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

anisotropy and crustal thickness of northern tibet new constraints for tectonic modelling in 1993 a seismological array was installed on a profile along the golmud lhasa highway from the qaidam basin to central tibet a travel time analysis for local and regional earthquakes was performed in order to better constrain the crustal structure obtained by teleseismic tomography more information about the crustal thickness and anisotropy comes from the analysis of moho converted p to sv waves the two methods supplement each other and confirm the existence of crust thinner than expected accross the northern edge of the tibetan plateau and qaidam p to sv splitting analysis yields a striking similarity between the fast polarization direction and the velocity field of present day deformation as inferred from kinematic modelling the two observations the thinning of the crust and the clockwise rotation of the fast polarization direction emphasize the leading role of eastward extrusion for the absorption of the large amount of crustal shortening occuring in central asia

mantle discontinuity structure beneath china mantle reflectivity profiles obtained from long period recordings of multiple scs reverberations from intermediate and deep focus earthquakes surrounding china reveal remarkably sharp images of china deep structure the tibetan plateau and fold belts to the north are underlain by a midlithospheric low velocity zone lvz evidenced by coincident g lid lvz boundary and l base of the lvz reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north south deformation the mantle beneath northeastern china mongolia and southeastern russia has a typically cratonic reflectivity signature with h hales and deep l lehmann discontinuities the sino korean paraplatform of eastern china is a region of active extensional tectonics as expressed by an oceanic reflectivity pattern at shallow depths a g discontinuity at 80 km but a more continental structure below the southern sea of japan korean peninsula yellow sea and bordering regions of the china mainland are underlain by a 5 4 impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410 km discontinuity produced at the subduction zones to the east regional inversion of the reflection coefficient of the 410 km discontinuity r 410 reveals a pronounced high beneath mongolia and north to northwestern china which is best explained by increased upper mantle olivine content signifying extensive basalt depletion in a tectosphere that extends below the 410 km discontinuity a low value of r 660 associated with topographic depression of the 660 km discontinuity beneath eastern asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity reflection defocusing is an alternative but requires 38 km of intermediate scale topography within the depressed region exceeding the amplitude of the depression by a factor of 1 5 2 either mechanism requires strong heterogeneity far west of seismically active slabs two lower mantle reflectors one beneath southern honshu kyushu and the sea of japan the other below shantung province china and the yellow sea correlate with horizontally elongated velocity anomalies in western pacific seismic tomography models we speculate that they may represent slabs stagnating below the 660 km discontinuity

geochemistry of crustally derived leucocratic igneous rocks from the ulugh muztagh area northern tibet and their implications for the formation of the tibetan plateau this is the published version copyright 2012 american geophysical union all rights reserved

fault plane solutions of earthquakes and active tectonics of the tibetan plateau and its margins summary fault plane solutions of earthquakes within and on the margins of the tibetan plateau show diverse styles of faulting and deformation with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike slip faulting resulting in roughly east west crustal extension within the plateau the direction of overthrusting of the himalaya onto the indian shield is radially outward varying from southwest in the western himalaya to south southeast in the east assuming that the indian shield behaves rigidly this requires a west northwest divergence of western tibet from southeastern tibet at a rate of 18 9 mm yr 1 comparable with the rate of convergence at the himalaya fault plane solutions of earthquakes in the southern portion of the tibet plateau consistently show large components of normal faulting on roughly north striking planes and corroborate such extension within the high plateau where elevations exceed 5000 m normal and strike slip faulting occur so that an overall east southeast west northwest extension of the region at about 10 mm yr 1 is partitioned into roughly equal parts of crustal thinning and north northeast south southwest crustal shortening about 5 mm yr 1 in general strike slip faulting characterizes solutions for earthquakes within eastern tibet where mean elevations drop below 4500 5000 m but the orientations of the strike slip faults vary across the region in central tibet left lateral slip occurs on planes trending roughly northeast but for earthquakes farther east the orientations of that plane become progressively east west and then southeast this variation in orientation implies a rotation of material along curved left lateral shear zones thus the eastward extrusion of tibet appears to be facilitated not only by rapid left lateral shear but also by large clockwise rotations of the material in eastern tibet the rate of eastward extrusion of material in eastern tibet relative to the tarim basin to its north is roughly 30 40 mm yr 1 fault plane solutions of earthquakes in the northern and eastern margins of tibet show large components of thrust faulting with the p axes oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau the orientation of this crustal shortening is northeast southwest on the northeastern margin east west on the eastern margin and northwest southeast in the longmenshan on the southeastern margin thus at least some of the extrusion of eastern tibet out of india s northward path into asia is absorbed by crustal shortening on the margins of the plateau the variation from normal faulting in the high tibetan plateau where elevations exceed 5000 m to dominantly strike slip faulting farther east where elevations are lower and then to thrust faulting on the margins of the plateau where elevations drop below 3000 m surely results at least in part from a decrease in the value of the vertical stress the magnitude of the east west compressive stress need not vary across the plateau

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

atmospheric sulfur in the orogenic gold deposits of the archean yilgarn craton australia the origin of sulfur and gold in archean orogenic gold systems should provide significant insights into the dynamics of fluid movement in the crust of the early earth but is poorly constrained and highly debated our natural laboratory to address this knowledge gap is the metal endowed yilgarn craton western australia where we measured the multiple sulfur isotope signatures of representative sulfide bearing auriferous samples from 24 archean orogenic gold deposits varying in size and geological setting utilizing chemically conservative mass independent fractionated sulfur mif s isotope signatures we fingerprinted a major source of sulfur in these deposits contrary to previous studies our data show that they display mif s isotope anomalies with 33 s values ranging from 1 18 to 2 04 most of the studied deposits show a sulfur signature that is consistent with a crustally derived source unlike smaller deposits which may form with sulfur derived from a single sedimentary sulfur source therefore providing a coherent archean atmospheric signal in their 33 s 36 s slope 0 9 1 5 the formation of giant deposits may require sourcing from a wider range of sulfur reservoirs and using different processes as reflected in their apparently random 33 s 36 s slopes

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

erosional control on the dynamics of low convergence rate continental plateau margins s u m m a r y over the last decade thermomechanical models have revealed the control of both tectonics and erosion on the morphology of continental plateaus margins however unravelling the specific effects of these two coupled processes has been difficult in practice here to assess the control of erosion we investigate the dynamics of the eastern and the northern borders of the tibetan plateau which are characterized by a low convergence rate and a steep topographic escarpment adjacent to the sichuan and tarim basins respectively thermomechanical modelling of continental lithosphere coupled with fluvial denudation reveals that important crustal deformation with large scale horizontal displacements can occur without any convergence as a response to mass transfer due to gravitational collapse and to erosional unloading these processes are sensitive to crustal structure geothermal gradient as well as surface erosion at a timescale of several million years our results suggest that this denudation triggered deformation exerts a primary control on the evolution of those plateau margins by counterbalancing the mass removal due to erosion and stabilizing the topographic escarpment this finding supports a possible explanation for the morphology of the longmen shan eastern tibet in which the paradoxical combination of persistent high topographic gradients close to the foreland and low convergence rates can be related to the influence of erosion on deformation patterns

a study of azimuthal p residuals and shear wave splitting across the kunlun range northern tibetan plateau abstract the kunlun fault system is an east west transcurrent fault bordering the tibetan plateau that has experienced left lateral motion since 3 ma a linear array of one component and three component seismological stations was deployed across the kunlun range for 4 months p travel time residuals define three basic units along the profile the north kunlun unit the south kunlun unit and the bayan har unit p residuals are a function of azimuth p residuals are fast in the southern stations for southern azimuths and in the northern stations for northern azimuths measurements of shear wave splitting of the s scs and sks phases have been performed on deep events delay times and fast polarization directions have been computed by finding the maximum of the cross correlation between the two horizontal components rotated at different angles delay times from 0 40 s to 2 52 s remain in the range of those found by mcnamara et al 1994 j geophys res 99 13655 13665 at two sites along the same profile p residual travel times and the splitting of s waves are discussed in relation to models of the geological evolution of this region the northern tibetan plateau appears as a juxtaposition of different terranes the azimuthal p residual pattern implies that p wave anisotropy is about 3 in the lithosphere on both sides of the kunlun fault and that fast velocities are oriented outwards from the kunlun fault plane s wave splitting is observed far from the fault and implies that the lithosphere beneath northern tibet has undergone an important deformation

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

regional variation of attenuation and travel time in china from analysis of multple scs phases long period multiple scs waveforms from intermediate and deep focus earthquakes surrounding china recorded by the chinese digital seismograph network are used to estimate the multiple scs attenuation operator and differential travel times using both frequency phase equalization and stacking and time domain waveform inversion techniques theoretical considerations augmented by synthetic testing indicate that for continental regions the frequency domain approach yields a more stable estimate of whole mantle attenuation qscs than the time domain approach which is better for whole mantle travel time scs when teamed together the two afford unprecedented success with continental paths the results 16 path averaged estimates of qscs and scs constitute a significant addition to the catalog of multiple scs studies of subcontinental mantle and provide sufficiently dense coverage to invert path estimates of attenuation and travel time for regional quantities of direct tectonic relevance across the frequency band of 10 to 50 mhz we obtain high 215 values of regionalized qscs scs the tarim platform and the fold systems of northeastern china and low 170 values elsewhere on the continent defining a pattern that is well correlated with present day levels of tectonic activity the corresponding two way travel time residuals with respect to the jeffreys bullen j b tables scs scs 935 7 s vary from 1 3 1 4 to 0 8 4 3 s and are typical of other fast continental regions unlike qscs scs varies little from region to region and is correlated poorly with tectonic activity indicating that qscs is much more sensitive to changes in the current tectonic environment offshore scs for the sea of japan is 2 7 3 6 s and for the combined east china and philippine seas region is 9 6 2 7 s the latter very large residual likely reflects the low velocities associated with back arc spreading throughout the study region scs correlates well with the residuals predicted by recent upper mantle tomographic models although the observed variance exceeds the model predictions by roughly a factor of 3

lateral growth of ne tibetan plateau restricted by the asian lithosphere results from a dense seismic profile abstract we present high resolution receiver function images along a 700 km long dense seismic array extending from northern tibetan plateau to the alxa block crossing the entire qilian thrust belt qtb the dense stations with less than 2 km station intervals allow the receiver functions to unveil unprecedented details of crustal structures across the northern frontier of the growing tibetan plateau the migration image shows a thickened and strongly deformed qtb crust with an uneven moho and complex internal structures that are indicative of pure shear shortening the alxa block in contrast has a thinner crust a flat moho and little internal crustal deformation these results suggest that the lateral growth of ne tibetan plateau is restricted by the strong asian lithosphere which shows no visible subduction beneath the tibetan plateau as previously suggested

injection of indian crust into tibetan lower crust a two dimensional finite element model study we propose a model in which the stronger indian crust injects into the weaker lower crust of tibet if the viscosity of the tibetan lower crust is below a critical value the continuous advancing of the indian crust beneath tibet would produce negligible variation in stresses on the bottom of the stronger brittle tibetan upper crust and would result in a spatially uniform uplift and a virtually undisturbed tibetan plateau surface this process is analogous to that of a piston rising in a hydraulic jack as additional fluid is forced into the chamber we find that if the viscosity of the tibetan lower crust is assumed to be constant throughout the computed critical viscosity is 6 1018 pa s for the present day lower crustal thickness of 58 km in addition to uniform viscosity two hypothetical depth dependent viscosities are also tested which give rise to essentially the same distribution of stress along the bottom of brittle upper crust our model also interprets the 4 km elevation contrast between the tibetan plateau and the tarim block a 1 160 viscosity contrast between the tibetan and tarim lower crust is required in order to maintain the current thickening rate of the tibetan crust 1 2 mm yr while keeping the thickness of the tarim crust unchanged

evidence from gravity and topography data for folding of tibet bouguer gravity and topography data from tibet suggest that the surface of the plateau and the subsurface density interfaces are warped into two series of ridges and troughs trending parallel to the collision zone with wavelengths of 150 and 500 km these folds are superimposed on an overall state of isostatic compensation owing to crustal thickening such folding is predicted from models of compressional shortening of a rheologically layered plate the results thus suggest the presence of a weak decoupling zone between the tibetan crust and upper mantle

a model of the crust and mantle structure down to 700 km depth beneath the lhasa to golmud transect across the tibetan plateau as derived from seismological data abstract a 700 km deep seismic velocity cross section beneath the lhasa to golmud transect across the tibetan plateau is presented in contrast to the first version of this cross section which comprised an 800 km wide swath centred on the lhasa to golmud transect due to the recent proliferation of publications concerning the mantle structure beneath tibet this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect the results from the recent indepth iv project indicate that the crustal thickness change from 70 km beneath the songpan ganzi terrane and kunlun mountains to 54 km beneath the qaidam basin is located about 100 km north of the kunlun fault and almost 45 km north of the north kunlun thrust the qaidam basin moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath the stronger qaidam crust beneath the crust high velocity dense cold indian lithospheric mantle extends northwards until about the banggong nujiang suture northwards asian lithospheric mantle is overlain by a low velocity less dense warm tibetan plate consisting of an upper lithospheric and a lower asthenospheric part the apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north tibet is slower less dense and warmer than under south tibet in agreement with the observed uppermost mantle velocities this in turn could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

convergence of the indian and eurasian plates under eastern tibet revealed by seismic tomography 1 a three dimensional p wave velocity model of the crust and upper mantle down to 400 km depth beneath eastern tibet is obtained using many temporary seismic stations of the ascent project and the namche barwa broadband seismic network we collected 16 508 arrival times of p pn and pg phases from 573 local and regional earthquakes and 7 450 p wave arrivals from seismograms of 435 teleseismic events our high quality data set enables us to reconstruct the 3 d velocity structure under eastern tibet in more detail than the previous studies in the shallow depth our results show that the low velocity zones are not interconnected well no wide spread low velocity zones which may reflect the complex pattern of material flowing in the study region below the moho we find that the indian lithospheric mantle underthrusts sub horizontally under eastern tibet and the extent of the northward advancing indian lithosphere decreases from west to east in the north the asian lithospheric mantle is detected under the vicinity of the qaidam basin between the indian and asian lithospheric mantles there is an obvious low velocity anomaly which may reflect an upwelling mantle diapir

the configuration of asia prior to the collision of india cretaceous paleomagnetic constraints paleomagnetic data from central asia show that 1700 610 km of shortening of southern asia since cretaceous time have been absorbed by distributed deformation between southern tibet and the siberia craton this result is based on a compilation of cretaceous poles from the junggar tarim tibet indochina south china north china and mongolia blocks complementing the recent compilation of enkin et al 1992a we propose a paleogeographic reconstruction of asia in the cretaceous in which the position of siberia is derived from the synthetic apparent polar wander path of besse and courtillot 1991 the resulting map which likely represents asia as it remained throughout the cretaceous until the collision with india began features an unbent tibet with an east west trending andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from sichuan to tarim through tibet the map allows one to estimate continental shortening and rotations between the blocks which are attributed to the collision despite large uncertainties these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of avouac 1991 appendix is available with entire paper on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b93 007 2 50 payment must accompany order

seismic tomography of northern tibet and kunlun evidence for crustal blocks and mantle velocity contrasts although the crust and mantle of the tibet plateau reveal vital information for understanding the interplay of dynamic processes that has governed its recent uplift and growth their deep physical and thermal structure remains poorly understood in order to throw light on the structure and hence to constrain the processes and models tied to them we performed a teleseismic experiment on a 600 km long profile across the northern part of the plateau the kunlun range and the qaidam basin the 400 km deep tomographic image we obtain has a resolution 50 km over one order of magnitude better than achieved in previous broader scale studies at relatively shallow depth 100 km the tomographic cross section clearly reveals that the mid lower crust and upper lithosphere of northern tibet is an assemblage of blocks with different velocities and thicknesses hence different natures histories and ages the crust of the qiantang block appears to be thickest 70 km with the lowest velocity along the north edge of that block the jinsha suture although not remarkable in the local geology and topography stands as a particularly sharp and prominent crustal boundary the crust and lithosphere of the region between that suture and the qaidam bayan har songpan south kunlun appear to be thinnest 50 km and with the highest velocity at greater depth 150 km our experiment confirms the existence of a bulky low velocity zone in the mantle beneath the northernmost reaches of the plateau the tomogram helps assess with unprecedented accuracy the location and shape of this low velocity anomaly which forms a 250 300 km wide dome rising to about 150 200 km south of the kunlun range roughly coincident with the region where the strongest shear wave splitting hence horizontal anisotropy has recently been found that dome lies between two relatively high velocity zones with opposite dips one towards the north under the qaidam basin the other towards the south beneath the quangtang platform the implications that this new detailed image of the deep structure of northern tibet has upon the uplift and thermo mechanical evolution of the plateau are discussed

structural model of the lithosphere asthenosphere system beneath the qinghai tibet plateau and its adjacent areas abstract the deep structure of the lithosphere asthenosphere system as imaged from geophysical data of the qinghai tibet plateau the highest on the earth provides important clues in understanding its orogenic processes here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography in the qinghai tibet plateau and its adjacent areas in order to understand the modality of the convergence and collision process between the indian and eurasian plates based on rayleigh waves dispersion theory we collected long period and broad band seismic data from the global and regional seismic networks surrounding the study area 20 n 50 n 70 e 110 e after applying instrument response calibration and filtering group velocities of the fundamental mode of rayleigh waves are measured using the frequency time analysis ftan combining the published dispersion data a 2 d surface wave tomography method is applied to calculate the lateral variations of group velocity distribution at different periods in the range from 8 s to 150 s the hedgehog non linear inversion method is performed to obtain shear wave velocity vs versus depth models of the crust and upper mantle for 181 cells with size 2 2 in order to identify the cellular representative models we applied the local smoothness optimization method lso fairly detailed structural models of the lithosphere asthenosphere system have been defined the vs models demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the bangong nujiang suture bns and the west and east of tibet the variation in thickness of the metasomatic lid may suggest that the leading edge of the subducting indian slab reaches up to bns

shear velocity structure of the crust and upper mantle beneath the tibetan plateau and southeastern china this paper addresses the velocity structure of the crust and upper mantle beneath southern china with special emphasis on the tibet region waveform data from 48 earthquakes as recorded on the wwssn and gdsn are used in this detailed forward modelling study constraints on the upper crustal section are derived from modelling local love waves in the time domain applying the mode sum modelling technique lower crustal constraints are derived by modelling the p nl wavetrain with the reflectivity method an average crustal thickness of 70 km is obtained beneath the tibetan plateau with a modest increase of velocity with depth the lithospheric and upper mantle structure is deduced from modelling s and ss triplication waveform data and relative traveltimes by applying a combination of wkbj and generalized ray methods s ss seismograms chosen with bounce points directly under tibet allow remote sensing of this inaccessible region the resulting model is an averaged 1 d model where corrections for lateral variation have been applied we conclude that the upper mantle structure in the entire region is basically shield like below 200 km sna however the velocity of the lithosphere is abnormally slow roughly 5 per cent beneath tibet the model for tibet derived does not have a distinct lid and has a positive velocity gradient in the crust suggesting crustal shortening a preliminary velocity model for southeastern china is also suggested

crust and upper mantle tomography in tibet using surface waves we have obtained maps of lateral variations of structure in the crust and lithosphere in tibet and surrounding regions by inversion of single station measurements of fundamental mode rayleigh wave phase velocities in the period range 25 100 sec for earthquakes located in central asia and observed at teleseismic digital stations in eurasia phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries the lateral variations of s velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the chang tang region north central tibet this zone is bounded by rather steep velocity gradients to the south and east about 250 and 400 km north and west of the frontal thrusts of the himalayas and lungmen shan respectively the northern boundary of the zone crosses the northern limit of the tibet plateau altyn tagh and kunlun extending into the southwest tarim and qaidam basins the low crustal velocities are consistent with a thick hot crust in the central part of tibet consistent with quaternary volcanism in chang tang the steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south and suggest that a similar situation might exist in the north

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

geologic evolution of northern tibet results of an expedition to ulugh muztagh a reconnaissance expedition across the northern margin of the tibetan plateau revealed evidence of a late cenozoic northward progression of the locus of crustal shortening and therefore of a northward growth of the area encompassed by the plateau active reverse faults crop out at the foot of the altyn tagh on the northern edge of the plateau and at the bases of several ranges within the altyn tagh and kunlun where the elevations of the neighboring basins are less than 4000 meters farther south where elevations are higher there was no evidence of recent faulting but late cenozoic rock in the ayak kum kol basin has been strongly folded south of this basin ulugh muztagh apparently the highest mountain in the eastern kunlun is underlain by late miocene tourmaline bearing and two mica granite these rocks suggest that thickening of continental crust had begun in this area by late miocene time overlying quartz sanidine welded tuffs of pliocene age imply that uplift and erosion occurred between miocene and pliocene time but with little subsequent erosion in addition we found an east west trending belt of mafic and ultramafic rock that probably marks a suture of a crustal fragment with southern asia in triassic or more recent time

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

forecasting damaging earthquakes in the central and eastern united states analysis of seismograph network data earthquake catalogs from 1727 to 1982 and paleoseismic data for the central and eastern united states indicate that the poisson probability of a damaging earthquake magnitude 6 0 occurring during the next 30 years is at a moderate to high level 0 4 to 0 6 when differences in seismic wave attenuation are taken into account the central and eastern united states has approximately two thirds the likelihood of california to produce an earthquake with comparable damage area and societal impact within the next 30 years

crust and upper mantle structure beneath western china from p wave travel time tomography 1 three dimensional velocity images of the crust and upper mantle beneath western china are obtained through seismic tomography by using p wave arrival times of local regional and teleseismic events recorded in chinese and kyrgyzstan seismic networks at shallow depths most mountain belts and tectonically elevated areas are imaged as higher velocities relative to adjacent sediment filled basins and foreland depressions with lower velocities at mid lower crust depths the kyrgyz tian shan is underlain by lower velocity material and a distinct low velocity zone in the south north direction is observed between the eastern and western parts of the chinese tian shan the moho of the southern tarim basin has a greater depth over 50 km in front of the western kun lun compared to the northern and western tarim basin lateral heterogeneity within the upper mantle is apparent at depths from 71 to 571 km the uppermost mantle beneath the kyrgyz tian shan altay qi lian and eastern kun lun is characterized by low velocity material that is probably related to the upwelling of hot mantle the lithospheric thickness varies from 120 to 170 km beneath the chinese tian shan and northern tibetan plateau to 200 250 km beneath the tarim basin deep coupling pattern of the chinese tian shan with adjacent provinces can be classified into the embedding detaching and sinking and underthrusting of lithospheric roots in addition the lithosphere of the northern edge of the tibetan plateau seems to be thickened in contrast to the central part of the northern plateau north dipping low velocities exist in the upper mantle between the tibetan plateau and provinces to its north which might indicate mantle flow from the tibetan plateau toward surrounding areas but it has been blocked by the thick tarim lithosphere

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

new constraints on crustal structure and moho topography in central tibet revealed by sinoprobe deep seismic reflection profiling abstract from october 2009 to may 2010 through the support of the sinoprobe project a series of deep seismic reflection profiles were conducted in order to image deep crustal structure of central tibet these profiles extend from the northern lhasa terrane to the qiangtang terrane by crossing the bangong nujiang suture bns the moho depth varies from 75 1 km beneath the northmost lhasa terrane to 68 9 km beneath southmost qiangtang terrane there is a 6 2 km sharp moho offset across the bns within 25 km to the north of the bns the moho rises smoothly to 62 6 km distinct moho reflector lies at 62 6 67 3 km beneath the qiangtang terrane the boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from indepth velocity model correspond to the strong reflection horizons at about18 8 km and 31 3 km depth beneath south qiangtang terrane respectively a north dipping series of reflection packages in the mid to lower crust may mark subduction of the lhasa terrane beneath the qiantang terrane the central portion of the reflection profile exhibits an antiformal structure at 14 1 km in the upper crust which corresponds with the blueschist bearing metamorphic belt

s p wave travel time residuals and lateral inhomogeneity in the mantle beneath tibet and the himalaya the average s wave travel time residuals from 11 earthquakes each in tibet and the himalaya imply that there are marked lateral variations in structure beneath tibet and the himalaya average s p wave residuals measured with respect to the jeffreys bullen j b tables for each of 11 earthquakes in the himalaya are less than 1 s average j b s p residuals from 10 of 11 earthquakes in tibet however are greater than 1 s even when corrected for the large crustal thickness of tibet the largest values 2 5 3 2 3 8 4 5 and 4 9 s are for five events in central and northern tibet and they imply that the average velocities in the crust and upper mantle of these portions of tibet are 4 10 lower than those beneath the himalaya therefore it is unlikely that a shield structure underlies these parts of tibet unless these s p residuals are due to variations in structure deeper than 250 km s p residuals from two other events each in western and eastern tibet are 1 2 s late and hence suggest that velocities beneath these areas are also somewhat lower than beneath shields the mean s wave velocity seems not only to be different beneath tibet and the himalaya but also seems to increase from a minimum beneath northern tibet to values close to those of the average earth toward the east south and west appendix a is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 010 2 50 payment must accompany order

deep structure of the northeastern japan arc and its relationship to seismic and volcanic activity evidence for possible deep seated magmatic activity beneath northeastern japan can be obtained from seismic observations tomographic inversions of p wave velocity data show low velocity zones distributed in the crust and upper mantle beneath active volcanoes in or around these zones anomalously low frequency micro earthquakes perhaps caused by magmatic activity are found at depths of 25 40 km distinct s wave reflectors corresponding to the upper surfaces of magma bodies are found at depths of 10 18 km large crustal earthquakes also occur around the low velocity zones these observations reveal the distribution of magma reservoirs at depths and elucidate its relation with shallow seismicity beneath volcanic arcs

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

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

comparison of the upper mantle shear wave velocity structure of the indian shield and the tibetan plateau and tectonic implications summary travel times and waveforms of long period sh waves recorded at distances of 10 30 and some ss waveforms are used to constrain the upper mantle velocities down to a depth of 400km beneath both the indian shield and the tibetan plateau the shear velocity in the uppermost mantle beneath both the indian shield and the tibetan plateau is high and close to 4 7 km s 1 the indian shield has a fairly thick high velocity lid and the mean velocity between 40 and 250 km is between 4 58 and 4 68 km s 1 in contrast s wave travel times and waveforms of s waves as well as a few for ss show that the mean velocity between 70 and 250km beneath the central and northern part of the tibetan plateau is slower by 4 per cent or more than that beneath the indian shield and probably is between 4 4 and 4 5km s 1 no large differences in the structure of the two areas below 250 km are required to explain both the arrival times and the waveforms of sh phases crossing tibet or the indian shield these results show that the structure of tibet is not that of a shield and imply that the indian plate is not underthrusting the whole of the tibetan plateau at the present time

anisotropy and p wave tomography a new approach for inverting teleseismic data from a dense array of stations summary a new formulation is given for inverting teleseismic p wave residuals in terms of both 3 d variations of seismic velocity and anisotropy of the subcrustal lithosphere two restrictive assumptions are made on the anisotropic elastic tensor to reduce the number of model parameters first the upper mantle anisotropy is modelled by a preferred orientation of olivine crystals with b and c axes randomly distributed around the a axis and second the elastic properties of polycrystalline aggregates are computed according to the voigt averaging rule the latter assumption allows us to link linearly the observed p wave residuals to the proportion of oriented crystals in a realistic polycrystalline assemblage in the inversion process the orientation of the symmetry axis is fixed either by a trial procedure or by using a priori hypotheses from independent data a linear inversion scheme is then applied to the data to retrieve the 3 d variations of two scalar parameters giving the isotropic velocity perturbation and the amount of anisotropy the inversion algorithm is tested for different source receiver configurations using several idealistic ray geometries it is concluded that the teleseismic p wave residuals could potentially carry enough information for retrieving both the 3 d isotropic velocity variations and the amount of anisotropy however from a more practical point of view by looking at the distributions of rays for two recent tomographic experiments in the southern rhine graben area and in the pyrenees it is concluded that the distributions of rays in these experiments do not fulfil the necessary requirements for retrieving reliable information about the lithospheric anisotropy additional data are thus needed to constrain the anisotropic model if the symmetry axis is set as horizontal the inversion of the p wave residual of the pyrenees leads to an east west fast p wave velocity orientation the resulting anisotropy is compatible with sks observations made in the pyrenees whatever the anisotropy is the numerical tests performed with the actual ray configurations show the robustness of the isotropic components in the inverted model a long wavelength anisotropy in the lithosphere if present should not drastically alter the isotropic 3 d velocity distributions that are obtained from classical methods of teleseismic tomography

high frequency seismic wave propagation beneath the indian shield himalayan arc tibetan plateau and surrounding regions high uppermost mantle velocities and efficient sn propagation beneath tibet summary this paper reports the regional propagation characteristics of high frequhcy p p s and l waves 0 5 to 2hz and p pg and s velocities for several regions in central asia tibet and india an important result is that s waves propagate efficiently in the uppermost mantle beneath most of the tibetan plateau also s transmission in tibet is not affected by crossing many physiographic and tectonic boundaries such as the indus tsangpo suture these observations are similar to observations of s propagation in stable continental and shield regions efficient s propagation is also observed across the stable tarim basin along the tien shan and himalayan mountains and across the indian shield s is strongly attenuated across the northern tibetan plateau chang thang terrane such strong attenuation suggests the existence of a low q zone in the uppermost mantle beneath that region this interpretation is supported by recent geological observations of widespread quaternary to recent basaltic volcanism in the chang thang terrane l propagates efficiently across the tarim basin the indian shield and along the strike of the himalayan mountains l is also observed although less pronounced for paths crossing large portions of the complicated structures of tien shan karakoram and pamir l is not observed for paths crossing the tibetan plateau the southern boundary of the region of inefficient l propagation in tibet lies in the vicinity of the indus tsangpo suture zone the lack of l can be attributed to attenuation within the tibetan crust however other possibilities including scattering by structural discontinuities at the margins of tibet are equally plausible pg and s are the most prominent features on most short period wwssn seismograms at epicentral distances less than about 4 apparent pg velocity determined for the area of the himalayan mountains is 6 0 0 2 km s l apparent velocities of p and s waves that have at least 50 per cent of their paths in the uppermost mantle beneath the tibetan plateau are 8 42 it 0 10 and 4 73 f 0 06 km s respectively these velocities are quite similar to those determined for the himalayan mountains p 8 45 f 0 08 km s i s 4 74 0 05 km s and the indian shield p 8 40 0 08 km s s 4 69 k 0 04 km s p

uplift of tibet from eclogites to granulites implications for the andean plateau and the variscan belt abstract using a simplified petrogenetic grid and a one dimensional thermal model to compute the evolution of the density of the crust with time the contribution to the uplift of tibet of metamorphic transitions in the lower crust is shown to be large the computations are made for a lower crust of either mafic or intermediate composition a mafic lower crust would have a mantle like density incompatible with seismological results this conclusion implies that any pre thickening gabbroic lower crust has been removed during the orogenic process a lower crust of intermediate composition would be affected by about 3 km of post thickening uplift over a few tens of millions of years this uplift is essentially the result of the eclogite to granulite transformation the resulting altitude matches the present altitude and does not require any delamination of the mantle lithosphere such a process would also explain why the neogene uplift of tibet to an average elevation of 5 km appears to have occurred without crustal shortening the same model may explain the uplift of the altiplano puna plateau which also occurred without surface shortening since about 10 ma a comparison is made with the now collapsed variscan high mountain belt for which there is evidence of a progressive increase of the temperature in the crust over tens of million years