Comparative Mineralogical and Geochemical Compositions within the Fault Gouge in the Surface Exposures of the Mw7.9 Wenchuan Earthquake Fault and Their Implications for Mass Removal and Fluid-Rock Interactions

Geochemistry of the fault gouge record information on fault behaviors and environmental conditions.We investigated variations in the mineralogical and geochemical compositions of the fault gouge sampled from the margin zone （MZ） to the slip central zone （CZ） of the fault gouge in the Beichuan-Yingxiu surface rupture zone of the Wenchuan Earthquake.Results show that the clay minerals contents increase from the MZ to CZ,and the quartz and plagioclase contents slight decrease.An increasing enrichment in Al2O3,Fe2O3,and K2O are observed toward the CZ;the decomposition of quartz and plagioclase,as well as the depletion of Si O2,Ca O,Na2O,and P2O5 suggest that the alkaline-earth elements are carried away by the fluids.It can be explained that the stronger coseismic actions in the CZ allow more clay minerals to form,decompose quartz and plagioclase,and alter plagioclase to chlorite.The mass loss in the CZ is larger than that in MZ,which is maybe due to the more concentrated stress in the strongly deformed CZ,however other causes will not be excluded.