Abstract
Hypotheses of plate motion in which the plate plays an active role in the convection process are suspect because they require the relatively thin lithosphere to transmit horizontal stresses over long distances and because there seems to be no force to produce the return flow. To judge the seriousness of the first objection, we consider a model in which an elastic lithosphere is pulled by its sinking toe over a viscous asthenosphere. Limiting viscosities of the asthenosphere are calculated for which there is no failure in the plate according to the Mohr-Coulomb law, and these values are compared with those obtained from considering the Fennoscandian uplift. The strength of the lithosphere is seen not to be an impediment to the model if the asthenosphere is not too thick. To produce the return flow, we speculate that the plate could thicken away from the ridge and thus create a horizontal pressure gradient beneath it. The excess mass will not be observed by gravity measurements if it is compensated lower in the mantle.
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