Segmentation and rejuvenation of the Greater Himalayan sequence in western Nepal revealed by in situ U–Th/Pb monazite petrochronology

Abstract

In the western Nepal Himalaya, a major mountain building shear zone, the Main Central thrust, is exposed in multiple locations along its southward transport direction. The hanging wall of the Main Central thrust was sampled in both hinterland and foreland exposures, in the upper Karnali valley and around Jumla, respectively. In situ U–Th/Pb monazite petrochronology coupled with microstructural and petrographic observations reveals distinctly different timing of deformation, despite the geographic proximity and apparent structural continuity between the two regions. The Y content, the Gd to Yb ratio and the Eu anomaly in monazite constrain the timing of metamorphic reactions. Rocks in the upper Karnali valley record deformation associated with metamorphism at 18Ma, melt crystallization starting at ca. 14Ma and lasting until at least 11–10Ma. In contrast, the Jumla region records a more protracted series of events with metamorphism initiating as early as 47Ma and lasting until 18Ma, punctuated by at least two periods of melt crystallization. In both areas, the age of melt crystallization decreases down section through the hanging wall towards the Main Central thrust. Ductile deformation in the Jumla region ceased around ca. 18Ma, while rocks at similar structural positions in the hinterland in upper Karnali valley were still being actively deformed and melting at ca. 11Ma. The southern, foreland-most, hanging-wall rocks (Jumla region) were consequently exhumed above the ductile–brittle transition while the northern hinterland-most hanging-wall rocks (upper Karnali valley) were still undergoing ductile deformation and partial melting.