Shallow Lower Mantle Viscosity Modulates the Pattern of Mantle Structure

Abstract

Tomographic images of Earth's mantle reveal that structure, even at the largest spatial scales, does not remain correlated across all mantle depths. This is a surprising result given the relationship between a dominantly spherical harmonic degree-2 pattern in the lower mantle and the pattern of plate motions at the surface, that is, the fact that the two large, low-shear velocity provinces (LLSVPs) beneath Africa and the Pacific Ocean are located near regions of surface divergence and away from regions of recent and ongoing subduction. Here, we combine analysis of seismic tomography models and numerical simulations of global-scale mantle circulation to show that the change in correlation in mantle structure across the extended transition zone (400–1,000-km depth), as revealed by the mantle radial correlation function (RCF), can be attributed primarily to the influence of seismically fast regions in the transition zone beneath the western Pacific subduction zones. Similar changes in the correlation of structure in global models of mantle circulation can be explained by a relatively large increase in viscosity (of at least a hundred times) between the upper mantle and lower mantle. We also demonstrate that a stronger endothermic phase transition at a depth of 660 km helps explain the observed pattern of the mantle RCF and the correlation between geodynamic models and seismic tomography. We account for the differences in resolution and parameterization of geodynamic versus tomographic models through the application of a tomographic resolution operator.