祁连山北缘玉门-北大河断裂晚第四纪活动特征

通过卫星影像解译、野外实地调查并结合前人研究成果,对位于祁连山北缘的玉门—北大河断裂晚第四纪构造活动特征进行研究。结果表明,玉门—北大河断裂为一条全新世活动的逆冲断裂,该断裂西起玉门青草湾,向东经老玉门市、大红泉止于骨头泉,全长约80km,整体走向NWW。根据断裂的几何结构及活动习性可将其分为三段:东段构造形态简单连续,为逆冲断层陡坎为主的古地震地表破裂带;中段结构复杂,由多条次级断层组成,以逆冲扩展为主;西段未出露地表而成为盲断裂-褶皱带。通过对断层陡坎差分GPS测量及相应地貌面年代测试,得到断裂晚更新世以来逆冲速率约为(0.73±0.09)mm/a。

Characteristics of Late Quaternary Activity of Yumen-Beidahe Fault in North Margin of Qilian Mountain

As the boundary of the Qilian Mountain and Jiuxi Basin, the Yumen-Beidahe fault is located along the western part of the southern margin of the Hexi Corridor, and is an important part of the north Qilian Mountain fault system. The Yumen-Beidahe fault extends from Qingcaowan in the west in an ESE trending for 80 km to near Gutouquan. Considering its geometric structure and active behavior, the fault can be divided into three segments. The eastern segment, whose length is about 20 km, is located on the two sides of Beidahe and about 2 km from the Qilian Mountain. This fault segment is preserved in the form of continuous fault scarp, whose height ranges from less than 1 m to 7 m. At the most recent alluvial fan, some fault scarps still retain a fresh free surface, which suggests that the segment is an earthquake rupture zone. The structure of the middle segment is complex and mainly exhibits thrust characteristics. Its length is about 20 km and it comprises six discrete faults. The topography is expressed as a fault triangle and a fault scarp of different heights. The fault section suggests that the latest earthquake event occurred about 5 000 years ago. The western segment, whose length is about 40 km, does not appear on the ground and becomes a blind thrust-fold belt. Some rivers, such as the Shiyou and Baiyang Rivers, cross over this blind thrust-fold belt. The terraces of these rivers have been described as fold deformation in previous work. This fault segment is also the seismogenic structure of the 2002 Yumen M_S5.9 earthquake. Based on previous data and field surveys, we believe the Yumen-Beidahe fault to be an active thrust fault in the Holocene, and there is no geological or geomorphic evidence of any left-lateral slip along the fault. According to a differential GPS survey of the fault scarps and geomorphic surface dating, we infer the average thrust rate of the Yumen-Beidahe fault since the late Pleistocene to be about (0.73±0.09) mm/a.