第八届青年地学论坛

As one of key geomorphic archives, staircase sequences of terraces and sediments record the history of fluvial aggradation and incision, which are related to tectonic activity, climatic change, and fluctuated local base level. Although abundant studies have discussed their effects on the fluvial terrace formation, how a river response to different climatic transitions (the warm-cold and cold-warm transitions) in various tectonic and climatic backgrounds is still in dispute. Here, we address this issue by reconstructing terrace distribution and identifying sedimentary feature with OSL dating in the headwaters of the Yangtze River in southeastern Tibetan Plateau. Two sets of thick valley filling sequences at different elevations suggest two aggradational phases when the Yangtze River was in transport limited conditions during the penultimate and last glacial periods. The river incised sharply between these two filling sequences when the river was in sediment-supply limited conditions during the climatic transition from the last interglacial to last glacial period (from warm to cold). While during the transition from the last glacial to last deglacial (from cold to warm), the river generally incised into the filling sequences at the bottom of the valley, and developed cut-in-fill terraces due to increasing discharge by monsoon precipitation. It shows the diverse fluvial adjustments (the transport-limited and sediment-supply-limited conditions) to different climatic transitions. The supply-limited river incised the riverbed by sediment abrasion in the transition from interglacial to glacial transition, in contrast to the transport-limited river, in which the increasing discharge in the transition from glacial to interglacial drove the incision. Besides climate changes, tectonic also impacted fluvial processes. Different tectonic background led to diverse geomorphological features in different reaches. Filling sequences separated on different elevations in downstream gorge due to high tectonic activity and deep river incision, whereas filling sequences superimposed upstream shallow valley due to low tectonic activity and insufficient river incision.
 

Fluvial terrace, OSL dating, Aggradation and incision, Diverse erosion mechanisms, Climatic transition, the headwaters of the Yangtze River