No abstract

No abstract

S U M M A R Y Deep seismic sounding (DSS) studies have been carried out in the north Cambay and Sanchor sedimentary basins in western India along three lines covering about 350 km. Seismic refraction and wide angle reflection data, pertinent to the sedimentary basin as well as the deep crustal section, have been recorded from 41 shot points using a 60 channel DFS-V digital recording system with 200 m geophone spacing and 4ms data sampling. Extensive modelling and interpretation of a large number of seismic record sections reveal four sub-basins in the sedimentary section along these lines. Maximum depth to the granitic/Proterozoic basement (P-wave velocity 5.9-6.0kms-l) is about 5000m in the north Sanchor and the Patan sub-basins and about 5600 m in the south Sanchor sub-basin. The deepest part of the sedimentary basin is delineated within the Gandhinagar sub-basin where the basement depth reaches 7700m. The Deccan Traps (P-wave velocity 4.3-4.8 km s-l) form the base of the Tertiary sediments, almost in the entire study area except the extreme northern part. There is also some indication of the presence of sub-Trappean Mesozoic sediments along this profile. Within the sedimentary basin two horst features, one near Diyodar (the Diyodar ridge) and the other northwest of Mehsana (the Unhawa ridge), are indicated by the seismic data consistent with the tectonics of the region.The thickness of the upper crust in this region does not exceed 15 km (P-wave velocity reaching 6.3 km s-'). A prominent low-velocity zone (velocity 5.5 km s-') occurs in the depth range from 10.5 to 12.5 km. The lower crust consists of two layers of velocities 6.6-6.9 km sC1 and 7.3-7.4 km S K I , the discontinuity between them occurring at 23-25 km depth. The Moho discontinuity (PM velocity 8.0 km s-') lies at a depth of 31-33 km. The high-velocity (7.3-7.4 km s-') lower crustal layer represents underplating of the crust due to mantle upwelling and rifting with large-scale extrusion of the Deccan volcanics. The large thickness of the Tertiary sediments in the Cambay basin and a relatively thin crust in the region suggest further rifting during the Tertiary.

No abstract

SUMMARY 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.