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Karakoram and Afghanistan |
1 School of Geological Sciences, Kingston University, Kingston upon Thames, Surrey KT1 2EE, UK
2 British Gas Exploration and Production, 100 Thames Valley Park, Reading RG6 1PT, UK
A synthesis of the available tectonic, structural, magmatic and geochronologic data from Afghanistan and western Pakistan suggests that collision between the Indian Plate and the Afghan Block (a collage of blocks assembled during the Cimmeride orogeny), took place during the Pliocene. Prior to this, the thinned western margin of the Indian Plate, together with the transitional to oceanic crust which separated the Indian Plate from the Afghan Block, was overlain by a wedge of Tertiary clastic sediments deposited in a large flexural basin that included what is now the Katawaz basin as well as the Sulaiman foredeep. At least three phases of collision can thus be recognised in the northwest Himalaya: the first, at about 100 Ma involved the suturing of the Kohistan-Ladakh-East Nuristan island arc to southern Asia; the second, at about 55 Ma involved India-Asia collision and the third, during the Pliocene, involved India-Afghanistan collision. During the Eocene and Miocene, the Afghan block was displaced southwards in response to Indian indentation into Asia. Simultaneous counterclockwise rotation of the Afghan Block resulted in intracontinental extension causing alkaline magmatism and right lateral displacements along the Herat Fault. As the Indian Plate moved northward with respect to Afghanistan, some of the wedge of clastic sediments deposited on the Indian transitional crust were accreted to the leading edge of the Afghan block and displaced sinistrally with respect to the rest of the sediments in the wedge. These displaced sediments now form the Makran sequence. Collisional structures on both sides of the India-Afghan collision zone in the Helmand block and the Sulaiman-Kirthar Ranges probably represent the inversion of continental margin basins rather than thrust belts with large shortening amounts. After initial inversion, the dominant feature of this highly oblique collision has been transpressional tectonics. Prior to its final docking with the Afghan Block, the Kabul block was a continental slice, completely surrounded by oceanic crust, and located northwest of continental India. Palaeocene collision between India and the Kabul block could have led to thickening of the Indian Plate prior to initiation of India-Asia collision.
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