Crystallization ages of volcanic rocks, dredged or drilled from the Walvis Ridge (ten sites) and the Rio Grande Rise (one site), have been determined by the 40Ar/39Ar incremental heating technique. The fundamentally age‐progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of Tristan da Cunha, along the Walvis Ridge and Rio Grande Rise, and for the formation of the continental flood basalts located in Namibia (Africa) and Brazil (South America). The Walvis Ridge‐Rio Grande Rise volcanic system evolved along a section of the South Atlantic spreading‐axis, as the African and South American plates migrated apart, astride, or in close proximity to, an upwelling plume. Reconstructions of the spatial relationship between the spreading‐axis, the Tristan hot spot, and the evolving Walvis Ridge‐Rio Grande Rise volcanic feature show that, at about 70 Ma, the spreading‐axis began to migrate westward, away from the hot spot. The resulting transition to intraplate hot spot volcanism along the Walvis Ridge (and associated termination of Rio Grande Rise formation) also involved a northward migration of previously formed African seafloor over the hot spot. Rotation parameters for African motion over fixed hot spots (i.e., absolute motion) have been recalculated such that the predicted trail of the Tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the Walvis Ridge. African absolute motion has been extended to the South and North American plates, by the addition of relative motion reconstruction poles.

[1] Estimates of the relative motion between the Hawaiian and Louisville hot spots have consequences for understanding the role and character of deep Pacific-mantle return flow. The relative motion between these primary hot spots can be inferred by comparing the age records for their seamount trails. We report the seamount chain, including the HEB. This model predicts an age for the oldest Emperor Seamounts that matches published ages, implying that a linear age-distance relationship might extend back to at least 82 Ma. In contrast, Hawaiian age progression was much faster since at least $15 Ma and possibly as early as $27 Ma. Linear age-distance relations for the Hawaii-Emperor and Louisville seamount chains predict $300 km overall hot spot relative motion between 80 and 47.5 Ma, in broad agreement with numerical models of plumes in a convecting mantle, and paleomagnetic data. We show that a change in hot spot relative motion may also have occurred between $55 Ma and $50 Ma. We interpret this change in hot spot motion as evidence that the HEB reflects a combination of hot spot and plate motion changes driven by the same plate/mantle reorganization.

No abstract