All in Good time...
Just be patient dear Podrock. The plate tectonic engine grinds slowly, but it grinds surely. Perhaps we need to wait for subduction to initiate down the east coast of the USA and a chain of andesite volcanoes to spring up there, then a string of back-arc basins, like the present western Pacific. After that, conditions will probably be ripe for a Baikal-style intra-continental rift, perhaps using the New madrid alignment, or perhaps ripping a new hole through the flatlands.
50 or 100 million years should do it...