Centre for Earth, Ocean and Atomospheric Sciences (CEOAS)

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Browsing Centre for Earth, Ocean and Atomospheric Sciences (CEOAS) by Subject "85°E Ridge"
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    Advances in marine geophysical studies of the Indian Ocean: Contributions from India (2010-2015)
    ( 2016-07-01) Krishna, K. S. ; Kamesh Raju, K. A. ; Ramprasad, T. ; Chaubey, A. K. ; Dewangan, P. ; Yatheesh, V.
    Indian scientists carrying marine geophysical research at different Earth Science institutions have made significant advances during the last six years (2012-2015) for understanding evolution of continental margins of India and deep sea regions. The investigations were mostly focused on continental margins of India encompassing deep-water regions, mid-ocean ridges, aseismic ridges and Andaman Sea including the back-arc basin. The studies were aimed at unraveling the tectonic and physical processes associated with evolution of the margins, ocean basins, volcanic structures, etc. In addition, industryrelated research for mapping of energy resources such as gas hydrates and hydrocarbons was also carried out in rifted basins on Eastern Continental Margin of India. The investigations were successfully carried out with integration of several geophysical datasets available at National Institute of Oceanography, India, National Geophysical Data Center, USA and petroleum industries, and by acquiring new geophysical data and geological sampling. In this report we describe major outcomes of the investigations focused on specific geological aspects of the Indian Ocean. Investigations carried out by individual works are also included in the bibliography. Geophysical studies carried out over the Ninetyeast Ridge led to understand how the Kerguelen plume interacted with the spreading centers as well as with the lithosphere during emplacement of volcanic edifices, isostatic compensation mechanisms of the ridge and magma production rates in response to the Indian plate motion. The analyses of seismic data over the 85°E Ridge revealed the internal structure consisting of volcanic plug and several stratigraphic units such as lava-fed deltas and prograding clinoforms. The lava deltas along with intervening erosional surfaces and mass wasting on ridge flanks suggested that the ridge was built by both subaqueous and multiphase submarine volcanism during the late Cretaceous. Study of seismic reflection data of the Eastern Continental Margin of India and adjacent deep-water region revealed two different phases of sediment deposition piled in the Bay of Bengal. The sediment distribution pattern, thickness and the bay architecture unveiled an occurrence of major change in sediment source from the peninsular India to the Bengal Basin at ∼23 Ma as a result of uplift of Himalayas and onset of Asian monsoon system and this eventually led to the commencement and growth of the Bengal Fan. The gas hydrates studies in Krishna-Godavari (KG) offshore basin revealed that the hydrate distribution in the basin is controlled mainly by fault systems. The velocity analysis of sediment strata within the basin suggested occurrence of high concentration of gas hydrate close to the large-scale fault system. Thus, the fault system in KG Basin provides a conducive environment for the migration of methane, which upon entering the hydrate stability zone forms the fracture-filled gas hydrate deposits. Marine geophysical data of both the continental margins of India and western Andaman region were studied for extending the country's limit of the continental shelf beyond the Exclusive Economic Zone. The scientific and technical information extracted from the studies would serve its primary purpose of delineating the outer limits of India's continental shelf beyond 200 nautical miles. The teleseismic studies of the back-arc basin, Andaman Sea revealed that the back-arc spreading fabric is undergoing readjustment to be in equilibrium with the current plate motions. A passive Ocean bottom seismometer (OBS) experiment in the back-arc basin recorded several events including teleseismic, microseismic and local events. Geological and geophysical studies over the Carlsberg and Central Indian Ridges have brought out finer scale segmentation of the ridge system and delineated zones of magmatic and sparsely magmatic sections of the ridges. The water column and deep-tow investigations carried out over the Carlsberg Ridge provided evidences for a prominent plume signatures emanating from a seafloor hydrothermal vent system. A model for the early opening of the Arabian Sea for the period 88 to 56.4 Ma is proposed with the consideration of the Gop and Laxmi Basin spreading centers as two arms of a triple junction and Narmada Rift on the Indian peninsula in the east as third arm of the triple junction. The reconstruction model did provide a new view on dispersals of Madagascar, Seychelles and India, during their early drift period, wherein the Laxmi Ridge and the Laccadive Plateau have been accommodated as intervening continental slivers.
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    Dating of the 85°E Ridge (northeastern Indian Ocean) using marine magnetic anomalies
    ( 2011-05-10) Michael, Laju ; Krishna, K. S.
    The 85°E Ridge is a typical aseismic ridge in the northeastern Indian Ocean in view of the fact that it associates with exceptional gravity and magnetic signatures. The ridge possesses two different gravity anomalies: the north part (up to 5°N latitude) is associated with negative gravity anomaly, whereas the south part coincides with positive gravity anomaly. In contrast to this, the ridge consists of alternate streaks of positive and negative magnetic signatures distributed for asymmetrical extents. With the consideration of ridge seismic structure and geomagnetic polarity reversals, we modelled both positive and negative magnetic anomalies of the ridge. This shows that the 85°E Ridge was formed during the period of rapid changes in the Earth's magnetic field, earlier to that, the underlying oceanic crust was created in the Cretaceous superlong normal polarity phase. The results further reveal that the positive and negative magnetic signatures of the ridge have been created, in general, by a relief of the ridge and polarity contrast between the ridge material and adjacent oceanic crust, respectively. On correlation of the ridge's magnetization pattern to the geomagnetic polarity timescale, we believe that the 85°E Ridge volcanism started at anomaly 33r time (~80 Ma) in the Mahanadi Basin by a short-lived hotspot, thereafter the process continued towards south and finally ended at ~55 Ma in the vicinity of the Afanasy Nikitin Seamount.
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    Growth of the Afanasy Nikitin seamount and its relationship with the 85°E Ridge, northeastern Indian Ocean
    ( 2014-01-01) Krishna, K. S. ; Bull, J. M. ; Ishizuka, O. ; Scrutton, R. A. ; Jaishankar, S. ; Banakar, V. K.
    The Afanasy Nikitin seamount (ANS) is a major structural feature (400 km-long and 150 km-wide) in the Central Indian Basin, situated at the southern end of the so-called 85°E Ridge. Combined analyses of new multibeam bathymetric, seismic reflection and geochronological data together with previously described magnetic data provide new insights into the growth of the ANS through time, and its relationship with the 85°E Ridge. The ANS comprises a main plateau, rising 1200 m above the surrounding ocean floor (4800 m), and secondary elevated seamount highs, two of which (lie at 1600 and 2050 m water depths) have the morphology of a guyot, suggesting that they were formed above or close to sea-level. An unbroken sequence of spreading anomalies 34 through 32n.1 identified over the ANS reveal that the main plateau of the ANS was formed at 80-73 Ma, at around the same time as that of the underlying oceanic crust. The 40Ar/39Ar dates for two basalt samples dredged from the seamount highs are consistent, within error, at 67 Ma. These results, together with published results of late Cretaceous to early Cenozoic Indian Ocean plate reconstructions, indicate that the Conrad Rise hotspot emplaced both the main plateau of the ANS and Conrad Rise (including the Marion Dufresne, Ob and Lena seamounts) at 80-73 Ma, close to the India-Antarctica Ridge system. Subsequently, the seamount highs were formed by late-stage volcanism c. 6-13 Myr after the main constructional phase of the seamount plateau. Flexural analysis indicates that the main plateau and seamount highs of the ANS are consistent with Airy-type isostatic compensation, which suggest emplacement of the entire seamount in a near spreading-center setting. This is contrary to the flexural compensation of the 85°E Ridge further north, which is interpreted as being emplaced in an intraplate setting, i.e., 25-35 Myr later than the underlying oceanic crust. Therefore, we suggest that the ANS and the 85°E Ridge appear to be unrelated as they were formed by different mantle sources, and that the proximity of the southern end of the 85°E Ridge to the ANS is coincidental. © Indian Academy of Sciences.
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    Magnetic anomalies of offshore Krishna-Godavari basin, eastern continental margin of India
    ( 2009-08-01) Swamy, K. V. ; Radhakrishna Murthy, I. V. ; Krishna, K. S. ; Murthy, K. S.R. ; Subrahmanyam, A. S. ; Malleswara Rao, M. M.
    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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    Magnetic studies in the northern Bay of Bengal
    ( 1994-06-01) Ramana, M. V. ; Subrahmanyam, V. ; Krishna, K. S. ; Chaubey, A. K. ; Sarma, K. V.L.N.S. ; Murty, G. P.S. ; Mittal, G. S. ; Drolia, R. K.
    Total magnetic intensity and bathymetric surveys were carried out in the northern Bay of Bengal between 6° to 11° 45′ N latitudes and east of 84° to 93° 30′ E longitudes. The hitherto known 85° E Ridge is characterised as a subsurface feature by a large amplitude, positive magnetic anomaly surrounded by Mesozoic crust. A newly identified NE to NNESSW trending magnetic anomaly between 7° N, 87° 30′ E and 10° 30′ N, 89-90° E may be one of the unidentified Mesozoic lineations in the northern Bay of Bengal. The Ninetyeast Ridge is not associated with any recognizable magnetic anomaly. The Sunda Trough to the east of the Ninetyeast Ridge is characterised by a positive magnetic anomaly. A combined interpretation, using Werner deconvolution and analytical signal methods, yields basement depths ~ 10 km below sea level. These depths are in agreement with the seismic results of Curray (1991). © 1994 Kluwer Academic Publishers.
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    Structure and evolution of the Afanasy Nikitin seamount, buried hills and 85°E Ridge in the northeastern Indian Ocean
    ( 2003-04-30) Krishna, K. S.
    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. © 2003 Published by Elsevier Science B.V.