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.

Full Text

     

M. Radha Krishna ¹, P. John Kurian ¹, C. G. Nambiar ¹, S. Krishna Mohan ²

Affiliations
1 Department of Marine Geology and Geophysics, Cochin University of Science and Technology, Cochin - 682 016, IN
2 Department of Geophysics, Osmania University, Hyderabad - 500 007, IN

Abstract

A network of 30 gravity base stations have been established. The dynamic drift curves show an average drift rate of less than 0.006 mgal/min. The observed gravity values have repeatability of 0.08 mgal, and can be used for regional gravity surveys.

Keywords

Geophysics, Gravity Base Staions, Kerala.

Full Text

     

P. John Kurian ¹, M. Radha Krishna ¹, C. G. Nambiar ¹, B. V. S. Murthy ²

Affiliations
1 Department of Marine Geology and Geophysics, Cochin University of Science and Technology, Cochin - 682 016, IN
2 Department of Geophysics, Osmania University, Hyderabad - 500 007, IN

Abstract

Gravity surveys have been carried out over the Perinthatta anorthosite and the adjoining areas and the results are discussed. The Bouguer anomaly map reveals the absence of any conspicuous gravity anomaly corresponding to the pluton and thus precludes the existence of larger mafic portions of the pluton in the immediate subsurface region. The residual gravity map obtained by regional-residual separation method shows a minor positive gravity field of irregular shape with localised highs of 6 - 12 mgal within and around the anorthosite body. TWo-dimensional gravity interpretation of profiles across the pluton suggests subjacent nature and lensoid shape for the anorthosite which is underlain by small isolated cylindrical bodies (≃ 2.5 km diameter) of higher density (2.9 gm/cc) extending 3 - 7 km downwards. These cylindrical bodies are inferred as either the mafic/oxide rich portions of the pluton sagged down during crystallization or the individual feeders of the plagioclase-rich mushes, which coalesced in the upper crust to give rise to the massif.

Keywords

Geophysics, Gravity, Anorthosite, Structure, Perinthatta, Kerala.

Full Text