Plate Tectonics - 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/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:
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.
References:
- 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.