The Studies On Fluvial Terraces In Eastern Lanzhou Region During The Last 1, 240 Ka

The competing role of surface uplift and climatic change on terrace formation remains controversy in geomorphic community. It needs more geomorphic evidences to solve the problem. Lanzhou area is an acknowledged perfect site for studies of fluvial terraces. In the last century, a lot of people in succession have investigated the terraces in Lanzhou area and many of their work are successful. However, previous researches on terrace mapping, correlation and dating are not very exact, and especially, few people investigated the terraces of the tributaries of Yellow River. Therefore, we chose the Eastern Lanzhou basin and Yuzhong Basin as a field lab for study of terrace in order to provide new information about the total number, distribution and ages of the terraces, and then, on this basis, put new ideas about the model of terrace formation and the role of the Qinghai-Tibet plateau tectonic on development and deformation of Yellow River terrace.Based on field investigations, loess-paleosol sequences, paleomagnetic ¹⁴C and optically stimulated luminescence (OSL) dating, the terrace sequence of the Yellow River over the past 1,200 ka in Lanzhou basin has been revealed. The ages of Yellow River terraces focus only in two relative short time periods. One is 1,240—860ka BP when 4 terraces were formed, their ages are respectively 1,240 (T7), 1,050 (T6), 960 (T5) and 860ka BP (T4). The other is 130ka BP-the present when 3 terraces were formed, their ages are respectively 130 (T3), 60 (T2) and 10 (Tl)ka.BP. The 1,050 ka terrace (T6) and 960 ka terrace (T5) were not reported in previous research The age of the fourth Yellow River terrace is 860ka BP rather than 590ka BP which were the previously believed. Up to now 590ka Terrace was not found in East Lanzhou basin. This result answers a long-term question in the geomorphology community of whether there exists a ⁸00ka terrace in Lanzhou area. The research results of Wangchuang river terrace shows that 4 terraces have developed in Yuzhong basin, whose ages are 330, 130, 60 and 10ka BP, respectively.All the Yellow River terraces are strikingly similar. They have several meters of paleosol developed on the top of overbank silt, suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates and there may be a link between the production of terraces and glacial-interglacial climatic cycles. However, it is not every glacial-interglacial transition during the last 1,240ka that terrace can form and preserve. That implies climate change is not sufficient condition of terrace development but necessary condition. Comparison of the terrace sequence of Yellow River with its average incision rates that is used as the proxy of surface uplift reveals that the formation and preservation of Yellow River terrace was strongly related to surface uplift and the realization of effect of climatic changes on the formation of terraces depends on the rate of rock uplift. It is when the uplift reaches a certain rate (e.g. 1,240-860ka BP and 130-0ka BP), the climatic changes can result in river incision and the terraces are preserved. If the uplift is very slow, no terrace can be preserved. In conclusion, the formation of Lanzhou Yellow River terrace is attributed to coupling effect of climatic changes and uplift. We named this model of terrace formation as "climate-uplift coupling model" This model not only can explain the formation of Lanzhou Yellow River terraces but also can explain the terrace formation of other area in the reach of the Yellow River.The differential uplift due to the sinistral strike-slip movement of the northern marginal fault of Maxianshan Mountains is responsibility for the deformation of Yellow River terraces. The strike-slip movement of the fault is associated with the lateral extrusion of plateau terranes therefore, both the uplift of Qindhai-Tibet Plateau and the lateral extrusion of plateau terranes are recorded by Yellow River terraces in Lanzhou east basin.