Abstract

The shallow portion of a megathrust represents the zone of first contact between two colliding plates, and its rheological properties control the seismic and tsunami hazards generated by the fault. Unfortunately, underwater geodetic observations are sparse due to the high cost of obtaining geodetic data, meaning limited information is available on the interseismic behavior of this part of most megathrusts. The Rakhine-Bangladesh megathrust offers a unique opportunity to probe the behavior of the shallow megathrust as it is the only ocean-continent subduction zone where the near-trench region is fully accessible on land. Here, we use observations from ALOS-2 wide-swath imagery spanning 2015 to 2022 to conduct an InSAR timeseries analysis of the overriding plate within Bangladesh and the Indo-Myanmar Ranges. We identify a narrow pattern of alternating uplift and subsidence associated with mapped anticlines but show that it cannot be explained by plausible rates of slip on the megathrust or other fault structures. Instead, we argue that the deformation is likely caused by active aseismic folding within the wedge above a shallow decollement. We show that estimates of the decollement depth derived from a viscous folding model and the observed anticline spacing are in agreement with previous seismic observations of the decollement depth across the fold belt. We suggest that the role of ductile deformation in the overriding plate in subduction zones may be more important than previously recognized.