Lowering of the kinetic barrier of clay-mineral reactions along a seismogenic fault: Example from the Nankai subduction zone, Japan

An analysis of clay minerals in core samples around the megasplay fault of the Nankai Trough, Japan, recovered during the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment reveals a local progression of the smectite-to-illite conversion (S-I) reaction in the slip zone, possibly associated with local thermogenesis caused by seismic slip on the fault. Our semiquantitative X-ray diffraction analysis demonstrates that the illite content in mixed-layer illite/smectite is -15% higher in the gouge than in the host sediments. The thermal history of the fault, constrained in a previous study, requires a reduction of the apparent activation energy by 20-30% compared with the literature value to explain the observed illitization. The Shirako Fault in the Miura-Boso accretionary complex, which is thought to have been activated in a tectonic setting similar to that of the megasplay fault of the Nankai Trough, also shows a 20-30% reduction in the apparent activation energy. These results suggest that the kinetic barrier of the smectite-illite reaction is lowered as a result of mechanochemical processes in seismogenic fault zones.