crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

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

crustal and mantle velocity models of southern tibet from finite frequency tomography 1 using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays we carried out finite frequency tomographic inversions to image the three dimensional velocity structure beneath southern tibet to examine the roles of the upper mantle in the formation of the tibetan plateau the results reveal a region of relatively high p and s wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the higher himalaya we interpret this high velocity anomaly as the underthrusting indian mantle lithosphere there is a strong low p and s wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the yadong gulu rift suggesting that rifting in southern tibet is probably a process that involves the entire lithosphere intermediate depth earthquakes in southern tibet are located at the top of an anomalous feature in the mantle with a low vp a high vs and a low vp vs ratio one possible explanation for this unusual velocity anomaly is the ongoing granulite eclogite transformation together with the compressional stress from the collision eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the indian mantle lithosphere beneath the higher himalaya our tomographic model and the observation of north dipping lineations in the upper mantle suggest that the indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north south trending rifts and subducted in a progressive piecewise and subparallel fashion with the current one beneath the higher himalaya

ambient noise tomography of north tibet limits geological terrane signature to upper middle crust 1 we use ambient noise tomography to map regional differences in crustal rayleigh wave group velocities with periods of 8 40 s across north tibet using the international deep profiling of tibet and the himalaya phase iv arrays 132 stations deployed for 10 24 months for periods of 8 24 s sensitive to midcrustal depths of 5 30 km we observe striking velocity changes across the bangong nujiang and jinsha suture zones as well as the kunlun qaidam boundary from south to north we see higher velocities beneath the lhasa terrane lower velocities beneath the qiangtang higher velocities in the songpan ganzi and kunlun terranes and the lowest velocities beneath the qaidam basin maps at periods of 34 and 40 s sensitive to the middle and lower crust at depths of 30 60 km do not show evidence of changes across those boundaries any differences between the tibetan terrane lower crusts that were present at accretion have been erased or displaced by cenozoic processes and replaced almost ubiquitously by uniformly low velocities

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

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho 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 stronger qaidam crust

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

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

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

seismic velocities in southern tibet lower crust a receiver function approach for eclogite detection summary beneath the tibet plateau the deficit of crustal thickening with respect to what is expected from the plate tectonic constraints is thought to be absorbed either by lateral extrusion or by vertical rock mass transfer to nourish the unsettled debate of the relative importance of these two processes we propose a new approach based on the s to p and the p to s wave conversions enabling the precise determination of the seismic velocities the weighted amplitudes of the direct conversion and of reverberations are stacked at their predicted arrival times for various values of layer thickness and vp vs ratio separately for two sets of p and s receiver functions for each set of receiver functions coherent stack gives the vp vs ratio and thickness for the considered layer the grid search stacking method the values of vp vs ratio and layer thickness are functions of the velocity used for stacking the set of receiver functions but using the p and s receiver functions allows us to solve this indetermination and to find the effective parameters of the layer velocity vs vp vs ratio and thickness we use a bootstrap resampling of the receiver function data sets to estimate the parameters uncertainties for the southern lhasa block the migrated sections of both p and s receiver functions hi climb experiment data show a layer in the lower crust that may be related to the lower indian crust underplated beneath tibet with the grid search stacking method high shear wave velocities vs 4 73 km s 1 and low vp vs ratios 1 69 are detected in this layer such values are typical for high grade eclogites and the low vp vs ratio precludes the confusion with mafic granulites there is no evidence for partial eclogitization near and south of the yarlung tsangpo suture and the about 19 km thick eclogitic layer extends northwards only to about the middle of the lhasa terrane

surface wave dispersion across tibet direct evidence for radial anisotropy in the crust recordings in western tibet of rayleigh and love waves at periods less than 70 s from aftershocks of the 2008 sichuan earthquake cannot be matched by an isotropic velocity model beneath tibet these intermediate period rayleigh and love waves require marked radial anisotropy in the middle crust of tibet with the vertically polarized s waves propagating more slowly than s waves with horizontal polarization the magnitude of anisotropy inferred using paths entirely within tibet is slightly greater than that obtained previously from a tomographic inversion of a dataset covering a larger region anisotropy in the middle crust likely reflects deformation of the middle crust and is consistent with the notion of mid crustal flow and thinning of the crust

crustal structure of the northeastern tibetan plateau from the southern tarim basin to the sichuan basin china abstract we present a new crustal cross section of the northeastern tibetan plateau based on active source seismic data recorded along a 1600 km long profile crossing the southern tarim basin the western flank of the south qilian shan the northeastern margin of qaidam basin east kunlun shan songpan ganzi terrane and sichuan basin the crustal p and s wave velocity structure and poisson s ratio outline the seismic characteristics of the crust and provide constraints on the crustal composition the derived crustal cross section shows several significant features 1 the crustal thickness varies considerably along this profile from 48 km to 70 km 2 north of the kunlun fault variations in total crustal thickness are mainly caused by variations in lower crustal thickness whereas south of the kunlun fault they are caused by thickness variations throughout the crust 3 north of the kunlun fault we detect a mid crustal low velocity zone that is not apparent south of the fault 4 the kunlun fault seems to act as a compositional boundary for the lower crust with a poisson s ratio of 0 29 north of the fault kunlun qaidam terrane and 0 26 south of the fault songpan ganzi terrane measured poisson s ratio and p wave velocity values suggest that the lower crust throughout the tibetan plateau south qilian shan margins of the qaidam basin east kunlun shan songpan ganzi terrane is of intermediate composition thus the ne tibetan plateau along our profile is missing a mafic lower crustal layer

crustal structure across the altyn tagh range at the northern margin of the tibetan plateau and tectonic implications wepresentnewseismicrefraction wide angle reflection dataacrossthealtyntaghrangeanditsadjacentbasins wefindthatthe crustal velocity structure and by inference the composition of the crust changes abruptly beneath the cherchen fault i e 100 km north of the northern margin of the tibetan plateau north of the cherchen fault beneath the tarim basin a platform type crust is evident in contrast south the cherchen fault the crust is characterized by a missing high velocity lower crustal layer our seismic model indicates that the high topography 3 km of the altyn tagh range is supported by a wedge shaped region with a seismic velocity of 7 6 7 8 km s that we interpret as a zone of crust mantle mix we infer that the altyn tagh range formed by crustal scale strike slip motion along the north altyn tagh fault and northeast southwest contraction over the range the contraction is accommodated by 1 crustal thickening via upper crustal thrusting and lower crustal flow i e creep and 2 slip parallel swdirected underthrusting of only the lower crust and mantle of the eastern tarim basin beneath the altyn tagh range d 2005 elsevier b v all rights reserved

deep seismic reflection waves and structure of the crust from dangxung to yadong on the xizang plateau tibet abstract in this paper we study the travel times velocities and spectral distributions of reflected waves generated by underwater explosions in lakes on the xizang plateau the crustal structure on both sides of the yarlung zangbo river has been estimated by the inverse solution known as damped least squares the depth of the moho is 70 73 km in the region north of the yarlung zangbo river the depth of the moho in the southern region is 68 45 km there is a low velocity 5 47 km s 1 layer in the crust and the crustal thickness decreases from north to south in the xizang plateau

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

water weakening of clinopyroxene in the dislocation creep regime we performed a series of triaxial compressive creep experiments at two different water fugacities to investigate the effect of water on the creep strength of a natural clinopyroxenite samples were deformed under water saturated conditions at temperatures between 1373 and 1473 k confining pressures of 150 and 300 mpa and differential stresses from 34 to 261 mpa strain rates were in the range 10 7 to 10 5 s 1 water fugacity was controlled at either 140 or 280 mpa the creep results yield a stress exponent of 2 7 0 3 and an activation energy of 670 40 kj mol compared to dry clinopyroxene wet samples creep over 100 times faster at a given temperature confining pressure water fugacity and differential stress the creep rate of clinopyroxene is proportional to the water fugacity to the 3 0 0 6 power with an activation volume of 0 m 3 mol one possible water weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions compared to other major minerals in earth s lower crust specifically olivine and plagioclase the water weakening effect is most significant for clinopyroxene under hydrous conditions the strengths of clinopyroxene and anorthite are comparable over the investigated stress range and both phases are weaker than olivine since the mineral assemblages in earth s lower continental crust are often dominated by plagioclase and pyroxene in places where a wet flow law applies the mechanical behavior of clinopyroxene will have a substantial effect on creep strength

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

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

seismic evidence for stepwise thickening of the crust across the ne tibetan plateau we performed a receiver function analysis on teleseismic data recorded along two 550 km long profiles crossing the northeastern tibetan plateau results from time to depth migration grid search vp vs determination and simulated annealing inversion of waveforms reveal that the crust thickens from 50 km near the northern edge of the plateau to 80 km south of the jinsha suture in the qiang tang block crustal thickening occurs in staircase fashion with steps located beneath the main reactivated sutures the vp vs ratio close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern tibetan crust

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

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

west east variation in crustal thickness in northern lhasa block central tibet from deep seismic sounding data 5 30 km depth 5 0 vp 6 4 km s a middle crust 33 45 km depth 6 5 vp 6 8 km s and a lower crust depths below 48 km 7 0 vp 7 4 km s the west to east increase in crustal thickness is accomplished by a 50 thickening of the middle and lower crust larger vertical velocity gradients separate these three layers and bound them above surficial and sedimentary rocks and below moho transition zone the most notable low velocity zone in the crust lies at the base of the upper crustal layer s wave velocity structure is less well constrained but parallels the p wave structure except that vp vs ratios may decrease from west to east in the lower crust our data suggest considerable variation in structure along strike of the tibetan plateau and show that interpretations of tibet as a purely two dimensional orogen are overly simplistic the west east increase in crustal thickness may occur across the karakorum jiali fault system and be an indicator of lateral tectonic escape of the qiangtang terrane

midcrustal reflector on indepth wide angle profiles an ophiolitic slab beneath the india asia suture in southern tibet the wide angle seismic experiment of project indepth international deep profiling of tibet and the himalayas focused on the structure of the india asia continental collision suture yarlung zangbo suture in southern tibet three component portable seismographs recorded the explosive sources of the indepth seismic reflection profile across the suture zone we image a prominent subhorizontal reflector the yarlung zangbo reflector yzr dipping 4 n at 20 km depth beneath the outcrop position of the yarlung zangbo suture with a total area of at least 90 80 km possible geological interpretations for the yzr include fault shear zones fluids magmatic metamorphic or hydrothermal or lithotectonic contacts high amplitude reflections off the yzr out to postcritical offsets and refracted phases with a velocity of 7 km s 1 provide evidence that it is a solid solid interface and the top of a mafic to ultramafic lithological unit we suggest that the yzr is a tethyan ophiolitic slab involving several kilometers of mafic ultramafic rocks we interpret the origin of the slab and its position in the midcrust in the context of the emplacement history of oceanic lithosphere and accreted material onto the northwestern and northern indian passive margin and the tectonic history of the yarlung zangbo suture zone the origin of the yzr body and its position in the midcrust are explained by the emplacement of an ophiolite nappe accretionary wedge complex onto india s passive margin prior to continental collision the subduction of this nappe complex beneath the forearc basin and the asian gangdese magmatic arc and its further burial by the gangdese arc from the north and the indian passive margin sedimentary sequence from the south our model of the yzr constituting an ophiolitic sheet can explain several features of southern tibetan tectonics namely the pre collisional deformation within the northern indian passive margin the lack of strong deformation during collision and the bivergent thrust belt along the suture zone and doming within central southern tibet e g kangmar dome our interpretation implies that the yarlung zangbo suture is subhorizontal in the middle crust in southern tibet

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

directionality of the ambient noise field in an arctic glacial bay the directionality of ambient noise in an arctic tidewater glacier bay was measured using two horizontally spaced broadband hydrophones segments of noise were divided into two frequency bands and analyzed for arrival angle these data show that different classes of source radiate noise in distinct spectral bands and are spatially diverse a previously unidentified source the interaction of surface gravity waves with underside of ice ledges at the periphery of icebergs is described the generation of noise by ice wave interaction suggests that surface waves should be measured if ambient noise is to be used to monitor ice dynamics in glacial fjords

crustal thickening and variations in architecture from the qaidam basin to the qang tang north central tibetan plateau from wide angle reflection seismology abstract wide angle reflection refraction profiles at crustal scale have been obtained along a 700 km transect of northeastern tibet from six shotpoints a combination of sampling by broadside fan recordings and in line profiling allows us to image increases in moho depth towards the centre of the plateau these occur with different styles across the two block boundaries between north kun lun bayan har and qang tang as shown also by the changes resolved in the internal architecture and layer velocities in the crust clear s waves are observed together with p waves they bring rare constraints on the composition of crustal units which reveals an abnormally felsic average composition for the whole crust and its lower half along the whole length of the transect across the north kun lun qaidam block variations of crustal thickness are detected with moho depth between 55 and 65 km and an inverse correlation with basement topography in the north of the bayan har block a strong heterogeneity in the middle of the 70 km thick crust is shown as suggested from synthetic seismograms modelling it may result from thin layer inclusion marking the northward tectonic superposition of the bayan har crust on the north kun lun crust as imaged by a fan profile the corresponding upper part could have been transported northwards to thicken also the upper crust of the present north kun lun block in the north of qang tang a model is obtained with a 75 km thick crust and a lower half of the crust with average velocities characteristic of felsic composition from bayan har to qang tang variations of deep architecture are documented across the jinsha suture and xianshuihe fault from recording in line and broadside to several shotpoints together with the felsic composition of the lower part of the crust a moho offset suggests the imbrication of the bayan har crust in the qang tang lithosphere between a south dipping moho above and a north dipping moho below a model of indentation of the qang tang by an originally thinner bayan har crust and lithosphere with part of the qang tang brought to greater depth would also be consistent also with the moho topography across the transition from the qang tang to bayan har as imaged by fan profile

changes in the pelagic fine fraction carbonate sedimentation during the cretaceous paleocene transition contribution of the separation technique to the study of bidart section abstract the bidart section southwestern france displays an almost complete late maastrichtian early danian transition as the cretaceous paleocene k p crisis mainly affects carbonate producers the aim of this study is to precisely evaluate the evolution of pelagic sediments composition across the boundary a new protocol for separation of fine carbonates into discrete size fractions was used 1 to precisely evaluate in weight of carbonate and not only number of particles per gram of rock the contribution of each carbonate component type foraminifera calcareous nannofossils and nonbiogenic carbonates calcite microparticles and calcareous macrocrystals and 2 to measure their isotopic signature and thus constrain the origin of these carbonate particles the maastrichtian pelagic carbonates are mostly biogenic 70 of the bulk carbonate but also contain nonbiogenic carbonates calcite microparticles 4 m due to their isotopic signature calcite microparticles cannot result from fragmentation and partial dissolution of biogenic remains and the most likely origin appears to be early diagenetic processes and or export from shallow water platforms the calcareous macrocrystals derive from an early diagenetic process the lowermost danian sediments are dominated by nonbiogenic carbonates between 50 and 80 of the bulk carbonate nevertheless two kinds of biogenic components are present thoracosphaera operculata and maastrichtian like calcareous nannofossils thoracosphaera operculata is an opportunistic calcareous dinoflagellate which frequently occurs at high abundances in basal danian sediments and is certainly in situ this is confirmed by it showing a danian isotopic signature the maastrichtian like calcareous nannofossils are nannofossils of taxa that indisputably occur in the maastrichtian but which do not occur consistently in later danian strata they show a maastrichtian type isotopic signature which strongly supports the inference that they are mainly reworked from the maastrichtian this study shows that nonbiogenic carbonates are important components of the pelagic carbonate even during periods of normal sedimentation in the crisis interval the biogenic carbonate production drops sharply while calcite microparticles and carbonate macrocrystals remain almost unaffected and represent the major part of the carbonate in the earliest danian sediments finally the sediments return to a composition similar to that of the maastrichtian in m pseudobulloides foraminifera zone

group velocity tomography of the indo eurasian collision zone we present results of a rayleigh and love wave group velocity dispersion study of the indo eurasian collision zone group velocity dispersion curves are measured and combined to produce dispersion maps for 10 70 s period rayleigh waves from 4054 paths and for 15 70 s love waves from 1946 paths group velocity maps benefit from the inclusion of data recorded at a large number of stations within india an advantage over previous global studies this has the largest impact at short periods as a result of the improved path length distribution synthetic tests are used to estimate resolution which ranges from 3 to 5 on the continents for rayleigh wave maps and from 5 to 7 5 for love wave maps group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods the cratons of the indian shield can be distinguished in the short period and midperiod group velocities group velocities are slow across tibet until 70 s whereas the cratonic cores of the indian shield appear as a high velocity anomaly at 70 s dispersion curves extracted from the rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across india and tibet the relationship between shear velocity contours and the moho indicated by receiver function studies has been used to obtain a first order estimate of crustal thickness across the collision zone results suggest a slow tibetan midcrust and low sub moho velocities beneath the central and northeastern tibetan plateau

crustal shear s velocity and poisson s ratio structure along the indepth iv profile in northeast tibet as derived from wide angle seismic data summary from the s wave data collected along a 270 km long profile spanning the kunlun mountains in ne tibet 14595 sg phase arrivals and 21 sms phase arrivals were utilized to derive a whole crustal s velocity model and together with a previously derived p velocity model a poisson s ratio model beneath the profile the final tomogram for the upper 10 15 km of the crust reveals the lower velocities associated with the predominantly neogene quaternary sediments of the qaidam basin to the north and the higher velocities associated with the predominantly palaeozoic and mesozoic upper crustal sequences of the songpan ganzi terrane and kunlun mountains to the south this study finds no evidence that the kunlun mountains are involved in large scale northward overriding of the qaidam basin along a shallow south dipping thrust the in the upper 10 15 km of the crust are often lower than 0 25 indicating a preponderance of quartz rich rocks in the upper crust beneath the profile below 10 15 km depth the remainder of the crust down to the moho has an average of 0 24 beneath the songpan ganzi terrane and kunlun mountains and 0 25 below the qaidam basin these low are similar to other low found along other profiles in the northeastern part of the plateau assuming an isotropic situation and no significant variation in between 10 15 km depth and the moho then the lower crust between 25 30 km depth below sea level and the moho with p velocities varying from 6 6 km s 1 at the top to around 6 9 km s 1 at the base and of 0 24 0 25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part as the pre cenozoic qaidam basin crust has probably not lost any of its lower crust during the present himalayan orogenic cycle in the cenozoic and only has a of 0 245 0 25 then it appears that the pre cenozoic qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average of 0 265 0 27 and the tarim and sichuan basin crusts this situation then probably facilitates the collision and promotes the formation of new high plateau crust at the ne margin of tibet south of the qaidam basin the crust of the songpan ganzi terrane and kunlun mountains has an even lower average crustal of 0 23 0 24 and is thus presumably even weaker and more easily deformable than the crust beneath the qaidam basin this then supports the hypothesis of karplus et al that the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

fragmented indian plate and vertically coherent deformation beneath eastern tibet 1 using fundamental mode rayleigh waves from the indepth iv and namche barwa seismic experiments for periods between 20 and 143 s we have investigated the lithospheric structure beneath eastern tibet we have found a 200 km wide high velocity body starting at 60 km depth and roughly centered beneath the bangong nijuang suture which is most likely a piece of the underthrusting indian continental lithosphere the sub horizontal underthrusting of the indian lithosphere beneath eastern tibet appears to be accompanied by its lateral tearing into at least two fragments and subsequent break off of the westernmost portion at 91 e 33 n the uppermost mantle low velocity zone we observe beneath the n qiangtang and songpan ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern tibet we attribute the low velocity zones concentrated along the northern and southern branches of the eastern kunlun fault at lithospheric depths to strain heating caused by shearing the azimuthal fast directions at all periods up to 143 s 200 km peak sensitivity depth beneath the n qiangtang and songpan ganzi terranes are consistent suggesting vertically coherent deformation between crust and uppermost mantle furthermore the low velocity zone below the kunlun shan reaching down to 200 km argues against a present southward continental subduction along the southern margin of qaidam basin

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere