Abstract
Advances in our understanding of Earth’s thermal evolution and the style of mantle convection rely on robust seismological constraints on lateral variations of density. The large-low-shear-wave velocity provinces (LLSVPs) atop the core–mantle boundary beneath Africa and the Pacific are the largest structures in the lower mantle, and hence severely affect the convective flow. Here, we show that anomalous splitting of Stoneley modes, a unique class of free oscillations that are perturbed primarily by velocity and density variations at the core–mantle boundary, is explained best when the overall density of the LLSVPs is lower than the surrounding mantle. The resolved density variations can be explained by the presence of post-perovskite, chemical heterogeneity or a combination of the two. Although we cannot rule out the presence of a ∼100-km-thick denser-than-average basal structure, our results support the hypothesis that LLSVPs signify large-scale mantle upwelling in two antipodal regions of the mantle.
Highlights
Advances in our understanding of Earth’s thermal evolution and the style of mantle convection rely on robust seismological constraints on lateral variations of density
For reasonable estimates of the velocity structure, we consistently find that Stoneley mode splitting functions are fitted best by overall low-density large-low-shear-wave velocity provinces (LLSVPs)
The resolved density variations cannot be uniquely interpreted in terms of purely thermal or thermochemical structures in the Earth’s deep mantle, nor can we rule out that the LLSVPs are dense at their very base (o100 km)
Summary
Advances in our understanding of Earth’s thermal evolution and the style of mantle convection rely on robust seismological constraints on lateral variations of density. Some previous inversions[14] and statistical analyses[15,16] of spheroidal normal-mode splitting data have suggested that the density of the LLSVPs is relatively high, which cannot be reconciled with a purely thermal origin[17] These studies relied primarily on observations of the splitting of modes with frequencies below 3 mHz (refs 18,19) and with a sensitivity to both the upper and lower mantle. For reasonable estimates of the velocity structure, we consistently find that Stoneley mode splitting functions are fitted best by overall low-density LLSVPs. We conclude that the LLSVPs signify large-scale positively buoyant regions in the Earth’s mantle, consistent with other geophysical observables[25,26]. The resolved density variations cannot be uniquely interpreted in terms of purely thermal or thermochemical structures in the Earth’s deep mantle, nor can we rule out that the LLSVPs are dense at their very base (o100 km)
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