Abstract
A two-dimensional time-dependent method that allows large viscosity variations is used to numerically simulate upper mantle convection, including lithospheric plate motion. The numerical operator developed for viscous flow is in self-adjoint form, so that the conjugate gradient iteration method can be used. Convergence is faster than with relaxation methods. The method is applied to sea-floor spreading with the objective of examining the driving mechanism of mid-ocean ridges. In accordance with this objective, deep convection is suppressed in the model. Counterflow below the plates is confined to depths less than 340 km. The model is fit to observed topography at four different ridge locations. It is found that, for a spreading velocity of 1.2 cm/yr, the ridge can produce compressive stress in the lithosphere out to a distance of 1600 km. For a spreading velocity of 6 cm/yr, this model is clearly excluded, for it requires excessively large stress in the lithosphere. Therefore, upwelling material must cross the seismic discontinuity at 400-km depth.
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