Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low‐temperature thermochronology and numerical modeling

The Himalayan range is commonly presented as largely laterally uniform from west to east. However, geological structures, topography, precipitation rate, convergence rates, and low‐temperature thermochronological ages all vary significantly along strike. Here, we focus on the interpretation of thermochronological data sets in terms of along‐strike variations in geometry and kinematics of the main crustal detachment underlying the Himalaya: the Main Himalayan Thrust (MHT). We report new apatite fission track (AFT) ages collected along north‐south transects in western and eastern central Nepal (at the latitudes of the Annapurna and Langtang massifs, respectively). AFT ages are consistently young (<3 Ma) along both N‐S transects in the high‐relief zone of the Higher Himalaya and increase (4 to 6 Ma) toward the south in the Lesser Himalaya. We compare our new data to published low‐temperature thermochronological data sets for Nepal and the Bhutan Himalaya. We use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the Pecube code in order to constrain potential along‐strike variations in the kinematics of the Himalayan range. Our results show that lateral variations in the geometry of the MHT (in particular the presence or absence of a major crustal‐scale ramp) strongly control the kinematics and exhumation history of the orogen.