Exhumation and Uplift of the Ladakh Range, NW Himalaya: New Constraints from Low-Temperature Thermochronology

Abstract:

The trans-Himalayan magmatic belt is a 2500 km long zone that hosts igneous rocks generated along plate boundaries during the course of Tethys Ocean closure and India-Eurasia collision. In the NW Himalaya, the Ladakh arc developed through late Mesozoic to Cenozoic time until at least ~45 Ma. During the ongoing collision, the Ladakh arc was exhumed and uplifted, leading to exposure of the Ladakh Batholith as one of the most prominent mountain ranges in the western Himalaya (elevation over 5.8 km). The exhumation history of the Ladakh Range has been studied with low temperature thermochronology. Existing apatite fission track ages are generally young along the northern range (as young as ~5-6 Ma) while AFT ages from the south are older, spanning from ~22 to 35 Ma. The exhumation history of the Ladakh Range is further constrained with other geological data. A depositional contact between the batholith and the overlying coarse-grained Indus molasse has been identified along the southern side of the range, ~ 35 km west of Leh. Accumulation of the molasse is constrained by paleontological and radiometric data to the latest Oligocene.

This study documents new zircon and apatite fission-track ages, and apatite U-Th/He ages from the southern Ladakh Range, along the contact between the batholith and overlying molasse deposits. Three types of samples were analyzed, granodiorites, intruding dikes, and granitic clasts in the molasse. Zircon fission-track ages of ~ 50-55 Ma and ~30-40 Ma were obtained for plutonic rocks and intruding dikes, respectively. Apatite fission-track ages are from ~25-30 Ma with bimodal track length distribution, implying a phase of re-heating to the ~120-60 °C temperature range. When modelled with geological constraints, our data yield new constraints on the emplacement depth for the intruding dikes and the overall exhumation history of the Ladakh Range.