We report the results of field observations and microstructural analyses of meso-scale faults that cut Cretaceous granitoids along and near lineaments of the Ikoma active fault zone. Based on measurements of the slip direction on fault planes, a paleostress of N-S extension is determined using the multiple inverse method. This stress field differs from the present-day regional stress field of predominant E-W compression, inferred from the seismic data inversion and the multiple inverse method applied to the active faults. Illite crystals of various sizes collected from a fault gouge, give K-Ar ages of .. Ma for the . -. µm fraction and .. Ma for the . -. µm fraction. High-and low-temperature components of illite are quantified by decomposing X-ray diffraction patterns to extrapolate the authigenic and detrital end-members. The ratios of low-temperature illite ( M d ) to high-temperature illite ( M and M) from a fault gouge are indistinguishable from one another. Based on the relative K-Ar ages of illite composites and of biotites from the host granitoid, the upper age limit of fault gouge formation is ca. Ma. These results suggest that formation of the fault gouge occurred between ~ and Ma, indicating that the meso-scale faults in basement rocks formed by N-S extension during the late Eocene to early Oligocene, rather than active tectonics under E-W compression.