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.

Text reading: p. 269-282.

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.
• What is the difference between 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?

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. 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.

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?

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).

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

• About 150 Ma for the time of assembly of North America:
  • 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 smaller plates prior to this.

Some conclusions:

• a majority of the continental crust had differentiated out of the 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 occuring by 1.1 Ga (Keweenawan).

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.