A case for hot slab surface temperatures in numerical viscous flow models of subduction zones with an improved fault zone parameterization

Viscous flow models play a fundamental role in our understanding of the dynamics and thermal structure of subduction zones. For example, the relatively cool slabs produced in 2D viscous flow models have been central to the argument against slab and sediment melting at subduction zones except in special cases, such as near slab edges. Because flow models have such an important role in our insight to the subduction process, it is imperative to understand how various assumptions in the models affect their results. The fault zone is a key part of subduction models, but the effects of different parameterizations on the results have been largely ignored. The two main fault parameterizations used in the literature are weak (low viscosity) nodes imposed at the fault surface and an imposed rigid overlying plate. Each formulation has pros and cons, but their only physical justification is that they keep the overlying mechanical lithosphere from deforming in unrealistic ways. Because the assigned thickness of the imposed plate or the dimensions of the weak zone governs the amount of ablation that occurs in the corner and the temperature of the material advected to the subducting slab, different arbitrary implementations can result in significantly different flow patterns and thermal structures. A fault zone parameterization with a rigid plate defined by temperature and strain rate rather than by depth or other geometry could ameliorate the predicament. This formulation removes much of the arbitrariness from the modeling, as it is based on a physical process. This formulation has the advantages of restricting viscous deformation in the overriding plate without limiting processes such as ablation by imposed boundary conditions. Model results with this parameterization show hotter slab surface temperatures than previously shown in the literature. The results also agree as well as or better than other formulations with geophysical observations such as heat flow, seismic velocity, and seismic attenuation. The calculated higher slab surface temperatures could lead to a reappreciation of sediment melting at subduction zones without requiring an atypical tectonic environment.