MONAZITE FISSION-TRACK DATING OF RIVER INCISION IN THE NEPALESE HIMALAYA

The modern topography represents the culmination of coupled tectonic and surface processes over geological time, posing challenges in isolating specific mechanisms, their temporal characteristics, and durations. Presently, it is challenging to record low-magnitude erosional events which play a significant role in connecting tectonic and surface relief forming processes. Monazite fission-track (MzFT) has a laboratory assessed closure temperature of <50 – 25 °C and presents itself as a useful proxy for documenting exhumation in the upper 1 – 2 km of the crust. Despite it’s promise, interpreting an individual MzFT age remains uncertain due to the relative contributions of fault-driven denudation, surface processes, surface temperatures, and grain geochemistry. Thus, MzFT requires empirical calibration to fully understand its capabilities and limitations. Here, we apply MzFT to the Nepalese Himalaya, an actively uplifting, highly erosive orogenic system, to connect tectonic and relief-forming processes and meaningfully determine exhumation associated with such a low closure temperature. We focus on the well-studied Kali Gandaki in western Nepal and collected two MzFT age-elevation profiles (1540 m – 2230 m; 2498 m - 2966 m) from within the Lesser Himalayan and Greater Himalayan across the Main Central Thrust. We integrate our MzFT profiles with existing low-temperature thermochronology data, including Zircon fission-track, Zircon (U-Th)/He, Muscovite ⁴⁰Ar/³⁹Ar (Pye et al., 2022), Biotite ⁴⁰Ar/³⁹Ar, and Apatite (U-Th)/ He (Pye et al., 2023) and single-grain monazite geochemistry in order to isolate incision and/or faulting processes.