Terrace sequences and its evolution since the Late Pleistocene in Bier River of Daliang Mountain area

The detailed information of regional active structures and geomorphic evolution were recorded by river terraces, which can provide evidence for the phased uplift of the Quaternary geomorphology. Zhaojue River Basin and its tributaries in Daliang Mountain of Sichuan Province are located in the transitional zone between Hengduan Mountains in the southeast of Qinghai-Tibet Plateau and the southwestern edge of Sichuan Basin, with the development of many river terraces and other layered landforms. The authors took a tributary of Zhaojue River- Bier River in the area as a case study to identify the three river terraces of Bier River, through the investigation of the terrace sequences, river alluvial sediments and ¹⁴C dating results. The T₂ and T₃ are the rock-seated terraces and T₁ is a constructional terrace, with formation times of 27.75 ka B.P., 12.20 ka B.P., and 6.65 ka B.P.. The river incision rates of T₃ and T₂ are 0.59-1.29 mm/a and 1.26-4.50 mm/a respectively. The landform in the Daliangshan area of Western Sichuan has experienced at least two intermittent uplifts in the past 30 000 years, and the crustal uplift rate in the upper reaches of the river is faster than that in the middle and lower reaches of the river during the overall uplift process, with characteristics of tilting uplift. The terraces formation of T₃, T₂ and T₁ has a certain correlation with the climate change in the study area, but the neotectonic activity is the dominant factor in the terraces formation of Bier River, which is of great significance for the river terraces sequence distribution and evolution in Western Sichuan since the Late Pleistocene.