Geology of Nepal
The Himalayan arc extends about 2400 km from Nanga Parbat (8,138 m) by the Indus River in northern Pakistan eastward to Namche Barwa (7,756 m) by the gorge of the Tsangpo-Brahmaputra in eastern Tibet. About 800 km of this extent is in Nepal; the remainder includes Bhutan and parts of Pakistan, India, and China. Since 55 Ma the Himalayan orogeny beginning with the collision of Indian subcontinent and Eurasia at the Paleocene/Eocene epoch, has thickened the Indian crust to its present thickness of 70 km. The northwest tip of India after colliding with Asia seems to have met along the full length of the suture by about 40 Ma. Immediately prior to the onset of the Indo—Asian collision, the northern boundary of the Indian shield was likely a thinned continental margin on which Proterozoic clastic sediments and the Cambrian ±Eocene Tethyan shelf sequence were deposited.
Tectonostratigraphic division of Nepal Himalaya
Heim & Gansser 1939, and Gansser 1964 divided the rocks of the Himalaya into four tectonostratigraphic zones that are characterised by distinctive stratigraphy and physiography. From south to north, these are the Sub Himalayan, Lesser Himalayan, Greater Himalayan, and Tibetan Himalayan zones.
The Terai is a rich, fertile and ancient land in the southern part of Nepal. In Nepal it is the Nepalese extension of the Indo-Gangetic Plains, which covers most of northern and eastern India, the most populous parts of Pakistan, and virtually all of Bangladesh. The Plains get their names from the rivers Ganges and Indus.
The vast alluvial plains of the Indo-Gangetic Basin evolved as a foreland basin in the southern part of the rising Himalaya, before breaking up along a series of steep faults known as the Himalayan Frontal Fault (Nakata 1989) or the Main Frontal Thrust (Gansser 1981). It comprises several sub-basins and all of them are quite shallow towards the south, but rather deep in the northern sections.
The Sub-Himalayan Sequence borders the Indo-Gangetic Floodplain along the Himalayan Frontal Fault and is dominated by thick Late Tertiary mollassic deposits known as the Siwaliks that resulted from the accumulating fluvial deposits on the southern front of the evolving Himalaya. In Nepal, it extends throughout the country from east to west in the southern part. It is delineated by the Himalayan Frontal Thrust (HFT) and Main Boundary Thrust (MBT) in south and north respectively. The youngest sediments on the top are the conglomerates, and the sandstones and mudstones are dominant in the lower portions. The upward coarsening sequence of the sediments obviously exhibit the time-history in the evolution and growth of the Himalaya during the early Tertiary time (Gansser 1964).
The Sub Himalayan zone is the 10 to 25 km wide belt of Neogene Siwaliks (or Churia) group rocks forming the topographic front of the Himalaya. It rises from the fluvial plains of the active foreland basin, and this front generally mapped as the trace of the Main Frontal Thrust (MFT). The Siwaliks Group consists of upward-coarsening successions of fluvial mudstone, siltstone, sandstone, and conglomerate. 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). Palaeocurrent and petrographic data from the sandstone and conglomerate indicate that these rocks were derived from the fold-thrust belt, and deposited within the flexural foredeep of the Himalayan foreland basin (Tokuoka et al. 1986; DeCelles et al. 1998).
The Lesser Himalayas lies in between the Sub-Himalayas and Higher Himalayas separated by the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT) respectively. The total width ranges from 60–80 km. The Lesser Himalayas is made up mostly of the unfossiliferous sedimentary and metasedimentary rocks; such as shale, sandstone, conglomerate, slate, phyllite, schist, quartzite, limestone and dolomite. The rocks range in age from Precambrian to Miocene. The geology is complicated due to folding, faulting and thrusting and are largely unfossiliferous. Tectonically, the entire Lesser Himalayas consists of two sequences of rocks: allochthonous, and autochthonous-paraautochthonous units; with various nappes, klippes and tectonic windows.
The northernmost boundary of the Siwaliks Group is marked by the Main Boundary Thrust (MBT), over which the low-grade metasedimentary rocks of the Lesser Himalaya overlie. The Lesser Himalaya, also called the Lower Himalaya, or the Midlands, is a thick (about 7 km) section of para-autochthonous crystalline rocks made up of low- to medium grade rocks. These lower Proterozoic clastic rocks (Parrish & Hodges 1996) are subdivided into two groups. Argillo-arenaceous rocks dominate the lower half of the succession, whereas the upper half consists of both carbonate and siliciclastic rocks (Hagen (1969); Le Fort 1975; Stöcklin 1980). The Lesser Himalaya thrust over the Siwaliks along the MBT to the south, and is overlained by the allochthonous thrust sheets of Kathmandu and HHC along the MCT. The Lesser Himalaya is folded into a vast post-metamorphic anticlinal structure known as the Kunchha-Gorkha anticlinorium (Pêcher 1977). The southern flank of the anticlinorium is weakly metamorphosed, whereas the northern flank is highly metamorphosed.
Main Central Thrust Zone
The Main Central thrust (MCT) is the single largest structure within the Indian plate that has accommodated Indian-Asian convergence. It extends for nearly 2500 km along strike and has been the site of at least 140 and perhaps more than 600 km of displacement (Schelling & Arita 1991; Srivastava & Mitra 1994). Heim & Gansser 1938 defined the MCT in Kumaon based on the difference in metamorphic grade between low to medium-grade rocks of the Lesser Himalaya and higher-grade rocks of the Greater Himalaya. However, the fault originally defined by Heim & Gansser 1938 is not the MCT, but a fault within Lesser Himalaya rocks (Valdiya 1980; Ahmad et al. 2000). This misidentification symbolizes the challenge that workers have faced in locating the MCT. The metamorphic grade within the Lesser Himalaya increases towards the MCT and at higher structural levels.
In central Nepal, the metamorphic grade increases from low (chlorite + biotite) to medium (biotite + garnet + kyanite + staurolite) towards the MCT over a north-south distance. The highest-grade rocks (kyanite and sillimanite gneisses) are found within the MCT shear zone, i.e. upper Lesser Himalaya. Arita (1983) places two thrusts (MCT I and MCT II) on each side of the MCT shear zone.
This zone extends from the MCT to Tibetan-Tethys Zone and runs throughout the country. This zone consists of almost 10 km thick succession of the crystalline rocks, commonly called the Himal Group. This sequence can be divided into four main units, as Kyanite-Sillimanite gneiss, pyroxenic marble and gneiss, banded gneiss, and augen gneiss in the ascending order (Bordet, Colchen & LeFort 1972).
The Higher Himalayan sequence has been variously named. French workers used the term Dalle du Tibet (Tibetan Slab) for this unit (Le Fort 1975; Bordet, Colchen & LeFort 1972). Hagen (1969) called them Khumbu Nappes, and Lumbasumba Nappes. Arita (1983) calls it the Himalayan Gneiss Group, and it lies above the MCT II, or the upper MCT.
The High Himalayan Crystalline units (HHC) are mainly composed of kyanite- to sillimanite-grade gneisses intruded by High Himalayan leucogranites at structurally higher levels (Upreti 1999). Throughout much of the range, the unit is divided into three formations (Pêcher & Le Fort 1986). In central Nepal (Guillot 1999), the upper Formation III consists of augen orthogneisses, whereas the Middle Formation II are calcsilicate gneisses and marbles, and the basal Formation I are kyanite- and sillimanite bearing metapelites, gneisses, and metagreywackes with abundant quartzite.
The gneiss of Higher Himalayan zone (HHZ) is a thick continuous sequence of about 5 to 15 km (Guillot 1999). The northern part is marked by North Himalayan Normal fault (NHNF), which is also known as the South Tibetan Detachment system (STDS). At its base, it is bounded by the MCT. The protolith of the HHC is interpreted to be Late Proterozoic clastic sedimentary rocks deposited on the northern Indian margin (Parrish & Hodges 1996).
The Tibetan-Tethys Himalayas generally begins from the top of the Higher Himalayan Zone and extends to the north in Tibet. In Nepal these fossiliferous rocks are well developed in Thak Khola (Mustang), Manang and Dolpa area. This zone is about 40 km wide and composed of fossiliferous sedimentary rocks such as shale, sandstone and limestone etc.
The area north of the Annapurna and Manaslu ranges in central Nepal consists of metasediments that overlie the Higher Himalayan zone along the South Tibetan Detachment system. It has undergone very little metamorphism except at its base where it is close to the Higher Himalayan crystalline rocks. The thickness is currently presumed to be 7,400 m (Fuchs, Widder & Tuladhar 1988). The rocks of the Tibetan Tethys Series (TSS) consist of a thick and nearly continuous lower Paleozoic to lower Tertiary marine sedimentary succession. The rocks are considered to be deposited in a part of the Indian passive continental margin (Liu & Einsele 1994).
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