Kinematics of the Dadeldhura klippe shear zones (W Nepal): Implications for the foreland evolution of the himalayan metamorphic core

摘要

Mapping combined with structural analyses in the foreland edge of the metamorphic core of the Himalayas in SW Nepal highlights the existence of two north-dipping shear zones with opposite sense of shear. Here, the metamorphic core is mainly affected by non-coaxial top-to-the-south sense of shear at temperatures between 450 °C and 550 °C that switch to a top-to-the-north sense of shear at the top of the metamorphic core. We regionally correlate this upper shear zone with the South Tibetan detachment system. Ar-dating on white mica indicates that both shear zones operated between 23 Ma and 17 Ma. Restoration of the folded South Tibetan detachment in far western Nepal yields a minimum dip-slip distance of 190 km, compatible with predictions made by models of extrusion of a weak mid-crustal channel. Our results support an orogenic model in which channel flow in the hinterland coexisted with thrust wedge mechanics in the foreland.