The opening of the Japan Sea, one of the Oligocene to Miocene back-arc formations in the western Pacific, was the most significant Cenozoic event in the Japan arc. Extensive magmatism occurred during this event in the Japan Sea and the circum-Japan Sea province, including the Hokuriku region where volcanism was initiated with basalts and andesites (Konosuyama Formation) in the Noto peninsula in the early Oligocene (ca. 33 Ma?). In the Early Miocene, andesitic volcanism (Kamiwazumi and Matsunagi formations) followed the deposition of late Oligocene dacitic pyroclastic flows (Goroku Formation). Rhyolitic pyroclastic rocks including moonstone rhyolites were deposited widely as pyroclastic flows in the Hokuriku region from ca. 23-22 Ma. The moonstone rhyolites are alkalic in composition, suggesting a continental-arc rift origin. From ca. 21-17 Ma, basaltic to andesitic volcanic rocks (Iwaine and Ito-o formations and Besshodake andesites) of various geochemical compositions (tholeiitic and calc-alkalic basalts to andesites, adakite, and high-Mg andesite) were effused widely in the Hokuriku region. This basaltic to andesitic volcanism was followed from ca. 17-16 Ma by active dacitic to rhyolitic volcanism (Iozen and Fukuhira formations) with geochemical compositions indicating a continental-arc to island-arc origin. This rhyolitic volcanic episode, and minor basalts and andesites from 16 Ma, mark the end of the magmatism related to the opening of the Japan Sea, coinciding with the termination of the clockwise rotation of southwest Japan. Previous petrological and geochemical studies suggest that magmatism related to the opening of the Japan Sea in the Hokuriku region was caused by upwelling of the asthenosphere into the mantle wedge below the margin of the eastern Eurasian continent.

The empirical “laws of convergence rate of plates” proposed by Otsuki (1989) were updated based on a large amount of new data accumulated during the past 30 years, and the parameters of plate kinematics were revised to accord with new plate motion models. The configurations of deeply subducting slabs are described accurately, not only by seismic slab models but also by seismic tomography results. The deformation rates of overriding plates, which have been difficult to calculate precisely, were revised by new paleoseismological and geological investigations as well as the World Stress Map and GPS geodetic research. Excepting the zones of buoyant subduction, 28 subduction zones in total were reexamined. Four subduction zones with slab tips shallower than 200 km (Western Aleutian, West Honshu, South Solomon, and Puysegur) and four subduction zones in some abnormal tectonic situations (Lesser Antilles, North Kamchatka, Tonga, and Hellenic) were excluded. The remaining 20 subduction zones follow the very simple equations V_a = V_o//a + V_s//a − 69 (R² = 0.912) and V_t//a = 69 − V_s//a (R² = 0.924), in mm/y, where V_a denotes the deformation rate of the overriding plate (shortening is positive); V_s//a and V_o//a are subducting and overriding plate velocity components, respectively, parallel to V_a (trenchward is positive); and V_t//a is the migration rate of the trench axis parallel to V_a (oceanward is positive). These updates confirm that the “laws of convergence rate of plates” are robust.

Obirakiyama Tuff (210 km³ DRE), erupted from Yunosawa Caldera in southern part of Hakkoda-Towada caldera cluster, consists of subaerial welded tuff and subaquatic unwelded tuff facies. This study reports zircon U-Pb and fission-track (FT) double dating for five samples collected from the two facies to reconstruct the eruptive ages of each facies. The resulting U-Pb ages for three pumices from the unwelded tuff facies were concentrated at 3.9 Ma, whereas the U-Pb ages of both the welded tuff exposed at the mid-point of Mt. Obiraki and obsidian in Daibutsu Park yielded U-Pb ages of 8.72±0.08 Ma and 5.31±0.05 Ma, respectively. The resultant U-Pb ages corresponded to the double-dated FT ages within the range of the analytical uncertainties, indicating that we succeeded to obtain highly precise and accurate eruptive ages. Thus, this study determined the unwelded tuff facies of the Obirakiyama Tuff were ejected at 3.9 Ma. In strike contrast, remarkably older ages were obtained from two samples of the welded tuff facies. We suggest that 5.3 Ma obtained from the rhyolite (welded tuff facies) may also indicate the eruptive age of the Obirakiyama Tuff but cannot rule out the possibility that there are some welded tuff facies erupted at 3.9 Ma. Moreover, the welded tuff mainly composing Mt. Obiraki (8.7 Ma) can be rather correlated to late Miocene formation. Thus, reexaminations of the eruptive age of the welded tuff facies and their distributions are further required.

The lower-middle Miocene Myojin Formation (Kuma Group) distributed in northwestern Shikoku contains gravels of granitoids and hypabyssal rocks. Based mainly on the southward paleocurrent directions of the Kuma Group, these gravels are considered to have originated from the Ryoke and San-yo igneous rocks. The Myojin Formation, therefore, provides information on the Inner Zone of this area at that time, although the Kuma Group is now distributed mainly in the Outer Zone. Examination of these igneous rock gravels through petrological studies and zircon U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry provides more certainty on their origin. The U-Pb ages of 99-95 Ma for the granitic gravels and ca. 90 Ma for the hypabyssal gravels are consistent with previously reported ages of the Ryoke (and San-yo) granitoids, especially those in the Takanawa Peninsula and Yanai area, which are located north and northwest of the Kuma Group distribution. Petrological characteristics of these igneous rock gravels are also comparable with the Ryoke (and San-yo) granitoids and hypabyssal rocks in the Chugoku and Shikoku regions. These results suggest-and further support the conventional view-that igneous rock gravels clasts of the Myojin Formation were derived mainly from the Ryoke igneous rocks.

The paleoceanography of the Japan Sea has revealed environmental shifts during glacial and interglacial periods of the early Pleistocene; however, detailed paleoenvironments in the shallow seas have not been fully reconstructed. In this study, detailed environmental changes were investigated by studying fossil ostracods in the Pleistocene Kakuma and Minagawa formations distributed in the Niitsu Hills, Niigata Prefecture, Central Japan. At least 211 species belonging to 83 fossil ostracod genera were recognized in 57 samples from the two formations. Four biofacies were identified based on cluster analysis: lower sublittoral, upper sublittoral, bay, and cold water lower sublittoral. Lower sublittoral biofacies are dominant in the Kakuma Formation, whereas upper sublittoral biofacies are dominant in the Minagawa Formation. This shallowing upward trend over a few million years was caused by uplift related to compressive tectonics within the northeastern Japanese archipelago. It is evident that at 1.4 Ma during the Pleistocene, cold water covered the lower sublittoral and upper bathyal environments in the Japan Sea.