Age distribution of Ocean Drill sites across the Central Walvis Ridge indicates plate boundary control of plume volcanism in the South Atlantic

The Tristan–Gough hotspot trail on the African plate consists of the Walvis Ridge and a younger province of seamounts and islands. In order to determine the relative motion between the African plate and the Tristan–Gough hotspot it is essential to resolve changes in the age and morphology of the Walvis Ridge. A significant problem is, however, to establish how the vigor and flow of hotspot material to the mid-ocean ridge constructed the Walvis Ridge. We have addressed this issue by measuring an40Ar/39Ar stratigraphy at three sites across the central Walvis Ridge sampled by Ocean Drilling (DSDP Leg 74). The age–distance relation of volcanism, together with geophysical, geochemical and paleodepth information, suggestscollectively that hotspot volcanism was occurring locally c. 72Ma on an elevated segment of the mid-ocean ridge located close to the Tristan–Gough hotspot. As the mid-ocean ridge migrated away from the hotspot (c. 36km/Ma) between c. 72Ma and 68Ma, hotspot material continued flowing to the mid-ocean ridge and the Walvis Ridge shoaled rapidly (c. 500m/Ma) to the west, on seafloor that might have been subsiding at a rate consistent with normal crustal cooling. This apparent correlation points to the possibility of an inverse relation between the volume flux of hotspot volcanism and the distance between the mid-ocean ridge and the Tristan–Gough hotspot. We infer that since c. 93Ma the geometry and motion of the mid-ocean ridge determined where the hotspot material that built the Walvis Ridge was channeled to the plate surface. Furthermore, interplay between hotspot flow, and the changing geometry of the mid-ocean ridge as it migrated relative to the Tristan–Gough hotspot, might explain the age and morphology of the Walvis Ridge. Our finding provides further evidence that the distribution of hotspot volcanism in the southeast Atlantic expresses interaction between deep mantle (plume) and shallow plate tectonic and asthenosphere processes.