Detection of tectonometamorphic discontinuities within the Himalayan orogen: Structural and petrological constraints from the Rasuwa district, central Nepal Himalaya

Abstract

A detailed structural, lithological and petrological study of different transects in the Rasuwa district of central Nepal Himalaya allows the characterization of the tectonostratigraphic architecture of the area. It also facilitates constraining the P-T evolution of the different units within the Lesser (LHS) and Greater (GHS) Himalayan Sequences. Peak P–T conditions obtained for the studied metapelite samples using the pseudosection approach and the Average PT method highlight the existence of four different T/P ratio populations in different tectonometamorphic units: 80 ± 11 °C/kbar (LHS), 66 ± 7 °C/kbar (RTS), 73 ± 1 °C/kbar (Lower-GHS) and 101 ± 12 °C/kbar (Upper-GHS). Integration of structural and petrological data emphasizes the existence of three tectonometamorphic discontinuities bounding these units, characterized by top-to-the-south sense of shear: the Ramgarh Thrust, which separates the LHS (peak metamorphism at ∼600 °C, 7.5 kbar) from the overlying RTS (peak metamorphism at ∼635 °C, 10 kbar); the Main Central Thrust, which separates the RTS from the Lower-GHS (peak at 700–740 °C, 9.5–10.5 kbar with a prograde increase in both P and T in the kyanite stability field), and the Langtang Thrust, which juxtaposes the Upper-GHS (peak at 780–800 °C, 7.5–8.0 kbar with a nearly isobaric heating in the sillimanite stability field) onto the Lower-GHS. An increase in the intensity of deformation, with development of pervasive mylonitic fabrics and/or shear zones, is generally observed approaching the discontinuities from either side.Overall, data and results presented in this paper demonstrate that petrological and structural analysis combined together, are reliable methods adequate to identify tectonometamorphic discontinuities in both the LHS and GHS. Geochronological data from the literature allow the interpretation of these discontinuities as in-sequence thrusts.