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

magnetic hysteresis properties of synthetic monodomain titanomaghemites titanomagnetites of near stoichiometric composition fe 2 4 al o 4 0 0 1 and 0 2 were oxidized in air to form compositions in the corresponding titanomaghemite system the magnetic hysteresis properties saturation magnetization isothermal remanence coercive force initial susceptibility were determined as a function of degree of oxidation in some cases being measured at elevated temperatures the titanomagnetites of the submarine crust are believed to become progressively maghemitized and the measured properties are related to the way in which the intensity and stability of an initially acquired thermoremanence will be modified during submarine weathering the results from the present monodomain synthetic analogues suggest that the intensity of remanence would fall by up to 50 during weathering this fall in intensity might be increased by a fraction of the grains becoming superparamagnetic although this was not so with the grain size range of the synthetics the stability of remanence would at first be increased by maghemitization and subsequently fall the rise in susceptibility with maghemitization would lead to an enhancement of the component of magnetization induced in an ambient field although the study probably simulates the compositions produced during submarine weathering the changes in microstructure e g due to grain cracking or heterogeneities in larger grains which may occur in nature are probably not reproduced in the synthetic analogues

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

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

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

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

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

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

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

new seismic model of the upper mantle beneath africa we present a seismic model of the upper 400 km of the mantle beneath africa and surrounding regions this model is constructed by inverse modeling of fundamental mode rayleigh wave phase velocities 40 200 s for about 8000 propagation paths among the most pronounced anomalies are high shear velocity structures as much as 6 higher than in the preliminary reference earth model beneath the west african congo and kalahari cratons that extend to about 250 km depth these structures have near vertical margins across which the shear velocity changes by as much as 6 over 500 km distance anomalous low shear velocities 3 4 lower than in the preliminary reference earth model structures are observed beneath the east african red sea and gulf of aden rifts and beneath the northwestern indian ocean these structures extend to a depth of at least 250 km our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere nor does our seismic model indicate that high temperature low density material beneath the lithosphere is responsible for the uplift of southern africa

teleseismic tomographic evidence for contrasting crust and upper mantles in south indian archaean terrains abstract we compare the velocity structure of the crust and upper mantle of the archaen granulite terrain of southernmost india with that of the archaean granite greenstone dharwar craton terrain just to the north through p wave teleseismic travel time measurements the teleseismic rays recorded by a coarse network of 11 portable seismic stations show an anomalous pattern of late arrivals delays over the granulite terrain in contrast to fast arrivals on the dharwar craton such a pattern of time residuals amongst these archaean terrains which have remained inert in the last 2 ga may indicate the presence of compositional heterogeneity within the crust and upper mantle beneath them three dimensional p wave velocity tomography using teleseismic rays from a variety of azimuths indicates the existence of contrasting p wave crust and upper mantle velocity patterns 1 in the crust 0 40 km the western dharwar craton and the granulite terrain have lower velocity up to 3 compared with the higher velocity 1 3 observed over the eastern dharwar craton and 2 in the upper mantle 40 177 km there is 2 3 lower velocity beneath the granulite terrain compared with the western dharwar craton the existence of such lateral velocity variation in the crust and upper mantle and its preservation since late archaean times points to the presence of possibly thick and chemically distinct lithospheres that did not participate in mantle convection

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

oxygen isotope analyses of co existing carbonate and phosphate in biogenic apatite a way to monitor diagenetic alteration of bone phosphate abstract the oxygen isotopic composition of phosphate 18 op and structural carbonate 18 oc of hydroxylapatite was determined in 31 bone and tooth samples of modern mammals from different countries these two variables are highly correlate r 2 0 98 and the calculated best fit of linear regression is very similar to the equation calculated from the phosphate and carbonate palaeotemperature equations 1 2 according to previous measurements 3 6 on fossils of different ages from different areas it seems quite improbable to find isotopically altered skeletal remains showing a good correlation between 18 op and 18 oc as is the case with modern samples it therefore seems possible at least in some cases to use these measurements for monitoring fossil bone and tooth diagenetic alteration when a set of points lie on the equilibrium line or close to it the 18 o values could be considered close to the original values in contrast when the points lie to the left or to the right of this line this probably means that the values are diagenetically modified due to interaction with meteoric water or 18 o enriched water respectively

contrasts in lithospheric structure within the australian craton insights from surface wave tomography abstract contrasts in the seismic structure of the lithosphere within and between elements of the australian craton are imaged using surface wave tomography new data from the wacraton and tigger experiments are integrated with re processed data from previous temporary deployments of broad band seismometers and permanent seismic stations the much improved path coverage in critical regions allows an interpretation of structures in the west of australia and a detailed comparison between different cratonic regions improvements to the waveform inversion procedure and a new multi scale tomographic method increase the reliability of the tomographic images in the shallowest part of the model 75 km a region of lowered velocity is imaged beneath central australia and confirmed by the delayed arrival times of body waves for short paths within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model in western australia differences are seen beneath and within the archaean cratons at depths greater than 150 km faster velocities are imaged beneath the yilgarn craton than beneath the pilbara craton in the complex north australian craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton suggesting the possibility of archaean lithosphere underlying areas of dominantly proterozoic surface geology

lithospheric electrical imaging of the deccan trap covered region of western india 1 we present here the results of magnetotelluric mt studies carried out along a 330 km long ne sw trending traverse from daulatabad at the northeastern end to koyna on the southwestern end across the deccan volcanic province dvp in western india the 2 d mt modeling results obtained along this traverse together with those from other mt traverses in this area are examined and integrated with the results from deep seismic sounding 3 d teleseismic tomography and heat flow studies the deep geoelectric section as deduced from mt models point out to a two layered lithospheric mantle structures with an upper very high resistive layer several thousands of ohm m and a bottom relatively conductive one a few tens of ohm m to a few hundred ohm m the lithospheric electrical structure is inferred to be linked to compositional changes associated with a depleted lithospheric upper mantle in the dharwar craton an impressive feature that the study has brought out is the presence of a well defined lithospheric upper mantle conductor in the depth range of 80 120 km limited to the middle segment of the mt traverse from sangole to partur over the dvp the electrical characterization of the region including the presence of upper mantle conductor shows close similarities with that of other similar cratonic regions like the slave and superior cratons in northwestern canada

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

evidence for plate motion related strain in the indian shield from shear wave splitting measurements 1 the predominantly precambrian indian lithospheric plate experienced a long history of deformation ever since its breakup from gondwanaland and its passage over four hot spots the nature of mantle deformation of this fast moving plate has hitherto remained elusive in view of the paucity of measurements of shear wave anisotropy in this paper we present the first results of 182 measurements of azimuthal ansiotropy obtained by analyzing the sks and skks waveforms from 85 earthquakes recorded at 35 broadband seismic stations located on various geological units of the indian shield results show that the delay times t between the fast and slow axes of anisotropy are close to 1s which is the global average for continental shield regions the fast axis azimuths do not reveal coherent patterns that suggest dominance of asthenospheric flow where a basally flat indian lithosphere is dragged over an inactive sublithospheric mantle the northern part of the indian shield where the lithosphere is thick shows a strong correlation between plate motion and azimuths of fast polarization due to the effect of the basal cratonic keel plowing through the sublithospheric mantle most of the fast axes azimuths can be explained by the absolute plate motion apm related strain considering the basal topography of the lithosphere the consistent ns orientations along the west coast may be due to edge flow associated with a transition from a thicker to a thinner lithosphere the southern granulite terrain and the cuddapah basin shows evidences for fossilized anisotropy particularly in regions close to continental shear zones

lithospheric s wave velocity structure of the bastar craton indian peninsula from surface wave phase velocity measurements abstract interstation phase velocities of surface waves are measured through a cross correlation method across the bastar craton in the eastern part of the indian peninsula the periods of love waves lie between 15 and 91 sec and those of rayleigh waves lie between 13 and 104 sec the observed phase velocities are close to the theoretical dispersion curves for the crust and upper mantle structure ip11 obtained earlier for central india in the indian peninsula through inversion of surface wave group velocities however for periods higher than 70 sec the theoretical curves remain above the observed data in ip11 the upper mantle low velocity zone lvz starts downward at a depth of 140 km the s wave velocity v s in the lvz of ip11 is decreased based on the recent studies and the modified structure ip11l is obtained the theoretical dispersion curves of ip11l improve the fit to the observed data particularly at higher periods the lithospheric part of ip11l is then improved through genetic algorithm inversion with more stratification in the structure for a better fit to the observed data such inversion generated the model ip11ba for the bastar craton the crustal thickness of this craton is found to be 40 1 0 6 km with the thickness of the upper crust as 15 3 0 7 km which is less than the corresponding thickness of 20 km in southern india in the upper crust v s shows a small increase with depth having an average value of 3 508 km sec and in the lower crust v s 3 934 km sec these values of v s for the crust of the bastar craton are nearly the same as those found in southern india below the moho down to a depth of 70 km ip11ba shows a constant v s of 4 577 km sec and then increases slowly to 4 609 km sec at the top of the lvz

continental scale body wave tomography of india evidence for attrition and preservation of lithospheric roots we assemble p and s waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the indian plate from the southern tip of india to the himalayan collision belt and generate an accurate data set of 52 050 p and 30 423 s arrival times through the multichannel cross correlation approach these traveltimes are then inverted to obtain 3 d p and s velocity structures of the subcontinent at a 2 2 lateral resolution the heterogeneous nature of the indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale the key cratonic segments of the indian shield are characterized by pockets of high velocity anomalies 3 at shallow depths 300 km with the diamondiferous regions like wajrakarur revealing high shear wave anomalies down to 300 km in contrast to the southern deccan volcanic province dvp the northwestern dvp is underlain by low velocity anomalies at similar depths suggesting that the upper mantle retains imprints of deccan volcanism which was facilitated by the reactivation of the rift systems

imaging the lithosphere asthenosphere boundary of the indian plate using converted wave techniques 1 the lithospheric structure of the indian plate has been investigated using converted wave techniques p and s receiver functions and a novel stacking analysis technique without using deconvolution applied to a large seismological data set from permanent and temporary broadband seismic stations we observe coherent energy from at least two seismic discontinuities i e the crust mantle moho and lithosphere asthenosphere boundary lab in the uppermost mantle here we provide a novel seismic image of the indian lithosphere showing definitive evidence of its flexure which is interpreted to be primarily caused by the hard collision at 55 myr resulting in the world s highest mountain chain the himalayas and the tibetan plateau results from geoidal and gravity studies do suggest postcollisional flexuring of the indian plate however the flexure lacks observational constraints the observed wavelength of the flex is 1000 km with the thickness of the indian shield lithosphere varying from 70 km to 140 km such a low value for a continent implies that the indian plate has been reworked in the past the plate deepens in the himalayan region to a depth of 170 km further the converted phases are interpreted to be resulting from the bottom of the lithosphere we clearly demonstrate that these are distinct and different from the midlithospheric discontinuity for a large number of stations the midlithospheric discontinuity and lab are clearly separated in depth our observations suggest that the archaean lithosphere is no longer intact and is prone to deformation

first multi scale finite frequency tomography illuminates 3 d anatomy of the tibetan plateau 1 with a new multi scale parameterization 3 d images from finite frequency seismic tomography including the very first images beneath central tibet from s waves reveal that regions of low electric resistivity in the crust previously observed along active rifts in southern tibet correlate well with regions of low p and s wave speeds v however such regions are not interconnected indicating that a prevailing south directed channel like crustal flow seems inactive or this popular geodynamic model needs modification in the upper mantle there is no clear indication of regional down welling between depths of 100 to 400 km instead a strong lateral boundary between high and low v extends up to 33 n marking the northern limit of sub horizontally advancing indian lithospheric mantle

seismic signatures of an altered crust and a normal transition zone structure beneath the godavari rift abstract the godavari rift valley in the south indian shield located over a paleosuture between the dharwar and singhbhum cratons is a very less studied region in terms of its deep seismic structure in the present study a total of 1559 receiver functions from 777 earthquakes registered by a 10 station network are utilized to investigate crust and mantle structure broadly the crustal architecture underneath the rift appears very complex with the presence of intracrustal layers and a weak moho compared to a sharp moho and a simple crust outside it detailed modelling of receiver functions at 6 stations using the nearest neighbourhood algorithm approach reveals a 40 km thick crust within the rift compared to a 33 km thick crust in the adjacent dharwar craton also the lower crustal character is found to be distinctly different with a faster lower crust beneath the rift a weak moho beneath the rift may be due to magmatic underplating the transition zone 660 410 km structure inferred from the common conversion point stacks of receiver functions can be interpreted in terms of the absence of a thermal anomalies within and above the mantle transition zone mtz b thin lithosphere or high temperatures in the uppermost mantle magmatic underplating within the crust and a normal mtz argue in favour of a rift formed due to lithospheric stretching without the influence of a mantle plume

seismic heterogeneities in the indian lithosphere abstract rayleigh wave group velocity distributions over the indian region were obtained by the backus gilbert method for time periods 15 25 40 50 and 60 s the observed velocity variations reveal sharp lithospheric heterogeneities in this region southern peninsular india is marked by high velocities in contrast to the northern indo gangetic plain and the himalaya highest surface wave velocities are observed over the southern part of the deccan traps and the dharwar craton whereas the metamorphic terrains and rift zones exhibit anomalous low velocities the nepal himalaya and indo burmese collision zones are characterised by contrasting velocities which reflect their diverse orogenesis the bay of bengal is marked by an anomalous 6 12 higher velocity as compared with the indian continental region for the periods 25 50 s

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

high velocity anomaly under the deccan volcanic province abstract tomographic modelling of p wave residuals at 22 vertical component seismograph stations operated in the deccan volcanic province dvp in west central india and the dharwar craton point to the existence of a large deep at least 300 km high velocity anomaly 1 3 faster directly beneath most of the dvp and dharwar craton there is also marginal evidence that the westernmost part of the dvp may be characterised by low velocity up to 1 5 to a depth of about 200 km this coherent high velocity anomaly under the dvp and its extension into the south indian shield is viewed as a lithospheric root an architecture characteristic of precambrian shields

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

time corrections to teleseismic p delays derived from sks splitting parameters and implications for western u s p wave tomography 1 upper mantle anisotropy will affect teleseismic p wave arrival times and create artifacts in isotropic travel time tomography compiled sks splitting results indicate variations in the strength and orientation of azimuthal anisotropy beneath the western u s we use sks splitting parameters and a hexagonally anisotropic elastic tensor fast symmetry axis to estimate azimuthal anisotropy contributions to p wave delay times and evaluate the effects on isotropic p wave tomography estimated anisotropy correction times have a root mean square rms value of 0 16 s which is 37 of p wave delay time rms the magnitude of azimuthal anisotropy rather than its azimuth has the strongest effect on p delay times however if the anisotropy symmetry axis has a plunge p wave travel times may be strongly affected by both magnitude and azimuth applying azimuthal anisotropy corrections significantly increases tomographic p wave velocity beneath the high lava plains and central california coast and decreases velocity beneath the great basin

amplification of seismic waves in the central indo gangetic basin india abstract although the indo gangetic basin is adjacent to rupture areas of largehimalayan earthquakes m 6 a quantitative study of the amplification of seismicwaves in the region is still lacking to obtain a first estimate of the amplification fortwo years we operated an array of 10 broadband seismographs that crossed the centralindo gangetic basin in a north south direction using earthquake recordings ofshallow earthquakes at soft sites and hard reference sites we computed standard spec tralratios ssr s ssr satsitesnearthehimalayanfoothills wherethesedimentthick ness is 4 km reveal a broadband amplification with a fundamental frequency of0 13 hz the amplification at this frequency varies between 20 and 60 the funda mental frequency increases to the south as the thickness of the sediments decreases becoming 0 8 hz at the southernmost site the amplification at the fundamentalfrequencies exceeds 10 at all eight soft sites calculations based on reasonable earth quake source and attenuation models and application of random vibration theorysuggest that peak ground acceleration and peak ground velocity at soft sites nearthe foothills located 100 km from the epicenter would be amplified by a factorof2 4and6 12 respectively all ourresultsassumelinear behaviorofthesediments although this assumption would not be valid during intense motions resultingfrom large earthquakes our results nevertheless provide basic building block forincorporating nonlinear behavior online material supplementary figures of amplification spectral ratio andacceleration spectra introduction

evidence for thick continental roots beneath south indian shield teleseismic p wave arrivals recorded at south indian seismological stations have been used to construct a 3 d velocity image of the upper mantle beneath this region analysis of a suite of models lead us to conclude that an anomalously high velocity region 1 6 contrast exists in the upper mantle beneath the whole of south indian shield including the deccan traps in the depth range of 60 300 km in contrast the westernmost part of the deccan traps and its subjacent mantle in the northwest is characterised by a relatively low velocity this observed low velocity zone may be a still warm remnant of the spreading centre offset eastwards from the carlsberg ridge which might have been the dominant source of the flood basalts that flowed over the deccan plateau 65 ma ago the high velocity upper mantle of the entire south indian shield is viewed as a continental root formed by a process of geochemical reordering which happens to be more stable

inversion of receiver functions without deconvolution application to the indian craton s u m m a r y receiver functions are a powerful tool to isolate and interpret receiver side structure effects in teleseismic seismic records they are easily constructed by deconvolving one component of a seismogram by another deconvolution is the inverse of convolution and hence can be mathematically viewed as an inverse problem it is a numerically unstable procedure that needs to be stabilized i e regularized this points to a recurring problem in geophysical imaging there is a trade off between variance and resolution where the user needs to arbitrarily define a level of compromise here we propose a novel misfit function for inversion of converted phases that avoids deconvolution in this way the choice of regularization parameters e g water level width of a low pass filter is avoided and statistics of data errors can be correctly accounted for we use this misfit measure to construct a likelihood probability function and carry out a transdimensional bayesian inversion for shear wave structure after illustrating the method with a synthetic test a real data application is shown where teleseismic signals recorded at hyb station hyderabad india and surface wave dispersion measurements are jointly inverted to provide a probabilistic 1 d seismic model beneath the station the results help address the debate on the thickness of the lithosphere in this region we show that the sharp negative velocity jump at 110 km that was previously interpreted as the lithosphere asthenosphere boundary lab is actually a mid lithospheric discontinuity the actual lab is seen deeper as a milder gradient between 150 and 200 km