Mantle flow in subduction systems: The subslab flow field and implications for mantle dynamics

The character of the mantle flow field in subduction zones remains poorly understood, despite its importance for our understanding of subduction dynamics. In particular, little attention has been paid to mantle flow beneath subducting slabs. In order to identify processes that make first-order contributions to the global pattern of subslab mantle flow, we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region. Globally, the subslab region is overwhelmingly dominated by trench-parallel fast splitting directions. We tested for relationships between splitting delay time, a measure of the strength of anisotropy, and parameters that are indicators of tectonic processes, such as trench migration velocity, convergence velocity and obliquity, age of subducting lithosphere, and slab dip, curvature, seismicity, and thickness. We used several different plate motion models to describe plate and trench motions and evaluated the differences among the models. We find that only one parameter, namely, the trench migration velocity in a Pacific hot spot reference frame, appears to correlate well with the strength of the subslab splitting signal. This finding supports a model in which the mantle beneath subducting slabs is dominated by three-dimensional flow induced by trench migration. We explore several implications of our model for various aspects of mantle dynamics, including the choice of a suitable reference frame(s) for mantle flow, the existence of a thin decoupling zone between slabs and the subslab mantle, and consequences for mass transfer between the upper and lower mantle.