关键词: 火山活动, 断层活动, ESR测年

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