A three-dimensional gravity study of the Rodrigues Triple Junction and Southeast Indian Ridge

Abstract

Three-dimensional analysis of gravity and bathymetry data is used to investigate the density structure of the Rodrigues Triple Junction and the first segment of the Southeast Indian Ridge south of the triple junction. The distribution of mantle Bouguer and residual gravity anomalies suggests that focused upwelling of mantle is occurring along the nearly co-linear Central Indian and Southeast Indian Ridge limbs of the triple junction. In contrast, the mantle Bouguer anomaly over the Southwest Indian limb of the triple junction shows little variation despite nearly 4 km of topographic relief within this segment of the Southwest Indian Ridge. The absence of a significant mantle Bouguer anomaly over the Southwest Indian Ridge near the triple junction suggests that the rift valley observed on this segment is not completely compensated by thinning oceanic crust; only ∼ 1 km of crustal thinning within the axial valley is expected, based on our gravity analysis. Further, no clear long-wavelength thermal signal associated with lithospheric growth can be associated with the Southwest Indian Ridge in this region. These observations imply that focused upwelling is not occurring on this segment of the Southwest Indian Ridge and require a primarily dynamic compensation mechanism for the extreme axial topography. Our analysis indicates that the Rodrigues Triple Junction system is dominated by mantle upwelling associated with the Southeast and Central Indian Ridges.