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EVOLUTION AND DIFFERENTATION OF THE PHYSICO-GEOGRAPHICAL ENVIRONMENT OF QINGHAI-XIZANG PLATEAU

Zheng Du Li Bingyuan(Institute of Geography, Chinese Academy of Sciences and State Planning Commission of the People's Republic of China)  
The Qinghai- Xizang region with an elevation of 1000 m asl, was characterized by landscapes of the subtropical montane forest and forest-steppe in Pliocene. The intense uplifting of Qinghai-Xizang plateau. either the Himalayas in the south or the Kunlun in the north,began at the end of Pliocene and the beginning of Early Pleistocene.By a rough estimation, the accumulative uplifting of the Plateau has ammounted to 3500 m in elevation since Early Pleistocene, Three well-defined stages with alternating violent uplifts and relative stabilities are identified in the block faulted uplifting of the Plateau at the ends of pliocene, Early Pleistocene and Mid-Pleistocene, It may be manifested by three great knick points on the longitudinal profiles of rivers in the peripheries of the Plateau,In accordance with the intense uplifting there were four Quaternary gla-ciations with the largest one at Mid-Pleistocene, Because of obvious spatial differentiation of climate and snowline location between the periphery and the interior of the Plateau, there has never emerged a vast unified ice sheet on the Plateau,Due to strong fluvial dissection the exterior drainage systems have been changed into the interior ones. Most of the interior lakes have dwindled and disintegrated,the lake surface descended and the evaporites deposit occurred. The aolian loess,deposited widely on the northern flanks in the western Kunlun Mts, ,bear relationship with the formation of the sandy desert in Tarim Basin. Cold-andization tendencies developed and the interior lakes were withdrawn in Holocene.It was favourable for forest growing and the peat-bog occurred widely in Middle Holocene with optimum climate, Microlothics can also be found in uninhabitated area on the Qiangtang Plateau.Based on the spectrum-structure, base-belt, dominating belt and the pattern of the altitudinal belt, two systems of structure-type of the altitudinal belt may be identified. the monsoona] and the continental. The monsoonal system of the altitudinal belt mainly borders the Plateau 01 the south and the southeast. It may be subdivided into three groups of the structure-type, the humid, subhumid and the high-cold subhumid.By contrast , the continental system of the altitudinal belt appears in the interior and north w estern part of the Plateau, It may be subdivided into several groups of the structure-type, the high-cold semiarid,high-cold arid,high-cold super arid, super-arid, arid and semiarid. The regional dif ferentiationof the altitudinal belt is discussed and generalized as shown in the distributional model diagram.As a whole, the obvious spatial differentiation of the temperature-moisture regimes of the Plateau, from warm-humid in the SE to cold-arid in the NW, caused by the combined effect of the topographic pattern and the atmospheric circulation, is mirrored in succession of the montane coniferous forest--alpine meadow--alpine setppe--the alpine desert.As concerns the regional differentiation of the Qinghai-Xizang Plateau, a number of striking geo-ecological phenomena and areas,such as a vapour channel in the lower reaches of the Yarlung Zangbo, the dry valley in the eastern southern and western parts of the peripheral area and the high-cold arid core area in the interior of the Kunlun Mts. are investigated and discussed in the present paper.
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