Two-dimensional velocity model of the crust beneath the South Cambay Basin, India from refraction and wide-angle reflection data

Abstract

SUMMARY A deep seismic sounding study was carried out in western India, in the southern part of the Cambay Basin, along a 230-km-long profile between the cities of Mehmadabad (22 ◦ 50 � N, 72 ◦ 46 � E) and Billimora (20 ◦ 46 � N, 72 ◦ 58 � E) during the year 1976–1977. Seismic refraction, as well as near- and wide-angle reflection, data were obtained from various boundaries from the sedimentary section to the Moho. Kaila et al. published models based on analogue versions of these data. In this study, we digitized, reprocessed, modelled, and reinterpreted the data from the Mehmadabad–Billimora seismic survey using modelling techniques that were not available at the time that Kaila et al. published their results. Our extensive modelling and interpretation of a large number of seismic record sections in south Cambay Basin reveal three Tertiary sedimentary subbasins: the Mehmadabad–Jambusar basin, the Jambusar–Kosamba basin, and the Kosamba–Billimora basin. These basins are vertically offset relative to the each other along high-angle faults, and the maximum depth (∼9.5 km) to the granitic/Proterozoic basement (5.8–6.0 km s −1 ) in the region occurs in the Jambusar–Broach region. Near-surface sedimentary rocks within the subbasins are underlain by Deccan Trap volcanics (4.75–5.2 km s −1 ), which have also been vertically offset throughout the study area. Although a prominent traveltime delay in first-arrivals, which is often indicative of low-velocity layers, is not observed on many of the seismograms, our modelling of secondary reflected arrivals indicate the presence of lower-velocity Mesozoic sediments below the Deccan Trap volcanics within the subbasins. The upper crust (6.0–6.3 km s −1 ), which is about 15 km thick, also appears to be stratified in places, with a low-velocity zone (5.5 km s −1 ) in the depth range of 12.5–14.5 km between the cities of Nadiad and Kathana. However, this low-velocity zone tapers and merges with the top of lower crust (6.5–6.8 km s −1 ) south of the city of Jambusar. The lower crust consists of two layers, an upper layer (6.5–6.8 km s −1 ) and a lowermost layer (7.2–7.5 km s −1 ), with the discontinuity between the two lower crustal layers occurring at 23–25 km depth. We interpret the higher velocity (7.2–7.5 km s −1 ) lowermost crustal layer as rocks that were magmatically underplated to the crust during rifting associated with the large-scale extrusion of the Deccan volcanics. The Moho discontinuity (8.0 km s −1 ) is about 31–33 km deep between the cities of Mehmadabad and Jambusar and increases to 37 km from south of the city of Ankleshwar. The large thickness of Tertiary sediments in the Cambay Basin and a relatively thin crust, as compared to the surrounding regions, suggests that rifting in the region occurred from the Cretaceous until the Tertiary.