Multi-stage evolution of the South Tibetan Detachment System in central Himalaya: Insights from carbonate-bearing rocks

Abstract

Low-angle normal faults (LANFs) are increasingly described in collisional orogens and are getting attention in modelling in regard to mass distributions, exhumation history, and the rheology of the continental crust during plate collision. Their kinematic evolution, however, is still not fully understood. We present a microstructural study of a key example of a LANF in a syn-collisional setting, i.e., the ductile portion of the South Tibetan Detachment System (STDS) in the Lower Dolpo region (central Himalaya). New data are provided on microstructural characteristics, crystallographic preferred orientations of calcite and quartz as well as of paleopiezometric and kinematic vorticity analyses. Two main stages of non-coaxial shearing from lower to higher crustal levels, here referred as D2early and D2late, are recognized. D2early developed higher temperatures (c. 600–400 °C traced up-section within the STDS) and at low differential stress (<15 MPa). D2late occurred at lower temperatures (T < 350 °C) and differential stress larger than 100 MPa at comparable strain rates with D2early. Cooling and differential stress increased from D2early to D2late, indicating progressive exhumation following typical lithospheric strength profiles. Vorticity data highlight a pure shear component that can account for the up-section telescoping of the paleo-isotherms. Our findings support a strain hardening of the STDS, where ductile shearing persisted to shallower levels, without necessarily localizing a prominent upper brittle fault.