Mid-Miocene initiation of orogen-parallel extension, NW Nepal Himalaya

Abstract

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 40Ar/39Ar 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.