Miocene intra-arc rifting associated with the opening of the Japan Sea formed grabens in several areas in Southwest (SW) Japan, but the extensional tectonics of the arc are still not well understood. In this study, we first document the tectonostratigraphy of the Hokutan Group in the northwestern part of the Kinki district, and demonstrate the termination of extensional tectonics at ca 16.5 Ma, as inferred from grabens in the lower part of the group being unconformably overlain by sediments of the upper part. Second, we review early Miocene grabens in SW Japan to suggest that intra-arc rifting was abandoned at ca 16 Ma, essentially simultaneously with the end of rotation of the SW Japan arc as evidenced by paleomagnetic studies. The lesser numbers of grabens and reduced thicknesses of graben fills suggest that extensional deformation of the SW Japan arc was significantly weaker than that of the Northeast (NE) Japan arc, which was broken into blocks, indicating various degrees of paleomagnetic rotation within NE Japan. The weak deformation has allowed paleomagnetic studies to infer the coherent rotation of the SW Japan arc.

Saruodaki Falls, which is about m high, is one of the highlights of the San in Kaigan Geopark, northern Hyogo Prefecture, SW Japan. The quartz diorite exposed at the falls has previously been thought to represent a wide dike. However, we found that it is a laccolith with a horizontal diameter of < km and a thickness of > m. The base of the laccolith is not exposed. The host of the intrusive body consists of a lower Middle Miocene shaley formation, which is subhorizontal in this region. However, the formation makes a culmination centered by the body. In addition, the interface between the shaley formation and the diorite is concordant with the domal structure of the surrounding shale. Fracture patterns observed at the falls suggest that the laccolith is a composite sill made up of at least four sheets. Fission-track and U-Pb dating of zircon from the lower part of the laccolith yields ages of . . Ma and . . Ma, respectively. These ages are concordant with fossil data from the host rocks.

Significant advances were made in the last century in the investigations of the Neogene stress history of the NE Japan arc. However, previous studies have failed to fully resolve middle Miocene post‐rift stress conditions owing to their assumption of Andersonian faulting and an inability to determine maximum and intermediate stress axes from dike orientations. We applied the latest methods of paleostress analysis in this study to igneous dikes and mesoscale faults in the Kakunodate area of the NE Japan arc to elucidate post‐rift stress conditions. Stratigraphic constraints and U–Pb dating indicate that the doleritic and dacitic dikes were formed at 16–12 Ma and 15–12 Ma, respectively. Dolerite and dacite dikes yielded NW–SE extensional stresses with intermediate and low stress ratios, respectively. Mesoscale faults in the middle Miocene formations of the studied area indicated similar stresses. We suggest the sluggish deformations resulting in the dike intrusion and faulting in the normal‐faulting stress regime after the termination of intra‐arc rifting at ca. 15 Ma.

The recent revision of the stratigraphy of the Aniai district (northern Honshu, Japan) showed the necessity of re-examination of the Paleogene and Neogene stratigraphy in other areas of the Dewa Hills, northeast Japan. We carried out a detailed field survey in the eastern Dewa Hills, around the city of Kakunodate. In addition, we conducted zircon fission-track (FT) and U-Pb double dating for samples from formations. Based on our results, the volcanic and sedimentary succession in the study area were divided into the Yamayakawa (the Late Oligocene to early Early Miocene), Katsurabuchi (ca. Ma), Shiotezawa (ca. -Ma), Hachiwari (ca. -Ma), Onnagawa (ca. -Ma) and Yamaya (ca. Ma) Formations in ascending order. Granitic rocks of the Cretaceous make up the basement of the Yamayakawa Formation. The contact between the Yamayakawa Formation and the overlying Katsurabuchi Formation (ca. Ma) is a sharp angular unconformity and is newly discovered in this study. The Shiotezawa and Hachiwari Formations unconformably overlie the Katsurabuchi Formation; and the lower part of the Hachiwari Formation interfingers with the Shiotezawa Formation. The Hachiwari Formation is conformably overlain by the Onnagawa Formation. The youngest unit, the Yamaya Formation (ca. Ma), overlies the Yamayakawa and Katsurabuchi formations by angular unconformity. The results demonstrate that the stratigraphic succession in the study area is similar to that of the Aniai district.

Previous research has shown that stress conditions in the SW Japan arc changed from extension to compression during the middle Miocene. However, the compressional stress regime has not been clearly determined. To reconstruct the middle Miocene stress regime in the region, we analyzed basaltic intrusions in steeply northwest dipping units of the Shimanto accretionary complex around Cape Muroto. Analysis of orientation data for coherent units applying the Bingham distribution method and tilt correction indicates a compressive paleostress regime with a vertical σ σ axis, which is supported by orientation data collected from mélange units. This paleostress regime is consistent with buoyant subduction predicted from late Cenozoic plate reconstructions of SW Japan.

The Setouchi volcanic rocks include high-Mg andesites (HMAs) and garnet-bearing dacite-rhyolite, and are sporadically distributed along the Median Tectonic Line, Japan. New U-Pb zircon ages and geological and geochemical data are presented for those rocks in the Western Setouchi region (W-Setouchi). Previous studies referred to the altered andesite in the W-Setouchi as "pre-Setouchi volcanic rocks." However, on the basis of the new U-Pb age (14.4 Ma ± 0.3 Ma) and geochemical characteristics, we redefine it as the Jikamuro Formation, part of the Setouchi volcanic rocks.Incompatible elements are more enriched in the Jikamuro Formation rocks than in the Setouchi HMAs. The characteristic element compositions may be explained by mixing of compositionally different magmas, including subducted sediment melts, plus a contribution from crustal contamination. A stress-inversion technique with Bingham distribution method was applied to the orientations of felsic and mafic dikes within the Setouchi volcanic rocks, and indicates paleo-stress conditions during the period of Setouchi volcanism in the W-Setouchi. The analysis reveals NNWextensional stresses and a strike-slip stress. We infer that the former represents extensional conditions during the main period of volcanism and the latter represents a stress transition during the most recent period of volcanism (after 12 Ma).

The Nedamo and North Kitakami belts are composed of Paleozoic-Mesozoic accretionary complexes, forming part of the Kitakami Massif in Sotoyama district, east of Morioka City, Iwate Prefecture. These accretionary complexes are intruded by abundant dikes classified into nine rock-types. Mela-quartz diorite, porphyritic fine melaquartz diorite, and hornblende andesite account for half of the dikes, with the remainder composed of quartz diorite, porphyritic fine quartz diorite, dacite, porphyritic fine tonalite, rhyolite, and megaporphyritic rhyolite. The dikes are typically several meters thick, mostly high-angle, and trend roughly NNE-SSW. Hornblende andesite and fine quartz diorite dikes yield hornblende with K-Ar ages of 131±3 Ma and 122±6 Ma, respectively, and a rhyolite dike yields zircon with a U-Pb age of 120±1 Ma. Therefore, the dikes in the Sotoyama district are approximately 130-120 Ma (Barremian-early Aptian), and would have been intruded just prior to intrusion of the Early Cretaceous Kitakami Granitoids. Paleostress analysis of dike attitudes indicates that they formed in a WNW-ESE trending extensional stress field, marking a temporary change in the generally E-W compressional stress field within the Kitakami Massif during the Early Cretaceous.

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.