lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

crustal velocity structure of the northeastern tibetan plateau from ambient noise surface wave tomography and its tectonic implications abstract broadband seismic data from the regional seismic network operated by the china earthquake administration and 32 temporary seismic stations are used to image the crustal velocity structure in the northeast tibetan plateau empirical rayleigh and love wave green s functions are obtained from interstation cross correlation of continuous seismic records group velocity dispersion curves for rayleigh and love waves between 10 and 50 s are obtained using the multiple filter analysis method with phase matched processing the group velocity variations of rayleigh and love waves overall correlate well with the major geologic structures and tectonic units in the study region shear wave velocity structures were then inverted from rayleigh and love wave dispersion maps the results show that the songpan ganzi terrane is associated with a low velocity at depth greater than 20 km the northern qilian orogen with higher elevation and thicker crust compared to the southern qilian orogen is also dominated by low velocity at depth greater than 25 km however there is no clear evidence of the low velocity mid to lower crust beneath the southern qilian orogen as the crustal flow model predicts the low velocity zone lvz beneath the northern qilian orogen may suggest that the crustal thickening and surface uplift of the northern qilian orogen are related to the lvz and the lvz may be considered as an intracrustal response to bear the ongoing deformation in the northern qilian orogen online material figures of crustal topography number of group velocity measurements checkerboard tests for nets stations and 1d velocity models

anisotropy and crustal thickness of northern tibet new constraints for tectonic modelling in 1993 a seismological array was installed on a profile along the golmud lhasa highway from the qaidam basin to central tibet a travel time analysis for local and regional earthquakes was performed in order to better constrain the crustal structure obtained by teleseismic tomography more information about the crustal thickness and anisotropy comes from the analysis of moho converted p to sv waves the two methods supplement each other and confirm the existence of crust thinner than expected accross the northern edge of the tibetan plateau and qaidam p to sv splitting analysis yields a striking similarity between the fast polarization direction and the velocity field of present day deformation as inferred from kinematic modelling the two observations the thinning of the crust and the clockwise rotation of the fast polarization direction emphasize the leading role of eastward extrusion for the absorption of the large amount of crustal shortening occuring in central asia

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

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

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

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

insights into the dynamics of an orogenic wedge from lubrication theory implications for the himalayan tectonics abstract using lubrication theory we develop a mechanical model to evaluate the dynamic relation between an orogenic wedge and the overriding plate the model suggests that the subducting plate motion produces a dynamic pressure in the wedge which supports the gravity load of the overriding plate lying above it stable condition a drop in the dynamic pressure results in a rotational collapse of the overriding plate unstable condition we present an analytical solution to provide an estimate of the dynamic pressures in orogenic wedges the lubrication model is also implemented in computational fluid dynamics cfd simulations to perform real scale numerical experiments considering mainly three variables 1 plate convergence angle 2 subduction rate us and 3 viscosity ratio r between the overriding plate and wedge the overriding plates attain mechanical stability when us exceeds a threshold value us otherwise they become unstable as us

mechanics of v shaped conjugate strike slip faults and the corresponding continuum mode of continental deformation v shaped conjugate strike slip faults occur widely on earth venus and mars in the solar system they commonly lie at 60 75 in map view from the maximum compressive stress 1 direction this fault pattern cannot be explained directly by the coulomb fracture criterion which predicts the formation of x shaped shear fractures at 30 from the 1 direction possible explanations of this odd fault geometry include rotation of early formed coulomb fractures or reactivation of preexisting weakness here we show that none of these mechanisms is feasible for the formation of a late cenozoic conjugate strike slip fault system in central tibet instead its initiation can be best explained by distributed deformation during the formation of two parallel and adjoining shear zones that have opposing senses of shear our suggestion is based on the current global positioning system gps velocity field in tibet which can be divided into two east trending shear zones a northern left slip zone consisting of active ene striking left slip faults and a southern right slip zone consisting of active wnw striking right slip faults the correlation between the gps strain field and the fault pattern suggests that the central tibet conjugate faults may have initiated as two sets of riedel shears in the two parallel but separate shear zones because the two east trending shear zones also experience north south contraction we refer to this mechanism of conjugate fault formation as paired general shear pgs deformation assuming a newtonian fluid the observed tibetan gps velocity field requires the paired shear zones to have formed either by gravitational spreading of the tibetan lithosphere or a horizontal shear at the base of the upper crust or mantle lithosphere we demonstrate the feasibility of the two inferred mechanisms for the formation of v shaped conjugate faults using simple sandbox experiments our paired general shear pgs model implies that the combined effect of the state of strain and the state of stress favors only one set i e riedel shear of coulomb conjugate shear fractures under general shear flow it also requires continuum deformation rather than discrete extrusion tectonics as the most dominant mode of deformation during the late cenozoic development of the central tibetan plateau

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

structural heterogeneity of the longmenshan fault zone and the mechanism of the 2008 wenchuan earthquake ms 8 0 we determined detailed 3 d images of p and s wave velocity vp vs and poisson s ratio in and around the longmenshan lms fault zone by using a large number of p and s wave arrival times from the aftershocks of the 2008 wenchuan earthquake ms 8 0 and other local events our results show that the structure of the lms fault zone north of the wenchuan main shock is very different from that south of the main shock the southern section of the lms fault zone contains a broad zone with low vp low vs and high anomalies while the northern segment exhibits more scattered heterogeneities corresponding to most of the aftershocks which occurred there a prominent low vp low vs and high anomaly exists directly beneath the wenchuan main shock hypocenter suggesting that in addition to compositional variations fluid filled fractured rock matrices may exist in the lms fault zone which may have influenced the generation of the large wenchuan earthquake our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the lms fault zone in addition most small earthquakes before the 2008 wenchuan main shock occurred around the guanxian jiangyou fault while the wenchuan aftershocks were mainly concentrated on the yingxiu beichuan fault suggesting that the rupture process of the wenchuan earthquake may belong to an out of sequence thrusting event a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 chi chi earthquake m 7 5 and the 2005 kashmir earthquake m 7 6 a few aftershocks occurred close to the blind guangyuan dayi fault in the sichuan basin suggesting that this blind fault was also ruptured by the wenchuan earthquake consistent with geological surveys

reconciling lithospheric deformation and lower crustal flow beneath central tibet comment and reply comment bendick and flesch 2007 1 model a band of high topography in northern tibet qiangtang bulge fig 1 2 as caused by a high viscosity rigid lower crustal indenter present below 45 km depth their fig dr4 that extends 120 km north of the banggong nujiang suture their fig dr3 they

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

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

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

reconciling lithospheric deformation and lower crustal flow beneath central tibet a viscous region with deformable boundaries is used to model simultaneous crustal flow and lithospheric coupling in northern tibet this model suggests that 1 the deformation of northern tibet is different from that of southern tibet because of structural differences between the regions 2 the viscosity contrast between the crust and the mantle lithosphere is relatively small beneath northern tibet and 3 crustal flow is compatible with crust mantle coupling under these conditions

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

a study of azimuthal p residuals and shear wave splitting across the kunlun range northern tibetan plateau abstract the kunlun fault system is an east west transcurrent fault bordering the tibetan plateau that has experienced left lateral motion since 3 ma a linear array of one component and three component seismological stations was deployed across the kunlun range for 4 months p travel time residuals define three basic units along the profile the north kunlun unit the south kunlun unit and the bayan har unit p residuals are a function of azimuth p residuals are fast in the southern stations for southern azimuths and in the northern stations for northern azimuths measurements of shear wave splitting of the s scs and sks phases have been performed on deep events delay times and fast polarization directions have been computed by finding the maximum of the cross correlation between the two horizontal components rotated at different angles delay times from 0 40 s to 2 52 s remain in the range of those found by mcnamara et al 1994 j geophys res 99 13655 13665 at two sites along the same profile p residual travel times and the splitting of s waves are discussed in relation to models of the geological evolution of this region the northern tibetan plateau appears as a juxtaposition of different terranes the azimuthal p residual pattern implies that p wave anisotropy is about 3 in the lithosphere on both sides of the kunlun fault and that fast velocities are oriented outwards from the kunlun fault plane s wave splitting is observed far from the fault and implies that the lithosphere beneath northern tibet has undergone an important deformation

measuring rill erosion using structure from motion a plot experiment abstract in this paper the results of a plot experiment on rill erosion are reported the rill network manually incised on the soil and further shaped by a clear inflow discharge was surveyed using the three dimensional photo reconstruction 3d pr technique which allows to obtain a digital terrain model dtm by a large series of oblique images of the channel from consumer un calibrated and non metric cameras the three dimensional 3d dtm and the quasi tridimensional 2 5d model were generated by agisoft photoscan software for a single rill channel the reliability of the 3d image based ground measurements of morphological and hydraulic variables was positively tested by the corresponding measurements carried out on the rill gypsum cast moreover the morphological and hydraulic variables were measured at 11 transects of the plot by a profilometer p the 2 5d and 3d models using the 3d model as reference the analysis showed that the reliability of the measurements by p and 2 5d methods was comparable and the error was generally lower than 15 the rill measurements carried out by the three methods agreed with the available literature measurements and supported the applicability of both the empirical relationship between rill length and its eroded volume and the theoretical dimensionless relationship among the morphological variables describing the channelized erosion process the analysis also showed that the effect of the distance between two consecutive cross sections on the rill volume measurement by 3d model is negligible finally the shaping effect of different flow discharges on three straight rills incised into the plot was tested in particular three increasing flow discharges 0 15 0 35 and 0 5 l s 1 were used sequentially in two rills while the maximum discharge of the sequence 0 5 l s 1 was used into the third rill both the grain size distribution of the eroded sediment and the total amount of soil loss were affected by the discharge sequence

correlated crust and mantle strain fields in tibet vertical to subvertical planes of shear within the active crustal deformation field in tibet align with fast directions of shear wave polarization this observation suggests that the present day velocity gradient tensor field within the crust of tibet correlates with the lithospheric mantle velocity gradient tensor field beneath tibet this inference requires the following 1 the 100 axes of olivine are aligned within established zones of lithospheric mantle fabric or anisotropy built up through finite strain 2 present day shear in the mantle lithosphere is occurring within these established zones of fabric parallel to the direction of olivine 100 axis alignment 3 the zones of mantle lithospheric fabric or zones of mantle shear are aligned with zones of crustal shear faults the correlation of crustal and mantle strain fields most simply results from the fact that both crust and mantle lithosphere are under the influence of similar velocity boundary conditions furthermore the observations confirm the distributed nature of lithospheric mantle deformation beneath east central tibet and suggest that left lateral shear has been the dominant component of finite shear there

extensional extrusion insights into south eastward expansion of tibetan plateau from magnetotelluric array data abstract despite extensive effort over many decades to understand the tectonic evolution of the tibetan plateau the geodynamic processes creating the iconic south eastward expansion of the plateau at the eastern himalayan syntaxis ehs are still unclear and are hotly debated two popular but not necessarily exclusive geodynamic models namely crustal flow at mid to lower crustal depths and coherent deformation between the crust and lithospheric mantle are commonly invoked to explain the expansion mechanism however neither of these is able to reconcile all of the abundant geological and geophysical data here we present a three dimensional 3d geo electrical model derived from new sinoprobe magnetotelluric mt array data that reveals the geo electrical and by inference rheological structure of southeast tibet instead of nw se conductive channels proposed in prior two dimensional 2d mt studies distinct nne ssw directed quasi linear conductive anomalies are identified in the mid to lower crust which are separated by a large scale electrically resistive structure that extends from the crust to the upper mantle this argues against the prior proposed model of south eastward conductive anomalies and hence against the southeast lower crust flow of material to interpret our observations and resultant model a new mechanism of extensional extrusion is proposed to address the lithospheric deformation of the south eastward expansion of tibetan plateau

evidence for mechanical coupling and strong indian lower crust beneath southern tibet how surface deformation within mountain ranges relates to tectonic processes at depth is not well understood the upper crust of the tibetan plateau is generally thought to be poorly coupled to the underthrusting indian crust because of an intervening low viscosity channel here however we show that the contrast in tectonic regime between primarily strike slip faulting in northern tibet and dominantly normal faulting in southern tibet requires mechanical coupling between the upper crust of southern tibet and the underthrusting indian crust such coupling is inconsistent with the presence of active channel flow beneath southern tibet and suggests that the indian crust retains its strength as it underthrusts the plateau these results shed new light on the debates regarding the mechanical properties of the continental lithosphere and the deformation of tibet

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

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

constraints on the seismic wave velocity structure beneath the tibetan plateau and their tectonic implications we combine observations of group and phase velocity dispersion of rayleigh waves of the waveform of a long period pl phase of pn and sn velocities from unreversed refraction profiles using earthquakes and of teleseismic s p travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under tibet from surface wave measurements alone the tibetan crustal thickness can be from 55 to 85 km with corresponding uppermost mantle shear wave velocities of about 4 4 to 4 9 km s respectively the pn and sn velocities were determined to be 8 12 0 06 and 4 8 0 1 km s respectively using travel time data at lhasa from earthquakes in and on the margins of tibet combining these results the crustal thickness is most likely to be between 65 80 km with an average shear wave velocity in the upper crust less than 3 5 km s a synthesis of one pl waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above in contrast to observations obtained for eight earthquakes in the himalaya measurements of both teleseismic s and p wave arrival times for nine earthquakes within tibet show unusually large intervals between p and s compared with the jeffreys bullen tables thus the pn and sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath tibet from the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured pn and sn velocities beneath tibet and beneath shields and platforms the velocities at the moho beneath tibet are compatible with the temperature being 250 300 higher than beneath shields and platforms i e 750 c if the temperature beneath the platforms is close to 500 c such a temperature could reach or exceed the solidus of the lower crust simple one dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0 9 hfu at the base of the crust if the distribution of radioactive heat production elements were not concentrated at the top of the crust radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations appendices are available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 004 1 00 payment must accompany order

seismic anisotropy beneath the southern himalayas tibet collision zone shear wave phases scs sks and skks collected from the project international deep seismic sounding of tibet and himalayas ii indepth ii array in the eastern himalayas and tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern himalayas tibet collision zone no evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the indus tsangpo suture the lack of shear wave splitting beneath the stations in the vicinity of suture and the tethyan himalayas indicates that there is no appreciable large scale mantle polarization seismic anisotropy beneath this part of the collision zone one explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened indian lower continental lithosphere north of the gandese belt the fast directions of the split shear waves are oriented in a northeast southwest direction approximately parallel to the trend of a major left lateral strike slip fault zone that bounds the gulu rift the coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric wide coherent deformation

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

crustal deformation of the lhasa terrane tibet plateau from project indepth deep seismic reflection profiles international deep profiling of tibet and the himalaya indepth deep reflection data in the yangbajain damxung graben of southern tibet yield evidence of magmatism and deformation beneath the southern lhasa terrane and yarlung zangbo suture shallow reflections and low velocity first arrivals indicate a thin quaternary graben fill of generally less than a few hundred meters thickness underlying stratified reflections extending to a maximum of about 11 5 km depth likely originate from deformed paleozoic cenozoic supracrustal strata of the lhasa terrane a prominent undulatory band of reflections within the crystalline basement extends beneath the length of the yangbajain damxung graben depth ranges from 12 km to 18 km this horizon has been interpreted to mark the top of a midcrustal partial melt zone underlying southern tibet the undulatory character of this horizon additionally suggests that it may be tectonically deformed a somewhat deeper subhorizontal wide angle reflection extends southward beneath the outcrop of the yarlung zangbo suture indicating that the suture is cut off or superposed by a younger structure at depth three gently north dipping reflections at 19 km 24 km and 27 km depth beneath the gangdese batholith are suggestive of a midcrustal duplex and may mark the northward downdip extension of the late oligocene early miocene gangdese thrust system a prominent 40 north dipping reflection imaged in the deep crust beneath the gangdese batholith between 40 and 60 km depth might mark the downdip expression of the yarlung zangbo suture or alternatively a younger reverse fault in the lower crust or both viewed in aggregate the reflection data are suggestive of moderate postcollisional shortening of the upper crust of the lhasa terrane accompanied by melting of the middle crust although the data are permissive of wholesale underthrusting or fluid injection of indian continental crust beneath the lhasa terrane they show no direct evidence for this having occurred

surface wave dispersion across tibet direct evidence for radial anisotropy in the crust recordings in western tibet of rayleigh and love waves at periods less than 70 s from aftershocks of the 2008 sichuan earthquake cannot be matched by an isotropic velocity model beneath tibet these intermediate period rayleigh and love waves require marked radial anisotropy in the middle crust of tibet with the vertically polarized s waves propagating more slowly than s waves with horizontal polarization the magnitude of anisotropy inferred using paths entirely within tibet is slightly greater than that obtained previously from a tomographic inversion of a dataset covering a larger region anisotropy in the middle crust likely reflects deformation of the middle crust and is consistent with the notion of mid crustal flow and thinning of the crust

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

evidence from gravity and topography data for folding of tibet bouguer gravity and topography data from tibet suggest that the surface of the plateau and the subsurface density interfaces are warped into two series of ridges and troughs trending parallel to the collision zone with wavelengths of 150 and 500 km these folds are superimposed on an overall state of isostatic compensation owing to crustal thickening such folding is predicted from models of compressional shortening of a rheologically layered plate the results thus suggest the presence of a weak decoupling zone between the tibetan crust and upper mantle

moho depth and vp vs ratio in peninsular italy from teleseismic receiver functions 1 we present a new data set of moho depth estimates for peninsular italy obtained from receiver functions analysis and a grid search approach teleseismic records from 174 broadband seismic stations were used to compute the receiver function data set in general our results agree well with previous knowledge adding original information in many regions where previous data were lacking or uncertain we find broad regions with consistent moho depth namely 1 the peri tyrrhenian areas bordering the homonymous sea have a 20 to 25 km deep moho 2 the foreland regions adriatic apulia iblean with 30 35 km deep moho 3 the apennines belt where we find westward deepening of the adriatic moho down to 50 km depth that well delineate the westward adria subduction overlayed by the shallow tyrrhenian moho 4 calabria shows a flat 40 km deep moho suddenly dipping at high angle from the tyrrhenian coast westward defining the ionian subduction and 5 a clear separation between northern and southern apennines is marked by a region of almost flat moho possible evidence of a detached slab in between we also find that in the northern apennines the shallow tyrrhenian moho is continuous for more than 200 km from the sea to under the northern apennines with a generally flat geometry and a few undulations suggesting either the boudinage or episodic pulses of extension of the tyrrhenian crust during its pliocene quaternary stretching the eastward extent of the tyrrhenian moho below the apenninic belt approximately corresponds to the eastward extent of the observed extension in the upper crust as mapped by the normal faulting earthquakes in this portion of the apennines we interpret an intracrustal discontinuity sw dipping below the belt parallel to the adria moho as the top of the lower crust being subducted beneath the apennines the subcrustal earthquakes of this region down to 80 km depth seem to be located within this subducted crustal layer the southern apennines show the same general geometry of subduction as the northern arc although the former have a less defined on land extended region and no earthquakes in the lower crust uppermost mantle in between the central apennines present a different moho geometry no clear doubling of the two plates but rather a constant bilateral deepening possibly associated to a crustal root with no evidence of a subduction setting this reinforces the hypothesis of a detached slab in this area we also present the first large scale mapping of average crustal vp vs ratios for peninsular italy the results although representing averages over the whole crust enhance some interesting features the most evident are belts of high vp vs in the thinned crust of the apennines where extension has long been active in the quaternary probably as an effect of highly fractured rocks with abundant fluid circulation strong correlation exists between areas of high vp vs and quaternary volcanoes suggesting the presence of fluids at depth

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

source side shear wave splitting and upper mantle flow in the romanian carpathians and surroundings we present shear wave splitting measurements from 5 earthquakes that occurred in the vrancea seismic zone of the carpathian arc s waves from these events all with magnitudes 5 4 mw and deeper than 88 km were recorded at broadband stations of the global seismic network and the geoscope and geofon networks and used by us to measure shear wave splitting corrected for sub station splitting and anisotropy in order to carry out these corrections we used published shear wave splitting parameters thus isolating contributions to observed splitting from the vrancea source region and upper mantle surrounding the carpathian arc the resulting 32 good observations of source side shear wave splitting along with 54 null splitting observations which yield two potential splitting directions clearly show that upper mantle anisotropy is strongly variable in the region of the tightly curved carpathian arc shear waves taking off from vrancea along paths that sample the east and southern carpathians have fast anisotropy axes parallel to these ranges whereas those leaving the source region to traverse the upper mantle beneath the transylvanian basin i e mantle wedge side trend ne sw shear waves sampling the east european and scythian platforms are separable into two groups one characterized by fast shear trends to the ne sw and a second deeper group with trends to nw se also the majority of null splits occur along paths leaving vrancea in these ne e azimuths we interpret these results to indicate the presence of at least three distinct upper mantle volumes in the carpathians region the upper mantle beneath the carpathian arc is strongly anisotropic with fabrics parallel to the local arc strike the transylvanian basin upper mantle fabrics trend ne sw and the anisotropy beneath the westernmost east european platform may be characterized by a shallow nw se trending fabric concentrated in the cratonic lithosphere of the east european platform and a second deeper fabric with e w trend marking asthenospheric flow beneath the craton s base this more complex anisotropy beneath the western edge of the east european platform would account for both the variability of observed splitting of waves that sample this volume and also the strong prevalence of nulls observed along eastward departing azimuths

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

comparison of shear wave splitting and finite strain from the india asia collision zone we investigate whether observations of shear wave splitting in asia may be related to distributed strain of the lithosphere or to strain caused by motion of the asian lithosphere over the deep mantle the hs2 nuvel angular velocity for eurasia and that of minster and jordan 1978 predict absolute plate motions that are a factor of at least 4 slower than the nne relative motion of india with respect to stable eurasia shear of the upper mantle beneath the south asian lithosphere is therefore likely to be dominated by the velocity field caused by the internal deformation of asia rather than by plate motion if that shear causes sks splitting fast polarization directions in tectonic asia should be aligned approximately in the direction of the motion of asian lithosphere relative to the deep mantle which for southern asia is nne in general however fast polarization directions are not aligned with this direction and in some cases are nearly orthogonal to it we calculate finite deformation of the asian lithosphere in response to its indentation by india and from the deformation predict the orientations of fast polarization directions aligned with the principal axes of elongation these predictions agree with the gross features of the anisotropy in tibet and the tien shan orientations of anisotropy in the baikal region of siberia are consistent with the orientations of the local principal axes of active strain orientations of anisotropy in mongolia however are inconsistent both with the azimuth of the velocity of this region with respect to the deeper mantle and with the orientation of the calculated maximum elongation of the region we conclude that sks splitting observations from asia agree better with those predicted by calculations of internal lithospheric deformation due to indentation by india than with strain predicted from motion of the lithosphere over the deeper mantle these results place constraints on the depth range where shear wave splitting occurs which we propose lies predominantly within the lithospheric mantle

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

joint imaging by teleseismic converted and multiple waves and its application in the indepth iii passive seismic array 1 a joint imaging by teleseismic converted and multiple phases is put forward to image the crustal structure beneath the indepth iii passive seismic profile in the central tibet a depth dependent vp vs ratio is adopted to map common conversion point ccp via a vp vs scanning and stacking approach our new crustal image shows that the crust is divided into two sections with different moho depth distributions and amplitudes with the boundary nearby the bangong nujiang suture bns a negative p to s amplitude band is located in the upper to middle crust indicating a low velocity zone possibly caused by partial melting beneath the negative amplitude band the average crustal vp vs ratio estimated by our joint imaging method is higher than previous study but is consistent with a partial melting middle to lower crust implied by many other results

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

crustal thickening and lateral extrusion during the indo asian collision a 3d viscous flow model abstract the 2000 km crustal shortening from the indo asian collision in the past 40 70 million years has been accommodated mainly by crustal thickening in the himalayan tibetan plateau and lateral extrusion of blocks of asian continents however the spatiotemporal evolution of crustal thickening and lateral extrusion hence the far field impact of the indo asian collision on asian tectonics remains controversial here we present a 3d viscous flow model that simulates the partitioning of crustal material between thickening and lateral extrusion during the indo asian collision the model assumes conservation of crustal mass and is constrained by the history of the indo asian plate convergence and by the present day topography of the himalayan tibetan plateau the results show that much of the early collision was absorbed by crustal thickening within the himalayan tibetan plateau however lateral extrusion of crustal material has gradually become dominant in the past 10 20 myr indicating increasing influence of the indo asian collision on asian tectonics

crustal structure and deformation under the longmenshan and its surroundings revealed by receiver function data abstract the convergence of india and eurasia and the obstruction from the rigid sichuan basin cause the longmenshan lms to have the steepest topographic gradient at the eastern margin of the tibetan plateau however the mechanisms of surface uplift are still controversial in this paper we estimate the crustal structure and deformation under the lms and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the china earthquake administration we apply a comprehensive splitting measurement technique on ps conversion phase at the moho moho ps splitting to calculate crustal anisotropy from azimuthal variations of receiver functions our results show that most of the seismic stations beneath the lms area exhibit significant seismic anisotropy with the splitting time of 0 22 0 94 s and a fast polarization direction of nw se while less or even no crustal anisotropy has been observed under the sichuan basin comparing the fast polarization directions of moho ps splitting with the indicators of lithospheric deformation such as shear wave splitting absolute plate motion and global positioning system imply a consistent tendency of deformation between the lower crust and upper mantle but decoupling deformation in the crust beneath the lms area we further compare moho ps splitting time to that estimated from previous sks splitting indicating that crustal anisotropy is an important source of the sks splitting time in this study area in addition a thick crust 50 km with high vp vs values 1 74 1 86 is also observed using the h stacking method these seismic observations are consistent with the scenario that the lms area has been built by the lower crustal flow combined with the seismic reflection refraction profile and geology studies we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the lms area resulting in the surface uplift of the lms

a review of heterogeneous materials and their implications for relationships between kinematics and dynamics in continents 1 the variation of mechanical properties as a function of depth in the lithosphere controls the relationship between surface deformation and whole lithosphere deformation where mechanical competence elastic strength or viscous stiffness is a continuous function with depth surface deformation can be used to constrain either force balance or rheological parameters for the lithosphere however where competence is vertically discontinuous the surface deformation and deformation at depth may have a range of relationships including complete independence therefore both inversions and forward models designed to test either rheology or stress state are highly sensitive to the a priori choice of a mechanical model

evolution of orogenic wedges and continental plateaux insights from crustal thermal mechanical models overlying subducting mantle lithosphere the links between an early phase of orogenesis when orogens are commonly wedge shaped and a later phase with a plateau geometry are investigated using coupled thermal mechanical models with uniform velocity subductionboundary conditions applied to the base of the crust and simple frictional plastic and viscous rheologies models in which rheological properties do not change with depth or temperature are characterized by growth of back to back wedges above the subduction zone wedge taper is inversely dependent on the ramberg number rm gravity stress basal traction increasing convergence velocity or crustal strength produces narrower and thicker wedges models that are characterized by a decrease in crustal viscosity from c to b with depth or temperature leading to partial or full basal decoupling of the crust from the mantle display more complex behaviour for models with a moderate viscosity ratio b c 10 1 the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust a redaction in the viscosity ratio b c 10 2 is sufficient to cause a transition of the central wedge region to a plateau this transition depends on the basal traction therefore the thickness of the weak basal layer also affects the transition further reduction of the viscosity ratio b c 10 4 leads to full basal decoupling and the development of plateaux in all cases considered in most models the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust where the crust is fully decoupled large scale model geometries for both depth and temperature dependent rheologies are similar with gravity driven flow concentrated in the low viscosity region however strong lateral temperature gradients within these models controlled by the interaction of horizontal and vertical thermal advection diffusion and heterogeneous thickening of the radioactive crustal layer lead to differences in the velocity and deformation fields between the two cases particularly at the plateau margins the results suggest that simple depth dependent viscosity models may be reasonable approximations for describing the large scale geometry of fully developed plateaux but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry

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

deep thermal state on the eastern margin of the lhasa gangdese belt and its constraints on tectonic dynamics based on the 3 d electrical model abstract since the mesozoic the lhasa gangdese block has undergone several stages of tectonism magmatism caused by the closure of the new tethys ocean and the collision between the indian and eurasian plates which led to the lithospheric shortening and crustal uplift through the careful processing and analysis of the magnetotelluric data on the eastern margin of the lhasa gangdese block near 92 e a reliable 3d electrical model was obtained to examine the relationship between the lithospheric electrical structure and geodynamics some conductive and resistive layers were obtained which may be formed in some tectonic processes the thermal model of the lithospheric uppermost mantle 75 100 km were calculated using the arrhenius equation and the hashin shtrikman hs boundary condition combined with the terrestrial heat flow point and curie isothermal in the study area the temperature distribution at the 30 100 km depths was calculated the melt fractions of the mid lower crust calculated by the classical archie s law pressure calculated by density and temperature at different depths can be obtained in a meantime the resistor beneath the tethys himalaya terrane represented the subduction of the indian plate with a deep angle the subduction of the indian crust may not exceed the indus yarlung zangbo suture while the indian lithospheric mantle may not reach the luobadui milashan fault additionally our electrical and thermal structure indicated that the indian lithospheric mantle probably detached from the indian crust our study further discussed the underplating of the mantle derived materials and the southern extrusion along the main himalaya trust and offered a subsidiary evidence for the possible south eastward crustal flow in a local area finally the relationship between the electrical structure and the deep metallogenic mechanism was briefly established

channel slope reversal near the martian dichotomy boundary testing tectonic hypotheses abstract faults along the martian dichotomy boundary are evidence of tectonic activity which by analogy with terrestrial tectonism may cause changes in or even reversal of fluvial longitudinal profiles in the eastern hemisphere between 30 e and 150 e this tectonic activity has been hypothesized to result from lower crustal flow or from lithospheric flexure for which loading e g by material deposition of the northern lowlands is a possible cause watters 2003a nimmo 2005 the topographic slope changes resulting from these two different mechanisms are distinct and can provide a means for distinguishing between them although other processes may complicate interpretations of reversed longitudinal profiles two fan shaped networks of inverted channels are located near 150 e in the aeolis dorsa region just north of the dichotomy boundary their original flow directions inferred from planform morphology suggest flow to the northeast in contrast to their current longitudinal profiles sloping down to the southwest this contrast indicates slope reversal we investigate the lower crustal flow and flexure mechanisms for slope reversal by testing three different hypotheses 1 lower crustal flow 2 flexure caused by material erosion from the highlands and deposition in the lowlands and 3 flexure caused by highland erosion and deposition in the lowlands plus deposition of the medusae fossae formation in the lowlands we test these three hypotheses by comparing the inferred magnitudes of the slope reversals with predicted slope changes from geophysical models for these processes taking the possibility of non tectonic i e collapse processes into account our results suggest that among these three models the slope reversal is most consistent with the predicted tectonic response to erosion and deposition of highland material in conjunction with deposition of the medusae fossae formation contrary to previous findings our results do not support the mechanism of lateral crustal flow although they do not rule it out because lower crustal flow and erosion may both have been operating but on different time scales the result of this hypothesis testing provides insight into the evolution of the dichotomy boundary and martian crust in this location

late pliocene onset of the cona rift eastern himalaya confirms eastward propagation of extension in himalayan tibetan orogen abstract several competing hypotheses have been proposed to explain east west extension observed across the himalayan orogen based primarily on observations from the western and central himalaya they make predictions for the temporal and spatial patterns of deformation in the eastern himalaya these tectonic models include radial spreading or oroclinal bending of the himalayan arc oblique convergence of the indian continent tearing or lateral detachment of the indian slab and eastward flow of lithosphere here for the first time we constrain the activity history of the cona rift the only north trending rift currently recognized in the eastern himalaya based on biotite and k feldspar 40ar 39ar and zircon and apatite u th he thermochronology to evaluate these proposed rifting mechanisms low temperature thermochronological results suggest that the cona rift is the youngest rift system in the himalayas after a slow phase of exhumation since 14 ma 0 2 0 13 mm yr normal faulting initiated here at 3 0 2 3 ma with a fault slip rate of 3 8 1 6 mm yr and a horizontal extension magnitude of 2 5 km analysis of rifting across the himalayas shows that rift initiation ages young eastward which is matched by eastward decreasing rift extension magnitudes monotonically eastward younging rift development is consistent with the tectonic model involving eastward lithospheric flow driven by indian slab dynamics and coupled asynchronous gravitational potential energy gradients

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

trace elemental and sulfur lead isotopic variations in metamorphosed volcanogenic massive sulfide vms mineralization systems an example from the keketale pb zn ag deposit nw china abstract the effects of post vms volcanogenic massive sulfide deformation metamorphism on sulfide trace element and sulfur lead isotopic compositions remain unclear the greenschist to lower amphibolite facies metamorphosed keketale vms pb zn ag deposit located in the devonian maizi volcanic sedimentary basin in the chinese altay nw china provides an opportunity to resolve the abovementioned issue two mineralization stages are recognized at keketale including 1 primary banded massive and disseminated ores derived from sea floor hydrothermal mineralization and 2 ore remobilization as represented by intensively deformed ores and quartz polymetallic sulfide ore veins that crosscut the primary ores sulfide trace elements data suggest that stage ii pyrite and pyrrhotite have higher contents of cu zn ag sb and pb whereas contents of these elements are higher in stage i sphalerite and galena than their stage ii counterparts such elemental remobilization was probably caused by the rapid intragrain diffusion and subsequent fluid mediated liberation with trace elements re precipitated as particles within the same grain and nearby minerals respectively the 34scdt values in both stages show a trimodal distribution 25 9 to 22 7 17 0 to 10 6 and 0 6 to 2 1 sulfide mineral pairs formed during primary vms mineralization are not in sulfur isotopic equilibrium and only localized re equilibration occurs during deformation and metamorphism in addition the lead isotopic values of galena from remobilized sulfide veins overlap with those of the stage i galena and pyrite these data indicate that regional deformation and metamorphism of the keketale deposit can be approximated as a closed system in terms of sulfur lead isotopes whilst the presence of the anomalously low 34s values minimum 25 9 and the distinct lead isotopic ratios of sulfides from the calcite quartz veins argue for that in the late part of the metamorphic event some exotic lead and light sulfur were sourced from outside the ore field in situ 34s values of stage i sulfides vary widely 17 0 to 2 1 suggesting that the sulfur was originated from bacterial sulfate reduction bsr of marine sulfates with probably some magmatic input lead isotopes of the sulfides and mineralized meta sedimentary volcanic rocks display a linear distribution between the mantle and upper crust line in the 206pb 204pb vs 207pb 204pb diagram the lead source may be derived from both felsic volcanic rocks and mafic igneous rocks and or magmatic volatiles and the sulfur lead isotopes may result from a shallow level seawater circulating process an additional group of lead isotopic compositions of the meta volcanic rocks show elevated present day 206pb 204pb 207pb 204pb and 208pb 204pb values probably due to decay of th and u carried by deeply penetrative circulation 13 km of co2 f cl rich hydrothermal metamorphic fluids both syn vms shallow seawater and post vms deep metamorphic fluids circulation processes are critical for the metal concentration at keketale

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

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

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

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure

crustal strength in central tibet determined from holocene shoreline deflection around siling co abstract controversial end member models for the growth and evolution of the tibetan plateau demand quantitative constraints of the lithospheric rheology direct determinations of bulk crustal rheology however remain relatively sparse here we use the flexural rebound of lacustrine shorelines developed during the lingtong highstand around siling co in central tibet to place bounds on the effective elastic thickness t e and viscosity of tibetan crust shoreline features associated with the lingtong highstand complex 60 m above present lake level are deflected from horizontal by 2 4 m over wavelengths of 200 km optically stimulated luminescence dating of aggradational shoreline deposits indicates that these lake levels were reached at 6 4 ka assuming that surface loads were entirely supported by an elastic layer overlying an inviscid fluid the range and spatial distribution of variations in shoreline elevation are consistent with deflections predicted by a uniform elastic plate with thickness t e of 20 30 km if viscoelastic relaxation in response to lake withdrawal is complete our data suggest an average viscosity 10 19 pa s these results imply that the apparent viscosity of the lower crust inferred over millennial timescales is comparable with that estimated from post seismic relaxation over decadal timescale