祁连山东段冲积扇的发育时代及其成因

第四纪期间祁连山东段山麓地带广泛发育了多级冲积扇,它们是研究区域构造抬升与气候变化最为直接的载体.本文主要通过ESR,IRSL以及14C等绝对测年手段对该区冲积扇的形成年代进行研究,结果表明祁连山东段北麓最高级冲积扇形成于0.85 MaBP,而毛毛山南麓的最高级冲积扇形成于0.43 MaBP.随后各地又相继发育了0.25,0.16,0.06,0.01 MaBP等多级冲洪积台地.根据主要冲积扇的形成年代及其与区域构造气候事件的对比,认为它们是构造运动与气候变化共同作用的产物.

Age and genesis of the alluvial fans in the east Qilian Mountains

The Qilian Mountain is a Caledonian orogenic belt that has been reactivated by the collision between the Indian and the Asian continent during the Cenozoic. As the northeast boundary of the Tibetan Plateau, Qilian Mountain records, to some extent, the uplift process of the plateau. A combination of stepped geomorphic surfaces, i.e. an erosion surface, several alluvial fans and river terraces formed in the northern Qilian Mountain, are viewed as direct indications of episodic, dramatic uplifts of mountains, therefore having important implications for inferring the ages of mountain uplifts. Several alluvial fans are identified in the east Qilian Mountain during the Quaternary period, which are wonderful archives for research on regional tectonic activities and paleoclimatic changes. From ESR, IRSL and ^14C chronology on gravel beds, our results show that the uppermost fans near the northern piedmont of Yongchang Nanshan yield the age of 0.85 MaBP. The uppermost alluvial fans in the southern piedmont of Maomao Mountain emerged at 0.43 MaBP. Then the 3-4 fans can be generally found and identified and the ages of each fan is 0.25, 0.16, 0.06 and 0.01 MaBP. After careful analyses on the ages of gravel bed accumulations, we find that alluvial gravels accumulated during the glacial periods, indicating that climate variations have played a significant role in controlling regional fluvial geomorphic evolutions even within the tectonic active regions.