APATITE FISSION-TRACK AND (U-TH)/HE DATING REVEAL NO EVIDENCE FOR ACTIVE FAULTING AT A LARGE KNICKZONE ON THE MODI KHOLA, CENTRAL NEPAL

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 Greater and Lesser 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 has been associated with modern faulting near the ancient Main Central thrust, but more recently was proposed to result from river incision following landslide dam breaching (Walsh et al., 2011). Apatite fission track ages from the transect are generally ca. 1 Ma, and apatite He ages, selected by considering He and effective uranium concentrations, are 0.5–0.8 Ma. There is no discernable age difference between any of the samples within 2σ uncertainty. HeFTy modeling of ages and fission-track lengths reveals cooling rates of ~140 °C/M.y., which also are uniform across the sample transect. We calculate a maximum exhumation rate of 5.6 mm/y across the region, with rates deduced from the fission-track data nearly twice as fast as those calculated from the He ages. According to our new data, no fault with a time-averaged slip rate faster than 3 mm/y has existed in our study area during the past one million years.