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

Abstract

S U M M A R YA 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.

Sub-basalt Imaging of Hydrocarbon-Bearing Mesozoic Sediments Using Ray-Trace Inversion of First-Arrival Seismic Data and Elastic Finite-Difference Full-Wave Modeling Along Sinor–Valod Profile of Deccan Syneclise, India

“…3), are correlative with findings coming from a well drilled nearby Ankleswar (Fig. 1) having 5.7 km total depth (TD) in which trap thickness obtained is 3.2 km (Roy 1991;Tewari et al 1995;Dixit et al 2010). This also explains the velocity inversion documented by the ray-tracing response derived from first-arrival traveltime data along the Sinor-Valod seismic profile due to presence of LVLs hidden below the HVLs (Fig.…”

“…This region is also topographically highly undulated surrounded by deep valleys and hillocks confined by three major rivers called Narmada, Tapti and Mahi flowing from east to west and confluence in the Arabian Sea. The Deccan basalts were also encountered at different depths below the Tertiary sediments ranging in age from the Palaeocene to Recent confirmed from the well drilled in the Cambay basin (Roy 1991;Tewari et al 1995;Dixit et al 2010). The hydrocarbon-bearing Mesozoic sediments hidden below the Deccan trap, which correspond to upper Jurassic to middle Cretaceous Bagh and Lameta beds/formations are exposed toward east of Sinor in the Cambay basin (Fig.…”

Geoelectric structure of northern Cambay rift basin from magnetotelluric data

“…This conductive feature (300-1000 S) is attributed to the Tertiary and Quaternary sediments deposited over the Cambay rift basin and particularly its western region. DSS and gravity studies (Kaila et al 1990;Mishra et al 1998;Dixit et al 2010) have reported a maximum sediment thickness of 2-3 km over the ridges and 5-6 km over the depressions, and seismic velocities (5.5-5.8 km/s) have suggested a past accumulation of sediments. The present MT study shows the presence of thick sediments (1-5 km) beneath the Cambay rift zone as denoted by C2, corroborating the DSS and gravity studies.…”

“…The extensive deep seismic sounding (DSS) studies carried out over the Cambay basin for hydrocarbon exploration (Kaila et al 1990;Tewari et al 1991Tewari et al , 1997Dixit et al 2010) revealed major structural trends in the basin and supported the explanation of magmatic underplating beneath the Kutch and Cambay rift basins. The Cambay basin is complex in nature, containing horst structures and E-W directional faults that divide the basin into four sub-basins.…”

“…The second-stage rifting of the Cambay basin is considered to be coeval with the breakup of the Indian plate and the Seychelles as well as the eruption of Deccan basalts in the Late Cretaceous (Raval and Veeraswamy 2000;Veeraswamy and Raval 2004) by generating significant variations in volcanic intensity as observed in the Baikal rift zone, a Rio Grande rift system. Several geophysical studies were carried out to understand the tectonics, sediment thickness, and rifting processes of the Cambay basin (Kaila et al 1990;Tewari et al 1991Tewari et al , 1995Mishra et al 1998;Singh et al 2003;Dixit et al 2010;Kumar et al 2016). Most authors have reported ~ 1-5-km-thick Tertiary and Quaternary sediments, inferring that lower crustal magmatic underplating occurred associated with plume interaction and rifting events.…”