SEGMENTATION OF THE HIMALAYAN THRUST WEDGE BY AN ACTIVE CROSS-OROGEN TRANSTENSIONAL FAULT SYSTEM

Field mapping in the High Himalaya of western Nepal reveals a regionally extensive, active right-slip fault system termed the Western Nepal fault system (WNFS). Right and normally offset Quaternary features exist along the extent of the WNFS. The WNFS accommodates right-slip along two strike-slip faults striking N40-50˚W(Tibrikot and Dogari faults) that are linked via an extensional right stepover striking N10-20˚E(Tarakot extensional stepover). Detailed neotectonic mapping documents a 63 km-long earthquake surface rupture, likely to have been produced by two seismic events. Radiocarbon dating of fault-related sediments indicates that it locally ruptured between AD 1165-1400, a time interval that overlaps with large range-front earthquakes. Fault slip along strike-slip segments is dominantly right-slip and oriented N70˚W with a minor amount of dip-slip. Decomposition of the arc-parallel velocity component from station DLP0 along the geometry of the WNFS shows a strong correlation to the observed style of faulting which supports our interpretation that the WNFS formed as a result of arc-parallel strain. On the basis of geometry, kinematics, and structural position we correlate the WNFS to active faults along the Karakoram fault, the Gurla Mandhata-Humla fault, the Dhaulagiri Southwest fault, and Bari Gad fault. This suggests an ~ 350 km-long transtensional fault system extending obliquely across the Western Nepal Himalaya that appears to intersect the Main Frontal thrust (MFT) near 83°30’E.This location coincides with a large gradient in the arc-parallel component of GPS velocities. Based on the geometry, kinematics, and position of the WNFS in the thrust wedge we interpret that it belongs to a class of strike-slip faults that work in concert with subduction to accommodate obliquely convergent plate motion. This implies that the region lying between the MFT and the WNFS is a continental version of a forearc sliver bounded at its base by the Main Himalayan thrust.