PRE-PANGEAN SUPERCONTINENTS COME OF AGE

Twenty-five years ago, the seeds of Rodinia were sprouting, fed by growing recognition of correlatable mid-Neoproterozoic (0.8-0.7 Ga) rifted passive margins, many of which were established on the eroded remnants of late Mesoproterozoic (1.3-1.0 Ga) orogenic belts. The 1990s witnessed a surge of interest in Rodinia, with many regional studies of tectonostratigraphy and U-Pb geochronology generally conforming to the “inside-out” reconstruction model: juxtaposition of W Laurentia with E Australia/Antarctica, N Laurentia with Siberia, and E Laurentia with Baltica and cratons that would later form West Gondwana-Land. This standard model of Rodinia appeared to be converging toward a solution with only minor variations by the turn of the Millennium, but new paleomagnetic data and tectonostratigraphic information obtained in the succeeding decade chipped away at various aspects of the reconstruction; several cratons seemed to require exclusion from the supercontinent (thus questioning its very validity), or the landmass might have assembled much later (< 0.9 Ga) than originally envisaged (thus breaking the link to global Mesoproterozoic orogenesis). Although a consensus model of Rodinia’s assembly and fragmentation has arisen from the IGCP Project 440 working group, the reconstruction is supported by rather sparse data of definitive quality.

As the quest for Rodinia matures to a third decade of scrutiny, the search for its predecessor Nuna (a.k.a Hudsonland or Columbia) is only now reaching a stage of global synthesis between tectonostratigraphic and paleomagnetic data. According to most definitions, Nuna assembled at 1.9-1.75 Ga, and fragmented during the interval 1.4-1.2 Ga. Because mafic dike swarms are ideal targets for paleomagnetic study, and because they are now amenable to routine dating by U-Pb on baddeleyite, the global abundance of Paleo-Mesoproterozoic dike swarms might make Nuna more imminently solvable than Rodinia.