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- Title
Middle Neoproterozoic (Tonian) Polar Wander of South China: Paleomagnetism and ID‐TIMS U‐Pb Geochronology of the Laoshanya Formation.
- Authors
Tonti‐Filippini, Justin A. D.; Robert, Boris; Muller, Élodie; Paul, André N.; Dellefant, Fabian; Wack, Michael; Meng, Jun; Zhao, Xixi; Schaltegger, Urs; Gilder, Stuart A.
- Abstract
Paleomagnetic records of middle Neoproterozoic (820 to 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 (TPW), 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 hypotheses, we collected 1,057 oriented cores over a 85 m stratigraphic succession in the Laoshanya Formation (Yangjiaping, Hunan, China). High precision U‐Pb dating of two intercalated tuff layers constrain the age of the sediments between 809 and 804 Ma. Thermal demagnetization isolates three magnetization components residing in hematite which are not time‐progressive but conflated throughout the section. All samples possess a north and downward directed component in geographic coordinates at temperatures up to 660°C that is ascribed to a Cretaceous overprint. Two components isolated above 660°C reveal distinct directional clusters: one is interpreted as a depositional remanence, while the other appears to be the result of a mid‐Paleozoic (460 to 420 Ma) remagnetization, which is likely widespread throughout South China. 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. 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 TPW hypothesis (ca. 805 Ma), are better explained by mixtures of primary and remagnetized components, and/or vertical axis rotations. Plain Language Summary: Paleomagnetic directions recorded in 820 to 780 million year old rocks from South China exhibit large amplitude changes that vary rapidly, which have been interpreted to indicate extraordinarily fast motion of Earth's crust and mantle, up to 90° within a 5 million year span, with respect to the spin axis of the core. This hypothetical phenomenon, called rapid true polar wander (TPW), could be responsible for dramatic global environmental change at that time. To test this theory, we collected over 1,000 samples from a well exposed section where the incongruous directions are found. Our measurements suggest that some of the rocks acquired a new magnetic signal during a pervasive remagnetization event in South China around 440 million years ago, long after original deposition of the rocks. New hematite growth has a demagnetization spectrum that partially overlaps or completely obscures the original magnetic signal, which was previously unrecognized. This implies that rapid TPW is likely an artifact of magnetic overprinting in ancient rocks from South China. Our results suggest that South China was in a relatively stable position at high latitudes 809 to 804 million years ago. We find no evidence to support exceptionally fast continental drift or an abnormal geomagnetic field geometry during that time. Key Points: High‐precision ID‐TIMS U‐Pb dating yields deposition ages of the Laoshanya Formation (Yangjiaping, South China) between 809 and 804 MaDiscordant paleomagnetic directions in the red beds are a conflation of three temporally distinct field records residing in hematiteOur results do not support rapid true polar wander or abnormal magnetic field geometry around 805 Ma
- Subjects
CHINA; POLAR wandering; PALEOMAGNETISM; GEOLOGICAL time scales; CONTINENTAL drift; RED beds; HEMATITE; PALEOGEOGRAPHY; GEOMAGNETISM
- Publication
Journal of Geophysical Research. Solid Earth, 2024, Vol 129, Issue 4, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2023JB027634