Geometry, Kinematics, and Exhumation Pathways in the Himalaya: Thermokinematic Models for Western Nepal and Arunachal Pradesh, India

Exhumation pathways of rocks in the Himalayan orogen are constrained using thermokinematic forward models of balanced cross-sections in Nepal and Arunachal Pradesh, India. Integration of balanced cross-section geometries, kinematic order of faults, and measured datasets including geologic mapping, basin accumulation data, peak temperature estimates, and cooling ages requires more complicated exhumation pathways than simple in-sequence (forelandward-propagating) deformation proposed for the Himalaya. In both Nepal and Arunachal Pradesh, shortening is initially accommodated on large displacement, low angle faults like the Main Central thrust, before transitioning to formation of the Lesser Himalayan duplex when the strata in front of the active ramp cools below ~375°C. Fast shortening rates (~20-45 mm/yr) are required during large displacement, low angle faulting at ~25-12 Ma. A distinct slowdown in shortening rates is observed after ~10 Ma, though the specific rates, detailed timing, and drivers of the slowdown differ along strike. Thermokinematic models highlight variations in the exhumation pathways that result from the different geometries and kinematics of each transect. Sections in western Nepal and Arunachal Pradesh require a complex kinematic sequence involving early foreland propagation of Lesser Himalayan faults to reproduce older (~15 Ma) zircon fission track and (U-Th)/He cooling ages in the more frontal portions of the fold-thrust belt and young (~5-10 Ma) muscovite 40Ar/39Ar and zircon fission track cooling ages in the hinterland. Reproducing these young cooling ages in the hinterland also requires out-of-sequence thrusting on formerly active, large displacement faults like the Main Central and Ramgarh-Munsiari thrusts to produce young (