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 ( projections, 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 plate motion reconstruction, and then on the results in the context of the debate on the supercontinent hypothesis.
Suggested 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 geometric 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 .
What is best line to try and match continental margins with? Ostensibly it 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 have taken a certain bathymetric level along the continental slope as the division between continent and ocean. 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)?
Such reconstruction/fitting of continental margins only provides constraint at more or less one point in geologic history.
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 of paleomagnetic methods. For Proterozoic times the data's resolution is such that more than 1500 km of relative motion is required before the offset is clear.
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. Multiple traits need to be considered. If one simply matched 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. What might be the traits that would use?
E) Matching zircons with Precambrian provenance: A relatively 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, a fingerprint of sorts.
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.
The results: 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 Gondwana 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 - a bit of a puzzle.
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 formed, 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 less 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
More detailed reconstruction from Stewart, Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins; USGS source: pubs.usgs.gov/of/2009/1191/of2009-1191_text.pdf
Importance of Grenville rocks and orogen in reconstructions of Rodinia supercontinent: marks Grenvillian sutures in part.
Places Grenville rocks exist (see handout):
Not all one continuous belt (Fitzsimmons, 2000).
Schematic diagram showing supercontinent rifting/breakup from Rodinia to Pangea. Image source: http://pubs.usgs.gov/pp/p1386a/gallery2-fig30.html
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."
This hypothesis has fundamental implications for how the earth works. One view of the earth might be that it is more like a machine that hums along at a globally consistent pace, along the lines that James Hutton proposed. In this view, while what happens locally can and has shifted dramatically, and while the earth is losing heat and must be slowing down some, on average it has been working in a fairly consistent fashion through time. For example, the global rate of magma production would be relatively consistent, although diminishing long term. This is perhaps a case of applying Occum's razor - a simplifying assumption to start with. The supercontinent hypothesis suggest that this basic model is wrong. Instead, there were periods of time when the earth was operating in a different mode than others. For example, during the life of the supercontinent continental rifting would be more or less absent, but would blossom during the time of supercontinent destruction and dispersal.
Various steps in the cycle (reading the diagrams literally, see handout):
About a 500 million year time span for the completion of teh entire cycle. We are presently at the end of step b, just starting c (?).
What is evidence for super continent cycles?
In-class writing exercise: One can build a predictive model given the basic framework of the super continent cycle. Possibilities to include are as follows. How should the nature/vigor of igneous activity change through the cycle? How should global climate change through the cycle and why? How should global sedimentation patterns change and why? Then there is the question of how do you establish true global episodicity? There is a tremendous suite of geochronologic data out there, and a simple histogram can tell a lot, but what are various types of biases that one should consider?
People have proposed distinct episodes of an increase in mountain building at a global scale, representing supercontinent assembly:
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?
Some additional material on supercontinent cycles:
Episodic resurfacing of Venus and planetary 'cycles'
The surface of Venus 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 widely disributed. In addition, the distribution of craters is statistical one distribution, instead of having different age crusts with different crater 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.