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

towards realistic aquifer models three dimensional georadar surveys of quaternary gravel deltas singen basin sw germany abstract understanding sedimentary heterogeneities has nowadays become crucial for sophisticated aquifer management the methods used so far for aquifer characterization e g pump test coring analysis cannot provide the needed resolution for a sufficient prediction of the subsurface properties e g sudicky 1986 this calls for the application of the outcrop analogue approach to aquifers e g macfarlan et al 1994 aigner 1995 in this project we used georadar ground penetrating radar or gpr to acquire the subsurface information for high resolution three dimensional images and radar facies mapping of glaciolacustrine delta sediments in the singen basin sw germany a three dimensional reconstruction of sedimentary heterogeneities for a detailed aquifer characterisation becomes possible by combining gpr profiling and outcrop information a case study is presented showing a complex three dimensional architecture of a quaternary gravel body consisting of the following three types of architectural elements 1 glaciolacustrine delta foresets 2 erosional surfaces with scour pool fills and 3 horizontal glaciofluvial gravel sheets although this seems a relatively simple architectural inventory the three dimensional georadar profiling shows considerable internal heterogeneities the radar data are interpreted following the seismic stratigraphic approach i e the delineation of radar facies types and of radar sequences the three dimensional heterogeneities are visualized by means of time slices 3 d block or chair diagrams or series of radar facies maps

deep burial of asian continental crust beneath the pamir imaged with local earthquake tomography an inclined zone of intermediate depth seismicity beneath the pamir orogen in central asia has been interpreted as southward subduction of a slab of asian lithosphere however it is not known whether asian lithosphere subducts intact or only partially we used arrival times of shallow and intermediate depth earthquakes recorded with a temporary 2008 2010 seismic network in this region to invert for 3d models of seismic velocities in an attempt to answer this question with local seismicity reaching depths of up to 240 km the deep structure of the pamir could be illuminated with high resolution the resulting velocity models show a north south contrast in crustal seismic velocities in the pamir with very low p velocities 5 7 5 9 km s at 15 30 km depth coupled with relatively low vp vsvp vs 1 70 at mid crustal levels in the southern part of the orogen at sub moho depths we image an arcuate high velocity 8 2 8 6 km s slab dipping south in the eastern pamir and east in the pamir s southwest underlying the intermediate depth earthquakes on top of the high velocity slab and just above the onset of deep seismicity between a depth of 60 to 100 km very low compressional wavespeeds around 7 1 km s and high vp vsvp vs ratios 1 80 attest to subducted crustal rocks additionally we carried out 2d numerical thermomechanical modeling of the continental collision in the pamir focusing on the fate of the crust and mantle lithosphere of the asian and indian plates seismic velocities were computed from the modeling results and the resulting images were compared with the velocity distributions obtained from seismic traveltimes combining tomography and modeling results we infer that a substantial amount of crustal material is pulled down beneath the pamir by cold mantle lithosphere to depths of at least 80 100 km from there on only lower crust and mantle lithosphere continue their subduction and earthquakes occur inside the lower crustal layer probably due to metamorphic reactions the buoyant asian upper and middle crust does not penetrate deeper into the mantle but pools at this depth level from where it might eventually exhume or relaminate

cenozoic right slip faulting along the eastern margin of the pamir salient northwestern china the pamir salient defines the western end of the himalayan tibetan orogen and has overthrust the tarim tajik basin to the north by 300 km along a late cenozoic south dipping intracontinental subduction zone field mapping structural measurements and analysis of mesoscale structures along a 32 km long reach of the yarkand river document the tectonic evolution of the east flank of this salient between the north pamir to the west and the western kunlun shan to the east the study area is cut by a set of four north northwest striking steeply dipping brittle faults microstructures and asymmetric outcrop to map scale folds indicate right slip along these faults between these structures fault bounded panels of phanerozoic strata are deformed by en echelon folds with axes that trend more westerly than the adjacent faults consistent with dextral transpression the fault system described here extends for 350 km along the eastern flank of the pamir salient transpressional right slip along this set of faults here called the kashgar yecheng transfer system appears to have accommodated late cenozoic separation of the north pamir from the western kunlun shan during south directed intracontinental subduction beneath the leading edge of the pamir salient correlation of major faults suggests total slip along the kashgar yecheng transfer system is likely on the order of 280 km this offset estimate implies long term slip rates of 7 15 mm a along the kashgar yecheng transfer system when combined with previous sedimentologic stratigraphic and thermochronologic data that indicate deformation along the east flank of the pamir started between the late eocene and early miocene these results imply that the first order structures on the western and eastern flanks of the pamir are asymmetric previous work has shown that deformation in the west was accommodated by anticlockwise vertical axis rotation of the pamir over the eastern margin of the tajik basin this rotation is generally interpreted to reflect northwest directed radial thrusting in contrast to the transpressional right slip transfer faulting on the east side reported here

building the pamir tibetan plateau crustal stacking extensional collapse and lateral extrusion in the central pamir 2 timing and rates geothermochronologic data outline the temperature deformation time evolution of the muskol and shatput gneiss domes and their hanging walls in the central pamir prograde metamorphism started before 35 ma and peaked at 23 20 ma reflecting top to n thrust sheet and fold nappe emplacement that tripled the thickness of the upper 7 10 km of the asian crust multimethod thermochronology traces cooling through 700 100 c between 22 and 12 ma due to exhumation along dome bounding normal sense shear zones synkinematic minerals date normal sense shear zone deformation at 22 17 ma age versus elevation relationships and paleoisotherm spacing imply exhumation at 3 km myr south of the domes mesozoic granitoids record slow cooling and or constant temperature throughout the paleogene and enhanced cooling 7 31 c myr starting between 23 and 12 ma and continuing today integrating the central pamir data with those of the east chinese pamir kongur shan and muztaghata domes and with the south pamir shakhdara dome implies i regionally distributed paleogene crustal thickening ii pamir wide gravitational collapse of thickened crust starting at 23 21 ma during ongoing india asia convergence and iii termination of doming and resumption of shortening following northward propagating underthrusting of the indian cratonic lithosphere at 12 ma westward lateral extrusion of pamir plateau crust into the hindu kush and the tajik depression accompanied all stages deep seated processes e g slab breakoff crustal foundering and underthrusting of buoyant lithosphere governed transitional phases in the pamir and likely the tibet crust

experiments on urban flooding caused by a levee breach breached levees often cause extensive flood damage to urban and residential areas there has been considerable interest in the numerical modeling of flooding of urban areas in recent years however comprehensive data that describe the complex flow field in residential areas are not available for validating the numerical models or for determining the extent of such damage to fill this gap results are presented of experiments on a scale model used as a test setup that examined the flow field in an urban area attributable to flooding caused by a levee breach steady state flow depths and flow velocity were measured by a point gauge and by an array of ultrasonic velocity profilers respectively in addition the surface velocities were recorded in the entire flooded area by using a digital particle tracking velocimetry technique the details of these measurements limited flow data and the observed flow characteristics are presented in this study full data sets are provided on the internet the results of these investigations show that closely spaced buildings act like a single obstacle with a common wake zone and there is little flow between these buildings the results also show that the strongest flow is in the prebreach direction and that the local topography plays an important role in distributing the flow in the flooded area the complete data set may be utilized to validate numerical models of free surface flow

geology geochronology and geochemistry of the siruyidie er prospect taxkorgan a possible miocene porphyry mo cu deposit in the central pamir abstract the pamir and tibetan plateaus which have a similar terrane structure and evolutionary history since the phanerozoic are both host abundant eocene to miocene intrusions however unlike the tibetan plateau coeval porphyry deposits appear to be devoid in the pamir plateau in this study we first report a potentially economic cu bearing sulfide mineralization at siruyidie er prospect in the central pamir the siruyidie er prospect is characterized by the development of 14 breccia pipes in the north block and of cu bearing sulfide veins in the south block field evidence indicates that the brecciation and mineralization are intimately related to the intrusion of granite porphyry zircon u pb dating of the granite porphyry yields an age of 13 74 0 21 ma this age overlaps with the hydrothermal muscovite 40ar 39ar data of 13 68 0 17 ma which confirms the hydrothermal activity and mineralization at siruyidie er prospect are induced by the granite porphyry the granite porphyries are shoshonitic in composition and have high k2o 6 03 6 46 wt k2o na2o 1 70 1 92 ba 2983 3316 ppm lree 317 363 ppm and sr 331 378 ppm and low mgo 0 54 0 63 wt cr 6 73 8 28 ppm and y 7 88 9 91 ppm contents the rocks are lree enriched la yb n 53 66 and display weakly negative eu eu eu 0 79 0 87 positive liles ba rb and k and negative hfses th u nb ta ti and y anomalies they have enriched hf isotopic compositions with hf t values ranging from 8 78 to 5 01 the combined major and trace elements and hf isotopic characteristics of the granite porphyries suggest that the primitive magmas were produced by high extent partial melting of an ancient crust source possibly a thickened lower crust in the garnet stability field a comparison of the granite porphyries at siruyidie er prospect with eocene to miocene ore forming granitic porphyries of the tibetan plateau reveals that the granite porphyries have lower water contents and originated from a continental crust rather than a depleted mantle source based on geological compositional and age data we propose that the siruyidie er prospect has a potential to form porphyry mo cu deposit this study provides the first direct evidence for the possible presence of porphyry mo cu deposit in the pamir plateau

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

mantle flow entrained by the hindu kush continental subduction inferred from source side seismic anisotropy abstract the intermediate depth seismicity below the hindu kush orogen is thought to mark the indian plate subduction with the bottom half of the slab currently breaking off unique features of this continental subduction are the near vertical slab and the roughly stationary convergence boundary how this subduction affects the mantle flow patterns remains to be understood in this study we measured source side shear wave splitting on the s waves from hindu kush intraslab events to sample the surrounding mantle the observed fast polarization directions exhibit a circular pattern around the slab resembling that predicted for the toroidal flow driven by slab rollback however the rollback scenario is not favored because it hardly sustains in dynamic models without a considerable retreat of convergence boundary we propose that the observed pattern is produced by the sub vertical shear flow entrained by the steep descent of the slab and the ongoing breakoff this scenario requires the existence of a type or ag type olivine fabrics with strong orthorhombic anisotropy in mid to lower upper mantle which is consistent with the global models of azimuthal and radial anisotropy this interpretation circumvents the debate on the cause of trench parallel anisotropy in some oceanic subduction zones where slab entrainment and rollback may coexist and supports the notion that orthorhombic anisotropy of olivine may play an important role in shaping mantle anisotropy

a simple raster based model for flood inundation simulation in this paper the development of a new model for simulating flood inundation is outlined the model is designed to operate with high resolution raster digital elevation models which are becoming increasingly available for many lowland floodplain rivers and is based on what we hypothesise to be the simplest possible process representation capable of simulating dynamic flood inundation this consists of a one dimensional kinematic wave approximation for channel flow solved using an explicit finite difference scheme and a two dimensional diffusion wave representation of floodplain flow the model is applied to a 35 km reach of the river meuse in the netherlands using only published data sources and used to simulate a large flood event that occurred in january 1995 this event was chosen as air photo and synthetic aperture radar sar data for flood inundation extent are available to enable rigorous validation of the developed model 100 50 and 25 m resolution models were constructed and compared to two other inundation prediction techniques a planar approximation to the free surface and a relatively coarse resolution two dimensional finite element scheme the model developed in this paper outperforms both the simpler and more complex process representations with the best fit simulation correctly predicting 81 9 of inundated and non inundated areas this compares with 69 5 for the best fit planar surface and 63 8 for the best fit finite element code however when applied solely to the 7 km of river below the upstream gauging station at borgharen the planar model performs almost as well 83 7 correct as the raster model 85 5 correct this is due to the proximity of the gauge which acts as a control point for construction of the planar surface and the fact that here low lying areas of the floodplain are hydraulically connected to the channel importantly though it is impossible to generalise such application rules and thus we cannot specify a priori where the planar approximation will work simulations also indicate that for this event at least dynamic effects are relatively unimportant for prediction of peak inundation lastly consideration of errors in typically available gauging station and inundation extent data shows the raster based model to be close to the current prediction limit for this class of problem q 2000 elsevier science b v all rights reserved

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

an enhanced image of the pamir hindu kush seismic zone from relocated earthquake hypocentres summary we determine the shape of the seismic zone in the pamir hindu kush region de ned by 30 42 0 n 68 78 0 e by obtaining improved hypocentral locations with 90 per cent con dence limits of less than 30 km the depth error bar for most of the earthquakes of about 6000 shallow and intermediate depth earthquakes available s and depth phase arrival times are also used together with the p wave arrival times in the joint hypocentre determination technique to obtain the best possible hypocentral locations the study region is divided into three depth ranges 0 60 60 160 and 160 km the 0 60 km depth zone is then subdivided laterally into 19 blocks with the deeper regions divided into two blocks each the improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate depth seismicity in the pamir hindu kush region is most simply explained by a single s shaped seismic zone 700 km long and no more than 30 km wide and with most activity concentrated at 100 300 km depth the main features observed are 1 the eastward steepening of the north dipping hindu kush seismic zone through to its overturning at its eastern end beneath the pamirs where it dips to the southeast 2 the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the hindu kush seismic zone 3 the very abrupt cut oi in intermediate depth seismicity at 90 110 km depth with no extension to shallower depths under the pamirs and with a persistent gap between the intermediate and shallow seismicity in the northern pamirs and 4 the unusual horizontal t axes for intermediate depth earthquakes of the pamir seismic zone which align with its curvature this study shows that the seismic zone under the hindu kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity whereas the pamir region has horizontal t axes that follow the trend of the contorted seismic zone this suggests that the pamir seismic zone is a slab deformed due to ow in the upper mantle

interaction of the indian and asian plates under the pamir and hindu kush regions insights from 3 d shear wave velocity and anisotropic structures

recurring large deep earthquakes in hindu kush driven by a sinking slab hindu kush subduction zone produces large intermediate depth earthquakes within a small volume every 10 15 years here we study the last three m 7 events within the cluster and find complex and diverse rupture processes however their main subevents appear to recur on the same fault patch dipping 70 to the south this recurrence requires an average of 9 6 cm yr slip rate on the patch much higher than the 1 cm yr surface convergence rate measured geodetically the high slip rate is likely caused by significant slab internal deformation such as localized slab stretching necking we infer that the hindu kush subducted slab below 210 km is sinking through the mantle at a vertical rate of 10 cm yr

an upper mantle s wave velocity model for east asia from rayleigh wave tomography abstract we present a new shear velocity model of the upper mantle beneath the east asia region derived by inverting rayleigh wave group velocity measurements between 10 and 145 s combined with previously published rayleigh wave phase velocity measurements between 150 and 250 s rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2d dispersion maps for 10 145 s periods the group velocity maps benefit from the inclusion of new data recorded by the china national seismic network and surrounding global stations the increase in available data has resulted in enhanced resolution compared with previously published group velocity maps the horizontal resolution across the region is about 3 for the periods used in this study the new shear wave velocity models indicate varying velocity structure beneath eastern china which yields estimates of a lithosphere asthenosphere boundary depth from around 160 km beneath the yangtze block to approximately 140 km beneath the western part of the north china craton ncc up to depths of 70 100 km beneath the eastern ncc northeast china and the cathaysia block the models reveal the subduction of two opposite facing continental plates under the southern and northern margin of tibet an obvious low velocity anomaly appears in the top 200 km of the upper mantle beneath northern tibet which is inconsistent with the presence of subducted asian or indian mantle lithosphere beneath northern tibet the cenozoic volcanism fields in the mongolian plateau are characterized by an obvious upper mantle negative anomaly but no signature of deep seated plume was observed

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 2 timing of dome formation 1 cenozoic gneiss domes exposing middle lower crustal rocks cover 30 of the surface exposure of the pamir western india asia collision zone they allow an unparalleled view into the deep crust of the asian plate we use titanite monazite and zircon u th pb mica rb sr and 40ar 39ar zircon and apatite fission track and zircon u th he ages to constrain the exhumation history of the 350 90 km shakhdara alichur dome southwestern pamir doming started at 21 20 ma along the gunt top to n normal shear zone of the northern shakhdara dome the bulk of the exhumation occurred by nnw ward extrusion of the footwall of the crustal scale south pamir normal shear zone along the southern shakhdara dome boundary footwall extrusion was active from 18 15 ma to 2 ma at 10 mm yr slip and with vertical exhumation rates of 1 3 mm yr it resulted in up to 90 km n s extension coeval with n s convergence between india and asia erosion rates were 0 3 0 5 mm yr within the domes and 0 1 0 3 mm yr in the horst separating the shakhdara and alichur domes and in the southeastern pamir plateau rates were highest along the dome axis in the southern part of the shakhdara dome incision along the major drainages was up to 1 0 mm yr thermal modeling suggests geothermal gradients as high as 60 c km along the trace of the south pamir shear zone and their strong n s variation across the dome the gradients relaxed to 40 45 c km since the end of doming

depth averaged model of open channel flows over an arbitrary 3d surface and its applications to analysis of water surface profile a new set of depth averaged equations is introduced to study the flow over an arbitrary three dimensional 3d surface these equations are derived based on a generalized curvilinear coordinate system attached to the 3d bed surface therefore it allows us to include the effect of centrifugal force due to the bottom curvature these general equations make it possible to analyze flows over complex terrain without the limitation of mild slope assumption used in conventional depth averaged models this new model is then applied to calculate the water surface profiles of 1 flow over a cylindrical surface 2 flow over a circular surface and 3 flow with an air core vortex at a vertical intake a simple hydraulic experiment is conducted in the laboratory to observe the water surface profile of flow over a circular surface the results obtained from the model are in good agreement with experimental measurements and calculation by an empirical formula consequently it demonstrates the applicability of the model in cases of flow over a highly curved bottom

zooming into the hindu kush slab break off a rare glimpse on the terminal stage of subduction abstract the terminal stage of subduction sets in when the continental margin arrives at the trench and the opposite forces of the sinking slab and buoyant continent extend and ultimately sever the subducted lithosphere this process although common in geological history is short lived and therefore rarely observed the deep seismicity under the hindu kush central asia including the 2015 m w 7 5 event is a rare case that testifies to this process here we use new seismological data to create a high resolution picture of slab break off and infer its dynamics high precision earthquake locations and tomographic images show subduction of continental crust down to 180 km a large dataset of source mechanisms indicates sub vertical extension in the entire slab but a strain rate analysis showed that the deeper seismogenic portion of the slab below the subducted crust extends at higher rates 40 km ma most m w 7 earthquakes between 1983 2015 relocated relative to our new well constrained earthquake catalog cluster in a small volume below 180 km and indicate shearing on an overturned interface a slip model for the latest 2015 m w 7 5 event suggests that it ruptured into a seismic gap on this interface from this configuration we conclude that a horizontal slab tear develops along strike of the hindu kush seismic zone at the base of the subducted continental crust below the subducted crust the deepest and also largest earthquakes 180 265 km are likely associated with deformation in the mantle lithosphere from the seismicity distribution and the rupture mechanisms we further deduce that the dominant deformation mechanism in this deeper portion of the slab changes along strike from simple to pure shear the fastest detachment rates and largest earthquakes occur during the simple shear dominated stage earthquakes in the upper part 60 180 km above the rapidly extending slab might be triggered by processes related to the subduction of crustal rocks

continental lithospheric subduction and intermediate depth seismicity constraints from s wave velocity structures in the pamir and hindu kush abstract the pamir has experienced more intense deformation and shortening than tibet although it has a similar history of terrane accretion subduction as a primary way to accommodate lithospheric shortening beneath the pamir has induced the intermediate depth seismicity which is rare in tibet here we construct a 3d s wave velocity model of the lithosphere beneath the pamir by surface wave tomography using data of the tipage tien shan pamir geodynamic program and other seismic networks in the area we imaged a large scale low velocity anomaly in the crust at 20 50 km depth in the pamir overlain by a high velocity anomaly at a depth shallower than 15 km the high velocity anomalies colocate with exposed gneiss domes which may imply a similar history of crustal deformation partial melting and exhumation in the hinterland as has occurred in the himalaya tibet system at mantle depths where the intermediate depth earthquakes are located a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the hindu kush and eastern pamir respectively moreover the geometry of the low velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting asian and indian lithospheric mantle beneath the pamir and hindu kush respectively metamorphic processes in the subducting lower crust may cause the intermediate depth seismicity down to 150 180 km depth beneath the pamir and hindu kush we inverted focal mechanisms in the seismic zone for the stress field differences in the stress field between the upper and lower parts of the indian slab imply that subduction and detachment of the indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep hindu kush

receiver function tomography of the central tien shan abstract to obtain an image of the deep structure of the tien shan in central asia we invert p and s receiver functions jointly for almost 40 local broad band seismograph stations the inversion is performed using a simulated annealing technique the combined inversion is an improvement on earlier studies where p and s receiver functions were inverted separately using this approach we deal with structural imaging problems that are usually investigated with teleseismic body wave and surface wave tomography techniques we demonstrate that the uppermost mantle in the north of the central tien shan is composed of a high velocity lid a few tens of kilometers thick above a pronounced low velocity zone the crustal structure in this region provides evidence of magmatic underplating these features are likely related to a small plume that is manifested by basaltic eruptions of cretaceous paleogene age the low velocity layer is also found in a southeast trending corridor which may correspond to the bachu uplift in the tarim basin crustal thickness beneath the orogen varies from about 45 to about 70 km the smallest values most likely inherited from the pre orogenic era are found in a neighborhood of the talas fergana fault similar values are characteristic of the kazakh shield in the north and the tarim basin in the south the largest values are found beneath the bounding ranges we infer that uplift of the central tien shan is unlikely to be caused by crustal shortening alone

slip partitioning in the northeast pamir tian shan convergence zone abstract based on a detailed analysis of satellite imagery combined with field geologic and geomorphic observations we have mapped late cenozoic folds and faults in the northeastern pamir tian shan convergence zone it is a unique example to understand intracontinental ongoing mountain building within india eurasia collision system in the front of northeastern pamir our investigations reveal that the nw wnw trending folds display a right stepping en echelon pattern and nw wnw striking faults are mainly characterized by south dipping thrusts with an extensive dextral strike slip component drainage systems across the active faults show a systematic right lateral offset in contrast structural style of the ene trending fold and thrust belts are predominated by south north directed shortening southwest of the tian shan our results also infer that oblique thrusting accommodates as long term dextral slip rate of ca 4 0 mm yr during the late cenozoic time north of the pamir topographic front tectono stratigraphic evidence suggests that the tectonic deformation was initiated at ca 3 5 ma in the study area we suggest that intracontinental mountain building in the pamir tian shan convergence zone should be attributed to the crustal shortening caused by folding and thrusting as well as block rotation related to strike slip faulting within the india eurasia collision system

observations of guided waves from the pamir seismic zone provide additional evidence for the existence of subducted continental lower crust abstract as part of the tipage tien shan pamir geodynamic program project passive seismological observations were made along an approximately n s profile crossing the pamir seismic zone for about one year from these observations guided waves were recognized these guided waves occur as a single continuous secondary compressional p wave phase behind the first p wave arrivals an equivalent phase in the shear s wavefield is hardly recognizable modelling of the phase shows that an approximately 10 km thick low velocity zone lvz between the moho and about 160 km depth reproduces the guided waves as a single continuous phase much better than a 15 20 km thick lvz modelling of the arrival times of the guided waves reveals that a model with a p wave velocity of 6 3 km s above about 100 km depth and a velocity of 7 6 km s between this depth and the deep cluster of earthquakes at about 150 km depth provides the best fit to the observed travel time data one plausible way to explain the low velocity of 6 3 km s is to invoke the presence of melts in the lvz then taking a velocity of 6 9 km s for the lower crust being subducted about 10 13 melt is required to obtain a velocity of about 6 3 km s in the lvz between the moho and about 100 km depth this would be in keeping with the estimated burial depths from xenoliths of gondwana terrane affinity brought to the surface in the southeastern pamir around 11 million yr ago the present day lvz is interpreted to comprise continental lower crust although guided waves are known to exist associated with subducted oceanic crust or fault zones this is the first time to the knowledge of the authors that guided waves have been observed resulting from a lvz associated with subducted continental lower crust

architectural element analysis within the kayenta formation lower jurassic using ground probing radar and sedimentological profiling southwestern colorado abstract a well exposed outcrop in the kayenta formation lower jurassic in southwestern colorado was examined in order to delineate the stratigraphy in the subsurface and test the usefulness of ground probing radar gpr in three dimensional architectural studies two fluvial styles are present within the kayenta formation sandbodies within the lower third of the outcrop are characterized by parallel laminations that can be followed in the cliff face for well over 300 m these sandbodies are sheet like in appearance and represent high energy flood deposits that most likely resulted from episodic floods the remainder of the outcrop is characterized by concave up channel deposits with bank attached and mid channel macroforms their presence suggests a multiple channel river system the gpr data collected on the cliff top together with sedimentological data provided a partial three dimensional picture of the paleo river system within the kayenta formation the 3 d picture consists of stacked channel bar lenses approximately 50 m in diameter the gpr technique offers a very effective means of delineating the subsurface stratigraphy its high resolution capabilities easy mobility and rapid rate of data collection make it a useful tool its shallow penetration depth and limitation to low conductivity environments are its only drawbacks

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

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

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 1 geometry and kinematics 1 cenozoic gneiss domes comprise one third of the surface exposure of the pamir and provide a window into the deep crustal processes of the india asia collision the largest of these are the doubly vergent composite shakhdara alichur domes of the southwestern pamir tajikistan and afghanistan they are separated by a low strain horst top to sse noncoaxial pervasive flow over the up to 4 km thick south pamir shear zone exhumed crust from 30 40 km depth in the 250 80 km shakhdara dome the top to nne alichur shear zone exposed upper crustal rocks in the 125 25 km alichur dome the gunt shear zone bounds the shakhdara dome in the north and records alternations of normal shear and dextral transpression it contributed little to bulk exhumation footwall exhumation along two low angle normal sense detachments resulted in up to 90 km syn orogenic n s extension extension in the southwestern pamir opposes shortening in a fold thrust belt north of the domes and in particular in the tajik depression where an evaporitic decollement facilitated upper crustal shortening gravitational collapse of the pamir plateau margin drove core complex formation in the southwestern pamir and shortening of the weak foreland adjacent to the plateau overall this geometry defines a vertical extrusion scenario comprising frontal and basal underthrusting and thickening and hanging gravitationally driven normal shear in contrast to the himalayan vertical extrusion scenario erosion in the pamir was minor preserving most of the extruded deep crust including the top of the south pamir shear zone at peak elevations throughout the dome

crustal and uppermost mantle velocity structure along a profile across the pamir and southern tien shan as derived from project tipage wide angle seismic data summary utilizing seismic refraction wide angle reflection data from 11 approximately in line earthquakes 2 d p and s velocity models and a poisson s ratio model of the crust and uppermost mantle beneath the southern tien shan and the pamir have been derived along the 400 km long main profile of the tipage tien shan pamir geodynamic program project these models show that the crustal thickness varies from about 65 5 km close to the southern end of the profile beneath the south pamir through about 73 6 km under lake karakul in the north pamir to about 57 7 km 50 km south of the northern end of the profile in the southern tien shan average crustal p velocities are low with respect to the global average varying from 6 26 to 6 30 km s 1 the average crustal s velocity varies from 3 54 to 3 70 km s 1 along the profile and thus average crustal poisson s ratio varies from 0 23 beneath the central pamir in the south central part of the profile to 0 265 towards the northern end of the profile beneath the southern tien shan the main layer of the upper crust extending from about 2 km below the earth s surface to 27 km depth below sea level b s l has average p velocities of about 6 05 6 1 km s 1 except beneath the south central part of the profile where they decrease to around 5 95 km s 1 this is in contrast to the s velocities which range from 3 4 to 3 6 km s 1 and exhibit the highest values of 3 55 3 6 km s 1 where the p velocity is lowest thus for the main layer of the upper crust is 0 26 beneath the profile except beneath the south central part of the profile where it decreases to 0 22 the low value of 0 22 for under the central pamir the along strike equivalent of the qiangtang terrane in tibet is similar to that within the corresponding layer beneath the northern lhasa and southern qiangtang terranes in central tibet and is indicative of felsic rocks rich in quartz in the state the lower crust below 27 km b s l has p velocities ranging from 6 1 km s 1 at the top to 7 1 km s 1 at the base further for this layer is 0 27 0 28 towards the northern end of the profile but is low at about 0 24 beneath the central and southern parts of the profile which is similar to the situation found in the northeast tibetan plateau the low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite facies and possibly also eclogite facies metapelites in the lower part within the uppermost mantle the average p velocity is about 8 10 8 15 km s 1 and is about 0 26 assuming an isotropic situation then a relatively cool 700 800 c uppermost mantle beneath the profile is indicated this would in turn indicate an intact mantle lid beneath the profile an upper mantle reflector dipping from 104 km b s l 120 km from the southern end of the profile to 86 km b s l 155 km from the northern end of the profile has also been identified the preferred model presented here for the crustal and lithospheric mantle structure beneath the pamir calls for nearly horizontal underthrusting of relatively cool indian mantle lithosphere the leading edge of which is outlined by the pamir seismic zone this cool indian mantle lithosphere is overlain by significantly shortening warm asian crust the moho trough that is a feature seen beneath some other orogenic belts for example the alps and the urals beneath the northern pamir may mark the southern tip of the actively underthrusting tien shan crust along the main pamir thrust

kinematics and vorticity in kangmar dome southern tibet testing midcrustal channel flow models for the himalaya 1 kinematic kinematic vorticity wm and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the kangmar gneiss dome southern tibet record ductile deformation patterns of a frozen segment of a southward flowing midcrustal channel microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top north and top south shear in garnet zone rocks to dominantly top north shear in staurolite kyanite zone and deeper rocks kinematic vorticity values indicate an increase in pure shear component with depth from 48 pure shear in chloritoid zone rocks through 62 in garnet zone to staurolite kyanite zone rocks to 68 pure shear in an orthogneiss the deepest exposed rocks deformation temperatures inferred from grain scale microstructures and quartz lattice preferred orientations increase from 300 c 400 c in chloritoid zone rocks to 600 c in the deepest exposed rocks these temperatures are equivalent to temperatures derived from garnet biotite thermobarometry indicating that wm was recorded during peak metamorphism this ductile deformation zone was cut by the brittle southern tibetan detachment system stds that juxtaposed metasedimentary rocks upon the orthogneiss on the basis of these relations midcrustal rocks in the core of kangmar dome record 1 general shear vertical thinning and n s horizontal extension with a component of top north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the stds 2 an increase in pure shear with structural depth a consequence of an increase in lithostatic load and 3 displacement of the high temperature shear zone by the brittle stds our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

seismic imaging of subducting continental lower crust beneath the pamir abstract exhumation of ultra high pressure metamorphic rocks testifies that the continental crust can subduct to significant depth into the mantle despite its buoyancy however direct observation of ongoing subduction of continental crust is rare the pamir is regarded as a possible place of active continental subduction because of the intermediate depth seismicity crustal xenoliths and estimates of crustal shortening versus convergence rates here we present for the first time receiver function images from a passive source seismic array traversing the tien shan and the pamir plateau showing southward subduction of eurasian continental crust in the eastern pamir we observe a southerly dipping 10 15 km thick low velocity zone lvz that extends from 50 km depth near the base of the crust to more than 150 km depth with a dip angle increasing to subvertical while the upper and mid crustal material seems to be shortened and incorporated into the pamir the lower eurasian crust detaches and subducts in its deeper part 80 km the lvz envelopes the intermediate depth earthquakes our observations imply that the complete arcuate intermediate depth seismic zone beneath the pamir traces a slab of subducting eurasian continental lower crust

simplified versus detailed two dimensional approaches to transient flow modeling in urban areas simplified and detailed two dimensional modeling approaches to transient flows in urban areas based on finite volume solution of the shallow water equations are compared through the example of a dam break flow in a simplified urban district for which accurate laboratory data exist various methods are compared 1 the solution of the two dimensional shallow water equations with a detailed meshing of each street 2 the use of a porosity concept to represent the reduction of water storage and conveyance in the urban area and 3 the representation of urban areas as zones with higher friction coefficient accuracy and adequacy of each method are assessed through comparison with the experiments among the simplified models the porosity approach seems to be the most adequate as head losses at the entrance and the exit of the city are considered

group velocity tomography of the indo eurasian collision zone we present results of a rayleigh and love wave group velocity dispersion study of the indo eurasian collision zone group velocity dispersion curves are measured and combined to produce dispersion maps for 10 70 s period rayleigh waves from 4054 paths and for 15 70 s love waves from 1946 paths group velocity maps benefit from the inclusion of data recorded at a large number of stations within india an advantage over previous global studies this has the largest impact at short periods as a result of the improved path length distribution synthetic tests are used to estimate resolution which ranges from 3 to 5 on the continents for rayleigh wave maps and from 5 to 7 5 for love wave maps group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods the cratons of the indian shield can be distinguished in the short period and midperiod group velocities group velocities are slow across tibet until 70 s whereas the cratonic cores of the indian shield appear as a high velocity anomaly at 70 s dispersion curves extracted from the rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across india and tibet the relationship between shear velocity contours and the moho indicated by receiver function studies has been used to obtain a first order estimate of crustal thickness across the collision zone results suggest a slow tibetan midcrust and low sub moho velocities beneath the central and northeastern tibetan plateau

dynamics of slab detachment 1 we investigate the dynamics of slab detachment around the detachment zone and evaluate the amount of time necessary for slabs to detach the study combines results of two dimensional 2d state of the art thermomechanical numerical simulations and a 1d analytical solution for viscous necking under gravity we show that the dominant deformation mechanism during slab detachment is viscous necking independent of the depth of slab detachment when the slab dip is moderate 35 70 slab detachment is partly affected by localized simple shearing in the colder parts of the slab brittle fracturing breaking plays a minor role during slab detachment our 2d thermomechanical models indicate that the duration of slab detachment quantified from the onset of slab thinning until the actual detachment i e vanishing of slab pull force is relatively short 250 km can occur within a short time interval 1 ma which has implications for geodynamic interpretations using slab detachment as explanation for processes such as melting exhumation or surface uplift the thinning of the slab during detachment observed in 2d simulations agrees well with predictions from a 1d analytical solution indicating that the 1d solution captures the first order features of the detachment process we also evaluate the impact of shear heating on the duration of slab detachment the predictions of a simple semi analytical solution based on dimensionless parameters agree well with our and previously published results

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

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

a tomographic image of indian lithosphere break off beneath the pamir hindukush region summary the p and s seismic anomalies in the upper mantle beneath the pamir hindukush collision zone and surrounding areas are investigated using more than 4 10 5 traveltimes from 5775 events located in the study region recorded by the stations of the worldwide seismic network and reported in the isc catalogue all available epicentral distance ranges of seismic rays are considered all sources are relocated using an algorithm which includes the double difference method in addition source parameters are corrected in the main inversion step simultaneously with the velocity models the high reliability of the resulting p velocity images is confirmed by various tests the s model provides much smoother patterns which are however generally consistent with p anomalies our model shows a complex image of lithospheric plate under the study region under sw hindukush the northwards subducting indian plate is imaged throughout the upper mantle and transition zone further to the ne within a 300 km wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at 250 km depth the tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern tien shan down to at least 250 km depth to the north of tarim we observe a northward dipping of the tarim plate down to 500 km depth at the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults which indicate complex crustal structure in this region

shear wave velocity structure beneath the central tien shan nw china from seismic ambient noise tomography abstract a new shear wave velocity model of the central tien shan is determined using seismic ambient noise tomography continuous seismic data are used to generate the three dimensional shear wave velocity structure these data are recorded at 38 broadband seismic stations from the ghengis and knet seismic networks in the central tien shan during june 1999 and august 2000 our new tomographic model reveals prominent lateral heterogeneities in the central tien shan region the shallow structure beneath the central tien shan shows a good spatial correlation with surface tectonics low and high velocity anomalies are imaged beneath basins and surrounding mountain ranges respectively most large earthquakes are mainly distributed in the low velocity anomalies or the transition zones from high to low velocity anomalies suggesting most large earthquakes are not only associated with active faults but also related to the low velocity anomalies pronounced low velocity anomalies are imaged in the middle crust beneath the high elevated mountain ranges whereas a reverse structure can be seen beneath low elevated basin ranges which implies that the low velocity anomalies caused by the upwelling of the hot and wet material into the crust from the upper mantle could have played an important role in the mountain building beneath the tien shan region all these results provide seismic evidence for better understanding the dynamics process of the tien shan mountain building

geometry of the pamir hindu kush intermediate depth earthquake zone from local seismic data 1 we present new seismicity images based on a two year seismic deployment in the pamir and sw tien shan a total of 9532 earthquakes were detected located and rigorously assessed in a multistage automatic procedure utilizing state of the art picking algorithms waveform cross correlation and multi event relocation the obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the pamir hindu kush intermediate depth seismic zone with improved detail and resolution the relocated seismicity clearly defines at least two distinct planes one beneath the pamir and the other beneath the hindu kush separated by a gap across which strike and dip directions change abruptly the pamir seismic zone forms a thin approximately 10 km width curviplanar arc that strikes east west and dips south at its eastern end and then progressively turns by 90 to reach a north south strike and a due eastward dip at its southwestern termination pamir deep seismicity outlines several streaks at depths between 70 and 240 km with the deepest events occurring at its southwestern end intermediate depth earthquakes are clearly separated from shallow crustal seismicity which is confined to the uppermost 20 25 km the hindu kush seismic zone extends from 40 to 250 km depth and generally strikes east west yet bends northeast toward the pamir at its eastern end it may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth in the upper part events form a plane that is 15 25 km thick in cross section and dips sub vertically north to northwest seismic activity is more virile in the lower part where several distinct clusters form a complex pattern of sub parallel planes the observed geometry could be reconciled either with a model of two sided subduction of eurasian and previously underthrusted indian continental lithosphere or by a purely eurasian origin of both pamir and hindu kush seismic zones which necessitates a contortion and oversteepening of the latter

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

deep india meets deep asia lithospheric indentation delamination and break off under pamir and hindu kush central asia abstract subduction of buoyant continental lithosphere is one of the least understood plate tectonic processes yet under the pamir hindu kush at the northwestern margin of the india asia collision zone unusual deep earthquakes and seismic velocity anomalies suggest subduction of asian and indian lithosphere here we report new precise earthquake hypocenters detailed tomographic images and earthquake source mechanisms which allow distinguishing a narrow sliver of indian lithosphere beneath the deepest hindu kush earthquakes and a broad arcuate slab of asian lithosphere beneath the pamir we suggest that this double subduction zone arises by contrasting modes of convergence under the pamir and hindu kush imposed by the different mechanical properties of the three types of lithosphere involved while the buoyant northwestern salient of cratonic india bulldozes into cratonic asia forcing delamination and rollback of its lithosphere india s thinned western continental margin separates from cratonic india and subducts beneath asia this torn off narrow plate sliver forms a prominent high velocity anomaly down to the mantle transition zone our images show that its uppermost section is thinned or already severed and that intermediate depth earthquakes cluster at the neck connecting it to the deeper slab providing a rare glimpse at the ephemeral process of slab break off

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