First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma

Resolving the time–space (and compositional) evolution of volcanism along long-lived South Atlantic hotspot trails is important to understanding the connection between hotspot volcanism and mantle plumes. Ar=Ar ages are reported here for rocks dredged from a line of five individual seamounts along an 3⁄4290 km northeast to southwest line extending from the vicinity of Saint Helena Island, and also for Circe Seamount. These seamounts were created in a midplate setting and could have formed rapidly ( 1 Myr). The St. Helena Seamount ages reveal a remarkably linear migration rate of volcanism of 20š 1 mm=yr for at least the past 19 Myr, which is interpreted as the absolute motion of the African plate. Because this is much slower than estimated for earlier African plate migration it also represents the first evidence based on seamount ages for a significant deceleration (3⁄433%) of the African plate since at least 19 Ma. However, this change could have occurred as early as 30 Ma when the limited data for the Tristan=Gough hotspot chain are also considered. This deceleration supports a relationship between African plate speed and the upsurge of hotspot volcanism on the African continent at 3⁄425 Ma. We suggest that the increased number of oceanic African hotspots between 3⁄419 and 30 Ma points to a link also between major changes in plate motion and the onset and continuation of oceanic hotspot volcanism. Our study supports the assumption that chains of individual, rapidly (?) formed seamounts have considerably more potential of providing clear insights into how mantle plumes interact with overriding lithosphere than do those consisting of uninterrupted, more massive lines of hotspot volcanism.  1999 Elsevier Science B.V. All rights reserved.