龙门山汶川-茂县断裂带的岩石磁学特征及其地震作用的指示意义  

Rock magnetic characteristics of the Wenchuan-Maoxian fault zone of the Longmen Shan fault and its earthquake faulting characteristics.

作　　者：张蕾 李海兵[1,2,3] 孙知明 葛成隆[1] 叶小舟 曹勇[4] 郑勇[1,2,3] ZHANG Lei;LI HaiBing;SUN ZhiMing;GE ChengLong;YE XiaoZhou;CAO Yong;ZHENG Yong(MNR Key Laboratory of Continental Dynamics,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China;Jiangsu Donghai Continental Deep Borehole Crustal Activity National Observation and Research Station,Lianyungang 222300,China;MNR Key Laboratory of Paleomagnetism and Tectonic Reconstruction,Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081,China)

机构地区：[1]中国地质科学院地质研究所,自然资源部大陆动力学重点实验室,北京100037 [2]南方海洋科学与工程广东省实验室(广州),广州511458 [3]江苏东海大陆深孔地壳活动国家野外科学观测研究站,连云港222300 [4]中国地质科学院地质力学研究所,自然资源部古地磁与古构造重建重点实验室,北京100081

出　　处：《岩石学报》2023年第12期3817-3832,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41830217、42172262、42072240、41972229);中国地质调查项目(DD20221630);南方海洋科学与工程广东省实验室(广州)人才团队引进重大专项(GML2019ZD0201)联合资助.

摘　　要：汶川-茂县断裂带是龙门山后山断裂,是松潘-甘孜褶皱带与龙门山断裂带之间的边界断层,然而,在2008年M_(W) 7.9级汶川大地震中并没有发生破裂。同时,汶川-茂县断裂带在汶川地震之前是否发生过历史大地震仍缺少确切的证据。这不仅制约着汶川地震发生机制的认识,而且还影响对龙门山形成演化过程的理解。因此,确定汶川-茂县断裂带的断裂作用环境对于认识龙门山断裂带的地震发生机制至关重要。断裂岩的岩石磁学可以有效地揭示断裂带的物理和化学环境。本文以汶川-茂县断裂带北部地表露头的断裂岩为研究对象,通过岩石磁学研究,并结合显微结构观察和地球化学分析,探讨汶川-茂县断裂带的断裂作用环境。断层泥和断层角砾岩最大磁化率值分别约为围岩的30倍和15倍,具有高磁化率值特征。断层泥的主要载磁矿物为磁铁矿、磁黄铁矿、针铁矿;断层角砾岩的主要载磁矿物为磁黄铁矿、针铁矿。断层泥的高磁化率值异常的主要原因是围岩中的含铁矿物在地震摩擦热和流体作用下新生成磁铁矿和磁黄铁矿。断层角砾岩的高磁化率值异常是围岩在含有大量硫元素的低温热液流体作用下生成了磁黄铁矿。大量针铁矿指示了震后期还原性的低温热液流体作用。断裂岩的高磁化率值异常和新生磁铁矿指示了汶川-茂县断裂带曾经发生了摩擦热温度>500℃的大地震活动,发震和震后期均为还原性的硫元素含量较高的低温热液流体环境。The Wenchuan-Maoxian Fault Zone(WMFZ),the boundary fault between the Songpan-Ganzi fold zone and the Longmen Shan Fault Zone(LSFZ),did not rupture during the 2008 M_(W) 7.9 Wenchuan earthquake.Furthermore,there were no records for the existence of the former large magnitude earthquakes had been found in this fault zone.These problems hinder progress in understanding both the earthquake mechanism of the Wenchuan earthquake and the formation and evolution of the LSFZ.Therefore,the faulting environment of the WMFZ is crucial in understanding the earthquake mechanism of the LSFZ.Rock magnetic study of fault rocks can effectively reveal the physical and chemical environment of the faults.Here,we carried out rock magnetic analyses combined with microstructural and geochemical analyses of fault rocks and wall rocks from the northern WMFZ.The highest magnetic susceptibility values of fault gouge and breccia are about 30 and 15 times of that of the wall rocks,respectively.Rock magnetic analysis results indicate that the magnetic minerals in the fault gouge are pyrrhotite,goethite together with some magnetite,while those in the fault breccia are pyrrhotite and goethite.The high temperature of frictional heating and fluids induced thermal decomposition of paramagnetic minerals,forming pyrrhotite and magnetite to a lesser extent,and thus contributing to a higher magnetic susceptibility values of the fault gouge.Higher magnetic susceptibility values of fault breccia results from the newly-formed pyrrhotite by reductive low-temperature hydrothermal fluids with large amounts of sulfur.Goethite reveals the presence of reductive low-temperature hydrothermal fluids after an earthquake.The high magnetic susceptibility values of fault rocks and the newly-formed magnetite indicate that large earthquakes with frictional heating(>500℃)did have occurred along the WMFZ in the past time,and this fault kept in a reductive environment with low-temperature hydrothermal fluids that contain high sulfur during and after the earthquakes.

关 键 词：断层泥 岩石磁学 断裂作用环境 大地震活动 汶川-茂县断裂带 

分 类 号：P315[天文地球—地震学] P542[天文地球—固体地球物理学]