Thin-layered clay-rich fault gouges were observed on the near-surface fault plane of the YingxiuBeichuan Fault during the 2008 MS 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,Rl-shears（the angle between Y-shears and Rl-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.
