Decoupled deformation between crust and mantle beneath Indo-Burmese Wedge: A new seismotectonic model

Abstract

Ongoing oblique convergence at the eastern margin of the Indo-Eurasian collision zone provides a natural laboratory for studying the deformation and dynamics of subduction beneath the Indo-Burmese Wedge (IBW). Here, we conduct the first comprehensive seismological investigations to understand the mechanical coupling between the crust and mantle beneath IBW using shear-wave splitting analysis and stress modeling. The deformation patterns in the crust signify a strong E-W compressional stress regime throughout IBW, with negligible influence from the major geological structures. These observations derived from local seismicity strongly support that the eastward active subduction of the Indian plate beneath the Burmese sliver is responsible for the crustal-scale deformation. Contrary to the crust, our splitting measurements from the mantle are in line with the major N-S trending arcs created by slip-partitioning due to transpressional oblique subduction. The splitting measurements with an N-S orientated fast axes and the estimated depth of the anisotropy source obtained from the spatial coherency of splitting parameters strongly suggest the presence of trench-parallel sub-slab flow system driven by slab retreat with westward trench migration, which can be the major controlling mechanism of the mantle deformation beneath IBW. Throughout the IBW, a significant change in the orientations of stress and splitting parameters between the crust and mantle supports a decoupled deformation scenario, implying the necessity of a new seismotectonic model. Our integrative study on the present stress patterns and decoupled deformation mechanism between crust and mantle combined with anisotropy measurements beneath the IBW suggests active subduction in the present scenario.