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.

Geologic field mapping surveys integrated with structural, thermochronological, and geochronological analyses confirm the existence of an orogen-parallel strike-slip-dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone obliquely cuts through the upper Greater Himalayan Sequence and is characterized by a S-dipping, high-strain foliation and intensely developed ESE-WNW-trending, shallow-plunging mineral elongation lineation. Monazite grains within the Greater Himalayan Sequence are deformed and transposed parallel to the orogen-parallel shear zone and ESE-WNW elongation lineations. In situ U-Th/Pb monazite geochronology constrains metamorphism between 19 and 15 Ma, which is consistent with the timing of Neohimalayan metamorphism and S-directed extrusion of the Greater Himalayan Sequence across the Himalaya, and it is therefore interpreted to have preceded orogen-parallel strike-slip deformation. Mineral deformation mechanisms and quartz c-axis patterns of orogen-parallel fabrics record a rapid increase in temperature of deformation from ~350 °C along upper levels of the shear zone to greater than 630 °C at ~2.5 km depth structurally below the shear zone. Symmetric quartz c-axis fabrics further suggest deformation included a significant component of pure shear. The 40 Ar/ 39 Ar thermochronology of foliation-defining muscovite indicates that orogen-parallel shearing was active in the area between ca. 13 and 10 Ma while temperatures cooled through the muscovite closure temperature for argon. By integrating these data with the current understanding of tectonic processes in the Himalaya, we interpret a transition from S-directed extrusion of the Greater Himalayan Sequence to orogen-parallel extension between ca. 15 and 13 Ma in the upper Karnali valley. Integration of our findings with chronological constraints from other migmatite-cored domes supports the growing recognition of a Himalayan-wide mid-Miocene initiation of orogen-parallel extension. GEOLOGIC SETTINGThe geology of the Himalaya consists of four distinct lithotectonic domains bounded by crustal-scale fault systems, all of which are laterally continuous across the length of the orogen (Fig. 1A). The two northernmost LITHOSPHERE GSA Data Repository Item 2015204