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

新元古代中晚期（850-542 Ma）的古地磁记录显示出异常快速的大幅度极移变化。激烈的争论围绕着这些数据的解释，特别是对他们的古地理意义或地球的领域是否偏离了稳定的地心轴向偶极子，例如，赤道偶极子准时主导的轴向偶极子或有超频繁的极性反转。另一方面，由于大的极移变化记录在几个大陆上相似年龄的岩石中，一些工作者提出了真极移（TPW）事件来解释它们。数值模拟表明，TPW的速度达到几个°/Ma，而磁场扰动预计将在°/ka的数量级。为了检验这些不同的假设，需要对连续剖面进行高分辨率的古地磁研究。我们已经确定了华南的一个剖面，其年代在810 - 820 Ma之间，我们在那里进行了初步的取样。当在同一层位收集多个样品时，这些样品上的逐步热退磁揭示了具有可重复的平行方向的非常稳定的磁化轨迹。然而，在我们采样的50米截面上，方向变化很大。因此，这一部分作为一个很好的目标，研究大极性漂移的变化之间观察到的其他工人820和800马，我们建议进行高分辨率（2000芯）的古地磁研究的完整部分（185米）。热退磁将隔离磁化组件。岩石磁性实验，光学和电子显微镜将确定磁性载体的性质和成因，目的是确定磁化采集机制，并评估古地磁方向是否是主要的和时间渐进的。我们将把结果在时间域中通过高精度U-Pb测年提取的凝灰岩层在该部分。由于这个高质量的数据集，我们将解决新元古代地球磁场几何形状的问题，这也将对TPW等机制带来约束。

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.