THE EVOLUTION OF THE DRAINAGE OF THE PAMIR INDENTER: THE INTERACTION OF TECTONICS, CLIMATE AND ACCIDENTS

During uplift of the Tibetan plateau and surrounding ranges, tectonic processes have interacted with climatic change and with local random effects (such as landslides) to determine the development of the major river systems of Asia. The Pamir arc developed only in the late Cenozoic (mid-Miocene onwards) and its drainage evolution can thus be related to relatively recent (and very large) tectonic displacements, uplifts, and climatic change as well as to accidental effects. Rivers draining the Pamir indenter have three distinctive patterns that are mainly controlled by different tectonic and climatic regimes. West of the Pamir, the rivers have moderate gradients and drain northwards to disappear into arid depressions. Relatively simple Cenozoic uplift of the early Mesozoic Hindu Kush in northern Afghanistan allowed rivers to cut across the rising ranges, modified by the shear along the Herat fault zone. In the Pamir itself, only one river, the Pyandzh appears to have kept its original northerly course with compression and uplift of the indenter, and its course strangely corresponds with a major geophysical boundary (a distorted subducted slab) but not a major geological boundary. The Pamir river sources are on, and north of, the Cretaceous Karakoram Andean margin. On the eastern flank of the Pamir, in the Kunlun and northern Tibetan plateau, the rivers rise similarly north of the Cretaceous Andean margin of southern Tibet, but then flow with low gradients across the plateau, before cutting and plunging steeply down across the Kun Lun to disappear into the arid Tarim. These steep profiles are the result of late Neogene uplift of the northern Tibetan plateau and Kun Lun, modified by glacial diversion and river capture and confirm a south to north migration of tectonic development of Tibet. The drainage history of the Pamir indenter can be reconstructed by restoring the gross movements of the plates and the tectonic displacement, uplift, and erosion of individual tectonic units. Most important changes in drainage took place in the last 10 million years (Pliocene to Quaternary), as the Pamir indenter developed, and include accidental diversions triggered by landslides, glacial advances and a large meteorite impact.