Observed seafloor depths from 12 kHz bathymetry coupled with seafloor spreading magnetic anomalies were used to obtain depth anomalies across the Mid-Atlantic ridge axis between 2ºS and 18ºS. The theoretical seafloor depths were calculated from the plate cooling model and crustal ages depicted from seafloor spreading magnetic anomalies between 0 to 25 Ma. We noticed that the seafloor spreading magnetic anomalies are symmetric with respect to the ridge axis between Ascension and St. Helena Islands, but the depth anomalies are quite asymmetric. The east side of the ridge axis is shallower (~ 1,000 meters) than the west side, probably as a result of a thermal upwelling of the lithosphere. We suggest that a mantle plume located on the African Plate, which could represent a larger swell linked to Ascension, St. Helena and Fernando Pó-Annobon Volcanic Lineament, might cause this type of anomaly.
depth anomalies; magnetic anomalies; Ascension Island; St. Helena Island; South Atlantic
