THE EVOLUTION OF THE GREAT RIVER SYSTEMS OF SOUTHERN ASIA DURING THE CENOZOIC INDIA-ASIA COLLISION - RIVERS DRAINING SOUTHWARDS

During uplift of the Tibetan plateau and surrounding ranges, tectonic processes have interacted with climatic change and with local random e ffects (such as landslides) to determine the development of the major river systems of Asia. Rivers draining southward have three distinctiv e patterns that are controlled by different tectonic and climatic regi mes. In central and southern Afghanistan, the rivers have moderate gra dients and fan out from northeastern sources to disappear into arid de pressions. Anti-clockwise rotation of southern Afghanistan, caused by differential compression and right-lateral shear, cut the rivers on th e north, while increasingly arid conditions developed on the south as are accretion in the Makran separated sources from the coastal rains. In Tibet and southeast Asia, the rivers are widely separated and have low gradients on the Tibetan plateau, higher gradients as they turn so uthwards into close and parallel gorges, before they fan out southeast to enter different seas. Differential shear and clockwise rotation be tween the compressing Tibetan plateau and Southeast Asia determined th e great sigmoidal bends of this river system which was accompanied by increasing aridity, with truncation of river systems in the north and river capture in the south. In the Himalaya and southern Tibet, the ma in rivers have steep gradients where they cut across the Himalayan ran ge and occasionally truncate former rivers with low gradients on the T ibetan plateau to the north. Southward thrusting and massive frontal e rosion of the Himalaya caused progressive truncation of longitudinal r ivers on the plateau, accompanied by river capture, and glacial and la ndslide diversions on the south. The drainage history of southern Asia can be reconstructed by restoring the gross movements of the plates a nd the tectonic displacement, uplift, and erosion of individual tecton ic units. Most important changes in drainage took place in Pliocene to Quaternary times. (C) 1998 Elsevier Science B.V.