models for midcontinent tectonism an update during the past decade significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical geologic data base which permits an integration of seismicity data crustal structures and current stress directions critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns the direction of the maximum horizontal compressive stress is generally directed east northeast which supports an origin associated with ridge push forces related to movement of the north american plate crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data these data have been used to relate current seismicity and associated tectonism to one of two models increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field the zone of weakness model as the dominant control on contemporary tectonism and the local basement inhomogeneity model as a mechanism for minor low energy release earthquake activity

estimation of ground motion for bhuj 26 january 2001 mw 7 6 and for future earthquakes in india only five moderate and large earthquakes mw 5 7 in india three in the indian shield region and two in the himalayan arc region have given rise to multiple strong ground motion recordings near source data are available for only two of these events the bhuj earthquake mw 7 6 which occurred in the shield region gave rise to useful recordings at distances exceeding 550 km because of the scarcity of the data we use the stochastic method to estimate ground motions we assume that 1 s waves dominate at r 100 km and lg waves at r 100 km 2 q 508f 0 48 is valid for the indian shield as well as the himalayan arc region 3 the effective duration is given by fc 1 0 05r where fc is the corner frequency and r is the hypocentral distance in kilometer and 4 the acceleration spectra are sharply cut off beyond 35 hz we use two finite source stochastic models one is an approximate model that reduces to the x 2 source model at distances greater that about twice the source dimension this model has the advantage that the ground motion is controlled by the familiar stress parameter dr in the other finite source model which is more reliable for near source ground motion estimation the high frequency radiation is controlled by the strength factor sfact a quantity that is phys ically related to the maximum slip rate on the fault we estimate dr needed to fit the observed amax and vmax data of each earthquake which are mostly in the far field the corresponding sfact is obtained by requiring that the predicted curves from the two models match each other in the far field up to a distance of about 500 km the results show 1 the dr that explains amax data for shield events may be a function of depth increasing from 50 bars at 10 km to 400 bars at 36 km the corresponding sfact values range from 1 0 2 0 the dr values for the two himalayan arc events are 75 and 150 bars sfact 1 0 and 1 4 2 the dr required to explain vmax data is roughly half the corresponding value for amax while the same sfact explains both sets of data 3 the available far field amax and vmax data for the bhuj mainshock are well explained by dr 200 and 100 bars respec tively or equivalently by sfact 1 4 the predicted amax and vmax in the epi central region of this earthquake are 0 80 to 0 95 g and 40 to 55 cm sec respectively

deep lower crustal earthquakes in central india inferences from analysis of regional broadband data of the 1997 may 21 jabalpur earthquake summary the only instance of a confirmed deep lower crustal earthquake occurrence in the indian shield region has been that of the 1938 satpura earthquake m 6 3 of central india reportedly at a depth of about 40 km moment tensor inversion of regional broadband waveform data of the 1997 may 21 jabalpur earthquake mw 5 7 confirms yet another such earthquake at about 35 km depth in the central part of the narmada son lineament nsl zone the study is based on a refined velocity model obtained using a traveltime grid search method a reverse fault mechanism is obtained which for a palaeo rift valley zone indicates the possibility of reactivation of a pre existing fault under the influence of the ambient stress field due to the india eurasia plate collision forces the occurrence of earthquakes at lower crustal depths quite unusual for the indian shield region indicates a possible causative mechanism related to crust mantle interaction based on the close proximity of the two deep earthquakes and their disposition with respect to the local trend of the central part of the nsl we suggest a model of stress accumulation due to horizontally elongated or elliptical possibly serpentinized mafic intrusives in the lower crust to explain the occurrence of deep earthquakes in the heart of the indian shield

passive seismological imaging of the narmada paleo rift central india abstract the narmada son lineament nsl is a prominent nearly e w trending tectonic feature dividing india into the peninsular and extra peninsular regions it marks the possible suture between two contrasting geological settings termed proto continents lying to the north and south of it various views prevail about the nature of the rift zone the present study delineates the seismic structure of the crust using p wave receiver functions at 10 broadband seismic stations installed along a profile across the nsl the results from common conversion point ccp stacking reveal a near continuous image of the moho boundary with the amplitudes of the ps conversions from the moho being weaker at stations within the rift modelling the receiver functions employing nearest neighborhood algorithm approach brings out a distinct 15 km thickening of the crust and a complex nature of the moho within the rifted zone we find that the backazimuthal variations of the sv and sh receiver functions at the permanent station jabalpur can be explained better by introducing azimuthal anisotropy in the middle and lower crust with the fast axes perpendicular to the rift axis also we find substantial evidence of post rifting magmatic underplating in the lower crust which could be reminiscence of deccan volcanism and or reunion plume path

crustal and upper mantle structure of peninsular india and source parameters of the 21 may 1997 jabalpur earthquake mw 5 8 results from a new regional broadband network abstract in the past two years a permanent network of 10 broadband seismographs has become operational in the indian peninsular shield region in this article we analyze the data recorded by this network with two goals 1 to lay ground for a quick estimation of source parameters of future earthquakes in the region and 2 to estimate the source parameters of the destructive jabalpur earthquake of 21 may 1997 mw 5 8 which occurred within the network toward these goals we measure rayleigh and love wave group velocity dispersion curves in the period range of 10 to 75 sec and invert these curves to estimate the crustal and the upper mantle structure below the indian shield region our best model consists of a two layer crust the upper layer is 14 km thick with a shear wave velocity v s of 3 55 km sec the corresponding values for the lower layer are 25 km and 3 85 km sec v s for the upper mantle is 4 65 km sec based on this structure we perform a moment tensor mt inversion of the bandpassed 0 05 0 02 hz seismograms of the jabalpur earthquake the best fit is obtained for a source located at a depth of 42 km with a seismic moment m 0 of 7 7 10 24 dyne cm and a focal mechanism with strike 61 dip 64 and rake 74 these source parameters are similar to those previously reported in the literature demonstrating that a routine mt inversion of moderate and large indian penisular earthquakes is now possible a careful examination of the seismograms and modeling of the two closest records reveal that the jabalpur earthquake nucleated in the lower crust at a depth of 36 km it consisted of two subevents separated in time by 0 65 sec with the second subevent located 2 km above the first one the total source duration was 1 4 sec

crustal density structure across the central indian shear zone from gravity data abstract the central indian shear zone is a distinct tectonic feature in the central part of india that separates the central indian tectonic zone from the bastar craton the complete bouguer anomaly map of the region encompassing central indian shear zone is characterised by a broad relative gravity high over central indian tectonic zone as compared to the bastar craton the paired gravity anomaly across the two crustal domains signifies that the central indian shear zone is a locus of density discontinuity along which the two crustal domains accreted horizontal gravity gradient analysis further demonstrates the northward subduction of the bastar craton beneath the bundelkhand craton 2 d gravity modelling along mungwani rajnandgaon profile constrained by seoni kalimati seismic section delineates a thick crust beneath the bastar craton as compared to the central indian tectonic zone northward dipping contact of the two crustal domains when projected on the surface coincides with the central indian shear zone with a well defined moho offset and crustal density discontinuity central indian shear zone represents a suture zone that separates the bastar craton from the central indian tectonic zone a low velocity 6 4 km s density 2 90 g cm 3 layer at the base of the crust and relatively lower density 3 21 g cm 3 subcrustal mantle may be the imprint of thermal remobilization beneath the central indian tectonic zone

deep crustal structure of the indian shield from joint inversion of p wave receiver functions and rayleigh wave group velocities implications for precambrian crustal evolution 1 the s wave velocity structure of the crust and uppermost mantle of the indian shield has been investigated by jointly inverting p wave receiver functions and rayleigh wave group velocities at 38 broadband stations in the subcontinent the indian shield is an amalgamation of several terranes of archean and proterozoic age that were partly flooded by deccan trap volcanism during cenozoic times and that make up a natural laboratory for assessing models of precambrian crustal evolution our results reveal significant variations in crustal thickness and deep crustal velocities 45 50 km thick under the archean west dharwar craton and southern granulite terrane with lower crustal velocities around 4 1 km s 32 35 km thick beneath the archean east dharwar and bundelkhand cratons with lower crustal velocities around 3 8 3 9 km s 50 65 km thick under the proterozoic bhandara craton with lower crustal velocities around 4 2 4 3 km s and 55 km thick under the proterozoic aravalli delhi belt with lower crustal velocities around 4 2 km s s velocities in the 4 1 4 3 km s range in the deep crust can be attributed to mafic lithologies suggesting there has been no secular change in the precambrian evolution of the south indian shield moreover pervasive mafic dike swarming throughout the indian shield suggests that the layer of mafic cumulates is 2 5 1 6 ga old and that it delaminated from some archean terranes our interpretation is that mafic underplating of the terranes making up the indian shield occurred in proterozoic times and that a refractory root developed under the archean terranes after the proterozoic event

seismic evidence for significant crustal thickening beneath jabalpur earthquake 21 may 1997 source region in narmada son lineament central india 1 we investigate crustal properties beneath the deep crustal 35 km jabalpur earthquake of 21 may 1997 narmada son lineament nsl central india in search of a possible cause of stress accumulation in the region teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests moho downwarp to 52 km across the width of the lineament in contrast with an average 40 km depth elsewhere in addition the crust beneath the nsl has higher vp vs of 1 84 compared to 1 73 in the surrounding suggestive of a high density mafic mass at depth that compensates the crustal root also supported by small topographic variation 200 m across the lineament presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre existing narmada son fault leading to the earthquake in deep crust

magnetotelluric studies in the central india tectonic zone implications for intraplate stress regimes and generation of shallow earthquakes abstract the central india tectonic zone citz dissects the indian peninsula into the northern and southern crustal blocks the citz has been a seismically active region since the precambrian whereas the relatively deep crustal earthquakes near the narmada faults in the eastern part of the citz have been well investigated the mechanisms for the shallow earthquakes in the western part remain unknown here we present results from a new magnetotelluric study to derive the crustal structure and to understand its implications our data show a thick and highly resistive 500 ohm m crust in the south of tapti river as against a less resistive one in the north these results in conjunction with heat flow values indicate that the crust below the southern part has stable continental cratonic signatures on the northern side of the tapti river we infer the ascent of basaltic magmas from the mantle into the shallow crust and crystallization into layered intrusions these mafic ultramafic bodies could be a potential cause for the shallow earthquakes in the western part of the citz the mafic ultramafic bodies below the crust of citz would locally modify the intraplate stresses which in turn would facilitate the occurrence of earthquakes due to reactivation of pre existing faults thus the large accumulation of strain energy in the deep crust beneath the region can be attributed to the presence of high stress bodies emplaced at depth during the deccan volcanic activity

a review of crust and upper mantle structure beneath the indian subcontinent this study has been supported by a grant from the ministry of earth sciences moes iitkgp ckh

two dimensional crustal velocity structure along hirapur mandla profile from seismic refraction and wide angle reflection data the seismic velocity structure of the upper and lower crusts along hirapur mandla pro fi le was obtained by 2 d forward modeling of the refraction and wide angle reflection data digitized record sections of the analog data were utilized for delineation of the crustal velocity model across the vindhyan basin and the narmada son lineament by travel time modeling and qualitatively matching the amplitudes of synthetic seismograms with seismic record sections an interface at a depth of 8 km has been traced from wide angle reflection data of shot points 160 and 235 between jabalpur and mandla the wide angle reflection phase from crust mantle boundary is very prominent in all the seismic record sections of shot points 0 40 80 120 and 235 the moho horizon along the entire study area was continuously traced depicting an upwarp between katangi and jabalpur below the narmada son lineament the three faults inferred near narsinghgarh katangi and jabalpur divide the crust into four blocks the number of faults and displacement patterns along them depicted along the profile indicates that block tectonics appear to be active in this region throughout the geological history

crustal velocity inhomogeneities along the hirapur mandla profile central india and its tectonic implications abstract wide angle seismic and gravity data across the narmada son lineament nsl in central india are analyzed to determine crustal structure velocity inhomogeneities and hence constrain the tectonics of the lineament we present the 2 d crustal velocity structure from deep wide angle reflection data by using a ray trace inverse approach the main result of the study is the delineation of fault bounded horst raised to a subsurface depth 1 5 km and the moho upwarp beneath the nsl the crust below the basement consists of three layers with velocities of 6 45 6 7 6 2 6 5 and 6 7 6 95 km s and interface depths of about 5 5 8 7 14 17 and 18 23 km along the profile the low velocity 6 2 6 5 km s layer goes up to a depth of 5 km and becomes the thickest part 13 km while the overlying high velocity 6 45 6 7 km s layer becomes the thinnest 3 km and upper boundary lies at a depth of 1 5 km beneath the nsl the overall uncertainties of various velocity and boundary nodes are of the order of 0 12 km s and 1 40 km respectively the up lifted crustal block and the up warping moho beneath the nsl indicate that the north and south faults bounding the nsl are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap the lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the narmada south fault during the 1997 jabalpur earthquake

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

breccias resembling melt bombs from the roter kamm crater irregular dark green to black glassy looking boulders are locally common in the northwest portion of the rim of the roter kamm impact crater in southern namibia their smooth surfaces flow like textures presence of folds and their similarity to melt bombs have led to the interpretation that these are molten impact ejecta however the samples we have examined are all fine grained breccias composed of angular fragments that show no evidence of melting meltlike surface textures of these rim breccias are largely a consequence of surface wind polishing of fine grained rocks in a desert environment these mono mict breccias were produced by impact related brecciation of gariep sediments carbonaceous siliceous shales previously modified by low grade damaran metamorphism secondary quartz within the breccias is attributed to fluid activity that postdates the impact

the evolution of the vindhyan basin vis a vis the narmada son lineament central india from deep seismic soundings abstract the evolution of the vindhyan basin has been studied from deep seismic soundings block tectonics appear to be active throughout the geological history in this region the crust is divided into four major blocks by deep faults at narsinghgarh katangi and jabalpur the crustal block between narsinghgarh and katangi forms a graben in the crystalline basement giving rise to the great vindhyan basin in which precambrian vindhyan sediments of about 5 5 km thickness are deposited the crustal block between katangi and jabalpur is a horst block which represents the narmada son lineament a crustal depth section obtained from wide angle reflections shows a large number of reflector segments up to the mono boundary indicating crustal heterogeneity the boundary faults at the northern and the southern ends of the vindhyan basin extend up to the moho the crustal thickness varies from 39 5 to 45 km

the central india tectonic zone a geophysical perspective on continental amalgamation along a mesoproterozoic suture abstract the central india tectonic zone citz is a prominent divide and a major suture zone between the north indian and south indian crustal blocks the resistive upper crust as modeled in the magnetotelluric data from citz suggests a dominant tonalite trdondhjemite granodiorite composition associated with an accretionary complex characterized by mainly felsic rock components the highly conductive bodies in this zone might represent mafic ultramafic layered intrusives derived from a deeper reservoir of underplated basaltic magma related to the formation of the cretaceous deccan flood basalts the uniformly thick mafic lower crust below the cratons on both sides of the suture is interpreted as the accreted remnants of archaean and paleoproterozoic subducted slabs we redefine the nature of deep faults traversing the citz which were described as steep and penetrating the moho by previous workers and classify them as listric faults with gentle dips at depth seismic reflection data from the eastern side of the suture suggest a northwestward subduction of the bhandara craton reflection data from the central part of the citz show northerly dip in the southern part suggesting northward subduction of the dharwar craton however an opposite trend is observed in the northern part of the suture with a southward dip of the bundelkhand craton based on these features and in conjunction with existing magnetotelluric models we propose a double sided subduction history along the citz this would be similar to the ongoing subduction accretion process in the western pacific region which possibly led to the development of paired collision type and pacific type orogens one important feature is the domal structure along the central part of the suture with a thick felsic crust occurring between mafic and intermediate crust the high resistivity felsic domain suggests underplated sediments felsic crust that would have caused the doming our model also accounts for the extrusion of regional metamorphic belts at the orogenic core and the occurrence of high pressure ultrahigh temperature paired metamorphic belts within the suture

approach to the volcanic tremor by covariance analysis application to the 1989 eruption of mt etna sicily polarization of the tremor wavefield recorded during three periods of the 1989 eruption of mt etna at a temporary array of three component stations is analyzed by use of a polarization filter the tremor shows steady horizontal near linear west east polarization at all stations such a behaviour which could be coherent with a p wavefield radiated by a vertical extended source with overall north south alignment excludes confinement of the source to the only active vent and is tentatively linked to activity of the main dyke system of mt etna

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

cenozoic epeirogeny of the indian peninsula peninsular india is a cratonic region with asymmetric relief manifest by eastward tilting from the 1 5 km high western ghats escarpment toward the floodplains of eastward draining rivers oceanic residual depth measurements on either side of india show that this west east asymmetry is broader scale occurring over distances of 2 000 km admittance analysis of free air gravity and topography shows that the elastic thickness is 10 3 km suggesting that regional uplift is not solely caused by flexural loading to investigate how indian physiography is generated we have jointly inverted 530 river profiles to determine rock uplift rate as a function of space and time key erosional parameters are calibrated using independent geologic constraints e g emergent marine deposits elevated paleosurfaces uplifted lignite deposits our results suggest that regional tilt grew at rates of up to 0 1 mm a 1 between 25 ma and the present day neogene uplift initiated in the south and propagated northward along the western margin this calculated history is corroborated by low temperature thermochronologic observations by sedimentary flux of clastic deposits into the krishna godavari delta and by sequence stratigraphic architecture along adjacent rifted margins onset of regional uplift predates intensification of the indian monsoon at 8 ma suggesting that rock uplift rather than climatic change is responsible for modern day relief a positive correlation between residual depth measurements and shear wave velocities beneath the lithosphere suggests that regional uplift is generated and maintained by temperature anomalies of 100 c within a 200 25 km thick asthenospheric channel this article is protected by copyright all rights reserved

nature of continental crust along the narmada son lineament inferred from gravity and deep seismic sounding data abstract the narmada son lineament nsl is a very prominent feature on the tectonic map of india and divides the indian peninsula into two parts the northern part characterised by lower elevations and more positive bouguer gravity anomalies as compared to the southern part the lineament is characterised by a gravity high practically all along its length four published deep seismic sounding dss profiles across the nsl give vital information concerning its origin an analysis of the bouguer gravity field along these profiles has been carried out using all available geological and velocity density information the analysis reveals that the areas immediately south of the nsl are characterised by higher density crust in comparison to the areas north of it this increase in density could have taken place during mesozoic to tertiary times when major tectonic volcanic activity took place over peninsular india the presence of deep seated faults sediments and anomalous crust shows that the lineament has some characteristics of a rift valley

distributed deformation in the lower crust and upper mantle beneath a continental strike slip fault zone marlborough fault system south island new zealand converted phases from teleseisms recorded by a seismic array spanning the northern half of the marlborough fault system south island new zealand show a continuous unbroken moho underlying a seismically anisotropic lower crust beneath the two north ernmost faults of the fault system these observations suggest that distributed deforma tion not slip on a narrow vertical fault accommodates displacement in the lower crust below the 120 480 km of right lateral slip across the wairau fault one splay of the marl borough fault system and the northward continuation of the alpine fault beneath the wairau fault the moho dips 258 308 southeast from a depth of 26 km northwest of the fault to a depth of 34 km southeast of the fault farther to the southeast ps conversions from the moho continue under the awatere fault 34 6 10 km of slip with a constant amplitude and depth of 34 km across the two faults converted energy from 16 20 km depth varies with back azimuth in a manner suggesting the presence of anisotropy in the lower crust these observations imply that one of the tenets of plate tectonics that faults defining plate boundaries pass through both crust and upper mantle does not apply to new zealand or to continents in general

deep structure of the baikal rift zone revealed by joint inversion of gravity and seismology 1 the question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas particularly in the baikal rift zone asia which is colder than other continental rifts as previous gravity and teleseismic studies support the dominance of opposing mechanisms in the baikal rift we reconsidered both data sets and jointly inverted them this more effective approach brings insight into location of the perturbing bodies related to the extension in this region our new joint inversion method allows for inverting the velocity density relationship with independent model parametrization we obtain velocity and density models that consistently show 1 crustal heterogeneities that coincide with the main tectonic features at the surface 2 a faster and denser cratonic mantle nw of lake baikal that we relate to the thermal contrast between old and depleted archean siberian platform and paleozoic orogenic belt sayan baikal belt 3 three dimensional topographic variations of the crust mantle boundary with well located upwarpings and 4 the lithosphere asthenosphere boundary uplift up to 70 km depth with a nw dip our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the baikal rift zone index terms 1234 geodesy and gravity regional and global gravity anomalies and earth structure 7218 seismology lithosphere and upper mantle 8122 citation tiberi c m diament j deverchere c petit mariani v mikhailov s tikhotsky and u achauer deep structure of the baikal rift zone revealed by joint inversion of gravity and seismology