The Atlantis Massif (Mid-Atlantic Ridge, 30¿N) is an example of an oceanic core complex (OCC) exposed by a major fault system. Our integrated field and analytical study of mafic and ultramafic rocks exposed on the south wall of the massif demonstrates the complex interplay of fluids, mass transfer, and metamorphism in strain localization associated with the evolution of a major detachment shear zone and development of this OCC. Extensive talc-amphibole-chlorite metasomatism as well as heterogeneous, crystal-plastic to cataclastic deformation characterize a strongly foliated, 100-m-thick zone of detachment faulting. The metasomatic fault rocks are key elements of this OCC and record a deformation and metamorphic history that is distinct from the underlying basement rocks. Talc-rich fault rocks preserve textural and geochemical characteristics of their ultramafic protoliths. Although primary textures and mineral parageneses are commonly obliterated in rocks dominated by amphibole, bulk rock data point to a mafic protolith. Major and trace elements indicate a complex mutual interaction between gabbroic and ultramafic rocks during metasomatism, which together with microstructures suggest localized circulation of oxidizing, Si-Al-Ca-rich fluids and mass transfer in high strain deformation zones. This type of flow was distinct from the more pervasive circulation that led to strongly serpentinized domains in the south wall. In contrast, cataclastic microfracturing is associated with a dominantly static metasomatism in less deformed domains, suggesting that a significant amount of metasomatism was controlled by diffuse flow and mass transfer associated with fractures that lack a strong preferred orientation. Distinct differences in lithologies, metamorphic overprinting, and degree of deformation between the south wall and central dome of the Atlantis Massif demonstrate the complex lateral and vertical heterogeneity in composition, alteration, and structure of this OCC.