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Yuan Baoyin;Wang Zhenhai(Institute of Geology, Chinese Academy of Sciences)  
The Qinghai-Xizang Plateau came into being as early as in the middle Himalayan movement in Late Miocene. After that, the earth crust had kept a secular steady state for 10 Ma, and a peneplain surface with an elevation about 1 000 metres formed.The Qinghai-Xizang Plateau was mainly formed in Quaternary. It rose to 2 000m in Early Pleistocene, and reached an elevation of more than 4 500m in Late Pleistocene. And it rose about 300-500m during Holocene. Recently, Ding Lin et al.calculated the elevation rate of the Qinghai-Xizang Plateau according to the result of ages by fission-track dating:12-11 Ma B.P. 0.06mm/a11-8 Ma B. P. 0.18mm/a8-3 Ma B. P. 0.1mm/a3 Ma B.P. 1.5mm/a0.31 Ma B. P. 14.19mm/aDing Lin's calculating results show that the Qinghai-Xizang Plateau has risen about 1 900 metres since 0.3Ma B. P. The data also show that its uplift had been a fluctuation process in which uplift sometimes were fast and sometimes were slow.The Uplift of the Qinghai-Xizang Plateau obviously affected its periphery areas and resulted in the changing of the pattern, speed and direction of tectogenesis and the alternation of erosion and sedimentation.Recent study shows that the development history of the Yellow River was obviously affected by the uplift of the Qinghai-Xizang Plateau, especially in its middle reaches. There were closely relationship between the developing of the river terraces and the stages of the uplift.According to the study on the river terraces, the development history of the Yellow River may be tentatively divided into three stages:1. The Embryonic Period The old broad valley formed in the Baode period was still retained in the Shanxi-Shaanxi segment of the Yellow River to the Shanmen gorge. It may be said that this segment formed at least in Pliocene and even in Late Miocene. In that period, the crust was stable and lateral erosion was dominant so as to form a broad river plain far to the Yinchuan basin. The paleoclimate was hot and humid but the division of arid and humid seasons was very obvious. So the sediment strata mainly characteristic of brown clay were formed. Though the Yellow River at that time was only at its initial stage, it was the largest river in North China.2. The Coexistence Period of Rivers and Lakes As a result of the uplift of the Qinghai-Xizang Plateau, a violent erosion occurred in the middle reaches of the Yellow River in Late Pliocene. It was the so-cal-led Fenhe Erosion Period. Both the Shanxi-shaanxi segment gorges and the Shanmen gorge were formed in this period. And the Yellow River from the Gonghe basin downward was formed.In the same period, on the other hand, some area sagged and formed lakes, such as the Gonghe paleolake, the Yinchuan paleolake, the Hetao paleolake, the Shanmen paleolake and the Fenhe paleolake. All of these paleolakes were distributed along the trunk stream and greater tributaries of the Yellow River, appearing a landscape of the coexistence of rivers and lakes. These lakes were different in lasting time. Some researchers thought that they were salt-water inland lakes without linking each other. The Yellow River in this period might be a continental river.Further study are needed to determine the characters and the development history of these lakes.3. The Large River Period A violent dissection occurred before accumulation of the Malan Loess in North China. It was the so-called Qingshui Erosion Period in physiography. In this period terraces with the height of 20 to 50m along the Yellow River formed and most of large lakes disappeared. The geoworphologic landscape which has now emerged along the Yellow River from the Gonghe basin to the river mouth has basically formed. All these are results of an important geological event in North China, which corresponded to the acceleration of uplift of the Qinghai-Xizang Plateau since 0.3-0.2Ma B. P. While the Yellow River was in downcutting, a landscape of crisscrossed gullies and ravines appeared in the Loess Plateau, especially west of the Liupan Mountains.
【CateGory Index】: P548.244
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