radioactivity heat flow and rifting of the indian continental crust the relationship between heat flow q and heat production a in india is quite different for the northern and southern parts of the peninsula in the north locations in a variety of tectonic settings including gondwana rifts show a linear relationship between q and a with a common reduced heat flow q about 40 mwm 2 in southern india the heat production of surface rocks is greater than is expected for the measured heat flow relative to other continental shields g is high in northern india and about average in the south apparently the indian crust evolved with a content of radioactive elements higher than in most shields and the southern part of the peninsula has undergone regional depletion in the lower crust and possibly upper mantle this depletion may coincide with a regional granulite event about 2 500 m y ago the high heat flow and heat production in northern india may explain several unique characteristics of the indian peninsula one feature is the continued reactivation of rif

lateral uniformity of india plate strength over central and eastern nepal the current understanding of the himalayan lithosphere stems mostly from cross sections through the range at the longitude of the kathmandu basin in this paper we laterally extend the analyses of structures and rheology along the nepal himalayas between the pokhara valley and the arun river we take advantage of available information and a new data set including gravity measurements and a receiver function profile it appears that the geometry of the moho inferred from seismological profiles and long wavelength gravity anomalies does not exhibit major east west variations within the 350 km wide study area using thermomechanical modelling we show that the northward deepening of the moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong india plate this result suggests a gradual mechanical decoupling between the crust and the mantle leading to a northward decrease of the effective elastic thickness of the indian lithosphere from 75 km to 25 km beneath the ganga basin and the tibetan plateau respectively our results also confirm partially eclogitized lower indian crust beneath southern tibet at shorter wavelengths the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east west variations of the thickness of subsurface geological structures such as the ganga basin and the tethyan sedimentary sequence

thin lithosphere asthenosphere boundary beneath eastern indian craton the lithosphere asthenosphere boundary lab separates the hard and rigid outer layer of the earth lithosphere and the weaker hotter and deeper part of the upper mantle asthenosphere and plays a pivotal role in plate tectonics however its definitive detection especially beneath the cratons is proving elusive one of the geophysical tools used to map the lab beneath the cratons is through magnetotelluric mt observations the resistivity at boundary falls in the range of 5 25 m and can be explained by the presence of a small amount of water in the asthenosphere possibly inducing partial melt here we report thickness of the lab in one of the oldest dated ancient cratons of india eastern indian craton eic of 3 3 gyr from mt studies the two prominent phase sensitive strike directions one each for crust and mantle and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath eic our results show that the lab beneath the eic is at 95 km the region is interesting as the electrical properties of the crust and mantle and the moho depth are similar to those of the slave craton canada 4 0 gyr but the depth of the lab beneath the eic is half that of the slave craton as cratonic signatures depicted by ultrapotassic rocks from gondwana coal fields close to eic are preserved at least till early cretaceous 117 ma it is likely that himalayan orogeny could have played a major role in delamination of the lithospheric roots of the eic in addition to attendant seismicity

lithospheric thinning in the eastern indian craton evidence for lithospheric delamination below the archean singhbhum craton abstract we herein present shear velocity structure extending down to 300 km depth below the archean singhbhum odisha craton soc and proterozoic chotanagpur granitic gneissic terrain cggt which has been obtained through the inversion modeling of p receiver functions we use three component broadband recordings of 200 teleseismic earthquakes 30 90 from a 15 station seismic network that has been operational in the eastern indian shield since february 2013 we obtain the thinnest crust of 35 km overlying a thin lithosphere of 78 km below the region near south singhbhum shear zone which could be attributed to the 1 6 ga plume activity associated with dalma volcanic however the thickest crust of 47 km overlying a thin lithosphere of 81 km is noticed below the region near the singhbhum granite of 3 6 ga this thinning of lithosphere could be attributed to the delamination of lithospheric root due to the himalayan orogeny with a shortening rate of 2 cm year this delamination model in soc gets further support from the densification of the lower crust which could result from repeated episodes of basaltic underplating associated with episodes related to dalma 1 6 ga and rajmahal 117 ma volcanisms this led to relatively more mafic heterogeneous and deformed crustal structure in soc as well as egmb with an average crustal vs of 4 0 km s in comparison to that in cggt with an average crustal vs of 3 9 km s as seen through our modeling results the thickest lithosphere of 100 km is observed in the southwestern soc as well as northeastern cggt we also notice that a sharp and flat moho in cggt which could be attributed to thermal reactivation and large volume melting of the mafic cratonic crust during the late archean subduction process and associated volcanism episodes this model gets further support from the estimated 169 km thick lower vs zone in the upper mantle below cggt our modeling results also support a northward subduction of archean soc under the proterozoic cggt during 1 0 1 6 ga as earlier proposed by the geological age data from the region

heterogeneous lithosphere and the underlying mantle of the indian subcontinent abstract p and s receiver functions prfs and srfs respectively for 21 broad band seismograph stations of the india meteorological department imd illuminate lithosphere and the underlying mantle of some previously poorly sampled regions of the indian sub continent our analysis demonstrates that the archean and early proterozoic lithospheric keel of the indian shield has been reworked by younger processes we find very low s wave velocities in the uppermost mantle from 4 0 to 4 3 km s to the north of the deccan volcanic province kutch region and aravalli craton 1 in the south southern granulite terrain and sri lanka 2 and in the north east gangetic plane bengal basin and singhbuhm craton 3 the anomalies 1 and 2 may extend into the transition zone early arrivals of the s410p seismic phase are indicative of anomalously high vp vs ratio 1 9 in the upper mantle of the low velocity regions whereas late arrivals in the western himalaya ladakh and western tibet are consistent with the previously found indications of anomalously low vp vs ratio a transition from the high s velocity mantle lid to a layer of slightly lower velocity is seen in part of the data but a straightforward interpretation of this transition as the lithosphere asthenosphere boundary is problematic a mafic s velocity in the upper crust and a pronounced low s velocity layer in the lower crust beneath the eruptive center is practically the only specific feature in the lithosphere that may be linked to the deccan traps a separation in depths between the 410 km and 660 km discontinuities varies laterally in a range from 240 to 270 km the largest uplift of the 410 km discontinuity up to 390 km is observed beneath the foothills of the himalaya where it is caused by cooling of the transition zone by the ongoing continental collision

unusual lithospheric structure beneath the hyderabad granitic region eastern dharwar craton south india abstract using multiparametric geological and geophysical data the evolutionary history of the lithosphere beneath the late archean early proterozoic hyderabad granitic region hgr of the eastern dharwar craton of south indian shield has been attempted our study reveals that the entire granitic region and the surroundings 200 km 200 km is being neotectonically uplifted possibly due to a major intrusive body situated at sub crustal depth leading to changes in river courses and also erosion of several kilometers 10 km from its upper crustal column beneath this region the moho is elevated 32 33 km the reduced heat flow is of the order of 28 mw m2 and the asthenosphere is located at shallow depths of about 124 km i e far less than 200 400 km depth found in precambrian shield areas of the globe the cratonic mantle lithosphere beneath hgr contains a highly conductive 15 m hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km where the estimated temperatures could be in the range of 900 1000 c the surface granitic layer containing unusually high radioactivity 5 25 w m3 appears very thin probably a kilometer or so beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies rocks of acid to intermediate composition persistent episodic thermal reactivations during the last 2 6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual

observation and inversion of surface wave group velocities across central india abstract surface wave group velocities across central india are measured from computer plots of frequency time analysis using multiple filter technique periods range from 5 to 56 sec for fundamental mode rayleigh wave 5 to 16 sec for first higher mode rayleigh wave and 5 to 49 sec for fundamental mode love wave the wave paths from koyna earthquakes to bokaro cross purely through the peninsular plateau the group velocities for these paths are found to be similar to those of surface waves recorded at poona from earthquake which lie to the northeast of bokaro in sub himalayan regions from this it is concluded that the shield structure of the peninsula extends to the sub himalayan regions to northeast of the peninsular plateu with a negligible part of sedimentary layer group velocities of fundamental mode rayleigh and love waves are also obtained for the paths from koyna earthquakes to shillong and are found to be similar to the above mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of bengal basin northern limit of this basin sedimentary layer has been identified mean and standard deviation of observed group velocity at each period are obtained for each of rayleigh fundamental rayleigh first higher and love fundamental modes the group velocities closely agree with those obtained earlier for the path between new delhi and kodaikanal and are also found to be similar to those observed in some shield regions monte carlo inversion method is applied and crust and upper mantle structure of the indian shield across central india is obtained crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

anomalous crustal and lithospheric mantle structure of southern part of the vindhyan basin and its geodynamic implications abstract tectonically active vindhyan intracratonic basin situated in central india forms one of the largest proterozoic sedimentary basins of the world possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin has led to surge in geological and geophysical investigations by various agencies an attempt to synthesize such multiparametric data in an integrated manner has provided a new understanding to the prevailing crustal configuration thermal regime and nature of its geodynamic evolution apparently this region has been subjected to sustained uplift erosion and magmatism followed by crustal extension rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle almost 5 6 km thick sedimentation took place in the deep faulted jabera basin either directly over the bijawar mahakoshal group of mafic rocks or high velocity high density exhumed middle part of the crust detailed gravity observations indicate further extension of the basin probably beyond nsl rift in the south a high heat flow of about 78 mw m2 has also been estimated for this basin which is characterized by extremely high moho temperatures exceeding 1000 c and mantle heat flow 56 mw m2 besides a very thin lithospheric lid of only about 50 km many areas of this terrain are thickly underplated by infused magmas and from some segments granitic gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks a 5 8 km thick retrogressed metasomatized zone with significantly reduced velocities has also been identified around mid to lower crustal transition

magnetotelluric studies across the kurduwadi gravity feature abstract the deccan traps have a predominantly negative bougner anomaly with two prominent regions of gravity low along the nasik koyna and nasik kurduwadi lines on the basis of these a rift zone was suggested along the nasik kurduwadi region this rift theory was not supported by later deep seismic sounding dss studies subsequent gravity modelling studies indicated that the observed anomalies could be explained in terms of undulations in the crust mantle boundary and the conrad discontinuity it was thus felt that the region should be reinvestigated to check for the rift valley using magnetotelluric mt techniques a linear profile about 100 km long was identified for these studies magnetotelluric data were collected at 11 stations with an average station spacing of 8 15 km the data at ten of these stations were found to be good for the purpose of analysis the rotation of the impedance tensor showed a regional rotation angle of 60 east of north with this rotation the direction of the major resistivity is approximately perpendicular to the direction of gravity low as well as the west coast of the indian peninsula one dimensional 1d analysis at all stations showed a five layered structure a regional geoelectric cross section is proposed using the 1d data the influence of the west coast was studied using two dimensional 2d modelling techniques the deccan basalts were observed to be about 500 m thick overlying the granitic upper crust a mid crustal conductor with a conductance of about 200 s was delineated at a depth of 12 18 km this shows good agreement with the conrad discontinuity reported from the dss and gravity studies the moho discontinuity was not seen in the data presumably because it is not accompanied by any detectable change of electrical resistivity the lithosphere asthenosphere boundary is shallow at a depth of about 80 100 km the asthenosphere is not well formed as is evident from the large resistivities 50 100 m signature of sediments below the traps could not be found in the survey area

group velocity dispersion characteristics and one dimensional regional shear velocity structure of the eastern indian craton abstract in the past three years a semi permanent network of fifteen 3 component broadband seismographs has become operational in the eastern indian shield region occupying the archean 2 5 3 6 ga singhbhum odisha craton soc and the proterozoic 1 0 2 5 ga chotanagpur granitic gneissic terrane cggt the reliable and accurate broadband data for the recent 2015 nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one dimensional regional shear velocity structure of the region first we measure fundamental mode rayleigh and love wave group velocity dispersion curves in the period range of 7 70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern indian craton eic we observe that group velocities of rayleigh and love waves in soc are relatively high in comparison to those of cggt this could be attributed to a relatively mafic rich crust mantle structure in soc resulting from two episodes of magmatism associated with the 1 6 ga dalma and 117 ma rajmahal volcanisms the best model for the eic from the present study is found to be a two layered crust with a 14 km thick upper crust uc of average shear velocity vs of 3 0 km s and a 26 km thick lower crust lc of average vs of 3 6 km s the present study detects a sharp drop in vs 2 to 3 at 120 260 km depths underlying the eic representing the probable seismic lithosphere asthenosphere boundary lab at 120 km depth such sharp fall in vs below the lab indicates a partially molten layer further a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88 103 km depths suggesting a 88 103 km thick thermal lithosphere below the eic this could also signal the presence of small amounts of partial melts thus this 2 3 drop in vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz back arc volcanism associated with the archean proterozoic subduction 1 6 ga dalma volcanism and 117 ma rajmahal volcanism the main result of our modeling provides evidences for the absence of keel or thick lithosphere below the eic

structure of the earth s crust and upper mantle below hindustan and the northern part of the indian ocean from geophysical data abstract the crustal cross section of the indian shield is refined on the basis of data obtained in the cooperative indian soviet deep seismic sounding dss research and from the use of a new interpretation technique crustal velocity parameters are studied and the main blocks and faults are identified the hypsometric position of the blocks is determined gravity field interpretation using dss and explosion seismology data has made it possible to characterize the structure of the entire tectonosphere for the area africa hindustan eastern coast of the bay of bengal it is confirmed that the continents have a normal continental crust that passes into the oceanic type under the ocean in the somali trough especially in the bay of bengal sediments are very thick the laccadiv isles have crust which is oceanic in composition the rift zone of the karlsberg ridge has deep seated asthenospheric roots the lithospheric density increases as the distance from the ridge grows the asthenosphere is developed everywhere but it is thinner and denser below the continent than below the ocean

insights into the crustal structure and geodynamic evolution of the southern granulite terrain india from isostatic considerations the southern granulite terrain of india formed through an ancient continental collision and uplift of the earth s surface was accompanied by thickening of the crust once the active tectonism ceased the buoyancy of these deep crustal roots must have supported the nilgiri and palani cardamom hills here the gravity field has been utilized to provide new constraints on how the force of buoyancy maintains the state of isostasy in the southern granulite terrain isostatic calculations show that the seismically derived crustal thickness of 43 44 km in the southern granulite terrain is on average 7 8 km more than that required to isostatically balance the present day topography this difference cannot be solely explained applying a constant shift in the mean sea level crustal thickness of 32 km the isostatic analysis thus indicates that the current topography of the southern granulite terrain is overcompensated and about 1 0 km of the topographic load must have been eroded from this region without any isostatic readjustment the observed gravity anomaly an order of magnitude lower than that expected 125 mgal however shows that there is no such overcompensation thermal perturbations up to pan african present day high mantle heat flow and low te together negate the possible resistance of the lithosphere to rebound in response to erosional unloading to isostatically compensate the crustal root compatible to seismic moho a band of high density 2 930 kg m 3 in the lower crust and low density 3 210 kg m 3 in the lithospheric mantle below the southern granulite terrain is needed a relatively denser crust due to two distinct episodes of metamorphic phase transitions at 2 5 ga and 550 ma and highly mobilized upper mantle during pan african thermal perturbation reduced significantly the root buoyancy that kept the crust pulled downward in response to the eroded topography

heat flow and heat generation in the archaean dharwar cratons and implications for the southern indian shield geotherm and lithospheric thickness abstract we report terrestrial heat flow density measurements from 11 new sites in the archaean eastern and western dharwar cratons of the southern indian shield the new values together with already published data have yielded a mean heat flow of 40 3 4 s d mw m 2 n 11 for the eastern dharwar craton which is a predominantly gneiss granite terrain the mean heat flow in the archaean supracrustal belts of western dharwar craton which is a granite greenstone terrain is 31 4 1 s d mw m 2 n 4 the observed difference in surface heat flow between the two cratons is ascribed to differences in radiogenic heat production in upper crustal layers inferred to be related to different crust forming processes it has been argued that low heat production appears to be an inherent characteristic of the crust similar to that of the western dharwar craton which evolved during the early archaean and suffered depletion of the lithophile elements of its lower crust prior to the late archaean radiogenic heat production measurements were carried out on borehole core samples for one heat flow site sortur and data have been compiled from the literature on three other sites and on a variety of rocks of the southern indian shield heat flow and radiogenic heat production for sortur when combined with the published data define a linear relation r 0 97 with a slope of 11 5 km and an intercept of 23 mw m 2 the thickness of the thermal lithosphere beneath the dharwar cratons is over 200 km thereby indicating beneath the cratons the existence of deep continental roots a concept which is also supported by teleseismic p wave travel time data the geothermal data has clearly demonstrated that the archaean dharwar cratons of the indian landmass are associated with low mean surface heat flow 37 7 mw m 2 reduced heat flow moho heat flux 12 17 mw m 2 and a cool and thick lithosphere all characteristics of many archaean early proterozoic terrains of other shields it is inferred that the thermal structure of the south indian lithosphere is identical with that of other cratons

viscous heating allows thrusting to overcome crustal scale buckling numerical investigation with application to the himalayan syntaxes article i nfo the eastern and western himalayan syntaxes are crustal scale antiforms whose metamorphic evolution is coeval in the india asia collisional history finite element modelling of lithospheric shortening and the resultingcrustal scale folds havepreviously been used todebate the structuralconditions for the development of such structures however numerical models could not account for the metamorphic evolution associated with folding in continuation of previous work we present two dimensional finite element modelling of lithospheric shortening inwhich thermal effects are implemented the new models are consistent with earlier interpretations that crustal folding is the plausible mechanism that shaped the two himalayan syntaxes they further lend support to the concept that lithospheric buckling is a basic response to large scale continental shorteningandan efficientmountainbuilding process introducingviscous shearheating shows however that the buckling mode shifts to a thrusting mode within few shortening depending on the initial thermal rheological structure of the deforming lithosphere the change to the thrusting mode prohibits further fold amplification and lateral fold propagation this mayexplainwhy in continental lithosphere crustal scale folds are isolated whereas regular periodic crustal fold trains are rare focusing deformation on through limbs thrust zones accompanies the establishment of inverted metamorphic gradients these results offer new working hypotheseson howlargethrusts like thehimalayamain centralthrustnucleate results further show that the thermal structure of the lithosphere strongly controls three fundamental deformation and metamorphic modes 1 a cold lithosphere mainly deforms by thermally activated thrusting and exhibits large areas with significant tectonic overpressures twice lithostatic 2 a warm lithosphere is essentially buckled and significant tectonic overpressure builds up in the upper crust and 3 a hot lithosphere tends to thicken homogenously and mainly records lithostatic pressures below the upper crust

seismogenesis of the lower crustal intraplate earthquakes occurring in kachchh gujarat india abstract large intraplate continental earthquakes like the 1811 12 new madrid m w 8 0 and the 2001 bhuj mw7 7 were highly destructive because they occurred in strong crust but the mechanisms underlying their seismogenesis are not understood here we show using local earthquake velocity tomography and joint inversion of receiver functions and surface wave group velocity dispersion that the crust and uppermost mantle beneath the 2001 bhuj earthquake region of western india is far more complex than hitherto known through previous studies a new image of the crust and underlying mantle lithosphere indicates the presence of a 18 km thick high velocity vp 7 15 8 11 km s differentiated crustal and mantle magmatic layer above a hot and thin lithosphere only 70 km in the epicentral region of 2001 bhuj earthquake this magmatic layer begins at the depth of 24 km and continues down to 42 km depth below this region brittle ductile transition reaches as deep as the moho 34 km due to the possible presence of olivine rich mafic magma our 1 d velocity structure envisages an initial phase of plume activity deccan plume at 65 m y ago resulting in basaltic magma in the eclogitic layers at sub lithospheric levels wherein they were subjected to crystallization under ultra high pressure conditions our study also delineates an updoming of moho 4 7 km as well as asthenosphere 6 10 km below the kachchh rift zone relative to surrounding areas suggesting the presence of a confined body of partial melts below the lithosphere asthenosphere boundary restructuring of this warm and thin lithosphere may have been caused due to rifting at 184 and 88 m y ago and tholeiitic and alkalic volcanism related to the deccan traps k t boundary event at 65 m y ago recent study of isotopic ratios proposed that the alkalic basalts found in kachchh are generated from a co 2 rich lherzolite partial melts in the asthenosphere that ascended along deep lithospheric rift faults into the lithosphere it appears that such kind of crust mantle structure deepening of brittle ductile transition and a high input of volatiles containing co 2 emanating from mantle control the seismogenesis of lower crustal earthquakes in the kachchh continental rift zone

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

depth extent of cratons as inferred from tomographic studies we address the question of the depth extent of mantle high velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies we divided old continents into two age provinces 800 1700 ma middle proterozoic and older than 1700 ma archean and early proterozoic the areas included in these age provinces are cross correlated with the tomography at different depths to quantify the global occurrence of high velocity anomaly hva extensions beneath cratons statistically significant hvas underlie the oldest cratons to a depth of 250 km there is no significant consistent correlation between tomography and younger cratons the older cratons have correlated hvas extending to depths varying from 100 to 450 km there is no significant difference in the average velocity anomaly beneath the early proterozoic part and the archean part of the canadian shield whereas the middle proterozoic part does not contain any hva the mantle under early precambrian crust is distinctly different from mantle elsewhere we propose two different hypotheses to explain these results 1 cratons of early proterozoic age or older contain ancient and deep physically attached roots or 2 these cratons contain a small permanent physical root that induces cold downwellings beneath it a root thickness of 200 km can be explained by an feo poor olivine rich mineralogy whereas a requirement of high strength probably demands in addition a volatile poor composition different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved and to their subsequent deformational history or they may represent different snapshots in time of transient roots such as thermal boundary layers

upwarped high velocity mafic crust subsurface tectonics and causes of intraplate latur killari m 6 2 and koyna m 6 3 earthquakes india a comparative study abstract a unique attempt is made to understand the genesis of intraplate seismicity in the latur killari and koyna seismogenic regions of india through derived crustal structure by synthesizing active and passive seismic magnetotelluric gravity and heat flow data it has indicated presence of relatively high velocity density intermediate granulite and amphibolite facies rocks underneath the deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since precambrian times these findings have been independently confirmed by detailed borehole geological geochemical and mineralogical investigations the crystalline basement rock is found to contain 2 wt of carbon di oxide fluid components the presence of geodynamic process associated with thermal anomalies at subcrustal depths is supported by a high mantle heat flow 29 36 mw m2 beneath both regions although some structural and compositional variations may exist as evidenced by p and s wave seismic velocities we suggest that the stress caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate these stresses appear to dominate over and above those generated by the india eurasia collision the role of fluids in stress generation as advocated through earlier studies appears limited

crustal and lithospheric thinning beneath the seismogenic kachchh rift zone gujarat india its implications toward the generation of the 2001 bhuj earthquake sequence a high resolution passive seismic experiment in the kachchh rift zone of the western india has produced an excellent dataset of several thousands teleseismic events from this network 500 good teleseismic events recorded at 14 mobile broadband sites are used to estimate receiver functions for the 30 310 back azimuth ranges which show a positive phase at 4 5 6 1 s delay time and a strong negative phase at 8 0 11 0 s these phases have been modeled by a velocity increase at moho i e 34 43 km and a velocity decrease at 62 92 km depth the estimation of crustal and lithospheric thicknesses using the inversion of stacked radial receiver functions led to the delineation of a marked thinning of 3 7 km in crustal thickness and 6 14 km in lithospheric thickness beneath the central rift zone relative to the surrounding un rifted parts of the kachchh rift zone on an average the kachchh region is characterized by a thin lithosphere of 75 9 5 9 km the marked velocity decrease associated with the lithosphere asthenoshere boundary lab observed over an area of 120 km 80 km and the isotropic study of xenoliths from kachchh provides evidence for local asthenospheric updoming with pockets of partial melts of co2 rich lherzolite beneath the kachchh seismic zone that might have caused by rifting episode at 88 ma and the associated deccan thermal plume interaction at 65 ma episodes thus the coincidence of the area of the major aftershock activity and the moho as well as asthenospheric upwarping beneath the central kachchh rift zone suggests that these pockets of co2 rich lherzolite partial melts could perhaps provide a high input of volatiles containing co2 into the lower crust which might contribute significantly in the seismo genesis of continued aftershock activity in the region it is also inferred that large stresses in the denser and stronger lower crust at 14 34 km depths induced by ongoing banni upliftment crustal intrusive marked lateral variation in crustal thickness and related sub crustal thermal anomaly play a key role in nucleating the lower crustal earthquakes beneath the kachchh seismic zone

super mobility of hot indian lithosphere negi j g pandey o p and agrawal p k 1986 super mobility of hot indian lithosphere tectonoph vsics 131 147 156 the great mobility of the indian subcontinent during the last 180 million years 9000 km of south north motion with an anticlockwise rotation of about 60 o has long been a major puzzle for theorists of plate tectonics a relative analysis of the available heat flow data the estimated temperature depth regime of india and the gravity anomalies reveals that the indian lithosphere does not have the characteristics of a typical shield zone and that it has higher radioactivity smaller viscous drag decidedly low viscosity of 10 p lower density and a well defined low velocity zone as compared to other shield areas of the world these factors appear to contribute significantly to the faster northward movement of the thin indian lithosphere as compared to the almost immobile african plate

estimating depth of investigation in dc resistivity and ip surveys in this paper the term depth of investigation refers generically to the depth below which surface data are insensitive to the value of the physical property of the earth estimates of this depth for dc resistivity and induced polarization ip surveys are essential when interpreting models obtained from any inversion because structure beneath that depth should not be interpreted geologically we advocate carrying out a limited exploration of model space to generate a few models that have minimum structure and that differ substantially from the final model used for interpretation visual assessment of these models often provides answers about existence of deeper structures differences between the models can be quantified into a depth of investigation doi index that can be displayed with the model used for interpretation an explicit algorithm for evaluating the doi is presented the doi curves are somewhat dependent upon the parameters used to generate the different models but the results are robust

seismic imaging of the proterozoic cuddapah basin south india and regional geodynamics abstract cuddapah basin has been believed to be one of the largest intra cratonic proterozoic sedimentary basins of india situated in the eastern part of the dharwar craton of the south indian shield which is magmatically infested and contain thick column of sediments its overall sedimentary thickness as well as the nature of crustal evolution has remained enigmatic based on deep seismic sounding and other geological studies in the past it was perceived that this basin may contain as much as 10 12 km thick sediments the results of our present analysis derived up to a depth of 12 km reveals five layered upper crust associated with velocities i 4 50 km s ii 5 20 5 30 km s iii 5 50 5 80 km s iv 5 85 6 00 km s and v 6 40 km s out of which second and third layers correspond to upper and lower cuddapah sediments the results suggest the presence of only 4 0 km thick sediments in the deepest part of the basin below the nallamalai fold belt which has its implications in the developmental history of the basin a thermal driving force was invoked by earlier workers to account for estimated 10 12 km thick sediments however the present estimate of only 4 km basement depth in the cuddapah basin shows that the role of the thermal driving force may be marginal particularly in the deeper eastern cuddapah as isostatic subsidence due to sedimentary accumulation alone is enough to explain the basin depth further a basement sag of about 10 km would have logically needed lateral extension of the order of several hundreds of kilometers however as our present estimate of the sediment thickness gets reduced from earlier 10 km to only 4 km the size of the basin 44 500 km 2 would be in conformity with isostatic subsidence due to sedimentary accumulation hypothesis the structural features derived from present analysis like maximum depth observed near the thrust suture on the basin margin from where it decreases away from it its association of shallow marine sediments the arcuate shape of the basin along with its areal dimension resembles foreland basin between continent continent collisions

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

rapid least squares inversion of apparent resistivity pseudosections by a quasi newton method1 a fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer this technique is based on the smoothness constrained least squares method and it produces a two dimensional subsurface model from the apparent resistivity pseudosection in the first iteration a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically for subsequent iterations a quasi newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times respectively compared to the conventional least squares method tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly this technique takes about one minute to invert a single data set on an 80486dx microcomputer

geophysical investigations identify hidden deposits with great potential for discovering peking man fossils at zhoukoudian china the mysterious loss of five calvaria of homo erectus and three skulls of homo sapiens sapiens unearthed from zhoukoudian a world cultural heritage site during world war ii is a great loss for paleoanthropological research although frontal and occipital bones of homo erectus were unearthed in 1966 additional important human fossils cannot be found due to the fact that most of the fossil bearing deposits were removed in early diggings at zkd locality 1 the so called peking man site from 1995 to 2004 a series of in situ geological and geophysical investigations using electromagnetic electric panel magnetic microgravity electric tomograms surface seismic waves ground penetrating radar and test drilling methods were conducted on dragon bone hill where the peking man site is located here we report that some geophysical anomalies have been clearly detected and there are still significant deposits suitable for archaeological excavations in hidden cavities in the hill which demonstrate how some civil engineering methods can be employed for archaeological purposes

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

long and short wavelengths of indian ocean geoid and gravity lows mid to upper mantle sources rapid drift and seismicity of kachchh and shillong plateau india abstract spectral analysis of the indian ocean geoid low provides depth to the large wavelength sources as 1300 700 and 340 km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels the three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine spinel transformation of successively increasing fe mg ratio as depth increases known as transition zones the first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field the low density rocks in this section appear to be related to the subducted indian tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments the relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85 km suggest sources along the lithosphere asthenosphere boundary lab under the indian continent and surrounding oceans respectively a low viscosity zone has been envisaged below 660 km discontinuity that may correspond to the low density rocks in this section which is popularly referred to as graveyards of the subducted rocks under geoid lows the subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid to upper mantle which is likely to set in baby plumes presence of fluid may transform part of olivine to serpentine further reducing the bulk density of rocks in this section they would make this region buoyant that appears to be responsible for the central indian ocean deformation zone with large scale folding faulting seismicity and high heat flow the same also appear to be responsible for the rapid drift of the indian plate short wavelength component of the indian ocean geoid low reflects most of the surface shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose kachchh and the shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries tectonics and residual geoid anomalies of the indian continent and adjoining regions suggest that the geoid highs of kachchh and the shillong plateau along the nw and the ne corners of india are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries geoid highs indicating high density mafic rocks suggest their connections extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries besides both these regions are affected by i prominent lithospheric flexure of the indian plate ii large scale mafic intrusions and iii interaction of proterozoic mobile belts with effects of present day himalayan orogeny that make them more vulnerable for seismic activity

shear wave splitting in eastern indian shield detection of a pan african suture separating archean and meso proterozoic terrains abstract based on fast axes orientation and delay time t measurements at fifteen broadband stations the archean eastern indian craton eic is classified into three main tectonic zones viz singhbhum odisha craton soc 82 8 and t 1 5 0 2 s chotanagpur granitic gneissic terrain cggt 63 9 and t 1 4 0 3 s and eastern ghats mobile belt egmb 70 10 and t 1 3 0 2 s the results in the archean province of eic depict a moderate to large delay times 1 0 1 9 s with azimuths that reveals an upper mantle anisotropy with a mean orientation of 74 5 5 6 sub parallel to the e w trending structural grain of the archean craton evidencing frozen lithospheric anisotropy the direction of anisotropy n39 e in northern meso proterozoic cggt is found to be consistent with the direction of current absolute plate motion of indian plate regional variation of observed orientation of fast axes near the flanking paleo and late archean orogenic belts is attributed to the tectonic control rather than current plate motion nevertheless estimated large mean delay times 1 41 0 27 s do also suggest a contribution from the anisotropy associated with asthenospheric mantle flow most interestingly our study detects a transition zone coinciding with the location of damodar graben separating south late archean and north meso proterozoic cggt across which the fast axes orientation changes from e w 71 to ne 39 which could be evidencing the signatures of the eastward extension of pan african 550 500 ma suture as also observed in antarctica

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

vertical electric profile at choutuppal hyderabad in peninsular india abstract natural electromagnetic e m field data form choutuppal hyderabad in the frequency range 2 10 1 1 10 4 hz are processed and resistivities in the principal directions determined a 1 d inversion of the resulting yx curve reveals the existence of three conductive layers 1 depth to the top 16 km thickness 9 km resistivity 35 m 2 depth to the top 90 km thickness 15 km resistivity 15 m and 3 depth to the top 340 km thickness 10 km resistivity 3 m whereas the mid crustal conductor has been visualized as a low velocity layer corresponding to the conrad discontinuity the second conductor has been ascribed to the uplifted asthenosphere as a result of the thinning of the lithosphere and the third has been related to the global conductivity zone arising from olivine to spinel phase transition

mantle derived fluids in the basement of the deccan trap evidence from stable carbon and oxygen isotopes of carbonates from the killari borehole basement maharashtra india we report here for the first time geochemical mineralogical and stable carbon and oxygen isotopic data on the crystalline basement rocks of the 1993 killari earthquake region of maharashtra india which is covered by a thick suite of deccan volcanics our results revealed the hitherto unknown amphibolite granulite nature of the 2 5 ga basement which contains 2 00 2 28 wt of co2 the stable carbon 13c and oxygen 18o isotopic measurements on carbonates separated from two basement samples kil 13 440 5 m depth and kil 20 499 6 m depth collected from the klr 1 borehole drilled in the epicentral region showed the respective values of 6 23 and 6 22 versus pdb for 13c and 7 94 and 8 11 versus smow for 18o the samples plot in the primary igneous carbonatite field indicating the mantle origin of the carbonates derived through the process of mantle metasomatism from the deep mantle carbon reservoir this would suggest large scale crust mantle thermal fluid interaction beneath the killari seismogenic region which is characterized by massive upwarping of the high velocity mafic crust and retrograde metamorphism

3d lithosphere density structure of southern indian shield from joint inversion of gravity geoid and topography data abstract we present the 3d crustal and lithospheric structure and crustal average density distribution of southern indian shield south of 18 n sri lanka and adjoining oceans the model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data the derived crustal thickness of 10 25 km in the oceanic region increases to 34 35 km along the coast a crustal thickness of 34 38 km is obtained beneath the eastern dharwar craton and 36 45 km beneath the western dharwar craton and the southern granulite terrain sri lanka has a thinner crust of 30 35 km the lithosphere asthenosphere boundary is located at depths of 70 120 km under oceanic regions and 150 180 km below the dharwar craton and the northern block of southern granulite terrain a notably thinned lithosphere of 130 km near bangalore in the eastern dharwar craton 140 km beneath the southern block of southern granulite terrain and 130 km in sri lanka is observed the thickness of the lithosphere 130 km near bangalore is inferred as the frozen in signature of a small fossil mantle plume and or tectono compositional effect of a rifted margin and a suture considerable stretching and or convective removal of pristine lithosphere in the southern block of southern granulite terrain and adjoining sri lanka before disappearing completely in the archaean northern block of southern granulite terrain and dharwar craton is suggested

the deccan traps an interpretation in terms of progressive lithospheric flexure in response to a migrating load abstract structural and geochemical data from the western ghats india have been used to construct an accurate north south section of the volcanic stratigraphy of the southwest part of the deccan province the characteristic style of the section is one of southerly dip reversing in the extreme south southward overstep of higher units onto pre deccan rocks and down dip thinning of individual formations in contrast an east west section shows continuity of essentially horizontal stratigraphic units without overstepping relationships previous authors have interpreted these data in terms of the progressive southward migration of the volcanic source as india drifted northwards over a hot spot now centered beneath reunion we have compared the observed stratigraphy to the predictions of a geophysical model based on the flexural response of the continental lithosphere to a migrating volcanic load the best fit to the observed section is for the structurally lowest part of the modelled sections with an elastic thickness of the lithosphere of 100 km this value explains the present day width of the traps in peninsula india but requires that up to 5 5 km of the upper part of the flank section has been removed by erosion the origin of the erosion is not clear but it may be related to flexural rebound following removal of the cone loads that formed along the trace of the plume the model cannot explain the subsidence of the traps beneath the shelf off bombay or the rim uplift of the traps in the western ghats we attribute these modifications of the stratigraphy to vertical movements uplift and subsidence that followed rifting of india from the mascarene plateau during the eocene

recent deformations of the deep continental root beneath southern africa seismic anisotropy manifested by the splitting of shear waves into two orthogonal polarizations provides evidence for deformation in the earth s upper mantle where such deformation has been inferred beneath precambrian cratons it has been interpreted either as related to recent plate motion1 2 or inherited from the precambrian3 4 here we present data from a portable seismograph array in the kaapvaal craton of south africa which indicate that the inferred direction of mantle flow underneath the array is close to the direction of absolute plate motion for southern africa since the end of the jurassic period such an alignment has also been reported for the north american craton2 suggesting that the flow in both regions is related mainly to shearing of the sublithospheric mantle by the plate above the old continental roots that are likely to exist in this depth range5 6 must therefore be deformed by the plate motion but the deformations are not strong enough to be seen in the presently available seismic tomography data

sub crustal lvz below dharwar craton india an evidence for mantle metasomatism and tectonothermal activity in the archean crust abstract improved knowledge of seismic structure of the continental crust provides deep insights into the mechanism of the crustal growth since the archean times reprocessing of deep seismic sounding data acquired over western dharwar craton india reveals presence of i a high velocity magmatic layer 7 30 km s at the base of the crust ii a higher pn velocity 8 35 km s at a deeper depth of 45 km than in an usual archean crust and iii an unusually low velocity layer 7 40 km s at a depth range of 48 60 km in the upper mantle such shallow subcrustal lithospheric lvzs are rarely found in a stable archean craton which is associated with low surface heat flow and devoid of recent tectonic activities considering various geological tectonic and geothermal scenarios occurrence of such a low velocity zone has been ascribed to chemical alteration caused by mantle metasomatism rather than mafic partial melts or high temperature sub solidus the derived velocity structure and distribution of volcanic rocks of different ages in the studied area indicate multiple crust mantle thermal interaction in the archean crust

shear wave splitting in the rajasthan craton india abstract the shear wave splitting parameters i e fast axes orientation and delay time t are measured at four broadband stations in rajasthan using sks skks core phases from three component broadband waveforms of 30 events recorded during 2014 16 the thicknesses of lithosphere at these four stations are also inferred from the estimated delays the average splitting parameters t at the northern most jai and southern most ban stations are modeled to be 36 44 08 36 and 1 48 0 32 s and 35 72 10 47 and 1 86 0 34 s respectively while these parameters at udi and bun stations are found to be 41 08 08 54 and 1 61 0 26 s and 43 86 08 96 and 1 68 0 27 s respectively thus our modeling reveals a mean fast axes orientation of 39 9 with a mean delay of 1 7 0 3 s for rajasthan which could be attributed to the frozen lithospheric anisotropy in the direction of current absolute plate motion of the indian plate a slight regional variation of observed orientation of fast axes ranging from 35 7 at ban to 43 9 at bun could be attributed to the tectonic control rather than current plate motion in addition the mean orientations of fast axes are measured to be 36 4o at udi and 41 1o at jai respectively thus this observed coherent variation of fast axes orientation across the rajasthan suggests a significant contribution from the asthenospheric flow further estimated mean delay time 1 7 0 3 s does also suggest a contribution from the anisotropy associated with asthenospheric flows the maximum delay 1 86 s is estimated at southernmost ban station suggesting a thick anisotropic layer of 214 km while minimum delay 1 48 s is modeled at northernmost jai station suggesting a 170 km thick anisotropic layer while asthenospheric layer thicknesses at udi and bun are found to be 185 and 193 km respectively this observation suggests a relatively more anisotropic and thick lithosphere in the south in comparison to that in the north the measured deviation of fast axis orientation from the absolute indian plate motion varies from 1 to 4 3 which could be related to the changes in asthenosphric flow pattern due to the presence of topography at the base of lithosphere

upper mantle shear velocity structure below northwestern india based on group velocity dispersion abstract we derive a well resolved 1d model of shear wave velocity v s through both linear and nonlinear joint inversions of rayleigh and love wave group velocity dispersion data our best model from the deterministic approach suggests a two layered crust over a half space the upper crust is 13 8 km thick with a shear velocity of 3 22 km s for the lower crust they are 24 9 km and 3 62 km s respectively finally we use a global optimization method very fast simulated annealing to estimate shear wave velocity multiple models give acceptable fits to the observed data a common feature of these models is a low velocity zone lvz with its upper boundary at 60 70 km depth at this depth the temperature depth profile of kachchh cuts the wet peridotite geotherm resulting in increased water content due to partial melting and thus the decreased shear velocity this lvz extends down to a depth of 120 130 km where the temperature depth profile of kachchh cuts the dry peridotite geotherm that leads to a sharp decrease in water content thereby a sharp increase in shear velocity the maximum v s perturbation of 3 in the upper mantle can be explained by the presence of 100 150 k excess temperature beneath northwestern india at 70 120 km depth relative to the adjacent mantle along with the presence of 1 penny shaped melt inclusions probably co 2 rich carbonatite melts in the upper mantle this is likely related to the imprints of the initial deccan reunion plume head that might have moved with india since the deccan volcanism at 65 ma

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

thermal evolution of the upper yangtze craton secular cooling and short lived thermal perturbations abstract the upper yangtze craton uyc has stayed in a long term stabilization state its thermal history can be portioned into two parts secular evolutionary trend and short lived thermal perturbations the former is simulated by a two dimensional forward transient thermal model and the latter is discussed by reviewing previous studies the numerical modeling indicates the uyc tended to be cooling as it adjusted to the asynchronous rates of change in the internal radiogenic heat production and in the convecting mantle temperature below the cratonic root the average cooling rate of the uyc has been 58 c ga since the end of archean faster than that of the convecting mantle the surface heat flow presents a decreasing trend from 82 mw m2 at 2 5 ga to 53 mw m2 at present day which constructs the long term evolutionary path the thermal perturbations derived from the occasional tectono thermal events such as mantle plume activity and regional lithosphere extension serves as paroxysmal factors the thermal effects of the emeishan mantle plume were very strong yielding to the maximum heat flow anomaly of 100 mw m2 at the late permian in contrast the influences of regional lithosphere extension during both the early paleozoic period and the early permian middle triassic period were rather weak resulting in a surface heat flow increase of

passive source seismic imaging of the crust and upper mantle underlying the archean singhbhum craton eastern india abstract an improved image of crust mantle structure has been obtained through the joint inversion of p to s receiver functions and fundamental mode group velocity data 7 70 s of rayleigh and love waves at fifteen broadband stations in the eastern indian craton our modelling reveals a marked crustal thinning of 5 10 km below the singhbhum odisha craton soc while the modelled moho depths show a flat crust with a thickness of 42 km below the chotanagpur granitic gneissic terrain cggt the modelled lithosphere is thin 90 km beneath the soc and deepens on both north and south sides to 100 km with a sharp drop of 4 4 in shear velocity across the lithosphere asthenosphere boundary the amount of melt required in the asthenosphere to account for this drop in shear velocity is 1 the isotopic constituents of the dalma volcanic rocks contains asthenospheric component we suggest that the modeled crustal and lithospheric thinning might have been resulted from the earlier magmatism or plume episodes that led to lithospheric delamination below the soc our modeling also detects a common elongated feature extending between latitude 21 23 n and longitude 85 86 4 e which is characterized by a marked crustal 5 10 km as well as lithospheric 10 15 km thinning and a significant drop in vs 2 1 to 4 4 in the upper mantle below the soc this mapped area coincides with the horse shoe shaped broad singhbhum synclinorium known as iron ore group iog basin that hosts huge iron ore and to some extent manganese deposits of precambrian age this lithospheric feature is inferred to have played a key role in forming the passage for melts to carry various iron rich minerals and metals to the uppermost oceanic crust during the archean plume associated with magmatism episode

low shear velocities in the sub lithospheric mantle beneath the indian shield 1 ever since its breakup from the gondwanaland 140 myr ago the indian plate was ravaged by four hot spots although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the deccan and the fast drift of the indian plate the modifications to the deep structure remain to be grasped in this study we investigate the mantle transition zone tz structure beneath the indian shield region using 14 000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains the arrival times of the p to s ps conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by 2 s in comparison with the times observed for other precambrian shield regions like africa australia and canada such delays in the conversions from the 410 km discontinuity below the indian shield suggest low shear wave speeds in the lithospheric and sub lithospheric mantles due to higher temperatures together with a thinner high velocity lid that contrasts with a thicker one found beneath most archean cratons a thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the tz itself also a further delay of the tz discontinuities is observed for stations on the southern granulite terrain which was under the influence of the marion plume that is responsible for the separation of madagascar from india although the data do not conclusively show evidence for a 520 km discontinuity an lvl atop the 410 cannot be ruled out beneath certain geological provinces of the indian shield

the purana basins of southern cratonic province of india a case for mesoproterozoic fossil rifts abstract since its cratonization in the palaeoproterozoic southern peninsular india witnessed the development of a number of large intracratonic sedimentary basins traditionally referred to as purana basins spanning in age from late palaeoproterozoic through neoproterozoic the localization of these intracratonic basins namely pranhita godavari pg chattisgarh and cuddapah basins is apparently controlled by pre existing sutures and or weak zones the pg basin and the cuddapah basin host unconformity bound thick sediment dominated successions attesting to several cycles of fluvial shallow marine to shelf slope basin sedimentation deposition was punctuated by block uplifts resulting in local hiatuses and or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites basin margin deep faults apparently played a role in the facies distribution in these basins based on these features we propose that these basins initiated as continental rifts which however never opened up into a full fledged ocean basin but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the chattisgarh succession spatial distribution of facies and sediment thickness in the cuddapah and pg basins suggest that an open seaway existed to the east of the south indian cratonic province during the mesoproterozoic while similar criteria point to the existence of an open seaway north of the chattisgarh basin development history including nature of inversion suggest that the southern cratonic province of india existed as a single large continental mass since the mesoproterozoic in spite of episodes of supercontinent build up and fragmentation involving india and east gondwana during the proterozoic

stochastic modeling of the thermal structure to decipher the lithospheric thickness application to dharwar craton a wide range of speculation regarding lithospheric thermal thickness in a region calls for quantifying the uncertainties associated with model parameters in this paper we present an attempt to decipher it by solving the differential equations governing the heat transfer through the lithosphere adequately quantifying the temperature field is very important since it is highly dependent on the controlling thermal parameters thermal conductivity is modeled as a random parameter with a known mean value and correlation structure we compute the temperatures in the crust and lithospheric mantle along with their error bounds using an analytical solution to the steady state heat equation and the adomian method of decomposition we apply these solutions in the archean dharwar craton which is divided into western dharwar craton wdc and eastern dharwar craton edc we obtain a moho temperature estimate of 304 58 c in wdc whilst in edc it is 375 74 c for a 20 coefficient of variability in thermal conductivity the range of lithospheric thickness obtained for wdc is 150 280 km and 110 160 km for the edc these values are in agreement with other geophysical and geological findings in the region the thermal lithosphere obtained from this study is based on the mantle solidus and the surface wave seismological studies considering radial anisotropy reveal that the lithosphere is characterized by higher shear wave velocities and decrease at the lithosphere asthenosphere boundary average shear velocity model for two different cratonic blocks edc and wdc is used to show the lab thickness for the eastern and western dharwar craton the two independent studies show a correlation in the lithospheric thickness

zhoukoudian in transition research history lithic technologies and transformation of chinese palaeolithic archaeology abstract zhoukoudian near beijing is where the homo erectus sinathropus pekinensis peking man fossils were found in the 1920s and has always been a focus for studies of palaeolithic archaeology in china this paper discusses two aspects of zhoukoudian in transition research histories and lithic technologies from a historical perspective the study considers the change in research paradigms and perspectives to examine how research at zhoukoudian has transformed chinese palaeolithic archaeology in the last two decades this paper will give an overview on lithic assemblages at zhoukoudian to illustrate the complexity of lithic industries in north china which are not yet fully understood in conclusion the study suggests more research problems rather than prestige collections that were inherited by the current generation of scholars who call for careful reviews of valuable historic data including lithic faunal sediment and human remains before any further interpretations are produced this study suggests that lithic industries in transition at zhoukoudian can provide another line of evidence to support the continuous development of pleistocene hominid evolutions especially leading to the origins of modern humans in china

crust and upper mantle velocity structure beneath north and central india from the phase and group velocity of rayleigh and love waves abstract crust and upper mantle velocity structure is determined using the fundamental mode rayleigh and love waves for 23 different paths across north and central india the earthquakes occurring in kashmir nepal himalaya burma and nearby regions and recorded at hyderabad new delhi and kodaikanal seismic stations are used for dispersion studies frequency time analysis ftan using the multiple filter technique is done for the fundamental mode surface waves to estimate group velocities from the time periods of 13 71 s the single station technique is used to estimate rayleigh and love wave phase velocities at time periods of 15 75 s velocity structure down to a depth of 120 km is determined using both rayleigh and love wave phase and group velocities the crustal thickness is found to be 50 km with 4 km sedimentary thickness for the northern india across burma to new delhi path and 44 km for the central india across kashmir nepal himalaya and northeast india to hyderabad and kodaikanal path a low velocity zone v p 8 4 km s v s 4 15 km s is found from 100 km below the surface velocity structure shows that the shield structure of the peninsula extends up to northernmost india below sub himalayan regions with an overlying sedimentary thickness of 4 km the lower value of observed group velocity and higher phase velocity across northern and central india may be caused by the presence of varying sedimentary thickness in the northernmost part of india the indo gangetic plain below the sub himalayan region the observed lower value of rayleigh wave group velocities below 40 s time periods can be explained by assuming a thin layer of a high contrast material with a p wave velocity of 4 6 km s an s wave velocity of 2 5 km s a density of 3 24 g cm3 and a thickness of 3 km centred above the moho

fine particle retention and deposition in regions of cyclonic tidal current rotation abstract benthic sediments in continental shelf seas control a variety of biogeochemical processes yet their composition especially that of fine sediment remains difficult to predict mechanisms for mud or fine sediment deposition and retention are not fully understood using sediment data and a hydrodynamic model of the northwest european shelf seas a relationship is shown to exist between fine benthic sediment composition and regions of cyclonic tidal current rotation the reduced thickness of cyclonic tidal benthic boundary layers compared with the anticyclonic case promotes deposition of fine sediment and trapping of resuspended material adding the effects of the benthic boundary layer thickness as influenced by ellipticity or not sheds some light on the limitations of approaches only focusing on bed shear stress and sediment pathways to predict the location of mud deposits a tidal boundary layer predictor that includes ellipticity alongside tidal current magnitude and depth was shown to spatially agree with maps of mud deposits

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

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

geochronology and tectonic evolution of karimnagar and bhopalpatnam granulite belts central india abstract the karimnagar granulite belt kgb and the bhopalpatnam granulite belt bgb occur along both flanks of the pranhita godavari pg rift basin we present a state of the art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains and report new age data on zircon monazite and uraninite recovered from granulite facies assemblages from kgb and bgb based on electron microprobe analyses epma zircons from kgb charnockites show core ages of up to 3 1 ga mantled by rims of 2 6 ga zircons from bgb have 1 9 ga cores mantled by 1 7 ga rims zircons with core ages of 1 6 to 1 7 ga in bgb rocks suggest new growth at this time monazites and uranitite from kgb show clear peaks with well defined ages in the narrow range between 2 42 0 08 ga and 2 47 0 03 ga rims of monazite show mean age of 2 21 0 08 ga monazites from bgb define sharp linear trend in pbo vs tho2 diagram delineating a clear isochron with age of 1 59 0 03 ga age data from kgb and bgb presented in this report negate current models linking these terrains to godavari granulite belt and considering them as single and contemporaneous entity the mid archaean to early palaeoproterozoic signature recognized from kgb is totally missing in bgb on the other hand kgb rocks do not record any evidence for major mesoproterozoic thermal regime the two granulite belts shouldering the pg rift basin have therefore evolved in different times under distinct p t conditions and thermal regimes our results have important implications in evaluating models of supercontinent assemblies particularly the older assemblies of ur columbia and rodinia while tectonothermal events in kgb broadly match with those of east dharwar we propose that bgb represents a 1 6 ga collisional mobile belt between the bastar and the dharwar cratons the 1 6 ga collisional mobile belt at the southern margin of the bastar craton was superposed by rift activity along the pg basin at 1 5 ga this sequence of events goes against the existence of a 3 0 ga old contiguous assembly of ur but closely matches with the history of accretion and break up of the columbia further parts of the pg basin located away from the influence of the eastern ghats mobile belt neither recorded any grenville ages 1 0 ga corresponding to the rodinia accretion nor late pan african ages ca 550 ma relating to the gondwana amalgamation indicating that the region did not witness any of these younger tectonic events

passive source seismic imaging of the crust and upper mantle beneath the 2001 mw 7 7 bhuj earthquake region gujarat india abstract variations in crustal and lithospheric thicknesses below the kachchh rift kr zone have been investigated employing the joint inversion of p receiver functions rf and rayleigh wave and love wave group velocity dispersion 5 70 s on the broadband waveforms of teleseismic events estimated crustal and lithospheric thicknesses at 15 broadband sites led to the delineation of an updoming of 4 7 km in the moho and 6 12 km in the asthenosphere beneath the central kr relative to the surrounding unrifted regions the moho depths vary from 35 to 42 km and lithospheric thicknesses range from 62 to 78 km the observed marked shear velocity reduction 2 6 at the lithosphere asthenosphere boundary lab over an area of 120 km 80 km suggests the presence of patches of partial melt in the asthenosphere below the kr evidencing the possible presence of imprints of the deccan reunion mantle plume the coincidence of the area of updoming of the moho and asthenosphere and the confined aftershock activity below the central kr implies that these patches could provide a high input of volatile co 2 into the lower crust which might play a key role in generating the continued aftershock activity in the region since the occurrence of the 2001 m w 7 7 bhuj mainshock

heat flow in the bastar craton central indian shield implications for thermal characteristics of proterozoic cratons measurements of surface heat flow density in various lithologies from the central parts of the proterozoic bastar craton of the central indian shield have yielded heat flow values ranging from 51 to 64 mw m 2 with a mean value of 56 mw m 2 standard deviation 6 1 mw m 2 these values are considerably higher than in the archaean and archaean to early proterozoic cratons of many precambrian shields but are compatible with the heat flow data from the proterozoic gawler craton central australian shield it is inferred that 1 the contrast in surface heat flow between the archaean and proterozoic cratons is a consequence of the difference in the radiogenic heat production in their upper 10 20 km thick crustal layers and 2 the proterozoic cratons as these could also have enclaves of basic archaean rocks show a wide range of surface heat flow values controlled mainly by the varying amounts of upper crustal radiogenic heat source

q structure beneath the north and central indian ocean from the inversion of observed love and rayleigh wave attenuation data abstract the fundamental mode love and rayleigh waves generated by 57 earthquakes which occurred in the north and central indian ocean extending to 40 s and recorded at indian seismograph and other wwssn stations such as how shl vis mdr hyb kod chg trd poo bom goa ndi nil and que are analysed love and rayleigh wave attenuation coefficients are estimated at periods of 15 100 s using the spectral amplitude of these waves for 98 different paths across the bay of bengal fan the arabian fan and the north and central indian ocean the large standard deviations observed in the surface wave attenuation coefficients may be a result of regional variation of the attenuative properties of the crust and upper mantle beneath these regions love wave attenuation coefficients are found to vary from 0 000 03 to 0 000 45 km 1 for the bay of bengal fan from 0 000 03 to 0 000 85 km 1 for the arabian fan and from 0 000 03 to 0 000 35 km 1 for the north and central indian ocean similarly rayleigh wave attenuation coefficients vary from 0 000 03 to 0 0004 km 1 for the bay of bengal fan from 0 000 06 to 0 0007 km 1 for the arabian fan and from 0 000 03 to 0 0007 km 1 for the north and central indian ocean backus and gilbert inversion theory is applied to these surface wave attenuation data to obtain average q 1 models for the crust and upper mantle beneath the bay of bengal the arabian fan and the north and central indian ocean inversion of love and rayleigh wave attenuation data shows a high attenuation zone centred at a depth of 120 km q 125 for the bay of bengal fan similarly a high attenuation zone q 40 70 occurs at a depth of 60 160 km for the arabian fan at 100 160 km q 115 for the indian ocean off ninetyeast ridge and at 80 160 km q 80 for the indian ocean across the ninetyeast ridge the q 1 models show a lithosphere thickness of 120 km beneath the bay of bengal fan similarly lithosphere thickness of 70 100 and 80 km is estimated beneath the arabian fan and the indian ocean off ninetyeast ridge and across ninetyeast ridge respectively the base of the lithosphere is identified as the depth at which there is a significant increase in the q 1 value which attains its maximum value in the asthenosphere the thinning of indian lithosphere beneath the arabian fan suggests high temperature below moho depth 60 km from surface which has caused a high attenuation zone at this shallow depth

geothermal studies in the hyderabad granitic region and the crustal thermal structure of the southern indian shield based on temperature measurements in three boreholes one specially drilled for the purpose and thermal conductivity determinations heat flow density values were determined for three sites in the archaean hyderabad granitic batholith a mean heat flow density value of 40 1 s d mw m 2 has been obtained the heat generation in its rocks 5 57 w m 3 is significantly higher than in average crustal rocks it has been proposed that the hyderabad batholith has a layered structure with a thin 1 km surface layer of high radioactivity these results together with the already reported data have been used to estimate the conductive steady state temperature within and at the base of the crust of the southern indian shield yielding values of the same order as found in the western australian shield