A single extensional, diffuse, ductile fault zone in the Goriganga section, western Himalaya: Part of the upper South Tibetan Detachment System?

Abstract

The Trans-Himadri Detachment Fault (T-HDF) of the Kumaon region of western Himalaya, a north-easterly steeply dipping discrete brittle/semi-brittle fault, had previously been regarded as a strand of the South Tibetan Detachment System (STDS). Yet, elsewhere in the Himalaya, the STDS is defined as a diffuse, ductile low-angle normal-sense shear zone. This aspect of the STDS remained conjectural and was even refuted by various workers in the Goriganga valley in the Kumaon. We have systematically documented the STDS as a low-angle, northerly dipping, ductile, diffuse shear zone, with a true thickness of >5 km in this transect. In the present scenario, the shear zone is located only within the psammo-pelitic basal part of the Tethyan Sedimentary Sequence (TSS), and bypassed the rock units of the Greater Himalayan Sequence (GHS). Multiple grain-statistical parameters of mylonitized micaceous quartzites point to an anastomosing architecture which displays a single precursory damage zone and a wider core for the entire STDS shear zone. The 40Ar*/39Ar geochronology and oxygen isotope analysis of the micas in mylonites demonstrated that the STDS evolved in weak lithologies, between c.14–11 Ma, at ∼300–350 °C ambient temperature, at or immediately below the brittle-plastic transition depth in the quartzo-feldspathic continental crust.Consideration of similar range of optimum ages of cessation of the extensional activity and already published timings of the end of brittle motion of the STDS in adjacent regions suggests that this wide diffuse shear zone is a deeper counterpart of the brittle upper STDS. This is in sharp contrast to the exposures of the detachment elsewhere in the Himalaya, where diffuse zone demonstrably constitutes the lower branch of this detachment. This finding has potential implications on overall understanding of the deformation related to the STDS and late Miocene geodynamic evolution of the GHS in this region.