Slab deformation in the Bengal basin due to uplift of the Shillong Plateau and the Indo-Burmese Ranges since the Pliocene, constrained by a Dynamic Topo-Tomographic technique

Earth’s topography is mainly controlled by the structures associated with density differences of the lithosphere and the crust. This is related to isostatic topographic processes which work in association with mantle-induced deformation that together leads to dynamic topography. In this study, the dynamic topographic model of Rubey et al. (2017) has been used. The model links sedimentary basin evolution with plate tectonics and mantle convection to deliver a quantitative framework to understand the combined roles of mantle convection and subduction processes in time and space. Dynamic topography is different from surface topographic variations and this difference can be used to explain past deformation. In the Bengal basin, sedimentation began in a deep basin and shelf region that endured continuous subsidence, and then became involved with crustal adjustments due to collision and uplift of the Himalayas and later on the Indo-Burmese Ranges (IBR). In this study, the dynamic topographic changes have been used to understand the past deformational history and plate dynamics beneath the Bengal Basin and IBR. The model has been run in a cloud-computing environment using the global mantle convection code TERRA along with the plate reconstruction Gplates software to reproduce dynamic topographic variations. In such conditions the shelf zones are the dynamic topographic representation. The results for Bengal basin region, 22.5° to 24.5°N latitude and 91.5° to 93.5° E longitude for the past 20Ma, showed that high sedimentation in the subducting basinal setting caused rising dynamic topography from 20 to 5 Ma continuously. A negative trend (i.e. subsidence) is seen for the past 5Ma. Moreover, when total change in subsidence in the last 5Ma is considered, it has been observed that the northern front of the Bengal Basin steeply plunged towards the north at a time when the Shillong Plateau was uplifted. While there has been overall subsidence of the region both the Shillong Plateau and IBR rose. Present day seismic tomographic study indicates the presence of denser magmatic mass beneath Shillong Plateau which might also be linked with Indian oceanic plate subduction. The Dynamic Topo-Tomographic Model suggests that slab bending associated with subduction caused detachment of the denser material zones and change in the slab setting above which the thick sedimentary column is stacked. The rise of the rigid Shillong Plateau caused a deformational front in the sedimentary zone, south of the Plateau, resulting in a steep plunging dynamic topography.