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Geology of Nepal Himalaya
The Himalayan arc extends about 2400 km from Nanga Parbat (8,138 m) in the west to Namche Barwa (7,756 m) in the east (Le Fort, 1996). This region includes Nepal, Bhutan and as well as parts of Pakistan, India, and China. Since 55 Ma, the Himalayan orogen which began with the collision of India and Eurasia at the Paleocene/Eocence epoch (Rowley, 1996), has thickened the Indian crust to its present thickness of 70 km (Le Fort, 1975). The northwest tip of India after colliding with Asia seems to have met along the full length of the suture by about 40 Ma (Dewey et. al., 1988). Immediately prior to the onset of the Indo-Asian collision, the northern boundary of the Indian shield was likely a thinned margin on which Proterozoic clastic sediments and the Cambrian±Eocene Tethyan shelf sequence were deposited (Le Fort, 1996).
Tectonostratigraphic division of Himalaya
Heim and Gansser (1939), and Gansser (1964) divided the rocks of the Himalaya into four tectonostratigraphic zones that are characterised by distinctive stratigraphy and physiography. From north to south, these are the Sub Himalayan, Lesser Himalayan, Greater Himalayan, and Tibetan Himalayan zones.
The Terai is the Nepalese portion of the Indo-Gangetic Plain that extends from the Indian Shield in the South to the Siwalik Fold Belt to the North. The plain is a few hundred metres above sea level and usually 400 to 600 m thick. it is composed of Recent of Quaternary alluvium, boulder, gravel, silt and clay. Terai Plain is underlain by a thick, relatively flat-lying sequence of Mid to Late Tertiary molasse (Siwalik Group) which uncomformably overlies subbasins of early Tertiary to Protorozoic sediments (Surkhet, Gondwana and Vindhyan Groups) and igeneous and metamorphic rocks of the Indian Shield (Agrawal, 1977; Acharya and Ray, 1982; Raiverman et.al, 1983).
The Sub Himalayan Zone or the Siwaliks of Nepal 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 Siwaliks consist of very thick (4000 to 6000m) molasses-like fluvial sedimentary deposits comprising a coarsening-upwards sequence as a whole, which reflects the rising history of the Himalayas (Gansser, 1964).
The Sub Himalayan zone is the 10 to 25 km wide belt of Neogene Siwaliks (or Churia) Group rocks, that forms 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 comprise 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 et al., 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 (Tokouka et al., 1986; DeCelles et al., 1998)
The Lesser Himalayas lies in between the Sub-Himalayas and Higher Himalayas separated by 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; like shale, sandstone, conglomerate, slate, phyllite, schist, quartzite, limestone, dolomite etc. Ranging in age from Precambrian to Miocene. The geology is complicated due to folding, faulting and thrusting and these complications added by the unfossiliferous nature. 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-autochtonous crystalline rocks comprising of low- to medium grade rocks. These lower Proterozoic clastic rocks (Parrish and 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 allochtonous 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 and Arita, 1991; Srivastava and Mitra, 1994). Heim and Gansser (1939) 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 and Gansser (1939) 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 10km 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 et al., 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 et al., 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 HHC are mainly comprised kyanite- to sillimanite-grade gneisses intruded by High Himalayan leucogranites at structurally higher levels (Upreti, 1999a). Throughout much of the range, the unit is divided into three formations (Pêcher and 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 and 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 40km 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 et al., 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 and Einsele, 1994).
Geological maps that are available in the market of Kathmandu
1. Geological map of Kathmandu area and Central Mahabharat Range, 1:250,000
2. Geological map of Nepal, 1:1,000,000 (NOT AVAILABLE)
3. Geotectonic Division and Mineral Deposits of Nepal, 1:100,000
4. Photogeological Map of a part of Central Nepal, 1:100,000
5. Geological map of Central Western Nepal, 1:250,000
6. Geological map of Eastern Nepal, 1:250,000
7. Geological map of Central Nepal, 1:250,000
8. Geological map of Mid - Western Nepal, 1:250,000
9. Geological map of Far - Western Nepal, 1:250,000
10. Geological map of Nepal, 1:1,000,000 (better quality)
11. Mineral resources of Nepal, 1:1,000,000 (MAY NOT BE AVAILABLE)
12. Geological Map of part of Tanahun & Kaski District (Sheet No. 71 D/4),
13. Geological Map of part of Tanahun, Gorkha & Nawalparasi District (Sheet
No. 71 A, 72A /1), 1:50,000
14. Miscroseismic Epicenter Map of Nepal Nepal and Adjoining Region (March
1994-Dec 1992) 1:2,000,00
15. Epicentre Map of Nepal (March 1994 - Dec 1997), 1:1,000,000
(MAY NOT BE AVAILABLE)
16. Engineering & Environmental Geological Map of Kathamandu Valley,1:50,000
17. Engineering & Environmental Geological Map of Pokhara Valley,1:50,000
18. Geological Map of part of Syangja, Kaski, and Tanahun District (Sheet
No. 62 P /16), 1:50,000
19. Geological Map of part of Tanahun & Nawalparasi District (Sheet No. 72
20. Geological Map of part of Syangja, Palpa, and Tanahun (Sheet No. 63