Tectono-sedimentary evolution of upper Assam “inter-orogenic” peripheral foreland basin, India

The northern periphery of the Indian craton is the leading edge of the under-thrusting continent in the continent–continent collision set up between the Indian and Eurasian plates. It is featured by the extensive peripheral foreland basin from Rajasthan in the west to Upper Assam valley in the east, bordering the foothills of the Himalaya. Upper Assam represents the northeastern corner of the Indian continental plate. This part of the craton is caught up between the Himalayan orogen on the northwest and the Assam–Arakan orogen on the southeast due to collision with the Eurasian and Burma microplate/Indosinian plates, respectively. Continued synchronous convergence of both the plates resulted in flexing of the narrow cratonic platform between them on either side and deposition of thick sediment wedges. This resulted in the development of two conjugate peripheral foreland basins sloping away from each other on either side of a central arch called the Bramhaputra Arch. This special type of foreland basin is described here as the “Upper Assam inter-orogenic peripheral foreland basin” (Biswas et al. 1993). Whereas the collision with the Eurasian plate is a straight-edge collision, that with the Burma microplate/Indosinian plate is oblique. In the southeastern flank, pre-orogenic marine shelfal and flysch sediments, which were deposited on the southeastern passive margin of the Indian plate, are stratigraphically overlain by syn-orogenic molasse in the northeastern sutured part of the obliquely disposed colliding plates. As the two plates converge progressively southward, the molasse sediments prograde southwestward and the inner tongue with the flysch sediments in the remnant ocean basin of the Bay of Bengal. These sediment dynamics are discussed here. The Indian plate drifted into a convergent setup as it is positioned opposite the Andaman-Sumatran arc in the Late Eocene–Oligocene. In this setup the northeastern part, which includes the Upper Assam–Tripura–Bengal basin, is characterized by shelfal sediments thrust over by the deep marine sediments in the subduction complex (Schuppen Belt) and overlain by the prograding packages of deltaic molasse sediments. In the northwestern Himalayan front, post-orogenic fluvial molasse sediments are deposited on the Proterozoic basement, and the autochthonous occurrence of pre-orogenic marine sediments is doubtful.