crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

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

structure of the crust beneath the southeastern tibetan plateau from teleseismic receiver functions abstract southeastern tibet marks the site of presumed clockwise rotation of the crust due to the india eurasian collision and abutment against the stable sichuan basin and south china block knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region here we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method we find that the crustal thickness decreases gradually from the tibetan plateau proper to the sichuan basin and yangtze platform but that significant intra crustal heterogeneity exists on shorter lateral scales 0 30 are detected beneath the southeastern margin of tibet but in the sichuan basin and southeastern yangtze platform the values are close to the global average combined with high regional heat flow and independent evidence for mid crustal layers of high electric conductivity the large intra crustal s wave velocity reduction 12 19 and the intermediate to high average crustal poisson s ratios are consistent with partial melt in the crust beneath parts of southeastern tibet these results could be used in support of deformation models involving intra crustal flow with the caveat that significant lateral variation in location and strength of this flow may occur

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

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

two crustal flowing channels and volcanic magma migration underneath the se margin of the tibetan plateau as revealed by surface wave tomography abstract the se margin of the tibetan plateau is an important area to develop a better understanding of the plateau uplift and the indian eurasian continental collision dynamics previous studies have reported widespread low velocity anomalies beneath this region particularly in the tengchong volcanic field tcvf however the spatial distribution and dynamic processes of these low velocity anomalies are not well constrained in this study a 3 d s wave velocity structure model of the crust and upper mantle 10 120 km in the region is constructed by the inversion of surface wave dispersion data a two step inversion procedure is adopted to generate the s wave velocity structure images the measured phase velocities and inverted s wave velocities jointly show a large scale low velocity anomaly distributed in the crust consistent with the view that the region is the passageway of the eastward migration of tibetan plateau material two crustal flowing channels are clearly observed at depths of 20 km and 30 km which connect and rotate clockwise around the eastern himalaya syntaxis beneath the tcvf there are two prominent low velocity anomaly zones at depths of 15 25 km and 50 80 km which indicate the existence of magma chambers one of the crustal flowing channels is connected with the magma chamber of the tcvf and the other has a short branch north of kunming toward the mile shizong fault at a depth of 20 km based on the distribution of the s wave velocities under the tcvf a dynamic model of the tengchong volcano magma system is proposed to explain the migration patterns of the volcanic material

mica dominated seismic properties of mid crust beneath west yunnan china and geodynamic implications abstract measurements of crystallographic preferred orientations cpo and calculations of p and s wave velocities v p and v s and anisotropy were conducted on three quartz mica schists and one felsic mylonite which are representative of typical metamorphic rocks deformed in the middle crust beneath the southeastern tibetan plateau results show that the schists have v p anisotropy av p ranging from 16 4 to 25 5 and maximum v s anisotropy av s max between 21 6 and 37 8 the mylonite has lower av p and av s max but slightly higher foliation anisotropy which are 13 2 18 5 and 3 07 respectively due to the lower content and cpo strength of mica with increasing mica content the deformed rocks tend to form transverse isotropy ti with fast velocities in the foliation plane and slow velocities normal to the foliation however the presence of prismatic minerals e g amphibole and sillimanite forces the overall symmetry to deviate from ti an increase in feldspar content reduces the bulk anisotropy caused by mica or quartz because the fast axis of feldspar aligns parallel to the slow axis of mica and or quartz the effect of quartz on seismic properties of mica bearing rocks is complex depending on its content and prevailing slip system the greatest shear wave splitting and fastest v p both occur for propagation directions within the foliation plane consistent with the fast pms s wave converted from p wave at the moho polarization directions in the west yunnan where mica amphibole bearing rocks have developed pervasive subvertical foliation and subhorizontal lineation the fast pms directions are perpendicular to the approximately e w orienting fast sks s wave traversing the core as p wave directions indicating a decoupling at the moho interface between the crust and mantle beneath the region the seismic data are inconsistent with the model of crustal channel flow as the latter should produce a subhorizontal foliation where vertically incident shear waves suffer little splitting

radial anisotropy in the crust of se tibet and sw china from ambient noise interferometry 1 we use rayleigh and love wave green s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the sichuan basin and around the eastern himalaya syntaxis in agreement with previous studies low velocity zones are ubiquitous in the mid lower crust with substantial variations both laterally and vertically discrepancies between 3 ds hear velocity from either rayleigh vsv o r love vsh waves are examined both in view of non uniqueness of tomographic solutions and radial anisotropy low shear wave speed and radial anisotropy with vsh vsv are most prominent in mid lower crust in area northwest to the lijiang muli fault and around the red river and xiaojiang faults this anisotropy could be caused by sub horizontal mica fabric and its association with low velocity zones suggests mica alignment due to flow in deep crustal zones of relatively low mechanical strength citation huang h h yao andr d vanderhilst 2010 radialanisotropyinthecrust of se tibet and sw china from ambient noise interferometry geophys res lett 37 l21310 doi 10 1029 2010gl044981

the crust and upper mantle structure beneath yunnan from joint inversion of receiver functions and rayleigh wave dispersion data national science foundation of china 40574040 40704011 science foundation for institute of geophysics ministry of science and technology of china dqjb06b07

crustal thickness and average vp vs ratio variations in southwest yunnan china from teleseismic receiver functions 1 recently many portable broadband three component seismic stations have been deployed in yunnan china we use teleseismic receiver functions to determine the crustal thickness h and the crustal average compressional to shear wave velocity vp vs ratio k variations beneath in southwest yunnan by comparing the synthetics generated from dipping moho models with those from a flat moho model it reveals that for the rays traveling along or around the updip direction the differential arrival times of ps and multiples generated at the dipping moho are comparable with those generated at the flat moho and the result from stacking with these synthetics is a better estimate of model structure for the synthetics traveling along or around the downdip direction the differential arrival times of ps and multiples generated at dipping moho are smaller than those generated at flat moho and make the h k stacking results deviate from the model structure thus the ambiguity induced by dipping moho can be reduced significantly by only stacking with the rays traveling along or around the updip direction applying this technique to 19 stations we obtained that h ranges from 35 42 km and the k ranges from 1 66 1 78 from the central simao basin h increases northward and northeastward k increases significantly northeastward across the ailao shan red river shear zone

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

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

carbonate dissolution in the guts of benthic deposit feeders a numerical model abstract we present a numerical model to quantify calcite dissolution in the guts of deposit feeding invertebrates deposit feeder guts were modeled as constantly stirred reactors cstrs following terminology from digestion theory saturation state and dissolution of calcium carbonate were calculated from changes in total dissolved carbon dioxide and alkalinity resulting from sediment passage through the digestive tract while accounting for dissolution of calcite and respiration of organic carbon typical dissolution rates for a gut volume of 1 ml ranged between 0 5 4 mg calcite d 1 sensitivity analysis revealed gut ph sediment organic matter om content and om reactivity to be the critical parameters determining calcite dissolution rate carbonate dissolution rate was inversely related to gut ph however calcite dissolution was found to be possible even at alkaline gut ph due to respiration by intestinal microbes the kinetics of calcite dissolution had only marginal influence on daily calcite dissolution rates varying the calcite dissolution rate constant by six orders of magnitude affected calcite dissolution rates by less than a factor of 10 calcite dissolution rates were calculated for 4 different hydrographic regimes that differed in their content of sedimentary calcite and om and furthermore in their om reactivity highest dissolution rates were calculated for the shallow water setting where relatively high om content facilitated high microbial respiration rates depressing gut ph however dissolution rates for the deep sea setting were only slightly lower due to greatly elevated ingestion rates resulting from low om content as a consequence of much higher faunal abundances shallow water benthos is likely to contribute the vast majority of gut mediated carbonate dissolution nevertheless the fraction of sedimentary calcite that dissolves during one gut passage is probably too small to be observable by simple gravimetric analysis this may explain the notable scarcity of evidence for gut mediated carbonate dissolution in the literature to date assuming depth dependent calcite dissolution rates and deposit feeder abundances we estimate gut mediated carbonate dissolution to contribute approximately 5 of the annual global sedimentary carbonate dissolution rate which corresponds to an average calcite dissolution rate of approximately 0 5 mg m 2 d 1 for the entire ocean floor

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

lateral variations in the crustal structure of the indo eurasian collision zone we thank michael ritzwoller and two anonymous reviewers for constructive comments that have helped improve the manuscript the majority of the seismic data used in this study were downloaded from iris dmc data for the ngri stations in india were provided by s s rai and zahid ra provided the pmd pakistan data kajal borah provided the ambient noise cross correlations for the uttaranchal network figures were prepared using generic mapping tools gmt software wessel and smith 1998 we thank robert herrmann for making the computer programs in seismology freely available

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

seismic anisotropy and mantle dynamics beneath china abstract we analyzed the shear wave splitting at 138 permanent seismograph stations to study seismic anisotropy and mantle dynamics under mainland china to obtain reliable results we used three different methods to measure the shear wave splitting parameters using core phases sks skks skiks and pks as well as the direct s waves from regional and distant earthquakes our results show that the fast orientations of the anisotropy wnw ese in eastern china are generally consistent with the absolute plate motion apm direction of the eurasian plate suggesting that the anisotropy is mainly located in the asthenosphere resulting from the lattice preferred orientation of olivine due to the shear deformation there the fast axes in western china generally agree with the strikes of the orogens and active faults while they are perpendicular to the direction of the maximum horizontal stress suggesting that the anisotropy in the lithosphere contributes significantly to the observed shear wave splitting the fast axes in western china are also consistent with the apm direction suggesting that the apm driven anisotropy in the asthenosphere is another source of the shear wave splitting there these results suggest that apm driven anisotropy commonly exists under continents similar to that under oceanic regions even though the continental lithosphere has suffered extensive deformation

shear wave splitting across the ailao shan red river fault zone sw china 1 a broad band seismic array was set up across the ailao shan red river asrr fault zone sw china the shear wave splitting parameters on sks and skks phases are calculated using the method of silver and chan 1991 and indicate the polarization direction of 105 6 and the delay time of 1 68 0 44 s beneath the asrr fault zone combined with previous studies the average polarization direction is 110 19 and the delay time is 1 58 0 44 s to the sw side of the fault while the values are 83 30 and 1 29 0 29 s to the ne side this result implies that the upper mantle anisotropy to two sides of the asrr fault zone are different compared with the se polarization direction observed in the p s phase and the n to s surface movement in gps data our results suggest decoupling of the crust and upper mantle beneath the study region

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

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

magmatic underplating and crustal growth in the emeishan large igneous province sw china revealed by a passive seismic experiment abstract in an attempt to characterize the subsurface structure that is related to fossil mantle plume activity a comprehensive geophysical investigation was conducted in the emeishan large igneous province elip the nature and geometry of the crust were examined within the scheme of the domal structure of elip which comprises the inner intermediate and outer zones which are defined on the basis of the biostratigraphy of pre volcanic sediments the bulk crustal properties within the inner zone are characterized by high density high p wave velocity high vp vs ratios and large crustal thickness a visible continuous seismic converter is present in the upper part of the crust in the whole intermediate zone and the eastern part of the inner zone but it is absent in the inner zone where another seismic converter is observed in the lower part of the crust the geometric configuration of these converters is attributable to the addition of mantle derived melts to the pre existing crust and subsequent interaction between them the crustal geometry which is delineated by the migrated image of receiver functions from the passive seismic experiment and the crustal properties collectively suggest that a mafic layer of 15 20 km thickness and 150 180 km width exists at the base of the crust in the inner zone such a mafic layer reflects a vertical crustal growth through magmatic underplating at the base of the crust and intraplating within the upper crust the salient spatial correlation between the deep crustal structure and the dome strongly supports a genetic link between crustal thickening and plume activity if the pre volcanic domal uplift is generated by the permian emeishan mantle plume this arrangement is further supported by the consistency of the extent of crustal uplift estimated by isostatic equilibrium modeling and sedimentary data this study therefore characterizes and provides evidence for a plume modified crust in a large igneous province

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

gulf stream s induced sea level rise and variability along the u s mid atlantic coast 1 recent studies indicate that the rates of sea level rise slr along the u s mid atlantic coast have accelerated in recent decades possibly due to a slowdown of the atlantic meridional overturning circulation amoc and its upper branch the gulf stream gs we analyzed the gs elevation gradient obtained from altimeter data the florida current transport obtained from cable measurements the north atlantic oscillation nao index and coastal sea level obtained from 10 tide gauge stations in the chesapeake bay and the mid atlantic coast an empirical mode decomposition hilbert huang transformation emd hht method was used to separate long term trends from oscillating modes the coastal sea level variations were found to be strongly influenced by variations in the gs on timescales ranging from a few months to decades it appears that the gs has shifted from a 6 8 year oscillation cycle to a continuous weakening trend since about 2004 and that this trend may be responsible for recent acceleration in local slr the correlation between long term changes in the coastal sea level and changes in the gs strength was extremely high r 0 85 with more than 99 99 confidence that the correlation is not zero the impact of the gs on slr rates over the past decade seems to be larger in the southern portion of the mid atlantic bight near cape hatteras and is reduced northward along the coast the study suggests that regional coastal sea level rise projections due to climate change must take into account the impact of spatial changes in ocean dynamics

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

crustal structure beneath se tibet from joint analysis of receiver functions and rayleigh wave dispersion new constraints on the pattern of crustal flow in se tibet are obtained from joint analysis of receiver functions and rayleigh wave dispersion with a newly deployed seismic array the crust in the sichuan yunnan diamond block has an average thickness of 45 km and gradually thins toward the indo china block to the west and the yangtze block to the east high vp vs ratios are detected to the west of the xiaojiang fault but not in the yangtze block to the east the s wave velocity profile reveals that intra crustal low velocity zones ic lvzs are strongly heterogeneous with two lvzs in the middle and mid lower crust respectively in marked contrast to previous observations of a single lvz combined with other observations the two ic lvzs are interpreted as isolated channels of crustal flow at different depths beneath se tibet resulting in the observed complex pattern of radial anisotropy and further elucidating patterns of flow and deformation

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

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

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

crustal flow pattern beneath the tibetan plateau constrained by regional lg wave q tomography abstract as a prominent geophysical anomaly unusually high seismic wave attenuation is observed in the crust and upper mantle of the tibetan plateau particularly along its northern area theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting which may decrease the viscosity of the material and cause it to flow thus seismic attenuation distribution may provide useful constraints to the crust flows if they exist using lg wave q tomography we construct a 0 05 10 0 hz broadband high resolution crust attenuation model for the tibetan plateau and its surrounding regions the maximum spatial resolution is approximately 1 0 1 0 in well covered areas and for frequencies between 0 05 and 1 5 hz this attenuation model reveals an apparent low qlg belt stretching along the northern and eastern tibetan plateau combining the lg wave q model with other geophysical data two possible crust flow channels are found in the tibetan plateau the main flow channel is from north to east and then turns to southeastern tibet along the western edge of the rigid sichuan basin while a second channel starts from southern tibet and crosses the eastern himalayan syntaxis