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 Indian shield in response to Gondwana rifting underlies a deep Gondwana trough covered by the Rajmahal (Cretaceous) basalts and younger sediments. Available crustal seismic data for four profiles in southern half of the basin, as well as other available seismic, drill-hole and gravity data, provide valuable information about the deep crustal structure and evolution of the WBB. The derived crustal model suggests significant mass anomalies in this part of the Indian palaeocontinental margin. The main results of the model are: (a) the Indian shield crust undergoes almost 33% thinning (36 to 24 km) across the WBB within a short distance of 130 km; (b) anomalous crustal layers appear in the top and middle parts of the crust below the shield-margin and aIso below the basin; (c) the top crust (8-12 km depth) at the shieldrnargin is less dense by an estimated amount of -0.14 g/cm³, corresponding to a low velocity layer as detected by the crustals eismic 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 between midcrustal depth (16 km) and the crustal base. Rapid crustal thinning occurs along its eastern flank, where a denser lower crustal block is located adjacent to the Eocene 'hinge zone' that separates the WBB from deeper parts of the Bengal delta. The intense zone of crustal necking below the WBB further bears a spatial relationship to the anomalous crustal layers and their attendant faults. The origin of these features is ascribed to the Gondwana rifting episode, resulting in the formation of a highly stretched crust at the Indian shield margin, where the Rajmahal volcanism eventually erupted.