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.

中部到上层新元古代（850-542 Ma）期间的古磁记录显示出大型放大器的极性徘徊的异常快速变化。激烈的辩论周围的解释这些数据 - 尤其是基于它们的古地理含义，还是地球场是否偏离了稳定的中心轴向偶极子；例如同样的偶极子准时地统治了轴向偶极子，或者存在超频的极性逆转。另外，由于在几个连续性的岩石中记录了较大的极地流浪变化，因此一些工人提出了真正的极性流浪（TPW）发作来解释它们。数值模拟表明TPW速度可达到几个°/mA，而磁场扰动预计将处于°/ka的顺序。为了检验这些不同的假设，需要在连续部分进行高分辨率的古磁研究。我们已经确定了中国南部的一个区域，其历史悠久在810至820 MA之间，在那里我们进行了初步抽样。这些样品上的逐步热消电化揭示了非常稳定的磁化轨迹，当在同一地平线中收集多个样品时，具有可重复的平行方向。但是，在我们采样的整个截面的50 m中，这些方向差异很大。因此，本节是研究820至800 Ma之间其他工人观察到的大型极地徘徊变化的绝佳目标，我们建议在完整的部分（185 m）上进行一项高分辨率（2000核）古磁研究。热消电化将隔离磁化成分。岩石磁实验，光学和电子显微镜将确定磁载体的性质和起源，目的是确定磁化采集的机理，并评估古磁方向是否主要且具有时间进展。我们将通过从本节中插入的凝灰岩层中提取的锆石的高精度U-PB日期将结果放在时域中。感谢这个高质量的数据集，我们将解决Neoperolesoic期间地球磁场几何形状的问题，这也将对TPW等机制产生限制。