Post‐rift stress history of Southwest Japan inferred from early to middle Miocene intrusions and meso‐scale faults in the Tajima–Myokensan area

Abstract

AbstractMiocene igneous dikes older and younger than 15 Ma in Southwest Japan are thought to be oriented parallel and perpendicular to the arc, respectively. This difference of orientations was referred to as significant evidence for termination of the opening of the Japan Sea at 15 Ma. The tightest constraint comes from ~60 dikes in the Tajima–Myokensan area, northern Hyogo Prefecture. Here we present orientations of 716 planar intrusive bodies and the directions of 143 meso‐scale faults, obtained using the latest stress inversion techniques from the lower to middle Miocene Hokutan Group in the Tajima–Myokensan area. The results contradict the 15 Ma hypothesis for the end of the opening of the Japan Sea. We find that intrusive bodies cannot be separated into two groups by their orientations as reported previously. Rather, the orientations of their poles comprise a horizontal girdle and a vertical cluster. The former indicates NE‐SW extensional stress, and the latter NW‐SE compression. However, the latter are interpreted as not representative of regional stress, based on common sill intrusions (the formation of which was not influenced by regional stress) in the well‐stratified Muraoka Formation resulting in the vertical cluster of pole orientations from which compression was recognized. The results of fault‐slip analysis are consistent with the extensional stress. Fission‐track and U–Pb ages of zircons were obtained from seven intrusive bodies. These and previously published ages suggest that the area underwent NW‐SE extension both before and after 15 Ma. In the main part of Southwest Japan, the weak extension was kept after 16 Ma when intra‐arc rifting was terminated. This is consistent with the hypothesis that the Japan Sea continued to open until 13.5 Ma.