Modelling central Nepal Himalayan tectonic from different temperature thermochronometers

The tectonic evolution of orogenic systems, such as the Himalayas, has been extensively studied using thermochronometers sensitive to temperatures above 120 °C. Landscape modelling and the inversion of these data provide estimates of deformation rates over timescales of millions of years and spatial scales of tens to hundreds of kilometres. For the Himalayas, these data generally support a Quaternary tectonic scenario dominated by duplexing, where the collision between the Indian and Eurasian plates is accommodated along the active Main Himalayan Thrust (MHT), expressed at the surface as the Main Frontal Thrust (MFT) at the southern front of the Himalayan range. With this model, the observed exhumation of the High Himalayas is driven by underplating beneath the topographic transition, which creates duplex structures and overthrusting. However, several studies challenge this model, highlighting the scarcity of data constraining deformation rates in the Lesser Himalayas, and the absence of thermochronometric data for recent (< 2 Ma) movements.

Trapped-charge thermochronometry, sensitive to ultra-low temperatures below 100 °C, offers new constraints on the final stages of exhumation of the Himalayas (last few kilometres), constraining rates of deformation on sub-Quaternary timescales. Analysis of trapped charge thermochronometry data (luminescence and electron spin resonance) indicate that the MFT has accommodated at least 62 % of the convergence since 200 ka, while also revealing localized fault activity within the Sub-Himalayan fold-and-thrust belt, suggesting strain partitioning. High exhumation rates in the Main Central Thrust (MCT) area, along with differing apparent ages and exhumation rates on each side of the MCT fault system during the late Quaternary point to potential out-of-sequence fault activity, challenging the in-sequence/duplexing model proposed by higher temperature thermochronometers. However, these findings alone cannot definitively favour one tectonic model over another, and further investigation through fault kinematics and landscape modeling is required.

To address this, we employ a 3-D thermo-kinematic landscape evolution model (Pecube), and perform a formal nonlinear inversion using the Neighborhood Algorithm. This approach couples a landscape evolution model with 2-D thermo-kinematic models to simulate regional landscape evolution of the Nepal Himalayas, assessing how different kinematics can explain the morphology of the region. By combining fault geometries, vertical and horizontal displacement trajectories, and surface processes simulations, we will differentiate between the in-sequence/duplexing and out-of-sequence deformation modes for the Quaternary period. This integrated modeling framework will help identify the relative roles of tectonics, climate, and geology in shaping the exhumation patterns in the foreland and hinterland, as well as across different valleys in Nepal. Ultimately, the thermo-kinematic model will also provide insights into the seismic behaviour of the Nepalese mountain belt during the Quaternary.