The tectonics would not change. The most a bomb is going to do is leave shallow craters. Nuclear blasts can't shake whole tectonic plates. However the dust that's throw up by the nuclear war will create a worldwide cloud layer that will block the sunlight and plunge the earth into a colder climate - possibly even n ice age. The cloud reflects the light and so cools the earth.
The fallout from the radiation would create mutations and kill huge portions of the population. Those weakened by the fallout will fall prey to the cold weather. The earth will be blasted and barren. It's possible that a new ice age would change the landscape through glaciation. You could set your world after the ice age lifted, or during the ice age itself.