Australia: The Land Where Time Began

A biography of the Australian continent 

Precambrian Tectonics

Archean crust, that formed some time between 4.4-2.5 Ga, is present in the relatively flat, stable regions of the continents. The transition from the molten early Earth to a condition where crust could be preserved is marked by the first appearance of the cratonic nuclei which were the original blocks around which the continents were built. Most cratons are attached to a mantle root, that is of high seismic velocity, extending down at least 200 km (King, 2005). The roots of cratons are chemically buoyant mantle material that is stiff and resistant, contributing to their survival from the Archean to the present (Carlson et al.2005).

The age of rocks at the beginning of the Archean Eon, when solid rock first formed that has been preserved, is not known for certain. The oldest generally accepted dates are for the Acasta gneisses of the Slave Craton, northwest Canada (Bowring & Williams, 1999). Detrital zircons have been found at Jack Hills, on the Yilgarn Craton, Western Australia (Wilde et al., 2001) that have been dated to 4.4 Ga. This interpretation has been disputed (Harrison et al., 2005, 2006; Valley et al., 2006). Because the age of the earliest rocks is being continually pushed back it cannot be said with confidence when the first solid rocks actually formed (Gradstein, 2004).

The end of the Archean - beginning of the Proterozoic Era coincides approximately with the inferred changes in tectonic style and petrologic characteristics of Precambrian rocks. The debate over the nature of tectonic activity in the Precambrian centres on these inferences. One of the issues is whether plate tectonics operated  in some form in the early Earth, and when it began. Based on current evidence it is believed the mechanisms of plate tectonics, such as subduction, were occurring by at least 2.8-2.6 Ga, and possibly much earlier (van der Velden et al., 2006; Cawood et al., 2006).

3 approaches have been used when considering Precambrian tectonic processes (Krner, 1981; Cawood, et al., 2006). These are:

  1. A strictly uniformitarian  approach - the same mechanisms of plate tectonics operating during the Phanerozoic are applied to cratons formed from the Precambrian. This approach is often used in the interpretation of Proterozoic belts, as well as to parts of the Archean cratons.
  2. A modified uniformitarian approach - in the Precambrian the processes of plate tectonics were probably somewhat different from those of the present, as the physical conditions affecting the crust have changed over geologic time. This approach has been applied to study of Archean and Early Proterozoic geology.
  3. A nonuniformitarian approach - an alternative to the mechanisms of plate tectonics to study Precambrian times. This method is not often applied to the Early and Middle Archean.

Informative results have been obtained from all 3 approaches.

See Source 1 for more detailed information on all aspects of plate tectonics

See Australian Proterozoic Evolution - Palaeomagnetic Constraints

Sources & Further reading

  1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.
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Last Updated 10/05/2012

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading