Spatio-temporal challenges in dating orogen-scale shear zones: The case of the Himalayan Main Central thrust

Abstract

The spatial extent and dynamic evolution of orogen-scale shear zones renders the determination of the timing and duration of ductile deformation and displacement challenging. The Himalayan Main Central thrust (MCT) is one of the most widely studied thrust-sense shear zones in the world, particularly regarding its timing, and yet ‘when was the MCT active’ remains an outstanding question. New and existing pressure-temperature-deformation-time (P-T -d-t) data from three well-exposed structural sections of the MCT in western Nepal, which together represent > 100 km in the direction of tectonic transport, are compared to examine how spatio-temporal variations in recorded shear complicate the answer to this question. Metamorphic P-T data, quartz microstructures and quartz crystallographic preferred orientation suggest that all three MCT transects underwent similar shear conditions and kinematic behaviour. However, each shear-zone transect reached peak T conditions during different windows of time. Furthermore, 40Ar/39Ar thermochronology on white mica reveals contrasting timing for the end of ductile deformation. When combined with published monazite and zircon dates, our data indicate that the record of ductile deformation on the MCT is diachronous in the transport direction and a single shear zone transect is thus not representative of the timing and duration of deformation on the MCT even for a narrow along-strike position. This study represents the first attempt to track ductile deformation in the direction of thrust propagation at this spatial extent on an orogen-scale thrust system. These results clearly point to the need to study the temporal evolution of shear zones over significant distances in the direction of tectonic transport, acknowledge potential gaps in the petrochronological record, and incorporate P-T -d data when using shear zone timing and duration results to make regional conclusions.