Quartz grain microstructures and microscopic shear sense indicators in the outer Lesser Himalaya, Barahakshetra-Tribeni-Raighat area of Arun Valley, eastern Nepal

Abstract

The outer Lesser Himalaya in eastern Nepal has gone through intense ductile shearing and recrystallization due to the emplacement of Higher Himalayan thrust sheet. It has resulted in the formation of various microstructures in the footwall rocks of the Lesser Himalaya. Present study focuses on the study of the quartz grain microstructures and shear sense indicators in the Lesser Himalayan metasedimentary rocks to interpret the deformation characteristics of the area. Thin sections were prepared from oriented rock samples collected from various formations and were studied under the petrographic microscope. The occurrence and diversity of quartz grain microstructures and shear sense indicators in the area manifests uneven distribution of stress regime and temperature conditions. Nevertheless, it suggests a progressive transition from the southern to the northern part. The vicinity of the Dharapani Thrust represents mechanical deformation of higher strain and lower temperature conditions. The Chimra Thrust Sheet exhibits mild- to medium deformation at the basal section while high-temperature high-strain deformation at the upper section in the vicinity of the MCT.