Two Paleozoic ophiolites, the Ordovician Oeyama and Permian Yakuno ophiolites, juxtaposed in the inner zone of southwestern Japan, record the tectonic evolution of the proto-East Asian continental margin during the Paleozoic. The Oura plutonic complex, located in the northern part of Maizuru City, Kyoto Prefecture, consists mainly of mafic to ultramafic rocks with minor amounts of intermediate intrusive rocks. Despite its key position for understanding the geotectonic framework of the Inner Zone of southwestern Japan, the geological attribution of the Oura plutonic complex has remained controversial. Zircon U-Pb dating of metagabbro and metadiorite from the Oura plutonic complex yields Permian ages of 273 and 292-254 Ma, respectively, indicating an affinity with the Permian Yakuno ophiolite of the Maizuru belt. Whole-rock and mineral geochemical data from metagabbro and ultramafic rocks indicate formation in an oceanic island arc setting, suggesting that the Oura plutonic complex represents lower crustal cumulates beneath a Permian island arc. The metadiorite is characterized by high Sr/Y and La/Yb ratios, typical of adakitic magmas derived from the melting of subducted oceanic crust. The Oura metadiorite is therefore interpreted to have originated from adakitic magma, most likely generated during backarc spreading. Late Paleozoic adakitic rocks are distributed not only in the Maizuru belt but also in northeast Japan and the Khanka-Jiamusi massif, and plausibly provide crucial clues to deciphering the tectonic evolution along the Permian proto-East Asian continental margin.
Roundness of gravel particles is widely used as an indicator of sediment transport, yet its potential for discriminating provenance between mainstem and tributary rivers has not been fully tested. In this study, we applied an image-analysis-based roundness measurement (Rgrains) to gravel samples from mainstem and tributary deposits in the Tama and Doushi river basins in central Japan. We used both modern riverbed sediments and terrace deposits across four grain-size fractions (2-4, 4-8, 8-16, and 16-32 mm) to examine suitable grain sizes for distinguishing mainstem and tributary deposits. The results suggest that terrace gravel clasts may be distinguished between mainstem and tributary sources by comparing their roundness distributions with those of modern riverbed sediments. Mainstem sediments consistently exhibited higher roundness than tributary sediments, whereas tributary deposits showed statistical characteristics biased toward specific roundness values, reflecting the restricted nature of their catchments and channels. The minimum grain size required for identification depended on the tributary gradient: in steep-gradient tributaries dominated by debris flows, clear differences were observed even in fine fractions (2-4 and 4-8 mm), whereas coarser fractions (16-32 mm) were generally required for low-gradient tributaries dominated by bedload transport. Furthermore, fluvial transport processes inferred from riverbed slopes appear to exert a stronger influence on clast roundness than lithology, which plays a relatively minor role. The roundness of terrace gravel clasts may preserve information on tributary gradients and transport modes at the time of deposition, and comparison with modern riverbed gravel clasts suggests that past tributary gradients may be reconstructed. These findings suggest that quantitative roundness analysis provides a useful approach for linking terrace gravel clasts to their provenance, offering insights into fluvial terrace development and sediment transport in mountainous rivers.
The Middle Miocene Takaiwa Rhyolite dike swarm in central Shikoku, southwest Japan, provides a key opportunity to evaluate whether arc-parallel dikes record the regional stress field in a crustal domain with a pervasive ENE-WSW structural grain. We measured 37 intrusive contacts and found that 36 dikes sharply crosscut schistosity, folds, and tectonic lines of the Sanbagawa Metamorphic Complex, demonstrating that their orientations were not governed by host-rock anisotropy. Using full three-dimensional dike-wall orientations and a mixed Bingham paleostress inversion, we obtained a single transtensional stress state with an NNW-SSE-trending sigma(3) axis. Although this sigma(3) direction is consistent with previous reconstructions for the Setouchi region, the Takaiwa stress regime differs from the extensional regimes documented at other Setouchi localities. The inferred transtensional configuration may reflect either depth-dependent modification of the Setouchi stress field or proximity to the compressional forearc domain. These results show that the ENE-WSW-striking Takaiwa dikes reliably record the regional Middle Miocene stress field, resolving long-standing concerns that arc-parallel dike trends in southwest Japan might simply reflect structural inheritance. The study highlights the importance of integrating outcrop-scale observations with three-dimensional orientation analyses when reconstructing paleostress fields in structurally anisotropic regions.
To gain new insights into non-marine diatom biostratigraphy, we provided taxonomic descriptions and examined the morphological evolution of the freshwater diatom genus Praestephanos from the Pliocene lacustrine deposits of the Kameyama Formation, Tokai Group in Mie Prefecture, Japan, which were deposited in Paleo-lake Anoutsu-ko. We also compared these results with the evolutionary record of Praestephanos from the contemporaneous lacustrine deposits of the Ueno Formation, Kobiwako Group, reported in a previous study, to establish a biostratigraphic correlation between the two formations; the deposits of the Ueno Formation accumulated in Paleo-lake Oyamada, which is thought to have been hydrologically connected to Paleo-lake Anoutsu-ko. In the Kameyama Formation, P. tokaiensis and P. miensis, originally described from the Ueno Formation, occur in intervals of approximately 3.9-3.8 Ma and 3.8-3.6 Ma, respectively. Around 3.8 Ma, during the transition between the two species, changes in the number of valve face fultoportulae were observed, suggesting a morphological shift from the ancestral P. tokaiensis to the descendant P. miensis. This stratigraphic distribution and morphological evolution were consistent with patterns observed in the Ueno Formation, indicating that the same evolutionary changes in Praestephanos occurred isochronously in both lakes. Therefore, the species turnover horizon can be regarded as a biostratigraphic datum for both the Kameyama and Ueno Formations. Notably, the two species were likely endemic to this paleo-water system. Although the biogeographic provinciality of freshwater diatoms can often be a limitation in biostratigraphic research, the results of this study demonstrate that biostratigraphic correlation based on evolutionary changes is possible between formations in regions belonging to the same paleo-water system, even for freshwater diatoms with strong provinciality. Furthermore, the finding that the same diatom species exhibited isochronous morphological evolution with an identical pattern in both regions within the same paleo-water system suggests that comparative analyses of diatom evolutionary changes and stratigraphic distributions between areas can potentially be developed into indicators for reconstructing paleo-water systems.
New zircon U-Pb ages are presented for Cretaceous to Paleogene volcanic and granitic rocks of Awaji Island in the Ryoke Belt, Arima area in the San'yo Belt, and Izushi-Kasumi areas in the San'in Belt. These areas are located on a transect across southwest Japan from the Median Tectonic Line to the Sea of Japan. So this is the first report of the zircon U-Pb ages of the volcano-plutonic complexes aligned across the Japan arc. The obtained U-Pb ages of the Sennan Rhyolite, foliated granite, and massive granite, all in the Ryoke Belt, are 94.0, 89.3-84.1, and 84.3 Ma, respectively. Those of the volcanic rocks of the Arima Group, Rokko Granite, and Nunobiki Granodiorite in the San'yo Belt are 79.9-74.1, 79.6-77.6, and 76.5 Ma, respectively. The volcanic rocks of the Yadagawa Group and Yadagawa Granite of the Izushi-Kasumi areas in the San'in Belt gave the ages of 68.2-64.9 and 64.0 Ma, respectively. The development of foliation and deformation of the foliated granite and the subsequent intrusion of massive granite on Awaji Island occurred at ca. 84 Ma. The volcanic rocks in the Arima and Yadagawa groups have been correlated to surrounding volcanic rocks based on lithological similarities, but the new U-Pb ages reveal that they each represent separate volcanic events. The durations of volcanic and plutonic activity, based on the zircon U-Pb ages, are 94-83 Ma for Awaji Island, 82-74 Ma for the Arima area, and 68-62 Ma for the Izushi-Kasumi areas, indicating a younging in the magmatic activity from south to north. The volcanic activity preceded the plutonic activity in all of the study areas.
