Geophysical signatures over and around the northern segment of the 85°E Ridge, Mahanadi offshore, Eastern Continental Margin of India: Tectonic implications

Abstract

The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.