Deflection of mantle flow beneath subducting slabs and the origin of subslab anisotropy

Global compilations of subslab shear wave splitting parameters show a mix of trench-parallel and trench-perpendicular fast directions that often directly contradict predictions from two-dimensional models of slab-entrained flow. Here we show that subslab anisotropy is consistent with three-dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow. Each model represents a specific region for which high-quality source-side shear wave splitting data are available. We compare the distribution of finite strain in the models with shear wave splitting observations, showing that both trench-parallel and trench-perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs. Subslab maximum elongation directions calculated from our models depend on a combination of geometry factors (such as slab dip angle and maximum depth), mechanical parameters (such as decoupling between the slab and the subjacent mantle), and the orientation and magnitude of the regional mantle flow.