Gao Jinyao

Magmatism and tectonic processes in Area A hydrothermal vent on the Southwest Indian Ridge

The hydrothermal vent in Area A (37.78°S, 49.65°E) is the first active hydrothermal vent discovered on the Southwest Indian Ridge (SWIR). Heat source and adequate bulk permeability are two necessary factors for the formation of a hydrothermal vent. Along the SWIR 49.3°E to 51.2°E, the gravity-derived crustal thickness is up to 9.0 km, much thicker than the average thickness of the global oceanic crust. This characteristic indicates that the magma supply in this area is robust, which is possibly affected by a hotspot. The large-scale residual mantle Bouguer anomalies (RMBA) reveal prominent negative-gravity anomalies between the first-order ridge segment (from Indomed to Gallieni, 46.0°E to 52.0°E) and the Marion-Del Cano-Crozet region. These anomalies indicate the channel of the hotspot-ridge interaction. The tomography data corrected with theoretical thermal model indicate that the low-velocity anomalies corresponding to this channel can reach the base of the lithosphere. Near the hydrothermal vent area, the topography and crustal thickness at the off-axis area are extremely asymmetrical. South of the SWIR, the high topography corresponds to the thinning crustal thickness. The residual isostatic topography anomalies indicate that Area A is a deviation from the local isostatic equilibrium, similar to the characteristics of the transform fault inside corner. The forward profiles of the magnetic data indicate that the thinning magnetic layer at the south side of Area A corresponds to the shallow, high-velocity area revealed by the OBS, which is the result of tectonic extension of a detachment fault. The active tectonic processes in Area A can provide sufficient crustal permeability to the hydrothermal circulation and may form massive sulfide deposits.