SW Japan Research Articles

Structural and paragenetic evolution of garnet–bearing barroisite schist from the Suo metamorphic complex, SW Japan

Structural and paragenetic evolution of garnet–bearing barroisite schist from the Suo metamorphic complex, SW Japan

Ancient crustal recycling in modern island arcs: A tale of the world's youngest charnockite from SW Japan

Modern accretionary orogens are considered as zones of extensive juvenile crustal growth on our planet with the Central Asian Orogenic Belt (CAOB) and the western Pacific domain as typical examples. Here we report magmatic charnockites (orthopyroxene-bearing anhydrous monzogranite) from the Shikoku Island in SW Japan intruding the Shimanto accretionary belt. Petrological and geochemical features of the intrusion are identical to Archean, Paleoproterozoic and Cretaceous charnockites from various parts of the world and show typical arc magmatic affinity and formation in a convergent margin setting. Zircon UPb data indicate that the pluton was emplaced at 13.75 Ma, revealing one of the youngest magmatic charnockite occurrences in the world. Zircon LuHf data yield epsilon Hf(t) values in the range of −3.2 to 2.8 suggesting that the magma source involved a substantial amount of reworked material, in addition to juvenile input. The Hf modal age (TDM) shows a major peak at 701 Ma, suggesting reworking of Neoproterozoic basement rocks beneath the young island arc. The data confirm the model of the birth of Proto-Japan along the periphery of the Cathaysia block in South China. The modal age peak corresponds to the timing of magmatism associated with Rodinia rifting. Our study provides evidence for the involvement of both ancient recycled components and juvenile material associated with crust building in a young island arc within an ongoing subduction system.

Characterization and correlation of the Hegawa‐Kasamori 5 tephra, a widespread tephra aged c. 450 ka associated with large‐scale pyroclastic flows from southern Kyushu, SW Japan

ABSTRACTA Middle Pleistocene widespread tephra, defined here as Hegawa‐Kasamori 5 tephra (Hgw‐Ks5), has been newly recognized over a broad area of Japan. Large‐scale pyroclastic flow deposits associated with co‐ignimbrite ash fall deposits (CAFDs) of Hgw‐Ks5 have been identified in the proximal southern Kyushu area, south‐west Japan. Hgw‐Ks5 possibly originated from the Aira caldera in southern Kyushu, and it is widely spread and intercalated with deposits of the Kasamori Formation, Honshu Island, more than 1000 km away from the source. In the north‐west area of the Aira Caldera, the tephra is sparsely distributed in the form of non‐welded ignimbrites, and is exposed stratigraphically above the well‐known Kobayashi‐Kasamori tephra. Hgw‐Ks5 is characterized through petrographic features, major element geochemistry of glass shards, and refractive indices of orthopyroxene. The results of previous stratigraphic isotope studies indicate that the eruptive age of Hgw‐Ks5 is 434–458 ka (Marine Isotope Stage 12). Assuming that the CAFDs originating from the Aira Caldera are distributed concentrically, the apparent volume of Hgw‐Ks5, estimated from the area of distribution and CAFD thickness, is ~100 km3. Therefore, a volcanic explosivity index of 7 is assigned to the Hgw‐Ks5 eruption.

Origin of the Tonaru body in the Sanbagawa metamorphic belt, SW Japan

AbstractZircon U–Pb dating of the Tonaru metagabbro body in the Sanbagawa metamorphic belt, southwest Japan, suggests that igneous events at ca 200–180 Ma were involved in the protolith formation. The trace element compositions of the Tonaru zircons are enriched in U (a fluid‐mobile element) and Sc (an amphibole‐buffered element), and depleted in Nb (a fluid‐immobile element), suggesting that the parental magmas related to the Tonaru metagabbros formed in an arc setting. Integration of our results with previous studies of the metasedimentary rocks in the Tonaru body clearly indicates that the protoliths of the Tonaru body were produced by oceanic‐arc magmatism. With the previous geochronological and geological studies, the tectono‐magmatic–metamorphic history of the Tonaru and other mafic bodies in the Sanbagawa metamorphic belt may be summarized as follows: (i) the protolith formation by the oceanic‐arc magmatic event had occurred at 200–180 Ma; (ii) the protoliths were accreted in the trench at ca 130–120 Ma; and (iii) they were completely subducted into the depth of the eclogite‐facies condition after 120 Ma.

Zircon U-Pb age of a lapilli tuff bed in the lower Miocene Yoka Formation, northern Hyogo, SW Japan

Zircon U-Pb age of a lapilli tuff bed in the lower Miocene Yoka Formation, northern Hyogo, SW Japan

Tracing dehydration and melting of the subducted slab with tungsten isotopes in arc lavas

Tungsten is strongly incompatible during magmatic processes and is fluid mobile in subduction zones. Here we show that W isotope fractionation in arc lavas provide a powerful new tool for tracing slab dehydration and melting in subduction zones. Geochemically well characterized, representative arc-lavas from three subduction zones were chosen for this study to evaluate W isotope fractionation under different sub-arc conditions. Arc-lavas from SW Japan are produced by subducting a young, hot slab, and lavas from the volcanic front and rear arc of the Sangihe and Izu arcs are produced during subduction of a cold slab. The heaviest W isotope compositions (δ184W∼0.06‰) are observed in fluid-rich samples from the volcanic fronts of the Sangihe and Izu arcs. With increasing distance from the volcanic front, more melt-rich samples are characterized by progressively lighter W isotope compositions. Enriched alkali basalts from SW Japan, thought to be the product of mantle melting at a slab tear, and adjacent shoshonites have the lightest W isotope compositions (δ184W∼0‰). The correlation of W isotope fractionation with various indices of fluid release (e.g., Ce/Pb, Ba/Th) suggests that the heavy W isotope signatures record fluid recycling near the volcanic front due to dehydration of the subducted slab. Upon release of the heavy W, the residual slab preferentially retains isotopically light W, which is released during subsequent melting of drier lithologies in hot subduction zones, such as SW Japan. These data suggest that W isotopes can be used as a tracer of slab dehydration, potentially helping to determine the onset of cold subduction zone magmatism and hence, modern-style plate tectonics.

High- and low-stress subduction zones recognized in the rock record

It is generally accepted that the main strength of subduction boundaries occurs in the shallower region where frictional deformation is dominant. However, estimates of absolute values—commonly expressed as apparent coefficient of friction, μ′—show great variation. Frictional shear heating is closely related to μ′, and estimates of the extra thermal energy supplied by shearing can in principle be used to estimate subduction zone strength. One such approach is based on surface heat flow measurements. However, heat flow in convergent margins shows large local scatter and even in the same area, different studies using this method show large variations in estimates of μ′ indicating large uncertainties. The thermal record of subduction conditions preserved in subduction-type metamorphic rocks is developed over geological time scales that average out local complexities in heat flow and therefore has good potential as an alternative indicator of the amount of shear heating and, hence, shear strength along subduction boundaries. Thermal models that incorporate shear heating were developed for two contrasting and well-known subduction-type metamorphic belts: the relatively warm Sanbagawa belt of SW Japan and the relatively cold Franciscan belt of western USA. High-grade rocks of the Sanbagawa belt show strongly curved P–T paths that display increasing P/T to about 2 GPa. Information on the rate of plate movement and the age of the subducting slab at the time of metamorphism can be combined with modelling results to show that relatively high shear stresses, equivalent to μ′∼0.13 are required to account for the observed curved P–T paths. In contrast, the high-grade rocks of the Franciscan belt show relatively cool P–T conditions that do not allow for strong shear heating with an appropriate upper bound for μ′ of ∼0.03. Modelling suggests subduction rate and lithology are potentially important controls on the development of high- versus low-stress subduction zones. High-stress subduction zones are likely to be associated with high aseismic/seismic slip ratios possibly related to slab roughness.

U–Pb ages of Miocene near-trench granitic rocks of the Southwest Japan arc: implications for magmatism related to hot subduction

AbstractWe present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, startedc. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex and the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan

AbstractIt is essential to clarify the lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex (MC) and the Cretaceous Shimanto Accretionary Complex (AC) for understanding the tectonic evolution of SW Japan. To this end, we carried out a detailed field survey of the Sanbagawa MC and the Cretaceous Shimanto AC on the central Kii Peninsula, where they are in direct contact with each other. We also conducted U–Pb dating of detrital zircons from these complexes. The field survey showed that the boundary between the Iro Complex of the Sanbagawa MC and the Mugitani Complex of the Shimanto AC, Narai Fault, shows a sinistral sense of shear with a reverse dip‐slip component, and there are significant differences in the strain intensity and the degree of recrystallization between the two complexes across this fault. Detrital zircon U–Pb dating indicates that the Iro Complex in the hanging wall of the Narai Fault shows a significantly younger maximum depositional age than the Mugitani Complex in the footwall of the fault, and an apparently large gap in the MDA of ca. 35 Myr exists across this fault. This large age gap across the Narai Fault suggests that this fault is an essential tectonic boundary fault within the Cretaceous accretionary–metamorphic complexes on the Kii Peninsula, and is considered to be an out‐of‐sequence thrust. In addition, a similar shear direction and a large age gap have been identified across the Ui Thrust, which marks the boundary between the Kouyasan and Hidakagawa belts of the Cretaceous Shimanto AC. The Cretaceous accretionary–metamorphic complexes record the large‐scale tectonic juxtapositions of complexes, and these juxtaposed structures had been caused by sinistral–reverse movements on the tectonic boundary faults such as the Narai Fault and the Ui Thrust.

THERMOGENIC PETROLEUM POTENTIAL OF THE NANKAI SUBDUCTION ZONE, OFFSHORE SW JAPAN

In the Kumano forearc basin, offshore SW Japan, the potential occurrence of hydrocarbons has been inferred from both geophysical surveys which point to the presence of natural gas hydrates, and geochemical analyses of gas samples recovered from boreholes and submarine mud volcanoes. The forearc basin is located in the inner part of the Nankai Trough where the Philippine Sea plate is subducting northwards beneath SW Japan. The basin succession unconformably overlies a deformed accretionary prism complex. Hydrocarbons with both microbial and thermogenic origins have been recorded in the Kumano Basin, but the thermogenic petroleum potential is poorly constrained because the regional‐scale geological architecture of the Nankai subduction zone is not well understood.In this paper, the regional‐scale petroleum potential of the Nankai Trough is investigated based on the geological interpretation of new seismic reflection images. The images were derived from legacy seismic survey data using advanced processing techniques such as reverse time migration, which make use of both primary and multiple reflections in wide‐angle seismic data collected with ocean‐bottom seismographs. In the study area, a seaward‐dipping reflector was identified which terminates at the plate boundary décollement and which is interpreted as part of the boundary between previously‐identified older (>14 Ma) and younger (<6 Ma) accretionary prisms. A triangle body at the base of the younger accretionary prism is interpreted to consist of underthrusted sediments including thick hemipelagic mudstones with source rock potential which were scraped off the down‐going oceanic plate. These tentatively identified, deeply‐buried mudstones may have the potential to generate thermogenic hydrocarbons which may then migrate upwards along dipping carrier beds or faults and fractures to the unconformity at the base of the Kumano forearc basin succession. The presence of hydrocarbons in the accretionary prism may be inferred from positive amplitude anomalies which have been identified in the reprocessed multi‐channel seismic data.The new reflection interpretations therefore allow the potential for thermogenic petroleum to be proposed in the study area. The hydrocarbon potential at this subduction margin may therefore rely not only on shallow microbial methane, but may also involve deeper, thermogenic petroleum generated within the underlying accretionary prism.

U-Pb and fission-track ages from the Hokutan Group, northern Hyogo Prefecture, SW Japan

U-Pb and fission-track ages from the Hokutan Group, northern Hyogo Prefecture, SW Japan

四国三波川変成帯・瀬場エクロジャイト中の泥質片岩に含まれるジルコンのU-Pb年代

四国三波川変成帯・瀬場エクロジャイト中の泥質片岩に含まれるジルコンのU-Pb年代

津和野地域の古原生代花崗岩類と高塩濃度深部流体

津和野地域の古原生代花崗岩類と高塩濃度深部流体

Factors controlling the geochemical differences between two types of rhyolites in the middle Okinawa Trough

The origin of the felsic volcanic rocks in the Okinawa Trough (OT) remains uncertain. In the middle Okinawa Trough (MOT), two types of rhyolites (type 1 and type 2 rhyolites) have been recognized. Research on the diverse volcanic rocks should be prioritized to determine the magmatic origins of these rocks. Consequently, in this article, the geochemical differences and controlling factors for these two types of MOT rhyolites are systematically discussed. Type 2 rhyolites have higher Dy/Yb and Nb/Ta ratios than type 1 rhyolites, implying that the former might be influenced by a greater contribution of amphibole. Quantitative models suggest that type 1 rhyolites could have formed via the remelting of andesites when amphiboles remain in the source, whereas type 2 rhyolites could have formed without residual amphiboles. These two distinct melting models effectively explain most discrepancies between the major and trace element compositions of type 1 and 2 rhyolites. Additionally, the lower Mg# and δEu values of type 2 rhyolites are partly caused by significant magmatic differentiation. Quasi-linear correlations between some major oxides, trace element ratios and Sr-Nd isotopic ratios for type 2 rhyolites suggest that these rhyolites might have been contaminated by crustal rocks or melts with compositions similar to those of the Kueishantao (KST) andesites or the Middle Miocene upper crustal rocks of SW Japan. Therefore, isotopically enriched crustal materials likely remain in the OT crust.

Multi-parameter imaging in the 2016 Mw 7.0 Kumamoto earthquake, Kyushu, Japan and its implications for fluids in seismogenesis

Fluids associated with structural heterogeneities in a seismogenic layer play a crucial role in triggering earthquakes and controlling their rupture processes, but the mechanism of fluid intrusion and the relationship with earthquake initiation are unclear. This study presents new seismic evidence of an earthquake mostly triggered by fluid intrusion associated with magmatism in Kyushu, SW Japan. Seismic imaging revealed two zones with slow seismic velocities beneath the Unzen and Aso Volcanoes with high Poisson's ratio, extending into the rupture belt of the 2016 Mw 7.0 Kumamoto earthquake. The following features, particularly the high Poisson's ratio anomalies, suggest the presence of fluids in the source area ascending from nearby magmatism. Anomalous crack density and saturation rate imply that the fractures/pores are not fully saturated with fluids in the crust of the Beppu-Shimabara Graben, possibly providing a path for fluids to penetrate into the source area, either through highly fracture-damaged faults or via incompletely saturated pores/cracks. Fluid penetrating into the hypocenter areas from deep magma increases the pore pressure and decreases the Coulomb failure function. We therefore speculate that a system such as this, with high pore pressure and fluids in the host faults might lower the threshold for fault failure, thus increasing the likelihood of failure of the mainshock fault. The results of this study suggest that the presence of fractures/pores within extensional rift zones around the world might be critical for fluid migration, and contribute to the initiation, development and migration of seismic activity.

Polycyclic aromatic hydrocarbons (PAHs) in sediment cores from lakes Shinji and Nakaumi, SW Japan: A proxy of recent fire events in the watershed

PAHs in sediment cores have been investigated as a useful proxy of historic fire events in the watershed of an estuary. Oxygen poor and anoxic lake bottom conditions in estuarine brackish lakes Shinji and Nakaumi could prevent the oxidization of polycyclic aromatic hydrocarbons (PAHs) and preserve a record of PAHs in sediments. PAHs emitted from wood and/or grass burning have been analyzed for composition and concentrations during the last ∼300 years. Carbonized plant material which remained after artificial agricultural fires and heating experiments have been used to estimate fire temperature. The equation relating maximum temperature and hydrogen to carbon (H/C) atomic ratio of the charcoal is as follows: Temp (°C) = 317 x (H/C)−0.708 (R2 = 0.924, 200–700 °C).These experiments and the gas chromatography - mass spectrometry analysis of PAHs on the charcoals suggest that PAHs were primarily produced at temperatures less than 500 °C in the burning of wild plants.The PAHs contents in the two sediment cores were not related to total organic carbon (TOC) contents and emission of industrial waste, and were directly due to fire events in this region. Although TOC contents of the sediments were constant at about 0.5% in Lake Shinji and 1.0% in Lake Nakaumi before ca. 1900 CE, and increased up to about 4% due to recent nutrient discharge, the profiles of PAHs contents were completely different. The sediment cores contained the following PAHs: phenanthrene (P), fluoranthene (Fla), pyrene (Py), benz[a]anthracene (BaAn), benzofluoranthenes (Bflas), benzo[e]pyrene (BePy), indeno[1,2,3-cd]pyrene (InPy) and benzo[ghi]perylene (BghiP). The contents of 6-ring InPy and BghiP, high temperature markers, are lower than those of the other low ring Py, BaAn, Bflas and BePy, showing low to medium intensity/temperature fire events. Relatively higher BaAn contents suggest that the PAHs were carried and deposited in the sediments for a short period after the fire. The PAHs peak groups B (1830–1870 CE) and C (1920 CE) may be attributed to the burning of wood buildings in Matsue City, and the PAHs peak group period D (1970–1980 and 2000–2007 CE) could be mainly due to forest fires. The general increase in PAHs after 1850 CE could be caused by climate change after the Little Ice Age. The PAHs with maximum total peak concentrations at 30–34 cm depth (1850 CE: 4890 ng g−1) in Lake Shinji and at 8–12 cm depth (1970–1980 CE: 2030 ng g−1) in Lake Nakaumi are not at a toxic level.

A “coarse-grained” metabasite body newly found in the Sambagawa metamorphic rocks in Kumakogen Town, Ehime Prefecture, SW Japan

A “coarse-grained” metabasite body newly found in the Sambagawa metamorphic rocks in Kumakogen Town, Ehime Prefecture, SW Japan

The progressive development of microfabrics from initial deposition to slump deformation: an example from a modern sedimenary mélange on the Nankai Prism

The progressive development of microfabrics from initial deposition to slump deformation and then a submarine slide was investigated in an active subduction zone using cores recovered during the Integrated Ocean Drilling Program Expedition 333. A Pleistocene–Holocene sequence was recovered at Site C0018A, which was located on a slope basin on the footwall of the megasplay fault in the Nankai Trough, SW Japan. Six mass-transport deposit units intercalated with coherent intervals were recovered from the upper 190 m of the drilled succession. The initial microfabrics in the undeformed hemipelagic sediments were characterized by random and porous fabrics composed predominantly of clay aggregations and connectors. The initial fabrics were cardhouse fabrics, which consist of clay flakes with edge-to-edge (E–E) and/or edge-to-face (E–F) contacts. These initial microfabrics developed into compacted microfabrics, which are random and consolidated fabrics (bookhouse fabrics) that consist of clay flakes with E–F and/or face-to-face (F–F) contacts and develop during burial as a pure shear deformation. During slumping, these fabrics were then deformed under simple shear to become predominantly F–F contacts and form clay chains. Thus, the microfabrics in these submarine slides are a sedimentary mélange that developed locally into a preferred clay orientation with F–F contacts. Supplementary material: A schematic illustration showing sedimentation processes and fabrics is available at https://doi.org/10.6084/m9.figshare.c.4483385 Thematic collection: This article is part of the Polygenetic mélanges collection available at: https://www.lyellcollection.org/cc/polygenetic-melanges

Interplate Coupling Distribution Along the Nankai Trough in Southwest Japan Estimated From the Block Motion Model Based on Onshore GNSS and Seafloor GNSS/A Observations

AbstractAlong the Nankai Trough of southwest Japan, large‐magnitude interplate earthquakes (M > 8) have occurred repeatedly, with recurrence intervals of approximately 100 years. It is essential to estimate more precisely the spatial distribution of interplate coupling; that is, along‐strike and along‐dip heterogeneity of the coupling ratio defined as the slip deficit rate divided by the plate convergence rate, for understanding the mechanisms and/or assessing the potential of future mega interplate earthquakes in this region. Recently, the seafloor Global Navigation Satellite System‐Acoustic (GNSS/A) combination technique observation network has been extended, and we can now obtain crustal displacement fields for both onshore and offshore regions. In this study, we estimate interplate coupling distribution along the Nankai Trough using both seafloor GNSS/A and onshore GNSS data with the block motion model, minimizing the spatial variations of modeling uncertainties of coupling estimates. Seafloor GNSS/A data depict very strong coupling near the trough axis off of Tokai, as well as trough‐parallel heterogeneity in interplate coupling at the shallow plate interface. Using the block motion model confirmed that observed crustal deformation above the deep plate interface in SW Japan is dominated by rigid block motion rather than elastic response to slip deficit on the deep plate interface of the subducting Philippine Sea plate. The minimization of spatial variations for the modeling uncertainties of coupling estimates helps us to estimate the coupling distribution with nearly homogeneous uncertainties between offshore and onshore regions, which is useful for evaluation of interplate earthquakes and/or tsunamis hazards.

Geological structure and detrital zircon U-Pb age around the boundary between the Jurassic Chichibu and Cretaceous Shimanto accretionary complexes in the central Kii Peninsula, SW Japan

Geological structure and detrital zircon U-Pb age around the boundary between the Jurassic Chichibu and Cretaceous Shimanto accretionary complexes in the central Kii Peninsula, SW Japan