Microstructural Characteristics of Sheared Gabbroic Rocks From the Mado Megamullion, Shikoku Basin, Philippine Sea

ABSTRACT 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.