Interpreting the past latitude and geography of the continents from palaeomagnetic data relies on the key assumption that Earth’s geomagnetic field behaves as a geocentric axial dipole. The axial dipolar field model implies that all geomagnetic reversals should be symmetric. However, palaeomagnetic data from volcanic rocks produced by the 1.1-billion-year-old Keweenawan Rift system in North America have been interpreted to show asymmetric reversals, which had led to the suggestion that there was a significant non-axial dipole contribution to the magnetic field during this time1,2. Here we present high-resolution palaeomagnetic data that span three geomagnetic field reversals from a well-described series of basalt flows at Mamainse Point, Ontario, in the Keweenawan Rift. Our data show that each reversal is symmetric. We thus conclude that the previously documented reversal asymmetry is an artefact of the rapid motion of North America during this time. Comparisons of reversed and normal populations that were time-averaged over entire polarity intervals, or from sites not directly on either side of a geomagnetic reversal, have previously led to the appearance of reversal asymmetry.
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We thank D. Jones, C. Rose and N. Eichelberger for field assistance; C. Bayne of Bay Niche Conservancy for land access; D. Kent and M. Jackson for assistance with hysteresis experiments; J. Kasbohm for help drafting the geological map; J. Kirschvink for support of pilot analyses at the Caltech Paleomagnetics Laboratory; and R. Mitchell and T. Kilian for support in the Yale palaeomagnetism facility, which was funded by NSF and the David and Lucile Packard Foundation. This work has benefited from discussions with D. Davis, J. Feinberg, H. Halls, D. Kent, B. Kopp, T. Raub and L. Tauxe. D. Bice encouraged N.L.S.-H. to begin study of Keweenawan volcanics when N.L.S.-H. was an undergraduate at Carleton College. This work was financially supported through a Sigma Xi Grant-In-Aid awarded to N.L.S.-H., A.C.M.’s Agouron Post-doctoral fellowship and Princeton University. B.P.W. acknowledges support from the NSF Geophysics Program.
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Swanson-Hysell, N., Maloof, A., Weiss, B. et al. No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts. Nature Geosci 2, 713–717 (2009). https://doi.org/10.1038/ngeo622
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