Geology of the Himalayas

The geology of the Himalayas is one of the most dramatic and visible creations of the immense mountain range formed by plate tectonic forces and sculpted by weathering and erosion. The Himalayas, which stretch over 2400 km between the Namcha Barwa syntaxis at the eastern end of the mountain range and the Nanga Parbat syntaxis at the western end, are the result of an ongoing orogeny — the collision of the continental crust of two tectonic plates, the Indian Plate thrusting into the Eurasian Plate. 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 2–12 mm/yr, the source of some of the greatest rivers and the highest concentration of glaciers outside of the polar regions. From south to north the Himalaya (Himalaya orogen) is divided into 4 parallel tectonostratigraphic zones and 5 thrust faults which extend across the length of Himalaya orogen. Each zone, flanked by the thrust faults on its north and south, has stratigraphy (type of rocks and their layering) different from the adjacent zones. From south to north, the zones and the major faults separating them are the Main Frontal Thrust (MFT), Subhimalaya Zone (also called Sivalik), Main Boundary Thrust (MBT), Lesser Himalaya (further subdivided into the "Lesser Himalayan Sedimentary Zone (LHSZ) and the Lesser Himalayan Crystalline Nappes (LHCN)), Main Central thrust (MCT), Higher (or Greater) Himalayan crystallines (HHC), South Tibetan detachment system (STD), Tethys Himalaya (TH), and the Indus‐Tsangpo Suture Zone (ISZ). North of this lies the Transhimalaya in Tibet which is outside the Himalayas. The Himalayas border the Indo-Gangetic Plain to the south, Pamir Mountains to the west in Central Asia, and the Hengduan Mountains to the east on the China–Myanmar border. From east to west the Himalayas are divided into 3 regions, Eastern Himalaya, Central Himalaya, and Western Himalaya, which collectively house several nations.

Making of the Himalayas

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 subcontinent 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 Late 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 the 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 Himalayan tectonics result in long term deformation. This includes shortening across the Himalayas that range from 900 to 1,500 km. Said shortening is a product of the significant ongoing seismic activity. The continued convergence of the Indian plate with the Eurasian plate results in mega earthquakes. These seismic events can reach greater than MW 8 and result in intense damage to infrastructure. The mid-crustal ramp in the Himalayas is a key geologic feature in the history for both long-term and short-term seismic processes linked to deformation and shortening. Over the last 15 Ma, the ramp has gradually moved south due to duplexing, accretion, and tectonic undercutting. The ongoing active collision of the Indian and Eurasian continental plates challenges one hypothesis for plate motion which relies on subduction.

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