New late Proterozoic paleomagnetic pole for the Rio de la Plata craton: Implications for Gondwana

Abstract

Major uncertainties remain on the precise kinematic and temporal aspects of Gondwana assembly. In order to provide new constraints on them, a paleomagnetic study was carried out on the Ediacaran succession of red claystones exposed in Sierra de los Barrientos (37.8°S, 59.0°W), Buenos Aires province, Argentina. This succesion belongs to the late Proterozoic-early Paleozoic sedimentary cover of the Rio de la Plata craton. Sixty-two oriented samples from 13 sites were submitted to standard stepwise demagnetization procedures yielding a characteristic remanence carried by hematite. A primary origin for the remanence is suggested by the apparent recording of secular variation along the stratigraphic column including a reversal of the earth magnetic field and an improvement in clustering of mean site directions after application of bedding corrections, although the latter is not statistically significant. Since the study samples show a significant anisotropy of magnetic susceptibility (AMS) degree (P up to 1.2) and an ellipsoid shape compatible with significant compaction during diagenesis, remanence inclination shallowing was investigated through oriented isothermal remanent magnetization (IRM) acquisition and demagnetization experiments. These permitted to confirm significant inclination shallowing and to compute a correction factor that was applied to the mean site directions producing a significantly further improvement in the statistical parameters. A paleomagnetic pole was computed for the Los Barrientos claystones at 15.1°S, 252.6°E, dp: 10.9°, dm: 14.2°. When rotated into a Gondwana framework, this new pole agrees with previous poles of ca. 550 Ma from several Gondwanan cratons, allowing the construction of a single APWP for the whole of Gondwana since that time, and suggesting that by ca. 550 Ma Gondwana was already assembled or in the final stages of that process.