Crustal Scale Thrusting and Continental Subduction During Himalayan Collision Tectonics on the NW Indian Plate

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Abstract

Following the subduction of Tethyan oceanic lithosphere beneath Asia and Kohistan the continued convergence between the upper plate and the Indian continent led to thrust stacking of Indian crust to form the Himalayas. This lasted from Oligocene to Recent times and in an attempt to evaluate displacements, a series of balanced cross-sections have been constructed across the belt. In Pakistan these illustrate that over 600km of relative convergence between India and the Kohistan complex north of the Eocene suture zone has occurred by SSE-directed thrusting. This deformation only involves Indian upper crust at present outcrop levels so that the lower crust and remaining lithosphere must have been subducted beneath Kohistan and Tibet. The northern edge of the Indian lower crust may lie beneath the Pamirs. Similarly large amounts of shortening (several hundred kilometres) are implied by other balanced crustal sections through the central Himalayas and western Pakistan. The continuity of thrust systems around the NW margin of the Indian continent is proposed so that thrusts which stack continental crust step off into oceanic lithosphere in the west. This thrusting mechanism accounts for a substantial fraction of the total, 1200–2000km relative convergence between stable India and Asia. Further shortening in the Tibetan region which developed after the Eocene continent-continent collision must be added to the displacements on thrusts which stack Indian lithosphere. Deformation within the entire collision zone approximates more closely to an essentially vertical plane strain model rather than to a process of lateral expulsion of Tibet towards the east.