Deep seismic sounding studies in the north Cambay and Sanchor basins, India

Abstract

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 4 ms 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 5.9–6.0 km s−1) is about 5000 m 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 7700 m. The Deccan Traps (P-wave velocity 4.3–4.8 km s−1) 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−1). A prominent low-velocity zone (velocity 5.5 km s−1) 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 s−1 and 7.3–7.4 km s−1, the discontinuity between them occurring at 23–25 km depth. The Moho discontinuity (PM velocity 8.0 km s−1) lies at a depth of 31–33 km. The high-velocity (7.3–7.4 km s−1) 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.