Re: Prior to Gondwana?

[George Engelmann <engelman@cwis.unomaha.edu>]

There is quite a bit of work on Plate Tectonics prior to Pangaea, but it is 
provides much less detailed reconstructions of continental positions. This is 
because virtually all of the continental masses that made up Pangaea still have
a continuous record of oceanic crust connecting them to some other continental
mass(es) and their relative motions since the breakup are therefore very 
tightly constrained. On the other hand, all of the oceanic crust that linked
continents prior to the assembly of Pangaea has been destroyed or broken by 
subduction, obduction, etc. It is possible to use paleomagnetism (pole 
position) to determine paleolatitude, but paleolongitude cannot be determined
directly. So, once the plaeolatitude and orientation have been worked out for
continental masses one must guess at their relative positions at any given 
time based on timing of collisions, past and future history, etc. Add to that
the possibility that the bit that carried the paleomag information may be 
rotated relative to the continent on which it occurs and you can see that there
is quite a margin for error. Nevertheless, most continental masses are pretty
well accounted for throughout the Phanerozoic. PT in the Precambrian is a much
more dicey proposition because so much has happened to most of the continents 
that were around then. There are even those who argue that PT didn't occur
during the Precambrian, but that some other tectonic mode. I think the 
evidence is persuading most that tectonics in the Proterozoic was much as it is
now, but the Archean remains open to question. Some good Historical Geology
texts such as Dott & Prothero show some attempted reconstructions.

George Engelmann
engelman@unomaha.edu