TOPOGRAPHIC EXPRESSION OF TRANSFORM FAULTING IN THE HIGH HIMALAYA OF WEST NEPAL

Splay faults resulting from convergence obliquity and slip partitioning are common along convergent margins but are often sparsely documented or omitted from seismic hazard and risk assessment. We hypothesize the active, right-slip Western Nepal Fault System (WNFS), is a northwest-striking splay fault that connects the Karakoram fault (KF) in the north to the Main Frontal Thrust in the south, forming a cross-range splay structure that segments the thrust wedge.

Rugged topography and deeply incised gorges pervade the southern topographic margin of the High Himalayas (commonly referred to as PT₂), which is commonly interpreted as rock uplift resulting from out-of-sequence thrusting or deformation of the orogenic wedge as it passes over a steep mid-crustal ramp in the Main Himalayan Thrust. In West Nepal, the PT₂ deviates from its range-parallel trend, forming a topographic embayment. Previous authors have noted the eastern bound of the embayment is collocated with 1) a bifurcation in the pattern of microseismicity, 2) a broadening of the coupling zone of the Main Himalaya Thrust, and 3) the WNFS. We present geodetic, seismological, and elevation data to investigate the role of active deformation, particularly the WNFS, on the embayment to distinguish between the effects of 1) transform faulting and 2) midcrustal structural architecture.

Fault parallel geodetic velocities across the WNFS indicate a velocity gradient of 4.3 ± 2.2 mm/yr. The velocity gradient across the WNFS is the same, within error, to that measured across the southern strand of the KF, suggesting present-day (10¹ yr time scale) slip on the KF is transferred south-east onto the WNFS, and slip does not continue eastward along the Indus-Yarlung suture zone. We combine observations of channel steepness (Ksn) values, relief, and mean elevations to define physiographic provinces. Physiographic boundaries correlate with active north-striking normal faults identified in satellite imagery and terrane models at extensional stepover segments of the WNFS, plausible mechanisms for creating physiographic boundaries. Our observations imply the WNFS plays a significant role in active deformation and modifying topography of West Nepal. Our results further indicate the WNFS should be considered in future seismic hazard and risk assessments.