关键词: 西山断裂组, 逆断层, 古地震识别, 乌鲁木齐

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