Page 390 - Global Tectonics

372 CHAPTER 11

American Grenville Province continues directly into (Tohver et al., 2002), the majority of assembly between
East Antarctica, and similar belts of this age can be 1.1 and 1.0 Ga, and minor collisions between 1.0 and
traced over most of the Gondwana fragments. The age 0.9 Ga (Li et al., 2008). Most current models of Rodinia

of the oldest sedimentary rock associated with break- also show a fit between the cratons at 750 Ma that differs
up, and the provinciality of certain animal groups across substantially from the older hypotheses (Wingate et al.,
the split, suggest that the supercontinent fragmented at 2002). Torsvik (2003) published a model (Fig. 11.20) that
about 750 Ma (Storey, 1993). During fragmentation the summarizes some of these changes. The position of the
blocks now making up East Gondwana (East Antarc- continents suggests that the break-up of Rodinia had
tica, Australia, and India) moved anticlockwise, opening begun by 850 or 800 Ma with the opening of the proto-

the proto-Pacific Ocean (Panthalassa), and collided with Pacific ocean between western Laurentia and Australia-
the blocks of West Gondwana (Congo, West Africa, and East Antarctica. The emplacement of mafi c dike swarms

Amazonia). The intervening Mozambique Ocean closed in western Laurentia at 780 Ma may reflect this frag-
by the pincer-like movements of these blocks and mentation (Harlan et al., 2003). The position of Austra-
Gondwana was created when they collided to form the lia-East Antarctica also suggests that India was not
Mozambique belt of East Africa and Madagascar. Gond- connected to East Antarctica until after ∼550 Ma. This
wana then rotated clockwise away from Laurentia model emphasizes that the internal geometry of Rodinia
about 200 Ma later. Southern Africa was located at the probably changed repeatedly during the few hundred
pivot of these movements and Baltica moved indepen- million years it existed.
dently away from Laurentia, opening the Iapetus The differences among the new and old models of

Ocean, which subsequently closed during the assembly Rodinia illustrate the controversial and fluid nature of
of Pangea (Section 11.5.5). Figure 11.19b shows a Precambrian reconstructions. Numerous uncertainties
postulated configuration at 500 Ma. in the relative positions of the continents exist, with the

The first paleomagnetic test of the SWEAT hypoth- paleolatitudes of only a few cratons being known for

esis was carried out by Powell et al. (1993) who showed any given time. It also must be remembered that paleo-
that paleomagnetic poles at 1055 Ma and at 725 Ma for magnetic methods give no control on paleolongitude
Laurentia and East Gondwana are in agreement when (Section 3.6), so that linear intercratonic regions whose
repositioned according to the Rodinia reconstruction, strike is directed towards the Eulerian pole used to bring
thereby lending support to the hypothesis. Between the cratons into juxtaposition are not constrained to
725 Ma and the Cambrian the APWPs diverge, suggest- have had any particular width. For these reasons, most
ing that East Gondwana broke away from Laurentia reconstructions rely on combinations of many different
after 725 Ma. The only fragment of Rodinia for which data sets, including geological correlations based on
a detailed Apparent Polar Wanderer (APW) path can be orogenic histories, sedimentary provenance, the ages of
defi ned for the period 1100–725 Ma is Laurentia (McEl- rifting and continental margin formation, and the
hinny & McFadden, 2000). This, therefore, has been record of mantle plume events (Li et al., 2008).
used as a reference path against which repositioned Another controversial aspect of the Rodinia super-
paleomagnetic poles from other Rodinian fragments continent concerns the effect of its dispersal on past
can be compared. However, many of the tests were climates. Some studies suggest that as Rodinia frag-
hindered by a lack of high quality geochronology. As mented the planet entered an icehouse or snowball Earth
new data were collected, the existence of a Late Pro- state in which it was intermittently completely covered
terozoic supercontinent gained acceptance, although by ice (Evans, 2000; Hoffman & Schrag, 2002). The geo-
numerous modifications have been proposed (Dalziel logic evidence for this intermittent but widespread gla-

et al., 2000b; Karlstrom et al., 2001; Meert & Torsvik, ciation includes glacial deposits of Late Proterozoic age
2003). There is now considerable geologic and paleo- that either contain carbonate debris or are directly over-
magnetic evidence that, except for Amazonia, the lain by carbonate rocks indicative of warm waters. In
cratons of South America and Africa were never part of addition, paleomagnetic data suggest that during at least
Rodinia, although they probably were close to it (Kröner two Late Proterozoic glacial episodes ice sheets reached
& Cordani, 2003). Newer models also indicate the piece- the equator. One possible explanation of these observa-
meal assembly of Rodinia beginning with Grenville-age tions is that periods of global glaciation during the Late
collisions in eastern Canada and Australia at 1.3–1.2 Ga, Proterozoic were controlled by anomalously low
followed by an Amazonia–Laurentia collision at 1.2 Ga atmospheric CO 2 concentrations (Hyde et al., 2000;