Sample records for subduction boundaries suggests

  1. Extending Alaska's plate boundary: tectonic tremor generated by Yakutat subduction (United States)

    Wech, Aaron G.


    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.

  2. Lower slab boundary in the Japan subduction zone (United States)

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


    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

  3. A model for the termination of the Ryukyu subduction zone against Taiwan: A junction of collision, subduction/separation, and subduction boundaries (United States)

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


    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.

  4. Influence of Initial Geometry and Boundary Conditions on Flat Subduction Models and Resulting Topography (United States)

    Nelson, P.; Moucha, R.


    Numerical investigations of surface deformation in response to flat slab subduction began with seminal papers by Bird (1988) and Mitrovica et al. (1989). Recently, a number of numerical studies have begun to explore the complexity in the dynamics of flat-slab subduction initiation and continuation, but did not address the corresponding surface deformation (English et al., 2003; Pérez-Campos et al., 2008; Liu et al., 2010; Jones et al., 2011; Arrial and Billen, 2013; Vogt and Gerya, 2014). Herein, we explore the conditions that lead to flat-slab subduction and characterize the resulting surface deformation using a 2D finite-difference marker-in-cell method. We specifically explore how initial model geometry and boundary conditions affect the evolution of the angle at which a slab subducts in the presence/absence of a buoyant oceanic plateau and the resulting surface topography. In our simulations, the surface is tracked through time as an internal erosion/sedimentation surface. The top boundary of the crust is overlaid by a "sticky" (viscous 10^17 Pa.s) water/air layer with correspondingly stratified densities. We apply a coupled surface processes model that solves the sediment transport/diffusion erosion equation at each time step to account for the corresponding crustal mass flux and its effect on crustal deformation. Model results show the initial angle of subduction has a substantial impact on the subduction angle of the slab and hence the evolution of topography. The results also indicate plate velocity and the presence of an oceanic plateau in a forced subduction only have a moderate effect on the angle of subduction.

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

    Energy Technology Data Exchange (ETDEWEB)

    Rotstein, Y.; Kafka, A.L.


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

  6. Seismotectonics of the southern boundary of Anatolia, eastern Mediterranean region: Subduction, collision, and arc jumping (United States)

    Rotstein, Yair; Kafka, Alan L.


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

  7. Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths (United States)

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


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

  8. Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths (United States)

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


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

  9. Evolving seismogenic plate boundary megathrust and mega-splay faults in subduction zone (Invited) (United States)

    Kimura, G.; Hamahashi, M.; Fukuchi, R.; Yamaguchi, A.; Kameda, J.; Kitamura, Y.; Hashimoto, Y.; Hamada, Y.; Saito, S.; Kawasaki, R.


    Understanding the fault mechanism and its relationship to the sesimo-tsunamigenesis is a key of the scientific targets of subduction zone and therefore NantroSEIZE project of IODP and future new drilling project of International Ocean Discovery Program keeps focusing on that. Mega-splay fault branched from plate boundary megathrust in subduction zone is located around the border between outer and inner wedges and is considered to cause great earthquake and tsunami such as 1960 Alaska earthquake, 1944 and 1946 Nankai-Tonankai earthquakes, and 2004 Sumatra earthquakes. Seismic reflection studies for the mega-splay fault in 2D and 3D in the Nankai forearc present the reflector with negative or positive polarities with various amplitudes and suggest complicated petrophysical properties and condition of the fault and its surroundings. The Nankai mega-splay fault at a depth of ~5km is going to be drilled and cored by NantroSEIZE experiments and is expected for great progress of understanding of the fault mechanics. Before drilling the really targeted seismogenic fault, we are conducting many exercises of geophysical and geological observations. The core-log-seismic integrated exercise for the exhumed mega-splay fault by drilling was operated for the Nobeoka thrust in the Shimanto Belt, Kyushu, Japan. The Nobeoka thrust was once buried in the depth >~10km and suffered maximum temperature >~300 dgree C. As the core recovery is ~99%, perfect correlation between the core and logging data is possible. Thickness of the fault zone is >200 m with a ~50 cm thick central fault core dividing the phyllitic hanging wall and the footwall of broken-melange like cataclasite. A-few-meter-thick discrete damage zones with fault cores are recognized by difference in physical properties and visual deformation textures at several horizons in the fault zone. Host rocks for those damaged zones are completely lithified cataclasites with abundant mineral veins, which record the older and deeper

  10. Reaction-induced grain boundary cracking and anisotropic fluid flow during prograde devolatilization reactions within subduction zones (United States)

    Okamoto, Atsushi; Shimizu, Hiroyuki; Fukuda, Jun-ichi; Muto, Jun; Okudaira, Takamoto


    Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (Δ V t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative Δ V t values. Coupled hydrological-chemical-mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative Δ V t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive Δ V t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.

  11. A satellite magnetic perspective of subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones (United States)

    Purucker, M. E.; Whaler, K. A.


    Large and intermediate-scale tectonic features such as subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones are often seen to have a magnetic signature visible from the perspective of near-Earth magnetic field satellites such as CHAMP and Orsted. Why do these tectonic features have a magnetic signature, while others do not? A new model of the lithospheric field (MF-6, Maus et al., 2008) extending to spherical harmonic degree 120 (333 km wavelength) has been used to evaluate the magnetic state of the lithosphere under the assumption that the magnetization is either induced (with a seismic starting model), or remanent (with a minimum norm approach). Some of the features identified from these images include the Tethyan and NE Siberian diffuse plate boundary zones, the Red Sea rift, and Cretaceous rift basins developed on the West African shield. Almost without exception, subduction zones exhibit a magnetic signature, as do many large igneous provinces. In this talk we discuss some of the new insights this magnetic perspective provides, and speculate on the controls which determine whether tectonic features will be expressed magnetically.

  12. Megathrust splay faults, forearc basins, and segment boundaries related to the Kodiak Islands segment of the Alaska subduction zone (United States)

    Ramos, M. D.; Liberty, L. M.


    We have compiled and interpreted a comprehensive upper-crustal seismic reflection dataset revealing spatiotemporal Cenozoic deformation across the Kodiak forearc and accretionary prism. Pervasive splay faults and forearc basin growth and positioning with respect to the prism record both accreting and eroding margin episodes through time. Seafloor bathymetry, in conjunction with seismic reflection data, show that post-glacial splay fault motion can exceed 40 meters offshore Kodiak Island. We observe considerable differences in splay fault uplift rates and activation spanning the near-shore region to the continental shelf, with significant tsunamigenic fault motion from the 1964 Great Alaska Earthquake concentrated along the Kodiak Island shelf fault zone system. We utilize potential fields data to highlight the continuity of megathrust splay faults that span the southern Gulf of Alaska and to identify geophysical signatures of segment boundaries that represent subducting Pacific plate morphology which may define megathrust rupture limits. Constraints on interseismic deformation from compiled geodetic GPS, focal mechanism, and earthquake datasets reveal the seismotectonic character of the Kodiak segment and are consistent with the structural heterogeneity at both the plate interface and upper plate. Upper plate geometry of the Kodiak segment shows Holocene fault motion has been accommodated along distinct fault zone segments. This knowledge informs tsunami risk modelers to allow for different alternatives of coseismic splay fault uplift during megathrust rupture. Our results suggest growth of forearc structures is contemporaneous with the subduction of major Pacific plate morphologies and provide clear geophysical evidence that can explain the persistence of the Kodiak segment, which is a major step towards a composite Alaska subduction zone deformation model.

  13. Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis (United States)

    Kameda, Jun; Inoue, Sayako; Tanikawa, Wataru; Yamaguchi, Asuka; Hamada, Yohei; Hashimoto, Yoshitaka; Kimura, Gaku


    The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels ( 320 °C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces.[Figure not available: see fulltext.

  14. Modeled temperatures and fluid source distributions for the Mexican subduction zone: Effects of hydrothermal circulation and implications for plate boundary seismic processes (United States)

    Perry, Matthew; Spinelli, Glenn A.; Wada, Ikuko; He, Jiangheng


    In subduction zones, spatial variations in pore fluid pressure are hypothesized to control the sliding behavior of the plate boundary fault. The pressure-temperature paths for subducting material control the distributions of dehydration reactions, a primary control on the pore fluid pressure distribution. Thus, constraining subduction zone temperatures are required to understand the seismic processes along the plate interface. We present thermal models for three margin-perpendicular transects in the Mexican subduction zone. We examine the potential thermal effects of vigorous fluid circulation in a high-permeability aquifer within the basaltic basement of the oceanic crust and compare the results with models that invoke extremely high pore fluid pressures to reduce frictional heating along the megathrust. We combine thermal model results with petrological models to determine the spatial distribution of fluid release from the subducting slab and compare dewatering locations with the locations of seismicity, nonvolcanic tremor, slow-slip events, and low-frequency earthquakes. Simulations including hydrothermal circulation are most consistent with surface heat flux measurements. Hydrothermal circulation has a maximum cooling effect of 180°C. Hydrothermally cooled crust carries water deeper into the subduction zone; fluid release distributions in these models are most consistent with existing geophysical data. Our models predict focused fluid release, which could generate overpressures, coincident with an observed ultraslow layer (USL) and a region of nonvolcanic tremor. Landward of USLs, a downdip decrease in fluid source magnitude could result in the dissipation in overpressure in the oceanic crust without requiring a downdip increase in fault zone permeability, as posited in previous studies.

  15. Imaging of the subducted Kyushu-Palau Ridge in the Hyuga-nada region, western Nankai Trough subduction zone (United States)

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


    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.

  16. Formation and stability of a double subduction system: a numerical study (United States)

    Pusok, Adina E.; Stegman, Dave


    al., 2016] capable of simulating lithospheric deformation while simultaneously taking mantle flow and an internal free surface into account. We start from a single subduction setup, where subduction is already initiated (mature) and we stress the system by controlling the convergence rate of the system (i.e. imposing influx/outflux boundary conditions). Under certain conditions, a second subduction may develop and transform into a stable double subduction system. Preliminary results suggest that the fate of the incipient secondary subduction depends on internal factors (i.e. buoyancy and rheology), but also on the dynamics of the primary subduction zone and the boundary conditions (i.e. convergence rate).

  17. On the initiation of subduction (United States)

    Mueller, Steve; Phillips, Roger J.


    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.

  18. Intrinsic and Extrinsic Factors in Subduction Dynamics (United States)

    Billen, Magali; Arredondo, Katrina


    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

  19. Kinematics of Late Cretaceous subduction initiation in the Neo-Tethys Ocean reconstructed from ophiolites of Turkey, Cyprus, and Syria (United States)

    Maffione, Marco; van Hinsbergen, Douwe; de Gelder, Giovanni; van der Goes, Freek; Morris, Antony


    Formation of new subduction zones represents one of the cornerstones of plate tectonics, yet both the kinematics and geodynamics governing this process remain enigmatic. A major subduction initiation event occurred in the Late Cretaceous, within the Neo-Tethys Ocean between Gondwana and Eurasia. Supra-subduction zone (SSZ) ophiolites (i.e., emerged fragments of ancient oceanic lithosphere accreted at supra-subduction spreading centers) were generated during this subduction event, and are today distributed in the eastern Mediterranean region along three E-W trending ophiolitic belts. Current models associate these ophiolite belts to simultaneous initiation of multiple, E-W trending subduction zones at 95 Ma. Here we report paleospreading direction data obtained from paleomagnetic analysis of sheeted dyke sections from seven Neo-Tethyan ophiolites of Turkey, Cyprus, and Syria, demonstrating that these ophiolites formed at NNE-SSW striking ridges parallel to the newly formed subduction zones. This subduction system was step-shaped and composed of NNE-SSW and ESE-WNW segments. The eastern subduction segment invaded the SW Mediterranean, leading to a radial obduction pattern similar to the Banda arc. Emplacement age constraints indicate that this subduction system formed close to the Triassic passive and paleo-transform margins of the Anatolide-Tauride continental block. Because the original Triassic-Jurassic Neo-Tethyan spreading ridge must have already subducted below the Pontides before the Late Cretaceous, we infer that the Late Cretaceous Neo-Tethyan subduction system started within ancient lithosphere, along NNE-SSW oriented fracture zones and faults parallel to the E-W trending passive margins. This challenges current concepts suggesting that subduction initiation occurs along active intra-oceanic plate boundaries.

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

    NARCIS (Netherlands)

    Schellart, W. P.; Rawlinson, N.


    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.

  1. Tsunami Hazard Posed to New Zealand by the Kermadec and Southern New Hebrides Subduction Margins: An Assessment Based on Plate Boundary Kinematics, Interseismic Coupling, and Historical Seismicity (United States)

    Power, William; Wallace, Laura; Wang, Xiaoming; Reyners, Martin


    We assess the tsunami hazard posed to New Zealand by the Kermadec and southern New Hebrides subduction margins. Neither of these subduction zones has produced tsunami large enough to cause significant damage in New Zealand over the past 150 years of well-recorded history. However, as this time frame is short compared to the recurrence interval for major tsunamigenic earthquakes on many of the Earth's subduction zones, it should not be assumed that what has been observed so far is representative of the long term. For each of these two subduction zones we present plate kinematic and fault-locking results from block modelling of earthquake slip vector data and GPS velocities. The results are used to estimate the current rates of strain accumulation on the plate interfaces where large tsunamigenic earthquakes typically occur. We also review data on the larger historical earthquakes that have occurred on these margins, as well as the Global CMT catalogue of events since 1976. Using this information we have developed a set of scenarios for large earthquakes which have been used as initial conditions for the COMCOT tsunami code to estimate the subsequent tsunami propagation in the southwest Pacific, and from these the potential impact on New Zealand has been evaluated. Our results demonstrate that there is a significant threat posed to the Northland and Coromandel regions of New Zealand should a large earthquake ( M w ≳8.5) occur on the southern or middle regions of the Kermadec Trench, and that a similarly large earthquake on the southern New Hebrides Trench has the potential to strongly impact on the far northern parts of New Zealand close to the southern end of the submarine Three Kings Ridge. We propose logic trees for the magnitude-frequency parameters of large earthquakes originating on each trench, which are intended to form the basis for future probabilistic studies.

  2. Seismicity and the subduction process (United States)

    Ruff, L.; Kanamori, H.


    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.

  3. Back-arc Extension: Critical Analisys of Subduction-related and Non Subduction-related Driving Mechanisms (United States)

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

    where the above boundary conditions are not recognized, as in the South American subduction zones, back arc extension does not occur. It is also suggested that the stop of extension in a number of basins, as the Kurile, Japan, Shikoku, Parece-Vela, Balearic and Pannonian was caused by the interruption of the boundary conditions which determined the lateral extrusion of the respective arcs.

  4. Trench dynamics: Effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems (United States)

    Yoshida, Masaki


    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

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

    Energy Technology Data Exchange (ETDEWEB)

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


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

  6. 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 (United States)

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


    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

  7. To what depth can continental crust be subducted: numerical predictions and critical observations (United States)

    Gerya, T.; Faccenda, M.


    We performed systematic two-dimensional numerical modeling of continental collision associated with subduction of the lithospheric mantle. Results of our experiments suggest that two contrasting modes of lithospheric subduction below an orogen can exist: one-sided and double-sided. One-sided subduction brings continental crust subducting atop the slab to the contact with hot asthenosperic mantle wedge below the overriding plate. This can result in strong heating, partial melting and rheological weakening of the crust triggering its delamination from subducting mantle lithosphere in form of compositionally buoyant structures (cold plumes) propagating away from subducting plate, passing through the hot mantle wedge, underplating the overriding lithosphere and producing large amount of relatively felsic syn-orogenic magmas at sub-lithospheric depths. One-sided subduction of the buoyant continental crust can also result in a transient "hot channel effect" triggering formation and exhumation of coesite- and diamond- bearing rocks metamorphosed at 700 to 900oC. Anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper-crustal rocks. Low effective viscosity of the channel subsequent to increased temperature and partial melting permits profound mixing of mantle and crustal rocks. The hot channel exists during few million years only but rapidly produces and exhumes large amounts of ultrahigh-pressure, high-temperature rocks within the orogen. Double-sided subduction can follow the one-sided mode at later stages of orogeny when significant rheological coupling between two plates occurs during the collision. In this case the orogen is characterized by double- verging structure, the layer of subducting continental crust is embedded between two negatively buoyant lithospheric slabs and delamination of the crust does not occur. This mode of subduction can bring crustal rocks from the bottom of an

  8. Reaction-induced rheological weakening enables oceanic plate subduction. (United States)

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


    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.

  9. Softening trigerred by eclogitization, the first step toward exhumation during continental subduction (United States)

    Jolivet, Laurent; Raimbourg, Hugues; Labrousse, Loïc; Avigad, Dov; Leroy, Yves; Austrheim, Håkon; Andersen, Torgeir B.


    Direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn-exhumation deformation. Most ultrahigh-pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation. Whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces, low viscosity in the subduction channel, overburden removal by erosion and normal faulting, a basic question is seldom considered: why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface? This event, seminal to exhumation processes, must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it. Our field observations in the Bergen arc show that Caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the Caledonian subduction. Thus, the sequence of deformation preserved in the Bergen Arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation. This observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions, i.e. underthrusting of the subducting slab. In this context of simple shear, metamorphic reactions assisted by fracturating, fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab. These tectonic units can then be incorporated into the channel circulation and start their upward travel.

  10. Effect of a weak layer at the base of an oceanic plate on subduction dynamics (United States)

    Carluccio, Roberta; Kaus, Boris


    The plate tectonics model relies on the concept of a relatively rigid lithospheric lid moving over a weaker asthenosphere. In this frame, the lithosphere asthenosphere boundary (LAB) is a first-order discontinuity that accommodates differential motions between tectonic plates and the underlying mantle. Recent seismic studies have revealed the existence of a low velocity and high electrical conductivity layer at the base of subducting tectonic plates. This thin layer has been interpreted as being weak and slightly buoyant and was suggested to affect the dynamics of subducting plates. However, geodynamically, the role of a weak layer at the base of the lithosphere remains poorly studied, especially at subduction zones. Therefore, we here use numerical models to investigate the first-order effects of a weak buoyant layer at the LAB on subduction dynamics. We employ both 2-D and 3-D models in which the slab and mantle are either linear viscous or have a more realistic temperature-dependent visco-elastic-plastic rheology. Results show that a weak layer affects the dynamics of the plates, foremost by increasing the subduction speed. The impact of this effect depends on the thickness of the layer and the viscosity contrast between the mantle and the weak layer. For moderate viscosity contrasts (1000), it can also change the morphology of the subduction itself, perhaps because this changes the overall effective viscosity contrast between the slab the and the mantle. For thinner layers, the overall effect is reduced. Yet, if seismological observations are correct that suggests that this layer is 10 km thick and partially molten, such that the viscosity is 1000 times lower than that of the mantle, our models suggest that this effect should be measurable. Some of our models also show a pile-up of weak material in the bending zone of the subducting plate, consistent with recent seismological observations.

  11. Subduction dynamics: Constraints from gravity field observations (United States)

    Mcadoo, D. C.


    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.

  12. 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

  13. Plume-induced subduction (United States)

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


    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.

  14. 3D geometry of a plate boundary fault related to the 2016 Off-Mie earthquake in the Nankai subduction zone, Japan (United States)

    Tsuji, Takeshi; Minato, Shohei; Kamei, Rie; Tsuru, Tetsuro; Kimura, Gaku


    We used recent seismic data and advanced techniques to investigate 3D fault geometry over the transition from the partially coupled to the fully coupled plate interface inboard of the Nankai Trough off the Kii Peninsula, Japan. We found that a gently dipping plate boundary décollement with a thick underthrust layer extends beneath the entire Kumano forearc basin. The 1 April 2016 Off-Mie earthquake (Mw6.0) and its aftershocks occurred, where the plate boundary décollement steps down close to the oceanic crust surface. This location also lies beneath the trenchward edge of an older accretionary prism (∼14 Ma) developed along the coast of the Kii peninsula. The strike of the 2016 rupture plane was similar to that of a formerly active splay fault system in the accretionary prism. Thus, the fault planes of the 2016 earthquake and its aftershocks were influenced by the geometry of the plate interface as well as splay faulting. The 2016 earthquake occurred within the rupture area of large interplate earthquakes such as the 1944 Tonankai earthquake (Mw8.1), although the 2016 rupture area was much smaller than that of the 1944 event. Whereas the hypocenter of the 2016 earthquake was around the underplating sequence beneath the younger accretionary prism (∼6 Ma), the 1944 great earthquake hypocenter was close to oceanic crust surface beneath the older accretionary prism. The variation of fault geometry and lithology may influence the degree of coupling along the plate interface, and such coupling variation could hinder slip propagation toward the deeper plate interface in the 2016 event.

  15. Investigating the Role of Dehydration Reactions in Subduction Zone Pore Pressures Using Newly-Developed Permeability-Porosity Relationships (United States)

    Screaton, E.; Daigle, H.; James, S.; Meridth, L.; Jaeger, J. M.; Villaseñor, T. G.


    Dehydration reactions are linked to shallow subduction zone deformation through excess pore pressures and their effect on mechanical properties. Two reactions, the transformation of smectite to illite and of opal-A to opal-CT and then to quartz, can occur relatively early in the subduction process and may affect the propagation of the plate boundary fault, the updip limit of velocity-weakening frictional paper, and tsunamigenesis. Due to large variations between subduction zones in heat flow, sedimentation rates, and geometries, dehydration location may peak prior to subduction to as much as 100 km landward of the deformation front. The location of the dehydration reaction peak relative to when compaction occurs, causes significant differences in pore pressure generation. As a result, a key element to modeling excess pore pressures due to dehydration reactions is the assumed relationship between permeability and porosity. Data from Integrated Ocean Drilling Program (IODP) drilling of subduction zone reference sites were combined with previously collected results to develop relationships for porosity-permeability behavior for various sediment types. Comparison with measurements of deeper analog data show that porosity-permeability trends are maintained through burial and diagenesis to porosities <10%, suggesting that behavior observed in shallow samples is informative for predicting behavior at depth following subduction. We integrate these permeability-porosity relationships, compaction behavior, predictions of temperature distribution, kinetic expressions for smectite and opal-A dehydration, into fluid flow models to examine the role of dehydration reactions in pore pressure generation.

  16. Geodetic Observations of Interseismic Strain Segmentation at the Sumatra Subduction Zone (United States)

    Prawirodirdjo, L.; Bock, Y.; McCaffrey, R.; Genrich, J.; Calais, E.; Puntodewo, S. S. O.; Subarya, C.; Rais, J.; Zwick, P.; Fauzi


    Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5 deg S and half as much to north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw greater than 8) thrust earthquakes. The rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle, suggesting that seismic segmentation is controlled by properties of the plate interface that persist occupied through more than one earthquake cycle. Structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north.

  17. Using open sidewalls for modelling self-consistent lithosphere subduction dynamics

    NARCIS (Netherlands)

    Chertova, M.V.; Geenen, T.; van den Berg, A.; Spakman, W.


    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

  18. Plate coupling across the northern Manila subduction zone deduced from mantle lithosphere buoyancy (United States)

    Lo, Chung-Liang; Doo, Wen-Bin; Kuo-Chen, Hao; Hsu, Shu-Kun


    The Manila subduction zone is located at the plate boundary where the Philippine Sea plate (PSP) moves northwestward toward the Eurasian plate (EU) with a high convergence rate. However, historically, no large earthquakes greater than Mw7 have been observed across the northern Manila subduction zone. The poorly understood plate interaction between these two plates in this region creates significant issues for evaluating the seismic hazard. Therefore, the variation of mantle lithospheric buoyancy is calculated to evaluate the plate coupling status across the northern Manila subduction zone, based on recently published forward gravity modeling constrained by the results of the P-wave seismic crustal structure of the TAIGER (Taiwan Integrated Geodynamic Research) project. The results indicate weak plate coupling between the PSP and EU, which could be related to the release of the overriding PSP from the descending EU's dragging force, which was deduced from the higher elevation of the Luzon arc and the fore-arc basin northward toward the Taiwan orogen. Moreover, serpentinized peridotite is present above the plate boundary and is distributed more widely and thickly closer to offshore southern Taiwan orogen. We suggest that low plate coupling may facilitate the uplifting of serpentinized mantle material up to the plate boundary.

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

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


    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.

  20. Subducting Plate Breakup by Plume-Lithosphere Interaction (United States)

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


    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.

  1. Reaction-induced rheological weakening enables oceanic plate subduction


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


    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...

  2. Geodynamic Modeling of the Subduction Zone around the Japanese Islands (United States)

    Honda, S.


    the Aleutian arc and the Kurile arc is generally weak and we have suggested the possible contribution of a hot anomaly in the sub-slab mantle as the origin of possible trench-parallel flow there. A 3D mantle flow model of the back-arc around the junction between the northeast Japan arc and the Kurile arc shows a trench-normal flow at a shallow depth. As a result, the expected seismic anisotropy shows the fast direction normal to the arc, even in the region of oblique subduction. This result is generally consistent with observations there. The existence of a hot anomaly in the sub-slab mantle under the Pacific plate was proposed from an analysis of the seismic tomography, and we have studied its possible origins. The origin of a hot anomaly adjacent to the cold downgoing flow, typically observed in internally heated convection, is preferable to that of a hot anomaly, such as a plume head, carried far from the subduction zone. The nature of the western edge of the stagnant slab under northeast China has been investigated with modeling studies, which take into account the subduction history and the phase changes in the mantle. It is likely to be a ridge-type plate boundary between the extinct Izanagi plate and the Pacific plate. Thus, we have concluded that the slab gap under northeast China is not a breakage of the stagnant slab. Further studies have suggested that the existence of the rheological weakening of the slab in the transition zone, and the additional effects of a hot anomaly in the sub-slab mantle under the Pacific plate, may explain the differences in slab morphology under the northern Okhotsk arc and the northeast Japan arc.

  3. Contrasting morphological and DNA barcode-suggested species boundaries among shallow-water amphipod fauna from the southern European Atlantic coast. (United States)

    Lobo, Jorge; Ferreira, Maria S; Antunes, Ilisa C; Teixeira, Marcos A L; Borges, Luisa M S; Sousa, Ronaldo; Gomes, Pedro A; Costa, Maria Helena; Cunha, Marina R; Costa, Filipe O


    In this study we compared DNA barcode-suggested species boundaries with morphology-based species identifications in the amphipod fauna of the southern European Atlantic coast. DNA sequences of the cytochrome c oxidase subunit I barcode region (COI-5P) were generated for 43 morphospecies (178 specimens) collected along the Portuguese coast which, together with publicly available COI-5P sequences, produced a final dataset comprising 68 morphospecies and 295 sequences. Seventy-five BINs (Barcode Index Numbers) were assigned to these morphospecies, of which 48 were concordant (i.e., 1 BIN = 1 species), 8 were taxonomically discordant, and 19 were singletons. Twelve species had matching sequences (Corophium multisetosum (18% divergence) and Dexamine spiniventris (16% divergence), which originated from sampling locations on the west coast of Portugal (only about 36 and 250 km apart, respectively). We also found deep divergence (4%-22%) among specimens of seven species from Portugal compared to those from the North Sea and Italy. The detection of evolutionarily meaningful divergence among populations of several amphipod species from southern Europe reinforces the need for a comprehensive re-assessment of the diversity of this faunal group.

  4. Subduction Mode Selection During Slab and Mantle Transition Zone Interaction: Numerical Modeling (United States)

    Shi, Yanan; Wei, Dongping; Li, Zhong-Hai; Liu, Ming-Qi; Liu, Mengxue


    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.

  5. Oceanic-style Subduction Controls Late Cenozoic Deformation of the Northern Pamir and Alai (United States)

    Sobel, E. R.; Chen, J.; Schoenbohm, L. M.; Thiede, R. C.; Stockli, D. F.; Sudo, M.; Strecker, M. R.


    The Pamir - Alai represents the preeminent example of an active intracontinental subduction zone in the early stages of continent-continent collision. Such zones are the least understood type of plate boundary because modern examples are few and of limited access, and ancient analogs have been extensively overprinted by subsequent continent-continent collision and erosion processes. In the Pamir, at least 300 km of convergence has apparently occurred between the North Pamir and the South Tien Shan. Published P-wave tomography and earthquake epicenters suggest subduction of a ~300 km-long slab. The MPT and Pamir Frontal Thrusts (PFT) correspond to the updip projection of this subduction zone. We have compiled ca. 260 published and 18 new apatite and zircon (U-Th)/He and fission track, and biotite and muscovite Argon cooling ages from basement samples as well as several detrital samples from key areas in the Pamir region. Our synopsis shows that the hanging wall of the MPT experienced relatively minor amounts of late Cenozoic exhumation. This is incompatible with a model of a huge overthrust such as the Himalayan Main Central Thrust. Rather, the bulk of the convergence is apparently accommodated by underthrusting. The Pamir orogen as a whole is an integral part of the overriding plate in a subduction system, while the remnant basin to the north constitutes the downgoing plate. Herein, we demonstrate that the observed deformation of the upper and lower plates within the Pamir-Alai convergence zone resembles highly arcuate oceanic subduction systems characterized by slab rollback, subduction erosion, subduction accretion, and marginal slab-tear faults. We suggest that the curvature of the North Pamir is genetically linked to the short width and rollback of the south-dipping Alai slab; northward motion (indentation) of the Pamir is accommodated by crustal processes related to slab rollback and intracontinental subduction. Our model relates late Oligocene - early Miocene

  6. 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.


    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

  7. Mapping subduction interface coupling using magnetotellurics: Hikurangi margin, New Zealand (United States)

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


    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.

  8. 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

  9. 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

  10. Seismotectonics in the Pamir: An oblique transpressional shear and south-directed deep-subduction model

    Directory of Open Access Journals (Sweden)

    Jiasheng Zhang


    Full Text Available The 3-D geometry of the seismicity in Hindu Kush–Pamir–western China region has been defined by seismic records for 1975–1999 from the National Earthquake Information Center, the U.S. Geological Survey, and over 16,000 relocated earthquakes since 1975 recorded by the Xinjiang seismic network of China. The results show that most Ms ≥ 5.0 hypocenters in the area are confined to a major intracontinental seismic shear zone (MSSZ. The MSSZ, which dips southwards in Pamir has a north-dipping counterpart in the Hindu Kush to the west; the two tectonic realms are separated by the sinistral Chaman transform fault of the India–Asia collisional zone. We demonstrate that the MSSZ constitutes the upper boundary of a south-dipping, actively subducting Pamir continental plate. Three seismic concentrations are recognized just above the Pamir MSSZ at depths between 45–65 km, 95–120 km, and 180–220 km, suggesting different structural relationships where each occurs. Results from focal mechanism solutions in all three seismological concentrations show orientations of the principal maximum stress to be nearly horizontal in an NNW–SSE direction. The south-dipping Pamir subduction slab is wedge-shaped with a wide upper top and a narrow deeper bottom; the slab has a gentle angle of dip in the upper part and steeper dips in the lower part below an elbow depth of ca. 80–120 km. Most of the deformation related to the earthquakes occurs within the hanging wall of the subducting Pamir slab. Published geologic data and repeated GPS measurements in the Pamir document a broad supra-subduction, upper crustal zone of evolving antithetic (i.e. north-dipping back-thrusts that contribute to north-south crustal shortening and are responsible for exhumation of some ultrahigh-pressure rocks formed during earlier Tethyan plate convergence. An alternating occurrence in activity of Pamir and Chaman seismic zones indicates that there is interaction between

  11. Modeling Diverse Pathways to Age Progressive Volcanism in Subduction Zones. (United States)

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


    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

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

    Butler, Jared P.; Beaumont, Christopher


    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

  13. 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.

  14. Subduction-driven recycling of continental margin lithosphere. (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


    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. Constraining the hydration of the subducting Nazca plate beneath Northern Chile using subduction zone guided waves (United States)

    Garth, Tom; Rietbrock, Andreas


    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

  16. Structure of the subducted Cocos Plate from locations of intermediate-depth earthquakes (United States)

    Lomnitz, C.; Rodríguez-Padilla, L. D.; Castaños, H.


    Locations of 3,000 earthquakes of 40 to 300 km depth are used to define the 3-D structure of the subducted Cocos Plate under central and southern Mexico. Discrepancies between deep-seated lineaments and surface tectonics are described. Features of particular interest include: (1) a belt of moderate activity at 40 to 80 km depth that parallels the southern boundary of the Mexican Volcanic Plateau; (2) an offset of 150 km across the Isthmus of Tehuantepec where all seismic activity is displaced toward the northeast; (3) three nests of frequent, deep-seated events (80 to 300 km depth) under southern Veracruz, Chiapas and the coast of Mexico-Guatemala. The active subduction process is sharply delimited along a NW-SE lineament from the Yucatan Peninsula, of insignificant earthquake activity. The focal distribution of intermediate-depth earthquakes in south-central Mexico provides evidence of stepwise deepening of the subduction angle along the Trench, starting at 15 degrees under Michoacan-Guerrero to 45 degrees under NW Guatemala. Historical evidence suggests that the hazard to Mexico City from large intermediate-depth earthquakes may have been underestimated.

  17. Topography and subduction geometry in the central Andes: Clues to the mechanics of a noncollisional orogen (United States)

    Gephart, John W.


    The central Andeean orogen between 12 deg and 32 deg S latitude exhibits a high degree of spatial order: principally an extraordinary bilateral symmetry that is common to the Earth's surface, the underlying Wadati-Benioff zone, and the Nazca/South America plate kinematics, which has been stable since the mid-Tertiary. This spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen. The shapes of the topography and subduction zone can be reduced to symmetric and antisummeric components relative to any verical symmetry plane; the particular plaen which minimizes the antisymmetry (and maximizes the symmetry) is well resolved and is essentially coincident with the stable Euler equator of Nacza/South America relative motion since the mid-Tertiary. That the topography, subduction geometry, and persistent mid-Tertiary plate kinematics share common spatial and geometric elements suggests that he distribution of topography in this orogen depends strongly on the dynamics of subduction. Other factors that might affect the topography and underlying tectonics, such as climate and inherited strutura fabric, which have different spatial characterisitcs, must be of less significance at a continental scale. Furthermore, the small components of asymmetry among the various elements of the orogen appear to be mutually relate in a simple way; it is possible that this coupled asymmetry is associated with a late Teriary change in plate kinematics. These observations suggest that there is a close connection between plate tectonics and the form of the Earth's surface in this noncollisional setting. It follows hta the distribution of topography near convergent plate boundaries may provide a powerful constraing for understanding the dynamics of subduction.

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

    Directory of Open Access Journals (Sweden)

    Fumiko Tajima


    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

  19. Buoyant subduction on Venus: Implications for subduction around coronae (United States)

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


    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.

  20. Splay fault branching along the Nankai subduction zone. (United States)

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


    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.

  1. Rheological property of mafic schist and geological interpretation to the subduction dynamics (United States)

    Okazaki, K.; Hirth, G.


    To understand the spatial and temporal distribution of deformation (e.g., underplating and exhumation of metamorphic rocks) and earthquakes in subduction zones, it is important to constrain the rheological properties of metamorphic rocks (i.e., altered oceanic crust and sediments), and how they evolve during metamorphic reactions following hydration, carbonation and dehydration of the down-going slab. Metamorphism of oceanic crust has stimulated hypotheses on the relationship between intra-slab earthquakes and slab-wedge coupling along plate boundaries in subduction zone. While it is well known that metamorphisms have important effects on material circulation and arc volcanisms at subduction system, it remains unclear how the formation of metamorphic minerals followed by fluid release on the subduction dynamics influences rheology. Past experimental studies on mafic metamorphic rocks were mostly concentrated on phase equilibrium of mineral, thus there are very few reports on the mechanical data for these metamorphic rocks. We conducted triaxial deformation experiments on a mafic greenschist using Griggs-type solid pressure-medium apparatus installed in Brown University. Mafic schist (chlorite - amphibole - epidote - albite schist) containing calcite and quartz veins from Sambagawa metamorphic belt (Japan), which is metamorphosed at the condition of nearly the corner of mantle wedge in hot subduction (1 GPa of pressure and 520C of temperature), was used as experimental samples for typical metamorphic rocks composing oceanic crust in warm subduction zones. Constant strain rete experiments and strain rate step experiment were conducted at 1.0 GPa of confining pressure, 400 ~ 500C of temperature and 10-5 ~ 5×10-7 1/s of strain rate. At stable conditions of samples (1 GPa of confining pressure and 400 and 500C of temperature), differential stresses were higher than 1 GPa. Microstructure of recovered samples showed backing and several localized shear zones. Although

  2. Finding the last 200Ma of subducted lithosphere in tomography and incorporating it into plate reconstructions (United States)

    Suppe, J.; Wu, J.; Chen, Y. W.


    Precise plate-tectonic reconstruction of the Earth has been constrained largely by the seafloor magnetic-anomaly record of the present oceans formed during the dispersal of the last supercontinent since 200Ma. The corresponding world that was lost to subduction has been only sketchily known. We have developed methodologies to map in 3D these subducted slabs of lithosphere in seismic tomography and unfold them to the Earth surface, constraining their initial size, shapes and locations. Slab edges are commonly formed at times of plate reorganization (for example bottom edges typically record initiation of subduction) such that unfolded slabs fit together at times of reorganization, as we illustrate for the Nazca slab at 80Ma and the western Pacific slabs between Kamchatka and New Zealand at 50Ma. Mapping to date suggests that a relatively complete and decipherable record of lithosphere subducted over the last 200Ma may exist in the mantle today, providing a storehouse for new discoveries. We briefly illustrate our procedure for obtaining slab-constrained plate-tectonic models from tomography with our recent study of the Philippine Sea plate, whose motions and tectonic history have been the least known of the major plates because it has been isolated from the global plate and hotspot circuit by trenches. We mapped and unfolded 28 subducted slabs in the mantle under East Asia and Australia/Oceania to depths of 1200km, with a subducted area of 25% of present-day global oceanic lithosphere, and incorporated them as constraints into a new globally-consistent plate reconstruction of the Philippine Sea and surrounding East Asia, leading to a number of new insights, including: [1] discovery of a major (8000 km x 2500 km) set of vanished oceans that we call the East Asia Sea that existed between the Pacific and Indian Oceans, now represented by flat slabs in the lower mantle under present-day Philippine Sea, eastern Sundaland and northern Australia and [2] the Philippine Sea

  3. Subducted oceanic relief locks the shallow megathrust in central Ecuador (United States)

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


    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.

  4. Interactions between strike-slip earthquakes and the subduction interface near the Mendocino Triple Junction (United States)

    Gong, Jianhua; McGuire, Jeffrey J.


    The interactions between the North American, Pacific, and Gorda plates at the Mendocino Triple Junction (MTJ) create one of the most seismically active regions in North America. The earthquakes rupture all three plate boundaries but also include considerable intraplate seismicity reflecting the strong internal deformation of the Gorda plate. Understanding the stress levels that drive these ruptures and estimating the locking state of the subduction interface are especially important topics for regional earthquake hazard assessment. However owing to the lack of offshore seismic and geodetic instruments, the rupture process of only a few large earthquakes near the MTJ have been studied in detail and the locking state of the subduction interface is not well constrained. In this paper, first, we use the second moments inversion method to study the rupture process of the January 28, 2015 Mw 5.7 earthquake on the Mendocino transform fault that was unusually well recorded by both onshore and offshore strong motion instruments. We estimate the rupture dimension to be approximately 6 km by 3 km corresponding to a stress drop of ∼4 MPa for a crack model. Next we investigate the frictional state of the subduction interface by simulating the afterslip that would be expected there as a result of the stress changes from the 2015 earthquake and a 2010 Mw 6.5 intraplate earthquake within the subducted Gorda plate. We simulate afterslip scenarios for a range of depths of the downdip end of the locked zone defined as the transition to velocity strengthening friction and calculate the corresponding surface deformation expected at onshore GPS monuments. We can rule out a very shallow downdip limit owing to the lack of a detectable signal at onshore GPS stations following the 2010 earthquake. Our simulations indicate that the locking depth on the slab surface is at least 14 km, which suggests that the next M8 earthquake rupture will likely reach the coastline and strong shaking

  5. Dynamics of double-polarity subduction: application to the Western Mediterranean (United States)

    Peral, Mireia; Zlotnik, Sergio; Fernandez, Manel; Vergés, Jaume; Jiménez-Munt, Ivone; Torne, Montserrat


    The evolution of the Western Mediterranean is a highly debated question by geologists and geophysicists. Even though most scientists agree in considering slab roll-back to be the driving mechanism of the tectonic evolution of this area, there is still no consensus about the initial setup and its time evolution. A recent model suggests a lateral change in subduction polarity of the Ligurian-Thetys oceanic domain to explain the formation and evolution of the Betic-Rif orogenic system and the associated Alboran back-arc basin. Such geodynamic scenario is also proposed for different converging regions. The aim of this study is to analyze the dynamic evolution of a double-polarity subduction process and its consequences in order to test the physical feasibility of this interaction and provide geometries and evolutions comparable to those proposed for the Western Mediterranean. The 3D numerical model is carried out via the Underworld framework. Tectonic plate behavior is described by equations of fluid dynamics in the presence of several different phases. Underworld solves a non-linear Stokes flow problem using Finite Elements combined with particle-in-cell approach, thus the discretization combines a standard Eulerian Finite Element mesh with Lagrangian particles to track the location of the phases. The final model consists of two oceanic plates with viscoplastic rheology subducting into the upper mantle in opposite direction and the problem is driven by Rayleigh-Taylor instability. We study the influence of the boundary conditions in the model evolution, and the slab deformation produced by the proximity between both plates. Moreover the case of asymmetric friction on the lateral sides of slabs is also considered. Simulations of single subduction models are used as a reference, to compare results and understand the influence of the second plate. We observe slight differences in the trench retreat velocity and the slab morphology near the contact area when plates are

  6. The Geodynamics of Continental Lithosphere Entering a Subduction Zone (United States)

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


    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.

  7. Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation (United States)

    Stanley, Dal; Villaseñor, Antonio; Benz, Harley


    buttress occurs under the North Cascades region of Washington and under southern Vancouver Island. We find that regional faults zones such as the Devils Mt. and Darrington zones follow the margin of this buttress and the Olympic-Wallowa lineament forms its southern boundary east of the Puget Lowland. Thick, high-velocity, lower-crustal rocks are interpreted to be a mafic/ultramafic wedge occuring just above the subduction thrust. This mafic wedge appears to be jointly deformed with the arch, suggesting strong coupling between the subducting plate and upper plate crust in the Puget Sound region at depths >30 km. Such tectonic coupling is possible if brittle-ductile transition temperatures for mafic/ultramafic rocks on both sides of the thrust are assumed. The deformation models show that dominant north-south compression in the coast ranges of Washington and Oregon is controlled by a highly mafic crust and low heat flow, allowing efficient transmission of margin-parallel shear from Pacific plate interaction with North America. Complex stress patterns which curve around the Puget Sound region require a concentration of northwest-directed shear in the North Cascades of Washington. The preferred model shows that greatest horizontal shortening occurs across the Devils Mt. fault zone and the east end of the Seattle fault.

  8. Boron cycling in subduction zones


    Palmer, Martin R.


    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...

  9. 3D Numerical modelling of topography development associated with curved subduction zones (United States)

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


    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

  10. Subduction-stage P-T path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction (United States)

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


    , but in a curve with dP/dT increasing with metamorphic pressure. In previous thermal models curved P-T paths of the same kind were predicted for subduction of a very young slab (<5 Ma) suggesting that the curved P-T paths will be formed just before oceanic-ridge subduction. A new model incorporating progressive approach of an oceanic ridge to a subduction zone shows that the series of the Sambagawa subduction P-T paths well fit the results for a setting where a ridge is close to being subducted at a slow rate relative to the subduction rate. The subduction P-T paths from the Sambagawa belt can, therefore, be regarded as a prophetic record of the subsequent ridge subduction. This suggests that exhumation of high-P/T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges.

  11. Impact-driven subduction on the Hadean Earth (United States)

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


    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.

  12. Subduction zone earthquakes and stress in slabs (United States)

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


    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.

  13. The hidden simplicity of subduction megathrust earthquakes (United States)

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


    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.

  14. Diverse melanges of an ancient subduction complex

    Energy Technology Data Exchange (ETDEWEB)

    Lash, G.G.


    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.

  15. Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra. (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


    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.

  16. Diffusion creep of fine-grained garnetite: Implications for the flow strength of subducting slabs (United States)

    Wang, Zichao; Ji, Shaocheng


    Creep experiments were performed on synthetic fine-grained garnetite to investigate the flow strength of the Earth's subducting slabs. Experiments were conducted at temperatures (T) of 1373-1543 K and total pressure (P) of 0.1 MPa in controlled atmospheres of fO2 =10-17-10-8 MPa. The mechanical data indicate a grain-size sensitive diffusion flow and the creep behavior can be described by an equation of the form: FD1 ɛ.=(5.32±3.10)×10-6Td2.5±0.3fO20σ1.1±0.2exp(-347±46kJ/molRT) where T in Kelvin, d in meter, σ and fO2 in MPa. Based on the diffusivities (D) calculated from creep and diffusion experiments, we proposed that grain boundary diffusion is the dominant mechanism for high temperature creep of the fine-grained garnetite. Normalized creep strength of the garnetite is found to be comparable to those of feldspar and olivine in diffusion creep regime, suggesting that garnetite may not form a strong layer in the subducted oceanic lithosphere if it deforms by grain boundary diffusion creep.

  17. The Hainan Lone Plume Prompted By Encircling Subduction Zones around the South China Sea (United States)

    Zhang, N.; Li, Z. X.


    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.

  18. Metallogeny of subduction zones

    Directory of Open Access Journals (Sweden)

    Sorokhtin N. O.


    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. Seamount subduction at the North-Ecuadorian convergent margin: Effects on structures, inter-seismic coupling and seismogenesis (United States)

    Marcaillou, Boris; Collot, Jean-Yves; Ribodetti, Alessandra; d'Acremont, Elia; Mahamat, Ammy-Adoum; Alvarado, Alexandra


    At the North-Ecuadorian convergent margin (1°S-1.5°N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake. This dataset highlights a subducted ∼ 30 × 40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits. The seamount subduction uplifted the margin basement by ∼1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion. In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8-1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts

  20. Investigating the Subduction History of the Southwest Pacific using Coupled Plate Tectonic-Mantle Convection Models (United States)

    Matthews, K. J.; Flament, N. E.; Williams, S.; Müller, D.; Gurnis, M.


    The Late Cretaceous to mid Eocene (~85-45 Ma) evolution of the southwest Pacific has been the subject of starkly contrasting plate reconstruction models, reflecting sparse and ambiguous data. Disparate models of (1) west-dipping subduction and back-arc basin opening to the east of the Lord Howe Rise, (2) east-dipping subduction and back-arc basin closure to the east of the Lord Howe Rise, and (3) tectonic quiescence with no subduction have all been proposed for this time frame. To help resolve this long-standing problem we test a new southwest Pacific reconstruction using global mantle flow models with imposed plate motions. The kinematic model incorporates east to northeast directed rollback of a west-dipping subduction zone between 85 and 55 Ma, accommodating opening of the South Loyalty back-arc basin to the east of New Caledonia. At 55 Ma there is a plate boundary reorganization in the region. West-dipping subduction and back-arc basin spreading end, and there is initiation of northeast dipping subduction within the back-arc basin. Consumption of South Loyalty Basin seafloor continues until 45 Ma, when obduction onto New Caledonia begins. West-dipping Tonga-Kermadec subduction initiates at this time at the relict Late Cretaceous-earliest Eocene subduction boundary. We use the 3D spherical mantle convection code CitcomS coupled to the plate reconstruction software GPlates, with plate motions and evolving plate boundaries imposed since 230 Ma. The predicted present-day mantle structure is compared to S- and P-wave seismic tomography models, which can be used to infer the presence of slab material in the mantle at locations where fast velocity anomalies are imaged. This workflow enables us to assess the forward-modeled subduction history of the region.

  1. Subduction related fluids fractionate Nb/Ta (United States)

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


    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

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

    Directory of Open Access Journals (Sweden)

    D. Arcay


    , and eventually jams the subduction process during incipient subduction of a young (20-Myr-old and soft lithosphere under a thick upper plate. Finally, both the BDT depth and the decoupling depth are a function of the subducting plate age, but are not influenced in the same fashion: cool and old subducting plates deepen the BDT but shallow the interplate decoupling depth. Even if BDT and kinematic decoupling are intrinsically related to different mechanisms of deformation, this work shows that they are able to interact closely. Comparison between modelling results and observations suggests a minimum friction coefficient of 0.045 for the interplate plane, even 0.069 in some cases, to model realistic BDT depths. The modelled zdec is a bit deeper than suggested by geophysical observations. Eventually, the better way to improve the adjustment to observations may rely on a moderate to strong asthenosphere viscosity reduction in the metasomatised mantle wedge.

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


    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.

  4. What's happening inside the subduction factory? (United States)

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


    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

  5. The dynamical control of subduction parameters on surface topography (United States)

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


    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.

  6. A discussion of numerical subduction initiation (United States)

    Buiter, Susanne; Ellis, Susan


    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

  7. A Dynamical Context for Small-scale Heterogeneity Throughout the Mantle Beneath Subduction (United States)

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


    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

  8. Topographic and sedimentary features in the Yap subduction zone and their implications for the Caroline Ridge subduction (United States)

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


    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.

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


    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

  10. Frictional behavior of carbonate-rich sediments in subduction zones (United States)

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


    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

  11. Subduction of a buoyant plateau at the Manila Trench: Tomographic evidence and geodynamic implications (United States)

    Fan, Jianke; Zhao, Dapeng; Dong, Dongdong


    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.

  12. Subduction on the margins of coronae on Venus: Evidence from radiothermal emissivity measurements (United States)

    Robinson, C. A.


    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.

  13. Extensive decarbonation of continuously hydrated subducting slabs (United States)

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


    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

  14. Processes and consequences of deep subduction

    NARCIS (Netherlands)

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


    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

  15. Transient uplift after a 17th-century earthquake along the Kuril subduction zone. (United States)

    Sawai, Yuki; Satake, Kenji; Kamataki, Takanobu; Nasu, Hiroo; Shishikura, Masanobu; Atwater, Brian F; Horton, Benjamin P; Kelsey, Harvey M; Nagumo, Tamotsu; Yamaguchi, Masaaki


    In eastern Hokkaido, 60 to 80 kilometers above a subducting oceanic plate, tidal mudflats changed into freshwater forests during the first decades after a 17th-century tsunami. The mudflats gradually rose by a meter, as judged from fossil diatom assemblages. Both the tsunami and the ensuing uplift exceeded any in the region's 200 years of written history, and both resulted from a shallow plate-boundary earthquake of unusually large size along the Kuril subduction zone. This earthquake probably induced more creep farther down the plate boundary than did any of the region's historical events.

  16. Seismic‐wave attenuation determined from tectonic tremor in multiple subduction zones (United States)

    Yabe, Suguru; Baltay, Annemarie S.; Ide, Satoshi; Beroza, Gregory C.


    Tectonic tremor provides a new source of observations that can be used to constrain the seismic attenuation parameter for ground‐motion prediction and hazard mapping. Traditionally, recorded earthquakes of magnitude ∼3–8 are used to develop ground‐motion prediction equations; however, typical earthquake records may be sparse in areas of high hazard. In this study, we constrain the distance decay of seismic waves using measurements of the amplitude decay of tectonic tremor, which is plentiful in some regions. Tectonic tremor occurs in the frequency band of interest for ground‐motion prediction (i.e., ∼2–8  Hz) and is located on the subducting plate interface, at the lower boundary of where future large earthquakes are expected. We empirically fit the distance decay of peak ground velocity from tremor to determine the attenuation parameter in four subduction zones: Nankai, Japan; Cascadia, United States–Canada; Jalisco, Mexico; and southern Chile. With the large amount of data available from tremor, we show that in the upper plate, the lower crust is less attenuating than the upper crust. We apply the same analysis to intraslab events in Nankai and show the possibility that waves traveling from deeper intraslab events experience more attenuation than those from the shallower tremor due to ray paths that pass through the subducting and highly attenuating oceanic crust. This suggests that high pore‐fluid pressure is present in the tremor source region. These differences imply that the attenuation parameter determined from intraslab earthquakes may underestimate ground motion for future large earthquakes on the plate interface.

  17. Building a Subduction Zone Observatory (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


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

  18. Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction (United States)

    Erkan, Kamil; Blackwell, David


    We examine the thermal relaxation of the Sierra Nevada and Baja California extinct outer arc blocks following the progressive cessation of Farallon subduction under western North America beginning at ˜30 Ma. Being parts of the same outer arc until the inland jump of the San Andreas transform fault at ˜5 Ma, these two regions show many similarities in their geology, geomorphology, rigid body behavior, and their relatively low seismicity. In the thermal model, we combine results of different geophysical and geophysical studies to constrain the thermal state and geometry of the outer arcs before the cessation of subduction and then model the postsubduction temperature responses in these regions using the results of the tectonic reconstructions. A well-constrained regional thermal model of these blocks using the results of many earlier studies in these regions confirms that the present low heat flow values in these regions are the remnants of the very cold outer arc thermal regime of the subduction zone even as long as 30 Ma after cessation of subduction. Thus the entire Pacific boundary of the North American plate is still in a transient thermal state. The calculated low lithospheric temperatures in the Sierra Nevada and Peninsular blocks correlate very well with their rigid body behavior obtained from geodetic studies, and seismogenic layer thicknesses obtained from seismological studies. This is in contrast with the fact that both regions are surrounded by intense deformation associated with the western North America intraplate and extraplate motions. These low-temperature islands play important roles in the present interaction of the North American and Pacific plates and contribute to the broad deformation of the transform boundary. The thermal relaxation of the extinct outer arcs includes both vertical heating from the underlying asthenosphere and the lateral heating from the extinct back arc (Basin and Range), which has remained as a high heat flow region after

  19. Thermal buoyancy on Venus - Underthrusting vs subduction (United States)

    Burt, Jeffrey D.; Head, James W.


    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.

  20. 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


    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.

  1. Technology for Boundaries

    DEFF Research Database (Denmark)

    Bødker, Susanne; Kristensen, Jannie Friis; Nielsen, Christina


    This paper presents a study of an organisation, which is undergoing a process transforming organisational and technological boundaries. In particular, we shall look at three kinds of boundaries: the work to maintain and change the boundary between the organisation and its customers; boundaries.......After analysing the history and the current boundary work, the paper will propose new technological support for boundary work. In particular the paper will suggest means of supporting boundaries when these are productive and for changing boundaries when this seems more appropriate. In total, flexible technologies...... seem a core issue when dealing with technology for boundaries....

  2. Tidal modulation of slow slip events in the Nankai trough subduction zone detected by borehole strainmeters (United States)

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


    Slow slip events (SSEs) often occur in the Nankai subduction zone, Japan, within a band-like zone extended from the center of Honshu to western Shikoku. SSEs are believed as shear slip on the plate interface, where the frictional property changes from velocity weakening to strengthening in the dip direction. Therefore the dynamics of SSEs may give some hints on the depth dependent friction and plate subduction. The tidal modulation of SSEs has been identified by statistical analysis using strain data of Plate Boundary Observatory, in the Cascadia subduction zone [Hawthorne & Rubin, 2010]. Here, we perform similar statistical analyses using strain data recorded at borehole stations maintained by National Institute of Advanced Industrial Science and Technology, in western Japan. The correlation between the oscillation in SSEs and tidal stress was confirmed statistically. In Nankai subduction zone, it is known that SSEs are accompanied with high activity of deep tectonic tremors [Hirose & Obara, 2006]. These tremors have been known to be sensitive to tidal stress [Nakata et al., 2008]. Therefore, the tidal modulation of SSEs is another representation of tidal modulation of tremors. To clarify the relation between SSEs and tremors, we investigate whether strain changes corresponding to SSEs can be explained only by tremors activity. For an SSE occurred in Aug. 2010 in Bungo channel, we assume that the seismic moment of the SSE is 1.6 × 1018 Nm (Mw 6.1) based on the inversion of GNSS data [Nishimura et al., 2013], and that this moment is released by 715 tremors that occur during this SSE [Idehara et al., 2014]. In this case, each tremor is assigned with seismic moment of 2.2 × 1015 Nm (Mw 4.2). Then the strain change at the observation station by these tremors is calculated using the Okada [1992] method, assuming a half space and focal mechanism consistent with the regional plate motion. The calculated strain is qualitatively similar with the observed strain

  3. Subducting characteristic of the Pacific slab beneath northeast China (United States)

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


    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.

  4. A {open_quotes}balanced{close_quotes} Y:16 translocation with the Y breakpoint just proximal to the Yq heterochromatin boundary associated with Turner-like neonatal lymphedema suggests the location of a potential anti-Turner gene

    Energy Technology Data Exchange (ETDEWEB)

    Erickson, R.P.; Hudgins, L. [Univ. of Arizona, Tucson, AZ (United States); Stone, J.F. [Southwest Biomedical Research Institute, Scottsdale, AZ (United States)] [and others


    A male patient with Turner-like hydrops in the newborn period (Bonnevie-Ullrich syndrome) was studied. The extensive nucchal cystic hygroma and hydrops resolved over several weeks. The karyotype was 46,X,t(Y;16)(q11.2;q24). The paternal karyotype was normal. Chromosome painting with the heterochromatic long arm repeat DYZ2 disclosed that all the hybridization was on the derivative 16. This was confirmed by chromosome painting with DYZ1, the other major Y long arm heterochromatic repeat, and DYZ3, the Y alphoid, centromeric repeat, which showed chromosomal separation of the 2 stained regions. A {open_quotes}FISHing trip{close_quotes} was performed using the Y YAC contig created in Dr. David Page`s laboratory. This disclosed 2 YACs located just proximal to the Y heterochromatin which {open_quotes}jumped{close_quotes} the translocation. The recent discovery of a candidate gene for the azoospermia factor (AZF) in this region suggests the possibility that there are several Y-expressed genes adjacent to the heterochromatin boundary as there are near the pseudoautosomal boundary.

  5. Kinematics of subduction and plate convergence under Taiwan and its geomorphic, geodetic and seismic expressions (United States)

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


    with the zone of flexure. Kinematic modeling of leveling and gps data is consistent with a migration rate of the hinge of ~50mm/y, which would be the subduction rate of Eurasian mantle lithosphere. This rate is somewhat faster than the long-term rate of ~30mm/y since ~15Ma, but less than the current slab-normal plate rate of ~80mm/y, which is thought to represent a speed-up in the last ~1-2Ma. This kinematic modeling also suggests that the main subduction interface under the eastern Central Mountains could be widely locked; if so it has substantial seismic potential at its ~12-13km depth.

  6. Experimental and observational evidence for plume-induced subduction on Venus (United States)

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


    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.

  7. Horizontal mantle flow controls subduction dynamics. (United States)

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


    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.

  8. Dynamics of subduction and continental collision: Influence of the nature of the plate contact. Geologica Ultraiectina (284)

    NARCIS (Netherlands)

    De Franco, R.


    At convergent plate boundaries, the properties of the actual plate contact are important for the overall dynamics. Convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain. We investigate two fundamental physical states of the subduction

  9. Viscous Dissipation and Criticality of Subducting Slabs (United States)

    Riedel, Mike; Karato, Shun; Yuen, Dave


    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

  10. Geodynamical Analysis of Plate Reconstructions based on Subduction History Models (United States)

    Quevedo, L. E.; Butterworth, N. P.; Matthews, K. J.; Morra, G.; Müller, R. D.


    We present a novel method to produce global subduction history models from plate reconstructions and use their predicted geodynamic behaviour as a quality metric for the physical consistency of absolute motions. We show that modelled slabs constructed by advecting material into the mantle according to absolute and relative plate motions given by a particular reconstruction are better correlated with the present day slab dips observed in mantle tomography than instantaneous kinematic quantities like present convergence rate. A complete simulation incorporating lithospheric thickness derived from oceanic age and a rheological model of the lithosphere was run using the Boundary Element Method-based software BEMEarth to infer the global pattern of mantle flow. The predicted plate motion orientations in the form of Euler pole location for the present day and mid-Cretaceous (125 Ma) were compared with the kinematic model for a set of rheologies and mantle structures, and found to be a robust and efficient indicator of the physical consistency of kinematic reconstructions based on their effect on the balance of plate driving forces. As an application example, during the Early Cretaceous, the predicted motion of the Farallon plate was found to be more consistent with the regional geology of the Western North American Cordillera system than the instantaneous motion suggested by a reconstruction at 125 Ma based on sparse hotspot track data on the Pacific Plate. This suggests that a methodology based on forward geodynamic modellling could be used to predict absolute plate motions in reconstructions for times that are ill-constrained by observations constraining absolute plate motions.

  11. Constraints on Subduction Zone Processes from Low Frequency Earthquakes (United States)

    Bostock, M. G.


    The discovery of tectonic tremor and constituent low-frequency earthquakes (LFEs) offers seismologists new opportunities to study both deformational processes and structure within the subduction zone forearc. This assertion is especially true for northern Cascadia where i) regular seismicity is sparse, and ii) a relatively transparent overriding plate inflicts minimal distortion upon direct P and S wave arrivals from LFEs. Despite low signal-to-noise ratios, LFEs are highly repetitive and signal can be enhanced through construction of stacked templates. Studies in both Cascadia and Nankai reveal an association between LFE hypocenters and a high Vp/Vs, low-velocity zone (LVZ) that is inferred to represent overpressured upper oceanic crust. Scattered signals within Vancouver Island templates, interpreted to originate at boundaries of the LVZ, place LFEs within the LVZ and suggest that this structure may define a distributed (several km) zone of deformation. A recent analysis of LFE magnitudes indicates that LFEs exhibit scaling relations distinct from both regular earthquakes and longer period (10's of seconds to days) phenomena associated with slow slip. Regular earthquakes generally obey a scaling of moment proportional to duration cubed consistent with self similarity, whereas long period slow slip phenomena exhibit a linear scaling between moment and duration that can be accommodated through constant slip or constant stress drop models. In contrast, LFE durations are nearly constant suggesting that moment is governed by slip alone and that asperity size remains approximately constant. The implied dimensions (~1 km2), the persistance of LFEs in time and their stationarity in space point to structural heterogeneity, perhaps related to pockets of upper oceanic crust impervious to hydrothermal circulation, as a fundamental control.

  12. Fore-arc deformation at the transition between collision and subduction: insights from 3D thermo-mechanical laboratory experiments.


    D. Boutelier; Onno Oncken; A. Cruden


    Three-dimensional thermomechanical laboratory experiments of arc-continent collision investigate the deformation of the fore arc at the transition between collision and subduction. The deformation of the plates in the collision area propagates into the subduction-collision transition zone via along-strike coupling of the neighboring segments of the plate boundary. In our experiments, the largest along-strike gradient of trench-perpendicular compression does not produce sufficiently localized ...

  13. Multiple subduction imprints in the mantle below Italy detected in a single lava flow (United States)

    Nikogosian, Igor; Ersoy, Özlem; Whitehouse, Martin; Mason, Paul R. D.; de Hoog, Jan C. M.; Wortel, Rinus; van Bergen, Manfred J.


    Post-collisional magmatism reflects the regional subduction history prior to collision but the link between the two is complex and often poorly understood. The collision of continents along a convergent plate boundary commonly marks the onset of a variety of transitional geodynamic processes. Typical responses include delamination of subducting lithosphere, crustal thickening in the overriding plate, slab detachment and asthenospheric upwelling, or the complete termination of convergence. A prominent example is the Western-Central Mediterranean, where the ongoing slow convergence of Africa and Europe (Eurasia) has been accommodated by a variety of spreading and subduction systems that dispersed remnants of subducted lithosphere into the mantle, creating a compositionally wide spectrum of magmatism. Using lead isotope compositions of a set of melt inclusions in magmatic olivine crystals we detect exceptional heterogeneity in the mantle domain below Central Italy, which we attribute to the presence of continental material, introduced initially by Alpine and subsequently by Apennine subduction. We show that superimposed subduction imprints of a mantle source can be tapped during a melting episode millions of years later, and are recorded in a single lava flow.

  14. Dynamics and Significance of Plume-Induced Subduction Initiation: Numerical Modeling (United States)

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


    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).

  15. Post-Eocene Subduction Dynamics and Mantle Flow beneath Western U.S. (United States)

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


    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

  16. Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone. (United States)

    Calvert, Andrew J


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

  17. Thermochronological response to rifting and subduction in the Corsica-Sardinia block (United States)

    Malusà, Marco Giovanni; Danišík, Martin; Kuhlemann, Joachim


    The linkage between deep-seated tectonic processes and surface processes provides a key to investigate the geological evolution of complex plate boundaries starting from the analysis of low-temperature geochronological systems. Here, we integrate published thermochronological data from Corsica (Danišík et al., 2007) with a new multi-thermochronological dataset (i.e., zircon and apatite fission track (ZFT and AFT), and apatite (U-Th)/He (AHe) data) from Sardinia, in order to tackle the Western Mediterranean tectonic issue and constrain the problematic transition in space and time between the opposite-dipping Alpine (European) and Apenninic (Adriatic) subductions. Mesozoic AFT ages (169-201 Ma) and AHe ages (133-204 Ma), found on mountain ridges of central Sardinia and on the eastern coast of the island, indicate that rocks now exposed at the surface have resided since Jurassic times at very shallow depth, i.e., above the partial annealing zone of the AFT system (~60-110°C) or even above the partial retention zone of the AHe system (~40-80°C). The observed age pattern and track length distributions are consistent with those predicted after rising of isothermal surfaces during rifting and subsequent thermal relaxation after continental break-up. We demonstrate that the crustal sections now exposed in central and eastern Sardinia were originally located closer to the Tethyan rift axis than crustal sections exposed in NW Sardinia and Corsica, pointing to a NNE trend for the continental crust isopachs of the northern Tethyan margin (ENE before Corsica-Sardinia rotation), with burial depth progressively increasing from SE to NW. In Alpine Corsica, the low-T geochronological evidence of Jurassic rifting was largely obliterated by Cenozoic metamorphism, but it is still recognized in high-T systems. AFT and AHe ages set after Tethyan rifting but not thermally affected by Neogene backarc extension, define a SE-NW trend of decreasing ages from southern Sardinia to northern

  18. Bulk rheology and simulated episodic tremor and slip within a numerically-modeled block-dominated subduction melange (United States)

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


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

  19. Reevaluating plate driving forces from 3-D models of subduction (United States)

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


    , 2004), owing to the analog slab having a lateral extent smaller than the width of the box. These experiments clearly show subduction of a finite-width slab will generate a flow of material from behind the slab around both the side edges and under the nose of the slab into the mantle wedge. This rollback induced flow establishes a positive feedback with backward hinge migration on the surface, and has significant consequences for the composition and dynamics of the mantle wedge. Here we present results of 3-D numerical experiments aimed to quantify the partitioning between different forces acting on such a slab. These experiments include a high viscosity slab (relative to background mantle), a high viscosity lower mantle and a computational domain large enough so that the flow induced by subduction of a finite-width slab is not constrained by the side or bottom boundaries. We provide a self-consistent force balance and integrate the forces acting over the different portions of the slab, thereby partitioning such forces into specific components. We quantify the force due to rollback-induced flow, and signify its importance as a driving force relative to the other forces present: a net slab pull force, a force responsible for bending the slab at the subduction hinge, and a resistive force due to shear traction on the upper, lower, and nose (if present) surfaces of the subducted slab.

  20. Seismic anisotropy and mantle flow below subducting slabs (United States)

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


    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.

  1. Fluid Release and the Deformation of Subducting Crust (United States)

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


    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.

  2. Cascadia subduction tremor muted by crustal faults (United States)

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


    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. Uplift in the Fiordland region, New Zealand: implications for incipient subduction. (United States)

    House, M A; Gurnis, M; Kamp, P J J; Sutherland, R


    Low-temperature thermochronometry reveals regional Late Cenozoic denudation in Fiordland, New Zealand, consistent with geodynamic models showing uplift of the overriding plate during incipient subduction. The data show a northward progression of exhumation in response to northward migration of the initiation of subduction. The locus of most recent uplift coincides with a large positive Bouguer gravity anomaly within Fiordland. Thermochronometrically deduced crustal thinning, anomalous gravity, and estimates of surface uplift are all consistent with approximately 2 kilometers of dynamic support. This amount of dynamic support is in accord with geodynamic predictions, suggesting that we have dated the initiation of subduction adjacent to Fiordland.

  4. Analyzing One-Sided vs. Two-Sided Subduction Arising from Mantle Convection Simulations (United States)

    Kaplan, M. S.; Becker, T. W.


    Purely thermal plate tectonic generation models struggle to consistently reproduce one-sided subduction as is observed on Earth (Tackley 2000; Van Heck and Tackley 2008; Foley and Becker 2009), and instead produce two-sided subduction where the subducting slab contains a significant flux of material from both plates. The models of Crameri et al. (2012) demonstrate that the implementation of a free upper surface boundary condition and the inclusion of a weak hydrated crust can facilitate one-sided subduction. We employ a similar model configuration to Crameri et al. (2012) to further investigate the dynamics and energetics which are associated with one-sided vs. two-sided subduction. We use a 2D finite difference code based off of the algorithms of I2ELVIS (Gerya and Yuen 2007) where material parameters are tracked on Lagrangian markers and the Stokes and Energy equations are solved on a Cartesian grid. A free surface is implemented by a low viscosity and density 'sticky air layer' (Schmeling et al., 2008; Crameri et al., 2012) with the stabilization routine of Duretz et al. (2011) to prevent the 'drunken seaman' instability (Kaus et al., 2010). The effects of a weak crust, shear heating, a free surface or free slip upper mechanical boundary condition, plasticity as a function of depth or pressure, and the sticky air layer thermal conductivity on one-sided vs. two-sided subduction are investigated. When we observe one-sided subduction it is transient and can smoothly evolve back to a two-sided configuration. In our models, 'sidedness' is a spectrum, rather than either discretely one or two sided, and the models move between the two regimes throughout the model runs. We observe that the thermal conductivity of the sticky air layer can influence the dynamics of the convective domain. Elevated values of thermal conductivity compared to those of rock must be implemented in the sticky air layer in order to maintain a constant temperature at the surface of the convective

  5. Subduction and Slab Advance at Orogen Syntaxes: Predicting Exhumation Rates and Thermochronometric Ages with Numerical Modeling (United States)

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


    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

  6. Geologic signature of early Tertiary ridge subduction in Alaska (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.


    ridge subduction, and changes in the strength of the prism as it was heated and then cooled. In this model, events in the Alaskan interior would have taken place above more distal, deeper parts of the slab window. Extensional (or transtensional) basin subsidence was driven by the two subducting plates that each exerted different tractions on the upper plate. The magmatic lull along the arc presumably marks a time when hydrated lithosphere was not being subducted beneath the arc axis. The absence of a subducting slab also may explain uplift of the Brooks Range and North Slope: Geodynamic models predict that longwavelength uplift of this magnitude will take place far inboard from Andean-type margins when a subducting slab is absent. Precise correlations between events in the accretionary prism and the Alaskan interior are hampered, however, by palinspastic problems. During and since the early Tertiary, margin-parallel strike-slip faulting has offset the near-trench plutonic belt - i.e., the very basis for locating the triple junction and slab window - from its backstop, by an amount that remains controversial.Near-trench magmatism began at 61 Ma at Sanak Island in the west but not until 51 Ma at Baranof Island, 2200 km to the east. A west-to-east age progression suggests migration of a trench-ridge-trench triple junction, which we term the Sanak-Baranof triple junction. Most workers have held that the subducted ridge separated the Kula and Farallon plates. As a possible alternative, we suggest that the ridge may have separated the Kula plate from another oceanic plate to the east, which we have termed the Resurrection plate.

  7. The expression pattern and inhibitory influence of Tenascin-C on the growth of spiral ganglion neurons suggest a regulatory role as boundary formation molecule in the postnatal mouse inner ear. (United States)

    Kwiatkowska, M; Reinhard, J; Roll, L; Kraft, N; Dazert, S; Faissner, A; Volkenstein, S


    Sensorineural hearing loss, as a consequence of acoustic trauma, aging, genetic defects or ototoxic drugs, is highly associated with irreversible damage of cochlear hair cells (HCs) and secondary degeneration of spiral ganglion (SG) cells. Cochlear implants (CIs), which bypass the lost HC function by direct electrical stimulation of the remaining auditory neurons, offer an effective therapy option. Several studies imply that components of the extracellular matrix (ECM) have a great impact on the adhesion and growth of spiral ganglion neurons (SGNs) during development. Based on these findings, ECM proteins might act as bioactive CI substrates to optimize the electrode-nerve interface and to improve efficacy of these implants. In the present study, we focused on the ECM glycoproteins Tenascin-C (TN-C), Laminin (LN), and Fibronectin (FN), which show a prominent expression along the growth route of SGNs and the niche around HCs during murine postnatal development in vivo. We compared their influence on adhesion, neurite length, and neurite number of SGNs in vitro. Moreover, we studied the expression of the chondroitin sulfate proteoglycan (CSPG) dermatan sulfate-dependent proteoglycan-1 (DSD-1-PG), an interaction partner of TN-C. In sum, our in vitro data suggest that TN-C acts as an anti-adhesive and inhibitory factor for the growth of SGNs. The DSD-1 carbohydrate epitope is specifically localized to HC stereocilia and SG fibers. Interestingly, TN-C and the DSD-1-PG exhibit a mutually exclusive expression pattern, with the exception of a very restricted region beneath the habenula perforata, where SG neurites grow through the basilar membrane (BM) toward the HCs. The complementary expression of TN-C, LN, FN, and the DSD-1 epitope suggests that TN-C may act as an important boundary formation molecule in the developing postnatal mouse inner ear, which makes it a promising candidate to regulate neurite outgrowth in the light of CIs. Copyright © 2016 IBRO. Published by

  8. Subduction and vertical coastal motions in the eastern Mediterranean (United States)

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


    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

  9. Why Do We Need 3-d Numerical Models of Subduction? (United States)

    Morra, G.; Faccenna, C.; Funiciello, F.; Giardini, D.; Regenauer-Lieb, K.

    We use a set of 2-D and 3-D numerical fluid dynamic experiments, modeled with different strain rate dependent rheologies (viscous, visco-plastic, power law) to ana- lyze the long-term dynamics of the subduction of an oceanic slab into an iso-viscous or stratified mantle. For the lithosphere a fluid dynamic approach has been bench- marked with our previous solid mechanical approach with the aim of overcoming the coherency problem of fluid dynamic calculations. The solid mechanical dichotomy Sstrong before failure and weak where it failsT has been cast into a specialized non- & cedil;linear fluid rheology. Analog 2-D and 3-D experiments are finally compared with the numerical experiments. 2-D numerical experiments are considered with and without free surface to investigate the limitations induced by a closed top boundary. The effect of asymmetric boundary conditions (with and without overriding plate) is analyzed with respect to the possibility of trench retreat. We clearly state the importance for the free surface analysis. 2-D experiments have inherent weaknesses: first they provide an unrealistic simulation of mantle flow (suppression of toroidal flow), second they give rise to the Sclosed boxT problem (interaction of the slab with a boundary, i.e. & cedil;660 km and the left and right box boundaries). 3-D numerical experiments permit to overcome these problems. A natural analysis of the behavior of the mantle flow during subduction and the three-dimensional behavior of the slab is thus possible. Physical observables like trench retreat and toroidal and poloidal flow are compared with the results of our companion analog 3-D experiments.

  10. An Investigation of the Accuracy of Coulomb Stress Changes Inferred From Geodetic Observations Following Subduction Zone Earthquakes (United States)

    Stressler, Bryan J.; Barnhart, William D.


    Earthquake clustering along plate boundaries suggests that earthquakes may interact, and static Coulomb stress change (CSC) is commonly invoked as one possible mechanism for stress transfer between earthquakes and nearby faults. Previous work has addressed the precision of CSC predictions that are influenced by observational noise, inversion regularization, and simplified modeling assumptions. Here we address the accuracy of CSC predictions informed by geodetic observations in subduction zones where inversion model resolution is poor. We conduct synthetic tests to quantify the degree to which the sign and magnitude of CSC can be reliably inferred from slip distributions inverted from various geodetic observations (interferometric synthetic aperture radar (InSAR), GPS, and seafloor observations). We find that in an idealized subduction zone, CSC can only be confidently inferred for receiver faults far (tens of kilometers) from the earthquake source, though this distance shortens with the addition of synthetic seafloor observations. We apply this methodology to the 2010 Mw8.8 Maule earthquake and identify 13 aftershocks from a population of 475 documented events for which we can confidently resolve coseismic stress changes. These results demonstrate that the low model resolution of fault slip inversions in subduction zones limits our ability to address fundamental questions about earthquake sources and stress interactions. Nonetheless, our results highlight that stress change predictions are considerably more accurate after the introduction of seafloor geodetic observations. Additionally, we show that InSAR observations are not required to substantially improve stress change approximations in regions where GPS may be the only viable observation, such as in island arcs settings.

  11. Deep subduction of hot young oceanic slab required by the Syros eclogites (United States)

    Flemetakis, Stamatis; Moulas, Evangelos; Kostopoulos, Dimitrios; Chatzitheodoridis, Elias


    The Cycladic islands of Syros and Siphnos, Aegean Sea, Greece, represent subducted IAT and BABB remnants of the Neotethyan Pindos Ocean. Garnet porphyroblasts (Ø=1mm) in a glaucophane-zoisite eclogite from Kini locality on Syros are compositionally zoned and display a unique prograde heating path from a high-pressure greenschist-facies core with high XSps and low Mg# via a blueschist-facies mantle with moderate XSps and Mg# to an eclogite-facies rim with low XSps and high Mg#. The outermost 35 μm of the garnet rims show flat XSps with rapidly increasing outwards Mg#. Na-Act-Chl-Ph rimmed by Gln mark the greenschist-blueschist facies transition, whereas Pg rimmed by Omp and the incoming of Rt at the expense of Ttn signify the blueschist-eclogite facies transition. Raman barometry of quartz inclusions in the eclogitic garnet rims coupled with elastic modelling of the garnet host [1], and Zr-in-Rt and Grt-Cpx-Ph thermobarometry revealed near-UHP P-T conditions of the order of 2.6 GPa/660°C (maximum residual pressure was 0.8-0.9GPa). By contrast, the greenschist-blueschist transition lies at ~0.75 GPa/355°C. This pressure is in excellent agreement with the position of the albite = jadeite + quartz boundary calculated at 350°C using the observed omphacite composition corrected for jadeite activity (Koons & Thompson, 1985) [2]. As a result, Cpx inclusions in garnet core signify the early entrance of garnet in the subduction zone history of the slab. Furthermore, the early growth of garnet (in lower pressures) observed in eclogites from Syros lies in great agreement with published slab-geotherms that indicate hot subduction and show a precocious garnet growth (Baxter and Caddick, 2013) [3]. The complete absence of lawsonite and the great abundance of zoisite crystals, based on the stability fields of both minerals (Poli et al., 2009) [4], further constrain the P-T trajectory of the slab. Our new P-T estimates match published T distributions on the slab surface

  12. Origin and dynamics of depositionary subduction margins (United States)

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


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

  13. Origin and dynamics of depositionary subduction margins (United States)

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


    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.

  14. Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation (United States)

    van Hinsbergen, Douwe J. J.; Vissers, Reinoud L. M.; Spakman, Wim


    The western Mediterranean recorded subduction rollback, slab segmentation and separation. Here we address the questions of what caused Oligocene rollback initiation, and how its subsequent evolution split up an originally coherent fore arc into circum-southwest Mediterranean segments. We kinematically reconstruct western Mediterranean geology from subduction initiation to present, using Atlantic plate reconstructions as boundary condition. We test possible reconstructions against remnants of subducted lithosphere imaged by seismic tomography. Transform motion between Africa and Iberia (including the Baleares) between 120 and 85 Ma was followed by up to 150 km convergence until 30 Ma. Subduction likely initiated along the transform fault that accommodated pre-85 Ma translation. By the 30 Ma inception of rollback, up to 150 km of convergence had formed a small slab below the Baleares. Iberia was disconnected from Sardinia/Calabria through the North Balearic Transform Zone (NBTZ). Subduction below Sardinia/Calabria was slightly faster than below the Baleares, the difference being accommodated in the Pyrenees. A moving triple junction at the trench-NBTZ intersection formed a subduction transform edge propagator fault between the Baleares and Calabria slab segments. Calabria rolled back eastward, whereas the Baleares slab underwent radial (SW-S-SE) rollback. After Kabylides-Africa collision, the western slab segment retreated toward Gibraltar, here reconstructed as the maximum rollback end-member model, and a Kabylides slab detached from Africa. Opening of a slab window below the NBTZ allowed asthenospheric rise to the base of the fore arc creating high-temperature metamorphism. Western Mediterranean rollback commenced only after sufficient slab-pull was created from 100 to 150 km of slow, forced subduction before 30 Ma.

  15. 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 (United States)

    Jean-Frederic, L.; DeMin, L.; Garrigou, J.; Münch, P.; Léticée, J. L.; Cornée, J. J.


    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.

  16. Observing mesoscale eddy effects on mode-water subduction and transport in the North Pacific. (United States)

    Xu, Lixiao; Li, Peiliang; Xie, Shang-Ping; Liu, Qinyu; Liu, Cong; Gao, Wendian


    While modelling studies suggest that mesoscale eddies strengthen the subduction of mode waters, this eddy effect has never been observed in the field. Here we report results from a field campaign from March 2014 that captured the eddy effects on mode-water subduction south of the Kuroshio Extension east of Japan. The experiment deployed 17 Argo floats in an anticyclonic eddy (AC) with enhanced daily sampling. Analysis of over 3,000 hydrographic profiles following the AC reveals that potential vorticity and apparent oxygen utilization distributions are asymmetric outside the AC core, with enhanced subduction near the southeastern rim of the AC. There, the southward eddy flow advects newly ventilated mode water from the north into the main thermocline. Our results show that subduction by eddy lateral advection is comparable in magnitude to that by the mean flow--an effect that needs to be better represented in climate models.

  17. Oblique subduction modelling indicates along-trench tectonic transport of sediments. (United States)

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


    Convergent plate margins are currently distinguished as 'accretional' or 'erosional', depending on the tendency to accumulate sediments, or not, at the trench. Accretion and erosion can coexist along the same margin and we have noticed that this mostly occurs where subduction is oblique. Here we show that at oblique subduction zones, sediments that enter the trench are first buried, and later migrate laterally parallel to the trench and at various depths. Lateral migration of sediments continues until they reach a physical barrier where they begin to accumulate. The accretionary wedge size decreases along the trench moving away from the barrier. We therefore suggest that the gradual variation of the accretionary wedge size and sediment amount at the trench along one single subduction zone, as observed in many active plate margins worldwide, can be explained by the lateral tectonic migration of sediments driven by obliquity of subduction as well.

  18. Dynamics of intraoceanic subduction initiation: 2D thermomechanical modeling (United States)

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


    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.

  19. SubductionGenerator: A program to build three-dimensional plate configurations (United States)

    Jadamec, M. A.; Kreylos, O.; Billen, M. I.; Turcotte, D. L.; Knepley, M.


    Geologic, geochemical, and geophysical data from subduction zones indicate that a two-dimensional paradigm for plate tectonic boundaries is no longer adequate to explain the observations. Many open source software packages exist to simulate the viscous flow of the Earth, such as the dynamics of subduction. However, there are few open source programs that generate the three-dimensional model input. We present an open source software program, SubductionGenerator, that constructs the three-dimensional initial thermal structure and plate boundary structure. A 3D model mesh and tectonic configuration are constructed based on a user specified model domain, slab surface, seafloor age grid file, and shear zone surface. The initial 3D thermal structure for the plates and mantle within the model domain is then constructed using a series of libraries within the code that use a half-space cooling model, plate cooling model, and smoothing functions. The code maps the initial 3D thermal structure and the 3D plate interface onto the mesh nodes using a series of libraries including a k-d tree to increase efficiency. In this way, complicated geometries and multiple plates with variable thickness can be built onto a multi-resolution finite element mesh with a 3D thermal structure and 3D isotropic shear zones oriented at any angle with respect to the grid. SubductionGenerator is aimed at model set-ups more representative of the earth, which can be particularly challenging to construct. Examples include subduction zones where the physical attributes vary in space, such as slab dip and temperature, and overriding plate temperature and thickness. Thus, the program can been used to construct initial tectonic configurations for triple junctions and plate boundary corners.

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

    NARCIS (Netherlands)

    Schellart, W. P.


    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

  1. Retrograde lawsonite formation in the Franciscan subduction complex (United States)

    Myers, S.; Mulcahy, S. R.


    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.

  2. The earthquake cycle in subduction zones (United States)

    Melosh, H. J.; Fleitout, L.


    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.

  3. Seismic coupling and uncoupling at subduction zones (United States)

    Ruff, L.; Kanamori, H.


    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.

  4. A numerical reference model for themomechanical subduction

    DEFF Research Database (Denmark)

    Quinquis, Matthieu; Chemia, Zurab; Tosi, Nicola


    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....

  5. Defining Incipient Subduction by Detecting Serpentenised Mantle in the Regional Magnetic Field (United States)

    Pires, Rui; Clark, Stuart; Reis, Rui


    Keywords: Subduction initiation, Incipient Subduction, Active Margins, Southeast Asia, Mantle wedge The mechanisms of subduction initiation are poorly understood. One idea is to look for incipient subduction zones in the present day and see what features are common in these zones. However, incipient subduction zones are very difficult to detect and debate surrounds particular cases as to whether they qualify as incipient or not. In the analysis conducted in this work, we use the signal of the presence of a mantle wedge in the magnetic anomaly field as an indicator of incipient subduction. Each subduction zone exhibits variations in the particular responses of the system, such as slab-dip angle, maximum earthquake depths and volcanism to various parameters. So far, attempts to reduce the system to a dominate controlling parameter have failed, probably as a result of the limited number of cases and the large variety of controlling parameters. Parameters such as down-going and overriding plate morphology and velocity, mantle flow, the presence of plumes or not, sediment transport into the trench are a few of the parameters that have been studied in the literature. However, one of the characteristics associated with a subduction zones is the presence of a mantelic wedge as a result of the partial melt of the subducting plate and the development of a mantle wedge between the subducting plate and the overriding plate. The wedge is characterised by the presence of water (coming from sediments in the down-going plate) as well as lower temperatures (because the wedge is between two relatively cold lithospheres). As a results a serpentinized mantle wedge is formed that contains hydrous minerals, of which magnetite is an example, that alter the composition and properties of this region. According to Blakely (2005), this region exhibits both higher magnetic susceptibility and lower densities than the surrounding medium. We analysed five active margin boundaries located

  6. IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

    Directory of Open Access Journals (Sweden)

    Sean Toczko


    Full Text Available The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay faultsystem—which may slip during plate boundary earthquakes—and initial drilling at Site C0011 (incoming oceanic plate for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and in situ measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide a the history of forearc basin development (including links to growth of the megasplay fault system and modern prism, b the first in situ hydrological measurements of the plate boundary hanging wall, and c integration of in situ stress measurements (orientation and magnitude across the forearc and with depth. A vertical seismic profile (VSP experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significantchanges in fault zone and hanging wall properties over short (<5 km along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatoryinstruments were tested for future operations.

  7. Thermobarometric and fluid expulsion history of subduction zones

    Energy Technology Data Exchange (ETDEWEB)

    Ernst, W.G. (Univ. of California, Los Angeles (United States))


    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).

  8. Deformation Patterns and Subduction Behavior of Continental Lithosphere Entering a Trench (United States)

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


    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

  9. Slow slip near the trench at the Hikurangi subduction zone, New Zealand. (United States)

    Wallace, Laura M; Webb, Spahr C; Ito, Yoshihiro; Mochizuki, Kimihiro; Hino, Ryota; Henrys, Stuart; Schwartz, Susan Y; Sheehan, Anne F


    The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world's largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist. Copyright © 2016, American Association for the Advancement of Science.

  10. Postglacial (after 18 ka) deep-sea sedimentation along the Hikurangi subduction margin (New Zealand): Characterisation, timing and origin of turbidites


    Pouderoux, Hugo; Proust, Jean-Noël; Lamarche, Geoffroy; Orpin, Alan; Neil, Helen


    International audience; Recent sedimentation along the Hikurangi subduction margin off northeastern New Zealand is investigated using a series of piston cores collected between 2003 and 2008. The active Hikurangi Margin lies along the Pacific-Australia subduction plate boundary and contains a diverse range of geomorphologic settings. Slope basin stratigraphy is thick and complex, resulting from sustained high rates of sedimentation from adjacent muddy rivers throughout the Quaternary. Turbidi...

  11. Diapir versus along-channel ascent of crustal material during plate convergence: Constrained by the thermal structure of subduction zones (United States)

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


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

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

    NARCIS (Netherlands)

    Schellart, W. P.


    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

  13. A Plate Tectonic Model for the Neoproterozoic with Evolving Plate Boundaries (United States)

    Merdith, Andrew; Collins, Alan; Williams, Simon; Pisarevsky, Sergei; Müller, Dietmar


    The Neoproterozoic was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana, during which, the planet experienced large climatic variations and the emergence of complex life. Here we present a topological plate model of the Neoproterozoic based on a synthesis of available geological and palaeomagnetic data. Subduction zones, which are well preserved in the geological record, are used as a proxy for convergent margins; evidence for mid-ocean ridges and transform motion is less clearly preserved, though passive margins are used as a proxy for spreading centres, and evidence for strike-slip motions are used to model transform boundaries. We find that the model presented here only predicts 70% of the total length of subduction active today, though it models similar lengths of both transform and divergent boundaries, suggesting that we have produced a conservative model and are probably underestimating the amount of subduction. Where evidence for convergent, divergent or transform motion is not preserved, we interpret the locations of plate boundaries based on the relative motions of cratonic crust as suggested through either palaeomagnetic data or the geological record. Using GPlates, we tie these boundaries together to generate a plate model that depicts the motion of tectonic plates through the Neoproterozoic. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, but tie them together due to similar Tonian aged accretionary patterns along their respective (present-day) north-western and northern margins, such that these two cratons act as a "lonely wanderer" for much of the Neoproterozoic, and form their own tectonic plate. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive

  14. Evidence for retrograde lithospheric subduction on Venus (United States)

    Sandwell, David T.; Schubert, Gerald


    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.

  15. Seismic Structure of the Subducted Cocos Plate (United States)

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


    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.

  16. 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...

  17. Thermal impact of magmatism in subduction zones (United States)

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


    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.

  18. Subduction trench migration since the Cretaceous (United States)

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


    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

  19. The “ophiolite rule”, chemostratigraphy of Teythan-type ophiolites and subduction initiation (United States)

    Whattam, S. A.; Stern, R. J.


    Characteristics of the classic late Jurassic and late Cretaceous Tethyan-type ophiolites of the Eastern Mediterranean - Persian Gulf region are consistent with formation in a supra-subduction zone (SSZ); the most robust model for their production entails formation upon subduction initiation in the forearc or proto-forearc region of a nascent intra-oceanic arc (IOA). The ‘ophiolite rule’ however, demonstrates that a co-existence of MOR-like (in addition to SSZ) lavas is the norm and not the exception. Specifically, the ‘normal’ chemostratigraphy of Tethyan-type ophiolites usually exhibits a thick, lower section of tholeiitic MOR-like lavas in addition to an upper section of dominantly calc-alkaline, HFSE-depleted, VAB-like lavas and intrusives and lesser boninites; latest stage dikes of boninitic affinity commonly cut both suites. This association testifies to changes in the ophiolitic melt source. To date, explanations for the near ubiquitous relationship of these two distinct tectonomagmatic suites usually infer that conventional MOR or marginal basin formation was followed by tectonic reorganization and consequent IOA construction above the (much older) first-formed MORB seafloor. While such ad hoc tectonic interpretations might apply to a few ophiolites, these explanations surely can’t explain what appears to be a global ophiolite phenomenon as this chemostratigraphy is now being recognized in other major ophiolitic belts (e.g., of the SW Pacific) and IOA forearcs (e.g., the Izu-Bonin-Mariana arc). A lack of hiatus between sequences is shown by boundaries between the two main ophiolitic units that are stratigraphic and not fault-bounded; there exists no significant temporal hiatus between formation of the lower tholeiitic and upper calc-alkaline suites (no sediment horizons or obvious unconformities); furthermore, lavas compositionally intermediate to MORB- and VAB-like occur stratigraphically between the sequences. We present a new model consistent

  20. Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

    Energy Technology Data Exchange (ETDEWEB)

    Van Keken, P.E. [Michigan Univ., Ann Arbor, MI (United States). Dept. of Earth and Environmental Sciences; Kita, S.; Nakajima, J. [Tohoku Univ., Sendai (Japan). Research Center for Prediction of Earthquakes and Volcanic Eruptions


    The cause of intermediate-depth (>40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The wellinstrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths >80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by timedependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity. (orig.)

  1. Metamorphic zirconology of continental subduction zones (United States)

    Chen, Ren-Xu; Zheng, Yong-Fei


    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

  2. Depth-dependent rupture mode along the Ecuador-Colombia subduction zone (United States)

    Yoshimoto, Masahiro; Kumagai, Hiroyuki; Acero, Wilson; Ponce, Gabriela; Vásconez, Freddy; Arrais, Santiago; Ruiz, Mario; Alvarado, Alexandra; Pedraza García, Patricia; Dionicio, Viviana; Chamorro, Orlando; Maeda, Yuta; Nakano, Masaru


    A large earthquake (Mw 7.7) occurred on 16 April 2016 within the source region of the 1906 earthquake in the Ecuador-Colombia subduction zone. The 1906 event has been interpreted as a megathrust earthquake (Mw 8.8) that ruptured the source regions of smaller earthquakes in 1942, 1958, and 1979 in this subduction. Our seismic analysis indicated that the spatial distribution of the 2016 earthquake and its aftershocks correlated with patches of high interplate coupling strength and was similar to those of the 1942 earthquake and its aftershocks, suggesting that the 2016 and 1942 earthquakes ruptured the same asperity. Our analysis of tsunami waveforms of the 1906 event indicated Mw around 8.4 and showed that large slip occurred near the trench off the source regions of the above three historical and the 2016 earthquakes, suggesting that a depth-dependent complex rupture mode exists along this subduction zone.

  3. Slow earthquakes linked along dip in the Nankai subduction zone. (United States)

    Hirose, Hitoshi; Asano, Youichi; Obara, Kazushige; Kimura, Takeshi; Matsuzawa, Takanori; Tanaka, Sachiko; Maeda, Takuto


    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.

  4. Effect of the Earth's rotation on subduction processes (United States)

    Levin, B. W.; Rodkin, M. V.; Sasorova, E. V.


    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.

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

    NARCIS (Netherlands)

    Schellart, Wouter P.


    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

  6. Stress orientations in subduction zones and the strength of subduction megathrust faults. (United States)

    Hardebeck, Jeanne L


    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.

  7. Stress orientations in subduction zones and the strength of subduction megathrust faults (United States)

    Hardebeck, Jeanne L.


    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.

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

    Zhang, Nan; Li, Zheng-Xiang


    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.

  9. Subduction and volatile recycling in Earth's mantle (United States)

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


    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.

  10. Earth's oldest mantle fabrics indicate Eoarchaean subduction. (United States)

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


    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.

  11. Necessity of the Ridge for the Flat Slab Subduction: Insights from the Peruvian Flat Slab (United States)

    Knezevic Antonijevic, S.; Wagner, L. S.; Beck, S. L.; Long, M. D.; Zandt, G.; Tavera, H.


    Flattening of the subducting plate has been linked to the formation of various geological features, 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]. However, the mechanism responsible for the slab flattening is still poorly understood. Here we focus on the Peruvian flat slab, where the Nazca plate starts to bend at ~80 km depth and travels horizontally for several hundred kilometers, at which point steep subduction resumes. Based on a 1500 km long volcanic gap and intermediate depth seismicity patterns, the Peruvian flat slab appears to have the greatest along-strike extent and, therefore, has been suggested as a modern analogue to the putative flat slab during the Laramide orogeny in the western United States (~80-55 Ma). Combining 3D shear wave velocity structure and Rayleigh wave phase anisotropy between ~10° and 18° S, we find that the subducting Nazca plate is not uniformly flat along the entire region, but fails to the north of the subducting Nazca Ridge. Our results show that, in combination with trench retreat, rapid overriding plate motion, and/or presence of a thick cratonic root, the subduction of buoyant overthickened oceanic crust, such as the Nazca Ridge, is necessary for the formation and sustainability of flat slabs. This finding has important implications for the formation of flat slabs both past and present.

  12. 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

  13. Subduction of hydrated basalt of the oceanic crust: Implications for recycling of water into the upper mantle and continental growth (United States)

    Rapp, R. P.


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

  14. Evidence for palaeo-Tethyan oceanic subduction within central Qiangtang, northern Tibet (United States)

    Liu, Yan; Santosh, M.; Zhao, Zhong Bao; Niu, Wen Chao; Wang, Gen Hou


    The mechanism of formation of blueschist-eclogite belts and their space-time distribution are important in understanding the tectonics associated with convergent plate boundaries. Here we investigate the garnet-bearing blueschists from Rongma area of central Qiangtang in northern Tibet. The mineral assemblage in these rocks is characterized by porphyroblastic garnet set within a matrix of fine-grained amphibole, white mica, epidote, chlorite, albite and quartz with accessory rutile, titanite and apatite. The garnet porphyroblasts exhibit core and rim portions, and the cores carry abundant inclusions of Na amphibole, quartz and rutile, as well as rhomb-shaped inclusions of paragonite and epidote which are interpreted as pseudomorphs after lawsonite. The rims are characterized by coarse-grained inclusions of epidote as well as the absence of paragonite and epidote aggregates, clearly suggesting that the transition from garnet core to rim marks a metamorphic transformation from lawsonite- to epidote-stability field. The Mn content of the garnet porphyroblasts decreases from core to rim, whereas the Fe and Mg contents show an increasing trend. In the matrix, we identify two stages of Na amphibole rimmed by Na-Ca amphibole and albite. Retrograde chlorite is rimmed by fine-grained biotite. Based on microstructural observations and pseudosection modelling, we trace the P-T path for the Rongma garnet blueschist from 1.92 GPa and 490 °C (lawsonite eclogite field) to about 1.68 GPa and 535 °C (epidote eclogite field), marking an initial increase in temperature and decrease in pressure. This stage is followed by a decrease of pressure through the blueschist facies down to P-T conditions of about 0.6 GPa and 530 °C. In combination with previous work including the available isotopic age data, the P-T path obtained in the present study suggests the deep subduction of palaeo-Tethyan oceanic crust between southern and northern Qiangtang blocks, supporting the model that the

  15. Thermal implications of the cessation of subduction in the Sierra Nevada and Baja- California arcs (United States)

    Erkan, K.; Blackwell, D. D.


    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

  16. Ground boundaries (United States)

    Balluffi, R. W.; Bristowe, P. D.

    The present document is a progress report describing the work accomplished on the study of grain boundaries in Ag, Au, Ni, Si, and Ge. Research was focused on the following four major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; grain boundary migration; short-circuit diffusion along grain boundaries; and development of Thin-Film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  17. The Elephants' Graveyard: Constraints from Mantle Plumes on the Fate of Subducted Slabs and Implications for the Style of Mantle Convection (United States)

    Lassiter, J. C.


    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 recycled material resides within or immediately above the thermo-chemical boundary layer(s) that generates mantle plumes. More recent Os- isotope studies of mantle xenoliths from OIB settings reveal the presence not only of recycled crust in mantle plumes, but also ancient melt-depleted harzburgite interpreted to represent ancient recycled oceanic lithosphere [1]. Thus, there is increasing evidence that subducted slabs accumulate in the boundary layer(s) that provide the source

  18. Mantle hydration and Cl-rich fluids in the subduction forearc (United States)

    Reynard, Bruno


    In the forearc region, aqueous fluids are released from the subducting slab at a rate depending on its thermal state. Escaping fluids tend to rise vertically unless they meet permeability barriers such as the deformed plate interface or the Moho of the overriding plate. Channeling of fluids along the plate interface and Moho may result in fluid overpressure in the oceanic crust, precipitation of quartz from fluids, and low Poisson ratio areas associated with tremors. Above the subducting plate, the forearc mantle wedge is the place of intense reactions between dehydration fluids from the subducting slab and ultramafic rocks leading to extensive serpentinization. The plate interface is mechanically decoupled, most likely in relation to serpentinization, thereby isolating the forearc mantle wedge from convection as a cold, potentially serpentinized and buoyant, body. Geophysical studies are unique probes to the interactions between fluids and rocks in the forearc mantle, and experimental constrains on rock properties allow inferring fluid migration and fluid-rock reactions from geophysical data. Seismic velocities reveal a high degree of serpentinization of the forearc mantle in hot subduction zones, and little serpentinization in the coldest subduction zones because the warmer the subduction zone, the higher the amount of water released by dehydration of hydrothermally altered oceanic lithosphere. Interpretation of seismic data from petrophysical constrain is limited by complex effects due to anisotropy that needs to be assessed both in the analysis and interpretation of seismic data. Electrical conductivity increases with increasing fluid content and temperature of the subduction. However, the forearc mantle of Northern Cascadia, the hottest subduction zone where extensive serpentinization was first demonstrated, shows only modest electrical conductivity. Electrical conductivity may vary not only with the thermal state of the subduction zone, but also with time for

  19. Geophysical detection of relict metasomatism from an Archean (approximately 3.5 Ga) subduction zone. (United States)

    Chen, Chin-Wu; Rondenay, Stéphane; Evans, Rob L; Snyder, David B


    When plate tectonics started on Earth has been uncertain, and its role in the assembly of early continents is not well understood. By synthesizing coincident seismic and electrical profiles, we show that subduction processes formed the Archean Slave craton in Canada. The spatial overlap between a seismic discontinuity and a conductive anomaly at approximately 100 kilometers depth, in conjunction with the occurrence of mantle xenoliths rich in secondary minerals representative of a metasomatic front, supports cratonic assembly by subduction and accretion of lithospheric fragments. Although evidence of cratonic assembly is rarely preserved, these results suggest that plate tectonics was operating as early as Paleoarchean times, approximately 3.5 billion years ago (Ga).

  20. Position and Life Span of Volcanic Arcs - What Link to Present-day Kinematic Parameters in Subduction Zones ? (United States)

    Schmidt, M. W.; Poli, S.


    Since slabs can be located at depth by geophysical methods, correlations are sought between the position of volcanic arcs above slabs and kinematic subduction parameters (subduction angle, velocity, burying rate...). However, the only (rough) correlation found so far, is between arc width and subduction angle, reflecting the projection of a slab depth-interval to a surface width. Volcanic arcs result from fluid and/or melt release from the slab (which occurs more or less continuously to >200 km depth), in most cases in combination with mantle wedge melting. Volcanic arcs may simply form above a zone with a sufficient extent of melting in the mantle and thus dominantly reflect the thermal structure in the mantle wedge which in turn mostly depends on mantle wedge convection. We argue that on geological time scales, pathways to actual (observable) subduction zone parameters are not unique as kinematic constraints in nature are fuzzy. Subduction systems do not converge to a steady state, implying that mantle wedge convection, the resulting thermal regime, loci of metamorphic reactions (i.e. fluid/melt release from the slab), and their surface manifestations, evolve continuously. As a consequence, present day kinematic parameters are not simply linked to the geometry of the volcanic arc (e.g. position of the volcanic front). An evolution of kinematic subduction parameters and their volcanic surface expression with time is documented in back arc basins which have an average life span of 11+/-6 Ma. About 48% of present intraoceanic subduction zones have or have had a Neozoic back arc and therefore they are highly dynamic systems strongly diverging from a steady state system with fixed kinematic boundaries. Other geologically short lived phenomena are aseismic ridge, oceanic plateau, and mid-ocean ridge subduction which modify buoyancy forces, slab geometry, and thermal structure within a subduction zone and thus cause changes in volcanic activity and melt type. In some

  1. Boundary issues (United States)

    Townsend, Alan R.; Porder, Stephen


    What is our point of no return? Caesar proclaimed 'the die is cast' while crossing the Rubicon, but rarely does modern society find so visible a threshold in our continued degradation of ecosystems and the services they provide. Humans have always used their surroundings to make a living— sometimes successfully, sometimes not (Diamond 2005)—and we intuitively know that there are boundaries to our exploitation. But defining these boundaries has been a challenge since Malthus first prophesied that nature would limit the human population (Malthus 1798). In 2009, Rockström and colleagues tried to quantify what the 6.8 billion (and counting) of us could continue to get away with, and what we couldn't (Rockström et al 2009). In selecting ten 'planetary boundaries', the authors contend that a sustainable human enterprise requires treating a number of environmental thresholds as points of no return. They suggest we breach these Rubicons at our own peril, and that we've already crossed three: biodiversity loss, atmospheric CO2, and disruption of the global nitrogen (N) cycle. As they clearly hoped, the very act of setting targets has provoked scientific inquiry about their accuracy, and about the value of hard targets in the first place (Schlesinger 2009). Such debate is a good thing. Despite recent emphasis on the science of human-ecosystem interactions, understanding of our planetary boundaries is still in its infancy, and controversy can speed scientific progress (Engelhardt and Caplan 1987). A few weeks ago in this journal, Carpenter and Bennett (2011) took aim at one of the more controversial boundaries in the Rockström analysis: that for human alteration of the global phosphorus (P) cycle. Rockström's group chose riverine P export as the key indicator, suggesting that humans should not exceed a value that could trigger widespread marine anoxic events—and asserting that we have not yet crossed this threshold. There are defensible reasons for a marine

  2. Shallow seismicity patterns in the northwestern section of the Mexico Subduction Zone (United States)

    Abbott, Elizabeth R.; Brudzinski, Michael R.


    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.

  3. Subduction zone forearc serpentinites as incubators for deep microbial life (United States)

    Plümper, Oliver; King, Helen E.; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P.; Rost, Detlef; Zack, Thomas


    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.

  4. Subduction zone forearc serpentinites as incubators for deep microbial life. (United States)

    Plümper, Oliver; King, Helen E; Geisler, Thorsten; Liu, Yang; Pabst, Sonja; Savov, Ivan P; Rost, Detlef; Zack, Thomas


    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.

  5. Stress and Strength of Seismogenic and Creeping Subduction Faults (Invited) (United States)

    Wang, K.; Bilek, S. L.; Wada, I.; Gao, X.; Brown, L.


    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

  6. Thermobarometric and fluid expulsion history of subduction zones (United States)

    Ernst, W. G.


    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

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

    NARCIS (Netherlands)

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


    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

  8. Numerical simulation of earthquake rupture sequences on the Manila thrust fault: Effects of seamount subduction (United States)

    Yu, H.; Liu, Y.; Ning, J.; He, C.; Zhang, L.


    The Manila subduction zone is located at the convergent boundary between the Philippine Sea Plate and the Sunda/Eurasian Plate from offshore Taiwan to northern Luzon of Philippines, where only infrequent M7 earthquakes were observed in modern seismological instrumentation history. The lack of great events (M8+) indicates the subduction fault is either aseismically slipping or is accumulating strain energy toward rapid release in a great earthquake. Here we conduct numerical simulations of earthquake rupture sequences in the framework of rate-state-friction along the 15-19.5ºN segment of the 3D plate boundary with subducted seamounts. Rate-state frictional properties are constrained by laboratory friction experiments conducted on IODP Expedition 349, South China Sea (SCS), drilling samples from the basaltic basement rock under 100ºC - 600ºC, effective normal stress of 50 MPa and pore pressure of 100 MPa. During the modeled 2000-year period, the maximum magnitude of earthquakes is Mw7. Each sequence repeats every ~200 years and is consisted of three sub-events, event 1 (Mw7) that can overcome the barrier, where dip angle changes most rapidly along the strike, to rupture the entire fault. Events 2 (Mw 6.4) and 3 (Mw 5.7) are of smaller magnitudes and result in north-south segmented rupture pattern. We further quantify the potential of earthquake nucleation by the S-ratio (lower S ratio means the initial stress is closer to peak strength, hence more likely to nucleate an earthquake). The subducted seamount shows higher S-ratios than its surroundings mostly, implying an unlikely nucleate area. Our results are qualitatively similar to 2D subduction earthquake modeling by Herrendörfer et al. (2015, 2-3 events per supercycle and median long-term S is 0.5-1). Finally, we plan to use our coseismic rupture model results as inputs for a tsunami propagation model in SCS. Compared to the kinematic seafloor deformation input, our physics-based earthquake source model and its

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


    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

  10. Long-term fore-arc basin evolution in response to changing subduction styles in southern Alaska (United States)

    Finzel, Emily S.; Enkelmann, Eva; Falkowski, Sarah; Hedeen, Tyler


    Detrital zircon U-Pb and fission track double-dating and Hf isotopes from the Mesozoic and Cenozoic strata in the southern Alaska fore-arc basin system reveal the effects of two different modes of flat-slab subduction on the evolution of the overriding plate. The southern margin of Alaska has experienced subduction of a spreading-ridge ( 62-50 Ma) and an oceanic plateau ( 40-0 Ma). When a subducting spreading ridge drives slab flattening, our data suggest that after the ridge has moved along strike retro-arc sediment sources to the fore arc become more predominant over more proximal arc sources. Spreading-ridge subduction also results in thermal resetting of rocks in the upper plate that is revealed by thermochronologic data that record the presence of young age peaks found in subsequent, thin sedimentary strata in the fore-arc basin. When a subducting oceanic plateau drives slab flattening, our data suggest that basin catchments get smaller and local sediment sources become more predominant. Crustal thickening due to plateau subduction drives widespread surface uplift and significant vertical uplift in rheologically weak zones that, combined, create topography and increase rock exhumation rates. Consequently, the thermochronologic signature of plateau subduction has generally young age peaks that generate short lag times indicating rapid exhumation. The cessation of volcanism associated with plateau subduction limits the number of syndepositional volcanic grains that produce identical geochronologic and thermochronologic ages. This study demonstrates the merit of double-dating techniques integrated with stratigraphic studies to expose exhumational age signatures diagnostic of large-scale tectonic processes in magmatic regions.

  11. Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska (United States)

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


    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

  12. Modeling of GPS velocities across the Ganges-Brahmaputra Delta - Burma Arc oblique subduction system (United States)

    Steckler, M. S.; Mondal, D. R.; Akhter, S. H.; Seeber, L.; Feng, L.; Gale, J.; Howe, M.; Masson, F.; Maurin, T.; Rangin, C.


    The Burma Arc is the northward continuation of the Sumatra-Anadaman subduction zone that gave rise to the 2004 M9.3 earthquake and tsunami. Near its northern end, it is colliding with the thick sediments of the Bengal Basin. The sediments of the Ganges-Brahmaputra Delta are folded and faulted, creating a subaerial 250-km wide accretionary prism. The deformation front is blind and reaches ½ way across the delta. Whether subduction is still occurring at this highly oblique plate boundary has been hotly debated. To investigate this, we combined our 25 continuous GPS receivers in Bangladesh with the campaign network in Myanmar, processing them together with GAMIT/GLOBK. We combined this data with 28 mostly campaign GPS sites in India (Gahalaut et al., 2012) by processing using the same IGS sites and performing a Helmert transformation to place all the data in same reference frame within ITRF2008. Published Indian plate poles yield a systematic residual for Bangladeshi sites believed to be on stable India. We estimated a new pole by combining 13 Indian stations (Mahesh et al., 2013) with 2 of our stations. Due to the rapid growth of the accretionary prism and the overthrusting by the Shillong Plateau, the Burma Arc changes shape from the backstop to the front folds. To project the GPS velocities into a profile, we have experimented with several projections that vary across the foldbelt, using the arc of the earthquakes and the topography as a guide. Strike-slip is mostly absorbed by the Sagaing and CMF faults, and some additional shear is distributed over the region. The best fitting suite of models for the shortening component yield 13-15 mm/y of shortening across the arc on a shallow-dipping megathrust. Additional shortening of ~4 mm/y is absorbed in the vicinity of the Kabaw Fault, where coverage is sparse. The dip of the megathrust and the depth of its downdip end trade off with one another in the different projections. The low range of dips of 6-10° is consistent

  13. The Melt Segregation During Ascent of Buoyant Diapirs in Subduction Zones (United States)

    Zhang, N.; Behn, M. D.; Parmentier, E. M.; Kincaid, C. R.


    Cold, low-density diapirs arising from hydrated mantle and/or subducted sediments on the top of subducting slabs may transport key chemical signatures from the slab to the shallow source region for arc magmas. These chemical signatures are strongly influenced by melting of this buoyant material during its ascent. However, to date there have been relatively few quantitative models to constrain melting and melt segregation in an ascending diapir, as well as the induced geochemical signature. Here, we use a two-phase Darcy-Stokes-energy model to investigate thermal evolution, melting, and melt segregation in buoyant diapirs as they ascend through the mantle wedge. Using a simplified 2-D axi-symmetric circular geometry we investigate diapir evolution in three scenarios with increasing complexity. First, we consider a case without melting in which the thermal evolution of the diapir is controlled solely by thermal diffusion during ascent. Our results show that for most cases (e.g., diapir radius ≤ 3.7 km and diapir generation depths of ~ 75 km) thermal diffusion times are smaller than the ascent time—implying that the diapir will thermal equilibrate with the mantle wedge. Secondly, we parameterize melting within the diapir, but without melt segregation, and add the effect of latent heat to the thermal evolution of the diapir. Latent heat significantly buffers heating of the diapir. For the diapir with radius ~3.7 km, the heating from the outside is slowed down ~30%. Finally, we include melt segregation within the diapir in the model. Melting initiates at the boundaries of the diapir as the cold interior warms in response to thermal equilibration with the hot mantle wedge. This forms a high porosity, high permeability rim around the margin of the diapir. As the diapir continues to warm and ascend, new melts migrate into this rim and are focused upward, accumulating at the top of the diapir. The rim thus acts like an annulus melt channel isolating the central part of

  14. An updated digital model of plate boundaries (United States)

    Bird, Peter


    A global set of present plate boundaries on the Earth is presented in digital form. Most come from sources in the literature. A few boundaries are newly interpreted from topography, volcanism, and/or seismicity, taking into account relative plate velocities from magnetic anomalies, moment tensor solutions, and/or geodesy. In addition to the 14 large plates whose motion was described by the NUVEL-1A poles (Africa, Antarctica, Arabia, Australia, Caribbean, Cocos, Eurasia, India, Juan de Fuca, Nazca, North America, Pacific, Philippine Sea, South America), model PB2002 includes 38 small plates (Okhotsk, Amur, Yangtze, Okinawa, Sunda, Burma, Molucca Sea, Banda Sea, Timor, Birds Head, Maoke, Caroline, Mariana, North Bismarck, Manus, South Bismarck, Solomon Sea, Woodlark, New Hebrides, Conway Reef, Balmoral Reef, Futuna, Niuafo'ou, Tonga, Kermadec, Rivera, Galapagos, Easter, Juan Fernandez, Panama, North Andes, Altiplano, Shetland, Scotia, Sandwich, Aegean Sea, Anatolia, Somalia), for a total of 52 plates. No attempt is made to divide the Alps-Persia-Tibet mountain belt, the Philippine Islands, the Peruvian Andes, the Sierras Pampeanas, or the California-Nevada zone of dextral transtension into plates; instead, they are designated as "orogens" in which this plate model is not expected to be accurate. The cumulative-number/area distribution for this model follows a power law for plates with areas between 0.002 and 1 steradian. Departure from this scaling at the small-plate end suggests that future work is very likely to define more very small plates within the orogens. The model is presented in four digital files: a set of plate boundary segments; a set of plate outlines; a set of outlines of the orogens; and a table of characteristics of each digitization step along plate boundaries, including estimated relative velocity vector and classification into one of 7 types (continental convergence zone, continental transform fault, continental rift, oceanic spreading ridge

  15. Seamount subduction underneath an accretionary wedge: modelling mass wasting and wedge collapse (United States)

    Mannu, Utsav; Ueda, Kosuke; Willett, Sean; Gerya, Taras; Strasser, Michael


    Seamounts (h >1 km) and knolls (h = 500 m-1000 m) cover about one-fifth of the total ocean floor area. These topographical highs of the ocean floor eventually get subducted. Subduction of these topographical features leads to severe deformation of the overriding plate and can cause extensive tectonic erosion and mass wasting of the frontal prism, which can ultimately cause a forearc wedge collapse. Large submarine landslides and the corresponding wedge collapse have previously been reported, for instance, in the northern part of the Hikurangi margin where the landslide is known as the giant Ruatoria debris avalanche, and have also been frequently reported in several seismic sections along the Costa Rica margin. Size and frequency relation of landslides suggest that the average size of submarine landslides in margins with rough subducting plates tends to be larger. However, this observation has not yet been tested or explained by physical models. In numerical subduction models, landslides take place, if at all, on a much larger timescale (in the order of 104-105 years, depending on the time steps of the model) than in natural cases. On the other hand, numerical models simulating mass wasting events such as avalanches and submarine landslides, typically model single events at a much smaller spatio-temporal domain, and do not consider long-term occurrence patterns of freely forming landslides. In this contribution, we present a multi-scale nested numerical approach to emulate short-term landslides within long-term progressive subduction. The numerical approach dynamically produces instantaneous submarine landslides and the resulting debris flow in the spatially and temporally refined inner model. Then we apply these convoluted changes in topography (e.g. due to the submarine landslide etc.) back to an outer larger-scale model instance that addresses wedge evolution. We use this approach to study the evolution of the accretionary wedge during seamount subduction.

  16. Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa. (United States)

    Moyen, Jean-François; Stevens, Gary; Kisters, Alexander


    Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0-2.5 Gyr ago) are disputed. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 degrees C km(-1) or less, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients are greater than 25 degrees C km(-1)). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites and circumstantial evidence for subduction processes, including possible accretion-related structures, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history. Here we report garnet-albite-bearing mineral assemblages that record pressures of 1.2-1.5 GPa at temperatures of 600-650 degrees C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12-15 degrees C-similar to those found in recent subduction zones-that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.

  17. Slab1.0: A three-dimensional model of global subduction zone geometries (United States)

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


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

  18. Progressive migration of slab break-off along the southern Tyrrhenian plate boundary: Constraints for the present day kinematics (United States)

    Chiarabba, Claudio; Palano, Mimmo


    The Ionian subduction in the central Mediterranean, just 200 km wide, is one of the narrowest in the world. Its evolution has involved a progressive disruption of the subducting slab, contemporaneous to the retreat and step-wise opening of back-arc basins. In this study, we analyse velocity anomalies of the upper mantle, together with the most comprehensive set of earthquake locations and kinematic indicators available for Italy, to reconstruct the geodynamics and tectonic evolution of the Ionian subduction system. Along the Sicilian boundary, we identify an eastward migration of the slab edge with detachment of the Ionian oceanic lithosphere. We hypothesize that the progressive detachment of the slab took place along lithospheric transform faults of the Neo-Tethys Ocean. Among the main active kinematic elements of the Ionian accretionary wedge, we suggest that a ∼400-km-long and highly segmented shear zone formed by the Aeolian-Tindari-Letojanni fault system and the Ionian fault represents the surface expression of such a lithospheric tearing. The present day convergence between the Eurasian and African plates is accommodated both at the frontal thrust of the flexed Hyblean margin in southern Sicily and offshore along the Tyrrhenian Sea. Lithospheric bending favors the wedging of the mantle underneath northern Sicily, while magmatic fluids are channeled along slab tears.

  19. Shear wave splitting and the dynamics of the hydrated mantle wedge in subduction regions constrained by the example of the Ryukyu subduction zone (United States)

    Nagaya, T.; Walker, A.; Wookey, J. M.; Wallis, S.; Ishii, K.; Kendall, J. M.


    H2O-rich subduction fluids are a key component of convergent plate margin dynamics, essential to earthquake initiation and magma formation. These fluids in the wedge mantle are dominantly derived from antigorite dragged down by plate motion. However, the accurate distribution of antigorite-rich serpentinite related to the fluid transport in subduction zones has thus far been difficult to determine. Our approach is to model the S-wave splitting of the Ryukyu arc in order to constrain the distribution, amount and orientation of antigorite, while taking into account the geometry of seismic ray paths and the elastic anisotropy of deformed antigorite-bearing mantle. We have also carried out a full assessment of uncertainties associated with our analysis including time delay estimates from the seismic waves themselves, crustal anisotropy, averaging schemes for CPO, and the strength of antigorite CPO patterns. The results suggest the presence of a large-scale flow in the hydrous mantle with a low viscosity and more than 54% of this domain consists of antigorite. Other geophysical observations in the forearc mantle including the low seismic velocity and gravity anomaly are also compatible with our inference of the presence of induced flow in an antigorite-rich, hydrated mantle wedge in the Ryukyu arc. We have also constructed a geodynamic model to examine flow patterns in the hydrated shallow wedge mantle using the distribution and proportion of serpentinite derived from our seismic model and subduction parameters that are close to those of the arc. The results clearly show that convection occurs in the serpentinized mantle wedge and that this domain is associated with a low surface heat flow. S-wave splitting observations in other subduction zones implies this large-scale serpentinization and hydrous mantle flow is likely to be more widespread than generally recognized and the view that the forearc mantle of cold subduction zones lacks significant zones of hydration needs

  20. Volcanism and Subduction: The Kamchatka Region (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.

  1. Regional differences in subduction ground motions

    CERN Document Server

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


    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...

  2. Ground boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.


    The present document is a progress report describing the work accomplished on the study of grain boundaries in Ag, Au, Ni, Si, and Ge. Research was focused on the following four major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; grain boundary migration; short-circuit diffusion along grain boundaries; and development of Thin-Film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals. 10 refs., 1 fig.

  3. Long-term Ocean Bottom Monitoring for Shallow Slow Earthquakes in the Hyuga-nada, Nankai Subduction Zone (United States)

    Yamashita, Y.; Shinohara, M.; Yamada, T.; Nakahigashi, K.; Shiobara, H.; Mochizuki, K.; Maeda, T.; Obara, K.


    The Hyuga-nada region, nearby the western end of the Nankai Trough in Japan, is one of the most active areas of shallow slow earthquakes in the world. Recently, ocean-bottom observation of offshore seismicity near the trench succeeded in detecting shallow tremor. The observed traces contained a complete episode lasting for one month exhibiting similar migration property of deep tremor [Yamashita et al., 2015]. This activity was associated with shallow very-low-frequency earthquake (VLFE) activity documented by land-based broadband seismic network. The coincidence between tremor and VLFE activities and similarity of their migration pattern show strong resemblance with the episodic tremor and slip episodes; this similarity suggests that the tremor activity in the shallow plate boundary may also be coupled with VLFE and short-term slow slip events in this area. It is important clarifying the seismicity including slow earthquakes to understand the slip behavior at a shallow plate boundary, and to improve assessments of the possibility of tsunamigenic megathrust earthquake that is anticipated to occur at the Nankai Trough. Motivated by these issues, we started long-term ocean-bottom monitoring in this area from May 2014 using 3 broadband and 7 short-period seismometers. In January 2015, we replaced the instruments and obtained the first data which includes minor shallow tremor and VLFE activity on June 1-3, 2014. Preliminary results of data processing show that the shallow tremor activity occurred at the northwestern part of the 2013 activity. The location corresponds the point where the tremors stopped migrating to further north direction and turned sharply eastward in the 2013 activity. On the other hand, clear tremor migration was not found in the 2014 activity. This local activity may imply that regional/small-scale heterogeneous structures such as a subducting sea mount affect the activity pattern. During the 2014 observation, many ordinary earthquakes also

  4. Introduction to the structures and processes of subduction zones (United States)

    Zheng, Yong-Fei; Zhao, Zi-Fu


    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

  5. Structural context and variation of ocean plate stratigraphy, Franciscan Complex, California: insight into mélange origins and subduction-accretion processes (United States)

    Wakabayashi, John


    the clastic OPS component. Such tectonic mélanges may include blocks and matrix derived from the olistostromes. Franciscan subduction and OPS accretion initiated in island arc crust at about 165-170 Ma, after which MORB and OIB were subducted and accreted following a long (tens of mega-ampere) gap with little or no accretion. Following subduction initiation, a ridge crest approached the trench but probably went dormant prior to its subduction (120-125 Ma), after which the subducted oceanic crust became progressively older until about 95 Ma. From 95 Ma, the age of subducted oceanic crust decreased progressively until arrival of the Pacific-Farallon spreading center led to termination of subduction and conversion to a transform plate boundary.

  6. Great earthquakes hazard in slow subduction zones (United States)

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


    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

  7. Subduction of oceanic plate irregularities and seismicity distribution along the Mexican Subduction Zone (United States)

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


    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.

  8. Velocities of Subducted Sediments and Continents (United States)

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


    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

  9. From transpressional to transtensional tectonics in Northern Central America controlled by Cocos - Caribbean subduction coupling change (United States)

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


    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.

  10. Negotiating boundaries

    DEFF Research Database (Denmark)

    Aarhus, Rikke; Ballegaard, Stinne Aaløkke


    to maintain the order of the home when managing disease and adopting new healthcare technology. In our analysis we relate this boundary work to two continuums of visibility-invisibility and integration-segmentation in disease management. We explore five factors that affect the boundary work: objects......To move treatment successfully from the hospital to that of technology assisted self-care at home, it is vital in the design of such technologies to understand the setting in which the health IT should be used. Based on qualitative studies we find that people engage in elaborate boundary work......, activities, places, character of disease, and collaboration. Furthermore, the processes are explored of how boundary objects move between social worlds pushing and shaping boundaries. From this we discuss design implications for future healthcare technologies for the home....

  11. Boundary Spanning

    DEFF Research Database (Denmark)

    Zølner, Mette

    The paper explores how locals span boundaries between corporate and local levels. The aim is to better comprehend potentialities and challenges when MNCs draws on locals’ culture specific knowledge. The study is based on an in-depth, interpretive case study of boundary spanning by local actors...... in the period of post-acquisition when their organization is being integrated into the acquiring MNC. The paper contributes to the literature on boundary spanning in three ways: First, by illustrating that boundary spanning is performed by numerous organizational actors in a variety of positions in MNCs......, inclusively by locals in subsidiaries. Second, by showing that boundary spanning is ‘situated’ in the sense that its result depends on the kind of knowledge to be transmitted and the attitude of the receivers. A third contribution is methodological. The study illustrates that combining bottom-up grounded...

  12. Control of high oceanic features and subduction channel on earthquake ruptures along the Chile-Peru subduction zone (United States)

    Contreras-Reyes, Eduardo; Carrizo, Daniel


    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

  13. Sharp Permeability Transitions due to Shallow Diagenesis of Subduction Zone Sediments (United States)

    James, S.; Screaton, E.


    The permeability of hemipelagic sediments is an important factor in fluid flow in subduction zones and can be affected by porosity changes and cementation-dissolution processes acting during diagenesis. Anomalously high porosities have been observed in cores from the Shikoku Basin sediments approaching the Nankai Trough subduction zone. These high porosities have been attributed to the presence of minor amounts of amorphous silica cement that strengthen the sediment and inhibit consolidation. The porosity rapidly drops from 66-68% to 54-56% at a diagenetic boundary where the amorphous silica cement dissolves. Although the anomalous porosity profiles at Nankai have received attention, the magnitude of the corresponding permeability change has not been addressed. In this study, permeability profiles were constructed using permeability-porosity relationships from previous studies, to estimate the magnitude and rate of permeability changes with depth. The predicted permeability profiles for the Nankai Trough sediment cores indicate that permeability drops by almost one order of magnitude across the diagenetic boundary. This abrupt drop in permeability has the potential to facilitate significant changes in pore fluid pressures and thus to influence the deformation of the sediment onto the accretionary prism. At the Costa Rica subduction zone, results vary with location. Site U1414 offshore the Osa Peninsula shows porosities stable at 69% above 145 mbsf and then decrease to 54% over a 40 m interval. A porosity drop of that magnitude is predicted to correlate to an order of magnitude permeability decrease. In contrast, porosity profiles from Site 1039 offshore the Nicoya Peninsula and Site U1381 offshore the Osa Peninsula show anomalously high porosities but no sharp drop. It is likely that sediments do not cross the diagenetic boundary due to the extremely low (<10°C/km) thermal gradient at Site 1039 and the thin (<100 m) sediment cover at Site U1381. At these locations

  14. 3-D imaging of two episodes of Hikurangi Plateau subduction in the southern South Island of New Zealand (United States)

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


    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.

  15. 3D receiver function Kirchhoff depth migration image of Cascadia subduction slab weak zone (United States)

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


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

  16. P-wave tomography of subduction zones around the central Philippines and its geodynamic implications (United States)

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


    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.

  17. Diapiric flow at subduction zones: a recipe for rapid transport. (United States)

    Hall, P S; Kincaid, C


    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.

  18. Coherent tilt signals observed in the Shumagin seismic gap Detection of time-dependent subduction at depth? (United States)

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


    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.

  19. Plume-induced roll back subduction around Venus large coronae (United States)

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


    On Venus, possible subduction trenches are mainly associated with large coronae, eventhough the latter are thought to be produced by hot mantle plumes. The mechanism of assocation between subduction and plume has long remained elusive. However, we recently observe the same association in laboratory experiments on thermal convection in colloidal aqueous dispersions of silica nanoparticles, which deform in the Newtonian regime at low solid particle fraction φp, and transition to strain-rate weakening, plasticity, elasticity, and brittle properties as φp increases. Hence, a dense skin akin to a planetary lithosphere grows on the surface when the system is dried from above. When a hot plume rises under the skin, the latter undergoes a flexural deformation which puts it under tension. Cracks then develop, sometimes using pre-existing weaknesses. Plume material (being more buoyant that the laboratory lithosphere) upwells through the cracks and spreads as a axisymmetric gravity current above the broken denser skin. The latter bends and sinks under the conjugate action of its own weight and the plume gravity current. The brittle character of the top experimental lithosphere forbids it to deform viscously to accomodate the sinking motions. Instead, the plate continues to tear as a sheet of paper would do upon intrusion. Several slabs are therefore produced, associated with trenches localized along partial circles on the plume, and strong roll-back is always observed. Depending on the lithospheric strength, roll-back can continue and triggers a complete resurfacing, or it stops when the plume stops spreading. Scalings derived from the experiments suggest that a weaker lithosphere than that present on Earth today is required for such a convective regime. We identified two candidates on Venus. At Artemis and Quetzelpetlatl Coronae, the radar image observations and subsurface density variations inferred from modeling the gravity and topography agree with the predictions from

  20. The temporal evolution of a subducting plate in the lower mantle (United States)

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


    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

  1. Investigation on subduction erosion of the Central Costa Rica margin with seismic wide- angle data (United States)

    Zhu, J.; Flueh, E. R.; Kopp, H.; Klaeschen, D.


    Seismic wide-angle investigations along the Pacific margin off Central Costa Rica were carried out using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines, intersecting at the mid slope. The structure and the P-wave velocities of the subducted oceanic Cocos Plate and overriding Carribean Plate were determined by modeling the wide-angle seismic data combined with the analysis of coincident reflection seismic data and the use of synthetic seismograms. Detailed velocity-depth distributions of two dip-lines and two strike-lines on the continental slope will be presented. Below the slope sediment, a wedge-shaped body, the margin wedge is defined by high velocities (4.3-6.1 km/s). This wedge shows a high velocity gradient zone in the uppermost one to two km, underlain by a low velocity gradient to the plate boundary. Between the subducted plate and overriding plate the low velocity zone including a lense-type structure is seen. This Megalens (4.0-4.3 km/s) and the subducted sediment comprise a low velocity zone (LVZ) all along the plate boundary. This LVZ is constrained by joint analysis of reflection seismic data and wide-angle data. The thickness of the wedge varies along the strike, this is associated with the subduction of the extension of Quepos Plateau, which also resulted in uplift of the margin. The extensional forearc environment is manifested by the normal faults indicated on the the multi-channel seismic (MCS) data. The Megalens is most probably comprised of material transferred from upper margin wedge at the tip of the wedge. The velocity structure within the Megalense resembles the velocities at the tip of the wedge, and is clearly lower than the oceanic crust, but higher thn subducted sediment. If this interpretation is valid, this material has been transported 16 km landward, which implies it was detached from the upper plate 0.2 Ma ago.

  2. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone. (United States)

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


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

  3. Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab. (United States)

    Kawamoto, Tatsuhiko; Yoshikawa, Masako; Kumagai, Yoshitaka; Mirabueno, Ma Hannah T; Okuno, Mitsuru; Kobayashi, Tetsuo


    Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure-temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

  4. Grain boundaries

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.


    The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990--February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: Study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  5. Grain boundaries (United States)

    Balluffi, R. W.; Bristowe, P. D.

    The present document is a progress report describing the work accomplished to date during the second year of our four-year grant (February 15, 1990 to February 14, 1994) to study grain boundaries. The research was focused on the following three major efforts: study of the atomic structure of grain boundaries by means of x-ray diffraction, transmission electron microscopy and computer modeling; study of short-circuit diffusion along grain boundaries; and development of a Thin-film Deposition/Bonding Apparatus for the manufacture of high purity bicrystals.

  6. Hypnosis, suggestion, and suggestibility: an integrative model. (United States)

    Lynn, Steven Jay; Laurence, Jean-Roch; Kirsch, Irving


    This article elucidates an integrative model of hypnosis that integrates social, cultural, cognitive, and neurophysiological variables at play both in and out of hypnosis and considers their dynamic interaction as determinants of the multifaceted experience of hypnosis. The roles of these variables are examined in the induction and suggestion stages of hypnosis, including how they are related to the experience of involuntariness, one of the hallmarks of hypnosis. It is suggested that studies of the modification of hypnotic suggestibility; cognitive flexibility; response sets and expectancies; the default-mode network; and the search for the neurophysiological correlates of hypnosis, more broadly, in conjunction with research on social psychological variables, hold much promise to further understanding of hypnosis.

  7. Metastability of Subducted Slabs in the Mantle Transition Zone: A Collaborative Geodynamic, Petrologic, and Seismological Approach (United States)

    Garber, J. M.; Billen, M. I.; Duncan, M. S.; Roy, C.; Ibourichene, A. S.; Olugboji, T.; Celine, C.; Rodríguez-González, J.; Grand, S. P.; Madrigal, P.; Sandiford, D.; Valencia-Cardona, J. J.


    Subducted slabs exhibit a range of geometries in the mantle transition zone. Studies of this phenomenon suggest that olivine and/or pyroxene metastability may profoundly alter the slab density profile, leading to slab flattening (e.g., King et al., 2015) and potentially yielding a resolvable seismological signature (e.g., Kawakatsu and Yoshioka, 2011; Yoshioka et al., 2015). Such metastability may also be critical for deep earthquake generation. Geodynamic modelling of this process is typically done with a simplified petrologic model of the downgoing slab, whereas petrologic studies of phase assemblages in subducted slabs typically impose an idealized geodynamic model with an unrealistic thermal structure. Connecting these two approaches should lead to a better understanding of the consequences of metastable assemblages on subducting slabs. Here, we present a new methodology that combines geodynamic, seismic and petrologic approaches to assess the impact of mineral metastability on dynamic subduction models, developed in a collaborative effort begun at the 2016 NSF CIDER summer program in Santa Barbara, CA. We use two parallel approaches to extrapolate equilibrium rock properties to metastable regions and impose these data on extracted time-slices from robust thermo-mechanical geodynamic models, allowing us to quantify the density and buoyancy changes in the slab that result from considering metastable phase assemblages. Our preliminary results suggest that metastable assemblages can yield a 10-30% density decrease over the subducted slab relative to an equilibrium reference model. We then generate a seismic velocity profile of the slab, and compute waveforms based on the 2D finite-difference method (e.g., Vidale & Helmberger, 1987) to determine whether metastable phases could reasonably be detected by different seismic approaches. Continuing analyses will be aimed at coupling the evolution of geodynamic models with phase metastability to model the feedback between

  8. Ablative subduction - A two-sided alternative to the conventional subduction model (United States)

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


    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.

  9. The melting of subducted banded iron formations (United States)

    Kang, Nathan; Schmidt, Max W.


    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.

  10. Organic matter cracking: A source of fluid overpressure in subducting sediments (United States)

    Raimbourg, Hugues; Thiéry, Régis; Vacelet, Maxime; Famin, Vincent; Ramboz, Claire; Boussafir, Mohammed; Disnar, Jean-Robert; Yamaguchi, Asuka


    The pressure of deep fluids in subduction zones is a major control on plate boundary strength and earthquake genesis. The record, by methane-rich fluid inclusions, of large ( 50-100 MPa) and instantaneous pressure variations in the Shimanto Belt (Japan) points to the presence of large fluid overpressure at depth (300-500 MPa, 250 °C). To further analyze the connection between methane and fluid overpressure, we determined with Rock-Eval the potential for a worldwide selection of deep seafloor sediments to produce methane as a result of organic matter (OM) cracking due to temperature increase during subduction. The principal factor controlling the methanogenesis potential of sediments is OM proportion, while OM nature is only a subordinate factor. In turn, OM proportion is mainly controlled by the organic terrigenous input. Considering a typical sediment from ocean-continent subduction zones, containing 0.5 wt% of type III OM, cracking of OM has two major consequences: (1) Methane is produced in sufficient concentration as to oversaturate the pore-filling water. The deep fluid in accretionary prisms is therefore a mechanical mixture of water-rich and methane-rich phases; (2) CH4 production can generate large fluid overpressure, of the order of several tens of MPa, The conditions for these large overpressure are a low permeability of the upper plate (z > 10 km) where OM thermal cracking occurs. At these depths, OM thermal cracking appears as a source of overpressure larger than the last increments of smectite-to-illite reaction. Such large overpressures play potentially a role in facilitating slip along the plate interface. Conversely, the scarcity of earthquakes in ocean-ocean subduction zones such as Marianna or Barbados may be related to the low influx of detrital OM and the limited methane/overpressure generation at depth.

  11. Fluid accumulation along the Costa Rica subduction thrust and development of the seismogenic zone (United States)

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


    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.

  12. Earth's first stable continents did not form by subduction. (United States)

    Johnson, Tim E; Brown, Michael; Gardiner, Nicholas J; Kirkland, Christopher L; Smithies, R Hugh


    The geodynamic environment in which Earth's first continents formed and were stabilized remains controversial. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite-trondhjemite-granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks; notably, these TTGs have 'arc-like' signatures of trace elements and thus resemble the continental crust produced in modern subduction settings. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust that is predicted to have existed if Archaean mantle temperatures were much hotter than today's. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG 'parents', and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents-coupled with the high geothermal gradients-is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust. Thus subduction was not required to produce TTGs in the early Archaean eon.

  13. MORB to supra-subduction geochemical transition in the extrusive sequences of major upper Cretaceous ophiolites of Iran (United States)

    Babaie, H. A.; Khalatbari Jafari, M.; Moslempour, M. E.


    We discuss the geochemical patterns and tectonomagmatic setting of the extrusive sequences in the Khoy, Kermanshah, Fannuj, Nosratabad, Dehshir, south and north Fariman, and Sabzevar ophiolite massifs of Iran. These sequences include pillow lava, sheet flow, hyaloclastite, hyaloclastic breccia, and interbeds of chert and pelagic limestone with Late Cretaceous micro fauna. The Khoy, north Fariman, and Sabzevar massifs also include Late Cretaceous-Early Paleocene supra-ophiolitic volcanic and volcano-sedimentary rocks that formed in a trough near the extrusive sequence. The Khoy pillow lava displays T-MORB characteristics but no chemical contribution from the components released from the subducted slab. On the other hand, the diabase dikes that cut the Khoy extrusive sequence show signatures of subduction zone magmatism and contribution from the melt released through the partial melting of the subducted slab. While lava in the Harsin (Kermanshah) extrusive sequence in west Iran displays E-MORB and P-MORB characteristics, the pillows in the Fannuj, north Fariman, Dehshir, and Sabzevar extrusive sequences indicate the contribution of both fluids and melt from the subducted slab. The Nosratabad and south Fariman ophiolites also show evidence for either melt or fluids, respectively. Partial melting of the subducted slab sedimentary cover may have formed the acidic pillow lava and sheet flow in the Fannuj and Nosratabad extrusive sequence, respectively. Some pillows in the Nosratabad, Sabzevar, north Fariman, and to a lesser extent, Dehshir extrusive sequence display the OIB geochemical characteristics. Mantle plumes or asthenospheric flow that probably moved up through weak zones of the subducted slab may have affected the partial melting of the mantle wedge above the slab. The combined OIB and supra-subduction characteristics suggest the role of the roll-back of the subducted slab in the magmatism of the northeast Iranian ophiolites. The clear MORB-like geochemical

  14. Melting carbonated epidote eclogites: carbonatites from subducting slabs (United States)

    Poli, Stefano


    Current knowledge on the solidus temperature for carbonated eclogites suggests that carbonatitic liquids should not form from a subducted oceanic lithosphere at sub-arc depth. However, the oceanic crust includes a range of gabbroic rocks, altered on rifts and transforms, with large amounts of anorthite-rich plagioclase forming epidote on metamorphism. Epidote disappearance with pressure depends on the normative anorthite content of the bulk composition; we therefore expect that altered gabbros might display a much wider pressure range where epidote persists, potentially affecting the solidus relationships. A set of experimental data up to 4.6 GPa, and 1000 °C, including new syntheses on mafic eclogites with 36.8 % normative anorthite, is discussed to unravel the effect of variable bulk and volatile compositions in model eclogites, enriched in the normative anorthite component ( An 37 and An 45). Experiments are performed in piston cylinder and multianvil machines. Garnet, clinopyroxene, and coesite form in all syntheses. Lawsonite was found to persist at 3.7 GPa, 750 °C, with both dolomite and magnesite; at 3.8 GPa, 775-800 °C, fluid-saturated conditions, epidote coexists with kyanite, dolomite, and magnesite. The anhydrous assemblage garnet, omphacite, aragonite, and kyanite is found at 4.2 GPa, 850 °C. At 900 °C, a silicate glass of granitoid composition, a carbonatitic precipitate, and Na-carbonate are observed. Precipitates are interpreted as evidence of hydrous carbonatitic liquids at run conditions; these liquids produced are richer in Ca compared to experimental carbonatites from anhydrous experiments, consistently with the dramatic role of H2O in depressing the solidus temperature for CaCO3. The fluid-absent melting of the assemblage epidote + dolomite, enlarged in its pressure stability for An-rich gabbros, is expected to promote the generation of carbonatitic liquids. The subsolidus breakdown of epidote in the presence of carbonates at depths

  15. Contrasting sediment melt and fluid signatures for magma components in the Aeolian Arc: Implications for numerical modeling of subduction systems (United States)

    Zamboni, Denis; Gazel, Esteban; Ryan, Jeffrey G.; Cannatelli, Claudia; Lucchi, Federico; Atlas, Zachary D.; Trela, Jarek; Mazza, Sarah E.; De Vivo, Benedetto


    The complex geodynamic evolution of Aeolian Arc in the southern Tyrrhenian Sea resulted in melts with some of the most pronounced along the arc geochemical variation in incompatible trace elements and radiogenic isotopes worldwide, likely reflecting variations in arc magma source components. Here we elucidate the effects of subducted components on magma sources along different sections of the Aeolian Arc by evaluating systematics of elements depleted in the upper mantle but enriched in the subducting slab, focusing on a new set of B, Be, As, and Li measurements. Based on our new results, we suggest that both hydrous fluids and silicate melts were involved in element transport from the subducting slab to the mantle wedge. Hydrous fluids strongly influence the chemical composition of lavas in the central arc (Salina) while a melt component from subducted sediments probably plays a key role in metasomatic reactions in the mantle wedge below the peripheral islands (Stromboli). We also noted similarities in subducting components between the Aeolian Archipelago, the Phlegrean Fields, and other volcanic arcs/arc segments around the world (e.g., Sunda, Cascades, Mexican Volcanic Belt). We suggest that the presence of melt components in all these locations resulted from an increase in the mantle wedge temperature by inflow of hot asthenospheric material from tears/windows in the slab or from around the edges of the sinking slab.

  16. Episodic tremor and slip in Northern Sumatra subduction zone (United States)

    Sianipar, Dimas; Subakti, Hendri


    The first reported observation of non-volcanic tremor in Sunda Arc in Sumbawa, Indonesia open a possibility of discovery of episodic tremor and slip (ETS) from out of Pacific Rim. Non-volcanic tremor gives some important information about dynamic of plate boundaries. The characteristics of these tremors are visually as non-impulsive, high frequency, long-duration and low-amplitude signals. Tectonic tremor occurred in a transition part of brittle-ductile of a fault and frequently associated with the shearing mechanism of slow slip. Tectonic tremor is a seismic case that also very interested, because it shows strong sensitivity to stress changes. Deep non-volcanic tremor is usually associated with episodic slow-slip events. Tectonic tremor is found in close association with geodetically observed slow-slip events (SSE) in subduction zones. One research found that there is possibility of SSE occurrence on Banyak Islands, North Sumatra revealed from coral observation. The SSE occurred on the Banyak Islands portion of the megathrust at 30-55 km depth, within the downdip transition zone. We do a systematic search of episodic tremor and its possible relationship with slow-slip phenomena in Northern Sumatra subduction zone. The spectrogram analysis is done to analyze the potential tremor signals. We use three component broadband seismic stations with 20, 25, and 50 sampling per second (BH* and SH* channels). We apply a butterworth 5 Hz highpass filter to separate the signal as local tremor and teleseismic/regional earthquakes. Before computing spectrogram to avoid high-frequency artifacts to remote triggering, we apply a 0.5 Hz filter. We also convert the binary seismic data into sound waves to make sure that these events meet the tectonic tremor criterion. We successfully examine 3 seismic stations with good recording i.e. GSI, SNSI and KCSI. We find there are many evidences of high frequency episodic tremor like signals. This include an analysis of potential triggered

  17. Noble gases recycled into the mantle through cold subduction zones (United States)

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


    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.

  18. GPS Monitoring of Subduction Zone Deformation in Costa Rica (United States)

    Lundgren, Paul


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

  19. The subduction dichotomy of strong plates and weak slabs (United States)

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


    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.

  20. 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 ...

  1. The Run-Up of Subduction Zones (United States)

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


    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.

  2. Mantle Wedge formation during Subduction Initiation: evidence from the refertilized base of the Oman ophiolitic mantle (United States)

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


    Although the Oman ophiolite is classically regarded as being the direct analog of oceanic lithosphere created at fast spreading ridges, the geodynamic context of its formation is still highly debated. The other alternative end-member model suggests that this ophiolite entirely formed in a supra-subduction zone setting. The latter one is supported by studies on volcanic sequences whereas studies dealing on the mantle section do not involve a significant influence of subduction processes on its structure and composition. We herein focus on basal peridotites from all along the ophiolite strike in order to decipher and characterize potential fluid/melt transfers relate to subduction processes. Samples were taken across the basal banded unit directly overlying the amphibolitic/granulitic metamorphic sole which represents an accreted part of the lower plate. We carried out a petrological, structural and geochemical study on these rocks and their constitutive minerals. Our results show that basal peridotites range from lherzolites to highly depleted harzburgites in composition. Clinopyroxenes (cpx) display melt impregnation textures and co-crystallized with HT/HP amphiboles (amph), spinels and sulfurs. Major and trace elements of the constitutive minerals indicate that these minerals represent trapped incremental partial melt after hydrous melting. Different cpx-bearing lithologies then result from varying degrees of partial melting and melt extraction. Combined with Boron isotopic data, we demonstrate that fluids responsible for hydrous melting of these ophiolitic basal peridotites are subduction-related, most likely derived from dehydration of the metamorphic sole during its formation in subduction initiation. From these observations and thermal constraints, we interpret the occurrence of these basal lherzolites as representing a freezing front developed by thermal re-equilibration (cooling) during subduction processes: subduction-related hydrous partial melts were

  3. Zooming into the Hindu Kush slab break-off: A rare glimpse on the terminal stage of subduction (United States)

    Kufner, Sofia-Katerina; Schurr, Bernd; Haberland, Christian; Zhang, Yong; Saul, Joachim; Ischuk, Anatoly; Oimahmadov, Ilhomjon


    The terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere. This process, although common in geological history, is short-lived, and therefore rarely observed. The deep seismicity under the Hindu Kush (Central Asia), including the 2015 Mw 7.5 event, is a rare case that testifies to this process. Here, we use new seismological data to create a high resolution picture of slab break-off and infer its dynamics. High precision earthquake locations and tomographic images show subduction of continental crust down to ∼180 km. A large dataset of source mechanisms indicates sub-vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab, below the subducted crust, extends at higher rates (∼40 km/Ma). Most Mw > 7 earthquakes between 1983-2015, relocated relative to our new well-constrained earthquake catalog, cluster in a small volume below 180 km, and indicate shearing on an overturned interface. A slip model for the latest 2015 Mw 7.5 event suggests that it ruptured into a seismic gap on this interface. From this configuration we conclude that a horizontal slab tear develops along-strike of the Hindu Kush seismic zone at the base of the subducted continental crust. Below the subducted crust, the deepest and also largest earthquakes (180-265 km) are likely associated with deformation in the mantle lithosphere. From the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along-strike from simple to pure shear. The fastest detachment rates and largest earthquakes occur during the simple shear dominated stage. Earthquakes in the upper part (60-180 km), above the rapidly extending slab, might be triggered by processes related to the subduction of crustal rocks.

  4. The zone of influence of the subducting slab in the asthenospheric mantle (United States)

    MacDougall, Julia G.; Jadamec, Margarete A.; Fischer, Karen M.


    Due to the multidisciplinary nature of combined geodynamics and shear wave splitting studies, there is still much to be understood in terms of isolating the contributions from mantle dynamics to the shear wave splitting signal, even in a two-dimensional (2-D) mantle flow framework. This paper investigates the viscous flow, lattice preferred orientation (LPO) development, and predicted shear wave splitting for a suite of buoyancy-driven subduction models using a non-linear rheology to shed light on the nature of the slab-driven asthenospheric flow and plate-mantle coupling. The slab-driven zone of influence in the mantle, LPO fabric, and resulting synthetic splitting are sensitive to slab strength and slab initial slab dip. The non-linear viscosity formulations leads to dynamic reductions in asthenospheric viscosity extending over 600 km into the mantle wedge and over 300 km behind the trench, with peak flow velocities occurring in models with a weaker slab and moderate slab dip. The olivine LPO fabric in the asthenosphere generally increases in alignment strength with increased proximity to the slab but can be transient and spatially variable on small length scales. The results suggest that LPO formed during initial subduction may persist into the steady state subduction regime. Vertical flow fields in the asthenosphere can produce shear wave splitting variations with back azimuth that deviate from the predictions of uniform trench-normal anisotropy, a result that bears on the interpretation of complexity in shear wave splitting observed in real subduction zones. Furthermore, the models demonstrate the corner flow paradigm should not be equated with a 2-D subduction framework.

  5. Effect of Aseismic Ridge Subduction on Volcanism in the NE Lesser Antilles Arc (United States)

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


    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.

  6. Three-dimensional structure and seismicity beneath the Central Vanuatu subduction zone (United States)

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


    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.

  7. Migration of teleseismically triggered tremor in southwestern Japan subduction zone (United States)

    Kurihara, R.; Obara, K.; Maeda, T.; Takeo, A.


    Deep low frequency tremor in subduction zone is sometimes triggered by surface waves from teleseismic earthquakes. In southwestern Japan, a sequence of triggered tremor was reported for the 2004 Sumatra-Andaman earthquake (Miyazawa and Mori, 2006). Such triggered tremor was observed in the ambient tremor zone where the short-term slow slip events episodically occur. However, the triggered tremor is not distributed in the entire source area of ambient tremor, but is concentrated in several fixed spots. In this study, we tried to reveal accurate location of triggered tremor and investigate the spatiotemporal characteristics for understandings of condition and occurrence mechanism of triggered tremor. We detected low frequency earthquakes in tremor sequence triggered by teleseismic wave by using matched filter technique. The data were obtained at 10 NIED Hi-net stations. We used low frequency earthquakes occurred in 2014 detected by JMA as template events. Time duration of the templates is five seconds. We analyzed continuous waveform data for one hour from the origin times of 2004 Sumatra, 2008 Wenchuan, 2012 Sumatra and 2015 Nepal earthquakes. In western Shikoku, detected triggered tremor is concentrated at distant fixed two spots with an average separation of 20 km for analyzed four teleseismic events. Particularly, southwestern spot has a streak-like distribution along the dip direction of the subducting plate. In this spot, we detected along-dip migration of triggered tremor. The migration speed is about 300 km/h for 2008 Wenchuan earthquake and about 20 km/h for 2015 Nepal earthquake. Shelly et al. (2007) reported similar along-dip migration of ambient tremor at velocity from 25 to 150 km/h. Therefore, migrations of triggered tremor detected in this study suggest that the triggered tremor is also associated by slow slip event like as ambient tremor.

  8. Controls on continental strain partitioning above an oblique subduction zone, Northern Andes (United States)

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


    simplified, generic subduction zone similar to the northern Andes. The upper surface is initially defined to resemble the Andes, but is free to deform during the experiments. We consider two main model designs, one with and one without a volcanic arc (weak continental zone). A relatively high angle of convergence obliquity is predicted to favor strain partitioning, but preliminary model results show no strain partitioning for a uniform continental crustal strength with a friction angle of Φ = 15° . However, strain partitioning does occur when including a weak zone in the continental crust resulting from arc volcanic activity with Φ = 5° . This results in margin-parallel northeastward translation of a continental sliver at 3.2 cm/year. The presence of the sliver agrees well with observations of a continental sliver identified by GPS measurements in the Northern Volcanic Zone with a translation velocity of about 1 cm/year, though the GPS-derived velocity may not be representative of the long-term rate of translation depending on whether the observation period includes one or more seismic cycles. Regardless, the observed behavior is consistent with the observed earthquake focal mechanisms and GPS measurements, suggesting significant northeastward transport of Andean crust along the margin of the northern Andes.

  9. Blurring Boundaries

    DEFF Research Database (Denmark)

    Neergaard, Ulla; Nielsen, Ruth


    This article builds on the results obtained in the so-called Blurring Boundaries project which was undertaken at the Law Department, Copenhagen Business School, in the period from 2007 to 2009. It looks at the sustainability of the Danish welfare state in an EU law context and on the integration...... of welfare functions into EU law both from an internal market law and a constitutional law perspective. The main problem areas covered by the Blurring Boundaries project were studied in sub-projects on: 1) Internal market law and welfare services; 2) Fundamental rights and non-discrimination law aspects......; and 3) Services of general interest. In the Blurring Boundaries project, three aspects of the European Social Model have been particularly highlighted: the constitutionalisation of the European Social Model, its multi-level legal character, and the clash between market access justice at EU level...

  10. Distribution of very low frequency earthquakes in the Nankai accretionary prism influenced by a subducting-ridge (United States)

    Toh, Akiko; Obana, Koichiro; Araki, Eiichiro


    We investigated the distribution of very low frequency earthquakes (VLFEs) that occurred in the shallow accretionary prism of the eastern Nankai trough during one week of VLFE activity in October 2015. They were recorded very close from the sources by an array of broadband ocean bottom seismometers (BBOBSs) equipped in Dense Oceanfloor Network system for Earthquakes and Tsunamis 1 (DONET1). The locations of VLFEs estimated using a conventional envelope correlation method appeared to have a large scatter, likely due to effects of 3D structures near the seafloor and/or sources that the method could not handle properly. Therefore, we assessed their relative locations by introducing a hierarchal clustering analysis based on patterns of relative peak times of envelopes within the array measured for each VLFE. The results suggest that, in the northeastern side of the network, all the detected VLFEs occur 30-40 km landward of the trench axis, near the intersection of a splay fault with the seafloor. Some likely occurred along the splay fault. On the other hand, many VLFEs occur closer to the trench axis in the southwestern side, likely along the plate boundary, and the VLFE activity in the shallow splay fault appears less intense, compared to the northeastern side. Although this could be a snap-shot of activity that becomes more uniform over longer-term, the obtained distribution can be reasonably explained by the change in shear stresses and pore pressures caused by a subducting-ridge below the northeastern side of DONET1. The change in stress state along the strike of the plate boundary, inferred from the obtained VLFE distribution, should be an important indicator of the strain release pattern and localised variations in the tsunamigenic potential of this region.

  11. Three-dimensional thermal structure and seismogenesis in the Tohoku and Hokkaido subduction system (United States)

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


    The Northern Japan arc is characterized by fast subduction of old oceanic lithosphere. The high density instrumentation and high seismicity make this an ideal natural laboratory to study the interplay between subduction zone dynamics, dehydration, migration of fluids, and seismogenesis. In this study we use high resolution finite element models to predict the thermal structure of the subduction slab below Tohoku (Northern Honshu) and Hokkaido. These models allow us to predict the pressure, temperature and mineralogy of the subducted crust and mantle. We use these models to predict the (p,T) conditions of earthquakes that are relocated with a precision of around 1 km by double difference techniques. Below Northern Hokkaido and Tohoku we find that the earthquake activity is strong in crust and the uppermost mantle for temperatures seismic moment. The strongest 3D variations in this arc occur below southern Hokkaido. This 200 km wide region is characterized by a change in trench geometry, anomalously low heatflow and an anomalous velocity structure in the mantle wedge. Tomographic imaging suggest that continental crust is subducted to significant depth, thereby insulating the subducting slab from the hot mantle wedge at least at intermediate depths. The thermal insulation is also suggested by the deepening of the earthquakes in the slab (Kita et al., EPSL, 2010). This region may be characterized by active crustal erosion which would lead to a further blanketing of the crust by a sedimentary layer. Further modifications in thermal structure are possible due to the 3D wedge flow that is generated by the along-arc variations in trench geometry. We quantitatively verify the relative importance of these processes using 2D and 3D dynamical models. Without the seismically imaged crustal structure the earthquake temperatures are significantly elevated compared to the Tohoku and (northern) Hokkaido sections. If we take the modified crustal structure into account we find a (p

  12. Open to Suggestion. (United States)

    Journal of Reading, 1987


    Offers (1) suggestions for improving college students' study skills; (2) a system for keeping track of parent, teacher, and community contacts; (3) suggestions for motivating students using tic tac toe; (4) suggestions for using etymology to improve word retention; (5) a word search grid; and (6) suggestions for using postcards in remedial reading…

  13. Gondwana breakup via double-saloon-door rifting and seafloor spreading in a backarc basin during subduction rollback (United States)

    Martin, A. K.


    attached to Southern Patagonia/West Antarctic Peninsula, while the Ellsworth Whitmore Terrane is combined with the Thurston Island Block; paleogeographies demonstrate rifting and extension in a backarc environment relative to a Pacific margin subduction zone/accretionary wedge where simultaneous crustal shortening occurs; a ridge jump towards the subduction zone from east of the Falkland Islands to the Rocas Verdes Basin evinces subduction rollback; this ridge jump combined with backarc extension isolated an area of thicker continental crust — The Falkland Islands Block; well-documented EW oriented seafloor spreading anomalies in the Weddell Sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback; the dextral strike-slip Gastre and sub-parallel faults form one boundary of the Gondwana subduction rollback, whereas the other boundary may be formed by inferred sinistral strike-slip motion between a combined Thurston Island/Ellsworth Whitmore Terrane and Marie Byrd Land/East Antarctica.

  14. Depositionary Margins: The Destruction and Renovation of Subduction Forearcs (United States)

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


    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.

  15. Deformation of the central Andes (15-27 deg S) derived from a flow model of subduction zones (United States)

    Wdowinski, Shimon; O'Connell, Richard J.


    A simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab. This formulation accounts for two forces that dominate the deformation in the overriding lithosphere: tectonic forces and buoyancy forces. Numerical solutions, obtained by using a finite element technique, are compared with observations from the central Andes (15-27 deg S). The model predicts the observed deformation pattern of extension in the forearc, compression in the Western Monocline (corresponding to magmatic activity), extension in the Altiplano, compression in the Eastern Monocline and Subandes, and no deformation in the Brazilian Shield. By comparing the calculated solutions with the large-scale tectonic observations, the forces that govern the deformation in the central Andes are evaluated. The approximately constant subduction velocity in the past 26 million years suggests that the rate of crustal shortening in the Andes has decreased with time due to the thickening of the crust.


    Directory of Open Access Journals (Sweden)

    Alexei V. Ivanov


    Full Text Available Geochemical features for volcanic rocks and petrologic data for deep-seated inclusions, which can be used to infer mass transfer between different geospheres, are reviewed. It is typically believed that slabs can subduct as deep as the core-mantle boundary with the following recycling by plumes coming up to the sublithospheric regions of magma generation. However, the petrologic evidence of the deepest accessible material is limited by the depth of the uppermost lower mantle (~650–700km, i.e. by the depth of the deepest earthquakes. Ferropericlase inclusions in some diamonds do not exclude involvement of deeper mantle horizons, yet do not unambiguously support it. No unambiguous confirmation of involvement of the lower mantle into magma generation underneath volcanically active regions is obtained from geochemical data either, while the geochemical data suggest complete chemical isolation of the Earth’s core from the upper mantle processes.

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

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


    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

  18. Plate Tectonic Consequences of competing models for the origin and history of the Banda Sea subducted oceanic lithosphere

    CERN Document Server

    Heine, Christian; McKay, Hamish; Müller, R Dietmar


    The Banda Arc, situated west of Irian Jaya and in the easternmost extension of the Sunda subduction zone system, reveals a characteristic bowl-shaped geometry in seismic tomographic images. This indicates that the oceanic lithosphere still remains attached to the surrounding continental margins of northern Australia and the Bird's Head microcontinent. Major controversies exist between authors proposing an allochthonous or autochthonous origin of the Bird's Head block. Either scenario has important implications for plate kinematic models aiming to reconstruct the tectonic evolution of the region and the late Jurassic seaoor spreading geometry of this now subducted Argo-Tanimbar-Seram (ATS) ocean basin. Wider implications affect the tectonic conguration of the Tethyan-Pacic realm, the distribution of plate boundaries as well as the shape and size of continental blocks which have been rifted off the northeastern Gondwana margin during the Late Jurassic and are now accreted to the SE Asia margin. We apply structu...

  19. Breaking the shell: Initiating plate tectonic-like subduction on Europa (United States)

    Bland, Michael T.; McKinnon, William B.


    Europa’s prominent bands have been proposed to form by a seafloor-spreading-like mechanism involving complete separation of Europa’s lithosphere and the emplacement of fresh ice from below [Prockter et al. 2002]. This formation mechanism poses a challenge for Europa’s strain balance: extensional rifting at bands must be offset by lithospheric shortening elsewhere, yet few obvious contractional features have been observed. Kattenhorn and Prockter [2014] suggested that extension on Europa is accommodated by subduction of the lithosphere at linear, tabular zones termed subsumption bands. Subduction of Europa’s lithosphere implicitly requires that lithospheric-scale thrust faults can develop. This contrasts with previous numerical modeling, which found that lithospheric shortening is instead primarily accommodated by folding or passive thickening [Bland and McKinnon 2012, 2013]. Here we reevaluate the conditions required to form large-scale thrust faults using a numerical model of lithospheric shortening on Europa that includes realistic localization of brittle failure (non-associated plasticity). In the absence of strain weakening (wherein brittle failure decreases the subsequent yield strength) essentially all shortening results in folding or thickening, consistent with previous results. With moderate strain weakening, deformation becomes localized within fault-like zones for surface temperatures ≤100 K; however, the resulting surface deformation suggests a complex interplay between folding and faulting. Only if the ice shell weakens very easily does faulting dominate. Large-scale faults preferentially form at cold surface temperatures and high heat fluxes. Cold temperatures promote faulting (as opposed to folding), and high heat fluxes result in a thinner lithosphere, which is more easily subducted. The subsumption bands identified by Kattenhorn and Prockter [2014] are at a relatively high latitude (cold temperature), and are associated with putative

  20. The initiation of subduction: criticality by addition of water? (United States)

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


    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.

  1. Deformation cycles of subduction earthquakes in a viscoelastic Earth. (United States)

    Wang, Kelin; Hu, Yan; He, Jiangheng


    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.

  2. Dynamic Linkages Between the Transition Zone & Surface Plate Motions in 2D Models of Subduction (United States)

    Arredondo, K.; Billen, M. I.


    While slab pull is considered the dominant force controlling plate motion and speed, its magnitude is controlled by slab behavior in the mantle, where tomographic studies show a wide range of possibilities from direct penetration to folding, or stagnation directly above the lower mantle (e.g. Fukao et al., 2009). Geodynamic studies have investigated various parameters, such as plate age and two phase transitions, to recreate observed behavior (e.g. Běhounková and Cízková, 2008). However, past geodynamic models have left out known slab characteristics that may have a large impact on slab behavior and our understanding of subduction processes. Mineral experiments and seismic observations have indicated the existence of additional phase transitions in the mantle transition zone that may produce buoyancy forces large enough to affect the descent of a subducting slab (e.g. Ricard et al., 2005). The current study systematically tests different common assumptions used in geodynamic models: kinematic versus free-slip boundary conditions, the effects of adiabatic heating, viscous dissipation and latent heat, compositional layering and a more complete suite of phase transitions. Final models have a complete energy equation, with eclogite, harzburgite and pyrolite lithosphere compositional layers, and seven composition-dependent phase transitions within the olivine, pyroxene and garnet polymorph minerals. Results show important feedback loops between different assumptions and new behavior from the most complete models. Kinematic models show slab weakening or breaking above the 660 km boundary and between compositional layers. The behavior in dynamic models with a free-moving trench and overriding plate is compared to the more commonly found kinematic models. The new behavior may have important implications for the depth distribution of deep earthquakes within the slab. Though the thermodynamic parameters of certain phase transitions may be uncertain, their presence and

  3. Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism. (United States)

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


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

  4. 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


    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

  5. 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.


    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

  6. Subduction Zone Dewatering at the Southern End of New Zealand's Hikurangi Margin - Insights from 2D Seismic Tomography (United States)

    Crutchley, G. J.; Klaeschen, D.


    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.

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

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


    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.

  8. Mw 8.6 Sumatran earthquake of 11 April 2012: rare seaward expression of oblique subduction (United States)

    Ishii, Miaki; Kiser, Eric; Geist, Eric L.


    The magnitude 8.6 and 8.2 earthquakes off northwestern Sumatra on 11 April 2012 generated small tsunami waves that were recorded by stations around the Indian Ocean. Combining differential travel-time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench-parallel component. The oblique plate convergence indicates that ~20-50 m of trench-parallel displacement could have accumulated since the last megathrust earthquake, only part of which has been taken up by the Great Sumatran fault. This suggests that the remaining trench-parallel motion was released during the magnitude 8.6 earthquake on 11 April 2012 within the subducting plate. The magnitude 8.6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the Sumatra-Andaman earthquake in 2004.

  9. Geophysics. Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface. (United States)

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


    Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events. Copyright © 2015, American Association for the Advancement of Science.

  10. The spatial distribution of earthquake stress rotations following large subduction zone earthquakes (United States)

    Hardebeck, Jeanne L.


    Rotations of the principal stress axes due to great subduction zone earthquakes have been used to infer low differential stress and near-complete stress drop. The spatial distribution of coseismic and postseismic stress rotation as a function of depth and along-strike distance is explored for three recent M ≥ 8.8 subduction megathrust earthquakes. In the down-dip direction, the largest coseismic stress rotations are found just above the Moho depth of the overriding plate. This zone has been identified as hosting large patches of large slip in great earthquakes, based on the lack of high-frequency radiated energy. The large continuous slip patches may facilitate near-complete stress drop. There is seismological evidence for high fluid pressures in the subducted slab around the Moho depth of the overriding plate, suggesting low differential stress levels in this zone due to high fluid pressure, also facilitating stress rotations. The coseismic stress rotations have similar along-strike extent as the mainshock rupture. Postseismic stress rotations tend to occur in the same locations as the coseismic stress rotations, probably due to the very low remaining differential stress following the near-complete coseismic stress drop. The spatial complexity of the observed stress changes suggests that an analytical solution for finding the differential stress from the coseismic stress rotation may be overly simplistic, and that modeling of the full spatial distribution of the mainshock static stress changes is necessary.

  11. The spatial distribution of earthquake stress rotations following large subduction zone earthquakes (United States)

    Hardebeck, Jeanne L.


    Rotations of the principal stress axes due to great subduction zone earthquakes have been used to infer low differential stress and near-complete stress drop. The spatial distribution of coseismic and postseismic stress rotation as a function of depth and along-strike distance is explored for three recent M ≥ 8.8 subduction megathrust earthquakes. In the down-dip direction, the largest coseismic stress rotations are found just above the Moho depth of the overriding plate. This zone has been identified as hosting large patches of large slip in great earthquakes, based on the lack of high-frequency radiated energy. The large continuous slip patches may facilitate near-complete stress drop. There is seismological evidence for high fluid pressures in the subducted slab around the Moho depth of the overriding plate, suggesting low differential stress levels in this zone due to high fluid pressure, also facilitating stress rotations. The coseismic stress rotations have similar along-strike extent as the mainshock rupture. Postseismic stress rotations tend to occur in the same locations as the coseismic stress rotations, probably due to the very low remaining differential stress following the near-complete coseismic stress drop. The spatial complexity of the observed stress changes suggests that an analytical solution for finding the differential stress from the coseismic stress rotation may be overly simplistic, and that modeling of the full spatial distribution of the mainshock static stress changes is necessary.[Figure not available: see fulltext.

  12. Electrical conductivity imaging in the western Pacific subduction zone (United States)

    Utada, Hisashi; Baba, Kiyoshi; Shimizu, Hisayoshi


    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

  13. 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 (United States)

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


    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.

  14. Impact of Mantle Wind on Subducting Plate Geometry and Interplate Pressure: Insights From Physical Modelling. (United States)

    Boutelier, D.; Cruden, A. R.


    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

  15. Albedo Boundary (United States)


    MGS MOC Release No. MOC2-510, 11 October 2003The sharp, nearly straight line that runs diagonally across the center of this April 2003 Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image is an albedo boundary. Albedois a term that refers to reflectance of sunlight. A surface with a low albedo is one that appears dark because it reflects less light than a high albedo (bright) surface. On Mars, albedo boundaries occur between two materials of differing texture, particle size, or composition, or some combination of these three factors. The boundary shown here is remarkable because it is so sharp and straight. This is caused by wind. Most likely, the entire surface was once covered with the lower-albedo (darker) material that is now seen in the upper half of the image. At some later time, wind stripped away this darker material from the surfaces in the lower half of the image. The difference in albedo here might be related to composition, and possibly particle size. This picture is located near the southwest rim of Schiaparelli Basin at 5.5oS, 345.9oW. The picture covers an area 3 km (1.9 mi) wide and is illuminated by sunlight from the left.

  16. Subduction, back-arc spreading and global mantle flow (United States)

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


    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.

  17. Manufacturer's Suggested Retail Prices

    NARCIS (Netherlands)

    Rosenkranz, S.|info:eu-repo/dai/nl/157222241


    Based on arguments of the `reference- dependent' theory of consumer choice we assume that a retailer's discount of a manufacturer's suggested retail price changes consumers' demand. We can show that the producer benefits from suggesting a retail price. If consumers are additionally sufficiently

  18. Slab-Forearc Density Structure and Rigidity Controlling the Seismogenic Behaviour Along the Peru-Chile Subduction Zone (United States)

    Tassara, A.; Hackney, R.; Legrand, D.


    The rupture area and recurrence time of historical earthquakes along the Peru-Chile subduction zone and seismicity recorded by modern networks show a distinctive spatiotemporal distribution defining a characteristic segmentation of the seismogenic zone. It is unclear what factors control this segmentation. Knowledge about this topic is urgent to understand the processes generating devastating subduction earthquakes and to improve its hazard assessment. We are studying this problem for the Peru-Chile subduction zone from two perspectives. First, we applied a wavelet-based spectral isostatic analysis of topography and gravity to compute a high resolution map of the flexural rigidity along the subduction zone. This parameter is a function of the thermo- mechanical structure of both converging plates and the frictional properties of the subduction channel between them. Spatial variations on this map show correlation with the seismogenic segmentation, suggesting that rigidity and the associated physical factors play a fundamental role for the seismogenic behaviour along the margin. Second, we used an existing 3D density model to derive a map of vertical stress acting on the subducting slab below the forearc. This stress is a function of the thickness and density structure of the forearc resulting from long-term geological processes, and is the main component of the normal stress that regulates the magnitude of shear stresses to be released during earthquakes. The spatial variations of vertical stresses show significant correlations with the seismogenic segmentation, implying that the geologically-inherited density structure of the forearc is an important parameter for sustaining a time-persistent seismic segmentation. Of particular interest is the analysis of the giant (Mw 9.5) 1960 Valdivia earthquake, which nucleated in a region of high rigidity and high vertical stress and propagated southward into a region of very low rigidity and vertical stress. This could have

  19. Dynamic Linkages Between the Transition Zone & Surface Plate Motion in 2D Models of Subduction (United States)

    Arredondo, K.; Billen, M. I.


    Subduction zones exhibit a wide range of behavior, from slab stagnation at 660 km to direct penetration into the lower mantle. Due to uncertainties in the tectonic history of individual subduction zones, such as trench velocities, potential mechanisms for controlling slab behavior in the transition zone are explored using numerical models. Numerical simulations have utilized a range of assumptions to improve computational efficiency, such as ignoring latent heat, ignoring compositional effects or fixing the trench location: the net effect of these assumptions resulting modeled dynamics remains unclear. Additionally the eight major, composition-dependent, phase transitions for pyrolite, harzburgite and eclogite may be an important influence on subducting slab dynamics due to the additional forces that are dependent on depth and compositional layering within the slab (e.g., Ricard et al., 2005). With the goal of developing more complete, self-consistent, and less idealized simulations, we test the importance of various factors on slab behavior: the presence of shear, adiabatic and latent heating, compositional layering, composition-dependent phase transitions and explicit plate speeds versus dynamically evolving plate and trench velocities. Preliminary results indicate that individual components have a relatively minor effect, but produce large changes when combined together. The extent of slab folding and stagnation is overestimated by only modeling the 410 and 660 km phase transitions. Dynamic models with all seven composition-dependent phase transitions are very sensitive to the plate strength and weak zone viscosity, causing large changes in plate speed and slab detachment. Changes to the overriding plate buoyance and strength investigate the origin and influence of trench movement on slab deformation. These feedbacks and parameter-sensitive behavior indicate that the wide range of observed slab behavior may result from subtle differences in plate and plate

  20. Intraplate volcanism influenced by distal subduction tectonics at Jeju Island, Republic of Korea (United States)

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


    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.

  1. The seismic cycle at subduction thrusts: Insights from seismo-thermo-mechanical models

    KAUST Repository

    van Dinther, Y.


    The underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics. To unravel their relation to associated subduction dynamics and long-term deformation, we developed a 2-D continuum viscoelastoplastic model that uses an Eulerian-Lagrangian finite difference framework with similar on- and off-fault physics. We extend the validation of this numerical tool to a realistic subduction zone setting that resembles Southern Chile. The resulting quasi-periodic pattern of quasi-characteristic M8–M9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters, albeit at very slow coseismic speeds. Without any data fitting, surface displacements agree with GPS data recorded before and during the 2010 M8.8 Maule earthquake, including the presence of a second-order flexural bulge. These surface displacements show cycle-to-cycle variations of slip deficits, which overall accommodate ∼5% of permanent internal shortening. We find that thermally (and stress) driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of ∼350°C and ∼450°C. Ruptures initiate above it (and below the forearc Moho), propagate within it, interspersed by small intermittent events, and arrest below it as ductile shearing relaxes stresses. Ruptures typically propagate upward along lithological boundaries and widen as pressures drop. The main thrust is constrained to be weak due to fluid-induced weakening required to sustain regular subduction and to generate events with natural characteristics (fluid pressures of ∼75–99% of solid pressures). The agreement with a range of seismological, geodetic, and geological observations demonstrates the validity and strength of this physically consistent seismo-thermo-mechanical approach.

  2. Fault and fluid systems in supra-subduction zones: The Troodos ophiolite (United States)

    Quandt, Dennis; Micheuz, Peter; Kurz, Walter; Krenn, Kurt


    The Troodos massif on the island of Cyprus represents a well-preserved and complete supra-subduction zone (SSZ) ophiolite. It includes an extrusive sequence that is subdivided into Upper (UPL) and Lower Pillow Lavas (LPL). These volcanic rocks contain mineralized fractures (veins) and vesicles that record fluid availability probably related to slab dehydration and deformation subsequent to a period of subduction initiation in the framework of a SSZ setting. Here, we present electron microprobe element mappings and cathodoluminescence studies of vein minerals as well as analyses of fluid inclusions entrapped in zeolite, calcite and quartz from veins and vesicles of the Pillow Lavas of the Troodos ophiolite. Two different zeolite type assemblages, interpreted as alteration products of compositional varying volcanic glasses, occur: (1) Na-zeolites analcime and natrolite from the UPL that require lower formation temperatures, higher Na/Ca ratios and pH values than (2) Ca-zeolites heulandite and mordenite from the LPL which indicate temporal or spatial varying fluid compositions and conditions. Calcite represents a late stage phase in incompletely sealed blocky type (1) assemblage and in syntaxial quartz veins. Additionally, calcite occurs as major phase in syntaxial and blocky veins of UPL and LPL. These syntaxial quartz and calcite veins are assumed to be related to tectonic extension. Chalcedony is associated with quartz and occurs in typical veins and vesicles of the LPL. In addition, the presence of neptunian dykes in veins suggests that seawater penetrated fractures throughout the extrusive sequence. Thus, circulation in an open system via advective transport is favored while diffusion in a closed system is a subordinate, local and late stage phenomenon. Calcite veins and quartz vesicles contain primary, partly re-equilibrated two phase (liquid, vapor) fluid inclusions. The chemical system of all studied inclusions in both host minerals is restricted to aqueous

  3. Evolution of the Archaean crust by delamination and shallow subduction. (United States)

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


    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.

  4. Differential subsidence of the forearc wedge of the Ryukyu (Nansei-Shoto) Arc caused by subduction of ridges on the Philippine Sea Plate (United States)

    Okamura, Yukinobu; Nishizawa, Azusa; Oikawa, Mitsuhiro; Horiuchi, Daishi


    The Philippine Sea Plate (PSP) carrying several ridges has been sudbucting under the Ryukyu (Nansei-Shoto) Arc since middle Miocene. Because no extensive accretionary prism has been growing along the Ryukyu Trench, the arc provides an opportunity to examine effects of ridge subduction on structure of the forearc wedge and a clue to reconstruct ancient plate motion of the PSP that is inferred to have changed between NW and NNW. To examine this perspective, we clarified structure of the Ryukyu forearc wedge based on seismic profiles and bathymetric data and related them to ridge subduction. An erosional unconformity between pre-Neogen and Neogene rocks is widely recognized through the Ryukyu Arc, and we divided the forearc wedges into Zones I to IV from southwest to northeast by difference of depth of the erosional unconformity. We correlated these four zones to the locations of ridge subduction that have been shifting NE or SW along the Ryukyu Trench. Zone I is underlain by the largely subsided unconformity and we attributed the structure to tearing of the Eurasia plate due to subduction of the western margin of the PSP including the Luzon Arc. Zone II consists of a wide terrace on the shallow erosional unconformity, and no ridge that was subducting in this zone is known. Zone III is characterized by the seaward descending unconformity covered with a thick sequence of Neogene sediments and is related to subduction of the NW-SE trending Daito Ridge and the E-W trending Amami Plateau. Zone IV has the deeper unconformity and slope geometry and corresponds to subduction of the NNW-SSE trending Kyushu-Palau Ridge. The structure of the Ryukyu forearc is consistently related to subduction of ridges on the PSP, suggesting that the change of the plate motion of the PSP can be precisely reconstructed by further detailed survey.

  5. Suggestive Objects at Work

    DEFF Research Database (Denmark)

    Ratner, Helene Gad


    In Western secular societies, spiritual life is no longer limited to classical religious institutions but can also be found at workplace organizations. While spirituality is conventionally understood as a subjective and internal process, this paper proposes the concept of ‘suggestive objects’, co...... scaffolding. This has deep implications for our understanding of the sacred, including a better appreciation of the way that suggestive objects make the sacred durable, the way they organize it....

  6. Suggestive techniques in advertising


    Sora, Olena


    In my thesis I focused on a detailed analysis of suggestive techniques that appear in contemporary advertising. The issue of the effects of advertising has existed for many years and still staying timely. On the one side there are entrepreneurs and advertising agencies that are trying to influence opinions and suggest motivation for consuming. On the other side there is a potential customer, who is trying to obtain information about the product he needs and at the same time not letting anybod...

  7. Evidences for recent plume-induced subduction, microplates and localized lateral plate motions on Venus (United States)

    Davaille, Anne; Smrekar, Suzanne


    Using laboratory experiments and theoretical modeling, we recently showed that plumes could induce roll-back subduction around large coronae. When a hot plume rises under a brittle and visco-elasto-plastic skin/lithosphere, the latter undergoes a flexural deformation which puts it under tension. Radial cracks and rifting of the skin then develop, sometimes using pre-existing weaknesses. Plume material upwells through the cracks (because it is more buoyant) and spreads as a axisymmetric gravity current above the broken denser skin. The latter bends and sinks under the combined force of its own weight and that of the plume gravity current. However, due to the brittle character of the upper part of the experimental lithosphere, it cannot deform viscously to accomodate the sinking motions. Instead, the plate continues to tear, as a sheet of paper would do upon intrusion. Several slabs are therefore produced, associated with trenches localized along partial circles on the plume, and strong roll-back is always observed. Depending on the lithospheric strength, roll-back can continue and triggers a complete resurfacing, or it stops when the plume stops spreading. Two types of microplates are also observed. First, the upwelling plume material creates a set of new plates interior to the trench segments. These plates move rapidly and expand through time, but do not subduct.. In a few cases, we also observe additional microplates exterior to the trenches. This happens when the subducting plate contains preexisting heterogeneities (e.g. fractures) and the subducted slab is massive enough for slab pull to become efficient and induce horizontal plate motions. Scalings derived from the experiments suggest that Venus lithosphere is soft enough to undergo such a regime. And indeed, at least two candidates can be identified on Venus, where plume-induced subduction could have operated. (1) Artemis Coronae is the largest (2300 km across) coronae on Venus and is bounded over 270° of

  8. [Psychoanalysis and suggestion]. (United States)

    Thomä, H


    In the history of psychoanalysis the problem of suggestion has been a central one. At first it involved the necessity to establish the psychoanalytic technique as independent scientific paradigm in contrast to persuasion and hypnosis. However, it was not only the symptom-oriented suggestion that had to be given up for scientific reasons and reasons of treatment technique. Since professional and human factors as well could have influenced the psychoanalytic situation to revert to the traditional "suggestion", Freud has given some technical considerations (e.g. the mirror-analogy), that were meant to counteract the confusion of the psychoanalytic technique with the persuasive one that had to come up to late. The discovery of the transference phenomena has further complicated the problem. It became obvious that the capacity of the analyst to exert an influence and to have impact, originated in very basic human categories and their specific psychogenetic developments and distortions. This understanding contributed to the development of psychoanalytic theories of suggestibility. Until the present day the discovery of the transference phenomena has determined the discussions of psychoanalytic technique in term of the relationship between the special and general therapeutic factors (i.e. interpretation versus relationship). The departure from the therapeutic mode of persuasive suggestion and the introduction of psychoanalytic technique signaled the revolutionary paradigm of Sigmund Freud, i.e. the active participation of the patient and the process of observation. Often scientific problems related to this pradigm and suggestion are discussed concurrently.

  9. State of stress and crustal fluid migration related to west-dipping structures in the slab-forearc system in the northern Chilean subduction zone (United States)

    Salazar, P.; Kummerow, J.; Wigger, P.; Shapiro, S.; Asch, G.


    Previous studies in the forearc of the northern Chilean subduction zone have identified important tectonic features in the upper crust. As a result of these works, the West Fissure Fault System (WFFS) has recently been imaged using microseismic events. The WFFS is the westward-dipping, sharp lower boundary of the northern Chilean forearc and is geometrically opposed to subduction of the Nazca plate. The present article builds on this previous work and is novel in that it characterizes this structure's stress distribution using focal mechanisms and stress tensor analysis. The results of the stress tensor analysis show that the state of stress in the WFFS is related to its strike-slip tectonic context and likely represents a manifestation of local forces associated with the highest areas in the Andes. Two seismic clusters have also been identified; these clusters may be associated with a blind branch of the WFFS. We studied these clusters in order to determine their sources and possible connection with fluid migration across the upper plate. We observed that the two clusters differ from one another in some regards. The central cluster has characteristics consistent with an earthquake swarm with two clearly identifiable phases. Conversely, the SW cluster has a clear main shock associated with it, and it can be separated into two subclusters (A and A΄). In contrast, similarities among the two clusters suggest that the clusters may have a common origin. The b-values for both clusters are characteristic of tectonic plate boundaries. The spatial spreading, which is approximately confined to one plane, reflects progressive growth of the main fracture underlying the swarm and subcluster A. We also find that earthquakes themselves trigger aftershocks near the borders of their rupture areas. In addition, the spatio-temporal migration of hypocentres, as well as their spatial correlation with areas that are interpreted to be fluid migration zones, suggest that there is a close

  10. Early Cretaceous wedge extrusion in the Indo-Burma Range accretionary complex: implications for the Mesozoic subduction of Neotethys in SE Asia (United States)

    Zhang, Ji'en; Xiao, Wenjiao; Windley, Brian F.; Cai, Fulong; Sein, Kyaing; Naing, Soe


    The Indo-Burma Range (IBR) of Myanmar, the eastern extension of the Yarlung-Tsangpo Neotethyan belt of Tibet in China, contains mélanges with serpentinite, greenschist facies basalt, chert, sericite schist, silty slate and unmetamorphosed Triassic sandstone, mudstone and siltstone interbedded with chert in the east, and farther north high-pressure blueschist and eclogite blocks in the Naga Hills mélange. Our detailed mapping of the Mindat and Magwe sections in the middle IBR revealed a major 18 km antiformal isocline in a mélange in which greenschist facies rocks in the core decrease in grade eastwards and westwards symmetrically `outwards' to lower grade sericite schist and silty slate, and at the margins to unmetamorphosed sediments, and these metamorphic rocks are structurally repeated in small-scale imbricated thrust stacks. In the Mindat section the lower western boundary of the isoclinal mélange is a thrust on which the metamorphic rocks have been transported over unmetamorphosed sediments of the Triassic Pane Chaung Group, and the upper eastern boundary is a normal fault. These relations demonstrate that the IBR metamorphic rocks were exhumed by wedge extrusion in a subduction-generated accretionary complex. Along strike to the north in the Naga Hills is a comparable isoclinal mélange in which central eclogite lenses are succeeded `outwards' by layers of glaucophane schist and glaucophanite, and to lower grade greenschist facies sericite schist and slate towards the margins. In the Natchaung area (from west to east) unmetamorphosed Triassic sediments overlie quartzites, sericite schists, actinolite schists and meta-volcanic amphibolites derived from MORB-type basalt, which are in fault contact with peridotite. Olivine in the peridotite has undulatory extinction suggesting deformation at 600-700 °C, similar to the peak temperature of the amphibolite; these relations suggest generation in a metamorphic sole. The amphibolites have U/Pb zircon ages of 119

  11. Post subduction thermal regime of the western North America and effects on the Great Valley, Sierra Nevada and northern Baja California provinces (United States)

    Erkan, Kamil

    Tectonic evolution of the transform margin of western North America has attracted great interest. This boundary formed as the fault-fault-trench type Mendocino triple junction has moved north. The tectonic transition is expected to have a significant thermal signature because the subduction zones are quite distinct from any other plate boundary zones with their depressed heat flow in the outer arc regions. There is a significant amount of heat flow data in most of California covering the Great Valley and Sierra Nevada regions where were part of the outer arc of the Farallon subduction was located. The heat flow in a very large area covering all over the Great Valley and western Sierra Nevada shows the consistent pattern of a recent subduction. The unique pattern of the heat flow distribution indicates the tectonic origin of the data which has a potential to reveal the nature of the tectonic transition after the cessation of subduction 30 Ma. In this collection, the first two papers deal with the interpretation of heat flow data using forward thermal models of the lithosphere. The two proposed tectonic scenarios, namely the slab window and the stalled slab models, were compared in terms of the thermal consequences. The low heat flow in the Great Valley and the narrow transition zone toward the Coast Ranges rules out opening of a slab window in a conventional sense and is therefore indicative of a stalled slab type transition. Therefore, the high heat flow in the Coast Ranges must be explained by a mechanism other than opening of a slab window beneath the Coast Ranges. The low (reduced) heat flow in the Western Sierra Nevada which extends to the south end of the region where the subduction ceased more than 15 My ago is important in understating the thermal history of the lithosphere after the cessation of subduction. The thermal data are also in close agreement with the seismic cut-out depth where sufficient seismic activity is present. The numerical thermal model of

  12. boundary dissipation

    Directory of Open Access Journals (Sweden)

    Mehmet Camurdan


    are coupled by appropriate trace operators. This overall model differs from those previously studied in the literature in that the elastic chamber floor is here more realistically modeled by a hyperbolic Kirchoff equation, rather than by a parabolic Euler-Bernoulli equation with Kelvin-Voight structural damping, as in past literature. Thus, the hyperbolic/parabolic coupled system of past literature is replaced here by a hyperbolic/hyperbolic coupled model. The main result of this paper is a uniform stabilization of the coupled PDE system by a (physically appealing boundary dissipation.

  13. Multiscale Architecture of a Subduction Complex and Insight into Large-scale Material Movement in Subduction Systems (United States)

    Wakabayashi, J.


    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

  14. The Subduction of Continental Crust, the Variscan Evolution of the Bohemian Massif, and the Origin of PO Granitoids (United States)

    Brueckner, H. K.


    east-northeast subduction of Moldanubia beneath Bohemia at 340-320 Ma1. The first subduction was followed by the intrusion of granitoids, including the Central Bohemian Batholith, between 370-340 Ma, all occurring south of the Saxo-Thuringian - Bohemia suture suggesting elements of the subducted Saxo-Thuringian continental crust melted and intruded the overlying Bohemian Craton. The second subduction event was accompanied and followed by multiple intrusions of granites (340 Ma synorogenic granitoids, 340-310 Ma post-orogenic S and high-K granitoids, and 310-290 I-type granitoids2) all occurring west of the of the Moldanubia-Bohemia suture suggesting elements of the subducted Moldanubian crust were melted and intruded the overlying Bohemian/Saxo-Thuringian Craton. Many of the HP/UHP terranes in the Bohemian Massif occur in domal structures suggesting exhumation and melting occurred in part by diapiric upwelling3. It is proposed that both the Saxo-Thuringian and Moldanubian terranes were exhumed by a combination of slab-return and diapiric upwelling and that elements of both terranes were stranded long enough in the mantle to heat up and melt to form most or all of the PO granitoids of the Bohemian Massif. This model involves successive re-distillation of the continental crust and may play a significant role in the evolution of both the continental crust and upper mantle. 1Medaris et al., 2005, Lithos 82. 2Finger et al., 1997, Min & Pet 61. 3Stípská et a., 04, J. Met. Geol. 22.

  15. A look inside of diamond-forming media in deep subduction zones. (United States)

    Dobrzhinetskaya, Larissa F; Wirth, Richard; Green, Harry W


    Geologists have "known" for many years that continental crust is buoyant and cannot be subducted very deep. Microdiamonds 10-80 microm in size discovered in the 1980s within metamorphic rocks related to continental collisions clearly refute this statement, suggesting that material of continental crust has been subducted to a minimum depth of >150 km and incorporated into mountain chains during tectonic exhumation. Over the past decade, the rapidly moving technological advancement has made it possible to examine these diamonds in detail, and to learn that they contain nanometric multiphase inclusions of crystalline and fluid phases and are characterized by a "crustal" signature of carbon stable isotopes. Scanning and transmission electron microscopy, focused ion beam techniques, synchrotron infrared spectroscopy, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they were crystallized from a supercritical carbon-oxygen-hydrogen fluid. These microdiamonds preserve evidence of the pathway by which carbon and water can be subducted to mantle depths and returned back to the earth's surface.

  16. Initiation of GPS-Acoustics Measurements on the Continental Slope of the Cascadia Subduction Zone (United States)

    Chadwell, C. D.


    Land-based GPS measurements suggest the megathrust is locked offshore along the Cascadia Subduction Zone. However, land-based data alone lack geometric resolution to constrain the how the slip is distributed. GPS-Acoustic measurements can provide these constraints, but using traditional GPS-Acoustic approaches employing a ship is costly. Wave Gliders, a wave- and solar-powered, remotely-piloted sea surface platform, provide a low cost method for collecting GPS-A data. We have adapted GPS-Acoustic technology to the Wave Glider. In July 2016, the GPS-A Wave Glider was launched on month-long mission to two sites on the continental slope of the Cascadia Subduction Zone. One site is approximately 45 NM offshore central Oregon and the other approximately 50 NM offshore central Washington State. We will report on initial results of the GPS-A data collection and operational experiences of the mission. Wave Glider based GPS-A measurement have the potential to significantly increase the number and frequency of measurements of strain accumulation in Cascadia Subduction Zone and elsewhere.

  17. Cascadia subducting plate fluids channelled to fore-arc mantle corner: ETS and silica deposition (United States)

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


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

  18. Source Parameters of Large Magnitude Subduction Zone Earthquakes Along Oaxaca, Mexico (United States)

    Fannon, M. L.; Bilek, S. L.


    Subduction zones are host to temporally and spatially varying seismogenic activity including, megathrust earthquakes, slow slip events (SSE), nonvolcanic tremor (NVT), and ultra-slow velocity layers (USL). We explore these variations by determining source parameters for large earthquakes (M > 5.5) along the Oaxaca segment of the Mexico subduction zone, an area encompasses the wide range of activity noted above. We use waveform data for 36 earthquakes that occurred between January 1, 1990 to June 1, 2014, obtained from the IRIS DMC, generate synthetic Green's functions for the available stations, and deconvolve these from the ­­­observed records to determine a source time function for each event. From these source time functions, we measured rupture durations and scaled these by the cube root to calculate the normalized duration for each event. Within our dataset, four events located updip from the SSE, USL, and NVT areas have longer rupture durations than the other events in this analysis. Two of these four events, along with one other event, are located within the SSE and NVT areas. The results in this study show that large earthquakes just updip from SSE and NVT have slower rupture characteristics than other events along the subduction zone not adjacent to SSE, USL, and NVT zones. Based on our results, we suggest a transitional zone for the seismic behavior rather than a distinct change at a particular depth. This study will help aid in understanding seismogenic behavior that occurs along subduction zones and the rupture characteristics of earthquakes near areas of slow slip processes.

  19. Volatile (Li, B, F and Cl) mobility during amphibole breakdown in subduction zones (United States)

    Debret, Baptiste; Koga, Kenneth T.; Cattani, Fanny; Nicollet, Christian; Van den Bleeken, Greg; Schwartz, Stephane


    Amphiboles are ubiquitous minerals in the altered oceanic crust. During subduction, their breakdown is governed by continuous reactions up to eclogitic facies conditions. Amphiboles thus contribute to slab-derived fluid throughout prograde metamorphism and continuously record information about volatile exchanges occurring between the slab and the mantle wedge. However, the fate of volatile elements and especially halogens, such as F and Cl, in amphibole during subduction is poorly constrained. We studied metagabbros from three different localities in the Western Alps: the Chenaillet ophiolite, the Queyras Schistes Lustrés and the Monviso meta-ophiolitic complexes. These samples record different metamorphic conditions, from greenschist to eclogite facies, and have interacted with different lithologies (e.g. sedimentary rocks, serpentinites) from their formation at mid-oceanic ridge, up to their devolatilization during subduction. In the oceanic crust, the initial halogen budget is mostly stored in magmatic amphibole (F = 300-7000 ppm; Cl = 20-1200 ppm) or in amphibole corona (F = 100-7000 ppm; Cl = 80-2000 ppm) and titanite (F = 200-1500 ppm; Cl glaucophane at the expense of magmatic and amphibole coronas. This episode is accompanied with a decrease of halogen concentrations in amphiboles (glaucophane (up to 600 ppm) whereas halogen concentrations are unaffected. At eclogite facies conditions, metagabbros display low halogens concentrations (< 20 ppm of F and < 100 ppm of Cl) relative to altered oceanic crust (F = 40-650 ppm; Cl = 40-1400 ppm) suggesting that these elements are continuously released by fluids during the first 30-80 km of subduction whatever the tectonic environment (e.g. slab, plate interface) and the considered fluid/rock interactions.

  20. Imprints of an "Arc" Signature onto Subduction Zone Eclogites from Central Guatemala (United States)

    Simons, K. K.; Sorensen, S. S.; Harlow, G. E.; Brueckner, H. K.; Goldstein, S. L.; Hemming, N. G.; Langmuir, C. H.


    High-pressure, low-temperature (HP-LT) rocks associated with the Motagua fault zone in central Guatemala occur as tectonic blocks in serpentinite mélange. Dismembered jadeitite and albitite veins within the melange are crystallization products of subduction fluids at glaucophane) in veins and overgrowths. The low temperatures recorded in these rocks indicate they have only seen an aqueous fluid, not a melt, and therefore, could provide a window into the acquisition of an arc signature at a cold margin. Trace-element patterns for both eclogite and jadeitite resemble arc lavas, with large enrichments in the most fluid mobile elements (e.g. Cs, Tl, Ba, Pb), moderate enrichments in U, Th, Be and LREE and generally little to no enrichment in HFSE and HREE, although enriched Nb in jadeitite indicates some HFSE mobility. Trace-element patterns also have similarities to average subducting sediment (GLOSS), with enrichments in Th, Be, Ba and Li that suggest a sediment contribution. Nd versus Sr isotopes lie to the right of the mantle array, indicating a hydrous fluid contribution from altered ocean crust or sediment. Overall, Guatemalan eclogites resemble counterparts from the Franciscan Complex (CA) and the Dominican Republic. Guatemalan and Franciscan eclogites are interpreted to have had a MORB protolith despite the arc trace element signature because of: 1) similarities in major elements to MORB; 2) HREE and HFSE abundances similar to MORB; and 3) high 143Nd/144Nd that overlap MORB values. The modifications that transformed these eclogites from a MORB trace element pattern to an arc one can be attributed to an aqueous subduction fluid at moderate depths (reactions, and an abundance of alkali-aluminosilicate components in subduction fluids. Together these may act to dissolve and transport trace elements (including elements considered insoluble like Nb) out of the slab and into the mantle wedge. The Guatemala data thus indicate that the arc geochemical fingerprint may be

  1. Dating Subduction Zone Metamorphism with Garnet and Lawsonite Geochronology (United States)

    Mulcahy, S. R.; Vervoort, J. D.


    Lawsonite [CaAl2Si2O7(OH)2 H2O] is a critical index mineral for high- to ultrahigh-pressure metamorphism associated with subduction. Lawsonite is an important carrier of water into the mantle, a likely contributor to subduction zone seismicity, and a bearer of trace elements that link metamorphism to arc magmatism. Due to its limited pressure-temperature stability, lawsonite can serve as a powerful petrogenetic indicator of specific metamorphic events. Lu-Hf dating of lawsonite, therefore provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist. Broad application of lawsonite Lu-Hf geochronology requires constraining the role of pressure-temperature path, lawsonite forming reactions, and the Lu and Hf systematics within lawsonite and other blueschist facies minerals. We are working to address the role of the metamorphic path on the applicability of lawsonite Lu-Hf geochronology within the Franciscan Complex of California. The Franciscan Complex preserves mafic high-grade exotic blocks in melange that underwent a counterclockwise pressure-temperature path wherein garnet, which strongly partitions heavy rare-earth elements, formed prior to lawsonite. Coherent mafic rocks within the Franciscan Complex, however, underwent a clockwise pressure-temperature path and lawsonite growth occurred prior to garnet. We sampled exotic blocks of garnet-hornblendite, garnet-epidote amphibolite, garnet-epidote blueschist, and lawsonite blueschist from the Berkeley Hills and Tiburon Peninsula of California. We collected four samples from coherent lawsonite blueschist across the lawsonite-pumpellyite-epidote isograds in Ward Creek, near Cazadero California. High-grade blocks give ages similar to existing Franciscan geochronology: multi-stage garnet in hornblendite gives the following ages: 171×1.3 Ma (MSWD 2.8) for the core and 159.4×0.9 Ma (MSWD 2.0) for the corresponding rim; 166

  2. Lithium Isotopic Fractionation in Subduction Zones: Clues From Clays (United States)

    Williams, L. B.; Hervig, R. L.


    Lithium isotope ratios show such large variations in nature (>30 per mil), that many areas of geosciences are exploring the usefulness of this system in explaining the evolution of particular rocks. Here we show how the lithium isotope ratios change during the transformation of smectite clay minerals to illite during burial metamorphism. Such a transition may be a common feature in the shallow regions of subduction zones and may ultimately affect the Li isotope compositions of fluids contributing to arc magmatism. Lithium is a ubiquitous trace element in natural formation waters that, like B, shows large isotopic fractionation especially during interactions with clay minerals. Lithium is adsorbed in the interlayer region of expandable clay minerals but is easily exchanged. Lithium is also incorporated into the octahedral sites. The substitutions of Li in two crystallographic sites of clay minerals may complicate interpretations of bulk Li-isotope ratios. We suggest that the magnitude of the isotopic fractionation of Li between fluid and clay is different in the interlayer sites of clay minerals than in the octahedral sites of clay minerals. Examination of Li contents and isotope variations in experimental reactions of smectite to illite (300C, 100MPa) shows changes with structural re-arrangement of the clay layers. The Li-isotope trend declines (from ~+6 to -13 per mil, expressed as ratios of 7/6) throughout R1-ordering of the mixed-layered illite smectite (I/S). However, the equilibrium end products of the reaction have R3-ordering and show a heavier isotope ratio (~0 per mil). This observation is very similar to the trends we observed for B-isotopes, where the interlayer B initially overprinted the tetrahedral-layer B isotope composition, but as the interlayer sites were collapsed during illitization, the equilibrium isotope composition was approached. The significant Li and B isotopic changes that occur during ordering of I/S coincides with the temperatures

  3. A recent deep earthquake doublet in light of long-term evolution of Nazca subduction. (United States)

    Zahradník, J; Čížková, H; Bina, C R; Sokos, E; Janský, J; Tavera, H; Carvalho, J


    Earthquake faulting at ~600 km depth remains puzzling. Here we present a new kinematic interpretation of two Mw7.6 earthquakes of November 24, 2015. In contrast to teleseismic analysis of this doublet, we use regional seismic data providing robust two-point source models, further validated by regional back-projection and rupture-stop analysis. The doublet represents segmented rupture of a ∼30-year gap in a narrow, deep fault zone, fully consistent with the stress field derived from neighbouring 1976-2015 earthquakes. Seismic observations are interpreted using a geodynamic model of regional subduction, incorporating realistic rheology and major phase transitions, yielding a model slab that is nearly vertical in the deep-earthquake zone but stagnant below 660 km, consistent with tomographic imaging. Geodynamically modelled stresses match the seismically inferred stress field, where the steeply down-dip orientation of compressive stress axes at ∼600 km arises from combined viscous and buoyant forces resisting slab penetration into the lower mantle and deformation associated with slab buckling and stagnation. Observed fault-rupture geometry, demonstrated likelihood of seismic triggering, and high model temperatures in young subducted lithosphere, together favour nanometric crystallisation (and associated grain-boundary sliding) attending high-pressure dehydration as a likely seismogenic mechanism, unless a segment of much older lithosphere is present at depth.

  4. Mesoscale subduction at the Almeria-Oran front. Part 1: Ageostrophic flow (United States)

    Allen, J. T.; Smeed, D. A.; Tintoré, J.; Ruiz, S.


    This paper presents a detailed diagnostic analysis of hydrographic and current meter data from three, rapidly repeated, fine-scale surveys of the Almeria-Oran front. Instability of the frontal boundary, between surface waters of Atlantic and Mediterranean origin, is shown to provide a mechanism for significant heat transfer from the surface layers to the deep ocean in winter. The data were collected during the second observational phase of the EU funded OMEGA project on RRS Discovery cruise 224 during December 1996. High resolution hydrographic measurements using the towed undulating CTD vehicle, SeaSoar, traced the subduction of Mediterranean Surface Water across the Almeria-Oran front. This subduction is shown to result from a significant baroclinic component to the instability of the frontal jet. The Q-vector formulation of the omega equation is combined with a scale analysis to quantitatively diagnose vertical transport resulting from mesoscale ageostrophic circulation. The analyses are presented and discussed in the presence of satellite and airborne remotely sensed data; which provide the basis for a thorough and novel approach to the determination of observational error.

  5. Aftereffects of Subduction-Zone Earthquakes: Potential Tsunami Hazards along the Japan Sea Coast. (United States)

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


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

  6. Shallow thermal structure constrained by seafloor temperature and heat flow estimated from BSRs in the Nankai subduction zone (United States)

    Ohde, A.; Otsuka, H.; Kioka, A.; Ashi, J.


    The Nankai Trough is a plate convergent boundary where earthquakes with a magnitude of 8 take place repeatedly. Thermal structure in subduction zones affects pore pressure and diagenesis such as consolidation, dewatering and cementation, and constrains physical properties of a fault-slip plane. In the Nankai subduction zone, existence of methane hydrate is confirmed from acoustic reflectors called the Bottom Simulating Reflectors (BSRs) which parallel the seafloor on seismic reflection images with high-amplitude and reverse-polarity waveforms. As a depth of BSR is theoretically constrained by subseafloor profiles of temperature and pressure, the BSR depths effectively produce subseafloor geothermal information over a wide area without heat flow probe penetration or in-situ borehole temperature measurement that is fragmentary. In this study, we aim at calculating precise two-dimensional shallow thermal structure. First, we investigate detailed distribution of the BSRs in the Nankai area ranging from offshore Tokai to Hyuga using two-dimensional multi-channel seismic reflection data. The BSR depths are then forwarded to estimate heat flow values. Second, we use a simple two-dimensional thermal modeling of Blackwell et al. [1980] that takes into account topographical effects of the seafloor roughness. We also employ additional boundary conditions constrained by seafloor temperature and the heat flow estimated from BSR depths. In order to confirm reliability of the modeled thermal structure, we additionally estimate the base of gas hydrate stability zone which is proved to almost equal to observational BSR depths. We find in the modeled thermal structure that the convex portions that are subject to cooling by cold bottom water, while depressions are less subject to the cooling from observational BSRs and theoretical calculation. The thermal structure gained here provides essential data for seismic simulations in subduction zones and for laboratory experiments as

  7. Open to Suggestion. (United States)

    Journal of Reading, 1986


    Offers (1) suggestions on how to teach students the importance of regular study habits for learning to spell, (2) story ideas to help students get started with creative writing, and (3) a model of a daily record assignment book to help students organize and remember their homework assignments. (SRT)

  8. Subduction initiation and Obduction: insights from analog models (United States)

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


    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

  9. Deep electrical resistivity structure of Costa Rican Subduction Zone (United States)

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


    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.

  10. Imaging the Lithospheric - Asthenosphere Boundary Structure of the Westernmost Mediterranean Using S Receiver Functions (United States)

    Butcher, A.; Miller, M. S.; Diaz Cusi, J.


    The Iberian microcontinent, in the westernmost portion of the Mediterranean is comprised of the Betic Cordillera Zone, the South Portuguese Zone, the Ossa-Morena Zone, the Central Iberian Zone, the Galicia-Tras Os Montes Zone, the West Asturian-Leonese Zone, and the Cantabrian Zone. These zones were created as a result of three primary stages of Iberian evolution, with the last being the collision of Iberia with in the Late Cretaceous. In northeastern Africa, Neogene convergence between the European and African plates created the Alboran System: comprised of the Gibraltar Arc, Rif-Betics, Atlas Mountains, and Alboran Sea. The primary purpose of this study is to advance our understanding of the structure and evolution of the lithosphere, as well as the lithosphere - asthenosphere boundary (LAB) of the Iberian microcontinent and surrounding areas. Of particular interest is improving our understanding of the evolution from ocean subduction to continental collision that has been taking place in the late stage convergence of this part of the Mediterranean., The region is a particularly complex three-dimensional settings and, several models have been suggested to explain the tectonics of this system including: continental lithospheric delamination and drips, slab breakoff, and subducting slab rollback. Here we use broadband seismic data from 272 broadband instruments deployed in Morocco and Spain as part of the PICASSO and IBERArray (Díaz, J., et al., 2009) projects to constrain lithospheric structure via identification of S-to-p conversions from S receiver functions (SRF). We use SRFs to image the characteristics and structure in terms of seismic velocity discontinuities, including the crust-mantle boundary (Moho) and the lithosphere-asthenosphere boundary (LAB) beneath the region. Our SRFs agree with previous work that suggests that the lithospheric thickness is shallow (~65 km) beneath the Atlas and thickest (~120 km) beneath the Rif. Additionally, LAB structures

  11. Imaging the Middle America subduction zone with body waves extracted from ambient noise by seismic interferometry (United States)

    Vargas, W.; Brown, L. D.; Cabolova, A.; Quiros, D. A.; Chen, C.


    Subduction zones have long been a prime target for seismic imaging with a variety of active and passive methodologies. Here we report an attempt to use seismic interferometry to extract body waves (P and S) from ambient noise recorded during a broadband experiment in southwestern Mexico for reflection imaging of the crust and subducting Cocos plate. The Middle America Subduction Experiment (MASE; Kim et al., 2010) included a quasi linear array of 100 broadband seismic instruments deployed at a nominal spacing of 6 km which continuously recorded for up to 30 months. Our focus was on using cross-correlation and autocorrelation of ambient noise along this array to 1) determine if useful body waves could be extracted, 2) assess which conditions were most favorable for such extraction, and 3) evaluate whether these waves could be used to image deep lithospheric structure, with particular interest in the seismogenic zone. While surface wave tomography using cross-correlation techniques have found widespread success in mapping crustal structure, examples of body wave imaging of crustal targets using this approach are still very few. In our analysis, we have found it necessary to suppress the surface wave energy to enhance body waves from virtual sources. Our pre-processing sequence includes bias removal, bandpass filtering, deconvolution (spectral whitening), and sign- bit conversion. The resulting data windows are cross-correlated and stacked until useful signals are apparent. The virtual shot gathers thus far produced show clear Rayleigh and Pg waves, with weaker but distinct Sg phases. We have also found arrivals with hyperbolic travel times that match those expected for deep reflections. Crustal imaging is limited by the large station spacing, which results in relatively few stations at sub-critical offsets. However several apparent reflections from sub-Moho depths suggest that key elements of the subduction process can be imaged using reflections derived from ambient

  12. Neoarchean Subduction Recorded in the Northern Margin of the Yangtze Craton, South China (United States)

    Zhang, S. B.; Zheng, Y. F.


    The Neoarchean is an important era during which plate tectonics began to operate widely on the earth and the continental crust compositions changed dramatically. However, reliable record of plate subduction has never been reported yet in the Yangtze Craton. Here we report geochemical studies on gneissic tonalite, trondhjemite and amphibolite in the Yudongzi Complex in the northern margin of the Yangtze Craton, which suggests that there is a plate subduction recorded in this area at about 2.7 Ga.The rocks in the Yudongzi Complex are gneissic granite, gneissic tonalite, amphibolite gneiss and magnetite quartzite. Most rocks are enriched in sodic. The gneissic granites show positive Eu anomalies, high (La/Yb)cn and Sr/Y ratios, low Ybcn and Y, resembling typical TTG. The amphibolite and tonalite gneiss show less fractionated REE patterns. SHRIMP zircon U-Pb dating on one gneissic trondhjemite, one amphibolite and one tonalite gave crystallization ages of 2667±21 Ma, 2701±10 Ma and 2697±9 Ma, respectively. They all recorded a metamorphic event at about 2.48 Ga. The SHRIMP zircon oxygen isotope analysis for a trondhjemite and an amphibolite gave δ18O values of 6.2±0.3‰ and 6.3±0.4‰, respectively. The oxygen isotope ratios higher than normal mantle values suggest a source experienced low temperature alteration. The laser fluoration analysis of bulk minerals gave δ18O values of 6.4-8.8‰ for zircon and 12.5-15.2‰ for quartz. The zircon Lu-Hf isotope analysis on the trondhjemite and amphibolite gave similar ɛHf(t) values of 0.08±0.48 and 0.07±0.63, respectively. Whole-rock ɛNd(t) values range from -1.5 to +1.0. These trondhjemite and tonalite can be interpreted as derivation from partial melting of subducted oceanic slab with a garnet-amphibolite residue.Considering the 2.67 Ga A-type granitic rocks at Huji in the interior of the craton, plate subduction took place in the northern edge of the Yangtze Craton. The Yudongzi trondhjemite and tonalite were

  13. Three-dimensional magnetotelluric imaging of Cascadia subduction zone from an amphibious array (United States)

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


    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

  14. Experimental Study of Slab-Mantle Geochemical Exchange in Subduction Zones (United States)

    Iizuka, Y.; Nakamura, E.; Kobayashi, K.


    Aqueous fluids derived from subducting oceanic crust play an important role in the material transport leading to the production or arc lavas, and in the long-term chemical evolution of the Earth's mantle and crust. In order to determine the geochemical evolution of both the subducting slab and the overlying mantle wedge, a series of dehydration/hydration experiments was carried out at conditions of 0.8-4.0 GPa and 650-900° C appropriate for subduction zones. Blueschist facies rocks/minerals, and olivine (Fo90) were used for starting materials, as analogue materials of slab and mantle, respectively. Finely ground metabasalt (H2O = 5.9 wt%) and glaucophane (H2O = 2.3 wt%) were separately sealed in gold capsules with an olivine grain (1mm diameter), and then run in a piston-cylinder apparatus. Polished sections of run products were observed and analyzed for major element compositions with an electron micro-probe. Trace elements of selected run-products were determined using an ion probe (Cameca-5f). At subsolidus conditions, the metabasalt was transformed into amphibolite-facies mineral assemblages containing Mg-ilmenite at 1.5 GPa. Glaucophane was transformed into the mineral assemblage of Na-Cpx, Opx +/- garnet. Garnets formed in the slab portion show low-LREE/HREE and higher-HREE contents when compared with the starting materials. In all subsolidus experiments, Al-rich silicate glasses, which could be quenched aqueous fluids, were observed between mineral grain boundaries in the slab portions. The fluids at 3.0 GPa show high-LREE/HREE, and higher-LILE and lower-HREE contents. In contrast, the quenched fluids for glaucophane experiments. The behavior of the HREE and HFSE is consistent with the existence of garnet and Ti-oxides (rutile and ilmenite) in the slab portion of the experiments. The fluids should therefore be enriched in SiO2, LILE and LREE. Mineral zones were observed on olivine grains near the initial olivine-slab interface. These reaction zones

  15. Breaking Boundaries

    DEFF Research Database (Denmark)

    produce desperate attempts to maintain old or create new differences. Political and sociological research into these complex processes has been mainly guided by structural and normative concerns. Faced with growing evidence about the instability of world order and domestic social structures alike, policy....... As a fundamental human experience, liminality transmits cultural practices, codes, rituals, and meanings in-between aggregate structures and uncertain outcomes. As a methodological tool it is well placed to overcome disciplinary boundaries, which often direct attention to specific structures or sectors of society....... Its capacity to provide explanatory accounts of seemingly unstructured situations provides an opportunity to link experience-based and culture-oriented approaches not only to contemporary problems but also to undertake comparisons across historical periods. From a perspective of liminality...

  16. 3D instantaneous dynamics modeling of present-day Aegean subduction (United States)

    Glerum, Anne; Spakman, Wim; van Hinsbergen, Douwe; Pranger, Casper


    To study the sensitivity of surface observables to subduction and mantle flow, i.e. the coupling of crustal tectonics and the underlying mantle dynamics, we have developed 3D numerical models of the instantaneous crust-mantle dynamics of the eastern Mediterranean. These models comprise both a realistic crust-lithosphere system and the underlying mantle. The focus for this presentation lies on the regional crustal flow response to the present-day Aegean subduction system. Our curved model domain measures 40°x40°x2900km with the Aegean subduction system taken as the geographic center. Model set-ups are based on geological and geophysical data of the eastern Mediterranean. We first create a 3D synthetic geometry of the crust-lithosphere system in a stand-alone program, including the present-day configuration of the plates in the region and crust and lithosphere thickness variations abstracted from Moho and LAB maps (Faccenna et al., 2014, Carafa et al., 2015). In addition we construct the geometry of the Aegean slab from a seismic tomography model (UU-P07; Amaru, 2007) and earthquake hypocenters (NCEDC, 2014). Geometries are then imported into the finite element code ASPECT (Kronbichler et al., 2012) using specially designed plugins. The mantle initial temperature conditions can include deviations from an adiabatic profile obtained from conversion of the UU-P07 seismic velocity anomalies to temperature anomalies using a depth-dependent scaling (Karato, 2008). We model compressible mantle flow for which material properties are obtained from thermodynamics P-T lookup-tables (Perple_X, Connolly, 2009) in combination with nonlinear viscoplastic rheology laws. Sublithospheric flow through the lateral model boundaries is left free via open boundary conditions (Chertova et al., 2012), while plate motion is prescribed at the model sides in terms of relative as well as absolute plate motion velocities (e.g. Doubrovine et al., 2012). So far, we used a free-slip surface, but

  17. Seismicity, topography, and free-air gravity of the Aleutian-Alaska subduction zone (United States)

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


    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

  18. Subduction processes related to the Sea of Okhotsk (United States)

    Zabarinskaya, Ludmila P.; Sergeyeva, Nataliya


    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. (United States)

    Debret, B; Sverjensky, D A


    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 (United States)

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


    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. Boron desorption and fractionation in Subduction Zone Fore Arcs: Implications for the sources and transport of deep fluids (United States)

    Saffer, Demian M.; Kopf, Achim J.


    At many subduction zones, pore water geochemical anomalies at seafloor seeps and in shallow boreholes indicate fluid flow and chemical transport from depths of several kilometers. Identifying the source regions for these fluids is essential toward quantifying flow pathways and volatile fluxes through fore arcs, and in understanding their connection to the loci of excess pore pressure at depth. Here we develop a model to track the coupled effects of boron desorption, smectite dehydration, and progressive consolidation within sediment at the top of the subducting slab, where such deep fluid signals likely originate. Our analysis demonstrates that the relative timing of heating and consolidation is a dominant control on pore water composition. For cold slabs, pore water freshening is maximized because dehydration releases bound water into low porosity sediment, whereas boron concentrations and isotopic signatures are modest because desorption is strongly sensitive to temperature and is only partially complete. For warmer slabs, freshening is smaller, because dehydration occurs earlier and into larger porosities, but the boron signatures are larger. The former scenario is typical of nonaccretionary margins where insulating sediment on the subducting plate is commonly thin. This result provides a quantitative explanation for the global observation that signatures of deeply sourced fluids are generally strongest at nonaccretionary margins. Application of our multitracer approach to the Costa Rica, N. Japan, N. Barbados, and Mediterranean Ridge subduction zones illustrates that desorption and dehydration are viable explanations for observed geochemical signals, and suggest updip fluid migration from these source regions over tens of km.

  2. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods (United States)

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


    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).

  3. Subduction and accretion of sedimentary rocks in the Yakutat collision zone, St. Elias orogen, Gulf of Alaska (United States)

    Van Avendonk, Harm J. A.; Gulick, Sean P. S.; Christeson, Gail L.; Worthington, Lindsay L.; Pavlis, Terry L.; Ridgway, Kenneth D.


    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. Sr, Nd, water and carbon dioxide input of altered Pacific MORB into the Tonga subduction zone (United States)

    Rosner, M.; Bach, W.; Erzinger, J.


    The hydrothermally altered and weathered uppermost section of subducting oceanic plates influences the budget of many fluid-mobile elements in supra-subduction zones. The characterisation of subducted upper basement is therefore an important parameter for the understanding of arc-magmatism. We present preliminary Sr and Nd isotope as well as H_2O^+ and CO_2 concentration data for altered basalts from ˜80 Ma Pacific crust drilled during DSDP Leg 91 (Site 595; 23^o49.3'S and 165^o31.6'W) 1000 km East of the Tonga trench. The sample set consists of 10 partially altered, aphyric microcrystalline basalts and subordinate breccia from the uppermost 55 m of basement. Smectite, celadonite, calcite, and Fe-oxides are the most abundant secondary phases, replacing igneous groundmass and filling fissures and void space. 87Sr/86Sr and 143Nd/144Nd isotope ratios vary between 0.70330 and 0.70445 and between 0.513059 and 0.513095, respectively. The H_2O^+ and CO_2 concentrations range from 2.2 to 4.3 wt.% and 0.21 to 3.13 wt.%, respectively. 87Sr/86Sr is positively correlated with H_2O^+ concentrations and alteration intensity (chiefly smectite abundance). Significant macroscopic authigenic carbonate is developed in only two samples, all others show slightly elevated concentrations between 0.21 and 0.82 wt.% related to trace calcite in veinlets. Interestingly, 143Nd/144Nd is negatively correlated with CO_2 abundance suggesting that the 143Nd/144Nd may by ascribed to seawater alteration rather than mantle source heterogeneity. Our preliminary data indicate that the intensity of alteration and the magnitude of chemical change may be greater at Site 595 than in the 6 Ma old eastern Pacific crust at Sites 504 and 896 (Alt et al., 1996) but somewhat smaller than at Sites 417/418 in 118 Ma Atlantic crust (Staudigel et al., 1995). In addition to Sites 801 and 1149 in old Pacific crust, Site 595 may provide insights into western Pacific subduction zone inputs with particular relevance for

  5. A wave equation migration method for receiver function imaging: 2. Application to the Japan subduction zone (United States)

    Chen, Ling; Wen, Lianxing; Zheng, Tianyu


    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

  6. Paleotethyan subduction process revealed from Triassic blueschists in the Lancang tectonic belt of Southwest China (United States)

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


    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.

  7. Grain Boundary Engineering of Electrodeposited Thin Films

    DEFF Research Database (Denmark)

    Alimadadi, Hossein

    of the favorable boundaries that break the network of general grain boundaries. Successful dedicated synthesis of a textured nickel film fulfilling the requirements of grain boundary engineered materials, suggests improved boundary specific properties. However, the textured nickel film shows fairly low......Grain boundary engineering aims for a deliberate manipulation of the grain boundary characteristics to improve the properties of polycrystalline materials. Despite the emergence of some successful industrial applications, the mechanism(s) by which the boundary specific properties can be improved...... to engineer new materials. In this study, one of the most widely used electrolytes for electrodeposition is chosen for the synthesis of nickel films and based on thorough characterization of the boundaries the potentials in grain boundary engineering are outlined. The internal structure of the nickel films...

  8. Mapping fluids to subduction megathrust locking and slip behavior (United States)

    Saffer, Demian M.


    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.

  9. A Silent Slip Event on the Deeper Cascadia Subduction Interface (United States)

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


    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.

  10. A silent slip event on the deeper Cascadia subduction interface. (United States)

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


    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.

  11. Friction and stress coupling on the subduction interfaces (United States)

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


    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

  12. Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: An integrated geodynamic and petrological modelling approach (United States)

    Palin, Richard M.; Reuber, Georg S.; White, Richard W.; Kaus, Boris J. P.; Weller, Owen M.


    The Tso Morari massif is one of only two regions where ultrahigh-pressure (UHP) metamorphism of subducted crust has been documented in the Himalayan Range. The tectonic evolution of the massif is enigmatic, as reported pressure estimates for peak metamorphism vary from ∼2.4 GPa to ∼4.8 GPa. This uncertainty is problematic for constructing large-scale numerical models of the early stages of India-Asia collision. To address this, we provide new constraints on the tectonothermal evolution of the massif via a combined geodynamic and petrological forward-modelling approach. A prograde-to-peak pressure-temperature-time (P-T-t) path has been derived from thermomechanical simulations tailored for Eocene subduction in the northwestern Himalaya. Phase equilibrium modelling performed along this P-T path has described the petrological evolution of felsic and mafic components of the massif crust, and shows that differences in their fluid contents would have controlled the degree of metamorphic phase transformation in each during subduction. Our model predicts that peak P-T conditions of ∼2.6-2.8 GPa and ∼600-620 ∘C, representative of 90-100 km depth (assuming lithostatic pressure), could have been reached just ∼3 Myr after the onset of subduction of continental crust. This P-T path and subduction duration correlate well with constraints reported for similar UHP eclogite in the Kaghan Valley, Pakistan Himalaya, suggesting that the northwest Himalaya contains dismembered remnants of what may have been a ∼400-km-long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie-Sulu belt, China. A maximum overpressure of ∼0.5 GPa was calculated in our simulations for a homogeneous crust, although small-scale mechanical heterogeneities may produce overpressures that are larger in magnitude. Nonetheless, the extremely high pressures for peak metamorphism reported by some workers (up to 4.8 GPa) are unreliable owing to conventional thermobarometry

  13. Middle Ordovician subduction of continental crust in the Scandinavian Caledonides - an example from Tjeliken, Seve Nappe Complex, Sweden (United States)

    Fassmer, Kathrin; Andersson, Barbro; Klonowska, Iwona; Walczak, Katarzyna; Froitzheim, Nikolaus; Majka, Jarosław; Fonseca, Raul


    identical ages within error suggests fast subduction. Brueckner & Van Roermund 2007. Journal of the Geological Society, London, 164, 117-128. Majka et al. 2014. In: Corfu et al., eds., New Perspectives on the Caledonides of Scandinavia and Related Areas: Geological Society of London Special Publication 390, p. 369-384. Root & Corfu 2012. Contributions to Mineralogy and Petrology, 163, 769-788.

  14. Did high Neo-Tethys subduction rates contribute to early Cenozoic warming?


    Hoareau, Guilhem; BOMOU, B.; van Hinsbergen, D. J. J.; Carry, N.; Marquer, D.; Donnadieu, Y.; Le Hir, G.; Vrielynck, Bruno; Walter-Simonnet, A.-V.


    The 58–51 Ma interval was characterized by a long-term increase of global temperatures (+4 to +6 °C) up to the Early Eocene Climate Optimum (EECO, 52.9–50.7 Ma), the warmest interval of the Cenozoic. It was recently suggested that sustained high atmospheric pCO2, controlling warm early Cenozoic climate, may have been released during Neo-Tethys closure through the subduction of large amounts of pelagic carbonates and their recycling as CO2 at arc volcanoes. To analyze the imp...

  15. Large trench-parallel gravity variations predict seismogenic behavior in subduction zones. (United States)

    Song, Teh-Ru Alex; Simons, Mark


    We demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench-parallel gravity anomaly (TPGA), whereas regions with strongly positive TPGA are relatively aseismic. These observations suggest that, over time scales up to at least 1 million years, spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore-arc. The correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench-parellel variations in fore-arc topography, gravity, and seismogenic behavior.

  16. The Mesozoic accretionary complex in Northeast China: Evidence for the accretion history of Paleo-Pacific subduction (United States)

    Zhou, Jian-Bo; Li, Long


    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).

  17. 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

  18. Review of subduction and its association with geothermal system in Sumatera-Java (United States)

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


    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.

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

    NARCIS (Netherlands)

    Cloetingh, Sierd


    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

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

    NARCIS (Netherlands)

    Cloetingh, S.A.P.L.


    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

  1. The course of water in Archean subduction systems (United States)

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


    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.

  2. Slab melting versus slab dehydration in subduction-zone magmatism. (United States)

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


    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.

  3. Surface deformation resulting from subduction and slab detachment

    NARCIS (Netherlands)

    Buiter, S.J.H.


    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

  4. Tensor-guided fitting of subduction slab depths (United States)

    Bazargani, Farhad; Hayes, Gavin P.


    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.

  5. 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


    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)

  6. Turbidite event history--Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone (United States)

    Goldfinger, Chris; Nelson, C. Hans; Morey, Ann E.; Johnson, Joel E.; Patton, Jason R.; Karabanov, Eugene; Gutierrez-Pastor, Julia; Eriksson, Andrew T.; Gracia, Eulalia; Dunhill, Gita; Enkin, Randolph J.; Dallimore, Audrey; Vallier, Tracy; Kayen, Robert; Kayen, Robert


    Turbidite systems along the continental margin of Cascadia Basin from Vancouver Island, Canada, to Cape Mendocino, California, United States, have been investigated with swath bathymetry; newly collected and archive piston, gravity, kasten, and box cores; and accelerator mass spectrometry radiocarbon dates. The purpose of this study is to test the applicability of the Holocene turbidite record as a paleoseismic record for the Cascadia subduction zone. The Cascadia Basin is an ideal place to develop a turbidite paleoseismologic method and to record paleoearthquakes because (1) a single subduction-zone fault underlies the Cascadia submarine-canyon systems; (2) multiple tributary canyons and a variety of turbidite systems and sedimentary sources exist to use in tests of synchronous turbidite triggering; (3) the Cascadia trench is completely sediment filled, allowing channel systems to trend seaward across the abyssal plain, rather than merging in the trench; (4) the continental shelf is wide, favoring disconnection of Holocene river systems from their largely Pleistocene canyons; and (5) excellent stratigraphic datums, including the Mazama ash and distinguishable sedimentological and faunal changes near the Pleistocene-Holocene boundary, are present for correlating events and anchoring the temporal framework. Multiple tributaries to Cascadia Channel with 50- to 150-km spacing, and a wide variety of other turbidite systems with different sedimentary sources contain 13 post-Mazama-ash and 19 Holocene turbidites. Likely correlative sequences are found in Cascadia Channel, Juan de Fuca Channel off Washington, and Hydrate Ridge slope basin and Astoria Fan off northern and central Oregon. A probable correlative sequence of turbidites is also found in cores on Rogue Apron off southern Oregon. The Hydrate Ridge and Rogue Apron cores also include 12-22 interspersed thinner turbidite beds respectively. We use 14C dates, relative-dating tests at channel confluences, and

  7. Recycling Revisited: Where did all the Subducted Sediments go? (United States)

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


    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

  8. Mapping seismic azimuthal anisotropy of the Japan subduction zone (United States)

    Zhao, D.; Liu, X.


    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

  9. Three-dimensional Thermal Model of the Mexican Subduction Zone (United States)

    Rosas, J. C.; Pimentel, F. D. C.; Currie, C. A.; He, J.; Harris, R. N.


    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

  10. Using paleomagnetism to expand the observation time window of plate locking along subduction zones: evidence from the Chilean fore-arc sliver (38°S - 42°S) (United States)

    Hernandez-Moreno, Catalina; Speranza, Fabio; Di Chiara, Anita


    Fore-arc crustal motion has been usually addressed by the analysis of earthquake slip vectors and, since the last twenty years, by velocity fields derived from Global Positioning System (GPS) data. Yet this observation time window (few decades) can be significantly shorter than a complete seismic cycle or constrained to interseismic periods where the postseismic deformation release, the vicinity of other important faults, and the slip partitioning in oblique subduction may hinder the finite deformation pattern. Paleomagnetic data may yield finite rotations occurring since rock formation, thus provide a much longer observation time span in the order of millions or tens of millions of years. The cumulative permanent or nonreversing deformation in function of the considered geological formation age can represent the average over many seismic cycles, thus significantly complement "instantaneous" information derived from seismic and GPS data. With the aim of evaluate the strike-variation and evolution of the plate coupling along the Chilean subduction zone, here we report on the paleomagnetism of 43 Oligocene-Pleistocene volcanic sites from the fore-arc sliver between 38°S and 42°S. Sites were gathered west of the 1000 km long Liquiñe-Ofqui dextral fault zone (LOFZ) that represents the eastern fore-arc sliver boundary. Nineteen reliable sites reveal that the fore arc is characterized by counterclockwise (CCW) rotations of variable magnitude, except at 40°S - 41°S, where ultrafast (>50°/Myr) clockwise (CW) rotations occur within a 30 km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (>10°/Myr) suggest that the observed block rotation pattern is related to NW-SE seismically active sinistral faults crosscutting the whole fore arc. According to previously published data, CW rotations up to 170° also occur east of the LOFZ and have been related to ongoing LOFZ shear. We suggest that the occurrence and width of the eastern

  11. Wind-induced subduction at the South Atlantic subtropical front (United States)

    Calil, Paulo H. R.


    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.

  12. Predicting SKS-splitting from 35 Myr of subduction and mantle flow evolution in the western Mediterranean (United States)

    Chertova, Maria; Spakman, Wim; Faccenda, Manuele


    We investigate the development of mantle anisotropy associated with the evolution of the Rif-Gibraltar-Betic (RGB) slab of the western Mediterranean and predict SKS-splitting directions for comparison with the recent observations compiled in Diaz and Gallart (2014). Our numerical model of slab evolution starts at 35 Ma and builds on our on recent work (Chertova et al., 2014) with the extension of imposing mantle flow velocities on the side boundaries of the model (Chertova et al., 2017). For the calculation of the evolution of finite strain deformation from the mantle flow field and for prediction of SKS-splitting directions we use the modified D-Rex program of Faccenda (2014). We test the predicted splitting observations against present-day shear wave splitting observations for subduction models with open boundary conditions (Chertova, 2014) and for models with various prescribed mantle flow conditions on the model side boundaries. The latter are predicted time-dependent (1 Myr time steps) velocity boundary conditions computed from back-advection of a temperature and density model of the present-day mantle scaled from a global seismic tomography model (Steinberger et al., 2015). These boundary conditions where used recently to demonstrate the relative insensitivity of RGB slab position and overall slab morphology for external mantle flow (Chertova et al., 2017). Using open boundaries only we obtain a poor to moderate fit between predicted and observed splitting directions after 35 Myr of slab and mantle flow evolution. In contrast, a good fit is obtained when imposing the computed mantle flow velocities on the western, southern, and northern boundaries during 35 Myr of model evolution. This successful model combines local slab-driven mantle flow with remotely forced mantle flow. We are in the process to repeat these calculations for shorter periods of mantle flow evolution to determine how much of past mantle flow is implicitly recorded in present-day observation

  13. Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: Initial report (United States)

    Haxel, Gordon B.; Jacobson, Carl E.; Wittke, James H.


    The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

  14. Subduction zone slip variability during the last millennium, south-central Chile (United States)

    Dura, Tina; Horton, Benjamin P.; Cisternas, Macro; Ely, Lisa L; Hong, Isabel; Nelson, Alan R.; Wesson, Robert L.; Pilarczyk, Jessica E.; Parnell, Andrew C.; Nikitina, Daria


    The Arauco Peninsula (37°-38°S) in south-central Chile has been proposed as a possible barrier to the along-strike propagation of megathrust ruptures, separating historical earthquakes to the south (1960 AD 1837, 1737, and 1575) and north (2010 AD, 1835, 1751, 1657, and 1570) of the peninsula. However, the 2010 (Mw 8.8) earthquake propagated into the Arauco Peninsula, re-rupturing part of the megathrust that had ruptured only 50 years earlier during the largest subduction zone earthquake in the instrumental record (Mw 9.5). To better understand long-term slip variability in the Arauco Peninsula region, we analyzed four coastal sedimentary sections from two sites (Tirúa, 38.3°S and Quidico, 38.1°S) located within the overlap of the 2010 and 1960 ruptures to reconstruct a ∼600-year record of coseismic land-level change and tsunami inundation. Stratigraphic, lithologic, and diatom results show variable coseismic land-level change coincident with tsunami inundation of the Tirúa and Quidico marshes that is consistent with regional historical accounts of coseismic subsidence during earthquakes along the Valdivia portion of the subduction zone (1960 AD and 1575) and coseismic uplift during earthquakes along the Maule portion of the subduction zone (2010 AD, 1835, 1751). In addition, we document variable coseismic land-level change associated with three new prehistoric earthquakes and accompanying tsunamis in 1470–1570 AD, 1425–1455, and 270–410. The mixed record of coseismic subsidence and uplift that we document illustrates the variability of down-dip and lateral slip distribution at the overlap of the 2010 and 1960 ruptures, showing that ruptures have repeatedly propagated into, but not through the Arauco Peninsula and suggesting the area has persisted as a long-term impediment to slip through at least seven of the last megathrust earthquakes (∼600 years).

  15. First results of high-resolution modeling of Cenozoic subduction orogeny in Andes (United States)

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


    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. Deformation Processes of Subduction and Exhumation in Alpine Eclogites with Focus on the Tauern Window (United States)

    Stipp, M.; Keppler, R.; Behrmann, J. H.; Heidelbach, F.


    Deformation processes inside subduction channels or during subduction and exhumation of HP and UHP nappes in collisional orogens are a matter of debate. Dehydration embrittlement, localized faulting and dissolution-precipitation creep have been suggested as major deformation processes. These processes, however, do not correspond to the field-based deformation record of penetrative foliations and stretching lineations in most HP and UHP nappes of the Central Alps, indicative of dislocation and diffusion creep processes including metamorphic reaction transfer. The Eclogite Zone of the Tauern Window (Eastern Alps) consists of fresh and retrogressed eclogites in a matrix of metasediments. Peak metamorphism was at 600°C and 2.0-2.5 GPa in the Oligocene followed by fast exhumation within a few million years. Eclogites and blueschists display a pronounced foliation and lineation fabric. Fresh and retrogressed samples have been investigated by neutron diffraction texture, electron backscatter diffraction and microprobe analysis. All investigated eclogites exhibit a pronounced crystallographic preferred orientation (CPO) of omphacite. In retrogressed eclogites the glaucophane CPO is strong as well and matches topologically always that of omphacite. Omphacite microstructures are characteristic of subgrain rotation recrystallization, identifying high-stress dislocation creep as main deformation mechanism. Diffusion and reaction transfer can be derived from the variable jadeite contents in omphacite porphyroclasts and recrystallized grains. The corresponding CPO of omphacite and glaucophane indicates that progressive CPO development spanned the whole high-pressure part of metamorphism and lasted until retograde blueschist grade. We propose that deformation in the Eclogite Zone was mostly distributed and continuous during subduction and exhumation and that dislocation and diffusion creep processes accommodated most of the strain below the frictional-viscous transition.

  17. Mantle convection, tectonics and the evolution of the Tethyan subduction zone (United States)

    Jolivet, Laurent; Sternai, Pietro; Menant, Armel; Faccenna, Claudio; Becker, Thorsten; Burov, Evguenii


    Mantle convection drives plate tectonics and the size, number and thermotectonic age of plates codetermines the convection pattern. However, the degree of coupling of surface deformation and mantle flow is unclear. Most numerical models of lithospheric deformation are designed such that strain is a consequence of kinematic boundary conditions, and rarely account for basal stresses due to mantle flow. On the other hand, convection models often treat the lithosphere as a single-layer stagnant lid with vertically undeformable surface. There is thus a gap between convection models and lithospheric-scale geodynamic models. The transmission of stresses from the flowing mantle to the crust is a complex process. The presence of a ductile lower crust inhibits the upward transmission of stresses but a highly extended crust in a hot environment such as a backarc domain, with no lithospheric mantle and a ductile lower crust in direct contact with asthenosphere, will be more prone to follow the mantle flow than a thick and stratified lithosphere. We review geological observations and present reconstructions of the Aegean and Middle East and discuss the possible role played by basal drag in governing lithospheric deformation. In Mediterranean backarc regions, lithosphere-mantle coupling is effective on geological time scale as shown by the consistency of SKS fast orientations in the mantle with stretching directions in the crust. The long-term geological history of the Tethyan convergent zone suggests that asthenospheric flow has been an important player. The case of Himalaya and Tibet strongly supports a major contribution of a northward asthenospheric push, with no persistent slab that could drive India after collision, large thrust planes being then decoupling zones between deep convection and surface tectonics. The African plate repeatedly fragmented during its northward migration with the separation of Apulia and Arabia. Indeed, extension has been active on the northern

  18. Trace-Element Mobility in Eclogite-Facies Subducted Crust: Garnet, Zircon, and Rutile Petrochronology of As Sifah, Oman (United States)

    Garber, Joshua M.; Rioux, Matthew; Kylander-Clark, Andrew R.; Vervoort, Jeff D.; Wilford, Diane; Hacker, Bradley R.; Searle, Michael P.; Waters, David J.; Warren, Clare


    Trace element flux in subduction zones is strongly affected by the chemistry and availability of an accompanying fluid, in addition to mineral partitioning behavior. These factors may be exacerbated in certain lithologies, suggesting a role for lithology-specific rock permeability, fluid fugacity, and/or trace-element partitioning during subduction. To assess lithological controls on elemental transport in subducted mafic crust and sediment, we obtained dates (Sm-Nd, Lu-Hf, and U-Pb) and major- and trace-element concentrations for garnet, zircon, and rutile in end-member mafic and metapelitic rocks from the ultrahigh-pressure As Sifah unit of Oman. The three phases record similar isotopic dates among all lithologies, but trace-element records for each phase are distinct for each rock type. For example, i) mafic rocks show expected garnet/zircon rare-earth element (REE) partitioning, but metapelitic garnet and zircon do not, and ii) mafic rutiles record lower intergranular solubilities for high-field-strength elements (HFSEs) than pelitic rutiles. Together, these data suggest that REE and HFSE equilibrium length-scales varied significantly between adjacent lithologies, implicating crucial differences in fluid flux during subduction. Further, Lu-Hf isotopic data are scattered and non-isochronous for all rocks - even in metapelites that exhibit cm- to outcrop-scale HFSE equilibrium length-scales - suggesting that achievement of elemental equilibrium does not imply isotopic equilibrium, even at the same scale. Our approach illustrates the power of multi-phase petrochronology in determining the behavior of distinct trace-element groups during metamorphism.

  19. Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles (United States)

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


    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

  20. Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles (United States)

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


    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.

  1. Plume-subduction interaction in southern Central America: Mantle upwelling and slab melting (United States)

    Gazel, Esteban; Hoernle, Kaj; Carr, Michael J.; Herzberg, Claude; Saginor, Ian; den Bogaard, Paul van; Hauff, Folkmar; Feigenson, Mark; Swisher, Carl


    The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~ 1380-1410 °C) in the Pliocene to ~ 1450 °C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0-2.7 GPa). The average final melting pressures range between 2.7 and 2.5 GPa, which is interpreted as the lithosphere-asthenosphere boundary at ~ 85-90 km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~ 10-8 Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the

  2. Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust (United States)

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


    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

  3. IODP Expedition 333: Return to Nankai Trough Subduction Inputs Sites and Coring of Mass Transport Deposits

    Directory of Open Access Journals (Sweden)

    Michael Strasser


    Full Text Available Integrated Ocean Drilling Program (IODP Expedition 333 returned to two sites drilled during IODP Expedition 322 on the ocean side of the Nankai Trough to pursue the characterization of the inputs to the Nankai subduction and seismogenic zone, as part of the Nankai Trough Seismogenic Experiment (NanTroSEIZE multi-expedition project. SiteC0011 is located at the seaward edge of the trench and Site C0012 on a basement high, Kashinozaki Knoll (Fig. 1. The main objectives of drilling again at these sites were to fill coring gaps in the upper part (<350 m of the sedimentary sequence, to measure heat flow, and to core the oceanic basement to a greater depth on the Knoll. New results include the observation of a diagenetic boundary within the Shikoku Basin sediments that may be compared to one documented further west by ODP Legs 131, 190 and 196 but occurs here at a lower temperature. Borehole heat flow measurements confirm spatial variations in the Shikoku Basin that were indicated by short probe surveys. Heat flow variations between topographic highs and lows may be related to fluid convection within the basement. This expedition also included the objectives of the Nankai Trough Submarine LandSLIDEhistory (NanTroSLIDE Ancillary Project Letter (APL and cored at Site C0018 a pile of mass transport deposits on the footwall of the megasplay fault, a major out of sequence thrust that presumably slips coseismically during large subduction earthquakes. This brought newinsight on the timing of these mass wasting events and on the deformation within the sliding slope sediments.

  4. Mineralogy of subducted clay and clay restite in the lower mantle (United States)

    Armstrong, L.; Skora, S. E.; Walter, M. J.


    Seismic tomography indicates that subducting oceanic lithosphere often penetrates the transition zone and eventually the lower mantle [e.g. 1, 2]. While mineralogical changes in the mafic and ultramafic portions of slabs have been well documented experimentally, the phase relations of overlying sediments at pressures above 25 GPa remain poorly studied. This is in part because sediments are expected to partially melt at sub-arc depth (P~2.5-4.5 GPa), and contribute to the genesis of arc magmas. Sediment restites left behind after the extraction of low pressure melts undergo major chemical changes, according to the melting reaction: Coe+Phen+Cpx+H2O = Grt+Ky+Melt [3]. However, sediments may not always melt depending on the thermal regime and volatile availability and composition [3]. Hence, chemically unmodified sediments as well as restites may be entrained to greater depths and contribute to compositional heterogeneity in the deep mantle. Indeed, mineral inclusions with compositions indicative of subducted sedimentary protoliths (CAS-phase; K-hollandite; stishovite) have been reported in 'ultradeep' diamonds and suggest that deep subduction and survival of sediments occurs to at least transition zone depths [4]. With this in mind, we have performed laser heated diamond anvil cell experiments at pressures of 8-80 GPa on two anhydrous glass starting materials: a marine clay and the restite that is left after 50% melt extraction of this clay at 3 GPa and 800 °C [3]. We chose to work with an anhydrous version of the marine clay given that the investigated pressure range exceeds that of phengite stability [5], and phengite is the only hydrous phase in subducted sediments at UHP conditions. The clay was heated along a P-T path representative of a cold subduction geotherm, whereas the clay restite was heated along a hotter subduction geotherm consistent with low pressure melting. Phases were identified by synchrotron X-ray micro-diffraction at beamline I15 of the Diamond

  5. Geochemical evidence for subduction in the early Archaean from quartz-carbonate-fuchsite mineralization, Isua Supracrustal Belt, West Greenland

    DEFF Research Database (Denmark)

    Pope, Emily Catherine; Rosing, Minik Thorleif; Bird, Dennis K.

    of this metasomatic-tectonic relationship requires that 1) Phanerozoic orogenic Au-deposits form in subduction-zone environments, and 2) the geochemical similarity of Precambrian orogenic deposits to their younger counterparts is the result of having the same petrogenetic origin. Hydrogen and oxygen isotope...... compositions of fuchsite and quartz from auriferous mineralization in the ca. 3.8 Ga Isua Supracrustal Belt (ISB) in West Greenland, in conjunction with elevated concentrations of CO2, Cr, Al, K and silica relative to protolith assemblages, suggest that this mineralization shares a common petro-tectonic origin......, are the result of seawater-derived fluids liberated from subducting lithosphere interacting with ultramafic rocks in the mantle wedge and lower crust, before migrating up crustal-scale vertical fracture zones. Thus, the presence of quartz-carbonate-fuchsite mineralization in the Isua supracrustal belt and other...

  6. Constraining the Rate of Water-Releasing Metamorphic Reactions in Subduction Zones (United States)

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


    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

  7. Dynamic Topography during Flat Subduction: Subsidence or Uplift? (United States)

    Davila, F. M.; Lithgow-Bertelloni, C. R.


    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

  8. Teleseismic P wave tomography of South Island, New Zealand upper mantle: Evidence of subduction of Pacific lithosphere since 45 Ma (United States)

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


    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.

  9. Detailed Structure and Thickness of Upper Mantle Discontinuities in the Tonga Subduction Zone From Regional Broadband Arrays (United States)

    Tibi, R.; Wiens, D. A.


    thickness is estimated to be at most ˜10 km outside as well as within the slab, and the 660 width ˜5 km. This suggests that the subduction process has probabily little effect if any on the sharpness of the discontinuities.

  10. Nb/Ta - Zr/Hf Fractionations during Subduction: Implications for the'Missing' Nb. (United States)

    Zateslo, T.; Bizimis, M.; Salters, V. J.; Stern, C.; Taylor, R. N.


    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. Here we report the first high precision HFSE data on products of the subduction processes thought to fractionate Nb from Ta: boninites (hydrous melting), adakites (slab melting), OIBs (Koolau, Walvis: plume with recycled oceanic crust), as well as kimberlites and lamproites. 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. The Koolau and Walvis Ridge lavas have subchondritic Nb/Ta for a given Zr/Hf, overlapping other OIB suites and show no evidence for a recycled, high Nb/Ta reservoir in their source. OIB, considered as a group, have relatively constant Nb/Ta (15-16) but more variable Zr/Hf (35-50). In contrast, boninites (Chichi Jima) have significantly subchondritic Nb/Ta (4-12) at near constant Zr/Hf (35), while adakites (South Andes) extend to near chondritic Nb/Ta (13-19) at more variable Zr/Hf (30-40). The adakites showing the least evidence for crustal contamination have the highest Nb/Ta. The arc lavas cross the OIB trend at near right angle on a Nb/Ta vs. Zr/Hf plot having larger Nb

  11. Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity (United States)

    Wannamaker, Philip E.; Evans, Rob L.; Bedrosian, Paul A.; Unsworth, Martyn J.; Maris, Virginie; McGary, R. Shane


    Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc.

  12. Transitional time of oceanic to continental subduction in the Dabie orogen: Clues from the Triassic age for oceanic eclogites (United States)

    Cheng, H.; Dufrane, S.; Nakamura, E.; Vervoort, J. D.


    Low-temperature and high-pressure eclogites with an oceanic affinity in the western part of the Dabie orogen have been investigated with combined Lu-Hf and U-Pb geochronology. These eclogites formed over a range of temperatures (482-565°C and 1.9-2.2 GPa). Three eclogites, which were sampled from the Gaoqiao country, yield Lu-Hf ages of 240.7 ± 1.2 Ma, 243.3 ± 4.1 Ma and 238.3 ± 1.2 Ma, with a corresponding lower-intercept U-Pb zircon age of 232 ± 26 Ma. Despite the well-preserved prograde major- and trace-element zoning in garnets, mineralogical and petrologic studies suggest that Lu-Hf ages mostly reflect a later garnet growth episode. These ages mark the termination of high-pressure eclogite-facies metamorphism instead of representing the early phase of garnet growth. An upper-intercept zircon U-Pb age of 765 ± 24 Ma is defined for the Gaoqiao eclogite, which is consistent with the weighted mean age of 768 ± 21 Ma for the country gneiss. This suggests an analogue protolith origin; however, the gneiss has not been subjected to successive high-pressure metamorphism. The new Triassic ages are thus either an estimate of the involvement of oceanic fragments in the continental subduction or a milestone of the termination of oceanic subduction. The latter implies that different oceanic crustal slices/fragments reached peak metamorphism and started to exhume at diverse times, rather than being subducted and exhumed as a whole. Despite these results, many fundamental questions regarding the multi-slices subduction and exhumation hypothesis remain unanswered.

  13. Thermal State, Slab Metamorphism, and Interface Seismicity in the Cascadia Subduction Zone Based On 3-D Modeling (United States)

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


    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.

  14. Grain boundaries: Progress report

    Energy Technology Data Exchange (ETDEWEB)

    Balluffi, R.W.; Bristowe, P.D.


    Quantitative measurements of grain boundary structure factors using x-ray diffraction have been performed on low angle (001) twist boundaries in gold. Also, a computer atomistic simulation program is being implemented to examine the equilibrium properties of a series of boundaries in gold. Simulation of boundaries at room temperature have been performed. Electron microscopy of grain boundary melting in aluminum was also performed. Results indicated an absence of melting. (CBS)

  15. Gravity and Magnetic Anomaly Interpretations and 2.5D Cross-Section Models over the Border Ranges Fault System and Aleutian Subduction Zone, Alaska (United States)

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


    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

  16. What favors the occurrence of subduction mega-earthquakes? (United States)

    Brizzi, Silvia; Funiciello, Francesca; Corbi, Fabio; Sandri, Laura; van Zelst, Iris; Heuret, Arnauld; Piromallo, Claudia; van Dinther, Ylona


    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

  17. Nonuniform subduction of the Indian crust beneath the Himalayas. (United States)

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


    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.

  18. The Community Boundary De-paradoxifyed

    DEFF Research Database (Denmark)

    Dragsdahl Lauritzen, Ghita; Salomo, Søren


    . In order to improve connections and collaborations across interfaces, it is therefore necessary to improve our understanding of the community boundary construct. Existing studies of community boundaries within the user innovation literature predominantly describe boundaries as incentives for user...... participation without a clear distinction of what is part of the community and what is not. This gap is intensified by the emergence of virtual communities, where the notion of boundary is even more distorted. The paper suggests a new construct of virtual community boundaries that sets up the distinction...... between community and its environment differently from existing studies of virtual communities. Instead of taking its starting point in the users, the paper takes an organizational approach and focuses on the function of the community boundary construct. Hereby, the paper shows how community boundaries...

  19. The dominant surface-topography contributions of individual subduction parameters (United States)

    Crameri, Fabio; Lithgow-Bertelloni, Carolina; Tackley, Paul


    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 (, 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.

  20. Complex Subduction Imaged by Diffractional Tomography of USArray Receiver Functions (United States)

    Zhou, Y.


    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.

  1. Crust and subduction zone structure of Southwestern Mexico (United States)

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


    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.

  2. Silicate dissolution boosts the CO2 concentrations in subduction fluids. (United States)

    Tumiati, S; Tiraboschi, C; Sverjensky, D A; Pettke, T; Recchia, S; Ulmer, P; Miozzi, F; Poli, S


    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.

  3. Ups and downs in western Crete (Hellenic subduction zone). (United States)

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


    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.

  4. Water and the oxidation state of subduction zone magmas. (United States)

    Kelley, Katherine A; Cottrell, Elizabeth


    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.

  5. An integrated approach to the seismic activity and structure of the central Lesser Antilles subduction megathrust seismogenic zone (United States)

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


    In order to increase the understanding of plate boundaries that show currently low seismic activity, as was the Sumatra-Andaman subduction before the major earthquake in 2004, a cluster of surveys and cruises has been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project on the Lesser Antilles subduction zone of the Carribean-America plate boundary. A segment of the corresponding transform boundary just tragically ruptured in the 2010 January 12, Haïti earthquake. This cluster is composed by the German cruise TRAIL with the vessel F/S M. A. MERIAN, the French cruise SISMANTILLES II with the IFREMER vessel N/O ATALANTE), and French cruise OBSANTILLES with the IRD vessel N/O ANTEA. During these cruises and surveys, 80 OBS, Ocean Bottom Seismometers, 64 of which with 3-components seismometers and hydrophones, and 20 OBH with hydrophones have been brought together from several pools (Geoazur Nice, INSU/IPGP Paris, IfM-GEOMAR Kiel, AWI Bremerhaven), with up to 30 land stations (CSIC Barcelona, IPG Paris, INSU-RLBM and -Lithoscope, ETH Zurich). The deployment of all these instruments has been supported principally in addition by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT. The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the subduction mega-thrust fault plane, and which is here the

  6. Shallow Low-frequency Tremor in the Hyuga-nada region, western Nankai Trough subduction zone, observed by ocean bottom seismographic experiment (United States)

    Yamashita, Y.; Yakiwara, H.; Shimizu, H.; Uchida, K.; Kamizono, M.; Nakamoto, M.; Fukui, M.; Fujita, S.; Aizawa, K.; Miyamachi, H.; Hirano, S.; Umakoshi, K.; Yamada, T.; Kanehara, H.; Aoshima, T.


    The Hyuga-nada region, locating western Nankai trough, is one of the most seismically active areas in Japan. Here, the Philippine Sea Plate subducts northwestward beneath the Eurasian Plate at an approximate rate of 5-7 cm/yr [e.g., Seno et al., 1993; Miyazaki and Heki, 2001]. Interplate earthquakes with magnitudes in the range of 6.5 to 7.5 repeatedly occur at intervals of decades. In the shallower part of the plate boundary in this region, the shallow very-low frequency earthquakes (dominant frequency 10~20 s) occur [Obara and Ito, 2005; Asano et al., 2008]. The shallow part of the plate boundary zone is very important for the generation of large interplate earthquakes and following tsunami. In order to reveal the detail of microseismicity from the shallower part of the plate boundary to seismogenic zone in the Hyuga-nada region, we have conducted Ocean Bottom Seismographic experiment from May 19 until July 6, 2013. We used 12 Ocean Bottom Seismometers (OBSs) with a three-component short-period (10 OBSs: 4.5Hz, 2 OBSs: 1Hz) seismometer. All OBSs were recovered but one OBS was no data because of the technical problem of the recorder. During this experiment, many earthquakes recorded by OBSs. In addition, many low-frequency signals were also recorded. From the characteristic of the waveform and estimated source location, these are the shallow low-frequency tremor which is recorded for the first time by close-in observation the Hyuga-nada region. Here, we report the result of preliminary analysis of these shallow low-frequency tremors. The tremor activity mainly occurred from end of May to end of July 2013. Dominant frequency range of these tremors are 1-8 Hz and long duration range (10 seconds ~ a few minutes), which is same character of low-frequency tremor observed in Kii-Peninsula, Nankai trough using short-period OBSs [Obana and Kodaira, 2009]. We estimated tremor source location using envelop correlation method [Obara, 2002]. Although we estimated only a few

  7. Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts. (United States)

    Elliott, Tim; Thomas, Alex; Jeffcoate, Alistair; Niu, Yaoling


    'Recycled' crustal materials, returned from the Earth's surface to the mantle by subduction, have long been invoked to explain compositional heterogeneity in the upper mantle. Yet increasingly, problems have been noted with this model. The debate can be definitively addressed using stable isotope ratios, which should only significantly vary in primitive, mantle-derived materials as a consequence of recycling. Here we present data showing a notable range in lithium isotope ratios in basalts from the East Pacific Rise, which correlate with traditional indices of mantle heterogeneity (for example, 143Nd/144Nd ratios). Such co-variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle. Contrary to many models, however, the elevated lithium isotope ratios of the 'enriched' East Pacific Rise lavas imply that subducted ocean crust is not the agent of enrichment. Instead, we suggest that fluid-modified mantle, which is enriched during residency in a subduction zone, is mixed back into the upper mantle to cause compositional variability.

  8. Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc (United States)

    Freymuth, Heye; Vils, Flurin; Willbold, Matthias; Taylor, Rex N.; Elliott, Tim


    The fate of crustal material recycled into the convecting mantle by plate tectonics is important for understanding the chemical and physical evolution of the planet. Marked isotopic variability of Mo at the Earth's surface offers the promise of providing distinctive signatures of such recycled material. However, characterisation of the behaviour of Mo during subduction is needed to assess the potential of Mo isotope ratios as tracers for global geochemical cycles. Here we present Mo isotope data for input and output components of the archetypical Mariana arc: Mariana arc lavas, sediments from ODP Sites 800, 801 and 802 near the Mariana trench and the altered mafic, oceanic crust (AOC), from ODP Site 801, together with samples of the deeper oceanic crust from ODP Site 1256. We also report new high precision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc lavas are enriched in Mo compared to elements of similar incompatibility during upper mantle melting, and have distinct, isotopically heavy Mo (high 98Mo/95Mo) relative to the upper mantle, by up to 0.3 parts per thousand. In contrast, the various subducting sediment lithologies dominantly host isotopically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas suggests a common source for these elements and we further use Pb isotopes to identify the origin of the isotopically heavy Mo. We infer that an aqueous fluid component with elevated [Mo], [Pb], high 98Mo/95Mo and unradiogenic Pb is derived from the subducting, mafic oceanic crust. Although the top few hundred metres of the subducting, mafic crust have a high 98Mo/95Mo, as a result of seawater alteration, tightly defined Pb isotope arrays of the Mariana arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean ridge basalts. This argues against a flux dominantly derived from the highly altered, uppermost mafic crust or indeed from an Indian

  9. An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle (United States)

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


    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.

  10. Foreland sedimentary record of Andean mountain building during advancing and retreating subduction (United States)

    Horton, Brian K.


    As in many ocean-continent (Andean-type) convergent margins, the South American foreland has long-lived (>50-100 Myr) sedimentary records spanning not only protracted crustal shortening, but also periods of neutral to extensional stress conditions. A regional synthesis of Andean basin histories is complemented by new results from the Mesozoic Neuquén basin system and succeeding Cenozoic foreland system of west-central Argentina (34-36°S) showing (1) a Late Cretaceous shift from backarc extension to retroarc contraction and (2) an anomalous mid-Cenozoic (~40-20 Ma) phase of sustained nondeposition. New detrital zircon U-Pb geochronological results from Jurassic through Neogene clastic deposits constrain exhumation of the evolving Andean magmatic arc, retroarc thrust belt, foreland basement uplifts, and distal eastern craton. Abrupt changes in sediment provenance and distal-to-proximal depositional conditions can be reconciled with a complex Mesozoic-Cenozoic history of extension, post-extensional thermal subsidence, punctuated tectonic inversion involving thick- and thin-skinned shortening, alternating phases of erosion and rapid accumulation, and overlapping igneous activity. U-Pb age distributions define the depositional ages of several Cenozoic stratigraphic units and reveal a major late middle Eocene-earliest Miocene (~40-20 Ma) hiatus in the Malargüe foreland basin. This boundary marks an abrupt shift in depositional conditions and sediment sources, from Paleocene-middle Eocene distal fluviolacustrine deposition of sediments from far western volcanic sources (Andean magmatic arc) and subordinate eastern cratonic basement (Permian-Triassic Choiyoi igneous complex) to Miocene-Quaternary proximal fluvial and alluvial-fan deposition of sediments recycled from emerging western sources (Malargüe fold-thrust belt) of Mesozoic basin fill originally derived from basement and magmatic arc sources. Neogene eastward advance of the fold-thrust belt involved thick

  11. Middle Miocene near trench volcanism in northern Colombia: A record of slab tearing due to the simultaneous subduction of the Caribbean Plate under South and Central America? (United States)

    Lara, M.; Cardona, A.; Monsalve, G.; Yarce, J.; Montes, C.; Valencia, V.; Weber, M.; De La Parra, F.; Espitia, D.; López-Martínez, M.


    Field, geochemical, geochronological, biostratigraphical and sedimentary provenance results of basaltic and associated sediments northern Colombia reveal the existence of Middle Miocene (13-14 Ma) mafic volcanism within a continental margin setting usually considered as amagmatic. This basaltic volcanism is characterized by relatively high Al2O3 and Na2O values (>15%), a High-K calc-alkaline affinity, large ion lithophile enrichment and associated Nb, Ta and Ti negative anomalies which resemble High Al basalts formed by low degree of asthenospheric melting at shallow depths mixed with some additional slab input. The presence of pre-Cretaceous detrital zircons, tourmaline and rutile as well as biostratigraphic results suggest that the host sedimentary rocks were deposited in a platform setting within the South American margin. New results of P-wave residuals from northern Colombia reinforce the view of a Caribbean slab subducting under the South American margin. The absence of a mantle wedge, the upper plate setting, and proximity of this magmatism to the trench, together with geodynamic constraints suggest that the subducted Caribbean oceanic plate was fractured and a slab tear was formed within the oceanic plate. Oceanic plate fracturing is related to the splitting of the subducting Caribbean Plate due to simultaneous subduction under the Panama-Choco block and northwestern South America, and the fast overthrusting of the later onto the Caribbean oceanic plate.

  12. Geometry and Evolution of the Cangdong Sag in the Bohai Bay Basin, China: Implications for Subduction of the Pacific Plate. (United States)

    Luo, Liang; Qi, Jiafu; Li, Hongxiang; Dong, Yueqi; Zhang, Shuai; Zhang, Xichen; Yu, Xiaoxia; Luo, Lingyan


    The Cangdong Sag is a complex Cenozoic rift basin at the center of the Bohai Bay Basin. Cenozoic structures in the Cangdong Sag can be subdivided into the Cangdong Fault System in the west and the Xuxi Fault System in the east. The geometry of the boundary faults varies along the axes of half-grabens. According to the cross-sectional strata geometry, unconformity and planar structural pattern, the Cenozoic structural evolution of the Cangdong Sag can be divided into four distinct stages: (1) major Paleocene initial rift, (2) latest Paleocene-early Eocene intensive rift, (3) late Eocene-Oligocene strike-slip superimposed rift, and (4) Neogene to present-day post-rift depression. The extensional deformation was mainly derived from horizontal stress induced by the upwelling of asthenosphere. The strike-slip structure of the Cangdong Sag provides important information related to the subduction of the Western Pacific Plate. It was found that the strike-slip movement of the southern Xuxi Fault Zone was activated during the deposition of the third member of Shahejie Formation to the Dongying Formation; therefore, ~43 Ma probably marks the time when the Western Pacific Plate initially changed its subduction direction from northwest to nearly west.

  13. Hidden Earthquake Potential in Plate Boundary Transition Zones (United States)

    Furlong, Kevin P.; Herman, Matthew; Govers, Rob


    Plate boundaries can exhibit spatially abrupt changes in their long-term tectonic deformation (and associated kinematics) at triple junctions and other sites of changes in plate boundary structure. How earthquake behavior responds to these abrupt tectonic changes is unclear. The situation may be additionally obscured by the effects of superimposed deformational signals - juxtaposed short-term (earthquake cycle) kinematics may combine to produce a net deformational signal that does not reflect intuition about the actual strain accumulation in the region. Two examples of this effect are in the vicinity of the Mendocino triple junction (MTJ) along the west coast of North America, and at the southern end of the Hikurangi subduction zone, New Zealand. In the region immediately north of the MTJ, GPS-based observed crustal displacements (relative to North America (NAm)) are intermediate between Pacific and Juan de Fuca (JdF) motions. With distance north, these displacements rotate to become more aligned with JdF - NAm displacements, i.e. to motions expected along a coupled subduction interface. The deviation of GPS motions from the coupled subduction interface signal near the MTJ has been previously interpreted to reflect clock-wise rotation of a coastal, crustal block and/or reduced coupling at the southern Cascadia margin. The geologic record of crustal deformation near the MTJ reflects the combined effects of northward crustal shortening (on geologic time scales) associated with the MTJ Crustal Conveyor (Furlong and Govers, 1999) overprinted onto the subduction earthquake cycle signal. With this interpretation, the Cascadia subduction margin appears to be well-coupled along its entire length, consistent with paleo-seismic records of large earthquake ruptures extending to its southern limit. At the Hikurangi to Alpine Fault transition in New Zealand, plate interactions switch from subduction to oblique translation as a consequence of changes in lithospheric structure of

  14. Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up. (United States)

    Kelemen, Peter B; Manning, Craig E


    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.

  15. Collapse risk of buildings in the Pacific Northwest region due to subduction earthquakes (United States)

    Raghunandan, Meera; Liel, Abbie B.; Luco, Nicolas


    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.

  16. Three-Dimensional Thermal Structure of the Middle-America Subduction Zone: Along-margin mantle flow and slab metamorphism (United States)

    Rosas, J. C.; Currie, C. A.; He, J.


    Temperature is the primary control parameter of several processes occurring at subduction zones, such as slab metamorphism and dehydration, arc volcanism and the rupture width of megathrust earthquakes. The thermal state depends on the temperature of the oceanic slab and the flow pattern of the overlying mantle wedge. In most previous studies, mantle flow was modeled as two-dimensional (2D) corner flow, driven by the subducting plate. However, recent studies have shown the limitations of the 2D corner flow scheme, as a three-dimensional (3D) oceanic plate structure can generate along-strike pressure gradients, producing a trench-parallel flow component. One region where 3D effects may be important is the Middle America Subduction Zone (MASZ). Here, the dip of the oceanic plate varies from 0 to 70 degrees along the margin, with abrupt changes in slab dip in Central Mexico and Costa Rica-Nicaragua. Seismic anisotropy and arc magma geochemistry variations suggest a significant along-margin component of flow in these areas. Further, offshore surface heat flow measurements show that there may be along-margin variations in the temperature of the subducting oceanic plate, due to variations in plate age and hydrothermal circulation. In this study, we quantify the changes in the thermal structure of a subduction zone that result from along-margin variations in oceanic plate structure. We use 3D numerical models that consist of kinematically-defined subducting and overriding plates, and a flowing mantle wedge driven by drag exerted by the subducting plate. The finite-element code PGCtherm-3D is used to solve the steady-state governing equations for mantle wedge flow and the 3D thermal structure of the subduction zone. The models employ an oceanic plate that smoothly dips into the mantle and has along-margin variations in the deep dip of 40 and 70 degrees over a distance of 50km to 300km, as observed in some regions of the MASZ. Using an isoviscous mantle wedge, our

  17. The relationship between orogenesis, terrane accretion and the subduction of oceanic ridges in the Ecuadorian andes (United States)

    Spikings, R.; Winkler, W.; Seward, D.; Hughes, R.; Handler, R.; Crowhurst, P.


    Oceanic hotspot activity, generating large oceanic igneous plateau provinces, plate rearrangements and the generation of new spreading centers since at least 90 Ma have formed large structural, thickness and density heterogeneities in the approaching and subducting oceanic crust offshore NW South America (SOAM). Various oceanic allochthonous terranes comprise western Ecuador and the relatively thick and buoyant Carnegie Ridge is being subducted. We present 40Ar/39Ar, fission track (FT) and (U-Th/He) data from i) the Eastern Cordillera and the Amotape Complex, which define the palaeo-continental margin, ii) the Western Cordillera, which is built upon allochthonous, oceanic crust and iii) a tectonic mélange at the ocean-continent suture. 40Ar/39Ar ages and FT data from exotic, Triassic blocks within the ocean-continent suture record elevated cooling rates of plateau basalts and the continental margin. 40Ar/39Ar ages and FT data from the palaeo-continental margin show that the entire contemporaneous continental margin was being cooled by rapid tectonic exhumation (combined with geochemical analyses, suggest that these periods of orogenesis were driven by stress imposed by the collision of terranes that originated at the Caribbean Plateau. Distinct periods of rapid cooling and exhumation of fault blocks in the W. Cordillera and the northern E. Cordillera occurred at ˜15 and ˜9 Ma. Cooling at ˜15 Ma was driven by the collision of the Carnegie Ridge with the trench at ˜15 Ma. The elevated, compressive stress field gave rise to a complex transcurrent system, resulting in uplift, exhumation and cooling in the northern E. Cordillera and extension in the southern E. Cordillera. Finally, FT and (U-Th)/He data record rapid cooling in the northern E. Cordillera and parts of the W. Cordillera at ˜6-5 Ma, suggesting that the middle Miocene transcurrent system was reactivated by thrust tectonics during the late Miocene, giving rise to the Interandean Valley, which split the

  18. State Agency Administrative Boundaries (United States)

    Kansas Data Access and Support Center — This database comprises 28 State agency boundaries and point of contact. The Kansas Geological Survey collected legal descriptions of the boundaries for various...

  19. Allegheny County Municipal Boundaries (United States)

    Allegheny County / City of Pittsburgh / Western PA Regional Data Center — This dataset demarcates the municipal boundaries in Allegheny County. Data was created to portray the boundaries of the 130 Municipalities in Allegheny County the...

  20. HUD GIS Boundary Files (United States)

    Department of Housing and Urban Development — The HUD GIS Boundary Files are intended to supplement boundary files available from the U.S. Census Bureau. The files are for community planners interested in...

  1. Political State Boundary (National) (United States)

    Department of Transportation — State boundaries with political limit - boundaries extending into the ocean (NTAD). The TIGER/Line Files are shapefiles and related database files (.dbf) that are an...

  2. Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays (United States)

    Tibi, Rigobert; Wiens, Douglas A.


    Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high-frequency content for precise mapping of discontinuity topography and sharpness. To enhance the low-amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660-km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660-km discontinuity in terms of local temperatures implies a thermal anomaly of -800°K to -1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660-km discontinuity region implies that the Tonga slab seems to penetrate the 660-km discontinuity with little deformation. Waveform modeling suggests that both the 410- and 660-km discontinuities are sharp. The 660-km discontinuity is at most 2 km thick in many parts of the region, and a first-order discontinuity cannot be precluded. The 410-km discontinuity thickness shows

  3. Phase equilibria in subducting basaltic crust: implications for H 2O release from the slab (United States)

    Forneris, Juliette F.; Holloway, John R.


    , is the most likely explanation for this difference, and suggests that chloritoid does not play an important role in the overall dehydration process of the basaltic layer in subduction zones. At pressures above the stability field of amphibole, zoisite/clinozoisite becomes the stable hydrous phase at temperatures above 645°C, whereas lawsonite is stable at lower temperatures. The positions of the zoisite-out and lawsonite-out reactions determined in this study indicate that, for an intermediate temperature subduction zone, the basaltic layer of the slab would be completely dehydrated between 90 and 110 km depth.

  4. Structures and microfabrics of the Franciscan Complex (California): Inferences on the rheology and kinematics of a subduction channel (United States)

    Krohe, A.; Wassmann, S.; Trepmann, C.; Stoeckhert, B.


    identified. Mechanical contrasts within the mélange are presumably governed by variations in grain sizes and the nature of interphase boundaries, which both control viscous deformation by dissolution precipitation creep. As such, huge viscosity contrasts between matrix and rigid blocks can persist during burial to HP metamorphic conditions and decompression, while the mélange is deformed to very high bulk strain. These findings pose constraints on the large scale properties of a subduction channel presently active at depth, to be identified by geophysical methods.

  5. Plate tectonics on the Earth triggered by plume-induced subduction initiation. (United States)

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


    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.

  6. Porosity and Salt Content Determine if Subduction Can Occur in Europa's Ice Shell (United States)

    Johnson, Brandon C.; Sheppard, Rachel Y.; Pascuzzo, Alyssa C.; Fisher, Elizabeth A.; Wiggins, Sean E.


    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.

  7. Challenging the Boundaries

    DEFF Research Database (Denmark)

    Nørgaard, Nina


    to explore in the study and teaching of foreign languages. Not only may linguistics and literature be employed to shed light on each other, the insights gained may furthermore prove useful in a broader context in our foreign language studies. The article begins with a brief introduction to literary...... linguistics in general and to Hallidayan linguistics in particular. The theoretical framework thus laid out, it is exemplified how Halliday's theory of language may be employed in the analysis of literature. The article concludes by considering the possible status of literary linguistics in a broader......To many people, challenging the boundaries between the traditional disciplines in foreign language studies means doing cultural studies. The aim of this article is to pull in a different direction by suggesting how the interface between linguistics and literature may be another fertile field...

  8. Did high Neo-Tethys subduction rates contribute to early Cenozoic warming? (United States)

    Hoareau, G.; Bomou, B.; van Hinsbergen, D. J. J.; Carry, N.; Marquer, D.; Donnadieu, Y.; Le Hir, G.; Vrielynck, B.; Walter-Simonnet, A.-V.


    The 58-51 Ma interval was characterized by a long-term increase of global temperatures (+4 to +6 °C) up to the Early Eocene Climate Optimum (EECO, 52.9-50.7 Ma), the warmest interval of the Cenozoic. It was recently suggested that sustained high atmospheric pCO2, controlling warm early Cenozoic climate, may have been released during Neo-Tethys closure through the subduction of large amounts of pelagic carbonates and their recycling as CO2 at arc volcanoes. To analyze the impact of Neo-Tethys closure on early Cenozoic warming, we have modeled the volume of subducted sediments and the amount of CO2 emitted along the northern Tethys margin. The impact of calculated CO2 fluxes on global temperature during the early Cenozoic have then been tested using a climate carbon cycle model (GEOCLIM). We show that CO2 production may have reached up to 1.55 × 1018 mol Ma-1 specifically during the EECO, ~ 4 to 37 % higher that the modern global volcanic CO2 output, owing to a dramatic India-Asia plate convergence increase. The subduction of thick Greater Indian continental margin carbonate sediments at ~ 55-50 Ma may also have led to additional CO2 production of 3.35 × 1018 mol Ma-1 during the EECO, making a total of 85 % of the global volcanic CO2 outgassed. However, climate modeling demonstrates that timing of maximum CO2 release only partially fits with the EECO, and that corresponding maximum pCO2 values (750 ppm) and surface warming (+2 °C) do not reach values inferred from geochemical proxies, a result consistent with conclusions arising from modeling based on other published CO2 fluxes. These results demonstrate that CO2 derived from decarbonation of Neo-Tethyan lithosphere may have possibly contributed to, but certainly cannot account alone for early Cenozoic warming. Other commonly cited sources of excess CO2 such as enhanced igneous province volcanism also appear to be up to 1 order of magnitude below fluxes required by the model to fit with proxy data of pCO2 and

  9. Isotopic investigation of metasomatism in subduction zones: the Franciscan Complex, California

    Energy Technology Data Exchange (ETDEWEB)

    Nelson, B.K.; Depaolo, D.J.


    Metasomatizing fluids associated with metamorphism in subduction zones may be an important agent for the chemical modification of subducted rocks and of the overlying mantle wedge, both of which may be sources for island arc magmas. To characterize the timing of metasomatism and the source and nature of the fluids, the authors measured Sm-Nd and Rb-Sr isotope ratios of vein-filling minerals and actinolitic rinds cross-cutting meta-basaltic blocks from the Franciscan Central belt. 12 of 14 unmetasomatized samples of greenstone, blueschist and eclogite blocks have measured element of/sub ND/=+8 to +11, indicating MOR protoliths. Measured element of/sub Sr/ values scatter, but initial element of/sub Sr/ for whole rocks and garnets at 150 m.y. are +1 to +16. In element of/sub Nd=element of/sub Sr/ space the initial ratios are shifted to higher element of/sub Sr/ values with respect to unaltered MORB, suggesting seawater-hydrothermal alteration of the protolith. Foliated actinolite +/- white mica rinds from an eclogite and blueschist, both with element of/sub Nd/=+9, have element of/sub Nd/ of +6 and +1 respectively. The Rb-Sr age of the aragonite-glaucophane pair is 153.6 +/- 2.4 m.y., with initial /sup 87/Sr//sup 86/Sr = 0.7052 and element of/sub Nd/ = +6.3. The Rb-Sr age of a white mica-zoisite pair from a vein in a Tiburon eclogite is 151.9 +/- 0.4 m.y., initial /sup 87/Sr//sup 86/Sr = 0.7050 and element of/sub Nd/=+6.7. The vein fluids had similar isotopic compositions, but some rind-forming fluids had element of/sub Nd/<+1 suggesting a component derived from continental crust. Jadeitic metagreywacke, such as that from Pacheco Pass may be a source for low element of/sub Nd/ fluids. Beyond alteration at a MOR, subduction-related metasomatism may further modify eclogite returned to the mantle.

  10. Geomorphologic Indices for Transition from Subduction to Arc-Continent Collision in Sumba Island, Indonesia (United States)

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


    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.

  11. Crustal Gravitational Potential Energy Change and Subduction Earthquakes (United States)

    Zhu, P. P.


    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

  12. Rock uplift above the subduction megathrust at Montague and Hinchinbrook Islands, Prince William Sound, Alaska (United States)

    Ferguson, Kelly M.

    Deformation related to the transition from strike-slip to convergent slip during flat-slab subduction of the Yakutat microplate has resulted in regions of focused rock uplift and exhumation. In the St. Elias and Chugach Mountains, faulting related to transpressional processes and bending of fault systems coupled with enhanced glacial erosion causes rapid exhumation. Underplating below the syntaxial bend farther west in the Chugach Mountains and central Prince William Sound causes focused, but less rapid, exhumation. Farther south in the Prince William Sound, plate boundary deformation transitions from strike-slip to nearly full convergence in the Montague Island and Hinchinbrook Island region, which is ˜20 km above the megathrust between the Yakutat microplate and overriding North American Plate. Montague and Hinchinbrook Islands are narrow, elongate, and steep, with a structural grain formed by several megathrust fault splays, some of which slipped during the 1964 M9.2 earthquake. Presented here are 32 new apatite (U-Th)/He (AHe) and 28 new apatite fission-track (AFT) ages from the Montague and Hinchinbrook Island regions. Most AHe ages are Hanning Bay and Patton Bay faults during the 1964 earthquake. AFT ages range from ˜5 Ma to ˜20 Ma and are also younger at the SW end of Montague Island. These ages and corresponding exhumation rates indicate that the Montague and Hinchinbrook Island region is a narrow zone of intense deformation probably related to duplex thrusting along one or more megathrust fault splays. I interpret the rates of rock uplift and exhumation to have increased in the last ˜5 My, especially at the southwest end of the island system and farthest from the region dominated by strike-slip and transpressional deformation to the northeast. The narrow band of deformation along these islands likely represents the northwestern edge of a broader swath of plate boundary deformation between the Montague-Hinchinbrook Island region and the Kayak Island

  13. The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary (United States)

    Schmerr, Nicholas


    The lithosphere-asthenosphere boundary (LAB) beneath ocean basins separates the upper thermal boundary layer of rigid, conductively cooling plates from the underlying ductile, convecting mantle. The origin of a seismic discontinuity associated with this interface, known as the Gutenberg discontinuity (G), remains enigmatic. High-frequency SS precursors sampling below the Pacific plate intermittently detect the G as a sharp, negative velocity contrast at 40- to 75-kilometer depth. These observations lie near the depth of the LAB in regions associated with recent surface volcanism and mantle melt production and are consistent with an intermittent layer of asthenospheric partial melt residing at the lithospheric base. I propose that the G reflectivity is regionally enhanced by dynamical processes that produce melt, including hot mantle upwellings, small-scale convection, and fluid release during subduction.

  14. Tectonics of the IndoBurma Oblique Subduction Zone (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.


    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.

  15. Slab2 - Updated subduction zone geometries and modeling tools (United States)

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


    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

  16. Variation of Seismic Velocity Structure around the Mantle Transition Zone and Conjecture of Deep Water Transport by Subducted Slabs (United States)

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


    structure beneath tectonic North America [Grand and Helmberger, 1984] and has a sharp discontinuity. Tajima and Nakagawa [2006] have already shown that SH waveforms which sampled the stagnant slab in the northwestern Pacific subduction zone could be modeled with TNA well. The reflectivity synthetic approaches attempted to model a large number of waveform data obtained from an array with a layered structure, and tend to average the discontinuity depth variation and other features. Our 3D finite difference waveform modeling clearly suggests variation of relatively sharp phase transformation depths beneath stagnant slabs. The results were interpreted with variation of geochemical properties under wet condition at the base of MTZ. Accordingly we explore how well the hydrous and dry conditions can be delineated for varying chemical properties from seismic waveform analysis.

  17. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Implications for the Dynamics of Flat-Slabs (United States)

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


    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

  18. Importance of initial buoyancy field on evolution of mantle thermal structure: Implications of surface boundary conditions

    Directory of Open Access Journals (Sweden)

    Petar Glišović


    Full Text Available Although there has been significant progress in the seismic imaging of mantle heterogeneity, the outstanding issue that remains to be resolved is the unknown distribution of mantle temperature anomalies in the distant geological past that give rise to the present-day anomalies inferred by global tomography models. To address this question, we present 3-D convection models in compressible and self-gravitating mantle initialised by different hypothetical temperature patterns. A notable feature of our forward convection modelling is the use of self-consistent coupling of the motion of surface tectonic plates to the underlying mantle flow, without imposing prescribed surface velocities (i.e., plate-like boundary condition. As an approximation for the surface mechanical conditions before plate tectonics began to operate we employ the no-slip (rigid boundary condition. A rigid boundary condition demonstrates that the initial thermally-dominated structure is preserved, and its geographical location is fixed during the evolution of mantle flow. Considering the impact of different assumed surface boundary conditions (rigid and plate-like on the evolution of thermal heterogeneity in the mantle we suggest that the intrinsic buoyancy of seven superplumes is most-likely resolved in the tomographic images of present-day mantle thermal structure. Our convection simulations with a plate-like boundary condition reveal that the evolution of an initial cold anomaly beneath the Java-Indonesian trench system yields a long-term, stable pattern of thermal heterogeneity in the lowermost mantle that resembles the present-day Large Low Shear Velocity Provinces (LLSVPs, especially below the Pacific. The evolution of subduction zones may be, however, influenced by the mantle-wide flow driven by deeply-rooted and long-lived superplumes since Archean times. These convection models also detect the intrinsic buoyancy of the Perm Anomaly that has been identified as a unique

  19. Examining the links between Slow Slip Events, crustal faults and subduction interface in Central Mexico (United States)

    Bigot, A.; Manighetti, I.; Vergnolle, M.; Campillo, M.


    subduction interface and the NE-trending faults that dissect the trench. Our work therefore suggests that the Mexican SSEs occur in a specific zone where the tectonics of the upper plate interacts with that of the down-going slab. It is not clear whether the SSEs induce the extension observed on the northern NC fault, or if extension on the NC fault induces the SSEs. By contrast, it is clear that the SSEs increase the stresses on and south the AC fault, and enhance the seismicity and hence the fault slips in this area.

  20. Full-waveform seismic tomography of the Vrancea, Romania, subduction region (United States)

    Baron, Julie; Morelli, Andrea


    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

  1. Subduction and slab tearing dynamics constrained by thermal anomalies in the Anatolia-Aegean region (United States)

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


    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

  2. A new integrated tectonic model for the Mesozoic-Early Cenozoic subduction, spreading, accretion and collision history of Tethys adjacent to the southern margin of Eurasia (NE Turkey) (United States)

    Robertson, Alastair; Parlak, Osman; Ustaömer, Timur; Taslı, Kemal; İnan, Nurdan; Dumitrica, Paulian; Karaoǧlan, Fatih


    Cretaceous age for the E Pontide ophiolites, with important implications for alternative tectonic hypotheses. The two-subduction-zone hypothesis is supported by sedimentological and structural studies of the volcanic-sedimentary melange and of the sedimentary thrust sheets within the suture zone. Geochemical studies of oceanic basaltic rocks in the melange and also new biostratigraphic dating of radiolarites and calcareous microfossils within pelagic and redeposited deep-sea/slope sediments add to the picture. Taken together, the evidence suggests the former existence of both an oceanic and a continental margin subduction complex that are now amalgamated within the suture zone. We propose the following tectonic hypothesis: Fragments of oceanic basaltic lithologies and their deep-sea sedimentary cover accreted to form a Jurassic-Cretaceous intra-oceanic subduction complex. Terrigenous and arc-derived volcaniclastic gravity flows and pelagic carbonates accumulated in a continental margin forearc basin, mainly during the Cretaceous. Subduction melange was first emplaced over the distal Eurasian margin during the Late Cretaceous owing to thickening of the accretionary prism. During suturing, the continental margin forearc basin was emplaced southwards over the oceanic-derived accretionary wedge. The Eurasian continental margin was imbricated and thrust northwards as collision proceeded. Final closure of the adjacent Tethys took place prior to late Middle Eocene. This was followed by marine transgression and the accumulation of non-marine to shallow-marine sediments, including Nummulitic limestones. Regional correlations suggest that the double subduction zone hypothesis, notably involving Jurassic intra-oceanic spreading, is applicable to >1000 km of the Eurasian margin, specifically the Lesser Caucasus and possibly also the Central Pontides.

  3. Lithospheric expression of cenozoic subduction, mesozoic rifting and the Precambrian Shield in Venezuela (United States)

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


    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.

  4. The Plasmasphere Boundary Layer

    Directory of Open Access Journals (Sweden)

    D. L. Carpenter


    Full Text Available As an inner magnetospheric phenomenon the plasmapause region is of interest for a number of reasons, one being the occurrence there of geophysically important interactions between the plasmas of the hot plasma sheet and of the cool plasmasphere. There is a need for a conceptual framework within which to examine and discuss these interactions and their consequences, and we therefore suggest that the plasmapause region be called the Plasmasphere Boundary Layer, or PBL. Such a term has been slow to emerge because of the complexity and variability of the plasma populations that can exist near the plasmapause and because of the variety of criteria used to identify the plasmapause in experimental data. Furthermore, and quite importantly in our view, a substantial obstacle to the consideration of the plasmapause region as a boundary layer has been the longstanding tendency of textbooks on space physics to limit introductory material on the plasmapause phenomenon to zeroth order descriptions in terms of ideal MHD theory, thus implying that the plasmasphere is relatively well understood. A textbook may introduce the concept of shielding of the inner magnetosphere from perturbing convection electric fields, but attention is not usually paid to the variety of physical processes reported to occur in the PBL, such as heating, instabilities, and fast longitudinal flows, processes which must play roles in plasmasphere dynamics in concert with the flow regimes associated with the major dynamo sources of electric fields. We believe that through the use of the PBL concept in future textbook discussions of the plasmasphere and in scientific communications, much progress can be made on longstanding questions about the physics involved in the formation of the plasmapause and in the cycles of erosion and recovery of the plasmasphere. Key words. Magnetospheric physics (plasmasphere; plasma convection; MHD waves and instabilities

  5. The Plasmasphere Boundary Layer

    Directory of Open Access Journals (Sweden)

    D. L. Carpenter


    Full Text Available As an inner magnetospheric phenomenon the plasmapause region is of interest for a number of reasons, one being the occurrence there of geophysically important interactions between the plasmas of the hot plasma sheet and of the cool plasmasphere. There is a need for a conceptual framework within which to examine and discuss these interactions and their consequences, and we therefore suggest that the plasmapause region be called the Plasmasphere Boundary Layer, or PBL. Such a term has been slow to emerge because of the complexity and variability of the plasma populations that can exist near the plasmapause and because of the variety of criteria used to identify the plasmapause in experimental data. Furthermore, and quite importantly in our view, a substantial obstacle to the consideration of the plasmapause region as a boundary layer has been the longstanding tendency of textbooks on space physics to limit introductory material on the plasmapause phenomenon to zeroth order descriptions in terms of ideal MHD theory, thus implying that the plasmasphere is relatively well understood. A textbook may introduce the concept of shielding of the inner magnetosphere from perturbing convection electric fields, but attention is not usually paid to the variety of physical processes reported to occur in the PBL, such as heating, instabilities, and fast longitudinal flows, processes which must play roles in plasmasphere dynamics in concert with the flow regimes associated with the major dynamo sources of electric fields. We believe that through the use of the PBL concept in future textbook discussions of the plasmasphere and in scientific communications, much progress can be made on longstanding questions about the physics involved in the formation of the plasmapause and in the cycles of erosion and recovery of the plasmasphere.

    Key words. Magnetospheric physics (plasmasphere; plasma convection; MHD waves and instabilities

  6. A thermo-mechanical model of horizontal subduction below an overriding plate

    NARCIS (Netherlands)

    Hunen, Jeroen van; Berg, A.P. van den; Vlaar, N.J.


    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

  7. Does subduction polarity changes below the Alps? Inferences from analogue modelling

    NARCIS (Netherlands)

    Luth, S.; Willingshofer, E.; Sokoutis, D.; Cloetingh, S.


    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

  8. Subduction in an Eddy-Resolving State Estimate of the Northeast Atlantic Ocean (United States)

    Gebbie, Geoffrey


    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.

  9. 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


    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.

  10. 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.


    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

  11. 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


    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

  12. Deepening sleep by hypnotic suggestion. (United States)

    Cordi, Maren J; Schlarb, Angelika A; Rasch, Björn


    Slow wave sleep (SWS) plays a critical role in body restoration and promotes brain plasticity; however, it markedly declines across the lifespan. Despite its importance, effective tools to increase SWS are rare. Here we tested whether a hypnotic suggestion to "sleep deeper" extends the amount of SWS. Within-subject, placebo-controlled crossover design. Sleep laboratory at the University of Zurich, Switzerland. Seventy healthy females 23.27 ± 3.17 y. Participants listened to an auditory text with hypnotic suggestions or a control tape before napping for 90 min while high-density electroencephalography was recorded. After participants listened to the hypnotic suggestion to "sleep deeper" subsequent SWS was increased by 81% and time spent awake was reduced by 67% (with the amount of SWS or wake in the control condition set to 100%). Other sleep stages remained unaffected. Additionally, slow wave activity was significantly enhanced after hypnotic suggestions. During the hypnotic tape, parietal theta power increases predicted the hypnosis-induced extension of SWS. Additional experiments confirmed that the beneficial effect of hypnotic suggestions on SWS was specific to the hypnotic suggestion and did not occur in low suggestible participants. Our results demonstrate the effectiveness of hypnotic suggestions to specifically increase the amount and duration of slow wave sleep (SWS) in a midday nap using objective measures of sleep in young, healthy, suggestible females. Hypnotic suggestions might be a successful tool with a lower risk of adverse side effects than pharmacological treatments to extend SWS also in clinical and elderly populations.

  13. Slab melting as a barrier to deep carbon subduction. (United States)

    Thomson, Andrew R; Walter, Michael J; Kohn, Simon C; Brooker, Richard A


    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.

  14. Dehydration-induced instabilities at intermediate depths in subduction zones (United States)

    Brantut, Nicolas; Stefanou, Ioannis; Sulem, Jean


    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.

  15. Deciphering the 3-D distribution of fluid along the shallow Hikurangi subduction zone using P- and S-wave attenuation (United States)

    Eberhart-Phillips, Donna; Bannister, Stephen; Reyners, Martin


    We use local earthquake velocity spectra to solve for the 3-D distribution of P- and S-wave attenuation in the shallow Hikurangi subduction zone in the North Island of New Zealand to gain insight into how fluids control both the distribution of slip rate deficit and slow-slip events at the shallow plate interface. Qs/Qp gives us information on the 3-D distribution of fluid saturation, which we can compare with the previously determined 3-D distribution of Vp/Vs, which gives information on pore fluid pressure. The Hikurangi margin is unusual, in that a large igneous province (the Hikurangi Plateau) is being subducted. This plateau has had two episodes of subduction—first at 105-100 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. We find that in the southern part of the subduction zone, where there is a large deficit in slip rate at the plate interface, the plate interface region is only moderately fluid-rich because the underlying plateau had already had an episode of dehydration during Gondwana subduction. But fluid pressure is relatively high, due to an impermeable terrane in the upper plate trapping fluids below the plate interface. The central part of the margin, where the slip rate deficit is very low, is the most fluid-rich part of the shallow subduction zone. We attribute this to an excess of fluid from the subducted plateau. Our results suggest this part of the plateau has unusually high fracture permeability, on account of it having had two episodes of bending—first at the Gondwana trench and now at the Hikurangi Trough. Qs/Qp is consistent with fluids migrating across the plate interface in this region, leaving it drained and producing high fluid pressure in the overlying plate. The northern part of the margin is a region of heterogeneous deficit in slip rate. Here the Hikurangi Plateau is subducting for the first time, so there is less fluid available from its

  16. Characterizations of boundary pluripolar hulls

    NARCIS (Netherlands)

    Djire, I.K.; Wiegerinck, J.


    We present some basic properties of the so-called boundary relative extremal function and discuss boundary pluripolar sets and boundary pluripolar hulls. We show that for B-regular domains the boundary pluripolar hull is always trivial on the boundary of the domain and present a “boundary version”

  17. The lithosphere-asthenosphere boundary beneath the South Island of New Zealand (United States)

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


    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.

  18. Grain Boundary Complexions (United States)


    deter- mine bulk materials behavior and properties such as superplasticity, creep, fatigue, corrosion , strength and conductivity [2]. Grain boundary...interface (i.e. lattice mismatch accommodated by interface dislocations ), wetting transitions will not occur. A wetting transition is possible in the case...melting only starts around dislocations at low- angle grain boundaries; the grain boundary structure con- sists of isolated liquid pools separated by

  19. Stable Boundary Layer Issues


    Steeneveld, G.J.


    Understanding and prediction of the stable atmospheric boundary layer is a challenging task. Many physical processes are relevant in the stable boundary layer, i.e. turbulence, radiation, land surface coupling, orographic turbulent and gravity wave drag, and land surface heterogeneity. The development of robust stable boundary layer parameterizations for use in NWP and climate models is hampered by the multiplicity of processes and their unknown interactions. As a result, these models suffer ...

  20. Oxygen boundary crossing probabilities. (United States)

    Busch, N A; Silver, I A


    The probability that an oxygen particle will reach a time dependent boundary is required in oxygen transport studies involving solution methods based on probability considerations. A Volterra integral equation is presented, the solution of which gives directly the boundary crossing probability density function. The boundary crossing probability is the probability that the oxygen particle will reach a boundary within a specified time interval. When the motion of the oxygen particle may be described as strongly Markovian, then the Volterra integral equation can be rewritten as a generalized Abel equation, the solution of which has been widely studied.

  1. Deepening Sleep by Hypnotic Suggestion (United States)

    Cordi, Maren J.; Schlarb, Angelika A.; Rasch, Björn


    Study Objectives: Slow wave sleep (SWS) plays a critical role in body restoration and promotes brain plasticity; however, it markedly declines across the lifespan. Despite its importance, effective tools to increase SWS are rare. Here we tested whether a hypnotic suggestion to “sleep deeper” extends the amount of SWS. Design: Within-subject, placebo-controlled crossover design. Setting: Sleep laboratory at the University of Zurich, Switzerland. Participants: Seventy healthy females 23.27 ± 3.17 y. Intervention: Participants listened to an auditory text with hypnotic suggestions or a control tape before napping for 90 min while high-density electroencephalography was recorded. Measurements and Results: After participants listened to the hypnotic suggestion to “sleep deeper” subsequent SWS was increased by 81% and time spent awake was reduced by 67% (with the amount of SWS or wake in the control condition set to 100%). Other sleep stages remained unaffected. Additionally, slow wave activity was significantly enhanced after hypnotic suggestions. During the hypnotic tape, parietal theta power increases predicted the hypnosis-induced extension of SWS. Additional experiments confirmed that the beneficial effect of hypnotic suggestions on SWS was specific to the hypnotic suggestion and did not occur in low suggestible participants. Conclusions: Our results demonstrate the effectiveness of hypnotic suggestions to specifically increase the amount and duration of slow wave sleep (SWS) in a midday nap using objective measures of sleep in young, healthy, suggestible females. Hypnotic suggestions might be a successful tool with a lower risk of adverse side effects than pharmacological treatments to extend SWS also in clinical and elderly populations. Citation: Cordi MJ, Schlarb AA, Rasch B. Deepening sleep by hypnotic suggestion. SLEEP 2014;37(6):1143-1152. PMID:24882909

  2. 2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone. (United States)

    Moreno, Marcos; Rosenau, Matthias; Oncken, Onno


    The magnitude-8.8 Maule (Chile) earthquake of 27 February 2010 ruptured a segment of the Andean subduction zone megathrust that has been suspected to be of high seismic potential. It is the largest earthquake to rupture a mature seismic gap in a subduction zone that has been monitored with a dense space-geodetic network before the event. This provides an image of the pre-seismically locked state of the plate interface of unprecedentedly high resolution, allowing for an assessment of the spatial correlation of interseismic locking with coseismic slip. Pre-seismic locking might be used to anticipate future ruptures in many seismic gaps, given the fundamental assumption that locking and slip are similar. This hypothesis, however, could not be tested without the occurrence of the first gap-filling earthquake. Here we show evidence that the 2010 Maule earthquake slip distribution correlates closely with the patchwork of interseismic locking distribution as derived by inversion of global positioning system (GPS) observations during the previous decade. The earthquake nucleated in a region of high locking gradient and released most of the stresses accumulated in the area since the last major event in 1835. Two regions of high seismic slip (asperities) appeared to be nearly fully locked before the earthquake. Between these asperities, the rupture bridged a zone that was creeping interseismically with consistently low coseismic slip. The rupture stopped in areas that were highly locked before the earthquake but where pre-stress had been significantly reduced by overlapping twentieth-century earthquakes. Our work suggests that coseismic slip heterogeneity at the scale of single asperities should indicate the seismic potential of future great earthquakes, which thus might be anticipated by geodetic observations.

  3. Evidence of Shear Failure at the Subduction Interface by VLF Earthquake Characterization in Guerrero, Mexico (United States)

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


    In Guerrero, Mexico, tremors and low frequency earthquakes have been observed to occur simultaneously, in the subduction interface, downdip where large slow slip events occur. To better understand the mechanism of tremors in Guerrero, we focus on the detection and mechanism estimation of very low frequency (VLF) earthquakes. Following the method of Ide and Yabe [2014], we first locate tectonic tremor (TT) by the envelope cross-correlation method [Obara, 2002]. Then, waveforms are stacked, in the VLF band between 0.02 and 0.05 Hz, at the time of occurrence of tremors. Finally, the stacked waveforms are inverted to better estimate the depth of these events and their moment tensor. This analysis is carried out for the periods of 2005-2007 and 2009-2013, at the time where two temporary networks were operating; the MASE and G-GAP network respectively. In addition, permanent broadband stations of the Servicio Seismológico Nacional (Mexico) are used. Our results show that the VLF sources are located at or close to the plate interface. These events have magnitudes of about 3 and very low-angle thrust mechanism in agreement with both the geometry of the nearly horizontal subduction interface and TT location determined with an independent technique, namely the Tremor Energy and Polarization (TREP) method. The slip directions of VLF earthquakes and TT are also consistent with the plates convergence vector. The MASE data from the first time period allows detection of VLF earthquakes in two different locations, the transient zone and "Sweet Spot" [Husker, 2012] but with similar mechanisms while the second dataset (G-GAP) gives more precise location in the main cluster (the "Sweet Spot"). In addition, some clear VLF signals are detected without any stacking. These events occur at the same time and areas as the TT thus suggesting a common origin of the two phenomena.

  4. The upliftment in the Mejillones peninsula during 1995-2015 with two subduction earthquakes (United States)

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


    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 th