A Numerical Study on the Effects of Surface Boundary Condition and Rheology on Slab Dynamics

A thin-shell program for modeling neotectonics of regional or global lithosphere with faults. Influence of fore-arc structure on the extent of great subduction zone earthquakes. 1994: Effects of multiple phase transitions in a three-dimensional spherical model of convection in Earth's mantle. 2000: Role of temperaturedependent viscosity and surface plates in spherical shell models of mantle convection. Summary Numerical models of free-slab subduction that employ viscous rheologies and a free-slip surface boundary condition result in slabs that 'drip-off' rather than subduct. Since this appears unrealistic, a depth-dependent Byerlee-type viscoplastic slab rheology is typically used instead. Here, we study the effects of rheology (viscous versus viscoplastic) and surface boundary condition (free-surface versus free-slip) on subduction dynamics. Results confirm confirm the ad hoc assumptions and conclusions from a previous visco-plastic study with free-slip. They also show that the presence of free-surface causes viscous slabs to subduct like observed in laboratory experiments, and that models with a free-slip/Byerlee rheology behave similar to models with a free surface and viscous rheology. Finally, we study the effects of varying slab/mantle viscosity contrasts and slab thickness on subduction dynamics, and show that the 178 rich behaviour known from laboratory experiments arises naturally in numerical models that have a free-surface.