Effect of lateral viscosity variations in the core-mantle boundary region on predictions of the long-wavelength geoid |
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Authors: | O ?adek L Fleitout |
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Institution: | (1) Department of Geophysics, Faculty of Mathematics and Physics, Charles University, V Holešovič kách 2, 180 00 Prague, Czech Republic;(2) Laboratoire de géologie, Ecole normale supérieure, 24, rue Lhomond, 75005 Paris, France |
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Abstract: | Seismic studies of the lowermost mantle suggest that the core-mantle boundary (CMB) region is strongly laterally heterogeneous
over both local and global scales. These heterogeneities are likely to be associated with significant lateral viscosity variations
that may influence the shape of the long-wavelength non-hydrostatic geoid. In the present paper we investigate the effect
of these lateral viscosity variations on the solution of the inverse problem known as the inferences of viscosity from the
geoid. We find that the presence of lateral viscosity variations in the CMB region can significantly improve the percentage
fit of the predicted data with observations (from 42 to 70% in case of free-air gravity) while the basic characterisics of
the mantle viscosity model, namely the viscosity increase with depth and the rate of layering, remain more or less the same
as in the case of the best-fitting radially symmetric viscosity models. Assuming that viscosity is laterally dependent in
the CMB region, and radially dependent elsewhere, we determine the largescale features of the viscosity structure in the lowermost
mantle. The viscosity pattern found for the CMB region shows a high density of hotspots above the regions of higher-than-average
viscosity. This result suggests an important role for petrological heterogeneities in the lowermost mantle, potentially associated
with a post-perovskite phase transition. Another potential interpretation is that the lateral viscosity variations derived
for the CMB region correspond in reality to lateral variations in the mechanical conditions at the CMB boundary or to large-scale
undulations of a chemically distinct layer at the lowermost mantle. |
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Keywords: | core-mantle boundary D″ lateral viscosity variations geoid post-perovskite phase transition |
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