Field mapping and structural analysis of an active oblique splay fault system within a subaerial thrust wedge: A case study of the Talphi segment along the Western Nepal Fault System in western Nepal Himalaya.

The Talphi fault (TF) segment is a NW-SE striking oblique-dextral fault located in northwest Nepal and is part of the seismically active Western Nepal Fault System (WNFS). The WNFS is a regionally extensive splay fault system that cuts obliquely across the seismic gap from the high Himalayas in the north to near Tansen in the south. It facilitates arc-parallel extension via strain partitioning as a result of varying convergence obliquity along strike of the orogen as well as varying ramp geometries at depth. The TF is a ~40 km segment that likely forms the valley of the village of Talphi, faulting the Ranimata formation (RF). The RF (~1.3 Ga) is a unit composed of quartzites, chloritic/sericitic phyllites, minor amounts of amphibolites, and is hypothesized to represent the ancient Indian passive margin. Field observations indicate that the RF can be classified into three members, tentatively named: Member P (crenulated phyllite with abundant quartz veins/lenses), Member QP (interbedded thin (<1m) beds of quartzite and phyllite), and Member Q (massive beds of quartzite (>1m) little to no phyllite). These members are pervasive throughout the RF and strike subparallel to the TF providing a minimum estimate that the TF has accumulated ~2 km of dextral offset. Layer parallel-to-subparallel shortening (LPS) is facilitated in different styles by each member. Member P accommodates LPS via minor faulting and folding. Member QP accommodates LPS via spaced cleavage, mode one/two fractures, minor folding, and faulting. Member Q accommodates LPS via large faults, and mode one/two fractures. Foliation/bedding measurements are consistent with a drag fold along strike of the TF. Fractures (n=58), faults (n=15), and their kinematic indicators define the damage zone of the TF and provide evidence for NW-SE dextral-brittle faulting. Along the TF, Riedel shear fractures are well developed and rotated with the progressive movement of the fault with the main NW-SE striking faults and fractures, indicating simple shear accommodation. The orogen-oblique strike of the TF and the orientation of the foliation within the RF members being subparallel to the TF suggests that this active segment may root into the megathrust via pre-existing shear zones taking advantage of previously weakened structures.