Geology of the Himalaya

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 between the Namche Barwa syntaxis in Tibet and the Nanga Parbat syntaxis in India, are the result of an ongoing orogeny â€" the result of a collision between two continental tectonic plates. This immense mountain range was formed by tectonic forces and sculpted by weathering and erosion. The Himalaya-Tibet region supplies fresh water for more than one-fifth of the world population, and accounts for a quarter of the global sedimentary budget. Topographically, the belt has many superlatives: the highest rate of uplift (nearly 10 mm/year at Nanga Parbat), the highest relief (8848 m at Mt. Everest Chomolangma), among the highest erosion rates at 2â€"12 mm/yr, 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".

The making of the Himalaya

During Late Precambrian and the Palaeozoic, the Indian Subcontinent, 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 Pan-African orogeny 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 covering a distance of about 6000 km, with the oceanic-oceanic subduction continuing until the final closure of the oceanic basin and the obduction of oceanic ophiolite onto India and the beginning of continent-continent tectonic interaction starting at about 65 Ma in the Central Himalaya. The change of the relative speed between the Indian and Asian plates from very fast (18-19.5 cm/yr) to fast (4.5 cm/yr) at about 55 Ma is circumstantial support for collision then. Since then there has been about 2500 km of crustal shortening and rotating of India by 45° counterclockwise in Northwestern Himalaya to 10°-15° counterclockwise in North Central Nepal relative to Asia (Fig. 4).

While 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 2500 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 & Tapponnier 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, Cande & Pitman 1989) or ~800 to ~1200 km) of the 2500 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.

The ongoing active collision of the Indian and Eurasian continental plates challenges one hypothesis for plate motion which relies on subduction.

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).

1. 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.
2. 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). 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.
3. 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.
   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 intruded in many places by granites of Ordovician (c. 500 Ma) and early Miocene (c. 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).
   2. 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 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 structure, the Central Himalayan Detachment System (also known as South Tibetan Detachment System or North Himalayan Normal Fault) which has indicators of both extension and compression (see 'ongoing geologic studies section below).
   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 (c. 480 Ma) 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 Late Permian to Eocene.
4. 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.

See also

Localized geology and geomorphology topics for various parts of the Himalaya are discussed on other pages:

• Zanskar is a subdistrict of the Kargil district, which lies in the eastern half of the Indian state of Jammu and Kashmir.
• Indus River - the erosion at Nanga Parbat is causing rapid uplifting of lower crustal rocks
• Mount Everest
• Sutlej River - similar small scale erosion to the Indus
• Tibetan Plateau to the North (also discussed in Geography of Tibet)
• Paleotethys
• Himalayas main page
• Karakoram fault system - major active fault system within the Himalayas

References

Notes

Bibliography

External links

• Catlos, Elizabeth Jacqueline (2000). Geochronologic and Thermobarometric Constraints on the Evolution of the Main Central Thrust, Himalayan Orogen (PDF). PhD Thesis. University of California. 
• "Geology and Petrographic study of the area from Chiraundi Khola to Thulo Khola, Dhading/Nawakot district, central Nepal". MS Thesis by Gyanendra Gurung
• Granitoids of the Himalayan Collisional Belt. Special Edition of "The Journal of the Virtual Explorer"
• Reconstruction of the evolution of the Alpine-Himalayan orogeny. Special Edition of "The Journal of the Virtual Explorer"
• "Engineering Geology of Nepal"
• Wadia Institute of Himalayan Geology, Dehradun,India, main page