Chapter Five - Deccan Volcanic Province Near the West Coast

Abstract

Three major Precambrian orogenic trends—the north-northwest to south-southeast Dharwar in the southern part, the northeast-southwest Aravalli-Delhi in the northeastern part, and east-northeast to west-southwest Satpura in the central part—converge on the west coast of India. Most of the area is covered with a thick layer of Deccan Trap volcanics. Deep seismic studies in the Koyna region, which is well known for its reservoir-triggered seismicity, show that the crust is 37–40km thick except near the west coast of India, where its thickness is about 31km. Most of the seismicity is concentrated at 4–5km depth above a region of low velocity in the upper crust. The upper mantle has alternating low-velocity layers. The hydrocarbon-bearing Cambay basin has Quaternary and Tertiary sediments, with maximum thickness of about 7700 m, overlying the Deccan Traps. The basin subsidence seems to have taken place in the late Cretaceous. Crustal thickness in most of the basin is 32–33km except in the southern part. The main features of the crust are a low-velocity zone in the upper crust and a high-velocity/high-density lower crust. Geologic considerations and deep wells drilled in the Saurashtra peninsula indicate that Mesozoic sediments could be present under the exposed Deccan Traps. The seismic data shows that the crustal thickness here is about 36km in the west and reduces sharply to 32km to the east, due to a deep fault. Similarly to the Cambay basin, the Saurashtra peninsula also has a high-velocity/high-density lower crust.