Late Proterozoic supercontinent

Similarities between the Late Proterozoic geologic record in western Canada and eastern Australia (Bell & Jefferson, 1987; Young, 1992) and between the southwestern USA and East Antarctica suggest that these areas were juxtaposed during Late Proterozoic times (Dalziel, 1991, 1995; Moores, 1991; Hoffman, 1991) (Fig. 11.19a). This seemingly radical suggestion was referred to as the SWEAT (South West US and East AnTarctica) hypothesis. The widespread Grenville orogenic belts, that immediately pre-date the Late Proterozoic, suggest that many other continental fragments can be added to this reconstruction to form a Late Proterozoic supercontinent called Rodinia (Fig. 11.19a). Laurentia (North America and Greenland) forms the core of the supercontinent and is flanked to the north by East Antarctica. The reconstruction shows that the North

Paleogeography Proterozoic
Canada Proterozoic Reconstruction
Fig. 11.19 (a) Reconstruction of the Late Proterozoic supercontinent Rodinia. (b) Late Cambrian paleogeography after the break-up of Rodinia and the formation of Gondwana (after Hoffman, 1991, with permission from Science 252, 1409-12 with permission from the AAAS).

American Grenville Province continues directly into East Antarctica, and similar belts of this age can be traced over most of the Gondwana fragments. The age of the oldest sedimentary rock associated with breakup, and the provinciality of certain animal groups across the split, suggest that the supercontinent fragmented at about 750 Ma (Storey, 1993). During fragmentation the blocks now making up East Gondwana (East Antarctica, Australia, and India) moved anticlockwise, opening the proto-Pacific Ocean (Panthalassa), and collided with the blocks of West Gondwana (Congo, West Africa, and Amazonia). The intervening Mozambique Ocean closed by the pincer-like movements of these blocks and Gondwana was created when they collided to form the Mozambique belt of East Africa and Madagascar. Gondwana then rotated clockwise away from Laurentia about 200 Ma later. Southern Africa was located at the pivot of these movements and Baltica moved independently away from Laurentia, opening the Iapetus Ocean, which subsequently closed during the assembly of Pangea (Section 11.5.5). Figure 11.19b shows a postulated configuration at 500 Ma.

The first paleomagnetic test of the SWEAT hypothesis was carried out by Powell et al. (1993) who showed that paleomagnetic poles at 1055 Ma and at 725 Ma for Laurentia and East Gondwana are in agreement when repositioned according to the Rodinia reconstruction, thereby lending support to the hypothesis. Between 725 Ma and the Cambrian the APWPs diverge, suggesting that East Gondwana broke away from Laurentia after 725 Ma. The only fragment of Rodinia for which a detailed Apparent Polar Wanderer (APW) path can be defined for the period 1100-725 Ma is Laurentia (McEl-hinny & McFadden, 2000). This, therefore, has been used as a reference path against which repositioned paleomagnetic poles from other Rodinian fragments can be compared. However, many of the tests were hindered by a lack of high quality geochronology. As new data were collected, the existence of a Late Pro-terozoic supercontinent gained acceptance, although numerous modifications have been proposed (Dalziel et al., 2000b; Karlstrom et al., 2001; Meert & Torsvik, 2003). There is now considerable geologic and paleo-magnetic evidence that, except for Amazonia, the cratons of South America and Africa were never part of Rodinia, although they probably were close to it (Kroner & Cordani, 2003). Newer models also indicate the piecemeal assembly of Rodinia beginning with Grenville-age collisions in eastern Canada and Australia at 1.3-1.2 Ga, followed by an Amazonia-Laurentia collision at 1.2 Ga

(Tohver et al., 2002), the majority of assembly between 1.1 and 1.0 Ga, and minor collisions between 1.0 and 0.9 Ga (Li et al., 2008). Most current models of Rodinia also show a fit between the cratons at 750 Ma that differs substantially from the older hypotheses (Wingate et al., 2002). Torsvik (2003) published a model (Fig. 11.20) that summarizes some of these changes. The position of the continents suggests that the break-up of Rodinia had begun by 850 or 800 Ma with the opening of the proto-Pacific ocean between western Laurentia and Australia-East Antarctica. The emplacement of mafic dike swarms in western Laurentia at 780 Ma may reflect this fragmentation (Harlan et al., 2003). The position of Australia-East Antarctica also suggests that India was not connected to East Antarctica until after ~550 Ma. This model emphasizes that the internal geometry of Rodinia probably changed repeatedly during the few hundred million years it existed.

The differences among the new and old models of Rodinia illustrate the controversial and fluid nature of Precambrian reconstructions. Numerous uncertainties in the relative positions of the continents exist, with the paleolatitudes of only a few cratons being known for any given time. It also must be remembered that paleo-magnetic methods give no control on paleolongitude (Section 3.6), so that linear intercratonic regions whose strike is directed towards the Eulerian pole used to bring the cratons into juxtaposition are not constrained to have had any particular width. For these reasons, most reconstructions rely on combinations of many different data sets, including geological correlations based on orogenic histories, sedimentary provenance, the ages of rifting and continental margin formation, and the record of mantle plume events (Li et al., 2008).

Another controversial aspect of the Rodinia supercontinent concerns the effect of its dispersal on past climates. Some studies suggest that as Rodinia fragmented the planet entered an icehouse or snowball Earth state in which it was intermittently completely covered by ice (Evans, 2000; Hoffman & Schrag, 2002). The geologic evidence for this intermittent but widespread glaciation includes glacial deposits of Late Proterozoic age that either contain carbonate debris or are directly overlain by carbonate rocks indicative of warm waters. In addition, paleomagnetic data suggest that during at least two Late Proterozoic glacial episodes ice sheets reached the equator. One possible explanation of these observations is that periods of global glaciation during the Late Proterozoic were controlled by anomalously low atmospheric CO2 concentrations (Hyde et al., 2000;

South China

Seychelles

Proto-Pacific Ocean

North China

Kalahari

Kalahari

Congo

Congo

Proto-Pacific Ocean

Seychelles

Paleogeography Proterozoic

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

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

Donnadieu et al., 2004). During break-up, the changing paleogeography of the continents may have led to an increase in runoff, and hence consumption of CO2, through continental weathering that decreased atmospheric CO2 concentrations (Section 13.1.3). The extreme glacial conditions may have ended when volcanic out-gassing of CO2 produced a sufficiently large greenhouse effect to melt the ice. The resulting "hothouse" would have enhanced precipitation and weathering, giving rise to the deposition of carbonates on top of the glacial deposits during sea-level (Hoffman et al., 1998). Alternatively, these transitions may have resulted mainly from the changing configuration of continental fragments and its effect on oceanic circulation (Sections 13.1.2, 13.1.3). Whichever view is correct, these interpretations suggest that the break-up of Rodinia triggered large changes in global climate. However, the origin, extent, and termination of the Late Proterozoic glaciations, and their possible relationship to the supercontinental breakup, remains an unresolved and highly contentious issue (Kennedy et al., 2001; Poulsen et al., 2001).

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