青藏高原的地貌演化与亚洲季风

STUDIES ON THE GEOMORPHOLOGICAL EVOLUTION OF THE QINGHAI XIZANG(TIBETAN)PLATEAU AND ASIAN MONSOON

青藏高原在新生代由于印度板块和欧亚板块的碰撞而发生三次上升和两次夷平，因而分别形成高低两级夷平面。较低夷平面形成于新第三纪，结束于３．６ＭａＢ．Ｐ．，其上常有红色风化壳保存，表明形成于气候温暖的低地环境，海拔不超过１０００ｍ。从３．６ＭａＢ．Ｐ．开始相继发生三次构造运动，分别命名为青藏运动（Ａ幕３．６ＭａＢ．Ｐ．，Ｂ幕２．６ＭａＢ．Ｐ．和Ｃ幕１．７ＭａＢ．Ｐ．），昆仑－黄河运动（１．２ＭａＢ．Ｐ．，０．８ＭａＢ．Ｐ．和０．６ＭａＢ．Ｐ．）以及共和运动（０．１５ＭａＢ．Ｐ．）。青藏运动Ｂ幕黄土开始堆积，高原达到２０００ｍ，冬季风稳定出现。昆仑－黄河运动使高原多数地面达到３０００ｍ或更高，许多地方冰期进入冰冻圈，气候转型可能与此有关。共和运动使高原达到现代高度，气候变干变冷。

The Qinghai Xizang(Tibetan)Plateau has experienced three stages of uplift and two stages of planation during Cenozoic,resulting in the formation of two planation surfaces in Early and Late Tertiary respectively.The older one is only preserved on the top of the mountain,called the summit surface.The later and low surface(main surface)is very common in the Qinghai Xizang Plateau with an area often up to several tens or even thousands of square kilometers.Small patches of ancient reddish weathering crust could be found everywhere on the planation surface,indicating a warm climate and low land condition(no more than 1 000 m)when they were formed.The calcite crystals collected from the karst cave gave dates of FT in 15～7Ma,presuming that the planation processes terminated at about 3 6Ma when a violent tectonic movement occurred.This tectonic movement is termed Qingzang movement and was divided into three phases:3 6Ma (phase A),2 6Ma (phase B) and 1 7Ma (phase C).Only after the Qingzang movement was the Qinghai Xizang plateau shaped into a huge tableland form and a new generation of drainage system such as Huanghe(Yellow) River appeared.Up to now, seven terraces have been discovered in Lanzhou Basin,dated to be 1 6Ma,1 5Ma,1 2Ma,0 6Ma,0 15Ma,0 05Ma and 0 015Ma respectively. Phase B(2 6Ma)of the Qingzang movement has a particular meaning in the Late Cenozoic environmental evolution,because the plateau was lifted to be about 2 000m a.s.l,and the thermal and dynamic affects upon the atmospheric circulation in Asia were strong enough to produce the modern Asian monsoonal system,especially the winter monsoon,reflected in the beginning of the accumulation of loess in North China. The tectonic events of 1 2～0 6Ma were called Kunlun Huanghe movement by which the plateau reached about 3 000m a.s.l.,on the average and most of the plateau land surface went into the cryosphere in the ice age and the atmospheric circulation in Asia changed dramatically,ushering in a new climate pattern characterized by 100ka cycle in loess records. There appeared another tectonic movement of 0 15Ma with the name of Gonghe movement by which the plateau finally approached its present elevation and the climate of Late Pleistocene and modern time became much drier and severe.