Coseismic fluid–rock interactions in the Yingxiu–Beichuan surface rupture zone of the Mw 7.9 Wenchuan earthquake and their implications for the structural diagenesis of fault rocks

Abstract

The mechanisms by which fluids modify the mineralogy and geochemistry of fault zones and the role of rock–fluid interactions in fault weakening are subjects of debate. We assessed the mineralogical and geochemical variations in fault rocks in the Shaba area of the Yingxiu–Beichuan surface rupture zone of the Mw 7.9 Wenchuan earthquake using analyses of their mineralogical compositions, major elements, and the microstructural characteristics of wall rocks, damage zones, and oriented fault gouge samples from the principal slip zones. The elemental and mineralogical compositions show a certain regularity across the fault zone from the wall rocks to the fault gouge so that (1) Al2O3, Fe2O3T, and K2O contents increase and exhibit significant enrichments in the fault gouge; (2) Na2O and P2O5 contents decrease gradually and exhibit significant depletions in the fault gouge; (3) quartz, feldspar, and carbonate mineral contents decline; and (4) clay mineral contents increase dramatically. Isocon analysis indicates that mass losses in the Shaba fault zone occur with losses in the damage zone < the fault gouge. The mechanisms of material loss and transformation in the fault zone are complex, and mechanical fracturing, dehydration reactions, and fluid–rock interactions are important factors in changing and controlling the material compositions and fault zone evolution, with material losses and transformation especially important within the fault core. Moreover, due to the damage zone having a higher permeability than the fault core, it was conducive to hydrothermal upwelling, fluid–rock interactions, and fracture healing.