Meso-scale brittle deformation structures in and around the Ikoma fault zone

Abstract

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.

Kinematic Analyses and Radiometric Dating of the Large‐Scale Paleogene Two‐Phase Faulting Along the Median Tectonic Line, Southwest Japan

“…Some sedimentary basins have a large depositional area with significant subsidence, which are formed by approximately EW trending normal faulting under the NS trending extensional stress condition (e.g., Sakai, ; Zhang et al, ). In the continental margin of the proto‐Southwest Japan arc, small sedimentary basins were formed with the extension axis of NE to NNE trend in some locations (e.g., Mitamura et al, ; Sakai, ) (referring to the reconstructed direction before the opening of the Japan Sea investigated by Otofuji & Matsuda, ), which was parallel to the trend of the subduction zone, owing to the almost orthogonal subduction of the Pacific oceanic plate underneath the proto‐Southwest Japan arc in the WNW direction. Some of the Miocene extensional paleostress field documented in the Kobe Group by Famin et al () might be correlated to that in this stage, because the Kobe Group, which used to be considered as Miocene, is now found to be Late Eocene to Early Oligocene (37–31 Ma; Ozaki et al, ).…”