till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

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

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

evolution of archaean crust in the dharwar craton the nd isotope record abstract a synthesis of the available nd isotope data from the dharwar craton puts important constrains on antiquity of crust extent of basement terranes and events marking juvenile crustal addition and crustal recycling the craton is divided into two blocks namely eastern dharwar craton edc and western dharwar craton wdc nd model ages trace crust extraction as far as 3 5 ga back in both the blocks although rock record of such antiquity is yet to be found in the craton in wdc 3 35 3 0 ga is the most significant period of juvenile crustal addition as well as crustal recycling significantly depleted mantle 1 5 to 6 4 existed below the wdc as early as 3 35 ga which was probably refertilized in a later event from the resultant chondritic to slightly enriched mantle 0 3 to 0 4 juvenile mafic crust was added to the wdc during 2 9 2 6 ga a widespread crustal recycling event at 2 6 ga marks the last major event in this block in edc 3 3 3 0 ga granitoids occur as vestiges and show nd model ages higher than crystallizations ages suggesting recycling of palaeoarchaean crust during 2 7 2 5 ga extensive juvenile magmatism took place in edc from variably depleted mantle 1 4 to 5 6 which is later than the global 2 7 ga peak of crustal growth attendant crustal recycling destroyed most of the earlier crust distribution of nd model ages suggests in contrast to general believe that palaeoarchaean crust was not only abundant in the wdc but also widespread in an area extending from the wdc edc boundary to further 60 150 km east up to the hungund ramagiri kolar hrk belt this belt appears to be a major crustal boundary along which terranes of different nd isotope signatures were amalgamated during the late neoarchaean by horizontal accretion major neoarchaean crustal recycling events documented in the dharwar craton are missing from nd isotope record of the eastern block eb of the north china craton this fact does not support correlation between the two cratons as suggested by some authors

on regional s the character of regional s wave trains is quite complex and consists of two major elements sn waves which principally travel in the upper mantle and lg waves guided in the earth s crust the effect of lateral heterogeneity is to modify the signals for these phases compared with the predictions for horizontally stratified models however complete synthetic seismogram calculations for simple stratified crustal models for a suite of distances out to 950 km including both sn and lg provide a range of insights into the nature of the s wave train in particular late picks of sn arrivals are likely to arise because the actual s onset has been missed in the p coda and a higher amplitude surface multiple phase with an extra crustal delay has been picked instead such a effect was first recognized by lapwood 1955 in a study of the iss readings for a sequence of japanese events recorded at regional distances but has usually not been taken into consideration in studies using regional networks lateral heterogeneity along the propagation path has the effect of inducing interconversion between sn and lg energy and thus alters the character of the s wave train the lower attenuation for s in the uppermost mantle compared with the crust frequently leaves sn as the most prominent phase at ranges greater than 1000 km long range propagation of high frequency lg requires a relatively undisturbed crust

fractal dimension and its translation into a model of gold spatial proxy abstract unraveling the spatial distribution of mineral deposits is an essential part of exploration and important for developing a better understanding of ore forming processes in this study gold assay results in g t of twelve boreholes completed from mysore block and eleven boreholes completed from nabhapur north block central lode system gadag gold field ggf india were analyzed using fractal analysis both the fractal correlation integral and rescaled range analysis were used to measure the fractal dimension from the gold concentration values the fractal dimension values obtained from the fractal correlation integral were used to better constrain the possible auriferous fluid flow pathways whilst results obtained from the rescaled range analysis were used to describe the behavior of auriferous veins across the boreholes the fractal dimension values were found to vary from 0 014 to 3 0 with the higher values taken to indicate the presence of well developed interconnected fault fracture networks representing steep permeable fluid flow pathways the lower fractal dimension values were taken to indicate unfavorable fluid flow pathways in other words higher grade gold ores were deposited along the structures represented by steep fractal gradients hurst exponent h values obtained from the rescaled range analysis were found to vary from 0 50 to 0 96 satisfying the condition 0 5

geoelectric structure of the dharwar craton from magnetotelluric studies archean suture identified along the chitradurga gadag schist belt summary broad band magnetotelluric data were collected at 50 stations over a 400 km long approximately east west profile over the granite greenstone terrain of dharwar southern india the tensor decomposed data were interpreted using a 2 d inversion scheme the geoelectric model is suggestive of a suture along the chitradurga gadag schist belt formed by the thrusting of the west dharwar craton beneath its eastern counterpart with an easterly dip of 20 30 the thrust proposed here pre dates the formation of these schists which occurred during the late archean 2600 ma the accretionary wedge of the thrust and the depressed part of the west dharwar craton may have controlled the emplacement of the chitradurga gadag and shimoga dharwar schists the subsequent weathering the several episodes of tectonic activity witnessed during the precambrian and the vertical block movements caused during the passage of the indian plate over the reunion hotspot may have modified the crust leading to the present day geological configuration despite its age and several tectonothermal episodes the signature of this thrust is adequately preserved in the dharwar craton several similarities with younger sutures as is evident from the observed relics of the oceanic rocks present along the chitradurga schist belt suggest that the tectonic processes leading to this archean event may have had a close resemblance to those witnessed in recent times magnetotelluric studies also image a zone of low resistivity at a depth of 40 km beneath the west dharwar craton this seems to be a regional feature extending to the north over a distance of at least 250 km beneath the deccan volcanics the low heat flow values and the high density associated with this feature make partial melting an unlikely explanation for the low resistivity the grain boundary graphites and the sulphides deposited in the form of pyrites may have caused the low resistivity in the lithospheric mantle of the west dharwar craton although the fluids generated and trapped in the mantle during the passage of the indian plate over the reunion hotspot in the waning phase of its outburst could also be a possibility the asthenosphere is delineated at a depth of about 100 km beneath the east dharwar craton

an integrated approach to modelling hydrology and water quality in glacierized catchments the results are summarized of an integrated investigation of glacier geometry ablation patterns water balance meltwater routing hydrochemistry and suspended sediment yield the ultimate objective is to evaluate the assumptions of lumped two component mixing models as descriptors of glacier hydrology and to develop a semi distributed physically based model as an alternative the results of the study demonstrate that a reconstruction of probable subglacial drainage alignments can be achieved through a combination of terrain modelling based on estimated potential surface and dye tracing experiments recession curve analysis evidence of the seasonal instability of the englacial and subglacial electrical conductivities assumed in a mixing model evidence of the non conservative behaviour of water chemistry in the presence of suspended sediment and evidence of the seasonal evolution of the subglacial drainage system based on dye tracing all indicate that an alternative to a lumped static model of the hydrology is necessary the alternative presented in this paper is based on the combination of an energy balance model for surface melt which operates on an hourly time step and accounts for the changing spatial distribution of melt through the day as shading patterns change and routing procedures that transfer surface melt to moulins on the basis of glacier surface gradients then route water through reconstructed conduit systems using a hydraulic sewer flow routing procedure

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

gravity interpretation of the dharwar greenstone gneiss granite terrain in the southern indian shield and its geological implications abstract the dharwar craton in the southern indian shield has a wide distribution of volcano sedimentary sequences surrounded by a vast gneissic complex both of which have been intruded by younger granites a gravity anomaly map of this craton compiled from all the available data is analysed here to study the structures and depths of the greenstone belts the mode of granite emplacements and the greenstone gneiss granite associations in general the anomaly map is a mosaic of well defined gravity highs and lows characterizing the dense volcano sedimentary sequences and exposed and or concealed granites respectively gravity modelling indicates that the shimoga belt has a limited depth range of only 3 4 km while the chitradurga and sandur belts have greater depths of over 10 km the structures inferred for the dharwar formations are alternating bands of synclines filled with dense schistose rocks separated by anticlinal ridges of gneisses and granites

translating the mineral systems approach into an effective exploration targeting system abstract holistic mineral systems approaches are being increasingly adopted in the study of ore deposits but application in industry has been suboptimal because of the lack of a conceptual framework to help translate the mineral system understanding to practical application a four step process is proposed for linking the conceptual mineral system with data available to support exploration targeting these steps include translation from 1 critical processes of the mineral system to 2 constituent processes of the mineral system to 3 targeting elements reflected in geology and 4 targeting criteria used to detect the targeting elements directly or by proxy this translation process is illustrated using examples from komatiite hosted ni cu pge and orogenic au mineral systems ranking of targets must be at the critical process level rather than the current practice of mixing processes elements and proxies the significance of targeting elements and their mappable criteria changes as a function of the scale at which the critical processes operate and the availability of relevant geoscience datasets this scale dependence is rarely recognised in current practice sources of uncertainty when translating the mineral system understanding into an effective exploration targeting system include 1 resolution of a mappable criterion in a dataset 2 how well the targeting criterion reflects the targeting element 3 how well the targeting element reflects the critical process and 4 whether the process being mapped is critical for the genesis of a mineral deposit the first three sources of uncertainty collectively are termed the representativeness of the targeting criteria i e how representative is a mappable criterion of the constituent process for which it is a proxy the last source of uncertainty reflects our lack of understanding of the processes that control mineralisation and highlights areas for further research

the nature and thickness of lithosphere beneath the archean dharwar craton southern india a magnetotelluric model abstract the dharwar craton in the southern indian shield region occupies a key position in reconstructions of the earliest supercontinent ur as well as in the global neoarchean configurations of continental assembly here we synthesize the available geophysical data that include heat flow seismic tomography temperature profile derived from xenoliths and magnetotellurics mt from the dharwar craton a thickness of more than 200 km is identified for the lithosphere beneath this craton by magnetotelluric model our inference lends support to the models from recent seismological investigations that argue for the presence of a thick lithosphere as well as the upper mantle structure derived from the analysis of mantle xenoliths a synthesis of recent geophysical and geochemical data together with our magnetotelluric model indicate the existence of a thick lithospheric root beneath the dharwar craton and suggest that not much of the lithosphere has been eroded since precambrian in this region leading to the conclusion that the dharwar craton preserves an archean tectosphere an increase in the lithospheric thickness from east to west in the dharwar craton and the lithosphere asthenosphere boundary lab migration to shallower levels observed from geophysical data towards the east is interpreted as a probable reflection of the mesoproterozoic subduction erosion of the archean cratonic keel

mapping of tectonic corridors through hidden parts of the greater dharwar terrane southern india abstract the dharwar craton india has been divided into western and eastern domains by a tectonic boundary between the closepet granite and chitradurga schist belt it has been noted that only the central portion of the dharwar craton is exposed its northern projection is buried beneath the deccan traps and its southern margin merges with a high grade metamorphic transition of late archean age resolution of structure and extent of the greater dharwar terrane gdt beneath its northern and southern end the deccan traps metamorphic transition zone and cuddapah basin thus remains unsolved results of deep geophysical probings have been examined along with associated geological and tectonic data the analysis leads to a delineation of palaeo accretionary corridors extending under the deccan trap cover along which two damaging stable continental region earthquakes koyna and latur occurred veeraswamy and raval 2005 b it is seen that the latur earthquake seem to lies within an unsuspected tectonic corridor which bifurcates the eastern dharwar craton edc into eastern dharwar craton 1 and eastern dharwar craton 2 passing through the cuddapah basin c possibility of the palghat cauvery shear zone connecting to the 2 5 ga metamorphic karimnagar granulite belt through the interface between napier nc and rayner rc complexes of east antarctica and d the indication that the bavali moyar bhavani mettur and palghat cauvery shear zones form the southern margins of the western dharwar craton eastern dharwar craton 1 and eastern dharwar craton 2 respectively

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

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

the karnataka late cretaceous dykes as products of the marion hot spot at the madagascar india breakup event evidence from 40ar 39ar geochronology and geochemistry two late cretaceous mafic dykes with an ene strike that is orthogonal to the west coast of india and located nearly 200 km inland around huliyardurga karnataka state yield 40ar 39ar plateau ages of 90 0 1 0 and 87 5 0 9 ma these fe ti enriched tholeiites are essentially co eval with at least four other igneous suites widely scattered in southern india namely the south and north kerala dykes the agali anaikatti dykes of central kerala tamil nadu and lavas of st mary islands off the west coast of india the karnataka cretaceous dykes are also co eval and compositionally very similar to the fe ti enriched tholeiitic lavas and dykes around mananjary a major phase of late cretaceous magmatism along the eastern rifted margin of madagascar which are believed to be products of the marion hot spot that extruded at 88 ma synchronous with the india madagascar break up event the age and compositional similarities between these late cretaceous magmatic rocks from india and madagascar constitute a clear evidence for extension of the thermal manifestations deep into the indian peninsula

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

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

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

magnetotelluric evidence on the southward extension of the eastern ghats mobile belt from ongole india abstract there has been a longstanding and scientifically contentious issue concerning whether or not the eastern ghats mobile belt egmb extends southward beyond ongole in the state of andhra pradesh in south india various geological and geophysical investigations carried out in different campaigns could not conclusively address this important issue some argued that the egmb that originates near mahanadi in the northeast of india and extends along the eastern continental boundary is abruptly cut near ongole in the south swerves eastward towards the bay of bengal and joins the erstwhile east antarctica boundary others opined differently and debated about its southward extension beyond ongole in the present study we provide a conclusive evidence for the first time on this perplexing issue by determining the deep earth electrical resistivity structure of the egmb region through broad band magnetotelluric mt investigations carried out along two different profiles laid out one near the ongole region profile aa of 35 km length and the other near the nellore region profile bb of 52 km length the latter is located at a distance of about 125 km south of the former the lengths of both these profiles are sufficiently far from the influences of the electrical nature of the subsurface of the adjoining geological terrains namely the east dharwar craton the cuddappah basin and the southern granulite terrain this enables to determine the electrical nature of the egmb alone and examine whether or not it extends south of ongole two dimensional inversion of mt data of both the profiles revealed a very thin top conductive layer of about 1 10 m with an average thickness of about 500 m representing the alluvium this layer is underlain by very high resistive subsurface with resistivity of about 107 m extending up to about 30 km and probably further beyond not shown in the present study since the major rock types in the study area namely khondalites calc granulites charnockites and granitic gneisses of archaean age have large overlapping resistivities mt data could not explicitly discern the nature of the multilayered crust beneath the egmb as a result the subsurface beneath both the regions shows up as a very thick single layer of very high resistivity the high grade rocks of the ongole domain and supracrustals of nellore schist belt nsb which lies south of ongole domain have undergone synchronous tectonic activity at ca 1 6 by this is evident by the kanigiri and kandra ophiolites within nsb that signify the proterozoic subduction of the egmb this geological evidence together with the similar nature of the high electrical resistivity of the subsurface observed at ongole and nellore regions confirm the southward extension of the egmb beyond ongole

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

subduction related tectonic evolution of the neoarchean eastern dharwar craton southern india new geochemical and isotopic constraints the neoarchean eastern dharwar craton edc is distinct from the mesoarchean western dharwar craton wdc in many aspects of its geology the important distinguishing features of the edc are the predominance of younger granitoids abundance of gold mineralization and the exposure at lower crustal depths no consensus exists on evolutionary models for edc mutually exclusive plume and subduction derived tectonic models have been proposed geochemical and radiogenic isotopic studies on the granitoids and volcanic rocks of three greenstone belts along a cross section in the northern part of edc are presented herein the evolved nd isotopic signatures radiogenic pb isotopic ratios and arc like geochemical signatures are suggestive of a subduction regime and the involvement of recycled older crust in the derivation of these rocks the proposed petrogenetic mechanism involves multi stage processes in a supra subduction regime involving slab dehydration formation of hydrous basaltic melts and re melting and interaction with sub arc basaltic crust at low pressures where amphibole plagioclase is the dominant residual phase there is a notable systematic decrease in the extent of older crustal involvement from west to east in the edc this is in concurrence with the younging of dharwar craton from west to east and eastward subduction the proposed petrogenetic model can efficiently explain the variations in older crustal involvement which is very common in other archean cratons c 2012 elsevier b v all rights reserved

decadal slowdown of a land terminating sector of the greenland ice sheet despite warming ice flow along land terminating margins of the greenland ice sheet gis varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet such inputs lubricate the ice bed interface transiently speeding up the flow of ice greater melting results in faster ice motion during summer but slower motion over the subsequent winter owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice sheet bed that have a high basal water pressure however the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained here we show that annual ice motion across an 8 000 km 2 land terminating region of the west gis margin extending to 1 100 m above sea level was 12 slower in 2007 14 compared with 1985 94 despite a 50 increase in surface meltwater production our findings suggest that over these three decades hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion not a speed up as previously postulated increases in meltwater production from projected climate warming may therefore further reduce the motion of land terminating margins of the gis our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought

co and postseismic characteristics of indian sub continent in response to the 2004 sumatra earthquake abstract the m w 9 3 sumatra earthquake of december 26 2004 caused extensive coseismic displacements globally measurements of which were made essentially using modern geodetic techniques this earthquake induced considerable perturbation in stress distribution as far as 8000 km away from the epicenteral region which is tending to relax to its normal rates as seen from postseismic transient deformation the monitoring of crustal displacements from strategically located sites using gps provides coseismic as well as postseismic deformation that facilitates the understanding of the fault geometry elastic thickness postseismic relaxation mechanisms rheology and earthquake recurrence time interval we investigated coseismic and postseismic gps derived displacements in indian region together with the gps data collected from andaman and sumatra region it is found that while ew displacements are significantly large in peninsular india those in the region to the north of central india tectonic zone citz are relatively small we could delineate the postseismic transients from position time series and interpreted them in terms of viscoelastic relaxation it is inferred that the postseismic deformation is characterized by a power law viscoelastic flow in the mantle in indian peninsula region the timescale parameter of the exponential decay 250 days would require an extremely low viscosity for the upper mantle relying on the prevailing coseismic and postseismic displacement fields the present study also reflects upon the contemporary litho tectonics of the indian sub continent

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

magnetotelluric images of paleoproterozoic accretion and mesoproterozoic to neoproterozoic reworking processes in the northern sao francisco craton central eastern brazil abstract broadband and long period magnetotelluric mt data were collected along an east west oriented 580 km long profile across the northern sao francisco craton where extensive proterozoic and phanerozoic sedimentary cover and lack of deep probing geophysical surveys have prevented to establish unequivocally the cratonic character of the archean paleoproterozoic lithosphere following dimensionality analyses the mt dataset was interpreted using both 2 d and 3 d inversion procedures the near surface structure is better resolved in the 2 d model due to its finer resolution a huge upper crustal conductor is found all along the shallow early neoproterozoic irece basin in the central domain of the craton extending laterally for approximately 150 km and restricting signal propagation below the basin its high conductance is explained by a combination of high porosity and high fluid salinity in the sedimentary package another upper crustal conductor is observed on the west side of the profile interpreted as fractured metasedimentary rocks of the rio preto belt thrusted on top of the craton basement during neoproterozoic marginal collision the 3 d model explains significantly better the measured data related to deep structure contrary to what is expected for a stable cratonic block the geoelectric model shows pronounced electrical complexity and heterogeneity an indication that the cratonic lithosphere was multiply reworked in the past by tectonothermal events different lithospheric resistive blocks bounded by major conductive zones are identified constrained by geochemical and isotopic data these vertical conductive interfaces are interpreted as cryptic suture zones due to large scale amalgamation of continents and microcontinents leading to the assembly of the sao francisco craton in the paleoproterozoic the conductivity enhancement is more likely explained by emplacement of sulfides along previous suture zones during mafic magmatism at upper mantle depths high conductivity observed below most of the profile indicates that metasomatism or refertilization processes with incompatible elements caused by the paleoproterozoic subducting slabs and mesoproterozoic to neoproterozoic upwelling of deep fluids and melts reworked this portion of the craton mantle

structural pattern in the dharwar craton high grade and low grade supracrustal belts sargur and dharwar set in a matrix of gneisses dotted with granitic plutons of different ages make up the dharwar craton the overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from nw se in the northern part to n s in the south both the high grade and low grade belts show three principal episodes of folding however the correlation between the deformation episodes in the sargur and the dharwar rocks is controversial and the structural relations between them are not unequivocally established parallelism of the first phase structures in the sargur and the dharwar rocks can be interpreted either as synchronous development or rotation of the earlier sargur structures into parallelism with the younger dharwar structures during later deformation the extension of the supracrustal belts is parallel to the f 2 axial traces of the second deformation granite diapirism and basement uplift further modified

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

magnetotelluric study to characterize kachchh mainland fault kmf and katrol hill fault khf in the western part of kachchh region of gujarat india abstract the kachchh mainland fault kmf is a major e w trending fault in the kachchh region of gujarat extending 150 km from lakhpat village in the west to the bhachau town in the east the katrol hill fault khf is an e w trending intrabasinal fault located in the central region of kachchh basin and the south of kmf the western parts of both of the faults are characterized and the sediment thickness has been estimated in the region using a magnetotelluric mt survey at 17 sites along a 55 km long north south profile with a site spacing of 2 3 km the analysis reveals that the maximum sediment thickness is 2 3 km quaternary tertiary and mesozoic in the region out of which the mesozoic sediments feature a maximum thickness of 2 km the estimated sediment thickness is found consistent with the thickness suggested by a deep borehole depth approx 2 5 km drilled by oil and natural gas corporation ongc at nirona northern part of the study area from 2 d inversion of the mt data three conductive zones are identified from north to south the first conductive zone is dipping nearly vertical down to 7 8 km depth it becomes north dipping below 8 km depth and is inferred as kmf the second conductive zone is found steeply dipping into the southern limbs near manjal village 28 km south of nirona which is inferred as the khf a vertical dipping down to 20 km depth conductive zone has also been observed near ulat village located 16 km north of manjal village and 12 km south of nirona village this conductive zone becomes listric north dipping beyond 20 km depth it is reported first time by a geophysical survey in the region

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

a magnetotelluric study from over dharwar cratonic nucleus into billigiri rangan charnockitic massif india abstract the electrical resistivity structure of the crust beneath the dharwar craton in southern india was investigated by magnetotelluric method in the present study a northwest southeast oriented 220 km long profile of 18 stations with a station spacing of 10 15 km is used the profile extends from dharwar cratonic nucleus in the north to billigiri rangan charnockitic massifs in the south time series data are processed to get the apparent resistivity and phase the dominant geoelectric strike direction 75 was calculated in a period range of 0 01 1000 s the data are rotated to 75 strike direction two dimensional inversion is carried out by using the non linear conjugate gradient scheme for both apparent resistivity and phase our inversion results show a conductor in the northern side of the profile and two distinct prominent conductors in southern part of the profile the mid lower crust in southern part of the profile shows less resistive 2 flushed terranes regional scale carbonation occurred in late archaean is associated with chitradurga shear zone and in late proterozoic is associated with salem attur shear zone the co 2 rich fluids derived during that time might have exhausted in dehydration reactions later events such as the indian plate passing over several hotspots and the metasomatized fluids associated with the cretaceous tertiary magmatism in the region is the reason for observed low resistivity near billigiri rangan massif and surrounding regions in the south