Subsurface deformation within the Bengal Basin due to oblique subduction of the Indian Plate at the IndoBurman Subduction Zone

The Bengal Basin, with sediments up to 16-20 km thick, is being obliquely subducted beneath the IndoBurma Sunduction Zone. The thick sediments have created an entirely subaerial subduction zone with an accretionary prism that is ~300 km wide. The prism contains a blind belt primarily consisting of buried detachment folds, an outer belt of emergent fault propagation folds, and an inner belt of greater deformation including out-of-sequence thrusts and dextral-transpressional faults that are rooted in a deeper décollement than the folds in the blind and outer belts. This study investigates the subsurface deformation in Bangladesh where the shallow décollement poses significant seismic risk. The deformation is imaged by a grid of twelve 2D seismic sections associated with gas fields located in the folds. Deformation in the blind belt is quantified using 2D seismic and logarithmic data in the Srikail and Bakhrabad structures. The images show that both structures have gentle limb dips (less than 2°) and are beginning to transition from detachment folding to fault propagation folding, with the development of localized discontinuities in the fore-limb that are developing into break-through thrusts. Seismic TWT sections were converted to depth using well VSPs and seismic RMS velocities. Excess area and line length calculations were used to estimate the horizontal shortening and depth of the décollement. Analyses indicate a shortening of 200-600 m (1-4%) and a horizontal décollement at a depth of ~5500 m. The décollement in the blind belt of Bangladesh is up to 2 km deeper than in the outer-belt where emergent fault-propagation folds are rooted in a 3-4 km deep décollement. This apparent deepening of the décollement toward the deformation front indicates that the shallow décollement is gently folded, either by subsidence with the Ganges-Brahmaputra Delta, uplift of the emergent structures in the outer belt by deformation above an additional deeper décollement horizon, or a combination of both. Ongoing geophysical and field-based research in the IBR will investigate the 3D geometry of the décollement(s) to help inform regional seismic hazard estimates.