Geometry, kinematics, and slip of the Talphi fault in the Western Nepal Fault System: Implications for strain partitioning during growth of a thrust wedge

Abstract Active deformation within the Himalayan thrust wedge in western Nepal is partitioned between slip along the Main Himalayan megathrust and splay faults within the wedge, most notably the Western Nepal Fault System (WNFS). The WNFS is a regional active fault system that obliquely cuts across the Himalayan thrust wedge from its hinterland in southwest Tibet to the frontal thrust in Nepal. Despite its potentially important role in active deformation of the thrust wedge, little is known about the total slip accommodated across the different segments of the WNFS. This study presents geologic mapping and structural analyses to quantify the geometry, kinematics, and magnitude of slip of the Talphi fault. Our field investigations document that the ~37-km-long Talphi fault, which is part of the northwestern segment of the WNFS, strikes NW-SE (N27°W–N40°W), dips steeply NE (89°–65°), and accommodates primarily dextral slip with a small normal component. To calculate displacement on the fault, we identified three mappable units in the Lesser Himalayan Ranimata Formation on either side of the Talphi fault. These units strike sub-perpendicular to the Talphi fault and are truncated and offset with ~3.2 km of dextral separation in map view. The intersection of the unit boundaries with the Main Central Thrust define piercing lines that yield 1.0–1.4 km of normal dip slip and 3.8–6.6 km of dextral strike-slip, for a total of 3.9–6.8 km of net slip on the Talphi fault. The data and observations presented in this paper suggest that the WNFS has been kinematically linked though time with the southward propagation of the active frontal thrust fault in the foreland as well as the location of crustal thickening. We interpret crustal thickening as a necessary condition to increase the vertical stress and drive the formation of strike-slip splay fault systems.