Structural drivers of exhumation in compressional orogens: examples from western Nepal

Abstract. The magnitude and location of vertical uplift in fold-thrust belts is a function of the geometry, duration, and timing of faults and how these structures have evolved over time. Yet in the Himalaya, uncertainties persist in whether vertical uplift and exhumation are driven by sustained displacement over mid-crustal ramps in the basal décollement, pulses of more rapid exhumation during periods of out-of-sequence fault displacement, or a combination of these drivers. In western Nepal, the well-defined zone of steep slopes and high relief that marks the high Himalaya in central Nepal splits into two zones: a northern zone ~10 km south of the Main Central thrust (MCT) and a southern zone ~80 km south of the MCT. While geomorphic metrics indicate active uplift in the southern zone, ~5–10 Ma apatite fission track and (U-Th)/He ages limit the amount of young exhumation. In the northern zone, <6 Ma muscovite 40Ar/39Ar ages indicate significant exhumation. We evaluate variations in ramp geometry and kinematic sequence, particularly the importance of out-of-sequence faults, necessary to reproduce the observed cooling ages, topography, and geomorphic metrics along the Simikot transect by integrating new and published cooling ages, basin accumulation data, and geomorphic uplift indicators with thermokinematic and landscape evolution models of three balanced cross-sections. Model results demonstrate that the northern zone of high relief and young exhumation is a combination of sustained uplift over an active ramp and recent motion on an out-of-sequence fault at ~5 km south of the MCT. The southern zone of high relief is produced by active (<0.6 Ma), but low displacement, surface breaking and subsurface faults. Thermokinematic model results emphasize the importance of a northernly ramp location, co-located with the youngest measured cooling ages at ~13 km north of the MCT, and of the out-of-sequence thrusting at ~6–5 Ma and <1 Ma.