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Tomographic filtering of high-resolution mantle circulation models: Can seismic heterogeneity be explained by temperature alone?

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Abstract

High-resolution mantle circulation models (MCMs) together with thermodynamic mineralogical models make it possible to construct 3-D elastic mantle heterogeneity based on geodynamic considerations. Recently, we have shown that in the presence of a strong thermal gradient across D″ and corresponding large temperature variations in the lower mantle, the heterogeneity predicted from isochemical whole mantle flow agrees well with tomographic models in terms of magnitudes of S wave velocity (vs) variations. Here, we extend the comparison of geodynamic and tomographic structures by accounting explicitly for the limited resolving power of tomography. We focus on lateral variations in vs and use the resolution operator equation image associated with S20RTS to modify our geodynamic models so that they reflect the long-wavelength (>1000 km) nature and the effects of heterogeneous data coverage and damping inherent to the tomographic inversion. Prior to the multiplication with equation image, the geodynamic models need to be reparameterized onto the basis of S20RTS. The magnitude reduction introduced by this reparameterization is significant and needs careful assessment. We attempt a correction of the reparameterization effects and find that the inherent tomographic filtering alone then leads to a magnitude reduction by a factor of ∼2 in the lower mantle. Our tomographically filtered models with strong core heating agree well with S20RTS, which resolves maximum negative anomalies of around −1.5% in the lowermost mantle. Temperature variations on the order of +1000 K, corresponding to perturbations of around −3% in vs in the unfiltered model, would be seen as −1.5% when “imaged” with the data and damping of S20RTS. This supports our earlier finding that isochemical whole mantle flow with strong core heating and a pyrolite composition can be reconciled with tomography. In particular, the large lateral temperature variations associated with lower mantle plumes are able to account for the slow seismic anomalies in the large low-velocity zones under Africa and the Pacific. We also find that strong gradients in shear wave velocity of 2.25% per 50 km in our unfiltered models compare well with the sharp sides of the African superplume.

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