Palaeo-landslide dams controlled the formation of Late Quaternary terraces in Diexi, the upper Minjiang River, eastern Tibetan Plateau

Abstract. Tectonic uplift and climate changes are the two critical factors controlling the evolution of river landscapes and the formation of terraces. However, little is known about the effect of river blockage events on terrace formation along valley areas. In this paper, we investigated the geomorphology, sedimentology, and chronology of Tuanjie (seven staircases) and Taiping (three staircases) Terraces in Diexi. These represent two typical fluvial terraces in the upper Minjiang River in the eastern Tibetan Plateau. These terraces are composed, from bottom to top, of lacustrine deposits, gravels, loess, and paleosol. Taiping Terrace 3 (T3) has two sets of mud-phyllite clasts sequences. Field investigation, Digital Elevation Model (DEM) data, lithofacies, and dating results confirm that terraces T1 to T3 in Taiping correspond to terraces T5 to T7 in Tuanjie. These findings suggest that two damming and four outburst events occurred in the area during the late Pleistocene. The palaeo-dam blocked the river around 32.40±1.91 ka, followed by the first outburst at 27.11±0.18 ka. Then, the palaeo-dam blocked the river again between 27 to 17 ka, and suffered a second dam-breaking event at 17 ka. The third and fourth gradual collapse events respectively occurred at ~10 ka and ~9.35 ka. Combined with the tectonic uplift rate, river incision rate, and high-resolution climate data, our analysis shows that the blockage and collapse of the palaeo-dam have been a major factor in the formation of tectonically active mountainous river terraces. Tectonic movement and climatic fluctuations, on the other end, play a minor role.