Abstract: Thin-layered clay-rich fault gouges were observed on the near-surface fault plane of the Yingxiu-Beichuan Fault during the 2008 M_S 8.0 Wenchuan earthquake. In this paper,the authors investigate the microstructural features and mineralogy of the clay-rich fault gouge samples based on the stereoscopic and optical microscopic observation and X-ray diffraction analysis. These fault gouge samples were collected from the trench excavated at the Beichuan segment of the Yingxiu-Beichuan Fault,where the largest vertical offsets were observed at the earthquake surface ruptures. The results show that the typical microstructures of localized brittle deformation are well developed in the co-seismic fault gouge,including Y-shears,R1-shears(the angle between Y-shears and R1-shears is larger than 14°),R2-shears,P-shears,tension cracks and stepped fragmented grains. All these microstructures are commonly accepted as the result of seismic slip event. In addition,there are also some microstructures similar to those representative of the distributed ductile deformations developed well within the gouges,including P-foliation,elongated clast grains and asymmetric trails of elongated clastic grains. However,all these microstructures are developed only in the area between two parallel Y-shears,indicating that they were induced by the same slip event along the two shears. The microstructures mentioned above indicate the thrust movement of the fault during the Wenchuan earthquake. Field investigation shows that the new fault gouge is very narrow with the thickness of about 3mm,which indicates that the slip movement of the fault was constrained in a very narrow slip zone. The results of analysis show that the contents of feldspar and quartz in the gouges are lower than those in the wall rocks,while the content of clay in the gouges is higher than that in the wall rock. This may indicate that feldspar and quartz were converted partly into clay or clay-particle sized materials because of the friction energy induced by fault slip. Moreover,the frictional heating produced by fault slip might cause the conversion of the I/S mixed layer in the clay-rich gouge into the illite(probably including some chemical reactions of solution),which made the differences of the contents of the illite or the I/S mixed layer in the gouge and in the wall rocks. These results present some criterions for identification of the features of the fault movement.

Key words: Wenchuan M_S 8.0 earthquake, fault gouge, microstructure, mineralogy

摘要： 汶川2008年M_S 8.0大地震同震地表破裂带中，近地表断层滑动面内多处分布了薄层断层泥。选取位于地表垂直位移量较大地段的北川沙坝探槽中的断层泥为研究对象，使用立体镜、光学显微镜和X光衍射等对断层泥微结构特征及其矿物成分进行详细的分析。结果表明：汶川地震地表破裂带的断层泥中发育Y-剪切、R1-剪切（与Y-剪切的交角>14°）、R2-剪切、P-剪切以及张裂隙和书斜式构造等局部化脆性变形的特征显微构造。此外，断层泥中还有发育良好的P-叶理，碎屑颗粒的拉长和不对称拖尾构造等类似于散布的韧性变形特征。但是，它们仅局限在2条平行的Y-剪切之间发育，应当是断层同一滑动事件的产物。显微构造特征揭示了汶川地震破裂带的逆冲滑动性质，强烈变形的断层泥带较窄，仅约3mm，表明同震滑动明显地局限在一个窄的滑动带内。断层泥的石英和长石含量小于围岩，而黏土矿物含量高于围岩，说明来自围岩的长石和石英由于同震断层滑动摩擦使其部分转变为黏土矿物或黏土粒级的物质。断层泥中的伊利石含量高于围岩，伊利石/蒙脱石混层则低于围岩，这种黏土矿物组成成分的差异，可能是由于地震同震断层滑动摩擦增温（不排除溶液化学作用的参与），使部分伊利石/蒙脱石混层转化为伊利石。这些研究结果为探讨断层滑动性质提供了一种判定标准。

关键词: 汶川地震, 断层泥, 显微构造, 矿物成分