The Neoproterozoic geomagnetic field: New insights from a high-resolution paleomagnetic study in South China

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The paleomagnetic record during the middle to upper Neoproterozoic (850-542 Ma) displays exceptionally fast polar wander variations of large amplitude. Heated debate surrounds the interpretation of these data—notably on their paleogeographic implications or whether the Earth’s field deviated from a stable geocentric axial dipole; e.g. that an equatorial dipole punctually dominated the axial dipole or that there were hyper-frequent polarity reversals. Alternatively, because the large polar wander variations were recorded in rocks of similar ages on several continents, some workers have proposed true polar wander (TPW) episodes to explain them. Numerical simulations suggest that TPW velocities reach up to several °/Ma, whereas magnetic field perturbations are expected to be on the order of °/ka. In order to test these different hypotheses, a high-resolution paleomagnetic study on a continuous section is needed. We have identified a section in South China, well dated between 810 and 820 Ma, where we carried out a preliminary round of sampling. Stepwise thermal demagnetization on these samples reveals very stable magnetization trajectories with repeatable, parallel directions when multiple samples were collected in the same horizon. However, the directions vary widely throughout the 50 m of section that we sampled. This section thus serves as an excellent target to study the large polar wander variations observed by other workers between 820 and 800 Ma and we propose to carry out a high-resolution (2000 cores) paleomagnetic study on the complete section (185 m). Thermal demagnetization will isolate the magnetization components. Rock magnetic experiments, optical and electron microscopy will ascertain the nature and genesis of the magnetic carriers, with the aim to determine the mechanism of magnetization acquisition and to assess whether the paleomagnetic directions are primary and time-progressive. We will place the results in the time domain through high-precision U-Pb dating of zircons extracted from tuff layers intercalated in the section. Thanks to this high-quality dataset, we will address the question of the Earth's magnetic field geometry during the Neoproterozoic, which will also bring constraints on mechanisms such as TPW.