Source: http://maps.unomaha.edu/Maher/plate/week12/super.html
Timestamp: 2019-04-23 15:58:06+00:00

Document:
Methods of reconstruction - how to piece the puzzle back together?
Episodic resurfacing of Venus and planetary 'cycles'.
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.
1920s Wegener noted fit and used the coast line. Bathymetric data to do otherwise with was still very limited.
1965, Bullard et. al used the continental margin, and produced a still well accepted fit.
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.
In the case of the South Atlantic you can trace the fracture zone to the margin (where it isn't covered by sediment). This should link points that were in common to start with.
Fracture/transform and spreading ridge geometry provides you with poles of rotation (remember that the fracture transform should trace out a small circle path aligned with the pole of rotation). Magnetic anomaly identification along with the estblished magnetic-polarity time scale allows one to calculate rates (see earlier material associated with seafloor spreading for review of this material).
Motions for the last 200 Ma are thus very well constrained, although the Cretaceous magnetic quiet period (120 Ma to 83 Ma), where there was magnetic field stability and a lack of reversals, provides some challenges as to details.
For greater than 200 Ma??
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?
tillites (or other distinctive paleo-climate related types of deposits).
orogenic features (e.g. Caledonides, Grenvillian). Specific raits to consider would be - timing and character of magmatism, metamorphism, basin infilling, deformation.
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.
Some terminology for past continents and oceans (as if present place name geography wasn't challenging enough - now there can be map quizzes where they move and reshape the pieces).
Pangea: short lived megacontinent, Laurasia and Gondwana together (450-250 Ma).
Gondwana: southern agglomeration of continental masses, longer lived.
Laurasia: northern agglomeration of continental masses (Laurentia, Europe and Asia).
Tethys: wedge of an ocean between Gondwana and Laurasia.
Rodinia: supercontinent that precedes Pangea (1800-1100 Ma).
Major continental masses involved: South America, Africa, Antarctica, India, Australia (see image below).
the various Permo-Carboniferous tillites (Tachir/India, Dwyka/South Africa, Itarare/S.America, Buckeye/Antarctica).
the Mesosaurus bearing shales in South Africa, South America and Antarctica.
the Permian Glossopteris coal measures on Antarctica and South Africa.
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.
India from Africa and Australia, c. 140 Ma.
Africa from S. America - c. 130 Ma initiation. Parana-Edenteka LIP involvement.
Australia from Antarctica - c. 60-55 Ma (Eocene).
To the right: timing of split-up from http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html .
matter of debate for Madagascar.
Arabia from Africa and formation of Red Sea - initiates some 30 Ma.
EAR - still ongoing. Some of the breakup boundaries follow young orogens, others older ones.
computer animation of break-up from Scotese lab.
their position suggests they mark a long lived subduction zone with accretionary tectonics along the southern edge of Gondwana (see handout).
Pan African orogen - a bit of a puzzle.
deformation and intrusions from 400-600 Ma.
found on all the Gondwana continents except India.
Damara orogen in South Africa a small part of - see handout. Not like Himalayas (show me the ophiolites)!
Dott and Batten suggest this orogeny was responsible for Gondwana's assembly, via the SWEAT hypothesis (Moores, see below).
in some contrast: Stanley "What is striking about this network of activity is that much of it seems to have taken place as regional metamorphism within rather than along the margins of Gondwanaland: thus far, no evidence of continental suturing has been found along the interior metamorphic zones. If suturing did not occur, however, the metamorphic zones can not be explained by conventional plate-tectonic processes, and their origin remains a mystery (p. 312)."
perhaps supercontinents exhibit more internal deformation (think about Asia at present).
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 .
Importance of Grenville rocks and orogen in reconstructions of Rodinia supercontinent: marks Grenvillian sutures in part.
originally defined in Canada, they extend as a continuous deformation belt down to Georgia (see Bartholomew, 1984).
found in Svalbard. Fit in supercontinent not clear, but likely represents subduction along the west side of Rodinia.
problem is there one long suture and collision zone during the assembly of Rodinia or multiple zones (seems a bit unlikely), or some of both.
image of reconstructed Grenvillian from Karlstrom et al. 1999: http://essayweb.net/geology/timeline/images/grenville.jpg .
Van Kranendork and Kirkland (2013) based on distribution of isotopic data through time, that Grenvillian a super orogen.
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."
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.
a) breakup of existing supercontinent over some 40 Ma.
b) development of Atlantic type ocean basins for about 160 Ma.
c) development of subduction zones in Atlantic type ocean to form Pacific like basin.
e) stable supercontinent for some 80 Ma with a heat accumulating underneath.sea level is static.
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?
650 Ma: Pan African Orogeny - Gondwanaland.
1.1 Ba: Grenvillian Orogeny (more on below), assoc. with Rodinia.
1.7 Ba: Penokean (assembly of North American craton).
Perhaps a 400-500 Ma cycle.
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?
What does your reading have to say on this?
Importance in planetary asymmetry of thermal properties (e.g. continental lids, spreading ridge release valves).
Driven from the top (conductive lids), from the bottom (core-mantle interactions), or from the middle (670 km breakthrough and cascade)?
Role of large igneous provinces?
This topic provides a natural transition into driving mechanisms for plate tectonics, which we will address next time. A model for super continent cycles with an overall evolution and consideration of what drives subduction (hot versus cold subduction) is explored by Hawkesworth et al. 2016.
As described the super continent cycle is far reaching. What does it not explain, or what might be difficulties?
Development of Laurentia and Nuna supercontinent - http://www.youtube.com/watch?v=K_ynW8q1JXs .
William Thomas - Eastern North America Through Two Supercontinent Cycles - http://www.youtube.com/watch?v=-dH7XLyeJ3Q .
Large Igneous Povinces, Kimberlites and the deep mantle and paleogeography - http://www.youtube.com/watch?v=Tbw3dVfbrUc .
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 .
Bartholomew, M. J., 1984, The Grenville Event in the Appalachians and Related Topics; GSA Special Paper, 194, 287p..
Dalziel, I. W. D., 1991, Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent. Geology, v. 19, p. 598-601.
Dalziel, I. W. D., 1995, Earth before Pangea. Scientific American, v. 272, p. 58-63.
Hawkesworth, C. J., Cawood, P. A. & Dhuime, B., 2016, Tectonics and crustal evolution; GSA Today, 26, # 9, 4-11, http://www.geosociety.org/gsatoday/archive/26/9/article/i1052-5173-26-9-4.htm .
Hoffman, P. 1989, Speculations on Laurentia's first gigayear (2.0-1.0Ga): Geology, v. 17, p. 135-138.
Hoffman, P. 1991,Did the breakout of Laurentia turn Gondwanaland inside out? Science, v. 253, p. 1409-1412.
Fitzsimons, I. C. W., 2000, Grenville-age basement provinces in East Antarctica: Evidence for three separate collisional orogens; Geology, v. 28, p. 879-882.
Moores, E. M., 1991 Southwest U.S. -- East Antarctica (SWEAT) connection: a hypothesis: Geology, v. 19, p. 425-428.
Nance, R. D., Worsley, T. R., & Moody, J. B., 1988, The Supercontinent Cycle; Scientific American.
Nance, R. D., Murphy, J. B. & Santosh, M., 2014, The supercontinent cycle: a retrospective essay; Gondwana Research, 25, 4-29. https://www.sciencedirect.com/science/article/pii/S1342937X13000506 .
Sankaram, A. V., 2003, The supercontinent medley: recent views; Current Science, 85, p.1121-1123.
Van Kranendonk, M. J. & Kirkland, C. L., 2013, Orogenic climax of Earth: The 1.2-1.1 Ga Grenvillian superevent; Geology, 41, 735-738.
Phillips and Bunge abstract on modeling mantle convection and implications for supercontinent cycles - http://geology.geoscienceworld.org/cgi/content/abstract/35/9/847 .
Worsley et al. AGU link - http://www.agu.org/pubs/crossref/1986/PA001i003p00233.shtml .
A review arguing that is somewhat critical of the hypothesis - www.iisc.ernet.in/currsci/oct252003/1121.pdf .
Article by Paul Hoffmann on supercontinents - www.eps.harvard.edu/people/faculty/hoffman/pdfs/supercontinents.pdf .
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.

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