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

A remarkable gap in seigmic 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 $ waves from deep earthquakes supports the idea that the deep earthquakes located at the northeast of the arc represent a detached piece of lithosphere. All except three of the deep earthquakes are located in a nearly horizontal zone between latitudes 12øS and 15øS. The earthquakes were. relocated by using travel times to 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 less 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 mantle above the inclined seismic zone. The computed travel times for this model explain the observed travel time residuals along the New Hebrides arc.