Green Tuff comprising altered Neogene volcanic and volcaniclastic rocks occurs on the Sea of Japan side of Hokkaido, Fossa Magna, and Southwest Japan, and is widely distributed in Northeast Japan. This paper presents a review of the history of research on the geological development of the Green Tuff in Northeast Japan in relation to the opening of the Sea of Japan. Research before 1970 was based mainly on the geosyncline paradigm; research after 1970, when the concept of plate tectonics was introduced, progressed in two stages. From early 1970 to about 2000, conventional stratigraphic data were summarized and tectonic models related to the opening of the Sea of Japan were presented. From about 2000 until now, new data have been provided by improved dating methods, paleomagnetic measurements, detailed restoration of paleoenvironments from sedimentary facies analysis, and restoration of stress fields, allowing the reconstruction of more advanced tectonic models.

The Neogene paleostress field around the Tanakura Fault Zone was investigated by fault analysis, such as mesoscale fault analysis and fault shear sense analysis. Since it is usually unknown when each fault was formed, these analyses are difficult to determine the age of paleostress field of the results. In this study, we studied rhyolite dikes of the Neogene Tochihara Rhyolites to obtain data of paleostress and its age. Age of Tochihara Rhyolites magmatism was constrained by zircon U-Pb dating, and paleostress field was revealed by analysis of dikes. The paleostress field revealed by analysis of the rhyolite dikes suggests a normal-faulting stress regime with a WNW-ESE-trending σ₃-axis, while zircon U-Pb dating of a rhyolite dike yielded a Miocene age of 17.2±0.2 Ma (2σ). The results show that the Tochihara Rhyolites were erupted under WNW-ESE extension when the Tanakura Basin began to form at ca. 17 Ma. These data provide clues to the development of the Tanakura Basin and the Neogene kinematic history of the Tanakura Fault Zone.

Highly scattered remanent magnetization directions were obtained from middle Miocene andesites of the Inase volcanic rocks on the western margin of the Kitakami Mountains, Northeast Japan. Oriented rock samples of lavas and intrusions were collected from 22 sites, and standard paleomagnetic analyses, including careful demagnetization experiments, were conducted to obtain reliable characteristic remanent magnetizations (ChRMs). Demagnetization data indicate that the main magnetic mineral that carries ChRMs is magnetite or Ti-poor titanomagnetite. Of 20 site-mean ChRM directions, 19 were successfully corrected for gentle post-emplacement tilt. Both positive and negative inclinations were determined, and though they show considerable scatter in declinations, many are northerly to easterly when directions with negative inclinations are flipped. Compared with the declinations, the scatter in inclinations is relatively small, and the inclinations lie along a small circle with an almost vertical axis. The mean inclination of ~47° is similar to the expected middle Miocene inclination value in Inase. A plausible explanation for the scattered directions aligning along the small circle is vertical-axis tectonic rotation, possibly related to island arc-scale crustal deformation. A recent study hypothesizes that, in the middle Miocene, the crust beneath the backarc side of Northeast Japan broke into several domino-like crustal blocks in a dextral shear regime along the eastern margin of the Japan Sea. The vertical-axis rotations identified in the ChRM data from Inase, which is located near the eastern end of the crustal blocks, may have resulted from rotations of these larger crustal dominos.

This study focuses on the biostratigraphy of diatoms, planktonic foraminifera, and ostracods from the Miocene Ayugawa, Tsuzuki, Yamabe, and Yamagasu groups in the Setouchi Geologic Province of Kinki District, southwestern Japan. The results of this study are as follows. The Ayugawa Group in Shiga Prefecture does not contain fossils of planktonic foraminifera and index species of diatoms, so a detailed depositional age of the group could not be determined; however, the first discovery of fossil ostracods, along with descriptions of fossil mollusks from previous studies, allow correlation of the Tsuchiyama and Kurokawa formations of this group with diatom biozone NPD2B (18.2-16.9 Ma). The Okuyamada Formation of the Tsuzuki Group in Kyoto Prefecture contains a few fossil ostracods and fossil diatom index species, the latter suggesting correlation with diatom biozone NPD2B. The first discovery of fossil diatoms in the Hayama Formation of the Yamabe Group in Nara Prefecture suggests correlation with the diatom biozone NPD2A (19.9-18.2 Ma) based on index species. Fossil planktonic foraminifera and ostracods were discovered in the Igami Formation of the Yamagasu Group in Nara and Mie Prefectures, but no index fossils were found. The first fossil diatoms discovered in the Nakatarō Formation of the Yamagasu Group suggest correlation with diatom biozone NPD2A. The Yamabe and Yamagasu groups are therefore the oldest marine Miocene strata in Setouchi Geologic Province. Based on these results and prior studies, the Miocene strata in Setouchi Geologic Province were deposited during four global sea-level rise intervals from 19 to 15 Ma.

The Nijikai Tuff was ejected in the late Cenozoic from the Ikarigaseki Caldera, located in the southwestern part of the Hakkoda-Towada caldera cluster. New U-Pb ages of zircons from the Nijigai Tuff reveal a bimodal age distribution with two peaks at 2.019±0.098 and 4.203±0.083 Ma, based on an Unmix Ages function. Considering the closure temperature of zircon, the younger peak age corresponds to the eruption of the Nijikai Tuff, whereas the older peak age may correspond to zircons inherited from massive assimilation by the Nijikai Tuff of underlying volcanic ejecta, likely the Obirakiyama Tuff. In this study the eruption age of the Nijikai Tuff is established with much higher precision than previously determined.

Illite K-Ar dating was conducted on phyllites from the Futase and Kawamata units of the Otaki Group in the Mitsumine area, Kanto Mountains. The K-Ar ages from the eastern part of the Futase Unit are 60-51 Ma, which are younger than previously reported K-Ar ages of 76-65 Ma. The illite fractions yielded K-Ar ages of 48-44 Ma in the western part of the Futase Unit and 46-42 Ma in the Kawamata Unit. Compared with the previous study, the measurement fractions contain relatively high K contents, and there is no significant difference in the ages obtained for each part. These results lead to the suggestion that the previous K-Ar ages are apparently older due to the influence of detrital mica and quartz grains. Integration of our results with previously published data for metamorphism of the Otaki Group indicates that the new K-Ar ages reflect the timing of the metamorphic peaks for each part of the units.