Microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the Godzilla Megamullion (site KH07-02-D18), located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a high-temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust. The deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite, and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene/amphibole porphyroclasts in a fine-grained matrix. Plagioclase crystallographic-preferred orientations vary from (010)100 and (001)100 patterns in the mylonites to a weak (001)100 pattern in the some mylonites and ultramylonite, suggesting a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep with increasing intensity of deformation. The chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts. Secondary amphibole is ubiquitous, consisting mainly of pargasite and magnesiohornblende (brown hornblende) and actinolite (green hornblende). The mineral assemblage is consistent with the hydrothermal metamorphic reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. Compared with deformed gabbroic rocks from the breakaway and termination areas of the Godzilla Megamullion, the samples record ductile shearing under high temperature conditions, possibly related to the development of the Godzilla Megamullion, although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas. ► Fe-Ti oxide gabbros at the medial area of the Godzilla Megamullion deformed under high temperature with hydrous fluid. ► Hydrothermal alteration resulted in retrograde metamorphism associated with deformation of the Fe-Ti oxide gabbros. ► The deformed gabbros in this study possibly relate to the development of the Godzilla Megamullion. ► Hydrothermal activity associated with the deformation of gabbroic rocks seems to be distinct from breakaway, medial to termination areas at the Godzilla Megamullion.