TEchTOT
GEOLOGY OF NEPAL HIMALAYA
Introduction
The Himalayan Range is the most impressive example on earth of an active collisional orogen and is highest, very highly active and youngest mountain system of the world. It combines rapid crustal shortening and thickening, intense denudation driven by monsoon climate, and frequent very large earthquake along an incomparably long and high mountain arc. The Himalaya was formed due to collision of two tectonic plates, Indian Plate and Eurasian Plate on the north at about 40 Ma (Dewey et al. 1988). Many scientists believe that at that time the northward moving Indian plate first touched the southern edge of Tibetan (Eurasian) plate. Before the collision of the plates, the place was occupied by Tethys Sea. It extends in 2400 km east-west direction. Its width varies from 230 to 320 km and bounded between the Indo-Gangetic Plain in the south and Tibetan Plateau in the north. Geographically, major part of Nepal (83%) falls within the mountainous region and the remaining portion lies in the northern edge of the alluvial plains of the Gangetic basin. Nepal Himalaya occupies the central 800km part. Geographically it is bounded by the north latitudes of 260 22′ and 300 27′ and the east longitudes of 800 11′ and 880 27′ and is divided into five distinct morpho-geotectonic zones , from south to the north as (1) Terai Plain (2) Sub Himalaya (Siwalik Range), (3) Lesser Himalaya (Mahabharat Range and mid valleys) (4) Higher Himalaya and (5) Inner Himalaya (Tibetan Tethys). Each of these zones is clearly identified by their morphological, geological and tectonic features. Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT) and South Tibetan Detachment Fault system (STDFS), from south to the north respectively are the major linear geological structures that act as the boundary line between the two consecutive units.
General sub-division of the Himalaya
Like the entire 2400 km long Himalayan Range, the Nepal Himalaya is also divided in to the five major tectonic zones from south to north, respectively (Gansser 1964, Hagan 1969 ).The tectonics are extending east to west and almost parallel to sub-parallel to each other are distinctive in their lithology, tectonics, structures, metamorphism, and geological history. The zones are as follows. (fig : The major tectonic subdivision of the Himalaya)
1. Terai(Indo-Gangatic Plane)
2. Sub Himalayas (Siwaliks or Churia zone)
3. Lesser Himalayan zone (Mahabharat Midland Zone)
4. Higher Himalayan zone (Central Crystalline Axis)
5. Tibetan-Tethys-zone
1. Terai (Indo-Gangatic Plane)
This zone is the southernmost tectonic division of Nepal and represents the northern edge of Indo–Gangetic alluvial plane and range in elevation from 100 to 200 m above MSL. The width of Terai varies from 10 to 50 km. It extends from the indo-Nepal boarder in the south to the base of churia or siwaliks hills in the north separated by Main Frontal Thrust (MFT) which is very well exposed at many places at the foot of Siwalik hills. Along this thrust, the Siwalik rock thrust over the alluvial sediments of the Indo-Gangetic plain. The Terai region is generally flat with very minor relief caused by river channel shifting and down warping of the basin. All the rivers emerging from the Nepal Himalaya flow straight to the south until they meet the Gangas River and finally head eastward towards the Bay of Bangal.
Geologically, the Terai plain is composed of Pleistocene to Recent alluvial sediments. Quaternary deposits mainly boulders, sand, mud’s with thickness of alluvium is 1500 m are present. Coarser Terai sediments lie close to the mountain front, which is known as the Northern Terai zone (Bhabhar zone), where they form large alluvial fans. Finer sediments are found farther to the south and the Recent Terai sediments represents the latest foreland basin deposits of the Himalaya.
2. Sub-Himalaya (Siwaliks or churia zone)
This zone represents the lower hill of the Churia range and is bounded to the north by the Main Boundary Thrust (MBT) and to the south by the Main Frontal Thrust (MFT). Lesser Himalayan metasedimentary rocks have been thrust southward over the churia rocks along the MBT and large part of the Churia zone rocks must be buried beneath the cover of the over thrust Lesser Himalayan rocks to the north. The Churia Hills abruptly rise from the plains of Terai and forms the southernmost mountain range of the Himalaya.The Siwaliks constitutes the narrow belt of 20 Km to 30 Km in width 5 Km to 6 Km in thickness runs east west. The age range is indicated as Middle Miocene to Early Pleistocene.
The Siwaliks Group in Nepal is composed of three units that are known as lower, middle and upper members. These units can be correlated with the Sub Himalaya of Pakistan and of northern India (Burbank, Beck & Mulder 1996).Sub-Himalaya is composed of fluvial sedimentary rocks like mudstone, sandstone, and conglomerate, shows the coarsening upward succession. Since middle Miocene (16-1 Ma) to lower Pleistocene, the sediments are accumulated in the basin due to rapid denudation of the Himalaya.
3. Lesser Himalaya Zone
The lesser Himalayan Zone lies in south of the Higher Himalaya that is separated by the Main Central Thrust and north of the Siwalik Range seperated by Main Boundary Thrust (MBT).The total width ranges from 60-80 km. From east to west, the Lesser Himalayan geology of Nepal shows much variation in stratigraphy, structures, magmatish and displays relatively subdued and mature geomorphology.
Tectonically, the entire Lesser Himalayas consists of two sequence of rock: allochthonous and autochthonous. The zone is made up of low-grade metamorphic rock units, with over riding crystalline nappes, klippen and tectonic windows. Geologically the Lesser Himalaya consists of low grade metamorphic rocks like slate, phyllite, schist, quartzite, marble and sedimentary rocks like limestone and dolomite, shale etc. in the south. In some region there is some minor volcanic and some granitic rocks of Proterozoic –Cambrian age (2000-500 Ma). The stratigraphic thickness of the rock sequence in this zone is 10-20 km. The sedimentary rocks in the Lesser Himalaya barely yield fossil partly because there are much older (dating back to the period when life form were not abundant or diverse) and partly because they have been metamorphosed.
Hagen (1969) and Stocklin&Bhattarai (1977) have divided the Lesser Himalayan zone into two main geological and tectonic units, which are outlined as follows:
- Kathmandu Complex
- Nawakot Complex
4. Higher Himalaya Zone
Geologically, the Higher Himalayan Zone includes the rocks lying north of the Main Central Thrust (MCT) and south of normal fault system called as South Tibetan Detachment System (STDS) below the highly fossiliferous Tibetan-Tethys Zone and runs throughout the country. This zone is made up of 10-20 km thick high grade metamorphosed coarse grained rocks (schist and gneiss) and granites situated at altitude of 3000 m to over 8000m.These rocks are proterozoic-Cambrian age (2,000-500 Ma) and belong to the continental crust of the Indianplate. In addition, white granites (leucogranites) of Miocene-are (24-17 Ma) in upper part of the Higher Himalaya is found. It extends continuously along the entire length of the country as in whole Himalaya, and its width varies from place to place. Vast area of the Higher Himalaya is occupied by the Precambrian rocks. This sequence can be divided into four main units, as KyaniteSillimanitegneiss, pyroxenic,marbleand gneiss, banded gneiss, and augengneiss in the ascending order (Bordet, Colchen&LeFort 1972). However, later Le Fort (1975) revised this classification and divided into three formations as Formation I, Formation II and Formation III in the ascending order. This zone is characterized by extremely high relief, steep topography, rocky cliff and outcrops with little soil covered terrain.
5. Tibetan- Tethys Himalaya Zone
The Tibetan-Tethys Himalayas generally begins from the top of the Higher Himalayan Zone separated by the South Tibetan Detachment System (STDS) and extends to the north in Tibet, represents the deformed remnants of the northern edge of the Indian subcontinent. It has undergone very little metamorphosed except at its base where it is close the Higher Himalayan Crystalline rocks of the Higher Himalayan Zone. This zone is about 40 km wide and consists of Cambrian through Eocene sediments (sandstone, shale and limestone) which were deposited on the continental shelf of the Tethys Ocean. These sedimentary rocks contain many kinds of fossils including the Cretaceous-age ammonite (‘Shaligramshilla’ in Nepal) fossils.In Nepal these fossiliferous rocks are well developed in ThakKhola (Mustang)Manangand 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.
Posts
