Constraints on the lateral strength of slabs from three-dimensional dynamic flow models

If the viscosity of subducted lithosphere is described purely by temperature and pressure, it should remain considerably more viscous than the surrounding material as it descends through the upper mantle. Many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored. Previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid. We present 3D finite element models of the regional geoid of the Western Pacific subduction zones. Slab buoyancies and viscosities are defined using the distribution of seismicity. The geoid is very sensitive to the lateral strength of the slab. Very viscous slabs penetrating a low viscosity mantle generate significant (10-50 m) long wavelength geoid lows: opposite to the geoid high which is observed over slabs. To obtain a geoid high comparable to that observed, the lower mantle viscosity must be 60-200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle. These strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface.