Review of the Geology of the Arun-Tamor Region, Eastern Nepal: Present Understndings, Controversies and Research Gaps

Abstract

Systematic study of the eastern Nepal Himalaya was started after 1950 when Nepal opened up for foreigners. Thereafter, several geological studies have been carried out in the Arun-Tamor region of eastern Nepal Himalaya. The Tibetan-Tethys sedimentary sequence, the Higher Himalayan amphibolite to granulite facies metamorphic crystalline sequence, the Lesser Himalayan sedimentary and greenschist facies metasedimentary sequences, and the Siwalik foreland molassic sedimentary sequence are the four major tectonic units of this area. The individual nomenclature schemes of stratigraphic units, the correlational dispute, the positions and interpretations of regional geological structures are some examples that have created controversies regarding the lithostratigraphy and structural arrangements. The difference in age and genesis of the Main Central Thrust and its effects in the metamorphism of the eastern Nepal Himalaya are the exemplification of the contradiction in the interpretation of the tectonometamorphic history. There is a gap in research in the tectonics and episodic metamorphic evolution of the area owing to the bare approach in the microstructural and geochronological investigation. Future investigations should be focused on solving the above mentioned controversies and narrowing down the research gaps in tectonic and metamorphic evolution.

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

“…The Lesser Himalaya in eastern Nepal consists of Pre-Cambrian metasedimentary rocks unconformably overlain by Permian to Miocene sedimentary rocks (Bashyal, 1980;Dhital, 1992;Bhandari et al, 2018;Baral et al, 2022). The Lesser Himalaya in eastern Nepal is limited within the inner window structures exposed as domal anticline in the deep gorges of the Arun, Tamor, and Dudhkoshi River valleys and outer metasedimentary front forming the belt of Mahabharat Hills (Bordet, 1961;Akiba et al, 1973;Maruo et al, 1979;Andrews, 1985;Schelling and Arita, 1991;Schelling, 1992;Meir and Hiltner, 1993;Dhital, 2015;Adhikari et al, 2021). The outer Lesser Himalaya of the Arun River valley section is exposed along the Barahakshetra-Tribeni-Raighat area.…”

Tectono-lithostratigraphy of the outer Lesser Himalaya in the Barahakshetra-Tribeni area, Arun valley section, eastern Nepal

“…The regional geological works covering the large areas of eastern Nepal have been carried out by several researchers (Auden, 1935;Bordet, 1961;Akiba et al, 1973;Maruo et al, 1979;Schelling and Arita, 1991;Schelling, 1992). A detailed review of regional geological work in eastern Nepal is given in Adhikari et al (2021). The geology of the Lesser Himalayan sequence in the tectonic windows of the inner Lesser Himalaya in east Nepal has been presented by various authors such as Hagen (1969), Akiba et al (1973), Maruo et al (1979), Schelling and Arita (1991), Schelling (1992), Upreti et al (2003), and Ambrose et al (2015).…”