No abstract

[1] 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.Citation: Robert, X., P. van der Beek, J. Braun, C. Perry, and J.-L. Mugnier (2011), Control of detachment geometry on lateral variations in exhumation rates in the Himalaya: Insights from low-temperature thermochronology and numerical modeling,

ABSTRACT

International audienceWe study the recent dynamics of the central Nepal Himalaya, focusing on possible reactivation of the footwall of the Main Central thrust, which is marked by an abrupt topographic transition. Different tectonic mechanisms, such as overthrusting of a major crustal ramp, underplating, or out-of-sequence thrusting, have been suggested to explain the morphology and exhumation patterns in this area. We present 25 new apatite fission-track ages collected along a north-south transect in central Nepal, as well as two age-elevation profiles. Ages are consistently younger than 3 Ma old in the Main Central thrust zone and increase continuously to 4-6 Ma old in the south. No jump in apatite fission-track ages is observed across the topographic transition. Apparent exhumation rates from age-elevation relationships vary from 0.46 + 0.13/-0.09 km/Ma in the Palung granite south of Kathmandu to 4.4 + 4.8/-1.5 km/Ma in the Main Central thrust zone; the latter rate is probably overestimated by a factor of two due to topographic effects. As shown by a new numerical model, these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp, which exerts a primary control on age patterns, and do not require out-of-sequence reactivation of thrusts in the Main Central thrust zone