新疆独山子的冲沟发育与断层新活动

Gullies Evolution and Fault Activities in Dushanzi,Xinjiang

独山子背斜是天山北麓三排褶皱带中西北端的背斜,其北翼发育一条逆冲断层。断层的活动在奎屯河阶地和冲积扇上形成了高度小于20m的断层陡坎。陡坎上游冲积物中发育多条冲沟,冲沟根据长度可分为7组。探槽揭示了独山子背斜北翼断层最新的两次活动,其中较早的断层活动时间为3110±40aBP。根据这次断层活动触发的冲沟的平均长度得到冲沟溯源侵蚀的速率为38mm/a。由该溯源侵蚀速率,计算出每组冲沟发育的时间,分别约为1170aBP,3110aBP,7200aBP,12570aBP,14990aBP,30570aBP,37750aBP,可能代表断层多次活动的时间。断层最新一次活动的时间与本文探槽研究限定时间段相符,其他最近的三次活动时间与前人探槽研究的结果相符。说明在特定条件下,用断层上升盘冲沟平均溯源侵蚀速率推算断层活动时间是可行的。

Dushanzi anticline is the northwestern fold of the three fold belts in north Tianshan Mountains. A thrust fault has developed in the north limb of the fold. The fault movement caused scarps with a height of less than 20 m in the terraces and alluvial fans of Kuitun River. Gullies have developed on the upper stream alluvium of the scarp, which can be divided into 7 groups by length. Trenches revealed two latest faulting events of the north limb fault in Dushanzi anticline, the former occurred at 3110-40 a BP. The headwater erosion rate is 38 mm/a, deduced from the average length of gullies triggered by faulting. Based on the headwater erosion rate, the time of formation can be calculated for each group of gullies （1169 a BP, 3110 a BP, 7203 a BP, 12573 aBP, 14992 a BP, 30568 a BP, and 37745 a BP）, which may coincide with occurrences of faulting events. The latest faulting age derived from headwater erosion rate is consistent with that discovered from trenches, and another three latest events are consistent with the previous studies on trenching. This result shows that under specific conditions it is applicable to estimate the time of faulting based on headwater erosion rate of gullies on uplift wall.