EVIDENCE FOR RECENT DEXTRAL SLIP ALONG THE WESTERN NEPAL FAULT SYSTEM IN NORTHWEST NEPAL

New mapping at two sites in northwestern Nepal documents geomorphic landforms that are truncated and offset by active faults along the Western Nepal Fault System (WNFS). The WNSF is comprised of three primary segments, 1) a northwestern right-slip segment that traces from southern Mugu district to Juphal; 2) a region comprised of right-slip faults and extensional stepovers including the Dhaulagiri Southwest fault, and the Tarakot extensional stepover near the Dhorpatan Hunting Reserve; and 3) a southern right-slip segment that includes the Bari Gad fault near Tansen that we hypothesize forms a branch-line with the Main Boundary Thrust. Neotectonic field investigations in spring, 2019 focused on the northwestern right-slip segment from Talphi to Tibrikot.

At site 1 near Talphi, prominent fault scarps, deflected and offset stream channels, and shutter ridges delineate a ~20 km long subvertical fault segment, striking ~N40W with a linear fault trace traversing areas of high topographic relief. Our field mapping documents two dextrally offset markers: an abandoned stream channel, consistent with 400 ± 91 m of dextral separation and a terrace riser, consistent with 52 ± 8 m of dextral separation. Approximately 60 km to the southeast at site 2 near Tibrikot, shutter ridges provide compelling evidence for dextral displacement along a ~N65W striking, steeply northeast dipping fault. Upstream of the main fault trace, our detailed field observations document a series of fluvial terraces, one of which is truncated and offset by two NW-striking fault splays, consistent with 38 ± 7 m and 47 ± 13 m of dextral separation across the south and north fault splays, respectively.

Results from this study will improve our understanding of seismic hazard and risk in Western Nepal, and the development of continental slivers and splay faults – all ubiquitous features of obliquely-convergent margins.