Influence of convergent plate boundaries on upper mantle flow and implications for seismic anisotropy

Shear‐wave splitting observations in the region of the upper mantle enveloping subduction zones have been interpreted as showing extensive regions of trench‐parallel flow, despite the difficulty of reconciling such behavior with a sound model based on the forces that drive mantle motion. To gain insight into the observations, we systematically investigate flow patterns around the cold downwelling sheets associated with consumed plate material in a three‐dimensional numerical mantle convection model. First, we compare results from calculations employing prescribed plate geometries and kinematic plate velocities where the convergent plate boundary morphology is varied while keeping the plate velocity and convective parameters fixed. Subsequently, we examine the flow around sheet‐like downwellings in a number of convection calculations featuring dynamically evolving plate velocities. All of the calculations include thick viscous plates and a stratified mantle viscosity. In all of the models examined, we find that at mid‐upper mantle depths, flow directions no longer align with plate motion and the influence of buoyancy‐driven downwellings clearly dominates flow solutions. In the first models analyzed, a pair of plates are included in the calculations, and the large‐scale flow is generally roll‐like. In the final model we investigate the interaction of four plates and a plate geometry characterized by triple junctions. We examine a sequence from this calculation that features a triple junction of convergent boundaries. In this model, large‐scale flow characterized by convection rolls is superseded by a complex flow solution where flow in the mid‐upper mantle neither aligns uniformly with the plate motion nor necessarily follows the forcing associated with local buoyancy sources. In this setting, upper mantle flow in the vicinity of the sheet‐like downwellings featured in the solution moves orthogonal, obliquely, and even parallel to different sections of the convergent plate boundaries. In the latter case our calculations of the deformation of a fixed volume parcel of upper mantle material suggest that an olivine lattice‐preferred orientation should develop that would result in a fast polarizing direction for seismic shear waves parallel to the slab. Our findings have implications for the interpretation of flow in the upper mantle based on seismic anisotropy.