关键词: 河流阶地, 洪积扇, 新构造运动, 酒泉盆地

Abstract: The Jiuquan Basin is a reverse-fault-controlled basin in the northwest part of the Hexi Corridor, a Cenozoicforeland basin system located in the northeastern margin of the Qinghai-Tibet Plateau. From the sout west to the northeast, the landforms in the Jiuquan Basin can be divided into three zones, i. e., the North Qilia Mountain, basin plain, and North Mountain. They are parallel with the active fault zones of NWW-SEE. The basinplain is around 2000 m alove sea level in the southwest and 1500 m above sea level in the northeast, which canbe subdivided into three zones, i. e., the folded zone, the deep downwarped zone and the monocline zone. Thepresent climate in this area is continental arid. The annual precipitation is only 86 mm in the Jiuquan Basin, butin the alpine zone of the Qilian Mountain over 4000 meters above sea level it can reach over 500 mm. The melt-water from the alpine zone is an important supply to the rivers.
The Beida, Hongshuiba, Fengle and Maying Rivers, rising from the Qilian Mountain flow through the Ji-uquan Basin from south to north, and disappear in deserts to the north. The longitudinal profiles of the rivershave been obtained based on 7~10 transverse sections on each river. The longitudinal profiles of the rivers areaffected by the faults. The fault in the front of the Qilian Mountains dislocated the terraces older than the T。terrace around 40 ka BP, and the fault at the north front of the folded zone dislocated the terraces older than theTterrace.
The river valleys are dissected into the Pleistocene gravel to the south of the fault in the front of the QilianMountain. The beginning of the highest terrace (T) development represents a transition from aggradation todegradation in this area. Based on the average dissecting rate of the Hongshuiba River, it is deduced that theTswas formed around 160 ka BP. The rivers incised through the gravel of the older fans between the F 1 and F 2.faults also reveal a transition from aggradation to degradation around 40 ka BP. These two transitions may repre-sent two periods of strong tectonic movement.
The two fans have been formed by each river at the front of the Qilian Mountains. The older fan is largerand closer to the Qilian Mountain. They are controlled by the fault in the front of the Qilian Mountair and the fault in the north boundary of the folded zone in the Jiuquan Basin. The movement of the latter fault caused theolder fan to be dssected, and the younger fan to be deposited more towards the centre of the Jiuquan Basin.

Key words: terrace, alluvial fan, neotectonics, Jiuquan Basin