Seismic velocity anomalies beneath the New Hebrides Island Arc: Evidence for a detached slab in the upper mantle

Abstract

A remarkable gap in seismic activity exists between depths of about 300 and 600 km beneath the New Hebrides island arc.A detailed study of travel time residuals of P and X waves from deep earthquakes supports the idea that the deep earthquakes located at the northeast of the arc represent a det.ached piece of lithosphere.All except three of the deep earthquakes are located in a nearly horizontal zone between latitudes 12"s and 15"s.The earthquakes were relocated by using travel times l o distant stations and excluding travel times to local and regional stations.Travel times for P waves from the deep earthquakes that travel through the dipping New Hebrides seismic zone are about 3 sec lem than those for waves that travel through the probably normal mantle beneath New Caledonia.A threedimensional seismic ray tracing program is used to compute P travel times for a velocity model beneath the New Hebrides arc.The model includes a 6% higher velocity relative to a, normal mantle inside a dipping lithospheric slab 300 km deep, a 6% higher velocity inside a detached slab at a depth of 600 km, and a 4% lower velocity in the wedge of mantIe above the inclined seisinic zone.The computed travel times for this model explain the observed travel time residuals along the New Hebrides arc.