Geophysical Studies of the Conjugate Margins of Eastern Gondwana to Understand the Early Cretaceous Evolution of the Bay of Bengal

Plate reconstruction models for the early separation of India from Antarctica are in circulation, and two theories are more predominant. One theory proposes the breakup of eastern Gondwana took place in the early Cretaceous, while the other favors at the beginning of middle Cretaceous. These two theories are severely constrained by inadequate magnetic data and age information in the conjugate margins i.e., the Bay of Bengal of the eastern continental margin of India, and the Enderby Basin, east Antarctica. Apart from the speculative breakup time, these two conjugate margins were also strongly influenced by the major plate reorganizations, hotspot activity and intraplate volcanism. We made an attempt to understand the evolution of these two conjugate margins through analyses of about 40,000 line kilometers of magnetic, gravity and bathymetry data that was acquired by the National Institute of Oceanography, India under the program "Crustal studies of the Bay of Bengal" and about 30,000 line km of magnetic and gravity data in the Enderby basin retrieved from the National Geophysical Data Center (NGDC), Colorado. Our interpretation reveals the following. The Bay of Bengal is characterised by oceanic crust and the breakup initiated prior to the formation of Mesozoic magnetic anomaly M11, and the ocean floor is associated with approximately NE-SW trending Mesozoic anomalies M11 through M0. The oceanic crust is also associated with the 85°E and Ninetyeast Ridges. These two ridges differ contrastingly in terms of their occurrence at varying depths and geophysical signatures. The physiography and geophysical anomalies of the Ninetyeast Ridge are akin to those ridges associated with known hotspot traces. On the other hand, the subsurface 85°E Ridge occurs at much deeper depths (>4 km) and is characterised by steep gradient negative gravity and well defined magnetic anomaly signatures. We interpreted the origin of the 85°E Ridge as an outpouring of magma through the lithosphere fracture during mid Cretaceous major plate reorganization. Analysis of geophysical data obtained from NGDC in the Enderby basin indicates the presence of magnetic anomaly M11 towards coast, and the younger sequence in deep offshore. Plate reconstructions under the new constraints of Mesozoic anomalies and fracture zones indicate that India separated from the contiguous Antarctica-Australia during early Cretaceous and the occurrence of M11 anomaly on both the margins unequivocally suggest that the breakup occurred prior to the formation of M11 crust with an initial average half-spreading rate of about 4.0 cm/yr. The continent ocean boundary in the eastern continental margin of India has been deciphered from the magnetic data in conjuncture with shipborne gravity data and satellite gravity images, and it coincides with the ~2500 m depth contour. Recently, some researchers proposed the presence of extinct spreading ridge based on few profiles in the southern ocean and suggested that no crust older to M2 could be present in the Bay of Bengal. However, our study shows the presence of complete sequence of Mesozoic anomalies M11 through M0 in the Bay of Bengal, south of Krishna Godavari offshore and in its conjugate the Enderby basin, east Antarctica, and strongly warrants for additional geophysical data to validate the presence of extinct spreading ridges.