Testing models of sub-slab anisotropy using a global compilation of source-side shear wave splitting data

Recently, a number of source-side shear wave splitting measurements that directly constrain anisotropy in the upper mantle beneath subducting slabs have been published. Such measurements have yielded an observational foundation on which to base our understanding of the dynamics of the sub-slab mantle. Here we compile measurements from recent studies of source-side splitting beneath slabs that employ identical measurement and processing techniques. We use this compilation to test the predictions of a number of recently proposed conceptual models for the dynamics of the sub-slab mantle, including those that invoke three-dimensional return flow beneath slabs, strong radial anisotropy in suboceanic asthenosphere that is entrained via subduction, and a model based on the correlation between sub-slab splitting behavior and the age of the subducting lithosphere. We find that a model in which fast splitting directions are determined by slab age matches the observations better than either the 3-D return flow or radial anisotropic models. Based on this observation, we propose that the sub-slab mantle is characterized by two distinct anisotropic and mantle flow regimes. Beneath younger lithosphere (<95Ma), we propose that the sub-slab mantle is characterized by 2-D entrained flow resulting in an entrained mantle layer. Beneath older lithosphere (>95Ma), the entrained layer is thin and effectively serves as decoupling layer; the dynamics of the sub-slab region beneath old lithosphere is therefore dominated by three-dimensional return flow. This variation in the amount of mechanical coupling may be facilitated by the onset of small-scale convection beneath older lithosphere.