Active Orogen Parallel Strike-Slip Faulting in the Lower Dolpa Region, Northwest Nepal: Implications for Expansion of the Himalayan Arc

We present preliminary results from geologic mapping and remote sensing in northwest Nepal that reveals the geometry and kinematics of a significant right-oblique slip fault we term the Tibrikot fault system. The Tibrikot fault system strikes approximately N70° W and a continuous well-exposed segment of the fault extends for approximately 40 km. The trace of the Tibrikot fault system lies in between the Thakkola graben to the east and Gurla-Mandhata fault system to the west, cutting obliquely across Greater Himalayan crystalline rocks and the Lesser Himalayan sequence. Where exposed, the Tibrikot fault is linear in regions of high relief suggesting it has a sub-vertical dip. Geologic mapping shows that deformation is mainly accommodated by right-slip motion with minor normal sense-of-shear. Brittle fault fabrics include fault striations and tool marks consistent with right-oblique normal slip, and are sub-parallel to the strike of the Himalayan orogen. Disrupted and right laterally offset landforms observed along the Tibrikot fault include fault scarps, river channels, terrace surfaces, and mountain fronts, all consistent with right-slip motion suggesting the Tibrikot fault system cuts across the Main Central Thrust at depth in the Lower Dolpa region of Nepal. Although our preliminary results do not demonstrate that the Tibrikot fault directly strikes into the Thakkola graben or Gurla Mandhata fault system, they may be linked via distributed right-stepping en echelon arrays of right-slip faults based on previous results of regional geologic mapping. If correct, this implies that transtensional structures within the Tibetan plateau are communicating with active strike-slip structures in the High Himalaya. We suggest that the function of this relationship serves to accommodate E- W stretching of the Himalayan thrust wedge driven by outward radial growth of the Himalayan arc.