State of the art numerical subduction modelling with ASPECT; thermo-mechanically coupled viscoplastic compressible rheology, free surface, phase changes, latent heat and open sidewalls

Abstract

Subduction dynamics exert great influence on surface processes, plate tectonics and mantle convection. Feedback between these processes in combination with the many parameters involved in numerical subduction modelling makes it a challenge to find out how a single variable affects the system. In this study we focus on three problems. Firstly, we try to find out how boundary conditions (particularly open boundary conditions) influence subduction evolution. Secondly, the effects of mantle phase transitions on subduction dynamics will be investigated. Thirdly, the role of compressibility in our models will be explored. Meanwhile, we try to find a relation between surface expressions and their causative mantle processes. Our research is performed in a two-dimensional domain with a free surface, either free slip, prescribed velocity or open sidewalls and a free slip bottom boundary. Up to seven compositional fields are involved which feature a thermo-mechanically coupled viscoplastic compressible rheology. Prescribed velocity boundaries are found to restrict the influences of the internal dynamics. As a result, the subduction geometry is largely defined by the boundary conditions. Open boundary conditions allow a multitude of new subduction geometries to evolve more naturally. The 410 km phase transition increases the slab pull force considerably and produces a surface depression. Open boundaries allow the slab to accelerate significantly after it has crossed the 410 km phase transition. Slabs in this study are probably too weak to penetrate through the 660 km phase transition. Instead they buckle on the phase transition and high stresses cause elevated surface expressions. Compressibility is found to have a second order effect on subduction evolution.