Evidence from gravity anomalies for interactions of the Marion and Bouvet hotspots with the Southwest Indian Ridge: effects of transform offsets

The ultra-slow spreading Southwest Indian Ridge (SWIR) presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry. Using recently available satellite free-air gravity and shipboard bathymetry data, we obtain mantle Bouguer (MBA) and residual mantle Bouguer anomalies (RMBA) by removing from free-air gravity the attractions of seafloor topography, sediment thickness variations, a reference crust, and theoretically predicted effects of lithospheric cooling. The Bouvet hotspot, previously observed to be associated with anomalous bathymetry and geochemistry near the Bouvet triple junction, has an MBA axial gravity low of ~100 mGal, implying pronounced localized crustal thickening. Off-axis, the RMBA lows along previously calculated Bouvet hotspot tracks are variable in amplitude, suggesting the possibilities that Bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center. Along-axis geophysical anomalies suggest that the Marion hotspot has a significant effect on accretionary processes in the central portion of the SWIR. Importantly, the Marion axial anomaly appears to be compartmentalized between the Andrew Bain and Discovery II fracture zones, implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented, ultra-slow spreading system.