From partial to full strain partitioning along the Indo-Burmese hyper-oblique subduction

Abstract

The Andaman–Nicobar Trench and its onshore prolongation—the Indo-Burmese wedge—is the least studied segment of the India–Australia subduction. New offshore geological and geophysical data have recently been collected along the Burma scarp during a marine survey conducted with the R/V Marion Dufresne (Andaman Cruise). Swath bathymetric mapping combined with shallow and deep seismic show that the dominant active tectonics is dextral strike-slip faulting accompanied by various amounts of shortening. The southern portion of the Burma scarp is transpressive (north Andaman Islands), the central portion is pure strike-slip on NNE-oriented segments (southern Myanmar), the northern portion shows sedimentary wedge growth along NW segments, both onshore and offshore (Arakan Yoma prism). Further north, the wedge bends to the west, while dextral shear faults more or less parallel to the Sagaing Fault develop within the internal part of the wedge and elongated NS folds form in its external part, abutting onto the Shillong Plateau. We propose a simple kinematic model involving evolution from partial to full partitioning from south to north along the West Burma Scarp (WBS), and we test it quantitatively using the most recent geodetic results. In the south, half of the 3.5 cm/year of India motion is taken at the trench itself, the other half being accommodated onto a single shear fault, the Sagaing Fault in Myanmar. In the north, where the Bangladesh fold system developed, dextral strike-slip faults are activated within the Arakan Yoma belt and at the accretionary prism–backstop contact, resulting in full partitioning there. Faults accommodating the oblique component of motion of India are progressively migrating in space from far field faults (Andaman–Sagaing) to near trench faults (Arakan Yoma belt and trench itself), the Burma sliver being “buttressed” by the Eastern Himalayas. This kinematic extends back to 4 Ma, which is the time of initiation of the last pulse of oceanic accretion in the Andaman Basin.