crustal structure and dynamics of the tien shan 1 p receiver functions of many seismograph stations in the tien shan in central asia show a ps phase converted from the bottom of a fractured surficial layer with a reduction in s velocity of around 30 in the northern tien shan the depth of the bottom of this layer increases sharply from a few kilometers beneath the foreland to 15 km beneath the kyrgyz range this low s velocity region extending to mid crustal depths is associated with a high level of ambient seismicity and is likely caused by fluids at high pore pressure that prevent closure of cracks the low velocity layer is absent beneath some stations in the central tien shan including a region of anomalously thin crust beneath the naryn basin the thin crust and reduced fracturing suggest a strong lithosphere that transfers compressive stress from the aksay plateau in the south to the kyrgyz and associated ranges in the north

source of metals and age constraints for epigenetic gold deposits in the skellefte and pohjanmaa districts central part of the fennoscandian shield the lead isotopic compositions of galena in early proterozoic gold deposits have been determined for three districts in northern sweden and central finland the deposits are hosted by a variety of 1870 1890 ma svecofennian host rocks including the volcanosedimentary succession within the skellefte district island arc in sweden as well as i type tonalites at jorn sweden and pohjanmaa finland the deposits are epigenetic in relation to these svecofennian rocks and are part of a goldbearing metallogenetic belt which can be followed for 600 km parallel to the southwestern margin of the archaean domain in spite of these epigenetic relationships the lead isotopic data indicate that the deposits are not dramatically younger than the 1870 1890 ma svecofennian host rocks probably not exceeding 10 20 million years two principal lead sources were activated when the gold deposits were formed the most significant source is represented by the i type tonalites which constitute a relatively primitive 9 3 and widely distributed source in the entire metallogenic belt in addition the volcanic components in the westernmost part of the skellefte district constitute an extremely primitive 9 0 source which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt the significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the skellefte district were not comagmatic with the i type tonalites recognized at jorn and central finland

geodynamics of the tarim basin and the tian shan in central asia the india asia collision has caused crustal thickening in tibet by at least a factor of 2 in the last 20 30 myr of this collision the tian shan mountain range has also been reactivated the tarim basin however shows little internal deformation we describe a series of numerical experiments which constrain the effective lithospheric strength parameters of the tarim basin and the tian shan in the context of a thin viscous sheet model we use a finite element representation of the thin viscous sheet model to approximate the deformation field in the india asia collision and compare model crustal thickness distributions with that inferred from a local isostatic model of topography the experiments show that a strong tarim basin while undergoing little internal deformation transfers strain to the tian shan producing significant crustal thickening in the tian shan region based only on diagnostic parameters such as the maximum thickening in the tian shan and the minimum in the tarim basin the principal features of the topography can be approximately reproduced using models in which either the tarim basin is strong or the tian shan is weak or both for a rheological model in which the stress versus strain rate exponent is n 3 the strength coefficient for the strong tarim basin model vtarim is between about 1 7 and 2 for the weak tian shan model the relative strength coefficient vtian is between about 0 65 and 0 75 if both strong tarim and weak tian shan are included there is a trade off between the required values of vtarim and vtian and the shape of the predicted crustal thickness profiles better matches the observed profiles the steep topographic slope on the southern margin of the tarim basin requires that it is anomalously strong while the rapid decrease of topographic height to the north of the tian shan requires that it is anomalously weak similar conclusions are obtained with a rheological model based on n 10 simplified rheological models of the lithosphere show that the variations in lithospheric strength may be explained by changes to the moho temperature of the order of 10 to 30 c

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

palaeozoic collisional tectonics and magmatism of the chinese tien shan central asia abstract the chinese tien shan range is a palaeozoic orogenic belt which contains two collision zones the older southern collision accreted a north facing passive continental margin on the north side of the tarim block to an active continental margin on the south side of an elongate continental tract the central tien shan collision occurred along the qinbulak qawabulak fault southern tien shan suture the time of the collision is poorly constrained but was probably in in the late devonian early carboniferous we propose this age because of a major disconformity at this time along the north side of the tarim block and because the youshugou ophiolite is imbricated with middle devonian sediments a younger probably late carboniferous early permian collision along the north tien shan fault northern tien shan suture accreted the northern side of the central tien shan to an island arc which lay to its north the north tien shan arc this collision is bracketed by the middle carboniferous termination of arc magmatism and the appearance of late carboniferous or early permian elastics in a foreland basin developed over the extinct arc thrust sheets generated by the collision are proposed as the tectonic load responsible for the subsidence of this basin post collisional but palaeozoic dextral shear occurred along the northern suture zone this was accompanied by the intrusion of basic and acidic magmas in the central tien shan late palaeozoic basic igneous rocks from all three lithospheric blocks represented in the tien shan possess chemical characteristics associated with generation in supra subduction zone environments even though many post date one or both collisions rocks from each block also possess distinctive trace element chemistries which supports the three fold structural division of the orogenic belt it is unclear whether the chemical differences represent different source characteristics or are due to different episodes of magmatism being juxtaposed by later dextral strike slip fault motions because the southern collision zone in the tien shan is the older of the two the tarim block sensu stricto collided not with the eurasian landmass but with a continental block which was itself separated from eurasia by at least one ocean the destruction of this ocean in late carboniferous early permian times represented the final elimination of all oceanic basins from this part of central asia

crustal thickness variations across the northern tien shan from teleseismic receiver functions variations in crustal thickness from the kazakh shield to the north central tien shan are determined by receiver function analysis of broadband teleseismic p waveforms recorded on the kyrgyzstan seismic telemetry network knet the receiver functions are calculated using a time domain deconvolution approach and are interpreted in terms of a single crustal layer with thickness determined by a grid search comparison of observed receiver functions with synthetics crustal thicknesses average 42 km in the southern kazakh shield and increase to approximately 60 km in the tien shan together with mass balance calculations these results are consistent with the hypothesis that the compensation of the central tien shan relative to the kazakh shield are accommodated by an airy type crustal root with a modest 300 kg m density contrast across the moho

lithosphere and asthenosphere of the tien shan imaged by s receiver functions 1 the s receiver function technique was applied to the recordings from 36 broad band stations in the tien shan the results reveal a pronounced difference in the properties of the lithosphere asthenosphere of the tien shan and those of the neighboring platforms beneath the tien shan an upper mantle lid with an s velocity of about 4 5 km s is underlain at a depth of about 90 km by a low velocity zone where the s velocity drops to 4 1 4 2 km s trends of p and s travel time residuals are consistent with the presence of melt or other liquids in this zone there is no evidence of any similar layer beneath the platforms the crust is 55 65 km thick beneath most of the tien shan and 45 km below the platforms although it thins to about 45 km near the naryn basin in the central tien shan

rayleigh wave group velocity tomography of siberia china and the vicinity rayleigh waves are used in a tomographic inversion to obtain group velocity maps of east asia 40 e 160 e and 20 n 70 n the period range studied is 30 to 70 seconds seismograms used for this study were recorded at cdsn stations at a temporary broadband seismic array in tibet at several sro stations and kirnos equipped stations established in asia by the former soviet union in siberia in the sakhalin and in mongolia altogether more than 1200 paths were available in the tomographic inversion the study area includes the angara craton the geologically ancient core of asia and the subsequently accreted units the altaids a paleozoic collision complex the sino korean platform a chain of archaen terranes separated by belts of active structures the south china platform a collage of precambrian paleozoic and mesozoic metamorphic and igneous terranes as well as the tibetan plateau an active tectonic feature created in late cenozoic through collision of the indian subcontinent and the asian continent many of these main units are recognizable in the tomographic images as distinctive units tibet appears as a prominent low velocity about 15 from the average structure with western and central tibet often appearing as the areas with the lowest velocities the central asian fold belt and the angara craton are consistently high group velocity areas some lesser tectonic features are also recognizable for example lake baikal is seen as a high velocity feature at periods greater than 40 seconds however the high group velocity feature does not stop near the southern end of lake baikal it extends south southwestward across mongolia the north china plain a part of the platform where extensional tectonics dominate is an area of high velocities as a result of relatively thin crust the south china block the least tectonically active region of china is generally an area of high velocity for periods longer than 40 seconds a nne trending high group velocity gradient clearly exists in eastern china the velocities are noticeably higher in the east from the group velocity maps average dispersion curves at twelve locations were determined and inverted to obtain velocity structures main results of group velocity inversion include 1 a tibetan crust of around 60 km thick with low crustal and upper mantle shear velocities at 3 3 km s and 4 2 km s respectively 2 with the moho constrained at 40 43 km the angara craton and the central asian foldbelt have a v s in excess of 4 6 km s 3 relatively low shear velocities are obtained for tectonically active areas in many parts of the study area where precambrian basement is exposed the process in the crust and upper mantle due to recent tectonic activities have modified the crust and upper mantle velocity structures under the precambrian terranes they are no longer underlain by high velocity crust and mantle

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

crustal and uppermost mantle s wave velocity structure beneath the japanese islands from seismic ambient noise tomography summary in this study the crustal and uppermost mantle shear wave velocities beneath the japanese islands have been determined by inversion from seismic ambient noise tomography using data recorded at 75 full range seismograph network of japan broad band seismic stations which are uniformly distributed across the japanese islands by cross correlating 2 yr of vertical component seismic ambient noise recordings we are able to extract rayleigh wave empirical green s functions which are subsequently used to measure phase velocity dispersion in the period band of 6 50 s the dispersion data are then inverted to yield 2 d tomographic phase velocitymapsand3 dshearwavevelocitymodels ourresultsshowthatthevelocityvariations atshortperiods 10s orintheuppermostcrust correlatewellwiththemajorknownsurface geological and tectonic features in particular the distribution of low velocity anomalies shows good spatial correlation with active faults volcanoes and terrains of sediment exposure whereas the high velocity anomalies are mainly associated with the mountain ranges we also observe that large upper crustal earthquakes 5 0 m 8 0 depth 25km mainly occurred in low velocity anomalies or along the boundary between low and high velocity anomalies suggesting that large upper crustal earthquakes do not strike randomly or uniformly rather they are inclined to nucleate within or adjacent to low velocity areas

localisation of deformation in the thermal contrast at a granite batholith margin new zealand abstract the wainui shear zone is a 1 2 km wide steeply dipping ductile shear zone that is intimately associated with the western margin of a very extensive granite batholith in central new zealand shear sense indicators and lineation orientations within the central high strain portion of shear zone consistently show a reverse dip slip east side up sense of motion shallower foliation in the margins of the shear zone is interpreted to pre date the central high strain zone however u pb and ar ar data reveal that all ductile deformation occurred between 114 and 109 ma and probably between 114 and 111 ma requiring 1 that deformation kinematics within the shear zone changed over a short time period and 2 the shear fabrics formed immediately after emplacement of the voluminous granite batholith the temporal and spatial link between the deformation and plutonism is hypothesised to be a result of thermal weakening of the cooler paleozoic crust by cretaceous granite intrusion which promoted localisation of deformation onto the intrusive contact deformation was initially distributed over a wide zone but became focused into a central mylonite zone continental scale shortening is recorded as a network of ductile shear zones that formed on thermal contrasts within the upper middle and lower crust along the new zealand gondwana margin

broadband ambient noise surface wave tomography across the united states 1 this study presents surface wave dispersion maps across the contiguous united states determined using seismic ambient noise two years of ambient noise data are used from march 2003 through february 2005 observed at 203 broadband seismic stations in the us southern canada and northern mexico cross correlations are computed between all station pairs to produce empirical green functions at most azimuths across the us coherent rayleigh wave signals exist in the empirical green functions implying that ambient noise in the frequency band of this study 5 100 s period is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements rayleigh and love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements a sufficient number of measurements 2000 is obtained between 8 and 25 s period for love waves and 8 and 70 s period for rayleigh waves to produce tomographic dispersion maps both phase and group velocity maps are presented in these period bands resolution is estimated to be better than 100 km across much of the us from 8 40 s period for rayleigh waves and 8 20 s period for love waves which is unprecedented in a study at this spatial scale at longer and shorter periods resolution degrades as the number of coherent signals diminishes the dispersion maps agree well with each other and with known geological and tectonic features and in addition provide new information about structures in the crust and uppermost mantle beneath much of the us

paleozoic multiple accretionary and collisional tectonics of the chinese tianshan orogenic collage abstract subduction related accretion in the junggar balkash and south tianshan oceans paleo asian ocean mainly in the paleozoic gave rise to the present 2400 km long tianshan orogenic collage that extends from the aral sea eastwards through uzbekistan tajikistan kyrgyzstan to xinjiang in china this paper provides an up to date along strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution the northern part of the orogenic collage developed by consumption of the junggar balkash ocean together with paleozoic island arcs northern ili issyk kul and chatkal located in the west which may have amalgamated into a composite arc in the paleozoic in the west and by addition of another two roughly parallel arcs dananhu and central tianshan in the east the western composite arc and the eastern dananhu and central tianshan arcs formed a late paleozoic archipelago with multiple subduction zones the southern part of the orogenic collage developed by the consumption of the south tianshan ocean which gave rise to a continuous accretionary complex kokshaal kumishi which separated the central tianshan in the east and other paleozoic arcs in the west from cratons tarim and karakum to the south cross border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early paleozoic to permian as recorded by successive southward juxtaposition of ophiolites slices of ophiolitic melanges cherts island arcs olistostromes blueschists and turbidites which are mainly paleozoic in age with the youngest main phase being late carboniferous permian the initial docking of the southerly tarim and karakum cratons to this complicated late paleozoic archipelago and accretionary complexes occurred in the late carboniferous early permian in the eastern part of the tianshan and in the late permian in the western part which might have terminated collisional deformation on this suture zone the final stages of closure of the junggar balkash ocean resembled the small ocean basin scenario of the mediterranean sea in the cenozoic in summary the history of the altaids is characterized by complicated multiple accretionary and collisional tectonics

gps velocity field for the tien shan and surrounding regions 1 measurements at 400 campaign style gps points and another 14 continuously recording stations in central asia define variations in their velocities both along and across the kyrgyz and neighboring parts of tien shan they show that at the longitude of kyrgyzstan the tarim basin converges with eurasia at 20 2 mm yr nearly two thirds of the total convergence rate between india and eurasia at this longitude this high rate suggests that the tien shan has grown into a major mountain range only late in the evolution of the india eurasia collision most of the convergence between tarim and eurasia within the upper crust of the tien shan presumably occurs by slip on faults on the edges of and within the belt but 1 3 mm yr of convergence is absorbed farther north at the dzungarian alatau and at a lower rate with the kazakh platform to the west the tarim basin is thrust beneath the tien shan at 4 7 mm yr with respect to eurasia the ferghana valley rotates counterclockwise at 0 7 myr 1 about an axis at the southwest end of the valley thus gps data place a bound of 4 mm yr on the rate of crustal shortening across the chatkal and neighboring ranges on the northwest margin of the ferghana valley and they limit the present day slip rate on the right lateral talas ferghana fault to less than 2 mm yr gps measurements corroborate geologic evidence indicating that the northern margin of the pamir overthrusts the alay valley and require a rate of at least 10 and possibly 15 mm yr

three dimensional seismic velocity structure of the crust beneath the central tien shan kyrgyzstan implications for large and small scale mountain building abstract the central tien shan kyrgyzstan is located approximately 2000 km from the main collisional suture between india and eurasia and is sandwiched between the kazakh shield to the north and the tarim basin to the south a three dimensional 3 d seismic velocity structure of the crust beneath the central tien shan was estimated to a depth of 50 km by means of seismic tomography using arrival time data of p and s waves from local earthquakes recorded by a temporary seismic network of the tien shan during 1998 2000 velocity structure images show a good correlation with the subsurface geology at shallow depths a low velocity zone is distributed beneath the basins probably due to the cenozoic deposition whereas a high velocity zone is visible beneath the mountain ranges likely due to paleozoic crystalline basement rocks at the surface layer results obtained at greater depths revealed the following features 1 a low velocity zone is clearly observed in the middle to lower crust and is probably caused by melt and or fluids conveyed through the flow of mantle upwelling 2 both north dipping and south dipping low velocity zones were imaged at locations where the kazakh shield and the tarim basin underthrust beneath the tien shan respectively and are probably caused by mantle materials somehow being intruded into the crust 3 the lateral variation of high and low velocity zones seen in the middle to lower crust beneath basins and surrounding mountain ranges respectively seems to be caused by melt and or fluids vertically penetrated and or laterally spread all these findings suggest that in process northward shortening and crustal thickening owing to collision and mantle convective motion i e upwelling and as a consequence partial melting are work horses in the mountain building beneath the tien shan

path specific velocity structure of western china from surface wave dispersion we develop one dimensional 1 d path specific velocity models in western china using new rayleigh and love wave group and phase velocity dispersion measurements for 20 events in the region the earthquakes were grouped into three geographic clusters from which we compute the average phase and group velocity dispersion we invert the average dispersion curves simultaneously for 1 d shear velocity models appropriate for the three central asian paths using three previous shear velocity models as initial models the models are validated by forward modeling waveforms of recent events the crustal thickness beneath western china in the vicinity of the lop nor test site is 50 60 km and our velocity models are consistent with major geologic features e g basins and mountain ranges and previous structural models for this region

analysis of seismic activity in the crust from earthquake relocation in the central tien shan using p and s wave arrivals from local earthquakes recorded by a temporary broadband seismic network ghengis and a kyrgyzstan broadband seismic network knet we determined the source parameters of 1938 earthquakes occurring from 1997 to 1998 in the central tien shan and adjacent areas based on one dimensional and three dimensional velocity models the results indicate that seismic activity is not restricted to only central tien shan but also penetrates into the edges of the kazakh platform and the tarim basin to the north and south respectively it implies that most range bounding faults in the central tien shan are active and they play a significant role in the tectonic activity of the mountain belt however seismic activity of the talas fergana fault in the study area is different its central segment seems more active than its eastern segment we infer that this part of the fault may be activated by neighboring faults whereas its eastern segment could be temporally locked the earthquakes at the depths of 30 40 km beneath the east central tien shan reveal an interaction between the upper and mid lower crust this suggests that brittle failure is possible to greater depth within the crust another possibility is that the seismic activity in the mid lower crust is affected by a positive buoyancy force created by mantle upwelling of this area these observations indicate that dynamic processes such as small scale mantle convection or mantle upwelling which resulted in lithospheric thinning or removal still occur beneath the central tien shan

crustal structure beneath the central tien shan from ambient noise tomography through analysis of seismic ambient noise recorded by the ghengis array we constructed a high resolution 3 d crustal shear wave velocity model for the central tien shan the obtained shear wave velocity model provides insight into the detailed crustal structure beneath the tien shan the results obtained at shallow depths are well correlated with known subsurface geological features low velocities are found mainly beneath sedimentary basins whereas high velocities are mainly associated with mountain ranges at greater depths of similar to 43 45 km high velocities were observed beneath the tarim basin and kazakh shield these high velocities extend forward in opposite directions and tilt down towards the central tien shan to a depth of in excess of 50 km most likely reflecting lateral variations in crustal thickness beneath the tien shan and surrounding platforms

tomographic image of the crust and upper mantle beneath the western tien shan from the manas broadband deployment possible evidence for lithospheric delamination abstract we combine teleseismic p arrival times from the recent manas deployment of broadband sensors with p and s arrival times from local events recorded by the genghis deployment and analog observations from the kyrgyz institute of seismology to generate a high resolution 20 km image of elastic wavespeeds in the crust and upper mantle beneath the western tien shan the total data set consists of 29 006 p and 21 491 s arrivals from 2176 local events recorded at 144 stations along with 5202 p arrivals from 263 teleseismic events recorded at 40 stations the most significant feature in our image of the mantle beneath the tien shan is a pair of large elongated high wavespeed regions dipping in opposite directions from the near surface to depths of at least 400 km these regions appear to be continuous and extend upwards to bounding range fronts where the tarim basin is being overthrust by the kokshal range on the south side and the kazach shield underthrusts the kyrgyz range on the north side while it is tempting to interpret these high wavespeed anomalies as evidence for contemporary subduction of continental lithosphere such a scenario is difficult to reconcile with both the timing of the orogen and the size of the wavespeed anomaly we suggest instead that they represent downwelling side limbs of a lithospheric delamination beneath the central part of the tien shan possibly by siphoning of the bordering continental lithosphere as the central part descends

midcrustal low velocity layer beneath the central himalaya and southern tibet revealed by ambient noise array tomography 1 ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the earth using 2 months of ambient noise data from 20 stations of the himalayan nepal tibet seismic experiment we investigate the upper and middle crustal structure in the central himalaya and southern tibet about 120 interstation rayleigh wave empirical green s functions with sufficient signal to noise ratio are obtained and used for group velocity dispersion analysis in the period range 6 25 s using frequency time analysis technique the obtained dispersion data are then used to construct 2 d group velocity maps at the short periods from 9 to 15 s the distribution of rayleigh wave velocities delineates several distinct low and high velocity zones separated mainly by geological boundaries the high group velocity zone is located mainly around regions with plutonic rocks and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks finally we invert for the shear velocity structure in the upper and middle crust along a n s trending cross section at the longitude 86 5 e we observe a clear low velocity layer in the middle crust about 10 25 km depth distributed on both sides of the indus yarlung suture zone the existence of this midcrustal low velocity zone suggests a mechanically weaker middle crust beneath the central himalaya and southern tibet which might decouple the upper crustal deformation from that of the lower crust in the tibetan himalayan orogenic processes

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

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