Long-lived detachment faults are now known to be important in tectonic evolution of slow-spreading mid-ocean ridges, and there is increasing evidence that fluid flow plays a critical role in development of detachment systems. Here we document a new manifestation of low-temperature hydrothermal venting associated with the detachment fault that formed Kane Megamullion ∼3.3–2.1 m.y. ago in the western rift-valley wall of the Mid-Atlantic Ridge. Hydrothermal effects on the detachment surface include (1) cemented mounds of igneous rock and chalk debris containing hydrothermal Mn oxides and Fe oxyhydroxides, and (2) layered deposits of similar Fe-Mn minerals ± interbedded chalks. Mounds are roughly conical, ∼1–10 m high, and contain primarily basalts with lesser gabbro, serpentinite, and polymict breccia. The layered Fe-Mn-rich sediments are flat-bedded to contorted and locally are buckled into low-relief linear or polygonal ridges. We propose that the mounds formed where hydrothermal fluids discharged through the detachment hanging wall near the active fault trace. Hydrothermal precipitates cemented hanging-wall debris and welded it to the footwall, and this debris persisted as mounds as the footwall was exhumed and surrounding unconsolidated material sloughed off the sloping detachment surface. Some of the layered Fe-Mn-rich deposits may have precipitated from fluids discharging from the hanging-wall vents, but they also precipitated from low-temperature fluids venting from the exposed footwall through overlying chalks. Observed natural disturbance and abnormally thin hydrogenous Fe-Mn crusts on some contorted, hydrothermal Fe-Mn-rich chalks on ∼2.7 Ma crust suggest diffuse venting that is geologically recent. Results of this study imply that there are significant fluid pathways through all parts of detachment systems and that low-temperature venting through fractured detachment footwalls may continue for several million years off-axis.