EDIACARAN CONTINENTAL MOTIONS: THE CONUNDRUM PERSISTS

For twenty years, it has been recognized that Ediacaran paleomagnetic data from several paleocontinents (Laurentia, Baltica, Siberia, Australia) show rapid variations that are not easily incorporated into a uniformitarian plate-tectonic framework. Recently, Bono and Tarduno (2015, Geology 43, 131-134) analyzed samples from the Sept-Iles complex in eastern Laurentia and concluded that one of its paleomagnetic components (steep) is biased by recent magnetic overprinting. They suggested that all of the anomalous Ediacaran paleomagnetic directions, in Laurentia and elsewhere, could be accounted for by similar mechanisms. However, Halls et al. (2015, Precambrian Research 257, 137-166) showed that the 585-575 Ma Grenville Dykes and related intrusions, also in eastern Laurentia, have robust paleomagnetic directions of both steep and shallow inclinations that appear unaffected by the recent magnetic field. According to those data, Laurentia would appear to have traversed a wide range of paleolatitudes at rates exceeding 200 cm/yr. Halls et al. considered such rates to be too rapid even for true polar wander (TPW), invoking instead an anomalous transitional state of the mid-Ediacaran geodynamo. The other expansive Ediacaran paleomagnetic dataset derives from Australia, which remained at low latitudes but experienced multiple oscillations in orientation. The Australian data are from redbeds with poor geochronological constraints. Schmidt and Williams (2010, Geophysical Journal International 182, 711-726) assigned numeric ages to the strata and concluded that the oscillations did not coincide with the paleolatitude shifts experienced by Laurentia. Nonetheless, an alternative age model for the Australian redbeds could permit synchroneity of the two continents' oscillations, consistent with TPW. Although the currently available data cannot distinguish between alternative models of TPW (with low enough mantle viscosity necessary to achieve such rates) and anomalous geomagnetic field behavior, the TPW option makes specific predictions of relative paleolongitude between continents that can be assessed by their tectonostratigraphic records. Either process could have substantially influenced Earth's surface paleoenvironment during the nascent stages of animal evolution.