Detecting sub-basalt Mesozoic sediments and active tectonics in the Narmada-Tapti Rift Zone, central India

Abstract

To obtain structural information below basalt cover using conventional seismic reflection has always been a challenge due to the high impedance of the basaltic layer. This study enumerates P-wave refraction tomography to image the shallow structure along three long offset profiles in the Deccan Volcanic Province, one of the world's largest flood basalt volcanic regions. The travel time inversion is performed to obtain a starting model for the wavefield inversion, and synthetic tests are used to estimate the resolution. Our results show an abrupt increase of crystalline basement depth from 1.5 to 3 km across the Narmada South Fault. The shallow basement depth coincides with a 70-km-wide sub-basalt (>1.5 km deep) seismic shadow zone between the Narmada and Tapti rivers, associated with the sub-basalt graben caused by a crustal extension that contains sediments or a combination of basalt and sediments. The basaltic layer appears to be highly heterogeneous in velocity and thickness, and the degree of heterogeneity increases laterally and vertically with depth within the layer. It is quite likely that these sub-basalt sediments could be the potential zone for hydrocarbon, similar to the exposed Vindhyan in the north and Gondwana sediments in the south of our study area. Another intriguing feature is the Barwani Sukta Fault, which controls the tectonics of the region by dividing the basement configuration. We also compare our tomographic model with those determined from previously obtained seismic, gravity, and surface mapping. The tomographic image clearly shows that numerous faults are reaching up to the basement and extending laterally. We suggest that these neo-tectonic faults may be capable of generating large-magnitude earthquakes, including the 21 May 1997 (mb 6.0) Jabalpur earthquake.