Physics of the Earth and Planetary Interiors, 172(3-4), 278-288, doi:10.1016/j.pepi.2008.10.018, 2009
Effects on the long-wavelength geoid anomaly of lateral viscosity variations
caused by stiff subducting slabs, weak plate margins and lower mantle rheology
Masaki Yoshida and Tomoeki Nakakuki
Abstract
Instantaneous flownumerical 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 103 or lower, andweak 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.