Geology of the Himalaya

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Fig 1: The earth in the Early Permian. At that time, India is part of Gondwana and bordered to the north by the Cimmerian Superterrane. Paleogeographic reconstructions. By Dèzes (1999), based on Stampfli and Borel (2002) and Patriat and Achache (1984). A more modern paleo-geographic reconstruction of the Early Permian can be found here

Fig 2: The earth at the Permian-Triassic boundary. The opening of the Neotethys separates the Cimmeridian Superterrane from Gondwana. Based on Stampfli and Borel (2002) and Patriat and Achache (1984). For a more modern paleo-geographic reconstruction of the same period, see this web-site (Stampfli et al.)

Fig 3: The earth in the Cretaceous. The Cimmeridian Superterrane has accreted to Mega Laurasia, the oceanic crust of the Neotethys is subducted to the north along the Dras volcanic arc , the Shigatze Ocean opens as a consequence of back-arc spreading, India is separated from Africa and E. Gondwana and the Indian Ocean opens. Paleogeographic reconstructions based by Dèzes (1999), on Stampfli and Borel (2002) and Patriat and Achache (1984).

Fig 4: The northward drift of India from 71 Ma ago to present time. Note the simultaneous counter-clockwise rotation of India. Collision of the Indian continent with Eurasia occurred at about 55 Ma. Source: www.usgs.org (modified)

Fig 5: Geologic - Tectonic map of the Himalaya, modified after Le Fort (1988).

Fig 6: Geological Map of the northwest Himalaya, compiled after the work of: Epard et al. 1995; Frank et al. 1997; Fuchs and Linner, 1995; Guntli, 1993; Herren, 1987; Kelemen et al. 1988; Kündig, 1988; Patel et al. 1993; Searle et al. 1988, 1997; Spring, 1993; Steck et al. 1993; Steck et al. 1998; Stutz, 1988; Thöni, 1977; Vannay, 1993; Vannay and Graseman 1998; Wyss 1999 and completed with personal observations by Dèzes (1999). for references, see bibliography. HHCS: High Himalayan Cristalline Sequence; ISZ: Indus Suture Zone; KW: Kishtwar Window; LKRW: Larji-Kulu-Rampur Window; MBT: Main Boundary Thrust; MCT: Main Central Thrust; SF: Sarchu Fault; ZSZ: Zanskar Shear Zone. (Download map in PDF format).

Fig 7: Simplified cross-section of the north-western Himalaya showing the main tectonic units and structural elements by Dèzes (1999). (Download in PDF format)

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 meaning in Sanskrit: «the abode of the snow».

1 The making of the Himalaya
2 Major tectonic subdivisions of the Himalaya
3 Notes
4 References
5 External links

[edit] 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 Paleotethys 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 is observed between the Indian continent and the Cimmerian Superterranes. During the Early Permian, this rift will develop into the Neotethys ocean (Fig. 2). From that time on, the Cimmerian Superterranes drift 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 splits Gondwana in two parts. The Indian continent becomes 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 only 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 the lower Eocene (48-52 Ma). Since that time and until today, the Indian continent continues its northwards ascent 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 these 2400 km of crustal shortening since collision 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.

[edit] 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 than 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 molasses known as 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...), which demonstrates that the Himalaya is still a very active orogen.
The Lesser Himalaya, LH is mainly formed by Upper Proterozoic to Lower Cenozoic detrital sediments from the passive Indian margin intercalated with some granites and acid volcanics (1840± 70 Ma, Frank et al., 1977). These low-grade 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 topographical relief. It is commonly separated into four zones.
1. 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 frequently intruded by granites of Ordovician (~ 500 Ma) and Lower Miocene (~ 22 Ma) age. Although most of the metasediments forming the HHCS are of Upper Proterozoic to Lower 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 admitted that the metasediments of the HHCS represent the metamorphic equivalent 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).
2. The Tethys Himalaya, TH is an approximately 100 km large synclinorium formed by strongly folded and imbricated, weakly metamorphosed sedimentary series. Several nappes, termed North Himalayan Nappes (Steck et al. 1993) have also been evidenced within this unit. An almost complete stratigraphic record ranging from the Upper Proterozoic to the Eocene is preserved within the sediments of the TH. The stratigraphic analyses 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. Yet, 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).
3. 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.
4. 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 Upper 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 flyschs and ophiolites from the Neotethys oceanic crust
the Dras Volcanics: which are relicts of an Upper Cretaceous to Upper 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.

[edit] Notes

Note 1: This paleogeographic reconstruction is mainly based on the papers of Besse et al. (1984), Patriat and Achache (1984), Dewey et al. (1989), Brookfield, (1993) Ricou (1994), Rowley (1996) and Stampfli et al. (1998). More information can be found on this website.

Note 2: The four-fold division of Himalayan units has been used since the work of Blanford and Medlicott (1879) and Heim and Gansser (1939).

[edit] References

Besse J., Courtillot V., Pozzi J.P., Westphal M., Zhou Y.X., (1984): Palaeomagnetic estimates of crustal shortening in the Himalayan thrusts and Zangbo Suture.: Nature (London), v. 311, p. 621-626.
Blanford W.T., Medlicott H.B., (1879): A manual of the geology of India: Calcutta.
Brookfield M.E., (1993): The Himalaya passive margin from Precambrian to Cretaceous times: Sedimentary Geology, v. 84, p. 1-35.
Dewey J.F., (1988): Extensional collapse of orogens: Tectonics, v. 6, p. 1123-1139.
Dewey J.F., Cande S., Pitman III W.C., (1989): Tectonic evolution of the Indian/Eurasia Collision Zone: Eclogae geologicae Helvetiae, v. 82, no. 3, p. 717-734.
Dèzes, p. (1999): Tectonic and metamorphic Evolution of the Central Himalayan Domain in Southeast Zanskar (Kashmir, India). Mémoires de Géologie (Lausanne) No. 32.
Frank W., Thoni M., Pertscheller F., (1977): Geology and petrography of Kulu - South Lahul area, in Ecologie et geologie de l’Himalaya, Paris, Dec. 7-10, p. 147-172.
Frank W., Gansser A., Trommsdorff V., (1977): Geological observations in the Ladakh area (Himalayas); a preliminary report: Schweiz. Mineral. Petrogr. Mitt, v. 57, no. 1, p. 89-113.
Girard, M. and Bussy, F. (1999) Late Pan-African magmatism in Himalaya: new geochronological and geochemical data from the Ordovician Tso Morari metagranites (Ladakh, NW India). Schweiz. Mineral. Petrogr. Mitt., v. 79, pp. 399-418.
Heim A., Gansser A., (1939): Central Himalaya; geological observations of the Swiss expedition 1936.: Schweizer. Naturf. Ges., Denksch., v. 73, no. 1, p. 245.
Molnar P., Tapponnier P., (1975): Cenozoic tectonics of Asia; effects of a continental collision.: Science, v. 189, p. 419-426.
Patriat P., Achache J., (1984): India-Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates.: Nature, v. 311, p. 615-621.
Ricou L.M., (1994): Tethys reconstructed: plates, continental fragments and their Boundaries since 260 Ma from Central America to South-eastern Asia: Geodinamica Acta, v. 7, no. 4, p. 169-218.
Stampfli, G.M. and Borel, G.D., 2002. A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrons. Earth and Planetary Science Letters, 196: 17-33.
Stampfli G.M., Mosar J., Favre P., Pillevuit A., Vannay J.-C., (1998): Permo-Triassic evolution of the westernTethyan realm: the Neotethys/east-Mediterranean basin connection: Peri Thetys, v. 3.
Steck A., Spring L., Vannay J.-C., Masson H., Stutz E., Bucher H., Marchant R., Tièche J.C., (1993): Geological Transect Across the Northwestern Himalaya in eastern Ladakh and Lahul (A Model for the Continental Collision of India and Asia): Eclogae Geologicae Helvetiae, v. 86, no. 1, p. 219-263.
Steck A., Spring L., Vannay J.C., Masson H., Bucher H., Stutz E., Marchant R., Tieche J.C., (1993): The tectonic evolution of the northwestern Himalaya in eastern Ladakh and Lahul, India, in Himalayan Tectonics, p. 265-276.