Lithostratigraphy and Metamorphism of the Lesser and Higher Himalaya in Trishuli and Langtang Valleys, North-Central Nepal

A thorough geological mapping and petrographic study was accomplished in the Trishuli and Langtang valleys, Rasuwa district, north-central Nepal, over a surface area of ca. 20×20 km². The area consists in several regional and local structures and exhibits the inverted metamorphic sequence associated with the movement of the Higher Himalayan Thrust Sheet over the Lesser Himalayan footwall along the Main Central Thrust (MCT). The area includes rocks from the Lesser Himalayan Sequence (LHS) and Greater Himalayan Sequence (GHS) to the south and north of Syabrubensi area, respectively. The metamorphic grade varies from chlorite to staurolite in the LHS and reaches to sillimanite in the GHS in the Langtang valley. Geology needs to be revisited in this area, where extensive metamorphic CO₂ discharge were identified near hydrothermal systems, substantially modified by the 2015 Gorkha earthquake (Girault et al., 2018).

The LHS rocks terminate into the MCT, indicating that MCT is a sharp structural boundary. The hinge zone of the Kunchha-Gorkha anticlinorium of the LHS passes through Ramche-Mulkharka area. The southern limb in Betrawati-Kalikasthan area is relatively less deformed whereas the northern limb in Dhunche-Syabrubensi area is highly deformed resulting in the formation of tight folds, S-C fabric showing show top-to-the-south sense of shearing (Ghimire et al., 2010), and stretching lineation usually oriented NNE to SSW. Though the area consists of several shear zones, it is delicate to ascertain whether every lithological unit is separated by the thrust as indicated by Macfarlane et al. (1992). However, a brittle shear zone of about 20 cm thick cross-cutting the foliation passes through Syabrubensi. Therefore, it must post date the ductile deformation event in the area.

The LHS and GHS in the area are divided into 6 and 3 lithostratigraphic units, respectively. The Langtang Thrust indicated by Kohn et al. (2005) is corroborated by the missing unit of Rasuwagadhi Migmatite in the Langtang valley. The LHS consists of three metamorphic zones viz., chlorite, biotite and garnet, whereas the GHS consists of kyanite and sillimanite zones. A better understanding of the metamorphic structures is essential to constrain the metamorphic CO₂ source and proper modeling of thermo-mechanics and post-seismic relaxation.