We have incorporated faults as plate margins into time-dependent mantle convection models by using a mixed Eulerian and Lagrangian finite element formulation. Plate margins in our models can migrate dynamically in response to variations in mantle buoyancy, and this enables us to study the dynamics of mantle and plates including changes in plate size. Convection models in a cylindrical geometry with such a faulted converging margin, non-Newtonian rheology, and continents indicate that the converging margin tends to migrate oceanward. The oceanward trench migration leads to an increase in the size of the overriding continental plate. ¿ American Geophysical Union 1995