Geology of Nepal Himalaya

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.

Terai
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).

Sub-Himalaya (Siwaliks)
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)

Lesser Himalaya
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.

Higher Himalaya
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).

Tibetan-Tethys
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).

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Major Earthquakes in Nepal

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Geology of the Himalaya

Mt. Everest (8848 m.)

The Geology of the Himalaya is a record of the most dramatic and visible creations of modern plate tectonic forces. The Himalayas, which stretch over 2400 km, are the result of an ongoing orogeny — the result of a collision between two continental tectonic plates. This immense mountain range was formed by huge tectonic forces and sculpted by unceasing denudation processes of weathering and erosion. The Himalaya-Tibet region is virtually the water tower of Asia: it supplies freshwater for more than one-fifth of the world population, and it accounts for a quarter of the global sedimentary budget. Topographically, the belt has many superlatives: the highest rate of uplift (nearly 1 cm/year at Nanga Parbat), the highest relief (8848 m at Mt. Everest Chomolangma), the source of some of the greatest rivers and the highest concentration of glaciers outside of the polar regions. This last feature earned the Himalaya its name, originating from the Sanskrit for "the abode of the snow".

Contents
1 The making of the Himalaya
2 Major tectonic subdivisions of the Himalaya
3 Future of the Himalaya

The making of the Himalaya
During Late Precambrian and the Palaeozoic, the Indian sub-continent, bounded to the north by the Cimmerian Superterranes, was part of Gondwana and was separated from Eurasia by the Paleo-Tethys Ocean (Fig. 1). During that period, the northern part of India was affected by a late phase of the so-called "Cambro-Ordovician Pan-African event", which is marked by an unconformity between Ordovician continental conglomerates and the underlying Cambrian marine sediments. Numerous granitic intrusions dated at around 500 Ma are also attributed to this event.

In the Early Carboniferous, an early stage of rifting developed between the Indian continent and the Cimmerian Superterranes. During the Early Permian, this rift developed into the Neotethys ocean (Fig. 2). From that time on, the Cimmerian Superterranes drifted away from Gondwana towards the north. Nowadays, Iran, Afghanistan and Tibet are partly made up of these terranes.

In the Norian (210 Ma), a major rifting episode split Gondwana in two parts. The Indian continent became part of East Gondwana, together with Australia and Antarctica. However, the separation of East and West Gondwana, together with the formation of oceanic crust, occurred later, in the Callovian (160-155 Ma). The Indian plate then broke off from Australia and Antarctica in the Early Cretaceous (130 - 125 Ma) with the opening of the "South Indian Ocean" (Fig. 3).

In the Upper Cretaceous (84 Ma), the Indian plate began its very rapid northward drift at an average speed of 16 cm/year, covering a distance of about 6000 km, until the collision of the northwestern part of the Indian passive margin with Eurasia in early Eocene time (48-52 Ma). Since that time and until today, the Indian continent continues its northwards movement at a slower but still surprisingly fast rate of ~ 5 cm/year, indenting Eurasia by about 2400 km and rotating by just over 33° in an anticlockwise direction (Fig. 4).

Whilst most of the oceanic crust was "simply" subducted below the Tibetan block during the northward motion of India, at least three major mechanisms have been put forward, either separately or jointly, to explain what happened, since collision, to the 2400 km of "missing continental crust". The first mechanism also calls upon the subduction of the Indian continental crust below Tibet. Second is the extrusion or escape tectonics mechanism (Molnar and Tapponier, 1975) which sees the Indian plate as an indenter that squeezed the Indochina block out of its way. The third proposed mechanism is that a large part (~1000 km; Dewey et al. 1989) of the 2400 km of crustal shortening was accommodated by thrusting and folding of the sediments of the passive Indian margin together with the deformation of the Tibetan crust.

Even though it is more than reasonable to argue that this huge amount of crustal shortening most probably results from a combination of these three mechanisms, it is nevertheless the last mechanism which created the high topographic relief of the Himalaya.

Major tectonic subdivisions of the Himalaya
One of the most striking aspects of the Himalayan orogen is the lateral continuity of its major tectonic elements. The Himalaya is classically divided into four tectonic units that can be followed for more than 2400 km along the belt (Fig. 5 and Fig. 7)2.

The Subhimalaya forms the foothills of the Himalayan Range and is essentially composed of Miocene to Pleistocene molassic sediments derived from the erosion of the Himalaya. These molasse deposits, known as the Muree and Siwaliks Formations, are internally folded and imbricated. The Subhimalaya is thrust along the Main Frontal Thrust over the Quaternary alluvium deposited by the rivers coming from the Himalaya (Ganges, Indus, Brahmaputra and others), which demonstrates that the Himalaya is still a very active orogen.
The Lesser Himalaya (LH) is mainly formed by Upper Proterozoic to lower Cambrian detrital sediments from the passive Indian margin intercalated with some granites and acid volcanics (1840± 70 Ma; Frank et al., 1977). These sediments are thrust over the Subhimalaya along the Main Boundary Thrust (MBT). The Lesser Himalaya often appears in tectonic windows (Kishtwar or Larji-Kulu-Rampur windows) within the High Himalaya Crystalline Sequence.
The Central Himalayan Domain, (CHD) or High Himalaya, forms the backbone of the Himalayan orogen and encompasses the areas with the highest topographic relief. It is commonly separated into four zones.
The High Himalayan Crystalline Sequence, HHCS (approximately 30 different names exist in the literature to describe this unit; the most frequently found equivalents are Greater Himalayan Sequence, Tibetan Slab and High Himalayan Crystalline) is a 30-km-thick, medium- to high-grade metamorphic sequence of metasedimentary rocks which are intruded in many places by granites of Ordovician (~ 500 Ma) and early Miocene (~ 22 Ma) age. Although most of the metasediments forming the HHCS are of late Proterozoic to early Cambrian age, much younger metasediments can also be found in several areas (Mesozoic in the Tandi syncline and Warwan region, Permian in the Tschuldo slice, Ordovician to Carboniferous in the Sarchu Area). It is now generally accepted that the metasediments of the HHCS represent the metamorphic equivalents of the sedimentary series forming the base of the overlying Tethys Himalaya. The HHCS forms a major nappe which is thrust over the Lesser Himalaya along the Main Central Thrust (MCT).
The Tethys Himalaya (TH) is an approximately 100-km-wide synclinorium formed by strongly folded and imbricated, weakly metamorphosed sedimentary series. Several nappes, termed North Himalayan Nappes (Steck et al., 1993) have also been described within this unit. An almost complete stratigraphic record ranging from the Upper Proterozoic to the Eocene is preserved within the sediments of the TH. Stratigraphic analysis of these sediments yields important indications on the geological history of the northern continental margin of the Indian continent from its Gondwanian evolution to its continental collision with Eurasia. The transition between the generally low-grade sediments of the Tethys Himalaya and the underlying low- to high-grade rocks of the High Himalayan Crystalline Sequence is usually progressive. But in many places along the Himalayan belt, this transition zone is marked by a major extensional structure, the Central Himalayan Detachment System (also known as South Tibetan Detachment System or North Himalayan Normal Fault).
The Nyimaling-­Tso Morari Metamorphic Dome, NTMD: In the Ladakh region, the Tethys Himalaya synclinorium passes gradually to the north in a large dome of greenschist to eclogitic metamorphic rocks. As with the HHCS, these metamorphic rocks represent the metamorphic equivalent of the sediments forming the base of the Tethys Himalaya. The Precambrian Phe Formation is also here intruded by several Ordovician (~480 Ma; Girard and Bussy, 1998) granites.
The Lamayuru and Markha Units (LMU) are formed by flyschs and olistholiths deposited in a turbiditic environment, on the northern part of the Indian continental slope and in the adjoining Neotethys basin. The age of these sediments ranges from Late Permian to Eocene.
The Indus Suture Zone (ISZ) (or Indus-Yarlung-Tsangpo Suture Zone) defines the zone of collision between the Indian Plate and the Ladakh Batholith (also Transhimalaya or Karakoram-Lhasa Block) to the north. This suture zone is formed by:
the Ophiolite Mélanges, which are composed of an intercalation of flysch and ophiolites from the Neotethys oceanic crust
the Dras Volcanics, which are relicts of a Late Cretaceous to Late Jurassic volcanic island arc and consist of basalts, dacites, volcanoclastites, pillow lavas and minor radiolarian cherts
the Indus Molasse, which is a continental clastic sequence (with rare interbeds of marine saltwater sediments) comprising alluvial fan, braided stream and fluvio-lacustrine sediments derived mainly from the Ladakh batholith but also from the suture zone itself and the Tethyan Himalaya. These molasses are post-collisional and thus Eocene to post-Eocene.
The Indus Suture Zone represents the northern limit of the Himalaya. Further to the North is the so-called Transhimalaya, or more locally Ladakh Batholith, which corresponds essentially to an active margin of Andean type. Widespread volcanism in this volcanic arc was caused by the melting of the mantle at the base of the Tibetan bloc, triggered by the dehydration of the subducting Indian oceanic crust.

Future of the Himalaya
Over periods of 5-10 million years, the plates will probably continue to move at the same rate. In 10 million years India will plow into Tibet a further 180 km. This is about the width of Nepal. Because Nepal's boundaries are marks on the Himalayan peaks and on the plains of India whose convergence we are measuring, Nepal will technically cease to exist. But the mountain range we know as the Himalaya will not go away.

This is because the Himalaya will probably look much the same in profile then as it does now. There will be tall mountains in the north, smaller ones in the south, and the north/south width of the Himalaya will be about the same. What will happen is that the Himalaya will have advanced across the Indian plate and the Tibetan plateau will have grown by accretion. One of the few clues about the rate of collision between India and Tibet before the GPS measurements were made was the rate of advance of Himalayan sediments across the Ganges plain. There is an orderly progression of sediments in front of the foothills. Larger boulders appear first, followed by pebbles, and further south, sand-grains, silts, and finally very fine muds. This is what you see when you drive from the last hills of the Himalaya southward 100 km. The present is obvious, but the historical record cannot be seen on the surface because the sediments bury all former traces of earlier sediments. However, in drill holes in the Ganges plain, the coarser rocks are always on the top and the finer pebbles and muds are on the bottom, showing that the Himalaya is relentlessly advancing on India.

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Geology Term

Geology (from Greek: γη, ge, "earth"; and λόγος, logos, "speech" lit. to talk about the earth) is the science and study of the solid matter that constitutes the Earth. Encompassing such things as rocks, soil, and gemstones, geology studies the composition, structure, physical properties, history, and the processes that shape Earth's components. It is one of the Earth sciences. Geologists have established the age of the Earth at about 4.6 billion (4.6x109) years, and have determined that the Earth's lithosphere, which includes the crust, is fragmented into tectonic plates that move over a rheic upper mantle (asthenosphere) via processes that are collectively referred to as plate tectonics. Geologists help locate and manage the Earth's natural resources, such as petroleum and coal, as well as metals such as iron, copper, and uranium. Additional economic interests include gemstones and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as sulfur, chlorine, and helium.
Planetary geology (sometimes known as Astrogeology) refers to the application of geologic principles to other bodies of the solar system. Specialised terms such as selenology (studies of the moon), areology (of Mars), etc., are also in use. Colloquially, geology is most often used with another noun when indicating extra-Earth bodies (e.g. "the geology of Mars").
The word "geology" was first used by Jean-André Deluc in the year 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in the year 1779. The science was not included in Encyclopædia Britannica's third edition completed in 1797, but had a lengthy entry in the fourth edition completed by 1809.[1] An older meaning of the word was first used by Richard de Bury to distinguish between earthly and theological jurisprudence.

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