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Interpretative cartoon of the final stage of the collision between the Laurentian and Gondwanan plates.

Missing here is the ocean part in the middle, left out to get to the point of interpreting what happened to the constituent parts of the two continental plates where they bashed into each other. What is postulated here is that the Laurentian plate was split into an upper and a lower portion (along the Mohorovicic discontinuity or Moho).
Remember the hot spots deep down below the crust? Supercontinents -- this one, by the way, went under the moniker Pangea -- are not meant to last as the convection-driven hot spots become hotter under the insulating package of the continental crust.

At any rate, Pangea ruptured along the lines which we now see as the mid-Atlantic ridge, and its offset branches to the northeast and northwest. But the break was not the same which earlier had ruptured the previous supercontinent, in that process forming the Iapetus Ocean. This time, the break was farther to the east, thus leaving much of the Appalachians, and even a piece of the continent opposite, on this side of the Atlantic.

The Atlantic split open during the interval ~180 to 90 Ma, and still is growing, as North America is moving away from Europe, and as the split has been cracking the ocean floor progressively northward.

The Appalachians still are there in all their splendor, having their most northeasterly position on this continent on Newfoundland, which probably is the best exposed part of this mountain belt.

Now, let's move with the continent itself, namely westward, and visit the last stop on our continental orientation tour, the Cordilleran system, one of Earth's younger offspring of plate tectonics. The West still is growing. Remember the Juan de Fuca plate subducting under the North American plate? And where else are we gaining new crust? Yes, you've got it, where volcanoes blow and plutons intrude, also along and above the subduction zone.

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