We present evidence for an uninterrupted continuation of Indian continental lithospheric mantle into the adjoining Bay of Bengal to a distance of 400-500 km away from the passive margin. The inference is based on the shear wave velocity image of the uppermost mantle beneath the Bay of Bengal, Bangladesh, and the adjoining Indian craton, computed using ambient noise and earthquake waveform data. The Indian lithospheric mantle is characterized by a shear wave velocity of ~ 4.1-4.3 km at the Moho depth of 35-40 km, progressively increasing to ~4.5-4.7 km/s at least up to a depth of 140 km. This velocity structure continues uninterrupted to about 86{deg} E in the Bay of Bengal. Further east, the thickness of the lithospheric lid decreases to ~90 km and is underlain by reduced shear wave velocity (~4.1-4.3 km/s) in the uppermost mantle. We postulate that the Indian craton is embedded in the western Bay of Bengal and the continent-ocean boundary lay around 86{deg} E. The craton possibly submerged soon after the India-Australia-Antractica rifting at around 136 Ma. The significantly reduced shear wave velocity beneath the eastern Bay of Bengal may be due to reheating of the mantle as a consequence of its interaction with the Kergulean hotspot around 90 Ma.
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