Kyoko Okino

Scarps along oceanic fracture zones provide key tectonic windows into the architecture of oceanic lithosphere. Recent investigations along the Mid-Atlantic Ridge (MAR) demonstrate that such scarps can expose both crust- and mantle-dominated sections. However, integrated datasets combining bathymetry, seafloor imagery, and systematic rock sampling remain spatially uneven, limiting direct comparisons among fracture-zone systems. Here, we address this issue at the Marie Celeste Fracture Zone (MCFZ) along the Central Indian Ridge. The MCFZ includes a ~215-km-long active transform fault with an ~11 Myr lithospheric age offset. The southern wall of the active transform valley comprises three morphological domains from west to east: abyssal hills, oceanic core complexes (OCCs), and a prominent transverse ridge. During the KH-24-4 cruise of the R/V Hakuho-maru in 2024, we conducted dredging at 10 sites distributed along depth transects ranging from the deepest part of the transform-fault valley to the top of the transform wall. These sites were spaced up to ~40 km apart along the transform fault. At each site, one to six dredge hauls were conducted, for a total of 27 dredge hauls. Descriptions of the dredged samples, bathymetric profiles, and seafloor photographs were combined to construct interpreted lithologic sections. Abyssal hill scarps expose pillow lava-dominated basalts, including plagioclase ultraphyric basalts typical of slow- to intermediate-spreading ridges. The OCC-adjacent scarps yield a mixed assemblage of basaltic, gabbroic, and ultramafic rocks, including strongly altered gabbros and epidosite. In contrast, the eastern transverse ridge appears to expose a crust-dominated section. Basalt is abundant across the shallower intervals, whereas dolerite and evolved olivine–oxide gabbro become more abundant at greater depths, and ultramafic rocks remain a minor component restricted to the deepest sampled interval. Comparisons with transverse ridges at the Vema and Vernadsky FZs of the MAR indicate that, despite similar spreading rates, the proportions and depth distributions of basaltic, gabbroic, and ultramafic rocks vary markedly among the systems. The MCFZ represents a crust-dominated end-member of oceanic fracture-zone architecture and highlights the structural diversity of transform lithosphere in slow-spreading settings.

The Mariana Trough is an active back‐arc basin formed by subduction of the Pacific Plate beneath the Philippine Sea Plate and provides an ideal setting for investigating the evolution of back‐arc basins. In this study, we compile bathymetric, magnetic, and gravity data to reconstruct the formation history of the entire basin. Based on bathymetry and backscatter intensity, the spreading axis is divided into 18 second‐order segments. Thirteen oceanic core complex–like structures are also identified both near axis and in off‐axis regions using topography and gravity anomaly data. Magnetic anomaly data indicate that seafloor spreading initiated at approximately 6.3 Ma in the central part of the basin near 17°30′N. During the initial stage (6.3–4 Ma), the oceanic crust was anomalously thick (∼8 km), likely reflecting enhanced melt production associated with slab‐derived fluids when the spreading axis was located close to the arc. Abyssal hill patterns and magnetic anomalies suggest that spreading was oriented NE–SW during this stage and began to change toward a trench‐normal direction between 4 and 3 Ma in each region. After 3 Ma, the basin evolved into its present crescent‐shaped geometry, with full spreading rates ranging from 25 to 45 km/Myr, which are higher in the southern region than in the north. Half‐spreading rates are generally higher on the western side of the spreading axis than on the eastern side, indicating persistent asymmetric spreading. Estimates of the Euler pole for the Mariana Plate suggest arc‐parallel extension, implying non‐rigid plate behavior during back‐arc basin opening.

Back-arc basins play a crucial role in the formation of oceanic crust, similar to mid-ocean ridges. While magmatic and tectonic processes in the mature spreading stage resemble those in mid-ocean ridges, the initiation and cessation of back-arc spreading remain unclear. The Shikoku Basin and its associated remnant arc in the Philippine Sea are among the most studied back-arc systems, providing an excellent opportunity to understand the detailed processes of oceanic arc rifting and the initial stages of back-arc opening. We have recently collected bathymetry, magnetic and gravity data from the southwestern part of the Shikoku Basin. This area is characterized by a deeper basin floor and numerous oceanic core complexes with high gravity anomalies, indicating that magma-poor spreading was predominant in the initial stage. The magnetic lineation pattern is only discernible in a limited abyssal hill area, but the identified geochrons align with previous studies in the northern part of the basin and new zircon U–Pb age data from a couple of minor gabbroic samples. The morphology of closely located oceanic core complexes suggests a ridge jump. The structural features of the basin floor correlate with the morphology of the remnant arc, the Kyushu-Palau Ridge, implying that the preexisting arc crustal structure influences the style of rifting/spreading and may affect the development of ridge segmentation.

Mylonitic mantle peridotites exposed at the Tosa Megamullion in the Shikoku Basin, Philippine Sea, provide direct evidence for amagmatic ductile shear deformation of the upper mantle beneath a back-arc spreading center. Oceanic core complexes (OCCs), or megamullions, are dome-shaped structures formed by detachment faulting and occur locally along slow-spreading mid-ocean ridges and back-arc basins, where they expose fault rocks derived from ductile shear zones in the lower crust and upper mantle. The Shikoku Basin hosts several OCCs, including the Tosa Megamullion, which formed during the early stage of back-arc spreading. In this study, nine ultramafic rocks were collected from the Tosa Megamullion using the submersible Shinkai6500 during cruise YK23-05S. Although all samples were highly serpentinized, several preserved primary peridotitic textures were composed mainly of olivine, orthopyroxene, with subordinate clinopyroxene, plagioclase, and spinel. Seven samples exhibit well-developed foliation and porphyroclastic textures dominated by orthopyroxene porphyroclasts, ranging from rounded to strongly elongated forms, commonly showing microkinks and undulose extinction. Crystallographic preferred orientations (CPOs) of three representative samples, analyzed using SEM-EBSD, reveal E-type-dominant olivine fabrics characterized by the (001)[100] slip system, with a subordinate contribution from C-type (100)[001] slip. These CPOs suggest deformation under non-dry conditions involving moderate hydration and/or elevated differential stress. These results indicate that the ultramafic rocks from the Tosa Megamullion represent mantle-derived mylonitic peridotites formed by ductile shear beneath the spreading axis and subsequently exhumed under strongly magma-poor, amagmatic conditions. The Tosa Megamullion thus represents an amagmatic end-member of the OCC formation in back-arc basins, dominated by tectonic strain localization rather than by magmatic accretion.

How to represent place names and geographical features in foreign languages is a complex and nuanced issue. The process that led to the realization that the location referred to in Japanese as Kutsurū Island is actually a reef at the Ngulu Atoll of the Yap Islands is described.

Back-arc basins (BABs) form an integral but often neglected part in the framework of subduction zone system. Many studies on the BAB have focused on BAB basalts because they are more accessible than to the lower crust and underlying mantle. The Shikoku and Parece Vela Basins formed behind arc systems in the Philippine Sea host numerous oceanic core complexes (OCCs), dome-like bathymetric highs composed of lower crust and mantle materials. One of these OCCs named Mado Megamullion is the study target emplaced in the Shikoku Basin, where dolerite dike intrusions are recognized as a late back-arc magma activity. Ilmenite U–Pb dating of a dolerite sample revealed that the dolerite formed at 9.7 ± 6.3 Ma (2 sigma error, n = 24), at which the Shikoku Basin ceased spreading. Geochemical tracers of Ba/Yb, Ba/Nb, and Th/Nb were used to evaluate the extent of subduction input to dolerite-forming magmas. The results demonstrated that the subduction components did not reach the back-arc spreading center and hence the dolerite-forming magma was dry as the back-arc spreading center was well apart from the fluid-supplying slab when the Mado Megamullion was formed. The Mado Megamullion has ample potential to decode mature back-arc spreading system, whereas the OCCs located in initial spreading modes of the Shikoku Basin can decipher immature back-arc spreading system. Accurate understanding of BAB evolution will come from further investigations of time-dependent emplacements of OCCs in the Shikoku and Parece Vela BABs.

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10–13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

Detachment faulting is one of the main styles of seafloor spreading at slow to intermediate mid-ocean ridges. However, we have limited insight into its role in back-arc basin formation. We surveyed a remnant back-arc spreading center in the Philippine Sea and determined the detailed features and formation processes of the Mado Megamullion (Mado MM) oceanic core complex (OCC). This was undertaken in the context of back-arc evolution, based on the shipborne bathymetry, magnetics, and gravity with radiometric age dating of the rock samples collected. The Mado MM OCC has a typical OCC morphology with prominent corrugations on the domed surface and positive gravity anomalies, suggesting that there has been an exposure of the lower crust and mantle materials by a detachment fault. The downdip side of the detachment continues to the relict axial rift valley, which has indicated that the Mado MM OCC was formed at the end of the back-arc basin opening. The spreading rate of the basin decreased once when the spreading direction changed after six million years of stable trench perpendicular spreading. The rate then further decreased immediately prior to the end of the spreading when the Mado MM OCC was formed. The existence of other OCC-like structures in the neighboring segment and the previously reported OCCs along the Parece Vela Rift have indicated that the melt-poor, tectonic-dominant spreading is a widespread phenomenon at the terminal phase of back-arc spreading. The decrease in spreading rate in the later stage is consistent with the previous numerical modeling because of the decrease in trench retreat. In the Izu–Bonin–Mariana arc trench system, the rotation of the spreading axis and the resultant axis segmentation have enhanced the lithosphere cooling and constrained mantle upwelling, which caused the tectonic-dominant spreading at the final phase of the basin evolution.

The back‐arc spreading at the southern Mariana is categorized as slow, but surface morphological and geophysical features of the spreading centers suggest that the spreading process is associated with enhanced melting in the upper mantle, due to water being derived from the subducted Pacific slab. A marine magnetotelluric experiment was performed along a transect across a segment at 13°N to reveal the key processes of melting, dehydration, and dynamics in the upper mantle, and their relationships to the surface characteristics. Our inversion model of electrical resistivity shows (a) a conductive body at 10–20 km depth beneath the spreading axis, and (b) another conductive area expanding asymmetrically under and around the conductive body. Away from the spreading center, there is (c) a resistive area thickening up to ∼40 km on the remnant arc side, and (d) another resistive area with a constant thickness of ∼150 km on the trench side. Implications of these model features are (a) a melt body beneath the spreading axis; (b) a hydrous mantle above the subducted slab and asymmetric passive decompression melting in the mantle wedge; (c) a residual mantle off from the spreading axis; and (d) a cold mantle wedge tip and the subducted Pacific mantle. The structure markedly contrasts with that in the central Mariana Trough at 18°N, suggesting that the horizontal distance between the location of the spreading center and the root of the buoyant upwelling above the subducted slab is a key parameter that controls the mantle dynamics beneath the back‐arc spreading.

The whole-rock trace-element compositions of igneous rocks provide primary geochemical information about their petrogenesis. Such compositions can be determined by LA–ICP–MS analysis of fused-glass samples as well as the conventional solution ICP–MS method. However, in contrast to basalt and andesite, the fused-glass method is not suitable for Si-rich rocks (granite and rhyolite) due to the difficulty of making homogeneous glasses. To extend the method to Si-rich rocks, we adopted an MgO dilution process to prepare fused glasses and evaluated the technique by analysis of granite and rhyolite reference materials. Dilution of 30 mg of powdered samples with 10 mg MgO facilitated the preparation of homogeneous fused-glass samples of felsic rocks. LA–ICP–MS analyses indicate that the fused glass is homogeneous in the contents of most incompatible elements considered in igneous petrology. The glass enables the analysis of these elements in felsic rocks within 20% deviation from reference data. The sample data are consistent with reference values for rhyolite although Zr and Hf data deviate significantly (by ~40%) from reference values for granitic rocks.