Estimation of the Moho depth in the Bay of Bengal using gravity data and understanding of its tectonic implications

At the beginning of the Cretaceous period, India and Antarctica started breaking apart. There were major changes to the seafloor in the Bay of Bengal (BOB) and geodynamic processes after this episode. Therefore, it is interesting to detailed understanding of the tectonics of the BOB. The BOB is surrounded by Bangladesh to the north, the Andaman-Sumatra arc to the east, and the eastern coast of India to the west. Bouguer gravity anomaly, elevation, and sediment thickness data are used in this study to determine the gravity Moho and Isostatic Moho topography of the BOB. The gravity effects of sediments are calculated by using the recent GlobSed model. Gravity Moho is derived from the inversion of sediments corrected gravity data using the Parker‐Oldenburg method. Generally, it is observed that the thin crust is associated with the BOB while the thicker crust is associated with two aseismic ridges: Ninetyeast and 85°E ridges, situated in the eastern and central parts of BOB, respectively. This suggests that these ridges may have formed due to the interaction of the plume-spreading centre. The thick depressed crust beneath the northernmost part of BOB, implies that it is due to a load of sediments, and abrupt ~12 km deepening of gravity Moho from eastern BOB (Sumatra trench) to Andaman Arc. The consequences of the difference between gravity and isostatic Moho for the isostatic state of the crust are examined in order to understand the geodynamics of the study area. The isostatic analysis of crust, which takes into account the difference between the two types of Moho, shows that all of the regions except for the north of Bengal fan, Ninetyeast ridge, and southern region of 85°E ridge are compensated. The Moho of the Andaman Arc and the north of Bengal fan, are overcompensated, which should be uplifted, while the Moho of the Sumatra trench, Ninetyeast ridge, and the southern region of 85°E ridge become depressed. In order to make isostatic compensation of the region, an additional upper mantle density variation between 47 to 62 kg/m³ has to be added. This implies an additional compensation mass is needed under the Ninetyeast ridge and the southern region of 85°E ridge is 47 kg/m³ and 56 kg/m³, respectively, for providing isostatic equilibrium.