Earlier supercontinents

The origin of the first supercontinent and when it may have formed are highly speculative. Bleeker (2003) observed that there are about 35 Archean cratons today (Plate 11.1a between pp. 244 and 245) and that most display rifted margins, indicating that they fragmented from larger landmasses. Several possible scenarios have been envisioned for the global distribution of the cratons during the transition from Late Archean to Early Proterozoic times (Fig. 11.21). These possibilities include a single supercontinent, called Kenorland by Williams et al. (1991) after an orogenic event in the Canadian Superior Province, and the presence of either a few or many independent aggregations called supercratons. Bleeker (2003) concluded that the degree of geologic similarity among the exposed cratons favors the presence of several transient, more or less independent supercratons rather than a single supercontinent or many small dispersed landmasses. He defined a

(a) Single supercontinent

("Kenorland solution") Slave

(b) Several supercratons

("supercraton solution")

(c) Many supercratons ("unlikely solution")

Fig. 11.21 Cartoons representing possible craton configurations during Late Archean-Early Proterozoic times. Three well-known cratons (Slave, Superior and Kaapvaal) are shown shaded in (a). These cratons may have been spawned by the larger supercratons shown in (b) (after Bleeker, 2003, with permission from Elsevier).

Fig. 11.21 Cartoons representing possible craton configurations during Late Archean-Early Proterozoic times. Three well-known cratons (Slave, Superior and Kaapvaal) are shown shaded in (a). These cratons may have been spawned by the larger supercratons shown in (b) (after Bleeker, 2003, with permission from Elsevier).

minimum of three supercratons, Sclavia, Superia, and Vaalbara, that display distinct amalgamation and breakup histories (Fig. 11.21b). The Sclavia supercraton appears to have stabilized by 2.6 Ga. Confirmation of these tentative groupings awaits the collection of detailed chronostratigraphic profiles for each of the 35 Archean cratons.

Diachronous break-up of the supercratons defined by Bleeker (2003) occurred during the period 2.5-2.0 Ga, spawning the 35 or more independently drifting cratons. Paleomagnetic evidence supports the conclusion that significant differences in the paleolatitudes existed between at least several of these fragments during the Early Proterozoic (Cawood et al., 2006). Following the break-up the cratons then appear to have amalgamated into various supercontinents. Hoffman (1997) postulated a Middle Proterozoic supercontinent called Nuna, which Bleeker (2003) considered to represent the first true supercontinent. Zhao et al. (2002) also recognized that most continents contain evidence for 2.1-1.8 Ga orogenic events (Section 11.4.3) (Fig. 11.12). They postulated that these orogens record the collisional assembly of an Early-Middle Proterozoic supercontinent called Columbia (Fig. 11.22). These studies, while still speculative, suggest that at least one supercontinent formed prior to the final assembly of Rodinia and after the Archean cratons began to stabilize during the Late Archean.

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