Crustal evolution and sedimentation history of the Bay of Bengal since the Cretaceous

Abstract

Multichannel seismic reflection, gravity, magnetic, and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of India to the Andaman shelf and four NW‐SE profiles in the Western Basin off Madras (seismics on three latitudinal profiles) in the Bay of Bengal. The trend of the fracture zones, the locations of the magnetic chron 34, and the Cretaceous Magnetic Quiet Zone suggest that Greater India separated from Antarctica after a period of transform motion in the early Cretaceous, that is, about polarity chron M0 (120 Ma) and drifted northwestward. Negative gravity anomalies are associated with basement rises including the 85°E Ridge between the continental margin of India and the Ninetyeast Ridge. Magnetic reversals and northward trend of the 85°E Ridge support a hotspot origin of the ridge and its emplacement most likely after the Cretaceous quiet period. Juxtaposition of high‐amplitude hyperbolic reflections, down‐faulted continental blocks buried under thick sediments, and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope, about 80 km seaward of the present continental shelf edge. Eight seismic sequences, as thick as 8.5 km, overlie the early Cretaceous oceanic basement and include four unconformities (lower Eocene, upper Oligocene, upper Miocene, and upper Pleistocene) which have been correlated to the major geologic/tectonic events. We also interpret the late Cretaceous/early Tertiary features on the eastern flank of the buried 85°E Ridge as carbonate reefs. The observation of steep subduction of older (cold) Indian plate beneath the Burmese plate near the Andaman Islands suggests the Sunda Arc in this region as low to intermediate stress subduction zone.