Southwest Japan Research Articles

Zircon U–Pb ages and Lu–Hf isotopes of the Jurassic Granites on the east coast of the Korean Peninsula and Southwest Japan: Petrogenesis and tectonic correlation between the Korean Peninsula and Japanese Islands

Jurassic granitoids in the Korean Peninsula and Japanese Islands are key for deciphering the tectonic evolution of East Asia and the tectonic correlation between them. Zircon U–Pb age dating of seven granitoids in Southwest Japan gives 247.3 ± 1.8 and 192–191 Ma for the Ebi Granites and 200.8 ± 2.1 and 184–183 Ma for the Hida Granites. On the east coast of the Korean Peninsula, 188–186 Ma, 183–177 Ma, 167.7 ± 1.5 Ma, and 173–166 Ma are obtained for the granitoids in the Gyeongsang Basin, Yeongnam Massif, Taebaeksan Basin, and Gyeonggi Massif, respectively. All granitoids show depletions in Nb, Ta, P, and Ti in whole-rock compositions, suggesting an arc environment. The studied granitoids can be divided into two types. The early-stage (∼201–183 Ma) granitoids from Southwest Japan and the Gyeongsang Basin are characterized by the absence of Precambrian inherited zircons, high εHf(t) values of +13.0 to −0.8, and higher crystallization temperatures of ∼750–830 °C. The later-stage (∼183–166 Ma) granitoids from the remaining areas have abundant Paleoproterozoic inherited zircons, low εHf(t) values of −13.9 to −25.0, and lower crystallization temperatures of ∼680–750 °C. The early-stage granitoids formed during a steep subduction regime under higher temperatures due to the heat supply from the asthenospheric mantle. In contrast, later-stage granitoids mostly have an adakitic nature and formed by the partial melting of garnet- and hornblende-bearing thickened Precambrian lower crust under lower temperature conditions during flat subduction. The Ebi and Hida Granites in Southwest Japan formed along the continental arc passing through the Gyeongsang Basin to the Dumangang Belt of the Korean Peninsula, indicating their paleopositions. A change in the subduction angle from steep to flat caused the migration of magmatism to the northwest or west throughout Northeast Asia, including Japan, Korea, and China.

Mantle deformation during opening of the Japan Sea back-arc: Insights from peridotite xenoliths, Kawashimo, southwest Japan

For a better understanding of plate dynamics such as the opening of Japan Sea back-arc basins, it is critical to know why and how back-arc spreading/rifting occurs adjacent to convergent plate boundaries. Two models have been proposed for the formation of Japan Sea back-arc basins: the “slab rollback model” and the “plume model”. We report here on 12 spinel peridotite xenoliths in the alkali basalt of the Cenozoic Kawashimo volcano in southwest Japan. These xenoliths are dominantly harzburgite with some lherzolite. The harzburgites and lherzolites show coarse-grained or porphyroclastic textures that record variable degrees of deformation. Olivine and spinel compositions indicate the samples are residual mantle peridotites formed under various degrees of partial melting. Olivine crystallographic preferred orientations in the xenoliths have orthorhombic patterns characterized by a strong [010] maximum, and this orthorhombic pattern implies that the olivine deformed by dislocation creep on the (010)[100] slip system. Using a sub-grain size piezometer, the maximum differential stress varies from 3 to 13 MPa in the samples. We also found that the peridotite xenoliths deformed under relatively high strain-rates, such as 10−13 to 10−10 s−1, using olivine flow laws and the obtained stress and grain size. The range of high strain-rates is comparable to that predicted using a thermomechanical model of back-arc spreading/rifting. Therefore, it is probable that the peridotite xenoliths preserve textures that formed during the mantle deformation that accompanied the Japan Sea back-arc spreading/rifting. We estimated an equilibrium temperature of 1238 ± 20 °C using the two-pyroxene geothermometer, and this temperature is higher than any temperature reported previously for peridotite xenoliths in southwest Japan. The obtained equilibrium P–T condition for the xenoliths implies that the hot mantle plume hypothesis best explains the mechanism of formation of Japan Sea back-arc basins.

Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin

During the middle Miocene, forearc alkaline magmatism occurred in the Outer Zone of Southwest Japan and the northern Ryukyu Arc, resulting in the formation of forearc alkaline basaltic rocks with ocean island basalt (OIB)-like chemical signatures. In contrast, subduction-related magmatism was present in the central Ryukyu Arc. However, the southwestern margin of the forearc alkaline magmatism was poorly constrained in the Ryukyu Arc. We examined two basaltic dikes in the Chichibu accretionary complex of Amami-Oshima Island, the central Ryukyu Arc. The dikes cut massive basalt, reddish chert, varicolored shale, and grey chert. The chemical compositions of basaltic dikes are characterized by the enrichment of incompatible trace elements, possibly representing a low degree of partial melting from a deep mantle source. 40Ar/39Ar dating analyses indicate that the basaltic dikes yield ages of 16.37 ± 0.14 Ma and 16.51 ± 0.10 Ma. The paleomagnetic analyses on stepwise thermal demagnetizations allowed extracting the direction of stable magnetizations with unblocking temperatures of 450–575 °C for the two dikes [(Dec, Inc) = (138.1°, − 13.3°), (124.0°, − 24.9°)]. The magnetization could be primary, acquired either as part of a secular variation or a geomagnetic excursion during reversed polarity chron/subchron. The 40Ar/39Ar ages and paleomagnetic directions within the error range imply that they intruded almost simultaneously during C5Cn.2r (16.532–16.434 Ma). The age and trace element patterns of the basaltic dikes are comparable to those of Miocene alkaline basaltic rocks, which resulted from forearc alkaline magmatism during the initial subduction of the young and warm Shikoku Basin. The discovery of alkaline basaltic dikes on Amami-Oshima Island suggests that the distribution of middle Miocene forearc alkaline magmatism may extend to the central Ryukyu Arc. Hence, the northern end of the Kyushu-Palau Ridge (i.e., southern end of Shikoku Basin) could have been located south of Amami-Oshima Island around 16.5–16.4 Ma, then moved eastward to the current location.Graphical

Infective endocarditis with systemic bacterial embolism caused by Staphylococcus aureus in a free-ranging Amami rabbit (Pentalagus furnessi)

The Amami rabbit (Pentalagus furnessi) is found only on the two islands of Amami-Ōshima and Tokunoshima in southwest Japan. It has a primitive appearance and ecology, is an evolutionarily valuable animal and has been assigned to the International Union for Conservation of Nature Red List of Threatened Species. We describe a case with mild purulent wounds on the distal digital skin of both forelimbs and multiple nodular lesions in various organs, including the heart and kidney. Microscopically, the heart lesions were characterized by disruption of the mitral valve and multifocal myocardial necrosis and abscesses due to infection with gram-positive cocci. Similar bacterial infarctions were also found in other organs, including the kidneys. The bacteria were identified as Staphylococcus aureus by immunohistochemical and molecular biological examinations. This first report of infective endocarditis and systemic infarctions caused by S.aureus in an Amami rabbit indicates the importance of monitoring purulent injuries, even if mild, to prevent secondary infections in this species.

Older magma at Aso caldera than at Unzen stratovolcano in south west Japan as recorded through helium isotopes

Stratovolcanoes form conical mountains by ejecta and lava, while caldera volcanoes have depressions caused by post caldera collapse. Few studies have investigated relationships between this morphology difference and discharged fluid geochemistry. Here we present helium isotope ratios of geothermal fluids and groundwater inside and outside the caldera of Mt. Aso, Japan, and compare these to helium ratios in groundwater at Mt. Unzen, a stratovolcano in the same tectonic field. We find that helium isotopes decrease in proportion to distance from the central part of the volcano. However, the helium isotopes of inferred magmas are lower in Mt. Aso relative to Mt. Unzen. We combine this result with seismic tomography data and suggest that Mt. Aso may not be receiving new mantle material to the magma reservoir. Thus, we conclude that the caldera volcano Mt Aso probably features older magma and more developed geothermal systems than the stratovolcano Mt. Unzen.

Zircon U-Pb ages of the Cretaceous gabbroic and granitic rocks from the Kajishima, northwest Shikoku, southwest Japan

Zircon U-Pb ages of the Cretaceous gabbroic and granitic rocks from the Kajishima, northwest Shikoku, southwest Japan

Marine osmium isotopic composition reconstructed from the early Permian umber deposit in the Japanese accretionary complex

• Re-Os isotopes of early Permian umbers in Japan were analyzed. • Oxidative Mn-oxides prevented Re fixation into the Kunimiyama umbers. • 187 Os/ 188 Os values of early Permian seawater ranged from 0.523 to 0.676. Strata-bound ferromanganese (Fe–Mn) deposits (umbers) are thought to have formed by precipitation of Fe–Mn oxyhydroxides derived from seafloor hydrothermal activity at a mid-oceanic ridge. Fe–Mn oxyhydroxides effectively adsorb various elements (e.g., P, V, As, rare earth elements, and Os) dissolved in seawater. Of these elements, osmium (Os) is particularly important for reconstructing the Earth’s surface environment during formation of the deposits. Here we report the Os and rhenium (Re) contents and marine Os isotopic ratio ( 187 Os/ 188 Os) i of the early Permian reconstructed from the Kunimiyama umber deposit in the Northern Chichibu Belt, central Shikoku, southwest Japan. The Kunimiyama umbers exhibit comparable to slightly lower Os (42–183 ppt) and much lower Re (1.3–49 ppt) contents than other umbers in Japanese accretionary complexes and modern seafloor metalliferous sediments, the latter of which are regarded as umber precursors. The ( 187 Os/ 188 Os) i ratio of the Kunimiyama umbers is 0.523–0.676, which is comparable to or slightly higher than the values of the middle to Late Cretaceous and early Paleogene. This work is the first report of seawater Os isotope ratios in the Panthalassa Ocean of the early Permian, representing an important achievement in understanding the marine Os isotopic record of the Paleozoic Era.

U-Pb zircon ages of metamorphic rocks and granitoids from the Nagato Tectonic Zone in Yamaguchi, southwest Japan: Implication for the geological correlation with the Kurosegawa Tectonic Belt

U-Pb zircon ages of metamorphic rocks and granitoids from the Nagato Tectonic Zone in Yamaguchi, southwest Japan: Implication for the geological correlation with the Kurosegawa Tectonic Belt

Frictional Behavior Downdip Along the Subduction Megathrust: Insights From Laboratory Experiments on Exhumed Samples at In Situ Conditions

AbstractSubduction megathrusts exhibit a spectrum of slip modes, including catastrophic earthquakes. Although the mechanical and frictional properties of materials sampled from subduction zones have been studied extensively, few datasets have been collected for compositions and at pressure and temperature conditions representative of those in situ. The Nankai subduction zone in southwest Japan is a well‐studied margin, and abundant data provide an opportunity to advance our understanding of fault and earthquake physics. Here, we use samples exhumed in the Shimanto and Sanbagawa Belts on Shikoku Island of southwest Japan that represent analogs for materials along the present‐day megathrust at depths of ∼5–>25 km, and we shear these at their peak in situ pressure‐temperature (P‐T) conditions. Effective normal stresses range from 28 to 192 MPa, and temperatures from 105°C to 470°C. We used pore fluid pressures of 45–240 MPa, corresponding to fluid overpressure ratios λ of 0.65 and 0.90. Slip velocities of 0.1–100 μm/s were used, in order to focus on the nucleation of instability and earthquakes. We found predominantly velocity‐strengthening (inherently stable) behavior under all conditions for λ = 0.65. For λ = 0.90, velocity‐weakening behavior was observed at 350°C, with velocity‐strengthening behavior at lower and higher temperatures. The rate/state frictional stability parameter (a‐b) increases with slip velocity at temperatures up to ∼200°C and remains constant or decreases with slip velocity at higher temperatures. Overall, our results demonstrate the potentially important roles of both temperature and slip velocity in controlling the distribution of stress and frictional rheology along subduction thrusts.

隠岐島後に分布する中新統久見層の放散虫化石層序

西南日本，島根県隠岐島後には中新統が広く分布しており，その中でも久見層は日本海拡大最初期の環境変遷を記録している可能性があることから重要である．本研究では久見層模式地に露出する久見層の放散虫化石分析を行った．その結果，久見層上部からはEucyrtidium inflatum帯a亜帯～Lychnocanoma magnacornuta帯下部が認められ，久見層下部からはMelittosphaera magnaporulosa帯が認められた．さらに，久見層最下部から下部にかけてPentactinosphaera hokurikuensisとCyrtocapsella tetraperaの2種によって構成されるP-C群集が産出した．この群集は非海水成層の直上でのみ確認されることから，日本海拡大最初期の特異的な海洋環境を指標する群集である可能性を指摘した．また，珪藻化石および放散虫化石に基づいて日本海側の中新統と対比を行った結果，久見層下部の群集は奥尻島の釣懸層と最も類似していると考えられる．

Export and dispersal of coastal macrophyte-derived organic matter to deep offshore sediment around the Tokara and Yaeyama Islands, southwest Japan: Evaluation using quantitative DNA probing techniques

Export and dispersal of coastal macrophyte-derived organic matter to deep offshore sediment around the Tokara and Yaeyama Islands, southwest Japan: Evaluation using quantitative DNA probing techniques

Groundwaters and deep-seated fluid circulation around Aso Volcano, Southwest Japan, revealed by multivariate statistical analysis of the geochemical data

To understand the fluid circulation under volcanoes and their surrounding area, geochemical compositions of 590 groundwater and spring/river water samples from the central Kyusyu in SW Japan, including hot and mineral spring waters around Aso and Kuju Volcanoes, have been analyzed for 12 major solutes as well as oxygen‑hydrogen and helium isotopic ratios. To extract independent processes and sources involved in formation of the groundwaters, whitened data-based k-means cluster analysis (KCA) has been applied to the data set of 12 major solutes. Eight clusters have been identified by KCA, which were mapped geographically and were compared with the isotopic compositions, temperature, and pH that were not included in KCA. By the geographical mapping, a concentric zonal structure of the clusters emerges in the Aso caldera, which resulted broadly from two fluid cycling systems from caldera rim and central cones. In the entire study area including Aso and Kuju Volcanoes, the mapping also shows geographical provenance, and the distribution of each cluster shows characteristics related to geological features, such as upwelling of deep-seated fluids along the prominent tectonic lines. On the other hand, the Piper diagram-based classification is significantly different from the KCA results and exhibits neither a concentric zonal structure within the Aso caldera nor a regional geographical provenance. These results suggest that multivariate statistical analysis such as KCA is potentially useful for investigating the origins of groundwaters and shows the advantages over classical Piper diagram-based evaluations.

Detrital zircon U–Pb ages and geochemistry of Devonian–Carboniferous sandstones and volcanic rocks of the Hida Gaien belt, Southwest Japan: Provenance reveals a Gondwanan lineage for the early Paleozoic tectonic evolution of proto-Japan

Devonian–Carboniferous strata in the Hida Gaien belt, Southwest Japan, are characterized by a change in the dominant lithology from clastic rocks and limestones to volcanic rocks. The sandstone compositions and detrital zircon U–Pb ages indicate that the Devonian sedimentary rocks were derived from various basement rocks, including granite and basalt, that were uplifted, exposed, and eroded during the Early–Middle Devonian (398–383 Ma). The source of sediment to these strata changed to basalts and rhyolites in the early Carboniferous (∼346 Ma). Early Carboniferous bimodal volcanic rocks are present in the Hida Gaien and South Kitakami belts, the southeastern margin of the South China block (Hainan), and the Jilin area (NE China) of the eastern Central Asian Orogenic Belt (CAOB). The Cambrian–Silurian zircon grains (540–480 Ma and 460–420 Ma) in the Hida Gaien belt were derived from arc basement of a similar age to the South Kitakami and Kurosegawa belts, the Jiamusi–Khanka–Bureya (NE China and the Russian Far East) and Songliao–Xilinhot (Inner Mongolia) blocks in the eastern CAOB, and the eastern South China and Indochina blocks. The Devonian strata also contain Precambrian zircon grains derived from Mesoarchean basement (∼3000 Ma) and the Grenvillian tectonothermal event (∼1000 Ma), suggesting sediment supply from NE Gondwana (the Thomson and Lachlan orogens of eastern Australia) or related continental remnants. Detrital zircon U–Pb age spectra similar to those of the Devonian strata of the Hida Gaien belt are also observed in Silurian–Devonian metasedimentary rocks in the Jilin area. During the early Paleozoic, the Hida Gaien belt, the eastern margin of the South China block, and the magmatic arcs and microcontinents in the eastern CAOB were located close to NE Gondwana along the western margin of the Paleo-Pacific Ocean.

Anelastic deformation in the lower crust and its implications for tectonic dynamics in Kyushu, Southwest Japan

Most of the seismicity in inland Kyushu, southwest Japan occur in the localized intraplate shear zones which are Beppu-Shimabara Graben (BSG) and left-lateral shear zone (LSZ). One of the largest fault systems in Japan (Median Tectonic Line—MTL), and subduction zone of the Philippine Sea Plate also exist near Kyushu. These suggest that the island arc, such as Kyushu has a complex deformation process. This study estimated the anelastic strain rate in the lower crust and the interplate coupling rate via inversion analysis using Global Navigation Satellite System data to elucidate the seismotectonics. Deformation in the lower crust and interplate coupling were modeled by cuboid volumetric sources and a back slip model, respectively. To stabilize the solution, a L2 regularization constraint was applied to the anelastic strain rate. Further, constraints on direction and smoothing to the slip deficit rate were applied. These strength of the constraints were determined via the minimum value of the Akaike Bayesian information criterion. The analyses showed that the anelastic strain rate of the BSG and LSZ was ∼0.6 ×10−6 yr−1, and BSG was related to MTL activity. The findings also suggest that this anelastic deformation in the lower crust created a stress concentration zone in the upper crust of BSG and LSZ, with a stress change rate of ∼6–10 kPa·yr−1. The value is consistent with the stress drop and recurrence interval of regional earthquakes, suggesting that the seismicity around BSG and LSZ is strongly influenced by anelastic deformation in the lower crust. It is also suggested that BSG and LSZ development is influenced by slab rollback, and a subducting ridge. Lastly, a comparison between the deviatoric stress field estimated using focal mechanisms and the anelastic strain rate suggests the existence of a shear zone consisting of weak faults south of BSG.

Forecasting tectonic tremor activity using a renewal process model

In many tectonically active regions of the world, a variety of slow deformation phenomena have been discovered and collectively termed slow earthquakes. Tectonic tremor is the high-frequency component of slow earthquakes and can be analyzed to monitor the overall slow deformation process, both spatially and temporally. Although tectonic tremor activity is complex, it does possess some characteristic patterns, such as spatial segmentation, a quasi-periodic recurrence, migration, and tidal modulation. These features are helpful for forecasting future activity if they are properly modeled in a quantitative manner. Here, we propose a stochastic renewal process to standardize and forecast tectonic tremor activity in the Nankai subduction zone, southwest Japan, using a 12.5-year tremor catalog that is divided into a 10-year estimation period and 2.5-year forecasting period. We group the tremor events into small rectangular 10-km regions and observe that the distribution of inter-event times is nearly bimodal, with the short and long inter-event times representing the characteristic times of nearby tremor interactions and long-term stress accumulation processes, respectively. Therefore, as the probabilistic distribution for the renewal process, we adopt a mixture distribution of log-normal and Brownian passage time distributions for the short and long inter-event times, respectively. The model parameters are successfully estimated for 72% of the entire tremor zone using a maximum likelihood method. This standard model can be used to extract anomalous tremor activity, such as that associated with long-term slow-slip events. We derive a scaling relationship between two characteristic times, the relative plate motion, episodicity of tremor activity, and tremor duration by characterizing the spatial differences in tremor activity. We confirm that the model can forecast the occurrence of the next tremor event at a given reference time for a certain prediction interval. This study can serve as a first step for implementing more complex models to improve the space–time forecasting of slow earthquakes.

Re-examination of the age of brackish-water deposit in the Miocene Tottori Group, Southwest Japan

Re-examination of the age of brackish-water deposit in the Miocene Tottori Group, Southwest Japan

Microscale evolution of reefal microbialites

Reefal microbialites are crusts and infills of microbialites developed within primary cavities of late Quaternary coralgal reef frameworks. In spite of many previous studies, factors controlling mesofabric transitions, particularly from laminated to digitate microbialites but as well as the microscale formation process in relation to microbial communities, have not yet been fully understood. Furthermore, there are still controversies regarding age differences between reef framework growth and microbialite formation. The authors have discussed the macroscale evolution of primary cavities as well as the microscale evolution of reefal microbialites obtained in a middle Holocene reef core drilled off Okinawa Island, south-west Japan, based on petrographic observations using light and scanning electron microscopy with energy dispersive X-ray spectroscopy and radiocarbon (14C) ages of corals and reefal microbialites. The authors’ petrographic observations show that coralgal reef framework growth was followed by encrusting foraminifers and metazoans, coevally with macro-bioerosion and microbioerosion of coralgal frameworks, then finally infilled with an intraskeletal and boring-filling microbialite (IBFM) and encrusted by reefal microbial crusts (RMCs), including structureless and digitate microbialites. The observed macroscale evolution was probably driven in response to progressive changes to reduced light and water circulation in the primary cavities associated with middle Holocene transgression and reef formation. The microfabric of reefal microbialites consists of primary in situ precipitated peloids (spherical micritic clots), voids (primary pores) and allochthonous detrital grains (bioclastic and siliciclastic) fallen from upper reef surfaces. IBFM geopetally infilled inside bioeroded cavities soon after the bioerosion of coralgal frameworks. RMCs have a two-layered microfabric succession, which is composed of the alternation of dense peloidal aggregates and laterally aligned voids in the lower part and the mixing of peloidal aggregates and irregularly and vertically developed voids in the upper part. The authors speculate that the repeated occurrences of laterally aligned voids in the lower part may imply the intermittent pauses of peloidal formation, while peloids accumulated on the irregular outer surfaces may result in the formation of vertically developed voids and digitate surfaces in the upper part. These peloids are probably formed by coccoid sulfate-reducing bacteria in anoxic environments developed by the degradation of bacterial biofilms and organic matters, allowing sulfate reduction and increasing alkalinity, therefore carbonate precipitation. The two-layered microfabric succession generally observed in RMCs may reflect spatial-temporal oxic/anoxic variations in interstitial water and the degree of sulfate ion supply by the gradual closing of primary cavities.

Why does Tetranychus evansi not threaten solanaceous vegetables in Japan?

The tomato red spider mite, Tetranychus evansi, was first found in Japan in Osaka and Kyoto, in 2001 (Ehara and Ohashi 2002). Since then, it has spread rapidly throughout southwest Japan from Kanto district (central Japan) to Okinawa (southernmost end of Japan) on solanaceous vegetables such as tomato and eggplant as well as solanaceous weeds such as Solanum carolinensis and S. nigrum. Although T. evansi frequently invades commercial fields of solanaceous vegetables, this species does not establish and is never an important pest nor a target of pest control. On the other hand, it is still a severe pest in southern Europe and Africa. Why does T. evansi not threaten solanaceous vegetables in Japan?

Fine divided method of active tectonic blocks based on adaptive density peak clustering and improved Euler model, revealed by GNSS velocities in Southwest Japan

The rational division of active tectonic blocks is an important premise for distinguishing active faults, discovering hidden faults, and an in-depth understanding of regional crustal deformation characteristics, seismicity, and tectonic dynamics. In recent years, clustering algorithms have been widely used in active block division owing to their high efficiency and reliability. However, the classical clustering algorithm sensitivity to the block boundary under different conditions is inconsistent. Additionally, the Euler model constraint (EMC) process involved in the classical clustering algorithm ignores the influence of the boundary points and gross error points of the active block. Therefore, in this study, a new active block division method based on adaptive density peak clustering (ADPC) and an improved Euler model constraint (IEMC) is proposed. The simulation test results show that compared with the traditional clustering algorithm, the mean error rate of the new method can be reduced by up to 40% in the simple single-fault model and by 10% in the complex three-segment model. The new method was further applied to an actual case of an active block division in southwest Japan. The results further show that the new method is less affected by the regional environmental conditions. Compared with previous results in southwest Japan, the proposed method has higher accuracy and higher reliability of the active block division results. The new method for active block division proposed in this study can be used to accurately explore the location and quantity of block boundaries.

Revision of Four Species of Turbo (Marmarostoma) (Gastropoda: Turbinidae) from the Lower-Middle Miocene of Japan

The turbinid gastropod species Turbo (Lunella) ozawai Otuka, 1938 was described based on two opercula from the lower to middle Miocene Korematsu Formation in Southwest Japan. This inadequate proposal of the new species without the shell character information has confused subsequent taxonomic and faunal studies on the early to middle Miocene species of Turbo in Japan. For the first time, we clarify the shell characters of T. (M.) ozawai based on the type series and newly obtained materials from the Korematsu Formation in the Shobara area, Hiroshima Prefecture, and the Yoshino Formation in the Tsuyama area, Okayama Prefecture, Southwest Japan. The clarification of the shell and opercular characters of T. (M.) ozawai has enabled a taxonomic revision of the previously described four nominal species from early to middle Miocene species of Turbo (Marmarostoma) from the Honshu Arc of the Japanese Islands. Consequently, these four species are reclassified into two species, T. (M.) ozawai and T. (M.) tochiyensis Kanno, 1958; T. (M.) parvuloides Nomura, 1940 and T. (M.) minoensis Itoigawa, 1960 are junior synonyms of T. (M.) ozawai and T. (M.) tochiyensis, respectively. Both species are rare examples of mollusks that thrived along the Honshu Arc throughout the Miocene Climatic Optimum (∼16.9 to 14.7 Ma). The species richness of Turbo (Marmarostoma) is similar to that of the modern species in the warm water region of the Japanese Islands.