Locked and loading megathrust linked to active subduction beneath the Indo-Burman Ranges

The Indo-Burman mountain ranges mark the boundary between the Indian and Eurasian plates, north of the Sumatra-Andaman subduction zone. Whether subduction still occurs along this subaerial section of the plate boundary, with 46 mm yr -1 of highly oblique motion, is contentious. About 21 mm yr -1 of shear motion is taken up along the Sagaing Fault, on the eastern margin of the deformation zone. It has been suggested that the remainder of the relative motion is taken up largely or entirely by horizontal strike-slip faulting and that subduction has stopped. Here we present GPS measurements of plate motions in Bangladesh, combined with measurements from Myanmar and northeast India, taking advantage of a more than 300 km subaerial accretionary prism spanning the Indo-Burman Ranges to the Ganges-Brahmaputra Delta. They reveal 13-17 mm yr -1 of plate convergence on an active, shallowly dipping and locked megathrust fault. Most of the strike-slip motion occurs on a few steep faults, consistent with patterns of strain partitioning in subduction zones. Our results strongly suggest that subduction in this region is active, despite the highly oblique plate motion and thick sediments. We suggest that the presence of a locked megathrust plate boundary represents an underappreciated hazard in one of the most densely populated regions of the world.