Tectonic plates as we know them don't sound very likely on a Flat Earth, at least a finite flat Earth with an edge. Either the plates would have to be fixed to the edge, because the edge is exposed and would therefore likely cool and harden, fixing the edge in place, or for some unknown reason it might still move, in which case the plates would slide off the edge, or magma would leak out. Since the Earth has apparently been around quite some time, and chunks of it haven't slid off into space, I think we have to conclude that tectonic plates can't really work on a flat Earth.
There have been several questions on this in Q&A, and usually the FE answer is, "why can't plates exist on a flat Earth?". Well there you have it - they can't because they would either slide off, which obviously hasn't happened, or they'd stick to the edge, which obviously isn't happening.