Ridge crest-trench interactions along continental destructive plate margins may result in the development of slab-free windows beneath the continental margin. Slab windows were generated at various locations along the Pacific margin of the Americas and the Antarctic Peninsula during the past 70 m.y. Slab-window formation is temporally and spatially associated with mafic, alkalic volcanism. Lavas erupted above the loci of slab windows are geochemically indistinguishable from some ocean-island, plume-related basalts. However, generation of slab-window basalts from deep-seated mantle plumes requires the fortuitous initiation of plume activity following cessation of subduction. Asthenospheric upwelling and associated decompressional melting following slab-window formation are probably promoted by removal of subducted oceanic lithosphere from beneath the continental margin following the cessation of subduction. Major lithospheric extension is not a prerequisite for alkalic volcanism in this case. The close association of subduction-related volcanism and within-plate alkalic volcanism within the geologic record may also be explained by this mechanism.

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