Monazite Fission-Track Dating of Erosion and Tectonics in the Nepalese Himalaya

Abstract

A modern orogenic system represents the culmination of tectonic and surface processes over geological time, posing challenges in isolating direct mechanisms and associated timing of relief formation. Modern relief-forming processes are typically dominated by more recent, low-magnitude erosion/exhumation events (<1 – 2 km), which are difficult to resolve with established low-temperature thermochronology and cosmochemistry methods. Monazite fission-track (MzFT) is a relatively new low-temperature thermochronometer with a laboratory assessed partial annealing zone of 140 – -70 °C. Field based estimates, however, have found MzFT to be a useful proxy for cooling through <50 – 25 °C and related to exhumation in the upper 1 – 2 km of the crust. Given the uncertainties between laboratory- and field-based estimates, interpreting a MzFT age can be challenging due to the relative contributions of fault-driven denudation, surface processes and surface temperatures. To assess the applicability of MzFT to resolving upper crustal exhumation, we apply MzFT to the Nepalese Himalaya, an actively uplifting and highly erosive orogenic system associated with a cool subtropical to temperate climate. We focus on the well-studied Kali Gandaki River in central Nepal, which has been previously targeted by a range of established low-temperature thermochronometers. We collected four MzFT samples, with three samples being part of an elevation profile (2498 – 2966 m) and two apatite fission-track (AFT) samples from within the Greater Himalayan Sequence across the Ramgarh Thrust, Main Central Thrust and South Tibetan Detachment. We integrate MzFT data with existing low-temperature thermochronology data and single-grain monazite and apatite geochemistry to isolate incision and/or faulting processes as the driver of rock cooling. MzFT data yield ages of ~0.4 – 0.09 Ma, which we interpret to correlate with recent movement of the Main Frontal Thrust since at least ~280 k.y., and concurrent river incision of the Kali Gandaki drainage.