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X-Ray Diffraction Analysis Result of Co-Seismic Fault Gouge in Carbon Mudstone at Outcrops of Bajiaomiao and Shenxigou in Hongkou
详细信息   
摘要
The co-seismic surface ruptures of the May 12,2008 Wenchuan earthquake in Bajiaomiao and Shengxigou were developed mainly at the outcrops of carbon mudstones of Xujiahe formation of Triassic system.The black color and textures of co-seismic gouge are similar to old gouge and bed rock.We excavated trenches along the surface ruptures and collected samples of wall rock,fault breccia,old fault gouge and co-seismic gouge.All samples were analyzed quantitatively by X-ray diffraction.The main rock-forming minerals and clay minerals of co-seismic gouge are similar to old gouge,but their content is different,which shows the co-seismic gouge was formed based on old gouge.The wall rock and fault breccias adjacent to co-seismic gouge are carbon mudstones.The mineral composition and texture of the fault zone are obviously simpler than that of the northern part of the surface ruptures of Yinxiu-Beichuan Fault.The major minerals of co-seismic gouge are quartz and clay minerals,containing a few amount of feldspar,without calcite;a small amount of dolomite was found in co-seismic gouge at Shenxigou,and the content of dolomite is much lower than that in bed rock and old gouge.The marked character of new gouge is abundant in clay minerals,and the content of clay minerals decreases in turn from co-seismic gouge to old fault gouge,fault breccia and wall rock.The main clay minerals are illite and illite /smectite(I /S) mixed layer,containing a few amount of chlorite;a few kaolinite was found in co-seismic gouge of Shenxigou,the bed rock and gouge of Bjiaomiao did not contain kaolinite.Mineral characters of co-seismic gouge are different from old gouge.The old gouge contains calcite and dolomite,and the co-seismic gouge contains a few amount of dolomite and without calcite;the old gouge does not contain illite.However,for co-seismic gouge,mineral characters are different between black and white gouges,the content of illite in black gouge is higher than that in white gouge.In this study,the main clay minerals are I /S mixed layer,illite and chlorite,which is similar with San Andreas Fault and Chelunpu Fault.However,kaolinite content is extremely low in this fault,only trace kaolinite was found in the co-seismic gouge of Shenxigou.The high content of I /S in co-seismic gouge shows that the rich K+fluid participated in the seismic fault slip.All of these characters show that minerals of co-seismic fault gouge in this study are somewhat different with that of San Andreas Fault and Chelunpu Fault.

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