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

Abstract

The distribution of intermediate-depth seismicity beneath the Vanuatu archipelago (South Pacific Ocean) includes several seismic clusters/alignments and gaps, including a remarkable 200 × 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 12 S Ridge (DER), and that another region of aligned seismicity to the south may correspond to another signicant, now-disappeared ridge. We suggest that regions of aligned intermediate- depth seismicity beneath Vanuatu are traces of subducted oceanic features on the downgoing 14 S 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.