The Slab Portal Beneath the Western Aleutians

Tomographic images of the distribution of shear wave speed beneath the northwestern Pacific delineate the configuration of the subducted oceanic lithosphere beneath the western Aleutian Arc. At ~100 km depth, a fast shear wave speed anomaly lies beneath the Aleutian Arc everywhere east of 173E. Between 164E and 173E, however, seismic velocities at this depth are slow relative to the surrounding mantle. The lateral termination of the fast shear wave speed anomaly at depth coincides with a gap in deep seismicity beneath the Aleutians. The absence of these two distinctive traits of subducting slabs leads us to conclude that this segment of the Aleutian Arc overlies a portal in the otherwise continuous lithospheric slab. The portal is likely to facilitate the production of distinctive volcanic rocks (adakites or high Mg # andesites) by partially melting the adjacent edges of the slab. The Miocene age of adakites above the westernmost Aleutians may indicate that the portal was formed relatively recently.