Evidence for a large-scale remnant of subducted lithosphere beneath Fiji

We combine spatial variations of P- and S-wave speeds, 1000 fault plane sol utions, and 6600 well-determined hypocenters to investigate the nature of s ubducted lithosphere and deep earthquakes beneath the Tonga back-are. We sh ow that perplexing patterns in seismicity and fault plane solutions can be accounted for by the juxtaposition of a steep-dipping Wadati-Benioff zone a nd a subhorizontal remnant of slab that is no Longer attached to the active ly subducting Lithosphere. The detached slab may be from a previous episode of subduction along the fossil Vitiaz trench about 5 to 8 million years ag o. The juxtaposition of slabs retains a large amount of subducted material in the transition zone of the mantle. Such a configuration, if common in th e past, would allow the preservation of a primordial component in the Lower mantle.