Formation And Evolution Of The Middle Reaches Of The Yellow River Since Late Cenozoic

The Yellow River is the second longest river in china, it is well known for "abundant sediment, channel migration, channel fill and levee break". There are two significant problems in the middle reaches of the Yellow River: firstly, the lack of water;secondly, the serious water and soil loss. So it is urgent to exploit and utilize the Yellow River.The Ordos Plateau is a steady block. Although the Ordos Plateau is far from Tibetan Plateau, the long-distance effect of Tibetan Plateau uplift transmits to it at least in two ways, that is, across the Altyn Tagh strike-slip fault to east and across the Kun-lun Mountains Fault System to northeast. The Altyn Tagh is one of the significant fault zones at the north part of Tibetan Plateau, it transmitted the stress to Qilian fault and then to Haiyuan-Liupanshan fault by intensive strike slip, which made the Ordos Plateau Block upheave and circumgyrate anticlockwise. In the other way, Tibetan Plateau stress had effect on the Ordos Plateau Block by Kunlun Mountains fault, Taibai fault, Qinling fault and at last speeded up the development and expansion of Weihe rift valley and Shanxi rift valley.So, studies about the formation and evolution of the Yellow River have a significant meaning in science for us to study the tectonic movement of the Ordos block, the long-distance effect of the Tibetan Plateau, the exploitation and utilization of the Yellow River. Whereas there are still many different opinions on the study about the formation of the Yellow River, it seriously blocked the development of earth science. An advanced study into the formation and evolution of the water system and landform in the middle reaches of the Yellow River has not only practical meaning but also academic value.This paper has made a primary study for the evolution of the geomorphologic evolution and the development of the Yellow River. It has built the process of the fluvial landform evolution in the middle reaches of the Yellow River based on numerous field survey, the analysis of particle size and magnetic susceptibility, reliable paleomagnetism data and TL data. Some important conclusions are drawn as following:(1) The planation surface of Tangxian developed at 22-3.6 Ma, which is the same with the main surface of Qinghai-Tibet Plateau. It shaped up at about 4.5-3.6 Ma and was covered with typical aeolian red clay. During the development of Tangxian surface, some tectonic deformations appeared in Ordos at about 8.4 Ma affected by the uplift of Tibetan Plateau. Italso formed many small endorheic lakes, around which some paleo-rivers flowed into these lakes. But this tectonic movement didn't collapse the development of Tangxian surface, Ordos was still eroded and planed, and the lakes as a depression received the deposition at the same time.(2) Affected by uplift of the Qingzang movement (phase A), otherness upheaval was strengthened, the difference of the surface relief appeared, Tangxian planation surface began to dismantled at 3.6 Ma, endorheic lakes and rivers into them began to disappear.(3) The topmost river terrace near Hequ indicates that a river had started to develop its stream about 4.9 Ma, and it flowed into the Baode paleolake. Another intensive incision happened at about 3.6 Ma and a terrace was formed. Companied by the breakup of the Tangxian surface, the Baode paleo-lake stopped to deposit fluvial sediment and the river disappeared, which was caused by the uplift of Qingzang movement (phase A).(4) At about 1.2Ma, the Yellow River run through Sanmen Gorges, cut off the Ordos Plateau and then the Jinshan canyon, which connected the Fenwei Basin and the Hetao Basin, was formed. Since then the water system of the middle reaches of the Yellow River was built.(5) At the setting of tectonic uplift, effected by climate and other factors, the river incised and formed the latest terrace at about 1.2 Ma, and formed others at 0.865 Ma, 0.787 Ma, 0.621 Ma, 0.412 Ma, 0.336 Ma, 0.245 Ma, 0.129 Ma, 60 ka, 5ka, respectively. Finally the landform was formed as it appears today.