Study On The Terrace Of The Yellow River At 0.8Ma B.P. In The Lanzhou Area And Surface Uplift Of The Loess Plateau

MPR(Mid-Pleistocene Revolution) that occurred at about 0.8Ma B.P. is a significant event of environment change, which is obviously responded by climate change, tectonic movement, etc.. A decisive uplift occurred on the Qinghai-Tibetan Plateau in this period, which made the Qinghai-Tibetan Plateau enter cryosphere. Since the Lanzhou Basin adjoins the Northeastern margin of the Qinghai-Tibetan Plateau, and also lies at the key location of China's Monsoonal Triangle, it is more sensitive in responding to the events of environment change.In the Lanzhou Basin, two fourth terraces of the Yellow River are selected as the study sections, namely the Wuyishan terrace and the fourth terrace of the Zaoshugou. Based on paleomagnetic dating, optically stimulated luminescence dating and loess-paleosol sequence matching, the result indicates that the two terraces have the same age, and both were formed at about 0.865Ma B.P.. Thus, this paper advances there is the terrace of the Yellow River at 0.8Ma B.P. in the Lanzhou Basin, and the fourth terrace of the Yellow River may be a geomorphic response to the event of environment change at 0.8Ma B.P.. Meanwhile, in view of the determined age of the Wuyishan terrace, it is improbable that there is the terrace of the Yellow River formed at 0.6Ma B.P. in the Lanzhou Basin. Therefore it is proposed that the Kunlun-Huanghe Movement might terminate at 0.8Ma B.P..River terrace is one of the direct geomorphic evidences of surface uplift. By analyzing the characteristics of terraces of the Yellow River in the Loess Plateau, it is concluded that the terraces are mainly tectonic genesis, and developing after the Yellow River reached a quasi-equilibrium state. So, it is reasonable to use terraces of the Yellow River for representing the surface uplift of the Loess Plateau. According to terraces of the Yellow River and its tributaries at 0.8Ma B.P. in the Loess Plateau, thispaper verifies that there was a large-scale surface uplift at about 0.8Ma B.P. on the Loess Plateau, and the surface uplift resulted in river incision and terrace formation. At the same time, based on the characteristic analysis of the terraces at the four sites on the Loess Plateau, it is suggested that the surface uplift of the Loess Plateau since 0.8Ma B.P. has obvious spatial and temporal features. The spatial feature is that the surface uplift of the Western Loess Plateau is more intense than that of the Eastern Loess Plateau, and the temporal feature is that the uplift gradually speeded up, especially since the late Pleistocene.