Dextral slip along the Western Nepal Fault System accommodates significant orogen-parallel shear

Abstract New mapping in northwestern Nepal documents geomorphic landforms that are truncated and offset by active faults along the Western Nepal Fault System (WNFS), a right-slip fault system that accommodates orogen-parallel strain in the West Nepal Himalaya between Simikot and Tansen. Neotectonic field investigations in this study focus on the northwestern segment of the WNFS in the Jumla and Dolpa districts, where field observations of dextral-oblique slip on the Talphi, Kuri Santha, and Tripurakot faults indicate active deformation within the Himalayan thrust wedge. The surface trace of the Talphi fault is expressed by prominent fault scarps, deflected and offset stream channels, and shutter ridges that delineate an ~40 km long, subvertical, ~N40°W striking fault that traverses areas of high topographic relief. Our field mapping documents truncated and offset fluvial terrace risers that, when combined with Quaternary geochronology, suggest a minimum dextral-oblique slip rate of 2.7 ± 0.6 mm/yr since ca. 130 ka. The Kuri Santha fault strikes subparallel to the Talphi fault, and mapping of offset fluvial terrace treads and Quaternary geochronology suggest a minimum fault slip rate of 1.0 ± 0.2 mm/yr since ca. 43 ka. Approximately 60 km to the southeast, near Tripurakot, shutter ridges provide compelling evidence for dextral displacement along an ~N65°W striking, steeply northeast dipping fault. Several geomorphic features documented along strike of the Talphi, Kuri Santha, and Tripurakot faults are consistent with surface expressions of active structures, suggesting all three faults reach >40 km in length. The similar geometry, kinematics, and proximity of the Talphi, Kuri Santha, and Tripurakot faults is consistent with a kinematic link between the structures, which would imply these three surface-rupturing faults could produce large magnitude earthquakes, a critical contribution to seismic hazard and risk analysis in western Nepal. Our data are consistent with a kinematic linkage between the Karakoram fault and a foreland-ward propagation of orogen-parallel strain. The Talphi, Kuri Santha, and Tripurakot faults also show geometric similarities with splay faults in marine subduction zones that accommodate intra-wedge deformation and produce repeated late Quaternary surface ruptures. Our study underscores the importance of understanding intra-wedge fault splays in mega-thrust fault systems, globally.