Deep crustal structure of the West Bengal basin deduced from gravity and DSS data

The West Bengal basin (WBB) which developed at the margin of the eastern In dian shield in response to Gondwana rifting underlies a deep Gondwana troug h covered by the Rajmahal (Cretaceous) basalts and younger sediments. Avail able crustal seismic data for four profiles in southern half of the basin, as well as other available seismic, drill-hole and gravity data, provide va luable information about the deep crustal structure and evolution of the WB B. The derived crustal model suggests significant mass anomalies in this pa rt of the Indian palaeocontinental margin. The main results of the model ar e: (a) the Indian shield crust undergoes almost 33% thinning (36 to 24 km) across the WEB within a short distance of 130 km; (b) anomalous crustal lay ers appear in the top and middle parts of the crust below the shield-margin and also below the basin; (c) the top crust (8-12 km depth) at the shield- margin is less dense by an estimated amount of -0.14 g/cm(3), corresponding to a low velocity layer as detected by the crustal seismic data, while its underlying crust (12-36 km depth) is significantly denser but (d) further east, a wider low-density crust occupies the central part of the basin betw een mid-crustal depth(16 km) and the crustal base. Rapid crustal thinning o ccurs along its eastern flank, where a denser lower crustal block is locate d adjacent to the Eocene 'hinge zone' that separates the WEB from deeper pa rts of the Bengal delta. The intense zone of crustal necking below the WEB further bears a spatial relationship to the anomalous crustal layers and th eir attendant faults. The origin of these features is ascribed to the Gondw ana rifting episode, resulting in the formation of a highly stretched crust at the Indian shield margin, where the Rajmahal volcanism eventually erupt ed.