penetration of mid crustal low velocity zone across the kunlun fault in the ne tibetan plateau revealed by ambient noise tomography abstract the ne tibetan plateau composed of the mesozoic accretions of lhasa qiangtang and songpan ganze terranes are bounded by the east kunlun qaidam block in the north with the boundary delineated by the kunlun fault the ne tibetan plateau is at a nascent stage of plateau growth resulting from the collision between the indian and eurasian plate starting 50 million years ago and is one of the best areas to study the growth mechanism of the tibetan plateau in this study we process continuous ambient noise data collected from 280 stations during 2007 and 2010 and generate rayleigh wave phase velocity maps at 10 60 s periods with a lateral resolution of 30 50 km for most of the study region by adopting a bayesian monte carlo method we then construct a 3 d vsv model of the crust using the rayleigh wave dispersion maps our 3 d model reveals that strong lvzs exist in the middle crust across the ne tibetan plateau and the lateral distribution of lvzs exhibit significant west east variations along the kunlun fault in the west of 98 e lvzs are confined to regions of the kunlun fault and the eastern kunlun ranges but absent beneath the qaidam basin while in the east of 98 e lvzs extend and penetrate northward into the east kunlun and qinling orogens over 100 km across the kunlun fault the strong contrast of the lvzs distribution along the kunlun fault may be related to the distinct neighboring tectonic units in the north a strong crust of the qaidam basin in the west blocking the penetration of lvzs but a weak crust in the qinling orogens facilitating the extrusion of lvzs the distribution of lvzs in the ne tibetan plateau is consistent with the crustal channel flow model which predicts a branch of north eastward mid crustal channel flow our 3d model clearly delineates the north extent of the mid crustal lvzs probably reflecting the status of channel flow in the ne tibetan plateau

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

lithospheric thermal structure and rheology of the eastern china abstract temperature distributions of the lithosphere beneath the eastern china were estimated by local isostasy equilibrium constrained geothermal calculation in this study maps of the lateral temperature variation at depths of 50 and 100 km as well as moho boundary are presented for the eastern china continent and the 600 c and 1100 c isotherm depth maps are also given meanwhile the thermal thickness of lithosphere is calculated as the depth of 1350 c isotherm the rheological strength of lithosphere is calculated based on a four layer model which consists of the upper lower lowermost crust layers and a wet lithospheric mantle layer the results show that the lithosphere of 160 180 km thick exists under sichuan basin in the western part of yangtze craton but keel is absent beneath the entire north china craton it is notable that the lithospheric thickness is only 130 140 km beneath ordos basin in the western part of north china craton accordingly there is no lithospheric keel beneath ordos basin these geothermal modeling results are in good agreement with those of seismic tomography and other geothermal studies the temperatures in the lower crust of the north china craton estimated are in range of 500 600 c consequently the composition of lower crust of the north china craton should be more mafic than that of previous estimation based on much cooler geotherms the spatial variation of lithospheric rheology in the eastern china is influenced by local geotherms and crustal compositions the creme brulee model which is represent by a strong crust portion but a weak lithospheric mantle portion in vertical strength profile approximates the lithospheric rheological layering for the regions of the western half of north china craton and the northern part of ne china which has thicker crust or moderate to high geotherm on the other hand the jelly sandwich model demonstrates the rheological layering of the western part of the yangtze craton and the eastern half of the north china craton which has low to moderate geotherm and or thinner crust in general thermal age of the tectonic province controls the present day thermal structure and rheology of the lithosphere of the eastern china

crustal structure across the ne tibetan plateau and ordos block from the joint inversion of receiver functions and rayleigh wave dispersions abstract we investigated the crustal structure at 34 stations using the h stacking method and jointly inverting receiver functions with rayleigh wave phase and group velocities these seismic stations are distributed along a profile extending across the songpan ganzi terrane qinling qilian terranes and southwestern ordos basin our results reveal the variation in crustal thickness across this profile we found thick crust beneath the songpan ganzi terrane 47 59 km that decreases to 45 47 km in the west qinling and qilian terranes and reaches its local minimum beneath the southwestern ordos block 43 51 km at an average crustal thickness of 46 7 2 5 km a low velocity zone in the upper crust was found beneath most of the stations in ne tibet which may be indicative of partial melt or a weak detachment layer our observations of low to moderate vp vs 1 67 1 79 represent a felsic to intermediate crustal composition the shear velocity models estimated from joint inversions also reveal substantial lateral variations in velocity beneath the profile which is mainly reflected in the lower crustal velocities for the ordos block the average shear wave velocities below 20 km are 3 8 km s indicating an intermediate to felsic lower crust the thick ne tibet crust is characterized by slow shear wave velocities 3 3 3 6 km s below 20 km and lacks high velocity material vs 4 0 km s in the lower crust which may be attributed to mafic lower crustal delamination or and the thickening of the upper and middle crust

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

crust structure of the north china craton from a long range seismic wide angle reflection refraction data abstract we present an interpretation of a 1530 km long wide angle seismic reflection refraction profile that extends across the north china craton ncc approximately from east to west this profile is the longest and the densest wide angle seismic reflection refraction profile in china up to now parameterizing the velocity model as layers with variable thickness we perform seismic travel time tomography to resolve the velocity variation within layers and velocity contrast across moho our model shows crustal thickness that varies from 35 km under the shandong peninsula to 30 km under the north china plain ncp and 32 40 km under central ncc to 45 km under the ordos plateau these results indicate obvious crustal thinning beneath the ncp in comparison with the 35 km crustal thicknesses typical of most cratonic regions in particular an abrupt moho step of 10 km is detected at the junction of the eastern block and western block of the ncc these crustal characteristics are consistent with studies of lithosphere asthenosphere boundary lab with the method of receiver function migration and they indicate a close correlation with lithospheric thickness and surface tectonics of the northeastern ncc these results provide robust seismological evidence that the lithosphere from ncc to the western edge of the taihang mountain might have been widely affected and thinned since its formation in the archean era

multiscale spatial distribution of crustal seismic anisotropy beneath the northeastern margin of the tibetan plateau and tectonic implications of the haiyuan fault abstract the haiyuan fault is a large strike slip fault in the northeastern ne margin of the tibetan plateau using new data recorded by a temporary seismic array this study obtains crustal seismic anisotropy measurements in different scales across the haiyuan fault by analyzing splitting of shear waves from local earthquakes parameters of shear wave splitting sws indicate clear zoning characteristics in seismic anisotropy in the upper crust the study area is divided into two different anisotropic subzones by the haiyuan fault the dominant orientation of fast polarizations is nne or ne north of the faults and wnw to ew south of the fault this pattern of spatial variation indicates the controlling impact of haiyuan fault to observed seismic anisotropy with a zone of dozens of kilometers from the haiyuan fault the dominant orientations of fast polarizations are wnw generally coincidental with the strike of the haiyuan fault except for several stations it indicates that the influence of the fault on stress could reach to about 10 km from the fault distance spatial pattern of anisotropy indicates multiple effects by stress faults and local tectonics it suggests different anisotropic mechanisms including stress induced anisotropy and tectonic or structural anisotropy the dominant orientation of fast polarizations can indicate in situ maximum horizontal principal compressive stress although there is always influence of tectonics the normalized time delays are higher around the haiyuan fault than those away from the fault suggesting stronger crustal seismic anisotropy in a narrow belt around the strike slip fault zone the sws parameters suggest that the haiyuan fault is the actual crustal boundary of ne tibetan plateau about 200 km north of the reported boundary of the tibetan plateau block combining with other studies in the crust and the lithosphere this paper infers possible two layer crustal anisotropy beneath the yinchuan graben north of the haiyuan fault the dominant orientation of fast polarizations are nne or ne in the upper crust probably originated from crack induced anisotropy but become wnw or nw in the middle lower crust possibly originated from deformation anisotropy

contemporary crustal deformation of northeast tibet from geodetic investigations and a comparison between the seismic and geodetic moment release rates abstract we processed the data recorded by 32 continuous gps stations from the crustal movement observation network of china and combined them with published gps observations in a common geodetic reference frame to better depict the ongoing crustal velocities and deformation field of northeast tibet the derived velocity field indicates that the deformation of northeast tibet is characterized mainly by crustal shortening coupled with the eastward extrusion of crustal material and clockwise block rotation we further used the gps velocity field to evaluate the strain rate tensors by the method of sandwell and wessel 2016 the strain rate field shows an obvious transition of the contraction rate from ne sw to nee sww and a decrease in the maximum shear strain rate from west to east was observed along the east kunlun fault and haiyuan fault indicating that the block rotation and lateral extrusion therein may be partitioned by ne sw oriented compression to some extent the inhomogeneity of the rotation strain rate implies that northeast tibet does not behave like a single rigid block but instead resembles a left lateral shear zone supporting the hypothesis that the northward transfer of motion along the east kunlun fault may play a significant role in the deformation style of this area the dilatation rate verified that most of northeast tibet experiences crustal thickening which coincides with the average uplift rate of 1 mm y revealed by gps velocities and channel steepness indices we also estimated the geodetic moment rates and compared them with the seismic moment rates estimated using the truncated gutenberg richter earthquake distribution the ratio of the seismic strain rate to the geodetic strain rate in the regions near the altyn tagh fault the qaidam basin adjoining the qilian mountains the haiyuan fault and the central part of the east kunlun fault are near unity indicating that the deformation in these areas is dominated by seismic release and may represent fully coupled seismogenic zones the occurrence of large earthquakes in the past estimated magnitudes m 6 coupled with a high geodetic deformation rate in these regions could indicate overdue m 6 earthquakes however other factors including the occurrence of creep and the long recurrence time of mw 8 0 earthquakes should be taken into consideration along the haiyuan fault by contrast in other regions including the southern and eastern parts of the haiyuan fault along the elashan fault and the northern qilian mountains the geodetic moment rate is higher than the seismic moment rate by a factor between 3 and 9 we propose that this phenomenon is related to the undersampling of long term earthquake rates within the seismic catalogue long term aseismic crustal deformation or a combination of these factors our study has implications for better comprehending the current seismotectonic pattern of northeast tibet

teleseismic p wave tomography and mantle dynamics beneath eastern tibet we determined a new 3 d p wave velocity model of the upper mantle beneath eastern tibet using 112 613 high quality arrival time data collected from teleseismic seismograms recorded by a new portable seismic array in yunnan and permanent networks in southwestern china our results provide new insights into the mantle structure and dynamics of eastern tibet high velocity high v anomalies are revealed down to 200 km depth under the sichuan basin and the ordos and alashan blocks low velocity low v anomalies are imaged in the upper mantle under the kunlun qilian and qinling fold zones and the songpan ganzi qiangtang lhasa and chuan dian diamond blocks suggesting that eastward moving low v materials are extruded to eastern china after the obstruction by the sichuan basin and the ordos and alashan blocks furthermore the extent and thickness of these low v anomalies are correlated with the surface topography suggesting that the uplift of eastern tibet could be partially related to these low v materials having a higher temperature and strong positive buoyancy in the mantle transition zone mtz broad high v anomalies are visible from the burma arc northward to the kunlun fault and eastward to the xiaojiang fault and they are connected upward with the wadati benioff seismic zone these results suggest that the subducted indian slab has traveled horizontally for a long distance after it descended into the mtz and return corner flow and deep slab dehydration have contributed to forming the low v anomalies in the big mantle wedge our results shed new light on the dynamics of the eastern tibetan plateau

trapping of aqueous fluids in the deep crust free aqueous fluids in the deep ductile crust have residence times there which are very short in geological terms if not consumed by retrograde metamorphic reactions they empty very rapidly upwards this can be shown by applying the theory of magma percolation in a ductile host to the problem of aqueous fluid percolation below the brittle ductile transition thermal activation of ductility is strong enough to facilitate the rapid evolution of porosity and consequently allow rapid fluid transport in the cooler crust above the brittle ductile transition however this mechanism vanishes as a consequence fluid overpressures resulting from the accumulation of fluid in the transition zone must induce hydraulic fracturing rather than pore inflation in regions of horizontally compressive regional stress the commonest situation in stable continental crust these fractures will be horizontal aqueous fluids rising through or liberated from the lower crust will therefore probably accumulate at least initially at the brittle ductile transition in horizontally extensive reservoirs with high horizontal permeability but low vertical permeability

shear wave structure in the northeastern tibetan plateau from rayleigh wave tomography 1 the northeastern tibetan plateau has undergone active shortening due to the india asia collision and has been an ideal place for studying the growth of the plateau to reveal deep structure that is responsible for geological observations in ne tibet we have constructed a 3 d shear wave velocity model from rayleigh waves recorded by 36 stations in the northeast tibetan plateau seismic experiment first the two plane wave tomography method was applied to compute the average and 2 d variations of phase velocities in a period range of 20 to 100 s then 1 d and 3 d shear wave velocity models were developed from the obtained rayleigh wave dispersions the entire region is characterized by a weak upper mantle with a relatively thin lithosphere of 80 100 km thick this observation helps to explain the active deformation at the kunlun mountains and the qinling orogen in ne tibet relative low velocity anomalies appear along and to the south of the kunlun fault kf in contrast to high velocity anomalies in the north and northeast one pronounced slow anomaly centered at 120 km depth is imaged right beneath the restraining bend of the kunlun fault which might be a northern extension of the large scale slow anomaly to the south of the fault we interpret it as a local feature due to its large magnitude and coincidence with the restraining bend it is probably related to anomalously high temperature and associated partial melting resulted from localized asthenosphere upwelling after the delamination of a thick lithosphere root the formation of the thick lithosphere root would require coherent shortening in the crust and mantle lithosphere caused by local compression due to the bend of the strike slip fault indicating that the kunlun fault is probably on lithosphere scale and has played an important role in accommodating the eastward extrusion of the tibetan plateau

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

crustal thickness map of the chinese mainland from teleseismic receiver functions abstract we present a new crustal thickness map of the chinese mainland and adjacent areas based on a compilation of previously published data more than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the chinese mainland the combined dataset provided good coverage especially over eastern and central china and our analysis revealed crustal thickness variations of the chinese mainland with considerably more detail than in previous models our crustal thickness map shows a large variation from a thickness of less than 30 km in the mesozoic basin of eastern china to over 80 km in the tibetan plateau the thickest crust was found in central tibet while the thinnest continental crust was observed beneath eastern china normal to slightly thick crust 38 51 km was found for the stable precambrian basins and orogenic belts comparison between our model and other models reveals that the models share similar large scale and smaller scale features over large portions of eastern china where the data coverage is good however large differences up to 25 km in crustal thickness occur in areas with complex crustal structure or poor data coverage we also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness

atlantic meridional overturning circulation and the prediction of north atlantic sea surface temperature highlights north atlantic sea surface temperature exhibits high decadal predictability potential model bias hinders exploiting the decadal predictability potential an innovative method was developed to overcome some of the bias problem north atlantic sea surface temperature will stay anomalously warm until about 2030 abstract the atlantic meridional overturning circulation amoc a major current system in the atlantic ocean is thought to be an important driver of climate variability both regionally and globally and on a large range of time scales from decadal to centennial and even longer measurements to monitor the amoc strength have only started in 2004 which is too short to investigate its link to long term climate variability here the surface heat flux driven part of the amoc during 1900 2010 is reconstructed from the history of the north atlantic oscillation the most energetic mode of internal atmospheric variability in the atlantic sector the decadal variations of the amoc obtained in that way are shown to precede the observed decadal variations in basin wide north atlantic sea surface temperature sst known as the atlantic multidecadal oscillation amo which strongly impacts societally important quantities such as atlantic hurricane activity and sahel rainfall the future evolution of the amo is forecast using the amoc reconstructed up to 2010 the present warm phase of the amo is predicted to continue until the end of the next decade but with a negative tendency

crustal structure beneath north west iberia imaged using receiver functions abstract in the last years the deep crustal structure of the north western part of the iberian peninsula has been explored using teleseismic receiver function rf analysis of p to s conversions at main crustal interfaces this area has been previously investigated by seismic reflection and refraction experiments and therefore provides an excellent opportunity to compare the results of both approaches the region shows the imprint of two orogenic events the variscan and alpine ones that exhibit different reverse intensities from west to east in a first stage a n s transect across the cantabrian mountains was instrumented to study the area affected by the alpine compressional tectonics later on the limit between the undisturbed variscan units and the reworked alpine zones was explored by n s and e w transects finally an array was deployed in the nw edge of iberia over the variscan hinterland zone the receiver functions are calculated by inverse filtering deconvolution of the l component from the q component and the resulting rf are processed using a simple form of migration to obtain images in depth of the lithosphere that can be compared to the 2 d velocity depth models from active seismic experiments the deep crustal structure constrained by both techniques is remarkably consistent and provides further evidence on the crustal doubling and wedging between the iberian and european crusts throughout the northern part of the iberian peninsula affected by the alpine tectonics the undisturbed variscan domains characterized by a clear subhorizontal moho and few intracrustal convertors show up at the southern edge of the n s transects southward of the alpine deformation front and over the variscan hinterland in the e w transect the crust resulting from alpine reworking presents a complex structure with short and frequently dipping convertors which in some cases seem to image preserved variscan structures

electrical resistivity structures between the northern qilian mountains and beishan block nw china and tectonic implications abstract controversies exist on the tectonic models at the northern margin of the qinghai tibetan plateau new magnetotelluric mt data with a bandwidth of 0 001 5827 5 s were collected along one profile starting from the northern qilian mountains in the southern end crossing the hexi corridor the kuantan shan hei shan uplift the huahai basin and coming into the beishan block two dimensional 2d and three dimensional 3d resistivity models of the data are derived using magnetotelluric inversion codes the 2d models and the electrical structures along the profile extracted from the 3d models are comparable and complementary in defining reliable scales and shapes of anomalies in electrical resistivity the crust upper mantle structures beneath the northern plateau are characterized by two imbricated southward underthrusted blocks of continental crust or lithosphere the northern qilian fault is interpreted as a gently south dipping decollement in the upper crust along the top of a high resistivity body below the northern qilian mountains the two high resistivity bodies below the kuantan shan hei shan uplift are disconnected the upper crustal one may connect with the high resistivity basement below the northern qilian mountains in that the attitudes and deformations of the strata at these areas are coherent whereas the lower crust upper mantle one may be the cold lithospheric materials of the block north to the hexi corridor and that forms a south dipping boundary between the hexi corridor and its northern neighbors the observed high electrical resistivity beneath the beishan block provides further support for the notion that the beishan block should be rigid as a whole only in this manner could it transfer stress from south to north the northern boundary of the qinghai tibetan plateau is below the kuantan shan hei shan fault and underthrusting of continental crust or lithosphere is the main cause for the thickening of the plateau crust in the north

mapping the moho with seismic surface waves a review resolution analysis and recommended inversion strategies abstract the strong sensitivity of seismic surface waves to the moho is evident from a mere visual inspection of their dispersion curves or waveforms rayleigh and love waves have been used to study the earth s crust since the early days of modern seismology yet strong trade offs between the moho depth and crustal and mantle structure in surface wave inversions prompted doubts regarding their capacity to resolve the moho here we review surface wave studies of the moho with a focus on early work and then use model space mapping to establish the waves sensitivity to the moho depth and the resolution of their inversion for it if seismic wavespeeds within the crust and upper mantle are known then moho depth variations of a few kilometres produce large 1 perturbations in phase velocities however in inversions of surface wave data with no a priori information wavespeeds not known strong moho depth shear speed trade offs will mask 90 of the moho depth signal with remaining phase velocity perturbations 0 1 only in order to resolve the moho with surface waves alone errors in the data must thus be small up to 0 2 for resolving continental moho with larger errors moho depth resolution is not warranted and depends on error distribution with period an effective strategy for the inversion of surface wave data alone for the moho depth is to first constrain the crustal and upper mantle structure by inversion in a broad period range and then determine the moho depth in inversion in a narrow period range most sensitive to it with the first step results used as reference prior information on crustal and mantle structure reduces the trade offs and thus enables resolving the moho depth with noisier data such information should be used whenever available joint analysis or inversion of surface wave and other data receiver functions topography gravity can reduce uncertainties further and facilitate moho mapping

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

an upper mantle s wave velocity model for east asia from rayleigh wave tomography abstract we present a new shear velocity model of the upper mantle beneath the east asia region derived by inverting rayleigh wave group velocity measurements between 10 and 145 s combined with previously published rayleigh wave phase velocity measurements between 150 and 250 s rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2d dispersion maps for 10 145 s periods the group velocity maps benefit from the inclusion of new data recorded by the china national seismic network and surrounding global stations the increase in available data has resulted in enhanced resolution compared with previously published group velocity maps the horizontal resolution across the region is about 3 for the periods used in this study the new shear wave velocity models indicate varying velocity structure beneath eastern china which yields estimates of a lithosphere asthenosphere boundary depth from around 160 km beneath the yangtze block to approximately 140 km beneath the western part of the north china craton ncc up to depths of 70 100 km beneath the eastern ncc northeast china and the cathaysia block the models reveal the subduction of two opposite facing continental plates under the southern and northern margin of tibet an obvious low velocity anomaly appears in the top 200 km of the upper mantle beneath northern tibet which is inconsistent with the presence of subducted asian or indian mantle lithosphere beneath northern tibet the cenozoic volcanism fields in the mongolian plateau are characterized by an obvious upper mantle negative anomaly but no signature of deep seated plume was observed

rayleigh wave tomography of the northeastern margin of the tibetan plateau abstract the convergence between the indian continent and the southern margin of eurasia has eventually led to the growth of the tibetan plateau eastward various models have suggested that a large scale flow in the lower crust or and the asthenosphere is responsible for the uplift of eastern tibet it has also been suggested that the continental flow may move around the thick lithosphere of the ordos plateau and the sichuan basin in order to investigate the validity of the crust mantle flow and its possible path we have used rayleigh wave tomography with sensitivity kernels to estimate both the phase velocity structure 20 143 s and the shear wave velocity structure 0 200 km across the northeastern margin of the tibetan plateau from the surface to 100 km our shear wave velocity model shows a prominent low velocity anomaly beneath northeastern tibet suggestive of a warm lithosphere its boundary lies at 105 e longitude which borders the western ordos plateau and sichuan basin from 125 km to 200 km our shear wave velocity model shows a low velocity channel beneath the qilian qinling orogen that lies between the ordos and sichuan blocks this channel may represent the asthenosphere given the fact that the lithosphere is 120 150 km thick in northeastern tibet and the qinling orogen we also inverted for the anisotropy parameters in the study area simultaneously with the velocity parameters the dominant fast direction is nww see generally consistent with the sks splitting results the calculated shear strain from gps and the fault slip rate measurements furthermore the fast directions in our anisotropic model vary only slightly with periods and the anisotropy magnitudes at long periods 100 143 s are large and exceed those at short medium periods 20 80 s indicating potentially although not necessarily vertically coherent deformation from the crust to the asthenosphere and possibly large deformation in the asthenosphere the deformation seems to be linked to the southeastward extrusion tectonics caused by the collision boundary force

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

distribution of the crustal low velocity zones beneath the central and northeastern tibetan plateau insights from joint analysis of receiver functions and surface wave dispersion observations abstract a crustal and uppermost mantle shear wave velocity model beneath the central and northeastern tibetan plateau is obtained from the joint analysis of rayleigh wave phase dispersions and teleseismic p wave receiver functions at 188 broadband seismic stations deployed in this region this newly presented model reveals wide spread low velocity zones in the middle crust and these zones are mainly confined to the region south of the kunlun fault the velocities in the low velocity zones are extremely low

high resolution body wave tomography models of the upper mantle beneath eastern china and the adjacent areas 1 we present new 3 d tomographic models of vp vs and vp vsratio anomalies in the upper mantle beneath ec and adjacent areas this data was collected and interpreted with the goal of clarifying geodynamic processes that caused spatially variable event histories throughout eastern china ec during mesozoic to cenozoic time the tomographic images were constructed by inverting body wave travel times recorded at 1300 stations within the upgraded china national seismic network and 9 temporary arrays resolution tests for different depths and the featured velocity anomalies verify that the tomographic images capture the velocity heterogeneities in the upper mantle to depths of 700 km the salient features of vp vs and vp vsratio anomalies can be clearly identified these include strong multiscale heterogeneities occupying the upper mantle beneath ec and differences in the spatial scale of anomalies found beneath northern and southern areas of ec these features demonstrate a degree of spatial variability in the geodynamic evolution of ec we propose two mechanisms to explain these patterns first the western front of the subducted slab may have imparted greater horizontal compressional stress in areas where it impinged further eastward into ec these areas would experience stronger convection and an altered stress regime in the upper mantle creating significant thermal anomalies beneath the south china block scb relative to the eastern north china craton ncc second differing thermal states and viscosities for the eastern ncc and the cathaysia block cab resulted in differing responses to regional deformation the archean hinterland of the eastern ncc specifically has a colder thermal state and higher viscosity and therefore exhibits only small scale heterogeneities due to the effect of shear localization the neoproterozoic cab has a relatively warm thermal state with lower viscosity and thus deformed more continuously

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

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

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

observations of frequency dependent sn propagation in northern tibet summary we present new observations of the frequency dependent propagation efficiency of the seismic phase sn over the tibetan plateau our measurements are the ratio of the sn amplitude to the pg coda amplitude on tibetan regional seismograms and we map the lateral variation in the maximum and mean values of this ratio across the plateau good path density and azimuthal coverage allow the area of sn blockage identified by previous studies to be better constrained an important result is that at low frequencies 0 2 hz sn propagates efficiently over the entire plateau whereas at higher frequencies 1h z sn is blocked for a region of the northern plateau as we take measurements at higher frequencies we find that the southern boundary of the region of inefficient propagation migrates to the south the observation that low frequency sn propagates efficiently across the whole plateau suggests that at the length scales sampled by these waves the upper mantle has an overall positive shear velocity gradient which could indicate that the high velocity seismic lid or seismic lithosphere beneath tibet is still intact and has not delaminated as some researchers have previously proposed the observation that high frequency sn does not propagate beneath northern tibet suggests that at the shorter length scales sampled by the higher frequency waves the upper mantle has a negative shear velocity gradient this could indicate that at shallow depths the sub moho upper mantle is hot and may contain some melt

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

petrology of gedrite bearing rocks in mid crustal ductile shear zones from the eastern ghats belt india a suite of metapelitic basic and quartzofeldspathic rocks intruded by enderbitic gneiss from the southernmost tip of the eastern ghats belt india and metamorphosed at c 750 800 c 6 kbar were subjected to repeated ductile shear deformation hydration cooling and accompanying alkali metasomatism along narrow shear zones gedrite bearing assemblages developed in the shear zones traversing metapelitic rocks interpretation of the reaction textures in an appropriate p t grid in the system fmash an isothermal isobaric h2o na2o grid in the system nfmash and geothermobarometric data suggest a complex evolutionary history for the gedrite bearing parageneses initially gedrite bearing assemblages were produced due to increase in na2o at nearly constant but high h2o accompanying cooling gedrite was partially destabilized to orthopyroxene albite due to progressively increasing na2o during further cooling and at increased h2o a second generation of gedrite appeared in the rocks

uppermost mantle velocities beneath china and surrounding regions 1 seismic p wave velocity variations in the uppermost mantle beneath china are tomographically imaged using pn wave travel time data the average velocity of the mantle lid beneath china is 8 05 km s with a range from 0 30 km s to 0 35 km s eastern china has slower mantle velocities and a thinner crust than western china different inversions performed for shorter ray path and longer ray path data sets separated the shallow and deep structure within the mantle lid velocities in the lower lid are no more than 0 2 km s faster than at the top the fastest velocities are beneath cratons which remained relatively undeformed despite orogenic events around their margins the pn velocity beneath southern tibet is faster than that beneath northern tibet suggesting that this contrast delineates the northern edge of the indian lithosphere on the southeastern margin of the tibetan plateau the panxi rift slow pn velocity exists in the lithosphere due to the transextensional tectonic environment in eastern china extension of the north china block produced a series of nne ssw trending rifts that cause slow mantle lid velocities these slow velocities extend to the base of the mantle lid suggesting that late cenozoic pacific subduction drove the rifting both high temperature and hydration may contribute to these low uppermost mantle velocities the shanxi rift along the east margin of the ordos platform is characterized by a slow mantle velocity in contrast the yinchuan rift on the west margin does not show any uppermost mantle velocity anomaly

investigation of the moho discontinuity beneath the chinese mainland using deep seismic sounding profiles abstract we herein describe the depth distribution of the moho beneath the chinese mainland determined via compilation and resampling of the interpreted results of crustal p wave velocity structures obtained from deep seismic soundings dsss performed since the pioneering dss work carried out in the qaidam basin in 1958 for the present study 114 wide angle seismic profiles acquired over the last 50 years were collated we included results for crustal structures from several profiles in japan and south korea to improve the reliability of the interpolation of the moho depth distribution our final moho map shows that the depth of the moho ranges from 10 to 85 km the deepest moho discontinuity at approximately 70 85 km beneath the tibetan plateau was formed by ongoing continent continent collision the moho beneath the eastern north china craton at a relatively constant 30 35 km has endured mantle lithosphere destruction the moho depths determined from active seismology are consistent within 3 5 km with results obtained from gravity inversion and surface wave tomography the spatial variation of the moho depth crustal formation and composition of different tectonic blocks contribute to controls on the spatial distribution of the seismicity and rheology in the crust beneath mainland china

joint inversion of surface waves and teleseismic body waves across the tibetan collision zone the fate of subducted indian lithosphere our study has included data from gsn including ic iu and ii china digital seismograph network geoscope iris ida pacific 21 kyrgyz digital network kyrgyz seismic telemetry network and iris usgs permanent seismic networks and the manas tien shan continental dynamics tibetan plateau broadband experiment indepth ii indepth iii indepth iv ascent himnt bhutan nanga parbat pakistan and ghengis passcal temporary seismic deployments we thank iiees and lgit for seismic data from iran and also seisuk for provision and assistance with instruments operated in northeast india cn was supported by a natural environment research council studentship grant ne h52449x 1 with case funding from awe blacknest we thank nick rawlinson and an anonymous reviewer for their constructive and helpful reviews figures were prepared with generic mapping tools gmt software wessel smith 1998

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

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

electrical resistivity structures northeast of the eastern kunlun fault in the northeastern tibet tectonic implications abstract only sparse deep geophysical data have been obtained in the region to the north of the eastern kunlun fault eklf in the tibetan plateau which leads to inconsistent and ambiguous descriptions of the area in various geodynamic models the detailed geophysical issues in the region including the northeast boundary of the plateau and the crust mantle coupling mode northeast of the eklf are under dispute to resolve these disputes we have collected magnetotelluric mt data along five profiles in the region to the north of the eklf since 2009 the interpretation of data from the latest profile labeled l5 which starts from the eastern margin of the qaidam basin crosses the qilian mountain and the hexi corridor and finally reaches the southern alashan block is the focus of this paper these new results are comparable with our previously determined structures the two dimensional 2d resistivity model supports three divisions of the qilian mountain a complex northern qilian a relatively simple sandwiched southern qilian and a transitional central qilian in between them our data indicate a relatively low resistivity layer at the middle lower crust below the eastern margin of the qaidam basin and the southern qilian mountain this result indicates a weak crustal zone wcz existing northeast of the eklf and is interpreted as a large north dipping thrust at this depth the presumed imbricated thrust in the crust in the northeastern plateau resulting from the indenting of the mantle wedge from the south is the primary process in the crustal thickening in the region the mt results indicate that the northeastern boundary of the tibetan plateau is not below the eklf instead we suggest that the northeastern boundary of the outgrowth of the tibetan plateau is along and below the northern rim of the hexi corridor

crust and upper mantle structure of the north china craton and the ne tibetan plateau and its tectonic implications abstract understanding the mesozoic cenozoic tectonic evolution of the north china craton ncc and the ne tibetan plateau tp requires detailed knowledge of the lithospheric structure using dense regional networks and temporary deployments as well as updated reference models we obtain the crust and upper mantle structure to 120 km depth our tomographic results show several major features which have particular implications for the weihe shanxi rift system wsrs deformation of the ne tp and lithospheric evolution of the ncc beneath the wsrs the crust gradually thickens from south to north the lithospheric mantle gradually becomes slower and the mid lower crustal velocities are lower in the weihe rift where rifting of the wsrs initiated we suggest that along strike variations of the lithospheric structures of the wsrs have played an important role in its multistage evolution a low velocity zone lvz in the mid crust beneath the qilian orogen is characterized by relatively higher velocities compared to lvzs in other parts of the tp thus coherent lithospheric deformation may occur due to the high viscosity of the lvz during early plateau growth causing strong anisotropy to develop the western ncc including the ordos block and part of the alashan block shows a high velocity cratonic root extending to the base of our model in contrast the lithosphere of the eastern ncc appears to have been completely modified during the mesozoic through cenozoic and presents a thin lithosphere of relatively low velocities underlain by hot asthenosphere we observed significant upper mantle heterogeneities in the ncc which may reflect its diachronous lithospheric modification

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

variations in crustal and uppermost mantle structures across eastern tibet and adjacent regions implications of crustal flow and asthenospheric upwelling combined for expansions of the tibetan plateau

surface wave tomography of eastern and central tibet from two plane wave inversion rayleigh wave and love wave phase velocity mapssurface wave tomography of eastern and central tibet from two plane wave inversion rayleigh wave and love wave phase velocity maps

lateral variation of crustal structure in the ordos block and surrounding regions north china and its tectonic implications abstract crustal thicknesses and poisson s ratios in the ordos block and surrounding regions were estimated by the use of the h k stacking method on teleseismic receiver functions the data came from 353 temporary and permanent seismic stations in 2006 2011 results show that the crustal thickness and poisson s ratio gently vary within the ordos block with an average of 41 3 km and 0 265 respectively consistent with a felsic composition of the crust crustal thicknesses predicted on the basis of airy isostasy are consistent with the estimated thicknesses implying that the topography is approximately compensated the reactivated portion of the north china craton that has undergone mesozoic cenzoic lithospheric thinning is also characterized by the thinning crust while the ordos block maintains normal crustal thickness and average crustal velocity inferred higher densities in the lower crust and the anti correlation between poisson s ratio and crustal thickness in the ordos block may be the result from underplating of mafic magmas in the precambrian around the ordos block the paleoproterozoic khondalite zone in the northern edge has higher poisson s ratio and thickened crust which is consistent with the lower crust being of more mafic composition the weihe shanxi graben in the southeastern edge has mid high poisson s ratio high heat flow and thinning crust which is consistent with the known transtensional tectonic setting in the liupanshan thrust belt along the southwestern edge of the ordos plateau significant variations in crustal thicknesses and poisson s ratios occur on two sides besides a thickened crust a concaved moho implies horizontal shortening of this edge of the ordos block due to its collision with the northeastern tibetan plateau the structural differences between the eastern and western edges of the ordos block reflect that the ordos block is in tectonic stress environment of the western compression and the eastern extension

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