Abstract: Xishan Fault group is distributed in the transition zone between the fold-reverse fault system along the front of the north Tianshan Mountains to the west and the thrust tectonics of Bogeda to the east.It is a tectonics that thrusts from the basin in the north to the mountains in the south,consisting of 4～5 faults that are more than 10 to about 30km long,showing low angles near surface and converging on the detachment surface at about 11km deep.We discovered that the activity of Xishan Fault group is distinct during late Quaternary by doing field investigation of geology and geomorphology,excavating trenches along faults and analyzing deep structure of the fault group.The faults offset the second and above terraces of Wanjiagou creek and created fault scarps of 0.5～5.4m high on the terraces.And traces of paleoearthquakes can be found easily.The younger two events on F₁,F₂ and F₃ are confined in(22.7±5.2)ka and 40ka BP by OSL samples dating,respectively and the traces of the youngest event on F₄ and the front fault of Xishan are covered by deposits whose ages of OSL samples are about(31.1±3.2)ka and(37.9±3.8)ka BP,respectively.It means that there was grouped faulting in late Quaternary in the Xishan Fault group.F₁,F₂ and F₃ or F₄ and the front fault of Xishan might rupture in a same event on near surface.Event traces on the Xishan Fault group and other reverse faults of low angle show that the deposits along the front of fault scarp,the offset relation between fault and deposit bed,and the abrupt increase and diminution of displacement on difference markers or unconformable surfaces on both sides of fault are important identification marks of paleoearthquakes along surface rupture-type reverse fault.The deposits along the front of fault scarp on reverse faults of low angle are much more different from those on normal fault.For ideal mode,the deposit in front of fault scarp of reverse fault of low angle is characterized with that the original structure of the collapsing thrust sheet front is not broken entirely on the lower part and the sloping deposit on the upper part may exist covering on both sides of the fault.We think that it is very important for reducing uncertainty of paleoearthquake identification to seek for evidences as many as possible and analyze the different influencing factors,such as tectonics,climate,environment and anthropic activities.

Key words: Urumqi, Xishan Fault group, reverse fault, paleoearthquake identification

摘要： 乌鲁木齐西山断裂层组展布于北天山山前断展褶皱系与博格达推覆构造系的转换部位,为盆地向S推覆的构造,由4～5条长度十几公里至近30km的断层组成,其滑脱面埋深约11km。通过地质地貌调查、探槽开挖,结合深部构造特征分析,西山断裂组晚更新世中晚期有明显的活动。断层F₁—F₃最年轻的2次事件基本都被限制在距今(22.7±5.2)ka和40ka左右。而F₄和西山断裂最年轻的事件被距今31.0ka和38.0ka的地层覆盖。这显示西山断裂组晚第四纪活动有分组和组合破裂的特征。断层F₄和西山南缘断裂为一组,F₁—F₃为另一组。坎前堆积地层、断层与堆积地层的切错关系和不同间断面或标志地层在断层两侧的累积位差的突变,是识别地表破裂型逆断层古地震事件的重要标志。降低逆断层古地震识别的不确定性,关键在于识别事件的标志需要综合分析各种影响因素,有多证据的支持。

关键词: 西山断裂组, 逆断层, 古地震识别, 乌鲁木齐