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

• Applicant: Professor Dr. Stuart Alan Gilder
• Subject Area: Palaeontology ; Geophysics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413587100

• Final Report Year: 2024

Final Report Abstract

Paleomagnetic records of middle Neoproterozoic (820-780 Ma) rocks display high amplitude directional variations that lead to large discrepancies in paleogeographic reconstructions. Hypotheses to explain these data include rapid true polar wander, a geomagnetic field geometry that deviates from a predominantly axial dipole field, a hyper-reversing field (>10 reversals/Ma), and/or undiagnosed remagnetization. To test these different hypotheses, a high-resolution paleomagnetic study on a continuous section was needed. We identified an appropriate section where we collected ~1200 samples in September 2019 in Hunan Province (China), ~300 km from Wuhan, the epicenter of the Covid-19 outbreak 4-5 months later. Stepwise thermal demagnetization of most samples revealed stable unblocking trajectories with reproducible directions when multiple samples were collected in the same horizon. However, the directions varied widely throughout the 85 m of section that we sampled. Three end-member magnetization components were identified, all carried by hematite. Similarities in unblocking spectra made it difficult to precisely identify the characteristic remanence in many samples, so our first task was to develop a method to unmix the complex magnetizations. This unique method was published in early 2023 and should become a standard technique to analyze and interpret paleomagnetic data. Besides stepwise demagnetization, the samples were subjected to a wide range of experiments to characterize their magnetic properties. The PhD student responsible for this project, Justin Tonti-Filippini, also spent 6 months in Geneva to learn high precision U-Pb dating, which was successfully applied to tuff layers collected in the upper and lower-middle parts of the section, thereby enabling us to date the stratigraphy to a very high level of precision between 809 and 804 Ma. The samples were analyzed for several geochemical parameters in collaboration with scientists at the Institut de Physique du Globe de Paris. The high-temperature directions are subtly dependent on lithology. Microscopic and rock magnetic analyses identify multiple generations of hematite that vary in concentration and distinguish the magnetization components. Thanks to the unmixing method, we could weight the rock magnetic and geochemical properties of the samples by the percentage of each of the three magnetization components. A comparison with other middle Neoproterozoic paleomagnetic studies in the region indicates that the sudden changes in paleomagnetic directions, used elsewhere to support the rapid true polar wander hypothesis (ca. 805 Ma), are better explained by mixtures of primary and remagnetized components, and/or vertical axis rotations. In sum, the outcome of this work suggests South China resided at a high latitude position, thereby refuting models requiring exceptionally fast, solid Earth rotation or complicated, higher order, magnetic field components other than a dipole.

Publications

• The Neoproterozoic geomagnetic field: new insights from a high-resolution paleomagnetic study in South China.
  Tonti-Filippini, Justin; Robert, Boris; Muller, Élodie; Wack, Michael; Zhao, Xixi & Gilder, Stuart
  
• Vector unmixing of multicomponent palaeomagnetic data. Abstract presented at the 17th Castle Meeting in Croatia, 28 August-3 September
  Tonti-Filippini, J. & Gilder, S.
  
• Vector unmixing of multicomponent palaeomagnetic data. Geophysical Journal International, 233(3), 1632-1654.
  Tonti-Filippini, Justin A. D. & Gilder, Stuart A.
  
• Challenging the hypothesis of rapid true polar wander in the Neoproterozoic: A multi-disciplinary study of the complex paleomagnetic record from Yangjiaping, South China, PhD Dissertation, LMU-Munich, 132 pp
  Tonti-Filippini, J.
  
• Middle Neoproterozoic (Tonian) polar wander of South China: Paleomagnetism and ID-TIMS U-Pb geochronology of the Laoshanya Formation.
  Tonti-Filippini, Justin; Robert, Boris; Muller, Elodie; Paul, André N.; Dellefant, Fabian; Wack, Michael R.; Meng, Jun; Zhao, Xixi; Schaltegger, Urs & Gilder, Stuart
  
• Middle Neoproterozoic (Tonian) Polar Wander of South China: Paleomagnetism and U-Pb Geochronology of the Laoshanya Formation. Abstract (GP14A-06) presented at the AGU Fall Meeting in San Francisco, 11-15 December
  Tonti-Filippini, J., Robert, B., Muller, E., Paul, A., Dellefant, F., Wack, M., Meng, J., Schaltegger, U., Zhao, X. & Gilder, S.
  
• Vector unmixing of multicomponent palaeomagnetic data. Abstract (GP23A-05) presented at the AGU Fall Meeting in San Francisco, 11-15 December
  Tonti-Filippini, J. & Gilder, S.
  
• Vector unmixing of multicomponent palaeomagnetic data. Abstract presented at the MagIC Workshop in San Diego, 28 February-2 March
  Tonti-Filippini, J. & Gilder, S.
  
• Deciphering paleomagnetic signals in Precambrian red beds. Abstract presented at Magnetic Interactions in Leeds, 4-5 January
  Tonti-Filippini, J.