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

magnetic basement in the bay of bengal through werner deconvolution abstract marine magnetic data from 16 well distributed profiles over bay of bengal were analysed to delineate the magnetic basement which in general matches with the sediment thickness pattern a series of high and low reliefs ride over a basement level of 11 12 km the high associated with 85 e ridge extends northward with an obliteration around 13 n parallel isolated depressions in this region and a nw se trending high between 11 and 15 n latitudes and 86 and 89 e longitudes are some of the features observed in the basement the basement obtained from available seismic data across 13 latitude shows a good agreement with that derived from the present analysis the deep basement in the northeast conforms to the high sediment thickness the undulations in the basement might be due to the stresses acting on the ocean floor at the time of break up of indian plate from gondwanaland

timing of intraplate deformation northeastern indian ocean abstract seismic stratigraphy and dsdp and odp drilling have demonstrated that two unconformities of early eocene and latest miocene age can be traced over much of the northeastern indian ocean the eocene event was a major hiatus in sedimentation following collision of india with asia and was the time of termination of deposition of a continental rise off the east side of india and the initiation of proximal bengal fan deposition the miocene event was the onset of north south compression in the indian plate during rapid uplift in tibet and the himalayas

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka

structural and tectonic interpretation of geophysical data along the eastern continental margin of india with special reference to the deep water petroliferous basins abstract the study area encompasses the eastern continental margin of india ecmi and the adjoining deep water areas of bay of bengal the region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the indian shield detailed analysis of total field magnetic and satellite derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features basement structure and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ecmi interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general the ene wsw trending faults dominate the structural configuration at the margin these maps also exhibit a clear density transition from the region of attenuated continental crust proto oceanic crust to oceanic crust based on which the continent ocean boundary cob has been demarcated along the margin basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the bengal fan in the north and reveal horst graben features related to rifting a comparison of basement depths derived from seismic data indicates that in general the basement trends and depths are comparable in cauvery and krishna godavari basins whereas in the mahanadi basin basement structure over the 85 e ridge is clearly revealed in seismic data further eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern rift geometries and depositional trends related to canyon fills and channel levee systems and provide a basic framework for future petroleum in this under explored frontier

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

exceptional preservation of angiosperm markers in miocene and eocene ambers the traditional organic geochemical view suggests that polar triterpenoids are degraded easily during fossilization of organic matter for the first time we show that dammarane type tetracyclic polar triterpenoids are preserved in the geosphere hydroxydammarenone 20 hydroxy 24 dammaren 3 one a typical constituent of extant dipterocarpaceae an angiosperm tree family derived dammar resin is preserved in miocene amber the oxidative products of hydroxydammarenone i e 20 24 epoxy 25 hydroxydammaran 3 one and hexakisnor dammaran 3 20 dione are found in both miocene and eocene ambers the presence of hydroxydammarenone in miocene amber clearly depicts that natural biotriterpenoid markers can be preserved in the geosphere for tens of millions of years this result demonstrates that the dammarane type oxygenated compounds which are extensively used in archaeological studies can also be utilized as diagnostic markers for paleochemosystematics

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

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

crustal evolution and sedimentation history of the bay of bengal since the cretaceous multichannel seismic reflection gravity magnetic and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of india to the andaman shelf and four nw se profiles in the western basin off madras seismics on three latitudinal profiles in the bay of bengal the trend of the fracture zones the locations of the magnetic chron 34 and the cretaceous magnetic quiet zone suggest that greater india separated from antarctica after a period of transform motion in the early cretaceous that is about polarity chron m0 120 ma and drifted northwestward negative gravity anomalies are associated with basement rises including the 85 e ridge between the continental margin of india and the ninetyeast ridge magnetic reversals and northward trend of the 85 e ridge support a hotspot origin of the ridge and its emplacement most likely after the cretaceous quiet period juxtaposition of high amplitude hyperbolic reflections down faulted continental blocks buried under thick sediments and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope about 80 km seaward of the present continental shelf edge eight seismic sequences as thick as 8 5 km overlie the early cretaceous oceanic basement and include four unconformities lower eocene upper oligocene upper miocene and upper pleistocene which have been correlated to the major geologic tectonic events we also interpret the late cretaceous early tertiary features on the eastern flank of the buried 85 e ridge as carbonate reefs the observation of steep subduction of older cold indian plate beneath the burmese plate near the andaman islands suggests the sunda arc in this region as low to intermediate stress subduction zone

gravity field and deep structure of the bengal fan and its surrounding continental margins northeast indian ocean abstract a revised gravity anomaly map for the northeast indian ocean shows that the shelf edge underlying the eastern continental margin of india is a rather narrow but extensively linear gravity low minimum free air 149 mgal the bengal fan seaward of the shelf has a depressed gravity field average free air 20 to 30 mgal in spite of the enormous thickness of sediments of as much as 10 15 km the two buried ridges below the bengal fan the 85 east and 90 east ridges have a large negative 75 mgal and a substantial positive 40 mgal free air anomaly respectively the andaman and burmese arcs lying along the east margin of the bengal fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mgal gravity interpretation for three regional traverses across the central and northern parts of the bengal fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of india the 85 east ridge that was created when the indian ocean lithosphere was very juvenile appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere while the 90 east ridge submarine topography buried load below the bengal fan is probably isostatically compensated by a low density mass acting as a cushion at the base of the crust the bengal fan crust with its thick sediment layer is carried down the andaman subduction zone to a depth of about 27 km where possibly phase transition takes place under higher pressure the maximum sediment thickness at the andaman burmese subduction zone is of the order of 10 12 km the gravity model predicts a low density zone about 60 km wide below the andaman burmese volcanic arc penetrating from crustal to subcrustal depths in the overriding burma plate a more complex density distribution is however envisaged for the andaman volcanic arc that is split by the neogene back arc spreading ridge the ocean continent crustal transition possibly occurs farther east of the volcanic arc below the shan plateau margin in burma or below the mergui terrace at the malayan continental margin east of the andaman sea

geophysical characteristics of the ninetyeast ridge andaman island arc trench convergent zone abstract the convergence tectonics of the ninetyeast ridge ner upon the andaman island arc trench system is examined through an analysis of etopo2 bathymetry satellite derived free air gravity and seismic data oblique subduction and the buoyancy forces arising from subduction of the ner render the subduction processes near the andaman arc highly complex the bathymetric expression of the ner is visible up to lat 10 n but seismic reflection data indicate that it extends up to about lat 17 n the gravity anomalies are strongly positive over the exposed segment of the ridge but are subdued over the buried portion there is a prominent break in the continuity of the trench gravity low where the ner seems to impinge upon the island arc further a strong curvilinear belt of negative anomalies just behind and running parallel to the island arc associated with the forearc basin is a dominant feature of the gravity map an offset in the continuity of this strong negative anomaly occurs at about the same latitude where the ner seems to be converging upon the island arc seismic reflection data indicate that the ner is very close to the trench flexural modeling of the gravity anomalies for the subducting indian ocean lithosphere loaded by sediments and the ner indicates that the ner is at the starting phase of its collision with the island arc and may not have started affecting the subduction process itself we infer that the en echelon block structure of the ner in the proximity of the convergent zone is a consequence of complex strike slip and subduction related tectonic forces

crustal architecture and deep structure of the ninetyeast ridge hotspot trail from active source ocean bottom seismology a 550 km long transect across the ninetyeast ridge a major indian ocean hotspot trail provided seismic refraction and wide angle reflection data recorded on 60 ocean bottom instruments about 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track two approaches have been chosen i a first arrival tomographic inversion yielding crustal properties and ii forward modelling of mantle phases revealing the structure at the crust mantle boundary region and of the uppermost mantle away from the volcanic edifice seismic recordings show the typical phases from oceanic crust that is two crustal refraction branches pg a wide angle reflection from the crust mantle boundary pmp and a wave group turning within the upper mantle pn approaching the edifice three additional phases have been detected we interpret these arrivals as a wide angle reflection from the base of material trapped under the pre hotspot crust pm2p and as a wide angle reflection pnp and its associated refraction branch pn from a layered upper mantle the resulting models indicate normal oceanic crust to the west and east of the edifice crustal thickness averages 6 5 7 km wide angle reflections from both the pre hotspot and the post hotspot crust mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere and hotspot volcanism has underplated the pre existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks 7 5 7 6 km s 1 in total the crust is up to 24 km thick the ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0 7 an important observation is that underplating continued to the east under the wharton basin during the shield building phase however ninetyeast ridge was located adjacent to the broken ridge and was subsequently pulled apart along a transform fault boundary therefore underplating eastwards of the fracture zone separating the edifice from the wharton basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield building stage this fact however requires mantle flow along the fossil hotspot trail the occurrence of pnp and pn arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere

structure and evolution of the afanasy nikitin seamount buried hills and 85 e ridge in the northeastern indian ocean abstract geophysical data of the afanasy nikitin seamount ans partly buried hills and 85 e ridge in the northeastern indian ocean were studied together with published seismic refraction results to understand genesis and evolution of the structures the ans joins the 85 e ridge through isolated buried hills and intervening subsurface structures and together they form a linear ridge system the gravity signature of the ridge system changes from negative to positive towards south of 5 n which seems to coincide with termination thinning of pre collision continental sediments in the bay of bengal thick pile of bengal fan sediments had great impact on underneath pre collision sediments as well as on basement rocks the process resulted in attaining higher velocities up to 6 6 and 7 1 km s respectively gravity model studies suggest that structures of the ridge system are compensated in different modes the ans is underlain by an 8 km thick deep crustal body of magmatic rocks while beneath other structures oceanic crust is down flexed up to 2 5 km the presence of metasediments more dense than volcanic rocks and flexure of the lithosphere would explain the negative gravity anomaly over the 85 e ridge whereas lack of metasediments and magmatic rocks at depth would explain the compensated positive anomaly over the ans the width of the 85 e ridge the wavelength 190 km and amplitude 2 5 km of the flexed oceanic crust and the intersection of the ridge with the mesozoic fracture zones suggest that the ridge was formed in intraplate position when the lithosphere underneath was approximately 35 myr old initial emplacement of the ans was coeval with the formation of oceanic lithosphere at 80 73 ma the hotspot forming the 85 e ridge had reactivated the ans during the paleocene and brought it to the sea surface then it underwent erosion and subsidence processes the deformation activity had converged the north and south parts of the ans at late miocene and again at late pleistocene and northward buried hills at early pliocene

finite extension across the south vietnam basins from 3d gravimetric modelling relation to south china sea kinematics abstract we derive a crustal thickness map over the south vietnam basins from a 3d inversion of gravity data that takes into account the sedimentary infill and its density variation with depth this map is used to estimate the total amount of stretching across these basins that varies from 190 km to the east at 111 e to 30 km to the west at 107 e comparison with south china sea kinematics amount and timing of oceanic opening implies large decoupling with respect to the indochina block and supports the hypothesis of formation of the south china sea in upper oligocene to lower miocene by southward subduction of the proto south china sea

deep sub crustal features in the bengal basin seismic signatures for plume activity the basaltic rajmahal traps 117 ma india are believed to be due to a mantle plume the plausible plume path on the continental side has not been traced probably because of the thick sedimentary cover these traps are located close to the bengal basin therefore a re examination of the deep seismic velocity structure should help in tracing the plume path on the continental side and its possible relationship to plume activity data obtained along four seismic profiles in the bengal basin were re examined which helped in identifying and modeling strong reflections from lower crust and moho the results indicate a 7 5 km s velocity zone as an underplated mantle material injected by the mantle plume or hotspot at the base of the continental crust the study also indicates the probable trace of plume in the continental region a nnw sse trending path east of 87 e with an up warp in moho

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

mesozoic anomalies in the bay of bengal abstract the analysis of 8200 line km of total magnetic intensity data in the bay of bengal northeastern indian ocean revealed the presence of approximately n30 e trending seafloor spreading type magnetic anomalies these anomalies resemble the mesozoic anomaly series m11 m0 reported elsewhere the oldest anomaly m11 132 5 ma identified close to the east coast of india is followed by the younger series of mesozoic anomalies towards the offshore some of the anomalies are offset by 60 80 km the configuration of the offsets of the isochrons allowed us to propose approximately n120 e trending oceanic fracture zones the mesozoic crust of 132 5 118 ma is estimated to evolve with an average half spreading rate of 3 5 cm yr except for the ocean floor between the m9 and m4 magnetic isochrons this part of the crust appears to be affected by the buried subsurface 85 e ridge

evidence for unusually thin oceanic crust and strong mantle beneath the amazon fan we used seismic and gravity data to determine the structure of the crust and mantle beneath the amazon fan seismic data suggest that the crust is of oceanic type and is unusually thin 4 km compared to elsewhere in the atlantic we attribute the thin crust to ultraslow seafloor spreading following the breakup of south america and africa during the early cretaceous gravity data suggest that the fan was emplaced on lithosphere that in creased its elastic thickness te and hence strength following rift ing the increase from 10 km to 40 km is greater however than would be expected if te were determined by a single controlling isotherm based on a cooling plate model hence we conclude that the amazon fan has been emplaced on and is supported by un usually thin oceanic crust and strong mantle

the negative gravity field over the 85 e ridge an isopach map made from seismic reflection and refraction data in the bay of bengal shows two prominent n s trending features in the basement topography one is the northernmost portion of the ninetyeast ridge which is totally buried by sediments north of 10on the other buried ridge trends roughly n s for 1400 km at 85oe to the latitude of sri lanka and then curves toward the west it has basement relief up to 6 km two free air gravity anomaly profiles across the region show a strong gravity low 60 mgal over the 85oe ridge while the ninetyeast ridge shows a gravity high to understand the negative free air gravity anomaly over the 85oe ridge we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load a plausible formation history for a buried ridge consists of at least two major episodes the first is the formation of the ridge on a lithosphere with a flexural rigidity of d i at some later time the ridge is buried by an influx of sediments the lithosphere is cooler and the flexural rigidity has increased to d 2 the character of the gravity field depends primarily upon the initial and final values of flexural rigidity these d1 and d2 values are varied to obtain good agreement between the model and observed gravity anomalies best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial an approximate relationship between flexural rigidity and crustal age shows that the 85oe ridge was formed on relatively young lithosphere 5 15 m y old and that it was buried when the lithosphere was 40 80 m y old

thin oceanic crust below bay of bengal inferred from 3 d gravity interpretation abstract the bay of bengal bob lithosphere evolved due to rifting between india and antarctica and subsequent seafloor spreading process since the early cretaceous is known for the presence of thick bengal fan sediments covering the entire bob with maximum sediment accumulation of 22 km in the offshore bangladesh three dimensional crustal structure of the bob is investigated by integrating the sediment isopach map seismic reflection refraction and wide angle reflection data with the 3 d gravity modelling large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function 3 d gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from 5 mgal over the ninetyeast ridge near the equator to as low as 210 mgal at the apex of bengal fan in the bangladesh shelf region the residual crustal bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6 km uniform crust display crustal thickness variations with depth to moho in the bob region varying from 11 km in the south to as much as 35 km in the bangladesh shelf with moho depressed locally below both 85 e and ninetyeast ridges the total crust map obtained after subtracting the water and sediment layers from the moho depth map shows an unusually thin oceanic crust of 2 4 km in the deep basinal areas of bob and 10 12 km of thick crust below 85 e and ninetyeast ridges due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85 e and ninetyeast ridges we infer its formation during plume spreading interactions in the eastern indian ocean further higher flexural rigidity of lithosphere and cold mantle in the bob region indicates that huge amount of bengal fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the amazon fan

structure of the lower crust beneath the carolina trough u s atlantic continental margin data from three large offset seismic profiles provide information on the crustal structure beneath the carolina trough the profiles obtained by the u s geological survey the naval oceanographic research development agency and the scripps institution of oceanography in 1985 were oriented parallel to the trough and were located 1 seaward of the east coast magnetic anomaly ecma which is generally thought to represent the boundary between oceanic and continental crust 2 along the axis of the trough between the ecma and the hinge zone which is thought to reflect the landward limit of highly stretched and altered transitional crust and 3 along the carolina platform landward of the basement hinge zone on crust thought to have been thinned only slightly during rifting these data constrain the velocity structure of the lower crust and provide evidence for a thick lens of high velocity 7 1 km s lower crustal material that extends beneath the carolina trough and the adjacent ocean basin this lens reaches a maximum thickness of about 13 km beneath the deepest part of the trough thins to about 5 km seaward of the ecma and is either very thin or absent landward of the hinge zone it is interpreted to represent material that was underplated beneath and or intruded into the crust during the late stage of continental rifting and that led to an anomalously thick plutonic layer during the early seafloor spreading phase these data thus support the recent conclusions of white et al 1987b and mutter et al 1988 that the initiation of seafloor spreading is attended in many if not most cases by the generation of an anomalously large volume of melt

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

a proposed super thick sedimentary basin bay of bengal a super thick 22km sedimentary basin under the northern bay of bengal is proposed the hypothesis is based on data from surface wave dispersion seismic refraction sn attenuation and geology we present new high frequency sn data which indicate a cold upper mantle beneath the bay of bengal we propose that the oceanic crust in this region is in fact nearly normal and that the sedimentary section is at least 6 km thicker than previously thought with velocities at the base of the sediments having been increased to near 6 5 km sec by high pressure metamorphism reinterpretation of the refraction data indicate a post india asia collision thickness of over 16 km

gravity interpretation of nonoutcropping sedimentary basins in which the density contrast decreases parabolically with depth the decrease in density contrast of sedimentary rocks with depth in many sedimentary basins can be approximated by a parabolic density function analytical gravity expression of an outcropping two dimensional vertical step along which the density contrast decreases parabolically with depth is derived in the space domain a modification ofbott s 1960 method of gravity interpretation is proposed by considering two outcropping vertical steps on either side of the first and last observation points in addition ton outcropping vertical prisms in order to interpret the gravity anomalies of nonoutcropping basins the thicknesses of the two outcropping vertical steps are made equal to the thicknesses of the two outcropping vertical prisms placed below the first and last observation points the initial depth estimates of the sedimentary basin are calculated by the infinite slab formula ofvisweswara raoet al 1993 the gravity effects of then outcropping prisms and the two outcropping vertical steps are calculated at each anomaly point and the depth to the floor of the basin are adjusted based on the differences between the observed and calculated anomalies a gravity anomaly profile of los angeles basin california is interpreted

gravity anomalies sediment loading and lithospheric flexure associated with the krishna godavari basin eastern continental margin of india abstract gravity anomalies constrained by seismic data have been used to examine the long term mechanical properties of the lithosphere beneath the krishna godavari basin a deltaic sedimentary basin on the eastern continental margin of india the gravity effect of the rift crust sedimentary loading and erosional unloading have been computed and the sum effect of these when compared with the observed gravity anomalies gives a best fitting te of 30 km at the margin the results suggest that the lithosphere which was weak during the early rifting stage has gained strength during sedimentation the onshore dipping doublet gravity anomaly at the margin reflects a weak rifted continental lithosphere abutting a strong oceanic lithosphere comparison with the results of similar investigations for the bengal basin in the north and cauvery basin in the south suggests increasing strength for the lithosphere from north to south

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

climatology of postsunset equatorial spread f over jicamarca 1 we use radar observations from 1996 to 2006 to study the climatology of postsunset equatorial 3 m spread f irregularities over jicamarca during all seasons we show that the spread f onset times do not change with solar flux and that their onset heights which occur near the altitude of the evening f region velocity vortex increase linearly from about 260 to 400 km from solar minimum to solar maximum higher onset heights generally lead to stronger radar echoes during the equinox spread f onset occurs near vertical drift evening reversal times while during the december solstice they occur near the drift reversal times close to solar minimum and near the time of the prereversal velocity peak for high solar flux conditions on average radar plume onset occurs earlier with increasing solar flux in all seasons plume onset and peak altitudes increase with solar activity and the peak heights are generally highest during the equinox the f region upward drift velocities that precede spread f onset increase from solar minimum to solar maximum and are approximately proportional to the maximum prereversal drift peak velocities

sediment volume and mass beneath the bay of bengal rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the himalayas and tibet can provide some insight into tectonics and geological history the objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments sedimentary rock and metasedimentary rock beneath the sea floor of the bay of bengal the sedimentary section in the bay of bengal is divided into two parts 1 eocene through holocene sediments and sedimentary rocks which post date the initial india asia collision volume 12 5 106 km3 mass 2 88 1016 t this is most of the bengal fan including its eastern lobe the nicobar fan plus some of the outer bengal delta 2 early cretaceous through paleocene pre collision sedimentary and metasedimentary rocks volume 4 36 106 km3 mass 1 13 to 1 18 1016 t these are interpreted as continental rise and pelagic deposits

structure and origin of the 85 e ridge the submerged 85 e ridge in the bay of bengal trends approximately n s between 19 n and 6 n latitudes off the southeast coast of sri lanka it takes an arcuate shape and seems to terminate with the northward extension of the afanasy nikitin seamounts situated around 2 s latitude the ridge is characterized by positive magnetic 100 400 nt and negative free air gravity 60 mgal anomalies with variable widths of 100 180 km magnetic model studies revealed that the rocks of the 85 e ridge are magnetized with reversed polarity the well defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin two alternative processes for the ridge emplacement have been suggested ridge emplacement may be 1 due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early cretaceous crust in the bay of bengal more precisely at m0 isochron or during the middle albian reversals within the cretaceous long normal polarity k t superchron epoch when the earth s magnetic polarity changed from normal to reversed polarity and or 2 due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin however more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model further the rajmahal traps normally polarized and the 85 e ridge reversely polarized appear to be associated with two different episodes of eruption that might have been triggered by the kerguelen mantle plume the 85 e ridge seems to extend into the onshore west bengal basin as a subsurface ridge and merges with the reported nne ssw trending zone of strong geophysical anomalies east of rajmahal traps up to 25 n latitude

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

three dimensional crustal structure of the voring margin ne atlantic a combined seismic and gravity image 1 the three dimensional 3 d crustal structure of the voring margin is investigated by integrating results from reflection and wide angle seismic data well log data and 3 d gravity modeling the reduced crustal bouguer anomaly obtained after computing the gravity effects of the water layer and cenozoic and cretaceous sedimentary cover displays an arcuate shaped gravity high that coincides with the cretaceous subbasins the pre cretaceous layer varies in thickness from 7 to 8 km below the voring marginal high to 2 km in the mesozoic depocenters and is overlying a relative high density upper middle crust 2850 kg m 3 underlying the upper middle crust there is a high velocity density body which is imaged along the marginal high and western region of the voring basin the high velocity body shallows to 14 km depth in the western end of the marginal high and thins out toward the eastern and southeastern regions of the basin regions of thick crust 24 km are located in the eastern and southern parts of the basin and northern and central parts of the marginal high whereas thinner crust is observed at the southwest region of the study area 16 km and in the fenris graben region and utgard high 18 km residual gravity anomalies are related to the distribution of interbedded volcanics and to the presence of sedimentary subbasins under the marginal high crustal thinning is not directly related to thickening of the hvb and or the location of cretaceous depocentres indicating a major rejuvenation of the cretaceous moho related to the maastritchian paleocene rifting episode and massive emplacement of igneous material during breakup

tectonics of the bay of bengal new insights from satellite gravity and ship borne geophysical data abstract recently released satellite derived free air gravity anomalies and the existing ship board geophysical data provide new insights into the tectonics of the bay of bengal with respect to the structure and regional extension of the buried 85 e ridge and the tectonics of the eastern continental margin of india ecmi the 85 e ridge can be visualized extending inland via the mahanadi basin volcanics to the rajmahal traps a large volcanic province in eastern india encompassing the rajmahal and sylhet traps and volcanics in the bengal and mahanadi basins almost on the scale of the deccan volcanic province along the west coast can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 ma the 85 e ridge represents the deep ocean volcanic trace of this magmatic activity towards the south the ridge continues in an arcuate manner to the afanasy nikitin seamount at equatorial latitudes in the central indian ocean gravity models of the ridge are indicative of hotspot related crustal underplating processes beneath the ridge the ecmi can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data which bear similarities with the conjugate east antarctica margin

an overview of the sedimentary geology of the bengal basin in relation to the regional tectonic framework and basin fill history abstract the bengal basin in the northeastern part of indian subcontinent between the indian shield and indo burman ranges comprises three geo tectonic provinces 1 the stable shelf 2 the central deep basin extending from the sylhet trough in the northeast towards the hatia trough in the south and 3 the chittagong tripura fold belt due to location of the basin at the juncture of three interacting plates viz the indian burma and tibetan eurasian plates the basin fill history of these geo tectonic provinces varied considerably precambrian metasediments and permian carboniferous rocks have been encountered only in drill holes in the stable shelf province after precambrian peneplanation of the indian shield sedimentation in the bengal basin started in isolated graben controlled basins on the basement with the breakup of gondwanaland in the jurassic and cretaceous and northward movement of the indian plate the basin started downwarping in the early cretaceous and sedimentation started on the stable shelf and deep basin and since then sedimentation has been continuous for most of the basin subsidence of the basin can be attributed to differential adjustments of the crust collision with the various elements of south asia and uplift of the eastern himalayas and the indo burman ranges movements along several well established faults were initiated following the breakup of gondwanaland and during downwarping in the cretaceous by eocene because of a major marine transgression the stable shelf came under a carbonate regime whereas the deep basinal area was dominated by deep water sedimentation a major switch in sedimentation pattern over the bengal basin occurred during the middle eocene to early miocene as a result of collision of india with the burma and tibetan blocks the influx of clastic sediment into the basin from the himalayas to the north and the indo burman ranges to the east rapidly increased at this time and this was followed by an increase in the rate of subsidence of the basin at this stage deep marine sedimentation dominated in the deep basinal part while deep to shallow marine conditions prevailed in the eastern part of the basin by middle miocene with continuing collision events between the plates and uplift in the himalayas and indo burman ranges a huge influx of clastic sediments came into the basin from the northeast and east throughout the miocene the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin from pliocene onwards large amounts of sediment were filling the bengal basin from the west and northwest and major delta building processes continued to develop the present day delta morphology since the cretaceous architecture of the bengal basin has been changing due to the collision pattern and movements of the major plates in the region however three notable changes in basin configuration can be recognized that occurred during early eocene middle miocene and plio pleistocene times when both the paleogeographic settings and source areas changed the present basin configuration with the ganges brahmaputra delta system on the north and the bengal deep sea fan on the south was established during the later part of pliocene and pleistocene and delta progradation since then has been strongly affected by orogeny in the eastern himalayas pleistocene glacial activities in the north accompanied sea level changes in the bay of bengal

seafloor spreading magnetic anomalies south off sri lanka abstract results obtained from compilation and reinterpretation of about 21 200 line km of bathymetry magnetic and satellite gravity data between 10 s to 10 n latitudes and 75 to 90 e longitudes south off sri lanka are presented here magnetic data and the synthetic seafloor spreading model reveal the presence of mesozoic anomaly sequence m11 through m0 south of sri lanka the oldest magnetic anomaly m11 134 ma occurs between 110 and 140 km away from the sri lankan coast the seafloor created during the early cretaceous is estimated to have evolved with variable half spreading rates ranging from 5 5 to 1 53 cm yr the trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic the cretaceous magnetic quiet zone cmqz 121 84 ma crust between the isochrons m0 and a34 has an unequal width and widens from about 170 km in the west 80 e longitude to about 500 km towards east 85 e longitude plate reconstruction models for 160 fit 134 m11 121 m0 and 84 ma a34 are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones the mismatch in the a34 reconstruction is attributed to the emplacement of the ob lena and marion dufresne seamounts at the spreading ridge axis and or the frequent ridge jumps in the middle cretaceous during the major plate reorganization

structural lineaments from the magnetic anomaly maps of the eastern continental margin of india ecmi and nw bengal fan abstract three major structural lineaments are inferred from the magnetic and bathymetry data collected over the eastern continental margin of india ecmi and nw bengal fan covering the area between 10 55 n to 20 15 n and 80 e to 88 e analysis of magnetic data associated with these trends suggests that 1 trend i located at the foot of the continental slope around 3000 m water depth represents the ocean continent boundary ocb of ecmi 2 trend ii represents the northern extension of 85 e ridge abutting the continental shelf off chilka lake and 3 trend iii located over the continental shelf slope between visakhapatnam and paradip represents a folded ridges and depressions nature of the continental basement the study also reveals the occurrence of dyke intrusions within the continental crust landward of ocb which might be related to the rift phase volcanism along the ecmi

modelling of sedimentary basins from gravity anomalies with variable density contrast summary the decrease of density contrast with depth in sedimentary basins could be approximated by a quadratic function the sedimentary basin is viewed as a number of outcropping vertical prisms put in juxtaposition at each observation point equations are derived for the gravity anomaly of a vertical prism for a quadratic density function the initial model is obtained by calculating the thicknesses of the prisms at each station by an infinite slab formula the differences between the observed anomalies and the anomalies of the initial model at each station are used to modify the initial model a computer program is developed for initial estimates and refinement of the model the method is applied to interpret the gravity anomaly of san jacinto graben california where sediments filling the graben have a density contrast that can be approximated to a quadratic function finally the convergence of the method is discussed

3 d gravity analysis of the dniepr donets basin and donbas foldbelt ukraine abstract the dniepr donets basin ddb is a linear nw se trending late palaeozoic and younger sedimentary basin on the east european platform separating the ukrainian shield from the voronezh massif its northwestern dniepr segment has the characteristics of a typical rift basin to the southeast through a transition zone of approximately 200 km length the ddb is progressively uplifted and compressionally deformed into the correlatable donbas foldbelt df along the axis of the dniepr segment a series of gravity highs has been previously explained by high density crystalline crust beneath the axis of the basin caused by intrusion of mafic and ultramafic rocks in this paper the results of a 3 d gravity analysis using a gravity backstripping technique is described that investigates the crustal and upper mantle structure in the region of the ddb df transition zone and df a residual gravity field i obtained by subtracting the gravity influence of the sedimentary succession of the ddb from the observed field reveals a distinct positive anomaly along the axis of the rift basin increasing in amplitude to the southeast in concert with increasing sedimentary thickness a residual gravity field ii derived by removing the gravity effects of a modelled homogeneous crystalline crust from residual field i reaches 200 and 100 mgal amplitude in the df for two respective moho models based on different interpretations of the published crust and upper mantle seismic velocity models the first of these model a assumes crustal thickening beneath the transition zone and df to a moho depth up to 50 km whereas the second model b assumes a moho shallowing to depths in the range 35 37 km along the whole basin axis for each residual anomaly ii the best fitting 3 d distribution of average density in the crystalline crust has been computed both models indicate the existence of a high density body in the crystalline crust along the ddb axis increasing in density from the dniepr segment to the df with higher average crustal density required in the case of moho model a 3 17 103 kg m 3 versus 3 06 103 kg m 3 for model b the preferred interpretation of the density models is one in which the denser crystalline crust underlying the ddb df transition zone and df is explained by intrusion of mafic and ultramafic rocks during late palaeozoic rifting processes invocation of processes related to the uplift and inversion in the df are not required to explain the observed gravity field although reworking of the crust during permian and younger tectonism in the df cannot be ruled out