Informatics Publishing Limited

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.

Full Text

     

J. N. Prasad ¹, K. V. V. Satyanarayana ¹, P. B. Gawali ¹

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

Abstract

Palaeomagnetic and Low-field Anisotropy of Magnetic Susceptibility (AMS) results are reported from 8 unmetamorphosed dolerite dykes, and from one site each of the Closepet Granite (CG), the Peninsular Gneiss (PG) and the Charnockite (Ch) in the Harohalli area of the Dharwar craton. The AMS results exhibit "normal" magnetic fabrics for three dykes. Rest of the dykes exhibit either "inverse" or "anomalous" magnetic fabrics, indicating the dominance of single domain (SD) uniaxial magnetite grains and a complex mixture of SD and multi-domain (MD) grains respectively. Almost similar AMS fabrics are exhibited by the Peninsular Gneiss, Closepet Granite and Charnockite. This magnetic fabric is in close conformity with the structural features of the terrain and indicates E-W compressional stress. Two dykes have yielded a mean characteristic remanent magnetization (ChRM) direction (D = 130°, I = -88°, α₉₅ =33°; N=2) which is similar to a previous result from the same dyke swarm and the Tiruvannamalai dykes in the adjacent South Indian Granulite Terrain (SIGT). Thus we support the earlier suggestion (Radhakrishna and Joseph, 1993) that no relative motion took place between the granite-greenstone terrain (GGT) and the SIGT, at least since the emplacement of Tiruvannamalai dykes ca. 1600 Ma ago. The palaeomagnetic directions from the PG (D = 285°, I = 43°), CG (D = 300°, I = 43°), and Ch (D = 300°, I = 43°) are statistically indistinguishable and the corresponding pole positions fall close to those reported for 1100-1000 Ma old rocks. Thus partial remagnetization/streaking in these sites occurred probably during the Eastern Ghats orogeny. It appears that the Eastern Ghats orogeny has penetrated selectively into the adjacent Dharwar craton.

Keywords

Palaemagnetism, AMS Studies, Proterozoic, Harohalli, Karnataka.

Full Text