Magnetic studies of basement off the coast of Bombay, west of India

Abstract

Bathymetric and total magnetic intensity data along 45 closely spaced profiles in 20–100 m of water totalling ~ 2400 line km were collected on the continental shelf off Bombay, west of India. Magnetic intensity anomaly maps were prepared. The anomalies show prominent trends in E-W, NNW-SSE and ENE-WSW directions. Correlations of the anomaly signatures with the geology known from the drill well logs and seismic results infer: 1. (1) the mostly basaltic nature of the basement overlain by the Tertiary sediments, except in the Murud Depression and isolated patches of the Bombay High and inner shelf where granites are interpreted; 2. (2) the lateral extent of the two NNW-SSE-trending regional faults between longitudes 70° 50′E and 71°40′E; 3. (3) the outlines of the magnetic signatures associated with the structural highs and depressions. Two-dimensional model and spectral studies of the anomalies under numerical constraints have revealed the structure of the basement overlain by 1.5–5.0 km thick sediments. A crustal section along a transect across the inner shelf, the Bassein High structure and the offshore basin in the middle shelf, the Bombay High structure in the middle-outer shelf and further west is interpreted. The section depicts the structure of the Tertiary sediments, the late Cretaceous-early Tertiary basalts and the Archaean-Precambrian granites. Thick basalt constructs occur in the paleoslopes of the Archaean-Precambrian basement. Down-faulting of parts of the Western Indian peninsula into the Arabian Sea along the regional faults is suggested, due to the lithologically similar sequences of the Tertiary sediments, the basalts/Deccan traps and granitic gneisses of the study and onshore areas. Therefore, the area is considered to be divided into three structural blocks consisting of the inner shelf, the offshore basin and the Bombay High, separated by NNW-SSE-trending deep-seated faults, which result from rifting of the crust. The basement faults, mostly extending to the overlying young sediments and the tectogenes, indicate a continued stress regime. The continued subsidence (both mechanical and thermal) of the crust in the study area under increased sediments, the drift of the Indian continent from the south to the present latitudes, and the dynamic interaction of the oceanic continental lithospheres appear to deform the crust by thinning and shearing.