s wave velocity images of the crust in the southeast margin of tibet revealed by receiver functions the southeast margin of tibet is the region in clockwise rotation around the eastern himalayan syntaxis due to the india eurasia collision and the resistance of the stable sichuan basin and south china block however the dynamic processes involved in the evolution and deformation of the region remain poorly understood due to a lack of reliable geophysical observations we collected abundant seismic data recorded by 108 permanent broadband stations deployed in the se margin of tibet since 2000 and obtained 4536 pairs of p wave receiver functions prfs with high signal to noise ratio in this study we have implemented a novel two step data inversion procedure that can reduce the dependence of the inversion results on the initial model we first use low frequency prfs obtained by iterative deconvolution in the time domain and then an initial model consisting of a series of 2 km thick isotropic layers to fit velocity models and thus determine an overall statistical solution by means of the bootstrap resampling technique this statistical solution is then regarded as a new initial model to adjust high frequency prfs hence the same resampling process is executed again to estimate the optimal s wave velocity structure below each station the results provide an accurate 3d image of the crust and uppermost mantle in the se margin of tibet we infer a wide intra crustal low velocity zone that varies laterally and in depth which is thinner or even absent in the most southern part of yunnan our hypothesis is that this low velocity zone is the result of the accumulation of lower crustal flow coming from central tibet furthermore we show that this lower crustal flow extends largely through the sichuan yunnan diamond shaped block and that there are significant variations in both crustal velocity structure and deformation mechanism across the great strike slip faults of the jinshajiang red river and xiaojiang fault systems

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

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

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

variscan inversion within the cheshire basin england carboniferous evolution north of the variscan front abstract a comprehensive seismic reflection and borehole database over the whole of the cheshire basin has allowed examination of the carboniferous succession beneath a thick permo triassic cover construction of a seismic sequence stratigraphy involving the recognition of unconformity bounded packages has allowed the examination of the control by tectonics of sedimentation within the pre permo triassic of the cheshire basin the carboniferous has been divided into two unconformity bound megasequences i e dinantian ms10 0 and namurian westphalian ms20 0 these are deformed by a hitherto unknown series of variscan structures interpretation of these structures has allowed assessment of the variscan orogeny in this area of the variscan foreland considerable variscan inversion in the area exists 100 150 km north of the variscan fold thrust front and is suggested to illustrate the effects of predominantly e w directed compression especially to the north of the northern apex of the adjacent midlands microcraton compression occurred during two late carboniferous pulses of thrusting

crustal structure across northeastern tibet from wide angle seismic profiling constraints on the caledonian qilian orogeny and its reactivation abstract the northeastern tibetan plateau results from the superposition of the caledonian qilian orogenic belt and cenozoic reactivation following continental collision between the indian and asian plates in order to identify the constraints on the qilian orogenic mechanism and the expansion of the plateau we carried out a 430 km long wide angle seismic experiment between jingtai and hezuo we herein present the crustal p wave velocity structure model resulting from the interpretation of this wide angle seismic dataset the principal characteristics of the crustal velocity model include 1 thinning of the crust from south to north within a range of about 48 54 km with some undulation of the moho beneath the northern qilian block 2 the thickness of the sedimentary layer and its average p wave velocity exhibit the obvious tectonic division of the qaidam kunlun west qinling belt the central and northern qilian and the alax blocks 3 the lower crustal layer has a p wave velocity of 6 6 6 8 km s beneath the qaidam kunlun west qinling belt northern qilian orogenic belt and alax block but has a very low value of 6 4 6 5 km s beneath the central qilian block and 4 the central qilian block shares a similar crustal p wave velocity depth relationship with the sierra nevada arc or the accretionary crust the qaidam kunlun west qinling and the northern qilian orogenic belts have a crustal p wave velocity depth relationship characteristic of the crust of the global average continent the particular vp depth relationship beneath the central qilian orogenic belt may result from a caledonian accretionary orogeny or from the delamination of the ecologized lower crust during the arc continental collisional orogeny the lower velocity layer lvl exists at the bottom of the upper crust to the north of the middle of the central qilian block we infer that this lvl plays an important role as an intracrustal decollement to expand the plateau

crustal structure and deformation of the se tibetan plateau revealed by receiver function data we analyze a large amount of receiver function data recorded by regional seismic networks of the china earthquake administration to estimate crustal structure and deformation beneath the southeast margin of the tibetan plateau and its surrounding areas we develop a comprehensive analysis method that facilitates robust extraction of azimuthal seismic anisotropy from receiver function data the method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data and an evaluation of measurement reliability by statistical and harmonic analysis we find significant seismic anisotropy with a splitting time of 0 5 0 9 s beneath the se margin of the tibetan plateau both the splitting time and fast polarization direction are comparable to those estimated from sks skks data suggesting that crustal anisotropy is the main cause of shear wave splitting of the sks skks wave this also suggests that deformation in the upper mantle is either weak or predominantly vertical and is obviously different from the one in the crust a vertical flow in the upper mantle combined with the observation of a thin lithosphere beneath the study area leads to the inference that part of the mantle lithosphere may have been removed and is descending into deep mantle stations located in the surrounding areas on the other hand exhibit very little to no crustal anisotropy the estimated moho depth and vp vs ratio also show a distinct difference between the se tibetan plateau and the surrounding regions stations on the tibetan plateau have a vp vs ratio of 1 79 which is substantially higher than those measured at the yunnan guizhou yungui plateau 1 69 our observations here are consistent with the scenario that the se tibet has been built by lower crustal flow they also suggest that the mantle lithosphere beneath the margin may have been mechanically decoupled from the upper crust

crustal structure across longmenshan fault belt from passive source seismic profiling 1 we analyse receiver functions from 29 broad band seismographs along a 380 km profile across the longmenshan lms fault belt to determine crustal structure beneath the east tibetan margin and sichuan basin the moho deepens from about 50 km under songpan ganzi in east tibet to about 60 km beneath the lms and then shallows to about 35 km under the western sichuan basin the average crustal vp vs ratios vary in the range 1 75 1 88 under songpan ganzi in east tibet 1 8 2 0 under the lms and decrease systematically across the nw part of the sichuan basin to less than 1 70 a negative phase arrival above the moho under songpan ganzi and sichuan basin is interpreted as a ps conversion from the top of a low velocity layer in the lower crust the very high crustal vp vs ratio and negative polarity ps conversion at the top of lower crust in east tibet are inferred to be seismic signatures of a low viscosity channel in the eastern margin of the tibetan plateau the lateral variation of moho topography crustal vp vs ratio and negative polarity ps conversion at the top of the lower crust along the profile seem consistent with a model of lower crust flow or tectonic escape

s wave velocity structure in southwest china from surface wave tomography and receiver functions using the surface wave records of 504 teleseismic events at 50 temporary and 92 permanent seismic stations in southwest china we obtain phase velocity maps at 10 15 25 40 60 and 75 s at a grid spacing of 0 5 0 5 from the interstation correlation method and surface wave tomography we also obtain the s wave velocity structures beneath three profiles using the joint inversion of receiver functions and surface waves at short periods 10 and 15 s high velocity zones hvzs are found in the panzhihua emeishan region the sichuan basin and the weixi lijiang region surrounding the low velocity zones lvzs from songpan ganzi block to the east of lijiang where the elevations are significantly higher at long periods 40 60 and 75 s hvzs are found in the weixi lijiang region the panzhihua chuxiong basin and the kunming tonghai region which form a belt in the center part of the study area the fast polarization directions on both sides of the belt defined by the shear wave splitting of teleseismic sks waves vary significantly and indicate that the flow of material from the plateau is blocked in two different depth intervals and leads to different horizontal extents the long period maps and the structures along the three profiles show that lvzs are present in the upper mantle beneath rapidly slipping fault zones such as the anninghe zemuhe xiaojiang fault zone the red river fault zone and the xiaojinhe fault zone implying that these faults penetrate deep into the mantle

combined co and cross polarized sar measurements under extreme wind conditions during summer 2016 the european space agency esa set up the satellite hurricane observations campaign a campaign dedicated to hurricane observations with sentinel 1 synthetic aperture radar sar in both vertical vertical vv and vertical horizontal vh polarizations acquired in wide swath modes among the 70 sentinel 1 passes scheduled by the esa mission planning team more than 20 observations over hurricane eyes were acquired and tropical cyclones were captured at different development stages this enables us to detail the sensitivity difference of vh and vv normalized radar cross section nrcs to the response of intense ocean surface winds as found the sensitivity of the vh nrcs computed at 3 km resolution is reported to be more than 3 5 times larger than in vv taking opportunity of sar high resolution we also show that the decrease in resolution up to 25 km does not dramatically change the sensitivity difference between vv and vh polarizations for wind speeds larger than 25 m s a new geophysical model function ms1a to interpret cross polarized signal is proposed both channels are then combined to get ocean surface wind vectors sar winds are further compared at 40 km resolution against l band soil moisture active and passive mission smap radiometer winds with co locations less than 30 min overall excellent consistency is found between smap and this new sar winds this paper opens perspectives for metop sg sca the next generation c band scatterometer with co and cross polarization capability

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

a review on active tectonics and deep crustal processes of the western sichuan region eastern margin of the tibetan plateau abstract one of remarkable geological and geomorphological features in continental china is the presence of a north south trending tectonic zone which separates the seismically active tibetan plateau to the west from the tectonically stable south china and ordos blocks to the east the zone also owns a name of north south seismic belt as a large number of great historical earthquakes occur on it the most recent one the 2008 wenchuan earthquake attests its recent tectonic activity in this review i take the western sichuan region as an example to probe the structural styles and kinematic pattern as well as deep geological process associated with tectonic deformation of the north south trending zone through integrated studies on active faults gps crustal deformation and geophysical structure we show that deformation in the western sichuan is governed by interactions among three crustal blocks songpan chuandian and south china of distinctive rheological properties under the tectonic framework that eastward growth of the soft eastern tibet is blocked by the hard lithosphere of the south china block the left lateral xianshuihe fault continues to the north south trending fault system without crustal shortening to form a bounding fault to limit the northern extend of the magnificent clockwise rotation of crustal material around the eastern himalaya syntax upper crust of the three blocks is dominated by brittle deformation whereas the ductile flow of lower crust would drag the brittle upper crustal blocks to move with respect to each other the relative motions among the brittle upper crustal blocks cause strain accumulations among their bounding faults to generate large earthquakes deformation of the western sichuan region can thus be described in terms of combined model of rigid block movement and continuous deformation we suggest this combined model can be applied to entire continental china

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

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

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

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

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

pn anisotropic tomography and dynamics under eastern tibetan plateau we present a new anisotropic tomographic model of the uppermost mantle around eastern tibet using pn traveltime data from a newly deployed temporary seismic array and recent observation bulletins of chinese provincial networks our results are generally consistent with previous results but provide new insights into the dynamics of tibetan plateau prominent high velocity high v anomalies are visible under alashan block and qaidam and sichuan basins which clearly outline their tectonic margins a distinct high v zone representing the double sided subduction of indo eurasian plates is imaged from lhasa block to the south of qaidam basin a pronounced low velocity low v zone is observed from songpan ganzi block to southern chuan dian diamond block suggesting the existence of hot material upwelling there crustal strong earthquakes frequently occurred around high v anomalies or transition zones from high v to low v anomalies suggesting that these earthquakes could be related to lateral heterogeneities in the mantle the pn fast direction approximately rotates around eastern himalayan syntaxis and it is tangential to the margins of sichuan basin suggesting that the mantle material flow of tibetan plateau may have affected east china in the yunnan region to the south of 26 n the pn fast direction is different from sks splitting results indicating that the mantle lithosphere could be mechanically decoupled at certain depth below the uppermost mantle which might be attributable to the subduction of indian or burma slab although the correlation between anisotropy and velocity is complicated anisotropy strength could be associated with the pattern of velocity anomalies in the region

three dimensional velocity structure of crust and upper mantle in southwestern china and its tectonic implications using p and s arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations this paper presents a three dimensional crustal and upper mantle velocity structure of southwestern china 21 34 n 97 105 e southwestern china lies in the transition zone between the uplifted tibetan plateau to the west and the yangtze continental platform to the east in the upper crust a positive velocity anomaly exists in the sichuan basin whereas a large scale negative velocity anomaly exists in the western sichuan plateau consistent with the upper crustal structure under the southern tibetan plateau the boundary between these two anomaly zones is the longmen shan fault the negative velocity anomalies at 50 km depth in the tengchong volcanic area and the panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle the red river fault is the boundary between the positive and negative velocity anomalies at 50 km depth the overall features of the crustal and the upper mantle structures in southwestern china are a low average velocity large crustal thickness variations the existence of a high conductivity layer in the crust or and upper mantle and a high heat flow value all these features are closely related to the collision between the indian and the asian plates

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

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

crustal structure and deformation under the longmenshan and its surroundings revealed by receiver function data abstract the convergence of india and eurasia and the obstruction from the rigid sichuan basin cause the longmenshan lms to have the steepest topographic gradient at the eastern margin of the tibetan plateau however the mechanisms of surface uplift are still controversial in this paper we estimate the crustal structure and deformation under the lms and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the china earthquake administration we apply a comprehensive splitting measurement technique on ps conversion phase at the moho moho ps splitting to calculate crustal anisotropy from azimuthal variations of receiver functions our results show that most of the seismic stations beneath the lms area exhibit significant seismic anisotropy with the splitting time of 0 22 0 94 s and a fast polarization direction of nw se while less or even no crustal anisotropy has been observed under the sichuan basin comparing the fast polarization directions of moho ps splitting with the indicators of lithospheric deformation such as shear wave splitting absolute plate motion and global positioning system imply a consistent tendency of deformation between the lower crust and upper mantle but decoupling deformation in the crust beneath the lms area we further compare moho ps splitting time to that estimated from previous sks splitting indicating that crustal anisotropy is an important source of the sks splitting time in this study area in addition a thick crust 50 km with high vp vs values 1 74 1 86 is also observed using the h stacking method these seismic observations are consistent with the scenario that the lms area has been built by the lower crustal flow combined with the seismic reflection refraction profile and geology studies we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the lms area resulting in the surface uplift of the lms

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

seismic upper mantle discontinuities beneath southeast tibet and geodynamic implications abstract in this study a total of 13 080 p receiver functions and 2800 s receiver functions recorded by 116 stations in southeast tibet are used to determine the depths of the moho the lithosphere asthenosphere boundary lab and the 410 km and 660 km discontinuities the results show that the lab depth increases from 80 100 km beneath southern yunnan to 140 180 km beneath sichuan most of the 410 km discontinuity is flat and at 410 km depth with a slight depression of 10 20 km beneath the longmenshan lms fault the central sichuan yunnan sy diamond shaped block and southeastern yunnan for the 660 km discontinuity the greatest depth occurs beneath the lms and southern yunnan reaching 670 680 km the deepening of 660 km discontinuity is attributed to compositional heterogeneity within the mtz or dynamic pressure on the discontinuity additionally the velocity of movement in the crust and upper mantle beneath sy is different in yunnan crustal movement is southeastward whereas across the red river fault part of the crustal motion is redirected southwest by the westward rollback of the mantle beneath myanmar the interaction between the asthenosphere moving northeastward from myanmar and southeastward from tibet near latitude 26 n causes the crust and lithospheric mantle beneath southern yunnan to have different movement directions

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

insight into the origin of the tengchong intraplate volcano and seismotectonics in southwest china from local and teleseismic data received 18 june 2008 revised 22 february 2009 accepted 10 march 2009 published 13 may 2009 1 a high resolution tomographic image of the crust and upper mantle under yunnan province in southwest china was determined by using a large number of teleseismic data measured precisely from digital seismograms as well as local earthquake arrival times recorded by the dense yunnan seismic network the grid spacing of the tomographic model is 70 km 78 km in the horizontal directions and 4 70 km in depth our resulting model shows a clear low velocity low v 0 3 with respect to the average one dimensional velocity model column extending from the surface down to about 400 km depth under the active tengchong volcano and some high velocity high v anomalies existing in the mantle transition zone furthermore the low vanomaly extends horizontally toward the northeast at 250 to 400 km depth combining our tomographic results with geochemical and geological investigations we infer that the tengchong volcano is a rift related volcano caused by the subduction and dehydration of the indian plate as well as corner flow in the mantle wedge though the subducted slab is a continental plate our results show that the upwelling flow under tengchong originates at 400 km depth a prominent low v anomaly along the red river fault zone extends down to the upper mantle 100 km depth reflecting that the red river fault zone may have cut through the crust to the upper mantle the two dayao earthquakes m 6 1 and m 6 2 in 2003 occurred on the margin of high v anomalies and are underlain by a prominent low v anomaly in the lower crust and upper mantle suggesting that the dayao earthquakes may be associated with fluids released from the dehydration of the subducted indian slab these results are significantly improved compared with previous studies and provide new seismic constraints on the dynamic processes of the india asia collision

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