Evidence for paleo-continental margin and folding of relic fragment of oceanic lithosphere, northeast Indian subcontinent

The processes of continental breakup, reassembling of landmasses, and huge deltaic sediment depositions contribute to reconfiguring the Earth's surface. Here, we show a classic example of contiguous structures of paleo-continental margin on the eastern Indian Shield and an ancient fragment of the oceanic lithosphere beneath Bangladesh. In the present work, we collated gravity data from Bangladesh and adjoined parts of the Indian shield and the northern Bay of Bengal to decipher rocks' nature and long-term behavior. The gravity map illustrates N-S oriented alternate bands of ∼150 km wide positive and negative anomalies in the Bangladesh region, besides showing signatures of prominent geomorphic features such as the Shillong Plateau, Sylhet Trough, Burmese Arc, etc. The crustal models reveal the presence of paleo-continental margin segments on the eastern edge of the Singhbhum craton and south of the Shillong Plateau, ∼150 km wide folded oceanic crust beneath Bangladesh, and a low slab dip of the subducting Indian plate beneath the Burma platelet. The inferred continental margin evolved after the breakup occurred between the Rajmahal-Sylhet Line and continental fragments (Elan Bank and southern parts of the Kerguelen Plateau) at about 120 Ma. Subsequently, the sediments carried by two great Himalayan river systems were excessively deposited on the margin and over an older fragment of the oceanic lithosphere, resulting in the progradation of the Bengal delta towards the ocean. The oceanic crust beneath Bangladesh was folded with approximately 150 km wavelength as the compressive stress regime is locally operative due to the eastward moment of the Indian plate and westward propagation of the Indo-Burman Wedge.