Island Arc最新文献

Forearc basin deposits on continental margins contain important information that can be used to reconstruct the tectonic setting, volcanism, and climate at the time of their deposition. Coal-bearing terrestrial to shallow marine strata in Northeast Japan were deposited in a forearc basin along the Eurasian continental margin during the Cretaceous–Paleogene. The rocks exposed in the Kado district, Iwate Prefecture, in the northern Kitakami Mountains include the Upper Cretaceous Yokomichi Formation and the Paleogene Kogawa Group; the latter is known for high-quality refractory clay (kaolin clay). However, their stratigraphy and sedimentological characteristics are not yet fully understood. To reconstruct the formation and filling of the basin, we investigated field observations, U–Pb dating of tuff samples, XRD analysis of tuff and mudstone samples, and vitrinite reflectance of coal samples in this sequence. The U–Pb ages of the Yokomichi Formation and the Kogawa Group are ∼86 Ma and 58–52 Ma, respectively. The U–Pb age of the kaolin-dominated tuffaceous rock (“red rocks”) is 56.1 ± 0.2 Ma. The mean random vitrinite reflectance (VR_r) ranges from 0.37% to 0.53% through the sequence. We propose the following sequence for the formation and filling of the basin. (1) The basin initially formed during the Coniacian–Santonian (Late Cretaceous) and was filled by fluvial–lacustrine sediment. (2) These sediments kept the shallow burial depths during a ∼28 Myr and made a hiatus (86–58 Ma). (3) The basin was reactivated and covered by tuffs during the Thanetian, and the tuffs were altered to kaolin clay during the Paleocene–Eocene Thermal Maximum. (4) The basin was filled rapidly by alluvial fan deposits and subsided <2300 m (assuming a geothermal gradient of >30 K/km). (5) The basin was uplifted and exhumed at a rate of >50 m/Myr faster than the mean exhumation rate of the Kitakami Mountains since the Paleogene.

The properties of host sediments and pore water considerably affect both the occurrence and formation processes of methane hydrate. In coarse-grained layers, hydrates are generally concentrated preferentially in the pore space, and their formation is influenced by pore water salinity. To understand how geophysical and geochemical factors control the distribution of methane hydrates, we conducted numerical simulations using a one-dimensional flow model under different reservoir and fluid conditions in the Kumano Forearc Basin, Nankai Trough, Japan. Assuming an estimated range of methane flux between 0.002 and 1.9 kg m⁻² year⁻¹, three flow scenarios were considered. When the methane flux was relatively small, the results coincided with the observed hydrate distribution. In general, a low-methane flux decreases the hydrate saturation upward from the bottom of the methane hydrate stability, whereas a high-methane flux increases the saturation downward. These results also suggest that the sediment structure, such as the fracture distribution, influences the sediment stress conditions and constrains the flow regime. We further examined the effects of permeability changes in the heterogeneous lithological units on the simulation results using typical permeabilities of 10⁻¹³ m² for sand and 10⁻¹⁵ m² for mud. The results showed that hydrate saturation sharply increased and decreased in adjacent high- and low-permeability units, respectively. The consideration of complex stratigraphic conditions and variable fluid configurations provides an understanding of the environmental factors controlling hydrate generation and distribution, which is important for hydrate resource extraction and geohazard prevention.

A nearly balanced carbon budget between subduction input and degassing output has likely controlled the long-term surface environment and habitability of Earth throughout geological history. However, the ongoing extensive hydration and carbonation of the mantle in trench outer-rise regions may affect the global carbon budget. In this study, we show that the carbon content of the lithospheric mantle can be inferred from geophysical data and thermodynamic modeling. Based on the seismic velocity anomaly in trench outer-rise regions, we estimated that the total carbon flux due to mantle carbonation is 7–31 Mt C/year, with possible fluid-to-rock mass ratios of 250–1000. These values are similar to the carbon uptake by altered oceanic crust, indicating that mantle carbonation has a significant effect on the subduction carbon budget. Although there are large uncertainties on the estimates of the subduction and degassing carbon fluxes, secular cooling of the mantle leads to the development of outer-rise faults associated with bending of the oceanic lithosphere and increased mantle carbonation, which may disrupt the self-regulating system of the global carbon cycle on geological timescales.

Fukutoku-Oka-no-Ba is a submarine volcano located at 24°17.1′ N/141°28.9′ E in the Izu–Bonin arc, and is one of the most active volcanoes in Japan. This volcano produced an explosive eruption in August 2021 that generated a large amount of volcaniclastic material, some of which drifted westward to Japan and the coastal area of East Asia as a pumice raft. The pumice clasts that drifted for >1000 km were mostly homogeneous and identical to those produced by past historical eruptions. The clasts have trachytic compositions (SiO₂ = 61–63 mass% and Na₂O + K₂O = 8.6–10.0 mass%) and contain augite, plagioclase, olivine (Mg# ~65), and magnetite, along with a small number of mafic enclaves containing diopside and high-Mg olivine (Mg# ~ 92). We undertook a research cruise to investigate the proximal volcaniclastic materials by dredging. The proximal materials include pumice, weakly vesiculated lapilli, and volcanic blocks, which have trachytic composition (SiO₂ contents up to 64.5 mass%). The main minerals in the proximal material are similar to those in the drift pumice, although remnants of mafic magma do not occur in the SiO₂-rich samples. The petrographic and geochemical characteristics of the proximal and drift ejecta from Fukutoku-Oka-no-Ba suggest the magma reservoir was stratified into two parts. The major part experienced magma mixing with a limited volume of mafic magma, whereas the other part was more differentiated. The differentiated high-SiO₂ magma accumulated in the upper part of the magma reservoir and avoided the mixing with and feed of volatile from the mafic magma, then were pushed out from the volcanic vent without extensive bubbling to sunk in the proximal area.

Oki metamorphic rocks have long been considered as a constituent of the Hida Belt based on their geographic proximity, lithology, and Permo-Triassic metamorphism. However, recent geochronological studies have demonstrated that both para- and ortho-gneisses in the Oki-Dogo Island display Paleoproterozoic protolith formation and two separate phases of metamorphism at 1.85 Ga and 250–230 Ma. Consequently, the Oki metamorphic rocks are closely connected to the Paleoproterozoic massifs in the Korean Peninsula, although little is known about their pressure (P)–temperature (T) history. Here, we provide petrological data on mafic metamorphic rocks in the Oki-Dogo Island. The mafic lithologies are classified into mafic granulite, amphibolitized granulite and amphibolite. In addition, we first discover a garnet-bearing variety of amphibolite from the Oki-Dogo Island. The texture and composition of Ca amphibole suggest these rock types share a common P–T history but the dominant mineral assemblage in each rock type records different stages of metamorphism. The inferred P–T history includes two distinct events. The first event includes a low-P granulite facies stage (~900°C, 0.7–0.8 GPa) and subsequent amphibolite facies retrogression. This event is linked to the continuous compositional change of Ca amphibole from Ti-rich pargasite to hornblende/actinolite. The second event is prograde amphibolite facies metamorphism, which is associated with the formation of tschermakitic hornblende and calcic plagioclase. In high Fe/(Mg + Fe) rocks, garnet was formed at ~550–580°C, 0.45–0.50 GPa in this stage. Depending on the age of the first event, the low-P granulite facies metamorphism is likely to have occurred in a similar tectonic setting as the Paleoproterozoic crustal metamorphism in the Yeongnam Massif or the Permo-Triassic ultrahigh-T metamorphism in the northern Gyeonggi Massif.

The chemical composition of coastal sediments and river sediments is influenced by the geological constitution of provenance and fractionation of clastic particles during sedimentary processes. The intricate nature of the geology in active subduction zones has complicated the comprehension of detrital sediments. More geochemical case studies in such areas are necessary to enhance the understanding of the coastal detrital sediments. This study presents the results of a geochemical and particle size analysis of sediment samples from the Sanriku coastal area of Northeast Japan, including inside and outside of Otsuchi Bay and the three rivers that flow into the bay. Otsuchi Bay area is known to have been severely affected by the 2011 off the Pacific coast of the Tohoku Earthquake and tsunami, and tsunami deposits on its seabed have been reported. The present geochemical analysis indicates that the sediments along those three rivers are well explained by the mixing of rocks occurring in their respective provenance. The chemical composition and particle size distribution of the sediments within Otsuchi Bay suggested the removal of coarse-grained quartz. They increased the abundance of fine-grained mafic minerals from Unosumai River having the largest catchment area among the three rivers. During shell debris in the seabed sediment outside of the bay made interpretation of the results was difficult; the sediments collected closer to the bay mouth contained more coarse-grained material with a composition closer to granitoid and sandstone in the provenance. In contrast, offshore sediments consisted of finer grains with a composition closer to mudstone. Notably, the sediments within Otsuchi Bay did not exhibit the characteristic compositional fractionation or particle size distribution associated with tsunami deposits. These findings underscore the importance of considering the geological diversity of provenances and the particle size distribution of minerals in understanding coastal sediments in subduction zones, including tsunami deposits.

Vertebrate fossils usually consist only of mineralized skeletons and rarely preserve soft body parts. However, the buried soft body releases the degraded material into the surrounding sediments. The degraded material can be preserved as invisible signals of some chemicals or authigenic minerals, which can be revealed by the elemental distribution of the fossils' surface and their surrounding rock. As a method for surface elemental mapping, X-ray fluorescence (XRF) measurement requires the samples to be ground and polished as a mirror-like plane that keeps a stable distance of X-ray source-sample-detector. This destructive pretreatment is undesirable for precious fossils. To rectify this problem, a nondestructive XRF for element mapping of the fossils' 3D surface is newly invented. The new equipment can follow the 3D shape model to keep the distance for every XRF scanned point. We performed the element mapping of Ca and Fe on the surface of a marine sauropterygian fossil (Keichousaurus hui) and its surrounding sediments, and found that the soft tissue around the thoracic cage is thicker than the abdomen. The abnormal elemental distribution on the area of the angular and surangular of the skull can be explained as unprofessionally prepared. In addition, this research found that elemental mapping revealed the invisible signals of the paleoenvironment around fossils and information for the reconstruction of soft-tissue anatomy. We also give an example that based on the wide application of the “Internet of Things” and “Industrial Network,” the design and development of specialized equipment for paleontologists' requirements are going simple.

The origins of early Paleozoic orogen in South China have two different models: subduction model and intra-continental model. Here we report two new identification of ~440 Ma arc-related ultramafic intrusions in Tingzifan (TZF) and Fomuting (FMT) along Jiangshan-Shaoxing fault (JSF) in South China, respectively. The Silurian ultramafic intrusions are composed of olivine pyroxenite, the SiO₂, MgO and TiO₂ contents of olivine pyroxenites are 39.67–41.25 wt%, 28.98–31.38 wt% and 0.23–0.51 wt%, respectively. The geochemical compositions of the olivines, clinopyroxenes and hornblendes suggest an arc-related environment for these intrusions. As for the whole-rock trace elements, the ultramafic intrusions contain low total rare earth element (REE) contents (27.59–34.26 μg/g) and high field strength elements (HFSEs), such as Nb, Zr, Hf, Ti, and are systematically enriched in large ion lithophile elements and light rare earth elements (LREEs). Trace element compositions share most features of Alaskan-type ultramafic-mafic intrusions. Isotopically, the TZF and FMT ultramafic intrusions are characterized by negative Zircons ε_Hf(t) values (0.38–7.54). Combined with their whole-rock and mineral chemistry as well as zircon Hf isotope, we suggest that the Alaskan-type TZF and FMT pyroxenite were formed at the root of the continental arc by underplating and fractional crystallization of mafic magma which derived from subduction metasomatized mantle source. Thus, we proposed that the early Paleozoic ultramafic–mafic along Jiangshan-Shaoxing fault were most likely related to early Paleozoic subduction of the Paleo-South China Ocean between Cathaysia and Yangtze blocks, arguing that the origins of early Paleozoic orogen in the South China Block is a typical subduction-accretionary collisional-type orogenic belt rather than an intraplate belt.

Middle to Late Miocene organic-rich siliceous mudstones (Onnagawa Formation) in the Akita Basin, Northeast Japan have been an important target for both paleoceanographic studies and hydrocarbon exploration in Japan. However, the reliable age of their formation has remained poorly constrained. Here, we report new zircon U–Pb and fission-track ages of the Onnagawa Formation from a previously well-studied outcrop route in the Yashima area, central Akita Basin. The thin tuff bed in the lower Onnagawa Formation was dated at around 11.6 Ma, whereas thin tuff beds in the upper Onnagawa Formation was dated at 10.4–9.6 Ma. The new age model constrains the base of the succession as older than 15.6–13.8 Ma and the top of the succession as 8.7–8.2 Ma. The results suggest that the lowest part of the succession was deposited before the Onnagawa Stage. The new age model indicates a rapid deposition in the lower Onnagawa Stage. The new age model also clarifies a temporary decrease in the sedimentation rate during 10.9–9.4 Ma, which coincided with a hiatus or slow deposition reported from other areas along the Sea of Japan coast. The new age model also revises the timing of paleoceanographic changes, of the best hydrocarbon source horizon, and of hydrothermal activity responsible for seafloor chemoautotrophic communities in the Akita Basin. This revised timing reveals that the onset of paleoceanographic changes from oxidizing to anoxic bottom environments favorable for hydrocarbon source rock formation was closely related to the tectonic uplift of the Northeast Japan Arc at ~12 Ma, whereas the timing of hydrothermal activity was related to the following extensional tectonics at ~9 Ma. The results of this study thus shed light on hitherto unclarified relationships between tectonics, volcanism, and paleoceanographic changes in the Sea of Japan.

The Qushandao Granite, mainly composed of alkali-feldspar granite, is situated in the eastern Zhejiang province of coastal southeast China. In this paper, we present whole-rock geochemistry, zircon U–Pb geochronology, and Hf isotopes to constrain the age, magma sources, and geodynamic setting of the Qushandao Granite. LA-ICP-MS zircon U–Pb dating results revealed that the Qushandao Granite was emplaced in the Late Cretaceous (101–98 Ma). Geochemically, the Qushandao Granite exhibits relatively high silica and alkali contents, metaluminous to weakly peraluminous (A/CNK = 0.98–1.02), and low abundances of phosphorus, titanium, magnesium, and calcium. It is also characterized by enrichment in Rb, K, Th, and depletion in Nb, Ta, P, Ti, and Sr with moderately to weakly negative europium anomalies (Eu/Eu* = 0.71–0.87). Furthermore, the Qushandao Granite displays lower FeO^T/MgO, 10⁴ × Ga/Al, and Zr + Nb + Ce + Y values relative to typical A-type granites. Therefore, we classify the Qushandao Granite as calc-alkaline I-type granite based on a synthesis of geological and geochemical characteristics. The Qushandao Granite shows variable zircon Hf isotopic compositions (ε_Hf(t) = −7.6 to +2.3) and T_DM2 model ages of 1.40–0.83 Ga with a mean value of 1.17 Ga. We argue that the Qushandao Granite was most likely generated by mixing of mantle-derived mafic magma and crust-derived felsic magma in the lower crust, and that it was formed during post-collisional extension in the Late Cretaceous, related to the gradually increasing subduction angle of the Paleo-Pacific plate.