evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

deep reflection surveying in central tibet lower crustal layering and crustal flow summary in the summer of 1998 a series of multichannel deep seismic reflection tests were conducted in central tibet around 89 e as part of the third phase of project indepth a multidisciplinary study of the structure of the core of the tibetan plateau short seismic reflection sections from four locations spanning the jurassic banggong nujiang suture show several features thin 2 km sedimentary cover sequences an unreflective upper crust down to approximately 25 km depth strongly reflective lower crust down to 65 km depth 22 s traveltime and a distinct change in seismic character from layered reflectivity to unreflective that we interpret as marking the reflection moho these profiles are unlike those in parts of the yadong gulu rift to the south where few deep reflections exist and reflection profiles are dominated by highamplitude reflection bright spots at 6 s which have been interpreted as fluid bodies either water or magma analyses of seismic amplitude decay on individual shot records confirm sourcegenerated energy at traveltimes beyond 22 s thus indicating that the change in reflection character at the moho is not the result of a loss of signal penetration the depths to the top and bottom of the lower crustal reflectivity match both a mid crustal velocity increase and the moho determined for this area from refraction profiling lower crustal reflectivity is consistent with current ductile deformation as suggested by models for middle and lower crustal flow in tibet however cretaceous age magmatic intrusions may explain at least part of the observed reflectivity

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

crustal structure of northern latium central italy from receiver functions analysis new evidences of a post collisional back arc margin evolution abstract the crustal velocity structure in a region of central apennines of italy at the hinge between the highly stretched portion of the monte argentario promontory and the magmatic province of the tolfa domes complex northern latium is discussed in this study s wave velocities at depth have been constrained by the modeling of p wave receiver functions rf from both temporary and permanent broadband seismic stations the computer 3d vs models show a thin crust 19 25 km made of a shallow and thin sedimentary cover a very high velocity and anisotropic layer related to a metamorphic basement and a low vs anisotropic layer in the middle lower crust above a shallow moho discontinuity modeled at about 20 km depth the volcano tectonic evolution of this portion of tyrrhenian back arc margin has been strongly influenced by its peculiar crustal architecture the low vs layer acted as a shear zone in the middle lower crust during the tyrrhenian extension also helping the development of plio quaternary magmatism our findings potentially give new constraints on the evolution of the area and to the general comprehension of back arc development in collisional regions

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

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

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

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

p sh conversions in layered media with hexagonally symmetric anisotropy a cookbook reflectivity synthetic seismograms demonstrate that the type layering and orientation of 1 d anisotropy influences strongly the coda of teleseismic p waves at periods t 1 sec particularly p sh converted waves we assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis of arbitrary orientation the resulting phase velocities vary as cos 2 with respect to that axis using three families of simple crustal models we compare the effects of an anisotropic surface layer with reverberations caused by both thick and thin layers of anisotropy at depth if anisotropy in the surface layer is significant the polarization of direct p can be distorted to generate a transverse component followed by ps and a prominent shear reverberation converted from direct p at the free surface if the anisotropic layer is buried the first and often the most prominent arrival on the transverse component is the p to sh conversion at its upper surface if the anisotropic layer is sufficiently thin p to sh conversions from its boundaries interfere to form a derivative pulse shape on the transverse component which could be mistaken as the signature of shear wave splitting if is horizontal compressional p and shear s anisotropy both produce similar waveform perturbations with four lobed azimuthal patterns suggesting that a weighted stack of p coda from different back azimuths would improve signal to noise for tilted between the horizontal and vertical however the effects of p and s anisotropy differ greatly the influence of p anisotropy on p to s conversion is greatest for a symmetry axis tilted at 45 to the vertical where its azimuthal pattern has two lobes rather than four combinations of p and s anisotropy typically lead to a composite azimuthal dependence in the p coda reverberations

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

seismic anisotropy in eastern tibet from shear wave splitting reveals changes in lithospheric deformation abstract knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions here we focus on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china we measured shear wave splitting of core refracted phases sks and skks at a temporary array of 25 iris passcal stations we calculated splitting parameters using a multi channel and a single record cross correlation method multiple layers of anisotropy cannot be ruled out but are not required by the data a fresnel zone analysis suggests that the shallow mantle between 60 and 160 km depth is the most likely source of anisotropy the polarization directions reveal a pronounced transition from primarily north south in the north sichuan to mostly east west orientations in the south yunnan in the southern part of the study region that is south of 26 n the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields whereas gps campaigns provide evidence suggesting north south crustal flow across the red river fault the pattern of anisotropy argues against such flow in the upper mantle these observations support models that allow differential movement of upper crust relative to lithospheric mantle in the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded the interpretation of the shear wave splitting results is non unique but we suggest that the observed n s transition reflects a fundamental change in deformation regime across our study region it may be related to lateral variations in lithospheric rheology or may mark a transition from the direct impact of the continental collision to dominance of the far field strain field associated with regional subduction processes understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of eastern tibet

neighbourhood inversion of teleseismic ps conversions for anisotropy and layer dip summary the inversion of teleseismic receiver functions for lithospheric structure is difficult due to the non linearity of the problem which is greatly increased in the presence of dipping interfaces and layer anisotropy given an efficient ray theoretical tool for forward modelling teleseismic seismograms we perform a directed monte carlo search technique using the neighbourhood algorithm of sambridge enabling us to search 20 30 parameters in a reasonable amount of computer time tests on synthetic data reveal inherent velocity depth trade offs in typical data sets due to the limited moveout present in teleseismic ps the azimuth of the anisotropic symmetry axis and the strike of a dipping interface prove to be well resolved given adequate backazimuthal coverage we apply this technique to two single station data sets the first from permanent station pgc vancouver island british columbia displays dipping low velocity sediment layers in the mid crust the second from a station at the northern end of the tibetan plateau operating in 1991 and 1992 requires a sequence of thick crustal anisotropic layers to explain the observed pattern of receiver function arrivals

seismic velocities and anisotropy of the lower continental crust a review seismic anisotropy is often neglected in seismic studies of the earth s crust since anisotropy is a common property of many typically deep crustal rocks its potential contribution to solving questions of the deep crust is evaluated the anisotropic seismic velocities obtained from laboratory measurements can be verified by computations based on the elastic constants and on numerical data pertaining to the texture of rock forming minerals for typical lower crustal rocks the influence of layering is significantly less important than the influence of rock texture surprisingly most natural lower crustal rocks show a hexagonal type of anisotropy maximum anisotropy is observed for rocks with a high content of aligned mica it seems possible to distinguish between layered intrusives and metasediments on the basis of in situ measurements of anisotropy which can thus be used to validate different scenarios of crustal evolution

correction to conjugate strike slip faulting along the bangong nujiang suture zone accommodates coeval east west extension and north south shortening in the interior of the tibetan plateau correction

crustal and upper mantle structure of southeast tibet from rayleigh wave tomography 1 we have applied the two plane wave inversion technique to fundamental mode rayleigh waves in southeast tibet phase velocity variations are obtained at 15 periods from 20 to 143 s and are used to construct three dimensional shear wave velocity structure a low velocity zone is imaged in the middle crust of southeast tibet in which velocity perturbation varies from 4 to 3 the slow area might be associated with high temperature and partial melt and can flow ductilely while the high velocity region could be relatively cold and mechanically strong enough to pass crustal strain implying that coherent lithospheric deformation is possible with the presence of complicated crustal flow strong negative anomalies in the shallow crust are imaged at 92 e correlating with the location of a north south rift zone a fast mantle lid to a depth of 120 km is present beneath most of the study area the lithosphere is significantly slower in the central part of the study area than in the west and east to the east of 96 e a subvertical high velocity zone is imaged at 120 240 km depths under the bangong nujiang suture bns which is more like the subduction of the asian lithosphere we interpret the high velocity anomalies to the south of the bns as the subducted indian lithosphere and those to the north as the asian lithosphere a vertical low velocity column is observed in the central part of the region from the lower crust to the midupper mantle which is probably in part responsible for the rift at 92 e the slow anomaly is likely associated with asthenosphere upwelling following the opening of a slab window that could have developed when the slab was torn apart vertically because of the significant change of subducting direction at the eastern syntaxis

thinning and flow of tibetan crust constrained by seismic anisotropy intermediate period rayleigh and love waves propagating across tibet indicate marked radial anisotropy within the middle to lower crust consistent with a thinning of the middle crust by about 30 the anisotropy is largest in the western part of the plateau where moment tensors of earthquakes indicate active crustal thinning the preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy the middle to lower crust of tibet appears to have thinned more than the upper crust consistent with deformation of a mechanically weak layer that flows as if confined to a channel

phase relations in andalusite sillimanite type fe rich metapelites tono contact metamorphic aureole northeast japan mineral assemblages in metapelites of the contact aureole of the tono granodiorite mass northeast japan change systematically during progressive metamorphism along an isobaric path at 2 3 kbar the bulk rock compositions of metapelites are aluminous with a values on an afm projection larger than that of the chlorite join the metapelites commonly contain paragonite in the low grade zone with increasing temperatures andalusite is formed by the breakdown of paragonite the importance of pyrophyllite as a source of al2sio5 polymorphs is limited in typical pelitic rocks the most common type of metapelite in the study area has feo feo mgo 0 5 0 6 and develops assemblages involving chlorite andalusite biotite cordierite k feldspar sillimanite and almandine with paragenetic changes similar to other andalusite sillimanite type aureoles rocks with feo feo mgo 0 8 progressively develop chloritoid bearing assemblages from bt chl cld and bt cld to and bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole the paragenetic relations are explained by a kfmash univariant reaction of chl cld and bt located to the low temperature side of the formation of cordierite by the terminal equilibrium of chlorite a p t model depicting the relative stability of chloritoid and staurolite at low and medium pressure conditions respectively is proposed based on the derived location of the chl cld and bt reaction combined with the theoretical phase relations among biotite chlorite chloritoid garnet and staurolite

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

conjugate strike slip faulting along the bangong nujiang suture zone accommodates coeval east west extension and north south shortening in the interior of the tibetan plateau this is the published version copyright 2002 american geophysical union all rights reserved

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

crustal seismic anisotropy in central tibet implications for deformational style and flow in the crust 1 receiver functions obtained at indepth iii stations located near the bangong nujiang suture in central tibet display a weak moho signal and strong p to s conversions within the first 5 s that vary systematically with back azimuth a single station with representative azimuthal variations located at the sharp onset of strong sks splitting is modeled for both dipping layers and seismic anisotropy by using a global minimization technique inversion results indicate strong anisotropy 10 near the surface and in the middle crust separated by a south dipping 25 layer possibly related to the earlier phase of crustal shortening near surface anisotropy has a fabric dipping steeply southward and trending wnw ese that correlates with the suture and younger strike slip faults in contrast mid crustal anisotropy occurs in a low velocity zone and has a fabric dipping gently 18 northward that might be related to a well developed near horizontal rock fabric induced by crustal flow