汶川地震假玄武玻璃的特征、成因及其构造意义  

Characteristics,genesis and tectonic significance of the pseudotachylyte produced by the Wenchuan earthquake

作　　者：王焕[1,2,3] 李海兵 司家亮[1,2] 裴军令 孙知明[5] 张蕾 WANG Huan;LI HaiBing;SI JiaLiang;PEI JunLing;SUN ZhiMing;ZHANG Lei(Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Jiangsu Donghai Continental Deep Hole Crustal Activity National Observation and Research Station,Lianyungang 222300,China;SinoProbe Laboratory of Ministry of Natural Resources,Beijing 100094,China;State Key Laboratory of Nuclear Resources and Environment,East China University of Technology,Nanchang 330013,China;Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Natural Resources,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]江苏东海大陆深孔地壳活动国家野外观测研究站,连云港222300 [3]自然资源部深地科学与探测技术实验室,北京100094 [4]东华理工大学,核资源与环境国家重点实验室,南昌330013 [5]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3833-3847,共15页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41972229,42372266,41830217);第二次青藏高原综合科学考察研究项目(2019QZKK0901)联合资助.

摘　　要：富流体的断层泥是浅部地震断层带中的特征岩石。一般认为,地震过程中摩擦热会导致粒间孔隙流体热膨胀增压,形成同震断层弱化(热增压机制),从而抑制摩擦熔融的发生。然而我们研究发现,在2008年汶川大地震(M_(W) 7.9)中断层浅部发生了摩擦熔融。汶川地震发生一年后,我们在汶川地震断裂带科学钻探项目一号钻孔(WFSD-1)732.6m深处的断层泥中发现了厚度约2mm的假玄武玻璃(凝固的摩擦熔融物)。该假玄武玻璃形成的位置极浅,且产生于非固结的、富流体的断层泥中。从岩心来看,断层面可见镜面构造和同震擦痕。微构造分析显示,该假玄武玻璃主要由石英碎屑和由长石与黏土矿物熔融的非晶质基质组成,基质中发育众多不规则的微裂隙,并可见流动构造。化学成分分析显示,其基质富Ba且被重晶石(BaSO 4)小细脉切割,为同震及震后流体存在的证据。由于假玄武玻璃在流体存在的条件下会快速蚀变,且龙门山地区大地震复发周期为3000~6000年,因而这些完全未被蚀变的新鲜假玄武玻璃可能是最近一次大地震,即2008年汶川大地震的产物。针对钻孔中断层泥进行的高速摩擦试验,证实了在钻孔732m深度发生地震滑动的条件下确实会产生假玄武玻璃。因此WFSD-1钻孔732.6m的假玄武玻璃被认为是汶川地震的产物,代表了汶川地震主滑动带位置。这是首次在自然界中发现浅部含水断层泥摩擦熔融形成假玄武玻璃的实例,不仅对认识汶川大地震的发震机制、断层强度、应力迁移与破裂传播具有重要意义,而且有助于我们认识断裂带内浅部脆性区域的力学属性、岩石变形环境和变形机制,促进对地震滑动机制和破裂过程的认识。Fluid-rich gouges are typical rocks at shallow depth in seismogenic faults.It is generally recognized that frictional heat during earthquakes leads to thermal expansion and pressurization of pore fluids,resulting in co-seismic fault weakening(thermal pressurization mechanism),which inhibits the occurrence of frictional melting.However,we show that frictional melting,rather than thermal pressurization,occurred at shallow depths during the 2008 M_(W) 7.9 Wenchuan earthquake,China.One year after the Wenchuan earthquake,an~2mm-thick,glass-bearing pseudotachylyte(solidified frictional melt)was found at 732.6m depth in the first borehole of the Wenchuan Earthquake Fault Scientific Drilling Project(WFSD-1).This pseudotachylyte was formed at an extremely shallow depth and was generated in unconsolidated,fluid-rich fault gouge.Mirror-like structures and co-seismic striations are visible on the fault surface,viewed from the drilling core.The pseudotachylyte consists mainly of quartz fragments and an amorphous matrix.Irregular microcracks are pervasive in the matrix where flow structures are visible.Chemical composition analysis shows that the matrix is Ba-rich and cut by small veins of barite,which provide evidence of co-and postseismic fluid percolation.Because pseudotachylyte can be rapidly altered in the presence of percolating fluids,its preservation suggests that gouge melting occurred in a recent large earthquake.Moreover,since the recurrence interval of large earthquakes in the Longmen Shan area is~3000 years,the completely unaltered pseudotachylyte likely produced in the most recent large earthquake,i.e.the 2008 Wenchuan earthquake.High-velocity frictional experiments on the fault gouge from the drillcore deformed at conditions expected for seismic slip at borehole depths showed the generation of pseudotachylytes.Therefore,the pseudotachylyte at 732.6m in WFSD-1 is considered to be a product of the Wenchuan earthquake and represents the location of the principal slip zone of the Wenchuan earthquake.This is the firs

关 键 词：假玄武玻璃 显微构造 摩擦熔融 富流体断层泥 汶川地震 

分 类 号：P315[天文地球—地震学] P585.2[天文地球—固体地球物理学]