Western Kii Peninsula Research Articles

Complex Interaction, Striations, and Periodicity of Deep Tremor Surrounding a Tremor Gap in the Central Nankai Subduction Zone, Japan

AbstractThe Nankai subduction zone is characterized by a belt of deep tremor extending from western Shikoku to the Tokai region and a tremor seismicity gap beneath the Kii Channel between Shikoku and the Kii Peninsula. Tremor activities near the gap are not well understood because of poor seismic network coverage. In this study, we conducted an observation campaign in eastern Shikoku to investigate the detailed tremor activity across the tremor gap. The new tremor catalog shows high detectability and resolution, with tremor clusters in both eastern Shikoku and the western Kii Peninsula having distinct shapes. We discovered characteristic time intervals between tremor activity on both sides of the gap, suggesting the occurrence of undetected slow slip events (SSEs) within the gap that propagated approximately 10 km/day. We also found a relationship between the tremor activities and a shallower long‐term SSE. Our examination of the detailed distribution of tremor events showed a distinct striation pattern that may be associated with the history of the subducting plate motion and the plate geometry, and our examination of their periodicity showed spatial variations in the dominant period of tremor activity that likely reflect the heterogeneous physical properties of the plate interface.

紀伊半島西部の三波川帯・中央構造線・領家帯

紀伊半島西部の三波川帯・中央構造線・領家帯

High‐Density Seismic Refraction Imaging of Plate‐Boundary Structures in the Slow Earthquake Gap Zone off Western Kii Peninsula, Nankai Trough

AbstractDespite recent observations of slow earthquakes along the Nankai subduction zone, none have been reported in the central Nankai Trough between the Kii Channel and Cape Shionomisaki. In November 2018, a very dense array of 96 ocean‐bottom seismometers were deployed by JAMSTEC to acquire active‐source seismic refraction dataset (supplemented by a multichannel seismic reflection profile) from the seaward side of the subduction trough to the accretionary prism off Cape Shionomisaki. We applied traveltime tomography to the refraction data to constrain the P wave velocity down to the upper mantle, coordinating with a migrated seismic reflection profile to confirm the depth of the Moho and interpret shallower structural features. From a comparison with a transect across the Kumano basin, we conclude that structural and physical differences between these two locations, especially the geometry of the subducting plate surface, lead to different slow earthquake activities.

Special Edition: Geological outline in the western Kii Peninsula, Japan and excursion guide

Special Edition: Geological outline in the western Kii Peninsula, Japan and excursion guide

Finding Paleogene beds in the uppermost Izumi Group in western Kii Peninsula, SW Japan

Finding Paleogene beds in the uppermost Izumi Group in western Kii Peninsula, SW Japan

Bi-Directional Movement of Deer between Tomogashima Islands and the Western Part of the Kii Peninsula, Japan, with Special Reference to Hybridization between the Japanese Sika Deer (Cervus nippon centralis) and the Introduced Exotic Deer

Deer have been artificially introduced into many countries that are not within their native range. In some cases, they have disturbed the natural ecosystem in the invaded area. On Okinoshima Island in Wakayama Prefecture, Japan, exotic deer were introduced from Taiwan in 1955 and have since bred in the wild, raising concerns about their invasion into the habitats of, and hybridization with, the native Japanese sika deer. To determine the extent of gene introgression, we investigated the population structure of deer on Okinoshima Island and the surrounding areas, including the islands of Awajishima and Jinoshima, and the mainland area in the western Kii Peninsula. Genetic analyses using microsatellite and mitochondrial DNA markers revealed that both exotic deer and Japanese sika deer inhabited Jinoshima Island. On the other hand, one of the individuals captured on the mainland was a first-generation hybrid between the exotic and native deer. These results suggest the bi-directional movement of the exotic and native deer between the islands and the mainland, with the potential for hybridization.

紀伊半島西部，松原地域に分布する上部白亜系の層序とフィッション・トラック年代

紀伊半島西部，松原地域に分布する上部白亜系の層序とフィッション・トラック年代

四国東部および紀伊半島西部の三波川帯の苦鉄質・超苦鉄質貫入岩について : (その1)四国東部の三成川帯の苦鉄質・超苦鉄質貫入岩について

四国東部および紀伊半島西部の三波川帯の苦鉄質・超苦鉄質貫入岩について : (その1)四国東部の三成川帯の苦鉄質・超苦鉄質貫入岩について

Thermal structure of the Kebara Formation and its proximal areas in the western Kii Peninsula, SW Japan: Application of the carbonaceous material Raman geothermometry

Thermal structure of the Kebara Formation and its proximal areas in the western Kii Peninsula, SW Japan: Application of the carbonaceous material Raman geothermometry

Mid-Paleozoic arc granitoids in SW Japan with Neoproterozoic xenocrysts from South China: New zircon U–Pb ages by LA-ICP-MS

The Kurosegawa belt in SW Japan preserves fragments of Early–Middle Paleozoic granitoids traditionally called the Mitaki igneous rocks and previously dated ca. 470–435Ma by several isotope analyses such as Rb–Sr, K–Ar and U–Pb, together with high-grade metamorphic rocks and Silurian strata, in a narrow belt. However, the timing of the oldest arc-related plutonism in Japan is constrained by laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U−Pb ages of zircon from Mitaki granitoids in 3 areas in SW Japan; i.e. Kyushu, Shikoku, and Kii Peninsula. Weighted mean U–Pb ages of these zircons are 431.8±3.9 for a granodiorite from the Mt. Mitaki area in Shikoku, 444.1±5.8Ma for a granodiorite from Kuraoka area in central Kyushu, and 444.5±7.6Ma for a quartzdiorite from the Nabaenohana area in western Kii peninsula, respectively. These ages confirm that the Mitaki igneous rocks have ca. 445–435Ma (late Ordovician to mid-Silurian) ages. They are some of the oldest subduction-related plutonic rocks in SW Japan. Particularly noteworthy is a “tonalite” from the Nabaenohana area, which has a unique spectrum of zircon U–Pb ages with distinct 3 clusters; ca. 700–500Ma (Neoproterozoic–Cambrian), ca. 1350–830Ma (Meo–Neoproterozoic), and ca. 3230–1560Ma (Paleoarchean–Mesoproterozoic). Of the 44 dated zircon grains, the youngest (possibly xenocrystic) grains are ca. 500Ma. Older zircon grains with ages >500Ma range up to 3230Ma and are interpreted as inherited xenocrysts in the “tonalite”. The Mitaki igneous rocks are interpreted to have been derived by melting of post-500Ma terrigenous sedimentary rocks that yielded a unique tonalitic S-type granitoid magma. From the presence of abundant 1350–700Ma (Meso- to Neoproterozoic) zircon grains in the “tonalite” we conclude that during the Early Paleozoic, proto-Japan was located close to the Cathaysian margin of South China.

High-pressure garnet amphibolite from the Funaokayama unit, western Kii Peninsula and the extent of eclogite facies metamorphism in the Sanbagawa belt

Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ∼ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The inferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ∼ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.

Sector-zoned aegirine in Sanbagawa quartz schist from the western Kii Peninsula, central Japan

Sector-zoned aegirine in Sanbagawa quartz schist from the western Kii Peninsula, central Japan

紀伊半島西部，中央構造線断層破砕帯から産出した花粉・胞子化石

紀伊半島西部，中央構造線断層破砕帯から産出した花粉・胞子化石

紀伊半島西部，松原地域に分布する秩父累帯上部白亜系の層序とフィッション・トラック年代

紀伊半島西部，松原地域に分布する秩父累帯上部白亜系の層序とフィッション・トラック年代

Re–Os geochronology of the Iimori Besshi-type massive sulfide deposit in the Sanbagawa metamorphic belt, Japan

Besshi-type Cu deposits are strata-bound volcanogenic massive sulfide deposits usually associated with mafic volcanic rocks or their metamorphic equivalents. Although there are numerous Besshi-type deposits in the Sanbagawa metamorphic belt, Japan, their tectonic settings and depositional environments remain controversial because of a lack of depositional age constraints. We report Re–Os data for the Iimori deposit, one of the largest Besshi-type deposits in western Kii Peninsula, in order to examine the robustness of the Re–Os isotope system for dating sulfide minerals in the high-P/T metamorphic belt and to elucidate the primary depositional environment of the Iimori sulfide ores. An 11-point Re–Os isochron plot yields an age of 156.8 ± 3.6 Ma. As this Re–Os isochron age is significantly older than the timing of the Sanbagawa peak metamorphism (110–120 or ∼90 Ma) and a well-defined isochron was obtained, the Re–Os age determined here is most likely the primary depositional age. Despite high-grade metamorphism at up to 520 ± 25 °C and 8–9.5 kbar, the Re–Os isotope system of the Iimori sulfides was not disturbed. Hence, we consider that the whole-rock Re–Os closure temperature for the Iimori sulfide ores was probably higher than 500 °C. As the accretion age of the Sanbagawa metamorphic belt is considered to be 120–130 or 65–90 Ma on the basis of radiolarian and radiometric ages, we estimated the time from the Iimori sulfide deposition on the paleo-seafloor to its accretion at the convergent plate boundary to be greater than 25 Myr. Consequently, the depositional environment of the Iimori sulfide ores was not in the marginal sea, but was truly pelagic.

砂質片岩中のジルコンの年代分布に基づく三波川帯再区分の試み

We measured the 206Pb/238U age distribution of detrital zircons in five psammitic schist samples from the Sanbagawa Belt in east-central Shikoku and the western Kii Peninsula to constrain their depositional age. The age-distribution diagrams for the five psammitic schist samples all show that detrital zircons of 100 to 90 Ma are most abundant and the age of the youngest zircon in each sample is less than 80 Ma. Considering the age of the retrogressive metamorphism of these psammitic schists, ca. 80-60 Ma, the protoliths age of the psammitic schists is constrained to 75-70 Ma, correlative to the age of the sandstone of the Middle Shimanto Belt (Yanai, 1984). A similar age-distribution has already been reported for two psammitic schist samples from the Central Unit of the Sanbagawa Belt in the Kanto Mountains (Tsutsumi et al., 2009). Thus the Sanbagawa Belt is most widely occupied by metamorphic rocks originating from rocks of the Middle Shimanto Belt. We also measured the 206Pb/238U age distribution of detrital zircons in Turonian sandstone from the Northern Shimanto Belt in the central Kii Peninsula. The age-distribution diagram shows that detrital zircons of around 128 Ma are most abundant and the age of the youngest zircon in the sample is about 100 Ma. A similar age-distribution has already been reported from a psammitic schist sample from the Southern Unit of the Sanbagawa Belt in the Kanto Mountains, overlying the Central Unit (Tsutsumi et al., 2009). The protolith age is still younger than the metamorphic age of the eclogites in central Shikoku, ca. 120-110 Ma (Okamoto et al., 2004), which occupy the uppermost portion of the Sanbagawa Belt. Although some previous studies suggested that the Sanbagawa Belt consists of metamorphosed Late Jurassic to Early Cretaceous accretionary complex, the present study shows that the belt is largely occupied by metamorphosed Late Cretaceous rocks: the Shimanto Metamorphic Rocks of Aoki et al. (2007). As a result, the Sanbagawa Belt consists of the following three units with different protolith ages: (1) Lower Unit of Shimanto Metamorphic Rocks with protoliths ages of 75-70 Ma and metamorphic ages of 70-60 Ma, (2) Upper Unit of Shimanto Metamorphic Rocks with protoliths ages of 95-85 Ma and metamorphic ages of 85-75 Ma, and (3) Sanbagawa Metamorphic Rocks (s.s.) with protoliths ages of Late Jurassic to Early Cretaceous and metamorphic ages of 120-110 Ma. The protoliths of the Upper and Lower units of the Shimanto Metamorphic Rocks are most likely rocks of the Northern Shimanto and Middle Shimanto belts, respectively.

The fossil spores and pollen from the Lower Cretaceous Arida Formation in western Kii Peninsula, southwestern Japan

The fossil spores and pollen from the Lower Cretaceous Arida Formation in western Kii Peninsula, southwestern Japan

和歌山県清水－美里地域の毛原層のローソン石を含む泥質片岩

This is the first report of lawsonite in pelitic schists from the Kebara Formation, a tectonometamorphic unit exposed between the Sambagawa metamorphic belt and the Chichibu belt in western Kii Peninsula. Lawsonite and pumpellyite have previously been reported from basic schists in the Kebara Formation, suggesting high-P/T metamorphic conditions. The pelitic schists consist mainly of quartz, albite and chlorite along with minor carbonaceous matter and rare phengite, lawsonite, calcite and titanite. The mineral assemblage of the lawsonite-bearing pelitic schists is chlorite+phengite +lawsonite+albite+quartz. This assemblage is basically the same as that reported for lawsonite-bearing pelitic schists from the chlorite zone in the Sambagawa metamorphic belt, i.e. in the Ise area in eastern Kii Peninsula and in the Besshi nappe complex exposed along the Asemigawa River in central Shikoku. These various lawsonite-bearing pelitic schists probably experienced similar high-P/T metamorphic conditions suggesting that the entire Kebara Formation experienced similar high-P/T metamorphism to that of the chlorite zone in the Sambagawa belt.

Crustal heterogeneity beneath southwest Japan estimated from direct and Moho-reflected waves

In this study, we have used later phases reflected at the Moho discontinuity in addition to direct arrival time data to study the 3-D crustal velocity structure beneath southwest (SW) Japan. We have collected more than 6000 direct and Moho-reflected phases and used them to obtain P- and S-wave velocity structures in the study area. Our results revealed significant low-velocity anomalies beneath the active and Quaternary volcanoes in SW Japan. Results obtained using each data set ( P, PmP, S, and SmS) are generally consistent with each other and all showed low-velocity anomalies beneath most of the active and Quaternary volcanoes, western Kii Peninsula and eastern Shikoku. Ray-path analysis showed that the data set we used has a good ray criss-crossing that is sufficient to accurately locate the velocity anomalies in the study area. In addition, results of the checkerboard resolution test indicate that these structures are generally well recovered and that adding later phases could significantly improve the resolution scale of the obtained tomographic images in comparison with those from first arrivals alone.

Reexamination of radiolarian biochronology of the Shimizu Formation (Northern Chichibu Belt) in the Shimizu-Misato area, western Kii Peninsula,Southwest Japan

Reexamination of radiolarian biochronology of the Shimizu Formation (Northern Chichibu Belt) in the Shimizu-Misato area, western Kii Peninsula,Southwest Japan