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No asymmetry in geomagnetic reversals recorded by 1.1-billion-year-old Keweenawan basalts

A Corrigendum to this article was published on 13 April 2010

This article has been updated


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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Figure 1: Mid-continent rift geology and palaeomagnetic data.
Figure 2: Summary of palaeomagnetic data from Mamainse Point.
Figure 3: The Logan Loop and Mamainse Point poles.
Figure 4: Example palaeomagnetic data.

Change history

  • 13 April 2010

    In the version of this Letter originally published, Supplementary Tables S2, S3 and S4 contained several errors. Consequently, the estimates of the rate of motion on page 716 should have read: "The estimates for the rate of motion from 1,108 Myr to 1,097 Myr range between 21.5±7.1 cm yr−1 for Osler Volcanics reversed→North Shore normal, to 33.6±3.5 cm yr−1 for Coldwell Complex reversed→North Shore normal." Furthermore, the position of the Duluth Complex reversed pole in Fig. 3a and the positions of the lower normal and upper normal palaeopoles in Fig. 3b were slightly incorrect. These errors have been corrected in the HTML and PDF versions of the text.


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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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Fieldwork was conducted by N.L.S.-H. (2 field seasons), A.C.M. (2 field seasons) and B.P.W. (1 field season) following initial project planning by A.C.M. and subsequent project planning by A.C.M. and N.L.S.-H. Samples were analysed by N.L.S.-H. in the laboratories of D.A.D.E. and B.P.W. N.L.S.-H. analysed the data, wrote the manuscript and drafted Figs 23 with input from A.C.M., B.P.W. and D.A.D.E. Figure 1 was drafted by A.C.M. and N.L.S.-H.

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Correspondence to Nicholas L. Swanson-Hysell.

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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).

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