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C. Subrahmanyam ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

Geological implications of the Bouguer gravity anomalies over the various structural provinces and their boundaries in the South Indian shield are discussed. The widely distributed greenstone belts and granites of Karnataka and Bastar craton show belts of positive and negative anomalies respectively, and control in a significant manner the Bouguer anomaly contour pattern over these low-grade terrains. The high grade Eastern Ghat terrain is divisible into two sectors whose geological and geophysical characteristics differ, as also their relationships with the adjacent low-grade terrains. While both represent deep crustal level ischolar_main zones, the Tamil Nadu-Kerala granulite terrain appears to be exposed due to crustal tilting and continues northward underneath the Archean Karnataka craton. On the other hand, the Coastal granulite terrain is in faulted contact with the adjoining cratons. A strong negative anomaly flanking the western margin of this terrain is caused by downfiexuring of the crust along the cratonic margin.

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C. Subrahmanyam ¹, C. S. Venkatachalam ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500 007, IN

Abstract

No Abstract.

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Sheena V Dev ¹, M. Radhakrishna ¹, C. Subrahmanyam ²

Affiliations
1 Department of Marine Geology and Geophysics, School of Marine Sciences, Cochin University of Science and Technology, Fine Arts Avenue, Cochin - 682 016, IN
2 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

The southwest continental margin of India has evolved due to rifting between India and Madagascar at 88 Ma and subsequent seafloor spreading. During this period, several major ridges and horst-graben structures were formed at the margin. In the southern most part, the Comorin Ridge aligned along the margin, is another topographic feature believed to be related to the earliest phase of margin evolution. In the present study, Coherence analysis of gravity and bathymetry data of the region based on Maximum Entropy Spectral Estimation (MESE) method is used to understand the regional geodynamic processes and lithospheric strength that controls the flexure and basin evolution. The study brings out effective elastic thickness (Te) values of 5-10 km along the Konkan and Kerala basins, and the Comorin Ridge at the margin and 5-8 km along the Chagos Laccadive Ridge north of the equator. The elastic thickness values increase southward and maximum Te value of 10 km is observed below the Comorin Ridge. These results, combined with the earlier estimates of Te indicate low strength < 15 km for the lithosphere below the western margin, but not as low as that observed below plume affected margins. The observed low elastic strength at the margin will have implications on long term topographic evolution of the Western Ghats.

The Marion Plume seems to have played limited role in the rifting and evolution process of the western margin. In the absence of such an active plume role, the substantial partial melt zone observed in the Comorin Ridge region based on geomagnetic induction models can be alternatively explained by invoking the convective partia1 melting model. The Comorin Ridge might have been emplaced as an oceanic basement high due to large-scale volcanism during the onset of seafloor spreading. Absence of diagnostic magnetic signature associated with the isostatic gradient over the Comorin Ridge that marks the Ocean Continent Boundary indicates that active volcanism due to strong convection might have blurred the crustal transition in the Comorin Ridge region.

Keywords

Gravity, Coherence, Effective Elastic Thickness, Continental Margins, Comorin Ridge, Konkan And Kerala Basin, Western Ghats, India.

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C. Subrahmanyam ¹, S. I. Reddi ¹, N. K. Thakur ¹, T. Gangadhara Rao ¹, Kalachand Sain ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

Large-scale occurrence of gas-hydrates in the marine sediments of outer continental margins is by now well-known. While the methods of tracing gas-hydrate enriched zones, like identifying markers such as bottom simulating reflectors and blanking zones on seismic reflection sections, are well-established, quantification and resource assessment of gas hydrates still remain hazy. Investigations over the classical gas-hydrate enriched Blake outer ridge on the U.S. Atlantic margin have provided new insights into the environs of gas-hydrate occurrences. Deep sea drilling results in this area indicate that the estimates of gas-hydrate from geophysical measurements could be less by an order of three in comparison with those obtained from drilling. Several factors like bathymetry, sediment thickness and sedimentation rates, sea bottom temperature and total organic carbon content, which control gas-hydrate formation, indicate that the shallow sediments in the offshore regions of India could be potentially enriched in gas hydrates. National agencies like the Oil & Natural Gas Corporation, Gas Authority of India Ltd., National Geophysical Research Institute, National Institute of Oceanography and others are now actively involved in assessing the true potential of this anticipated future energy resource. The environmental aspects of gas hydrates like large-scale release of methane gas into the air and slope instabilities and submarine landslides in the continental margins resulting in catastrophic events, however, need to be kept in mind.

Keywords

Energy Resources, Gas-Hydrates, Global Distribution, Indian Offshore Occurrences.

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