2.2 The making of the Himalaya

This paleogeographic reconstitution is mainly based on the following papers: Besse et al., 1984; Patriat and Achache, 1984; Dewey et al. 1989; Brookfield, 1993; Ricou, 1994; Rowley, 1996; Stampfli et al. 1998 (see also http://www-sst.unil.ch/research/plate_tecto/index.htm)

During Late Precambrian and the Palaeozoic, the Indian continent, bounded to the north by the Cimmerian Superterranes, was part of Gondwana and was separated from Eurasia by the Paleotethys Ocean (Fig. 2.1 A). 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. 2.1 B). 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. 2.1 C).

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. 2.1 D).

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.