Mantle dynamics constrained by degree 6 surface topography, seismic tomography and geoid: Inference on the origin of the South Pacific Superswell

Abstract

The relationship between global geophysical observables, i.e., seismic tomography, geoid and Earth surface topography, is investigated. This study focuses on the high correlation at degree 6 which, in addition to degree 2, characterizes these global fields. Surface topography is found to be highly correlated at degree 6 to seismic velocity anomalies between the 150 km and 450 km depths in the upper mantle. Several highs of the degree 6 topography coincide with particular provinces of the Earth surface, among others the South Pacific Superswell and the Afar region. These provinces overlie low-velocity (hot) upper mantle. By developing a global circulation model in a compressible, radially viscosity stratified mantle, we show that the degree 6 global fields can be explained as the result of the mantle convective flow driven by density anomalies. The viscosity structure that best fits the degree 6 observables also explains the low-degree (2 and 3) components of the dynamical topography and geoid. The best-fit model has a viscosity increase of factor 30 at the upper mantle-lower mantle discontinuity and a low-viscosity layer extending from the 100 to the 400 km depth in the upper mantle, with a viscosity drop of factor 30 with respect to the transition zone. These values are in good agreement with most previously published results based on the low-degree geoid alone. From this study we infer that the South Pacific Superswell, which lies over a broad zone of hot upper mantle extending down to the 450 km depth, is generated by the degree 6 flow of the mantle circulation.