geochemistry and geochronology of the amphibolite blocks in ophiolitic melanges along bangong nujiang suture central tibet abstract amphibolites occur as blocks within serpentinite matrix melange at dong tso and lagkor tso in the gertse area of central tibet where they are associated with the bangong nujiang suture bns geochemical data shows amphibolites have similar morb or arc like morb geochemical characteristics lagkor tso amphibolites show greater arc affinity whereas those from dong tso amphibolites are more morb like these characteristics are similar to basalts from continental back arc basin settings hornblendes from the amphibolites dated by the 40 ar 39 ar single grain laser fusion technique show similar metamorphic ages 170 177 ma which is interpreted to represent the timing of the main metamorphic event experienced in this region we suggest that the western and middle sectors of the bns contain the remnants of a short lived back arc basin the bangong nujiang ocean which partly opened and closed in the middle jurassic

electrically conductive crust in southern tibet from indepth magnetotelluric surveying the crust north of the himalaya is generally electrically conductive below depths of 10 to 20 km this conductive zone approaches the surface beneath the kangmar dome dipping north and extends beneath the zangbo suture a profile crossing the northern yadong gulu rift shows that the high conductivity region extends outside the rift and its top within the rift coincides with a bright spot horizon imaged on the indepth cmp common midpoint profiles the high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region

temporal variations of mesozoic sandstone compositions in the qiangtang block northern tibet china implications for provenance and tectonic setting abstract one hundred and forty four and fifty two mesozoic sandstone samples from the qiangtang block ca 32 20 35 n 85 93 e northern tibet china were conducted for analysis of detrital modes and heavy mineral spectra respectively in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited the lower middle triassic sandstones in central qiangtang are characterized by complex derivations from both recycled orogen and continental block provenances most of the upper triassic and jurassic sandstones are dominated by a recycled orogen origin however sandstones derived from a continental block source also occur in upper triassic strata in the southern margin of southern qiangtang and in lower jurassic strata in the southern margin of northern qiangtang although the heavy mineral spectra of the upper triassic and jurassic sandstones are occupied mostly by the stable heavy minerals zircon tourmaline and rutile ztr most of the samples are generally accompanied by unstable heavy minerals such as apatite chlorite chrome spinel amphibole epidote garnet and pyroxene except for those from upper triassic strata in the southern margin of southern qiangtang and from lower jurassic strata in the southern margin of northern qiangtang the detrital modes and heavy mineral spectra of the mesozoic sandstones along with the lithologic evidence indicate that the central qiangtang anticlinorium must have been elevated and eroding due to orogenesis and most likely has contained rolling topographic relief at least since early triassic time it could have contained abundant ophiolitic fragments as evidenced by rich chrome spinel amphibole and other readily decomposed ferromagnesian minerals across the entire anticlinorium these mafic minerals may point to the presence of the shuanghu suture along the entire central qiangtang anticlinorium the blueschist bearing metamorphic complexes in central qiangtang could be accommodated in a large scale enclave in an irregular central qiangtang continental margin the upper triassic jurassic recycled orogen derived sandstones in northeastern qiangtang suggest continued orogenesis due to the suturing between qiangtang and asia such a contractional background does not support models that relate a tensile mechanism for back arc spreading along the bangong nujiang belt

details of the doublet moho structure beneath lhasa tibet obtained by comparison of p and s receiver functions the collision of the indian and asian plates since about 55 ma has created the largest plateau with the thickest crust on earth there is however no general agreement on the modes of the crustal thickening the crustal thickness of the tibetan plateau still remains poorly determined it is generally accepted that the tibetan crust has roughly double normal thickness and that it thins somewhat toward the north but individual observations of moho depth vary spatially and for different techniques at the same place by greater than 20 km in this work we compare p and s receiver functions at station lsa located in the southern lhasa terrane to determine the crustal thickness beneath the station a doublet moho structure with two significant interfaces at about 60 and 80 km depth in the lower crust is seen clearly in the p receiver functions the deeper phase moho is however absent in the s receiver function data here we model the observed p and s receiver functions by a strong moho topography that dips to the east at an angle of 30 this result may indicate that the moho structure beneath tibet can be very complicated and has strong lateral variations and suggests that a detailed map of moho depth is only possible with 2d dense seismic experiments the moho dips locally perpendicular to the direction of the indian plate motion suggesting that the lower crustal deformation is decoupled from the underlying indian mantle lithosphere the extremely strong moho variation beneath and east of station lsa may also imply a crust mantle interaction specifically delamination or foundering of lower crust down to the upper mantle

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

structure of the crust in the vicinity of the banggong nujiang suture in central tibet from indepth magnetotelluric data 1 magnetotelluric data from a 150 km long profile crossing the banggong nujiang suture bns central tibet acquired as part of the international deep profiling of tibet and the himalaya indepth project have been examined for crustal and upper mantle structure strike and dimensionality analyses demonstrate that regional scale electrical structures are two dimensional and oriented approximately parallel to surface geological strike as seen elsewhere in tibet the double thickness crust is generally characterized by resistive upper crust hundreds to thousands of ohm meters overlying conductive middle and lower crust tens to hundreds of ohm meters but in detail there are lateral variations at all levels regionally a northward transition from thick 45 km to thin 15 km resistive upper crust coincides with 1 the surface trace of the bns 2 a prominent strand of the karakorum jiali fault system 3 northward decrease in upper mantle seismic velocities and increase in attenuation and 4 pronounced northward onset of seismic polarization anisotropy the latter two seismological features have been taken to mark the northern limit of indian mantle lithosphere thrust beneath southern tibet on the basis of our electrical model we speculate that 1 the resistive upper crustal root beneath the neogene lunpola and duba basins was produced by crustal shortening localized along the northern edge of the lhasa terrane 2 the low midcrustal resistivity beneath the bns reflects enhanced neogene melting and or metamorphic dewatering of relatively fertile subduction zone complex rocks 3 observed steep upper crustal low resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the bns and 4 these strike slip and extensional fault arrays are surface manifestations of lithosphere penetrating shear localized along the northern edge of the underthrust indian plate

northward channel flow in northern tibet revealed from 3d magnetotelluric modelling abstract the kunlun fault defines one of the major northern tectonic boundaries of the tibetan plateau previous geophysical studies have detected a major change in rheology across this boundary but it is not clear how this is related to models that have invoked crustal flow the lithospheric resistivity structure of the kunlun fault has been investigated by both the indepth iii and iv magnetotelluric mt transects all the mt data were processed using modern statistically robust methods and have been analysed for directionality and dimensionality in order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the indepth iii mt data a variant approach on 3d inversion of 2d profiles was investigated to explore and improve lateral resolution in addition to the apparent surficial deformation associated with the sinistral strike slip kunlun fault the 3d modelling of the indepth mt data reveals that complex deformation processes are occurring at mid crustal depths in northern tibet the 3d mt inversion results supported by synthetic modelling particularly confirm and highlight the presence of separate north south intrusions of conductive material crossing the kunlun fault into the more resistive kunlun qaidam block these north south intrusions are interpreted to be associated with the horizontal channel flow of partially molten songpan ganzi crust into two or more separated channels moving northwards and crossing the surficial trace of the kunlun fault at mid to lower crustal depths

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

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

spatial gap between lhasa and qiangtang blocks inferred from middle jurassic to cretaceous paleomagnetic data abstract we present paleomagnetic results from middle jurassic red sandstones of the sangba formation orientated samples were collected at 14 sites on north side of the basu 30 1 n 96 9 e northeastern border of the lhasa block after stepwise thermal demagnetization high temperature component with unblocking level of 680 c is isolated from 7 sites primary nature of this magnetization is ascertained through positive fold and reversal tests at 95 confidence level combined with previously reported paleomagnetic results tilt corrected mean direction of d 353 4 i 15 1 95 14 1 n 8 is calculated indicating that during middle jurassic the study area was located at 7 7 n 7 4 comparison with the jurassic paleomagnetic data reported from eastern tibet indicates that the lhasa block was located south of the qiangtang block at a latitudinal difference of 31 11 compilation of paleomagnetic data further suggest that until the cretaceous large spatial gap of more than 1200 km remained open between the lhasa block and the qiangtang block of eastern himalayan syntaxis and eventually vanished during tertiary cretaceous paleolatitude corresponding to this gap 13 0 n 31 0 n is almost identical to that reported from shan thai block 25 4 n 5 2 furthermore the permo carboniferous sequences of gondwanic affinity are distributed in western part of the shan thai block as well as in the lhasa and qiangtang blocks keeping in view these similarities we propose a new pre collision india vs asia reconstruction model in which the lhasa shan thai and qiangtang blocks are shown in their respective order from south to north along southern margin of the asian continent the aftermath of gigantic india asia collision however introduced north south compressional regime in the asian continent that eventually played a role in eastward extrusion of the shan thai block from an area between the qiangtang and lhasa blocks

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

tectonics of western tibet between the tarim and the indus a tarim to indus traverse provides insight into the tectonics of western tibet the kunlun was the site of a mid paleozoic collision at least three phases of post paleozoic accretion have thickened the blanket of sediments that covers western tibet sizeable parts of western qiangtang have remained stable however since the mid mesozoic since the neogene deformation and volcanism have been localized near the edges of the plateau strike slip motion along the karakorum and altyn tagh faults has been coeval with overthrusting in the himalayas and kunlun such slip partitioning and the volcanism appear to result simply from northward subduction of india and southward subduction of the tarim as tibet is extruded eastwards by india s penetration into asia

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

cenozoic evolution of the eastern pamir implications for strain accommodation mechanisms at the western end of the himalayan tibetan orogen detailed field mapping geochronologic and thermochronologic analyses and petrologic investigations conducted along the southern segment of the late cenozoic kongur shan extensional system provide new information on the cenozoic tectonic evolution of the eastern pamir at the western end of the himalayan tibetan orogen field relations and cooling age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east west extension along the southern kongur shan extensional system from 20 km or less along the muztaghata massif in the north to l3 km along the tashkorgan fault in the south these results in conjunction with previously published work on the northern segment of the kongur shan extensional system show a general southward decrease in east west extension along the entire length of the extensional system consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the pamir petrologic data 40 ar 39 ar cooling ages and monazite th pb ages from schists and gneisses in the footwall of the southern kongur shan normal fault along the muztaghata massif record two tectonic events that immediately preceded late miocene initiation of east west extension 1 high grade schists and gneisses experienced upper amphibolite facies metamorphic conditions 9 10 kbar 700 750 c dated as late oligocene to middle miocene by in situ ion microprobe analyses of monazite inclusions in garnet and 2 high grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late miocene with 40 ar 39 ar biotite cooling ages of 7 5 9 ma rapid exhumation was accommodated in part by the east west striking south dipping shenti normal fault field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the central pamir gneiss domes and the antiformal muztaghata massif is the eastward continuation of the sares dome of the central pamir these observations suggest that the antiformal gneiss domes of the central pamir have not been offset across the karakorum right slip fault from the qiangtang anticlinorium in tibet instead we propose that the development of the central pamir gneiss domes may have been related to oligocene miocene northward underthrusting and thickening of crust beneath the pamir

xainxa ultramafic rocks central tibet china tectonic environment and geodynamic significance the xainxa ultramafic rocks 200 km south of the donqiao ophiolite bangong nu jiang suture zone central tibet form a 1 km thick sheet in tectonic contact with paleozoic sediments to the south and with mesozoic sediments to the north through roughly east west south dipping reverse faults the ultramafic rocks consist of depleted harzburgites with minor dunite patches and 1 m sized deposits of podiform chromite mafic rocks represented by a few isotropic gabbros and dolerites occur in only minor amounts these rocks which may have underlined a suture zone within the lhasa block have lithologies and tectonics very similar to those occurring in the donqiao area along the bangong nu jiang suture zone and therefore can be considered as a klippe of this ophiolite the xainxa ultramafic rocks were emplaced in early cretaceous time the main tectonic event observed in that area northward folding and overthrusting occurred after late early cretaceous time

seismic attenuation characteristics along the hi climb profile in tibet from lg q inversionseismic attenuation characteristics along the hi climb profile in tibet from lg q inversion

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

erratum to velocity structure of the tibetan lithosphere constraints from p wave travel times of regional earthquakes using data from regional earthquakes recorded by the hi climb array in tibet we model p wave arrival times to constrain the velocity structure in the crust and the upper mantle in central and western tibet of more than 30 high quality regional seismic profiles that have been assembled we have selected 10 that show excellent crustal and pn arrivals for further analysis travel times from four events along the hi climb array provide details on crustal velocities and six events at regional distances to the array provide further constraints on moho structure and upper mantle lid velocities we use three dimensional ray tracing to model the travel times and the results indicate that the moho beneath the lhasa terrane of southern tibet is over 73 km deep with a pn speed of about 8 2 km s the qiangtang terrane north of the bangong nujiang suture bns shows a thinner crust by up to 10 km and a lower pn speed of 7 8 7 9 km s travel times from events to thewest and east of the array indicate that both moho structure and mantle lid velocities in the region are three dimensional in nature but approximately follow the trend of the bns although only a limited number of events were used for the travel time modeling the results are consistent with earlier results from teleseismic imaging using the hi climb array

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

crustal structure of central tibet as derived from project indepth wide angle seismic data summary in the summer of 1998 project indepth recorded a 400 km long nnw sse wide angle seismic profile in central tibet from the lhasa terrane across the banggong nujiang suture bns at about 89 5 e and into the qiangtang terrane analysis of the p wave data reveals that 1 the crustal thickness is 65 5 km beneath the line 2 there is no 20 km step in the moho in the vicinity of the bns as has been suggested to exist along strike to the east based on prior fan profiling 3 a thick high velocity lower crustal layer is evident along the length of the profile 20 35 km thick 6 5 7 3 km s 1 and 4 in contrast to the southern lhasa terrane there is no obvious evidence of a mid crustal low velocity layer in the p wave data although the data do not negate the possibility of such a layer of modest proportions combining the results from the indepth iii wide angle profile with other seismic results allows a cross section of moho depths to be constructed across tibet this cross section shows that crustal thickness tends to decrease from south to north with values of 70 80 km south of the middle of the lhasa terrane 60 70 km in the northern part of the lhasa terrane and the qiangtang terrane and less than 60 km in the qaidam basin the overall northward thinning of the crust evident in the combined seismic observations coupled with the essentially uniform surface elevation of the plateau south of the qaidam basin is supportive of the inference that northern tibet until the qaidam basin is underlain by somewhat thinner crust which is isostatically supported by relatively low density hot upper mantle with respect to southern tibet

crustal lithospheric structure and continental extrusion of tibet crustal shortening and thickening to c 70 85 km in the tibetan plateau occurred both before and mainly after the c 50 ma india asia collision potassic ultrapotassic shoshonitic and adakitic lavas erupted across the qiangtang c 50 29 ma and lhasa blocks c 30 10 ma indicate a hot mantle thick crust and eclogitic root during that period the progressive northward underthrusting of cold indian mantle lithosphere since collision shut off the source in the lhasa block at c 10 ma late miocene pleistocene shoshonitic volcanic rocks in northern tibet require hot mantle we review the major tectonic processes proposed for tibet including rigid block continuum and crustal flow as well as the geological history of the major strike slip faults we examine controversies concerning the cumulative geological offsets and the discrepancies between geological quaternary and geodetic slip rates low present day slip rates measured from global positioning system and insar along the karakoram and altyn tagh faults in addition to slow long term geological rates can only account for limited eastward extrusion of tibet since mid miocene time we conclude that despite being prominent geomorphological features sometimes with wide mylonite zones the faults cut earlier formed metamorphic and igneous rocks and show limited offsets concentrated strain at the surface is dissipated deeper into wide ductile shear zones

cambrian to triassic geodynamic evolution of central qiangtang tibet abstract central qiangtang is a tectonic melange between the northern qiangtang terrane nqt and the southern qiangtang terrane sqt and it preserves a complete cambrian to triassic geological record related to tethyan evolution e g high pressure metamorphic rocks and ophiolites thus this region is the key to understanding the geodynamic processes that occurred during the dispersion of gondwana and the accretion of laurussia given previous studies on sedimentary paleomagnetic and seismic records this paper presents an integrated interpretation of ophiolites magmatism metamorphism and tectonism and reconstructs the cambrian triassic tectonic evolution of central qiangtang as described below 1 following the neoproterozoic early cambrian pan african orogeny and associated postcollisional extension the southward subduction of the proto tethys oceanic plate produced andean type magmatism along the northern margin of gondwana a back arc basin between the nqt and sqt further opened during the late cambrian early ordovician accompanying with trench retreat and overriding plate extension resulting from slab rollback 2 with the middle ordovician late silurian successive incorporation of micro continental terranes e g qaidam kunlun and tarim terranes into the northern margin of gondwana the back arc basin closed through subduction polarity reversal from southward to northward 3 during the late devonian early carboniferous the southward subduction of an oceanic plate to the north of the nqt separated the nqt and sqt through back arc extension consequently the nqt drifted northward rapidly and the back arc basin gradually evolved into a mature ocean basin i e longmu co shuanghu paleo tethys ocean lspto 4 subsequently a late carboniferous early permian continent continent collision between the nqt and the south china block scb resulted in the northward subduction of the lspto and the ongoing subduction probably induced the opening of a back arc basin between the nqt and the scb during the late permian 5 finally diachronous collision between the nqt and sqt closed the lspto during the early middle triassic and the termination of the collisional orogenic event was evidenced by strong late triassic magmatism related to postcollisional extension in the qiangtang block in this paper we propose that the opening and closure of back arc basins controlled the cambrian triassic tectonic evolution of central qiangtang which provides significant insights into the dispersion of gondwana and the accretion of laurussia

detection of the deep crustal structure of the qiangtang terrane using magnetotelluric imaging abstract to determine the deep electrical structure of the qiangtang terrane northern tibetan plateau we reanalysed three broadband magnetotelluric mt profiles collected by china university of geosciences wuhan in 1993 1994 and derived corresponding 2 d resistivity models in these models a remarkable high conductivity layer that was divided into northern and southern parts and that had formed an antiformal zone of high conductivity beneath the central qiangtang terrane was visible this high conductivity layer corresponds well with the trace of the southward subduction of the melange from the jinsha river suture inferred from surface geology according to our resistivity models the crust of the northern qiangtang terrane is mostly composed of melange the underplated melange may be closely related to the formation of the high conductivity layer and provide a reasonable explanation for the cause of special geophysical features in this area in addition the similarity between these resistivity models provides evidence that the qiangtang anticlinorium extends eastward to at least 92 e whereas the differences between them may offer an explanation for the gradual narrowing of the metamorphic belt from west to east in the central qiangtang terrane

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

moho undulations beneath tibet from grace integrated gravity data summary knowledge of the variation of crustal thickness is essential in many applications such as forward dynamic modelling numerical heat flow calculations seismologic applications and geohistory reconstructions we present a 3 d model of the moho undulations over the entire tibetan plateau derived from gravity inversion the gravity field has been obtained by using the gravity recovery and climate experiment grace potential field development which has been integrated with terrestrial data and is presently the best available in the studied area for the effective use of the global geopotential model that has no height information of observation stations upward continuation is applied the moho model is characterized by a sequence of troughs and ridges with a semi regular pattern which could reflect the continent continent collision between the indian and eurasian plates the three deep moho belts troughs and shallow moho belts ridges between them are clearly found to have an e w directional trend parallel to the border of the plateau and tectonic lines while variation of the directionality is observed in central to southeast tibet to describe the distinctive shape of the moho troughs beneath tibet we introduce the term moho ranges the most interesting aspects of the moho ranges are 1 that they run in parallel with the border and tectonic sutures of the plateau 2 that the distances between ranges are found at regular distances of about 330 km except in northeast tibet and 3 that the splitting of the ranges into two branches is found as the distance between them is increasing from our study we conclude that the distinctive undulations of the tibetan moho have been formed by buckling in a compressional environment superimposed on the regional increase in crustal thickness according to our analysis the grace satelliteonly data turns out to have good enough resolution for being used to determine the very deep moho beneath tibet our moho model is the first one that covers the entire plateau

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

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

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

seismicity of central tibet from project indepth iii seismic recordings as part of project international deep profiling of tibet and the himalaya iii a 400 km long densely spaced array of 57 broadband and short period seismic stations was deployed in central tibet from august 1998 through may 1999 although originally designed to image the lithosphere with teleseismic events the array also recorded numerous local and regional seismic events more than 900 local and regional events were detected on at least 10 stations during the 1 year deployment and we were able to locate 267 local earthquakes a substantial number of the events were found to cluster in or near large grabens and along known strike slip faults while other events show no obvious correlation with known structures in addition to spatial clustering at least one of the large clusters also exhibits temporal clustering that may be associated with magmatic or geothermal activity in the upper crust the average v p and v s are estimated to be 5 85 and 3 35 km sec for the upper crust and 7 0 and 3 9 km sec for the lower crust respectively the 50 focal mechanisms computed from this set of events are consistent with north south shortening and east west extension there are no clear indications of significant local perturbations in the regional stress field induced by the collision between india and eurasia the majority of the focal mechanisms indicate normal and strike slip faulting at least six of the newly computed focal mechanisms however indicate thrust faulting which is a phenomenon not well documented previously ninety nine percent of the local earthquakes have focal depths shallower than 25 km and the locations of the few deeper events are poorly constrained the shallow earthquake focal depths are consistent with high temperatures and proposed ductile or aseismic behavior in the middle to lower crust of central tibet figure 6 1 figure 6 a earthquake locations computed from this study shown on shaded topographic data triangles are indepth iii seismic stations circles are events and are scaled by magnitude graphic 2 a color version of this figure is available online at the ssa web site b earthquake locations shown with major geologic structures from taylor et al 2003 3 reproduced with permission m h taylor personal comm 2002 triangles are indepth iii seismic stations geologic structures are as in figure 2 4 1 pending yes 2 f1 embed inline graphic 1 gif 3 ref 46 4 f3

influence of mantle dynamics on the topographic evolution of the tibetan plateau results from numerical modeling 1 we investigate numerically the evolution of crustal and lithospheric thickness thermal structure topography and strain rate of the tibetan plateau through time using the thin viscous sheet approach we show that lithospheric mantle must have been removed from beneath tibet to explain the present surface elevation and lack of regional surface slope in the absence of this removal the modeled topography reaches a maximum elevation of 4000 m for weak rheology or the surface slopes significantly northward for strong rheology the crust must have been warmed and weakened by an increase of radiogenic heat production at depth due to crustal thickening in the absence of this warming viscous stresses associated with plate convergence exceed stresses produced by topography and the present pattern of vertical thinning and east west extension would not have developed continuous removal of lithosphere by delamination or convection does not allow sufficient crustal warming and fails to reproduce either the present topography or the pattern of active deformation on the plateau in a reasonable time geologically rapid removal of the lithospheric root beneath the thickened crust of tibet successfully explains the current elevation of the plateau its lack of surface slope the steep south and north margins and the pattern of the present deformation including vertical thinning e w extension and extrusion and vertical axis rotation on the eastern margin our modeling suggests that this removal took place within the last 12 m y

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

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

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

crustal thickening and uplift of the tibetan plateau inferred from receiver function analysis the india eurasia collision crustal thickening and northward convergence leading to uplift of the tibetan plateau have been topics of multidisciplinary studies here we employ the h k stacking technique and depth domain receiver function to constrain the crustal thickness and p and s wave velocity ratio as well as the upper mantle discontinuity our results identify the signature of slab break off and lower crustal delamination in southern tibet which might have induced the mantle upwelling and crustal melting the convergence of the indian plate inducing northward crustal flow might be a major factor that contributes to the uplift and crustal thickening of tibet we also evaluate the e w trending cumulative compression as another dominant factor that contributes to the increase in crustal thickness in the northern tibet 2015 elsevier ltd all rights reserved

lateral variations of crustal seismic attenuation along the indepth profiles in tibet from lg q inversion 1 fourier spectra of crustal guided lg waves are collected to study seismic attenuation or q along the indepth profiles the 1 hz lg q q0 values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying q0 models along the indepth iii profile in central tibet the q0 model consists of low and nearly constant values of 90 these low values are consistent with abnormally high temperature as well as partial melts in the crust along the indepth ii profile in southern tibet the q0 values increase southward they are extremely low 60 over the northernmost segment but increase by a factor of 2 over a distance of 100 km between the indus yalong suture and the kangmar dome farther south and into the high himalayas q0 values are higher than 300 regional rayleigh waves observed along the northern indepth ii profile are used to infer a low velocity low q layer at midcrustal depths between 15 and 30 km in southern tibet the aqueous fluid trapped in the upper crust and a midcrust partial melting zone associated with the underthrusting of the indian lithosphere are the likely causes of the high lg attenuation in southern tibet low q rather then a systematic lg conversion along a dipping moho is the main cause of the previously observed lg blockage over paths crossing the southern tibetan boundary

petrogenesis of the majiari ophiolite western tibet china implications for intra oceanic subduction in the bangong nujiang tethys abstract the majiari ophiolite lies in the western bangong nujiang suture zone which separates the qiangtang and lhasa blocks in central tibet the ophiolite consists of peridotite gabbro diabase and basalt zircon u pb dating yielded an age of 170 5 1 7 ma for the gabbro whereas 40 ar 39 ar dating of plagioclase from the same gabbro yielded ages of 108 4 2 6 ma plateau age and 112 2 ma isochron age indicating that the ophiolite was formed during the middle jurassic and was probably emplaced during the early cretaceous zircons from the gabbro have e hf t values ranging from 6 9 to 10 6 and f lu hf values ranging from 0 92 to 0 98 mafic lavas plot in the tholeiitic basalt field but are depleted in nb ta and ti and enriched in rb ba and th in the n morb normalized trace element spider diagram these lavas have whole rock e nd t values of 5 9 to 6 6 suggesting that they were derived from a depleted mantle source which was probably modified by subducted materials the majiari ophiolite probably formed in a typical back arc basin above a supra subduction zone ssz mantle wedge intra oceanic subduction occurred during the middle jurassic and collision of the lhasa and south qiangtang terranes likely occurred in the early cretaceous thus closure of the bangong nujiang tethys ocean likely occurred before the early cretaceous

undulating moho beneath a near uniform surface of central tibet abstract we present 805 reliable estimates of crustal thickness in central tibet revealing an undulating moho beneath a near uniform surface of the highest and the largest plateau in the world our results are derived from wide angle reflections of p waves from virtual seismic sources near seismographs virtual deep seismic sounding vdss the virtual sources are produced by large earthquakes well distributed in back azimuth and we also utilize obvious changes of measured arrival times as a function of ray parameter to constrain independently bulk thickness and p wave speed of the crust about 6 3 km s so there is little trade off between the two parameters additionally we consider effects of known heterogeneities in the upper mantle in regions surrounding the hi climb linear array there is an eastward trend in addition to the previously proposed northward trend of crustal thinning respectively these trends are approximately parallel and normal to the indian mantle front imf or the northern leading edge of underthrust lithospheric mantle of india deduced from a collection of additional geophysical evidence the moho near the imf also shows large 3 d variations in depths prompting us to propose that the leading edge of intact indian lower crust or the indian crustal front icf that accompanied the underthrusting indian lithospheric mantle beneath tibet lies along the zone of disturbed moho based on results from about a dozen permanent broadband seismographs of the china national seismic network more or less flat lying and thus presumably stable cratonic indian moho south of the icf apparently extends eastward for another 500 km to about 93 e

geophysical constraints on the link between cratonization and orogeny evidence from the tibetan plateau and the north china craton abstract understanding the geodynamic process of orogeny and cratonization and their transition is among the key topics of research in evaluating the link between plate tectonics and continental dynamics the tibetan plateau and the north china craton ncc two key areas in mainland china offer excellent laboratories to understand continental tectonics over a broad span of earth history particularly the deep structure of the lithosphere as imaged from geophysical data on the tibetan plateau and the ncc provide important clues in understanding orogeny and cratonization the tibetan plateau is the largest and highest plateau on earth in terms of mean altitude and is an important region for understanding the mechanisms of continent continent collision and cenozoic plateau uplift the ncc is an archean craton that underwent lithospheric disruption during the mesozoic here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography and gravity modeling in tibet and its neighboring regions in order to understand the modality of the convergence and collision process between the indian and eurasian plates and the influence of this process on the uplift of the plateau in the ncc geological geochemical geophysical and tectonic investigations demonstrate that lithospheric destruction mainly occurred in the eastern block the crustal structure of the ncc is reconstructed from ambient noise surface wave tomography and the different possible disruption mechanisms are evaluated the vs shear wave velocity tomography results and the density structure of the crust and upper mantle to about 350 km depth 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 which suggest that the leading edge of the subducting indian slab reaches the bns the subduction angle of indian plate indicates a transition from steep to shallow from the west to east tibet sections depicting the gravitational potential energy suggest that mantle flow contributes to the subduction of the indian plate as far as the bns and the transition from the asthenospheric layer to the metasomatic lid overlaps with the transition from north south shortening in south tibet to eastward tectonic escape in north tibet qiangtang and songpan ganzi blocks both vs and models suggest the following 1 north southward lower crust flow beneath the eastern ncc and interaction between the westward mantle flow and eastward escape flow beneath the central ncc in addition to the earlier proposed mechanisms of delamination and thermal erosion played important roles in the lithospheric disruption of the archean craton 2 mantle flow plays an important role in the continental tectonic transition between neighboring tectonic blocks and within the cycle between orogeny and cratonization

submarine volcanic activity ocean acoustic waves and internal ocean tides 1 submarine volcanic event often generates acoustic waves t waves traveling over long distances through the low velocity channel sofar of the ocean by a method of coherent stacking of t waves from a submarine volcanic activity in northern mariana we found a significant semidiurnal variation of t wave travel times the amplitude of variation is an order of larger than those reported in the previous ocean sound transmission experiments ray theoretical consideration for the numerically simulated ocean tides indicates that such large t phase travel time variation is a consequence of large up and down movement of seawater around the axis of the sofar channel due to the m2 internal tide effectively converted from external tidal forcing t phases a ubiquitous feature of the ocean acoustic noise field can be used to infer internal tidal motion and the associated ocean mixing

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

lateral structure variations and transient swarm revealed by seismicity along the main himalayan thrust north of kathmandu abstract the midcrustal seismicity along the main himalayan thrust in nepal presents lateral variations along the rupture of the 2015 gorkha earthquake in order to resolve these variations we relocate the seismicity north of kathmandu during a period well covered by the nepal national seismological network using a double difference algorithm the 550 relocated events highlight a complex pattern of clustered seismicity within the unstable stable transition zone part of the seismicity is densely clustered on a southward dipping plane which ruptured on january 31st 1997 ml 5 8 activating a backthrust with a geometry consistent with the centroid moment tensor of this event calculated in this study at its eastern end the midcrustal cluster is offset by 20 km to the south suggesting the presence of a tear fault the analysis of the time sequence allows constraining a scenario involving stress transfer between these local midcrustal structures beginning more than one month before the 1997 main shock the temporal evolution of the seismicity is strikingly similar for two other transient seismic swarm episodes which developed hundreds of kilometers apart along the main himalayan thrust at the same time the local stress field appears responsible for the higher sensitivity of these regions to subtle strain transients developing along the main himalayan thrust

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

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

the upper crustal structure of the qiangtang basin revealed by seismic reflection data abstract the qiangtang basin located in central tibet is a thick and widely developed jurassic marine sedimentary stratum and it is the largest marine basin on the chinese mainland without a breakthrough in oil or gas exploration various forms of crustal movement related to the convergence between the eurasian and indian plates in the cenozoic have played significant roles in the formation and preservation of the oil and gas resources of the qiangtang basin to determine the shallow crustal structure of the qiangtang basin and forecast its prospects for oil and gas extraction we reprocessed the seismic reflection data 0 6 s twt from 11 reflection sections acquired at different times by different groups and connected them to form a 350 km shallow seismic reflection profile across the qiangtang basin this profile provides reliable data on the north to south changes of the basement and the upper crustal structures of the qiangtang basin we speculated that the reflective events at 3 4 s twt in the qiangtang basin represent the paleozoic basement which runs shallow beneath the central anticline the location of the proterozoic basement was determined from the discontinuous reflection events at approximately 4 5 s twt the data indicate that the basements of the qiangtang basin are deeper in the south and shallower in the north the shallow crustal deformations approximately 0 3 s twt are quite different between the north and south qiangtang basin in the north qiangtang basin there are strong fold deformations alternating between uplifts and depressions while the deformations are relatively flat in the south qiangtang basin continuous arc reflections interpreted as paleozoic strata were found beneath the central anticline of the qiangtang block a half graben on the north side of the central anticline represents a possible location for oil and gas resources

continental scale body wave tomography of india evidence for attrition and preservation of lithospheric roots we assemble p and s waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the indian plate from the southern tip of india to the himalayan collision belt and generate an accurate data set of 52 050 p and 30 423 s arrival times through the multichannel cross correlation approach these traveltimes are then inverted to obtain 3 d p and s velocity structures of the subcontinent at a 2 2 lateral resolution the heterogeneous nature of the indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale the key cratonic segments of the indian shield are characterized by pockets of high velocity anomalies 3 at shallow depths 300 km with the diamondiferous regions like wajrakarur revealing high shear wave anomalies down to 300 km in contrast to the southern deccan volcanic province dvp the northwestern dvp is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of deccan volcanism which was facilitated by the reactivation of the rift systems

early mesozoic deep crust reworking beneath the central lhasa terrane south tibet evidence from intermediate gneiss xenoliths in granites abstract understanding the rheological behavior of the tibetan plateau and its response to geodynamic processes requires a clear knowledge of the composition evolution and lithological properties of the deep crust here we present u pb hf isotopes of zircons bulk rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host early mesozoic granites 205 6 ma in the central lhasa terrane to probe the deep crust beneath southern tibet the xenoliths contain plagioclase amphibole titanite allanite quartz biotite and muscovite with accessory fe ti oxides apatite and zircon bulk rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite facies metamorphism equilibration at pressures of 0 46 0 83 gpa and temperatures of 650 c before they were captured by the host granite at 205 ma zircons in these xenoliths show complex microstructures including inherited cores magmatic or metamorphic bands and high u th hydrothermal rims inherited zircon cores record u pb ages from 2277 ma to 517 ma igneous zircons show a range of concordant u pb ages suggesting a protracted magmatism from 236 ma to 203 ma metamorphic zircon zones record the timing of amphibolite facies metamorphism from 224 to 192 ma while the high u th hydrothermal rims show a subsequent fluid activity until 150 ma unradiogenic hf isotopic compositions of both xenoliths and host granites xenolith e hf t 11 2 to 0 host granite e hf t 17 3 to 3 3 indicate that the early mesozoic deep crust in the central lhasa terrane originated mainly from ancient i e proterozoic crust with little or no interaction with juvenile magmas this study suggests a possible continental differentiation mechanism during crustal reworking progressive melting may initiate from the lower mafic crust at ca 236 ma and gradually migrate into the sediment rich upper crust until ca 203 ma the reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous s type granite in a convergent depth

the moho structure beneath the yarlung zangbo suture and its implications evidence from large dynamite shots abstract the yarlung zangbo suture yzs is the collisional front between the indian and eurasian plates the deep structure beneath the yzs has long been a focus of research its particular tectonic location and formation time provide a natural laboratory for the study of ongoing continental collisional processes lateral variations in the moho discontinuity provide evidence of basic processes governing plate tectonic evolution the depth and geometry of the moho provides first order information for restoration of complex geodynamic systems this paper presents a studying result of the moho both adjacent to and beneath the yzs obtained using five large dynamite shots along two profiles these five large shots were combined to construct two single fold profiles yzs a and yzs b and obtain high resolution images of the moho beneath the yzs both profiles show that continental crust thickness beneath the yzs is twice the global average of continental crust and the depth of the moho along profile yzs b is deeper than yzs a yzs a 81 e indicates a relatively flat moho without offset related to continent continent convergence beneath the collision zone conversely a moho structure with offset and overlapping reflections was imaged beneath profile yzs b 88 e significant lateral variations in the geometry of the moho discontinuity along the yzs may indicate that the indian lithosphere is thrusting beneath tibet at a different angle from west to east

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

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

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

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet comment and reply comment kapp et al 2000 1 suggested that the qiangtang metamorphic belt consists of underplated blueschist bearing melange and was exhumed by low angle normal faults and that both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the

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

late cenozoic right lateral strike slip faulting in southern tibet field observations collected during the sino french himalaya tibet cooperation project 1980 1982 show that right lateral faulting between southern and northern tibet roughly along the chord joining the eastern and western syntaxes of the himalayan arc is a major component of the active tectonics north of the himalayas just north of the well developed rifts of southern tibet we studied three right lateral 100 200 km long strike slip faults forming an en echelon array along that chord evidence for earthquake surface breaks was found on each of these faults for the m 8 beng co or damxung strike slip earthquake of november 18 1951 rupture parameters could be constrained to be 8 2 m and l 90 km offset glacial and postglacial morphological markers 105 years and 104 years b p imply displacement rates of the order of 1 2 cm yr on the three faults the en echelon fault zone appears to extend westward 2000 km across the plateau to the karakorum fault and is thus referred to as the karakorum jiali fault zone kjfz fast rates of slip on it and on faults along the northern rim of tibet appear to contrast with the slower rates of deformations in the interior of the plateau our estimates of slip rates on the strike slip and normal faults of southeastern tibet suggest that eastward extrusion of north tibet between the right lateral kjfz and the left lateral altyn tagh and kun lun faults absorbs at least 30 of the present convergence between india and asia we propose an instantaneous velocity model of the active tectonics of asia compatible with such faulting rates and geometries evidence for tertiary right lateral strike slip movements in southern tibet and around the eastern syntaxis of the himalayas is also found in the structural record offsets of various large scale geological features imply that these movements have been large

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

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

petrogenesis and tectonic implications of late cretaceous highly fractionated i type granites from the qiangtang block central tibet abstract the tectonic evolution of the lhasa and qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central tibet in this study we present zircon u pb ages whole rock major and trace elements and sr nd pb isotopic data for the newly discovered chuburi pluton in the southern qiangtang block zircon u pb data reveal that the chuburi pluton was emplaced during the late cretaceous ca 73 ma bulk rock analyses show that this intrusion is characterized by high sio2 na2o and k2o but low mgo cao and p2o5 the granites are enriched with large ion lithophile elements and light rare earth elements with marked eu anomalies and there are few heavy rare earth elements and high field strength elements these geochemical features indicate that the chuburi pluton is a highly fractionated i type granite the nine granitic samples show high 87sr 86sr i ratios 0 7064 0 7089 negative end t values 1 94 to 5 59 and have nd isotopic model ages ranging from 0 76 to 0 97 ga the initial pb isotopic ratios of the chuburi pluton is uniform 206pb 204pb t 18 774 18 884 207pb 204pb t 15 695 15 713 208pb 204pb t 39 362 39 613 based on geochemical characteristics of the chuburi pluton and consideration of the region geological setting we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust and then undergoing extensive fractional crystallization with removal of k feldspar plagioclase mafic mineral and minor accessory mineral the crust beneath the central tibet may have been significantly thickened following the final lhasa qiangtang amalgamation we argue the generation of the chuburi pluton can be interpreted as a result of regional lithospheric delamination

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

crustal structure of northern and southern tibet from surface wave dispersion analysis 1 group and phase velocities of fundamental mode rayleigh waves in the period range of 10 to 70 s are obtained for southern and northern tibet significant variations in crustal velocity structure are found the group velocity minimum for tibet occurs at 33 s and the minimum is 0 12 km s lower for southern tibet than for northern tibet at periods greater than 50 s however group velocities are up to 0 2 km s faster in southern tibet the group and phase velocities are inverted for layered s wave models the dispersion observations in southern tibet can only be fit with a low velocity layer in the middle crust in contrast the velocity models for northern tibet do not require any lowvelocity zone in the crust the s wave velocity of the lower crust of southern tibet is 0 2 km s faster than the lower crust of northern tibet in southern tibet the sub moho velocity increases with a positive gradient that is similar to a shield while there is no velocity gradient beneath northern tibet the high velocity lower crust of southern tibet is consistent with the underthrusting of indian continental lithosphere the most plausible explanation of the mid crustal low velocity zone is the presence of crustal melt resulting from h2o saturated melting of the interplate shear zone between the underthrusting indian crust and overflowing asian crust the lack of a pronounced crustal low velocity zone in northern tibet is an indication of a relatively dry crust the low s wave velocity in the lower crust of northern tibet is interpreted to be due to a combination of compositional differences high temperatures presumably caused by a high mantle heat flux and possibly small amounts of partial melt combined with all available observations in tibet the new surface wave results are consistent with a hot and weak upper mantle beneath northern tibet index terms 7205 seismology continental crust 1242 7218 seismology lithosphere and upper mantle 7255 seismology surface waves and free oscillations 8102 tectonophysics continental contractional orogenic belts keywords tibet crustal velocity structure surface wave rayleigh waves continental collision

origin and pre cenozoic evolution of the south qiangtang basement central tibet abstract the qiangtang terrane in central tibet with its high pressure rocks is a key area to unravel the evolution of the paleo tethys we present a new 1 50 000 map of the rongma area in the central qiangtang terrane of which relatively little is known so far we show that the qiangtang metamorphic belt can be separated in a paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction melange that contains high pressure rocks and permian sediments detrital zircon ages youngest peak at 591 ma and an 470 ma granite intrusion age constrain the age of the qiangtang basement to be between the late precambrian to middle ordovician this is in agreement with the observed unconformity between basement and overlying mid late ordovician strata this unconformity and the zircon age spectra are comparable to the himalaya area the qiangtang terrane must thus have been part of the gondwana supercontinent during the early paleozoic the occurrence of the late triassic eclogite and glaucophane bearing schists in the central qiangtang terrane indicates the existence of a suture zone between the north and south qiangtang terranes before the late triassic this suture zone resulted from closure of the paleo tethys between the north and south qiangtang terranes late triassic syn collisional granites and rapidly exhumed high pressure rocks resulted from the closure of this ocean and final amalgamation of the re united qiangtang terranes with eurasia collision of the lhasa and qiangtang terranes is correlated with north dipping reverse faults in the south qiangtang terrane which were probably related to exhumation of the qiangtang basement and formation of the qiangtang culmination

mapping the moho beneath the southern alps continent continent collision new zealand using wide angle reflections 1 a wide angle stack using all onshore offshore and onshore seismic data from the sight 96 experiment provides a direct three dimensional 3 d image of the moho below the continental collision zone through the south island of new zealand a bright reflector sequence up to 4 s thick the base of which coincides with the pmp modeled moho extends throughout most of the lower crust and bends downward asymmetrically beneath the southern alps the crustal root reaches a maximum depth at 15 s 45 km beneath the regional 80 mgal bouguer gravity low and is flanked east and west by shallower moho where the average crustal thickness is 20 km the 3 d structure of the crustal root in the central south island is moderately well resolved by the sight 96 experiment

insar measurement of the deformation around siling co lake inferences on the lower crust viscosity in central tibet the siling co lake is the largest endorheic lake in central tibet altimetric measures combined with lake contours show that in 1972 1999 its water level remained stable while it increased by about 1 0 m yr in the period 2000 2006 the increased rate gradually stepped down to 0 2 m yr in 2007 2011 the ground motion associated with the water load increase is studied by interferometric synthetic aperture radar insar using 107 ers and envisat sar images during the period 1992 2011 the deformation amplitude closely follows the lake level temporal evolution except that subsidence continues in 2008 2011 while the lake level stagnated this temporal evolution suggests a non elastic relaxation process taking place at a decade timescale phase delay maps are used to constrain possible layered viscoelastic rheological models an elastic model could partly explain the observed subsidence rate if elastic moduli are about twice lower than those extracted from vp vs profiles the surface deformation pattern is also extracted by projecting the phase delay maps against the best fit model temporal behavior it shows that deep relaxation in the asthenosphere is negligible at the decade timescale and favors the existence of a ductile channel in the deep crust above a more rigid mantle overall the best fit model includes a ductile lower crust with a viscosity of 1 3 1018 pa s between 25 and 35 km and the moho at 65 km overlying a rigid mantle

seismic structure and evidence for eclogitization during the himalayan convergence abstract reflections on the deep moho of the tethyan himalayas have been obtained from vertical seismics at the time predicted by wide angle reflection interpretations supporting the previous inference of a low average crustal velocity a mostly felsic composition is then required through the 70 km of the thickened crust the moho appears as a heterogenous transition layer more than 10 km thick the high velocity measured beneath the moho from pn refractions cannot be attributed any more to e w oriented olivine since sks observations do not confirm such an anisotropy instead eclogite just under the seismic moho could account for the observations these three results from independent parts of the seismic data set support a model of the himalayan convergence in which the indian crust has undergone phase transformation in the granulite eclogite system upon burial the intermediate and basic composition rocks of the middle and lower layers of an initial crust of average platform type can be transformed to such high velocities that they are lost across the himalayas into the level of the moho or just under the moho defined by seismic velocity mostly upper crustal type material then remains to form the thickened crust above consistent with the observed low velocity the variation in moho depth across the himalayas as suggested from wide angle seismics accounts for the first order variation in teleseismic transit times variations in the velocity depth structure of the mantle hence do not dominate but a change in the orientation of sks splitting anisotropy would suggest deformation within the lithosphere to change across the high himalayas and the indus tsangpo suture

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

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

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

seismic attenuation characteristics along the hi climb profile in tibet from lg q inversion we investigate the q of crustal lg waves along the himalayan tibetan continental lithosphere during mountain building hi climb profile in tibet using a reliable two station method four earthquakes of m 5 5 recorded along a 800 km long densely spaced seismic profile are used to extract lg wave spectra one hundred seven high quality interstation paths that allow us to use the standard two station method for lg q measurement are selected from 2369 possible pairs by using those 107 interstation q0 1 hz lg q values as input lateral variations in attenuation char acteristics are derived through inversion the region has q0 estimates ranging from 88 5 to 165 15 with the lowest value in the qiangtang terrane and the highest across some part of the lower himalaya the lowest value is attributed to the high poisson s ratio higher temperatures as well as partial melts in the crust the results do not contain any detectable variation in q0 either across the himalaya tibet collision zone i e the indus tsangpo suture zone or along the major bangong nujiang suture zone bnsz consistent low q0 values along the hi climb profile manifest the presence of omnipresent partial melting of the crust throughout the plateau

development of spit lagoon complexes in response to little ice age rapid sea level changes in the central guilan coast south caspian sea iran the central guilan coast along the iranian caspian coastline is characterized by sandy beaches and the development of spit lagoon complexes which are prone to preserve past sea level fluctuations the morphology of three spit lagoon complexes along the central guilan coast was studied using ground penetrating radar gpr and sediment sequences to understand the effects of past sea level changes on spit lagoon development the results showed the prominent role of coastal setting in conditioning the development of spit lagoon formation in response to sea level change when the caspian sea experienced a highstand in the little ice age the coast of central guilan recorded fluctuations in sedimentation which are reflected for example by river avulsion and beach ridge formation depending on physical setting in the western half of the central guilan eastward longshore currents and strong wave action on a w e coastline coupled with sea level changes shaped the anzali spit lagoon complex while in the eastern part of the studied area river avulsion and changing the coastline orientation are responsible for development of the amirkola and kiashahr spit lagoon complexes under the same sea level fluctuations although sea level change has a major role in spit lagoon development an increase in the frequency of storms changes in sediment supply due to more precipitation and river avulsion are other players in spit lagoon development in the central guilan during the little ice age and more recent times

isostatic gravity anomaly lithospheric scale density structure of the northern tibetan plateau and geodynamic causes for potassic lava eruption in neogene abstract this paper will help to further constrain geodynamic models for the evolution of northern tibet through two techniques that employ bouguer gravity anomaly data the first is isostatic correction of bouguer gravity anomalies and the second is 2 d density modeling along a profile at 88 5 e from 30 n to 37 n within the tibetan plateau areas dominated by volcanic rocks exhibit ultra low bouguer gravity anomalies and low isostatic residual anomalies the 2 d density structure beneath profile 88 5 e shows a giant upward dome distributed body of low density in the middle lower crust and mantle beneath the high to ultra potassic areas suggesting that the original middle and lower crusts were reformed and replaced extensively in the area based on existing regional geology and tectonics geochemical data from magmatic rocks and geophysical information this paper proposes that the sustained northward subduction of the indian plate has resulted in breaking off of the frontal margin of the asian lithospheric mantle subsequently the original lithosphere was reactivated by thermal tectonic changes high to ultra potassic volcanoes erupted extensively in the northern tibetan plateau during neogene and the plateau experienced rapid ne trending uplift

an eocene magmatic belt across central tibet mantle subduction triggered by the indian collision mountain domes rising to 6600 m along tibet s tanggula range herald the eocene intrusion of calcalkaline granites into terranes accreted much earlier together with coeval cogenetic volcanics such intrusives which have similar crystallization and cooling ages may be part of an ese trending belt cutting central tibet in half this magmatic belt may have marked a former northern boundary of the plateau testifying to s directed subduction of the asian mantle such subduction would have developed soon after india s collision but long before the rise of the plateau s present rims along one of central tibet s mesozoic sutures

tibetan satellite magnetic low evidence for widespread melt in the tibetan crust the tibetan plateau is associated with a pronounced satellite magnetic low forward modeling of this anomaly demonstrates that the crust underlying the tibetan plateau is weakly magnetic compared to the crust on either side and that the boundaries of the weakly magnetic region coincide closely with the topographic edges of the plateau himalaya kun lun because there are no obvious changes in the bulk magnetization of the crust associated with the principal terrane boundaries within the plateau banggong and yarlung zangbo sutures the low likely manifests hot crust beneath tibet forward modeling of the magnetic anomaly with simple assumptions suggests that the curie isotherm is likely to reside in the upper crust across the tibetan plateau 15 km depth implying that granite minimum melt temperature 600 650 c is also achieved in the upper crust across the plateau 16 18 km depth this inference is consistent with the shallow depth extent of earthquakes in tibet and with the recent suggestion that melt may be areally widespread within the tibetan crust

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