A 3-DIMENSIONAL GRAVITY STUDY OF THE RODRIGUES TRIPLE JUNCTION AND SOUTHEAST INDIAN RIDGE

Three-dimensional analysis of gravity and bathymetry data is used to i nvestigate the density structure of the Rodrigues Triple Junction and the first segment of the Southeast Indian Ridge south of the triple ju nction. The distribution of mantle Bouguer and residual gravity anomal ies suggests that focused upwelling of mantle is occurring along the n early co-linear Central Indian and Southeast Indian Ridge limbs of the triple junction. In contrast, the mantle Bouguer anomaly over the Sou thwest Indian limb of the triple junction shows little variation despi te nearly 4 km of topographic relief within this segment of the Southw est 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 compensate d by thinning oceanic crust; only similar to 1 km of crustal thinning within the axial valley is expected, based on our gravity analysis. Fu rther, no clear long-wavelength thermal signal associated with lithosp heric growth can be associated with the Southwest Indian Ridge in this region. These observations imply that focused upwelling is not occurr ing on this segment of the Southwest Indian Ridge and require a primar ily dynamic compensation mechanism for the extreme axial topography. O ur analysis indicates that the Rodrigues Triple Junction system is dom inated by mantle upwelling associated with the Southeast and Central I ndian Ridges.