River aggradation or incision at different spatial-temporal scales are governed by tectonics, climate change, and surface processes which all adjust the ratio of sediment load to transport capacity of a channel. But how the river responds to differential tectonic and extreme climate events in a catchment is still poorly understood. Here, we address this issue by reconstructing the distribution, ages, and sedimentary process of fluvial terraces in a tectonically active area and monsoonal environment in the headwaters of the Yangtze River in the eastern Tibetan Plateau, China. Field observations, topographic analyses, and optically stimulated luminescence dating reveal a remarkable fluvial aggradation, followed by terrace formations at elevations of 55−62 m (T7), 42−46 m (T6), 38 m (T5), 22−36 m (T4), 18 m (T3), 12−16 m (T2), and 2−6 m (T1) above the present floodplain. Gravelly fluvial accumulation more than 62 m thick has been dated prior to 24−19 ka. It is regarded as a response to cold climate during the last glacial maximum. Subsequently, the strong monsoon precipitation contributed to cycles of rapid incision and lateral erosion, expressed as cut-in-fill terraces. The correlation of terraces suggests that specific tectonic activity controls the spatial scale and geomorphic characteristics of the terraces, while climate fluctuations determine the valley filling, river incision and terrace formation. Debris and colluvial sediments are frequently interbedded in fluvial sediment sequences, illustrating the episodic, short-timescale blocking of the channel ca. 20 ka. This indicates the potential impact of extreme events on geomorphic evolution in rugged terrain.
Evolution of the Yangtze River network, southeastern Tibet: Insights from thermochronology and sedimentology