MANTLE DYNAMICS CONSTRAINED BY DEGREE-6 SURFACE-TOPOGRAPHY, SEISMIC TOMOGRAPHY AND GEOID - INFERENCE ON THE ORIGIN OF THE SOUTH-PACIFIC SUPERSWELL

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.