Linear volcanism in French Polynesia

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The island chains of French Polynesia form subparallel line segments whose southeasterly extensions are perpendicular to the East Pacific Rise, the site of present sea-floor spreading in the eastern Pacific Ocean. Samples collected from island members of the Society and Austral Islands chains are used, together with previously reported age determinations for the Marquesas and Pitcairn-Gambier Islands, in a geochronological study of the southeastward migration of volcanism in each of those four lineaments. The suggestion from geomorphologic evidence that island ages increase to the northwest within each island chain, is confirmed by KAr whole-rock ages. The linear volcanism which built the islands of French Polynesia began in the Miocene and continues today. Rates of migration of volcanism are calculated from the nearly linear relationship between average island ages and distance from the southeast ends of the four island lineaments. The four rates are indistinguishable, within limits of detection, at 11 ± 1 cm/year. These rates are consistent with the model of rigid Pacific plate movement over four fixed sources of volcanism, be they dynamic as in the hot spot/plume models or passive as in models of propagating lithospheric fractures. If it is accepted that these volcanic sources trace the motion of the lithosphere over the mantle and thus define the “absolute” frame of reference for plate movement, Pacific plate motion may be fixed to the geometry and volcanic migration rates of French Polynesia. This allows calculation of the absolute motion of all other plates, providing an accurate relative motion model is known (Minster et al., 1974). Such a calculation predicts that Africa is virtually stationary and that the Mid-Atlantic Ridge and East Pacific Rise are moving slowly to the west.