S U M M A R YDeep seismic sounding data were acquired in the West Bengal basin, India, along two profiles: (i) Bishnupur-Palashi-Kandi, along a line about 227 km long in the northsouth direction and (ii) Taki-Arambagh, along a line about 120 km long in the east-west direction. Seismic refraction and wide-angle reflection data were recorded by continuous profiling using two 60-channel digital seismic units (DFS-V) with an 80 m geophone group interval and 4 ms sampling rate. These data were interpreted in order to delineate the basement configuration. The 2-D models of the seismic data both indicate a five-layer velocity structure above the Archaean crystalline basement (5.9-6.2 km s-I). A low-velocity layer (4.0 km s-') is inferred immediately above the basement in the shelf region of the basin corresponding to the Gondwana sediments (Upper Carboniferous to Lower Triassic) below the Rajmahal Traps (Upper Jurassic to Lower Cretaceous) of 4.6 to 4.8 km s-' velocity, which is also confirmed from the nearby well data. The results along the Taki-Arambagh profile and the drilling results at the Jaguli (J-1) well are used to investigate whether Gondwana sediments and the Rajmahal Traps exist in the deep part of the Bengal basin. An additional layer of velocity 5.2-5.3 km s-', delineated in the Palashi-Kandi profile overlying the basement, may correspond to the Singhbhum group of rocks of the Proterozoic.A structural contour map of the basement prepared from the present results indicates a south-easterly dip of the basement in general. The depth of the basement on the stable shelf of the basin gently increases to about 8 km and dips steeply, plunging to a maximum depth of 14 km in the deep basin. No structural high that can be related to the 'Calcutta gravity high' is found in the basement around the Hooghly River.

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S U M M A R YThe results of deep reflection profiling studies carried out across thy palaeo-mesoProterozoic Delhi Fold Belt (DFB) and the Archaean Bhilwara Gneissic Complex (BGC) in the northwest Indian platform are discussed in this paper. This region is a zone of Proterozoic collision. The collision appears to be responsible for listric faults in the upper crust, which represent the boundaries of the Delhi exposures. In these blocks the lower crust appears to lie NW of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks. A fairly reflective lower crust northwest of the DFB exposures appears to be the downward continuation of the DFB upper crust. The poorly reflective lower crust under the exposed DFB may be the westward extension of the BGC upper crust at depth. Thus, the lower crust in this region can be divided into the fairly reflective Marwar Basin (MB)-DFB crust and a poorly reflective BGC crust. Vertically oriented igneous intrusions may have disturbed the lamellar lower-crustal structure of the BGC, resulting in a dome-shaped poorly reflective lower crust whose base, not traceable in the reflection data, may have a maximum depth of about 50 km, as indicated by the gravity modelling.The DFB appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. This has resulted in three Moho reflection bands, two of which are dipping SE from 12.5 to 15.0 s two-way time (TWT) and from 14.5 to 16.0 s TWT.Another band of subhorizontal Moho reflections, at = 12.5 s TWT, may have developed during the crustal perturbations related to a post-Delhi tectonic orogeny. The signatures of the Proterozoic collision, in the form of strong SE-dipping reflections in the lower crust and Moho, have been preserved in the DFB, indicating that the crust here has not undergone any significant ductile deformation since at least after the Delhi rifting event.

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