gravity anomalies and segmentation of the continental margin offshore west africa the free air edge effect anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions like many margins the edge effect high offshore gabon west africa is located at the shelf break in the region of maximum sediment thickness gabon differs however from other margins in that the edge effect low is displaced from over the continental rise to the slope the origin of the low has been investigated by backstripping the sediments at the margin we have used the backstrip to calculate the amount of crustal thinning assuming airy isostasy such a model implies zero strength during rifting alternatively this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting we have developed a method which takes into account variations in the strength during rifting and the depth of necking to compute the thinning directly from the sediment backstrip the backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly differences between the backstrip and the present day water depth give the geometry of the sediment load and hence the contribution to the gaavity anomaly of sedimentation by comparing the combined rifting and sedimentation anomaly to the observed free air gravity anomaly we have been able to constrain the flexural rigidity and equivalent elastic thickness te of the lithosphere during and after rifting the best fitting model is one in which the low is caused by a 125 km wide zone of relatively thin i e 15 km continental crust which was weak i e 0 te 10k during rifting and has remained so since then we speculate that the weak zone extends for 350 400 km along strike of the gabon margin and that passive margins may be highly segmented as regards their long term strength

india madagascar paleo fit based on flexural isostasy of their rifted margins the present study contributes new constraints on and definitions of the reconstructed plate margins of india and madagascar based on flexural isostasy along the western continental margin of india wcmi and the eastern continental margin of madagascar ecmm we have estimated the nature of isostasy and crustal geometry along the two margins and have examined their possible conjugate structure here we utilize elastic thickness te and moho depth data as the primary basis for the correlation of these passive margins we employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness gravity inversion and flexure inversion techniques are used to estimate the configuration of the moho crust mantle interface that reveals regional correlations with the elastic thickness variations these results correlate well with the continental and oceanic segments of the indian and african plates the present study has found a linear zone of anomalously low te 1 5 km along the wcmi similar to 1680 km which correlates well with the low te patterns obtained all along the ecmm we suggest that the low te zones along the wcmi and ecmm represent paleo rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the marion hotspot and lithospheric extension due to rifting we have produced an india madagascar paleo fit representing the initial phase of separation based on the te estimates of the rifted conjugate margins which is confirmed by a close fit correlation of moho geometry and bathymetry of the shelf margins the matching of tectonic lineaments lithologies and geochronological belts between india and madagascar provide an additional support for the present plate reconstruction c 2014 international association for gondwana research published by elsevier b v all rights reserved

evolution of the passive continental margins of india a geophysical appraisal abstract the passive continental margins of india have evolved as india broke and drifted away from east antarctica madagascar and seychelles at various geological times in this study we have attempted to collate and re examine gravity and topographic bathymetry data over india and the adjoining oceans to understand the structure and tectonic evolution of these margins including processes such as crustal lithosphere extension subsidence due to sedimentation magmatic underplating and so on the eastern continental margin of india ecmi seems to have evolved in a complex rift and shear tectonic settings in its northern and southern segments respectively and bears similarities with its conjugate in east antarctica crustal extension rates are uniform along the stretch of the ecmi in spite of the presence or absence of crustal underplated material variability in lithospheric strength and tectonic style of evolution ranging from rifting to shearing the krishna godavari basin is underlain by a strong 30 km elastic lithosphere while the cauvery basin is underlain by a thin elastic lithosphere 3 km the coupling between the ocean and continent lithosphere along the rifted segment of the ecmi is across a stretched continental crust while it is direct beneath the cauvery basin the western continental margin of india wcmi seems to have developed in an oblique rift setting with a strike slip component unlike the ecmi the wcmi is in striking contrast with its conjugate in the eastern margin of madagascar in respect of sedimentation processes and alignment of magnetic lineations and fracture zones the break up between eastern india and east antarctica seems to have been accommodated along a proterozoic mobile belt while that between western india and madagascar is along a combination of both mobile belt and cratonic blocks

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

gravity anomalies and segmentation at the east coast usa continental margin summary the free air gravity edge effect anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust while crustal thinning is a major contributor recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it at some margins to be highly segmented along their strike in this paper we use a combined 3 d flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of atlantic type continental margins we focus on the east coast usa since a substantial amount of high quality seismic reflection and refraction gravity and magnetic data already exists for this margin by calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free air anomaly we have determined the best fit elastic thickness te structure of the margin we show that 0 10 5 a integrated strength of the lithosphere these results imply that weak regions abut strong ones at the east coast usa margin te generally decreases with increase in the amounts of crustal thinning and the flexed basement curvature k suggesting it is controlled at least in part by the mechanical structure of the pre rift lithosphere and yielding due to flexural loading however there is considerable scatter suggesting other factors such as along strike changes in crustal composition irrespective we show that an isostatic anomaly that takes into account the best fit te distribution is significantly reduced in spectral power compared with one which is computed assuming only airy compensation i e te 0 km this is not to imply that rifting and sediment loading completely accounts for the anomalies along strike and across strike the east coast usa margin to the contrary significant isostatic anomaly highs and lows persist especially in inner and middle shelf regions one of the most prominent is an arcuate 670 km long high with flanking lows offshore carolina that we attribute to magmatism during the initial stages of continental break up

structure and isostatic compensation of the comorin ridge north central indian ocean summary bathymetry gravity and magnetic data about 9200 lkm of the comorin ridge north central indian ocean were investigated using the transfer function and forward model techniques to understand the mode of isostatic compensation and origin of the ridge the ridge extends for about 500 km in nnw sse direction and associates with low amplitude gravity anomalies ranging from 25 to 30 mgal compared to the ridge relief suggesting that the anomalies are compensated at deeper depths from admittance analysis an airy model or local compensation with an elastic plate thickness t e of about 3 km and crust thickness t of 15 20 km are suggested for the southern part of the comorin ridge south of 5 n whereas for the northern part a flexural plate model with an elastic thickness of about 15 km is obtained admittance analysis together with the results from gravity forward modelling reveal that the south part was emplaced on relatively weak oceanic crust with both surface and subsurface loading while the north part was emplaced on the continental crust based on present studies and published plate kinematic models we interpret that the comorin ridge was evolved at about 90 ma during the rift stage of madagascar from the southwest of india we have also demarcated the continent ocean boundary cob west of sri lanka and southern tip of india which runs across the strike of the ridge placing the northern part of the ridge on continent and southern part on oceanic crust on the southern part of the ridge eastern flank is steep faulted up to 0 6 km and is controlled by the 79 e fz and then by cob

synoptic systems interacting with the rocky mountain barrier observations and theories abstractusing a composite analysis for strong sea level pressure perturbations off the west coast of north america the evolution of large amplitude synoptic systems upstream of the rocky mountains is investigated for the winter season corresponding previous analyses are refined by avoiding multiple counting of events and extended by including potential vorticity vertical motion and deformation in the analysis cyclonic and anticyclonic anomalies behave similarly with weak local extrema forming in the lee of the mountain range southeast of the parent systems however neither the geopotential anomaly nor the associated potential vorticity anomaly cross the mountain range nevertheless these anomalies contribute to the sea level pressure anomaly in the lee for both positive and negative anomalies potential vorticity exhibits a bipolar structure with lobes over the reference point and over the cordillera respectively the relevance of several theories describing the interaction between synoptic syste

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

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

relating cenozoic north sea sediments to topography in southern norway the interplay between tectonics and climate abstract about 482 000 km 3 of sediment ca 24 m ma accumulated in the north sea during the cenozoic early cenozoic sedimentation was likely due to uplift of the circum north atlantic landmasses related to continental break up kilometre scale transient uplift and in some areas permanent uplift generated sources for progradational influx of clastic sediments from scotland the shetland platform and to a lesser degree southwestern norway the eocene sedimentation pattern was similar to the palaeocene with lower rates of accumulation associated with flooding and tectonic quiescence sediment influx from the shetland platform continued throughout the cenozoic while supply from southern norway increased markedly around the eocene oligocene coeval with the greenhouse icehouse transition mass balance calculations of sediment and eroded rock volumes suggest that while some topography along the western margin of norway may be pre cenozoic significant uplift of the main paleic surface in southern norway occurred around the early oligocene sedimentation rates were almost ten fold higher than the cenozoic average in the plio pleistocene slightly higher than the global average mass balance calculations indicate that plio pleistocene erosion over deepened a pre existing topography

madagascar a continental fragment of the paleo super dharwar craton of india the morphological kinship of madagascar to its immediate neighbors on the west african continent and east indian subcontinent during the early and middle cretaceous has been debated for the past two decades on the basis of available geologic tectonic and paleomagnetic information most of the paleoreconstructions of madagascar have shown its attachment to the east african continent we present magnetic satelite and gravity data and morphological geophysical and geotectonic similarities to hypothesize that in the period before the breakup of gondwana madagascar was a continental fragment of the paleo super dharwar craton of india

the basement of the deccan traps and its madagascar connection constraints from xenoliths paleogeographic reconstructions of india and madagascar before their late cretaceous rifting juxtapose the antongil block of madagascar against the deccan traps of india indicating that the western dharwar craton extends below the deccan lavas some recent studies have suggested that the south maharashtra shear zone along the northern konkan coast of india limits the northern extent of the western dharwar craton implying that the craton does not extend below the deccan traps raising a question mark on paleogeographic reconstructions of india and madagascar the continuity of the western dharwar craton north of the south maharashtra shear zone below the deccan traps or its lack thereof is critical for validating tectonic models correlating madagascar with india in this study zircons in tonalitic basement xenoliths hosted in deccan trap dykes were dated in situ using the u pb isotope system the data furnish u pb ages that define three populations at 2527 6 2456 6 and 2379 9 ma the 2527 6 ma ages correspond to the igneous crystallization of the tonalites whereas the 2456 6 and 2379 9 ma ages date metamorphic overprints the results help to establish for the first time that the basement is a part of the neoarchean granitoid suite of the western dharwar craton which extends northward up to at least talvade in central and kihim beach in the western deccan by implication the south maharashtra shear zone cannot be the northern limit of the western dharwar craton the granitoids are correlated with the neoarchean felsic intrusions 2 57 2 49 of the masaola suite in the antongil block of madagascar supporting the existence of a neoarchean greater dharwar craton comprising the western dharwar craton and the antongil masora block

depositional environments of campanian maastrichtian successions in the kirthar fold belt southwest pakistan tectonic influences on late cretaceous sedimentation across the indian passive margin abstract the creation of a complex submarine morphology on the passive margin of the indo pakistan continental plate during later stages of its northward drift just prior to collision with eurasia is clearly demonstrated by the nature architecture and regional distribution of sandstone bodies within the campanian maastrichtian successions mughal kot and pab formations exposed within the north south trending kirthar fold belt of southwest pakistan these successions range in thickness from 7 m to more than 500 m and are dominated by fine to coarse sometimes pebbly thin to thick bedded sandstones with subordinate mudstones and marls all of which were deposited on the north facing passive margin of the indo pakistan plate twelve facies have been identified in these sequences and these have been grouped into nine facies associations which were formed within two broadly coeval depositional systems that occupied the contiguous southern kirthar and central kirthar sub basins it is argued here that these sub basins and their fill sequences developed mainly in response to major tectonically induced changes in both the morphology of this sector of the palaeo margin and in the sediment supply systems the more proximal sequences in the east and southeast sectors of the southern kirthar sub basin are dominated by fluviodeltaic deposits while more distal areas to the northwest are characterized by turbidite sandstone bodies formed in channel levee and lobe complexes within slope and basin floor settings of a relatively confined deep marine trough in this southern kirthar sub basin the mughal kot formation comprises basin floor lobes submarine channel fill sand bodies and base of slope mud rich lobes whereas the succeeding pab formation reflects overall progradation of the system being dominated by submarine slope fan lobes channels and levee deposits the central kirthar sub basin displays a broadly east to west transition from shoreface inner shelf through shelfal delta lobes middle shelf and deeper shelf ramp outer shelf facies associations all formed on a gently inclined storm and flood dominated clastic ramp synsedimentary normal faults observed in this study were controlled the depositional architecture and creation of closed deep marine southern kirthar sub basin during late cretaceous

role of flexure in earthquake triggering along the western ghat escarpment india the occurrence of earthquakes near reservoirs in the region of the western ghat escarpment far exceeds the reported rate in other areas of peninsular india reservoir loading and induced pore pressure are generally thought to be triggering earthquakes but in order to explain the anomalous rates of occurrence in the western ghat we invoke the additional effects of elastic plate flexure the height of the escarpment is usually considered to be maintained by flexure in the east west direction intense erosion and sediment loading further control its evolution we propose that this promotes failure at shallow depth by a reduction of the normal stress on escarpment parallel sub vertical planes in the region the same mechanism has been proposed to operate globally in several other escarpment regions along passive margins

gravity anomalies crustal structure and flexure of the lithosphere at the baltimore canyon trough seismic and gravity anomaly data have been used to examine the long term mechanical properties of the lithosphere beneath the baltimore canyon trough a 15 km deep sedimentary basin in the rifted u s atlantic margin seismic data constrain the crustal structure at the time of rifting as well as the amount of sedimentation and erosion that has occurred the gravity effect of the initial crustal structure sedimentary loading and erosional unloading have been computed and compared to the observed free air gravity anomaly the best fitting model is one in which the elastic thickness of the lithosphere te changes at the east coast magnetic anomaly landward of the anomaly te is significantly smaller than previously reported values for continental or oceanic lithosphere of the same thermal age seaward of the anomaly however te follows the depth to the 450 c oceanic isotherm which also describes the response of oceanic lithosphere to seamount and oceanic island loads these results are interpreted as indicating that the shelf part of the trough is underlain by continental lithosphere which following rifting did not acquire any significant long term strength

correlations between silicic volcanic rocks of the st mary s islands southwestern india and eastern madagascar implications for late cretaceous india madagascar reconstructions abstract the st mary9s islands southwestern india expose silicic volcanic and sub volcanic rocks rhyolites and granophyric dacites emplaced contemporaneously with the cretaceous igneous province of madagascar roughly 88 90 ma ago the st mary9s islands rocks have phenocrysts of plagioclase clinopyroxene orthopyroxene and opaque oxide moderate enrichment in the incompatible elements e g zr 580 720 ppm nb 43 53 ppm la yb n 6 9 7 2 relatively low initial 87 sr 86 sr 0 7052 0 7055 and near chondritic initial 143 nd 144 nd 0 51248 0 51249 they have mineral chemical whole rock chemical and isotopic compositions very close to those of rhyolites exposed between vatomandry ilaka and mananjary in eastern madagascar and are distinctly different from rhyolites from other sectors of the madagascan province we therefore postulate that the st mary9s and the vatomandry ilaka mananjary silicic rock outcrops were adjacent before the late cretaceous rifting that split madagascar from india if so they provide a valuable tool to check and aid traditional cretaceous india madagascar reconstructions based on palaeomagnetism matching precambrian geological features and geometric fitting of continental shelves supplementary material mineral analyses mass balance calculations and locality information are available at http www geolsoc org uk sup18332

backstacking apatite fission track stratigraphy a method for resolving the erosional and isostatic rebound components of tectonic uplift histories measuring tectonic uplift of the mean surface of earth using thermochronological data to document uplift histories is often plagued by confusion arising from the independent nature of the thermal and sea level frames of reference the technique of backstacking apatite fission track stratigraphy differentiates between vertical movement of rocks toward earth9s surface due to erosional or tectonic denudation and tectonic uplift of earth9s surface apatite fission track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists in addition to providing information about the timing magnitude and geographic distribution of erosion

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

admittance signatures of rifted and transform margins examples from eastern canada examination of the gravity and bathymetry data across the rifted scotian margin and the sheared grand banks margin off the canadian east coast using a spectral analysis or admittance approach yielded different results the admittance for the scotian margin best fitted a regional compensation model in contrast a local compensation mechanism reproduced the transfer function for the grand banks margin the results for the longer wavelengths were verified using satellite altimetry data the effect of a hidden or subsurface load was investigated for both margins and this was also found to differ the differences in the compensation mechanism between rifted and sheared margins was attributed to their formation mechanisms

evolution of the continental margin of western india new evidence from apatite fission track dating apatite fission track dating in southwestern india yields middle triassic to middle jurassic ages that correlate with the breakup of eastern gondwanaland and africa the later india madagascar split is not recorded in the apatite ages the apatite ages are not a reflection of thermal events in the present location of this region but are associated with uplift and erosion in the upper paleozoic early mesozoic eastern gondwana africa separation track length measurements indicate that cooling through the 100 c isotherm began in the upper paleozoic time temperature modeling suggests that a period of slow cooling during the mesozoic was followed by relatively rapid cooling to the present

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

structure mechanical properties and evolution of the lithosphere below the northwest continental margin of india the continental breakup history at the northwest continental margin of india remained conjectural due to lack of clearly discernable magnetic anomaly identifications and the presence of several enigmatic structural basement features whose structure was partly obscured by the late cretaceous deccan magmatic event in this study a detailed analysis of the existing seismic and seismological data covering both onshore and offshore areas of the northwest indian margin along with 3 d 2 d constrained potential field gravity magnetic and geoid modeling has been carried out the crustal structure and lithosphere asthenosphere boundary lab delineated across the margin provided valuable insights on the mechanism of continental extension an analysis of the residual geoid anomaly degree 10 map and the modeled lab below deccan volcanic province dvp revealed significant variation in upper mantle characteristics between the northwest nw and south central sc parts of dvp having thinner lithosphere in the nw part the depth to lab ranges 80 130 km at the margin with gradual thinning towards the western offshore having sharp gradient in the south sc part of dvp and gentle gradient in the north nw part of dvp the moho configuration obtained from seismically constrained 3 d gravity inversion reveals that moho depths vary 34 42 km below dvp and gradually thins to 16 20 km in the western offshore the effective elastic thickness te map computed through 3 d flexural modeling indicates that the te values are in general lower in the region and range 12 25 km such lower te values could be ascribed to the combined effect of the lithosphere stretching during gondwana fragmentation in the mesozoic and subsequent thermal influence of the reunion plume based on the crustal stretching factors te estimates and the modeled lithosphere geometry at the margin in this study we propose that the lithosphere below laxmi gop basin region 3 0 had undergone continuous stretching since india madagascar rifting 88 ma much prior to this event however this continuous stretching did not lead to breakup due to syn rift cooling the developed necking zone brittle ductile deformation got ceased and led to the development of a new necking zone between seychelles and laxmi ridge subsequent stretching between seychelles and the laxmi ridge contemporaneous with the deccan flood basalts eruption led to the seafloor spreading in the western basin anomaly c28n thus the laxmi ridge became a continental sliver

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

interpretation of the lithospheric structure beneath the indian ocean from gravity gradient data abstract global geoidal undulations are often used as the constraining parameter to model the mantle convection pattern while gravity data are typically applied to interpret the lithospheric structure in more recent studies especially after launching the gravity field and steady state ocean circulation explorer goce satellite gravity gradiometry mission gravity gradient measurements have also been used in various geoscience applications the main motivation of using gravity gradients is to provide a more conclusive information about a spatial localization of investigated phenomena in geophysical studies such implications are particularly related to a probably better detection of investigated lithospheric structures such as active tectonic margins in this study we address this aspect by comparing interpretational characteristics from gravity and gravity gradient information the bouguer and mantle gravity gradients are used for this purpose to interpret the lithospheric structure beneath the indian ocean as well as the adjacent continental lithospheric structures such as the east african rift system our results confirm that gravity gradient more closely reproduces some tectonic and volcanic features such as oceanic subductions volcanic island arcs and convergent continental tectonic margins similarly mid oceanic spreading ridges are localized slightly better in the gravity gradient map our findings also indicate that the interpretational quality cannot be improved by using directly observed gravity gradients the reason is that these interpretations require applying gravimetric forward modelling techniques in order to enhance a particular lithospheric structure

shoulder uplift of the western ghats passive margin india a denudational model the paper presents a denudational rift flank uplift model of the western ghats of karnataka india the cenozoic denudation of the deccan plateau was constrained by relative dating of regional planation surface levels in combination with preliminary apatite fission track results the denudational history of the western ghats escarpment coastal foreland was constrained both by onshore fission track and offshore sediment data methods are briefly described results were used as reference data to elaborate a computer simulation model of both flexural rift flank upwarp and escarpment retreat from the k t boundary to the present depending on the assumptions concerning the attributes of the thin elastic lithospheric sheet infinite or semi infinite the shape and elevation of the initial deccan topography flat lying or affected by lithospheric updoming due to the reunion mantle plume and the position of the continental divide upon rifting the flexural response to denudational unloading is shown to yield either a concave up flexure of the dharwar craton or a convex monoclinal downwarp of the margin the relative merits of each model are discussed in the light of auxiliary field evidence particularly from the neighbouring deccan traps however the diverse assumptions on boundary conditions fail to entirely avoid geomorphological equifinality 1998 john wiley sons ltd

hydrothermal oxidation in the biwabik iron formation mn usa precambrian iron formations throughout the world notably in australia brazil and south africa show evidence of hypogene 110 c mostly 250 c oxidation alteration and silica dissolution as a result of tectonic or magmatic activity although hydrothermal oxidation has been proposed for the prototype lake superior type iron formation the biwabik iron formation in minnesota usa it has not been documented there by examining oxidized and unoxidized biwabik iron formation in three mines including material from high angle faults that are associated with oxidation we document an early hypogene oxidation event 175 c involving medium salinity aqueous fluids 8 4 4 9 wt nacl equiv that infiltrated iron formation along high angle faults at the hibbing taconite mine hydrothermal fluids oxidized iron carbonates and silicates near faults producing goethite quartz in contrast with much of the oxidized iron ores on the mesabi range silica was not removed but rather recrystallized during this event perhaps lying in a rock dominated system at low cumulative fluid flux during the hydrothermal oxidation event in the hibbing taconite deposit quartz filled microfractures and irregular inclusions commonly formed in coarse variably oxidized magnetite currently the ore mineral these inclusions degrade the ore by introducing excess silica in magnetic concentrate hydrothermal oxidation at hibbing taconite mine is overprinted by later relatively minor supergene oxidation both along faults and near the surface which locally dissolved quartz at the fayal reserve mine widespread silicate and carbonate gangue dissolution and iron oxidation was followed by precipitation of pyrite mn siderite apatite and other minerals in void spaces which prevented post oxidation compaction and significant volume loss in the sampled rocks although definitive temperature data for this assemblage are needed the weight of evidence indicates that this mineralization is hypogene the association of oxidation with epithermal conditions constrains the oxidation and subsequent mineralization to have taken place during the precambrian the only time when these rocks would have experienced the necessary temperatures the mineralization at fayal reserve shows little supergene overprint pyrite is largely unoxidized hydrothermal oxidation in both mines was likely produced by basinal fluids that were expelled during the 1 83 1 87 ga penokean orogeny and mixing with meteoric fluids along faults although a 1 1 ga rift related fluid flow event is also possible later supergene overprinting of the iron formation was minor

madagascar s climate at the k p boundary and its impact on the island s biotic suite abstract the k p boundary 66 ma was a critical time in the history of the planet s biota on the island of madagascar few late cretaceous species survived the associated extinction and distance from the various mainland sources then acted as a strong filter for the number of over water arrivees reconstructing the climate of the early paleogene is vital for understanding the environments to which new colonizers landed and established a toe hold on the island beginning with a global air sea coupled climate model simulation using the land sea distribution at 66 ma we used dynamical downscaling to construct a scenario for how the atmospheric and oceanic fluid envelopes washed over and around the madagascar at the start of the paleocene dynamical downscaling of the global model yields a climate model with much finer resolution we used this method to reconstruct the habitats for known fossil localities from that general period better understand the ecological diversity of plants and animals that have been hypothesized to have then been present and evaluate hypotheses regarding the evolutionary history of lemurs a vast clade of primates endemic to madagascar whose last common ancestor dates to this time our results show an island with a climate notably different from that of today but not nearly as arid as others have suggested the spiny thicket biome did not exist temperatures were lower rainfall less seasonal

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

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

effective elastic thickness along the conjugate passive margins of india madagascar and antarctica a re evaluation using the hermite multitaper bouguer coherence application abstract gondwana correlation studies had rationally positioned the western continental margin of india wcmi against the eastern continental margin of madagascar ecmm and the eastern continental margin of india ecmi against the eastern antarctica continental margin eacm this contribution computes the effective elastic thickness te of the lithospheres of these once conjugated continental margins using the multitaper bouguer coherence method the results reveal significantly low strength values te 2 km in the central segment of the wcmi that correlate with consistently low te values 2 3 km obtained throughout the entire marginal length of the ecmm this result is consistent with the previous te estimates of these margins and confirms the idea that the low te segments in the central part of the wcmi and along the ecmm represents paleo rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the marion hotspot and lithospheric extension during the rifting the uniformly low te value 2 km along the eacm indicates a mechanically weak lithospheric margin probably due to considerable stretching of the lithosphere considering the fact that this margin remained almost stationary throughout its rift history in contrast the ecmi has comparatively high te variations 5 11 km that lack any correlation with the regional tectonic setting using gravity forward and inversion applications we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere the study concludes that the thick pile of the bengal fan sediments in the ecmi masks and has erased the signal of the original load induced topography and its gravity effect has biased the long wavelength part of the observed gravity signal the hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling which likely accounts a relatively high te estimate

tertiary palaeosurfaces of the sw deccan western india implications for passive margin uplift two genetically distinct lateritized palaeosurfaces of different ages are recognized in the southwest deccan traps region of western india using a combination of geochemical topographical and satellite image data the deccan traps were erupted at the cretaceous tertiary boundary c 65 ma and comprise a huge area of originally near horizontal basalt lavas covering much of northwest peninsular india and topographically forming the coast parallel western ghats escarpment and elevated maharashtra plateau to the east remnants of the older palaeosurface currently exist as a series of isolated laterite capped plateaux forming the highest elevations along the western ghats 15 30 18 15 n this surface is of late cretaceous early tertiary age and originally developed upon flows which lay at or near to the top of the lava sequence this lateritization phase was terminated by a period of uplift and extensive erosion in lower to mid tertiary times during which the low lying low relief coastal konkan plain developed through the eastward recession of the ghats scarpline a second phase of lateritisation occurred upon this coastal pediplain during mid to late tertiary times since the earlier uplift had gently deformed the lava pile prior to the development of the pedimented surface the low level konkian laterite lies with marked angular unconformity upon the lava stratigraphy both surfaces have been subject to further large scale distortion resulting from continuing uplift effects development and evolution of these deccan palaeosurfaces is important since together they provide a record of uplift effects in western india moreover they offer a datum against which the uplift erosional history may be further constrained and demonstrate that uplift effects have acted upon the indian margin throughout the tertiary since such longevity of uplift is difficult to reconcile with the commonly cited thermal and dynamic post rift mechanisms known to act upon passive margins the morphological and structural evolution of the rifted deccan margin is better described in terms of denudational isostasy

crustal underplating and effective elastic plate thickness of the laxmi ridge northern arabian sea abstract laxmi ridge a prominent basement high feature with negative gravity anomaly in the northern arabian sea is generally considered as stretched continental crust however recently it is proposed that the ridge could be an extinct spreading ridge ship borne and satellite altimetry derived bathymetry gravity and geoid data over the laxmi ridge are analysed and modelled to determine effective elastic plate thickness te and crustal structure beneath the ridge the estimation of te and compensation mechanism of the ridge are examined based on 3d coherence and admittance analysis considering surface and subsurface loads which involve computation of coherence and admittance between mantle bouguer anomaly mba and sediment corrected bathymetry the best fit model reveals that te is about 3 4 km and amplitude of subsurface load at moho is approximately equal to the surface load low value of te implies elastically weak lithospheric plate which can be attributed to either weakening of lithosphere by re heating and underplating caused by the then nearby reunion hotspot or volcanic emplacement on the young oceanic crust further all along the length of the laxmi ridge the mba is negative and its magnitude reaches up to 115 mgal which indicates either presence of thickened crust or less dense mantle beneath the ridge 2d crustal models suggest that underplating material is prevalent all along the laxmi ridge high amplitude 400 nt magnetic anomaly over the southern part of the ridge reveals several lineations in wnw ese direction suggesting magma outpouring along week zones geoid to topographic ratio over the ridge is negative 1 98 m km possibly indicating shallow subsurface load

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