Non‐metamorphosed autochthonous Kuncha‐Naudanda‐Heklang Formations and their differences from those of the Kuncha nappe: A multichronological approach

Abstract

Non-metamorphosed, autochthonous Lesser Himalayan sediments (LHS), which are correlated to the Kuncha and Naudanda Formations, were found in a narrow belt between the Main Boundary Thrust and the Lesser Himalayan Thrust at the base of the Kuncha nappe in southeastern Nepal. The autochthonous Naudanda Formation is comprised of cross-bedded and rippled orthoquartzite and yielded a maximum depositional age of 1795.1 Ma ±5.1 Ma using detrital zircons. Low-grade metamorphosed quartzite in the Kuncha nappe yielded a maximum depositional age of 1867.4 Ma ±3.4 Ma, although it is totally recrystallized. These ages and age distribution patterns of detrital zircon grains indicate that the meta-quartzite of the nappe is originally Naudanda Formation. A zircon fission-track age of the autochthonous Naudanda Formation shows partially annealed age of 864 Ma ±56 Ma, in contrast, that of the Kuncha nappe shows a totally annealed age of 11.9 Ma ±1.6 Ma. These results suggest that the autochthonous LHS have never undergone metamorphism during the Himalayan orogeny. We also discovered a non-metamorphosed Heklang Formation that rests on the Naudanda Formation, and designated it as a sub-type section on the basis of detailed lithostratigraphic study. It is characterized by black and light green slate with dolerite sills and ill-sorted quartzose sandstone, and correlated to the metamorphosed Dandagaon Phyllites in the Kathmandu area. Nonmetamorphosed autochthonous formations distributed to the south of the nappe front suggest that they escaped from thermal metamorphism by hot nappe.

Thematic issue: Evolution of Nepal Himalaya

“…Nakajima et al (2020aNakajima et al ( , 2020b deals with northward lateral cooling of Kathmandu and Kuncha nappe revealed by zircon fission-track (ZFT) dating. Distribution of nonmetamorphosed LHS along the southern margin of the Lesser Himalaya was confirmed by the zircon fission -track and Pb-Pb dating by Kobayashi et al (2021). Uplift and erosion history of the Himalaya was discussed on the basis of heavy mineral composition of sandstone of the Siwalik Group and sediment core of the Bengal Deep Sea Fan in Yoshida et al (2021).…”

Denudation Process of Crystalline Nappes in a Continental Collision Zone Constrained by Inversion of Fission‐Track Data and Thermokinematic Forward Modeling: An Example From Eastern Nepalese Himalaya

“…The LHS distributed in eastern Nepal are divided into three geotectonic units; the Lesser Himalayan (LH) para‐autochthon, the Kuncha nappe, and the MCT zone. The LH para‐autochthon is composed of the Paleoproterozoic sediments of the Lower Nawakot Group (∼2,500 m thick: Kobayashi et al., 2021; Rai et al., 2016) and the Gondwana and post‐Gondwana foreland basin sediments of the Tansen Group (∼2,400 m thick: Sakai, 1983). The Kuncha nappe is separated from the LHS para‐autochthon by the Lesser Himalayan Thrust (known as the Ramgarh Thrust: Pearson & DeCelles, 2005) at its base.…”

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

“…The similar kind of study made by Baral et al (2022) has correlated the Sapt Kosi Formation with the Taltung Formation and the Tamrang Formation with the Dumri Formation of western Nepal. Kobayashi et al (2021) has correlated the Mulghat Formation and the Chiuribas Formation with the Kuncha Formation based on multichronological approach. The Sangure Quartzite has been correlated with the Naudanda Quartzite and the Raguwa Formation along with the Phalametar Quartzite has been correlated with the Heklang Formation (Sakai, 1985) of western Nepal.…”