Late Cenozoic Transient Diversion Of The Yalu River(Yarlung Tsongpo) In Southern Tibet

How large rivers (>1000km in length) evolve as a result of interactions betweentectonic and climatic changes is a fundamental problem in Earth Sciences. In thisstudy we investigated the Yalu-Brahmaputra River system in the active Himalayaorogen in terms of modern transport process and sedimentary record from bothforeland basin and past river course, to reveal how it has responded to Himlaayanuplift since late Cenozoic.We conducted U-Pb dating of detrital zircon from modern sand of theYalu-Brahmaputra River system, focusing on its main trunk and right-bank tributaries.Together with pulished data for this drainage system, we determined thezircon-population distribution in the Yalu-Brahmaputra River and its tributaries:(1)its left-bank short rivers dectect50Ma age peak with varying Hf isotopic valuescharacterized by the corresponding segment of the southern Lhasa terrane;(2) itsleft-bank long rivers also carry110-120Ma age clusters and mostly negative Hfisotopic signatures of the Northern Lhasa terrrne;(3) its main trunk is dominated by50Ma age peak and varying Hf signals from the adjacent southern Lhasa terrane; and(4) the proportion of young component (<300Ma) for its main trunk variesdownstream, from11-25%to45-70%for the Yalu reach, at45-55%for the Siangupstream even after crossing the eastern Himalayan syntaxis, and then a decrease to25-30%for the Siang downstream and~10%for the Brahmaputra reach. Our workindicates both the location of Yalu-Brahmaputure River course location relative to theIndus-Tsangpo suture zone and the spatial coverage of its tributaries exert a first-ordercontrol on whether or not main turnk carry specific age component and itsconcentration; meanwhile, the erosion rates controlled by both topographic relief andmonsoon precipitation play a key role on the timeliness of its conconstration variation.Quantertative comparison of detrital-zircon ages between the Yalu-Brahmaputra Riversand and Neogene sediments detect from specific stratigraphic horizons its transientdiversion into the Subansiri River (one of the Himalayan transerver rivers) during lateMiocene. We further investigated the valley fill preserved at the drainage divide between theYalu and Subansiri Rivers by field mapping, stratigraphic and sedimentologicanalyses, and age determination of Quaternary sediments using the opticallystimulated luminescence (OSL) and radiocarbon dating methods. Our work revealsthe occurrence of a major aggradational event in the study area, which started after~24-20ky BP and was on-going at9.8-8ky BP. During the aggradation, the channelfloors of the Yalu River and its tributaries were raised up, and thus rebuilt the thelandscape in eastern Himalaya. As a result, the Yalu River was diverted and linkedwith the south-flowing Subansiri River across the eastern Himalaya, possibly assistedby damming at a rift-induced gorge along the Yalu River. The timing of theaggradational event correlates with a regional glacier-advance event and a transitionperiod from a cold-humid environment to a warm-humid environment in the easternHimalaya. Our work indicates that rift-related narrow gorges along the Yalu Rivermake the geometric configuration of the eastern Himalayan drainage system highlysensitive to climate fluctuations and the related glacier advances.