identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

reconciling subduction dynamics during tethys closure with large scale asian tectonics insights from numerical modeling we use three dimensional numerical models to investigate the relation between subduction dynamics and large scale tectonics of continent interiors the models show how the balance between forces at the plate margins such as subduction ridge push and far field forces controls the coupled plate margins and interiors evolution removal of part of the slab by lithospheric break off during subduction destabilizes the convergent margin forcing migration of the subduction zone whereas in the upper plate large scale lateral extrusion rotations and back arc stretching ensue when external forces are modeled such as ridge push and far field forces indentation increases with large collisional margin advance and thickening in the upper plate the balance between margin and external forces leads to similar convergent margin evolutions whereas major differences occur in the upper plate interiors here three strain regimes are found large scale extrusion extrusion and thickening along the collisional margin and thickening only when negligible far field forces ridge push and larger far field forces respectively add to the subduction dynamics the extrusion tectonics develops a strong asymmetry toward the oceanic margin driven by large scale subduction with no need of preexisting heterogeneities in the upper plate because the slab break off perturbation is transient the ensuing plate tectonics is time dependent the modeled deformation and its evolution are remarkably similar to the cenozoic asian tectonics explaining large scale lithospheric faulting and thickening and coupling of indentation extrusion and extension along the asian convergent margin as a result of large scale subduction process

the dynamics of extrusion tectonics insights from numerical modeling to elucidate the dynamics of extrusion tectonics i investigate the relation between subduction processes and intraplate deformation three dimensional numerical models of subducting upper plate systems show that subduction of a heterogeneous buoyancy slab that is hosting continental and oceanic lithosphere causes the retreat of the oceanic trench and the localization of deformation in the back arc similar trench evolution is found when the subducting slab partially detaches from the impinging continent however deformation in the upper plate interiors widens with indentation plastic yielding and rotation of a large block extruding laterally toward the retreating margin indentation stresses are consequence of the partial slab breakoff at depth and vanish when the detached slablet reaches the mantle transition zone resulting in transient extrusion tectonics the distance to which indentation deformation propagates depends on the ability of subducting lithosphere to propagate stress laterally and is linear with the lithospheric strength while in the stretching back arc the plate deformation and the underlying mantle flow are coupled they are uncoupled in the extrusion tectonics where mostly stresses propagated through the margin drive rotations the potential role of these diverse subduction processes in the eastern mediterranean is discussed suggesting that different mechanisms likely drive the tectonics of this area

a lithosphere dynamics constraint on mantle flow analysis of the eurasian plate 1 we present a method to estimate the poorly understood mechanical coupling between lithosphere and underlying mantle and apply it to the eurasian plate mechanical equilibrium of tectonic plates requires the torque from mantle tractions m to be balanced by the torques from edge forces e and lithospheric body forces b the direction of e proves tightly constrained by plate boundary nature but b is affected uncertainties in the density structure of continents we consistently find that the non zero torque required from mantle tractions does not agree with the orientation of any published absolute motion model we conclude that mechanical balance of the eurasian plate requires an actively convecting mantle which should result in a torque on the eurasian plate located in the southwest pacific

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

the coupling of indian subduction and asian continental tectonics abstract in order to understand the potential controls on asian tectonics during the subduction of the tethys and indian lithospheres we reconstruct the coupled subduction continent deformation history using tomographic imaging kinematics constraints and numerical modeling the global p waves tomographic images of the mantle below the india asia collision zone provide constraints on the deep structure of continents and subduction history linking the slab positions in the mantle to the asian tectonics reconstructions and the indian plate kinematics we reconstruct the timing and location of successive subduction and breakoff events showing one major breakoff occurred between india and the tethys ocean 45 ma in the western syntax a vertical slab continuous to the continent is shown to override the deeper detached tethys slab in the central region similar structure is found with a detached slab yet closer to the tethys slab in the eastern syntax no slab is imaged it is inferred that after tethys slab had broke off subduction only resumed in the center of the margin while underthrusting took place at both extremities of the convergent margin during following convergence a second breakoff event detached the central indian slab from the margin 15 ma ago which renewed indian lithosphere underthrusting below asia this most probably occurred when the tibetan plateau was already uplifted implying that uplift is not a direct result of underthrusting numerical models of breakoff during subduction illustrate the controls of slab detachment on the complexities of the indian margin in these models the subduction of continental lithosphere resumes after breakoff only where this is entrained by the mantle flow associated with the long lasting oceanic slab sinking that is in the center of the margin while converging continent edges underthrusts the upper plate furthermore the breakoff during subduction has profound implications on the asian intra plate tectonics in the models the breakoff is rapidly followed by large stresses in the upper plate interiors propagating at large distance from the margin along a belt oriented at 45 from the trench the long term evolution of the asian continental tectonics shows drastic changes in the fault pattern with successive strike slip faulting across the asian continent which are in agreement with the mechanisms illustrated by the models transient large coupling at the trench caused by the breakoff events during india asia convergence offers an explanation for episodic nucleation of lithospheric faults within the asian continent and their link to deep processes

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

comparison of azimuthal seismic anisotropy from surface waves and finite strain from global mantle circulation models summary we present global models of strain accumulation in mantle flow to compare the predicted finite strain ellipsoid fse orientations with observed seismic anisotropy the geographic focus is on oceanic and young continental regions where we expect our models to agree best with azimuthal anisotropy from surface waves finite strain derived models and alignment with the largest fse axes lead to better model fits than the hypothesis of alignment of fast propagation orientation with absolute plate motions our modelling approach is simplified in that we are using a linear viscosity for flow and assume a simple relationship between strain and anisotropy however results are encouraging and suggest that similar models can be used to assess the validity of assumptions inherent in the modelling of mantle convection and lithospheric deformation our results substantiate the hypothesis that seismic anisotropy can be used as an indicator for mantle flow circulation derived models can contribute to the establishment of a quantitative functional relationship between the two

present day stress field of southeast asia article i nfo it is now well established that ridge push forces provide a major control on the plate scale stress field in most of the earth s tectonic plates however the sunda plate that comprises much of southeast asia is one of only two plates not bounded by a major spreading centre and thus provides an opportunity to evaluate other forces that control the intraplate stress field the cenozoic tectonic evolution of the sunda plate is usually considered to be controlled by escape tectonics associated with india eurasia collision however the sunda plate is bounded by a poorly understood and complex range of convergent and strike slip zones and little is known about the effect of these other plate boundaries on the intraplate stress field in the region we compile the first extensive stress dataset for southeast asia containing 275 a d quality 177 a c horizontal stress orientations consisting of 72 stress indicators from earthquakes located mostly on the periphery of the plate 202 stress indicators from breakouts and drilling induced fractures and one hydraulic fracture test within 14 provinces in the plate interior this data reveals that a variable stress pattern exists throughout southeast asia that is largely inconsistent with the sunda plate s approximately ese absolute motion direction the present day maximum horizontal stress in thailand vietnam and the malay basin is predominately north south consistent with the radiating stress patterns arising from the eastern himalayan syntaxis however the present day maximum horizontal stress is primarily oriented nw se in borneo a direction that may reflect plate boundary forces or topographic stresses exerted by the central borneo highlands furthermore the south and central sumatra basins exhibit a ne sw maximum horizontal stress direction that is perpendicular to the indo australian subduction front hence the plate scale stress field in southeast asia appears to be controlled by a combination of himalayan orogeny related deformation forces related to subduction primarily trench suction and collision and intraplate sources of stress such as topography and basin geometry

continental collision slowing due to viscous mantle lithosphere rather than topography the rate of india s convergence with eurasia has decreased exponentially since their collision which suggests that the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved the forces that control plate motion are fundamental to plate tectonic theory it has generally been thought that deforming and thickening continental crust is responsible for slowing down the convergence between two continental plates marin clark now shows however that the rate of penetration of the indian subcontinent into eurasia has decreased exponentially since the plates collided which suggests that contractional strain across tibet has been constant throughout the collision she infers that this reflects deformation of the mantle lithosphere and that under conditions of constant stress and strength the deforming continental lithosphere creates viscous resistance that affects plate motion irrespective of how topography evolved because the inertia of tectonic plates is negligible plate velocities result from the balance of forces acting at plate margins and along their base1 observations of past plate motion derived from marine magnetic anomalies provide evidence of how continental deformation may contribute to plate driving forces2 3 4 5 6 7 8 a decrease in convergence rate at the inception of continental collision is expected because of the greater buoyancy of continental than oceanic lithosphere2 3 but post collisional rates are less well understood slowing of convergence has generally been attributed to the development of high topography that further resists convergent motion7 8 9 10 however the role of deforming continental mantle lithosphere on plate motions has not previously been considered here i show that the rate of india s penetration into eurasia has decreased exponentially since their collision the exponential decrease in convergence rate suggests that contractional strain across tibet has been constant throughout the collision at a rate of 7 03 10 16 s 1 which matches the current rate a constant bulk strain rate of the orogen suggests that convergent motion is resisted by constant average stress constant force applied to a relatively uniform layer or interface at depth this finding follows new evidence that the mantle lithosphere beneath tibet is intact11 which supports the interpretation that the long term strain history of tibet reflects deformation of the mantle lithosphere under conditions of constant stress and strength the deforming continental lithosphere creates a type of viscous resistance that affects plate motion irrespective of how topography evolved

effects of plate bending and fault strength at subduction zones on plate dynamics for subduction to occur plates must bend and slide past overriding plates along fault zones because the lithosphere is strong significant energy is required for this deformation to occur energy that could otherwise be spent deforming the mantle we have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology by increasing the effective viscosity of either the plate or the fault zone we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy we have developed a simple physical theory that predicts this slowing by estimating a convecting cell s total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone the energy required to bend the slab is proportional to the slab s viscosity and to the cube of the ratio of its thickness to its radius of curvature the distribution of dissipation among the mantle lithosphere and fault zone causes the speed of a plate to depend on its horizontal length scale using the observation that earth s plate velocities are not correlated to plate size we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity with higher values required if the fault zone can support shear tractions 50 mpa over 300 km these subduction zone strengths imply that the mantle fault zone and lithosphere dissipate about 30 10 and 60 of a descending slab s potential energy release if the slab is 100 km thick the lithospheric component is highly dependent on slab thickness it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the earth cools because the details of convergent plate boundaries are so important to the dynamics of plate motion numerical models of mantle flow should treat subduction zones in a realistic way

inference of mantle viscosity from grace and relative sea level data summary gravity recovery and climate experiment grace satellite observations of secular changes in gravity near hudson bay and geological measurements of relative sea level rsl changes over the last 10 000 yr in the same region are used in a monte carlo inversion to infer mantle viscosity structure the grace secular change in gravity shows a significant positive anomaly over a broad region 3000 km near hudson bay with a maximum of 2 5 gal yr 1 slightly west of hudson bay the pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ice 5g deglaciation history strongly suggesting a postglacial rebound origin for the gravity change we find that the grace and rsl data are insensitive to mantle viscosity below 1800 km depth a conclusion similar to that from previous studies that used only rsl data for a mantle with homogeneous viscosity the grace and rsl data require a viscosity between 1 4 1021 and 2 3 1021 pa s an inversion for two mantle viscosity layers separated at a depth of 670 km shows an ensemble of viscosity structures compatible with the data while the lowest misfit occurs for upper and lower mantle viscosities of 5 3 1020 and 2 3 1021 pa s respectively a weaker upper mantle may be compensated by a stronger lower mantle such that there exist other models that also provide a reasonable fit to the data we find that the grace and rsl data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle

reconstructing the ocean interior from high resolution sea surface information abstractwhen evaluated against the 1 30 resolution submesoscale resolving ofes model outputs the previously published interior surface quasigeostrophic method from the 2013 study by wang et

india asia convergence driven by the subduction of the greater indian continent the most spectacular example of plate convergence on earth was the motion of the indian plate towards eurasia and the subsequent collision density estimates of the greater indian continent after its upper crust is scraped off at the himalayan front suggest that this continental plate is readily subductable potentially explaining why the convergence did not halt on collision the most spectacular example of a plate convergence event on earth is the motion of the indian plate towards eurasia at speeds in excess of 18 cm yr 1 ref 1 and the subsequent collision continental buoyancy usually stalls subduction shortly after collision as is seen in most sections of the alpine himalayan chain however in the indian section of this chain plate velocities were merely reduced by a factor of about three when the indian continental margin impinged on the eurasian trench about 50 million years ago plate convergence accompanied by eurasian indentation persisted throughout the cenozoic era1 2 3 suggesting that the driving forces of convergence did not vanish on continental collision here we estimate the density of the greater indian continent after its upper crust is scraped off at the himalayan front and find that the continental plate is readily subductable using numerical models we show that subduction of such a dense continent reduces convergence by a factor similar to that observed in addition an imbalance between ridge push and slab pull can develop and cause trench advance and indentation we conclude that the subduction of the dense indian continental slab provides a significant driving force for the current india asia convergence and explains the documented evolution of plate velocities following continental collision

shaping mobile belts by small scale convection claudio faccenna and thorsten becker compute global mantle flow based on a recent high resolution seismic tomography model to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean they show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region and conclude that vigorous small scale convection in the uppermost mantle may also hold the key for understanding other complex mobile belts such as those in the north american cordillera and the himalayan tibetan collision zone here global mantle flow is computed on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation in the mediterranean region the findings show that mantle flow explains much of the observed dynamic topography and microplate motion in the region small scale convection in the uppermost mantle may also hold the key to understanding complex mobile belts elsewhere mobile belts are long lived deformation zones composed of an ensemble of crustal fragments distributed over hundreds of kilometres inside continental convergent margins1 2 the mediterranean represents a remarkable example of this tectonic setting3 the region hosts a diffuse boundary between the nubia and eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence4 surface expressions of mediterranean tectonics include deep subsiding backarc basins intraplate plateaux and uplifting orogenic belts although the kinematics of the area are now fairly well defined the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions however the effects of mantle convection well established for continental interiors5 6 7 should be particularly relevant in a mobile belt and modelling may constrain important parameters such as slab coherence and lithospheric strength here we compute global mantle flow on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean we show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region more generally vigorous small scale convection in the uppermost mantle may also underpin other complex mobile belts such as the north american cordillera or the himalayan tibetan collision zone

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

development of mantle seismic anisotropy during subduction induced 3 d flow 1 the dynamics of subduction can be indirectly constrained by studying the induced mantle flow however inferring the circulation of the mantle around subducting plates from the interpretation of shear wave splitting patterns remains elusive we calculated the strain induced lattice preferred orientation lpo developed in 3 d models of subduction where retreat motions are maximized and found that in the mantle layer entrained with the downgoing slab the seismic anisotropy is trench perpendicular and becomes trench parallel deeper where the toroidal flow accommodates slab retreat synthetic sks splitting shows that in the fore arc slab rollback favors trench parallel polarization of the fast shear wave component while plate advance enhances trench perpendicular seismic anisotropy the interference between these two competing mechanisms yields subslab delay times of 0 5 to 1 3 sec comparable with those observed at most natural subduction zones the magnitude of the subslab trench parallel splitting is independent of the rate at which the slab migrates instead it is proportional to the amount of retreat

mineralogical textural sulfur and lead isotope constraints on the origin of ag pb zn mineralization at bianjiadayuan inner mongolia ne china degao zhai jiajun liu nigel j cook xilong wang yongqiang yang anli zhang yingchun jiao

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

mantle conveyor beneath the tethyan collisional belt abstract collisional belts are generated by the arrival of continental lithosphere into a subduction zone the tethyan suture from the bitlis to the himalayas is a prime example where the arabian and indian plates collided with eurasia during the cenozoic while the kinematics of this process are well established its dynamics are more uncertain india and arabia intriguingly keep advancing in spite of large collisional resisting forces and in the absence of a substantial upper mantle slab driving force at present day we perform global mantle circulation computations to test the role of deep mantle flow as a driving force for the kinematics of the tethyan collisional belt evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models our results show that mantle drag exerted on the base of the lithosphere by a large scale convective conveyor belt with an active upwelling component is likely the main cause for the ongoing indentation of the indian and arabian plates into eurasia

a dynamical model for generating eurasian lithospheric stress and strain rate fields effect of rheology and cratons for most continents stress models driven by plate boundary forces have successfully reproduced the main characteristics of the stress field however eurasia has remained a challenge due to its large areas of intraplate deformation we present a set of three dimensional models of the upper mantle lithosphere system for a simplified geometry of the eurasian plate where we try to match the first order characteristics of the stress and strain rate fields simultaneously for typical elastic viscous or plastic rheologies high stress levels are required in order to produce realistic convergence rates between india and asia our models show robustly that such stresses are transmitted throughout most of the plate dominating locally generated stresses even in distal regions such as europe in a manner that is not compatible with observations cratons with roots that extend deep into the mantle are unable to provide a significant stress shielding effect unless the viscosity contrast between the asthenosphere and the underlying mantle is around 100 or greater a damage rheology for the lithosphere with history dependent behavior and material softening by a viscosity reduction of several orders of magnitude is shown to eliminate this conundrum continental convergence at high velocity but low stress is facilitated by the formation of long lived shear zones similar to those observed north of the himalayas the low stress associated with the collision together with the decoupling effect of the shear zones causes the distal stress field in europe to be controlled by the effects of the neighboring boundaries in agreement with observations

insights on the kinematics of the india eurasia collision from global geodynamic models the eocene india eurasia collision is a first order tectonic event whose nature and chronology remains controversial we test two end member collision scenarios using coupled global plate motion subduction models the first conventional model invokes a continental collision soon after 60 ma between a maximum extent greater india and an andean style eurasian margin the alternative scenario involves a collision between a minimum extent greater india and a neotethyan back arc at 60 ma that is subsequently subducted along southern lhasa at an andean style margin culminating with continent continent contact at 40 ma our numerical models suggest the conventional scenario does not adequately reproduce mantle structure related to tethyan convergence the alternative scenario better reproduces the discrete slab volumes and their lateral and vertical distribution in the mantle and is also supported by the distribution of ophiolites indicative of tethyan intraoceanic subduction magmatic gaps along southern lhasa and a two stage slowdown of india our models show a strong component of southward mantle return flow for the tethyan region suggesting that the common assumption of near vertical slab sinking is an oversimplification with significant consequences for interpretations of seismic tomography in the context of subduction reference frames

continental deformation in asia from a combined gps solution 1 after decades of research on continental tectonics there is still no consensus on the mode of deformation of continents or on the forces that drive their deformation in asia the debate opposes edge driven block models requiring a strong lithosphere with strain localized on faults to buoyancy driven continuous models requiring a viscous lithosphere with pervasive strain discriminating between these models requires continent wide estimates of lithospheric strain rates previous efforts have relied on the resampling of heterogeneous geodetic and quaternary faulting data sets using interpolation techniques we present a new velocity field based on the rigorous combination of geodetic solutions with relatively homogeneous station spacing avoiding technique dependent biases inherent to interpolation methods we find 1 unresolvable strain rates 3 109 yr over a large part of asia with current motions well described by block or microplate rotations and 2 internal strain possibly continuous limited to high elevation areas

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

the bending mechanics in a dynamic subduction system constraints from numerical modelling and global compilation analysis abstract lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate motions to understand the mechanics of plate bending we derive scaling laws for the deflection at the margin i e radius and dip from numerical models of a subducting viscoelastic plate in such dynamic system we find that the buoyancy and the stiffness of the plates control the radius and the dip as well as the plate motions toward the trench this mechanical model successfully predicts the curvature of published three dimensional laboratory and numerical models for a thorough comparison with the observable we have also implemented forces additional to the slab pull such as the suction force and far field stresses by increasing or resisting the torque applied at the trench by the slab these forces can largely rearrange the dip and the radius of slabs and the inherent plate motions although they do not alter the observed anticorrelation between radius and dip similar inverse correlation relationship and dip radius ranges are shown by most of the subduction zones analysed from a global compilation radii in the range of 100 350 km and dips of 30 70 for slabs that extends to the bottom of the upper mantle are compatible with the models and allow estimating an average lithospheric viscosity contrast of 200 in the bending with respect to the ambient mantle radii and dips outside of this range are in good agreement with the trends and the magnitudes of models that include suction and far field forces in all these subduction zones the correlation between dip radius and plate velocity is found to be compatible with that of the models showing how relevant bending is for the dynamics of earth

lithosphere mantle coupling and the dynamics of the eurasian plate mechanical equilibrium of tectonic plates implies that lithospheric edge and body forces are balanced by forces arising from interaction with the underlying mantle we use this quantitative physical relation to integrate existing modelling approaches of lithosphere dynamics and mantle flow into a new combined approach applied to the eurasian plate by combining a thorough analysis of lithospheric forces with the requirement of torque balance we constrain the orientation of the torque on eurasia arising from mantle tractions we use this constraint to evaluate convective mantle flow models driven by tomographic or subduction history model anomalies and observed plate motion mantle forcing is considered through both shear at the bottom of the plate and induced dynamic topography we find that instantaneous semi analytic flow models without lateral viscosity variations generate tractions that meet the constraint from eurasian lithosphere dynamics but only for specific ranges of mantle flow parameters of the explored set of mantle anomaly models only mantle flow models based on s wave tomography anomalies can balance eurasia for realistic viscosity profiles and velocity density scaling choices in mantle density forcing and viscosity are crucial in that they govern the relative magnitude of tractions due to convective mantle flow active tractions and resistive tractions due to plate motion passive tractions we find mechanical balance is only achieved for similar torque magnitudes of active and passive shear the two shear contributions do however in no case balance each other and a considerable dominant net torque from edge forces is required to balance total mantle tractions and lithospheric body forces lbfs our analysis provides a range of mechanically consistent total force sets acting on the eurasian plate using this result we find that mantle buoyancy forces and lbfs acting on eurasia itself are important driving forces but do not drive eurasia in the observed direction continental collision at eurasia s southern boundary significantly deviates eurasia northwards our combined torque balance approach in which mantle tractions from convective mantle flow modelling are combined with explicitly applied edge forces thus emphasizes the role of plate interactions to the dynamics of tectonic plates

orocline and syntaxes formation during subduction and collision 1 the present work investigates the formation of curved ranges and syntaxes with scaled laboratory experiments we simulated subduction and collision processes comparable to india asia configuration involving a continental upper plate and a subducting plate composed of an oceanic lithosphere and a continental indenter the experiments reveal that the shape of the mountain range concave straight or convex and the development of syntaxes are controlled by the subduction interface the buoyancy number fb of the upper plate i e thickness and viscosity and the boundary conditions four end members regimes of indentation can be defined depending on the range shape and dynamics of the upper plate the curvature of the range is convex toward the subducting plate with syntaxes for a weak subduction fault and concave without syntaxes for a strong subduction fault convex curvature and syntaxes form by overthrusting of upper plate material on the subducting plate which is faster at the center than at the extremities they are associated with a rather flat slab underthrusting during continental collision low fb experiments show less pronounced curvatures associated to thickening comparable to the early stages of the india asia collision in contrast a thick and weak upper plate high fb leads to gravity collapse that increases the amplitude of the curvature and lateral escape similar to the late evolution of the himalaya tibet system important lateral decoupling on the sides of the indenter enhances the indentation and produces sharper syntaxes

role of lithospheric strength heterogeneities in the tectonics of tibet and neighbouring regions unlike rigid oceanic plates continents show diffuse deformation for example the tibetan plateau that has accommodated much of the convergence of india with asia we interpret the tertiary deformation of asia as the three dimensional strain of a non linear viscous continental lithosphere lateral strength contrasts modify this deformation stress concentration around the relatively strong tarim basin gives rise to local crustal thickening in the tien shan to the north and to strike slip in the altyn tagh along its southern margin

timescales for the evolution of seismic anisotropy in mantle flow 1 we study systematically the relationship between olivine lattice preferred orientation and the mantle flow field that produces it using the plastic flow recrystallization model of kaminski and ribe 2001 in this model a polycrystal responds to an imposed deformation rate tensor by simultaneous intracrystalline slip and dynamic recrystallization by nucleation and grain boundary migration numerical solutions for the mean orientation of the a axes of an initially isotropic aggregate deformed uniformly with a characteristic strain rate show that the lattice preferred orientation evolves in three stages 1 for small times recrystallization is not yet active and the average a axis follows the long axis of the finite strain ellipsoid 2 for intermediate times the fabric is controlled by grain boundary migration and the average a axis rotates toward the orientation corresponding to the maximum resolved shear stress on the softest slip system 3 for the fabric is controlled by plastic deformation and average a axis rotates toward the orientation of the long axis of the finite strain ellipsoid corresponding to an infinite deformation the infinite strain axis in more realistic nonuniform flows lattice preferred orientation evolution depends on a dimensionless grain orientation lag parameter x defined locally as the ratio of the intrinsic lattice preferred orientation adjustment timescale to the timescale for changes of the infinite strain axis along path lines in the flow explicit numerical calculation of the lattice preferred orientation evolution in simple fluid dynamical models for ridges and for plume ridge interaction shows that the average a axis aligns with the flow direction only in those parts of the flow field where 1 calculation of provides a simple way to evaluate the likely distribution of lattice preferred orientation in a candidate flow field at low numerical cost

collision of continental corner from 3 d numerical modeling abstract continental collision has been extensively investigated with 2 d numerical models assuming infinitely wide plates or insignificant along strike deformation in the third dimension however the corners of natural collision zones normally have structural characteristics that differ from linear parts of mountain belt we conducted 3 d high resolution numerical simulations to study the dynamics of a continental corner lateral continental oceanic transition zone during subduction collision the results demonstrate different modes between the oceanic subduction side continuous subduction and retreating trench and the continental collision side slab break off and topography uplift slab break off occurs at a depth 100 km to 300 km that depends on the convergence velocity the numerical models produce lateral extrusion of the overriding crust from the collisional side to the subduction side which is also a phenomenon recognized around natural collision of continental corners for instance around the western corner of the arabia asia collision zone and around the eastern corner of the india asia collision zone modeling results also indicate that extrusion tectonics may be driven both from above by the topography and gravitational potentials and from below by the trench retreat and asthenospheric mantle return flow which supports the link between deep mantle dynamics and shallower crustal deformation

combined effects of eurasia sunda oblique convergence and east tibetan crustal flow on the active tectonics of burma it is widely accepted that deformation of the india sunda plate is the result of partitioned hyper oblique convergence presently sub meridian dextral strike slip faulting accommodates this india sunda motion in a buffer zone the burma platelet this wide dextral strike slip shear zone is complicated by the side effect of the tibet plateau collapse that can be described in term of crustal flow and gravity tectonics the loss of potential energy related to this plateau collapse affects most of the burmese platelet particularly in its northernmost part interaction of these two distinct geodynamic processes is recorded in the gps based regional strain field the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore myanmar and bangladesh we propose the apparent e w shortening component of this so called partitioned hyper oblique subduction is only the effect of regional gravitational forces related to the tibet plateau collapse whereas the ns strike slip faulting accommodates the india sunda motion

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

an integrated global model of present day plate motions and plate boundary deformation summary in this paper we present a global model gsrm 1 of both horizontal velocities on the earth s surface and horizontal strain rates for almost all deforming plate boundary zones a model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field in our model we perform a least squares fit between model velocities and observed geodetic velocities as well as between model strain rates and observed geological strain rates model velocities and strain rates are interpolated over a spherical earth using bi cubic bessel splines we include 3000 geodetic velocities from 50 different mostly published studies geological strain rates are obtained for central asia only and they are inferred from quaternary fault slip rates for all areas where no geological information is included a priori constraints are placed on the style and direction but not magnitude of the model strain rate field these constraints are taken from a seismic strain rate field inferred from normalized focal mechanisms of shallow earthquakes we present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones generally the strain rate tensor field is consistent with geological and seismological data with the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs we find that in general there is a good agreement between the present day plate motions we obtain and longterm plate motions but a few significant differences exist the rotation rates for the indian arabian and nubian plates relative to eurasia are 30 13 and 50 per cent slower than the nuvel1a estimate respectively and the rotation rate for the nazca plate relative to south america is 17 per cent slower on the other hand caribbean north america motion is 76 per cent faster than the nuvel 1a estimate while crustal blocks in the india eurasia collision zone move significantly and self consistently with respect to bounding plates only a very small motion is predicted between the nubian and somalian plates by integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in for instance redefining a no net rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

tethyan collision forces and the stress field of the eurasian plate resistive forces along convergent plate boundaries have a major impact on surface deformation most visibly at collisional plate boundaries although quantification of these forces is key to understanding the evolution and present state of mountain belts they remain highly uncertain due to the complexity of plate boundary structures and rheologies in previous studies of the eurasian plate we have analysed the balance of plate boundary forces tractions resulting from lithosphere mantle coupling and intraplate variations in topography and density structure this yielded a range of acceptable force distributions in this study we investigate to which extent the observed present day stress field provides further constraints on the distribution of forces we address the dynamics of the eurasian plate as a whole this enables us to base our analysis on mechanical equilibrium of a tectonic plate and to evaluate all forces as part of an internally consistent set of forces driving and deforming eurasia we incorporate tractions from convective mantle flow modelling in a lithospheric model in which edge and lithospheric body forces are modelled explicitly and compute resulting stresses in a homogeneous elastic thin shell intraplate stress observations used are from the world stress map project eurasia s stress field turns out to be particularly sensitive to the distribution of collision forces on the plate s southern margin and to a much lesser extent to lithospheric density structure and tractions from mantle flow stress observations require collision forces on the india eurasia boundary of 7 0 10 5 tn m 1 and on the arabia eurasia boundary of 1 3 2 7 tn m 1 implication of mechanical equilibrium of the plate is that forces on the contacts with the african and australian plates amount to 1 0 2 5 and 0 1 3 tn m 1 respectively we use our results to assess the validity of the classical view that the mean elevation of an orogenic plateau can be taken as a measure of the magnitude of the compressive in this case collision related forces involved for both the tibetan and the iranian plateaus two plateaus with significantly different average elevations we find that the horizontal force derived from the excess gravitational potential energy collapse force is in balance with the collision force

development of topography in 3 d continental collision models understanding the formation and evolution of high mountain belts such as the himalayas and the adjacent tibetan plateau has been the focus of many tectonic and numerical models here we employ 3 d numerical simulations to investigate the role that subduction collision and indentation play on lithosphere dynamics at convergent margins and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper mantle scale model distinct dynamics are obtained for the oceanic subduction side trench retreat slab rollback and the continental collision side trench advance slab detachment topographic uplift lateral extrusion we show that slab pull alone is insufficient to generate high topography in the upper plate and that external forcing and the presence of strong blocks such as the tarim basin are necessary to create and shape anomalously high topographic fronts and plateaus moreover scaling is used to predict four different modes of surface expression in continental collision models i low amplitude homogeneous shortening ii high amplitude homogeneous shortening iii alpine type topography with topographic front and low plateau and iv tibet himalaya type topography with topographic front and high plateau results of semianalytical models suggest that the argand number governs the formation of high topographic fronts while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate applying these results to natural examples we show that the alps belong to regime iii the himalaya tibet to regime iv whereas the andes altiplano fall at the boundary between regimes iii and iv

the role of crustal strength variations in shaping orogenic plateaus with application to tibet 1 the tibetan plateau is the type example of a large orogenic plateau formed as a result of continent continent collision the morphology of the plateau and its margins suggests that preexisting variations in crustal strength have influenced the growth of the plateau we have developed a three dimensional numerical model of deformation in a viscous crust in order to investigate the effects of lateral heterogeneities on plateau growth the model includes a two layer crust and allows for lateral variation of viscosity in both the upper and lower layers model results indicate that crustal strength variations have a dramatic effect on the morphology and dynamics of a developing plateau a region of strong crust is characterized by a very steep plateau margin that propagates extremely slowly does not accommodate significant shortening strain and is subparallel to local upper and lower crustal velocities a weak crustal region develops a gently sloping margin uplift propagates rapidly across the weak zone and crustal material within the plateau is diverted toward the low strength region with a relatively simple distribution of strength variations corresponding to strong tarim and sichuan basin crust and a weak southeastern corner our model produces a plateau with many similarities to the tibetan plateau including the overall morphology rotation around the eastern syntaxis and e w extension analysis of model results suggests that e w extension of the central plateau may be related to the rapid eastward flow of crustal material into a weak zone in the se corner without significant change in plateau elevation

how collision triggers backarc extension insight into mediterranean style of extension from 3 d numerical models the formation and evolution of a backarc basin are linked to the dynamics of the subduction system the opening of the central mediterranean basins is a well documented example of backarc extension characterized by short lived episodes of spreading the underlying reasons for this episodicity are obscured by the complexity of this subduction system in which multiple continental blocks enter the subduction zone we present results from three dimensional numerical models of laterally varying subduction to explain the mechanism of backarc basin opening and the episodic style of spreading our results show that efficient backarc extension can be obtained with an along trench variation in slab buoyancy that produces localized deformation within the overriding plate we observe peaks in the trench retreating velocity corresponding first to the opening of the backarc basin and later to the formation of slab windows we suggest that the observed episodic trench retreat behavior in the central mediterranean is caused by the formation of slab windows

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

gps velocity field for the tien shan and surrounding regions 1 measurements at 400 campaign style gps points and another 14 continuously recording stations in central asia define variations in their velocities both along and across the kyrgyz and neighboring parts of tien shan they show that at the longitude of kyrgyzstan the tarim basin converges with eurasia at 20 2 mm yr nearly two thirds of the total convergence rate between india and eurasia at this longitude this high rate suggests that the tien shan has grown into a major mountain range only late in the evolution of the india eurasia collision most of the convergence between tarim and eurasia within the upper crust of the tien shan presumably occurs by slip on faults on the edges of and within the belt but 1 3 mm yr of convergence is absorbed farther north at the dzungarian alatau and at a lower rate with the kazakh platform to the west the tarim basin is thrust beneath the tien shan at 4 7 mm yr with respect to eurasia the ferghana valley rotates counterclockwise at 0 7 myr 1 about an axis at the southwest end of the valley thus gps data place a bound of 4 mm yr on the rate of crustal shortening across the chatkal and neighboring ranges on the northwest margin of the ferghana valley and they limit the present day slip rate on the right lateral talas ferghana fault to less than 2 mm yr gps measurements corroborate geologic evidence indicating that the northern margin of the pamir overthrusts the alay valley and require a rate of at least 10 and possibly 15 mm yr

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback 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 characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

plate motions and stresses from global dynamic models delineating the driving forces behind plate motions is important for understanding the processes that have shaped earth throughout its history however the accurate prediction of plate motions boundary zone deformation rigidity and stresses remains a difficult frontier in numerical modeling we present a global dynamic model that produces a good fit to such parameters by accounting for lateral viscosity variations in the top 200 kilometers of earth together with forces associated with topography and lithosphere structure as well as coupling with mantle flow the relative importance of shallow structure versus deeper mantle flow varies over earth s surface our model reveals where mantle flow contributes toward driving or resisting plate motions furthermore subducted slabs need not act as strong stress guides to satisfy global observations of plate motions and stress

gravitational potential energy of the tibetan plateau and the forces driving the indian plate we present a study of the vertically integrated deviatoric stress field for the indian plate and the tibetan plateau associated with gravitational potential energy gpe differences although the driving forces for the indian plate have been attributed solely to the mid oceanic ridges that surround the entire southern boundary of the plate previous estimates of vertically integrated stress magnitudes of 6 7 x 1012 n m in tibet far exceed those of 3 x 1012 n m associated with gpe at mid oceanic ridges calling for an additional force to satisfy the stress magnitudes in tibet we use the crust 2 0 data set to infer gravitational potential energy differences in the lithosphere we then apply the thin sheet approach in order to obtain a global solution of vertically integrated deviatoric stresses associated only with gpe differences our results show large n s extensional deviatoric stresses in tibet that the ridge push force fails to cancel our results calibrate the magnitude of the basal tractions associated with density buoyancy driven mantle flow that are applied at the base of the lithosphere in order to drive india into tibet and cancel the n s extensional stresses within tibet moreover our deviatoric stress field solution indicates that both the ridge push influence 1 x 1012 n m and the vertically integrated deviatoric stresses associated with gpe differences around the tibetan plateau 3 x 1012 n m have previously been overestimated by a factor of two or more these overestimates have resulted from either simplified two dimensional approximations of the thin sheet equations or from an assumption about the mean stress that is unlikely to be correct

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

feedback between mountain belt growth and plate convergence while it is generally assumed that global plate motions are driven by the pattern of convection in the earth s mantle the details of that link remain obscure bouyancy forces associated with subduction of cool dense lithosphere at zones of plate convergence are thought to provide significant driving force but the relative magnitudes of other driving and resisting forces are less clear as are the main factors controlling long term changes in plate motion the ability to consider past as well as present plate motions provides significant additional constraints because changes in plate motion are necessarily driven by changes in one or more driving or resisting forces which may be inferred from independent data here we present for the first time a model that explicitly links global mantle convection and lithosphere models to infer plate motion changes as far back as miocene time by accurately predicting observed convergence rates over the past 10 m y we demonstrate that surface topography generated at convergent margins is a key factor controlling the long term evolution of plate motion specifically the topographic load of large mountain belts and plateaus consumes a significant amount of the driving force available for plate tectonics by increasing frictional forces between downgoing and overriding plates

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

seismic anisotropy around subduction zones insights from three dimensional modeling of upper mantle deformation and sks splitting calculations 1 inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive to assist the interpretation of the seismic signal and its relation with the mantle circulation pattern we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3 d numerical models of earth like subduction this computational strategy accounts for the non steady state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered in the subduction models a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate the sub slab upper mantle is characterized by two domains with different fabrics at shallow depth the mantle entrained with the subducting slab develops trench perpendicular directed anisotropy due to simple shear deformation while in the deeper mantle slab rollback induces pure shear deformation causing trench parallel extension and fast seismic directions subducting plate advance favors the development of the fabric in the entrained mantle domain while slab retreat increases the trench parallel anisotropy in the deeper upper mantle in the deeper domain the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center as such strong trench parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates the synthetic sks splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain with trench perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges instead above relatively narrow retreating slabs 600 km and low subduction partitioning ratio spr azimuths are trench parallel due to the strong anisotropy in the lower sub slab domain in all models the anisotropy in the back arc and on the sides of the subducting plate is respectively trench perpendicular and sub parallel to the return flow at depth results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

evidence of lower mantle slab penetration phases in plate motions it is well accepted that subduction of the cold lithosphere is a crucial component of the earth s plate tectonic style of mantle convection but whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate which has substantial implications for the chemical and thermal evolution of the mantle here we identify lower mantle slab penetration events by comparing cenozoic plate motions at the earth s main subduction zones with motions predicted by fully dynamic models of the upper mantle phase of subduction driven solely by downgoing plate density whereas subduction of older intrinsically denser lithosphere occurs at rates consistent with the model younger lithosphere of ages less than about 60 myr often subducts up to two times faster while trench motions are very low we conclude that the most likely explanation is that older lithosphere subducting under significant trench retreat tends to lie down flat above the transition to the high viscosity lower mantle whereas younger lithosphere which is less able to drive trench retreat and deforms more readily buckles and thickens slab thickening enhances buoyancy volume times density and thereby stokes sinking velocity thus facilitating fast lower mantle penetration such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle thus we identify a direct expression of time dependent flow between the upper and lower mantle

structure of the upper mantle and transition zone beneath southeast asia from traveltime tomography 1 tomographic images of the mantle beneath east asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the tibetan plateau our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large scale tectonic framework of the continental india asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east in the realm of continental collision west of 100 e a relatively slow p wave speed characterizes the upper mantle beneath much of the tibetan plateau but the wave speed is high beneath cratonic india the southern and western part of the tibetan plateau hindu kush and the tian shan in the subduction realm east of 110 e the main structures are i pronounced low wave speed anomalies at a depth of between 100 and 400 km beneath asia s southeastern seaboard and the back arc regions of ongoing subduction ii narrow fast anomalies in the upper mantle beneath major subduction zones and iii widespread fast anomalies at a depth of 500 700 km beneath the sea of japan the northern part of the philippine sea plate and southeastern china if the latter anomalies represent stagnant slabs their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western pacific island arcs along the old se margin of asia or during the mesozoic collision of cratonic units in southeast asia attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for at least several tens of millions of years shallow slow anomalies beneath the red river fault region connect to deep anomalies beneath the south china fold belt and south china sea suggesting a causal relationship between the evolution of the continental lithosphere of sw china and deeper mantle processes between the collision and the subduction realms tomography reveals high wave speed continental roots beneath the western part of the north china craton ordos block and the south china or yangtze craton sichuan basin to a depth of 300 km