Small-scale convection in the subduction zone mantle wedge

Small-scale convection in the upper mantle can appreciably influence large-scale mantle dynamics and has important implications for geophysical observables such as seismic anisotropy and surface heat flow. We develop numerical models to evaluate the likelihood of small-scale convection in the mantle wedge above subducting slabs. The characteristics of small-scale convection are analyzed using the 3-D single-mode approximation, in which one characteristic wavenumber represents small-scale convective motions with rotational axes parallel to the convergence direction. Numerical simulations are run for a range of characteristic wavenumbers, Rayleigh numbers, and subduction parameters (e.g., slab dip angle, convergence velocity, and downgoing plate age). For each simulation, we quantify the excitation due to small-scale convection and determine under what conditions and characteristic wavenumbers this excitation is maximized. We find that of the parameters examined, mantle wedge viscosity plays the most significant role in dictating the occurrence and strength of small-scale convective motions in the mantle wedge. Numerical models run with subduction parameters similar to that of northeast Japan, where it has been proposed that small-scale convection may be occurring in the mantle wedge, require a mantle wedge viscosity of ∼ 10 18 Pa s for significant small-scale convection to occur.