40Ar/39Ar ages of samples dredged and drilled from along the Walvis Ridge (O'Connor and Duncan, 1990) and the St. Helena Seamount Chain (O'Connor and le Roex, 1992) have been used previously to reconstruct African plate motion since the opening of the S. Atlantic. However, because of the limited number of samples recovered from the St. Helena Chain prior to this study, these reconstructions have relied primarily on age data for the Walvis Ridge. In order to more rigorously combine the time-space histories of these chains, the F/S SONNE (Devey et al., 1994) and F/S METEOR have been used to dredge-sample five seamounts extending to the west of the island of St Helena. The broadness (O'Connor and Le Roex, 1992; Brozena and White, 1990) of the zone of volcanism associated with the impacted head of the St. Helena plume was investigated by sampling Circe Seamount (Schilling et al., 1985; Hanan et al., 1986; Graham et al., 1992; Chaffey et al., 1989), located 900 km to the NW of St. Helena Island.
New 40Ar/39Ar results show that the ages of the intraplate St. Helena seamounts decrease systematically from near St. Helena Island towards the S. Atlantic spreading-axis. Combining these new ages with those published Ridge (O'Connor and Duncan, 1990; O'Connor and le Roex, 1992) for St. Helena seamounts and the Walvis Ridge leads to a more robust estimate of African plate speed since the opening of the S. Atlantic. This increases our confidence in both the fixity of the St Helena and Tristan plumes and in the reconstruction African plate motion based on the combined time-space information from their respective volcanic trails. The fixity of the St Helena plume supports the suggestion that the distinctive high-Pb isotopic signature associated with the St. Helena plume (Schilling et al., 1985; Hanan et al., 1986; Graham et al., 1992; Chaffey et al., 1989) originates from a deep region in the mantle.
The age and location of the rock sample dredged from Circe Seamount shows that it erupted in an intraplate tectonic setting and that it was formed by the motion of the African plate over the St Helena plume. We conclude therefore that Circe Seamount is part of the St. Helena Seamount Chain, which can now be defined as an approximately
900 km-wide zone of scattered seamounts. This supports earlier suggestions that the impacted heads of St Helena (O'Connor and le Roex, 1992; Brozena and White, 1990) and Walvis (O'Connor and le Roex, 1992) plumes extend over broad regions under the lithosphere, so leading to the formation of wide volcanic trails. The compositional anomalies occurring along an approximately 1200 km section of the S. Atlantic spreading-axis could reflect the westward flow of material (Schilling et al., 1985; Hanan et al., 1986; Graham et al., 1992) from a broad St Helena plume head. This is probably occuring via a series of sub-lithospheric conduits (Schilling et al., 1985; Hanan et al., 1986; Graham et al., 1992). Tomographic studies in the St. Helena region confirm the existence of a broad region of warm mantle under St Helena Island that is flowing to the S. Atlantic spreading-axis. A broad Tristan plume can similarly explain the 550 km distance between Tristan da Cunha and Gough islands and the compositional anomalies along the spreading-axis to the west (Schilling et al., 1985; Hanan et al., 1986; Graham et al., 1992).
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