Dave Evans: Yale U

The Western Churchill Province owes its existence to late Paleoproterozoic (1.9-1.8 Ga) assembly of older cratonic elements as part of the hypothesized supercontinent Nuna, predecessor of better-known Rodinia. Because many proposed cratonic juxtapositions for Rodinia involve Paleoproterozoic similarities and are not joined by 1.3-1.0 Ga sutures, the search for a precise Nuna configuration is intimately related to the quest for a precisely quantified Rodinia. For example, placement of the North Australian craton against various regions of western North America (e.g., SWEAT, AUSWUS, and AUSMEX) invokes, among other similarities, a common origin for the U-rich Kombolgie and Athabasca basins. Most published reconstructions of Rodinia and Nuna rely primarily upon qualitative geological comparisons, lacking quantitative constraints from paleomagnetism.

I present a new model of Rodinia, which is a radical departure from all previous reconstructions. The new model is motivated by recent paleomagnetic refutations of all previous Australia-Laurentia juxtapositions, as well as intriguing tectonostratigraphic similarities between western Laurentia and the eastern margin of the Amazon craton in South America. This new pairing of the proto-Cordilleran rift margin of Laurentia with its proposed proto-Brasiliano rift conjugate is abbreviated, COBRA. West Africa occupies a position adjacent to western Canada. These connections are hypothesized to have originated by latest Archean suturing of the Carajas and Wyoming cratons, followed by juvenile accretion to the north of these provinces at 2.3-2.1 Ga, and to the south at 1.8-1.6 Ga (in present orientations of both regions). COBRA rifted at 780 Ma, via dextral transtension that allowed Laurentia to vacate the Brasiliano margins of Amazon and West Africa in time for 620-Ma convergence (also with a strong dextral component of motion) in both of those latter regions as part of Gondwanaland assembly. The model thus requires Congo-Sao Francisco to have lain in the neighborhood of arctic Canada in Rodinia.

Two or more paleomagnetic poles from a given craton define an apparent polar wander (APW) path for that continent. If two cratons' APW paths share a precisely similar length and form across a given interval of geological time, then it is permissible that those two cratons were part of the same supercontinent; their relative paleolatitude and paleolongitude are defined by rotating the APW paths into superposition, and applying the same rotations to the cratons. If the cratons are then in direct juxtaposition their geological histories may be used to assess the plausibility of the paleomagnetic match. COBRA's first successful prediction arises from quantitative placement of Congo adjacent to arctic Laurentia by superposition of pairs of paleomagnetic poles from ca.1270 and 750 Ma. In so doing, the "other side" of the giant Muskox-Mackenzie large igneous province is at last discovered as the precisely coeval Kabanga-Musongati layered mafic-ultramafic intrusive province straddling the Kibaran belt of eastern Africa. Other intriguing similarities, from an economic perspective, between east-central Africa and Canada include a large evaporite basin at ca.800 Ma (Roan Supergroup of the Zambian Copperbelt and Amundson basin in Canada) and post-orogenic (post-1860 Ma) epicratonic sandstone successions (Mporokoso basin of the Bangweulu block in eastern Zambia, and Athabasca-Thelon basins in Canada).

The remainder of Rodinia is assembled by similar comparisons of pairs of the most reliable paleomagnetic poles from other cratons with the Laurentian APW path. Most notably, Australia finds itself in a cartographically inverted position about 2000 km distant from the Greenland margin of Laurentia, as determined by superposition of paleomagnetic poles from 1070 and 755 Ma. This reconstruction also nearly aligns ca.1750-1700 Ma poles from the McArthur basin with those from the Trans-Hudson (s.s.) and Nagssugtoqidian orogens, and suggests that Australia's "inverted" position in Rodinia may have also existed within Nuna. Baltica may have lain adjacent to Greenland, as previously published in several Rodinia reconstructions, and would thus provide a Paleo-Neoproterozoic link between Laurentia and Australia. India and the Chinese cratons adorn the western and northern sides of Australia, as recently proposed by Z.X. Li et al. (EPSL v.220 p.409-421), and Kalahari is placed between India, Mawsonland, and Congo. Siberia appears to be surrounded by Mesoproterozoic ("Riphean") passive margins, thus may have been excluded from Rodinia--while on the other hand perhaps near the center of Nuna.

Quantitative constraints upon Nuna's configuration are in their infancy, yet great strides may be achieved in the near future if it is appreciated that the Nuna problem overlaps substantially with the Rodinia puzzle. Insights toward cratonic connections within one of those supercontinents may help constrain the same juxtapositions within the other. Likewise, constraints on Nuna will certainly assist endeavors to assemble its predecessor, latest Archean Kenorland.