Tectonometamophic evolution of the Greater Himalayan sequence, Karnali valley, northwestern Nepal

Abstract

In the Karnali valley of west Nepal, detailed mapping, thermobarometry, quartz-petrofabrics, vorticity analysis, and thermochronology delineate three tectonometamorphic domains separated by structural and metamorphic discontinuities. The lowest domain, the Lesser Himalayan sequence, is weakly metamorphosed and preserves evidence of primary sedimentary features and a polydeformational history. The Greater Himalayan sequence (GHS) is pervasively sheared and metamorphosed and overlies the Lesser Himalayan sequence along the Main Central thrust. The Greater Himalayan sequence is sub-divided into two tectonometamorphic domains that display contrasting metamorphic histories. The lower portion of the Greater Himalayan sequence contains garnet- to kyanite-grade rocks whose peak metamorphic assemblages developed during top-to-the-south directed shear and a metamorphic pressure gradient that increases up structural section. The upper portion of the Greater Himalayan sequence contains kyanite and sillimanite-grade migmatites that preserve polymetamorphic assemblages and a metamorphic pressure gradient that decreases up structural section. The upper and lower portions of the Greater Himalayan sequence are separated by a metamorphic discontinuity that roughly coincides with the bottom of the lowest migmatite unit. Vorticity estimates indicate roughly equal contributions of pure and simple shear during deformation of the upper and lower portions of the GHS. Quartz petrofabrics suggest deformation temperatures are equivalent to peak metamorphic temperatures in the lower Greater Himalayan sequence. These observations are consistent with channel flow tectonic models whereby the upper portion of the Greater Himalayan sequence is ductily extruded to the south while ductily accreting the subjacent lower portion of the Greater Himalayan sequence across a metamorphic discontinuity. 40Ar/39Ar thermochronology indicates Miocene homogeneous cooling of the Greater Himalayan sequence. Cooling rates of the GHS and the homogeneous cooling profile suggest east-west extensional exhumation followed peak-metamorphism and south-directed shearing and supports the hypothesis of the southeast propagation of the Gurla-Mandhata-Humla fault system into the Karnali valley.