Effects of plate tectonic simulations on mantle convection and mixing

The mantle rheology has significant impacts on the mantle convection mechanism and the resulting geochemical mixing. For instance, a simple Arrhenius viscosity model has difficulty with producing plate deformation because it does not have shear localization and weakening of the lithosphere. In contrast, geodynamic models that consider brittle and ductile deformation have successfully reproduced plate-tectonics type mobile lid convection. These models include a model with force-balanced plates (e.g., Brandenburg et al., 2008) and a depth-dependent yield stress model (e.g., Tackley, 2000; Nakagawa and Tackley, 2014). Alternatively, grain size reduction by deformation (e.g., Bercovici & Richard, 2012; Foley et al., 2014) or mantle hydration (e.g., Korenaga, 2011) can also produce plate tectonics motions. Another example is that the convection mode of the early Earth, which would have higher temperatures than today, would be sensitive to the rheology model and it could range from stagnant-lid to fully plate-tectonics type mobile lid convection. Thus, understanding the mantle rheology is essential to study the evolution of planetary interiors. Here, we investigate the effects of the viscosity on the mantle convection mechanism by performing a number of geodynamical calculations. Our basic model settings are described in Brandenburg et al. (2008) and we have implemented various viscosity models, including the models described in Tosi et al. (2015) and Nakagawa and Tackey (2004). Our models are benchmarked against previous studies. We use various parameters such as the yield stress, radiogenic heating, and convective vigor to quantify their effects on the outcome. The mixing efficiency in these yield stress models is compared to that of previous force-balance models.