Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

P. B. V. Subba Rao ¹, B. P. Singh ¹, P. B. Gawali ¹

Affiliations
1 Indian Institute of Geomagnetism, Colaba, Mumbai 400 005, IN

Abstract

Ocean bottom magnetometer (OBM) investigations were made across the Ninetyeast Ridge in the Bay of Bengal and geomagnetic field variations were recorded for a period of about one month. Analysis of data indicates that the thickness as well as resistivity of the oceanic crust/upper mantle reduces from 17 - 14 km and 439 - 308 Ohm-m respectively, as one moves from the Bay of Bengal to the Andaman arc region. The highly conducting Andaman arc region may have formed due to subduction related processes at the Andaman trench.

Analysis of OBM data collected earlier across the Barren Island in the Andaman Sea indicates the presence of two conductivity zones, at 17 - 27 km and 80 - 100 km depth. The enhanced conductivity at a shallow depth may be due to the conducting material/magma that has been emplaced by upwelling of the mantle material at the tectonic zone. The thickness of this material increases from north to south of the Barren Island and seems to concentrate the induced currents within the island. The enhanced conductivity at greater depths may be due to the H₂O and CO₂, that may be released by metamorphic reactions of the suhducting lndian plate beneath the Burmese plate.

Keywords

Geophysics, Ocean Bottom Magnetometer Investigations, Oceanic Crustal Structure, Partial Subduction, Low Resistivity Layer, Andaman Sea, Bay of Bengal.

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R. K. Verma ¹, M. Mukhopadhyay ¹, P. B. V. Subba Rao ¹, Y. Satyanarayana ¹

Affiliations
1 Indian School of Mines, Dhanbad, IN

Abstract

A regional gravity survey has been carried out over the area lying between latitude 20°30' to 21°30'N, and longitude 85° to 86°15'E, forming a part of south Singhbhum geological province and northern part of the Eastern Ghats. The area is well-known for Sukinda ultramafic complex, Diateri and Malayagiri iron-ore deposits as well as Sukinda thrust.

Nearly 350 gravity stations were established along main roads, forest roads and cart tracks. Bouguer anomaly map of the area shows gravity highs over the Diated iron-ore group, the Malayagiri volcanics, the Sukinda ultramafic complex as well as over the Eastern Ghats province. Two-dimensional models. have been prepared to show the nature of the ultramafic complex, iron-ore group (lOG) of rocks and the Sukinda thrust. The lOG appear to form narrow but steeply dipping sedimentary troughs, while the Sukinda thrust appears to be steeply dipping to the south.

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