Page 364 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 347

11.1 INTRODUCTION 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,
The relatively flat, stable regions of the continents although it also has been applied to parts of the Archean

contain remnants of Archean crust that formed some cratons. Second, a modified uniformitarian approach
4.4 to 2.5 billion years ago (Plate 11.1a between pp. can be postulated in which plate tectonic processes in
244 and 245). The formation of these cratonic nucleii the Precambrian were somewhat different from present
marks the transition from an early Earth that was because the physical conditions affecting the crust and
so hot and energetic that no remnants of crust were mantle have changed throughout geologic time. This
preserved, to a state where crustal preservation became approach has been used in studies of both Archean and
possible. Most of the cratons are attached to a high Early Proterozoic geology. Third, alternatives to plate
velocity mantle root that extends to depths of at tectonic mechanisms can be invoked for Precambrian
least 200 km (King, 2005) (Plate 11.1b,c between times. This latter, nonuniformitarian approach most
pp. 244 and 245). These cratonic roots are composed often is applied to the Early and Middle Archean. Each
of stiff and chemically buoyant mantle material of these three approaches has yielded informative
(Section 11.3.1) whose resistant qualities have con- results.
tributed 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 11.2 PRECAMBRIAN
4.0 Ga, which coincides with the age of the oldest rocks
found so far on Earth: the Acasta gneisses of the Slave HEAT FLOW
craton in northwestern Canada (Bowring & Williams,
1999). However, >4.4 Ga detrital zircon minerals found
in the Yilgarn craton of Western Australia (Wilde et One of the most important physical parameters to
al., 2001) suggest that some continental crust may have have varied throughout geologic time is heat fl ow.
formed as early as 4.4–4.5 Ma, although this interpre- The majority of terrestrial heat production comes
tation is controversial (Harrison et al., 2005, 2006; Valley from the decay of radioactive isotopes dispersed
et al., 2006). Because evidence for continental crust throughout the core, mantle, and continental crust
and the ages of the oldest known rocks and minerals (Section 2.13). Heat flow in the past must have been

continually are being pushed back in time, the Archean considerably greater than at present due to the expo-

has no defined lower boundary (Gradstein et al., 2004). nential decay rates of these isotopes (Fig. 11.1). For
The end of the Archean, marking the beginning of an Earth model with a K/U ratio derived from mea-
the Proterozoic Eon, approximately coincides with surements of crustal rocks, the heat flow in the crust

inferred changes in the tectonic style and the petrologic at 4.0 Ga would have been three times greater than
characteristics of Precambrian rocks. It is these infer- at the present day and at 2.5 Ga about two times
ences that are central to a debate over the nature of the present value (Mareschal & Jaupart, 2006). For
tectonic activity in Precambrian times. Among the most K/U ratios similar to those in chondritic meteorites,
important issues are whether some form of plate tec- which are higher than those in crustal rocks,
tonics was operating in the early Earth and, if so, the magnitude of the decrease would have been
when it began. Current evidence (Sections 11.3.3, greater.
11.4.3) suggests that plate tectonic mechanisms, includ- The increased heat flow in Archean times implies

ing subduction, were occurring at least by 2.8–2.6 Ga that the mantle was hotter in the younger Earth than
and possibly much earlier (van der Velden et al., 2006; it is today. However, how much hotter and whether
Cawood et al., 2006). a hotter mantle caused young continental lithosphere
In considering the nature of Precambrian tectonic to be much warmer than at present is uncertain.
processes, three approaches have been adopted (Kröner, This uncertainty arises because there is no direct way
1981; Cawood et al., 2006). First, a strictly uniformitar- to determine the ratio of heat loss to heat produc-