantle and slab dynamics. However, subduction zones always generate topography (trenches, uplifts, etc), which is likely to be influenced by surface processes. Hence, surface processes may also influence the evolution of subduction zones. We focus on different kinds of subduction systems initiated by plume-lithosphere interactions (single slab subduction/multi-slab subduction) and scrutinize their surface expression. We use numerical modeling to examine large-scale subduction initiation and three-dimensional slab retreat. We perform two kinds of simulations: 1) large scale subduction initiation with the 3D-thermomechanical code I3ELVIS (Gerya and Yuen, 2007) in an oceanic domain and 2) large scale subduction initiation in oceanic domain using I3ELVIS coupled with a robust new surface processes model (SPM). One to several retreating slabs form in the absence of surface processes, when the conditions for subduction initiation are reached (c.f. Gerya et al., 2015), and ridges occur in the middle of the extensional domain opened by slab

  16. Subduction initiation for the formation of high-Cr chromitites in the Kop ophiolite, NE Turkey

    Science.gov (United States)

    Zhang, Peng-Fei; Uysal, Ibrahim; Zhou, Mei-Fu; Su, Ben-Xun; Avcı, Erdi

    2016-09-01

    The Kop ophiolite in NE Turkey is a forearc fragment of Neo-Tethys ocean, consisting mainly of a paleo-Moho transition zone (MTZ) and a harzburgitic upper mantle unit. Locally, the Kop MTZ contains cumulate dunites and high-Cr chromitites (Cr# up to ca. 79), which are cut by pyroxenites. Dunites and chromitites in the MTZ have REE concentrations that are 1-2 orders of magnitude lower than those of chondrite; they are either depleted in LREE or have concave REE shapes. The LREE depleted patterns are interpreted to reflect production of cumulate rocks by magmas derived from a depleted mantle, the concave patterns the modification of these rocks by LREE-enriched fluids. Clinopyroxenes from pyroxenites are diopsidic and characterized by high Mg#s (ca. 92-96) and high CaO contents (ca. 24-25 wt.%); their Al2O3 contents (1.0-3.0 wt.%) fall between those of clinopyroxenes in N-MORB and komatiite/boninite, suggesting that the parental melts originated from more refractory mantle than abyssal lherzolites. However, these clinopyroxenes display LREE depleted patterns consistent with those of clinopyroxenes in abyssal lherzolites, indicating their genetic connection with decompression melting of asthenosphere. The cross-cutting relationship between pyroxenite veins and chromitiferous rocks suggests that depleted mantle remained beneath the proto-forearc after chromitite formation; it had not been significantly modified by slab-derived components and continued interacting with the upwelling asthenosphere until pyroxenite crystallization. This study provides a temporal constraint on the formation of high-Cr chromitites; they possibly began to be produced during the transition between early and late proto-forearc spreading, during which subduction dehydration had not well developed.

  17. Earthquake Directivity, Orientation, and Stress Drop Within the Subducting Plate at the Hikurangi Margin, New Zealand

    Science.gov (United States)

    Abercrombie, Rachel E.; Poli, Piero; Bannister, Stephen

    2017-12-01

    We develop an approach to calculate earthquake source directivity and rupture velocity for small earthquakes, using the whole source time function rather than just an estimate of the duration. We apply the method to an aftershock sequence within the subducting plate beneath North Island, New Zealand, and investigate its resolution. We use closely located, highly correlated empirical Green's function (EGF) events to obtain source time functions (STFs) for this well-recorded sequence. We stack the STFs from multiple EGFs at each station, to improve the stability of the STFs. Eleven earthquakes (M 3.3-4.5) have sufficient azimuthal coverage, and both P and S STFs, to investigate directivity. The time axis of each STF in turn is stretched to find the maximum correlation between all pairs of stations. We then invert for the orientation and rupture velocity of both unilateral and bilateral line sources that best match the observations. We determine whether they are distinguishable and investigate the effects of limited frequency bandwidth. Rupture orientations are resolvable for eight earthquakes, seven of which are predominantly unilateral, and all are consistent with rupture on planes similar to the main shock fault plane. Purely unilateral rupture is rarely distinguishable from asymmetric bilateral rupture, despite a good station distribution. Synthetic testing shows that rupture velocity is the least well-resolved parameter; estimates decrease with loss of high-frequency energy, and measurements are best considered minimum values. We see no correlation between rupture velocity and stress drop, and spatial stress drop variation cannot be explained as an artifact of varying rupture velocity.

  18. Testing the intraplate origin of mega-earthquakes at subduction margins

    Directory of Open Access Journals (Sweden)

    Prosanta K. Khan

    2012-07-01

    Full Text Available The disastrous Mw 9.3 (seismic moment 1.0×1030 dyn/cm earthquake that struck northwest Sumatra on 26 December 2004 and triggered ∼30 m high tsunami has rejuvenated the quest for identifying the forcing behind subduction related earthquakes around the world. Studies reveal that the strongest part (elastic core of the oceanic lithosphere lie between 20 and 60 km depth beneath the upper (∼7 km thick crustal layer, and compressive stress of GPa order is required to fail the rock-layers within the core zone. Here we present evidences in favor of an intraplate origin of mega-earthquakes right within the strong core part (at the interface of semi-brittle and brittle zone, and propose an alternate model exploring the flexing zone of the descending lithosphere as the nodal area for major stress accumulation. We believe that at high confining pressure and elevated temperature, unidirectional cyclic compressive stress loading in the flexing zone results in an increase of material yield strength through strain hardening, which transforms the rheology of the layer from semi-brittle to near-brittle state. The increased compressive stress field coupled with upward migration of the neutral surface (of zero stress fields under non-coaxial deformation triggers shear crack. The growth of the shear crack is initially confined in the near-brittle domain, and propagates later through the more brittle crustal part of the descending oceanic lithosphere in the form of cataclastic failure.

  19. Carbonation of Subduction Interface Ultramafic Rocks and Implications for Deep Carbon Cycling: Evidence from Hybrid Serpentinite-Marble in the Voltri Massif, Italy

    Science.gov (United States)

    Scambelluri, M.; Bebout, G. E.; Gilio, M.; Belmonte, D.; Campomenosi, N.; Crispini, L.

    2015-12-01

    Release of COH fluids from hydrous minerals and carbonates influences element recycling and magmatism at subduction zones. Contradictory interpretations exist regarding the retention/storage of C in subducting plates and in the forearc to subarc mantle. Multiple lines of evidence indicate mobility of C in forearcs; however, the magnitude of this loss is highly uncertain[1-5]. A poorly constrained fraction of the 40-115 Mt/y of C initially subducted is released into fluids (e.g., by decarbonation, carbonate dissolution), and 18-43 Mt/y is returned at arc volcanoes[2-5, refs. therein]. The imbalance could reflect subduction into the deeper mantle or forearc/subarc storage[4-7]. We examine the fate of C in slab/interface ultramafic rocks, and by analogy serpentinized mantle wedge, via study of fluid-rock evolution of marble and variably carbonated serpentinite (Ligurian Alps). Based on petrography, and major/trace element and C and O isotope compositions, we demonstrate that serpentinite dehydration at 2-2.5 GPa, 550°C released aqueous fluids triggering breakdown of dolomite in nearby marbles, thus releasing C into fluids. Carbonate + olivine veins document flow of COH fluids; interaction of these COH fluids with serpentinite led to formation of high-pressure carbonated ultramafic-rock domains, thus resulting in retention of C in some rocks at an ancient subduction interface. We stress that lithologically complex interfaces could contain sites of both C release and C addition, further confounding estimates of net C loss at forearc and subarc depths [cf 4,5]. Sites of C retention, also including carbonate veins and graphite as reduced carbonate[7], could influence the transfer of slab C to at least the depths beneath volcanic fronts. 1. Poli S et al. 2009 EPSL; 2. Ague and Nicolescu 2014 Nat Geosci; 3. Cook-Collars et al. 2014 Chem Geol; 4. Collins et al. 2015 Chem Geol; 5. Kelemen and Manning 2015 PNAS; 6. Sapienza et al. 2009 CMP; 7 Galvez et al. 2013 Nat Geosci

  20. Mineralogy, geochronology and geochemistry of early Triassic blueschists in the Lancang metamorphic zone of Southwest China: fingerprints of the Paleotethyan subduction

    Science.gov (United States)

    Wang, Y.; Fan, W.; Zhang, Y.; Cai, Y.; Jourdan, M.

    2013-12-01

    The subduction of the Paleotethyan Ocean and subsequent continental collision along the Lancang tectonic belt of the southeastern Paleotethyan belt is a major tectonic event for Southwest China, but the timing of the subduction and associated thermal structure preceding the final collision are still unknown. The mafic blueschists exposed in the Lancang accretionary complex provide the crucial records of the kinematics and thermal dynamics for the subduction zone. In this paper, we present a set of new mineralogical, geochronological and geochemical data for the Xuyi blueschists and report the presence of high-pressure Na-amphibole in the Lancang metamorphic zone. The mineral assemblage of these blueschists consists of zoned sodic amphibole (25-30 %), albite (15-20 %), epidote (25-30 %), phengite (5-10 %), chlorite (~5-10 %), and minor amounts of actinolite, apatite, sphere, zircon, ilmenite, quartz and secondary limonite. They display a progressive metamorphism from ~0.5 GPa to ~0.9 GPa and retrograde metamorphic overprinting (back to ~0.6 GPa) within the temperature range 300-450°C. The glaucophane and phengite developed during progressive metamorphism yielded the Ar-Ar ages of 246-251 Ma, and the glaucophane overprinted by retrograde metamorphism gave the Ar-Ar plateau ages of 233.2 ×3.1 Ma. The Xuyi blueschists yielded the zircon U-Pb age of 254 × 3 Ma, and are compositionally identical to the subalkaline basalt with typical OIB-type REE and multi-elemental patterns and ɛNd(t) values of +3.35 to +4.85. Based on all the available data, it is inferred that the protolith originated from the basaltic seamount with shallow-marine carbonate cap in the Paleotethyan Ocean till 254 Ma. The seamounts subducted eastward down to 30-35 km depths beneath the Lincang arc and became metamorphosed to form the epidote blueschists at ~246 -251 Ma. The blueschists were subsequently transported to shallower crustal levels in response to the continuous underthrust of the

  1. Dynamic Topography during Flat Subduction: Subsidence or Uplift?

    Science.gov (United States)

    Davila, F. M.; Lithgow-Bertelloni, C. R.

    2011-12-01

    Since the first studies on dynamic topography and basin evolution, low-dipping subduction has been related to intracontinental, long-wavelength and high-amplitude subsidence, whereas retreating to normal subduction systems to uplift. This was proposed to explain the Cretaceous-early Cenozoic topographic evolution of the western US. However, modern flat-slab and slab-retreating segments of South America do not record such a subsidence and uplift patterns. For example, the flat slab of Peru at ˜10°SL, related to the subduction of the Nazca Ridge, underlies an elevated promontory known as the Fitzcarrald Arch. The Argentine flat-slab at ˜31°SL associated to the subduction of the Juan Fernandez Ridge underlies a high-elevated intermontane system known as the Pampean broken foreland. Both upwarping features are younger than 7 Ma and contemporaneous with the arrival of flat subduction to these segments. In order to shed light into this controversy, we calculate dynamic topography along the Andean flat-slab segments using the Hager and O'Connell (1981) instantaneous flow formulation, an accurate reconstruction of the slab geometry along the central Andes and a density contrast between the flat slabs and the country mantle close to zero (△δ≈0) in order to simulate a buoyant oceanic lithosphere. We demonstrate that dynamic subsidence develops only at the leading edge of flat subduction, where the slabs plunge >30°, whereas the flatter slabs reproduce minor or no dynamic topography signals. These results agree with geological and geophysical proxies. Along the Argentine Plains, the account for a accumulated relief of ˜200 m, which might be considered as an "observed dynamic subsidence" signal (given that no tectonic activity has been recorded in this region since the Cretaceous to explain this surface topography). This gives a ˜0.03 mm/yr dynamic subsidence rate that are curiously similar to the exhumations estimated by low-temperature thermochronology along the

  2. The Modern Gorda-Juan de Fuca Subduction System and the Inherited Stratification of the Transition Zone below Western US

    Science.gov (United States)

    Tauzin, B.; van der Hilst, R. D.; Wittlinger, G.; Ricard, Y. R.

    2012-12-01

    We provide insights on the deep geodynamic processes that have shaped western North America (NA) during the past 100 Myr. We use data of P-to-S converted waves recorded at seismic stations of the US Transportable Array to image main discontinuities in shear-wave velocity within the transition zone (TZ). We measure the topography and the reflectance (i.e. the amplitude of conversion) of the `410' and `660' interfaces. We image also minor seismic discontinuities around 350, 590 and 630 km depths. Idaho is the location of strong structural anomalies with a sharp topography of the `660', a thick TZ, a reduced reflectance of the `410' and a positive interface at 630 km depth. The thick TZ and the `630' discontinuity suggest low temperatures and a possible garnet to ilmenite transition. Water could also reduce locally the `410' Vs contrast. A remnant of the Farallon plate, anchored 55 Myr ago in the mantle below Idaho [Schmandt and Humphreys, 2011] and providing cool temperatures and water to the TZ, is a possible explanation. At the same location, a layer with 4% shear-wave speed reduction (the `350') and 70 km thickness is present atop the `410' discontinuity. In addition, an almost continuous interface, marking shear wave velocity reductions, spreads at 590 km depth roughly from southern Idaho to southeastern western US. Dehydration induced partial melting [Bercovici and Karato, 2003] can explain a melt layer atop the `410'. However, it does not explain a widespread negative `590' discontinuity within the TZ. It is possible that eclogitic material has progressively accumulated at the base of the TZ due to crustal delamination from the subducted Farallon oceanic lithosphere. With low Vs, the top of a garnetite layer could produce sharp negative velocity gradients around 590 km depth. The geometry of the Gorda-Juan de Fuca subduction system is also subject to debate. Tomography has shown the plate to subduct in the mantle along the whole western margin of NA except

  3. Subduction and accretion of sedimentary rocks in the Yakutat collision zone, St. Elias orogen, Gulf of Alaska

    Science.gov (United States)

    Van Avendonk, Harm J. A.; Gulick, Sean P. S.; Christeson, Gail L.; Worthington, Lindsay L.; Pavlis, Terry L.; Ridgway, Kenneth D.

    2013-11-01

    The collision of the Yakutat Block with the continental margin of North America in the Gulf of Alaska has intensified exhumation and erosion in the Chugach-St. Elias orogen over the last few million years. The resultant sediment flux and deposition of the glaciomarine Yakataga Formation on the continental shelf has filled a deep sedimentary basin offshore, where the Pamplona fold-thrust belt first deforms these strata. It is presently unclear whether the older sedimentary rocks of the Poul Creek and Kulthieth Formations are also accreted in the Pamplona Zone, or whether they are underthrusting the margin. In this paper we use marine seismic and well logging data to show that in the offshore Yakataga strata, porosity loss and lateral compaction can account for half of the convergence between the Yakutat Block and North America over the last 2 Myr. A lateral seismic velocity gradient in these syn-orogenic strata suggests that this layer-parallel shortening starts approximately 100 km outboard of the deformation front. Beneath the fold-and-thrust belt, where the seismic velocity is as high as 4.7 km/s, we image a large low-velocity zone (2.0-2.5 km/s) at 5 km depth. The dramatic decrease in seismic velocity with depth coincides with the boundary between the Yakataga and Poul Creek Formations in well data. Fine-grained and organic-rich Poul Creek strata possibly accommodate slip, such that older sedimentary rocks are entrained with the subducting Yakutat Block. Alternatively, the imaged low-velocity zone may have formed by increased fluid pressures in the hanging wall. In that case the décollement would lie beneath this low-velocity zone, possibly within the coal-bearing layers of the older and deeper Kulthieth Formation.

  4. What favors the occurrence of subduction mega-earthquakes?

    Science.gov (United States)

    Brizzi, Silvia; Funiciello, Francesca; Corbi, Fabio; Sandri, Laura; van Zelst, Iris; Heuret, Arnauld; Piromallo, Claudia; van Dinther, Ylona

    2017-04-01

    Most of mega-earthquakes (MEqs; Mw > 8.5) occur at shallow depths along the subduction thrust fault (STF). The contribution of each subduction zone to the globally released seismic moment is not homogenous, as well as the maximum recorded magnitude MMax. Highlighting the ingredients likely responsible for MEqs nucleation has great implications for hazard assessment. In this work, we investigate the conditions favoring the occurrence of MEqs with a multi-disciplinary approach based on: i) multivariate statistics, ii) analogue- and iii) numerical modelling. Previous works have investigated the potential dependence between STF seismicity and various subduction zone parameters using simple regression models. Correlations are generally weak due to the limited instrumental seismic record and multi-parameter influence, which make the forecasting of the potential MMax rather difficult. To unravel the multi-parameter influence, we perform a multivariate statistical study (i.e., Pattern Recognition, PR) of the global database on convergent margins (Heuret et al., 2011), which includes seismological, geometrical, kinematic and physical parameters of 62 subduction segments. PR is based on the classification of objects (i.e., subduction segments) belonging to different classes through the identification of possible repetitive patterns. Tests have been performed using different MMax datasets and combination of inputs to indirectly test the stability of the identified patterns. Results show that the trench-parallel width of the subducting slab (Wtrench) and the sediment thickness at the trench (Tsed) are the most recurring parameters for MEqs occurrence. These features are mostly consistent, independently of the MMax dataset and combination of inputs used for the analysis. MEqs thus seem to be promoted for high Wtrench and Tsed, as their combination may potentially favor extreme (i.e., in the order of thousands of km) trench-parallel rupture propagation. To tackle the

  5. The dominant surface-topography contributions of individual subduction parameters

    Science.gov (United States)

    Crameri, Fabio; Lithgow-Bertelloni, Carolina; Tackley, Paul

    2017-04-01

    It is no secret, not any longer, that dynamic processes below the plate exert a significant contribution to the elevation of the plate at the surface (e.g., Flament et al., 2013). We have therefore studied* the individual impact each and every major subduction parameter has on surface topography. This allows us to qualitatively compare the different sources amongst each other, and to quantify their actual potential to vertically deflect the surface. The gained knowledge from this compilation is crucial: We might finally be able to link the directly-observable surface topography to the dynamics (buoyancy, rheology, and geometry) of the subduction system. *This study is made possible by the efficient convection code StagYY (Tackley 2008), the largely-automated post-processing and visualisation toolbox StagLab (www.fabiocrameri.ch/software), and crucial model developments (Crameri and Tackley, 2015; Crameri et al., G-cubed, submitted, Crameri and Lithgow-Bertelloni, Tectonophysics, submitted). REFERENCES 
Flament, N., M. Gurnis, and R. D. Müller (2013), A review of observations and models of dynamic topography, Lithosphere, 5(2), 189-210. Crameri, F., and P. J. Tackley (2015), Parameters controlling dynamically self-consistent plate tectonics and single-sided subduction in global models of mantle convection, J. Geophys. Res. Solid Earth, 120(5), 3680-3706. Crameri, F., C. R. Lithgow-Bertelloni, and P. J. Tackley (submitted), The dynamical control of subduction parameters on surface topography, Geochem. Geophys. Geosyst. Crameri, F., and C. R. Lithgow-Bertelloni (submitted), Dynamic Mantle-Transition-Zone Controls on Upper-Plate Tilt, Tectonophysics. Tackley, P.J (2008) Modelling compressible mantle convection with large viscosity contrasts in a three- dimensional spherical shell using the yin-yang grid. Physics of the Earth and Planetary Interiors 171(1-4), 7-18.

  6. Retrograde lawsonite formation in the Franciscan subduction complex

    Science.gov (United States)

    Myers, S.; Mulcahy, S. R.

    2016-12-01

    Lawsonite [CaAl2Si2O7(OH)2·H2O] is an index mineral of low-temperature subduction zones, contains a significant amount of water, and is an important host of rare-earth and trace elements in mafic protoliths. For these reasons, numerous studies have investigated the consequences of lawsonite breakdown during prograde subduction. In the Franciscan subduction complex, however, lawsonite in mafic blueschist largely formed along a retrograde path from pre-existing eclogite. In order to asses the conditions and significance of retrograde lawsonite formation we examined the petrology and geochemistry of lawsonite-bearing assemblages in Franciscan mafic rocks. All of the samples have the common assemblage: lawsonite, glaucophane, and sphene. Quartz is generally absent. Muscovite, chlorite, and relict epidote and rutile are variably present. Different index minerals calcite, aragonite, albite, and jadeitic pyroxene are present within lawsonite assemblages. Garnet occurs in equilibrium with lawsonite, as a relict mineral in lawsonite and the matrix, or is completely absent. Major element compositions vary from typical basalts and are strongly correlated with one another. Chondrite normalized REE compositions are variably LREE depleted or enriched, MREE are flat to enriched, and HREE are generally flat. Trace elements normalized to NMORB show variably enriched and depleted LILE. The petrology suggests lawsonite, glaucophane, and sphene formed from multiple retrograde reactions involving garnet, clinopyroxene, epidote, and rutile, together with significant hydration. Important index minerals imply lawsonite formed over a wide range of pressures within the subduction zone. The major, REE, and trace element compositions suggest lawsonite assemblages were derived from different protoliths or experienced variable amounts of metasomatism and interaction with crustally derived material and serpentinite.

  7. Crawling beneath the free surface: Water snail locomotion

    OpenAIRE

    Lee, Sungyon; Bush, John W. M.; Hosoi, A. E.; Lauga, Eric

    2008-01-01

    Land snails move via adhesive locomotion. Through muscular contraction and expansion of their foot, they transmit waves of shear stress through a thin layer of mucus onto a solid substrate. Since a free surface cannot support shear stress, adhesive locomotion is not a viable propulsion mechanism for water snails that travel inverted beneath the free surface. Nevertheless, the motion of the freshwater snail, Sorbeoconcha physidae, is reminiscent of that of its terrestrial counterparts, being g...

  8. Slow upper mantle beneath Southern Norway from surface waves

    Science.gov (United States)

    Weidle, C.; Maupin, V.

    2009-04-01

    A recent regional surface wave tomography for Northern Europe revealed unprecedented images of the upper mantle beneath the (Tertiary) North Atlantic and the bordering Fennoscandian craton of Archean-Proterozoic age. With respect to the circum-Atlantic regions of uplift, no common mantle pattern supporting the uplift of these regions is observed. The western boundary of the thick cratonic lithosphere follows the trend of the continental margin offshore northern Norway (i.e. the northern Scandes are underlain by thick lithosphere) whereas further south the boundary of the craton is located further east beneath southwestern Sweden. SV shear wave velocities beneath southern Norway are 10% slower than ak135 (at 70-115 km depth) and these low-velocities are clearly connected to the North Atlantic low-velocity regime through a ~ 400 km wide "channel". The low-velocity anomaly beneath Southern Norway coincides in geometry roughly with the dome-like high topography of the southern Scandes and may thus have a non-negligible contribution to the isostatic balance of the region. The amplitude and depth-distribution of this anomaly are due to be further constrained by new data that were acquired during the MAGNUS experiment in 2006-2008. The temporary seismic network, consisting of 40 broadband seismometers covers to a large extent the location of the anomaly as imaged by the regional tomography. This enables us to get unique control on the tomographic model at improved lateral and vertical resolution. Preliminary analysis of surface wave phase velocities yields an average 1-D shear wave velocity profile for southern Norway as a first step to constrain the presence and depth extent of this low-velocity anomaly.

  9. On Irrotational Flows Beneath Periodic Traveling Equatorial Waves

    Science.gov (United States)

    Quirchmayr, Ronald

    2017-06-01

    We discuss some aspects of the velocity field and particle trajectories beneath periodic traveling equatorial surface waves over a flat bed in a flow with uniform underlying currents. The system under study consists of the governing equations for equatorial ocean waves within a non-inertial frame of reference, where Euler's equation of motion has to be suitably adjusted, in order to account for the influence of the earth's rotation.

  10. The extent of continental crust beneath the Seychelles

    Science.gov (United States)

    Hammond, J. O. S.; Kendall, J.-M.; Collier, J. S.; Rümpker, G.

    2013-11-01

    The granitic islands of the Seychelles Plateau have long been recognised to overlie continental crust, isolated from Madagascar and India during the formation of the Indian Ocean. However, to date the extent of continental crust beneath the Seychelles region remains unknown. This is particularly true beneath the Mascarene Basin between the Seychelles Plateau and Madagascar and beneath the Amirante Arc. Constraining the size and shape of the Seychelles continental fragment is needed for accurate plate reconstructions of the breakup of Gondwana and has implications for the processes of continental breakup in general. Here we present new estimates of crustal thickness and VP/VS from H-κ stacking of receiver functions from a year long deployment of seismic stations across the Seychelles covering the topographic plateau, the Amirante Ridge and the northern Mascarene Basin. These results, combined with gravity modelling of historical ship track data, confirm that continental crust is present beneath the Seychelles Plateau. This is ˜30-33 km thick, but with a relatively high velocity lower crustal layer. This layer thins southwards from ˜10 km to ˜1 km over a distance of ˜50 km, which is consistent with the Seychelles being at the edge of the Deccan plume prior to its separation from India. In contrast, the majority of the Seychelles Islands away from the topographic plateau show no direct evidence for continental crust. The exception to this is the island of Desroche on the northern Amirante Ridge, where thicker low density crust, consistent with a block of continental material is present. We suggest that the northern Amirantes are likely continental in nature and that small fragments of continental material are a common feature of plume affected continental breakup.

  11. Silicate dissolution boosts the CO2 concentrations in subduction fluids.

    Science.gov (United States)

    Tumiati, S; Tiraboschi, C; Sverjensky, D A; Pettke, T; Recchia, S; Ulmer, P; Miozzi, F; Poli, S

    2017-09-20

    Estimates of dissolved CO2 in subduction-zone fluids are based on thermodynamic models, relying on a very sparse experimental data base. Here, we present experimental data at 1-3 GPa, 800 °C, and ∆FMQ ≈ -0.5 for the volatiles and solute contents of graphite-saturated fluids in the systems COH, SiO2-COH ( + quartz/coesite) and MgO-SiO2-COH ( + forsterite and enstatite). The CO2 content of fluids interacting with silicates exceeds the amounts measured in the pure COH system by up to 30 mol%, as a consequence of a decrease in water activity probably associated with the formation of organic complexes containing Si-O-C and Si-O-Mg bonds. The interaction of deep aqueous fluids with silicates is a novel mechanism for controlling the composition of subduction COH fluids, promoting the deep CO2 transfer from the slab-mantle interface to the overlying mantle wedge, in particular where fluids are stable over melts.Current estimates of dissolved CO2 in subduction-zone fluids based on thermodynamic models rely on a very sparse experimental data base. Here, the authors show that experimental graphite-saturated COH fluids interacting with silicates at 1-3 GPa and 800 °C display unpredictably high CO2 contents.

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

    Science.gov (United States)

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

    2014-07-14

    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.

  13. Water and the oxidation state of subduction zone magmas.

    Science.gov (United States)

    Kelley, Katherine A; Cottrell, Elizabeth

    2009-07-31

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

  14. Thermobarometric and fluid expulsion history of subduction zones

    Energy Technology Data Exchange (ETDEWEB)

    Ernst, W.G. (Univ. of California, Los Angeles (United States))

    1990-06-10

    Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite {r arrow} prehnite-pumpellyite {r arrow} glaucophane schist {r arrow} eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogites at depth. Inclusion studies suggest that two-phase immiscible volatiles are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses (e.g., Western Alps) have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Less common blueschist terranes (e.g., Franciscan belt of western California) preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression approximately retraced the prograde path or, for early formed high-grade blocks, reflect somewhat higher pressures and lower temperatures. The ease with which volatiles are expelled from a subduction complex and migrate upward along the plate junction zone is roughly proportional to the sandstone/shale ratio: low-permeability mudstones tend to maintain P{sub fluid} values approaching lithostatic, lose strength, and deform chaotically (forming melange belts), whereas permeable sandstone-rich sections retain structural/stratigraphic coherence and fail brittlely (forming coherent terranes).

  15. Lithospheric radial anisotropy beneath the Gulf of Mexico

    Science.gov (United States)

    Chu, Risheng; Ko, Justin Yen-Ting; Wei, Shengji; Zhan, Zhongwen; Helmberger, Don

    2017-05-01

    The Lithosphere-Asthenosphere Boundary (LAB), where a layer of low viscosity asthenosphere decouples with the upper plate motion, plays an essential role in plate tectonics. Most dynamic modeling assumes that the shear velocity can be used as a surrogate for viscosity which provides key information about mantle flow. Here, we derive a shear velocity model for the LAB structure beneath the Gulf of Mexico allowing a detailed comparison with that beneath the Pacific (PAC) and Atlantic (ATL). Our study takes advantage of the USArray data from the March 25th, 2013 Guatemala earthquake at a depth of 200 km. Such data is unique in that we can observe a direct upward traveling lid arrival which remains the first arrival ahead of the triplications beyond 18°. This extra feature in conjunction with upper-mantle triplication sampling allows good depth control of the LAB and a new upper-mantle seismic model ATM, a modification of ATL, to be developed. ATM has a prominent low velocity zone similar to the structure beneath the western Atlantic. The model contains strong radial anisotropy in the lid where VSH is about 6% faster than VSV. This anisotropic feature ends at the bottom of the lithosphere at about the depth of 175 km in contrast to the Pacific where it extends to over 300 km. Another important feature of ATM is the weaker velocity gradient from the depth of 175 to 350 km compared to Pacific models, which may be related to differences in mantle flow.

  16. Why are there few seedlings beneath the myrmecophyte Triplaris americana?

    Science.gov (United States)

    Larrea-Alcázar, Daniel M.; Simonetti, Javier A.

    2007-07-01

    We compared the relative importance of chemical alellopathy, pruning behaviour of resident ants and other non-related agents to ant-plant mutualism for seedling establishment beneath Triplaris americana L. (Polygonaceae), a myrmecophyte plant. We also included a preliminary analysis of effects of fragmentation on these ecological processes. Seeds and seedlings of Theobroma cacao L. (Sterculiaceae) were used as the target species in all experiments. Leaf-tissue extracts of the myrmecophyte plant did not inhibit germination of cacao seeds. Resident Pseudomyrmex triplarinus Weddell (Pseudomyrmecinae) ants did not remove seeds under the canopy of their host plants. The main seed consumer was the leaf-cutting ant Atta sexdens L. (Myrmicinae). Leaves of cacao seedlings were partially or totally pruned by Pseudomyrmex ants mainly in forest fragments studied. We offer evidence pointing to the possibility that the absence of seedlings beneath Triplaris may result from effects of both ant species. We discuss the benefits of pruning behaviour for the resident ant colony and the effects of ant-ant interactions on seedling establishment beneath this ant-plant system.

  17. Understanding the nature of mantle upwelling beneath East-Africa

    Science.gov (United States)

    Civiero, Chiara; Hammond, James; Goes, Saskia; Ahmed, Abdulhakim; Ayele, Atalay; Doubre, Cecile; Goitom, Berhe; Keir, Derek; Kendall, Mike; Leroy, Sylvie; Ogubazghi, Ghebrebrhan; Rumpker, Georg; Stuart, Graham

    2014-05-01

    The concept of hot upwelling material - otherwise known as mantle plumes - has long been accepted as a possible mechanism to explain hotspots occurring at Earth's surface and it is recognized as a way of removing heat from the deep Earth. Nevertheless, this theory remains controversial since no one has definitively imaged a plume and over the last decades several other potential mechanisms that do not require a deep mantle source have been invoked to explain this phenomenon, for example small-scale convection at rifted margins, meteorite impacts or lithospheric delamination. One of the best locations to study the potential connection between hotspot volcanism at the surface and deep mantle plumes on land is the East African Rift (EAR). We image seismic velocity structure of the mantle below EAR with higher resolution than has been available to date by including seismic data recorded by stations from many regional networks ranging from Saudi Arabia to Tanzania. We use relative travel-time tomography to produce P- velocity models from the surface down into the lower mantle incorporating 9250 ray-paths in our model from 495 events and 402 stations. We add smaller earthquakes (4.5 image structures of ~ 100-km length scales to ~ 1000 km depth beneath the northern East-Africa rift (Ethiopia, Eritrea, Djibouti, Yemen) with good resolution also in the transition zone and uppermost lower mantle. Our observations provide evidence that the shallow mantle slow seismic velocities continue trough the transition zone and into the lower mantle. In particular, the relatively slow velocity anomaly beneath the Afar Depression extends up to depths of at least 1000 km depth while another low-velocity anomaly beneath the Main Ethiopian Rift seems to be present in the upper mantle only. These features in the lower mantle are isolated with a diameter of about 400 km indicating deep multiple sources of upwelling that converge in broader low-velocity bodies along the rift axis at shallow

  18. 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)

    Science.gov (United States)

    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.

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

    Science.gov (United States)

    Kelemen, Peter B; Manning, Craig E

    2015-07-28

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

  20. Collapse risk of buildings in the Pacific Northwest region due to subduction earthquakes

    Science.gov (United States)

    Raghunandan, Meera; Liel, Abbie B.; Luco, Nicolas

    2015-01-01

    Subduction earthquakes similar to the 2011 Japan and 2010 Chile events will occur in the future in the Cascadia subduction zone in the Pacific Northwest. In this paper, nonlinear dynamic analyses are carried out on 24 buildings designed according to outdated and modern building codes for the cities of Seattle, Washington, and Portland, Oregon. The results indicate that the median collapse capacity of the ductile (post-1970) buildings is approximately 40% less when subjected to ground motions from subduction, as compared to crustal earthquakes. Buildings are more susceptible to earthquake-induced collapse when shaken by subduction records (as compared to crustal records of the same intensity) because the subduction motions tend to be longer in duration due to their larger magnitude and the greater source-to-site distance. As a result, subduction earthquakes are shown to contribute to the majority of the collapse risk of the buildings analyzed.

  1. Geodynamic Modeling of the Subduction Zone around the Japanese Islands

    Science.gov (United States)

    Honda, S.

    2017-06-01

    In this review, which focuses on our research, we describe the development of the thermomechanical modeling of subduction zones, paying special attention to those around the Japanese Islands. Without a sufficient amount of data and observations, models tended to be conceptual and general. However, the increasing power of computational tools has resulted in simple analytical and numerical models becoming more realistic, by incorporating the mantle flow around the subducting slab. The accumulation of observations and data has made it possible to construct regional models to understand the detail of the subduction processes. Recent advancements in the study of the seismic tomography and geology around the Japanese Islands has enabled new aspects of modeling the mantle processes. A good correlation between the seismic velocity anomalies and the finger-like distribution of volcanoes in northeast Japan has been recognized and small-scale convection (SSC) in the mantle wedge has been proposed to explain such a feature. The spatial and temporal evolution of the distribution of past volcanoes may reflect the characteristics of the flow in the mantle wedge, and points to the possibility of the flip-flopping of the finger-like pattern of the volcano distribution and the migration of volcanic activity from the back-arc side to the trench side. These observations are found to be qualitatively consistent with the results of the SSC model. We have also investigated the expected seismic anisotropy in the presence of SSC. The fast direction of the P-wave anisotropy generally shows the trench-normal direction with a reduced magnitude compared to the case without SSC. An analysis of full 3D seismic anisotropy is necessary to confirm the existence and nature of SSC. The 3D mantle flow around the subduction zone of plate-size scale has been modeled. It was found that the trench-parallel flow in the sub-slab mantle around the northern edge of the Pacific plate at the junction between

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

    Science.gov (United States)

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

    2015-11-12

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

  3. Porosity and Salt Content Determine if Subduction Can Occur in Europa's Ice Shell

    Science.gov (United States)

    Johnson, Brandon C.; Sheppard, Rachel Y.; Pascuzzo, Alyssa C.; Fisher, Elizabeth A.; Wiggins, Sean E.

    2017-12-01

    Motivated by recent evidence for subduction in Europa's ice shell, we explore the geophysical feasibility of this process. Here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct. We also vary the initial salt content in Europa's ice shell and determine the buoyancy of our simulated subducting slab. We find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in Europa's ice shell is actually possible. Generally, we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction. If salt contents are laterally homogenous, and Europa has a reasonable surface porosity of ϕ0 = 0.1, the conductive portion of Europa's shell must have salt contents exceeding 22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of ϕ0 = 0.1 and overall salt contents of 5%. Thus, we argue that under plausible conditions, subduction in Europa's ice shell is possible. Moreover, assuming that subduction is actively occurring or has occurred in Europa's recent past provides important constraints on the structure and composition of the ice shell.

  4. THE POTENTIAL OF TSUNAMI GENERATION ALONG THE MAKRAN SUBDUCTION ZONE IN THE NORTHERN ARABIAN SEA. CASE STUDY: THE EARTHQUAKE AND TSUNAMI OF NOVEMBER 28, 1945

    Directory of Open Access Journals (Sweden)

    George Pararas-Carayannis

    2006-01-01

    Full Text Available Although large earthquakes along the Makran Subduction Zone are infrequent, the potential for the generation of destructive tsunamis in the Northern Arabian Sea cannot be overlooked. It is quite possible that historical tsunamis in this region have not been properly reported or documented. Such past tsunamis must have affected Southern Pakistan, India, Iran, Oman, the Maldives and other countries bordering the Indian Ocean.The best known of the historical tsunamis in the region is the one generated by the great earthquake of November 28, 1945 off Pakistan's Makran Coast (Balochistan in the Northern Arabian Sea. The destructive tsunami killed more than 4,000 people in Southern Pakistan but also caused great loss of life and devastation along the coasts of Western India, Iran, Oman and possibly elsewhere.The seismotectonics of the Makran subduction zone, historical earthquakes in the region, the recent earthquake of October 8, 2005 in Northern Pakistan, and the great tsunamigenic earthquakes of December 26, 2004 and March 28, 2005, are indicative of the active tectonic collision process that is taking place along the entire southern and southeastern boundary of the Eurasian plate as it collides with the Indian plate and adjacent microplates. Tectonic stress transference to other, stress loaded tectonic regions could trigger tsunamigenic earthquakes in the Northern Arabian Sea in the future.The northward movement and subduction of the Oman oceanic lithosphere beneath the Iranian micro-plate at a very shallow angle and at the high rate is responsible for active orogenesis and uplift that has created a belt of highly folded and densely faulted coastal mountain ridges along the coastal region of Makran, in both the Balochistan and Sindh provinces. The same tectonic collision process has created offshore thrust faults. As in the past, large destructive tsunamigenic earthquakes can occur along major faults in the east Makran region, near Karachi, as

  5. Geodynamic evolution of the lithosphere beneath the Eastern Anatolia region: Constraints from geodynamic modeling

    Science.gov (United States)

    Memis, Caner; Hakan Gogus, Oguz; Pysklywec, Russell; Keskin, Mehmet; Celal Sengor, A. M.; Topuz, Gultekin

    2016-04-01

    The east Anatolian orogenic plateau is characterized by an average elevation of 2 km, and is delimited by the Bitlis-Zagros collision zone to the south and the Pontide arc to the north. Stratigraphic evidence suggests that the high plateau attained its current elevation since the Serravallian (about 12 million years ago), but probably did not reach its present height until at least the latest Pliocene. While the crustal shortening following the Arabia-Eurasia collision in the south enabled its relatively rapid rise and regional tectonic evolution, the presumed removal of the downgoing slab beneath east Anatolia has potentially played a significant role in this geodynamic configuration. According to the proposed scenario, the northward subducting slab of Neo-Tethys peels away from the overlying crust similar to the lithospheric delamination model. In this work, we performed a series of lithospheric removal models by varying rheological, physical and mechanical properties by using 2D numerical geodynamic experiments, (e.g. plate convergence rate, crustal thickness, mantle lithosphere yield-stress). Our model results show that the average amount of delamination hinge motion is maximum (18 km/my) when the lower crustal rheology is felsic granulite. The slab break-off only occurs at lower convergence rates (≤ 2 cm/yr), and is imposed on the margin of delaminating mantle lithosphere. The surface uplift takes place above the asthenospheric column (or plateau gap) through isostatic and thermal support of asthenospheric upwelling, and varies dependent on the width of the asthenospheric column. However; with higher plate convergence rates (≥3 cm/yr), the asthenospheric column does not widen enough and the continental collision occurs rather than delamination/peeling away. In this case, the average uplift appears in the central section of the crust, and this exceeds a surface elevation of 3 km. All model results are consistent with the observations from the Eastern

  6. Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts, Lesser Antilles

    Science.gov (United States)

    Melekhova, Elena; Blundy, Jon; Martin, Rita; Arculus, Richard; Pichavant, Michel

    2017-12-01

    St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure-temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An97 and An95, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions ( XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950-1025 °C, and fO2 = NNO - 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt KdCa-Na and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H2O) basaltic andesites that crystallised over a wide pressure range (1.5-6 kbar). Comparison of experimental and natural (lava

  7. Constraints on the Dip of the Anisotropic Symmetry Axis Beneath Japan From Shear Wave Splitting

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    Long, M. D.; Chevrot, S.; van der Hilst, R.

    2001-12-01

    Shear wave splitting is widely used as a tool to characterize deformational signatures in the upper mantle. However, we nearly always make the simplifying assumption that the axis of symmetry of the anisotropy is horizontal, which may not always be correct. Due to the nearly vertical incidence angles of the SKS phases typically used in splitting studies, the dip of the symmetry axis is difficult to characterize. However, if phases with varying angles of incidence are used, constraints may be placed on the dip of the symmetry axis. Splitting measurements made on upgoing shear waves with different incidence angles should exhibit discrepancies if the axis of symmetry is not horizontal. Therefore, recordings of SKKS, S, and ScS phases in addition to SKS may be used to constrain the dip of the axis of anisotropic symmetry. Japan is an excellent candidate region to potentially exhibit such discrepancies; deformation associated with the subduction beneath Japan could reasonably be expected to produce a dipping axis of symmetry. In addition, Japan is favorably located with respect to suitable source regions and data from several dense broadband seismic networks are available. We examine data from several high-quality, low-noise stations from the FREESIA network, a network of 62 broadband stations in Japan. We search for good recordings of events in the 0o-60o distance range for ScS, from 40o-80o for S, from 90o-130ofor SKS, and beyond 105o for SKKS. Only deep (>200km) events are used for S and ScS to eliminate contamination from source-side anisotropy. The multichannel method of Chevrot (JGR 2000) is used to determine splitting parameters (φ , δ t) at each station for each phase. Splitting parameters are determined from the azimuthal dependence of the splitting intensity; the method is therefore limited by the azimuthal coverage. Japan has relatively poor azimuthal coverage for SKS and SKKS, and this limits the usefulness of the multichannel method for this dataset

  8. Crustal Gravitational Potential Energy Change and Subduction Earthquakes

    Science.gov (United States)

    Zhu, P. P.

    2017-05-01

    Crustal gravitational potential energy (GPE) change induced by earthquakes is an important subject in geophysics and seismology. For the past forty years the research on this subject stayed in the stage of qualitative estimate. In recent few years the 3D dynamic faulting theory provided a quantitative solution of this subject. The theory deduced a quantitative calculating formula for the crustal GPE change using the mathematic method of tensor analysis under the principal stresses system. This formula contains only the vertical principal stress, rupture area, slip, dip, and rake; it does not include the horizontal principal stresses. It is just involved in simple mathematical operations and does not hold complicated surface or volume integrals. Moreover, the hanging wall vertical moving (up or down) height has a very simple expression containing only slip, dip, and rake. The above results are significant to investigate crustal GPE change. Commonly, the vertical principal stress is related to the gravitational field, substituting the relationship between the vertical principal stress and gravitational force into the above formula yields an alternative formula of crustal GPE change. The alternative formula indicates that even with lack of in situ borehole measured stress data, scientists can still quantitatively calculate crustal GPE change. The 3D dynamic faulting theory can be used for research on continental fault earthquakes; it also can be applied to investigate subduction earthquakes between oceanic and continental plates. Subduction earthquakes hold three types: (a) crust only on the vertical up side of the rupture area; (b) crust and seawater both on the vertical up side of the rupture area; (c) crust only on the vertical up side of the partial rupture area, and crust and seawater both on the vertical up side of the remaining rupture area. For each type we provide its quantitative formula of the crustal GPE change. We also establish a simplified model (called

  9. Stress and Strength of Seismogenic and Creeping Subduction Faults (Invited)

    Science.gov (United States)

    Wang, K.; Bilek, S. L.; Wada, I.; Gao, X.; Brown, L.

    2013-12-01

    Force balance studies of subduction zone forearcs constrained by earthquake focal mechanisms, active faulting, and topography suggest very weak subduction megathrusts. If represented by an effective coefficient of friction μ', the ratio of shear to normal stress at failure, the average μ' value of most megathrusts is about 0.03, seldom exceeding 0.06, an order of magnitude lower than fault strengths predicted by the Byerlee's law with hydrostatic pore fluid pressure. The μ' value required to explain heat flow observations using megathrust frictional heating modeling is usually also about 0.03, regardless of whether the megathrust is seismogenic or creeping. The mechanism for the weakness is not fully understood, although it must be a combined consequence of fault zone material, fault zone fabric, and pore fluid pressure. Prior to March 11, 2011, the Japan Trench was a rare exception where pervasive margin-normal compression of the upper plate made it difficult to infer megathrust strength. But wholesale stress reversal in much of the forearc due to the M 9 Tohoku earthquake dramatically verified the low-strength (μ' = 0.03) prediction of Wang and Suyehiro (1999, GRL 26(35), 2307-2310). This value translates to depth-dependant shear strength of roughly 10 MPa at 10 km and 30 MPa at 30 km. With regard to how fault strength and stress affect earthquake processes, several issues deserve special attention. (1) There is little doubt that no megathrust is 'strongly' locked, but creeping megathrusts can be either weaker or stronger than locked faults. In fact, subduction of extremely rugged seafloor causes creeping, despite strong resistance caused by geometrical incompatibilities. Physical meanings of regarding locked and creeping faults as 'strongly coupled' and 'weakly coupled', respectively, are in serious question. (2) A μ' value of 0.03-0.05 is a spatial average. For a smooth fault, even small changes in pore fluid pressure alone can cause local deviations from

  10. Thermobarometric and fluid expulsion history of subduction zones

    Science.gov (United States)

    Ernst, W. G.

    1990-06-01

    Phanerozoic, unmetamorphosed, weathered, and altered lithotectonic complexes subjected to subduction exhibit the prograde metamorphic facies sequence: zeolite → prehnite-pumpellyite → glaucophane schist → eclogite. Parageneses reflect relatively high-P trajectories, accompanied by semicontinuous devolatilization. The thermal evolution of convergent plate junctions results in early production of high-rank blueschists, high-P amphibolites, and eclogues at depth within narrow subduction zones while the hanging wall lithosphere is still hot. Protracted underflow drains heat from the nonsubducted plate and, even at profound depths, generates very low-T/high-P parageneses. Inclusion studies suggest that two-phase immiscible volatiles (liquid H2O, and gaseous high-hydrocarbons, CH4 and CO2) are evolved in turn during progressive metamorphism of the subducted sections. Expulsion of pore fluids and transitions from weathered and altered supracrustal rocks to zeolite facies assemblages release far more fluid than the better understood higher-grade transformations. Many blueschist parageneses, such as those of the internal Western Alps, have been partially overprinted by later greenschist and/or epidote-amphibolite facies assemblages. Alpine-type postblueschist metamorphic paths involved fairly rapid, nearly adiabatic decompression; some terranes even underwent modest continued heating and fluid evolution during early stages of ascent. Uplift probably occurred as a consequence of the underthrusting of low-density island arc or microcontinental crust along the convergent plate junction, resulting in marked deceleration or cessation of lithospheric underflow, decoupling, and nearly isothermal rise of the recrystallized subduction complex. Other, less common blueschist terranes, such as the eastern Franciscan belt of western California, preserve metamorphic aragonite and other high-P minerals, and lack a low-pressure overprint; physical conditions during retrogression

  11. The ionospheric heating beneath the magnetospheric cleft revisited

    Directory of Open Access Journals (Sweden)

    G. W. Prölss

    2005-03-01

    Full Text Available A prominent peak in the electron temperature of the topside ionosphere is observed beneath the magnetospheric cleft. The present study uses DE-2 data obtained in the Northern Winter Hemisphere to investigate this phenomenon. First, the dependence of the location and magnitude of the temperature peak on the magnetic activity is determined. Next, using a superposed epoch analysis, the mean latitudinal profile of the temperature enhancement is derived. The results of the present study are compared primarily with those obtained by Titheridge (1976, but also with more recent observations and theoretical predictions.

  12. The structure of the crust and uppermost mantle beneath Madagascar

    Science.gov (United States)

    Andriampenomanana, Fenitra; Nyblade, Andrew A.; Wysession, Michael E.; Durrheim, Raymond J.; Tilmann, Frederik; Julià, Jordi; Pratt, Martin J.; Rambolamanana, Gérard; Aleqabi, Ghassan; Shore, Patrick J.; Rakotondraibe, Tsiriandrimanana

    2017-09-01

    The lithosphere of Madagascar was initially amalgamated during the Pan-African events in the Neoproterozoic. It has subsequently been reshaped by extensional processes associated with the separation from Africa and India in the Jurassic and Cretaceous, respectively, and been subjected to several magmatic events in the late Cretaceous and the Cenozoic. In this study, the crust and uppermost mantle have been investigated to gain insights into the present-day structure and tectonic evolution of Madagascar. We analysed receiver functions, computed from data recorded on 37 broad-band seismic stations, using the H-κ stacking method and a joint inversion with Rayleigh-wave phase-velocity measurements. The thickness of the Malagasy crust ranges between 18 and 46 km. It is generally thick beneath the spine of mountains in the centre part (up to 46 km thick) and decreases in thickness towards the edges of the island. The shallowest Moho is found beneath the western sedimentary basins (18 km thick), which formed during both the Permo-Triassic Karro rifting in Gondwana and the Jurassic rifting of Madagascar from eastern Africa. The crust below the sedimentary basin thickens towards the north and east, reflecting the progressive development of the basins. In contrast, in the east there was no major rifting episode. Instead, the slight thinning of the crust along the east coast (31-36 km thick) may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including average thickness (40 km versus 35 km), Poisson's ratio (0.25 versus 0.26), average shear-wave velocity (both 3.7 km s-1), and thickness of mafic lower crust (7 km versus 4 km), show weak evidence of secular variation. The uppermost mantle beneath Madagascar is generally characterized by shear-wave velocities typical of stable lithosphere (∼4.5 km s-1). However

  13. Subduction zone forearc serpentinites as incubators for deep microbial life

    Science.gov (United States)

    Plümper, Oliver; King, Helen E.; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P.; Rost, Detlef; Zack, Thomas

    2017-04-01

    Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu-Bonin-Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni-Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ˜10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.

  14. Subduction zone forearc serpentinites as incubators for deep microbial life.

    Science.gov (United States)

    Plümper, Oliver; King, Helen E; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P; Rost, Detlef; Zack, Thomas

    2017-04-25

    Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu-Bonin-Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni-Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth's largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth's history such as the late heavy bombardment and global mass extinctions.

  15. Slow earthquakes linked along dip in the Nankai subduction zone.

    Science.gov (United States)

    Hirose, Hitoshi; Asano, Youichi; Obara, Kazushige; Kimura, Takeshi; Matsuzawa, Takanori; Tanaka, Sachiko; Maeda, Takuto

    2010-12-10

    We identified a strong temporal correlation between three distinct types of slow earthquakes distributed over 100 kilometers along the dip of the subducting oceanic plate at the western margin of the Nankai megathrust rupture zone, southwest Japan. In 2003 and 2010, shallow very-low-frequency earthquakes near the Nankai trough as well as nonvolcanic tremor at depths of 30 to 40 kilometers were triggered by the acceleration of a long-term slow slip event in between. This correlation suggests that the slow slip might extend along-dip between the source areas of deeper and shallower slow earthquakes and thus could modulate the stress buildup on the adjacent megathrust rupture zone.

  16. Tectonics of the IndoBurma Oblique Subduction Zone

    Science.gov (United States)

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

    2016-12-01

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

  17. Effect of the Earth's rotation on subduction processes

    Science.gov (United States)

    Levin, B. W.; Rodkin, M. V.; Sasorova, E. V.

    2017-09-01

    The role played by the Earth's rotation is very important in problems of physics of the atmosphere and ocean. The importance of inertia forces is traditionally estimated by the value of the Rossby number: if this parameter is small, the Coriolis force considerably affects the character of movements. In the case of convection in the Earth's mantle and movements of lithospheric plates, the Rossby number is quite small; therefore, the effect of the Coriolis force is reflected in the character of movements of the lithospheric plates. Analysis of statistical data on subduction zones verifies this suggestion.

  18. Slab2 - Updated subduction zone geometries and modeling tools

    Science.gov (United States)

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

    2016-12-01

    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 for others to reproduce the models it describes. Now near completion, Slab2 will update and replace Slab1.0 by: (1) extending modeled slab geometries to the full extent of all known global subduction zones; (2) incorporating regional data sets (e.g., tomography models) that may describe slab geometry more comprehensively 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) adding further layers to the base geometry dataset, such as historic moment release, earthquake tectonic providence, and interface coupling; (6) migrating the slab modeling code base to a more universally distributable language, Python; and (7) 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 will describe our progress made in creating Slab2, and provide information on

  19. Transformation from Paleo-Asian Ocean closure to Paleo-Pacific subduction: New constraints from granitoids in the eastern Jilin-Heilongjiang Belt, NE China

    Science.gov (United States)

    Ma, Xing-Hua; Zhu, Wen-Ping; Zhou, Zhen-Hua; Qiao, Shi-Lei

    2017-08-01

    onset of the subduction of the Paleo-Pacific Plate beneath the East Asian continental margin probably took place during the latest Triassic (ca. 215 Ma); (3) changes in the drifting direction of the Paleo-Pacific Plate were responsible for the intermittent magmatic activities; and (4) roll-back of the subducted plate resulted in the oceanwards migration of the magmatic arc and large-scale back-arc extension throughout NE China during the Early Cretaceous.

  20. IODP Expedition 362: Initial results from drilling the Sumatra subduction zone - the role of input materials in shallow seismogenic slip and forearc plateau development

    Science.gov (United States)

    McNeill, L. C.; Dugan, B.; Petronotis, K. E.; Expedition 362 Scientists, I.

    2016-12-01

    IODP Expedition 362, August-October, 2016, plans to drill two boreholes within the input section of the Indian oceanic plate entering the North Sumatran subduction zone. In 2004, a Mw 9.2 earthquake ruptured the Sunda subduction zone from North Sumatra to the Andaman Islands, a length of 1500 km. The earthquake and tsunami devastated coastal communities around the Indian Ocean. This earthquake and the 2011 Tohoku-Oki Mw 9.0 earthquake showed unexpectedly shallow megathrust slip. In the case of North Sumatra, this shallow slip was focused beneath a distinctive plateau of the accretionary prism. This intriguing seismogenic behavior and forearc structure are not explained by existing models or by observations at other margins where seismogenic slip typically occurs farther landward. Expedition 362 will use core and log data in conjunction with in situ temperature and pressure measurements to document the lithology, structures, and physical and chemical properties of the input sediments. The input materials of the North Sumatran subduction zone are a distinctive, thick (up to 4-5 km) sequence of primarily Bengal-Nicobar Fan-related sediments. This sequence geophysically shows strong evidence for induration and dewatering and has probably reached the temperatures required for sediment-strengthening diagenetic reactions, and input materials may be key to driving the distinctive slip behavior and long-term forearc structure. The plate boundary fault (décollement) originates within the lower pelagic and submarine fan sediments so sampling this interval will help determine what controls décollement development and how its properties evolve. Initial results from the Expedition and plans for post-expedition experiments and modeling will be presented. These methods will be used to predict physical, thermal, fluid, and mechanical properties and diagenetic evolution of the sediments as stresses and temperatures increase due to burial and subduction. Results will be used to

  1. New GPS velocity field in the northern Andes (Ecuador - Colombia) : partial locking along the subduction and northeastward escape of the Northern Andean Block

    Science.gov (United States)

    Nocquet, Jean-Mathieu; Mothes, Patricia; Mohamed, Chlieh; Jarrin, Paul; Martin, Vallee; Rui, Gorki; Regnier, Marc

    2010-05-01

    Rapid subduction of the Nazca plate beneath the Ecuador-Colombia margin (~58 mm/yr) results in two different processes: (1) elastic stress is accumulating along the Nazca/South America plate interface which is responsible of one of the largest megathrust earthquake sequence during the last century (1906, Mw = 8.8, 1942 Mw = 7.8, 1958 Mw = 7.7, and 1979 Mw = 8.2) (2) the Northern Andean Block (NAB) moves northeastward with respect to Stable South America. However, kinematics of the NAB and its level of internal deformation has yet to be quantified. We present a new GPS velocity field covering the northern Andes from south of the Gulf of Guyaquil to the Carribean plate. Our velocity field includes new continuously-recording GPS stations installed along the Ecuadorian coast, together with campaign sites observed since 1994. The observed velocity field confirms that the current surface deformation results from the superimposition of a NNE motion the crustal North Andean Block occurring at ~8 mm/yr and the elastic deformation induced by partial locking of the subduction interface. We first estimate the long-term kinematics of the North Andean block in a joint inversion including GPS data, earthquake slip vectors and quaternary slip rates on major faults. The inversion provides an Euler pole located at long. -107.8°E, lat. 36.2°N, 0.091°/Ma and indicates little internal deformation of the North Andean Block (wrms of residual velcoties is 1.3 mm/yr). As a consequence, 30% of the obliquity of the Nazca/South America motion is accommodated by transcurrent deformation along the eastern boundary of the NAB. Residual velocities with respect to the North Andean Block are then modelled in terms of elastic locking along the subduction interface. Models indicate that the subduction interface is partially locked (50%) up to a depth of 40 km over the area of rupture of the 1906 earthquake. Further south, coupling decreases with latitude, with no coupling detected at the latitude

  2. Flat-slab subduction, orogenesis, intraplate deformation, and glacial erosion in southern Alaska: A tectonic-glacial progression from STEEP

    Science.gov (United States)

    Pavlis, T. L.; Gulick, S. S.; Bruhn, R. L.; Christeson, G. L.; Enkelmann, E.; Freymueller, J. T.; Hallet, B.; Horton, B. K.; Hansen, R. A.; Koons, P. O.; Pavlis, G. L.; Ridgway, K. D.; Spotila, J. A.; Van Avendonk, H. J.

    2012-12-01

    The ST. Elias Erosion and tectonics Project (STEEP) is a Continental Dynamics multidisciplinary collaboration involving offshore and onshore studies of a modern example of an oceanic plateau collision with a continental margin in southern Alaska. These studies constrain erosion-tectonic interactions and clarify the timeline of northern Cordilleran orogenesis. At ~55 Ma an oceanic plateau formed on either the Kula-Farallon or Farallon-Resurrection spreading center. From 50-46 Ma, this plateau attempted to subduct beneath an accretionary complex, the Yakutat Group (YG), near offshore British Columbia. The YG was thrust onto the plateau to form what is now the Yakutat Terrane. From ~40 to as late as 33 Ma the Yakutat Terrane was part of North America and the proto-Transition Fault was active moving the remnant Kula Plate towards the Aleutian Trench, slicing off the southern edge of the Yakutat Terrane, and emplacing Pacific crust adjacent to the Terrane. From ~33 to 6 Ma the Yakutat Terrane moved northward with the Pacific Plate. There is some Oligocene paleogeographic uncertainly, but upper plate deformation and basin development starting ~ 20-25 Ma may be Yakutat related. At ~6 Ma the Pacific Plate underwent a clockwise shift in motion reactivating the Transition Fault, albeit at a slow rate, and this motion drove a component of oblique convergence along the Fairweather Fault and orogenesis in the St. Elias. Rejuvenation of the Transition Fault formed a stable triple junction with the Aleutian Trench and the Yakutat-North American subduction front. Uplift in the orogen seeded glacial systems that reached tidewater by ~5.5 Ma and the sediments produced were the glaciomarine, syn-orogenic Yakataga Formation. The eastern syntaxis of the St. Elias orogen began to focus exhumation as thickened crust generated along the transpressive Fairweather system was fed into the fully contractional core of the orogen. Between 4 and 3 Ma, the thicker portions of the Yakutat Terrane

  3. The Baltic Klint beneath the central Baltic Sea and its comparison with the North Estonian Klint

    Science.gov (United States)

    Tuuling, Igor; Flodén, Tom

    2016-06-01

    Along its contact with the Baltic Shield, the margin of the East European Platform reveals a well-developed, flooded terraced relief. The most striking and consistent set of escarpments at the contact of the Lower Palaeozoic calcareous and terrigenous rocks, known as the Baltic Klint (BK), extends from northwest of Russia to the Swedish island of Öland. Marine seismic reflection profiling in 1990-2004 revealed the central Baltic Sea Klint (BSK) section in detail and enabled comparison of its geology/geomorphology with a classical klint-section onshore, namely the North Estonian Klint (NEK). The conception of the BK onshore, which is based on the land-sea separating terraced relief in northern Estonia, is not fully applicable beneath the sea. Therefore, we consider that the BSK includes the entire terraced Cambrian outcrop. We suggest the term "Baltic Klint Complex" to include the well-terraced margin of the Ordovician limestone outcrop, which is weakly developed in Estonia. Because of a steady lithological framework of the bedrock layers across the southern slope of the Fennoscandian Shield, the central BSK in the western and the NEK in the eastern part of the Baltic Homocline have largely identical morphologies. The North Estonian Ordovician limestone plateau with the calcareous crest of the BK extends across the central Baltic Sea, whereas morphological changes/variations along the Klint base occur due to the east-westerly lithostratigraphic/thickness changes in the siliciclastic Cambrian sequence. The verge of the NEK, located some 30-50 m above sea level, starts to drop in altitude as its east-westerly course turns to northeast-southwest in western Estonia. Further westwards, the BK shifts gradually into southerly deepening (0.1-0.2°) layers as its crest drops to c. 150 m below sea level (b.s.l.) near Gotska Sandön. This course change is accompanied by a considerable decrease in thickness of the platform sedimentary cover, as below the central Baltic Sea the

  4. Azimuthal anisotropy in the D″ layer beneath the Caribbean

    Science.gov (United States)

    Maupin, ValéRie; Garnero, Edward J.; Lay, Thorne; Fouch, Matthew J.

    2005-08-01

    The lowermost mantle beneath Central America has anisotropic seismic velocity structure manifested in shear wave splitting of signals from South American earthquakes recorded at North American broadband recording stations. Prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy (VTI), which is sufficient to explain a general trend of early tangential (SH) component arrivals. However, VTI models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region. After accounting for splitting effects of upper mantle anisotropy beneath the recording stations, we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic D″ layer. This is the first attempt to quantitatively model a large data set including azimuthal anisotropy in D″. The models include transverse isotropy with either a vertical or tilted symmetry axis, the latter resulting in azimuthal anisotropy. For some initial shear wave polarizations, tilted transverse isotropy (TTI) produces small, reversed polarity arrivals on the SV components at the arrival time of SH, consistent with the data. Geographical variations in the azimuth of the TTI symmetry axis are indicated by the data. The lack of azimuthal coverage prevents unique resolution of the TTI orientation and also precludes distinguishing between TTI and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals. Nonetheless, our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than VTI for this region.

  5. Electromagnetic Mapping of Electrical Conductivity Beneath the Columbia Basalts

    Energy Technology Data Exchange (ETDEWEB)

    Morrison, H. Frank; Shoham, Yoram; Hoversten, G. Michael; Torres-Verdin, Carlos

    1992-01-02

    Sedimentary rocks beneath the Columbia River Basalt Group are recognized as having potential for oil and gas production, but the overlying layered basalts effectively mask seismic reflections from the underlying sediments. Four electromagnetic (EM) methods have been applied on profiles crossing Boylston Ridge, a typical east-west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments. The EM surveys detected strong variations in resistivity within the basalts, and in particular the continuous magnetotelluric array profiling (EMAP) revealed resistivity lows beneath the surface anticlines. These low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile. The controlled-source EM surveys (in-loop transient, long-offset transient, and variable-offset frequency-domain) were designed in anticipation of relatively uniform high resistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, the resistivity sections derived from 1D inversions were found to be inconsistent and misleading. The EMAP survey provided the most information about the subsurface resistivity distribution, and was certainly the most cost-effective. However, both controlled-source and EMAP surveys call for accurate 2D or 3D inversion to accommodate the geological objectives of this project. [References: 18

  6. Electromagnetic mapping of electrical conductivity beneath the Columbia basalts

    Energy Technology Data Exchange (ETDEWEB)

    Morrison, H.F.; Hoversten, G.M [Lawrence Berkeley Lab., CA (United States); Shoham, Y. [Shell Development Corp., Houston, TX (United States); Torres-Verdin, C. [Schlumberger-Doll Research, Ridgefield, CT (United States)

    1996-11-01

    Sedimentary rocks beneath the Columbia River Basalt Group are recognized as having potential for oil and gas production, but the overlying layered basalts effectively mask seismic reflections from the underlying sediments. Four electromagnetic (EM) methods have been applied on profiles crossing Boylston Ridge, a typical east-west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments. The EM surveys detected strong variations in resistivity within the basalts, and in particular the continuous magnetotelluric array profiling (EMAP) revealed resistivity lows beneath the surface anticlines. These low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile. The controlled-source EM surveys (in-loop transient, long-offset transient, and variable-offset frequency-domain) were designed in anticipation of relatively uniform high resistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, the resistivity sections derived from 1D inversions were found to be inconsistent and misleading. The EMAP survey provided the most information about the subsurface resistivity distribution, and was certainly the most cost-effective. However, both controlled-source and EMAP surveys call for accurate 2D or 3D inversion to accommodate the geological objectives of this project.

  7. A plate tectonics oddity: Caterpillar-walk exhumation of subducted continental crust

    NARCIS (Netherlands)

    Tirel, C.; Brun, J.-P.; Burov, E.; Wortel, M.J.R.; Lebedev, S.

    2013-01-01

    Since plate tectonics began on Earth, grandiose "subduction factories" have continually shaped the continents, accreting continental blocks and new crust at the convergent plate boundaries. An enigmatic product of subduction factories is the high-pressure to ultrahigh-pressure (HP-UHP) metamorphic

  8. Experimental and observational evidence for plume-induced subduction on Venus

    Science.gov (United States)

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

    2017-04-01

    Why Venus lacks plate tectonics remains an unanswered question in terrestrial planet evolution. There is observational evidence for subduction--a requirement for plate tectonics--on Venus, but it is unclear why the features have characteristics of both mantle plumes and subduction zones. One explanation is that mantle plumes trigger subduction. Here we compare laboratory experiments of plume-induced subduction in a colloidal solution of nanoparticles to observations of proposed subduction sites on Venus. The experimental fluids are heated from below to produce upwelling plumes, which in turn produce tensile fractures in the lithosphere-like skin that forms on the upper surface. Plume material upwells through the fractures and spreads above the skin, analogous to volcanic flooding, and leads to bending and eventual subduction of the skin along arcuate segments. The segments are analogous to the semi-circular trenches seen at two proposed sites of plume-triggered subduction at Quetzalpetlatl and Artemis coronae. Other experimental deformation structures and subsurface density variations are also consistent with topography, radar and gravity data for Venus. Scaling analysis suggests that this regime with limited, plume-induced subduction is favoured by a hot lithosphere, such as that found on early Earth or present-day Venus.

  9. A thermo-mechanical model of horizontal subduction below an overriding plate

    NARCIS (Netherlands)

    Hunen, Jeroen van; Berg, A.P. van den; Vlaar, N.J.

    2000-01-01

    Subduction of young oceanic lithosphere cannot be explained by the gravitational driving mechanisms of slab pull and ridge push. This deficiency of driving forces can be overcome by obduction of an actively overriding plate, which forces the young plate either to subduct or to collide. This

  10. Subduction on the margins of coronae on Venus: Evidence from radiothermal emissivity measurements

    Science.gov (United States)

    Robinson, C. A.

    1993-01-01

    Retrograde subduction has been suggested to occur at three coronae on Venus: Latona, Artemis, and Eithinoha. Using the mineralogical arguments of Klose to explain surface emissivity, a study of radio thermal emissivity of Venus coronae showed that emissivity changes associated with Latona, Artemis, and Ceres imply the same crustal movements predicted by the subduction model of Sandwell and Schubert.

  11. Does subduction polarity changes below the Alps? Inferences from analogue modelling

    NARCIS (Netherlands)

    Luth, S.; Willingshofer, E.; Sokoutis, D.; Cloetingh, S.

    2013-01-01

    The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the

  12. Subduction in an Eddy-Resolving State Estimate of the Northeast Atlantic Ocean

    Science.gov (United States)

    Gebbie, Geoffrey

    2004-01-01

    Are eddies an important contributor to subduction in the eastern subtropical gyre? Here, an adjoint model is used to combine a regional, eddy-resolving numerical model with observations to produce a state estimate of the ocean circulation. The estimate is a synthesis of a variety of in- situ observations from the Subduction Experiment, TOPEX/POSEIDON altimetry, and the MTI General Circulation Model. The adjoint method is successful because the Northeast Atlantic Ocean is only weakly nonlinear. The state estimate provides a physically-interpretable, eddy-resolving information source to diagnose subduction. Estimates of eddy subduction for the eastern subtropical gyre of the North Atlantic are larger than previously calculated from parameterizations in coarse-resolution models. Furthermore, eddy subduction rates have typical magnitudes of 15% of the total subduction rate. Eddies contribute as much as 1 Sverdrup to water-mass transformation, and hence subduction, in the North Equatorial Current and the Azores Current. The findings of this thesis imply that the inability to resolve or accurately parameterize eddy subduction in climate models would lead to an accumulation of error in the structure of the main thermocline, even in the relatively-quiescent eastern subtropical gyre.

  13. Using open sidewalls for modelling self-consistent lithosphere subduction dynamics

    NARCIS (Netherlands)

    Chertova, M.V.; Geenen, T.; van den Berg, A.; Spakman, W.

    2012-01-01

    Subduction modelling in regional model domains, in 2-D or 3-D, is commonly performed using closed (impermeable) vertical boundaries. Here we investigate the merits of using open boundaries for 2-D modelling of lithosphere subduction. Our experiments are focused on using open and closed (free

  14. A two-way interaction between the Hainan plume and the Manila subduction zone

    NARCIS (Netherlands)

    Mériaux, Catherine A.; Duarte, João C.; Schellart, Wouter P.; Mériaux, Anne Sophie

    2015-01-01

    The interaction between mantle plumes and subducting slabs is well accepted, but the influence of slabs on plumes has more often been portrayed than the reverse. Here we present three-dimensional upper mantle laboratory models in which a compositional plume rises underneath a subducting plate.

  15. Molecular modelling of rare earth element complexation in subduction zone fluids

    NARCIS (Netherlands)

    van Sijl, J.; Allan, N.L.; Davies, G.R.; van Westrenen, W.

    2009-01-01

    Complexation of (trace) elements in fluids plays a critical role in determining element mobility in subduction zones, but to date, the atomic-scale processes controlling elemental solubilities are poorly understood. As a first step towards computer simulation of element complexation in subduction

  16. Rapid fore-arc extension and detachment-mode spreading following subduction initiation

    NARCIS (Netherlands)

    Morris, Antony; Anderson, Mark W.; Omer, Ahmed; Maffione, Marco; van Hinsbergen, Douwe J.J.|info:eu-repo/dai/nl/269263624

    2017-01-01

    Most ophiolites have geochemical signatures that indicate formation by suprasubduction seafloor spreading above newly initiated subduction zones, and hence they record fore-arc processes operating following subduction initiation. They are frequently underlain by a metamorphic sole formed at the top

  17. Slab melting as a barrier to deep carbon subduction.

    Science.gov (United States)

    Thomson, Andrew R; Walter, Michael J; Kohn, Simon C; Brooker, Richard A

    2016-01-07

    Interactions between crustal and mantle reservoirs dominate the surface inventory of volatile elements over geological time, moderating atmospheric composition and maintaining a life-supporting planet. While volcanoes expel volatile components into surface reservoirs, subduction of oceanic crust is responsible for replenishment of mantle reservoirs. Many natural, 'superdeep' diamonds originating in the deep upper mantle and transition zone host mineral inclusions, indicating an affinity to subducted oceanic crust. Here we show that the majority of slab geotherms will intersect a deep depression along the melting curve of carbonated oceanic crust at depths of approximately 300 to 700 kilometres, creating a barrier to direct carbonate recycling into the deep mantle. Low-degree partial melts are alkaline carbonatites that are highly reactive with reduced ambient mantle, producing diamond. Many inclusions in superdeep diamonds are best explained by carbonate melt-peridotite reaction. A deep carbon barrier may dominate the recycling of carbon in the mantle and contribute to chemical and isotopic heterogeneity of the mantle reservoir.

  18. Dehydration-induced instabilities at intermediate depths in subduction zones

    Science.gov (United States)

    Brantut, Nicolas; Stefanou, Ioannis; Sulem, Jean

    2017-08-01

    We formulate a model for coupled deformation and dehydration of antigorite, based on a porosity-dependent yield criterion and including shear-enhanced compaction. A pore pressure and compaction instability can develop when the net volume change associated with the reaction is negative, i.e., at intermediate depth in subduction zones. The instability criterion is derived in terms of the dependence of the yield criterion on porosity: if that dependence is strong, instabilities are more likely to occur. We also find that the instability is associated with strain localization, over characteristic length scales determined by the hydraulic diffusivity, the elasto-plastic parameters of the rock, and the reaction rate. Typical lower bounds for the localization length are of the order of 10 to 100 for antigorite dehydration and deformation at 3 GPa. The fluid pressure and deformation instability is expected to induce stress buildup in the surrounding rocks forming the subducted slab, which provides a mechanism for the nucleation and propagation of intermediate-depth earthquakes.

  19. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra.

    Science.gov (United States)

    Hüpers, Andre; Torres, Marta E; Owari, Satoko; McNeill, Lisa C; Dugan, Brandon; Henstock, Timothy J; Milliken, Kitty L; Petronotis, Katerina E; Backman, Jan; Bourlange, Sylvain; Chemale, Farid; Chen, Wenhuang; Colson, Tobias A; Frederik, Marina C G; Guèrin, Gilles; Hamahashi, Mari; House, Brian M; Jeppson, Tamara N; Kachovich, Sarah; Kenigsberg, Abby R; Kuranaga, Mebae; Kutterolf, Steffen; Mitchison, Freya L; Mukoyoshi, Hideki; Nair, Nisha; Pickering, Kevin T; Pouderoux, Hugo F A; Shan, Yehua; Song, Insun; Vannucchi, Paola; Vrolijk, Peter J; Yang, Tao; Zhao, Xixi

    2017-05-26

    Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records. Copyright © 2017, American Association for the Advancement of Science.

  20. The role of frictional strength on plate coupling at the subduction interface

    KAUST Repository

    Tan, Eh

    2012-10-01

    At a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc. In this paper, we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins. In numerical models, we vary thickness of a serpentinized layer in the mantle wedge (15 to 25km) and the frictional strength of both the sediments and serpentinized mantle (friction angle 1 to 15, or static friction coefficient 0.017 to 0.27) to control the amount of frictional coupling between the plates. With plastic strain weakening in the lithosphere, our numerical models can attain stable subduction geometry over millions of years. We find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths. In the case of low sediment and serpentinite friction, the subduction interface has a shallow dip, while the subduction zone develops an accretionary prism, a broad forearc high, a deep forearc basin, and a shallow trench. In the high friction case, the subduction interface is steep, the trench is deeper, and the accretionary prism, forearc high and basin are all absent. The resultant free-air gravity and topographic signature of these subduction zone models are consistent with observations. We believe that the low-friction model produces a geometry and forearc structure similar to that of accretionary margins. Conversely, models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin. We find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones. © 2012. American Geophysical Union. All Rights Reserved.

  1. The link between great earthquakes and the subduction of oceanic fracture zones

    Directory of Open Access Journals (Sweden)

    R. D. Müller

    2012-12-01

    Full Text Available Giant subduction earthquakes are known to occur in areas not previously identified as prone to high seismic risk. This highlights the need to better identify subduction zone segments potentially dominated by relatively long (up to 1000 yr and more recurrence times of giant earthquakes. We construct a model for the geometry of subduction coupling zones and combine it with global geophysical data sets to demonstrate that the occurrence of great (magnitude ≥ 8 subduction earthquakes is strongly biased towards regions associated with intersections of oceanic fracture zones and subduction zones. We use a computational recommendation technology, a type of information filtering system technique widely used in searching, sorting, classifying, and filtering very large, statistically skewed data sets on the Internet, to demonstrate a robust association and rule out a random effect. Fracture zone–subduction zone intersection regions, representing only 25% of the global subduction coupling zone, are linked with 13 of the 15 largest (magnitude Mw ≥ 8.6 and half of the 50 largest (magnitude Mw ≥ 8.4 earthquakes. In contrast, subducting volcanic ridges and chains are only biased towards smaller earthquakes (magnitude < 8. The associations captured by our statistical analysis can be conceptually related to physical differences between subducting fracture zones and volcanic chains/ridges. Fracture zones are characterised by laterally continuous, uplifted ridges that represent normal ocean crust with a high degree of structural integrity, causing strong, persistent coupling in the subduction interface. Smaller volcanic ridges and chains have a relatively fragile heterogeneous internal structure and are separated from the underlying ocean crust by a detachment interface, resulting in weak coupling and relatively small earthquakes, providing a conceptual basis for the observed dichotomy.

  2. Dynamics and Significance of Plume-Induced Subduction Initiation: Numerical Modeling

    Science.gov (United States)

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

    2014-12-01

    How did the first subduction zone form? Most present-day subduction initiation mechanisms require acting plate forces and/or preexisting zones of lithospheric weakness, which are themselves the consequence of plate tectonics (Stern 2004). In contrast, spontaneous plume-induced subduction initiation - suggested on the basis of numerical thermo-mechanical experiments (Ueda et al., 2008) and supported by data re-interpretation of how subduction started in Late Cretaceous time around the Caribbean LIP (Whattam and Stern, 2014) - does not require pre-existing lithospheric fabric, such as are created by active plate tectonics and is viable for both stagnant lid and mobile/deformable lid conditions. Here, we present first results of high-resolution 3D numerical thermo-mechanical modeling of plume-induced subduction resulting from mechanical-magmatic interaction of an ascending thermal mantle plume with old, cold, dense oceanic lithosphere. We demonstrate that weakening of the strong lithosphere by plume-induced magmatism is the key factor enabling subduction initiation around the plume head. A large plume head is required to overcome ring confinement, and subduction initiation is further favored when plume activity and lithospheric weakening continues for several tens of Ma. We further discuss possible implications of this scenario for modern plate tectonics as well as for plate tectonics initiation in Precambrian time. ReferencesStern, R.J., 2004. Subduction initiation: spontaneous and induced. EPSL 226, 275-292.Ueda, K., Gerya, T., Sobolev, S.V., 2008. Subduction initiation by thermal-chemical plumes. PEPI 171, 296-312.Whattam, S.A., Stern, R. 2014. Late Cretaceous plume-induced subduction initiation along the southern margin of the Caribbean and NW South America: The first documented example with implications for the onset of plate tectonics. Gondwana Research, (accepted).

  3. Petrology and geochemistry of volcanic rocks from the island of Panarea: implications for mantle evolution beneath the Aeolian island arc (southern Tyrrhenian sea)

    Science.gov (United States)

    Calanchi, N.; Peccerillo, A.; Tranne, C. A.; Lucchini, F.; Rossi, P. L.; Kempton, P.; Barbieri, M.; Wu, T. W.

    2002-06-01

    Stromboli has a more radiogenic Sr-isotope signature, and shows trace element abundances and ratios that are intermediate between arc and intraplate compositions. Panarea mafic rocks have geochemical and isotopic signatures that are intermediate between those observed in the two sectors of the arc. The late-erupted CA scoriae of Panarea have trace element and isotopic compositions similar to those of the mafic rocks from the western islands of Filicudi and Alicudi, whereas the HKCA and shoshonitic mafic rocks have isotopic and trace element signatures that are closer to those of Stromboli. This reflects the particular position of Panarea, which is sited midway between the western-central arc and Stromboli. According to some recent views, subduction of the Ionian sea plate is actively occurring beneath the eastern Aeolian arc, with rollback of the subduction zone toward the southeast. The Tindari-Letoianni-Malta Escarpment fault zone is considered to be the boundary between the active subducting plate in the east and the African plate and western Aeolian arc in the west. It is suggested that the rollback of the Ionian plate generated inflow of mantle material from below the western arc into the mantle wedge above the subducting Ionian slab. This situation generated a hybrid mantle beneath Panarea, which resulted in a mixture of western-type and resident eastern-arc mantle materials; the latter had a composition akin to the source of Stromboli magmas. Early HKCA and shoshonitic magmatism tapped such a hybrid source, whereas the younger CA activity has been derived from melting of unmodified western-type mantle material. The late eruption of CA rocks with a composition similar to western arc can be explained by assuming that a continuing inflow process had increased the amount of western-type mantle with time, thus favouring the late appearance of CA magmas. This hypothesis accounts for the overall decrease of potassium with time, which is the opposite of the trend observed in

  4. The Elephants' Graveyard: Constraints from Mantle Plumes on the Fate of Subducted Slabs and Implications for the Style of Mantle Convection

    Science.gov (United States)

    Lassiter, J. C.

    2007-12-01

    The style of mantle convection (e.g., layered- vs. whole-mantle convection) is one of the most hotly contested questions in the Geological Sciences. Geochemical arguments for and against mantle layering have largely focused on mass-balance evidence for the existence of "hidden" geochemical reservoirs. However, the size and location of such reservoirs are largely unconstrained, and most geochemical arguments for mantle layering are consistent with a depleted mantle comprising most of the mantle mass and a comparatively small volume of enriched, hidden material either within D" or within seismically anomalous "piles" beneath southern Africa and the South Pacific. The mass flux associated with subduction of oceanic lithosphere is large and plate subduction is an efficient driver of convective mixing in the mantle. Therefore, the depth to which oceanic lithosphere descends into the mantle is effectively the depth of the upper mantle in any layered mantle model. Numerous geochemical studies provide convincing evidence that many mantle plumes contain material which at one point resided close to the Earth's surface (e.g., recycled oceanic crust ± sediments, possibly subduction-modified mantle wedge material). Fluid dynamic models further reveal that only the central cores of mantle plumes are involved in melt generation. The presence of recycled material in the sources of many ocean island basalts therefore cannot be explained by entrainment of this material during plume ascent, but requires that recycle