Dynamic changes in fluid redox state associated with episodic fault rupture along a megasplay fault in a subduction zone

Although redox state plays an important role in chemical reactions, it has received little attention in terms of processes of fluid–rock interaction in fault zones. The results of geochemical analyses (major element mapping and trace element analyses) of carbonate veins along the Nobeoka Thrust, a fossilized megasplay fault within the Shimanto Accretionary Complex, Japan, suggest dynamic sudden change in redox state associated with seismic faulting and fluid flow. Fluids migrating along the fault zone were relatively oxidized during the pre-failure fluid pressurization stage of the seismic cycle, but were relatively reduced during the immediate post-failure stage. The generation of a reducing post-failure hydrothermal regime may be associated with seismogenic rupture along the megasplay fault, involving the advection of exotic reducing fluid from deeper levels or in situ generation of hydrogen by co-seismic mechanochemical reactions.