Bedrock Mapping Of The Talphi Fault Segment Of The Western Nepal Fault System, Western Nepal Himalaya.



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The Western Nepal Fault System (WNFS) is a regionally extensive fault system that cuts obliquely across the high Himalayas in the north to near Tansen, Nepal, in the south. The Himalayan orogen is experiencing strain partitioning via arc-parallel extension, as a result of varying convergence obliquity and varying ramp geometries at depth. The WNFS is the structure accommodating this strain partitioning. The Talphi Fault (TAF) is an important segment of the WNFS and is the focus of this research. The TAF is a NW-SE (~325° – 321°) striking, steeply NW dipping (~87° – 80°), oblique (dextral-normal) fault located in the high Himalayas of western Nepal. The TAF is a ~37 km segment that forms the valley of the village of Talphi, and locally cuts across the Ranimata Formation (RF). The Paleoproterozoic RF (~1.9 – 1.8 Ga) is composed of quartzites, chlorite/sericite phyllites, and minor amounts of amphibolites. The RF can be broken out into three members, tentatively named: Member QP (interbedded thin beds (0.5 m – cm) of yellow quartzite and pelitic chlorite/sericite phyllite), Member P (crenulated pelitic chlorite/sericite phyl-lite with abundant quartz veins/lenses), and Member Q (thick beds of white quartzite (1 m – 0.5 m) and thin beds (~1 cm) of phyllite). Strain is facilitated in different styles by each member. These members are exposed within the RF throughout the study area and strike perpendicular to sub-perpendicular to the TAF with ~2.8 km of dextral separation in map view. The intersection of these members with the Main Central Thrust (MCT) define piercing lines that yield 10.5 – 5.5 km of net slip on the TAF. Foliation/bedding measurements are consistent with a drag fold along strike of the TAF. Faults, mode II fractures, and joints define the width of the damage zone (~10 m) and provide evidence for NW-SE dextral-brittle faulting. Along the TAF, Riedel shear fractures are well developed within the damage zone of the TAF. The orogen-oblique strike of the TAF and the orientation of the foliation within the RF members subparallel to the TAF suggests that this active segment may root into the megathrust by taking advantage of pre-existing shear zones (e.g., MCT).



Structural Geology, Bedrock Mapping, Net Slip, Talphi Fault, Western Nepal Fault System