Temperature, Deformation, and Mass Transfer in a Hot Orogen: Insights From Thermokinematic Forward Models for Far Western Nepal

Abstract

Exhumation and cooling pathways of mid‐crustal metamorphic rocks in the western Nepal Himalaya can be replicated by fold‐thrust belt structures with displacement localized along discrete décollements. New and published muscovite ⁴⁰Ar/³⁹Ar, zircon U‐Th/He, and apatite fission track cooling ages, peak temperature estimates, geologic mapping, and basin data are integrated with thermokinematic forward models to constrain the geometry, kinematics, and rates of shortening in far western Nepal. The best fit to peak temperatures, cooling ages, and basin accumulation data is achieved with a largely in‐sequence kinematic order, with out‐of‐sequence motion on the Ramgarh‐Munsiari thrust. Fast rates (∼20–40 mm/yr) are required during shortening on early, large displacement faults at ∼23–12 Ma and decrease to ∼10–15 mm/yr during formation of the Lesser Himalayan duplex until ∼1 Ma. Thermokinematic models highlight the relationship between peak temperature, geometry, and shortening on the large displacement Main Central and Ramgarh‐Munsiari thrusts. In the thermokinematic models, we observe a relationship between the location of frontal ramps for the faults that displace lower Lesser Himalayan units and the ∼375°C isotherm, immediately before the ramp becomes active. These correlations suggest that temperature exerts a first‐order control on thrust geometry in a hot orogen. Viable models highlight the position of active ramps, kinematic order of faults, timing of fault motion, and reduction in shortening rates that are required to reproduce the surface geology, basin accumulation, peak temperature conditions, and timing of exhumation. Cooling ages are far more sensitive to the age of fault motion than the rate of fault motion.