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Newtonian versus non‐Newtonian upper mantle viscosity: Implications for subduction initiation

The effect of rheology on the evolution of the slab‐tip during subduction initiation is analyzed using 2‐D numerical flow models. Experimentally determined flow laws have both strong temperature‐ and stress‐dependence, which leads to large local variations in viscosity with direct consequences for subduction initiation. We find that models with Newtonian viscosity lead to flat or coupled subduction due to hydrodynamic stresses that pull the slab‐tip up towards the overriding plate. Non‐Newtonian rheology reduces these hydrodynamic stresses by decreasing the wedge viscosity and the slab coupling to wedge‐corner flow, rendering the small negative‐slab buoyancy of the slab‐tip sufficient to maintain its dip during the early stages of subduction.