Short- and long-term fault-slip kinematics of the Churachandpur-Mao fault zone, Indo-Burman Ranges.

Abstract:

The Indo-Burman Ranges (IBR) are a ~375 km-wide accretionary prism, located at the northern end of the Sunda subduction zone, which accommodates oblique convergence (~70º; ~46 mm/yr) of the Ganges-Brahmaputra delta on the Indian Plate with the Shan Plateau of SE Asia. GPS measurements indicate that the northward component of convergence is mostly partitioned along two north-striking dextral faults, known as the Sagaing and Churachandpur-Mao faults (CMF). The eastward component of convergence (~13-17 mm/yr) is absorbed by the westward propagating IBR fold-thrust belt. While the fold-thrust belt, Sagaing fault, and northern segment of the CMF are well constrained by GPS measurements and field studies, the extent, structure and significance of active faulting along CMF south of ~24ºN is largely unknown. We combine geologic mapping, tectonic geomorphology, fault-slip analysis, and InSAR to evaluate faulting models. Using a 12m shaded relief DEM, we identified several NW-striking structural lineaments which cross-cut the IBR fold-thrust belt within an ~35 km wide zone along-strike of the CMF between 24ºN and 23ºN. We mapped geomorphic indicators of active deformation including shutter ridges, flat irons, sag ponds, and fault gouge. Fault-slip data indicate that the NW-striking cross-faults are sinistral. SW-NE shortening axes from fault-slip data are parallel to the local shortening direction from geodetically modeled regional strain rate maps. Within fault blocks, fold axes and minor thrusts appear to have rotated ~30-45º clockwise from the northerly structural trend of the Indo-Burman fold-thrust belt. Within fault zones fold axes trend NW, subparallel to the cross faults and indicating some fault-normal shortening. InSAR results from one cross-fault reveal some dip-slip motion, however, basin subsidence and hydrological effects must also be considered. Our results document NW-striking sinistral-reverse faults that cross cut the fold-thrust belt. We present a tectonic model in which fault blocks rotate clockwise around a vertical axis to partly accommodate the NE-SW convergence compatible with the dextral motion on the CMF. Our study contributes new mapping of the potentially seismogenic CMF and describes how oblique convergence is accommodated across a subduction-accretion wedge.