Page 366 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 349

the Archean mantle could be dissipated by increasing
the length of ocean ridge systems or by increasing the 11.3 ARCHEAN
rate of plate production with respect to the present
(Bickle, 1978). Hargraves (1986) concluded that heat TECTONICS
loss through the oceanic lithosphere is proportional to
the cube root of the total length of the mid-ocean ridge.
Assuming a nonexpanding Earth (Section 12.3), the 11.3.1 General characteristics
increased rate of plate production implies a similar
increase in plate subduction rate. These computations of cratonic mantle lithosphere
suggest that some form of plate tectonics was taking

place during the Precambrian at a much greater rate A defining characteristic of the cratonic mantle litho-
than today. The fast rates suggest an image of the solid sphere is a seismic velocity that is faster than normal
surface of the early Earth where the lithosphere was subcontinental mantle to depths of at least 200 km and
broken up into many small plates that contrasts with locally to depths of 250–300 km (Plate 11.1b,c between
the relatively few large plates that exist presently. This pp. 244 and 245). Many Proterozoic belts lack these fast
interpretation is consistent with the results of numeri- velocity anomalies at similar depths. In addition,
cal models of mantle convection, which show that small Archean cratons are characterized by the lowest surface

plates are capable of releasing more heat from the heat flow of any region on Earth, with a heat fl ux that
Earth’s interior than large plates (Lowman et al., is lower than adjacent Proterozoic and Phanerozoic
−2
2001). crust by some 20 mW m (Jaupart & Mareschal, 1999;
More recent calculations have disputed this con- Artemieva & Mooney, 2002). Isotopic age determina-
ventional view, at least for the Late Archean. Van tions and Re-Os studies of mantle nodules (Pearson et
Thienen et al. (2005) suggested that the increased al., 2002; Carlson et al., 2005) confirm that the mantle

heat flux from the Archean mantle could have been roots are Archean in age and indicate that most have
dissipated by thinning the lithosphere and thereby remained thermally and mechanically stable over the

increasing the heat flow through the lithosphere. past 2–3 Ga. These observations indicate that the roots
These authors concluded that for a steadily (expo- of the Archean cratons are cool, strong, and composi-
nentially) cooling Earth, plate tectonics is capable tionally distinct from the surrounding mantle.
of removing all the required heat at a plate tectonic In the ocean basins, the base of the oceanic litho-
rate comparable to or lower than the current rate sphere is marked by a strong decrease in the velocity of
of operation. This result is contrary to the notion shear waves at depths generally less than 100 km beneath
that faster spreading would be required in a hotter the crust (Sections 2.8.2, 2.12). Similar low velocity
Earth to be able to remove the extra heat (e.g. zones occur under tectonically active continental
Bickle, 1978). It also suggests that reduced slab pull regions, such as the Basin and Range Province, but
and ridge push forces in a hotter mantle would beneath the stable cratons low velocity zones are either
result in a slower rate of plate tectonics compared extremely weak or entirely absent (Carlson et al., 2005).
to the modern Earth. Korenaga (2006) showed that Consequently the base of the continental lithosphere is
a more sluggish style of plate tectonics during not well defined by seismological data. With increasing

Archean times satisfies all the geochemical constraints depth the high seismic velocities of the stable litho-

on the abundance of heat-producing elements in sphere gradually approach those of the convecting
the crust and mantle and the evidence for a gradual mantle across a broad, ill-defined transition zone below

cooling of the mantle with time in the framework 200 km. Thermal modeling and geochemical data from
of whole mantle convection. This result removes mantle xenoliths have helped to define the location of

the thermal necessity of having extensive ocean the lithosphere–asthenosphere boundary. The results
ridges and/or rapid spreading and subduction. It suggest that the base of the continental lithosphere is
must be appreciated, however, that thermal condi- deepest (∼250 km) in undisturbed cratonic areas and
tions during Archean times are quite conjectural, shallowest (∼180 km) beneath Phanerozoic rifts and
so that these and other alternative interpretations orogens (O’Reilly et al., 2001). This determination is in
remain speculative. general agreement with seismic observations.