Kohei Nimura

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.

Subcritical cracking propagates fractures via stress-assisted chemical reactions at crack tips under stresses below the critical threshold. Although this mechanism is increasingly recognized as a driver of time-dependent damage accumulation and permeability evolution in the Earth’s crust, direct natural constraints on the lower oceanic crust remain limited. Here, we document plagioclase microstructural and compositional features consistent with subcritical cracking in a deformed oxide gabbro from the Mado Megamullion oceanic core complex, Shikoku Basin, Philippine Sea. We combine co-registered scanning electron microscopy, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy mapping with MATLAB MTEX Toolbox processing to link plagioclase crystallographic orientation, misorientation, and anorthite content (An = Ca/(Ca + Na + K)). Anorthite content maps show heterogeneous low-An domains within plagioclase porphyroclasts. These low-An domains are locally subparallel to the trace of the (001) cleavage plane and locally contain subgrain boundaries. We interpret the low-An domains as possible crack–seal-related features formed by stress-corrosion-assisted subcritical cracking along cleavage planes and subgrain boundaries, followed by host-controlled epitaxial precipitation of sodic plagioclase. These grain-scale crack–seal features may indicate that permeability creation by cracking can be followed by permeability reduction through sealing in gabbros from oceanic core complexes at slow-spreading ridges.

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.

This paper investigated the deformation microstructures of gabbroic rocks from the breakaway region of the Mado Megamullion (site YK19‐04S‐6K‐1536), Shikoku Basin, Philippine Sea to constrain the processes that contribute to the mechanical weakening of the lower oceanic crust. The deformed gabbroic rocks are characterized by porphyroclastic textures consisting mainly of coarse plagioclase + clinopyroxene ± amphibole porphyroclasts and a fine‐grained matrix of plagioclase + clinopyroxene + amphibole + Fe‐Ti oxides ± talc. Based on macroscopic foliation development, the samples are divided into low‐ and high‐strain domains. The low‐strain domain shows weakly developed foliations and a granoblastic texture with partially preserved magmatic textures, whereas the high‐strain domain shows well‐developed foliation defined by monomineralic and mixed mineral layers and porphyroclastic textures. In the low‐strain domains, plagioclase grains show similar crystallographic orientations and chemical compositions between porphyroclasts and surrounding matrix, consistent with deformation predominantly via dislocation creep and subgrain rotation recrystallization. Clinopyroxene porphyroclasts and adjacent brown amphibole at porphyroclast rims share similar crystallographic orientations, suggesting topotactic replacement of clinopyroxene by amphibole associated with fluid infiltration. In the high‐strain domains, fine‐grained plagioclase shows distinct compositions from porphyroclasts and fabric development with [100] aligned to lineation and (001) to foliation, indicating that grain nucleation occurred during deformation via fluid‐assisted reactions. It suggests that the deformation could have been accommodated by a combination of dislocation creep and dissolution–precipitation creep. Fine‐grained clinopyroxene within monomineralic layers exhibits a clear (010)[001] crystallographic‐preferred orientation indicative of dislocation glide. In contrast, fine‐grained clinopyroxene in polymineralic layers shows weaker or nearly random orientations, suggesting enhanced grain boundary sliding at clinopyroxene–amphibole interfaces. This behavior was facilitated by grain‐size reduction, abundant phase mixing, and inhibition of grain growth by Zener pinning. Brown amphibole within the high‐strain matrix exhibits strong crystallographic and shape preferred orientations parallel to foliation, suggesting syn‐tectonic growth and alignment. Ti‐rich brown amphibole formed synchronously with deformation through clinopyroxene breakdown at temperatures of 755°C–871°C, whereas Ti‐poor green amphibole formed during subsequent lower‐temperature (686°C–770°C) hydrothermal alteration. A comparison with the Godzilla Megamullion suggests similar fluid‐mediated deformation processes are active in megamullions throughout back‐arc basin environments. Our results demonstrate that melt‐ and fluid‐assisted reactions, coupled with grain size–sensitive deformation mechanisms, contribute significantly to long‐term rheological weakening and strain localization in ductile shear zones within oceanic core complexes.