TIME-SPACE MAPPING OF EASTER CHAIN VOLCANISM

New Ar-40/Ar-39 and published K-Ar ages show that the locus of volcani sm along the Easter Volcanic Chain (EVC) has shifted systematically fr om the Nazca Ridge, at about 26 m.y., to the recently active Sala y Go mez Island/Easter Island region. This indicates a plume rather than a hotline (i.e., mantle roll) origin for the EVC. The time-space distrib ution of ages, combined with published ages for the Galapagos and Juan Fernandez volcanic chains, is used to reconstruct Nazca plate velocit ies over the past 26 m.y. A plume now located in the region of Sala y Gomez Island is most compatible with these data. West of the plume, th e EVC records neither Nazca nor Pacific plate motions. This section of the EVC may be related to westward channeling of plume material to th e Pacific-Nazca spreading boundary region.