Comment on the article "Probability of radial anisotropy in the deep mantle" by Visser et al. (2008) EPSL 270:241-250
Résumé
Based on the inversion of a large surface waves dataset, including both fundamental and higher modes, Visser et al. (2008) constructed new maps of the probability of surface wave polarization (or radial) anisotropy. These maps bring new constraints on the variation of radial anisotropy with depth as a function of the geodynamic environment. However, the interpretation of the seismic anisotropy distribution in terms of mantle deformation is oversimplified, since it is essentially based on data of the evolution of olivine crystal preferred orientations and seismic anisotropy at high temperature, but low pressure conditions, neglecting recent findings on the deformation of mantle minerals under high-pressure in-situ mantle conditions. Three points, in particular, deserve a throughout discussion, since they highlight the limitations of using basic concepts that have been established from the analysis of shallow mantle rocks and low pressure deformation experiments, as for instance: (i) lack of anisotropy implies deformation by diffusion creep or (ii) fast polarization (or propagation for P or SV waves) directions mark systematically the flow direction, to interpret seismic anisotropy data below depths of 200–250 km in the mantle.
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