Apatite thermochronometry within a knickzone near the Higher Himalaya front, central Nepal: No resolvable fault motion in the past one million years

We combine apatite fission track ages and track-length measurements with apatite (U-Th)/He ages to test the hypothesis that active near-surface faulting is absent at the transition between the Higher and Lower Himalayan physiographic provinces in central Nepal. Fifteen samples define a 2.5-km-long, nearly constant-elevation transect that crosses the largest knickzone on the Modi River in central Nepal. This knickzone could have resulted from modern faulting near the ancient Main Central thrust or from river incision following landslide dam breaching. Apatite fission track ages from the transect are generally ca. 1 Ma, and apatite He ages, selected by considering He concentrations, are 0.5-0.8 Ma. There is no discernable age difference between any of the samples outside 2 sigma uncertainty. Cooling rates are > 100 degrees C/Myr and are uniform across the sample transect. We calculate an average exhumation rate of similar to 3 mm/yr across the region, with rates deduced from the fission track data equal to those extracted from the He ages. According to our new data, no fault in our study area has undergone a time-averaged slip rate faster than 4 mm/yr during the past 1 Myr.