the distribution of the mid to lower crustal low velocity zone beneath the northeastern tibetan plateau revealed from ambient noise tomography we collected continuous seismic data recorded between 2007 and 2010 by 208 broadband stations from the chinese provincial digital seismic networks a seismic collaborative experiment of northern tibet and the northeastern tibet seismic experiment cross correlations of vertical component records are computed to extract the rayleigh wave empirical green s functions group and phase velocities are then constructed from the empirical green s functions in 8 to 50 s period at periods 25 s more than 10 lower velocities are imaged beneath the qaidam basin and high velocities are observed beneath the nonbasin regions at periods 30 s up to 10 lower velocities are imaged in the qiangtang and songpan ganze terranes from these group and phase velocity maps a three dimensional 3 d vsv model of the crust is derived the model shows that the qiangtang and songpan ganze terranes have a very thick crust with a prominent low velocity zone lvz in the middle crust the lvz thins out in the vicinity of the eastern kunlun mountains providing a new constraint on the mode of deformation across the tibetan plateau the northwestern qilian orogen where receiver functions reveal a moho deeper than the surrounding areas also features a relatively weak midcrustal lvz which we interpret as an intracrustal response associated with the shortening between the north china craton and the tibetan plateau

plate tectonic history basin development and petroleum source rock deposition onshore china abstract china comprises a mosaic of distinct continental fragments separated by fold belts these fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust the major sedimentary basins onshore china can be classified into four groups those in western china are flexural developing as a result of north south compression in contrast those in the east are extensional and related to development of the pacific oceanic margin in central china basins have a more problematic origin those of north central china ordos sichuan are flexural basins controlled by eastward directed thrusting along their western margin in contrast basins further south chuxiong shiwandashan are predominantly extensional and related to major strike slip movements by synthesizing basin stratigraphies across china in tectonostratigraphic terms and in particular comparing the nature and timing of unconformities it is possible to formulate a coherent model for the palaeoreconstruction of china we identify five major tectonostratigraphic breaks which equate with the collision of the following continental fragments tarim north china carboniferous permian south china block permian triassic qiantang late triassic early jurassic lhasa block late jurassic early cretaceous and india early tertiary prior to permian times the southern margin of eurasia ran approximately along the northern border of modern china the late carboniferous collision of tarim north china with eurasia resulted in the development of a flexural basin junggar and deposition of non marine clastics to the south of the suture shallow marine deposition continued in the late permian early triassic the progressive collision from east to west of the south china block with the north china block resulted in a change to fluvial lacutrine sedimentation across the entire north china tarim block open marine carbonate deposition in the north of the south china block passed southward into a deeper marine clastic sequence deposited in a backarc basin further south a subduction zone existed along the southeastern margin of the south china block in western china northward subduction throughout the triassic resulted in the development of the songban ganzi accretionary prism with retroarc thrusting resulting in flexure and the first development of the tarim basin oblique collision of the qiantang block in the late triassic along the east of the south china block resulted in east west directed thrusting which initiated the suchuan and ordos basins continued strike slip deformation along the south western margin of the south china block resulted in the development of basins with a significant extensional component such as chuxiong the collision of the qiantang block with the southern edge of the tarim basin early jurassic resulted in a renewed clastic influx in both the tarim and junggar basins along the eastern pacific margin a compressional arc and retroarc basin in the south passed northwards into an extensional arc system subduction rollback of the extensional arc initiated rifting in the late jurassic in the eren and songliao basins the late jurassic early cretaceous collision of the lhasa block in the west rejuvenated the thrust systems bordering the western basin and resulted in a renewed clastic influx in the southeast the compressional arc phase culminated in widespread thrusting and folding of early cretaceous age in the northeast extension continued with the progressive migration of the rift system southward with time the arrival of the indian block in the early tertiary rejuvenated the bounding thrust belts of all the western basins in the east the change in convergence of the pacific plate to a more westerly direction is marked by extension and widespread rifting along the entire length of eastern china throughout most of china mesozoic and cenozoic deposition occurred in predominantly non marine environments source rocks in such settings comprise principally mudstones deposited in lakes organic rich mudstones these can accumulate in both deep and shallow lakes in order to accumulate substantial volumes the lake must be significant in space and time in china lacustrine orms occur in both rift and flexural basins lacustrine orms deposited under humid climatic conditions are restricted to the period of maximum tectonic subsidence in the flexural basins of western china source rock deposition follows basin initiation by 20 30 ma in the extensional basins of eastern china source rock deposition takes place 5 15 ma after basin initiation by contrast semi arid and arid climate lacustrine orms whilst being best developed during the period of maximum tectonic subsidence occur at all stages in the basin history

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

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data summary we developed an integrated technique for estimating crustal anisotropy with a horizontal axis using radial r and transverse t receiver functions the technique includes computing three individual and one joint objective function jof and a reliability analysis of the estimated anisotropy the individual objective functions iofs are designed to 1 maximize the peak energy of the stacked r receiver function after a cosine moveout correction in the ps arrival time 2 to maximize the correlation of the radial receiver functions after a full correction of anisotropy or 3 to minimize the total energy of transverse receiver functions stacked after a removal of crustal anisotropy the jof was computed by a weighted average of the three iofs and the reliability analysis uses the principle that stacking coherent signals can lead to an increase of signal to noise ratio we applied the technique to synthetic receiver functions generated with 30 60 per cent white noise from a variety of anisotropic and heterogeneous models the synthetic tests indicate that the proposed technique has good capability to recover the input models despite the presence of random and other coherent noises such as those caused by inhomogeneous structures in the data the technique can always provide accurate estimates of crustal anisotropy we applied the technique to two permanent seismic stations in western china and found significant crustal anisotropy beneath one station located at the northern edge of the tibetan plateau the observed fast polarization direction at this station follows the direction of the maximum horizontal tensile stress suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust the station situated in the sichuan basin on the other hand shows little to no seismic anisotropy which may suggest that the crust beneath the basin is nearly rigid with very little deformation the developed technique can be applied to any broadband seismic stations that have a good backazimuthal coverage of teleseismic events

depth variant azimuthal anisotropy in tibet revealed by surface wave tomography azimuthal anisotropy derived from multimode rayleigh wave tomography in china exhibits depth dependent variations in tibet which can be explained as induced by the cenozoic india eurasian collision in west tibet the e w fast polarization direction at depths 100 km is consistent with the accumulated shear strain in the tibetan lithosphere whereas the n s fast direction at greater depths is aligned with indian plate motion in northeast tibet depth consistent nw se directions imply coupled deformation throughout the whole lithosphere possibly also involving the underlying asthenosphere significant anisotropy at depths of 225 km in southeast tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the himalaya and burma respectively the multilayer anisotropic surface wave model can explain some features of sks splitting measurements in tibet with differences probably attributable to the limited back azimuthal coverage of most sks studies in tibet and the limited horizontal resolution of the surface wave results

complicated crustal deformation beneath the ne margin of the tibetan plateau and its adjacent areas revealed by multi station receiver function gathering abstract we use a multi station stacking method to analyze receiver function data recorded by a large scale dense seismic array covering the northeastern ne margin of the tibetan plateau and its adjacent regions to estimate crustal thickness h vp vs ratio and crustal anisotropy fast polarization direction and splitting time for each station of the array we gathered all the receiver functions recorded by the station and its nearby stations located inside a circle with a radius of 0 5 we applied the h stacking technique and the joint inversion scheme to a total of 654 station clusters to measure the h and respectively the measured moho depth varies from a peak value of 67 km beneath the northern central plateau to 39 45 km beneath the surrounding blocks in the northeast the front of the depressed moho beneath the plateau margin exhibits a complicated geometry which suggests that when the tibetan plateau encroached upon the surrounding terranes the front expanded unevenly depending on the strength of the encountering terranes the thickened crust beneath the margin also has a very low vp vs ratio indicating that the crust is composed largely of felsic minerals more than one third of the station clusters 221 have a splitting time significantly larger than 0 2 s the average splitting times of the 221 measurements is 0 68 s which is comparable to those measured from the sks skks xks phases 0 94 s the fast directions estimated from the moho ps and xks phases recorded by the station clusters within the margin are very similar and are also approximately parallel to the strikes of major faults sutures and thrust fronts in the area the low vp vs ratio together with the observed fast polarization directions from the moho ps and xks phases suggests that shortening of the entire lithosphere orthogonal to the compressional direction is likely the main cause for the observed crustal thickening occurring at the margin we also found that the moho ps and xks recorded by stations inside the western hetao basin and the yinchuan basin present very different anisotropy which may suggest that the crust and mantle beneath the two basins have very different deformation patterns

a comparison of predicted and observed ocean tidal loading in alaska we investigate the elastic and anelastic response of the crust and upper mantle across alaska to mass loading by ocean tides gps inferred surface displacements recorded by the plate boundary observatory network are compared with predictions of deformation associated with the redistribution of ocean water due to the tides we process more than 5 yr of gps data from 131 stations using a kinematic precise point positioning algorithm and estimate tidal contributions using harmonic analysis we also forward calculate load induced surface displacements by convolving ocean tide models with load green s functions derived from spherically symmetric earth models we make the comparisons for dominant tidal harmonics in three frequency bands semidiurnal m diurnal o and fortnightly m f vector differences between predicted and observed ocean tidal loading otl displacements are predominantly sub mm in magnitude in all three frequency bands and spatial components across the network with larger residuals of up to several mm in some coastal areas accounting for the effects of anelastic dispersion in the upper mantle using estimates of q from standard earth models reduces the residuals for the m harmonic by an average of 0 1 0 2 mm across the network and by more than 1 mm at some individual stations for the relatively small m f tide the effects of anelastic dispersion 0 03 mm are undetectable within current measurement error incorporating a local ocean tide model for the northeastern pacific ocean reduces the m vertical residuals by an average of 0 2 mm with improvements of up to 5 mm at some coastal stations estimated rms observational uncertainties in the vertical component for the m and o tides are approximately 0 08 mm at the two sigma level 0 03 mm in the horizontal components and 0 21 mm for the m f harmonic 0 07 mm in the horizontal components for the m harmonic discrepancies between predicted and observed otl displacements exceed observational uncertainties by about one order of magnitude none of the ocean tide and earth model combinations is found to reduce the m residuals below the observational uncertainty and no single forward model provides a best fit to the observed displacements across all tidal harmonics and spatial components for the o harmonic discrepancies between predicted and observed displacements are generally several fold larger than the observational uncertainties for the m f harmonic the discrepancies are roughly within a factor of two of the observational uncertainties we find that discrepancies between predicted and observed otl displacements can be significantly reduced by removing a network uniform tidal harmonic displacement and that the remaining discrepancies exhibit some regional scale spatial coherency particularly for the m harmonic we suggest that the remaining discrepancies for the m o and m f tides cannot be fully explained by measurement error and instead convey information about deficiencies in ocean tide models and deviations from spherically symmetric earth structure

seismic anisotropy and implications for mantle deformation beneath the ne margin of the tibet plateau and ordos plateau the northeastern tibet and the ordos plateau connects east china and west china which are dominated by horizontal extension and compression respectively knowledge of seismic anisotropy beneath this transitional region can provide important constraints on deformation pattern of the crust and lithosphere mantle during an orogeny process we measured sks wave splitting parameters from the new installed regional networks and obtained a detailed anisotropy map for 119 broadband stations beneath the ne margin of tibet the observed wnw fast polarization direction is parallel to the surface geological features the coherence between the observed geodetic motion of the crust and fast axis direction suggests that the vertical coherent deformation of the lithosphere is the dominate source for the observed seismic anisotropy further east to the qinling orogenic belt the fast axis direction tracks change in strike of the faults with the fast direction aligning almost ew we suggest that both the lithosphere mantle and east extrusion of mantle material contribute to the observed anisotropy small value of delay time is observed interior the thick rooted ordos plateau suggesting a rigid and stable lithosphere with little deformation the deviation of fast polarization direction beneath the adjacent rifting regions might be caused by the edge flow induced by sharp changes in the thickness of lithosphere our results propose that the upper crust and mantle lithosphere beneath the northeastern tibet are at least partly coupled or subjected to the same boundary conditions which is inconsistent with the channelized lower crustal flow model

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

extension in the graben systems around the ordos china and its contribution to the extrusion tectonics of south china with respect to gobi mongolia the graben basins around the ordos block are major tectonic features of late cenozoic extension in north china this paper gives a synthetic view of the basic characteristics of the graben basins and of the kinematics of the graben bounding faults this view is based on the available geological data spot imagery analysis field observations and focal mechanisms of earthquakes from this region these graben basins have been grouped into two major systems the yinchuan hetao graben system along the northern and northwestern margins of the ordos and the weihe shanxi graben system along the southern and eastern margins of the ordos crustal deformation along these graben systems involves normal slip on ne sw striking graben bounding faults normal right lateral and normal left lateral slip on nne ssw and wnw ese striking faults respectively this fault pattern is consistent with nw se trending extension the amount and rate of deformation resulting from this extension have been estimated from geological data using a cross sectional area balance and the extensional factor method deduced from the tilt of faulted blocks the results of these estimates show that the uppermost pliocene quaternary extension rate in a nw se direction is in the order of 1 6 mm yr 1 across the weihe graben 0 5 mm yr 1 across the central shanxi graben on the southeastern margin of the ordos block and 3 1 mm yr 1 across the linhe graben on the northwestern margin of the ordos block the extension across these graben systems is accompanied with counterclockwise rotation of continental blocks in north china ordos taihang shan north china plain a kinematic model is proposed in which the late cenozoic extension and block rotation in north china are interpreted as the effect of the eastward termination of the left lateral slip haiyuan fault and kunlun taibai fault system the ordos block rotates counterclockwise as a result of the push of tibet through the left lateral strike slip haiyuan fault and the liupan shan thrust and fold belt the extension along the weihe shanxi graben system may represent the effect of the left lateral slip on the kunlun taibai qinling fault system the rough estimates of the extensional deformation around the ordos block indicate that this contributes to about 5 2 mm yr 1 of left lateral movement in a n125 e direction between south china and gobi mongolia this motion adds to the left lateral slip on the qinling fault zone about 7 2 mm yr 1 in a n90 e direction between the qinling shan and the hua shan north china thus the eastward motion of south china with respect to gobi mongolia is partitioned between normal faulting around the ordos and strike slip faulting in the qinling shan it is estimated to be in the order of 12 4 mm yr 1 in a n105 e direction during the quaternary

receiver function analysis of crustal structure beneath the eastern tibetan plateau abstract we measure the crustal structure beneath the eastern tibetan plateau by analyzing three component teleseismic data recorded at 252 stations from january 1 2008 to march 31 2011 lateral variations of the crustal thickness and poisson s ratio are obtained by using receiver function method the thinnest crust is 27 km in the south yangtze craton and the deepest moho 70 km is estimated in the eastern margin of the tibetan plateau the distribution of poisson s ratio shows the heterogeneity that high poisson s ratios 0 27 are found in the chuandian plateau intermediate values 0 24 0 27 exhibit in the ordos block the qilian orogen the qinling dabie orogen the songpan ganzi terrane and the sichuan basin and low values 0 30 in the southeastern margin of the tibetan plateau can be considered as the evidence of the dominantly mafic composition or locally partial melting in crust our observations of the crustal thickness and poisson s ratio beneath the eastern tibetan plateau not only reveal the lateral inhomogeneity of the crustal structure but also provide some constraints on the mechanism of uplift and crustal thickening of the tibetan plateau

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

poisson s ratio behaviour in various crystalline rocks application to the study of the earth s interior the study of poisson s ratio behaviour in various crystalline rocks under different temperatures and pressures shows this parameter to depend upon the rock composition rather than upon p t conditions the results of this study are presented in the form of a comparison of z distributions within the consolidated crust and continental upper mantle and the specific variations of in crust and mantle rocks underlying the voronezh crystalline massif vcm these investigations which are based upon seismic and seismological data as well as high pressure experiments should clarify in particular the composition and petrology of the earth s interior

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

receiver function study of the crustal structure of the southeastern caribbean plate boundary and venezuela 1 we have investigated crustal thickness and composition across the southeastern caribbean plate boundary with the receiver function technique we used teleseismic data recorded by a temporary broadband array deployed under the bolivar project and the permanent national seismic network of venezuela we used the primary p to s conversion and crustal reverberations to estimate crustal thickness and average crustal vp vs ratio over the region we observe large variations in crustal thickness and poisson s ratio estimated moho depth ranges from 16 km beneath the southeastern caribbean sea to 52 km beneath northeastern venezuela and the venezuelan andes there is a good correlation between crustal structure and tectonic terranes data from the precambrian guayana shield suggest that the underlying crustal structure is relatively uniform with a moderate thickness 37 km and an intermediate composition a thick crust is found below the foreland basins the two mountain systems in northern venezuela the serrania del interior and the serrania del falcon have a thin crust with arc composition and are likely dynamically supported by elastic rebound or underthrusting of the oceanic plateau that characterizes the southern caribbean on the other hand the venezuelan andes and perija range on the western side of the country are probably isostatically balanced by thick crustal roots

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

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

seismic evidence for the north china plate underthrusting beneath northeastern tibet and its implications for plateau growth abstract lithospheric deformation of the tibetan plateau is caused by subduction of the indian northward and asian southward plates the effects of this interaction on inner reaches of the asian continent between the tibetan plateau and the north china craton ncc for example remain uncertain due to poor geophysical data coverage in northeastern tibet ne tibet we provide here detailed knowledge of the lithospheric structure beneath ne tibet as determined from a dense broadband seismic profile traversing ne tibet to the westernmost ncc receiver function imaging reveals several significant features including a north dipping intracrustal converter nc moho offset overlap beneath major fault zones and a low velocity layer lvl in the middle lower crust the lithosphere asthenosphere boundary lab is clearly defined and appears as a south dipping interface that runs continuously from the alxa interior to the qilian orogen interpretation of these observations combined with other seismic evidence implies that the ncc lithospheric mantle has been persistently underthrust beneath the qilian orogen this process forms the thick skinned crustal accretionary wedges which develop above a middle lower intracrustal decollement our results provide further deep geophysical constraints on the cenozoic post collisional evolution of the convergent boundary between ne tibet and the ncc and help clarify the mechanism of plateau growth in this boundary area

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

holocene paleomagnetic records from the st lawrence estuary eastern canada centennial to millennial scale geomagnetic modulation of cosmogenic isotopes two long holocene piston cores md99 2220 and md99 2221 were raised from the st lawrence estuary eastern canada because of the expanded holocene sediment sequence this location provides a u channel based paleomagnetic study supported by an accelerator mass spectrometry ams 14c chronology rock magnetic and sedimentological data indicates that these sediments provide a paleomagnetic directional secular variation psv and relative paleointensity rpi proxy records for the last 8500 cal bp sedimentation rates vary from 1 2 to 4 2 m kyr comparison of inclination and declination features with other north american holocene psv records are generally consistent and temporal offsets are within chronological uncertainties the normalized natural remanent magnetization intensity record a rpi proxy from the postglacial sediments of core 2220 compares favorably with north american and european holocene rpi records at millennial and even some centennial timescales comparisons between core 2220 rpi proxy and the 10be flux record from the greenland summit gisp2 ice core finkel and nishiizumi j geophys res 102 1997 26699 26706 and a 14c production rate record bond et al science 294 2001 2130 2136 suggest that geomagnetic modulation may control the millennial and even some centennial scale variability within cosmogenic isotope records this implies that the core 2220 rpi record reflects changes in global scale geomagnetic field at these timescales

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

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