Precambrian tectonics and the advent of plate tectonics
Above is a metamorphosed and folded banded iron formation (BIF) from the Lake Superior craton. BIFs are one of the more distinctive geologic assemblages that characterize the Precambrian. They played a role in the evolution of our atmosphere and of life by representing a large oxygen sink/reservoir. Tectonically, they mainly formed on shallow stable shelfs, although the detailed mechanisms of formation are still debated. Their concentration in time to Precambrian rocks may reflect surface conditions and geochemical systems more than they do tectonic processes, but they are a clear indication that geologically the Precambrian is unique. This is hardly surprising, as we expect all systems to evolve, especially over a 4 billion year time span.
Introduction and guiding questions
A planetary perspective: For both Mars and Venus there were early attempts to argue that plate tectonics had occurred on these planets, but these largely failed as we learned more. The tectonic style of each planetary body appears distinctive. This would suggest that the array of possible planetary tectonic behaviors is rich, and that 'earthly' plate tectonics has some distinct requirements (e.g. of lithospheric thickness, and geothermal gradients). What we see on other planets also suggests that planets change tectonic style with time. In such a context it is not unreasonable to think that at some earlier time in the earth's history plate tectonics as we know it was not operating, or was at least substantially different.
Precambrian time scales:
Basic framing questions:
Striking image of NW Australia (source: http://visibleearth.nasa.gov/view_rec.php?id=3022). Since this part of Australia is dry and relatively barren, the pattern one sees is that of the basement geology. The lighter colored 'splotches' in the upper half to the right are granite/gneiss bodies with greenstone belt synclinal keels (described further below) sandwiched between them. Note how the map pattern changes below this to a more east-west linear pattern.
Precambrian lithotectonic associations
An ophiolite is a distinct assemblage of rocks with distinct tectonic significance that characterizes 'modern' plate tectonics. Craton basements are characterized by other distinct rock assemblages, the tectonic significance of which is debated.
Map exercise: Please identify Precambrian tectonic elements/patterns on the maps that are provided, using what you have learned.
Continental assembly (Hoffmann, 1988)
There is about a 150 Ma span for the time of assembly of the North America craton (we touched upon this topic a bit earlier when we discussed terrane accretion):
Image showing some of the major components that North America was assembled from. Image source was orginally from USGS material and now resides at https://commons.wikimedia.org/wiki/File:North_america_basement_rocks.png .
USGS gravity anomaly map showing the mid-continent gravity high. In the Lake Superior Region the exposed, thick sequence Keweenawan Rift basalts clarifies the reason for the gravity high (basalts being relatively dense). This rift was the last significant challenge to North America cratonic stability. The continuation of the rift in the subsurface down into Iowa has been confirmed by drilling. The gravity low area in Wisconsin is partly associated with the Wolf River batholith. Map source: https://www.mngs.umn.edu/nicegeo/pdfs/boug_grav.pdf .
Enigmatic anorogenic granite-rhyolite province
These were very widespread granite and rhyolite bodies of extensive volume, which mostly intruded otherwise stable continental crust of the time. The basic question is as to what generated them, especially given the large volumes involved.
Histogram of ages of anorogenic granites from du Bray et al (2018).
Research on this topic (the nature of Precambrian tectonics) is still ongoing. It is not a simple task to look back billions of years! Below are some thoughts that play into understanding our planet's early tectonic history.
USGS tapestry map with the PC highlighted in color. Map source: http://tapestry.usgs.gov/ages/precambrian.html. This maps does not show all of the myriad of drill holes and geophysical data that also give us information on the Precambrian basement.
This is an image of white columnar stromatalites preserved in growth position, mantling a vesicular volcanic clast in the Copper Harbor conglomerate of the upper peninsula of Michigan. Stromatalites are one of the more common fossil structures in the Precambrian. The history of life and how it influenced the composition of the atmosphere, and surface geochemical systems on the earth is another major, fascinating story within Precambrian history.
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.