Readings:
A) Fit of continental margins:
Classic example is the fit of Africa and South America:
The best line to try and match is the contact between continental and oceanic crust. However, that is deeply buried underneath the passive margin sediments. In addition, the contact can be either fairly sharp or transitional. Typically people doing computer reconstructions take a certain bathymetric level as the division between continent and ocean. It might be better to try to geophysically mark the margin, but there is probably little extra learned for the effort. The goodness of the fit can be described mathematically.
What will be modification processes that create gaps or overlaps?
Take home exercise: Take
the paper with the outlines of continents on it and cut out the
various continental pieces. Note that they show boundaries between
provinces of different deformational age. Piece them back together.
Coloring the different age province can help. Note that you can
cut hinges into pieces (like at central America) to produce a
better fit. This would mimic some intraplate deformation. When
you have them in the best fit you can attain tape them together.
Note specific areas of gaps or overlaps. How might they be explained?
How well do contacts trace across the existing continental margins?
The way to do this rigorously is mathematically on a sphere using
the computer.
B) Reversal of seafloor spreading
history. In the case of the South Atlantic you can trace the
fracture zone into the margin (where it isn't covered by sediment).
This should link points that were in common to start with.
C) Paleomagnetism: One can ask - when did these two continents have a parallel APW path. This would indicate they were traveling together as part of one plate. A problem can be the resolution. For Proterozoic times the data's resolution can permit up to 1500 km of relative motions that wouldn't be evident in data (text, p. 279).
D) Matching geologic provinces: This is the method of last resort used for the older reconstructions where type A, B, and C data (of above) have been destroyed. If simply matching age of deformation and intrusion, think of how you could match Alaska and the Himalayas at present because they both exhibit such activity at present.
E) Matching zircons with Precambrian provenance: A new method that has been utilized a lot in the last decade is the dating of detrital zircons in sediments or metasediments. The frequency distribution of the zircon populations can then be correlated with likely basement sources, and in this way different continental fits can be tested.
All sorts of plate animations and maps.
Some terminology for past continents
and oceans (as if place name geography wasn't challenging enough
- now there can be map quizzes where they move and reshape the
pieces).
Major continental masses involved: South America, Africa, Antarctica, India, Australia.
Gondwana stratigraphy (see handout). Major common elements include:
Common stratigraphy suggests it was a coherent block from the Cambrian to the Cretaceous (some 400 Ma). Common APW paths for these masses are consistent with this history. That is a big chunk of time.
Break-up of Gondwana (overall better known story than assembly because much of the oceanic crust of this age is still around):
Samfrau 'geosyncline' of du Toit becomes the Gondwanides of Triassic age: 3 components
Pan African orogen
In the climatic Alleghanian Orogeny of the southern Appalachians Africa and North America were welded together. At about the same time the Urals were forming uniting eastern Asia with Europe and North America. Together they formed a supercontinent named Pangea that persisted only for a brief 70 million years or so before Africa and North America parted ways.
Animation of its assembly by Scotese lab.
SWEAT hypothesis (Moores, 1991; Hoffman, 1991) - South West US and East Antarctica connection (see handout). Note that North American craton is the center of this land mass.
Break up of Rodinia occurred some 700-500 Ma. Pieces reshuffled as broke up and formed Gondwana in the Pan African event (one interpretation), with the expulsion of Laurentia from the middle.
Importance of Grenville rocks in reconstructions of Rodinia supercontinent: assembly mark.
Places Grenville rocks exist (see handout):
Not all one continuous belt (Fitzsimmons, 2000).
Nance et al. "it suggests that the processes of plate tectonics on the largest scale are primarily governed not by chance but by a regular cyclic process."
Various steps in the cycle (reading the diagrams somewhat literally, see handout):
What is evidence for super continent cycles?
Distinct episodes of mountain building;
Should see effects on sea level as described above. Problem is the noise in the signal; i.e. other things that effect sea level (such as glaciation).
S and C isotopes in marine sediments. For example, due to precipitation in closed basins such as the Red Sea heavy sulfur (S-34) should be preferentially taken out of sea water during the early dispersal phase. Nance claims such lows are seen at 200 and 600 Ma.
As described the super continent cycle is far reaching. What does it not explain, or what might be difficulties?
Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.