Interseismic uplift of anticlines above the Rakhine-Bangladesh Megathrust from ALOS-2 InSAR

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.