青藏高原东缘龙门山断裂带孕震区域流体行为  

Fluid behaviors from the seismogenic region of Longmenshan fault zone,eastern Tibetan Plateau

作　　者：李春锐 李海兵[1,3] 司家亮 王焕[1,3] 吴琼 张进江[2] LI ChunRui;LI HaiBing;SI JiaLiang;WANG Huan;WU Qiong;ZHANG JinJiang(Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China)

机构地区：[1]中国地质科学院地质研究所,自然资源部深地动力学重点实验室,北京100037 [2]北京大学地球与空间科学学院,北京100871 [3]南方海洋科学与工程广东省实验室(广州),广州511458

出　　处：《岩石学报》2022年第11期3559-3577,共19页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41830217、41972229);中国地质调查局项目(DD20190059);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：震间期、同震期和震后期流体对断裂带物质的强度和运动性质起到重要作用。前人已识别出断裂带浅部区域流体对断层的弱化以及矿物沉淀导致的断层愈合,然而对于断裂带深部流体的研究鲜有报道。为深入了解孕震区流体行为以及地震成核过程中流体对断层的影响,本文以龙门山断裂带的映秀-北川断裂南段虹口乡八角庙村附近碎裂岩滑动带中石英和方解石脉为研究对象,通过对断裂带脉体的显微构造、碳氧同位素和主量元素含量等分析,开展地震相关脉体的特征结构、流体来源和矿物沉淀环境的研究。结果表明,碎裂岩主滑移带由颜色结构不同的三层断层泥和细小的方解石条带组成,在主滑动带边部和上盘碎裂岩中则分别发育了指示震间期、同震期和震后期三个阶段断层活动的脉体:(1)沿阶步生长的纤维状方解石脉和拉伸型柱状颗粒方解石脉;(2)断层泥楔入脉;(3)近等粒状方解石脉以及具有横向竞争生长结构的非等粒状方解石脉和石英脉。它们分别代表了震间期封闭的还原环境下的微滑动、同震外源高压流体注入以及震后开放的氧化环境至还原环境下的矿物沉淀。碳氧同位素结果表明主滑动带和碎裂岩方解石脉δ^(18)O V-PDB值为-20.5‰~-20.3‰,低于围岩碳酸钙胶结物,表明方解石脉具有大气水来源特征。方解石沉淀温度结合地温梯度表明方解石脉的形成深度大于4km,与碎裂岩形成深度及龙门山断裂带震源深度一致。该地区方解石脉和石英脉的研究深化了关于龙门山断裂带孕震区流体行为的理解,并且对进一步认识震间期、同震期和震后期断层的强度变化机制具有重要意义。Fluid circulation controls the fault zone strength and motion properties during inter-seismic,co-seismic,and post-seismic periods.It has been recognized that fluids in the shallow parts of the fault zone not only weaken the fault but also heal the fault by mineral precipitation.However,research on fluid behavior at deep fault depths is rarely investigated.In this paper,we study an outcrop located in Bajiaomiao Village,Hongkou Town,southern segment of the Longmenshan Yingxiu-Beichuan fault through microstructure analyses,carbon and oxygen isotopes,as well as contents of major elements within the veins.We aim to deepen our understanding of fluid behavior in this seismogenic region,as well as how fluids affect faults in the earthquake nucleation processes.The main structure,fluid source,and environment of mineral precipitation of earthquake-related veins are clarified.Results show that the principal slip zone consists of three fault gouge layers and one calcite vein layer.The fault gouges have different colors and structures,with formation of fault gouge wedge veins.Three types of veins were formed in inter-seismic,co-seismic,and post-seismic periods respectively along the edge of the principal slip zone and in the cataclastic hanging wall:(1)calcite slickenfibres along fault steps and stretching calcite veins;(2)injection gouge veins;and(3)blocky calcite veins,elongated calcite veins,and quartz veins.These veins represent micro-slips in the closed reduction environment during the inter-seismic period,high-pressure fluid injection during the co-seismic period,and mineral precipitation in the open oxidation environment to the reduction environment after the earthquake,respectively.The carbon and oxygen isotope analysis results show that δ^(18)O V-PDB values of calcite veins in the principal slip zone and cataclastic rocks are with the range of-20.5‰~-20.3‰,which are lower than that of the carbonate cements in the host rocks,suggesting that these calcite veins contain meteoric water source.Calcite precipitation t

关 键 词：龙门山断裂带 孕震区 流体行为 方解石脉 石英脉 地球化学 

分 类 号：P542.3[天文地球—构造地质学] P597.2[天文地球—地质学]