SW Japan Research Articles

Detrital zircon U–Pb geochronology of the trench-fill sandstone on the Cretaceous Shimanto accretionary complex in SW Japan: Implications for provenance and igneous activity in the eastern edge of East Asia

Cretaceous igneous rocks are widely distributed in the East Asian margin, and their formation is due to the large-scale migration of igneous activity areas from the continental interior to near the subduction margin. Detrital zircon U–Pb ages of trench-fill sandstones of the Shimanto accretionary complex in the Akaishi Mountains of southwest Japan can provide information on eroded igneous rocks, and their location at the eastern edge of East Asia makes them essential for better understanding the intensity and migration of the igneous activity. As a result of the dating, the Cretaceous zircons in the trench-fill sandstones were found to increase dramatically in the middle Cretaceous and to be abundant in the Late Cretaceous. In addition, these large amounts of Cretaceous zircons were found to be derived from igneous rocks such as the Abukuma Granites and Ryoke Granites distributed in the proto-Japan arc. These results indicate that migration of igneous activity from the continental side reached the eastern edge of East Asia in the middle Cretaceous and that large-scale igneous activity continued to occur in the eastern edge of East Asia until the Late Cretaceous. Thus, the detrital zircon age spectra in trench-fill sediments provide a clear picture of the degree of igneous activity in the source area and migration of the igneous activity on active continental margins.

Subducted carbon weakens the forearc mantle wedge in a warm subduction zone

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

Geology of the Paleogene Kumage Group distributed around the Tanowaki–Okigahamada area in the northern part of Tanegashima Island, Kagoshima, SW Japan

Geology of the Paleogene Kumage Group distributed around the Tanowaki–Okigahamada area in the northern part of Tanegashima Island, Kagoshima, SW Japan

Reconstruction of the Cretaceous continental arc–trench system of the Japanese Islands: a basis for Cretaceous palaeoenvironmental studies

Abstract Spatiotemporal distributions of Cretaceous rocks differ markedly between the SW and NE Japan arcs. However, four parallel zonal arrangements of rocks are recognized broadly throughout both arcs: mostly non-marine sedimentary rocks in back-arc/intra-arc basins; granitic and volcanic rocks in magmatic arcs; predominantly marine and subordinately fluvial sedimentary rocks in forearc basins; and sedimentary rocks of turbiditic and mélange facies in accretionary complexes. These zones constituted a palaeo-Japan continental arc–trench system during the Cretaceous. We describe and correlate 71 Cretaceous back-arc/intra-arc and forearc basinal successions from Kyushu (south) to Hokkaido (north) islands, including a southern Sakhalin and two Kuril Arc (eastern Hokkaido) successions. Stratigraphic ranges and major sedimentary facies are generally similar between the SW and NE Japan arcs, except for the pre-Aptian Lower Cretaceous in Hokkaido of NE Japan, suggesting continuity throughout the two arcs during the Cretaceous. Although Cretaceous strata are sporadically exposed in northern Honshu, NE Japan, interpretation of seismic sections suggests that Cretaceous forearc sedimentary rock measuring several tens of kilometres laterally are developed offshore beneath the present Pacific forearc. In contrast, Cretaceous forearc strata in southern SW Japan are distributed along two narrow belts that may have been deformed by post-Early Miocene tectonism.

Inside the Ryoke Magmatic Arc: Crustal Deformation, High-T Metamorphism, and Magmatic Pulses

The Ryoke belt represents the root of a volcanic arc exposed across SW Japan. It records successive deformation phases, high-temperature metamorphism, and several magmatic pulses that occurred during the Late Cretaceous. Successive magma intrusions at different crustal levels raised the overall geothermal gradient of the arc crust, and their thermal influence was contrastingly recorded in metamorphic zircon and monazite. Despite a broadly similar duration of magmatic activity (20–30 My) along the belt, the timing and periodicity of magma pulses varied. An along-arc variation in lower crustal magma generation together with a fluctuating crustal stress regime likely controlled the formation and evolution of this magmatic arc section.

Paired Metamorphism of SW Japan and Implications for Tectonics of Convergent Margins

The Sanbagawa-Ryoke pair of geological units in southwest Japan is the classic example of paired metamorphism originally identified by Akiho Miyashiro. Together these belts represent an important study area for developing and testing ideas about how convergent margins behave over geological time based on studies of the rock record including petrology, geochemistry, deformation, and geochronology. The two sides of the pair represent ancient examples of a subduction zone in the Sanbagawa belt and an associated volcanic arc in the Ryoke belt. This issue of Elements brings together the results of a wide range of different approaches summarizing the current state of knowledge about the Sanbagawa-Ryoke pair and how this informs our understanding of convergent margins in general.

Geochronology of the Sanbagawa Belt: Younger and Faster than Before

Recent advances in geochronological studies have helped establish the Sanbagawa belt as an important site for studying metamorphism related to subduction. Application of several dating methods yield the following important results. 1) Metamorphism and deformation related to subduction started ~120 Ma and were complete by ~50 Ma. 2) Subduction to eclogite facies, followed by return to the surface, was rapid and occurred within a few million years or less (at ~89 Ma), indicating exhumation rates of at least 1–2 cm·y−1. 3) The age of the slab during the peak eclogite facies metamorphism was ~60 My. These results help redefine the geological history of SW Japan and provide important constraints for mechanical and thermal models of subduction zones in general.

Dominant slip systems of quartz under lower amphibolite-facies conditions identified from microstructures and CPOs in quartz phenocrysts

Identifying the dominant slip systems in naturally deformed quartz is crucial for understanding continental crust rheology. Although the basal <a> slip system is considered dominant in quartz under upper to middle crustal conditions, its activity is controversial because an oriented nucleation and growth model has been proposed for the c-axis distribution near the minimum strain axis. This study examined the crystallographic orientation and shape of quartz phenocrysts in a deformed granitic porphyry in the Ryoke belt, SW Japan, to clarify the dominant slip systems in naturally deformed quartz at temperature conditions of ∼400–500 °C utilizing optical and electron backscatter diffraction (EBSD) observations. Identified active slip systems include prism <a>, basal <a>, prism [c], and rhomb <a> through misorientation analyses. The aspect ratios of phenocrysts with dominant prism <a> and basal <a> slip systems are higher than prism [c] and rhomb <a> slip systems, indicating similar strengths between prism <a> and basal <a> slip systems, which are weaker than prism [c] and rhomb <a> slip systems under lower amphibolite-facies conditions. The c-axis of phenocrysts with dominant basal <a> distribute at pole figures peripheries, indicating basal <a> activation over the proposed oriented nucleation and growth model.

Implications of Surface Heat Flux for Shear Stress and Temperature on the Plate Interface Beneath Northern Honshu

AbstractMeasurements of surface heat flux from boreholes of the NIED Hi‐net network of seismometers are used to determine temperature and shear stress on the plate interface beneath northern Honshu. At the maximum depth of thrust‐faulting earthquakes, about 60 km, the temperature is 660 ± 240°C and the average shear stress during slip on the interface is 100 ± 45 MPa. The equivalent quantities from the oceanic setting of the Kermadec trench are 630 ± 140°C and 115 ± 25 MPa. The shear stresses are substantially higher than commonly assumed in models of convergent plate interfaces, but are consistent with measurements made above other convergent plate interfaces where heat flux data are more sparse. They are also consistent with the level of shear stress required to explain measurements of pressure and temperature from high‐pressure‐low‐temperature terrains that experienced metamorphism on subduction interfaces, in particular those from the Cretaceous Sanbagawa belt of SW Japan.

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting.

Petrogenesis of Josoji rhyolite and intrusive rocks of NE Shimane Peninsula, SW Japan: implication on Miocene volcanism related to back-arc rifting in San’in region

Petrogenesis of Josoji rhyolite and intrusive rocks of NE Shimane Peninsula, SW Japan: implication on Miocene volcanism related to back-arc rifting in San’in region

Eocene extensional tectonics in the Amakusa region, northern Ryukyu arc

AbstractPaleogene surface tectonics in Japan is not well understood because of the paucity of onshore Paleogene stratigraphic records except for those from accretionary complexes. Paralic Paleogene formations remaining in SW Japan are usually so thin that it is difficult to decipher the tectonics from them. However, the Eocene paralic sedimentary package with a thickness of kilometers indicates syn‐depositional tectonic subsidence by a few kilometers in the Amakusa archipelago, west of Kyushu Island. Thus, we made a detailed geological map of the Eocene formations in an area of ~50 square kilometers in the northwestern part of the archipelago. We identified NE‐SW and NW‐SE trending normal faults, most of which were recognized by previous researchers, and also discovered low‐angle faults. NW‐SE trending ones are known to be of the Miocene. NE‐SW trending and low‐angle normal faults are the oldest map‐scale structures in the Eocene ones. It is not obvious within the above‐mentioned area whether those normal faults are accompanied by growth strata. However, the significant southeastward thickening of the Eocene formations across the Amakusa archipelago suggests that they filled a large half graben with the basin margin fault along the eastern side of the archipelago. This basin model is consistent with the N‐S to NW‐SE transport directions of the low‐angle and NE‐SW trending normal faults. Since many NE‐SW to EW trending Eocene grabens were formed in the offshore regions west of Kyushu Island and in the East China Sea, the Amakusa region was probably a northeastern branch of the rift system. The geologic structures and depositional ages of the Eocene formations indicate that the Eocene extensional tectonics removed the overlying strata to some extent for the high‐P/T Takahama Metamorphic Rocks which crops out to the south of our study area.

Microstructural comparison in fault gouges and slip surfaces of active and inactive faults: A case study at Watarigawa area in northeastern Yamaguchi Prefecture, SW Japan

Microstructural comparison in fault gouges and slip surfaces of active and inactive faults: A case study at Watarigawa area in northeastern Yamaguchi Prefecture, SW Japan

Thermochronology of hydrothermal alteration zones in the Kii Peninsula, southwest Japan: an attempt for detecting the thermal anomalies and implications to the regional exhumation history

Fluid-inclusion and thermochronometric analyses have been applied to hydrothermal alteration zones and their host rocks outcropping in the Hongu area of the Kii Peninsula, southwestern Japan in an attempt to detect thermal anomalies related to hydrothermal events and quantify the thermal effects on the host rocks. Hydrothermal events at ~ 150 °C and ~ 200 °C were identified by fluid-inclusion microthermometry of quartz veins in the alteration zones. For the host rocks and alteration zones, in the youngest population zircon yielded U–Pb dates ranging between ~ 74.7–59.2 Ma, fission-track dates of ~ 27.2–16.6 Ma, and (U–Th)/He single-grain dates of ~ 23.6–8.7 Ma. Apatite yielded pooled fission-track ages of ~ 14.9–9.0 Ma. The zircon U–Pb dates constrain the maximum depositional ages of the sedimentary samples. However, the fission-track and (U–Th)/He dates show no clear trend as a function of distance from the alteration zones. Hence, no thermal anomaly was detected in the surrounding host rocks based on the thermochronometric data patterns. The fission-track and (U–Th)/He dates are rather thought to record regional thermal and exhumation histories rather than a direct thermal imprint of fluid flow, probably because the duration of such activity was too short or because fluid flow occurred before regional cooling events and were later thermally overprinted. Apatite fission-track ages of ~ 10 Ma may reflect regional mountain uplift and exhumation related to the obduction of the SW Japan lithospheric sliver onto the Shikoku Basin, or the rapid subduction of the Philippine Sea slab associated with the clockwise rotation of the Southwest Japan Arc.Graphical

Clockwise rotation of SW Japan and timing of Izanagi–Pacific ridge subduction revealed by arc migration

Igneous rocks associated with the Cretaceous to Paleogene volcanic arc in SW Japan show ages that young from west to east in a direction parallel to the Median Tectonic Line suggesting corresponding translation of a heat source traditionally interpreted in terms of oblique subduction of a spreading ridge. However, recent oceanic plate reconstructions suggest ridge subduction may be younger than the main arc activity. Age compilations of 1227 points of felsic to intermediate Cretaceous and Cenozoic igneous rocks from the Japan arc show arc magmatism that can be separated into an early active period 130–60 Ma (stage 1), a subsequent period of quiescence 60–46 Ma (stage 2), which is followed by a resumption of igneous activity from 46 Ma onward (stage 3). In southwest Japan, the orientations of the magmatic arcs of stages 1 and 3 show and angular discordance of about 20°. The lack of active arc magmatism and the occurrence patterns of adakitic and high-Mg andesitic magmas indicate that ridge subduction occurred during stage 2. The arc age distribution pattern of stage 1 is explained by the slab shallowing related to a younging of the subducting slab as the ridge approaches. Furthermore, the obliquity of the arcs formed at stages 1 and 3 is explained by a 20° clockwise rotation of the inner zone of southwest Japan during the ridge-subduction phase. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase, and coupling of the subducting oceanic plate with the upper plate would support microplate rotation in the inner zone. The new proposed tectonic reconstructions provide a framework to related Paleogene subduction of an active spreading ridge along the east Asia margin not only to the distribution of granitic bodies but also to rift-related basin formation on the eastern margin of the Eurasian continent and to rotation of crustal blocks indicated by paleomagnetic data of Cretaceous terranes.

Assessing the activity of mud volcanism using boron isotope ratios in pore water from surface sediments of mud volcanoes off Tanegashima (SW Japan)

Mud volcanoes can cause various geohazards, so it is very important to know their activity level and their distribution. Surface sediments were collected from four submarine mud volcanoes (MVs) off Tanegashima (SW Japan), namely, MV1, MV2, MV3, and MV14. We extracted pore water from the surface sediments and investigated its chemical and isotopic compositions. The sodium (Na) and chloride (Cl-) concentrations decreased and the boron (B) and lithium (Li) concentrations increased with increasing depth, suggesting that some fluids with lower Na and Cl- concentrations and higher B and Li concentrations than seawater were supplied upward from the deep sub-seafloor. The fluid advection velocities estimated from the pore-water profiles differed for each MV, and those of MV3 were the fastest (14 cm/yr) in this study. The estimated equilibrium temperature with clay minerals using Na and Li concentrations were 93-134°C, corresponding to the temperature of environments around 3.7 to 5.3 km below the seafloor. This indicates that these components originated from these depths and that the origin depth did not reach the plate boundary in this area. The B isotope ratio in the pore water was extremely high up to +57 ‰, suggesting that it was strongly affected by adsorption onto the surface of the sediments. A higher B isotope ratio (+57 ‰) was detected in MV3, which was considered to be more active, indicating that more B was adsorbed onto clay minerals supplied from deeper depths.

Parent magma of the Cretaceous Northern Kyushu batholith and heat source of large-scale igneous activity: Petrological constraints on magmatism of the Northeastern Asian continental margin in the Cretaceous period, example for the Kita-Taku mafic complex in northern Kyushu, SW Japan

The Japanese island in the Northeastern Asian continental margin is located in a subduction zone, and granitoid covers about 30% of the island arc. The Cretaceous Northern Kyushu batholith (CNKB) is classified as 19 bodies based on previous studies and consists of a granitic body and a small amount of associated mafic bodies. Of these, the Kita-Taku mafic complex body (KMC) mainly consists of gabbroic rocks and can be classified into cumulus textures (Cumulus group: Types 1–4) and those without (Non-cumulus group: Types 5–7), which can provide information on the heat source of the large-scale igneous activity in northern Kyushu from the Northeastern Asian continental margin at Cretaceous period.Melt compositions inferred from the high Mg# pyroxene compositions of the olivine-bearing pyroxene hornblendite (Type 1) member of KMC were estimated using melt-pyroxene partition coefficients. The clinopyroxene-bearing equilibrium melts are similar to the Sanukitic high-Mg andesite (HMA) in multi-element and rare-earth element patterns. Besides, the trace element and rare earth element patterns of the Cretaceous high-Mg diorites (HMDs) in northern Kyushu have similar patterns to the clinopyroxene-bearing equilibrium melts of the KMC and Sanukitic HMA magma. This implies that Sanukitic HMA magmas were substantially activated in northern Kyushu during the Cretaceous period.Analysis of the parent magma (heat source) and the small-scale plutonic bodies that consist of CNKB indicates that the essential granitic magma is formed by partial melting of the lower crust and mixing with mantle-derived Sanukitic HMA magma. The Sanukitic HMA magma could be the parent magma for the heat source and crustal growth. The lithofacies of CNKB can be reconstructed from the examination of small-scale plutonic bodies due to the mixing and assimilation of various end-member magmas and crustal materials, and their associated differentiation and accumulation processes. As the Northeastern Asian continental margin during the Cretaceous is assumed to be situated within the extensional setting due to slab rollback, the results of this study provide new insights into the complicated batholith formation processes and heat source.

Evolved Late Mesozoic continental arc: Constraints of detrital zircons from the western East China Sea

ABSTRACT The Late Mesozoic subduction of Izanagi slab beneath East Asia formed large-scale intraplate magmatism in SE South China and subduction mélanges outcropped in SW Japan to eastern Taiwan Island, but the accompanying arc remains uncertain. We conducted LA-ICP-MS U–Pb zircon dating and trace element analyses of proximal sandstones from the SW East China Sea to trace a Jurassic to Cretaceous magmatic arc. Newly acquired data reveal that arc magmatism mostly developed in episodes of 150–124 and 124–102 Ma, exhibiting characteristics of low-T, high Th/U, U/Yb, Sc/Yb, Th/Nb, and low Nb/Yb, Nb/Hf ratios. This magmatic arc, combined with the SE South China intraplate and residual subduction mélanges, spatially forms a Late Mesozoic trench–arc–intraplate architecture responding to the Izanagi subduction. Its identified tectonic scenarios mainly include slab strike-slip subduction (200–170 Ma), slab stagnation and intraplate foundering (170–150 Ma), slab rollback and arc-root removal (150–102 Ma), and arc migration (102–86 Ma). Both the western East China Sea and Cathaysia block share a unified Paleoproterozoic basement that formed at ca. 1.85 Ga, and the Cathaysia-based magmatic arc occurred then.

Low-temperature thermochronology of Ryoke belt granitoids, SW Japan: New insights into the recent cooling history from monazite fission-track dating

Traditional low-temperature thermochronology techniques such as apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) have so far produced limited information on the neotectonic (late Miocene and younger) evolution of Japan. However, the development of monazite fission track (MFT) dating, which has an ultra-low temperature sensitivity (<∼25 – 46°C), provides a fresh opportunity to directly analyse recent denudation histories. Low-temperature thermochronology methods have been applied to granitoid samples from the Ryoke belt, located in eastern Yamaguchi and Nara Prefectures, SW Japan. Zircon (U-Th)/He (ZHe), AFT and AHe data and modelled thermal histories reveal Late Cretaceous - Pliocene cooling related to granitoid thermal relaxation combined with paleo-Izanagi and Pacific plate subduction along the eastern Eurasian continental margin. MFT dating reveals Plio – Pleistocene cooling ages, coincident with elastic loading caused by Philippine Sea plate subduction since the Middle - Late Miocene, along with the Quaternary collision of NE and SW Japan at the Itoigawa-Shizuoka Tectonic Line. Silica concentration, non-thermal “radiation assisted” annealing, and changing climatic conditions are proposed as possible caveats on MFT annealing kinetics since the samples are residing in an “open” system. Taking all these factors into consideration, the apparent MFT ages are interpreted as minimum cooling ages, being slightly younger than the thermal events that cooled the samples through the MFT nominal closure temperature. Estimated denudation rates based on MFT dating are higher in Nara relative to eastern Yamaguchi Prefecture, with the difference likely reflecting variations in the tectonic regime, timing of uplift and uplift mechanisms of the two regions.

Miocene lamprophyre and felsic volcanics in the outer zone of SW Japan: arc building through multiple sources in an active convergent margin

ABSTRACT Subduction-related magmatism is a significant contributor to arc building and continental growth. In the active convergent margin of the SW Japan arc, voluminous volcanic rocks and mafic dykes formed through subduction and back-arc rifting during Miocene. In this study, we investigate a suite of felsic volcanic rocks (rhyolite and dacite) and lamprophyres from central Shikoku Island, which occur in the north-central part of the Chichibu belt and Sanbagawa belt in the Outer zone of the SW Japan arc. Whole-rock geochemistry results show that the rhyolite and dacite samples are peraluminous, belong to the high-K calc-alkaline series, show LREE and LILE enrichment, HREE and HFSE depletion that suggest arc volcanic affinity. The lamprophyre samples belong to alkali lamprophyre, show LREE and HFSE enrichments, and display OIB (oceanic island basalt)-like geochemical features. The 206Pb/238U weighted mean ages of the zircons from the rhyolite and dacite samples range from 14.39 ± 0.21 Ma to 13.92 ± 0.30 Ma, whereas the lamprophyre shows 206Pb/238U weighted mean age of 12.21 ± 0.35 Ma. Zircon Lu-Hf isotope data suggest that the rhyolite and dacite were derived from the remelting of the arc crustal rocks and that the lamprophyre was derived from depleted mantle sources and was contaminated by crustal materials. The felsic volcanic rocks (rhyolite and dacite) can be correlated to Miocene (15.5 Ma to 13.5 Ma) magmatism in the Outer Zone of the SW Japan arc. The formation of lamprophyre dykes in the SW Japan arc may be related to the subhorizontal mantle flows beneath the SW Japan arc associated with the opening of the Japan Sea back-arc basin or asthenosphere upwelling through the slab-tear of the Philippine Sea Plate beneath the SW Japan arc. Our study shows that the SW Japan Arc was built by multiple magmatism from diverse sources in an active convergent margin.