RIVER TERRACES AND NEOTECTONIC EVOLUTION AT NORTH MARGIN OF THE QILIANSHAN MOUNTAINS
Yang Jingchun; Tan Lihua; Li Youli; Duan Fengjun(Department of Geography, Peking University, Beijing 100871)
Based on field measurement and study on the terraces of 14 rivers at the northmargin of the Qilianshan Mountains, data of distribution, structure , height age, anddevelopmentS of river terraces have been obtained (Table 1), and the relationshipbetween river terraces and neotectonic evolution has been discussed.The north margin of the Qilianshan Mountains has been under a NE-SWcompression during the Cenozoic. Tectonic landforms, separated by active reversefaults trending WNW-ESE, can be divided into geomorphic units , such as tectonictableland, alluvial tableland, alluvial fan zone, and plain in lower basin fromsouthwest to northeasL The ages of the geomorphic units are getting younger from theQlianshan Mountains to basin. For example, the tectonic tablelands were formed inthe Middle Pleistocene, the alluvial tablelands were formed in fare Middle Pleistocene and Late Pleistocene, and the alluvial fans were formed in Holocene.Rivers run from the Qilianshan Mountains to basin, and pass through thegeomorphic units mentioned above. The amount of river terraces is different indifferent geomorphic units because the units formed in different periods. The largestnumber of the river terraces in the tectonic tableland and alluvial tableland are 7 and15, and the age of the oldest terrace is around 0.45 Mab. P. and 0.15MaB. P.,respectively . Longitudinal profiles of rivers have been displaced by many faults. Thedislocated profiles can be divided into 4 types (Fig.1): 1)terraces only developed on theuplifted block, and disappeared at the locality of the fault 2)higher terraces onlydeveloped on the uplifted block, and disappeared at the fault lower terraces developedcontinually on both blocks; 3) terraces comparably developed on both blocks, but theyare dislocated by the fault, the higher the terraces is, the more it is displaced; 4)higher terraces only developed on the uplifted block and disappeared near the faultlower terraces developed on both sides, but dislocated by the fault Movements of thefaultS also curved the river terraces . The older the terrace is, the more it is curved.Some terraces were upwarped, some were down-warped , and some terraces weredeformed because the superimPOse of upwarping and down-warping. The curvedterraces usually occur where the river valley is composed of sandstone and shale, orCenozoic soft clay , sand and gravel.According to the deformation of the terraces, activities of the faults, includingfrequences, vertical dislocation , age, and migration have been studied. Terraces at thenorth margin of the Qilianshan Mountains were formed around 0.45MaB.P.,0.35MaB.P.,0.25MaB.P.,0.15MaB.P.,0.10MaB.P. 70 000aB.P., 40 000- 30 000aB.P. and 10 000aB.P.iiis sequence can be divided into tWo groups, 0.45-0.15MaB.P. and 0.15MaB.P. -present .The interval of the adjacent terrace is about 0.10Ma in the former group and30 000a in the later groups. According to the heights between every two adjacentterraces , the down-cutting rate of different periods can be obtained. Comparing thethes of different areas, it can be found that the down-cutting rates are greater in thewest than in the east The rates of same river in different Periods indicate that thedown-cutting of the river became greater in the west while no obvious change in theeast 0.15MaB.P. - 30 000aB.P. Combining with the climatic changes, we believe thatthe tectonic movement was more intensive in the west than in the east especiallysince 30 000aB.P. This may due to that the width of the deformation field is narrowerin the west than in the east.
【CateGory Index】： P546
【CateGory Index】： P546