Isostasy of aseismic tectonic units in the South Atlantic Ocean and geodynamic implications

Mechanical isostasy of the lithosphere in the South Atlantic Ocean was studied using information on gravity anomalies and bathymetry with additional constraints imposed by the altimetric geoid. The isostatic responses (admittances) over the Walvis Ridge, Rio Grande Rise and Trindade Seamount Chain were computed using a three-dimensional algorithm. The eastern Walvis Ridge and the Rio Grande Rise have the same response, which is well explained by an Airy model of isostasy. The other features are regionally supported. A variation in the thickness of the elastic plate was found along the western Walvis Ridge. A high value of the elastic plate thickness (20 km) was found under the Trindade Chain. Geodynamic implications are discussed in the light of these results.