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A new type of intra‐plate volcanism; Young alkali‐basalts discovered from the subducting Pacific Plate, Northern Japan Trench

Alkali pillow basalts were collected from the toe of the oceanward slope of the northern Japan Trench. These alkali‐basalts formed as a result of a low degree of partial melting of Pacific Ocean mantle and rapid rise of the magma (no fractionation in shallow magma chambers). Reconstructing Pacific Plate motion based on 40Ar‐39Ar age dates of 5.95±0.31 Ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the Japan Trench in the NW Pacific. We suggest that these alkali‐basalts represent a new form of intra‐plate volcanism, whereby magmatic activity occurs off the forebulge of the downgoing Pacific slab, perhaps using conduits related to fracturing of the slab during bending prior to subduction.