MIDDLE MIOCENE CLIMATE OPTIMUM RESULTS IN RAPID DOWNCUTTING OF THE MEKONG RIVER (Invited Presentation)

The timing of rapid kilometer-scale fluvial incision is widely used to infer the timing of uplift of plateaus. The Mekong River, which drains the eastern Qiangtang Terrane and southeastern Tibetan Plateau, is one of the 10 largest rivers in the world by water and sediment discharge. When the Mekong River was established is critical in understanding the elevation history of the Tibetan Plateau but is highly debated (ranging from >55 to <5 Ma). Here based on new low-temperature thermochronology data, we reveal a phase of rapid >700 m downward incision in the Mekong River during the middle Miocene, when the East Asian summer monsoon was intensified several-fold compared with the early Miocene. Using stream profile modeling, we demonstrate that such a middle Miocene increase in precipitation can produce the observed incision in the Mekong River. In the absence of an obvious tectonic contribution, this provides evidence for climatic control on kilometer-scale fluvial incision in the Tibetan Plateau.