Mantle viscosity inferred from geoid and seismic tomography by genetic algorithms: Results for layered mantle flow

Until recently, the long-wavelength geoid has almost been exclusivly analysed with whole mantle flow models. Such models may not reflect the actual flow situation across the transition zone, where the flow conditions may vary regionally from whole mantle flow models to partially layered conditions. We have investigated whether the observed geoid signal can be compatible with a layered flow model. By using the genetic algorithm, a non-linear global optimization technique, we demonstrate that it is indeed possible to find such parameters of the mantle, namely the viscosity and the seismic velocity-to-density scaling factor, which allow the long wavelength geoid to be fit with the same accuracy as for the whole mantle models. A more reliable estimate of the local flow situation can be obtained from analysis of the long-wavelength geoid together with observation of the surface dynamic topography at different wavelengths. Both the layered and the whole-mantle flow models produce similar dynamic topographies at intermediate and high degrees. At the lowest degrees, however, the topographies are strikingly different, showing opposite signs and very different amplitudes in many tectonic regions, in particular behind the subduction zones. We propose that comparing the sign and amplitudes of the observed topography in different portions of the spectrum would serve as a useful diagnostic tool for quantifying the local permeability across the 660-km boundary.