Modeling High‐Resolution Pressure‐Temperature Paths Across the Himalayan Main Central Thrust (Central Nepal): Implications for the Dynamics of Collision

Abstract

AbstractHigh‐resolution, garnet‐based pressure‐temperature (P‐T) paths were obtained for nine rocks across the Himalayan Main Central Thrust (MCT) (Marsyangdi River transect, central Nepal). Paths were created using garnet and whole rock compositions as input parameters into a semiautomated Gibbs free‐energy‐minimization technique. The conditions recorded by the paths, in general, yield similar T but lower P compared to estimates from mineral equilibria and quartz‐in‐garnet Raman barometry. The paths are used to modify a model based on a two‐dimensional finite difference solution to the diffusion‐advection equation. In this model, P‐T paths recorded by the footwall garnets result from fault motion at specified times, thermal advection, and alteration of topography. The best fit between the high‐resolution P‐T paths and model predictions is that from 25 to 18 Ma, samples within the MCT footwall moved at 5 km/Ma, while those in the hanging wall moved at 10 km/Ma. Under these conditions, topography grew to 3.5 km. A pause in activity along the MCT between 18 and 15 Ma allows heat to advect and may be due to a transfer of tectonic activity to the structures closer to the Indian subcontinent. During this time, the topography erodes at a rate of 1.5 km/Ma. Thrusting within the MCT footwall reactivates between 8 and 2 Ma with exhumation rates up to 12 mm/yr since the Pliocene. The results suggest the potential for the highest‐resolution garnet‐based P‐T paths to record both the thermobarometric consequences of fault motion and large‐scale erosion.