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

new aeromagnetic data from the western enderby basin and consequences for antarctic india break up 1 in order to enhance our understanding of the continental break up between east india and antarctica a japanese german aerogeophysical survey was conducted off eastern dronning maud land and western enderby land antarctica the systematic survey with a line spacing of 20 km provides strong constraints on the timing of the initial break up of india antarctica compared to the conjugate margin south of sri lanka while the continent ocean transition is very well marked by pronounced positive and negative magnetic anomalies no hints on the existence of magnetic spreading anomalies north of this zone has been found in the new data set thus in contrast to most recently published models the data only support kinematic models which include a final break up of sri lanka india from our investigated area some 90 118 ma within the cretaceous normal superchron

provenance of lower cretaceous wolong volcaniclastics in the tibetan tethyan himalaya implications for the final breakup of eastern gondwana abstract lower cretaceous volcanic lithic arenites widely distributed in the tethyan himalaya provide insights into the continental breakup of eastern gondwana in southern tibet they are represented by the wolong volcaniclastics the volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts whereas rhyolitic dacitic volcanism becomes the predominant source of the upper strata geochemical analyses of basaltic grains and of detrital cr spinels from the wolong volcaniclastics demonstrate the alkaline character of the volcanism and suggest within plate tectonic setting for the volcanism zircon u pb ages confirm that this volcanism continued from 140 ma to 119 ma hf isotope data on these early cretaceous zircons indicate that their parental magmas were mantle derived but in the later stage of magmatic activity mantle derived magmas were mixed with partial melts derived from the continental crust the lower cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of greater india the onset of volcaniclastic deposition in the himalayas appears to become progressively younger toward the west but it ended synchronously during the late albian 102 ma the low volume of volcanic rocks and their intra plate tectonic setting suggest that they are the result of decompressional melting along extensional deep seated fractures cross cutting the continental crust and reflect changes in regional intra plate tectonic stresses when greater india began to separate from the australia antarctica supercontinent

the geochronology structure and metamorphism of early archaean rocks at fyfe hills enderby land antarctica abstract the fyfe hills of enderby land antarctica are composed of archaean cratonic rocks of the napier complex initial crustal formation possibly of plutonic volcanic and sedimentary origin was in the early archaean but probably significantly later than the 4000 ma ages previously reported at 3100 ma the rocks underwent granulite facies metamorphism and an intense regional strain d1 which rotated into parallelism pre existing discordances and produced a recumbent gneissic pile approximate sr isotopic homogenisation over at least a metre and new zircon growth and or major resetting apparently occurred at this time subsequent ductile deformation d2 also characterised by tight to isoclinal folding produced characteristic microstructure only within f2 folds but no axial plane foliation was developed peak granulite facies metamorphic conditions close to 900 c and 9 11 kbars were reached during d1 d2 considerable pb loss occurred in zircon during asymmetric open to tight folding d3 and upper amphibolite to granulite facies metamorphism 650 c 7 kbars at 2500 ma rb sr equilibration on the scale of centimetres occurred at that time after which the napier complex was essentially cratonised a subsequent semiductile deformation did not affect the whole napier complex but produced upright shear zones in the fyfe hills area which lies close to the boundary between the napier complex and the younger rayner complex to the south

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 evolution of the east coast of india abstract the evolution of the east coast of india from the permian up to the end of the cretaceous is traced peninsular india had an imprint of hercynian epeirogeny manifested in the formation of some regional geofracture and rift systems the east coast basins mahanadi godavari krishna palar and cauvery basins are pericratonic the structural style and regional trends during the jurassic are ne sw the basin architecture during the late jurassic is of horst graben type the close of the mesozoic was marked by regional easterly regression basinal tilt and formation of monoclinal post mesozoic sedimentary cover the oldest sequence in these basins is of permian age and is known in the palar basin the triassic sediments are not known in the land area from the faunistic and floristic evidence the uppermost jurassic lower cretaceous gondwana sediments are paralic the occurrence of marine permian palynofossils in the indian east coast sediments points to the presence of a narrow paleo shelf to the east prior to the break up and drift of gondwanaland different continental fits offered by various authors are analysed to understand the evolution of the indian ocean the structural grain and basin architecture of the pre drift and post drift basins of the east coast of india are similar to that of the western and northwestern coastal basins of australia it is postulated that permian sediments were extensive and have since been eroded such a pattern of erosion of permian beds is also observed in northwestern australia the most favoured fit of western australia against northern india does not conform to the regional geotectonic pattern and the nature of sediments that are present in india and the australian counterparts in addition to geological evidence palaeo biological evidence has been considered which favours the juxtaposition of the east coast of india with western australia the authors are of the opinion that palaeomagnetic interpretations which have of late gained importance fail to explain the pre drift geological similarities of the east coast of india and western coast of australia

proterozoic to mesozoic east gondwana the juxtaposition of india sri lanka and antarctica jurassic and cambro ordovician paleomagnetic pole positions deduced from rocks of sri lanka and antarctica generally conform when sri lanka is juxtaposed with the coast of lutzow holm bay east antarctica in a manner similar to a current gondwana reassembly the highland and southwest groups mostly proterozoic of sri lanka are correlated with the ongul and skallen groups of east antarctica on the basis of similar lithology structural characteristics structural trend tectonothermal history ranging from early proterozoic to mesozoic and the pressure temperature time path of the main metamorphism some neighboring lithotectonic units are also comparable between the two areas fracture lineament systems of sri lanka and antarctica formed after the latest silurian and during or before the jurassic conform in this reconstruction an improved fit of the 2000 m isobaths of india sri lanka antarctica and continuation of the latest archaean mobile belt from enderby land to peninsular india is obtained by fitting sri lanka into lutzow holm bay and enderby land into the embayment in the east coast of peninsular india this juxtaposition of peninsular india to east antarctica involves about 80 km northward movement and about 30 clock wise rotation of sri lanka relative to india the reassembly thus obtained accords well with data supporting the idea that east gondwana existed from the earliest proterozoic to middle mesozoic although some disruption collision events of limited scale are considered possible

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

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

dominant symmetry of a conjugate southern australian and east antarctic magma poor rifted margin segment synthesis and modeling of published deep seismic and potential field data from the conjugate magma poor rifted margins of the great australian bight southern australia and central wilkes land east antarctica show that there is pronounced symmetry of structures in a 300 km wide zone straddling the axis of final breakup this symmetry is observed consistently for a distance of some hundreds of kilometers along strike from inboard to outboard both margins comprise a narrow zone of attenuation of the crystalline continental crust an approximately 4 km high basement ridge interpreted as unroofed peridotites at the location of maximum thinning of the continental crust and a 60 70 km wide continent ocean transition zone that contains a sedimentary basin that may be underlain by altered mantle and fragments of crystalline continental crust the marked breakup symmetry described here is in contrast to the asymmetry of the iberia newfoundland margin and is consistent with the operation of a symmetrical extensional detachment system deforming the whole crust in the center of the rift as envisaged by some numerical models for the continental rifting process

seafloor spreading magnetic anomalies in the enderby basin east antarctica abstract the timing of the early separation of india from the contiguous antarctica australia is still an unresolved problem although it is well established that antarctica and india formed a single indo antarctic platform prior to the fragmentation of eastern gondwanaland inadequate age information either in the form of magnetic anomaly isochrons or dating of oceanic rocks from the conjugate margins of antarctica and india perhaps led several authors to propose incomplete plate reconstruction models particularly for the early separation of india from antarctica analysis of magnetic and satellite derived gravity data in the enderby basin east antarctica reveals the presence of seafloor spreading type linear magnetic anomalies and eight new fracture zones the observed magnetic anomalies can be interpreted as the younger sequence of mesozoic anomalies m11 m0 half spreading rates range from 6 5 to 2 8 cm yr and are comparable with those measured in the bay of bengal these similarities in the mesozoic magnetic anomaly sequence and in the spreading rates provide evidence that these two basins are conjugate and contemporary a consistent plate reconstruction model can be derived from the identified conjugate patterns of mesozoic magnetic anomalies and fracture zones the occurrence of the oldest magnetic anomaly m11 close to the coasts in these two offshore basins unequivocally suggests that the break up of india from antarctica occurred before 134 ma

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

revisiting the structure age and evolution of the wharton basin to better understand subduction under indonesia understanding the subduction processes along the sunda trench requires detailed constraints on the subducting lithosphere we build a detailed tectonic map of the wharton basin based on reinterpretation of satellite derived gravity anomalies and marine magnetic anomalies the wharton basin is characterized by a fossil ridge dated 36 5 ma offset by n s fracture zones magnetic anomalies 18 to 34 38 84 ma are identified on both flanks although a large part of the basin has been subducted we analyze the past plate kinematic evolution of the wharton basin by two plate india australia and three plate india australia antarctica reconstructions despite the diffuse plate boundaries within the indo australian plate for the last 20 ma we obtain finite rotation parameters that we apply to reconstruct the subducted wharton basin and constrain the thickness buoyancy and rheology of the subducting plate the lower subductability of younger lithosphere off sumatra has important consequences on the morphology with a shallower trench forearc islands and a significant inward deviation of the subduction system this deviation decreases in the youngest area where the wharton fossil spreading center enters subduction the discontinuous magmatic crust and serpentinized upper mantle consequences of the slow spreading rates at which this area was formed weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism deeper effects include the possible creation of asthenospheric windows beneath the andaman sea in relation to the long offset fracture zones and east of 105 e as a result of subduction of the spreading center

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

the breakup of east gondwana assimilating constraints from cretaceous ocean basins around india into a best fit tectonic model published models for the cretaceous sea oor spreading history of east gondwana result in unlikely tectonic scenarios for at least one of the plate boundaries involved and or violate particular constraints from at least one of the associated ocean basins we link east gondwana spreading corridors by integrating magnetic and gravity anomaly data from the enderby basin off east antarctica within a regional plate kinematic framework to identify a conjugate series of east west trending magnetic anomalies m4 to m0 126 7 120 4 ma the mid ocean ridge that separated greater india from australia antarctica propagated from north to south starting at 136ma northwest of australia and reached the southern tip of india at 126 ma sea oor spreading in the enderby basin was abandoned at 115 ma when a ridge jump transferred the elan bank and south kerguelen plateau to the antarctic plate our revised plate kinematic model helps resolve the problem of successive two way strike slip motion between madagascar and india seen in many previously published reconstructions and also suggests that sea oor spreading between them progressed from south to north from 94 to 84 ma this timing is essential for tectonic ow lines to match the curved fracture zones of the wharton and enderby basins as greater india gradually began to unzip from madagascar from 100 ma in our model the 85 east ridge and kerguelen fracture zone formed as conjugate anks of a leaky transform fault following the 100ma spreading reorganization our model also identi es the afanasy nikitin seamounts as products of the conrad rise hotspot

seismic evidence of continental margin influence on the ninetyeast ridge in the bay of bengal the ninetyeast ridge ner one of the most enigmatic features in the indian ocean is covered by thick bengal fan sediment north of 9 n we present seismic reflection data on the eastern flank of the ner at 10 n that show the presence of 4 5 km thick sediments beneath the bengal fan sediments these sediments can be imaged up to 60 km beneath the andaman fore arc accretionary wedge suggesting that the decollement lies above these sediments the presence of thick sediments above the northernmost segment of the ner suggests that this segment was close to a continental margin during its emplacement we propose that these sediments were deposited soon after the breakup of india and antarctica between 130 and 100 ma and might act as source rocks for oil and gas generation beneath recent bengal fan sediments furthermore subducting thick sediments can significantly change the seismogenic behavior of the andaman subduction zone

a global scale plate reorganization event at 105 100 ma abstract a major plate reorganization is postulated to have occurred at approximately 100 ma however this reorganization has received limited attention despite being associated with the most prominent suite of fracture zone bends on the planet and many other geological events we investigate tectonic events from the period 110 to 90 ma and show that the reorganization occurred between 105 and 100 ma was global in scale and affected all major plates seafloor evidence for plate motion changes is abundant during this period with either fracture zone bends or terminations preserved in all ocean basins long lived eastern gondwanaland subduction ended along a 7000 km long section of the margin while elsewhere around the proto pacific rim subduction continued and there is evidence that compressional stresses increased in the overriding plates thrusting in western north america transpression and basin inversion in eastern asia and development of the present day andean style margin along western south america occurred contemporaneous with the development of an extensional regime in eastern gondwanaland basin instability in africa and western europe further demonstrates that lithospheric stress regime changes were widespread at this time considering the timing of the reorganization and the nature of associated plate boundary changes we suggest that eastern gondwanaland subduction cessation is the most likely driving mechanism for the reorganization subduction is the dominant driver of plate motion and therefore this event had the potential to strongly modify the balance of driving forces acting on the plates in the southwestern proto pacific and neighboring plates whereby producing widespread changes in plate motion and continental lithospheric stress patterns we propose that major changes in ridge trench interaction triggered the cessation of subduction the progressive subduction of two closely spaced perpendicular mid ocean ridges at the eastern gondwanaland subduction zone to the east of australia and new zealand respectively resulted in very young crust entering the trench and we suggest that by 105 100 ma there was insufficient negative buoyancy to drive subduction finally we propose that the plume push force of the bouvet plume that erupted near the african antarctic south american triple junction contributed to plate motion changes in the southern atlantic region

evidence of paleo cold seep activity from the bay of bengal offshore india 1 we report evidence of paleo cold seep associated activities preserved in methane derived carbonates in association with chemosynthetic clams calyptogena sp from a sediment core in the krishna godavari basin bay of bengal visual observations and calculations based on high resolution wet bulk density profile of a core collected on board r v marion dufresne may 2007 show zones of sharp increase in carbonate content 10 55 vol within 16 20 meters below seafloor mbsf the presence of calyptogena clam shells chimneys shell breccias with high mg calcite cement and pyrite within this zone suggest seepage of methane and sulfide bearing fluid to the seafloor in the past highly depleted carbon isotopic values 13c ranges from 41 to 52 vpdb from these carbonates indicate carbon derived via anaerobic oxidation of methane extrapolated mean calendar age 58 7 ka b p of the clastic sediments at a depth of 16 mbsf is close to the upper limit of the u th based depositional age 46 2 3 7 and 53 0 1 6 ka of authigenic carbonates sampled from this level thereby constraining the younger age limit of the carbonate deposition methane expulsion events the observed carbonate deposition might have resulted from the flow of methane enriched fluids through the fracture network formed because of shale diapirism

sedimentation rates from calcareous nannofossil and planktonic foraminifera biostratigraphy in the andaman sea northern bay of bengal and eastern arabian sea abstract in this study we determined calcareous nannofossil and planktonic foraminifera events in sediments recovered during the 2006 indian national gas hydrate program nghp expedition 01 selected bioevents permitted the assignment of orbitally calibrated ages derivative sedimentation rate estimates and identification of sedimentary discontinuities in the andaman sea at hole nghp 01 17a a late miocene to recent record was recovered but with a significant hiatus during the latest miocene and pliocene sedimentation rates here vary from 50 m myr during the pleistocene to 130 m myr for the late miocene in the northern bay of bengal at hole nghp 01 19a the base of the cored record also reaches the late miocene and extends to recent but the same hiatus missing the pliocene is present at this hole a the early pleistocene at hole nghp 1 19a shows sensible variation in sedimentation rates ranging between 130 m myr and 10 m myr during the last 0 5 ma for the late miocene the sedimentation rate was ca 50 m myr in the western bay of bengal at hole nghp 01 10b d calcareous nannofossils were sparse and only foraminifera datums are available the base of the section is latest early pliocene to late pliocene and the majority of the record is considered younger than late pliocene due to the scarcity of calcareous nannofossils and the few foraminifera datums sedimentation rates were not determined here however hole nghp 01 16a in the same region recorded the highest occurrence of pseudoemiliania lacunosa an event that confirms a pleistocene age allowing us to estimate a sedimentation rate between 110 m myr and 450 m myr along the western peninsular indian margin at hole nghp 01 01a a continuous record from the holocene to the early oligocene indicates sedimentation rates of ca 25 m myr until the middle miocene and a sensible reduction downward reaching 4 m myr in the early miocene to the early oligocene no hiatuses were observed the large changes in sedimentation rates observed in these cores reflect long timescale changes in the environment likely induced by changes in the strength of the indian monsoon system

middle cretaceous geomagnetic field anomalies in the eastern indian ocean and their implication to the tectonic evolution of the bay of bengal abstract the middle cretaceous period is largely known for its stable normal polarity in the earth s magnetic field a few reversals isea m 3r m 2r and m 1r have been postulated during this period but are yet to be accepted in total recently two anomalies q1 92 ma and q2 108 ma have been identified globally and proposed as internal time markers useful to trace the evolution of the world oceans while the evolutionary history of the indian ocean from late cretaceous to present is well established the older middle to early cretaceous record is still ambiguous the occurrence of a major plate reorganization during the middle cretaceous period has added to the dilemma in understanding the early evolution of the eastern indian ocean the detailed evolution of the bay of bengal and its conjugate enderby basin has remained speculative to date due to various constraints such as lack of good geophysical dataset and drill sites and the presence of thick sedimentary load in the present study an attempt is made to validate the occurrence of the middle cretaceous internal time markers in the eastern indian ocean these time markers are used to provide additional constraints for tracing the evolution of the eastern indian ocean since late jurassic identification of these markers aided to confirm the timing of spreading ridge extinction in the perth basin as 102 ma in the bay of bengal these markers facilitated to infer the evolution of the middle cretaceous crust east of the buried 85 e ridge the age of the oceanic crust increases from south to 115 ma at the northern tip of the bay of bengal seafloor spreading occurred in nne ssw direction between 115 ma and 102 ma and thereafter it changed its direction to n s half spreading rates ranging from 4 0 to 4 2 cm yr between 108 ma and 84 ma in the bay of bengal are higher than its conjugate enderby basin this spreading system belongs to the second rifting episode between antarctica and india and was caused by the kerguelen mantle plume activity the 85 e ridge south of 12 n is emplaced on a transform fault that constitutes the northern extension of the 86 e fracture zone and active since 102 ma the study suggests that excess crustal accretion occurred on the indian plate since the middle cretaceous

anisotropic amplitude variation of the bottom simulating reflector beneath fracture filled gas hydrate deposit 1 for the first time we report the amplitude variation with angle ava pattern of bottom simulating reflectors bsrs beneath fracture filled gas hydrate deposits when the effective medium is anisotropic the common depth point cdp gathers of two mutually perpendicular multichannel seismic profiles located in the vicinity of site nghp 01 10 are appropriately processed such that they are fit for ava analysis ava analysis of the bsr shows normal incidence reflection coefficients of 0 04 to 0 11 with positive gradients of 0 04 to 0 31 indicating class iv pattern the acoustic properties from isotropic rock physics model predict class iii ava pattern which cannot explain the observed class iv ava pattern in krishna godavari basin due to the anisotropic nature of fracture filled gas hydrate deposits we modeled the observed class iv ava of the bsr by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic hti medium after accounting for anisotropic wave propagation effects on bsr amplitudes the effective medium properties are estimated using backus averaging technique and the ava pattern of bsrs is modeled using the properties of overlying hti and underlying isotropy hti media with or without free gas anisotropic ava analysis of the bsr from the inline seismic profile shows 5 30 gas hydrate concentration equivalent to fracture density and the azimuth of fracture system fracture orientation with respect to the seismic profile is close to 45 free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system f1

composition and origin of authigenic carbonates in the krishna godavari and mahanadi basins eastern continental margin of india abstract the mineralogical and stable isotopic composition of authigenic carbonates from the krishna godavari kg and mahanadi basin provide a deeper insight into the processes inducing carbonate formation in the sediments of the eastern continental margin of india in the bay of bengal authigenic carbonate cements micro nodules bioturbation casts and tubes from 12 core locations drilled during the indian national gas hydrate program nghp expedition 01 were investigated for this study three main processes responsible for authigenic carbonate precipitation are identified organoclastic sulfate reduction anaerobic oxidation of methane and methanogenesis evidence of vigorous methane seepage is indicated in carbonates recovered at sites 7 10 12 kg basin and 19 mahanadi basin these methane derived carbonates display typical paragenetic carbonate mineralogies aragonite high mg calcite with 15 mol mgco 3 ca rich dolomite two separate horizons of methane derived carbonates are correlated between 4 drill holes up to 16 km apart the upper horizon has been dated with 14 c 40 100 51 600 a bp 1950 and clearly indicates that methane seepage has been much more vigorous in the past possibly due to the effect of lower sea level on the gas hydrate stability zone across the margin

ostracod biodiversity from shelf to slope oceanic conditions off central bay of bengal india abstract thirty seven surface sediment samples collected through a multi corer in seven transects of orv sagar kanya sk 308 leg 1 from off central bay of bengal shelf to slope environment were analysed for living and dead ostracoda the samples which ranged in water depth from 29 m to 2540 m yielded a total of 46 species belonging to 32 genera the ostracod fauna of the bay of bengal exhibit a depth related distribution that is controlled by substrate and hydrographic conditions sandy substrates of shallow water depth 28 m are characterized by propontocypris sp and cytherelloidea malaccaensis these species are replaced by neonesidea cracenticlavula and bairdoppilata paraalcyonicola in high population which shows positive linear correlation with salinity temperature and total calcium carbonate tcc in the outer shelf 55 73 m paijenborchella sp dominates at 763 m in the bathypelagic zone giving way to parakrithella sp at greater depths of 2540 m on soft bottom substrates the fauna represents an interesting mixture of displaced species and others known from bathyal and abyssal depths in the bay of bengal including some pandemic deep sea species the ostracod faunal diversity is higher in shallower water than in the deeper water which may be due to the optimum temperature ph salinity and substrate the fauna are comprised of three associations shelf upper slope slope lower slope abyss a marked faunal turnover occurs at the outer shelf factor analysis was performed to determine the correlation of water and sediment parameters with the ostracod species this study provides a detailed account of bathymetry and ostracod biodiversity recorded in the study area

evolution of the late cretaceous crust in the equatorial region of the northern indian ocean and its implication in understanding the plate kinematics summary analysis of 3100 km of newly acquired marine magnetic data constrained by satellite and shipborne free air gravity anomalies in the corridor between the 86 e fracture zone and ninetyeast ridge north of the equator reveals the evolutionary history of the late cretaceous crust characterised by anomaly 34 through 31 83 5 68 7 ma under complex tectonic settings seafloor spreading model studies suggest that the crust particularly between the chrons 33r and 33 79 0 73 6 ma was formed with variable and slightly higher half spreading rates 4 8 7 1 cm yr than the crust of similar age either in the regions west of 86 e fracture zone east of the ninetyeast ridge or the southern crozet basin further the interpretation of magnetic anomalies suggests the presence of fossil spreading ridge segments and extra oceanic crust on the indian plate that has been transferred from the antarctica plate by discrete southward ridge jumps these ridge jumps are caused by thermal instability of the spreading center as the indian plate moved northward over the kerguelen mantle plume the present study indicates that the spreading ridge plume interaction is the prime mechanism for these ridge jumps which have occurred since 75 8 ma the newly identified magnetic anomalies 34 through 31 and the inferred n3 e trending fracture zones refined the plate reconstruction models for that period

the baja california peninsula borderland structural and sedimentological characteristics abstract structural and sedimentological data from three oceanographic cruises define the peninsular margin of the gulf of california as a borderland similar to the california continental borderland bathymetric and high resolution seismic profiles show some active normal faults with a lateral strike slip component which are parallel and oblique at low angle to the peninsular coast and delimit horst and graben structures preliminary conclusions are that active faults are related to the movement of the peninsular block towards the northwest as part of the opening of the gulf circa 6 5 my ago grabens and half grabens separated by highlands islands and banks have formed some of the basins are now inland such as santa rosalia loreto and san jose del cabo and are filled with pliocene and possibly miocene marine rocks other basins are modern shallow bays like concepcion 30 m deep others are slope basins with a range of depths such as alfonso and cochimie basins at about 400 m and la giganta and la paz basins at 500 and 700 m respectively sedimentation in these basins is varied inland basins pliocene are filled with shallow marine sedimentary rocks clastics and evaporites concepcion is a starved basin filled with some terrigenous but mainly biogenic sediments the modern deep basins alfonso la giganta and la paz are filled with terrigenous sediments on their peninsular margin biogenic if they are limited by islands and laminated sediments if they are stagnant or have slopes or sills intersecting the oxygen minimum layer of the water column

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

tectonic fabric map of the ocean basins from satellite altimetry data abstract satellite altimetry data provide a new source of information on the bathymetry of the ocean floor the tectonic fabric of the oceans i e the arrangement of fracture zones ridges volcanic plateaus and trenches is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements seasat and geosat altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features the uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship track data this map is also the first global map to incorporate the results of the geosat mission and as a result new tectonic features are revealed at high southerly latitudes this map permits the extension of many tectonic features well beyond what was previously known for instance various fracture zones such as the ascension tasman and udintsev fracture zones can be extended much closer to adjacent coninental margins the tectonic fabric map also reveals many features that have not been previously mapped these features include extinct ridges minor fracture zone lineations and seamounts in several areas especially across aseismic plateaus or along the margins of the continents the map displays broad gravity anomalies whose origin may be related to basement structures

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

paleogeographic maps of the continents bordering the indian ocean since the early jurassic paleomagnetic data from the african plate from the early jurassic onward are critically re evaluated and updated resulting in a set of 30 paleopoles from which an apparent polar wander path is derived using 20 ma intervals this path is then used in conjunction with kinematic models of the indian and atlantic oceans to place other continents bordering the indian ocean eurasia india islands and plateaus madagascar seychelles mascarene and blocks involved in the collision of india with eurasia south tibet indochina in an absolute paleogeographic frame fixed with respect to africa paleogeographic maps are presented for the early jurassic pre breakup the jurassic cretaceous boundary anomaly ml5 the early cretaceous m2 the late cretaceous a 34 the cretaceous tertiary boundary a 29 the early eocene a 24 the middle eocene a 21 and the oligocene miocene boundary a 6 observed and predicted paleomagnetic directions for landmasses other than africa i e for western europe china korea india and australia which are not used in deriving the maps are found to be in very satisfactory agreement the quality of reconstructions as measured by rms discrepancies between observed and predicted paleolatitudes is of the order of 5 the main features of the paleogeographic maps are discussed with the aim of relating oceanic and on land geodynamic events the constraints provided by tibetan data on the india asia collision and the 20 ma reorganization of geology and plate boundaries in a band extending from the south china sea through tibet to the red sea system are discussed in more detail we emphasize the episodic nature of the breakup of gondwanaland and formation of the indian ocean and subsequent interaction with eurasia in which six major discrete events are identified separated by 30 ma phases of rather uniform plate motion

seismic and heat flow constraints from the gas hydrate system in the krishna godavari basin india abstract high resolution multi channel seismic reflection data acquired in the offshore domain of the krishna godavari kg basin provide seismic indicators of gas hydrate including bottom simulating reflectors bsr and high reflectivity zones below the bsr as an indication of free gas the recent recovery of gas hydrate by drilling across various sites in the kg basin together with the seismic indicators suggests overall widespread favourable gas hydrate stability conditions the bsrs provide indirect estimates of geothermal gradients and heat flow a simple conductive model is used to calculate heat flow from the depth of the bsr bsr derived heat flow values range from about 30 to 102 mw m2 across the kg basin we also modeled the base of gas hydrate stability zone bghsz to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is analysed in relation to both geological formations and tectonics our analysis suggests a highly variable ghsz in the kg basin controlled by local variations in the parameters of stability condition and faulting the bsr derived heat flow increases towards deeper water depths across the basin consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs this variation may be due to focusing and defocusing effects of the topography however local bsr depths patterns may also be due to dynamic effects including the displacement of isotherms by thrust faulting additionally recent mass wasting events and normal faulting resulted in the bghsz being too shallow relative to the predicted model which assumed equilibrium conditions

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

paleomagnetic and geochronological results from the zhela and weimei formations lava flows of the eastern tethyan himalaya new insights into the breakup of eastern gondwana