Plate reconstructions and tomography reveal a fossil lower mantle slab below the Tasman Sea

The Southwest Pacific region is tectonically complex and is home to numerous fossil and active subduction zones. At the Earthʹs surface, there remains a geological controversy regarding the polarity and continuity of fossil subduction zones in New Zealand and New Caledonia, the origin of obducted ophiolites, the presence of high-pressure metamorphism, the occurrence of widespread Cenozoic magmatism, and the potential disappearance of one or more ocean basins. This controversy can be solved by looking at the lower mantle rather then at the Earthʹs surface. New P-wave and S-wave mantle tomography models from the Southwest Pacific are presented, which identify a previously unrecognized lower-mantle high-velocity anomaly that cannot be linked to Pacific subduction. The anomaly is located below the Tasman Sea at ~ 1100 km depth, strikes NW–SE and is ~ 2200 km by 600–900 km in lateral extent. By combining relative and absolute plate motions it is demonstrated that when the geological structures at the surface are reinterpreted as a single northeast-dipping 2500-km middle Cenozoic subduction zone (the so-called New Caledonia subduction zone) the lower mantle anomaly can be accounted for, as it is found at the predicted location and depth. Discovery of the lower mantle slab anomaly thereby solves a long-standing geological controversy in the New Zealand–New Caledonia region. Finally, reconstructions and analytical calculations predict a lower mantle slab sinking velocity of ~ 1.5 cm/yr and a lower mantle viscosity of ~ 1022 Pa·s.