Abstract
<title>Abstract</title> <p>Fluid-assisted mass transfer strongly influences permeability, pore fluid pressure, and strength recovery along seismogenic faults in subduction zones. Quartz dissolution, transport, and precipitation can promote fracture sealing and fault healing after seismic fragmentation. Although quartz veins are used to infer fluid pressure variations along seismogenic faults in subduction zones, quartz redistribution in seismogenic fault rocks remains poorly understood. In this study, we investigated ultracataclasites associated with pseudotachylytes from the Minami-Awa Fault in the exhumed Paleogene Shimanto accretionary complex in southwest Japan using scanning electron microscopy–cathodoluminescence (SEM–CL) imaging and electron probe microanalysis (EPMA) mapping. SEM–CL images reveal abundant healed microcracks and grain boundaries in quartz. Crack-filling quartz with high-cathodoluminescence displays Al-enrichment, whereas low-cathodoluminescence in quartz is characteristic of Al-poor conditions, demonstrating a positive correlation between cathodoluminescence intensity and Al concentration. Cross-cutting relationships of the quartz-filled microcracks indicate repeated crack sealing and mutual overprinting of high- and low-cathodoluminescence quartz indicating repeated alternation between different quartz precipitation regimes. High-cathodoluminescence, Al-rich quartz likely records relatively rapid precipitation under disequilibrium conditions during transient permeability enhancement, whereas Al-poor quartz reflects slower precipitation under near-equilibrium associated with local diffusion-dominated mass transfer at low permeability. Hence, the investigated ultracataclasites preserve direct microstructural evidence for repeated quartz redistribution, fracture sealing, and fault healing. Repeated alternation between these contrasting precipitation regimes may provide a mechanism for cyclic fault healing and permeability evolution along seismogenic faults in subduction zones.</p>