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
Highlights
► Significant variation in crustal structure along the margin indicates lateral segmentation of K–G basin during breakup. ► Study reveals emplacement of high density proto-oceanic crustal rocks along the continent-ocean transition. ► Basement high in the deep offshore K–G basin is the continental fragment formed during the breakup of Elan Bank from India.
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.
Section snippets
Regional geotectonic setting
Regional geophysical investigations at the Eastern Continental Margin of India (ECMI) suggest that the margin is a composite with two segments, the northern part (north of 16° N), which has a rifted margin character, and the southern part, which developed because of shearing during the early stages of continental separation (Subrahmanyam et al., 1999; Chand et al., 2001; Krishna et al., 2009). Different views exist on the timing of the breakup and rifting between India and East Antarctica, and
Methodology and data analysis
In the present study, an integrated geological interpretation of various geophysical datasets, such as gravity, magnetic, multi-channel seismic reflection, and refraction data in the onshore and offshore regions of the K–G basin is conducted in terms of the structural setup of the basin. The DNSC08 version of the high-resolution 1′ × 1′ satellite-derived free-air gravity and bathymetry data (Andersen and Knudsen, 2008) in the offshore and the GTOPO30 topography and Bouguer anomalies (Kumar, 1983
Crustal architecture
The seismically constrained gravity models that were presented above provided new information about the regional variations in the crustal structure below the K–G basin. The models indicate that the crust at the eastern Indian shield margin is 39–41 km thick, and thins to nearly 20–23 km at the Ocean Continent Transition (OCT) in the offshore area. A sudden rise in the Moho, which is correlated with a significant change in the gradient of the CBA anomaly, is observed. From the crustal models,
Conclusions
The combined analysis of gravity and seismic data along the central part of the ECMI provided valuable insights about the crustal architecture and early breakup history of the K–G basin at the margin. Several key results of the study are summarized below:
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Gravity-derived crustal models indicate that the crust at the eastern Indian shield margin is 39–41 km thick and thins to as little as 20–23 km at the Ocean Continent Transition (OCT) in the offshore region.
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Significant changes in the nature of
Acknowledgments
We are thankful to Reliance Industries Ltd. for the permission to publish this paper. This work was carried out as part of IRCC-IITB funded research project (07IR025). MR is thankful to IITB for the research grant, and DT thanks CSIR, New Delhi for awarding the research fellowship (SRF). Critical comments and suggestions from two anonymous reviewers and the editor helped to improve the manuscript.
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