River incision results from interactions among tectonics, climate change, and surface processes, and yet the role of each process operating at different time scales remains poorly understood. In this study, we address this issue by reconstructing the late Quaternary spatiotemporal variation of aggradation and incision rates along the Lancang River (Upper Mekong River) in southeast Tibet. Our work combined field observations, topographic data analysis, and optically stimulated luminescence (OSL) and cosmogenic radionuclide (CRN) dating of geologically well-defined fluvial terrace deposits, and it reveals five levels of fluvial terraces with strath heights up to 200–240 m and a 300-km-wide knickzone along the Lancang River. The new data indicate that: (1) the Lancang River has experienced four aggradation events at >120–100 ka, 90–70 ka, 25–15 ka, and <9 ka, with each event followed by rapid incision at ca. 100 ka, ca. 45 ka, ca. 15 ka, and ca. 6 ka; (2) river incision rates since the late Pleistocene decrease upstream across the knickzone from <2.8–2.3 and <2.1–1.7 to <0.5 mm/yr; and (3) they decrease with time at the knickzone from <2.1 mm/yr at ca. 100 ka to <1.1 mm/yr at 15–6 ka. The terrace-derived incision rates since the late Pleistocene from this study are more than an order of magnitude higher than the existing landscape-scale erosion rates derived from both thermochronological dating of bedrock bounding the river valley at million-year scales and cosmogenic nuclide concentrations of river sand at millennial scales. These findings imply decoupling of hydrologically induced river incision rates since the late Pleistocene from regional erosion rates on million-year and millennial time scales. Specifically, the hydrologically driven incision in a large fluvial system like the Lancang River in southeast Tibet, most likely related to local climate conditions, is much more efficient than tectonically driven erosion at a time scale of 100–10 k.y.

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