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 l ong-lived South Atlantic hotspot trails is important to understanding the c onnection between hotspot volcanism and mantle plumes. Ar-40/Ar-34 ages are reported here for rocks dredged from a line of five individual seamounts a long an similar to 290 km northeast to southwest line extending from the vi cinity of Saint Helena Island, and also for Circe Seamount. These seamounts were created in a midplate setting and could have formed rapidly (less tha n or equal to 1 Myr). The St. Helena Seamount ages reveal a remarkably line ar migration rate of volcanism of 20 +/- 1 mm/yr for at least the past 19 M yr, which is interpreted as the absolute motion of the African plate. Becau se this is much slower than estimated for earlier African plate migration i t also represents the first evidence based on seamount ages for a significa nt deceleration (similar to 33%) of the African plate since at least 19 Ma. However, this change could have occurred as early as 30 Ma when the limite d data for the Tristan/Gough hotspot chain are also considered. This decele ration supports a relationship between African plate speed and the upsurge of hotspot volcanism on the African continent at similar to 25 Ma. We sugge st that the increased number of oceanic African hotspots between similar to 19 and 30 Ma points to a Link also between major changes in plate motion a nd the onset and continuation of oceanic hotspot volcanism. Our study suppo rts the assumption that chains of individual, rapidly (?) formed seamounts have considerably more potential of providing clear insights into how mantl e plumes interact with overriding lithosphere than do those consisting of u ninterrupted, more massive lines of hotspot volcanism. (C) 1999 Elsevier Sc ience B.V. All rights reserved.