So, for a few hundred million years we had this Precambrian surface which was not flat, as some people would have you believe. Then the plates shifted over a cold spot and the whole thing sank beneath the sea. Although silicates are less dense than the ocean basalts, they will sink if cold enough. They just sank enough for shallow seas, which built up sediments and sank more.
This is all the limestone and such, full of little critters. The Appalachian collision shoved up more sediments, and everybody forgot about the Precambrian, which held us up. :)
The amazing thing about the Precambrian is that it takes all the strain of the plate moving about. There have been several collisions and separations, and that rock keeps us alive. Of course, we humans are only a blip on the geological time scale, but we should respect our elders. The last Atlantic collision had all the continents together and it was a great time for the Earth's heat flow to cook out all that buried carbon dioxide and water. The result was the dinosaurs and a very hot world. The Precambrian was stretched to its limit.
All that heat, and then the continents separated to form the Atlantic. NA slid to a colder spot, which is like pressing a thumb on a beachball, and it settled into a dimple. It uniformly compressed the Precambrian on all sides, yet only a strong NE-SW trending horizontal stress remains. What could have happened? Does anybody in the USGS think of this?
It's all in the grain of ENA, which is strong down the spine of the megathrusts, and weak in the NW direction. In order for the stress to be so strongly polarized, a lot of displacement had to happen in the NW direction. This crunched up the Paleozoic over the megathrusts, and caused things such as the gigantic Paleo-collapse up at the Bruce Black Hole, which has conveniently been left out in the reports.
Now we are left with the strong NE horizontal stress, which kills us when we have a water seep such as New Madrid.....
--Christmas dinner - too much to drink...have to stop.