Island Arc最新文献

Paleogene 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.

Internal microtextures of ternary alkali feldspars in sanidine trachyte from Oki-Dogo Island were examined using an electron microprobe analyzer, a scanning electron microscope, a transmission electron microscope and cathodoluminescence instruments, to develop the understanding of volcanic processes of alkaline magmas related to feldspar crystallization. The examined trachyte is an evolved rock of the Oki-Dogo Pliocene trachyte group. Its phenocryst feldspars are commonly associated with lamellar-wavy-domain textures with scales approximately from 100 nm up to several hundreds of μm that show complex and gradual variations in composition: however, anti-rapakivi zoning textures common in other Oki-Dogo alkaline rocks are almost completely absent in the trachyte. These textures are produced by extensive magmatic ion-exchange replacement reactions progressively advanced in the evolved magma. Characteristic braided fluorite alignments are developed consistently with lamellar-wavy-domain textures in phenocryst feldspars, and similar braided alignments are also present in groundmass feldspars with complicated microtextures. Most of fluorite grains are <100 nm in diameter, and the patterns of braided fluorite alignments vary greatly in individual feldspars. The whole occurrence of the feldspar microtextures represents an extreme example of diffusion-controlled replacement reactions, progressively advanced in the dry (relatively anhydrous) trachyte magma. The genetic processes forming fluorite alignments in feldspars are related to magma compositions, especially F and P contents, and the crystallization of F-bearing minerals, especially of fluorapatite.

The crystallization of nanolites within magma chambers has recently raised a strong interest due to their impact on increasing melt viscosity and triggering magmatic eruptions. In 2021, the Fukutoku-Oka-no-Ba (FOB) underwater eruption produced large quantities of pumices that eventually formed rafts drifting at the surface of the ocean to the East coasts of Japan. Pumices collected along the shore shortly after grounding show various colors, microscopic and Raman analyses made by Yoshida et al. (Island Arc, 31, 1, 2022) revealed the presence of magnetite nanolites in some of them. In this study, we explore the magnetic properties of a batch of pumices of different colors from the FOB eruption, aiming to refine characterization of iron oxide nanolites. We used various analytical techniques such as SEM and FEG-SEM observations, EDS-X analyses, and rock magnetic experiments, including thermomagnetic analyses, hysteresis curves, coercivity analyses and FORC measurements. Our findings reveal that the iron oxides present in the FOB samples are Ti-magnetite, with minor amounts of Mg and Al. The magnetic crystals show a wide range of sizes, from extra small iron oxide nanolites (ESION) in the pumices with the lighter colors, to more bulky grains reaching the micrometer size in some of the dark color samples, significant diffusion is inferred in that case. Consequently, the magnetic characterization of iron oxide crystals within the Fukutoku-Oka-no-Ba pumices reveals varying stages of nucleation, dissolution, growth, and diffusion processes, providing evidences for the heterogeneous state of the magma during the eruption.

The opening of the Japan Sea led to the separation of southwest Japan from the Eurasian continent. Subsequent to this event, a diverse range of igneous activities occurred in southwest Japan. On the back-arc side of the region, igneous activity commenced at approximately 22 Ma and persisted for an extended period. In the trench-proximal region of southwest Japan, magmatism initiated around 15.6 Ma, immediately following the cessation of the Japan Sea opening, in correlation with the subduction of the Philippine Sea plate beneath southwest Japan. The Amakusa Islands in western Kyushu host felsic to intermediate igneous rocks with Miocene radiometric ages. There has been a debate regarding the attribution of the igneous rocks in Amakusa Island among the Miocene igneous rocks in southwest Japan. To address this issue, we conducted zircon U–Pb dating and analyzed the major- and trace-element compositions of felsic igneous rocks in the Amakusa Islands to elucidate their characteristics. The obtained U–Pb ages range from 14.5 to 14.8 Ma, suggesting contemporaneity between magmatism in the Amakusa Islands and the Setouchi Volcanic Rocks in the trench-proximal region of southwest Japan. The major and trace element compositions of the felsic igneous rocks exhibit similarities to the dacites of the Setouchi Volcanic Rocks. These findings support previous suggestions that the magmatism in the Amakusa Islands can be correlated with the Setouchi Volcanic Rocks, based on the discovery of a high-Mg andesite dike and paleo-stress analysis utilizing the direction of dikes and sills. Therefore, the Setouchi Volcanic Belt is proposed to extend further west than the previously identified Ohno volcanic rocks in eastern Kyushu. The subduction of the Shikoku Basin of the Philippine Sea plate toward western Kyushu supports the hypothesis that the Kyushu-Palau Ridge was positioned west of Kyushu at ~15 Ma.

Understanding the Miocene tectonics of the southwest Japan Arc is key to elucidating the history of the opening of the Japan Sea, and important advances have been made in the last 10 years, such as clarification of the timing of arc rotation. Syn-rift successions occur in the eastern San'in basins, but they have received little attention because the basic stratigraphy and development of the basins are poorly understood. This paper provides details of a field survey of the Miocene geology of the Kinbusan area in eastern Tottori Prefecture, Japan. The lithofacies suggest alluvial fan and plain environments, which correlate with the syn-rifting horizon of the southwest Japan Arc. A syn-depositional graben, which is newly named the Kinbusan Graben, can be inferred from the geometry of the basin fill, and this graben consists of two sub-basins bounded by two ENE–WSW-trending normal faults. Miocene dikes also trend ENE–WSW, indicating that the graben was formed in response to extension oriented perpendicular to the strike of the faults. The age of the Iwami Formation shows that extension was initiated before arc rotation. Fault-slip data, collected from meso-scale faults in the basin fill, indicate axial compressive stress with the maximum principal stress being vertical. The stress field suggests that the basin fill was accommodated not only in fault-perpendicular extension but also in fault-parallel extensions, and fault-parallel extension was the probable cause of differential subsidence of the basin fill during faulting. The results show that the basic stratigraphy and structures of the eastern San'in basins provide important constraints on the timing and style of deformation in the southwest Japan Arc and the Japan Sea.

Even though no basalts have erupted at Asama volcano, its large, felsic magma chamber standing beneath the summit is frequently replenished by cryptic injections of mafic magma of basalt-basaltic andesite composition. This mafic magma is preserved within melt inclusions trapped in olivine phenocrysts collected from the old Itabana pumice of the Kurofu stage and the Tenmei pumice of the current Maekake stage. The trapped mafic melts provide a detailed and vivid record of cryptic mafic magma injection into the felsic chamber where they admix to erupt andesite magma. The main phenocrysts of plagioclase, orthopyroxene, and clinopyroxene, commonly seen in the erupted products, are derived from the felsic magmas and trapped felsic melts of dacite-rhyolite compositions, containing low sulfur contents (0.3 wt% of SO₃ or less; most are lower than 0.10 wt%). In contrast, the mafic melts, entrapped by olivine phenocrysts, show high concentrations of SO₃, up to 1.06 wt% for Itabana and SO₃ 0.65 wt% for Tenmei. In addition, the olivine phenocryst commonly encloses early crystalline phases precipitated from the sulfur-rich mafic magma, such as Cr-spinel, Mg-rich orthopyroxene, Fe, Cu, and Ni-bearing sulfides, and often Al-rich clinopyroxene. The olivine-hosted mafic melt inclusions always contain numerous vesicles. Furthermore, they are often included as hourglass-shaped trapped melts, exhibiting snapshots of intense foaming and gas phase exsolution during the olivine growth and probably in the process of the cryptic injection into the felsic magma beneath the Asama summit. Our new data contributes to a better understanding of the magmatic system of the Asama volcano but also highlight the importance of the “Excess sulfur problem”, especially concerning the 1783 Tenmai eruption, which was contemporary to the Laki Fires.

Euphorbia mauritanica is a succulent shrub that is indigenous to South Africa and widely distributed throughout the country. Dying plants have been observed in their natural habitat in the Northern and Western Cape Provinces of South Africa in recent years. Stems displaying lesions were collected and the emerging cultures were identified based on ITS, LSU, ACT, RPB2, TEF1 and/or TUB2 sequence data. Four filamentous fungi were consistently observed and isolated. One was identified as Alanphillipsia (Ala.) aloes, and the other three were new to science and are described here as Cytospora euphorbiicola sp. nov., Nothomicrosphaeropsis namakwaensis sp. nov. and Austrophoma (Aus.) euphorbiae gen. et sp. nov. These new species and Ala. aloes were the most commonly encountered, and their pathogenicity was tested on E. mauritanica plants in a greenhouse trial. All four species gave rise to lesions that were significantly larger than those associated with the controls, but they were not significantly different to each other. Although the lesions associated with the inoculations were well-developed, they did not give rise to plant death, suggesting that they are not responsible for the large-scale die-off of E. mauritanica in the field. The primary cause of the death of E. mauritanica in the studied area remains unknown and could be due to environmental factors such as has been found with the die-off of Euphorbia ingens in South Africa. Citation: Marincowitz S, Pham NQ, Wingfield BD, Roets F, Wingfield MJ (2023). Microfungi associated with dying Euphorbia mauritanica in South Africa and their relative pathogenicity. Fungal Systematics and Evolution 12: 59-71. doi: 10.3114/fuse.2023.12.04.

Pelagic clay that contains rare-earth elements and yttrium (REY) at over 2000 ppm (termed “highly REY-rich mud”) has been reported from a few areas in the western North Pacific Ocean through focused geochemical analyses. Those REY are carried by biogenic calcium phosphate, suggesting that the formation of highly REY-rich mud involves enhanced biological productivity. However, detecting REY enrichment in sediment is time-consuming, and the global significance of the formation of REY-rich mud is still under debate. Here we perform x-ray computed tomography (CT) analyses on cores recovered around Minamitorishima Island and demonstrate the positive correlation with highly REY-rich mud and high CT numbers of the sediment. The variation of the CT number matches the wet bulk density. Further quantitative analyses using direct measurements of density and geochemistry suggest that a grain density increase and a porosity decrease due to the REY-carrying biogenic apatite are essential to explain the high CT numbers in the REY-enriched layer. While the chemical composition itself is of minor importance to elevate the CT numbers, our results suggest that x-ray CT can be a proxy for highly REY-rich mud.

Sedimentological and biochronological analyses were undertaken on carbonate deposits from 20 seamounts belonging to the Marcus–Wake Seamount Group, the Magellan Seamounts, and the Marshall Islands Seamounts in the JA area, northwestern Pacific Ocean. Deposition of carbonates on the JA seamounts varied markedly with age. The oldest carbonate deposits are Lower to middle Cretaceous shallow-water limestones containing mollusks (including rudists), scleractinian corals, and calcareous sponges. Upper Cretaceous and Paleocene carbonates are rare, and no Oligocene carbonates may exist. In contrast, Eocene foraminiferal packstones are widespread, and Miocene–Pleistocene foraminiferal ooze covers the JA seamounts. The limited occurrence of Paleogene carbonate deposits on the JA seamounts is consistent with global observations (i.e., a paucity of Paleogene carbonates). The Cretaceous–Eocene carbonates have been phosphatized, whereas Miocene and later limestones have not. This fact, along with the results of previous studies, suggests that carbonate rocks on seamounts were phosphatized globally during the Oligocene. Upwelling of nutrient-rich bottom waters during this time is likely responsible for the limited occurrence of Oligocene carbonate rocks on the JA seamounts. The thicknesses of the pelagic caps, which consist mainly of Miocene and younger foraminiferal oozes, varies among the seamounts and depends at least partly on the topography of the top of the seamount.