Coseismic fluid–rock interactions in the Beichuan-Yingxiu surface rupture zone of the Mw 7.9 Wenchuan earthquake and its implication for the fault zone transformation

Abstract. Mechanism of fluids in modifying mineralogy and geochemistry of the fault zone and the role of rock-fluid interaction in the faulting weakening is still debatable. Through analyzing mineralogical compositions, major elements as well as micro-structural characteristics of outcrop samples including wall rocks, low damage zone, high damage zone and oriented fault gouge samples from principal slip zone gouges, mineralogical and geochemical variations of the fault-rocks is observed from Shaba outcrop of Beichuan-Yingxiu surface rupture zone of the Mw 7.9 Wenchuan earthquake, China. The element enrichment/depletion pattern of fault rock shows excellent consistency with the variation pattern of minerals in terms of the notable feldspar alteration and decomposition, decarbonization, coseismic illitization, and chloritization that occurs in the fault zone. The Isocon analysis indicates that the overall mass loss amount of the Shaba fault zone is ranked as low damage zone < high damage zone < fault gouge, while the mass removal within the fault gouge causes the greatest loss amount in the centeral strong-deformation region. The mechanism of material loss and transformation in the fault zone, analyzed by comprehensive study, is found to be complicated: (1) during the coseismic period, the mechanical fracturing, the dehydration reaction and thermal pressurization are likely the main factors; (2) during the postseismic period, infiltration by the postseismic hydrothermal fluids is the key factor. Therefore, the coseismic mechanical fracturing, chemical reaction related to coseismic frictional heating, and postseismic fuild-rock interaction are important factors to change and control the material composition and the fault zone evolution.