PROTEROZOIC SUPERCONTINENTS AND GEOMAGNETIC SUPERCHRONS

Paul Hoffman’s mobilistic perspective of Proterozoic tectonics, once less readily accepted than now, has inspired generations of paleomagnetists to interpret their Precambrian data in the context of drifting plates. We compile a quality-filtered global paleomagnetic dataset for two recently proposed kinematic models of the supercontinent Nuna evolving toward Rodinia (1.8-0.9 Ga), with extensions to older times for well constrained pre-Nuna cratons (Slave and Superior). As Hoffman earlier noted, the stratigraphic record of Slave-marginal tectonism is consistent with a Northern Hemisphere location, and “inverted” orientation, of the newly assembling Laurentia continent. This sets the absolute polarity of the global Proterozoic paleomagnetic dataset, which cannot otherwise be joined confidently to Phanerozoic apparent polar wander paths. We identify as many as 10 possible magnetic superchrons, of predominant polarity bias (N or R) over at least 15 million years duration, between 2.1 and 0.9 Ga, which we name according to the best-quality representative data or regions: Superior R (2105-2069 Ma), Sudbury N (1863-1838), Cleaver N (1755-1727), McArthur R (1648-1630), Lower Belt N (1476-1452), Middle Belt R (1449-1415), Mackenzie N (1291-1235), Middle Gardar N (1200-1150), Keweenawan N (1100-1050), and Maya N (1035-1000 Ma). Most of these are derived merely from compilations of site-based sampling methods, but some are also bolstered by detailed magnetostratigraphic studies of well preserved sedimentary basins. An average of one superchron per 100 million years is about twice the Phanerozoic rate; the number may be reduced by future data of opposing polarity bias within any of the proposed intervals, but it may be increased by additional data within about 10 temporal gaps in the global compilation. The number of Proterozoic superchrons determined empirically, as in this study, can test theoretical models of geodynamo behavior through the eras of steady inner core growth. Because paleomagnetic data are a central pillar of the Snowball Earth hypothesis and figure prominently in the interpretation of cap carbonate accumulation rates, a full understanding of Earth’s geodynamo evolution through Proterozoic time thus returns full-circle to Paul Hoffman’s remarkable scientific expertise and legacy.