Rio Grande Rise Hot Spot System' Implicationsfor African and South American Plate Motions Over Plumes
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. BI1, PAGES 17,475-17,502,OCTOBER 10, 1990 Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System' Implicationsfor African and South American Plate Motions Over Plumes JOHN M. O'CONNOR AND ROBERT A. DUNCAN College of Oceanography,Oregon State University,Corvallis Crystallizationages of volcanicrocks, dredged or drilled from the Walvis Ridge (ten sites)and the Rio GrandeRise (one site), have been determined by the 40Ar/39Ar incremental heating technique. The fundamentallyage-progressive distribution of thesebasement ages suggests a commonhot spot sourcefor volcanismon 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 WalvisRidge-Rio Grande Rise volcanic system evolved along a sectionof the SouthAtlantic spreading-axis, as the African and South American plates migratedapart, astride,or in closeproximity to, an upwelling plume. Reconstructionsof the spatialrelationship between the spreading-axis,the Tristan hot spot, and the evolvingWalvis Ridge-Rio Grande Rise volcanic feature show that, at about70 Ma, thespreading-axis began to migratewestward, away from the hot spot. The resultingtransition to intraplatehot spotvolcanism along the Walvis Ridge (and associatedtermination of Rio GrandeRise formation)also involveda northward migrationof previouslyformed African seafloor over the hot spot. Rotationparameters for Africanmotion overfixed hot spots(i.e., absolutemotion) have been recalculated such that the predictedtrail of the Tristan hotspot agrees with the distribution of radiometricand fossil basement ages along the Walvis Ridge. African absolutemotion has been extended to the Southand North American plates, by the additionof relativemotion reconstructionpoles.
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