火山岩地区断层活动时代鉴定中的问题及其讨论
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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。
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 Q1p(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 Q1p 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.
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 Q1p(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 Q1p 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.
引文
[1]吉林省地质局,1979,靖宇县幅1/20万区域地质调查报告。
[1]中国地震局地质研究所,1997,吉林省靖宇平岗-赤松核电站厂址火山安全性评价报告。
[2]辽宁省地震研究所,1996,吉林核电厂平岗、赤松候选厂址初步可行性研究地震、地质调查报告。
陈以健.1988.ESR测年方法概述[J].核技术,11(11):59—60.CHEN Yi-jian.1988.General aspects of ESR dating[J].Nuclear Techniques,11(11):59—60(in Chinese).
范俊喜,彭土标,袁建新,等.2007.工程区断层最新强烈活动年代确定方法探讨[J].水力发电,23(10):26—30.FAN Jun-xi,PENG Tu-biao,YUAN Jian-xin,et al.2007.Discussions on the dating method of the most recent faultmovement[J].Water Power,23(10):26—30(in Chinese).
黄培华.1994.用电子自旋共振(ESR)测年法测定断层活动年代的探讨[J].地震地质,16(3):269—274.HUANG Pei-hua.1994.Study on electron spin response(ESR)dating of fault movement[J].Seismology and Geolo-gy,16(3):269—274(in Chinese).
计凤桔,马胜利,刘明达.1994.断层物质测年的热释光研究[J].地震地质,16(2):167—175.JI Feng-ju,MA Sheng-li,LIU Ming-da.1994.Study of fault gouge dating using TLtechnique[J].Seismology and Ge-ology,16(2):167—175(in Chinese).
计凤桔,高平.1988.热释光断代技术测定断层活动时代可能性的研究[J].地震地质,10(4):207—213.JI Feng-ju,GAO Ping.1988.Application of thermoluminescent technique to dating of fault[J].Seismology and Geol-ogy,10(4):207—213(in Chinese).
刘若新主编.2000.中国的活火山[M].北京:地震出版社.67—74.LIU Ruo-xin(ed).2000.Active Volcanoes in China[M].Seismological Press,Beijing.67—74(in Chinese).
杨坤光,梁兴中,谢建磊,等.2006.ESR定年:一种确定脆性断层活动年龄的方法原理与应用[J].地球科学进展,21(4):430—435.YANG Kun-guang,LIANG Xing-zhong,XIE Jian-lei,et al.2006.ESR dating,the principle and application of a methodto determine active ages of brittle faults[J].Advances in Earth Science,21(4):430—435(in Chinese).
赵俊香.2006.断层最新活动年龄测定方法的探讨[A].见:中国地震局地壳应力研究所编.地壳构造与地壳应力文集(18).北京:地震出版社.62—71.ZHAO Jun-xiang.2006.Investigation of the dating method of most recent activities of faults[A].In:Institute ofCrustal Dynamics,China Earthquake Administration(ed).Collection of Crustal Structure and Crustal Dynamics(18).Seismological Press,Beijing.62—71(in Chinese).
周本刚,沈得秀.2006.地震安全性评价中若干地震地质问题探讨[J].震灾防御技术,1(2):113—120.ZHOU Ben-gang,SHEN De-xiu.2006.Discussion on some geological problems in earthquake safety evaluation[J].Technology for Earthquake Disaster Prevention,1(2):113—120(in Chinese).
Chen Y,Feng J,Gao J,et al.1997.Observations on the micro-texture and ESR spectra of quartz from fault gouge[J].Quaternary Science Reviews,16(3—5):487—493.
Kanaori Y.1983.Fracturing mode analysis and relative age dating of faults by surface textures of quartz grains fromfault gouges[J].Engineering Geology,19(3):261—281.
Kanaori Y,Miyakoshi K,Kakuta T,et al.1980.Dating fault activity by surface textures of quartz grains from fault gou-ges[J].Engineering Geology,16(3—4):243—262.
Macdonald G A.1972.Volcanoes[M].Englewood Cliffs,New Jersey:Prentice-Hall.1—510.
[1]中国地震局地质研究所,1997,吉林省靖宇平岗-赤松核电站厂址火山安全性评价报告。
[2]辽宁省地震研究所,1996,吉林核电厂平岗、赤松候选厂址初步可行性研究地震、地质调查报告。
陈以健.1988.ESR测年方法概述[J].核技术,11(11):59—60.CHEN Yi-jian.1988.General aspects of ESR dating[J].Nuclear Techniques,11(11):59—60(in Chinese).
范俊喜,彭土标,袁建新,等.2007.工程区断层最新强烈活动年代确定方法探讨[J].水力发电,23(10):26—30.FAN Jun-xi,PENG Tu-biao,YUAN Jian-xin,et al.2007.Discussions on the dating method of the most recent faultmovement[J].Water Power,23(10):26—30(in Chinese).
黄培华.1994.用电子自旋共振(ESR)测年法测定断层活动年代的探讨[J].地震地质,16(3):269—274.HUANG Pei-hua.1994.Study on electron spin response(ESR)dating of fault movement[J].Seismology and Geolo-gy,16(3):269—274(in Chinese).
计凤桔,马胜利,刘明达.1994.断层物质测年的热释光研究[J].地震地质,16(2):167—175.JI Feng-ju,MA Sheng-li,LIU Ming-da.1994.Study of fault gouge dating using TLtechnique[J].Seismology and Ge-ology,16(2):167—175(in Chinese).
计凤桔,高平.1988.热释光断代技术测定断层活动时代可能性的研究[J].地震地质,10(4):207—213.JI Feng-ju,GAO Ping.1988.Application of thermoluminescent technique to dating of fault[J].Seismology and Geol-ogy,10(4):207—213(in Chinese).
刘若新主编.2000.中国的活火山[M].北京:地震出版社.67—74.LIU Ruo-xin(ed).2000.Active Volcanoes in China[M].Seismological Press,Beijing.67—74(in Chinese).
杨坤光,梁兴中,谢建磊,等.2006.ESR定年:一种确定脆性断层活动年龄的方法原理与应用[J].地球科学进展,21(4):430—435.YANG Kun-guang,LIANG Xing-zhong,XIE Jian-lei,et al.2006.ESR dating,the principle and application of a methodto determine active ages of brittle faults[J].Advances in Earth Science,21(4):430—435(in Chinese).
赵俊香.2006.断层最新活动年龄测定方法的探讨[A].见:中国地震局地壳应力研究所编.地壳构造与地壳应力文集(18).北京:地震出版社.62—71.ZHAO Jun-xiang.2006.Investigation of the dating method of most recent activities of faults[A].In:Institute ofCrustal Dynamics,China Earthquake Administration(ed).Collection of Crustal Structure and Crustal Dynamics(18).Seismological Press,Beijing.62—71(in Chinese).
周本刚,沈得秀.2006.地震安全性评价中若干地震地质问题探讨[J].震灾防御技术,1(2):113—120.ZHOU Ben-gang,SHEN De-xiu.2006.Discussion on some geological problems in earthquake safety evaluation[J].Technology for Earthquake Disaster Prevention,1(2):113—120(in Chinese).
Chen Y,Feng J,Gao J,et al.1997.Observations on the micro-texture and ESR spectra of quartz from fault gouge[J].Quaternary Science Reviews,16(3—5):487—493.
Kanaori Y.1983.Fracturing mode analysis and relative age dating of faults by surface textures of quartz grains fromfault gouges[J].Engineering Geology,19(3):261—281.
Kanaori Y,Miyakoshi K,Kakuta T,et al.1980.Dating fault activity by surface textures of quartz grains from fault gou-ges[J].Engineering Geology,16(3—4):243—262.
Macdonald G A.1972.Volcanoes[M].Englewood Cliffs,New Jersey:Prentice-Hall.1—510.
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