Coupled Detachment Faulting and Hydrothermal Circulation at 49.7°E Southwest Indian Ridge Revealed by Seafloor Magnetism.

Detachment faults at slow‐ and ultraslow‐spreading mid‐ocean ridges are essential for lithosphere‐hydrosphere interactions. However, their coupling with subseafloor hydrothermal circulation is poorly understood. Here, we investigated shallow hydrothermal fluid circulations in the active Dragon Horn detachment fault system of the Southwest Indian Ridge using combined near‐bottom magnetic anomaly data and rock magnetic analyses. We observed enhanced magnetic anomalies related to the high magnetization of basaltic country rocks and highly serpentinized (>80%) peridotite, whereas the hydrothermally altered basalts and sulfide deposits had weak magnetization and reduced magnetic anomalies. Therefore, the low‐magnetization zone was interpreted as a proxy for the hydrothermal fluid channel. The spatial distribution of the low‐magnetization zone revealed that the inactive Suye field expanded more than 500 m in diameter horizontally and ∼100 m vertically, and the active Longqi hydrothermal field had a >1 km fluid channel, both centered around the detachment fault fracture zone. Our results establish a robust link between detachment faulting and shallow hydrothermal circulation in which the detachment fault system provides critical fluid paths. Therefore, detachment tectonics are important elements for evolving seafloor hydrothermal fields along the Southwest Indian Ridge and possibly other slow‐ and ultraslow‐spreading mid‐ocean ridges globally. Plain Language Summary: The detachment faults of slow‐ and ultraslow‐spreading mid‐ocean ridges are geological structures motivated by seafloor spreading. Intermitted movements of detachment faults exhume deep oceanic crust or mantle rocks and provide pathways for seafloor hydrothermal systems. The magnetic properties of rocks in seafloor detachment fault systems are sensitive to hydrothermal fluids and can be used to study fluid circulation within seafloor detachment fault systems. Using magnetic methods, we found that magnetic minerals in fresh basalt and highly serpentinized peridotite created positive magnetic anomalies, whereas the absence of magnetic minerals in hydrothermally altered basalts and sulfides decreased the magnetic anomalies. As represented by the low‐magnetization zones, the shallow hydrothermal fluid channels sit on the detachment fault, highlighting the close relationship between detachment tectonics and the evolution of seafloor hydrothermal systems. We conclude that the detachment fault system primarily controls the formation of black‐smoker chimneys along the magma‐poor mid‐ocean ridges. Key Points: Altered basalt and sulfide deposits attenuated the magnetic anomaly of the normal‐polarity zone in the Dragon Horn areaSerpentinized peridotite (>80%) produced significant positive magnetic anomalies in the Dragon Horn areaDetachment faulting controls the formation of fluid channels for hydrothermal circulation [ABSTRACT FROM AUTHOR]