So Inoue

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.

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.