extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

transverse tectonics in the sikkim himalaya evidence from seismicity and focal mechanism data abstract in the present study about 356 local earthquakes in the region of the sikkim himalaya have been accurately located and analyzed using 2181 p travel times and 2161 s travel times from a network of 11 broadband seismic stations operated by the national geophysical research institute during january 2006 to november 2007 further refinement of the hypocentral parameters using the hypodd relocation program resulted in 198 well constrained locations interestingly this study reveals several characteristic features that distinguish sikkim from the rest of the himalaya the seismicity distribution is found to be confined mostly between the main boundary thrust mbt and the main central thrust mct but not quite associated with either while the entire himalayan front is generally characterized by shallow angle thrust faulting focal mechanisms in this region are predominantly of strike slip type in conformity with a right lateral strike slip mechanism along the northwest trending tista and gangtok lineaments the p axis trends of earthquake focal mechanisms are clearly oriented north northwest marking a clear transition from the ambient north northeast trending direction of indian plate motion with respect to the eurasian plate all along the himalayan front moderate sized earthquakes occur down to 70 km depth in this region compared to an average focal depth of 15 20 km in the rest of the himalaya also a high average crustal p velocity of 6 66 km sec and a fairly low b value of 0 83 0 04 are obtained indicating the probability of occurrence of a higher magnitude earthquake in the future a north south section in the sikkim region shows a relatively flat topography unlike in the rest of the himalayan mountain chain and suggestive of lower rates of convergence in the recent geologic past it is proposed that crustal shortening in the sikkim himalaya has been substantially accommodated by transverse tectonics rather than underthrusting in recent times

shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

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

significant late neogene east west extension in northern tibet field mapping in northern tibet reveals that the normal slip along late cenozoic northsouth trending faults is comparable to that estimated for equivalent structures in southern tibet the orientation of fault striations in two north south trending rifts suggests an eastnortheast west northwest direction of extension in northern tibet which in turn implies that northeast striking active faults in northern tibet have significant left slip components initiation of rifting in northern tibet postdates the early oligocene and possibly occurred after 4 ma the broad similarities in the magnitude of slip and the direction of extension for normal faults in both northern and southern tibet imply that the entire plateau has been extending this precludes significant eastward extrusion of north tibet relative to south tibet and requires a regional boundary condition as the cause of east west extension for the entire tibet plateau

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

source parameters for 11 earthquakes in the tien shan central asia determined by p and sh waveform inversion the tien shan mountain belt of central asia was formed in late paleozoic time and reactivated in tertiary time following the collision of india with eurasia more than 1500 km to the south to study the style and distribution of faulting occurring today in the tien shan we digitized long period world wide standard seismograph network p and sh waveforms of 11 of the largest tien shan earthquakes between 1965 and 1982 and then used a least squares inversion routine to constrain their fault plane solutions and depths four of the earthquakes occurred near the southern edge of the tien shan and two occurred in the intermontane fergana basin these earthquakes occurred at depths of 10 20 km and are associated with thrust faulting on east west to southwest northeast striking fault planes that dip moderately 35 55 the other five earthquakes in the northern tien shan were mostly deeper 15 44 km and the fault plane solutions of four of them are similar to the events farther south the exception an event on june 2 1973 probably occurred on a gently northward dipping east west striking fault plane all events occur within the basement at depths of 10 km or greater waveforms for earthquakes near the edges of the dzungarian fergana and kucha basins are fit best using velocity structures with thick sediment layers implying that sediments from these basins have been and are being underthrust beneath the neighboring mountains we examined only the relatively small mb 5 5 6 2 events after 1965 and did not study any of the largest events ms 8 that occurred in the tien shan earlier in this century still we can conclude that north south shortening is presently occurring in the tien shan with the formation of basement uplifts flanked by moderately dipping thrust faults the present day tectonics of the tien shan seem to be analogous to those of the rocky mountains in colorado wyoming and utah during the laramide orogeny in late cretaceous and early tertiary time

kinematics of the pamir and hindu kush regions from gps geodesy 1 gps velocities measured in the pamir and surrounding regions show a total of 30 mm yr of northward relative motion between stable pakistan and eurasia the convergence budget is partitioned into 10 15 mm yr of localized shortening across the trans alai thrust which bounds the pamir on the north consistent with southward subduction of intact lithosphere another 10 15 mm yr of shortening is distributed across the chitral himalaya and hindu kush suggesting that hindu kush seismicity might be related to northward subduction of indian lithosphere modest shortening at 5 mm yr occurs north of the trans alai thrust across the south tien shan and between the ferghana valley and eurasia negligible north south shortening occurs within the high pamir but as much as 5 mm yr and perhaps 10 mm yr of east west extension occurs within this region this extension is matched by a comparable amount of east west shortening in the tajik depression the localization of shortening to the margins of the pamir combined with observations of distributed internal extension implies that the east west vertically averaged horizontal compressive normal stress is smaller than the north south compressive stress

strain and rotation rate from gps in tibet anatolia and the altiplano 1 deformation measured by regional gps networks in continental plateaus reflects the geologic and tectonic variability of the plateaus for two collisional plateaus tibet and anatolia and one noncollisional the altiplano we analyze the regional strain and rotation rate by inverting gps velocities to calculate the full two dimensional velocity gradient tensor to test the method we use gridded velocities determined from an elastic block model for the eastern mediterranean middle east region and show that to a first order the deformation calculated directly from the gps vectors provides an accurate description of regional deformation patterns principal shortening and extension rate axes vertical axis rotation and two dimensional 2 d volume strain dilatation are very consistent with long term geological features over large areas indicating that the gps velocity fields reflect processes responsible for the recent geologic evolution of the plateaus differences between geological and gps descriptions of deformation can be attributed either to gps networks that are too sparse to capture local interseismic deformation or to permanent deformation that accrues during strong earthquakes the altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2 d dilatation everywhere vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau in contrast the collisional plateaus have large regions of quasi rigid body rotation bounded by strike slip faults with the opposite rotation sense from the rotating blocks tibet and anatolia are the mirror images of each other both have regions of positive dilatation on the outboard sides of the rotating blocks positive dilatation in the aegean correlates with a region of crustal thinning whereas that in eastern tibet and yunnan province in china is associated with an area of vertical uplift rollback of the hellenic trench clearly facilitates the rotation of anatolia rollback of the sumatra burma trench probably also enables rotation about the eastern syntaxis of tibet

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

present day crustal thinning in the southern and northern tibetan plateau revealed by gps measurements gps measurements from sites within the tibetan plateau show not only east southeast west northwest extension but also more importantly horizontal dilation throughout the interior of the plateau assuming conservation of volume vertical thinning strain rates equal horizontal dilation rates and they 8 9 0 8 nanostrain a 1 and 7 4 1 2 nanostrain a 1 in northern and southern tibet and 12 0 3 2 nanostrain a 1 in its southwestern part suggest no measureable difference principal extensional strain rates also are similar in magnitude and orientation if crustal thinning began at 10 15 ma and the current rates of horizontal dilation applied both to the entire crust and to that period the crust should have thinned by 5 5 8 5 km if isostatic equilibrium applied the mean elevation of the plateau would have dropped 1 km the similar rates for northern southern and southwestern tibet suggest that the processes dictating crustal extension normal faulting and crustal thinning in the three regions differ little from one another

three dimensional velocity field of present day crustal motion of the tibetan plateau derived from gps measurements 1 using the measurements of 750 gps stations around the tibetan plateau for over 10 years since 1999 we derived a high resolution 3 d velocity field for the present day crustal movement of the plateau the horizontal velocity field relative to stable eurasia displays in details the crustal movement and tectonic deformation features of the india eurasia continental collision zone with thrust compression lateral extrusion and clockwise rotation the vertical velocity field reveals that the tibetan plateau is continuing to rise as a whole relative to its stable north neighbor however in some subregions uplift is insignificant or even negative the main features of the vertical crustal deformation of the plateau are the following a the himalayan range is still rising at a rate of 2 mm yr the uplift rate is 6 mm yr with respect to the south foot of the himalayan range b the middle eastern plateau has a typical uplift rate between 1 and 2 mm yr and some high mountain ranges in this area like the longmen shan and gongga shan have surprising uplift rates as large as 2 3mm yr c in the middle southern plateau there is a basin and endorheic subregion with a series of ns striking normal faults showing obvious sinking with the rates between 0 and 3 mm yr d the present day rising and sinking subregions generally correspond well to the cenozoic orogenic belts and basins respectively e at the southeastern corner of the plateau there is an apparent trend that the uplift rate is gradually decreasing from between 0 8 and 2 3 mm yr in the inner plateau to between 0 5 and 1 6 mm yr outside the plateau with the decrease of terrain height

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

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

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

evidence for mechanical coupling and strong indian lower crust beneath southern tibet how surface deformation within mountain ranges relates to tectonic processes at depth is not well understood the upper crust of the tibetan plateau is generally thought to be poorly coupled to the underthrusting indian crust because of an intervening low viscosity channel here however we show that the contrast in tectonic regime between primarily strike slip faulting in northern tibet and dominantly normal faulting in southern tibet requires mechanical coupling between the upper crust of southern tibet and the underthrusting indian crust such coupling is inconsistent with the presence of active channel flow beneath southern tibet and suggests that the indian crust retains its strength as it underthrusts the plateau these results shed new light on the debates regarding the mechanical properties of the continental lithosphere and the deformation of tibet

large scale deformation in the india asia collision constrained by earthquakes and topography analysis of earthquake focal mechanisms allows division of the india asia collision into kinematic domains that strongly correlate with topography these kinematic domains indicate strain partitioning dominated by oblique slip deformation the kunlun and south tibetan fault systems mark discontinuities in the strain field and bound the high flat topography of the plateau which deforms by transtension the northern and southern margins of tibet deform by transpression or contraction and are topographically steep correlations between seismicity and topography are due to mohr coulomb wedge mechanics at the northern and southern plateau margins which produce naturally steep surface slopes whereas the flat interior and eastern margin of the tibetan plateau is underlain by viscous crust which supports subdued topography further muted by cenozoic basin fill these data indicate that the long wavelength topography of the india asia collision is controlled by seismically caused surface displacements which are linked to deep crustal deformation mechanics

correlated crust and mantle strain fields in tibet vertical to subvertical planes of shear within the active crustal deformation field in tibet align with fast directions of shear wave polarization this observation suggests that the present day velocity gradient tensor field within the crust of tibet correlates with the lithospheric mantle velocity gradient tensor field beneath tibet this inference requires the following 1 the 100 axes of olivine are aligned within established zones of lithospheric mantle fabric or anisotropy built up through finite strain 2 present day shear in the mantle lithosphere is occurring within these established zones of fabric parallel to the direction of olivine 100 axis alignment 3 the zones of mantle lithospheric fabric or zones of mantle shear are aligned with zones of crustal shear faults the correlation of crustal and mantle strain fields most simply results from the fact that both crust and mantle lithosphere are under the influence of similar velocity boundary conditions furthermore the observations confirm the distributed nature of lithospheric mantle deformation beneath east central tibet and suggest that left lateral shear has been the dominant component of finite shear there

geodynamic inversion to constrain the non linear rheology of the lithosphere a common method to determine the strength of the lithosphere is through estimating its effective elastic thickness from the coherence between gravity and topography this method assumes a priori that the lithosphere is a thin elastic plate floating on a viscous mantle whereas this seems to work well with oceanic plates it has given controversial results in continental collision zones usually continental collisions zones are well studied areas for which additional geophysical datasets such as receiver functions and seismic tomography exist that constrain the geometry of the lithosphere and often show that it is rather complex yet lithospheric geometry by itself is insufficient to understand the dynamics of the lithosphere as this also requires knowledge of the rheology of the lithosphere experimental results show significant variability between various rock types and there are large uncertainties in extrapolating laboratory values to nature which leaves room for speculation

microearthquake activity in some parts of the himalaya and the tectonic model abstract microearthquake data from temporary permanent networks in different parts of the himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region the microearthquake activity in arunachal pradesh northeastern himalaya is found to be pronounced at the main boundary thrust mbt and to its south the subcrustal earthquakes depth 50 80 km occur much below the plane of detachment of the tectonic models proposed by seeber et al 1981 ni and barazangi 1984 the mbt is not the seismogenic fault the earthquakes are generated by strike slip movement on deep seated hidden faults transverse to the mbt the high seismic activity in the shillong plateau about 200 km south of the mbt in the northeast region is due to the influence of himalayan collision tectonics to the north and burmese arc subduction tectonics to the east the activity in the plateau is not directly related to the himalayan thrust belt or seismic belt these are mostly crustal earthquakes depth 10 30 km and are caused by local active faults lineaments in the eastern himalaya in the sikkim and darjeeling area the seismic activity is found to be clustered mostly to the north of the mbt the earthquakes occur at a depth range 0 50 km the majority of them occur below the detachment plane by thrust faulting in the central part in the nepal himalaya lateral variations of the seismic activity are observed which represent lateral segmentation of the mbt by transverse tectonic features in the western himalaya however the tectonic models fit well with the microearthquake data in the himachal pradesh of the western himalaya the microearthquakes are mostly recorded in the mbt zone and the hypocentres depth 0 20 km are confined above the plane of detachment or on the basement thrust the earthquakes mostly occur to the south of the main central thrust mct which suggests that the mct is not seismogenic it is rather a dormant fault no single tectonic model explains the himalayan earthquakes

lithospheric thickness of the chinese continent abstract we invert for the upper mantle temperatures of the chinese continent in the depth range of 70 240 km from a recent s velocity model the depth where temperatures intersect a mantle adiabat with a potential temperature of 1300 c is in close correspondence with the top of the seismic low velocity zone for most regions this correspondence implies that seismic lithosphere estimated from short time scale seismic information may be equivalent to the long time scale geodynamical lithosphere defining the 1300 c adiabat as coinciding with the lithospheric base we estimate the seismic thermal lithosphere thickness the estimated thickness shows obvious dependence on the tectonic settings beneath eastern china which mainly belongs to the circum pacific tectonic domain it has a thickness of 100 km and beneath the qinghai tibet plateau and south to the tarim craton which belong to the tethyan tectonic domain it has a thickness of 160 220 km the lithospheric thicknesses of the three large para platforms cratons range from 170 km for the western yangtze 140 km for tarim and 100 km for sino korean the three cratons may have been reshaped by phanerozoic tectonic activities and are thinner than most cratons in other continents

seismic moments of major earthquakes and the rate of shortening across the tien shan three twentieth century earthquakes that richter assigned m 8 and two of comparable magnitude in the nineteenth century imply rapid deformation within the tien shan seismic moment tensors of major earthquakes in this century suggest an average shortening rate of 7 2 mm yr 1 across the tien shan in the western part however where three of the five largest earthquakes occurred the calculated rate is consistent with the value of 20 mm yr 1 measured using gps in that area by abdrakhmatov et al as avouac et al suggested the high rate in the western part apparently is a manifestation of counter clockwise rotation of the tarim basin relative to eurasia about an axis near the east end of the tien shan

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

orogenesis at the southern tip of the americas the structural evolution of the cordillera darwin metamorphic complex southernmost chile abstract new detailed lithologic and structural data are presented from three separately mapped areas along the southern boundary of the cordillera darwin metamorphic complex of southernmost chile cordillera darwin is a unique uplift because it exposes the highest grade rocks in the andes south of peru and averages 1 km higher in elevation than adjacent areas the structural data indicate that cordillera darwin experienced mid late cretaceous trans pressional deformation with a partitioned strike slip component localized along the beagle channel that forms the southern boundary to the range foliation lineation and fold axis trends indicate ne sw directed contraction and nw se strike slip shearing present directions during progressive d1 d2 andean deformation d2 deformation is marked by outcrop to 10 km scale south southwest vergent folds late cretaceous tertiary brittle ductile and brittle left lateral strike slip faults and shear zones crosscut all d1 d2 structures although limited structural evidence for extensional tectonics was documented in this study apparent normal offsets across both arms of the beagle channel and previously documented field evidence for extension from other areas in cordillera darwin suggest that transtensional displacements also may have occurred in southern cordillera darwin during the late cretaceous early tertiary cordillera darwin s position within the evolving patagonian orocline adjacent to an evolving mesozoic cenozoic left lateral transform boundary between the south american and antarctic plates and later the south american and scotia plates necessitates consideration of the possible effects of regional counterclockwise rotation on development of structures regional counterclockwise rotation of cordillera darwin may have controlled the temporal and spatial transition of deformational regimes within cordillera darwin exhumation of the metamorphic core of cordillera darwin during the late cretaceous early tertiary is interpreted to have been due to rapid uplift and normal rates of erosion rapid uplift may have been due to migration of contraction to northern cordillera darwin and crustal relaxation in southern cordillera darwin as contractional structures rotated into parallelism with 1 and either ceased to be active or were reactivated as strike slip or oblique slip faults rapid isostatic rebound of cordillera darwin may have also been a response to aborted continental subduction of southern cordillera darwin and detachment of a subducted mafic root during mid cretaceous closure of the rocas verdes marginal basin

the indo asian continental collision a 3 d viscous model abstract the large scale physical process of the indo asian continental collision and the formation of the himalayan tibetan plateau have been simulated in various viscous thin sheet models but the thin sheet simplification also kept some important issues from being fully explored among these issues are the role of strike slip fault zones in facilitating large scale lateral translation of lithospheric blocks the escaping tectonics during the collision and the speculated lateral flow of the ductile middle lower crust under the tibetan plateau here we present a fully three dimensional finite element model to simulate the indo asian continental collision the model includes major boundary faults to simulate the escaping tectonics and vertically variable rheological structures to model lower crustal flow the collisional process is constrained by the history of the indo eurasian plate convergence and the present crustal thickness and topography of the tibetan plateau our results indicate that the restrictive boundaries of the tibetan plateau including the rigid tarim and south china blocks largely control the spatiotemporal patterns of crustal deformation in the collision zone as the indian indenter moves toward the tarim block higher strain rates and topography developed in the western part of the collision zone than in the eastern part causing the northward migration of the deformation front to gradually change to eastward migration in the past 10 20 myr broadly consistent with the initiation of widespread e w extension in the plateau these restrictive boundary blocks also force the crustal and mantle materials in the collision zone to flow coherently hence providing an alternative explanation for the apparently vertically coherent deformation in tibet assuming that the crust weakens as it thickens our model predicts the lateral expansion of the tibetan plateau an important feature of the tibetan tectonics that is missing in previous models with constant rheology

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

why subduction zones are curved 1 we give an explanation for the polarity localization shape size and initiation of subduction zones on earth by considering a soft thin curved lithospheric cap with either elastic or viscous rheology supported by a thick nearly incompressible mantle we find two different characteristic subduction geometries arise depending on boundary conditions 1 plate boundaries where subduction results primarily from the gravitational body force free subduction have characteristic plate lengths and form arc shaped dimpled segments resulting from the competition between bending and stretching in edge buckling modes of thin spherical shells and 2 subduction zones due to localized applied loads that push one slab of thin positively buoyant lithosphere beneath an overriding plate forced subduction form localized straight segments consistent with the deformation of indented spherical shells both types of subduction are nonlinear subcritical instabilities so small perturbations in the mechanical properties of the lithosphere have pronounced effects on subduction initiation and evolution yet in both cases geometric relationships determined by the shape of the earth itself play the most critical role in controlling the basic morphology and characteristic length scales of subduction zones

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

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

high resolution lithospheric structure beneath mainland china from ambient noise and earthquake surface wave tomography abstract we present a new high resolution shear velocity model of the lithosphere down to about 160 km beneath china using rayleigh wave tomography we combined ambient noise and earthquake data recorded at 1316 seismic stations the largest number used for the region to date more than 700 000 dispersion curves were measured to generate group and phase velocity maps at periods of 10 140 s the resolution of our model is significantly improved over previous models with about 1 2 in eastern china and 2 3 in western china we also derived models of the study region for crustal thickness and averaged s velocities for upper and mid lower crust and uppermost mantle these models reveal important lithospheric features beneath china and provide a fundamental data set for understanding continental dynamics and evolution different geological units show distinct features in the moho depth lithospheric thickness and shear velocity in particular the north china craton ncc lithosphere shows strong east west structural variations with thin and low velocity lithosphere in eastern ncc and thick and high velocity lithosphere beneath western ncc and the lithosphere of the ordos block seems to have undergone strong erosion the results support the progressive destruction of the ncc lithosphere from east to west at least partly caused by the thermal chemical erosion of the cratonic lithosphere from the asthenosphere another pronounced feature of our model is the strong lateral variations of the mantle lithosphere beneath the tibetan plateau tp the indian lithosphere beneath the tp shows variable northward advancement with nearly flat subduction in western and eastern tp and steep subduction in central tp with evidence for the tearing of indian lithosphere beneath central tp which may be important for the riftings at the surface in himalayas and southern tp the low velocity zone in northern tp shows strong correlation with the region of the mid miocene to quaternary potassic magmatism suggesting that delamination of lithosphere may have played an important role in the rise of the tp

thinning and flow of tibetan crust constrained by seismic anisotropy intermediate period rayleigh and love waves propagating across tibet indicate marked radial anisotropy within the middle to lower crust consistent with a thinning of the middle crust by about 30 the anisotropy is largest in the western part of the plateau where moment tensors of earthquakes indicate active crustal thinning the preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy the middle to lower crust of tibet appears to have thinned more than the upper crust consistent with deformation of a mechanically weak layer that flows as if confined to a channel

evidence of active mantle flow beneath south china 1 the india eurasia collision is responsible for producing the himalayan mountains and tibetan plateau and has been hypothesized to have significant far field influences including driving the baikal rift and the eastward extrusion of south china however quantification of lithospheric buoyancy forces and integrated effect of tractions acting at base of the lithosphere are unable to explain the observed surface motions within south china we present 198 new sks shear wave splitting observations beneath south china and invert these data along with published gps data to solve for the subasthenospheric flow field beneath south china to assess the role of small scale convection here we find a 15 20 mm yr southwestward directed mantle flow toward the burma slab this flow is consistent with the mantle response of slab retreat over the past 25 ma and counter flow due to subduction of burma sunda slabs demonstrating the importance of localized mantle convection on present day plate motions

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

significant and vertically coherent seismic anisotropy beneath eastern tibet 1 shear wave splitting measurements using teleseismic sks and skks phases recorded by the indepth iv arrays has revealed a strong upper mantle anisotropic fabric in northeastern tibet with large delay times of up to 2 2 s suggesting that anisotropy exists in both the lithospheric and asthenospheric mantle the coherence among fast polarization orientations of split core phases and the left lateral slip on eastern striking southern striking faults in eastern tibet and the surface deformation fields calculated from both gps observations and quaternary fault slip rates support the idea that left lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics we suggest the bending of the eastern himalayan syntaxis around the foundering burma andaman sumatra slab also contributes to the observed seismic anisotropy in the eastern himalayan syntaxis region two plausible competing processes are proposed for the flow of asthenosphere in eastern tibet in the first the deforming lithosphere glides over the passive asthenosphere inducing flow in the asthenospheric mantle in the second the asthenosphere beneath northeastern tibet is squeezed between the advancing indian continental lithosphere and the thick tarim and qaidam lithospheric blocks to the north a westward retreat of the burma slab from eurasia may induce flow that is toroidal and located exclusively around the northern edge of the slab the rotation of fast orientations for stations in the eastern himalayan syntaxis region are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere

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

coupling of rock uplift and river incision in the namche barwa gyala peri massif tibet geodynamic modeling demonstrates the strong potential for erosion to influence the pattern and style of deformation in active mountain belts but field studies yield conflicting views on the importance of erosion in influencing orogenesis here we compare patterns in river power inferred excess fluvial transport capacity topographic relief precipitation and mineral cooling ages to assess the coupling between surface erosion and rock uplift within the vicinity of the namche barwa gyala peri massif an active antiformal structure within the eastern himalayan syntaxis our rich and dense data set reveals a tight spatial correspondence of fluvial incision potential high relief and young cooling ages the spatial coincidence is most easily explained by a sustained balance between rock uplift and denudation driven by river incision over at least the last 1 m y the yarlung tsangpo brahmaputra river is the largest and most powerful river in the himalaya and two lines of evidence point to its active role in the dynamic interaction of local erosion rock uplift thermal weakening of the lithosphere and deformation 1 whereas along the rest of the himalayan front high relief and high rock uplift rates are essentially continuous the high relief and rapid exhumation in the syntaxis is restricted to a bull s eye pattern exactly where the largest river in the himalaya the yarlung tsangpo brahmaputra has the most energy per unit area available to erode its channel and transport sediment 2 the location of rapid incision on the yarlung tsangpo brahmaputra has been pinned for at least 1 m y and without compensatory uplift of the namche barwa gyala peri massif during this time the river would have eroded headward rapidly incising deeply into tibet

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

mapping the descent of indian and eurasian plates beneath the tibetan plateau from gravity anomalies the collision of india with asia has produced a complicated continental continental plate boundary involving folding and faulting of variable trends and styles within and along the margins of the tibetan plateau numerous lines of evidence including the development of two scales of folding in tibet suggest that the lowermost crust is behaving in a ductile fashion this weak lower crust might then decouple the fundamental plate tectonic motions in the uppermost mantle from the complex pattern of surface faulting in this study we use bouguer gravity anomalies to map out the geometry of indian and eurasian plate interactions in the mantle beneath the plateau based on both the inferred geometry of the moho and lateral variations in lithospheric strength determined from mechanical modeling in our preferred model the lithosphere beneath tibet consists of two distinct units 1 the underthrust to the north indian plate which sutures with the eurasian plate in the upper mantle below the yarlung zangpo suture or the gangdese igneous belt 200 400 km north of the main boundary thrust and 2 the underthrust to the south eurasian plate a subducting slab of indian upper mantle extends about 200 km into the asthenosphere north from the mantle suture and exerts a bending moment of about 3 5 1017 n on the indian plate thus the mantle lithosphere appears to be behaving in the simple fashion of converging oceanic plates while the more buoyant continental crust deforms under high gravitational potential in a complex pattern controlled by its lateral and vertical strength heterogeneity

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

seismic images of the biggest crash on earth in plate tectonics the upper layer of earth the lithosphere consists of rigid plates that shift over geological time scales data from global positioning surveys have estimated that the entire indian subcontinent has moved over the past 50 years about 2 m to the north diving slowly underneath tibet this giant collision has been ongoing for 50 million years and has thrown up the highest mountains as well as the largest and highest plateau on earth not only is the world climate strongly influenced by this massive plateau average elevation of 5000 m but the collision also causes catastrophic earthquakes in southern central and eastern asia

crustal thickening and lateral extrusion during the indo asian collision a 3d viscous flow model abstract the 2000 km crustal shortening from the indo asian collision in the past 40 70 million years has been accommodated mainly by crustal thickening in the himalayan tibetan plateau and lateral extrusion of blocks of asian continents however the spatiotemporal evolution of crustal thickening and lateral extrusion hence the far field impact of the indo asian collision on asian tectonics remains controversial here we present a 3d viscous flow model that simulates the partitioning of crustal material between thickening and lateral extrusion during the indo asian collision the model assumes conservation of crustal mass and is constrained by the history of the indo asian plate convergence and by the present day topography of the himalayan tibetan plateau the results show that much of the early collision was absorbed by crustal thickening within the himalayan tibetan plateau however lateral extrusion of crustal material has gradually become dominant in the past 10 20 myr indicating increasing influence of the indo asian collision on asian tectonics

subduction and slab breakoff controls on asian indentation tectonics and himalayan western syntaxis formation 1 we test the link between large scale asian continent deformations and indian slab subduction and breakoff during convergence by means of three dimensional numerical models of the subducting upper plates system we find that the subduction of the buoyant continent results in the reduction of convergence velocity comparable to that observed in the indian motions yet the upper plate deformation remains accommodated in a narrow belt along a straight margin comparable rates are measured when the subducting slab breakoff is modeled although the convergent margin deforms and curves markedly with large underplating contiguous to ongoing subduction similar to what observed along the himalayan range the models support the interpretation of the himalayan western syntaxis evolution the progressive curvature of the indian margin and the underthrusting as a consequence of the indian slab breakoff the modeled slab detachment is followed by short lived large stresses in the upper plate interiors propagating at large distance from the margin with a trend similar to several major asian lithospheric faults such localized stress has likely provided the conditions for the formation of the central asian intracontinental faulting the bangong red river and the altyn tagh faults that followed successive indian slab breakoff episodes continent subduction and breakoff during india asia convergence offer an explanation for the different deformation mechanisms as the long lived understhrusting and the episodic lithospheric faulting in the asian continent and their link to deep processes

an overview of the crustal structure of the tibetan plateau after 35 years of deep seismic soundings abstract since the pioneer wide angle seismic profile along the yadong gulu rift acquired in 1974 by the ex institute of geophysics chinese academy of sciences cas several research programs aimed to deep geophysics performed thanks to the participation of chinese national and international institutions have been developed during last 35 years including 23 wide angle seismic profiles with total length of about 6000 km these profiles are unevenly distributed most of them in eastern tibet and few profiles in western tibet in this paper we make a summarized presentation of all these wide angle seismic profiles and provide an overall view of the seismic velocity structure of the crust beneath the broad tibetan plateau which is the product of the continuous convergence and collision of the indian and eurasian plates since about 50 ma ago different patterns of crustal thickness variation related to the tectonic blocks and along suture zones of the region are displayed the crust thickness is confirmed to be about 70 75 km under southern tibet and 60 65 km under northern northeastern and southeastern tibet the leading edge of the subducted lithosphere reaches the northern margin of the plateau and directly contacts with tarim basin westward of the 90 e boundary the indian crust is moving towards the northern edge of the plateau and collides with tarim basin at 80 e while reach the bangong nujiang suture belt at 88 e eastward of the 90 e boundary the northern edge of the crust should be at 50 100 km south of bangong nujiang suture the results supply helpful constrains to understand the mechanism of the continent continent collision and its consequences in the plateau and neighbouring areas

structural evolution of orogenic wedges interplay between erosion and weak decollements orogenic wedges commonly display an inner wedge where crystalline units have been exhumed and an outer wedge formed by imbricated sedimentary units detached from the basement analog experiments have shown that similar structures can emerge naturally in the presence of weak decollements due to the interplay between erosion and deformation in this study we further investigate this hypothesis using two dimensional visco elasto plastic numerical models our experiments assume a basal and an intermediate decollement within the wedge experiments with a frictional strength of the basal decollement lower or equal to that of the intermediate decollement show a structural evolution of fold and thrust belts dominated by out of sequence thrusting conversely when the intermediate decollement is weaker than the basal decollement distinct outer and inner wedges are formed this process leads to episodic migration of midcrustal ramps tectonic underplating and antiformal stacking facilitated by erosion comparison between our models and the himalayan wedge suggests a low effective friction 0 10 which is probably due to dynamic weakening during large m8 himalayan earthquakes the deeper decollement along which the lower plate thrusts beneath the high himalaya must be a thermally activated ductile shear zone with an apparent friction of 0 18 fold and thrust belts worldwide exhibit various architectures in which different decollement levels might be activated thus our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise

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

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

accretionary tectonics of burma and the three dimensional geometry of the burma subduction zone the geometry of the burma wadati benioff zone wbz has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well located hypocenters the dip of this surface which passes through the middle of the wbz varies from about 50 in the north near the eastern himalayan syntaxis to about 30 in the bay of bengal area the eastern edge of the indo burman ranges closely follows the map projection of the 60 km depth contour of the wbz the curvature of the indo burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the indian plate and the leading edge of the overriding burma platelet shallow earthquakes beneath the indo burman ranges are primarily confined to the underthrusting indian plate their focal mechanisms indicate strike slip faulting and north south shortening parallel to the eastern margin of the indian plate

comparing thin sheet models with 3 d multilayer models for continental collision summary various models have been proposed to explain tectonic deformations during continent collision a frequently applied model is the thin viscous sheet model which is however not fully 3 d and assumes a priori diffuse thickening as the dominant deformation style we compare a fully 3 d multilayer numerical model with a corresponding thin viscous sheet numerical model for the scenario of continent indentation in our comparison we focus on the three basic viscous deformation styles thickening buckling folding and lateral crustal flow both numerical models are based on the finite element method fem and employ either a linear or power law viscous rheology the 3 d model consists of four layers representing a simplified continental lithosphere strong upper crust weak lower crust strong upper mantle and weak lower mantle the effective viscosity depth profile in the 3 d model is used to calculate the depth averaged effective viscosity used in the thin sheet model allowing a direct comparison of both models we quantify the differences in the strain rate and velocity fields and investigate the evolution of crustal thickening buckling and crustal flow resulting from the two models for two different phases of deformation 1 indentation with a constant velocity and 2 gravitational collapse after a decrease of the indenting velocity by a factor of 5 the results indicate that thin sheet models approximate well the overall large scale lithospheric deformation especially during indentation and for a linear viscous rheology however in the 3 d model additional processes such as multilayer buckling and lower crustal flow emerge which are ignored in the thin sheet model but dominate the deformation style in the 3 d model within a range of a few hundreds of kilometres around the collision zone and indenter corner differences between the 3 d and thin sheet model are considerably larger for a power law viscous than for a linear viscous rheology buckling and lower crustal flow are significant in the 3 d model with power law viscous rheology for example fibre strain rates due to buckling can be several hundred per cent different to the depth averaged strain rate and the lateral mass flow of lower crustal material can be up to six times more than the flow of upper crustal material our results also show that the horizontal velocity fields of the upper crust and upper mantle remain nearly identical in the 3 d model during indentation despite their mechanical decoupling due to an intermediate weak lower crust this result questions the validity of using similarities between velocities from the surface global positioning system gps and mantle shear wave splitting data as evidence for a mechanically coupled lithosphere 3 d multilayer models provide a more complete picture of continental collision than thin sheet models as they enable studying the timing locality and relative importance of different processes simultaneously which is especially important for the hundreds of kilometre scale around the collision zone and indenter corners 3 d models are however still computationally challenging and we therefore also present results of a computational performance test of several solution algorithms

spatial coincidence of rapid inferred erosion with young metamorphic massifs in the himalayas a spatially distributed rate of erosion index ei based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the himalayas the ei is derived from slopes and drainage areas calculated from a hydrologically cor rected digital elevation model gtopo30 combined with precipitation data iiasa to generate synthetic annual stream discharges these variables drive three generalized pro cess models to produce ei maps that while differing in detail provide an internally con sistent spatially continuous index of large scale erosion rates the large spatial variation in potential erosion rates in the himalayas suggested by the ei patterns contrasts with the uniform convergence of the indian subcontinent if these ei gradients persist through time they support the emerging view of a positive feedback between localized rapid ero sion and upward advection of lower crust

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

the relationship between surface kinematics and deformation of the whole lithosphere the variation of mechanical properties with depth in the lithosphere determines the relationship between surface deformation and whole lithosphere deformation hence between surface deformation and whole lithosphere dynamics where viscosity or elastic strength is a continuous function with depth surface deformation can be used to constrain both force balance and rheological parameters where viscosity is discontinuous but the upper crust and mantle lithosphere have comparable maximum values surface deformation can be used to approximate force balance and rheological parameters but tradeoffs mean that estimates of stress and viscosity are effective equivalent values rather than actual values where viscosity is both discontinuous and differs by much more than an order of magnitude between the upper crust and mantle lithosphere information about both force balance and rheology are absent from the surface deformation so surface observations alone are insufficient to estimate either the dynamic or mechanical state of the lithosphere

extreme localized exhumation at syntaxes initiated by subduction geometry some of the highest and most localized rates of lithospheric deformation in the world are observed at the transition between adjacent plate boundary subduction segments the initiating perturbation of this deformation has long been attributed to vigorous erosional processes as observed at nanga parbat and namche barwa in the himalaya and at mount st elias in alaska however an erosion dominated mechanism ignores the 3 d geometry of curved subducting plates here we present an alternative explanation for rapid exhumation at these locations based on the 3 d thermomechanical evolution of collisions between plates with nonplanar geometries comparison of model predictions with existing data reproduces the defining characteristics of these mountains and offers an explanation for their spatial correlation with arc termini these results demonstrate a bottom up tectonic rather than top down erosional initiation of feedbacks between erosion and tectonic deformation hence the importance of 3 d subduction geometry

crustal structure beneath se tibet from joint analysis of receiver functions and rayleigh wave dispersion new constraints on the pattern of crustal flow in se tibet are obtained from joint analysis of receiver functions and rayleigh wave dispersion with a newly deployed seismic array the crust in the sichuan yunnan diamond block has an average thickness of 45 km and gradually thins toward the indo china block to the west and the yangtze block to the east high vp vs ratios are detected to the west of the xiaojiang fault but not in the yangtze block to the east the s wave velocity profile reveals that intra crustal low velocity zones ic lvzs are strongly heterogeneous with two lvzs in the middle and mid lower crust respectively in marked contrast to previous observations of a single lvz combined with other observations the two ic lvzs are interpreted as isolated channels of crustal flow at different depths beneath se tibet resulting in the observed complex pattern of radial anisotropy and further elucidating patterns of flow and deformation

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

crustal seismic anisotropy in central tibet implications for deformational style and flow in the crust 1 receiver functions obtained at indepth iii stations located near the bangong nujiang suture in central tibet display a weak moho signal and strong p to s conversions within the first 5 s that vary systematically with back azimuth a single station with representative azimuthal variations located at the sharp onset of strong sks splitting is modeled for both dipping layers and seismic anisotropy by using a global minimization technique inversion results indicate strong anisotropy 10 near the surface and in the middle crust separated by a south dipping 25 layer possibly related to the earlier phase of crustal shortening near surface anisotropy has a fabric dipping steeply southward and trending wnw ese that correlates with the suture and younger strike slip faults in contrast mid crustal anisotropy occurs in a low velocity zone and has a fabric dipping gently 18 northward that might be related to a well developed near horizontal rock fabric induced by crustal flow

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

bending stress and dissipation in subducted lithosphere 1 oceanic lithosphere undergoes permanent deformation during subduction once the stresses exceed the elastic limit departures from elastic behavior occur by brittle failure in the shallow lithosphere and by a combination of low and high temperature creep at greater depths we combine laboratory based rheological models with estimates of slab shape from earthquake hypocenters to quantify the bending stress and dissipation in subduction zones the peak stress occurs at the depth of the brittle ductile transition which is controlled mainly by lithospheric age integrals of the stress over the thickness of the plate are used to evaluate the resistive bending force and the bending moment a representative value for the resistive force on old oceanic lithosphere is 3 1012 n m 1 which is comparable in magnitude to ridge push but opposite in direction both the bending force and moment are remarkably insensitive to the rate of subduction in fact the bending moment can be approximated using a simple power law rheology and a stress exponent of n 14 such a large exponent implies that the lithosphere behaves like a perfectly plastic solid for most subduction zones the bending moment saturates along the entire plate as a consequence the bending stress does not influence the development of curvature during subduction this behavior may explain why the curvature of subducted lithosphere is nearly independent of age

eastward expansion of the tibetan plateau by crustal flow and strain partitioning across faults the tibetan plateau is expanding eastwards but the modes of deformation are poorly understood high resolution seismic images from the region identify localized zones of weak crustal rocks as well as deep faults implying that deformation occurs through a combination of crustal flow and movement of rigid blocks of crust