The relatively flat, stable regions of the continents contain remnants of Archean crust that formed some 4.4 to 2.5 billion years ago (Plate 11.1a between pp. 244 and 245). The formation of these cratonic nucleii marks the transition from an early Earth that was so hot and energetic that no remnants of crust were preserved, to a state where crustal preservation became possible. Most of the cratons are attached to a high velocity mantle root that extends to depths of at least 200 km (King, 2005) (Plate 11.1b,c between pp. 244 and 245). These cratonic roots are composed of stiff and chemically buoyant mantle material (Section 11.3.1) whose resistant qualities have contributed to the long-term survival of the Archean continental lithosphere (Carlson et al., 2005).

The beginning of the Archean Eon approximately coincides with the age of the oldest continental crust. A conventional view places this age at approximately 4.0 Ga, which coincides with the age of the oldest rocks found so far on Earth: the Acasta gneisses of the Slave craton in northwestern Canada (Bowring & Williams, 1999). However, >4.4 Ga detrital zircon minerals found in the Yilgarn craton of Western Australia (Wilde et al., 2001) suggest that some continental crust may have formed as early as 4.4-4.5 Ma, although this interpretation is controversial (Harrison et al., 2005, 2006; Valley et al., 2006). Because evidence for continental crust and the ages of the oldest known rocks and minerals continually are being pushed back in time, the Archean has no defined lower boundary (Gradstein et al., 2004). The end of the Archean, marking the beginning of the Proterozoic Eon, approximately coincides with inferred changes in the tectonic style and the petrologic characteristics of Precambrian rocks. It is these inferences that are central to a debate over the nature of tectonic activity in Precambrian times. Among the most important issues are whether some form of plate tectonics was operating in the early Earth and, if so, when it began. Current evidence (Sections 11.3.3, 11.4.3) suggests that plate tectonic mechanisms, including subduction, were occurring at least by 2.8-2.6 Ga and possibly much earlier (van der Velden et al., 2006; Cawood et al., 2006).

In considering the nature of Precambrian tectonic processes, three approaches have been adopted (Kroner, 1981; Cawood et al., 2006). First, a strictly uniformitar-

ian approach is taken in which the same mechanisms of plate tectonics that characterize Phanerozoic times are applied to the Precambrian cratons. This approach is common in the interpretation of Proterozoic belts, although it also has been applied to parts of the Archean cratons. Second, a modified uniformitarian approach can be postulated in which plate tectonic processes in the Precambrian were somewhat different from present because the physical conditions affecting the crust and mantle have changed throughout geologic time. This approach has been used in studies of both Archean and Early Proterozoic geology. Third, alternatives to plate tectonic mechanisms can be invoked for Precambrian times. This latter, nonuniformitarian approach most often is applied to the Early and Middle Archean. Each of these three approaches has yielded informative results.

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