Earth, Ocean and Atomospheric Sciences - Publications
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ItemLate Quaternary seismic sequence stratigraphy of the Gulf of Kachchh, northwest of India( 2009-03-01)High-resolution seismic reflection and bathymetric data in 10 to 50 m water depth in the Gulf of Kachchh, northwest of India, have been analysed together with earlier results from the area. They have revealed geomorphic features and Late Quaternary seismic sequence stratigraphy of 25-m-thick sediments. Seabed topography is uneven except in the east, carpeted by thick acoustically transparent to semitransparent sediment clays and stratified limestone beds. The sediments consist of (i) the transparent unit "A" up to 12 m thick, (ii) thick 8 to 10 m diffuse seismic reflection free unit "D," and (iii) 4 to 5 m thick hummocky reflections unit "K." The geomorphic/subsurface features are pinnacles (1 to 2 m high) and mounds (4 to 5 m high) on the seafloor and subsurface in the south and west, valleys (1 to 3 m) on the seafloor in the south and subsurface in the north and onlap of reflectors in the north. They mark corals, seafloor incisions, and sediment influx. Spatial shifts of valleys that mark a high energy tidal regime incising the seafloor in paleo and present times appear to be due to neotectonics: subsidence and uplift. Based on the seismic images and on correlation with the sea level curve published by other authors, the pinnacles, mounds, and the hummocky reflectors are interpreted as corals (live and relict) formed in subaqueous conditions. The reflection-free sediments are coral debris, sands mostly derived during the interglacial period of late Pleistocene and Holocene when the gulf was exposed to arid climates during lowered sea levels, around Last Glacial Maximum (centered ∼ 18 ka). Access to unique sedimentary records of the Late Quaternary climates/sea level changes even of decadal scale can be retrieved by collecting sediments/corals from shallow drill wells in the gulf.
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ItemGeoid 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( 2009-03-04)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 MacRdbertson Land and in the south between southwest Sri Lanka and Gunnerus Ridge region of East Antarctica. Approximately during the period between the anomalies Ml and MO 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. Copyright 2009 by the American Geophysical Union.
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ItemMagnetic anomalies of offshore Krishna-Godavari basin, eastern continental margin of India( 2009-08-01)The marine magnetic data acquired from offshore Krishna-Godavari (K-G) basin, eastern continental margin of India (ECMI), brought out a prominent NE-SW trending feature, which could be explained by a buried structural high formed by volcanic activity. The magnetic anomaly feature is also associated with a distinct negative gravity anomaly similar to the one associated with 85°E Ridge. The gravity low could be attributed to a flexure at the Moho boundary, which could in turn be filled with the volcanic material. Inversion of the magnetic and gravity anomalies was also carried out to establish the similarity of anomalies of the two geological features (structural high on the margin and the 85°E Ridge) and their interpretations. In both cases, the magnetic anomalies were caused dominantly by the magnetization contrast between the volcanic material and the surrounding oceanic crust, whereas the low gravity anomalies are by the flexures of the order of 3-4 km at Moho boundary beneath them. The analysis suggests that both structural high present in offshore Krishna-Godavari basin and the 85°E Ridge have been emplaced on relatively older oceanic crust by a common volcanic process, but at discrete times, and that several of the gravity lows in the Bay of Bengal can be attributed to flexures on the Moho, each created due to the load of volcanic material. © Printed in India.
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ItemEarly (pre-8 Ma) fault activity and temporal strain accumulation in the central India Ocean( 2009-09-08)The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere, where shortening between the Indian and Capricorn plates is manifest as reverse faulting (5-10-km-spaced faults) and long-wavelength (100-300 km) folding. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. There has been disagreement concerning the likely timing for the onset of deformation between plate-motion inversions and seismic refl ection-based studies. In the present study, fault displacement data from seismic-refl ection profiles within the central Indian Ocean demonstrate that compressional activity started much earlier than previously thought, at around 15.4-13.9 Ma. From reconstructions of fault activity histories, we show that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0-7.5 Ma. There is no evidence for any regional unconformity before 8.0-7.5 Ma; early shortening was accommodated by activity on single isolated fault blocks. Total strain estimates derived are more variable and complex than those predicted from plate inversion, and they do not show simple west to east increase. © 2009 The Geological Society of America.
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ItemUtilization of high resolution satellite geoid data for estimation of lithospheric thickness in the Bay of Bengal( 2009-12-01)Very high-resolution database generated from Seasat, Geosat GM, ERS-1 and TOPEX/ POSEIDON altimeters data of the northern Indian Ocean has been used for the first time for preparation of geoid and free-air gravity maps. In the present work, geoid height anomalies have been analyzed across the Ninetyeast and 85°E Ridges within the Bay of Bengal. Present data sets are more accurate and detailed (off-track resolution: about 3.33 km and grid size: about 3.5 km). Observed geoid height - age and geoid height derivative (with respect to age) - age relationships have been established and compared with the plate model of lithospheric cooling to determine. The present endeavor is to determine the lithospheric plate thickness beneath both ridge structures. Attempts have been made to match the observed value with the computed value over the Bay of Bengal. The lithospheric plate cooling model correlates convincingly with the observed value. It has shown the efficacy of a plate model according to which geoid observations of the Bay of Bengal are better explained by a larger plate thickness of 90 - 125 km for the oceanic crust of age older than 30 Ma.