SEGMENTATION OF THE HIMALAYAN ARC – THE EFFECTS OF COLLISION OBLIQUITY ON THE DEVELOPMENT OF ACTIVE FAULTS AND EARTHQUAKE POTENTIAL

The Western Nepal Fault System (WNFS) occurs in the High Himalaya of western Nepal. The WNFS is kinematically linked to the Gurla Mandhata-Humla fault system to the northwest, where it cuts obliquely across the Himalayan thrust wedge, linking at its southeast extent with the Main Frontal Thrust forming a branch-line geometry. The WNFS is an active structure with continuous fault scarps for at least 65 km, documenting a series of linear NW-striking fault segments cutting across areas of high relief, with dominantly right-slip motions, and with a lesser component of north-directed slip. The linear fault segments are connected in between by ~N-striking normal faults forming extensional step-overs. Trenching at the base of the normal fault scarps reveals at least 10 m of displacement that occurred between AD 1165 and 1400. We suggest that paleo seismic events to the west of the 2015 Gorkha earthquake should be considered in more detail when evaluating seismic hazards for the Himalaya, and more generally, the three dimensional kinematics of thrust wedges.