Page 391 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 373

South
China
Seychelles India Proto-Pacific Ocean 30°N
North
Madagascar Australia China

Kalahari Laurentia Baltica

East
Antarctica Siberia
Rio
Plata
Amazonia Avalonian 30°S
Congo Timanian
Brazilide
Ocean
West
Africa Cadomian 60°S

1.3–1.0 Ga Grenville orogenic belts
Cratons with paleomagnetic data ( 750 Ma)

Fig. 11.20 Reconstruction of Rodinia at ∼750 Ma (after Torsvik, 2003, with permission from Science 300, 1379–81, with
permission from the AAAS).

Donnadieu et al., 2004). During break-up, the changing 11.5.4 Earlier supercontinents
paleogeography of the continents may have led to an
increase in runoff, and hence consumption of CO 2 ,
through continental weathering that decreased atmo- The origin of the fi rst supercontinent and when it may
spheric CO 2 concentrations (Section 13.1.3). The extreme have formed are highly speculative. Bleeker (2003)
glacial conditions may have ended when volcanic out- observed that there are about 35 Archean cratons today
gassing of CO 2 produced a suffi ciently large greenhouse (Plate 11.1a between pp. 244 and 245) and that most
effect to melt the ice. The resulting “hothouse” would display rifted margins, indicating that they fragmented
have enhanced precipitation and weathering, giving rise from larger landmasses. Several possible scenarios have
to the deposition of carbonates on top of the glacial been envisioned for the global distribution of the
deposits during sea-level (Hoffman et al., 1998). Alterna- cratons during the transition from Late Archean to
tively, these transitions may have resulted mainly from Early Proterozoic times (Fig. 11.21). These possibilities
the changing configuration of continental fragments include a single supercontinent, called Kenorland by Wil-

and its effect on oceanic circulation (Sections 13.1.2, liams et al. (1991) after an orogenic event in the Cana-
13.1.3). Whichever view is correct, these interpretations dian Superior Province, and the presence of either a few
suggest that the break-up of Rodinia triggered large or many independent aggregations called supercratons.
changes in global climate. However, the origin, extent, Bleeker (2003) concluded that the degree of geologic
and termination of the Late Proterozoic glaciations, and similarity among the exposed cratons favors the pres-
their possible relationship to the supercontinental break- ence of several transient, more or less independent
up, remains an unresolved and highly contentious issue supercratons rather than a single supercontinent or
(Kennedy et al., 2001; Poulsen et al., 2001). many small dispersed landmasses. He defi ned a