Can lower mantle slab-like seismic anomalies be explained by thermal coupling between the upper and lower mantles?

Below subduction zones, high resolution seismic tomographic models resolve fast anomalies that often extend into the deep lower mantle. These anomalie s are generally interpreted as slabs penetrating through the 660-km seismic discontinuity, evidence in support of whole-mantle convection. However, th ermal coupling between two flow systems separated by an impermeable interfa ce might provide an alternative explanation of the tomographic results. We have tested this hypothesis within the context of an axisymmetric model of mantle convection in which an impermeable boundary is imposed at a depth of 660 km. When an increase in viscosity alone is imposed across the impermea ble interface, our results demonstrate the dominant role of mechanical coup ling between shells, producing lower mantle upwellings (downwellings) below upper mantle downwellings (upwellings). However, we find that the effect o f mechanical coupling can be significantly weakened if a narrow low viscosi ty zone exists beneath the 660-km discontinuity. In such a case, both therm ally induced 'slabs' in the lower mantle and thermally activated plumes tha t rise from the upper/lower mantle boundary are observed even though mass t ransfer between the shells does not exist.