Effects on the long-wavelength geoid anomaly of lateral viscosity variations caused by stiff subducting slabs, weak plate margins and lower mantle rheology

Instantaneous flow numerical calculations in a three-dimensional spherical shell are employed to investigate the effects of lateral viscosity variations (LVVs) in the lithosphere and mantle on the long-wavelength geoid anomaly. The density anomaly model employed is a combination of seismic tomography and subducting slab models based on seismicity. The global strain-rate model is used to represent weak (low-viscosity) plate margins in the lithosphere. LVVs in the mantle are represented on the basis of the relation between seismic velocity and temperature (i.e., temperature-dependent rheology). When highly viscous slabs in the upper mantle are considered, the observed positive geoid anomaly over subduction zones can be accounted for only when the viscosity contrast between the reference upper mantle and the lower mantle is approximately 103103 or lower, and weak plate margins are imposed on the lithosphere. LVVs in the lower mantle exert a large influence on the geoid pattern. The calculated geoid anomalies over subduction zones exhibit generally positive patterns with quite high amplitudes compared with observations, even when the low activation enthalpy of perovskite in the lower mantle is employed. Inferred weak slabs in the lower mantle may be explained in terms of recent mineral physics results, highlighting the possibility of grain-size reduction due to the postspinel phase transition.