Collision tectonics in the Himalaya as evidenced by the Indus and Shyok rock assemblages

Abstract

The Himalayan orogeny is closely associated with the tectonic developments which occurred along the Indus Suture Zone. During the last decade many new data on sediment stratigraphy and volcanic-plutonic suites have been obtained. These data have been interpreted in terms of plate tectonics. The Meso-Tethys was generated following the Permo-Triassic rifting and spreading during the Jurassic. The Tethys Ocean is postulated, in accordance with geophysical data, to have been consumed during northward subduction along the Indus Suture, resulting in the Himalayan orogeny through collision between India and Eurasia. However, the nature of tectonic processes and their chronology appear to be different in the Western and Eastern Himalayan segments. In the Western Himalaya, subduction in an island-arc setting and Eocene collision are evident mainly by the occurrence of the island-arc Dras volcanics, the Sangeluma Group (Srikantia and Rajdhan, 1980) trench deposits, ophiolitic blocks, older (>60 m.y.) calc-alkaline plutons and blueschist facies rocks. The Indus Group (Srikantia and Rajdhan, 1980) continental deposits and the younger (<49 m.y.) ages from the Ladakh plutons are interpreted as being due to southward subduction of the back-arc (?) crust along the Shyok Suture Zone. The chronology of these events is discussed. In the Eastern and Central Himalaya, the lack of island-arc volcanism, the dominance of calc-alkaline plutonism and the epicontinental deposits to the north of the Indus Suture Zone are in favour of subduction in a Cordillaran type tectonic setting. The younger granite ages (ca 40 m.y.) indicate that continental collision occurred in the Eastern Himalaya at least 10-12 m.y. after the collision in the Western Himalaya.