#Fold mountains
Hi, Jon–
The author of the article (Ian W. D. Dalziel) seemed to have the strongest
conviction that Antarctica was connected to Laurentia before the Cambrian; the
various collisions of Laurentia (proto-N.America) with the S. American part of
Gondwana seemed to be worded as more speculative, or perhaps that was more
because he was referencing other people's notions that fit in well.
The article has a series of maps showing Baltica and Laurentia starting to
collide between the mid-Ordovician and mid-Silurian maps, not a bad fit for the
Taconic orogeny. In the next late-Devonian map they are more firmly connected
and the southeastern U.S. is scraping along Peru – this is the event they
associated with the Acadian orogeny, thus I guess Laurentia is getting mashed
from two directions at nearly the same time (it is comforting that you describe
the Acadian as long-lasting). The Canadian/Greenland coast is clearly affected
mainly by Baltica in these events. Perhaps that Avalon terrane was far enough
south to be involved in that presumed Peru contact?
If all of the fold mountains from the pre-Pangaean continental collisions wore
away, we have been barking up the wrong tree in trying to explain the
Appalachians in such terms. Were they completely worn away all up and down the
east coast, or did significant parts survive? Perhaps the modern Appalachians
are still mainly shaped by the folds of those older mountains? This would help
explain the frequency of missing/fallen anticline tops we spoke of by adding a
potential mechanism; as weathering exposed a tough layer at the top, it would
be attacked before the flanks of that tough layer became exposed.
–Doug