Contrasted interactions between plume, upper mantle, and lithosphere: Foundation chain case

The Foundation Seamounts are a volcanic chain formed during the last 21 m.y. by the action of a hot spot, presently located near the axis of the Pacific‐Antarctic Ridge. The western part of the chain is formed by volcanoes with ages ranging from 21 to 16 m.y., spatially distributed along two parallel lines roughly 200 km apart. The shape and distribution of the edifices are strongly suggestive of an emplacement along preexisting fissures or fractures, resulting from lithospheric deformation during the major kinematic reorganization of 25 m.y. Their chemical and isotopic composition becomes enriched toward young volcano ages, resulting from the progressive contamination of the upper mantle by the arrival of a plume pulse. The eastern part of the chain is formed by volcanoes younger than 5 m.y., also distributed along two subparallel lines. The distance between these two lines diminishes toward the Pacific‐Antarctic Ridge axis. A geoid high is located very near the Pacific‐Antarctic Ridge axis, over the volcanoes of the north line, which lies in the prolongation of the older Foundation Seamounts, and possibly marks the location of the Foundation hot spot. Both the geoid anomaly and the morphology of the edifices show that the north line is the main locus of the volcanism. The southern line was probably formed on top of the flexural arch resulting from the emplacement of the north line. The chemical and isotopic compositions of the basalts reveal a growing influence of the ridge on the off‐axis plume magmatism. The pattern is coherent with a mixing between two sources, occurring when the two melting zones merge and overlap.