镇江地区主要NW向断裂的第四纪活动性

RESEARCH ON THE CHARACTERISTIC OF QUATERNARY ACTIVITIES OF NW-TRENDING FAULTS IN ZHENJIANG AREA

五峰山-西来桥断裂和丹徒-建山断裂是镇江地区2条主要的NW向断裂,可能与镇江多次破坏性地震相关。文中通过浅层地震勘探和钻孔联合剖面探测方法,对五峰山-西来桥断裂和丹徒-建山断裂的展布特征及第四纪活动性进行了系统研究。五峰山-西来桥断裂在浅层地震剖面上倾向NE,倾角约为60°,断距约为5~9m,以正断活动为主;大路镇场地上,该断裂断错的最新地层为中更新统底部,位错量为2m,判断五峰山-西来桥断裂的最新活动时代为中更新世早期。丹徒-建山断裂在浅层地震剖面上倾向SW,倾角约为50°~55°,断距约为2~7m,以正断活动为主;访仙镇场地上,中更新统之上的地层没有被断错的迹象,中更新统底部可能被断层影响,判断丹徒-建山断裂的最新活动时代为早更新世—中更新世早期。

Running across the east of Zhenjiang city,the Wufengshan-Xilaiqiao Fault and Dantu-Jianshan Fault are two important NW-trending faults in Zhenjiang area.They controlled the Cretaceous stratigraphic deposition and Mesozoic volcanic activities,and also have obvious control effects on modern geomorphology and Quaternary stratigraphic distribution.There have been many destructive earthquakes in Zhenjiang area,most of which occurred at the intersection of NW-trending faults and near EW faults.It is of great significance to study the nature,characteristics and the latest active age of the NW-trending faults in Zhenjiang area for the prevention and reduction of earthquake disaster in Zhenjiang City,but the past targeted research work and the knowledge of activity of the faults are very limited.Based on the project of"Urban active fault exploration and seismic risk assessment in Zhenjiang City",a series of shallow seismic exploration work has been carried out on the two major NW-trending faults in Zhenjiang area,and representative points were selected to carry out drilling joint profiling to study the Quaternary activity characteristics of these two faults.The results are of great significance for urban earthquake disaster reduction,urban planning and land use.The results of shallow seismic exploration show that the Wufengshan-Xilaiqiao Fault is dominated by normal faulting,dipping to the northeast,with a dip angle of about 60°and a displacement of 5~9 m on the bedrock surface.The Dantu-Jianshan Fault is dominated by normal faulting,dipping to the southwest,with a dip angle of about 50°~55°and a displacement of 2~7 m on the bedrock surface.All breakpoints of Wufengshan-Xilaiqiao Fault and Dantu-Jianshan Fault reveal that only the bedrock surface was dislocated,not the interior stratum of Quaternary.On the Dalu site,there is no sign of dislocation in the stratum above the Middle Pleistocene,and the bottom boundary of the Middle Pleistocene has been dislocated,with a displacement of 2 m.The dislocation of the bottom boundary of the lower Pleistocene is 3.2 m on both sides of the fault,and the maximum displacement of the bedrock surface is 9.1 m.The characteristics of the fault surface developed in the drill cores indicate that the latest activity of the fault is of sinistral normal faulting.According to the characteristics of dislocated stratum,the latest active age of Wufengshan-Xilaiqiao Fault is early Middle Pleistocene.On the Fangxian site,there is no sign of fault in the stratum above the Middle Pleistocene,and the bottom of the Middle Pleistocene may be affected by the fault.The displacement of the bottom boundary of Baishan Formation on both sides of the fault is 2 m,and the maximum displacement of the bedrock surface is 6.7 m.Due to the insufficient evidence of dislocation of Baishan Formation,the latest active age of Dantu-Jianshan Fault is estimated to be between early Pleistocene and early Middle Pleistocene.The NW-trending Su-Xi-Chang Fault is an important regional fault in the Yangtze River Delta region.Its latest active age is the early Middle Pleistocene,and the displacement in the Quaternary is about 3 m.The Wufengshan-Xilaiqiao Fault and the Dantu-Jianshan Fault can be regarded as spatial extension of the Su-Xi-Chang Fault to the northwest,and their activities are also consistent.This study shows that the two NW-trending faults in the Zhenjiang area have significant activity since the Quaternary,and are the main faults with relatively high earthquake risk in this area.Therefore,the intersection of these two faults with EW-trending faults and NE-trending faults should be the focus of attention for earthquake damage prevention in the Zhenjiang area.The bedrock depth in the Zhenjiang area is relatively shallow,and the stratification difference within the cover layer is small,resulting in an unsatisfactory effect by the geophysical exploration methods.The Lower Pleistocene of the Quaternary system is basically missing,and the boundaries of the Middle and Upper Pleistocene are difficult to distinguish.Developed mainly in the bedrock and the bottom of the Quaternary,the stratum displacement is difficult to judge whether it was caused by sedimentary difference or fault activity.Therefore,the quantitative study of fault activity in this paper is still insufficient.