新疆天山北轮台断裂阿克艾肯段晚第四纪活动特征

LATE-QUATERNARY ACTIVITY CHARACTERISTICS OF AKEAIKEN SEGMENT OF BEILUNTAI FAULT BELT AT SOUTH OF TIANSHAN, IN XINJIANG

北轮台断裂是一条全新世活动断裂，构成了南天山与塔里木盆地的界线。为清楚断裂晚第四纪以来的活动特征，进行地震危险性评价，对北轮台断裂阿克艾肯段开展了细致的工作。在野外地质调查和局部微地貌变形的测量基础上，开挖2个古地震探槽，采集确定地貌面和古地震年代的光释光（OSL）年代样品，得到了阿克艾肯断裂段的基本定量参数。阿克艾肯断裂段是一条全新世活动断裂，全长8km，分布宽度约4km，属于南天山与塔里木盆地的分界断裂。该断裂在晚第四纪期间仍具有较强的活动性，形成了一系列的地质地貌现象。断层陡坎主要分布在F3a和F3b地貌面上，其年龄分别为14.64±1.81ka和9.55±1.48ka，垂直位移量分别为29m和10.7m，阿克艾肯断裂段的平均垂直滑动速率为0.97～1.32mm/a。全新世中期以来，该断裂段发生过2次古地震事件，事件Ⅰ发生在6.60～7.15ka之间，靠近6.60ka；事件Ⅱ发生在距今4.05ka至今，更加靠近4.05ka。事件Ⅱ的同震位移为2.5～3.5m，计算得到该断裂段特征地震复发周期为2.7ka。

Large-scale thrust nappe system is distributed in the front of Tianshan. All of this thrust tectonic system are controlled by the reverse fault, such as the southern margin of the Junggar Fault in the north of Tianshan and the Beiluntai Fault in the south of Tianshan. The Beiluntai fault zone is the boundary between the Tarim Basin and the south Tianshan, and is a micro plate boundary polymerization, with a length of ca.120km, the latest deformation located in the proluvial fan and made different fault scarp. In order to accurately evaluate the fracture characteristics of Late Quaternary activity, the activities paragraph length, fault slip rate, the maximum total displacement and the latest activity of the era and a series of important quantitative data must be clarified. We carried out field geological investigation and measurement, excavated 2 large palaeoearthquake trench and collected some OSL samples in the Akeaiken Fault segment of Beiluntai Fault. Based on this data we have a new understanding of the characteristics of activities to provide the basic data for the judge with the regional seismic hazard analysis. The Akeaiken Fault segment is located in the piedmont alluvial fan of south of Tianshan, and the surface deformation traces is 8km. The strike of fault is EW, and the tendency is N and dip is 40～50 degrees. Since the Holocene, the activity of Akeaiken Fault segment is still strong, and it has formed a series of geological phenomena. There has been developed multi periods alluvial fan, and its major material source is intrusive granite in Cenozoic. The alluvial fan of most widely distributed is F3, and the fault scarps are mainly distributed in this alluvial fan. According to the result of OSL samples, the age of F3 alluvial fan is divided into two small periods（F3a and F3b）about 14.64±1.81ka and 9.55±1.48ka. The average vertical dislocations of the two fans are 29.0m and 10.7m. So, the average vertical slip rate since the Late Quaternary of F3a is 1.71～2.07mm/a and F3b is 0.97～1.32mm/a. We have evaluated the stratigraphic relationship of trench, and found two paleoseismic event occurred in the Akeaiken Fault segment since the Middle Holocene, and the event Ⅰ is dating from 6.60ka to 7.15ka and the other event is after 4.05ka. The coseismic displacement of the event Ⅱ is 2.5～3.5m. According to the sliding rate method（T=D/V）, the characteristic earthquake recurrence period is 2.7ka.