crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

tectonic geomorphology of the southeastern mississippi embayment in northern mississippi usa we analyzed drainage basin geometry in part of the southeastern mississippi embayment near the new madrid seismic zone in order to detect areas of lateral stream migration that could indicate recent tectonism toward this goal a field of drainage basin asymmetry vectors was generated from a digital terrain model and compared to geologic structure lithofacies seismicity remotely sensed lineaments and major drainage divides transverse topographical drainage basin asymmetry t index results define geomorphic domains in northern mississippi 33 to 35 n 88 5 to 90 25 w that may reflect deep crustal blocks bounded by active faults or flexures t index measurements a proxy for lateral stream migration of more than 4500 second order drainage basins produced a two dimensional field of 282 spatially averaged vectors from which we interpreted 12 domains some domain boundaries correspond to mapped faults of two paleozoic fault systems in our study area the northeast striking mississippi valley graben system and the northwest striking ouachita thrust belt and foreland faults several other prominent domain boundaries may be previously unmapped faults flexures parallel to these systems this interpretation is supported by satellite image analysis of the northeastern study area which reveals lineaments consistent with our domain boundaries that have strikes that reflect the regional subsurface structural grain

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

buckling of the lithosphere in western himalaya constraints from gravity and topography data the deformation of the lithosphere in western himalaya and karakoram is analyzed by correlating bouguer gravity anomalies and topographic heights sampled along profiles in the direction of the indo asian collision the spectral features of the coherence linear admittance and correlation coefficients between topography and deformation at depth support the hypothesis that the flexural mode of lithospheric deformation is complemented by a folding mode with typical wavelengths concentrated in bands centered at 250 and 120 km when combined with realistic models of a rheologically stratified lithosphere with two strong layers upper crust and upper mantle sandwiching a weak very ductile layer the lower crust the data are consistent with a model of response to horizontal deformation in which a coupled biharmonic folding instability develops the lithosphere as a whole warps with the longer wavelengths and the upper layer alone also with the shorter wavelengths major faults such as the main boundary thrust or the main karakoram thrust and the computed folds of the upper layer are phased as if there were a causal relationship between the position of the faults and the zones of highest strain in the crust

grace measurements of mass variability in the earth system monthly gravity field estimates made by the twin gravity recovery and climate experiment grace satellites have a geoid height accuracy of 2 to 3 millimeters at a spatial resolution as small as 400 kilometers the annual cycle in the geoid variations up to 10 millimeters in some regions peaked predominantly in the spring and fall seasons geoid variations observed over south america that can be largely attributed to surface water and groundwater changes show a clear separation between the large amazon watershed and the smaller watersheds to the north such observations will help hydrologists to connect processes at traditional length scales tens of kilometers or less to those at regional and global scales

continental lithosphere folding in central asia part ii constraints from gravity and topography abstract periodical sub parallel fold like structures observed in the western gobi have two characteristic dominant wavelengths of approximately 50 and 300 360 km we explain these observations in terms of independent quasi viscous folding of the crustal and upper mantle parts of the lithosphere due to horizontal transpressional stresses in central asia induced by the collision with the indian plate we derive a model for the mechanical response of the continental lithosphere to horizontal stresses for a rheologically layered plate with non newtonian power law rheology overlying a low viscosity asthenosphere the differentiation in the effective viscosity and thickness of the strong upper crust and upper mantle lithosphere along with the presence of a low viscosity lower crust between them leads to their partial decoupling during compressional deformation these features result in the appearance of different wavelengths of the folds in the western gobi region we derive simple semi analytical estimates for the dominant wavelengths and rates of growth of surface undulations constrained by data from experimental rock mechanics topography and gravity

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

moho undulations beneath tibet from grace integrated gravity data summary knowledge of the variation of crustal thickness is essential in many applications such as forward dynamic modelling numerical heat flow calculations seismologic applications and geohistory reconstructions we present a 3 d model of the moho undulations over the entire tibetan plateau derived from gravity inversion the gravity field has been obtained by using the gravity recovery and climate experiment grace potential field development which has been integrated with terrestrial data and is presently the best available in the studied area for the effective use of the global geopotential model that has no height information of observation stations upward continuation is applied the moho model is characterized by a sequence of troughs and ridges with a semi regular pattern which could reflect the continent continent collision between the indian and eurasian plates the three deep moho belts troughs and shallow moho belts ridges between them are clearly found to have an e w directional trend parallel to the border of the plateau and tectonic lines while variation of the directionality is observed in central to southeast tibet to describe the distinctive shape of the moho troughs beneath tibet we introduce the term moho ranges the most interesting aspects of the moho ranges are 1 that they run in parallel with the border and tectonic sutures of the plateau 2 that the distances between ranges are found at regular distances of about 330 km except in northeast tibet and 3 that the splitting of the ranges into two branches is found as the distance between them is increasing from our study we conclude that the distinctive undulations of the tibetan moho have been formed by buckling in a compressional environment superimposed on the regional increase in crustal thickness according to our analysis the grace satelliteonly data turns out to have good enough resolution for being used to determine the very deep moho beneath tibet our moho model is the first one that covers the entire plateau

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

geophysical constraints on the link between cratonization and orogeny evidence from the tibetan plateau and the north china craton abstract understanding the geodynamic process of orogeny and cratonization and their transition is among the key topics of research in evaluating the link between plate tectonics and continental dynamics the tibetan plateau and the north china craton ncc two key areas in mainland china offer excellent laboratories to understand continental tectonics over a broad span of earth history particularly the deep structure of the lithosphere as imaged from geophysical data on the tibetan plateau and the ncc provide important clues in understanding orogeny and cratonization the tibetan plateau is the largest and highest plateau on earth in terms of mean altitude and is an important region for understanding the mechanisms of continent continent collision and cenozoic plateau uplift the ncc is an archean craton that underwent lithospheric disruption during the mesozoic here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography and gravity modeling in tibet and its neighboring regions in order to understand the modality of the convergence and collision process between the indian and eurasian plates and the influence of this process on the uplift of the plateau in the ncc geological geochemical geophysical and tectonic investigations demonstrate that lithospheric destruction mainly occurred in the eastern block the crustal structure of the ncc is reconstructed from ambient noise surface wave tomography and the different possible disruption mechanisms are evaluated the vs shear wave velocity tomography results and the density structure of the crust and upper mantle to about 350 km depth demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the bangong nujiang suture bns and the west and east of tibet which suggest that the leading edge of the subducting indian slab reaches the bns the subduction angle of indian plate indicates a transition from steep to shallow from the west to east tibet sections depicting the gravitational potential energy suggest that mantle flow contributes to the subduction of the indian plate as far as the bns and the transition from the asthenospheric layer to the metasomatic lid overlaps with the transition from north south shortening in south tibet to eastward tectonic escape in north tibet qiangtang and songpan ganzi blocks both vs and models suggest the following 1 north southward lower crust flow beneath the eastern ncc and interaction between the westward mantle flow and eastward escape flow beneath the central ncc in addition to the earlier proposed mechanisms of delamination and thermal erosion played important roles in the lithospheric disruption of the archean craton 2 mantle flow plays an important role in the continental tectonic transition between neighboring tectonic blocks and within the cycle between orogeny and cratonization

crustal structure across the altyn tagh range at the northern margin of the tibetan plateau and tectonic implications wepresentnewseismicrefraction wide angle reflection dataacrossthealtyntaghrangeanditsadjacentbasins wefindthatthe crustal velocity structure and by inference the composition of the crust changes abruptly beneath the cherchen fault i e 100 km north of the northern margin of the tibetan plateau north of the cherchen fault beneath the tarim basin a platform type crust is evident in contrast south the cherchen fault the crust is characterized by a missing high velocity lower crustal layer our seismic model indicates that the high topography 3 km of the altyn tagh range is supported by a wedge shaped region with a seismic velocity of 7 6 7 8 km s that we interpret as a zone of crust mantle mix we infer that the altyn tagh range formed by crustal scale strike slip motion along the north altyn tagh fault and northeast southwest contraction over the range the contraction is accommodated by 1 crustal thickening via upper crustal thrusting and lower crustal flow i e creep and 2 slip parallel swdirected underthrusting of only the lower crust and mantle of the eastern tarim basin beneath the altyn tagh range d 2005 elsevier b v all rights reserved

hydrothermal silicification along faults affecting carbonate sandstone units and its impact on reservoir quality potiguar basin brazil abstract we analyzed hydrothermal silicification along the afonso bezerra strike slip fault system in the potiguar basin equatorial margin of brazil to assess the role of si rich fluids in fault geometry properties and evolution the fault cut across the whole basin including its crystalline basement a late cretaceous siliciclastic unit and a lower carbonate unit and a paleogene neogene upper carbonate unit intense fault controlled silicification caused by the upward migration and diffusion of hydrothermal fluids occurred repeatedly the fault zone is characterized by multiple events of syntectonic dynamic silicification with hydraulic brecciation at all scales in the fault core and static silicification in which no or little lattice orientation or deformation occurred in the damage zone in the carbonate units silicification resulted in the complete replacement of the carbonate mineralogy by quartz chalcedony and opal and the sio2 content increased from 3 to 15 in the host carbonate units to as much as 94 97 in the silicified portions of the fault the silicified fault zone presents an abrupt widening from 150 m in the siliciclastic unit to as much as 800 m in the lower carbonate unit the matrix of the siliciclastic unit exhibits a porosity reduction from 27 to 4 by contrast the matrix porosity of the lower carbonate unit exhibits a primary porosity of approximately 5 10 which is either reduced to less than 1 or increased to more than 15 in the silicified zone in the upper and lower carbonate units centimeter scale vuggy porosity along fractures occurs in the silicified zone this anomalous vuggy porosity could serve as a reservoir pore space the collective field and petrological evidence suggests that volcanic intrusion plays an important role in silicification this work may shed light on the origin and likely geometry of the fault controlled silica distribution causing heterogeneities within carbonate and siliciclastic reservoirs

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

a global earthquake discrimination scheme to optimize ground motion prediction equation selection we present a new automatic earthquake discrimination procedure to determine in near real time the tectonic regime and seismotectonic domain of an earth quake its most likely source type and the corresponding ground motion prediction equation gmpe class to be used in the u s geological survey usgs global shakemap system this method makes use of the flinn engdahl regionalization scheme seismotectonic information plate boundaries global geology seismicity cat alogs and regional and local studies and the source parameters available from the usgs national earthquake information center in the minutes following an earthquake to give the best estimation of the setting and mechanism of the event depending on the tectonic setting additional criteria based on hypocentral depth style of faulting and regional seismicity may be applied for subduction zones these criteria include the use of focal mechanism information and detailed interface models to discriminate among outer rise upper plate interface and intraslab seismicity the scheme is validated against a large database of recent historical earthquakes though developed to assess gmpe selection in global shakemap operations we anticipate a variety of uses for this strategy from real time processing systems to any analysis involving tectonic classification of sources from seismic catalogs

moho geometry gravity inversion experiment moggie a refined model of the australian moho and its tectonic and isostatic implications abstract at the continent scale models of moho depth based on seismic estimates alone can be inadequate due to irregular or sparse data gravity based moho modelling provides better coverage however the methods used are typically hampered by an inability to explicitly honour seismic constraints and are also limited by over simplistic model conditions e g laterally homogenous layering i present a new method to generate a continent scale moho model based on the constrained inversion of free air gravity data this method explicitly honours seismic moho estimates and accounts for a laterally heterogeneous crust and mantle resolution and sensitivity testing shows that for wavelengths greater than 200 km crustal density and moho depth are recovered with reasonable accuracy 30 kg m 3 and 3 km respectively moggie uses a six layer model incorporating ocean sedimentary basin upper crust lower oceanic crust eclogitised crust and mantle inversion variables were the density of the crustal layers constrained by a standard density model and the depths to intra crustal boundaries and the moho constrained by 230 seismic depth estimates the results demonstrate that a balanced approach to seismically constrained gravity inversion has the capability to generate detailed and well constrained models of the moho and crustal density at the continent scale for australia this is a clear improvement on the sparse and irregular resolution of the moho provided by seismic estimates of crustal thickness which fail to resolve short wavelength features newly defined tectonic features include extensive magmatic underplates crustal scale shear zones and the boundaries between tectonic blocks isostatic analysis reveals that little of the continent is close to isostatic equilibrium with isostatic disequilibria preserved at multiple scales from hundreds of kilometres to the entire continent these disequilibria are interpreted to indicate long wavelength flexure of highly competent continental lithosphere

lithospheric folding based understanding on the origin of the back arc basaltic magmatism beneath jeju volcanic island 1 compression near collisional plate boundaries causes lithospheric folding which results in the decrease of the pressure beneath the ridge of the fold while the pressure beneath the trough increases the decompression beneath the lithosphere is likely to bring on basaltic magmatism along and below the ridge we investigate the subsurface structure beneath jeju cheju volcanic island and its vicinity and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by episodic lithospheric folding unlike the prevailing hypothesis of the intraplate basaltic magmatism that requires extending lithosphere our hypothesis can explain how the basaltic magma could be generated at the back arc regions without the extension

tarim underthrust beneath western kunlun evidence from wide angle seismic sounding abstract this paper describes the results of a wide angle seismic profile over 700 km long across the southern margin of the tarim basin a crustal structure that best fits the observed data that tests and verifies the earlier discovery that the crystalline basement dips southward from the area of the petroleum industry seismic reflection profiling and geological investigation is presented the profile also reveals that the moho dips southward beneath the southern tarim at an angle in concordance to that of the crystalline basement this result is thought to be the first evidence for the underthrusting of the tarim crust beneath the western kunlun additional evidence of underthrusting is that the relatively weak western kunlun crust has contracted and expressed by a lower crust which is more than 20 km thick with an uplift of the kunlun basement accompanied by northward directed thrusts

mapping the descent of indian and eurasian plates beneath the tibetan plateau from gravity anomalies the collision of india with asia has produced a complicated continental continental plate boundary involving folding and faulting of variable trends and styles within and along the margins of the tibetan plateau numerous lines of evidence including the development of two scales of folding in tibet suggest that the lowermost crust is behaving in a ductile fashion this weak lower crust might then decouple the fundamental plate tectonic motions in the uppermost mantle from the complex pattern of surface faulting in this study we use bouguer gravity anomalies to map out the geometry of indian and eurasian plate interactions in the mantle beneath the plateau based on both the inferred geometry of the moho and lateral variations in lithospheric strength determined from mechanical modeling in our preferred model the lithosphere beneath tibet consists of two distinct units 1 the underthrust to the north indian plate which sutures with the eurasian plate in the upper mantle below the yarlung zangpo suture or the gangdese igneous belt 200 400 km north of the main boundary thrust and 2 the underthrust to the south eurasian plate a subducting slab of indian upper mantle extends about 200 km into the asthenosphere north from the mantle suture and exerts a bending moment of about 3 5 1017 n on the indian plate thus the mantle lithosphere appears to be behaving in the simple fashion of converging oceanic plates while the more buoyant continental crust deforms under high gravitational potential in a complex pattern controlled by its lateral and vertical strength heterogeneity

how fast is the middle lower crust flowing in eastern tibet a constraint from geodetic observations various geophysical observations including seismological and magnetotelluric imaging have implied that the deep crust beneath eastern tibet may be partially melted and flowing faster than the brittle upper crust however it is still unclear how much faster the deep crust is flowing geodetic observations which are more sensitive to dynamic changes provide constraints on the flow rate of the middle and lower crust mlc three dimensional gps velocities show that deformation within the brittle upper crust contributes little 0 02 0 02 mm yr to the overall surface uplift 2 7 0 3 mm yr therefore two plausible models for the surface uplift are discussed which are consistent with the geodetic constraints in the deep crustal flow model crustal thickening requires the horizontal flow rate of the mlc to be 330 710 of the rate of motion of the upper crust and the deepening of the moho is only up to 35 of that required to maintain isostatic balance isostasy may not be maintained over the geodetic timescale in the hybrid model of deep crustal flow and convective lithospheric detachment the moho is uprising and only weak or moderate 130 250 deep crustal flow is required which results in moderate present day crustal thickening beneath eastern tibet this result improves our understanding of the plateau construction and dynamics and also offers advice for numerical simulations

three dimensional fold structure of the tibetan moho from grace gravity data 1 although the prevailing wavelength of the moho fold has been estimated from the spectral analysis of gravity and topography there has not been a suggested method developed to reveal its structure here we present a threedimensional 3d moho fold structure beneath tibet which clearly reflects the continental collision for the structure estimation a new method has been introduced based on the gravity inversion and flexural model the estimated direction and wavelength of the moho fold are consistent with the velocities calculated from global positioning system gps and with an elastic plate model under horizontal compression the prevailing wavelength of the moho fold is estimated to be 300 to 420 km which corresponds to an elastic plate with effective elastic thickness eet of about 35 km and much smaller than the prior estimates of 500 to 700 km citation shin y h c k shum c braitenberg s m lee h xu k s choi j h baek and j u park 2009 three dimensional fold structure of the tibetan moho from grace gravity data geophys res lett 36 l01302 doi 10 1029 2008gl036068

comparing thin sheet models with 3 d multilayer models for continental collision summary various models have been proposed to explain tectonic deformations during continent collision a frequently applied model is the thin viscous sheet model which is however not fully 3 d and assumes a priori diffuse thickening as the dominant deformation style we compare a fully 3 d multilayer numerical model with a corresponding thin viscous sheet numerical model for the scenario of continent indentation in our comparison we focus on the three basic viscous deformation styles thickening buckling folding and lateral crustal flow both numerical models are based on the finite element method fem and employ either a linear or power law viscous rheology the 3 d model consists of four layers representing a simplified continental lithosphere strong upper crust weak lower crust strong upper mantle and weak lower mantle the effective viscosity depth profile in the 3 d model is used to calculate the depth averaged effective viscosity used in the thin sheet model allowing a direct comparison of both models we quantify the differences in the strain rate and velocity fields and investigate the evolution of crustal thickening buckling and crustal flow resulting from the two models for two different phases of deformation 1 indentation with a constant velocity and 2 gravitational collapse after a decrease of the indenting velocity by a factor of 5 the results indicate that thin sheet models approximate well the overall large scale lithospheric deformation especially during indentation and for a linear viscous rheology however in the 3 d model additional processes such as multilayer buckling and lower crustal flow emerge which are ignored in the thin sheet model but dominate the deformation style in the 3 d model within a range of a few hundreds of kilometres around the collision zone and indenter corner differences between the 3 d and thin sheet model are considerably larger for a power law viscous than for a linear viscous rheology buckling and lower crustal flow are significant in the 3 d model with power law viscous rheology for example fibre strain rates due to buckling can be several hundred per cent different to the depth averaged strain rate and the lateral mass flow of lower crustal material can be up to six times more than the flow of upper crustal material our results also show that the horizontal velocity fields of the upper crust and upper mantle remain nearly identical in the 3 d model during indentation despite their mechanical decoupling due to an intermediate weak lower crust this result questions the validity of using similarities between velocities from the surface global positioning system gps and mantle shear wave splitting data as evidence for a mechanically coupled lithosphere 3 d multilayer models provide a more complete picture of continental collision than thin sheet models as they enable studying the timing locality and relative importance of different processes simultaneously which is especially important for the hundreds of kilometre scale around the collision zone and indenter corners 3 d models are however still computationally challenging and we therefore also present results of a computational performance test of several solution algorithms

spatial variations of flexure parameters over the tibet quinghai plateau we investigate the tibet quinghai plateau and the tarim basin in terms of spatial variations of the elastic thickness te in the frame of the thin plate flexure model the method of investigation makes use of a convolutive method which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance coherence methodologies we study the relation between the topographic and subsurface loads and the observed crust mantle interface cmi undulations the latter having been obtained from gravity inversion the gravity data used for the inversion are a unique set of high quality data available over the chinese part of the plateau and constitute the highest resolution grid today available in this impervious area the gravity inversion is constrained by results from the study of the propagation of seismic waves the two extensive sedimentary basins the tarim and the qaidam basins are modeled by forward gravity modeling the oscillations of the cmi obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads it is found that the modeling of the sedimentary basins is essential in the flexure analysis the spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau most of the tibet plateau has low te varying in the bounds 10 30 km with lower values in the qiangtang terrain where the te reaches 8 km the tarim and the qaidam basins precambrian platforms overlain by sediments are rigid and have a te of up to 110 km and 70 km respectively the flexural analysis distinctly discerns the tibet plateau with thick crust part of which is molten from the cratonic areas the tarim and qaidam basins which though of thinner crust act as undeformable rigid blocks