TECTONIC CONFIGURATION OF THE BENGAL BASIN: MIOCENE JUXTAPOSITION WITH ASSAM, INDIA

The Bengal basin, a remnant ocean basin south of the eastern Himalayas, exhibits dramatic variability in Neogene sediment thickness that reflect a complicated depositional and tectonic history. Sedimentary and seismic data show evidence of thickening of basin fill toward the south, opposite of the pattern seen in foreland basins. This is presumably due to sedimentary loading of voluminous deltaic sediments near the continent-ocean boundary and basinward downfaulting analogous to that on the Gulf of Mexico. Isopach data show that there is considerable vertical relief along the base of the Miocene stratigraphic sequence, probably due to down-to-the-basin faulting caused by focused deltaic sedimentation and associated crustal flexure. In contrast, when viewed in east-west profile, basin shape is more typical of a foreland basin, with sediment thickening eastward toward the Indo-Burman ranges, which reflect an east-west component of convergence. Along the northernmost margin of the basin, Neogene strata thicken locally northward into the Sylhet trough. This is attributed to southward thrusting of the Shillong Plateau along the Dauki fault. In the northwest, the buoyant continental crust has allowed very little accommodation space for sedimentation.

Non-orogenic detritus in pre-Miocene strata in the Bengal basin was apparently derived from the neighboring Indian craton from the west, accumulating on crust of the Indian plate prior to arrival of the clastic wedge shed from the approaching orogen. The Bengal basin was protected from orogenic sedimentation during pre-Miocene time, either by a barrier to sediment transport (a peripheral forebulge or a marine basin?) or simply by distance. If “distance” was the cause, then the part of the Indian plate represented by the Bengal basin was still far to the south of Asia until the Miocene when it arrived close to Assam, India, to receive detritus from the orogenic highlands fringing Asia’s southern boundary. Motion of this part of the Indian plate relative to now-adjacent SE Asia was most likely accomplished along strike-slip faults, like the N-S trending Kaladan fault, located just east of the Bengal basin. Ongoing study is directed toward constraining provenance history, paleolatitudinal position of the Bengal and Assam basins during the Tertiary, and structural details of the Kaladan fault.