LATE PLIOCENE COOLING RECORDED BY MONAZITE FISSION-TRACK DATING OF THE LOWER TILA RIVER, WESTERN NEPALESE HIMALAYA

Low-temperature thermochronology (<200 °C) documents cooling in the Earth’s upper crust and provides a powerful tool for investigating tectonic, erosional, and magmatic processes. However, many low-temperature thermochronometric methods lose resolution at the lower end of the temperature range <50 °C making it difficult to resolve more recent relief forming processes. Monazite fission-track (MzFT) is a novel thermochronometer with a laboratory constrained partial annealing zone of <50 – 25 °C, suggesting that MzFT is sensitive to cooling in the upper 1 – 2 km of the Earth’s crust. Despite its promise, the applicability of MzFT remains outstanding due to uncertainties on the natural annealing of fission-tracks in monazite, geochemical control, and possible influence of surface temperature. The Nepalese Himalaya is an ideal natural laboratory to test MzFT due to its high topographic relief, active tectonics, and climate-enhanced erosion. In this study, we present MzFT and Th-Pb ages from the Tila River which incises the western Nepalese Himalaya. Traditional low-temperature thermochronology (apatite fission-track and [U-Th-Sm]/He) from the lower Tila River document cooling at ca. 14 – 8 Ma. A preliminary MzFT central age from the lower Tila River yields a central age of 2.57 ± 0.1 Ma and a Th-Pb age of 31.6 ± 0.8 Ma. The monazites are sourced from a Miocene leucogranite, thus, the Th-Pb age may represent a zone of inheritance. Whereas our MzFT age suggests a more recent cooling mechanism such as enhanced incision by the Tila River, exhumation related to movement along the Main Frontal Thrust, or combination therein.