LEVERAGING SIGNALS OF UPLIFT AND EXHUMATION TO CONSTRAIN RAMP LOCATION AND PERIODS OF OUT-OF-SEQUENCE FAULTING

The magnitude and location of fault-driven uplift and erosional exhumation in a fold-thrust belt like the Himalaya are controlled by the size and location of décollement ramps and the extent of displacement over the ramp. Thus the observable signals of uplift and exhumation at the surface, like young cooling ages and geomorphic indicators (e.g. increased slope, relief, and river steepness), provide insight into the geometry and kinematic order of faults. Traditionally, décollement geometries are constrained using balanced cross-sections and positioning ramps to minimize the amount of shortening required in the forelandward portions of the fold-thrust belt. This focus on minimizing the shortening amount ultimately disconnects the ramp locations from the signals of uplift and exhumation at the surface and may cause a disparity between the proposed ramp locations and distribution of young cooling ages. In far western Nepal, previous studies have proposed fault geometries with active ramps at ~120 km to ~40 km south of the Main Central thrust (MCT). While geomorphic metrics indicate recent uplift in the region of the proposed ramps, the ~5-10 Ma apatite fission track and (U-Th)/He ages suggest limited amounts of young exhumation. Additionally, muscovite ⁴⁰Ar/³⁹Ar ages decrease from ~14-18 Ma at ~100-30 km south of the MCT to <6 Ma north of ~25 km south of the MCT, indicating significant young uplift of rocks at ~20 km south to ~20 km north of the MCT. We evaluate the changes in ramp geometry and kinematic sequence, particularly the importance of out-of-sequence (OOS) faults, that are necessary to reproduce the measured signals of uplift and exhumation along the Simikot transect by integrating new and published cooling ages, basin accumulation data, and geomorphic uplift indicators with thermokinematic forward models of balanced cross-sections. The best fit to the measured datasets is produced by models with the active ramp positioned at ~13 km north of the MCT and a kinematic sequence that incorporates young OOS motion on the Ramgarh-Munsiari thrust and a fault immediately south of the Lesser Himalayan duplex, beneath the Dadeldhura klippe, at ~6-5 Ma and <1 Ma. Thermokinematic model results emphasize the importance of a northernly ramp location, OOS faulting in the high Himalaya, and the requirement that the active ramp is co-located with the youngest measured cooling ages.