The Takada Granodiorite, in the central part of the San’in batholith in southwest Japan, is thought to have undergone prolonged magmatic activity based on zircon U–Pb dating. The pluton consists mainly of hornblende–biotite granodiorite that shows textural evidence for mingling with mafic magma (Ogi Quartz diorite). Oscillatory zoning in minerals (e.g., anorthite content in plagioclase, Zr content in titanite, and Ba content in alkali feldspar) and remelting textures are common in the Takada Granodiorite. Hornblende and titanite geothermobarometers indicate emplacement pressures of 180–200 MPa with temperature fluctuations near the solidus. Moreover, whole-rock Rb–Sr isotopic compositions did not form a well-defined isochron. These observations suggest multiple injections of granitic magma with isotopically heterogeneous source material in addition to mafic magma, which account for the prolonged magmatic activity that produced the Takada Granodiorite.

This study updates the mid-Cretaceous (100 Ma) paleomagnetic pole for the inner arc of the southwestern Japanese islands, referred to as southwest Japan, to investigate potential stagnation in its apparent polar wander path (APWP). The updated pole interpolates the Cretaceous APWP for southwest Japan, providing insights into the geodynamic characteristics of the region during this period. For paleomagnetic analysis, red mudstone samples were collected from four sites in the 100 Ma Hayama Formation in the Hayama area. A high-temperature remanent magnetization component, carried by hematite, was isolated from the four sites, passing the fold test. The primary nature of the magnetization was confirmed by the detrital origin of the magnetic carrier. The primary directions, combined with previously reported results, were used to define a paleomagnetic pole representing the 100 Ma pole for southwest Japan, located within the group of Cretaceous poles. The APWP for southwest Japan between 110 and 70 Ma was revised to evaluate stagnation in polar motion in this region.

Zircon double-dating (U–Pb and (U–Th)/He) was performed on a tephra in the Pleistocene Chiraigawa Formation, Oshamambe district (Kuromatsunai lowland), southwestern Hokkaido. The obtained ages were 1.035 ± 0.048 Ma and 1.030 ± 0.054 Ma, respectively, which are mutually consistent. These results indicate that the zircons from this tephra crystallized in a magma at ca. 1.0 Ma and an eruption producing the tephra happened instantly. The applied methodology appears to be robust for dating Quaternary volcanic activity, similarly to another zircon double-dating (U–Pb and fission-track) approach. Furthermore, U–Pb dating of a tephra in the Pleistocene Setana Formation yielded an age of 1.063 ± 0.037 Ma. These results indicate that the depositional age of the Setana Formation is in close agreement with previous estimates, but that the Chiraigawa Formation, which was previously assumed to be Middle Pleistocene, is Early Pleistocene. The ages also indicate that the Setana and Chiraigawa formations were deposited in a very short time interval.

Cretaceous–early Cenozoic tectonic and slab–mantle processes in Far East Asia have been investigated mainly using granitic rocks. However, most granitic magmas are generated by assimilation and/or partial melting of crust. Therefore, it is important to use mafic igneous rock chemistry for investigating such processes in this period, because these rocks are less crustally contaminated than granitic rocks. A core of >4000 m in length was obtained from Moneron Island, southwest of Sakhalin Island, Far East Asia. The core consists mainly of Lower and Upper Cretaceous volcanic rocks, with basaltic–andesitic compositions. Whole-rock K–Ar ages of 101.9 ± 2.3 and 98.2 ± 2.2 Ma were obtained for the Lower Cretaceous rocks. Based on the location and eruption age, the Cretaceous volcanic rocks from Moneron Island belong to the Rebun–Kabato zone in Hokkaido. We also determined the mineral chemistry, and whole-rock major and trace element and Sr–Nd–Hf isotopic compositions of these rocks. The Moneron volcanic rocks have subduction-related compositional features with similar HFSE contents as N-MORB. Furthermore, these rocks exhibit a decreasing trend of Th/Yb and La/Nb, which are signatures of a slab-derived component, in the shallow part of the core. The Nd–Hf isotopic compositions also suggest a decreasing flux of slab-derived sediment from the subducting Izanagi Plate. In the western Pacific, recent studies have proposed that subduction of the Farallon–Izanagi Ridge occurred at ca. 127 Ma, a volcanic hiatus occurred at 56–46 Ma due to subduction of the Izanagi–Pacific Ridge, and depletion of the mantle occurred after 46 Ma, based mainly on zircons from granitic rocks. However, our results imply that the Cretaceous mantle wedge beneath the study area was already depleted at ca. 100 Ma.

The “Upper Miocene” Mitoku Formation in the south–central part of Tottori Prefecture, southwest Honshu, Japan, is a stratigraphic type of Mitoku-type flora, which is thought to represent the origins of the modern temperate forest vegetation on the islands of Japan. In this study, zircon U–Pb dating and palynostratigraphic analyses were undertaken to determine the age of the Mitoku Formation at the Naru site, north of the famous Mitoku-san historic site and place of scenic beauty, where Mitoku-type Misasa–Naru flora have been previously reported. The ages obtained for the pumice lapilli–tuff and tuff beds, which are intercalated with plant-fossil-bearing beds at the Naru site, are ca. 4.7 Ma (mid-Early Pliocene). The preliminary palynological results reveal that the flora comprise both cool and warm temperate elements, with the common and continuous occurrence of Cathaya pollen, which can be correlated with the NP-5 Neogene palynostratigraphic zonation (ca. 5.5–3.0 Ma). Warm elements in the Misasa–Naru flora were once considered to be remnants of mid-Miocene flora. However, our results show that the flora represent a warm temperate climate during the Early Pliocene, and might necessitate revision of the sequential changes in the Mitoku-type floras in this area, and even to the regional stratigraphy.

Despite the recent diversification of Tenthredinid sawflies in Japan, fossil records of this family are notably lacking. Here, we describe specimens belonging to this family from the Chibanian (Middle Pleistocene) lacustrine deposits of the Shiobara Group. These specimens are identified as Tenthredinidae gen. et sp. indet A and B and Nematinae gen. et sp. indet. This study represents the first report of fossil tenthredinids in the Shiobara Group and also an incontrovertible Cenozoic fossil record of this family from Japan.

Determining the depositional ages of the Early–Middle Miocene forearc basin deposits in southwestern Japan is essential for understanding the Neogene deformation and stress history of the forearc region. To constrain the depositional age of the Tanabe Group, an Early–Middle Miocene forearc basin sedimentary sequence, we analyzed the U–Pb ages of detrital zircon separated from eight sedimentary rock samples, and obtained 221 concordant ages from 330 zircon grains. The age of the youngest grain is 19.4 ± 0.6 Ma, which constrains an upper limit for the depositional age. However, the youngest grain is much older than a foraminiferal age (16.3–15.1 Ma) for the deposition of the group. The absence of zircon ages corresponding to the widespread mid-Miocene felsic volcanic activities on Kii Peninsula at ~15 Ma suggests that the deposition of the Tanabe Group ended before these volcanic events. The discrepancy between the youngest zircon age and depositional age can be attributed to volcanic quiescence in the provenance area during the deposition of the Tanabe Group. We also suggest that the E–W tensional stress recorded by synsedimentary clastic dikes was exerted before ca. 15 Ma in the southern part of Kii Peninsula.

The coastal lowlands of Japan are mostly covered by unconsolidated sediments deposited since the last glacial maximum (LGM). The sedimentary facies and physical properties of post-LGM deposits filling small incised valleys with catchments in upland areas are distinct from those of the major alluvial lowlands, due mainly to a low rate of sediment supply. We studied two borehole cores (GS-SUM-1 and GS-SMS-1) to investigate the depositional environments, facies, and physical properties of the post-LGM fills in a small incised valley in the Shibakawa Lowland, one of the largest valleys in the Ōmiya Upland in Saitama Prefecture, central Japan. Both cores consist of Pleistocene deposits, post-LGM deposits, and artificial fill in an ascending order. Post-LGM deposits in the two cores consist of fluvial, inner-bay to brackish–fresh water, and fresh-water swamp deposits. The inner-bay deposits are mainly mud with little to no sand and yield very low S-wave velocities (~100 m/s). We reconstructed the post-LGM depositional environments in the Shibakawa Lowland using the sedimentary facies and radiocarbon ages. After 9 ka, seawater entered the incised valley. The inner bay extended furthest upstream at ~6 ka, forming a ravinement surface. The valley floor transformed into a freshwater swamp by 4 ka. In addition to the low rate of sediment supply due to the small catchment area, the progradation of deltaic to fluvial deposits in the Arakawa Lowland downstream blocked the valley outlets at the time of the highstand, resulting in the deposition of very muddy soft sediments in the Shibakawa Lowland. On the margins of the uplands, similar soft post-LGM deposits filling the incised valleys are likely to be present beneath the alluvial plain; consequently, attention should be paid to the ground strength in such areas.

We propose the name ‘Obaneo Lava’ for Pliocene mafic volcanic rocks at Cape Haneobana, northeastern Tottori Prefecture. We undertook a geological survey of these rocks, measured their whole-rock major element compositions, and subjected them to plagioclase K‒Ar dating. The mafic volcanic rocks comprise at least three lava flows of clinopyroxene–olivine basalt and olivine basalt. Their chemical composition indicates they are alkali basalt. Their uniform compositions and the lack of evidence for dormancy indicate that they were derived from continuous volcanic activity. The plagioclase K‒Ar age of 4.28±0.11 Ma for the upper lava is older than the ages of surrounding Pliocene volcanic units (i.e., the Inabayama Basalt and rocks of Hamasaka Volcano). The Obaneo Lava is distinguished from these surrounding rocks on the basis of chemical composition and radiometric age.

To increase the number of radiometric dates of Jurassic accretionary complexes in Japan, we conducted zircon U–Pb dating of a sandstone sample from the Sawando Complex in the eastern Mino Terrane using a multi-collector inductively coupled plasma mass spectrometer coupled with a multi-spot femtosecond laser ablation system (msfsLA-MC-ICP-MS). Thirty two concordant dates from 72 analyzed grains are divided into seven groups: 1920–1850 Ma (Paleoproterozoic), 1110 Ma (Mesoproterozoic), 492 Ma (Cambrian), 442 Ma (Silurian), 262–260 Ma (Permian), 250–218 Ma (Triassic), and 186–168 Ma (Jurassic). The youngest single grain date (YSG) is 168.0±6.9 Ma (±2σ). The YSG is within uncertainty of radiolarian ages reported in previous studies (late Callovian to middle Oxfordian).

The depositional age and sedimentary environment of the Pleistocene succession beneath the Tokushima Plain in southwestern Japan remain poorly understood, even though it includes key marker beds and age constraints that are crucial for understanding the history of the basin. We identified brackish–marine beds and constructed the chronostratigraphy of the subsurface Pleistocene strata using analyses of tephra layers, the electric conductivity (EC) and pH of muddy sediments, and diatom and pollen fossils in borehole cores from the southeastern Tokushima Plain. We correlated four tephra layers to the G9L , Naruohama-IV (Nh-IV), Kakuto (Kkt), and Ata-Torihama (Ata-Th) tephra layers, based on major and trace element compositions and the refractive index values of glass shards. High EC, brackish–marine diatoms, and pollen assemblages highlighted five brackish–marine beds, which were deposited during highstands in marine isotope stages (MISs) 11 to 5e. The marine beds corresponding to MISs 11, 9, and 5e (or 7) occur in subsurface Pleistocene strata in the northern part of the plain, and thus provide age constraints on the evolution of the Tokushima Plain.

Sub-bottom profilers (SBPs) are essential tools for investigating geological structures beneath sea or lake floors. SBPs are widely deployed because they do not interfere with other onboard survey methods such as sediment sampling and seismic profiling. However, SBP data are underutilized because they are stored in multiple different proprietary formats and commercial software is expensive. To improve this situation, we initially targeted a SyQwest Bathy-2010 installed on the R/V Hakuho Maru (JAMSTEC) and developed a procedure for SBP data visualization. This note aims to be a practical manual, with step-by-step guidance and screenshots, allowing interested users to easily process SBP data. The procedure involves three key steps: onboard data conversion, running a Python script, and creation of an index map. To ensure accessibility, the workflow has been tailored for free platforms, including Google Colab, enabling users with limited technical resources or programming experience to process SBP data effectively. We hope this effort democratizes the use of SBP data and facilitates its broader application in marine geological research.