Miocene 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.

中文翻译：

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日本西南部裂谷后应力历史推断中新世早中新世侵入体和但马-妙见山地区中尺度断层

日本西南部年龄小于 15 Ma 的中新世火成岩脉被认为分别平行和垂直于弧。这种方向差异被称为日本海在 15 Ma 终止开放的重要证据。最严格的限制来自兵库县北部 Tajima-Myokensan 地区的约 60 条堤防。在这里，我们展示了 716 个平面侵入体的方向和 143 个中尺度断层的方向，这些断层的方向是使用最新的应力反演技术获得的，这些技术来自 Tajima-Myokensan 地区的中新世 Hokutan Group。结果与日本海开放结束的 15 Ma 假设相矛盾。我们发现侵入体不能按照之前报道的方向分为两组。相当，它们的极点的方向包括一个水平的腰带和一个垂直的集群。前者表示 NE-SW 拉伸应力，后者表示 NW-SE 压缩。然而，基于分层良好的 Muraoka 组中常见的基岩侵入体（其形成不受区域应力的影响），后者被解释为不代表区域应力，导致垂直方向的极点群，从中识别出压缩. 断层滑动分析结果与拉伸应力一致。从七个侵入体中获得了锆石的裂变径迹和 U-Pb 年龄。这些和以前发表的年龄表明该地区在 15 Ma 之前和之后都经历了 NW-SE 延伸。在日本西南部的主要部分，16 Ma 后弧内裂谷终止，弱伸展得以保持。这与日本海持续开放至 13.5 Ma 的假设一致。