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

M. Radha Krishna ¹, R. K. Verma ²

Affiliations
1 Department of Marine Geology and Geophysics, Cochin University of Science and Technology, Cochin-682 016, IN
2 Centre of Geo-Resources, University of Delhi, New Delhi, IN

Abstract

A detailed analysis of earthquakes along the slow-spreading Central Indian Ridge (CIR) is carried out by comparing the seismicity pattern with different morpho-tectonic segments of the ridge. The seismicity pattern is correlatable well with the bathymetric trend of major fracture zones and longer spreading ridge segments in the central part of the ridge between 7°S and 19°S. The frequencymagnitude relation of earthquakes along four major fracture zones and two spreading segments suggest that the spreading segments, in general, are characterised by higher b-values than the fracture zones. The difference in b-values can be attributed to changes in deformation pattern and faulting mechanism between the two regions.

Source mechanisms for 54 major earthquakes occurring along the ridge mostly show either normal faulting with one of the nodal planes parallel to the local spreading segment, or strike-slip motion on steeply dipping nodal planes parallel to the transform. However, a careful analysis of their locations with the morphotectonic details of the ridge reveals few anomalous mechanisms located near the ridge - transform intersections or along geometrically complex transform faults. The reverse faulting mechanisms at the ridge-transform intersections near Sealark F.Z. and Argo F.Z have been inferred to be due to thermal stresses associated with the differential cooling of the lithosphere, The normal faulting earthquakes along the 12°12'S F.Z. might be due to an extensional offset of the fault. Another normal faulting event located along the Marie-Celeste EZ, outside the Principal Transform Displacement Zone(PTDZ) indicates the geometrical complexity of the fault.

Keywords

Seismotectonics, Earthquake, Central Indian Ridge, Indian Ocean.

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R. K. Verma ¹, D. Ghosh ¹, S. K. Roy ¹, A. Ghosh ¹

Affiliations
1 Indian School of Mines, Dhanbad, IN

Abstract

Results of Gravity surveys conducted over Bankura anorthosite complex lying between latitude 23°28.5' to 23°33'N and longitude 86°50' to 87°15'E are discussed. The area is characterised by a gravity high of the order of 20 mgals in the eastern part and decreases to 4-5 mgals in the western part. The gravity high is found to be unrelated to the anorthosite and extends beyond the outcrop of this exposure. It is attributed to the presence of a gabbroic body underlying the anorthosite and its adjacent areas. The thickness of anorthosite layer is found to be less than 200 meters. Assuming a density contrast of +0.23 gm/cc between the underlying gabbro and the surrounding Bengal gneisses, the maximum thickness of the gabbro is found to be 5.0 km in the eastern part near Rajarampur (23°29.7'N, 87°11'E). The causative body is found to be elongated along the strike of the anorthosite exposure. It appears to be faulted underneath the Damodar river along the northern as well as the northeastern sides.

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R. K. Verma ¹, T. K. Bandyopadhyay ¹, Manol Mukhopadhyay ¹

Affiliations
1 Indian School of Mines, Dhanbad-826 064, IN

Abstract

A residual Bouguer gravity anomaly map of the Raniganj Coalfield, West Bengal, India, has been prepared using graphical method of regional and residual separation. The gravity anomaly map reveals a minimum residual gravity of the order of -27 mgal in the central part of the coalfield. The residual gravity field over the basin has been interpreted in terms of the Gondwana sediments overlying the crystalline metamorphic rocks, using a three dimensional model with the help of computer programme given by Strange and Woollard (1969). A density contrast of -0.37 gm/cm³ between Gondwana sediments and metamorphic rocks has been used for calculations. The maximum thickness of sediments filling the basin is estimated to be 2.9 km in the central part of the basin. The basement depths obtained by three dimensional analysis are correlated with available borehole data in the coalfield and are found to give a good indication of the reliability of the model constructed.

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R. K. Verma ¹, M. Mukhopadhyay ¹, M. H. Ashraf ¹

Affiliations
1 Indian School of Mines, Dhanbad 826004, IN

Abstract

Both coalfields are marked by significant gravity 'lows'. Maximum sedimentary thickness present is of the order of 1. 75km below the surface under the Karanpura and 0.8 km under the Daltonganj coalfields. The two coalfields, are characterized by prominent normal faults at their southern margin. The throw of the faults may be few hundred metres, Within the individual coalfields, basement faults appear to be associated with younger Gondwana sediments on the surface. The areas occupied by youngest Gondwanas do not necessarily represent the deepest part of the coalfields. This is noted, in general, for nearly all coalfields of the Damodar Valley. Differential movement appears to have occurred during Gondwana times under the coalfields resulting in either single-walled or double-walled grabens.

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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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