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/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

A planetary perspective: For both Mars and Venus there were early attempts to argue that plate tectonics had occurred on them, but these basically 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 suggest that planets also 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:

• Archean - 2.5 Ba - Proterozoic - .54 Ba - Paleozoic
• 2.5 Ba - Paleoproterozoic - 1.8 Ba - Mesoproterozoic - 1.1 Ba Neoproterozoic
• Eocambrian, Vendian: later part of Neoproterozoic. Known for the snowball earth episodes.
• What is the difference between the underlying basis for Precambrian and Paleozoic time scales?

Two basic framing question:

• When in the earth's history did plate tectonic processes similar to those today initiate?
• In what ways were Precambrian tectonics different from those operating today?

Why would you expect Precambrian tectonics to be any different from those of today?

How might of those differences been specifically manifest?

What would you look for in the geologic record to indicate that plate tectonics was operating at some point in time?

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 sandwiched between them. Note how the map pattern changes below this to a more east-west linear pattern.

Precambrian lithotectonic units or associations:

• greenstone belts.
• high grade (granulite) gneiss terranes.
• ensialic zones.
• supracrustal sequences on cratons.
• granite-rhyolite anorogenic granites (see Dr. Bob for a detailed description).
• large mafic intrustions - Great Dike and Bushveld complex in southern Africa.
• 
• Image of Great Dike of 'Rhodesia', now Zimbabwe. Image from NASA's Invisble Earth http://visibleearth.nasa.gov/view_rec.php?id=16793. Note the scale!! The dike is 530 km long. That is one huge tensile crack! Similar Paleozoic dikes at anywhere this scale are simply unknown. What do you think the internal layering is due to?

Map exercise: Please identify Precambrian tectonic elements/patterns on the maps that are provided, using what you have learned.

Penokean 'orogeny' and continental assembly (Hoffmann, 1988)

• About 150 Ma for the time of assembly of North America (we touched upon this topic a bit earlier when we discussed terrane accretion):
  • 1.96 Ga for Thelon orogen.
  • 1.92 & 1.85 Ga for Snowbird zone.
  • 1.85 & 1.83 Ga for the Trans-Hudson orogen.
  • 1.87-1.81 Ga for the New Quebec orogen.
  • 1.91 Ga start for Wopmay orogen accretion.
  • 1.85 Ga start for the Penokean orogen.
  • 1.81 for the Makkovik orogen.
  • 1.75 Ga in the Cheyenne belt.
• one sees younger additions as go south. Why did the continent grow preferentially on that side?
• was this the initiation of a supercontinent cycle?
• suggestive that at the very least the earth had more numerous smaller continental fragments prior to this.
• many are comfortable with the thought that at this time in earth history plate tectonics processes were fairly similar to those since the Paleozoic.

Some conclusions:

• a majority of the continental crust had differentiated out of the earth's interior by 2.5 Ba.
• komatiites indicate mantle heat flow was locally higher.
• clearly had cratons developed by circa 1.7 Ga.
• suggestion of 2.6 Ga old ophiolite in Wyoming (Harper), but sans mantle portion.
• continental rifting of modern character was cearly occuring by 1.1 Ga (Keweenawan rift).
• a transition period or oscillation may have existed between a permobile, vertical tectonics dominated regime and plate tectonics as we know it.
• there still exists significant debate.
• an equally fascinating, somewhat parallel questions and debate exist with respect to Precambrian surface conditions. By when did the atmosphere have significant oxygen, and how was this related to the history of the microbial ecosystem? With the absence of vegetation, how were terrestrial sedimentary processes different?

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.

• Bjornerud, M. G. & Austrheim, H. 2004, Inhibited eclogite formation: The key to the rapid growth of strong and bouyant Archean continental crust: Geology, 32, 765-768.
• Bowring, S. & Housh, T., 1995, The Earth's early evolution; Science, 269, 1535-1540.
• Davies, G. F., 1992, On the emergence of plate tectonics: Geology, v. 20, p. 963-966.
• Hoffman, P. F., 1988, United Plate of America, the Birth of a Craton: early Proterozoic Assembly and Growth of Laurentia; Ann. Rev. Earth Planet. Sci., v. 16, p. 543-603.
• Krogstad, E. J., Balakrishnan, S., Mukhopadhay, D. K., Rajamani, V & Hanson, G. N., 1989, Plate Tectonics 2.5 Billion Years Ago: Evidence at Kolar, South India; Science V. 243, p. 1337-1339.
• Nisbet, 1987, The Young Earth; Allen & Unwin, Boston, p. 318-325.
• Kerr, R. A., Plate Tectonics is the key to the distant past; Science, v. 234, p. 670-72.
• Zegers, t & van Keken, P, 2001, Middle Archean continent formation by crustal delamination: Geology, 29, 1083-1086.

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.