Altimetry data from repetitive orbits of the GEOSAT satellite are interpreted together with sea-floor depths over the Mid-Atlantic Ridge, South of Iceland and South of the Azores. Geoid / Topography ratios indicate a strong asymmetry of the upper mantle structure in oceanic areas created since 35 My, extending several hundreds kilometers from Iceland to the Azores. A model including interaction between the MAR and two layers of mantle convection seems adequate to produce the observed asymmetry.