Abstract: Longgang volcano group is located in Huinan and Jingyu counties of Jilin Province,which occupies the middle segment of the Longgang mountain ranges.The volcanic activity in Quaternary,especially in the early Pleistocene,is very intensive,and as a result,over 2/3 of the total area is covered by Q_p¹(Xiaoyizishan Group)basalt sheet.The study area of this paper lies in the northeastern part of the Longgang volcano group,where some small-scaled faults are developed and their activity in the Quaternary Period is critical to the potential earthquake risk evaluation for the local engineering.In order to study the activities of these faults,a comprehensive project has been carried out by means of field investigation and application of ESR dating technique to determine the last active time of each fault.However,an inconsistent phenomenon was found in the study area,in which the age of fault gouge from ESR dating method is much younger that the age of Q_p¹ basalt sheet that covers both fault plane and fault deformed materials.We propose two facts that would be responsible for this contravention.Firstly,the extensive volcanic activity in the study area in Quaternary may produce a significant amount of heat regionally.For example,the temperature of the molten basalt lava is between 1050℃ and 1200℃,which is high enough to cause the effect of annealing and resetting of ESR signals.Consequently,such effect may result in the age of fault gouge with ESR dating method younger than the time when this heat event occurred.Secondly,after a fault is formed,chemical weathering will occur at the fault zone,which may result in dissolution on the grain surface of fault gouge.Such effect may significantly increase the amount of annual ESR dose and make the sample of weathered material younger than the un-weathered ones.Therefore,in this study area with extensively volcanic activity,the age from fault gouge dated with ESR method should be considered as the "upper limit age" of the last fault activity rather than the last time when the fault was active.Even though the "upper limit age" does not represent the age of the last activity of a fault,it is very useful in the practice of determining the capable fault for earthquake safety evaluation of large engineering sites.

Key words: volcanic activity, fault activity, ESR dating method

摘要： 通过野外地质、地貌调查,对新生代玄武岩普遍发育的吉林省靖宇县龙岗火山群地区的断层活动性进行了研究,结合断层物质ESR测年分析结果以及本地区第四纪火山活动的特点,发现ESR测年数据与野外实际观察到的地质现象比较相对偏新。文中讨论了产生这一现象的两个可能原因:1)研究地区大规模的第四纪岩浆喷发活动作为一种重要的热事件,对于先存的断层物质中的ESR测年信号具有"退火"和零化作用;2)化学风化作用、地下水的淋滤作用、次生物质的混入和富硅热液作用而导致的次生石英颗粒的增长,都可能会导致断层带物质ESR测年剂量的增加,从而引起测年结果偏新。因此,研究区ESR测年结果反映出的并非是断层最后一次的活动年代,而可能是断层活动的上限年龄。尽管与实际的断层活动年龄有较大的差距,但对于大型工程场地的地震安全性评价,尤其是对于能动断层的鉴定,这种"上限年龄"仍然具有十分重要的应用价值。

关键词: 火山活动, 断层活动, ESR测年