CRUSTAL STRUCTURE OF THE WEST-BENGAL BASIN, INDIA FROM DEEP SEISMIC-SOUNDING INVESTIGATIONS

Basement and sub-basement crustal configuration of parts of the west Bengal basin has been delineated from inverse and forward modelling of refraction and wide-angle reflection data obtained along two east-west profiles: (a) Beliator-Burdwan-Bangaon (profile-1) about 180 km long and (b) Gopali-Tamluk-Port Canning (profile-2) about 140 km long. The study has revealed the existence of four groups of prominent refractors (velocities 1.7-2.1 km s-1, 2.7-3.2 km s-1, 3.7-4.2 km s-1, and 4.6-5.3 km s-1) overlying the crystalline basement with a velocity of 5.8-6.2 km s-1. Rajmahal Traps (velocity 4.6-5.3 km s-1, averaging 4.8 km s-1) occur beneath the base of the Tertiary sediments in the middle parts of the profiles. The presence of sub-Trappean Gondwanas is also indicated from the wide-angle reflection modelling supported by deep well lithological information. In general, depth to the basement is found to increase from west to east, with a maximum depth of around 10 to 11 km at the extreme eastern parts of the profiles. A basement upwarp, with a sharp flexure west of Bishnupur on profile-2 in combination with the fault west of Shantipur on profile-1 possibly brings into focus the regionally extending NNE-SSW 'Hinge zone'. A low velocity layer (LVL) of about 2 to 3 km thickness (velocity 5.6-5.8 km s-1) has been inferred in the upper crust in this area. In general the thickness of the upper (including the sedimentary column) and the middle (6.4-6.6 km s-1 velocity) parts of the crust is found to be larger (thickness varying between 22 and 27 km) than the lower crustal column (interval velocity 6.8-7.0 km s-1 with a thickness of 5 to 10 km). The Moho lies at varying depths of 36 to 26 km along profile-1, with a prominent domal feature of about 40 km width between Satgachia and west of Shantipur. In the middle part of profile-2 the Moho occurs at depths of 32 to 34 km, with a mild flexure east of Tamluk. Configuration of the Moho indicates that the crust is more akin to continental in nature in the western and middle parts of the basin, whereas significantly thinned continental crust is indicated in the eastern and southeastern parts.