Slip-partitioning above a shallow, weak décollement beneath the Indo-Burman accretionary prism

The Indo-Burman Ranges (IBR) are an ∼375 km wide accretionary prism that accommodates oblique convergence of the ∼13-20 km thick Ganges-Brahmaputra Delta on the Indian plate with the Shan Plateau to the east and Shillong massif to the north. The IBR are entirely subaerial and adjacent to one of the most densely populated (>140 M people) regions on the planet, with the potential to generate a Mw ≥ 8.2 megathrust earthquake. To determine the kinematic evolution, décollement geometry, and geologic deformation rates near the front of the IBR, we combined geologic field mapping, detrital thermochronology, and structural analysis of eight antiforms that define the ∼120 km wide outer belt. The antiforms are bivergent fault-propagation or detachment folds that record plane-strain and east-trending horizontal shortening perpendicular to the axial trace of the folds, indicating nearly full slip-partitioning with the fold-belt normal and parallel components of convergence dominating, respectively, the front and back of the IBR. At 24°N the antiforms have accommodated ∼38.4 ± 16 km of shortening since ∼8 Ma above a ∼3-4 km deep, weak, subhorizontal décollement. Results indicate a shortening rate of ≥4.8 mm/yr, at least 28-37% of the arc-normal geodetic rate, and thrust front propagation rate of ≥15 km/Myr. The shallow décollement is laterally continuous with a regional detachment previously imaged by industry seismic reflection profiles to the west, north, and south of the study area. We interpret this décollement to be the up-dip part of the megathrust that has the potential to accommodate large coseismic slip during a great earthquake.