Mass removal and clay mineral dehydration/rehydration in carbonate-rich surface exposures of the 2008 Wenchuan Earthquake fault: Geochemical evidence and implications for fault zone evolution and coseismic slip

We report variations in the mineralogical, geochemical, and isotopic (δ13C, δ18O) composition of fault rocks sampled in transects across the Zhaojiagou and Pingxi exposures of the Wenchuan Earthquake or Longmenshan Fault Zone, where the gouge-rich fault core and principal slip surface cuts through carbonate-rich strata. Pervasive fluid infiltration was found to modify the mineralogical and geochemical architecture of the fault zones studied. Enrichment/depletion patterns, element partitioning, and a very large implied volume loss are quite different from those characterizing faults in granites and clastic sedimentary rocks and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution. An increasing enrichment in smectite observed toward the principal slip surface, a high abundance of elements such as Ba, Mg, and F, the deposition of minerals such as barite and fluorapatite, as well as the distinct depletion in 13C in vein material consistently suggest reactions involving a hydrothermal fluid originating at depth. Illitization of black gouges, caused by coseismic frictional heating, was found to be widespread. We propose that coseismic frictional heating along with the action of postseismic hydrothermal fluids controlled the transformation and distribution of smectite and illite within the fault core of the Longmenshan Fault Zone. The coseismic dewatering reactions are expected to have been more extensive at depth, possibly helping generate excess pore pressure assisting dynamic slip weakening during the Wenchuan Earthquake.