Study Of The Step-like Landforms In Western Sichuan Plateau, China

Field observation and analysis of the DEM of Western Sichuan Plateau indicate that, there are two planation surfaces and one erosion surface in Western Sichuan Plateau. The higher planation surface about 5100m in height is only preserved on the crest of the mountains, so called the summit surface; the lower one about 4500-4300m in altitude is well preserved over the plateau, so called the main planation surface. This widely distributed surface was dissected by an intensive tectonic activity around 3.6 Ma, as indicated by the deposition of Dayi conglomerate in Western rim in Chengdu Basin, Luhuo conglomerate in Luhuo Basin, and Wenjiaci conglomerate in Songpan Basin. An erosion surface (pediment) gently tilted towards the modern drainage system, is identified below the main planation surface. This pediment is deeply incised to form V-shaped valley. Several fluvial terrace sequences within the V-shaped valley at upper reach of Minjiang, the Zagunao River at Lixian, the Xianshui River at Luhuo, and the Dadu River at Luding, were identified and dated applying ESR method. The result shows that all the terraces were developed during the Middle Pleistocene, and some of them, synchronously. The development of these terraces on Western Sichuan Plateau is prominently under the control of tectonic uplift, and climate change , such as glacial and interglacial circle , plays a assitant role in the forming of the river terraces , so the river terraces have recorded the coherently phased rise of the plateau during Quaternary. The incision rates of Zagunao River, Dadu River and Xianshui River were 0.37m/ka, 0.38m/ka and 0.39m/ka respectively, from the plateau margin to the interior, the incision rate has a slight increasing trend , to some extent, we can say that, from the margin to the interior , the uplift intensionof the plateau has a slight increasing trend.