Quantitative analysis of planation surfaces of the upper Yangtze River in the Sichuan-Yunnan Region, Southwest China

Identification of the planation surfaces (PSs) is key for utilizing them as a reference in studying the long-term geomorphological evolution of the Upper Yangtze River Basin in the Sichuan-Yunnan region, Southwest China. Using a combined method of DEM-based fuzzy logic and topographic and river profiles analysis and based on a comprehensive analysis of four morphometric parameters: slope, curvature, terrain ruggedness index, and relative height, we established the relevant fuzzy membership functions, and then calculated the membership degree (MD) of the study area. Results show that patches with a MD >80% and an area >0.4 km2 correspond well to the results of Google Earth and field investigation, representing the PS remnants. They consist of 1764 patches with an altitude, area, mean slope, and relief of mostly 2000–2500 m above sea level (asl), 0–10 km2, 4°–9°, 0–500 m, respectively, covering 9.2% of the study area’s landscape, dipping to southeast, decreasing progressively from northwest to southeast in altitude, and with no clear relation between each patch’s altitude and slope, or relief. All these results indicate that they are remnants of once regionally continuous PSs which were deformed by both the lower crust flow and the faults in upper crust, and dissected by the network of Upper Yangtze River. Additionally, topographic and river profiles analysis show that three PSs (PS1–PS3) well developed along the main valleys in the Yongren-Huili region, indicating several phases of uplift then planation during the Late Cenozoic era. Based on the incision amount deduced from projection of relict river profiles on PSs, together with erosion rates, breakup times of the PS1, PS2, and PS3 were estimated to be 3.47 Ma, 2.19 Ma, and 1.45 Ma, respectively, indicating appearance of modern Upper Yangtze River valley started between the Pliocene to early Pleistocene.