Cause of low velocity anomaly along the South Atlantic hotspots

Abstract

A recent upper‐mantle seismic velocity model shows a narrow strip of asthenospheric low velocity anomaly in the South Atlantic that connects four hotspots, Tristan da Cunha, St. Helena, Ascension and Cape Verde. This deep strip of slow velocity anomaly runs approximately along the seam where Africa and South America broke up, according to available plate reconstruction models. There is, thus, a possibility that the deep, slow velocity strip is preserving the shape of the boundary along which the break‐up occurred. If this is the case, it will be a useful constraint in the reconstruction of plates. A question arises, however, as to why there is a line of low velocity anomaly rather than localized low velocity anomalies near hotspots. We propose three possible causes for this observation, which are (1) upwelling along this line, (2) lithosphere‐controlled passive breakup and (3) plume controlled breakup with lateral channeled flow from plumes to the seam of breakup, when rifts were created between adjacent hotspots. We favor the last hypothesis since it naturally explains the local velocity minimums near hotspots as well as the connected low velocity anomaly and, furthermore, the same mechanism seems to be working under the Red Sea in the present Earth.