Out-of-sequence fault activity in the High Himalaya revealed by luminescence thermochronometry

Two end-member competing models have been proposed to describe the kinematics of the central Nepal Himalayas in the last few Myr. They differ in their interpretations of which surface breaking faults accommodate current shortening and the kinematics responsible for driving rapid exhumation in the topographic transition zone around the Main Central Thrust (MCT). These locally higher uplift and erosion rates in the High Himalaya could reflect (1) thrusting over a midcrustal ramp with the growth of a Lesser Himalaya duplex at midcrustal depth causing underplating along the Main Himalayan Thrust ramp, or (2) out‐of-sequence thrusting along the front of the High Himalaya, possibly driven by climatically controlled localized exhumation.

To decipher between the two tectonic models, we compare existing low and medium-temperature thermochronometric data (⁴⁰Ar/³⁹Ar on muscovite, apatite (U-Th)/He - AHe, zircon (U-Th)/He - ZHe, apatite fission track - AFT, and zircon fission track - ZFT), extracted from the world thermochronometric data file of Herman et al. (2013), to luminescence thermochronometry data from 61 newly collected rock samples along transhimalayan rivers between the Kali Gandaki and the Trisuli. The luminescence thermochronometry data provide a new perspective on Late Pleistocene exhumation rates (timescales of 10⁴ to 10⁵ years) of the Nepalese Himalayas, by offering quantitative high-resolution constraints of rock cooling histories within the upper kilometres of the Earth’s crust.

All of the thermochronometric data show younger ages and higher exhumation rates around the topographic transition and the MCT zone through central Nepal. For the higher temperature thermochronometers, there is a continuous trend towards younger ages from the Lesser Himalaya through the topographic transition and the MCT zone. These data suggest that the in-sequence model, with exhumation rates linked to increased erosion and the formation of a duplex below the Higher Himalayas, coincident with the MCT location in some areas, is the model that best describes the thermochronometric ages of this study area on Myr timescales. However, the luminescence thermochronometry data reveal a spatial and temporal variability of the higher exhumation rates at different timescales, suggesting an intermittency of exhumation signal due to geomorphological processes. The luminescence thermochronometry data also highlight a systematic sharp transition at the MCT, pointing to out-of-sequence activity at this tectonic boundary on 100-kyr timescales. Whether this difference in tectonic model between the two timescales is due to low resolution of the higher temperature thermochronometers, shallow isotherms deflected by fluid circulation and hot spring activity near the MCT, or to a change in tectonic regime during the last 200 kyr, out-of-sequence activity of the MCT needs to be considered in seismic hazard models as it could put the local population at risk.