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.