Thin oceanic crust below Bay of Bengal inferred from 3-D gravity interpretation

Abstract

The Bay of Bengal (BOB) lithosphere evolved due to rifting between India and Antarctica and subsequent seafloor spreading process since the early Cretaceous is known for the presence of thick Bengal Fan sediments covering the entire BOB with maximum sediment accumulation of 22 km in the offshore Bangladesh. Three-dimensional crustal structure of the BOB is investigated by integrating the sediment isopach map, seismic reflection, refraction and wide-angle reflection data with the 3-D gravity modelling. Large amount of sediment seismic velocities have been used to construct depth varying density contrast with respect to the underlying crustal layer due to deeper burial effect by a quadratic density function. 3-D gravity effect of the sedimentary layer computed for the derived density distribution shows large variations ranging from −5 mGal over the Ninetyeast Ridge near the equator to as low as −210 mGal at the apex of Bengal Fan in the Bangladesh shelf region. The residual crustal Bouguer anomaly map obtained after subtracting the effect of water and sediment layers and normalised for 6 km uniform crust display crustal thickness variations with depth to Moho in the BOB region varying from 11 km in the south to as much as 35 km in the Bangladesh shelf with Moho depressed locally below both 85° E and Ninetyeast ridges. The total crust map obtained after subtracting the water and sediment layers from the Moho depth map shows an unusually thin oceanic crust of 2–4 km in the deep basinal areas of BOB and 10–12 km of thick crust below 85° E and Ninetyeast ridges. Due to the widespread nature of observed thin crust as well as the presence of plume traces that have emplaced 85° E and Ninetyeast ridges, we infer its formation during plume–spreading interactions in the eastern Indian Ocean. Further, higher flexural rigidity of lithosphere and cold mantle in the BOB region indicates that huge amount of Bengal Fan sediments were emplaced over the unusually thin oceanic crust and a strong mantle similar to that observed in the Amazon Fan.