Origin of localized fast mantle flow velocity in numerical models of subduction

The origin of localized fast mantle flow near subduction zones found in regional (Jadamec and Billen, 2010, 2012) and global (Stadler et al., 2010; Alisic et al., 2010) numerical models using non-Newtonian rheology is explored using 3D models with simple geometry. Two suites of models, using both a free slab and a slab attached to a lithospheric layer, are presented to determine (1) the origin of high magnitudes of mantle flow (>50 cm/yr) and (2) focusing of flow within the mantle wedge, with mantle wedge velocities up to two times higher than slab sinking velocities. The rheology in the models is either Newtonian with a prescribed low viscosity wedge (LVW) or non-Newtonian wherein low viscosity regions form in response to high stress surrounding the sinking slab. The results show that there are two aspects of the models that lead to localized fast velocities: reduction in the mantle viscosity surrounding the sinking slab leads to faster overall flow rates induced by the negative buoyancy of the sinking slab; and geometric effects that are caused by lateral variations in viscosity, including retrograde pivoting motion of the slab.