The earthquake mechanism is recorded in the thermal history of fault rocks, and magnetic mineralogical investigation is crucial for understanding this process. In this study, fault rocks from an exhumed accretionary complex in the Shimanto Belt, southwest Japan, were analyzed using scanning SQUID microscopy, bulk rock magnetic measurements, and microscopic observations. Pyrite grains in cataclastic fault zones were found to have transformed into pyrrhotite and hematite, indicating that localized thermal effects and oxidation occurred in association with faulting. These mineralogical changes are spatially associated with deformation zones characterized by cataclastic flow textures. Magnetic signals detected by SQUID microscopy revealed a clear correlation between high-coercivity components and these deformation zones. Hematite and pyrrhotite were identified as the main high-coercivity minerals, while low-coercivity signals, which were more widely distributed, were attributed to magnetite. Although cataclastic deformation can result from seismic cycles, the mineralogical and magnetic features observed here suggest that localized thermal effects and oxidation occurred during fault-related deformation. Therefore, these changes serve as important indicators of thermal events and redox condition changes associated with fault activity. They offer valuable insight into the reconstruction of faulting processes and deformation histories preserved in exhumed fault rocks.