Small-scale convection in the subduction zone mantle wedge

We investigate the occurrence of small-scale convection in the mantle wedge above subducting slabs. Although occurring on short length scales, small-scale convection can appreciably influence large-scale flow and have important implications for geophysical observables such as seismic anisotropy and surface heat flow. We have developed numerical models that allow for a systematic study of the likelihood of small-scale convection in a subduction zone setting. The characteristics of small-scale convection in the out-of-plane coordinate are analyzed using the 3-D single mode approximation, in which one characteristic wavenumber is used to represent small-scale convective motions aligned perpendicular to the sidewalls. Numerical simulations were run for a range of Rayleigh numbers, out-of-plane wavelengths, temperature-dependent viscosities, and subduction parameters (e.g. slab dip angle, convergence velocity, and downgoing plate age). For each model run, we measure the out-of-plane excitation and determine under what conditions and characteristic wavelengths this excitation is maximized. We can then predict the likelihood of small-scale convection in the mantle wedge as a function of several variables, and draw conclusions regarding the occurrence of small-scale convection in subduction zone mantle wedges globally.