One of the most ubiquitous boundaries within our planet is between sheeted dikes and gabbros in fast-spreading ocean crust. This boundary marks the brittle-ductile transition at the ridge crest, and is localized by a decametric conductive boundary layer between hydrothermal circulation in the sheeted dike layer and a shallow quasi-steady-state melt lens. In contrast, at slow-spreading ridges, the crustal structure appears chaotic, with no consistent sheeted dike layer and widespread occurrences of gabbro and serpentinized peridotite on the seafloor. Recent work suggests that as much as 50% of the Atlantic Ocean crust formed by a detachment mode of seafloor spreading, including the formation of oceanic core complexes capped by long-lived, convex-upward detachment faults. These detachment faults are often associated with large hydrothermal systems in which the location of any magmatic heat source is uncertain. Here we show that detachment faults can act as thermal boundaries between gabbroic melt in the fault footwall and hydrothermal circulation in the fault zone and hanging wall, thus explaining the link between faulting and black smoker systems. We suggest that interaction between magmatism and hydrothermal circulation means that detachment faults can act as the dike-gabbro transition in the detachment mode of spreading, inevitably leading to exposure of gabbros on the seafloor through continued faulting. This concept provides a means of unifying apparently contrasting processes and crustal structures at different spreading rates.