Complex morphology of subducted lithosphere in the mantle beneath the Tonga trench

Abstract

AT the Tonga trench, old Pacific sea floor subducts at a rapid rate below the Indo-Australia plate, generating most of the world's deep earthquakes (focal depth >300 km)1,2 and producing a deep slab of former oceanic lithosphere. The seismogenic part of the slab has been mapped in detail3,4, but its fate has remained enigmatic. Here I present evidence from seismic tomography that the Pacific plate descends deep into the Earth's mantle along a trajectory that is more complex than previously thought. In the north, the slab deflects in the transition zone (between about 400 and 700 km depth) before continuing into the lower mantle (below 700 km). Further south, penetration into the lower mantle occurs without a kink. The slab morphology can be explained in terms of the recent tectonic evolution of the subduction system, and reconciles pre-existing evidence from this region for both local horizontal flow in the transition zone2–8 and slab penetration into the lower mantle9–12.