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

seismic imaging of crust beneath the dharwar craton india from ambient noise and teleseismic receiver function modelling we use cross correlation of continuous 18 months 2009 february to 2010 august ambient noise data recorded over 35 broad band seismographs in the archean dharwar craton and the adjoining granulite terrain to generate rayleigh wave group velocity maps in the period 5 28 s this is supplemented with longer period data 40 70 s from earthquake source combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations includes 15 stations operated during 1998 2002 to produce shear velocity image of the crust the velocity image reveals thinner crust 34 38 km in the late archean 2 7 ga eastern dharwar craton edc while all other terrains mid archean and proterozoic have crustal thickness from 40 to over 50 km the mid archean 3 36 ga greenstone belt of the western dharwar craton wdc has the thickest crust 50 km the average crustal vs beneath the edc is 3 70 3 78 km s 1 as compared to 3 80 3 95 km s 1 beneath the wdc we observe significant lateral variation in the thickness of lower crust vs 3 8 4 2 km s 1 10 15 km in the edc compared to 20 30 km in the wdc the lowermost part of the crust vs 4 0 km s 1 is thin 5 km beneath the edc in contrast to more thickness 10 27 km beneath the wdc our analysis suggests intermediate composition for the crust beneath the edc similar to those for other cratons in contrast the mid archean exposed wdc crust has more mafic composition and exceptional thickness a scenario at variance with the global observations we interpret this thick mafic crust to represent undeformed geological segment of 3 36 ga the edc with a nearly flat moho felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late archean

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

moho geometry and upper mantle images of northeast india 1 images of the crust and mantle beneath northeast india obtained by 2d migration of 1000 broadband p receiver functions clearly trace a northward dipping moho from the himalayan foredeep reaching depths up to 50 km further north beneath the himalayan convergence zone also these images reveal presence of largely coherent 410 km and 660 km discontinuities that conform to the iasp91 model marginal variations in the depth of the 410 km interface are observed that appear region specific the thickness of the mantle transition zone does not deviate significantly from a global average of 250 km interestingly our results reveal consistent presence of a signal from an interface around 300 km origin of such a boundary known as x discontinuity and unrelated to the lehmann discontinuity is discussed possible presence of the x discontinuity from the indian region is reported here for the first time

uplift and convergence along the himalayan frontal thrust of india along the himalayan thrust front in northwestern india terrace deposits exposed 20 to 30 m above modern stream level are interpreted to have been uplifted by displacement on the underlying himalayan frontal thrust a radiocarbon age limits the age of the terrace to 1665 215 calendar bc 3663 215 radiocarbon years before present yielding a vertical uplift rate of 6 9 1 8 mm yr in combination with published studies constraining the dip of the himalayan frontal thrust fault to about 30 in the study area the observed uplift rate equates to horizontal shortening across the himalayan frontal thrust of 11 9 3 1 mm yr and the slip rate of the himalayan frontal thrust of 13 8 3 6 mm yr this is similar to previously reported rate estimates along the himalayan arc based on displacement of older plio miocene age rocks or the much shorter records of geodesy and historical seismicity the similarity is consistent with the idea that convergence across the himalayan front has occurred at a relatively steady rate through time the seismic expression of this deformation includes several great m 8 historical earthquakes which due to lack of surface rupture during those events have been attributed to their occurrence on blind thrusts yet the occurrence of a possible fault scarp in the field area indicates that past earthquakes have been sufficiently large to rupture to the surface and produce coseismic scarps these observations suggest a potential for earthquakes along the himalayan frontal thrust larger than those observed historically

structural elements of the southern tethyan himalaya crust from wide angle seismic data a deep seismic common midpoint cmp profile shot across the southern margin of the tethyan himalaya as the first stage of project international deep profiling of tibet and the himalaya indepth imaged the top main himalayan thrust or mht and bottom moho of the indian continental crust underthrusting the himalaya we used portable seismographs to record the cmp profile shots at a wide range of offsets up to 155 km our short offset data corroborate the cmp profile data while our large offset data are dominated by a band of reflectivity we interpret the bright onset of this reflective band as the basal detachment of the south tibetan detachment system and a phase at the end of the reflective band as the mht we used the cmp profile first break data to model the uppermost 2 km in detail the depth of young extensional basins of the yadong rift system along the cmp profile was constrained to a maximum of 2 km yielding a throw of 4 6 km across the eastern flank of the rift and a rough estimate of approximately 1 5 east west extension by normal faulting in the tethyan himalaya the wide angle data were used to construct a crustal seismic velocity model down to the mht the south tibetan detachment system stds basal detachment reflector is observed dipping 12 5 nne from a depth of about 6 km beneath the surface under the south end of the cmp profile to a depth of 22 km then flattening to a dip of only 2 5 nne thus our observations suggest that the stds basal detachment is a deep rooted basement fault for the mht we observe a dip of 7 5 nne from 20 km depth below sea level at the crest of the high himalaya crystalline sheet to 36 km below sea level 40 km beneath the surface at a distance of about 70 km south of the indus yarlung suture iys from geometrical arguments we suggest that indian crust underthrusts the surface expression of the iys within the crust but does not form the lower crust of central and northern tibet

crustal structure variations in northeast india from converted phases 1 teleseismic receiver functions from a ten station network deployed in northeast india region sampling the shillong plateau mikir hills himalayan foredeep and the himalayan convergence zone are analyzed to obtain the crustal structure in this seismically active but less studied region the shillong plateau and mikir hills away from the convergent margins reveal remarkably simple crust with thickness 35 km and poisson s ratio 0 25 akin to the indian shield values a surprisingly thin crust for the uplifted shillong plateau may be explained invoking presence of an uncompensated crust that popped up in response to tectonic forces in contrast crustal signatures from assam valley suggest a thicker crust and higher poisson s ratio with evidences for a dipping moho predictably the crust is much thicker and complicated in the eastern himalaya further north with values in excess of 50 km

development of the himalayan frontal thrust zone salt range pakistan the salt range is the active frontal thrust zone of the himalaya in pakistan seismic reflection data show that a 1 km offset of the basement acted as a buttress that caused the central salt range potwar plateau thrust sheet to ramp to the surface exposing mesozoic and paleozoic strata the frontal part of the thrust sheet was folded passively as it overrode the subthrust surface on a ductile layer of eocambrian salt lack of internal deformation of the rear part of the thrust sheet is due to decoupling of sediments from the basement along this salt layer early to middle pliocene 4 5 ma conglomerate deposition in the southern potwar plateau previously interpreted in terms of compressional deformation may instead document uplift related to basement normal faulting stratigraphic evidence paleomagnetic dating of unconformities and sediment accumulation rates suggest that the thrust sheet began to override the basement offset from 2 1 to 1 6 ma cross section balancing demonstrates at least 20 to 23 km of shortening across the ramp the rate of himalayan convergence that can be attributed to underthrusting of indian basement beneath sediments in the pakistan foreland is therefore at least 9 14 mm yr about 20 35 of the total plate convergence rate

collision tectonics in the nw himalayas west himalayan tectonics involve the collision of microplates between the indian and asian plates the kohistan complex consists largely of tightly folded basic volcanics and sediments generated as late jurassic to late cretaceous island arcs these were intruded by post folding mid cretaceous eocene plutonics produced from continued subduction of the indian plate after closure of a suture between kohistan and the karakorum the himalayan structures show major thrust sheets and the kohistan arc is essentially a crustal pop up with southward upright and northward verging structures developed above a thick ductile decoupling zone the indus suture which can be traced for 100 km beneath kohistan on large reentrants this pop up formed by a two stage process closure of the northern suture followed by closure of the southern indus suture granitic rocks of the kohis tan ladakh batholith dated at 100 40 ma post date most of the structures related to the northern suture but were deformed and carried southwards on shear structures related to the indus suture postcollisional deformation carried this kohistan complex on deep decoupling zones over the indian plate on a series of imbricated gneiss sheets the thrusts climbing up section in the movement direction so that in the far s some override their own molasse debris folds above these deep decoupling zones deformed their overlying thrust sheets into large antiforms i e the nanga parbat and hazara syntaxes the nanga parbat syntaxis probably formed due to a shear couple near a branch line where one of the main himalayan thrusts joined the indus suture beneath kohistan crustal delamination to produce the imbricated gneiss sheets could not account for all the displacement of india into asia suggested by palaeomagnetic data there must also have been lateral displacement as demonstrated by the large oblique slip shear zone in the hunza valley n of kohistan

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

crustal structure along the lawrencepur astor profile in the northwest himalayas during the pamir himalayan project in the year 1975 seismic refraction and wide angle reflection data were recorded along a 270 km long lawrencepur astor sango sar profile in the northwest himalayas the profile starts in the indus plains and crosses the main central thrust mct the hazara syntaxis the main mantle thrust mmt and ends to the east of nanga parbat the seismic data as published by guerra et al 1983 are reinterpreted using the travel time ray inversion method of zelt and smith 1992 and the results of inversion are constrained in terms of parameter resolution and uncertainty estimation the present model shows that the high himalayan crystallines hhc velocity 5 4 km s 1 overlie the indian basement velocity 5 8 6 0 km s 1 the crust consists of four layers of velocity 5 8 6 0 6 2 6 4 and 6 8 km s 1 followed by the upper mantle velocity of 8 2 km s 1 at a depth of about 60 km

interseismic strain accumulation on the himalayan crustal ramp nepal the departement of mines and geology has been monitoring the seismicity of the central himalayas of nepal since 1985 intense microseismicity and frequent medium size earthquakes ml 4 tend to cluster beneath the topographic front of the higher himalaya this 10 20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data both microseismic and geodetic data indicate strain accumulation on a mid crustal ramp that had been previously inferred from geological and geophysical evidence this ramp connects a flat decollement under the lesser and sub himalaya with a deeper decollement under the higher himalaya and probably acts as a geometric asperity where strain and stress build up during the interseismic period the large himalayan earthquakes could nucleate there and probably activate the whole flat and ramp system up to the blind thrusts of the sub himalaya

tomographic evidence for wholesale underthrusting of india beneath the entire tibetan plateau we analyzed a global tomographic model for the tibet himalayan collision zone which indicates that the indian lithospheric slab has been subducted subhorizontally beneath nearly the entire tibetan plateau to depths of 165 260 km tibetan velocity structure is low in its crust and high in its lithospheric mantle at depths between 75 and 120 km we interpret an asthenospheric layer positioned above the subducted indian slab at depths between 120 and 165 km beneath the tibetan plateau beneath the central portion of the plateau a lowvelocity anomaly exists from the crust down to 310 km depth indicating mantle upwelling through a weakened part of the subducted slab we present a model which explains that the uplift history and low relief of the tibetan plateau is a result of subhorizontal subduction and heating of indian lithosphere that is separated from tibetan lithosphere by a thin channel of asthenosphere two predictions made by our model are 1 the amount of shortening in the himalayas is equivalent to the amount of underthrusted indian mantle lithosphere and 2 a young mantle geochemical signature should be present along the entire southern portion of the tibetan plateau q 2004 elsevier ltd all rights reserved

crustal shear velocity structure of the south indian shield 1 the south indian shield is a collage of precambrian terrains gathered around and in part derived from the archean age dharwar craton we operated seven broadband seismographs on the shield along a n s corridor from nanded nnd to bangalore bgl and used data from these to determine the seismic characteristics of this part of the shield surface wave dispersion and receiver function data from these sites and the geoscope station at hyderabad hyb give the shear wave velocity structure of the crust along this 600 km long transect inversion of rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3 66 km s 1 12 km thick layer overlying a 3 81 km s 1 23 km thick lower crust at all sites the receiver functions are extremely simple indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds crustal thickness 35 2 km average shear wave speed 3 79 0 09 km s 1 and vp vs ratio 1 746 0 014 there is no evidence for a high velocity basal layer in the receiver function crustal images of the central dharwar craton suggesting that there is no seismically distinct layer of mafic cumulates overlying the moho and implying that the base of the dharwar crust has remained fairly refractory since its cratonization index terms 7203 seismology body wave propagation 7205 seismology continental crust 1242 7255 seismology surface waves and free oscillations keywords continental crust archean crust receiver function indian shield

seismicity and one dimensional velocity structure of the himalayan collision zone earthquakes in the crust and upper mantle 1 earthquakes beneath the himalayan collision zone occur at depths between near surface and around 100 km below sea level after relocating earthquakes with two one dimensional 1 d velocity models we found a clear bimodal depth distribution for earthquakes in the himalayas of eastern nepal and the southern tibetan plateau and evidence that some earthquakes originate at upper mantle depths seismicity in nepal shows an accumulation of earthquakes along the front of the himalayan arc with a seismic gap between longitudes 87 3 e and 87 7 e although upper crustal seismicity along the topographic front of the high himalaya is consistent with a region of high strain accumulation associated with convergence on the main himalayan thrust fault microearthquakes do not necessarily occur on this fault instead they concentrate in the hanging wall seismic activity in the sub himalaya and the terai plains is almost exclusively limited to the vicinity of the location of the magnitude 6 5 20 august 1988 udayapur earthquake with most of the earthquakes in the lower crust and the upper mantle clusters of earthquakes in the lesser and high himalayas and south tibet tethyan himalayas mark very well defined zones of seismicity at depths between 50 and 100 km confirming the presence of earthquakes in the upper mantle in the region of continental collision the occurrence of earthquakes at sub moho depths favors the idea that the continental upper mantle deforms by brittle processes

the himalayan foreland basin crust and upper mantle abstract modeling of multimode surface wave group velocity dispersion data sampling the eastern and the western ganga basins reveals a three layer crust with an average v s of 3 7 km s 1 draped by 2 5 km foreland sediments the moho is at a depth of 43 2 km and 41 2 km beneath the eastern and the western ganga basins respectively crustal v p v s shows a felsic upper and middle crust beneath the eastern ganga basin 1 70 compared to a more mafic western ganga basin crust 1 77 due to higher radiogenic heat production in felsic than mafic rocks a lateral thermal heterogeneity will be present in the foreland basin crust this heterogeneity had been previously observed in the north indian shield immediately south of the foreland basin and must also continue northward below the himalaya the high heat producing felsic crust underthrust below the himalayas could be an important cause for melting of midcrustal rocks and emplacement of leucogranites this is a plausible explanation for abundance of leucogranites in the east central himalaya compared to the west the uppermost mantle v s is also significantly lower beneath the eastern ganga basin 4 30 km s 1 compared to the west 4 44 km s 1

configuration of the indian moho beneath the nw himalaya and ladakh 1 teleseismic receiver function analysis of seismograms recorded on a 700 km long profile of 17 broadband seismographs traversing the nw himalaya shows a progressive northward deepening of the indian moho from 40 km beneath delhi south of the himalayan foredeep to 75 km beneath taksha at the karakoram fault similar studies by wittlinger et al 2004 to the north of the karakoram fault show that the moho continues to deepen to 90 km beneath western tibet before shallowing substantially to 50 60 km at the altyn tagh fault the continuity of the indian moho imaged in the receiver functions reported here along with those of wittlinger et al 2004 suggest that in this part of the himalayan orogen the indian plate may penetrate as far as the bangong suture and possibly as far north as the altyn tagh

reply to comment by ali and aitchison on restoration of cenozoic deformation in asia and the size of greater india in our recent paper van hinsbergen et al 2011a weprovide a kinematic restoration of cenozoic deformation inasia based on the currently available kinematic estimates onfault zones and fold thrust belts in tibet the pamir the tienshan mongolia siberia and indochina our reconstructionsuggests that approximately 1050 km in the pamir to600 km in eastern tibet of india asia convergence wasaccommodated by intraasian shortening in the last 50 ma by comparing this reconstruction with the respective posi tions of india and asia constrained by the eurasia northamerica africa india plate circuit using model a of vanhinsbergen et al 2011b we explored the implicationsfor the size for greater india as a function of collision age whereby we define greater india as the area of lithosphereconsumed by northward subduction beneath the asianmargin since collision of the tibetan himalaya with asia importantly wedonot aprioridefine thatall thatlithospheremust be continental in nature our reconstruction demon strated that if collision started by 50 ma greater india at thetime of initial collision must have been up to 2600 km wide such a 50 ma collision age follows from the timing of thefirst arrival of asia derived detritus in the tibetan himalaya najman et al 2010 wang et al 2011 hu et al 2012 andthe overlap of paleomagnetically determined paleolatitudesfrom the former southern margin of asia the lhasa terrane e g dupont nivet et al 2010a lippert et al 2011 withthose from the tibetan himalaya patzelt et al 1996 yiet al 2011 such a large n s width of greater india inlate cretaceous and paleocene time is consistent with thehigh quality paleomagnetic data from tibetan himalayanrocks of patzelt et al 1996 shown to have undergonenegligible inclination shallowing due to compaction bydupont nivet et al 2010b and recently corroborated by yiet al 2011 when compared to high quality paleomagneticpoles from india those paleomagnetic data demonstrate an s separation between the tibetan himalaya and theindian craton of 22 0 3 0

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

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

crustal structure and deformation in the northeast india asia collision zone constraints from receiver function analysis summary crustal structure across the india asia collision zone tidding suture in the northeast himalaya bounded by the eastern himalayan syntaxis ehs is investigated using the p wave receiver function rf method the analysed data included three component waveforms of teleseismic earthquakes recorded by a linear array of 11 broad band seismic stations the rfs and inverted shear wave velocity models reveal azimuthally varying crustal structure the rfs for earthquakes from the northeast back azimuths are conspicuous by absence of p to s converted phase at the moho discontinuity inverted velocity model ascribe this to absence of the typical step velocity jump at the moho in a narrow section of the ehs bordering the indenting indian plate and pierced by travelling waves in contrast teleseismic waves arriving from southeast back azimuth sample different litho tectonic blocks of the himalayan collision zone and inverted models show northeast dipping moho across the tidding suture compared to an overall thickness of 70 km in the northwest and central himalaya the crust across the tidding suture is only about 55 km thick this is attributed to a slower rate of convergence in this part of the collision zone the moho structure beneath indian plate and southeastern tibetan plateau reveal opposite dip directions with their colliding margin placed just east of the walong thrust the inverted shear wave velocity models show evidence of intracrustal low velocity layer whose strength varies across the tidding suture the magnitude of velocity reduction beneath lohit plutonic complex trans himalaya favour partial melt as a possible mechanism whereas south of the tidding suture where velocity reduction is comparatively less fluids generated by dehydration reactions appear to be the source for velocity reduction

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

electrical structure of the himalaya of central nepal high conductivity around the mid crustal ramp along the mht twelve broadband magnetotelluric mt soundings were performed across the himalaya of central nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics the mt impedance tensors were obtained for frequencies between 0 001 and 500 hz the 2 d section derived from joint inversion of te and tm mode after rri and groom bailey decomposition shows high conductivity in the foreland basin 30 m that contrasts with the resistive indian basement 300 m and lesser himalaya 1000 m in addition our mt sounding reveals a major conductive feature beneath the front of the higher himalaya also characterized by intense microseismic activity and the position of a mid crustal ramp along the major active thrust fault mht this high conductivity zone probably reflects metamorphic fluids released during underthrusting of the indian basement and pervading well connected microcracks induced by interseismic stress build up or distributed brittle deformation around the ramp

crustal structure in eastern kazakh u s s r from teleseismic receiver functions broadband receiver functions determined from teleseismic p waveforms recorded at two seismic stations in eastern kazakh u s s r are inverted for the vertical velocity structure beneath the stations the detailed broadband receiver functions are obtained by stacking source equalized radial components of teleseismic p waveforms a time domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat lying homogeneous layers the general features of the inversion results are a complex shallow crust velocities less than 6 km s in the upper crust a high velocity 6 9 7 5 km s lower crust and a moho that varies between 47 and 57 km depth these results compare favorably with a composite velocity model from deep seismic sounding data but show lower velocities in the upper crust and higher velocities in the lower crust the results indicate that the crust in this region is relatively uniform however the nature and depth of the moho changes significantly across the region the receiver function inversion structure for the kazakh sites is similar to a published receiver function structure for a site on the canadian shield

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

the lithosphere asthenosphere boundary revealed by s receiver functions from the hi climb experiment summary detailed high resolution images of the crust mantle and lithosphere asthenosphere boundaries the moho and the lab respectively have been well observed by applying an s receiver function technique to data collected by the hi climb himalayan tibetan continental lithosphere during mountain building experiment the moho depth variation in the range of 50 70 km is in good agreement with that from previous p receiver function results the significant variation in the lab depth indicates that the subducting indian lithosphere drops northwards from a depth of 80 km beneath the himalayas to 130 km just north of the bangong nujiang suture at 33 0 n and undergoes a transition from low angle to flat subduction beneath the yarlung zangbo suture our findings provide new seismic constraints on the 3 d subducting configuration of the indian lithosphere beneath tibet

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

geomorphology and volcanology of maat mons venus abstract full resolution fmidr magellan radar backscatter images have been used to characterize the geology and volcanology of the volcano maat mons on venus this volcano has often been identified by remote sensing techniques as one of the volcanoes on the planet that could have been recently active and is the highest volcano on venus with a relief of 9 km the summit of maat mons is characterized by a caldera complex 26 30 km in diameter with at least six remnant pit craters 10 km in diameter preserved in the walls of the caldera suggesting that multiple small volume 1 km in diameter on the flanks these pits appear to have formed by collapse with no effusive activity associated with their formation no evidence for explosive volcanism can be identified despite the relatively low atmospheric pressure 55 bar near the summit there is also a lack of evidence for lava channels deformation features within the caldera and thrust faults on the flanks indicating that the physical volcanology of maat mons is simpler than that of typical martian and terrestrial shield volcanoes preservation of fine scale 3 4 pixels structures within the pit craters and summit pits is consistent with geologically very recent activity but no evidence for current activity can be identified

sediment thickness beneath the indo gangetic plain and siwalik himalaya inferred from receiver function modelling abstract the indo gangetic plain and the adjoining siwalik himalaya are the seismically most vulnerable regions due to high density of human population and presence of thick sediments that amplify the seismic waves due to an earthquake in the region we investigate the sedimentary structure and crustal thickness of the region through joint inversion of the receiver function time series at 14 broadband seismograph locations and the available rayleigh velocity data for the region results show significant variability of sedimentary layer thicknesses from 1 0 to 2 0 km beneath the delhi region to 2 0 5 0 km beneath the indo gangetic plain and the siwalik himalaya as we progress from the delhi to the indo gangetic plain we observe a decrease in the shear velocity in sedimentary layer from 2 0 km s to 1 3 km s while the layer thickness increases progressively from 1 0 km in south to 2 0 5 0 km in the north average s velocity in the sedimentary layer beneath the siwalik himalaya is 2 1 km s crustal thicknesses varies from 42 in the delhi region 48 km in the indo gangetic plain 50 km in the western part of siwalik himalaya to 60 km in the kumaon region of siwalik himalaya

gravity anomalies flexure of the indian plate and the structure support and evolution of the himalaya and ganga basin bouguer gravity anomalies along four profiles across the western himalaya and ganga basin show large deviations from local isostatic equilibrium a deficit of mass characterizes the ganga basin and an excess underlies the lesser himalaya both can be understood if the indian plate is flexed down by the distributed load of part of the mountains the cross sectional shape of the ganga basin seems to be controlled by the deflection of the indian plate which we compute assuming the indian plate to overlie an inviscid fluid from the shapes of both bouguer anomaly profiles and the basement topography we place bounds on the flexural rigidity of such a plate if the ganga basin is a steady state feature then the age of the basal sediments in a given locality should be proportional to the distance of that locality from the southern edge of the basin if the rate of convergence of india and the himalaya were constant that rate should equal the distance divided by the corresponding age we find a rate of 10 to 15 mm a for the last 15 to 20 ma which is consistent with a large part of the 50 mm a rate of convergence between india and eurasia being absorbed by the eastward extrusion of parts of tibet profiles of bouguer gravity anomalies show only a small peak or plateau over the southern edge of the lesser himalaya implying that the boundary between the light sediments of the ganga basin and the heavier crustal rocks of the lesser himalaya is not sharp and that there exists some light material beneath the range we infer that some sediment deposited in the ganga basin has been underthrust beneath the lesser himalaya but the quantity is small most of this sediment probably is scraped off the indian plate to make the foothills of the range we find that the load of the high himalaya is too large to be supported solely by elastic stress in the indian plate if the flexural rigidity of the plate is constant and if no other external forces act on the plate the observed gradient in bouguer gravity anomalies increases from an average of about 1 mgal km over the ganga basin and lesser himalaya to about 2 mgal km over the high himalaya this increase in the gravity gradient implies that the moho dips more steeply 10 15 beneath the high himalaya than beneath the lesser himalaya 2 3 we interpret this steepening of the moho to be due to a weakening of the plate which allows it to bend more sharply beneath the high himalaya than farther south with the inclusion of a weak segment of indian plate beneath the high himalaya calculated anomalies show a somewhat increased gradient beneath the high himalaya but when the weight of the entire himalaya is used as a load on the plate calculated anomalies are more negative than observed therefore an external force system is needed to support much of the weight of the high himalaya as well as to bend the plate sufficiently beneath the high himalaya the magnitudes of the bending moment and the force per unit length that must be applied to the end of the plate are compatible with their sources being gravity acting on a part of india s mantle lithosphere stripped of its crust and underthrust beneath southern tibet

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

geological and hydrogeological environment in tianjin with potential geohazards and groundwater control during excavation this paper discusses the geological and hydrogeological features of quaternary deposits in tianjin as well as the geohazards related to groundwater hydrology in this region the soft soil deposits comprising silt sand silty clay and clay are composed of four aquifer groups in the first aquifer group one phreatic aquifer and two confined aquifers have relationships with underground construction in the urban area these three aquifers are separated by two aquitards and collectively form a multi aquifer system during geotechnical construction potential geohazards present are related to the groundwater which include water in rushing quicksand and piping hazards to prevent the aforementioned geohazards dewatering is conducted however groundwater pumping may result in large settlements of the surrounding ground to reduce pumping induced settlement the dewatering waterproofing system has been adopted according to the characteristics of the subsoil excavation depth and the surrounding environment the dewatering system can be divided into five patterns in the first four patterns when pumping is conducted in the excavation pit the groundwater head in the adjacent aquifers outside the pit decreases due to the leakage effect of the aquitards located between the aquifers in the fifth pattern waterproof curtain has cut off the aquifers completely and dewatering in the pit cannot result in settlement around excavation pit to avoid geohazards related to groundwater hydrology countermeasures recommended include construction of an effective waterproof curtain selection of a reasonable excavation dewatering pattern and withdrawal of required groundwater

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

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

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

the crustal structure of the western himalayas and tibet we present new high resolution shear velocity models for the western himalayas and west tibet from the joint inversion of p receiver functions recorded using seismic stations from four arrays in this region and fundamental mode rayleigh wave group velocity maps from 5 70 s covering central and southern asia the tibetan plateau is a key locality in understanding large scale continental dynamics a large number of investigations has examined the structure and processes in eastern tibet however western tibet remains relatively understudied previous studies in this region indicate that the western part of the tibetan plateau is not a simple extension of the eastern part the areas covered by these arrays include the karakoram and altan tagh faults and major terrane boundaries in west tibet and the himalayas the arrays used include broadband data collected by the west tibet array a u s china deployment on the western side of the tibetan plateau between 2007 and 2011 we use the shear wave velocity models to obtain estimates of moho depth the moho is deep 68 84 km throughout west tibet we do not observe significant steps within the moho beneath west tibet a large step in moho depth is observed at the altyn tagh fault where moho depths are 20 30 km shallower to the north of the fault compared to those to the south beneath the lhasa terrane and tethyan himalayas we observe a low velocity zone in the midcrust this feature is not interrupted by the karakoram fault suggesting that the karakoram fault does not cut through the entire crust

observations of frequency dependent sn propagation in northern tibet summary we present new observations of the frequency dependent propagation efficiency of the seismic phase sn over the tibetan plateau our measurements are the ratio of the sn amplitude to the pg coda amplitude on tibetan regional seismograms and we map the lateral variation in the maximum and mean values of this ratio across the plateau good path density and azimuthal coverage allow the area of sn blockage identified by previous studies to be better constrained an important result is that at low frequencies 0 2 hz sn propagates efficiently over the entire plateau whereas at higher frequencies 1h z sn is blocked for a region of the northern plateau as we take measurements at higher frequencies we find that the southern boundary of the region of inefficient propagation migrates to the south the observation that low frequency sn propagates efficiently across the whole plateau suggests that at the length scales sampled by these waves the upper mantle has an overall positive shear velocity gradient which could indicate that the high velocity seismic lid or seismic lithosphere beneath tibet is still intact and has not delaminated as some researchers have previously proposed the observation that high frequency sn does not propagate beneath northern tibet suggests that at the shorter length scales sampled by the higher frequency waves the upper mantle has a negative shear velocity gradient this could indicate that at shallow depths the sub moho upper mantle is hot and may contain some melt

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

crustal shear wave velocity structure beneath northeast india from teleseismic receiver function analysis abstract we investigated the seismic shear wave velocity structure of the crust beneath nine broadband seismological stations of the shillong mikir plateau and its adjoining region using teleseismic p wave receiver function analysis the inverted shear wave velocity models show 34 38 km thick crust beneath the shillong plateau which increases to 37 38 km beneath the brahmaputra valley and 46 48 km beneath the himalayan foredeep region the gradual increase of crustal thickness from the shillong plateau to himalayan foredeep region is consistent with the underthrusting of indian plate beyond the surface collision boundary a strong azimuthal variation is observed beneath shl station the modeling of receiver functions of teleseismic earthquakes arriving the shl station from ne backazimuth baz shows a high velocity zone within depth range 2 8 km along with a low velocity zone within 8 13 km in contrast inversion of receiver functions from se baz shows high velocity zone in the upper crust within depth range 10 18 km and low velocity zone within 18 36 km the critical examination of ray piercing points at the depth of moho shows that the rays from se baz pierce mostly the southeast part of the plateau near dauki fault zone this observation suggests the effect of underthrusting bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the ew ne striking dauki fault dipping 300 toward northwest

teleseismic imaging of subducting lithosphere and moho offsets beneath western tibet teleseismic images suggest that the tarim plate plunges 45 s down to 300 km depth beneath nw tibet the 410 km discontinuity shallows by 10 km under the plateau implying 100 c cooler upper mantle the deepest moho on record 90 km lies under w qiangtang it rises abruptly by 20 and 10 km beneath the altyn tagh fault and bangong suture respectively vp vs ratios are normal except in the yecheng flexural basin and deep under the south karakax volcanics 1 92 w kunlun s neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward extruding thrust wedge of the tarim crust

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

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

imaging the indian lithosphere beneath the eastern himalayan region summary lithospheric thickness is an important parameter to understand the nature of collision and subduction between the indian and asian tectonic plates in this study we apply the s receiver function technique to data from a network of broad band stations in the northeast india and eastern himalayan regions and image the geometry of indian plate collision this analysis reveals clear s to p conversions from the moho and lithosphere asthenosphere boundary lab in the various tectonic units of the study region the indian lithosphere is found to be only 90 km thick beneath the shillong plateau deepening to 135 km on either side suggestive of a lithospheric upwarp related to the plateau uplift the lithosphere thickens northward with values reaching 180 km beneath the eastern himalaya the trend of the lab north of the foredeep region indicates that the indian plate plunges beneath the eastern himalaya the consistent northward dipping character of the indian plate suggests that the indian plate is traceable until it gets subducted beneath tibet just south of bangong suture zone the deepening of the lab and its correlation with the topographic elevation is in agreement with homogeneous thickening of the lithosphere in response to compressive forces due to the continental collision of india with asia

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

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

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

crustal structure in the pakistan himalaya from teleseismic receiver functions subsurface structure in the pakistan himalaya is poorly known despite its geological importance in understanding the trend and growth of the himalaya in this paper we compute receiver functions from a broadband seismic network in northern pakistan and construct crustal structure beneath the stations p to s converted phases at the moho are clearly observed on stacked receiver functions we model the conversions from the moho and shallower interfaces using the trial and error method assuming a vp vs ratio of 1 75 the modeled crustal thickness is 52 53 km in the peshawar basin 55 62 km in the lesser himalaya and 65 76 km in the higher himalaya the variation of the moho depth correlates well with surface topography indicating an airy type of isostasy in the pakistan himalaya a large negative phase before the moho conversion is consistently observed at all three stations to the north of the india asia suture which is best modeled by a low velocity layer in the lower crust the slow lower crust is likely caused by partial melt resulted from crust thickening volatile in the crust of the kohistan island arc and the subduction and melting of the indian upper crust may also contribute to the low velocity layer like the weak middle lower crust in tibet this slow lower crust probably has an important role in the formation and growth of the western himalaya

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

the tectonic evolution of the kohistan karakoram collision belt along the karakoram highway transect north pakistan the kohistan arc terrane comprises an intra oceanic island arc of cretaceous age separating the indian plate to the south from the karakoram asian plate to the north within the indus suture zone of north pakistan the intra oceanic arc volcanics chalt dras group were built on a foundation of dominantly mid ocean ridge basalt morb related amphibolites of the kamila group the subarc magma chamber is represented by multiple intrusions of a huge gabbro norite complex chilas complex which includes some ultramafic assemblages of residual mantle harzburgite and dunite layered cumulates and hornblendites cut by late stage dikes of hornblende plagioclase pegmatites the chilas complex norites intrude the gilgit metasediments of lower amphibolite and greenschist facies in northern kohistan which also form xenolithic roof pendants within the top of the chilas complex along the southern margin of kohistan jijal and sapat complex ultramafics dunites harzburgites and websterites form remnant suprasubduction zone ophiolitic mantle rocks along the hanging wall of the main mantle thrust the cretaceous obduction plane along which kohistan was emplaced onto indian plate rocks garnet granulites of the jijal complex formed at 12 14 kbars represent original magmatic lower crustal rocks subducted to depths of at least 45 km and metamorphosed during high pressure and high temperature subduction of earlier arc related rocks obduction of the sapat ophiolite and kohistan arc occurred between 75 and 55 ma the closure of the shyok suture zone separating kohistan from the karakoram plate must have occurred prior to 75 ma the age of the jutal basic dikes which crosscut the closure related fabrics mainly late north directed backthrusting in the lower hunza valley andean type granitoid gabbrodiorite granodiorite granite emplacement along the kohistan ladakh batholith ended at the time of india asia collision 60 50 myr ago postcollisional crustal thickening along the karakoram led to multiple episodes of metamorphism from latest cretaceous and throughout the tertiary sillimanite grade metamorphism in hunza was actually pre india asia collision and may have resulted from the earlier kohistan collision localized and sporadic crustal melting episodes across northern kohistan indus confluence and parri granite sheets and the southern karakoram hunza dikes and sumayar and mango gusar leucogranites occurred from 51 to 9 ma and culminated in the huge baltoro monzogranite leucogranite intrusion 25 21 myr ago a vast network of leucogranitic and pegmatitite dikes containing gem quality aquamarine muscovite tourmaline garnet biotite quartz are younger than 5 ma and form the final phase of intrusion in the haramosh area and parts of the southern karakoram area

gravity anomalies crustal structure and thermo mechanical support of the himalaya of central nepal summary we use two gravity profiles that we measured across central nepal in conjunction with existing data to constrain the mechanical behaviour and the petrological structure of the lithosphere in the himalayan collision zone the data show 1 overcompensation of the foreland and undercompensation of the higher himalaya as expected from the flexural support of the range 2 a steep gravity gradient of the order of 1 3 mgal km x1 beneath the higher himalaya suggesting a locally steeper moho and 3 a 10 km wide hinge in southern tibet we compare these data with a 2 d mechanical model in which the indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland the model assumes brittle coulomb failure and nonlinear ductile flow that depends on local temperature which is computed from a steadystate thermal model the computed moho fits seismological constraints and is consistent with the main trends in the observed bouguer anomaly it predicts an equivalent elastic thickness of 40 50 km in the foreland the flexural rigidity decreases northwards due to thermal and flexural weakening resulting in a steeper moho dip beneath the high range residuals at short wavelengths over distances of 20 30 km are interpreted in terms of 1 sediment compaction in the foreland dr 150 kg m x3 between the lower and middle siwaliks 2 the contact between the tertiary molasse and the meta sediments of the lesser himalaya at the mbt dr 220 kg m x3 and 3 the palung granite intrusion in the lesser himalaya dr 80 kg m x3 finally if petrological transformations expected from the local p t are assumed a gravity signature of the order of 250 mgal is predicted north of the lesser himalaya essentially due to eclogitization of the lower crust which is inconsistent with the gravity data we conclude that eclogitization of the indian crust does not take place as expected from a steady state local equilibrium assumption we show however that eclogitization might actually occur beneath southern tibet where it could explain the hinge observed in the gravity data we suspect that these eclogites are subducted with the indian lithosphere

an overview of the sedimentary geology of the bengal basin in relation to the regional tectonic framework and basin fill history abstract the bengal basin in the northeastern part of indian subcontinent between the indian shield and indo burman ranges comprises three geo tectonic provinces 1 the stable shelf 2 the central deep basin extending from the sylhet trough in the northeast towards the hatia trough in the south and 3 the chittagong tripura fold belt due to location of the basin at the juncture of three interacting plates viz the indian burma and tibetan eurasian plates the basin fill history of these geo tectonic provinces varied considerably precambrian metasediments and permian carboniferous rocks have been encountered only in drill holes in the stable shelf province after precambrian peneplanation of the indian shield sedimentation in the bengal basin started in isolated graben controlled basins on the basement with the breakup of gondwanaland in the jurassic and cretaceous and northward movement of the indian plate the basin started downwarping in the early cretaceous and sedimentation started on the stable shelf and deep basin and since then sedimentation has been continuous for most of the basin subsidence of the basin can be attributed to differential adjustments of the crust collision with the various elements of south asia and uplift of the eastern himalayas and the indo burman ranges movements along several well established faults were initiated following the breakup of gondwanaland and during downwarping in the cretaceous by eocene because of a major marine transgression the stable shelf came under a carbonate regime whereas the deep basinal area was dominated by deep water sedimentation a major switch in sedimentation pattern over the bengal basin occurred during the middle eocene to early miocene as a result of collision of india with the burma and tibetan blocks the influx of clastic sediment into the basin from the himalayas to the north and the indo burman ranges to the east rapidly increased at this time and this was followed by an increase in the rate of subsidence of the basin at this stage deep marine sedimentation dominated in the deep basinal part while deep to shallow marine conditions prevailed in the eastern part of the basin by middle miocene with continuing collision events between the plates and uplift in the himalayas and indo burman ranges a huge influx of clastic sediments came into the basin from the northeast and east throughout the miocene the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin from pliocene onwards large amounts of sediment were filling the bengal basin from the west and northwest and major delta building processes continued to develop the present day delta morphology since the cretaceous architecture of the bengal basin has been changing due to the collision pattern and movements of the major plates in the region however three notable changes in basin configuration can be recognized that occurred during early eocene middle miocene and plio pleistocene times when both the paleogeographic settings and source areas changed the present basin configuration with the ganges brahmaputra delta system on the north and the bengal deep sea fan on the south was established during the later part of pliocene and pleistocene and delta progradation since then has been strongly affected by orogeny in the eastern himalayas pleistocene glacial activities in the north accompanied sea level changes in the bay of bengal

crustal structure beneath the sub himalayan fold thrust belt kangra recess northwest india from seismic reflection profiling implications for late paleoproterozoic orogenesis and modern earthquake hazard abstract most compressional orogens include salients and recesses along their strike the age and origin of which can be hard to ascertain in the kangra recess in the trace of the main boundary thrust mbt the largest such recess within the active himalayan orogen the sub himalayan sedimentary fold thrust belt increases in width to as much as 90 km the kangra basin but narrows to as little as 10 km in the adjoining nahan salient of the mbt the subathu basin to the southeast new seismic reflection profiling places the himalayan decollement at 6 8 km depth above thin but reflective meso to neoproterozoic vindhyan lesser himalayan series equivalent strata these data show that the vindhyan sedimentary rocks are thinner in the kangra recess than further southeast allowing the hypothesis that the width of the lesser himalayan thrust belt and the existence of the kangra recess could be related to the pre deformation basin thickness this hypothesis obviates the need for control of the kangra recess by a lateral ramp in the main himalayan thrust so making it more likely that the kangra segment could rupture as part of an earthquake far larger than the devastating 1905 m 7 8 kangra earthquake below the proterozoic sedimentary rocks our reflection data show a west southwest dipping reflective fabric spanning a 30 km crustal thickness which we infer corresponds to a widespread ulleri wangtu orogenic event at c 1850 ma affecting a pre tethyan indian continental margin thickening the basement by c 20 the deepest 10 km of this 50 km thick crust shows a more horizontal arguably younger reflectivity though the moho is not clearly marked by strong reflectors

geology of petroleum in kohat potwar depression pakistan the 36 000 km2 kohat potwar depression in northern pakistan has more than 5 000 m of marine deposition precambrian to eocene with a major break during ordovician to carboniferous more than 10 000 m of miocene to pleistocene alluvial sediments overlies the marine sequence the tectonic depression formed as a result of continent to continent collision at the northwestern margin of the indian plate although the region has been under active petroleum exploration since the mid nineteenth century it has been tested by only 60 exploratory wells ten oil fields have been discovered original recoverable reserves total 200 million bbl of which 112 million bbl has been produced the region has several attractive features for petroleum generation and accumulation i cluding source reservoir trap assemblages and thermal maturity regimes total ultimately recoverable hydrocarbon resource of the area is estimated at 2 4 billion bbl of oil equivalent many unexplored surface and subsurface prospects and stratigraphic leads are known parts of the region particularly the bannu depression and kohat salt zone can be regarded as frontier areas based on the exposed geology and available seismic information we believe these areas are promising recent discoveries of dakhni field 1983 and dhurnal field 1984 which were the first discoveries in the northern part of the platform zone of the potwar plateau and production from paleozoic horizons have added new dimension to petroleum prospects of the area the region needs careful reevaluation of available information exploratory drilling on selected targets and detailed exploration in its western part

himalayan seismic hazard the devastating earthquake in bhuj on 26 january 2001 highlighted the high earthquake risk in the indian region of gujarat in their perspective bilham et al warn that major earthquakes may be expected in another part of india the himalayan arc an analysis of historic earthquakes suggests that several major earthquakes are overdue in this region but that recent low activity has focused attention away from the himalayan arc projections from earlier earthquakes suggest that just one of the possibly overdue himalayan earthquakes may lead to 200 000 predictable fatalities