Cathodoluminescence and fluid inclusion analyses of mineral veins within major thrusts in the Shimanto accretionary complex: Evidence of hydraulic fracturing during thrusting

To elucidate fluid-rock interaction in a seismogenic zone along a plate-subduction boundary, we investigated the occurrence of mineral veins within the major thrusts in the Shimanto accretionary complex and examined their microstructures using a cathodoluminescence technique. We found discriminative structures, for example, a jigsaw-puzzle structure, within the quartz veins in the thrusts, which could indicate that hydraulic fracturing occurred by abnormal pore-fluid pressure during thrusting. Pore pressure values, estimated quantitatively by fluid inclusion analyses, were 3-27 MPa higher than the surrounding parts, which may be direct evidence of abnormal pore-fluid pressure. High pore-fluid pressures and subsequent hydraulic fracturing may play an important role within major thrusts along a plate-subduction boundary.