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We have studied the influences of depth‐varying rheologies on the structure of mantle convection and the near surface flow fields. Upwelling flows crossing the upper‐lower mantle boundary become considerably thinner and faster. Streamlines are concentrated near the surface and other plate‐like characteristics are enhanced by this type of rheological stratification in which the averaged effective viscosity of the upper mantle is about two orders of magnitude lower than that of the lower mantle. The presence of a lithosphere with a higher non‐linear dependence in the flow law further promotes plate‐like behavior. Plate‐like character of the top boundary‐layer is encouraged by a decreasing depth‐dependent viscosity in the non‐Newtonian upper‐mantle and by greater convective vigor.
Berg et al. (1991) studied this question.