Tracking subducted ridges through intermediate-depth seismicity in the Vanuatu subduction zone

The distribution of intermediate-depth seismicity beneath the Vanuatu archipelago (South Pacific Ocean) includes several seismic clusters/alignments and gaps, including a remarkable 200 x 150 km gap beneath central Vanuatu. We show that a zone of enhanced seismicity beneath this gap corresponds to the most likely trace of the subducted D'Entrecasteaux Ridge (DER), and that another region of aligned seismicity to the south may correspond to another significant, now-disappeared ridge. We suggest that regions of aligned intermediate-depth seismicity beneath Vanuatu are traces of subducted oceanic features on the down-going plate, probably caused by enhanced hydration beneath them prior to subduction. The seismic gap above the DER trace, previously interpreted as a tear in the slab, may instead be a region of reduced hydration due to thick/unfractured input crust, low bending of this crust before subduction, and/or greater slab residence time at shallower subducted depths, the latter two being directly related to DER collision. We propose that an uneven distribution of intermediate-depth seismicity at subduction zones may provide a window to the history and effects of subducted oceanic features.