2008年汶川地震断裂带北东段南坝地区岩石特征与内部结构  

Rock characteristics and internal structures of the Beichuan fault zone at Nanba outcrop in the northeastern segment of the 2008 Wenchuan seismogenic fault

作　　者：李成龙[1,2] 李海兵 王焕[1,3] 张蕾 孙知明[4] 张佳佳 云锟 张进江[2] LI ChengLong;LI HaiBing;WANG Huan;ZHANG Lei;SUN ZhiMing;ZHANG JiaJia;YUN Kun;ZHANG JinJiang(MNR Key Laboratory of Deep-Earth Dynamics,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,Chin;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)Guangzhou 511458,China;MNR Key Laboratory of Paleomagnetism and Tectonics Reconstruction,Institute of Geomechanics,Chinese Academy of Geological Sciences,ChineseAcademyofGeologicalSciences,Beijing 100081,China;Institute Exploration of Technology,Chinese Academy of Geological Sciences,Chengdu 611734,China;Chengdu University of Technology,Chengdu 610059,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 [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 [5]中国地质科学院探矿工艺研究所,成都611734 [6]成都理工大学,成都610059

出　　处：《岩石学报》2020年第10期3192-3208,共17页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41520104006、41830217);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);中国地质调查局项目(DD20190059)联合资助。

摘　　要：龙门山断裂带中的映秀-北川断裂带是2008年汶川地震的主发震断裂,它具有斜冲断裂的运动学特征(逆冲兼右旋走滑),其中南西段映秀断裂带以逆冲为主,而北东段北川断裂带以右旋走滑为主。断裂带的物质组成和内部结构的研究,有利于深入认识断裂活动性质、构造变形行为和地震发震机制。本文以出露于映秀-北川断裂带北东段南坝地区的断裂岩为研究对象,通过野外地质调查、显微构造观察、XRD和μXRF等多种分析手段,探讨映秀-北川断裂带北东段的岩石组成和内部结构。研究表明发育于寒武纪粉砂质板岩中的断裂带宽度约30m,断层中心发育厚20~40cm的黑色断裂物质,2008年汶川地震的同震位移沿黑色物质层中厚约10mm滑动带滑动。断层NW侧和SE侧表现出近于对称的结构特征,两侧的角砾岩宽~4m和~3m,破碎带宽~10m和~12m。黑色断裂物质中石英含量30%~50%,长石含量18%~25%,黏土矿物总含量30%~36%,主要由伊利石、伊蒙混层和绿泥石组成,少有蒙脱石。XRD结果显示黑色断裂物质具有非晶质成分特征,SEM观测结果可见熔融结构特征,显微结构与μXRF结果呈现后期流体作用明显,其表明黑色断裂物质主要由古地震滑动形成的假玄武玻璃蚀变而成,并发育不同蚀变程度的多期假玄武玻璃。上述研究揭示出断裂摩擦熔融是汶川地震断裂带北东段南坝地区的重要动态弱化机制,其内部结构和岩石特征与南西段映秀断裂带具有明显的差异。The Yingxiu-Beichuan fault zone is the main seismogenic fault of the 2008 Wenchuan earthquake in the Longmen Shan thrust belt.Most of the surface ruptures during the earthquake were oblique(thrusting with dextral strike-slip motion).Among them,the Yingxiu fault in the southwestern segment was dominantly characterized by thrusting,while the Beichuan fault in the northeastern segment was dominated by dextral slips.Studies upon the material compositions and internal structures of the fault zone are conducive to further understand the nature of fault activity,deformation behavior,and seismogenic mechanism.In this study,we focus on an outcrop of the Beichuan fault in Nanba area,and present material compositions and internal structures of the fault through field investigation,microstructural observation,X-ray diffraction(XRD)and micro X-ray fluorescence(μXRF)analyses.The results show that the fault zone developed in the Cambrian sandy slate with a width of^30 m.A black fault material layer with a thickness of 20~40 cm is developed in the center of the fault.In addition,the coseismic displacement during 2008 Wenchuan earthquake occurred along the^10 mm sliding zone which is composed of black materials.Both the NW and SE sides of the fault zone show almost symmetrical structural features.The breccia layers are 3~4 m thick,while the fracture layers are 10~12 m thick.The black material is composed of 30%~50%of quartz,18%~25%of feldspar,and 30%~36%clay minerals that mainly consist of illite,illite/smectite mixed layer,and chlorite.The XRD analyses show the occurrence of amorphous component,and the SEM observation exhibits melt structures.Both the microstructures andμXRF analysis show strong fluid influence,indicating that the black material is most likely altered from pseudotachylyte.Varying levels of alteration suggest that there are multiple generations of pseudotachylytes.These results show that frictional melting is the main dynamic weakening mechanism for the NE segment of the Wenchuan earthquake fault zone in Nanba

关 键 词：汶川地震 断裂岩 假玄武玻璃 北川断裂带 龙门山 

分 类 号：P315.2[天文地球—地震学]