Short concept on Himalaya: Focusing Nepal

    

en.wikipedia.org/wiki/Himalayas

      The Himalayan Range is a young mountain system of world. The Himalayan range is home to the planet's highest peaks, including the highest, Mount Everest. It is a broad continuous arc along the northern fringes of the Indian subcontinent, from the bend of the Indus River in the northwest to the Brahmaputra River in the east. The Himalayan mountain chain extends in an east-west direction between the wide plains of the Indus and Bramhaputra in the south and the vast expanse of the high Tibetan Plateau in the north. The limit of the Himalayas in the east and west is marked by the eastern and western arc of Himalayan bends. Between these bends the Himalayan range is approximately 2400 km long and 200 km to 300 km wide. The Himalayas cover an area of approximately 600,000 sq. km in south Asia.

The Himalayas cross five countries: India, Nepal, Bhutan, China and Pakistan. 

Longitudinally, Himalayan Range in Nepal is also divided into five tectonic zones (Gansser 1964): 

1.   Terai Zone or Gangetic Plain

2.   Sub-Himalayan Zone

3.   Lesser Himalayan Zone

4.   Higher Himalayan Zone 

5.  Tibetan-Tethys Himalayan Zone

These east-west extending zones run almost parallel to each other. They have different lithology, structure, and geological history. Geological Framework of Nepal Himalaya Nepal occupies the central sector of Himalayan arc. Nearly one third of the 2400 km long Himalayan range lies within Nepal. 

These all tectonic zones are separated from each other by the thrust faults. The southernmost fault, the Main Frontal Thrust (MFT) separates the Sub-Himalayan (Siwalik) Zone from Gangetic Plains. The Main Boundary Thrust (MBT) separates the Lesser Himalayan Zone from Siwalik. The Main Central Thrust (MCT) separates the Higher Himalayan Zone from the Lesser Himalayan Zone. The South Tibetan Detachment System (STDS) marks the boundary between the Higher Himalayan Zone and the overlying fossiliferous sequence of the Tibetan-Tethys Himalayan Zone. The Indo-Tsangpo Suture Zone is the contact knot between Indian plate and Tibetan (Eurasian) Plate in terms of plate tectonics. 

1.  Sub Himalaya

The Sub-Himalaya Zone is also called as Siwalik Zone and is delimited on the south by the Main Frontal Thrust (MFT) and on the north by the Main Boundary Thrust (MBT). It consists basically of fluvial deposits of the Neogene age (23 million years to 1.6 million years old). This Zone extends all along the Himalaya forming the southernmost hill range with width of 8 to 50 km. The Lesser Himalayan rocks thrust southward over the rocks of Siwalik along the MBT (Dahal, 2006). 

The general dip of beds of Siwalik has northward trend with varying angles and the overall strike is east-west. The Siwalik Zone has number of east-west running thrusts. Siwalik Zone is also rich with fossils. Fossils of plants, pisces, reptiles and mammals (Carnivora, Proboscidea, Artiodactyla, Rodentia and Primates) have been reported from Siwalik. 

The three-fold classification of Siwalik in Potwar region of Pakistan and western Indian Himalaya was freely applied to the equivalent Siwalik of Nepal (Burbank et al., 1996) from the beginning of the geological studies in Nepal. 

According to three fold classification, Siwalik can be classified as follow.

1.  Lower Siwalik

2.  Middle Siwalik and

3. Upper Siwalik

2. Lesser Himalaya

The Lesser Himalayan Zone is bounded to the north by the Main Central Thrust (MCT) and to the south by Main Boundary Thrust (MBT). MBT can be traced out in whole Nepal Himalaya and it can be also well observed in aerial photographs also. The rocks of Lesser Himalayan Zone have been transported southwards in several thrust slices. Generally two types of sequences namely autochthonous and allochthonous can be distinguished in this Zone throughout the Himalayas. The both sequences of the Lesser Himalaya mainly have unfossiliferous, sedimentary, and metasedimentary rocks such as slate, phyllite, schist, quartzite, limestone, dolomite, etc, ranging in age from Precambrian to Eocene. There are also some granitic intrusions in this zone.

From east to west, the Lesser Himalayan Zone of Nepal varies in rock type, age, structures, and igneous rock intrusion. Eastern Nepal is characterized by the development of extensive thrust sheets (allochthonous) of high grade metamorphic rocks (gneiss and schist) which have moved southwards. Below this sequence, due to erosion, large exposure of the low-grade metamorphic rocks (autochthonous) can be seen. In Central Nepal, a large thrust sheet called the Kathmandu Nappe (allochthonous) covers a wide area around the Kathmandu region. Whereas west of Kathmandu, between the Budhi Gandaki and Bheri rivers, amount of transported high grade metamorphic rocks (allochthonous) is very low and the area is generally covered by autochthonous sequence. But in west of the Bheri River, up to the western border of Nepal (Dadeldhura-Baitadi) high-grade metamorphic rocks reappear and cover much of the terrain. 

3.  Higher Himalaya

The Higher Himalayan zone mainly consists of huge pile of strongly metamorphosed rocks. Geologically, the Higher Himalayan Zone includes the rocks lying north of the Main Central Thrust (MCT) and below the highly fossiliferous Tibetan-Tethys Zone. This zone is separated with Tibetan-Tethys Zone by normal fault system called as South Tibetan Detachment System (STDS). Higher Himalayan Zone consists of an approximately 10 km thick succession of strongly metamorphosed coarse grained rocks. It extends continuously along the entire length of the country as in whole Himalaya, and its width varies from place to place. The kyanite - sillimanite minerals bearing gneisses, schists, and marbles of the zone form the basement of the Tibetan-Tethys Zones. Granites are found in the upper part of the unit. 

4.  Tibetan-Tethys Himalaya

The Tibetan-Tethys Zone lies in northern part of the country. It begins from the top of the STDS and extends to the north in Tibet. In Nepal, the fossiliferous rocks of the Tibetan-Tethys Zone are well-developed in Mustang, Manang and Dolpa area. In eastern part, amount of exposure of the Tibetan Tehys Zone is almost negligible and found only in top of the Mount Everest. Most of the other Great Himalayan peaks of Nepal such as Manaslu, Annapurna, and Dhaulagiri have rocks of Tibetan-Tethys Zone. This zone is composed of sedimentary rocks, such as shale, limestone, and sandstone, ranging in age from Cambrian to Eocene. This zone in some area is found as continuous deposits of Higher Himalayan Zone without normal fault.

Thanks to: Anish Dangol, Sanjay Singh, Sunil Man Singh, and Nirjan Gothe.

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