The Chikapupetsu and Shii-Tokachigawa formations in the Tomuraushi area in central Hokkaido, northern Japan, consist mainly of alternating mudstone and discontinuous sandstone facies previously assigned to the Hidaka Supergroup. Here, we used detrital zircon U-Pb geochronology using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and microfossil occurrence to constrain the depositional ages of the formations. The youngest detrital zircon grains from a turbidite sandstone and an acidic tuff yielded maximum depositional ages of 22.5±0.7 and 19.5±0.1 Ma, respectively. In addition, the occurrence of diatom fossils indicates an early Miocene depositional age. Based on these depositional ages, the Chikapupetsu and Shii-Tokachigawa formations should be excluded from the Hidaka Supergroup, which ranges from Paleocene to early Eocene, and correlated instead with the upper members of the Tsubetsu and Kawakami groups. Previous research on the Tatsuushi and Erimo formations has shown that the Hidaka Belt has been active as a right-lateral strike-slip fault system since the late Oligocene. The Chikapupetsu and Shii-Tokachigawa formations are marine deposits that formed during the early Miocene between deposition of the late Oligocene Tatsuushi and Erimo formations and the middle Miocene Kamishiyubetsu Formation, while the right-lateral strike-slip fault system was active.

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 σ3 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 orientations of 76 sheet intrusions in northern Amakusa-Shimoshima, western Kyushu, were inverted to understand paleostress condition(s) in the backarc behind the junction of the SW Japan and Ryukyu arcs. The radiometric ages of intrusive rocks in the Amakusa area suggest that the sheet intrusions are 14-17 Ma, contemporaneous with the formation of the Japan Sea. The Eocene host formations are folded about a NNE-trending axis with an interlimb angle of ~140°, but the relative timing of the folding and magmatism is not constrained. The sheet intrusions have various orientations, but E-W-trending dikes are dominant. The tilt correction did not significantly affect the E-W-trending intrusions, because they meet the fold axis at high angles. Stress inversion using the mixed Bingham distribution yielded two stresses from in-situ and tilt-corrected orientations. The stresses calculated from the in-situ set of orientations appeared to fit better than those from the tilt-corrected set of orientations, because the stresses of the normal faulting and strike-slip faulting regimes from the in-situ set of orientations had nearly vertical stress axes. We thus suggest that magmatism postdated the folding.

Detrital zircon U-Pb ages were measured for nine non-fossiliferous sandstones from the uppermost Izumi Group, a “Late Cretaceous” forearc clastic sequence in the eastern Izumi Mountains of western Kii Peninsula, SW Japan. Seven out of nine sandstones yielded Paleocene grains. These results confirm that the uppermost Izumi Group was deposited in the Paleogene (Selandian-Thanetian or younger), extending the total depositional duration of the group to ca. 27 myr, almost double the previous estimate. The new age data raise the possibility that a stratigraphic interval across the Cretaceous-Paleogene extinction boundary may be preserved within the group. The occurrence of Paleogene strata further constrains the onset of the low-angle Median Tectonic Line to post-Paleocene.

Miocene dikes and sills are widely exposed in the eastern Shitara area, Aichi Prefecture, central Japan. Whole-rock chemical composition, grain size, lightness, and the alteration ratio of the intrusive rocks from three outcrops in the Shitara area are described in this paper. The central part of the intrusive rock bodies, commonly dark gray, gradually changes into grayish white toward its marginal part. The lightness of the polished surface of the rocks was measured using the hue, saturation, lightness (HSL) tool in the Inkscape graphics editor. At the white part, plagioclase crystals are fine-grained and severely replaced by secondary minerals such as calcite, whereas plagioclase, at the dark gray part, is much larger than that in the white part and less altered. Turning to chemical composition, common feature cannot be found among three outcrops.

Vitrinite reflectance R_o is commonly used as a paleotemperature proxy for rocks. Among various models used to reconstruct paleothermal histories, EASY%R_o, a set of ordinary differential equations to describe the maturation of vitrinite, has been widely used since 1990. Approximations of EASY%R_o solutions employing specific paleothermal histories have been used in the field of structural geology to estimate maximum paleotemperatures T_max. However, comparisons of those approximations with results obtained with EASY%R_o have not been reported in scientific literatures, and the errors on the approximations and the limitations of their use to determine T_max remain undocumented. Here, we performed such comparisons and found that the use of those approximations can lead to considerable deviations from the results obtained with EASY%R_o. We then derived new approximations of EASY%R_o solutions for four representative paleothermal histories that provided estimates of T_max within 3 ^°C of those obtained with EASY%R_o. We also describe the Jacobian of EASY%R_o, which can be used to propagate errors from R_o to T_max. We expect that the four approximations provided herein will be useful, although we recommend using EASY%R_o itself if greater precision or particular paleothermal histories other than those assumed in this study are required.