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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.
