Survival times of subducted slab remnants in numerical models of mantle flow

We examine the rate of sinking of truncated subducted slabs in a series of idealized three-dimensional and two-dimensional simulations using a 3D Cartesian numerical convection model. Our goal is to account for the inference from seismic tomographic studies that some slab remnants have persisted at mid-mantle depths for 150 Myr or more while others have descended to the base of the mantle, where they have spread laterally for great distances along the core–mantle boundary. We consider the influence of initial slab dimensions, mantle viscosity stratification, slab dip angle and three-dimensionality on the ‘slab survival time’ — the time required to clear the upper half of the mantle of all measurable traces of a previous subduction zone. We find slab survival time is determined by two principal factors: the initial slab length and the viscosity stratification of the mantle. Survival time decreases with increasing slab length and increases with increasing viscosity stratification. For a viscosity jump by a factor of 100 at a depth of 660 km, shallow slab remnants remain in the upper 1500 km of the mantle for times well in excess of 150 Myr while deep slabs draw their upper portions out of the upper half of the mantle in about 150 Myr. The dependence of the age of slab remnants on both the total depth extent of the original slab and the degree of viscosity stratification has not been studied previously; it can account for the different rates of descent of slabs lying in close geographic proximity.