Crustal structure and rift architecture across the Krishna–Godavari basin in the central Eastern Continental Margin of India based on analysis of gravity and seismic data

Abstract

The Krishna–Godavari basin is a rifted passive margin basin that developed orthogonally to the NW–SE trending Pranhita–Godavari graben along the central Eastern Continental Margin of India in response to the continental rifting process and the subsequent seafloor spreading between India and eastern Antarctica during the early Cretaceous period. The 3-D gravity interpretation of both the onshore and offshore sections of the basin integrated with detailed seismic reflection and refraction data provided significant new information about the crustal architecture and the early breakup history of the basin. The gravity-derived crustal models indicate that the crust at the eastern Indian shield margin is 39–41 km thick and thins to as much as 20–23 km at the Ocean Continent Transition (OCT) in the offshore. There is a significant variation in the nature of crust and the configuration along the margin between the three crustal domains encompassing the basin: the Cuddapah basin, Pranhita–Godavari Graben, and the Eastern Ghat Mobile Belt that indicates the lateral segmentation of the margin. A zone of high density (3.0 g/cm³) crustal material at the OCT separates the pure continental and oceanic crusts on either side. The Moho is shallowest along this zone and brought the upper mantle notably close to the surface, which indicates that it could be comprised of Proto-Oceanic Crustal rocks. Furthermore, the present study highlights a major basement structural high in the deep offshore area of the margin. The geophysical signatures over this structure suggest that it is a crustal scale feature that is characterized by intrusive volcanic rocks, comprised of low density crust, and lies close to the OCT towards offshore. Based on the inferred tectonic reconstruction, we believe that this structural high could be a continental fragment that was left behind during the process of breakup between India and the Elan Bank.

Introduction

The East Coast of India is defined by the rifted passive margin that evolved in response to the continental rifting and seafloor spreading processes between India, Antarctica and Australia that occurred during the early Cretaceous period (Powell et al., 1988). It is observed that the Precambrian structural trends, namely, the Godavari and Mahanadi grabens, shear zones and granulite terrains of the Indian shield margin, have close linkages with similar features of the East Antarctica coast (Yoshida et al., 1999). Because of the intrinsic distinctions between different sub-crustal blocks (Radhakrishna, 1989; Lal et al., 2009), the East Coast of India is tectonically and geomorphologically segmented into several peri-cratonic rift basins following the rifting between India and Antarctica, and these basins are the Cauvery, Palar, Krishna–Godavari, Mahanadi and Bengal basins (Fig. 1). Detailed geophysical studies of the East Coast basins (Sastri et al., 1973, Sastri et al., 1981; Fuloria et al., 1992; Rangaraju et al., 1993; Prabhakar and Zutshi, 1993; Rao, 2001) revealed NE–SW trending horst-graben structural features in all of the basins formed during rifting.

The Krishna–Godavari (K–G) basin that lies between Visakhapatnam in the north and Nellore in the south (from 14° 30′ N and 17° 40′ N, including the Pennar river basin) developed along the bite of the East Coast of Peninsular India during the above rifting process (Rao, 2001). The K–G basin is one of the most important petroliferous basins of India (Fig. 1), and it comprises an area of approximately 28,000 km² onshore and 24,000 km²–49,000 km² offshore considering the areas with water depths up to 200 m and 2000 m, respectively (Rangaraju et al., 1993). This basin has been classified as a major intra-cratonic rift within the Gondwanaland until the Early Jurassic period (Rao, 2001), and it later transformed into a peri-cratonic rift basin (Biswas et al., 1993). The horst-graben structural configuration and other major basement trends in the onshore and the shallow offshore region of this basin are well-known because of previous geophysical studies and exploratory well drilling data (Sastri et al., 1973; Kumar, 1983; Prabhakar and Zutshi, 1993; Mohinuddin et al., 1993; Rao, 2001; Gupta, 2006; Bastia, 2006). However, the structural continuity in terms of the onshore-offshore tectonic linkages, the rift architecture and the deeper crustal structure has not been comprehensively studied to understand the development of the basin as a whole. Furthermore, the rift-drift history of India and East Antarctica might have left its imprints both at the coast and the adjoining Bay of Bengal (BOB) ocean floor in terms of the structural features in the crust which are not clearly understood. In the present study, we address these issues through an integrated interpretation of the onshore and offshore geophysical data (seismic, gravity and magnetic) across the margin with both 2-D and 3-D constrained potential field modeling. Knowledge of the structural architecture from the onshore to the deep offshore areas of the margin is useful for defining the geometry and structural parameters of the basin and for the delineation of the Continent-Ocean Boundary (COB). These elements contain a record of the early continental rift-drift history and are extremely important for evaluating the deepwater hydrocarbon potential. This study is also expected to provide deeper insight into the variations in structural styles and segmentation of the margin and the role of pre-existing structures during the development of the margin architecture.