An extensive deep-tow magnetic survey of the TAG ridge segment on the Mid-Atlantic Ridge reveals new information about the relationship between the magnetic anomaly field and the TAG hydrothermal deposits. Results show the strongest magnetization is located over the neovolcanic axis and asymmetrically toward the western side of the central Brunhes anomaly. A well-defined linear magnetization low is located over the eastern rift valley wall of the TAG segment. The near-bottom data show no direct correlation between this crustal magnetization low and the hydrothermal deposits. The magnetization low is explained by crustal thinning caused by 4 km of horizontal extension along a normal fault. Previous observations and sampling indicate exposures of gabbros and dikes in the eastern rift valley wall, suggesting slip along a normal fault has revealed this crust. Modeling suggests the fault has been active since 0.35 ¿ 0.1 Ma at a horizontal slip rate of roughly half the spreading rate of 22 km/Myr. The TAG hydrothermal system is located on the hanging wall of this fault within 3 km of its termination. Over the past several hundred thousand years, movement on the detachment fault may have episodically increased the permeability of the hanging wall reactivating the overlying hydrothermal systems. Significant vents like TAG may be typically associated with hanging walls of long-term detachment faults near seafloor spreading centers. This would imply that it is the reactivation of permeability in the hanging wall related to repeated fault movement that controls the longevity of these hydrothermal systems.