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

新元古代地磁场：华南高分辨率古地磁研究的新见解

• 批准号: 413587100
• 负责人: Professor Dr. Stuart Alan Gilder
• 金额: --
• 依托单位: Sektion Geophysik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/413587100?language=en
• 关键词: Neoproterozoic; geomagnetic; field; New; insights

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.

新元古代中上层（850-542 Ma）的古地磁记录显示出异常快速、大幅度的极移变化。围绕这些数据的解释展开了激烈的争论，特别是关于它们的古地理意义或地球磁场是否偏离稳定的地心轴偶极子；例如赤道偶极子准时支配轴向偶极子或存在超频繁的极性反转。另外，由于几大洲年龄相似的岩石中记录了巨大的极地漂移变化，一些工作人员提出了真正的极地漂移（TPW）事件来解释它们。数值模拟表明，TPW 速度高达几°/Ma，而磁场扰动预计为°/ka 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古地磁研究。我们在华南地区确定了一个年代为 810 Ma 至 820 Ma 之间的剖面，并在那里进行了初步采样。当在同一层位收集多个样品时，这些样品的逐步热退磁揭示了非常稳定的磁化轨迹，具有可重复的平行方向。然而，在我们采样的 50 m 截面中，方向变化很大。因此，该剖面是研究其他工作者在 820 至 800 Ma 之间观测到的大极移变化的绝佳目标，我们建议对整个剖面（185 m）进行高分辨率（2000 个核心）古地磁研究。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，旨在确定磁化强度获取机制并评估古地磁方向是否是主要的和时间渐进的。我们将通过对从剖面中插层的凝灰岩层中提取的锆石进行高精度 U-Pb 定年，将结果置于时域中。借助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，这也将为TPW等机制带来约束。