Geological and Petrological Evolution of the Lesser Himalaya between Mugling and Damauli, central Nepal

Abstract

The Himalaya is divided into four tectonic zones as the Sub-Himalaya, the Lesser Himalaya, the Higher Himalaya, and the Tethys Himalaya from south to north, respectively. The Lesser Himalaya is a fold-and-thrust belt bounded by the Main Central Thrust (MCT) in the north and the Main Boundary Thrust (MBT) in the south and comprises autochthonous unit made up of Late-Precambrian to Early Paleozoic (?) low- to medium-grade metasedimentary rocks discordantly overlain by Proterozoic metamorphic crystalline rocks transported by thrusts. Although many authors have worked in central Nepal Lesser Himalaya, many parts still lacks large scale geological map and there are a number of problems and controversies on stratigraphic classification, tectonic interpretation and metamorphism. Present study was carried out in the central Nepal Lesser Himalaya between Mugling in the east and Damauli in the west covering both the autochthonous and allochthonous units. Main objectives of the study were to clarify the stratigraphic classification proposed by the previous authors, to prepare geological map and its crosssections,

to use magnetic susceptibility for stratigraphic comparison, and to unravel the tectono-metamorphic history of the area. In the present study about 1000 square km area of the Lesser Himalaya between Mugling and Damauli was mapped in 1:25,000 scale. Lithostratigrpahy of the area was established and compared with the type-section by detailed route-mapping and preparation of columnar sections. Magnetic susceptibility was measured and analyzed in the autochthonous rocks along three sections in the Mugling-Damauli area and one section in the Malekhu area. Regional geological structures were measured and traced throughout the study area. Mesoscopic and microscopic structures were studied and analyzed both in the field and in thin sections. Metamorphic study was carried out using conventional petrographic microscope in thin sections. Illite and graphite crystallinities were measured ix

by X-ray diffractometer and Raman Spectrocopy of Carbonaceous Materials was carried out using Raman Spectroscope. The study shows that the stratigraphic classification of the Lesser Himalayan autochthon proposed by Stöcklin and Bhattarai (1977) and Stöcklin (1980) need some modificaton. Possibly there exists no disconformity between the Dhading Dolomite and the Benighat Slate. Therefore, whole autochthonous rocks should be named as the Nuwakot Group (not the Nawakot Complex). The Anpu Quartzite, the Labdi Phyllite and the Banspani Quartzite are found to be lateral extensions of the same units as the Fagfog Quartzite, the Dandagaon Phyllite and the Purebensi Quartzite, respectively. Therefore, the former units do not exit as separate members. The Nourpul Formation is very extensive and is divisible into four members and two beds in the present study area. Magnetic susceptibility (MS) measurement in the Nuwakot Group rocks shows that each stratigraphic unit has its own MS pattern and range of MS values. This pattern is uniform in all sections of the present study area as well as in the type locality of the Nuwakot Group (i.e., Malekhu section). Therefore, it supports the present lithostratigraphic correlation made by field mapping in the Lesser Himalaya. The rocks of the allochthonous unit (Kahun Klippe) are named as the Tanahun Group and can be divided into three formations as the Gwaslung Formation, the Musimarang Formation and the Shivapur Schist, from bottom to top, respectively. The lithology of the Shivapur Schist is similar to that of the Raduwa Formation of the Kathmandu Nappe and Chaurijhari Formation of the Jajarkot Klippe. Therefore, the Gwaslung and the Musimarang Formations should be units older than the Raduwa Formation. It indicates that the Kathmandu Nappe, Kahun Klippe and Jajarkot Klippe are a part of a single crystalline thrust sheet and the basal thrust of the Kahun Klippe (the Dubung Thrust) is equivalent to the Mahabharat Thrust (MT). There is no lithological similarity between the rocks of the Kahun Klipe and that of the MCT zone and Higher x

Himalaya. Therefore, the rocks of the klippe are probably units older than the Kunchha Formation deposited on top of the Higher Himalayan basement. The Mugling-Damauli area forms a part of a large duplex structure. The Dubung Thrust is the roof thrust, the MBT is the floor thrust, the Dewachuli Thrust is the imbricate fault and the Bhangeri Thrust is a back-thrust. The origin of the Lesser Himalayan crystalline nappes can be explained on the basis of single thrust model, i.e., the southward extension of the MCT. The area shows polyphase deformation (D xi

1 -D 5 ) and metamorphism (M 0 -M 3 ) as in the other parts of the Lesser Himalaya. At least two deformation events (D 1 and D 2 ) and one metamorphic event (M 0 ) are pre-Himalayan. The M 0 is normal burial metamorphism with grade increasing stratigraphically downwards and peak temperature reaching up to 370°C. The area suffered three deformation events (D 3 , D 4 and D 5 ) and three metamorphic events (M 1 , M 2 and M 3 ) after India-Eurasia collision. The second event (M 1 ) is Eohimalayan event causing garnet-grade prograde metamorphism in the Tanahun Group. This is pre-MCT event. The MCT-related Neohimalayan metamorphism (M 2 ) is inverted also in the low-grade zone of the Lesser Himalaya just below the Kahun Klippe. It is shown by both the illite and graphite crystallinity values. Key words: Lithostratigraphy, metamorphism, deformation, Kahun Klippe, root zone, central Nepal, Lesser Himalaya.