rifting between india and madagascar mechanism and isostasy abstract the nature of isostasy along the western continental margin of india wcmi and the eastern continental margin of madagascar ecmm and their conjugate nature are examined the first part of the study deals with the estimation of effective elastic thickness te of the lithosphere for the two margins through cross spectral analysis of gravity and bathymetry data the results bring out comparable te values of 8 15 km for the wcmi and 10 13 km for the ecmm despite the wcmi having been traversed along strike by a hotspot trace we have also compared topographic profiles across the two margins extending inland to examine the rift flank uplift experienced by the two landmasses abutting the margins which bear a striking similarity these results establish the symmetrical rifting mechanism resulting in the formation of the two margins and indicate that despite the presence of a hotspot trace along one margin i e the wcmi the two margins have comparable isostatic compensation mechanisms with low te values in the second part of the study we have examined the possible mechanism for the rift flank uplifts using a process oriented approach of backstripping the sediments along a traverse across the wcmi and reconstructing the original rift flank topography by flexurally backstacking the amount of eroded material on the present day topography of the western ghats the backstack load configuration is taken from the amount of erosion that took place over the western ghats obtained from afta apatite fission track analysis studies the reconstructed rift time topography and bathymetry are used to get the moho configuration which was used to calculate the gravity anomaly and is compared with the observed anomaly to derive a possible evolution model for the region we have tried to examine the observed gravity anomaly through two different processes in the first process a te 15 km for backstripping and backstacking and a compensated underplating for the topography give a best fit model to the observed gravity in the second process a necking model with a deep level of necking 20 km with te 15 km and without any underplating explains the topography as well as the observed gravity the nature and amount of the western ghat topography and cratonic age archaen of the pre rifted crust of western india and eastern madagascar are more in favor of the rifting mechanism necking model rather than due to the passage of the reunion hotspot underplating model this is evidenced by the observed increase in topography away from the influence of the reunion hotspot trace as well as the presence of a similar topography in madagascar

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

structure and emplacement of the nandurbar dhule mafic dyke swarm deccan traps and the tectonomagmatic evolution of flood basalts flood basalts such as the deccan traps of india represent huge typically fissure fed volcanic provinces we discuss the structural attributes and emplacement mechanics of a large linear tholeiitic dyke swarm exposed in the nandurbar dhule area of the deccan province the swarm contains 210 dykes of dolerite and basalt 1 km in length exposed over an area of 14 500 km2 the dykes intrude an exclusively basaltic lava pile largely composed of highly weathered and zeolitized compound pahoehoe flows the dykes range in length from 1 km to 79 km and in thickness from 3 to 62 m almost all dykes are vertical with the others nearly so they show a strong preferred orientation with a mean strike of n88 because they are not emplaced along faults or fractures they indicate the regional minimum horizontal compressive stress 3 to have been aligned n s during swarm emplacement the dykes have a negative power law length distribution but an irregular thickness distribution the latter is uncommon among the other dyke swarms described worldwide dyke length is not correlated with dyke width using the aspect ratios length thickness of several dykes we calculate magmatic overpressures required for dyke emplacement and depths to source magma chambers that are consistent with results of previous petrological and gravity modelling the anomalously high source depths calculated for a few dykes may be an artifact of underestimated aspect ratios due to incomplete along strike exposure however thermal erosion is a mechanism that can also explain this whereas several of the nandurbar dhule dykes may be vertically injected dykes from shallow magma chambers others particularly the long ones must have been formed by lateral injection from such chambers the larger dykes could well have fed substantial 1 000 km3 and quickly emplaced a few years flood basalt lava flows this work highlights some interesting and significant similarities and contrasts between the nandurbar dhule dyke swarm and regional tholeiitic dyke swarms in iceland sudan and elsewhere

crustal constraints on the uplift mechanism of the western ghats of india summary over 800 receiver functions rfs from three broadband and five short period stations deployed in the northern segment of the western ghats of india were analysed to examine the crustal constraints on the rift flank uplift mechanism of the ghats estimates of moho depths mean shear velocities and poisson s ratios were determined and used as constraints to model the rfs the study reveals that the crust beneath the ghats varies in thickness between 31 and 39 km and is neither uniformly thin nor thick as required by existing hypotheses for uplift mechanism the poisson s ratio is 0 26 and the mean shear velocity is 3 65 km s 1 the uncertainties in the estimates of the moho depths and poisson s ratios are 2k m and 0 01 respectively the crust beneath the ghats is 3 4 km thinner than that beneath the adjoining low lying coastal plains suggesting block tectonics modelling the rfs using crustal constraints reveals sub moho low velocity zones lvzs with velocity reductions of 0 5 0 65 km s 1 which are confined to shallow depths of about 50 km beneath the ghats the persistent relief of the ghats may be attributed to the buoyancy forces of the possibly rift related shallow upper mantle lvzs beneath the ghats

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

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

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

lithospheric and upper mantle structure of the indian shield from teleseismic receiver functions receiver functions derived from teleseismic p waveforms of 297 earthquakes recorded at broadband station hyderabad hyb are analysed to investigate the lithospheric and upper mantle structure in the vicinity of the station located in the northeastern part of the archaean dharwar craton the internal structure of the 33 2 km thick crust is simple it can be approximated by a single layer with s velocity linearly increasing with depth from 3 4 to 4 0 km s the average being 3 7 0 2 km s the crustal poisson s ratio is 0 25 0 01 a seismic discontinuity at 90 km depth is inferred from a signal particularly prominent on the sh component and possibly indicative of a layer of depth localized anisotropy in the lithospheric mantle the two principal mantle discontinuities are at depths of 406 and 659 km which are close to the global averages

structure and dynamics of plate boundaries in the indian ocean based on gravity anomalies the indian ocean has been the site of numerous geophysical investigations which commenced during the international indian ocean expedition in this paper gravity surveys are reviewed which have been undertaken since this international effort to elucidate the structure and evolution of the most complex of the three major oceans particular attention is paid to the interrelationship between the plate boundaries and the gravity field some selected boundaries are discussed in the framework of global plate tectonics these areas comprise the red sea gulf of aden and the indonesian deep sea trench in addition the continental margin structure south of india is included which provides some further constraints on the location of the initial break up of gondwanaland and consequently bears on pre drift reconstructions of india and antarctica

crustal structure of the indian shield new constraints from teleseismic receiver functions teleseismic earthquake waveform data from 10 broadband stations spread over the indian shield and in operation since 1997 were analyzed to infer the crustal structure using the receiver function technique the south indian shield is characterized by a 33 39 km thick and remarkably simple crust with an average poisson s ratio close to 0 25 the archaean crust is devoid of any prominent intra crustal discontinuities the velocity contrast at the well developed moho is large resulting in very clear p to s conversions as well as first order multiples in contrast the predominantly proterozoic crust in the northern part of the shield exhibits a complex character due to the presence of additional seismic discontinuities moho conversions which are considerably weaker compared to the archaean terrains indicate crustal thicknesses of more than 40 km

effect of random sea surface and bottom roughness on propagation in shallow water the potential difficulties associated with modeling acoustic propagation in shallow water environments are well documented larger scale deterministic features combine with random fluctuations in the water column sediment sea surface and water sediment interface to produce an extremely complicated propagation regime the extent to which each of these factors needs to be included in realistic propagation modeling remains to be quantified toward this end results from a series of detailed simulations generated using the parabolic equation method are presented beginning with a deterministic downward refracting sound speed profile and a known sloping bottom realizations of the random features are sequentially added to the simulation wind driven surface gravity waves and power law bottom roughness are considered the individual and cumulative effects of these scattering mechanisms on the acoustic wavefront are quantified successive interactions with the random interfaces are studied a modal decomposit

a receiver function perspective of the dharwar craton india crustal structure summary teleseismic data from six broad band stations on the south indian shield have been analysed primarily to examine the differences in the crustal structure between the eastern and western units of the archaean dharwar craton sv receiver functions for these stations have been computed and modelled down to the moho level in terms of poisson s ratio and shear wave velocities results show that the crust for the entire dharwar craton is mainly simple and has a low about 0 25 poisson ratio it is usually thinner and less complex than the adjacent proterozoic crust however the western dharwar craton crust thickness 41 km with a gradational moho boundary is substantially 7 km thicker than its eastern counterpart thickness 34 km no substantial differences in average crustal s velocities 3 6 3 8 km s 1 were found the eastern dharwar crust below the proterozoic cuddapah basin was also found to have remained relatively simple and undisturbed the continental margin to the west of the dharwar craton appears to have shifted further west off the coast where a possible west coast fault has down thrown a continental crustal block under the seas the crust constituting the deccan volcanic province in the western indian shield is found to be similar to that of the eastern dharwar craton the crust underneath the neighbouring godavari graben is significantly different from the dharwar crust and resembles that of a typical rift valley

petrophysical properties of the deccan basalts exposed in the western ghats escarpment around mahabaleshwar and koyna india abstract we present petrophysical properties density p and s wave velocity porosity and poisson s ratio of the deccan basalts from the western ghats escarpment around mahabaleshwar and koyna and characterize the dhawar basement rocks around goa the petrophysical properties of basaltic lava flows show significant variation in the bushe poladpur ambenali and mahabaleshwar formations that are widespread in the deccan volcanic province the bushe formation stands out distinctly from other formations because of its lower density and p and s wave velocity the porosity vesicles and amygdales plays a major role in controlling the variation of the petrophysical properties of the basalts the poisson s ratio of the deccan basalts is largely affected by the vesicular porosity aspect ratio of the amygdales and zeolite content the dharwar basement rocks greywackes and granites are found to be lower in density p and s wave velocity and poisson s ratio than the basaltic rocks the variation of the petrophysical properties with elevation in the western ghats sections roughly follows the geochemical stratigraphy high porosity of the bushe and mahabaleshwar formations constitute a multi layered aquifer system in the deccan volcanic province in the koyna earthquake zone these formations may provide an effective groundwater connectivity between the reservoir and earthquake focal areas the new petrophysical data of the deccan basalts and dharwar basement rocks will help to refine the geophysical models of the southwestern deccan basalt province

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

crustal velocity structure beneath saurashtra nw india through waveform modeling implications for magmatic underplating abstract the saurashtra peninsula in the northwestern segment of the deccan volcanic province of india is characterized by several high gravity and magnetic anomalies which correspond to mafic crustal intrusions this study attempts to quantify the alterations to the crust caused by the deccan volcanism by estimating the crustal poisson s ratio shear wave velocity vs structure and the shear velocity contrast across the moho through teleseismic waveform modeling the p receiver functions rfs for six broadband seismic stations were constructed using about 575 high quality s n 2 5 teleseismic waveforms of earthquakes m 5 5 recorded during the period 2004 2010 the moveout corrected rf summation stacks were inverted using the neighborhood algorithm na to estimate the shear velocity structure beneath each station the crustal thickness is estimated to range from 38 km in western saurashtra to 33 km close to the southern extension of the cambay rift a low velocity zone possibly corresponding to sub basaltic sediments is detected beneath all the stations the average crustal vs and poisson s ratio are estimated to be 3 68 km s and 0 276 respectively the crustal poisson s ratio indicates an intermediate to mafic composition for the crust modeling reveals a relatively high velocity lower crust with an average vs 3 88 0 1 km s which is consistent with the high vp 7 1 km s reported for this region through seismic refraction and wide angle reflection studies overlying a relatively low velocity vs 4 4 0 1 km s upper mantle the shear velocity contrasts across the moho derived from the amplitudes of the p to s pms conversions from the moho range from 0 08 to 0 17 which are much smaller than those 0 20 observed across the indian shield implying a gradational moho due to underplating this study reveals that the crust beneath saurashtra is distinctly different from that of the archean indian shield largely due to significant alterations by the deccan volcanism

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

seismological constraints on the structure and composition of western deccan volcanic province from converted phases 1 receiver function analysis using teleseismic data from one broadband and nine short period stations was carried out in a 50 km x 100 km area near mumbai bombay encompassing the coastal plains of the rifted margin of the western deccan volcanic province dvp of india this region is characterized by a thick 36 41 km shield like crust which is at variance with earlier inferences in this part of dvp the crustal composition as inferred from the poisson s ratio of 0 26 0 01 is felsic to intermediate in nature the continental crust exhibits structural controls similar to the offshore shelf region but does not exhibit any typical seismic signatures of magmatic underplating nor appears to be perturbed by the deccan volcanism

structure and evolution of the western continental margin of india deduced from gravity seismic geomagnetic and geochronological studies abstract available geochronological gravity seismic and geomagnetic data on the western continental margin of india between 8 20 n are presented based on the data three models describing the evolution of the crustal structure are developed model 1 describes the evolution of the crustal structure along 19 n the main basic igneous activity that accompanied the tectonic events since the late cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure the acid volcanic event that preceded the above activity at about 93 ma ago is responsible for the observed crustal structure along 13 30 n shown in model 2 this event resulted in the formation of the st mary group of islands along this latitude counterclockwise rotation of the indian plate during the tertiary period caused rifting of the continental blocks giving rise to the laccadive kerala graben bounded by the chagos laccadive ridge on the west and the west coast fault on the east vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault this tectonic event was accompanied by upwarping of the mantle crust thus giving rise to the present day structure shown in model 3

seismic properties of the central indian shield we use broadband seismic data at hyderabad to investigate average crustal properties of the central indian shield crustal receiver functions p to sv conversions based on data of excellent quality and azimuthal coverage show essentially no signal on the transverse component indicating laterally homogeneous structures near this station a joint analysis of receiver functions and the dispersion of fundamental mode rayleigh waves result in a simple model for the shear wave speed v s with a moderate gradient of 0 010 0 028 km sec km in the crust the moho is sharp with a contrast of approximately 0 7 km sec in v s across a small thickness of only 0 4 km measured from the middle of the moho region the crust is 30 34 km thick with an average v s of 3 58 0 10 km sec by modeling the timing and waveforms of sp and sspmp phases sv to p conversions we estimate the average crustal poisson9s ratio to be 0 26 0 01 this value is comparable to that found in the tanzanian craton which is also of archean age both values being somewhat lower than the global average of 0 29 0 02 for shield regions that include both proterozoic and archean crust a low poisson9s ratio and a slow v s in the crust indicate that on average archean crust beneath the central indian shield is intermediate to felsic in composition somewhat less mafic than globally averaged precambrian crust