Introduction: The reconstruction of plate movements back in time is a major scientific accomplishment. When viewing the myriad of reconstructions, complete with animations, available on the web these days the uninitiated may be forgiven for thinking these are 'artistic' renderings and impressions. The incredible wealth of data that inform and constrain these reconstructions is often not immediately evident in the reconstruction itself. Nor is the geometric rigor behind the depictions ( projects, poles of rotation, etc.). These reconstructions are now being used in testing and developing global climate models, and so arguably they are much more than an academic exercise. Additionally they are routinely used in resource exploration. This material focuses on the how of reconstruction creation, and then on the results in the context of the debate on the supercontinent hypothesis.
Reading: Nance, D., Worsley, T. and Moody, J., 1988, The Supercontinent Cycle, Scientific American.
Methods of reconstruction - how to piece the puzzle back together?
A) Fit of continental margins:
Classic example is the fit of Africa and South America:
To the right is a reproduction of Alfred Wegeners reconstruction of the continents. Remember that large portions of the geologic community rejected his hypothesis of contiental drift. Image from http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html .
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 modification processes will create gaps or overlaps in reconstructions (or should the fit be perfect, and why or why not)?
B) Reversal of seafloor spreading history.
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/history: This is the method 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. Part of the idea is that terranes shedding zircons through erosion have a fairly distinct signature.
Image to the right is from Gehrels et al., Constraints on the Age and Provenance of the Chugach Accretionary Complex from Detrital Zircons in the Sitka Graywacke near Sitka, Alaska as found at pubs.usgs.gov/pp/pp1709f/pp1709f.pdf . Note the distinct difference between the two histograms. They suggest that these sediments had distinctly different source terranes, in this case fairly young terranes.
All sorts of plate animations and maps.
Major continental masses involved: South America, Africa, Antarctica, India, Australia (see image below).
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
Break-up of Pangaea - From USGS site - http://pubs.usgs.gov/gip/dynamic/historical.html .
In the climatic Alleghanian Orogeny of the southern Appalachians Africa and North America were welded together. Just a bit later 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, as early start on this reconstruction. 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.
Earlier reconstruction of Rodinia from USGS Technical Report, as found at http://expertvoices.nsdl.org/connectingnews/files/2008/07/rodinia.gif .
Another version of Rodinia as displayed at NASA site The Earth as a Planet: http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html
Importance of Grenville rocks in reconstructions of Rodinia supercontinent: assembly mark.
Places Grenville rocks exist (see handout):
Not all one continuous belt (Fitzsimmons, 2000).
Animation of its break-up.
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 literally, see handout):
What is evidence for super continent cycles?
Fundamentally it is pulsing activity on the long term.
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 the amount of weathering changes (due to sea level changes and related exposure, and to changes in mountain building), climate should also change.
Why might super continent cycles exist?
As described the super continent cycle is far reaching. What does it not explain, or what might be difficulties?
Episodic resurfacing of Venus and planetary 'cycles'.
The surface of image is fundamentally different from that of earth's (see the image below). Instead of volcanic and tectonic features being fairly well constrained to linear belts. they are scattered. In addition, the distribution of craters is statistical one distribution, instead of having different age crusts with different age distributions as occurs for Mars, Mecury, Earth and the Moon. This has led some to conclude that some 500 Ma ago or so, Venus went through a global resurfacing event. So the question develops - could planetary bodies be prone to large scale cycles of tectonics activity?
Image from NASA at: http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.html .
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.