Plate Tectonics - Precambrian tectonics
and the advent of plate tectonics.
Why would you expect
Precambrian tectonics to be any different from those of today?
- at 3600 Ma, given the exponential character
of radioactive decay, one would expect the heat flow to be 3
times that of present day.
- at 2600 Ma heat flow still should have been
twice that of present day rates.
- continental crust is a chemical differentiate,
and at some time in the Archean there was a lot less of it,
and this would obviously effect the type of tectonics at that
- other terrestrial planetary bodies show distinctly different tectonic style, so other possibilities clearly exist.
How might of those
differences been specifically manifest?
- higher geothermal gradients with shallower
- different metamorphic signatures (perhaps
crucial for eclogites and olivine to spinel mantle transformation).
- thinner lithosphere and crust (thickness
and thermal structure should be significant in subduction mechanics).
- higher convection rates and associated higher
rates of tectonism (e.g. perhaps faster spreading and subduction rates and more hotspots).
- smaller and more mobile plates (permobile
regime) with a greater length of plate boundary.
- different or unique geologic associations (e.g. greenstone belts?).
What would you look
for in the geologic record to indicate that plate tectonics was
operating at that time?
- blueschists and/or eclogites and accretionary
- oldest blues schists .7 to .8 billion years ago (but could one still subduct but not produce blueschists?).
- calc-alkaline arc sequences?
- polar wander paths.
- significant horizontal shortening (thin-skinned tectonism).
- evidence of the 'surface' isotope recycling
that includes mantle material.
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.