Abstract:

The history of fault activity is the basis for understanding earthquake activity, evaluating earthquake risk and reducing earthquake hazards. At present, the determination of fault activity age can be divided into two categories according to different dating objects: One is the sediments related to fault activity to define the activity age, mainly through the dating of dislocated and non-displaced strata and landforms. For sediment dating, there are currently many dating techniques available with high reliability, such as ¹⁴C method, luminescence dating, etc. Therefore, those dating methods are currently the main ones for determining the chronology of fault movement. The other is the dating of materials and geomorphic surfaces directly related to fault activity. Such as fault gouges, bedrock slip surfaces(including associated or secondary substances on fault surfaces or slip surfaces), veins filled in fault zones, etc. However, the Quaternary activity of the faults in the carbonate rock area is difficult to determine due to the lack of Quaternary sediment cover. The latest research shows that recrystallized carbonate formed on the fault surface in carbonate rock area provides an ideal material for the study of such faults using ESR method. In the carbonate rock area, during the fault activity, the heat produced by friction leads to melting of the frictional surface of the rocks and produces a thin layer(~2mm)of recrystallized carbonates. The recrystallized carbonate with obvious fault scratches is the product of dynamic metamorphism of the frictional surface due to the local high temperature and pressure environment generated by the rapid friction of the two walls of fault during the fault activity, and its age coincides with the fault movement. The recrystallization experienced by recrystallized carbonate can clear the accumulated ESR signals before the fault occurs, and the new ESR signals accumulation begins after the fault movement. Therefore, the chronology of fault activity can be obtained by determining the age of recrystallized carbonate on the fault surface using the ESR method. The Napo fault zone controls the earthquake breeding and occurrence of the Youjiang seismic zone, so it is of great significance to study the tectonic activity in Guangxi region. However, due to the limitation of dating materials, the chronology of Napo fault zone is very scarce. In order to solve this problem, in this paper, three recrystallized carbonate samples from an R-shear fault in the north section of Napo fault system were collected for ESR analysis. The results show the chronology of this fault activity is approximately 200kaBP. Based on the Riedel shear model, it can be speculated that the main fault of the Napo fault system may have undergone tectonic activity after 200kaBP. Consequently, recrystallized carbonates produced by fault activity in the carbonate bedrock region have broad application prospects as dating materials that directly record fault activity information.

Key words: ESR dating, carbonate rock, bedrock fault, Riedel shear, Napo fault system

摘要：

在缺乏第四纪沉积物覆盖的碳酸盐岩基岩区, 难以确定断层的第四纪活动历史。碳酸盐岩基岩区断层活动往往会在断层面上形成重结晶碳酸盐, 这为利用ESR方法研究此类断层的第四纪活动历史提供了物质基础。文中采集了那坡断裂系北段一条R剪切断层基岩面的3个重结晶碳酸盐样品, 并开展ESR测年研究。结果表明, 该断层的活动时代大致为距今0.2Ma, 根据里德尔剪切模型推测, 那坡断裂的主干断层可能发生过晚于距今0.2Ma的构造活动; 同时, 碳酸盐岩基岩区断层活动产生的重结晶碳酸盐作为直接记录断层活动信息的测年材料, 具有广阔的应用前景。

关键词: ESR测年, 碳酸盐岩, 基岩断层, 里德尔剪切, 那坡断裂