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Deformation in the orogen-foreland system of the southern Central Andes between 33° and 36° S varies in style, locus, and amount of shortening. The controls that determine these spatially variable characteristics have largely remained unknown, yet both the subduction of the oceanic Nazca plate and the strength of the South American plate have been invoked to play a major role. While the parameters governing the subduction processes are similar between 33° and 36° S, the lithospheric strength of the upper plate is spatially variable due to structures inherited from past geodynamic regimes and associated compositional differences in the South American plate. Regional Mesozoic crustal horizontal extension generated a < 40-km-thick crust with a more mafic composition in the lower crust south of 35°S; north of this latitude, however, a more felsic lower crust is inferred from geophysical data. To assess the influence of different structural and compositional heterogeneities on the style of deformation in the southern Central Andes, we developed a suite of geodynamic models of intraplate lithospheric shortening for two E–W transects (33° 40′ S and 36° S) across the Andes. The models are constrained by local geological and geophysical information. Our results demonstrate a decoupled shortening mode between the brittle upper crust and the ductile lower crust in those areas characterized by a mafic lower crust (36° S transect). In contrast, a more felsic lower crust, such as in the 33° 40′ S transect, results in a coupled shortening mode. Furthermore, we find that differences in lithospheric thickness and the asymmetry of the lithosphere–asthenosphere boundary may promote the formation of a crustal-scale, west-dipping detachment zone that drives the overall deformation and lateral expansion of the orogen. Our study represents the first geodynamic modeling effort in the southern Central Andes aimed at understanding the roles of heterogeneities (crustal composition and thickness) at the scale of the entire lithosphere as well as the geometry of the lithosphere–asthenosphere boundary with respect to mountain building. Fil: Barrionuevo, Matías. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Liu, Sibiao. Universitat Potsdam; Alemania Fil: Mescua, Jose Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Yagupsky, Daniel Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina Fil: Quinteros, Javier. German Research Centre for Geosciences; Alemania Fil: Giambiagi, Laura Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Sobolev, Stephan V.. Universitat Potsdam; Alemania Fil: Piceda, Constanza Rodríguez. Universitat Potsdam; Alemania Fil: Strecker, Manfred R.. German Research Centre for Geosciences; Alemania

Subduction initiation induced by a hot and buoyant mantle plume head is unique among proposed subduction initiation mechanisms because it does not require pre-existing weak zones or other forces for lithospheric collapse. Since recognition of the first evidence of subduction nucleation induced by a mantle plume in the Late Cretaceous Caribbean realm, the number of studies focusing on other natural examples has grown. Here, we review numerical and physical modeling and geological-geochemical studies which have been carried out thus far to investigate onset of a new subduction zone caused by impingement of a mantle plume head. As geological-geochemical data suggests that plume-lithosphere interactions have long been important - spanning from the Archean to the present - modeling studies provide valuable information on the spatial and temporal variations in lithospheric deformation induced by these interactions. Numerical and physical modeling studies, ranging from regional to global scales, illustrate the key role of plume buoyancy, lithospheric strength and magmatic weakening above the plume head on plume-lithosphere interactions. Lithospheric/crustal heterogeneities, pre-existing lithospheric weak zones and external compressional/extensional forces may also change the deformation regime caused by plume-lithosphere interaction., Frontiers in Earth Science, 9, ISSN:2296-6463

Subduction zones are key components of plate tectonics and plate tectonics could not begin until the first subduction zone formed. Plume-induced subduction initiation, which has been proposed as triggering the beginning of plate tectonics (Gerya et al., 2015), is one of the few scenarios that can break the lithosphere and recycle a stagnant lid without requiring any pre-existing weak zones. So far, two natural examples of plume-induced subduction initiation have been recognized. The first was found in southern and western margins of the Caribbean Plate (Whattam and Stern 2014). Initiation of the Cascadia subduction zone in Eocene times has been proposed to be the second example of plume-induced subduction initiation (Stern and Dumitru, 2019).The focus of previous studies was to inspect plume-lithosphere interaction either for the case of stationary lithosphere (e.g., Gerya et al., 2015) or for moving lithosphere without considering the effect of lithospheric magmatic weakening above the plume head (e.g., Moore et al., 1998). In present study we investigate the response of moving oceanic lithosphere to the arrival of a rising mantle plume head including the effect of magmatic lithospheric weakening. We used 3D numerical thermo-mechanical modeling. Using I3ELVIS code, which is based on finite difference staggered grid and marker-in-cell with an efficient OpenMP multigrid solver (Gerya, 2010), we show that plate motion may affect the plume-induced subduction initiation only if a moderate size plume head (with a radius of 140 km in our experiments) impinges on a young but subductable lithosphere (with the age of 20 Myr). Outcomes indicate that lithospheric strength and plume buoyancy are key parameters in penetration of the plume and subduction initiation and that plate speed has a minor effect. We propose that eastward motion of the Farallon plate in Late Cretaceous time could play a key role in forming new subduction zones along the western and southern margin of the Caribbean plate. References:Gerya, T., 2010, Introduction to Numerical Geodynamic Modelling.. Cambridge University Press.Gerya, T.V., Stern, R.J., Baes, M., Sobolev, S.V. and Whattam, S.A., 2015. Plume-induced subduction initiation triggered Plate Tectonics on Earth. Nature, 527, 221–225.Moore, W. B., Schubert, G. and Tackley, P., 1998, Three-dimensional simulations of plume-lithosphere interaction at the Hawaiian swell. Science, 279, 1008-1011.Stern, R.J., and Dumitru, T.A., 2019, Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation? Geosphere, v. 15, 659-681.Whattam, S.A. and Stern, R.J., 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, 27, doi: 10.1016/j.gr.2014.07.011.

It has recently been demonstrated that the interaction of a mantle plume with sufficiently old oceanic lithosphere can initiate subduction. However, the existence of large lithospheric heterogeneities, such as a buoyant plateau, in proximity to a rising plume head may potentially hinder the formation of a new subduction zone. Here, we investigate this scenario by means of 3‐D numerical thermomechanical modeling. We explore how plume‐lithosphere interaction is affected by lithospheric age, relative location of plume head and plateau border, and the strength of the oceanic crust. Our numerical experiments suggest four different geodynamic regimes: (a) oceanic trench formation, (b) circular oceanic‐plateau trench formation, (c) plateau trench formation, and (d) no trench formation. We show that regardless of the age and crustal strength of the oceanic lithosphere, subduction can initiate when the plume head is either below the plateau border or at a distance less than the plume radius from the plateau edge. Crustal heterogeneity facilitates subduction initiation of old oceanic lithosphere. High crustal strength hampers the formation of a new subduction zone when the plume head is located below a young lithosphere containing a thick and strong plateau. We suggest that plume‐plateau interaction in the western margin of the Caribbean could have resulted in subduction initiation when the plume head impinged onto the oceanic lithosphere close to the border between plateau and oceanic crust., Geochemistry, Geophysics, Geosystems, 21 (8), ISSN:1525-2027

The classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. This process, however, is impeded by the high strength of passive margins, and it has never occurred in Cenozoic times. Here using thermomechanical models, we show that additional forcing, provided by mantle flow, which is induced by neighboring subduction zones and midmantle slab remnants, can convert a passive margin into a subduction zone. Models suggest that this is a long-term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the U.S. East Coast. Mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones. We propose that new subduction zones will preferentially originate where subduction zones were active in the past, thus explaining the remarkable colocation of subduction zones during at least the last 400 Myr.

Subduction is substantially multi-scale process where the stresses are built by long-term tectonic motions, modified by sudden jerky deformations during earthquakes, and then restored by following multiple relaxation processes. Here, we develop a cross-scale thermomechanical model aimed to simulate the subduction process from 1 minute to million years' time scale. The model employs elasticity, nonlinear transient viscous rheology, and rate-and-state friction. It generates spontaneous earthquake sequences and by using an adaptive time-step algorithm, recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles. The model predicts that viscosity in the mantle wedge drops by more than three orders of magnitude during the great earthquake with a magnitude above 9. As a result, the surface velocities just an hour or day after the earthquake are controlled by viscoelastic relaxation in the several hundred km of mantle landward of the trench and not by the afterslip localized at the fault as is currently believed. Our model replicates centuries-long seismic cycles exhibited by the greatest earthquakes and is consistent with the postseismic surface displacements recorded after the Great Tohoku Earthquake. We demonstrate that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in South Chile inferred from long-term geodynamic models and appearance of the largest earthquakes, like the Great Chile 1960 Earthquake.

Pre-existing weakness zones in the lithosphere such as transform faults/fracture zones and extinct mid-oceanic ridges have been suggested to facilitate subduction initiation in an intra-oceanic environment. Here, we propose that the additional forcing coming from the mantle suction flow is required to trigger the conversion of a fracture zone/transform fault into a converging plate boundary. This suction flow can be induced either from the slab remnants of former converging plate boundaries or/and from slabs of neighbouring active subduction zones. Using 2-D coupled thermo-mechanical models, we show that a sufficiently strong mantle flow is able to convert a fracture zone/transform fault into a subduction zone. However, this process is feasible only if the fracture zone/transform fault is very close to the mid-oceanic ridge. Our numerical model results indicate that time of subduction initiation depends on the velocity, domain size and location of mantle suction flow and age of the oceanic plate.

We study segregation of the subducted oceanic crust (OC) at the core-mantle boundary and its ability to accumulate and form large thermochemical piles (such as the seismically observed Large Low Shear Velocity Provinces (LLSVPs)). Our high-resolution numerical simulations of thermochemical mantle convection suggest that the longevity of LLSVPs for up to three billion years, and possibly longer, can be ensured by a balance in the rate of segregation of high-density OC material to the core-mantle boundary (CMB) and the rate of its entrainment away from the CMB by mantle upwellings. For a range of parameters tested in this study, a large-scale compositional anomaly forms at the CMB, similar in shape and size to the LLSVPs. Neutrally buoyant thermochemical piles formed by mechanical stirring—where thermally induced negative density anomaly is balanced by the presence of a fraction of dense anomalous material—best resemble the geometry of LLSVPs. Such neutrally buoyant piles tend to emerge and survive for at least 3 Gyr in simulations with quite different parameters. We conclude that for a plausible range of values of density anomaly of OC material in the lower mantle—it is likely that it segregates to the CMB, gets mechanically mixed with the ambient material, and forms neutrally buoyant large-scale compositional anomalies similar in shape to the LLSVPs.

An inclined zone of intermediate-depth seismicity beneath the Pamir orogen in Central Asia has been interpreted as southward subduction of a slab of Asian lithosphere. However, it is not known whether Asian lithosphere subducts intact or only partially. We used arrival times of shallow and intermediate-depth earthquakes, recorded with a temporary (2008–2010) seismic network in this region, to invert for 3D models of seismic velocities in an attempt to answer this question. With local seismicity reaching depths of up to 240 km, the deep structure of the Pamir could be illuminated with high resolution. The resulting velocity models show a north–south contrast in crustal seismic velocities in the Pamir, with very low P velocities (5.7–5.9 km/s at 15–30 km depth), coupled with relatively low vp/vsvp/vs (

Here, we study the 3-D subduction initiation process induced by the interaction between a hot thermochemical mantle plume and oceanic lithosphere using thermo-mechanical viscoplastic finite difference marker-in-cell models. Our numerical modeling results show that self-sustaining subduction is induced by plume-lithosphere interaction when the plume is sufficiently buoyant, the oceanic lithosphere is sufficiently old and the plate is weak enough to allow the buoyant plume to. pass through it. Subduction initiation occurs following penetration of the lithosphere by the hot plume and the downward displacement of broken, nearly circular segments of lithosphere (proto-slabs) as a result of partially molten plume rocks overriding the proto-slabs. Our experiments show four different deformation regimes in response to plume-lithosphere interaction: a) self-sustaining subduction initiation, in which subduction becomes self-sustaining; b) frozen subduction initiation, in which subduction stops at shallow depths; c) slab break-off, in which the subducting circular slab breaks off soon after formation; and d) plume underplating, in which the plume does not pass through the lithosphere and instead spreads beneath it (i.e., failed subduction initiation). These regimes depend on several parameters, such as the size, composition, and temperature of the plume, the brittle/plastic strength and age of the oceanic lithosphere, and the presence/absence of lithospheric heterogeneities. The results show that subduction initiates and becomes self-sustaining when the lithosphere is older than 10 Myr and the non dimensional ratio of the plume buoyancy force and lithospheric strength above the plume is higher than approximately 2. The outcomes of our numerical experiments are applicable for subduction initiation in the modern and Precambrian Earth and for the origin of plume-related corona structures on Venus. (C) 2016 Elsevier B.V. All rights reserved.

High-resolution three-dimensional thermomechanical simulations of Earth's lithosphere indicate that mantle plumes could have initiated the first subduction zones, but only in the hotter early Earth for old oceanic plates. Taras Gerya and co-authors use high-resolution three-dimensional numerical thermomechanical models to show that mantle plumes could have initiated the first subduction zones. They find that several factors combine to trigger self-sustained subduction: a strong, negatively buoyant oceanic lithosphere, focused magmatic weakening and thinning of lithosphere above the plume, and lubrication of the slab interface by hydrated crust. They also conclude that plume-induced subduction was only feasible in the hotter early Earth for old oceanic plates, as younger plates would have favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics. Scientific theories of how subduction and plate tectonics began on Earth—and what the tectonic structure of Earth was before this—remain enigmatic and contentious1. Understanding viable scenarios for the onset of subduction and plate tectonics2,3 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 tectonics4. However, plume-induced subduction initiation5,6,7,8,9 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.

The classic example of the not-well-understood rapid change of tectonic plate motion is the increase and then decrease of the convergence rate between the Nazca and South America plates during the past 25–20 m.y. that coincided with the growth of the Andes Mountains. Currently, the decrease in convergence rate is explained either by the increasing load of the Andes or by the appearance of flat slab segments beneath South America. Here, we present an alternative view based on a thermomechanical self-consistent (gravity driven) model of Nazca plate subduction. We explain the changes in the convergence rate as a natural consequence of the Nazca plate penetration into the transition zone and lower mantle after long-term oblique subduction of the Farallon plate. The model is consistent with seismic tomographic images of the Nazca plate beneath South America. Our model also shows that the presence of the Andes does not significantly affect the convergence rate between the Nazca and South America plates.

[1] We present a three-dimensional finite element thermomechanical model idealizing the complex deformation processes associated with evolution of the San Andreas Fault system (SAFS) in northern and central California over the past 20 Myr. More specifically, we investigate the mechanisms responsible for the eastward (landward) migrationof the San Andreas plate boundary over time, a process thathas largely determined the evolution and present structure of SAFS. Two possible mechanisms had been previously suggested. One mechanism suggests that the Pacific plate first cools and captures uprising mantle in the slab window, subsequently causing accretion of the continental crustal blocks. An alternative scenario attributes accretion to the capture of plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Here we test both these scenarios numerically using a recently developed lithospheric-scale code, SLIM3D, that employs free surface, nonlinear temperature- and stress-dependent elastoviscoplastic rheology and allows for self-generation of faults. Modeling suggests that microplate capture is the primary driving mechanism fortheeastwardmigrationoftheplateboundary,whiletheslabwindowcoolingmechanismaloneisincapable of explaining this phenomenon. We also show that the system evolves to the present day structure of SAFS only if the coefficient of friction at mature faults is low (0.08 for the best fit model). Thus, our model provides an independent constraint supporting the “weak fault in a strong crust” hypothesis for SAFS.

Transformation kinetics associated with the presence of a metastable olivine wedge in old and fast subducting slabs has been the subject of many studies in the last years. Even with improvements in kinetics models, many of the parameters are still not well constrained. In particular, there is no consensus on the blocking temperature that could inhibit the transformation from olivine to spinel. Recently, based on anomalous later phases in the P wave coda and differential P wave slowness, a wedge of metastable olivine was detected in the Marianas subduction zone, that is approximately 25 km wide, at a depth of 590 km and is truncated at 630 km. In this work, a thermomechanical model was used to mimic the subduction in the Marianas and try different blocking temperatures for the olivine/spinel transformation. The model includes, among other features, non-linear elasto-visco-plastic rheology based on laboratory data, phase transformations, latent heat, proper coupling between stress and thermal state of the slab and force balance of the system. The results show a positive correlation between the blocking temperature, depth of the wedge and its distance from the trench (or subduction angle). We compare these results to the situation in the Marianas and suggest that a blocking temperature for the olivine/spinel transformation of approximately 725 °C would be the most likely. The volume of the wedge presents some oscillations that we relate to a runaway effect of the transformation kinetics in the mantle transition zone. Namely, the interaction between latent heat release and the advection of the isotherms due to the subduction velocity. The inclusion of shear heating in the model was fundamental to modeling such a subduction zone. Without shear heating, the slab shows a higher level of internal stress and the necessary bending to mimic the Marianas subduction zone cannot be reached.

[1] The evolution of a subducting slab is strongly influenced by the viscosity of the mantle which it overlies. However, there is still no consensus about the viscosity in the transition zone and shallower lower mantle. We use a numerical self-consistent subduction model and run a set of experiments in order to find critical patterns of viscosity that would allow the evolution of the different subduction styles that can actually be found in nature. Our results show that a jump in viscosity of ∼5 times from transition zone to lower mantle gives the most reasonable results. Optimal values of viscosity in the transition zone are in the range of 3 * 1020–1021 Pa s. Higher values would produce piling up of the slab and later sinking or even slab flattening. Lower viscosities result in velocities (>30 cm/yr) that are too high, while the new slab subducts through the upper mantle and transition zone, a phenomenon that is rarely seen in nature. Reduction of the Clapeyron slope, related to the spinel-perovskite transition, variations in oceanic crustal thickness and in the age of the slab do not influence much the style of subduction. When overriding velocity is applied to the upper plate, the previously penetrating slab tends to lay at the 660 km boundary but does not substantially change the subduction velocity.

[1] Since the devastating earthquake and tsunami in 2004 offshore Sumatra, many source models have been put forward. Recent studies clearly show that modern GPS-processing could achieve high resolving power for slip in near real time, which is crucial for determining tsunami initial conditions, provided accurate GPS-processing and inversion. Here, we propose an inversion technique with improved representation of the subduction zone geometry and physically justified boundary conditions. We show that the discrepancy between the inversion of near- and far field GPS data for the 2004 event, which is often explained by postseismic slip, can be eliminated by using our inversion method and IASP91 earth model. Inverted source models, including versions with splay faulting, are shown to be consistent with satellite altimetry data of offshore tsunami wave height, suggesting that displacement at the splay fault might have been present but was likely a second order process.

We employ high-resolution 2D thermomechanical modelling with complex elasto-visco-plastic rheology to analyse stresses in subducting and overriding plates. The model contains a dynamic subduction channel, defined as a few km thick zone of specific (weak) visco-plastic rheology between the slab and the overriding plate. Our high-resolution model corresponds to the zoom-in of the large-scale thermomechanical model of the Central Andes, with the 4-fold resolution increase. Initial model configuration (material, temperature and stress distributions) was interpolated from the parent model. Boundary conditions including slab pull and push velocities as well as velocity of the overriding plate, were also re-mapped from the large scale model. We demonstrate that, for shallow-dipping overridden slabs, the dominant stress is unbending stress at 50–100 km depth. This stress reaches 1–1.5 GPa and strongly prevails over the slab bending stress near the trench, which is limited by the frictional plastic yield. Unbending creates double (5–10 km distance between maximums) compression zone in the upper part of the slab and a zone of extension some 20–30 km below. Pressure in these zones differs from the lithostatic pressure to as much as plus/minus 0.8 GPa. Details of the distribution and magnitude of bending and unbending stresses in the slab are mainly controlled by the overriding velocity and direction of the slab-pull force. Overpressure in the subduction channel in all models does not exceed 100 MPa. Large over- and under-pressures in the slab, do not, however, lead to significant (more than 10 km) offsets of main dehydration reactions in crust and mantle of the slab.

SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.

SUMMARY The P and S seismic anomalies in the upper mantle beneath the Pamir‐Hindukush collision zone and surrounding areas are investigated using more than 4 × 10 5 traveltimes from 5775 events located in the study region, recorded by the stations of the worldwide seismic network and reported in the ISC catalogue. All available epicentral distance ranges of seismic rays are considered. All sources are relocated using an algorithm, which includes the double-difference method. In addition, source parameters are corrected in the main inversion step, simultaneously with the velocity models. The high reliability of the resulting P-velocity images is confirmed by various tests. The S model provides much smoother patterns, which are, however, generally consistent with P anomalies. Our model shows a complex image of lithospheric plate under the study region. Under SW Hindukush, the northwards subducting Indian plate is imaged throughout the upper mantle and transition zone. Further to the NE, within a 300-km-wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at ∼250 km depth. The tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern Tien Shan down to at least 250 km depth. To the north of Tarim, we observe a northward-dipping of the Tarim plate down to ∼500 km depth. At the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults, which indicate complex crustal structure in this region.

The Andes, the world's second highest orogenic belt, were generated by the Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca plate. We use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening. From the modeling, we infer that the most important factor was accelerated westward drift of the South American plate; changes in the subduction rate were less important. Other important factors are crustal structure of the overriding plate and shear coupling at the plates' interface. The model with a thick (40–45 km at 30 Ma) South American crust and relatively high friction coefficient (0.05) at the Nazca–South American interface generates >300 km of tectonic shortening during 30–35 m.y. and replicates the crustal structure and evolution of the high central Andes. The model with an initially thinner (

[1] A 400-km-long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP'96)) and integrated geophysical experiments (wide-angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15–30 km depth. This reflectivity is associated with a low-velocity zone, P to S wave conversions, the upper limits of high conductivity and high Vp/Vs ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust-mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first-order features appear to be related to fluid-assisted processes in a subduction setting.

Seismic data from central Tibet have been combined to image the subsurface structure and understand the evolution of the collision of India and Eurasia. The 410- and 660-kilometer mantle discontinuities are sharply defined, implying a lack of a subducting slab beneath the plateau. The discontinuities appear slightly deeper beneath northern Tibet, implying that the average temperature of the mantle above the transition zone is about 300°C hotter in the north than in the south. There is a prominent south-dipping converter in the uppermost mantle beneath northern Tibet that might represent the top of the Eurasian mantle lithosphere underthrusting the northern margin of the plateau.

The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovicić discontinuity--generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

We have imaged the upper mantle discontinuities in a 30×20° large region at the active continental margin of the Japan subduction zone and neighboring areas, using P-to-S converted phases from teleseismic records of permanent broadband stations. The 410 km discontinuity is detected within ±10 km of its global average position. An interesting exception in its observation is a gap near 135°E, very close to the slab penetration of the 410 km discontinuity and in an area where we have rather high data density. The 660 km discontinuity reaches 700 km depth at two places where it is hit directly by the slab. These data generally show good agreement with tomographic results. Both, the 660 km discontinuity depression and the P-velocity anomalies, suggest about 400–500 K temperature deficit within the slab. However, no downward bending of the 660 km discontinuity is observed in eastern China where the flat lying slab is imaged by tomography. This suggests that the slab does not cool the 660 km discontinuity in this region. Therefore the positive buoyancy required to keep the slab lying flat cannot have been provided by the negative Clapeyron slope of the spinel–perovskite phase transition. Another mechanism is needed, which could possibly be metastable olivine in the cold core of the slab. We have also imaged the shallower portion of the slab down to about 150 km underneath some seismic stations, likely because metastable gabbro is still existing to this depth providing a sufficient velocity contrast. A strong negative converted phase is observed at 150–200 km depth underneath a volcanic region in Japan. If real (observed at one station only), the zone which produces this conversion and which may begin exactly above the slab would require fluid-fed melting.

Using isotropic inversion of teleseismic P-travel times computed for laterally varying anisotropic structures, we investigate how anisotropy may show up in classical tomographic studies based on isotropic 3-D inversions. Anisotropic bodies of several tens of kilometers in the upper mantle may significantly bias the estimation of the isotropic part of a P-wave velocity image. The largest artifacts are introduced by anisotropic bodies with a dipping olivine “a” axis which could be associated with fossil subduction zones. However, anisotropy-induced artifacts are negligible for anisotropy caused by pure shear deformation of the lithosphere due to horizontal extension or compression. The negative P-wave velocity anomaly arising from isotropic inversion beneath a rift zone may be enhanced if the olivine crystal's “a” axes are preferentially oriented parallel to the asthenospheric flow along the rift axis. Examination of recently published tomographic models shows that the image of the Rhine Graben may contain significant anisotropic artifacts.