modeling mountain building and the seismic cycle in the himalaya of nepal a host of information is now available regarding the geological and thermal structure as well as deformation rate across the himalaya of central nepal these data are reconciled in a two dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes over geological timescale 5 ma the 20 mm yr estimated shortening rate across the range is accommodated by localized thrust faulting along the main himalayan thrust fault mht the mht reaches the surface along the foothills where it is called the main frontal thrust fault mft the mht flattens beneath the lesser himalaya and forms a midcrustal ramp at the front of the higher himalaya consistent with the river incision and the anticlinal structure of the lesser himalaya farther northward the mht roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust increasing the coulomb stress beneath the front of the higher himalaya where most of the microseismic activity clusters negligible deformation of the hanging wall requires a low apparent friction coefficient less than 0 3 on the flat portion of the mht on the ramp might be as high as 0 6 sensitivity tests show that a rather compliant quartz rich rheology and a high radioactive heat production in the upper crust of 2 5 w m 3 is required erosion affects the thermal structure and interplays with crustal deformation a dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure topography and deformation field using a linear diffusion model of erosion we constrain the value of the mass diffusivity coefficient to 0 5 1 6 l0 4 m 2 yr this study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period

common conversion point stacking of receiver functions for estimating the geometry of dipping interfaces summary we develop a new method to estimate geometries of dipping seismic velocity discontinuities from common conversion point stacking of receiver functions the multistage fast marching method is applied to the ray tracing with refraction at dipping interfaces although geometry of interfaces dipping at 30 70 cannot be correctly estimated with a 1 d velocity model it can be correctly estimated when refraction at dipping interfaces is taken into account we apply this method to synthetic and observed data and confirm that geometries of dipping interfaces are successfully estimated this method will contribute to revealing the structures beneath subduction zones where dipping interfaces exist

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

evolving strain partitioning in the eastern himalaya the growth of the shillong plateau the shillong plateau is the only raised topography up to 2000 m elevation in the himalayan foreland it is proposed to have had a major influence on strain partitioning and thus tectonics in the eastern himalaya additionally its position on the trajectory of the summer monsoon means it has influenced the regional climate with reduced erosion rates proposed over geological timescales in its lee the timing of surface uplift of the plateau has been difficult to determine exhumation rates have been calculated over geological timescales but these seem at variance with estimates based upon extrapolating the present day velocity field measured with gps and it has thus been suggested that exhumation and surface uplift are decoupled we determine the timing of surface uplift using the sedimentary record in the adjacent surma basin to the south which records the transition from a passive margin with southward thickening sedimentary packages to a flexural basin with north thickening strata due to loading by the uplifting plateau our method involves using all available 2d seismic data for the basin coupled to well tie information to produce isochore maps and construct a simple model of the subsidence of the surma basin in order to assess the timing and magnitude of flexural loading by the shillong plateau we conclude that the major period of flexural loading occurred from the deposition of the tipam formation 3 5 2 ma onwards which is likely to represent the timing of significant topographic growth of the shillong plateau our isochore maps and seismic sections also allow us to constrain the timing of thinning over the north south trending anticlines of the adjacent basin bounding indo burman ranges as occurring over this same time interval the combined effect of the uplift of the shillong plateau and the westward encroachment of the indo burman ranges to this region served to sever the palaeo brahmaputra drainage connection between himalayan source and surma basin sink at the end of tipam formation times 2 ma

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

improved imaging with phase weighted common conversion point stacks of receiver functions summary broad band array studies frequently stack receiver functions to improve their signal to noise ratio while mapping structures in the crust and upper mantle noise may produce spurious secondary arrivals that obscure or mimic arrivals produced by p to s conversions at large contrasts in seismic impedance such as the moho we use a hilbert transform to calculate phase weights which minimize the constructive stacking of erroneous signal in receiver function data sets we outline this approach and demonstrate its application through synthetic data combined with different types of noise a previously published example of signal generated noise and a large data set from the sierra nevada earthscope project these examples show that phase weighting reduces the presence of signal generated noise in receiver functions and improves stacked data sets

crustal and mantle velocity models of southern tibet from finite frequency tomography 1 using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays we carried out finite frequency tomographic inversions to image the three dimensional velocity structure beneath southern tibet to examine the roles of the upper mantle in the formation of the tibetan plateau the results reveal a region of relatively high p and s wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the higher himalaya we interpret this high velocity anomaly as the underthrusting indian mantle lithosphere there is a strong low p and s wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the yadong gulu rift suggesting that rifting in southern tibet is probably a process that involves the entire lithosphere intermediate depth earthquakes in southern tibet are located at the top of an anomalous feature in the mantle with a low vp a high vs and a low vp vs ratio one possible explanation for this unusual velocity anomaly is the ongoing granulite eclogite transformation together with the compressional stress from the collision eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the indian mantle lithosphere beneath the higher himalaya our tomographic model and the observation of north dipping lineations in the upper mantle suggest that the indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north south trending rifts and subducted in a progressive piecewise and subparallel fashion with the current one beneath the higher himalaya

receiver function imaging of crustal suture steep subduction and mantle wedge in the eastern india tibet continental collision zone abstract to understand the along strike variation of crustal deformation and tectonic processes in the india tibet continental collision zone we deployed a linear array of broadband seismic stations along 92 e to image lithospheric structure our receiver function cross section reveals a prominent negative converter dipping 20 north from 10 55 km depth below sea level almost through the whole crust beneath the southern lhasa terrane we interpret it to be a manifestation of the yarlung zangbo suture zone yzs separating the continental crust of the indian and eurasian plates this implies the hypothesized channel flows of indian middle crust extruding southwards from tibet are limited at this longitude to the southernmost portion of the lhasa terrane a positive converter consistent with previous suggestions of eclogite formation is seen about 10 15 km above the moho and continuing 50 km north of the 20 dipping yzs converter we image this positive converter continuously from 60 km south of the surface trace of the yzs to the vicinity of the jiali fault supporting the interpretation of sub horizontal underplating of tibetan crust by indian crust to 31 n at 85 e on the hi climb transect however we also show a negative mantle converter sub parallel to the crustal yzs converter from the northern limit of the underplating indian lower crust to at least 140 km depth that we interpret as the base of tibetan lithosphere overlying an asthenospheric mantle wedge based on the lithospheric structure observed in this and other studies we infer that indian mantle lithosphere currently detaches from indian lower crust at the mantle suture that is nearly 50 km south of the surface trace of the yzs at 92 e south of the mantle suture suggested by indepth transect beneath the surface trace of the yzs at 90 e and far south of the mantle suture suggested to be at the 31 n northern limit of underthrusting indian lower crust suggested by hi climb transect at 85 e a change from underplating in the west to steep subduction in the east can reconcile all these observations

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

wide angle vibroseis test across the rhine graben summary as a joint operation of ecors and dekorp a deep wide angle seismic experiment using vibrators was carried out in the autumn of 1984 the object was to get information on the deep crust under the rhine graben without crossing through sedimentary layers offsets were in the range of 50 to 89 km in the first phase two vibration points were executed in the vosges mountains a signal was received in the black forest from solely the farthest vp in the second phase fourteen vps were executed in the black forest no stacking or correlation was performed in the field in france the quality of the results is good only if an equalization is applied before vertical stacking and correlation in the computing centre

crustal structure of the pannonian basin the alcapa and tisza terrains and the mid hungarian zone abstract the pannonian basin of central europe is one of the key examples of miocene continental extension that is easily accessible to surface seismological investigation it comprises two major crustal blocks alcapa and tisza which abut along a poorly understood structure referred to as the mid hungarian zone mhz the whole being surrounded by the arc of the carpathian mountains the alps and the dinarides using data from the cbp carpathian basins project temporary broadband seismic array of 46 stations deployed across the western pannonian basin in 2006 2007 we calculated receiver functions that constrain the variation of crustal thickness across the basin and derive a map of moho depth across a nw se oriented swath about 450 km long and 75 km wide the measured moho depths show no significant change in crustal thickness between alcapa and tisza terrains but the moho is not or very weakly imaged along a ca 40 km wide strip centred on the mhz moho depths within the pannonian basin are typically in the range 25 30 km and increase toward the periphery of the basin our measurements are generally consistent with earlier vp models from controlled source seismic surveys and recent vs models determined by tomographic analysis of ambient noise signals the lack of a sharp moho image beneath the mhz suggests that the crust mantle boundary in that zone may consist of a gradual increase in velocity with depth the relatively constant crustal thickness across the two domains of the pannonian basin suggests that thinning to the same final state is controlled thermally this structural characteristic seems to be governed by a large scale balance of gravitational potential energy that is insensitive to the separate prior histories of the two regions

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

geometric and kinematic evolution of the foreland thrust and fold belt in the southern urals the first balanced and restored cross section through the foreland thrust and fold belt of the southern urals along the belaya river of bashkortostan is presented this cross section is used to discuss the paleozoic structure and kinematic evolution of the area and to provide quantitative estimates of fault displacement and shortening the belaya river thrust stack can be divided into a late imbricate thrust system nygush thrust system the burzyan thrust sheet and an early duplex system timirovo thrust system overprinting relationships indicate two deformation events during which the foreland thrust and fold belt evolved by a forward propagating thrust sequence the first deformation event involved emplacement of the low metamorphic grade timirovo thrust system and the overlying zilair nappe during accretion of the magnitogorsk volcanic arc with the east european craton at the end of the devonian a deformational hiatus followed until the end of the carboniferous at which time a second deformation event related to final closure of the uralian ocean resulted in development of the nygush thrust system the calculated shortening in the belaya river area 16 7 km or 17 is significantly less than that in other foreland thrust and fold belts the basal thrust in the belaya river section is located within the basement and cuts continuously up section with a staircase like geometry instead of following an easy glide horizon

low seismicity in the bhutan himalaya and the stress shadow of the 1897 shillong plateau earthquake abstract the seismicity of the bhutan himalaya region is generally low as compared to its adjoining himalayan segments the topography in the region is slightly subdued and different from the neighbouring central nepal region low seismicity in the region may possibly be due to the postulated low convergence rate in the bhutan himalaya difference in subsurface structures or the aseismic nature of the region we suggest that the lower seismicity in the bhutan himalaya may also be due to stress change caused by the great 1897 shillong plateau earthquake as the low seismicity region of the bhutan himalaya coincides with the stress shadow of the earthquake

a data adaptive multiscale approach of finite frequency traveltime tomography with special reference to p and s wave data from central tibet 1 we discuss an innovation in traveltime tomography that combines wavelet based multiscale parameterization and finite frequency theory to solve two outstanding issues that inevitably arise from uneven source station distributions and from the three dimensional 3 d nature of wavefront healing how to objectively address the intrinsically multiscale nature of data coverage while simultaneously maintain model resolution at each scale level we apply the new integrated methodology to investigate 3 d variations of p and s wave speeds lnvp and lnvs beneath the himalayan tibetan orogen in particular we are able to constrain variations in the poisson s ratio via ln vp vs the formulation is naturally data adaptive resolving features at each scale only if the required data converge is available the very first long wavelength feature that emerges is a clear anomaly of high lnv that extends over more than 500 km beyond the northern edge of the lhasa terrane at places farther northward a strong negative anomaly underlies the region where recent volcanism occurs in northern tibet regions of negative ln vp vs delineate a slab like subhorizontal feature concentrated between depths of 100 250 km such characteristics are consistent with the notion that chemically refractory and therefore buoyant mantle lithosphere of the indian shield greater india has advanced subhorizontally northward far beyond the surficial bangong nujiang suture in the crust two isolated regions of low lnv each extending to depths near 100 km occur along the lunggar and the yadong gulu active rifts in southern tibet deep penetrating rifts imply that only a limited amount of horizontal displacement is being accommodated on subvertical structures

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

clockwise rotation of the brahmaputra valley relative to india tectonic convergence in the eastern himalaya naga hills and shillong plateau gps data reveal that the brahmaputra valley has broken from the indian plate and rotates clockwise relative to india about a point a few hundred kilometers west of the shillong plateau the gps velocity vectors define two distinct blocks separated by the kopili fault upon which 2 3 mm yr of dextral slip is observed the shillong block between longitudes 89 and 93 e rotating clockwise at 1 15 myr and the assam block from 93 5 e to 97 e rotating at 1 13 myr these two blocks are more than 120 km wide in a north south sense but they extend locally a similar distance beneath the himalaya and tibet a result of these rotations is that convergence across the himalaya east of sikkim decreases in velocity eastward from 18 to 12 mm yr and convergence between the shillong plateau and bangladesh across the dauki fault increases from 3 mm yr in the west to 8 mm yr in the east this fast convergence rate is inconsistent with inferred geological uplift rates on the plateau if a 45 n dip is assumed for the dauki fault unless clockwise rotation of the shillong block has increased substantially in the past 4 8 myr such acceleration is consistent with the reported recent slowing in the convergence rate across the bhutan himalaya the current slip potential near bhutan based on present day convergence rates and assuming no great earthquake since 1713 a d is now 5 4 m similar to the slip reported from alluvial terraces that offsets across the main himalayan thrust and sufficient to sustain a mw 8 0 earthquake in this area

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

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

crustal structure of the darjeeling sikkim himalaya and southern tibet analysis of data from nine temporary broadband seismic stations operated across west bengal and sikkim along with publicly available data from seismographs in the surrounding region provides the first image of the descending indian plate beneath the darjeeling sikkim himalaya the down going indian crust is imaged by receiver function common conversion point stacking using data from 32 sites in combination with more detailed analyses from simultaneous modelling of receiver function data and rayleigh wave group velocity dispersion at 13 stations compared to locations farther south on the indian shield our southernmost station shows evidence for thickened crust beneath the mahanadi rift basin with a possible mafic basal layer north of the mahanadi rift the indian moho is 38 km deep below the archean terranes of the northeast part of the indian shield the moho then dips gently northward beneath the himalayan foreland basin reaching a depth of 44 48 km below the himalayan foothills below sikkim the moho continues to deepen but there are indications of secondary structures in the receiver function image and modelling results suggesting some imbrication of the crust as it flexes downward the crust thickens further beneath the greater himalaya and southern tibet reaching depths of 65 70 km below the southern tibet detachment std below the lhasa terrane north of the std a double discontinuity exists with interfaces at 55 60 km and 80 km depth there is a significant reduction in the average shear wave velocity of the crystalline crust between sites to the south of and on the himalayan foreland basin and sites in the himalaya and to the north below the himalaya and southern tibet the p to s conversion ps has a lower amplitude compared to that observed at sites on the undeformed indian shield this decrease in amplitude of the moho ps phase could arise from a lower impedance contrast across the crust mantle boundary or from scattering due to deformation of the crust and moho a coherent negative arrival beneath the darjeeling sikkim himalaya indicate the presence of a low velocity zone lvz possibly associated with the main himalayan thrust this lvz can be traced beneath the darjeeling sikkim himalaya but disappears beneath the greater himalaya

seismic structure of the underthrusting indian crust in sikkim himalaya 1 this study presents the first results of the seismic character of the underthrusting indian crust in the sikkim himalaya deduced through an analysis of 3600 receiver functions rfs abstracted from waveforms registered at 11 broadband stations spanning a 110 km long n s profile from the foothills to the higher himalaya common conversion point stacks of receiver functions prominently trace the northward dipping geometry of the indian moho beneath the himalaya monte carlo inversion of the azimuthal variations of the rfs at individual stations adopting the nearest neighborhood algorithm approach reveals that the crustal thickness varies from 40 km to 61 km from south to north with a dip varying between 4 and 10 among stations a moho doublet prominently seen at a depth of 40 km in the higher himalaya to the north of main boundary thrust has been interpreted in terms of possible partial eclogitization of a granulitic indian lower crust akin to the finding just north of the study region beneath southern tibet a strong layer of anisotropy 17 localized within a low velocity layer between 20 and 30 km has a nw se oriented fast polarization direction counterintuitive to the convergence parallel and range perpendicular alignment expected in a convergent setting due to shear processes midcrustal transcurrent deformation in sikkim and bhutan evidenced by a conjugate system of strike slip faulting with nw to ne trending p axis orientations is the most feasible mechanism for causing a near strike parallel oriented fast axis of anisotropy in this segment of himalaya

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

stress buildup in the himalaya the seismic cycle on a major fault involves long periods of elastic strain and stress accumulation driven by aseismic ductile deformation at depth ultimately released by sudden fault slip events coseismic slip distributions are generally heterogeneous with most of the energy being released in the rupture of asperities since on the long term the fault s walls generally do not accumulate any significant permanent deformation interseismic deformation might be heterogeneous revealing zones of focused stress buildup the pattern of current deformation along the himalayan arc which is known to produce recurring devastating earthquakes and where several seismic gaps have long been recognized might accordingly show significant lateral variations providing a possible explanation for the uneven microseismic activity along the himalayan arc by contrast the geodetic measurements show a rather uniform pattern of interseismic strain oriented consistently with long term geological deformation as indicated from stretching lineation we show that the geodetic data and seismicity distribution are reconciled from a model in which microseismicity is interpreted as driven by stress buildup increase in the interseismic period the uneven seismicity pattern is shown to reflect the impact of the topography on the stress field indicating low deviatoric stresses 35 mpa and a low friction 0 3 on the main himalayan thrust arc normal thrusting along the himalayan front and east west extension in southern tibet are quantitatively reconciled by the model

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

seismic structure of the crust and the upper mantle beneath the himalayas evidence for eclogitization of lower crustal rocks in the indian plate 1 variations in the seismic velocity structure of the himalayan collision zone include significant differences between its north and south portions with transitions in physical properties across the greater himalaya we combined p and s wave traveltimes from a temporary broadband seismic network in eastern nepal and southern tibet with arrival times at the permanent station network of the department of mines and geology of nepal to determine the seismic velocity structure across the himalaya using local earthquake tomography and traveltimes of regional earthquakes the p to s velocity ratio vp vs structure marks the difference between the indian plate and the overlying materials with the vp vs ratios being high for the former and low for the latter we also found a significant increase in the uppermost mantle seismic velocities from south to north reaching p wave velocities vp over 8 4 km s north of the greater himalaya these high vp values do not seem to be the result of biases due to anisotropy in the upper mantle beneath the greater and tethyan himalayas instead we suggest that rocks in the lower crust of the underthrusting indian plate undergo metamorphism to eclogite as they plunge to greater depth beneath the mountain range explaining the high seismic velocities

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

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

crustal structure of the tethyan himalaya southern tibet new constraints from old wide angle seismic data summary a wide angle seismic profile between peigu tso 85 5 e and pumoyong tso 90 5 e in southern tibet acquired under a joint sino french program in 1981 passes through the south tibetan detachment system stds the eastern segment of the profile and the central part of the tethyan himalaya the western segment we herein reinterpret this wide angle seismic profile which has a total length of 480 km in order to improve our understanding of the crustal structure of the tethyan himalaya we identify multiple p and s wave wide angle reflections within the crystalline crust and from the moho but cannot identify any refracting waves beneath the moho we model the structure of p and s wave velocity throughout the whole crust while acknowledging the significant uncertainties that are inherent in many places a low wave speed cover sequence may be identified with the tethyan sedimentary cover vp 5 8 km s 1 from the surface to a depth of typically 5 10 km which is only absent where the profile crosses the kangmar dome it may be presumed that these sedimentary rocks overlay a felsic upper crustal basement 5 8 vp 6 3 km s 1 which in places extends as far down as 35 km and which itself overlays a 5 10 km thick mid crustal low velocity zone between a depth of 30 and 45 km at the base of the low velocity zone and the moho at a depth of 73 km the observed velocities gradually increase from 6 2 to 6 8 km s 1 the low velocity layer lvl may consist of a partial melt due to the high geothermal gradients found in the felsic rocks or of a fluid rich shear zone as in the ductile channel of a channel flow model the difference in depth of the lvl between the western central tethyan himalaya and eastern segments nearly along the stds at the boundary of the dingye mabjia fault leads to a north south dip estimate of the lvl of 8 5 which is consistent with the estimated dip of the mht obtained from the indepth i project and can support the channel flow model this north south low dip angle of subduction slab supports that the subducting indian lithosphere endured a transition from steep subduction to low angle subduction and the subduction slab flattening may be attributed to the break off of indian lithosphere slab beneath the indur zurpo suture

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

mountain building in the nepal himalaya thermal and kinematic model we model crustal deformation and the resulting thermal structure across the nepal himalaya assuming that since 20 ma shortening across the range has been primarily taken up by slip along a single thrust fault the main himalayan thrust mht fault and that the growth of the himalayan wedge has resulted mainly from underplating and to the development of a duplex at midcrustal depth we show that this process can account for the inverse thermal metamorphic gradient documented throughout the lesser himalaya lh the discontinuity of peak metamorphic temperatures across the mct as well as the distribution of age of exhumation across the range this study suggests that the metamorphic evolution of the range over about the last 20 million years is compatible with the kinematics of recent crustal deformation deduced from morphotectonic and geodetic studies

characterizing the main himalayan thrust in the garhwal himalaya india with receiver function ccp stacking abstract we use common conversion point ccp stacking of ps receiver functions to image the crustal structure and moho of the garhwal himalaya of india our seismic array of 21 broadband seismometers spanned the himalayan thrust wedge at 79 80 e between the main frontal thrust and the south tibet detachment in 2005 2006 our ccp image shows the main himalayan thrust mht the detachment at the base of the himalayan thrust wedge with a flat ramp flat geometry seismic impedance contrasts inferred from geologic cross sections in garhwal imply a negative impedance contrast velocity decreasing downward for the upper flat located beneath the lower himalaya and a positive impedance contrast velocity increasing downward for the ramp located beneath the surface trace of the munsiari thrust or mct i at the lower flat located beneath the higher himalaya spatially coincident measurements of very high electrical conductivities require the presence of free fluids and we infer a negative impedance contrast on the mht caused by ponding of these fluids beneath the detachment our seismic image indicates that the upper flat of the mht is 10 km below sea level and dips north at 2 connecting to a mid crustal ramp which is 10 km high and dips at 16 the lower flat is 20 25 km below sea level and dips at 4 the main central thrust mct appears as a negative impedance contrast dipping at 16 the moho is nearly horizontal at 35 45 km depth beneath the sub himalaya and lower himalaya deepening to 50 km or more beneath the higher himalaya this depth is 10 25 km shallower than in the nw indian himalaya and 5 10 km shallower than in central nepal requiring significant along strike variations in crustal thickness the observed thickness of subducted indian crust in garwhal is 20 28 km

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

thermal structure and exhumation history of the lesser himalaya in central nepal the lesser himalaya lh consists of metasedimentary rocks that have been scrapped off from the underthrusting indian crust and accreted to the mountain range over the last 20 myr it now forms a significant fraction of the himalayan collisional orogen we document the kinematics and thermal metamorphism associated with the deformation and exhumation of the lh combining thermometric and thermochronological methods with structural geology peak metamorphic temperatures estimated from raman spectroscopy of carbonaceous material decrease gradually from 520 550 c below the main central thrust zone down to less than 330 c these temperatures describe structurally a 20 50 c km inverted apparent gradient the ar muscovite ages from lh samples and from the overlying crystalline thrust sheets all indicate the same regular trend i e an increase from about 3 4 ma near the front of the high range to about 20 ma near the leading edge of the thrust sheets about 80 km to the south this suggests that the lh has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm yr for a convergence rate of about 20 mm yr this implies underthrusting of the indian basement below the himalaya by about 15 mm yr the structure metamorphic grade and exhumation history of the lh supports the view that since the mid miocene the himalayan orogen has essentially grown by underplating rather than by frontal accretion this process has resulted from duplexing at a depth close to the brittle ductile transition zone by southward migration of a midcrustal ramp along the main himalayan thrust fault and is estimated to have resulted in a net flux of up to 150 m 2 yr of lh rocks into the himalayan orogenic wedge the steep inverse thermal gradient across the lh is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

layered deformation in the taiwan orogen the underthrusting of continental crust during mountain building is an issue of debate for orogens at convergent continental margins we report three dimensional seismic anisotropic tomography of taiwan that shows a nearly 90 rotation of anisotropic fabrics across a 10 to 20 kilometer depth consistent with the presence of two layers of deformation the upper crust is dominated by collision related compressional deformation whereas the lower crust of taiwan mostly the crust of the subducted eurasian plate is dominated by convergence parallel shear deformation we interpret this lower crustal shearing as driven by the continuous sinking of the eurasian mantle lithosphere when the surface of the subducted plate is coupled with the orogen the two layer deformation clearly defines the role of subduction in the formation of the taiwan mountain belt

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

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

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

indepth international deep profiling of tibet and the himalaya multichannel seismic reflection data description and availability project indepth international deep profiling of tibet and the himalaya has collected over 300 km of multichannel deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the tibetan plateau the reflection profiles lie within the yadong gulu rift and were acquired in the summers of 1992 and 1994 data processing utilized typical industry tools and a new method was used to migrate the data both unmigrated and migrated sections are presented here in large format to facilitate further interpretations

intraplate deformation of the indian subcontinent 1 while deformation at the earth s surface primarily occurs along tectonic plate boundaries major earthquakes have shaken regions deep within continental interiors three of the largest m 7 5 historic intraplate earthquakes occurred within the indian subcontinent suggesting the possibility of significant intraplate deformation we consider surface velocities determined from new gps data collected at 29 continuous gps stations and 41 survey mode gps stations in india between 1995 and 2007 to find a north south shortening rate of 0 3 0 05 nanostrain yr 1 which may be accommodated by 2 1 mm yr of more localized convergence across central india southward motions at 4 7 mm yr of sites on the shillong plateau in northeast india reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau the width and magnitude of the elastic strain accumulation field across the himalaya varies little from 76 90 longitude but the strain is more broadly distributed and convergence rates are higher along the eastern 200 km of the range

imaging faults and shear zones using receiver functions the geometry of faults at seismogenic depths and their continuation into the ductile zone is of interest for a number of applications ranging from earthquake hazard to modes of lithospheric deformation teleseismic passive source imaging of faults and shear zones can be useful particularly where faults are not outlined by local seismicity passive seismic signatures of faults may arise from abrupt changes in lithology or foliation orientation in the upper crust and from mylonitic shear zones at greater depths faults and shear zones with less than near vertical dip lend themselves to detection with teleseismic mode converted waves receiver functions provided that they have either a contrast in isotropic shear velocity vs or a contrast in orientation or strength of anisotropic compressional velocity vp we introduce a detection method for faults and shear zones based on receiver functions we use synthetic seismograms to demonstrate common features of dipping isotropic interfaces and contrasts in dipping foliation that allows determination of their strike and depth without making further assumptions about the model we proceed with two applications we first image a laramide thrust fault in the western u s the wind river thrust fault as a steeply dipping isotropic velocity contrast in the middle crust near the surface trace of the fault further downdip and across the range where basin geometry suggests the fault may sole into a subhorizontal shear zone we identify a candidate shear zone signal from midcrustal depths the second application is the use of microstructural data from exhumed ductile shear zones in scotland and in the western canadian shield to predict the character of seismic signatures of present day deep crustal shear zones realistic anisotropy in observed shear fabrics generates a signal in receiver functions that is comparable in amplitude to first order features like the moho observables that can be robustly constrained without significant tradeoffs are foliation strike and the depth of the foliation contrast we find that an anisotropy of only a few percent in the shear zone is sufficient to generate a strong signal but that the shear zone width is required to be 2 km for typical frequencies used in receiver function analysis to avoid destructive interference due to the signals from the boundaries of the shear zone

mapping seismic anisotropy using harmonic decomposition of receiver functions an application to northern apennines italy 1 isotropic and anisotropic seismic structures across the northern apennines italy subduction zone are imaged using a new method for the analysis of teleseismic receiver functions rfs more than 13 000 p wave coda of teleseismic records from the 2003 2007 retreating trench extension and accretion tectonics retreat experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of mediterranean evolution a new methodology for the analysis of receiver functions is developed which combines both migration and harmonic decomposition of the receiver function data set the migration technique follows a classical common conversion point scheme and helps to focus on a crucial depth range 20 70 km where the mantle wedge develops harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of p to s converted phases the separation of the back azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures the application of our method to the retreat data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the northern apennines chain where previous studies failed to clearly retrieve the geometry of the subducted interfaces we present our results in closely spaced profiles across and along the northern apennines chain to highlight the convergence of the tyrrhenian and the adriatic microplates which differ in their crustal structure where the adriatic microplate subducts beneath tuscany and the tyrrhenian sea a signature of the dipping adriatic moho is clearly observed beneath the tyrrhenian moho in a large portion of the forearc region in the area where the two mohos overlap our new analysis reveals the presence of an anisotropic body above the subducted moho there is a strong ps converted phase with anisotropic characteristics from the top of the adriatic plate to a depth of at least 80 km because the ps conversion occurs much deeper than similar ps phases in cascadia and japan dehydration of oceanic crust seems unlikely as a causative factor rather the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting

the underthrusting indian crust and its role in collision dynamics of the eastern himalaya in bhutan insights from receiver function imaging most of the convergence rate between the indian and eurasian plate is assumed to be absorbed along a major basal thrust beneath the himalaya the main himalayan thrust mht deformation along this basal thrust in combination with frontal accretion results in the formation of the upper crustal fold thrust belt the role of the underthrusting indian crust and its impact on the long term growth of the himalaya are only poorly understood partly due to the lack of high resolution seismic images of the crust to improve the imaging of lithospheric structures we developed an 3 d migration scheme for receiver functions using seismic data from the temporary gansser network in bhutan extending the 2 d high frequency ray approximation and common conversion point stacking to 3 d including linear phase weighting and a quality assessment we reveal significant along strike differences in the lithospheric structure beneath bhutan in western bhutan the moho geometry shows an increased dip south of the higher himalaya reaching almost 70 km depth thereafter whereas in eastern bhutan the moho is almost subhorizontal at 50 km depth across our network the appearance of distinct listric structures beneath the mht indicates intra crustal deformation up to crustal imbrication down to the lower crust we propose that these variations in the crustal thickness and in intra crustal structures influence the upper crustal kinematics of the bhutan himalayan orogeny and are primarily driven by an indian mantle slab northwest of bhutan and its absence northeast of bhutan