Calcite fabric development in calc-mylonite during progressive shallowing of a shear zone: An example from the South Tibetan Detachment system (Kali Gandaki valley, Central Himalaya)

Abstract

Calcite-rich lithologies within the Annapurna Detachment zone, a strand of the South Tibetan Detachment System in central Himalaya (Kali Gandaki region, Western Nepal) have been characterized for the superimposition of microstructures to unravel deformation variations during syn-collisional exhumation. Finite-strain, grain size, twinning and crystallographic preferred orientations have been combined to define the contribution of dynamic recrystallization and twinning in calcite in the overall deformation. Dynamic recrystallization and twinning occurred respectively at temperatures of c. 400–550 °C and <250 °C. The comparison of calcite-based paleopiezometers indicates that cooling occurred along with an increase in differential stress from c. 4–19 MPa up to c. 118–154 MPa at decreasing strain rates. Flow estimates with the subsimple shear (30–50% of simple shear) regime for both fabrics support a single progressive deformation where the plastic regime progressively changed. We interpret the changes in intracrystalline deformation mechanisms in calcite and their differences in differential stress records, deformation temperature, and strain rate as linked to the exhumation of the rocks induced by the detachment, from deeper to upper crustal levels. Results for the Annapurna Detachment zone have been compared with the literature database for the regional continuation along the Himalaya, the South Tibetan Detachment System, which is defined by a lower main shear zone and an upper brittle fault. Data comparison led us to link our early stage of ductile shearing to that elsewhere recorded in the main lower branch of the system. Conversely, data for the late shearing are not found for the main branch when affecting quartz-bearing rocks, as strain hardening resulted in an upward partitioning of the deformation and in the localization of a brittle fault. Therefore, this study highlights how variations in lithology in regional-scale shear zones influences the exhumation path and the overall architecture of shear zones.