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Abstract:
This study is aimed at a development of numerical method to model the dynamic geoid and the surface plate velocities induced by global mantle flow with the effect of strong lateral viscosity variations (LVV) in conjunction with the effects of selfgravitation
and mantle compressibility. I employ the technique, which comprises the combination of the spherical harmonic method, the direct Godunov method used for solving the Stokes and Poisson equations in spherical harmonics with arbitrary boundary conditions, functions of density and radial viscosity, and the iterative method based on the principles suggested by Zhang and Christensen (1993) used for modeling the effect of LVV.
The 3-D mantle viscosity model is based on the global seismic tomography model S20a converted to temperature variations. The maximum lateral viscosity contrast in the lithosphere-asthenosphere zone modeled reaches four orders of magnitude. It is found that the influence of LVV on the dynamic geoid is extremely significant: an alteration of the geoid figure due to LVV exceeds 45% of the maximum geoid undulations. The detailed analysis showed that the geoid is affected by both, strong LVV induced in the
upper mantle and large-scale LVV induced in the lower mantle. According to the results of this study the separated effects of the upper- and lower-mantle LVV on the geoid figure are nearly additive with respect to the whole-mantle LVV and partly compensating with respect to each other. The mantle flows are strongly affected by LVV as well, especially by the long-wavelength viscosity variations in the lower mantle: global upwellings tend to intensify due to the effects of LVV, while downwellings become weaker. The alteration of the near-surface velocities reaches 30-40% in amplitude not only due to the LVV induced toroidal flow but also due to change in the spheroidal velocity component. I can conclude that the LVV presented in both, upper and lower mantle, play an important part in global modeling, therefore, an incorporation of 3-D viscosity structure into the next generation global dynamic models is a task of vital significance.
