upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

intracontinental subduction and palaeozoic inheritance of the lithosphere suggested by a teleseismic experiment across the chinese tien shan teleseismic tomography across the chinese tien shan shows that seismic wave speeds in the lithosphere beneath central tien shan are high and therefore the lithosphere is not weaker than that beneath the adjacent undeformed tarim and junggar basins there is evidence for significant velocity contrasts within the lithosphere that are presumably inherited from the palaeozoic collision history the high velocity thick yili block observed underneath the northern tien shan is a clue for shortening by a intracontinental subduction the observed geometry is consistent with a simple model of intracontinental subduction and suggests that during orogeny the lithosphere has remained heterogeneous and has deformed along existing planes of weakness rather than by homogeneous thickening of a particularly weak lithosphere

three dimensional thermo mechanical modeling of the cenozoic uplift of the tianshan mountains driven tectonically by the pamir and tarim abstract we develop a three dimensional thermo mechanical model as an approximate representation of the lithosphere of the tianshan orogenic belt and adjacent geological units for simulating the uplift mechanism of the tianshan mountains in the cenozoic we apply a velocity boundary condition approximating the northward push of the pamir and tarim against the tianshan block to derive lithospheric deformation of the model in particular we investigate systematically the influence of gravity induced creep deformation on mountain uplift our modeling results indicate that 1 the modeled topographic pattern coincides well with the actual topography implying that the tectonic push of the pamir and tarim can cause significant uplift of the tianshan mountains 2 the lithosphere of the tianshan mountains is mechanically weaker than that of the platforms to the south and north of the tianshan orogenic belt and 3 the gravity induced creep deformation has a significant influence on the process and elevation of mountain uplift therefore when subjected to the tectonic compression resulting from the northward push of the pamir and tarim the relatively weak lithosphere of the tianshan mountains would deform easily and hence absorb more shortening resulting in lithospheric thickening and consequent mountain uplift

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

rapid changes in the redox conditions of the western tethys ocean during the early aptian oceanic anoxic event the early aptian 125 to 121 ma records an episode of severe environmental change including a major perturbation of the carbon cycle an oceanic anoxic event oae 1a 122 5 ma a platform drowning episode and a biocalcification crisis we propose to trace changes in the oxygenation state of the ocean during the early aptian anoxic event using the redox sensitive trace element rste distribution phosphorus accumulation rates pars and organic matter characterization in three different basins of the western tethys the following sections have been investigated gorgo a cerbara central italy in the umbria marche basin glaise se france in the vocontian basin and cassis la bedoule se france located in the provencal basin in the gorgo a cerbara section rste distributions show a low background level along the main part of the section contrasted by different maxima in concentrations within the selli level in the glaise section the goguel level displays a weak increase in rste contents coeval with moderate toc values at cassis la bedoule no significant rste enrichments have been observed in sediments equivalent to the selli level these differences in the records of the geochemical proxies of the selli level or its equivalent indicate the deposition under different redox conditions probably related to the paleogeography our data indicate the development of anoxic euxinic conditions in the deeper part of the tethys during oae 1a whereas in the shallower environments conditions were less reducing moreover at gorgo a cerbara the selli level is characterized by rapid changes in the intensity of reducing conditions in the water column ocean eutrophication seems to be a major factor in the development and the persistence of anoxia as suggested by the par evolution higher par values at the onset of oae 1a suggest an increase in nutrient input whereas the return to lower values through the first part of the oae 1a interval may be related to the weakened capacity to retain p in the sedimentary reservoir due to bottom water oxygen depletion this general pattern is contrasted by the data of gorgo a cerbara where the sediments deposited during the oae 1a interval show p enrichments mainly authigenic p this is associated with maxima in toc values and corg ptot ratios suggesting that a part of the remobilized p was trapped in the sediments and as such prevented from returning to the water column

antigorite induced seismic anisotropy and implications for deformation in subduction zones and the tibetan plateau the present study which is a follow up of the journal of geophysical research paper by ji et al 2013a provides a new calibration for both seismic and fabric properties of antigorite serpentinites comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high t serpentinite which is essentially controlled by the antigorite c axis fabric is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock extrapolation of the experimental data with both pressure and temperature suggests that vp anisotropy decreases but shear wave splitting vs and vp vs increase with increasing pressure in either cold or hot subduction zones for a cold steeply subducting slab antigorite is most likely deformed by nearly coaxial flattening or trench parallel movements forming trench parallel seismic anisotropy for a hot shallowly subducting slab however antigorite is most likely deformed by simple shear or transpression trench normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30 the geophysical characteristics of the tibetan plateau such as strong heterogeneity in vp vs and attenuation shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks hydrated zones of subducting lithospheric mantle and the crust mantle boundary if the temperature is below 700 c in the region of interest

seismic azimuthal anisotropy beneath the pakistan himalayas teleseismic s sks and skks data collected from a temporary broadband array across the himalayan front in pakistan are analyzed for shear wave splitting parameters the sks and skks phases have ray paths originating from both the south pacific and colombia which have azimuths approximately 40 apart with respect to the pakistan array if significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths no evidence was seen for any shear wave splitting beneath any of the stations in the array teleseismic s waves were also used in order to provide better azimuthal coverage for the shear wave splitting measurements we were able to correct for any source side anisotropy when needed no receiver side splitting was observed in any of the s wave data the lack of shear wave splitting beneath the pakistan array indicates that there is no appreciable large scale azimuthal anisotropy beneath this part of the himalayas therefore if there is any significant strain in the upper mantle beneath this area it must either be vertically oriented or if horizontal vertically vary in such a way that the integrated effect on s wave splitting is null

seismic anisotropy in the java banda and philippine subduction zones and its implications for the mantle flow system beneath the sunda plate

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

gravity anomalies the deep structure and dynamic processes beneath the tien shan bouguer gravity anomalies over the tien shan and pamir are large and negative and therefore consistent with crustal thickening beneath both mountain ranges gravity anomalies over the western tien shan are within about 10 mgal of those expected for local airy isostatic equilibrium but those over the eastern tien shan are up to 50 mgal too large and negative thus they require static or dynamic support of an uncompensated mass deficit these gravity anomalies cannot be matched using regional compensation of the weight of the range on a continuous elastic plate a simple model of two separate elastic plates extending north and south from the axis of the tien shan can allow an adequate fit of observed and calculated anomalies the flexural rigidity of the northern plate must be quite small d 1023 n m corresponding to an equivalent effective elastic thicknesste 25 km but that of the southern plate must be larger d 1024 n m te 50 60 km the data for the eastern tien shan cannot be fit without a force per unit length of roughly 1 4 1012 n or by a bending moment applied to the end of the southern plate thus either a deep mass per unit length of about 1 4 1011 kg m must underlie the eastern part of the range or dynamic deviatoric tensional normal stresses of up to about 5 mpa due to flow in the asthenosphere must pull the tien shan down probably a combination of them resulting from the formation and sinking of a thickened lithospheric root causes the deflection of the moho and the deviations from local isostatic equilibrium

seismic tomographic imaging of the crust and upper mantle under the central and western tien shan orogenic belt 1 detailed 3 d tomographic images of p and s wave velocity vp vs and poisson s ratio under the central and western tien shan orogenic belt are determined by using a large number of high quality p and s wave arrival times from local earthquakes the results show that under the tien shan orogenic belt high vp high vs and low anomalies are revealed in the upper and middle crust possibly indicating the existence of the paleozoic crystalline basement rocks while low vp low vs and high anomalies appear in the lower crust and upper mantle perhaps suggesting that the hot and wet material is upwelling under the tien shan orogenic belt from the mantle some high vp high vs and low anomalies are tilted toward the tien shan along with the seismicity these are found in the collision zones between the tien shan and the tarim basin in the south and the kazakh shield in the north and suggest the underthrusting of the tarim and kazakh lithosphere beneath the tien shan meanwhile some low vp low vs and high anomalies are imaged in other parts of these collision zones perhaps indicating the intrusion of the hot and wet material into the crust from the upper mantle these results indicate that both the upwelling of the hot and wet material and the underthrusting of the tarim and kazakh lithosphere may have played an important role in the mountain building under the tarim and fergana basins low vp low vs and high anomalies are revealed in the upper crust while high vp high vs and low anomalies are visible in the lower crust and upper mantle these may reflect the existence of less compacted sedimentary material in the shallow crust and more highly compacted craton like structures in the deeper crust and upper mantle under the basins the talas fergana fault shows an obvious tectonic boundary between central and western tien shan the central tien shan displays high vp high vs and low anomalies in the upper and middle crust while western tien shan exhibits low vp low vs and high anomalies however the pattern of seismic structure between central and western tien shan reverses in the lower crust such a correlation may extend down to the upper mantle suggesting that the talas fergana fault may be a lithospheric scale boundary additionally a columnar low vp and low vs anomaly is clearly observed around the turning point of the talas fergana fault from the nwn to nww trending orientations and may indicate that the fault provides a channel for the hot and wet material upwelling from the mantle to the surface

crustal and upper mantle structure under the tien shan from surface wave dispersion long period seismograms of nuclear explosions and natural earthquakes in central asia recorded at three digital stations of the laboratoire de detection geophysique ldg network in france attest to a significant propagation effect in the tien shan area this allows for an investigation of the deep structures under the range which has been the site of particularly intense intracontinental deformation since about the middle miocene group velocity curves have been obtained by regionalization for pure paths across either the central or eastern tien shan they indicate a crust of approximately 50 55 km in thickness and particularly low shear wave velocities under the central tien shan our study suggests lateral variation of the deep structures under the range which could be related to a westward increase along the range of cumulative amount and rate of shortening

attenuation of high frequency p and s waves in the crust of central and western tien shan in order to better understand the attenuation characteristics in the crust of the central and western tien shan orogenic belt we investigate both qp and qs values by applying the extended coda normalization method we estimate the frequency dependent attenuation of both p and s waves in the frequency band of 1 0 20 0 hz using data from local networks the average frequency relations of qp and qs have been derived as qp 61 9 f 1 21 0 08 and qs 77 6 f 1 11 0 04 by fitting a power law frequency dependence model for the study region the low q0 and high values indicate that the central and western tien shan is a tectonically and seismically active region we also find lateral variations of both qp s and qs qp values reflecting complex tectonic structures in the study area in general the relatively high attenuation areas corresponding to tectonically active regions are found beneath mountainous ranges in the central tien shan whereas the relatively low attenuation areas are associated with stable regions such as fergana basin and issyk kul lake besides regions in the central tien shan with qs qp 1 are likely partially saturated with fluids or rich in scattering heterogeneities whereas regions in the western tien shan display qs qp 1 possibly suggesting almost complete fluid saturation

azimuthal anisotropy beneath the bohemian massif from broad band seismograms of sks waves abstract shear wave splitting in sks phases recorded by the two broad band stations khc south bohemia and ksp lower silesia is used to determine the anisotropic properties of the lithosphere beneath the marginal regions of the bohemian massif directions of fast velocity of 80 and 120 clockwise from north were obtained for khc and ksp respectively these directions are consistent with the predominantly e w direction of fast velocity found by different authors using similar techniques in some regions of western europe they disagree however with the directions found for the investigated region by other authors on the basis of teleseismic p wave traveltime residuals the value of the time delay between the two splitted shear waves is close to 1 s for both stations implying that the thickness of the anisotropic zone in the upper mantle essentially exceeds 80 km

depth variations of azimuthal anisotropy beneath the tian shan mt range nw china from ambient noise tomography abstract seismic anisotropy beneath the tian shan mt range has been well studied with shear wave splitting the origin of asthenospheric anisotropy is mainly attributed to the north south shortening however the crustal anisotropy and its depth variation are poorly constrained in this study we focus on inversion of rayleigh wave phase velocity azimuthal anisotropy in the 5 35 s period bands beneath the tian shan mt range through analysis of continuous seismic ambient noise we use approximately 7 months of available continuous seismic data recorded by 31 broadband seismic stations deployed in the tian shan mt range the tomographic results reveal substantial variations in the azimuthal anisotropy with an increase in the depth in the upper crustal depths the orogen parallel or orogen subparallel azimuthal anisotropy observed in the tian shan mt range likely originates from the lattice preferred orientation of crustal minerals in response to compression whereas the orogen perpendicular or orogen sub perpendicular azimuthal anisotropy in the southern and northern piedmont of the tian shan mt range is related to the stress alignment of cracks fractures due to ongoing north south shortening the pattern of azimuthal anisotropy in the middle to lower crustal and upper most mantle is different from that at the upper crustal depths an orogen perpendicular or orogen sub perpendicular fast direction associated with prominent lower velocities has been observed north and south of lake issyk kul the spatial correlation between the azimuthal anisotropy and the lower velocity anomalies at the middle to lower crustal and upper most mantle depths may suggest a warm melt rich upper mantle below the tian shan mt range which could be attributed to the upwelling asthenospheric mantle flow in that region

seismic studies of the upper mantle and transition zone in the last few years seismic studies of the upper mantle and transition zone have achieved significant new results in several areas progess has been driven largely by the increasing size quality and accessibility of digital seismic datasets from global and regional networks and from experiments using portable instruments this paper focuses on a few of the most interesting and important developments these include 1 measurements of upper mantle anisotropy obtained using a variety of techniques which can reveal details of the flow regimes and tectonic history of the upper mantle 2 improvements in models of upper mantle velocity and attenuation structure with some controversy arising regarding how deep slow velocity anomalies extend beneath oceanic ridges 3 direct seismic observations of topography on the 410 and 660 km discontinuities in the upper mantle providing constraints on mantle temperatures and dynamics and 4 observations suggesting secondary seismic discontinuities in some regions

observations of teleseismic shear wave splitting in the basin and range from portable and permanent stations observations of shear wave splitting were obtained from temporary and permanent stations in western nevada and southern california measurements of fast polarization azimuth and delay time t were made on both sks and direct s phases for stations in the northern basin and range the results were consistent yielding an average of t 75 8 0 9 0 3 s one station in the southern basin and range gave 40 11 0 4 0 1 s and one in the mojave desert gave 54 3 1 2 0 1 s t 0 9s is consistent with a 100km thick mantle layer characterized by 4 anisotropy for the northern stations the anisotropy appears unrelated to the present day extension or absolute plate motion rather we suggest that it is fossil anisotropy associated with pre miocene extension whose direction is about 68 for the california station is nearly parallel to the strike of the san andreas fault system and is attributed to shear strain associated with relative plate motion

upper mantle anisotropy a preliminary model seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion the physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral the dominant glide systems are assumed to be 100 0l0 for olivine and 001 100 for the pyroxenes these yield a predominantly orthorhombic fabric with the olivine 100 and the pyroxene 001 axes aligned in the upper mantle flow direction and the olivine 010 and the pyroxene 100 axes aligned normal to the upper mantle flow plane these glide systems have a threshold temperature of enhanced mobility of 1100 1200 k which yields a solid state thermally defined lithosphere asthenosphere boundary in an olivine pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere mantle anisotropy due to mineral alignment is then actively maintained below this boundary asthenosphere and mesosphere and is a fossil state above this boundary lithosphere we use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models pyrolite and piclogite for the upper mantle by an extrapolation of single crystal anisotropic mineral elastic properties to a depth of 400 km the real earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy isotropy from random mineral alignment the seismic anisotropy of the upper 220 km is best represented by the pyrolite model which reduces to quasi hexagonal symmetry with the unique axis in the direction of mantle flow the lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy the piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite the piclogite anisotropy model can appear quasi isotropic when measured by transverse isotropy parameterization if the mantle flow is mainly horizontal with a horizontal shear plane that is the shear flow gradient duh dr dominates

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

cretaceous paleogene basaltic rocks of the tuyon basin nw china and the kyrgyz tian shan the trace of a small plume abstract the tuyon basin nw china and the central portion of the kyrgyz tian shan preserve a volumetrically small series of basaltic extrusive and intrusive units emplaced primarily in mesozoic paleogene sedimentary rocks four units from the tuyon basin yield 40 ar 39 ar ages of 67 59 46 and between 110 and 120 ma major and trace element analysis combined with sr nd and pb isotopic data suggest that these units were derived from apparently ocean island basalts oib like mantle melts which have characteristics close to asthenospheric prema mixed with an enriched component in the continental mantle lithosphere or emplaced with some crustal contamination two basalts from kyrgyzstan were also dated by 40 ar 39 ar as 54 ma these results agree with previous k ar analysis which show that most kyrgyz basalts were emplaced during the eocene major element data from the kyrgyz basalts knauf et al 1980 fortuna et al 1994 show strong similarities with the tuyon data suggesting that the more extensive tuyon data can be taken as representative of the entire basaltic suite there is little geological data to support either late cretaceous paleogene rifting or the presence of a large plume beneath the region instead these magmas point to the presence of a small thermally anomalous region in the mantle beneath the central tian shan which might best be described as a small plume which probably rooted at a shallow level in the asthenosphere

shear velocity model for the kyrgyz tien shan from joint inversion of receiver function and surface wave data the tien shan is the largest active intracontinental orogenic belt on earth to better understand the processes causing mountains to form at great distances from a plate boundary we analyse passive source seismic data collected on 40 broad band stations of the manas project 2005 2007 and 12 stations of the permanent krnet seismic network to determine variations in crustal thickness and shear wave speed across the range we jointly invert p and s wave receiver functions with surface wave observations from both earthquakes and ambient noise to reduce the ambiguity inherent in the images obtained from the techniques applied individually inclusion of ambient noise data improves constraints on the upper crust by allowing dispersion measurements to be made at shorter periods joint inversion can also reduce the ambiguity in interpretation by revealing the extent to which various features in the receiver functions are amplified or eliminated by interference from multiples the resulting wave speed model shows a variation in crustal thickness across the range we find that crustal velocities extend to 75 km beneath the kokshaal range which we attribute to underthrusting of the tarim basin beneath the southern tien shan this result supports the plate model of intracontinental convergence crustal thickness elsewhere beneath the range is about 50 km including beneath the naryn valley in the central tien shan where previous studies reported a shallow moho this difference apparently is the result of wave speed variations in the upper crust that were not previously taken into account finally a high velocity lid appears in the upper mantle of the central and northern part of the tien shan which we interpret as a remnant of material that may have delaminated elsewhere under the range

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

variations in sks splitting across western canada measurements of sks splitting parameters have been made at stations of the canadian national seismograph network for events over the period 1993 mid1994 here we report on results obtained from western stations in the northern cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently splitting parameters in the southern cordillera however are oriented approximately perpendicular to the strike of major tectonic features notably the cascadia subduction zone this contrast in the character of fast polarization directions between the southern and northern cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern b c coast to transform north of the triple junction one plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the juan de fuca ridge subduction along the coast and associated back arc asthenospheric flow beneath the b c interior

late cenozoic tectonics of the kepingtage thrust zone interactions of the tien shan and tarim basin northwest china the kepingtage kalpin thrust zone northwest china is an actively deforming part of the india asia collision system it lies south of the tien shan has an area of 16 000 km2 and consists of arcuate emergent imbricates overall vergence is toward the interior of the tarim basin to the south thrust sheets typically expose upper cambrian to permian platformal strata thrusting is largely thin skinned thin upper cambrian evaporites are likely to be the main decollement horizon the kepingtage thrust zone is the only margin of the tarim basin to deform in this style in the cenozoic it is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the tarim basin thick up to 10 km mesozoic and cenozoic clastic successions are present and deformed in these regions such successions are not present in the kepingtage thrust zone or the adjacent bachu uplift within the tarim basin because of episodic activity on steep northwest southeast trending thrusts which define the margins of the bachu uplift these mesozoic cenozoic strata may have suppressed the ability of regions along strike from the kepingtage thrust zone to deform by thin skinned thrusting utilizing the upper cambrian decollement the along strike variation in active thrusting at the southern margin of the tien shan is an example of syntectonic sedimentation controlling thrust belt deformation style a balanced section across the thrust zone indicates 28 shortening equivalent to 35 km this is equivalent to an average slip rate of 1 8 mm yr 1 and a strain rate of 4 4 10 16 s 1 assuming deformation began at circa 20 ma active deformation is focused along the frontal thrust the kepingtage fault the northern boundary of the kepingtage thrust zone is formed by the south tien shan fault this major north dipping thrust is seismically active with published earthquake focal depths at midcrustal levels 14 and 18 km it juxtaposes upper carboniferous sedimentary rocks of different facies and different paleozoic deformation histories speculatively it represents a reactivation of a late paleozoic thrust which originated at the boundary between the platformal interior of the tarim block and a deeper water foreland basin to its north

velocity variations in the crust and upper mantle beneath the tien shan inferred from rayleigh wave dispersion implications for tectonic and dynamic processes we used data from nuclear explosions and earthquakes recorded by the kyrgyzstan telemetered broadband seismic network to examine crustal structure in the tien shan region of central asia seventy nine measurements of fundamental and first higher mode rayleigh waves were analyzed using the single station multiple filtering technique to determine group velocity dispersions we found that there was a distinct difference in rayleigh wave group velocity values between different regions but the data could be combined in three groups 1 paths from the west and south across the turan platform and western tarim basin 2 paths across the eastern tarim basin and 3 nuclear explosions from the lop nor test site group velocity values were inverted to derive one dimensional shear wave velocity models for the tien shan region results reveal crustal thickness varying from 50 to 60 km with thickness increasing toward the eastern tien shan region in agreement with published crustal thickness maps for the region array analysis indicates that paths along the eastern tien shan and tarim tien shan are affected by lateral refractions and multipathing but we argue that these have a small effect on the measured dispersion curves our results indicate that upper mantle velocities in the eastern tarim and tien shan are significantly lower than those beneath the western tien shan and turan platform our results suggest that the boundary between these two regimes lies somewhere west of the depocenter of the tarim basin and east of kashgar china

complex seismic anisotropic structure beneath the central tien shan revealed by shear wave splitting analyses summary shear wave splitting parameters fast polarization directions and splitting times obtained at the permanent broad band station wushi wus which is situated in central tien shan china demonstrate systematic azimuthal variations the fast polarization directions range from 26 to 85 and the delay times range from 0 4 to 1 6 s suggesting a complex anisotropic structure that cannot be adequately explained by a single anisotropic layer with horizontal axis of symmetry the azimuthal variations have an apparent 90 periodicity and thus can be best explained by a two layer anisotropic model with horizontal symmetry axes the optimal fast direction for the upper layer is in the 60 80 range that is subparallel to the strike of the tien shan orogenic belt and can consequently be attributed to the ongoing n s directed shortening the fast direction for the lower layer ranges from 85 to 105 and can be explained by lattice preferred orientation of olivine crystals in the asthenosphere associated with present day absolute plate motion of the eurasian plate

lithospheric structure of the central himalaya from 3 d tomographic imaging abstract the high himalaya are one of the most spectacular consequences of the ongoing collision between india and eurasia yet the role that the lithosphere beneath them play in this collision is not well understood to illuminate the influence of the himalaya on collisional processes we produced tomographic images of the subsurface beneath them from arrival times of p and s waves generated by 551 teleseisms and 1051 local regional events and recorded by the himalayan nepal tibet broadband seismic experiment himnt from 2001 to 2002 compared to those in initial 1d models the rms p and s arrival time misfits in our final 3d model were reduced by as much as 46 from 0 45 s to 0 24 s and 38 from 0 97 s to 0 6 s respectively our results show strong crustal variations perpendicular to but no significant lateral variation along the trend of the high himalaya within our study area in the ns direction the crust thickens from about 60 km under the lesser himalaya to 70 80 km under the tethys himalaya there is no localized crustal thickening beneath the high himalaya wavespeeds and seismicity patterns beneath the tethys himalaya suggest that the middle crust at 30 50 km depth is ductile and are consistent with the presence of a mid crustal reflector associated with the stronger gradients in the lower crust correlation between relocated earthquakes and the xainza dinggye rift indicates that the rift is not cutting through the crust at the present time shear wave splitting from core phases indicates that the upper mantle beneath the tethys himalaya is nearly isotropic the combination of isotropy and higher seismic wavespeed leads us to suggest that eclogites included in the lowermost crust and the uppermost mantle are responsible for the mid crustal reflector under the tethys himalaya as well as for a slightly high wavespeed anomaly just beneath the moho this interpretation is supported by the existence of granulitic eclogite in the ama drime massif 50 km northeast of mt everest

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

elevation of the 410 km discontinuity beneath the central tien shan evidence for a detached lithospheric root we analyzed p sv converted phases recorded by the kyrgyzstan broadband network knet to investigate the nature of phase transitions in the upper mantle beneath the central tien shan and the kazakh shield we find that p sv phases from the 410 km discontinuity recorded by several stations located along the range front are about 2 s earlier from beneath the tien shan than they are from beneath the kazach shield because the delay times at stations located on the kazakh shield are normal and seismic velocities in the upper mantle beneath this part of the tien shan apparently are low about 4 less than the kazakh shield to the north the early arrival of the conversion from 410 km implies that this discontinuity is elevated by about 20 km because the 410 km discontinuity can be elevated by the introduction of cold material our results are consistent with a model in which the lithosphere in this area developed a root during collision that later detached and is now residing near 410 km depth

more thoughts on convergent crustal plateau formation and mantle dynamics with regard to tibet evidence suggests that some 10 m y ago a large portion of thickened mantle lithosphere below the tibetan plateau was rapidly removed removal appears to have been abrupt and delayed from the time of substantial thickening fluid dynamical models of convective thermal boundary layer instability have had difficulties explaining abrupt lithospheric removal long after initial thickening such models have implied that either removal should be synchronous with thickening or should not occur at all due to boundary layer stabilization resulting from high lithospheric viscosity we present simple thermal chemical boundary layer convection models that suggest a compromise between previous end member results the models allow for delayed instability of a thickened thermal boundary layer under geologically reasonable parameter conditions instability delay relies on high viscosity of thickened mantle lithosphere stabilizing it against free convective removal and on lateral extent of thickened crust exceeding that of mantle lithosphere the latter condition leads to a peripheral region of thickened crust and relatively unthickened mantle lithosphere and an interior plateau region of both thickened crust and mantle lithosphere if the thermal resistance and or the rate of internal heat production of crust is greater than that of mantle then mantle lithosphere at the peripheral region can be thermally eroded to the point that the base of the crust comes into contact with bulk mantle this creates a peripheral weak zone that allows thickened mantle lithosphere of the interior plateau region to become mechanically detached and sink into bulk mantle the instability is akin to delamination as originally defined and its delay time is roughly the thermal diffusion time across the thinnest portion of mantle lithosphere

depth localized azimuthal anisotropy from sks and p receiver functions the tien shan summary shear wave splitting in the seismic sks phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations fast s wave polarization direction in collisional belts is often parallel to the trend of the belt although deformations of the mantle lithosphere in low angle thrusts would lead to the fast polarization direction normal to the trend of the belt these considerations suggested that the upper mantle in collisional belts is decoupled from the crust however sks technique is notable by a poor depth resolution and usually it assumes that the fast polarization direction is the same at any depth which is hard to justify here to investigate depth dependent azimuthal anisotropy in the mantle we invert jointly p receiver functions and sks particle motions at a number of seismograph stations the technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium a search for the optimum models is conducted with a technique similar to simulated annealing testing with synthetics demonstrates that this approach is robust the results for 10 seismograph stations in the tien shan the world s most active intracontinental collisional belt in central asia reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km in the mantle lithosphere at depths less than 100 km anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range this layer is not necessarily decoupled from the crust its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past in the lower layer asthenosphere the average azimuth of fast wave polarization is close to the trend of the belt whereas magnitude of s wave anisotropy is stable and large between 5 and 6 per cent this anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt at some stations the data require anisotropy in the crust there is no evidence for anisotropy at depths exceeding 150 250 km

lithospheric temperature estimates from seismic attenuation across range fronts in southern and central eurasia seismic attenuation 1 q changes abruptly between the stable eurasian craton and adjacent mountain belts associated with the alpine himalayan collision new attenuation measurements on local body and coda waves provide quantitative constraints on this contrast near the caucasus kopet dagh and tien shan mountains the 1 q estimates from all range fronts are similar the mountains being two to three times more attenuative than adjacent stable platforms these 1 q variations are a reasonable proxy for temperature variations and reveal crustal heating on the order of 100 c but probably not melting beneath the mountain ranges attenuation 1 q is high wherever high topography is seen implying that active mountains are typically hot hence processes that generate or transport heat dominate advective cooling due to lithospheric thickening even over the 10 20 m y lifetimes of these young mountain belts

directional variations in travel time residuals of teleseismic p waves in the crust and mantle beneath northern tien shan we study the directional variation in travel time residuals using 13 820 p wave arrivals from 1 998 teleseismic events 15 d 98 4 1 mb 7 3 recorded in 1991 1997 by the kyrgyz digital seismic network knet based on a modified version of the iasp91 model that accounts for the kyrgyz crustal thickness beneath knet we convert p wave travel times to travel time residuals dt the de pendence of dt on backazimuth is modeled as one two and four lobed variations in a horizontal plane backus 1965 a least squares fit of the azimuthal variation of dt indicates that the crust in the northern tien shan is about 11 15 km thicker than it is in the kazakh shield and the chu depression from nine knet stations the one lobe model estimates that the slowest p wave travel time direction is 5 0 4 8 almost directly north and the magnitude of variation is 1 71 0 13 sec this result is consistent with an upwelling lower mantle plume for the two lobe model the slowest p wave travel time directions anisotropy term are 89 7 and 269 7 4 7 i e trending east west we find p wave velocity anisotropy of 2 0 2 9 associated with a layer with a thickness of 440 km at the top of the lower mantle the fast direction of the p wave travel time north south azimuthal anisotropy at the top of the lower mantle is 1 parallel to the absolute motion of the india plate and 2 close to the direction of the upwelling hot mantle flow the last result suggests that the azimuthal aniso tropy of the travel time residuals is due to the shape preferred orientation of middle mantle material that results from plume intrusion shear wave splitting studies mak eyeva et al 1992 wolfe and vernon 1998 estimated the fast polarization direction to be parallel to the strike of the geological structures of the northern tien shan 71 29 thus the fast polarization direction determined from these shear wave splitting studies using knet data contradicts differs by 90 the fast travel time direction 0 3 and 179 7 4 7 we determine here using p wave travel time residuals using knet data this suggests that the azimuthal anisotropy determined from p wave travel time variations and from shear wave splitting in sks and skks have different sources

shear wave splitting beneath the central tien shan and tectonic implications 1 shear wave splitting analyses were performed at 30 seismic stations in the central tien shan and its vicinity fast orientation at most of the stations on the tien shan range is ene wsw parallel to the strike of the tien shan it largely reflects a coherent deformation of the lithosphere in response to the ns shortening anisotropy with nne ssw fast axes is observed within adjacent basins around the issyk kul and near the northwestern tarim where the lithosphere is presumably strong less deformed the nne ssw orientation which coincides with the surface velocity of the central tien shan can be best explained by shear between the lithosphere and asthenosphere on a regional scale rather than small scale convection as proposed in previous studies the dextral talasso fergana fault seems to have little effect on anisotropy at its nearby stations suggesting this fault is probably confined to the shallow part of the lithosphere

three dimensional numerical simulations of crustal deformation and subcontinental mantle convection abstract 3 d simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large scale mantle flow simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 1 1 cartesian domain convective stresses cause crust to thicken above a sheet like mantle downwelling for mild convective vigor an initial crustal thickness variation is required to induce 3 d lithospheric instability below the zone of crustal convergence the amplitude of the required variation decreases with increasing convective vigor morphologically instability is manifest in formation of drip like thermals that exist within the large scale roll associated with initial crustal thickening a strong surface signature of the drips is their ability to cause deviations from local airy compensation of topography after the initial thickening phase the crustal accumulation that forms serves as a model analog to a continent its presence leads to mantle flow patterns distinctly different from the steady state roll that results in its absence large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small scale thermal instabilities eventually these instabilities induce a restructuring of large scale mantle flow with the roll pattern being replaced by a square cell although preliminary and idealized the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2 d mountain chain by the formation of the chain itself the ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low order despite the seemingly trivial volume of crust relative to mantle

small scale convective instability and upper mantle viscosity under california thermal calculations and convection analysis constrained by seismic tomography results suggest that a small scale convective instability developed in the upper 200 kilometers of the mantle under california after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the san andreas transform boundary the upper bound for the upper mantle viscosity in the slab window 5 x 10 19 pascal seconds is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions these model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time dependent small scale convection that can generate localized areas of uplift and subsidence

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

crust and upper mantle structure beneath western china from p wave travel time tomography 1 three dimensional velocity images of the crust and upper mantle beneath western china are obtained through seismic tomography by using p wave arrival times of local regional and teleseismic events recorded in chinese and kyrgyzstan seismic networks at shallow depths most mountain belts and tectonically elevated areas are imaged as higher velocities relative to adjacent sediment filled basins and foreland depressions with lower velocities at mid lower crust depths the kyrgyz tian shan is underlain by lower velocity material and a distinct low velocity zone in the south north direction is observed between the eastern and western parts of the chinese tian shan the moho of the southern tarim basin has a greater depth over 50 km in front of the western kun lun compared to the northern and western tarim basin lateral heterogeneity within the upper mantle is apparent at depths from 71 to 571 km the uppermost mantle beneath the kyrgyz tian shan altay qi lian and eastern kun lun is characterized by low velocity material that is probably related to the upwelling of hot mantle the lithospheric thickness varies from 120 to 170 km beneath the chinese tian shan and northern tibetan plateau to 200 250 km beneath the tarim basin deep coupling pattern of the chinese tian shan with adjacent provinces can be classified into the embedding detaching and sinking and underthrusting of lithospheric roots in addition the lithosphere of the northern edge of the tibetan plateau seems to be thickened in contrast to the central part of the northern plateau north dipping low velocities exist in the upper mantle between the tibetan plateau and provinces to its north which might indicate mantle flow from the tibetan plateau toward surrounding areas but it has been blocked by the thick tarim lithosphere

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

teleseismic shear wave splitting and lithospheric anisotropy beneath and across the altyn tagh fault sixteen 3 component 5 sec stations along a profile crossing the altyn tagh fault atf at 90 e were used to measure shear wave splitting beneath tibet s northern edge splitting delay times t of 0 3 to 1 3 sec show a marked increase in a narrow zone centered on the fault implying anisotropy in the lithospheric mantle fast polarization directions fpd swing counterclockwise from a mean azimuth of 110 e in the qaidam basin south of the fault to a mean azimuth of 60 e in the altyn mountains this swing occurs in less than 40 km both the t increase and fpd swing are consistent with ductile sinistral shear along the 40 km wide low p wave velocity anomaly 6 8 imaged to a depth of 140 km beneath the fault by teleseismic tomography this confirms that the atf guides the northeastwards extrusion of the tibet qaidam s lithosphere relative to the tarim s

significant seismic anisotropy beneath the southern lhasa terrane tibetan plateau 1 shear wave splitting measurements using teleseismic pks skks and sks phases recorded by station lsa on the southern part of the lhasa terrane of the tibetan plateau reveal significant azimuthal anisotropy with a splitting time of up to 1 5 s a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy in addition systematic variations of the splitting parameters fast polarization direction and splitting time with the arriving azimuth of the seismic ray path are observed suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry the measurements are consistent with a model with two layers of anisotropy the top layer has a ne sw fast direction which is consistent with gps revealed direction of surface movement and can be associated with lattice preferred orientation of middle lower crustal minerals such as amphibole the lower layer has a nearly e w fast direction and can be the consequence of either the n s directed compressional stress originated from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia this study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy

inversion of teleseismic s particle motion for azimuthal anisotropy in the upper mantle a feasibility study summary we have developed a technique for the inversion of teleseismic s waveforms in terms of azimuthal anisotropy in the upper mantle we test different models of the earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component the optimum model is found by minimizing the difference between the synthetic vertical component and the observed one using this method we explore the possibility of constraining the distribution of azimuthal anisotropy with depth we present examples of seismic observations where the data are clearly in favour of an anisotropic model these observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes

mantle deformation in sikkim and adjoining himalaya evidences for a complex flow pattern abstract new measurements of azimuthal anisotropy within the himalayan collision zone and southern tibet obtained by applying the sks splitting technique to waveforms from 31 broadband seismic stations reveal significant variations in the strength and orientation of the fast axes stations in the sikkim himalaya show strong anisotropy south of main central thrust mct with delay times 1 s the fast polarization directions are nearly parallel to the strike of the himalaya mountain chain and vary consistently along the profile from 105 in the south to 130 in the north however in the adjacent nepal himalaya a predominance of plate motion related strain is observed at least up to the mct the western portion of southern tibet south of the indus tsangpo suture zone reveals characteristics of e w oriented anisotropy contrary to null anisotropy found in the eastern part the complex flow pattern revealed in the present study may be due a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression

teleseismic p wave tomography and the upper mantle structure of the central tien shan orogenic belt abstract in this study an updated version of the teleseismic tomographic method is applied to 7176 high quality first p wave arrivals from four combined seismic networks to determine the detailed deep structure under the central tien shan our high resolution tomographic model not only displays the general features contained in the previous models but also reveals some new features a much narrower tilted columnar high velocity anomaly with a diameter of about 60 km below 150 200 km depth under the tien shan extends down to the mantle transition zone suggesting the breaking and dropping off of the collided portions of the southward underthrusting kazakh shield and the northward underthrusting tarim basin prominent low velocity low v anomalies above 150 200 km depth beneath the tien shan may be related to two branches of ascending hot anomalies one rises upward from the lower mantle beneath the tarim basin while the other rises up from the top of the mantle transition zone beneath the western kazakh shield these results suggest that the underthrusting of the tarim basin and kazakh shield and the upwelling of hot anomalies indeed play an important role in the mountain building of the central tien shan these dynamic processes might be related to the collision of the indian plate with eurasian plate

shear wave splitting and small scale convection in the continental upper mantle the deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt but this deformation is difficult to observe seismic azimuthal anisotropy manifested by shear wave splitting provides the best geophysical evidence of deformation in the upper mantle here we use observations of shear wave splitting to investigate deformation in the upper mantle beneath the tien shan in central asia we find that underneath most of the tien shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt but it deviates by nearly 90 where the upper mantle is anomalously hot the former relationship previously observed in other collisional belts seems to be usual for this environment the anomalous directions however provide evidence of a rising plume beneath the range suggesting more generally that the along strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection

uppermost mantle structure and its relation with seismic activity in the central tien shan 1 using arrival data from ghengis and kyrgyz network knet stations we determined the uppermost mantle structure of the central tien shan from pn tomography pn velocities in the north central tien shan are slower than normal for stable lithosphere they are very slow in the south central tien shan where a hot mantle previously has been postulated the pn velocity variation correlates well with seismicity most earthquakes are concentrated in the north central tien shan but almost no earthquakes in the south central tien shan even though both geologic and gps observations show active convergence there this correlation suggests a genetic relationship between seismicity and the state of lithospheric mantle specifically we propose that the very slow pn velocity beneath the south central tien shan is caused by high temperatures produced by heating from mantle upwelling and that this heat has conducted well up into the crust inducing a ductile rheology that prevents significant seismicity

morphology of the uv aurorae jupiter during juno s first perijove observations on 27 august 2016 the nasa juno spacecraft performed its first close up observations of jupiter during its perijove here we present the uv images and color ratio maps from the juno uvs uv imaging spectrograph acquired at that time data were acquired during four sequences three in the north one in the south from 5 00 ut to 13 00 ut from these observations we produced complete maps of the jovian aurorae including the nightside the sequence shows the development of intense outer emission outside the main oval first in a localized region 255 295 system iii longitude and then all around the pole followed by a large nightside protrusion of auroral emissions from the main emission into the polar region some localized features show signs of differential drift with energy typical of plasma injections in the middle magnetosphere finally the color ratio map in the north shows a well defined area in the polar region possibly linked to the polar cap