Could the magnetic field influence tectonic forces
The electromagnetic force and related field is a strong force at very small distances (governs the way the proton and electron are held to an atom) but is relatively weak over large distances.  I don't see how that small force could act on processes involved in the convection of the mantle.  If memory serves, the iron in magmas preserves the force of the electromagnetic field on earth at the time the magma cools to form rock. It leaves its imprint but has no affect on the process or origin of magmatism much less the convection that drives plate tectonics