Effects of the postperovskite phase change on the observed geoid

Abstract

In the lowermost mantle, seismic velocity variations beneath Pacific margins have been related to the perovskite to postperovskite (pPv) phase transition. We investigate the influence of this phase transformation on the geoid using 3-D spherical mantle circulation models based on a seismic tomography model and strong lateral viscosity variations in the lower mantle. We demonstrate that the geoid anomalies are strongly affected by the presence of pPv because of phase-dependent viscosity changes relative to the surrounding mantle. Whereas geoid heights above subduction zones are increased for high-viscosity pPv, the presence of weak pPv reduces them, thereby improving the fit to the observed geoid. An investigation using two different tomography models, different pPv density contrasts, and the presence or absence of a global thermal boundary layer and of lateral viscosity variations in the lower mantle demonstrates the various effects of weak pPv on the geoid.