Geodynamic modeling of the South Pacific superswell

Abstract

The South Pacific superswell is a broad region situated in the south central Pacific, characterized by numerous geophysical anomalies including very shallow seafloor compared to the depth predicted for its age by classical seafloor subsidence models, a negative geoid anomaly, a mantle characterized by slow seismic velocities, and a high volcanism concentration. Until recently, the image of the mantle provided by the seismic tomography models was rather blurry due to the sparse distribution of seismometers. This shortcoming has been lately overcome by regional seismic observations on islands and seafloor. The new P-wave seismic tomography model derived from these regional data in addition to global data provides a more reliable and precise image of the mantle, in particular beneath the French Polynesia region. We use it to perform numerical simulations of the instantaneous flow occurring in the mantle, using realistic laws for converting velocity anomalies into density anomalies and for describing the viscosity variations. We compute the associated dynamic topography and geoid anomaly. We show that the superswell could be caused by the large-scale slow velocity anomalies in the lower mantle, which are recognized as the South Pacific superplume. The surface geodetic observations are explained by a model including a low viscosity asthenosphere situated immediately beneath the lithosphere, and a lower mantle viscosity 100 times greater than the upper mantle one. Our study assumes a purely thermal origin of the velocities anomalies. Although the existence of compositional heterogeneities is often invoked to explain the dynamics of the South Pacific superplume in previous numerical and laboratory experiments, and are important to account for plume/superplumes phenomenology, we cannot definitively conclude the presence of such compositional heterogeneities from our geodynamic modeling.