LATE CENOZOIC FLOW REVERSAL AND CAPTURE OF THE YALU RIVER (YARLUNG TSANGPO) CAUSED BY RIFT-DAMMING AND CLIMATE CHANGE

Reversal of the Yalu River is indicated by west-flowing tributaries that make near U-turns at the confluence with the main stream. The reversal event may be related to occasional capture of the Yalu as suggested by the presence of Cretaceous-early Tertiary Gangdese zircons in the foreland basin of the eastern Himalaya. To explain the above observations, we suggest that the development of north-trending Late Cenozoic rifts across the Yalu in southern Tibet has periodically disrupted its east-flowing course. Specifically, the disruption and restoration were controlled by four competing processes. (1) Normal faulting orthogonally across the Yalu River raises the local channel elevation to discourage eastward flow. (2) Channel incision overcomes tectonic uplift to maintain the east-flowing course. As climate fluctuates, down-cutting may not always keep up with tectonic uplift, causing the Yalu to pond and then divert southward across the Himalayan crest. (3) Uplift and northward tilting of the Himalaya may push the locally ponded south-flowing river back to its east-flowing course. (4) Headward erosion of the beheaded segment of the east-flowing Yalu River could recapture the south-flowing river. To test the above model, we conducted a preliminary study across the Zedong rift zone in southeastern Tibet. The rift has created a 70-km long Zedong gorge that has a local relief of ~3 km and a local width of only 100-200 m. The western end of the gorge is linked with the northwest-flowing Sewu River from the south, which cuts through a 200-m thick lacustrine-fluvial sequence. Paleo-current indicators from fluvial strata suggest southeastward flow, opposite to the direction of the Sewu River. Detrital zircon dating reveals the presence of zircons with ages < 250 Ma, which may have derived from the Lhasa terrane. Terraces at elevations of 4400-4800 m are also preserved on the banks of the Sewu River, with the highest preserved level only ~100 m below the drainage divide that separates the Sewu River from the southeast-flowing Subansiri River. The above observations led us to suggest that the Sewu and Subansiri Rivers were once connected, diverting the Yalu to the Himalayan foreland. Climate change, Himalayan uplift, or headward erosion of the Brahmaputra River may have assisted the reestablishment of its east-flowing course.