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

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-yearold 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. Extrusive and intrusive igneous rocks of the Keweenawan province form part of a failed mid-continent rift (MCR) system that was active from 1.11 to 1.09Gyr ago. The largest areal exposure of these rocks is in the Lake Superior region, but aeromagnetic and gravity surveys show that the relatively dense and magnetic volcanic rocks of the rift span more than 3,000 km, largely beneath sedimentary cover (Fig. 1)3. Palaeomagnetic data from Keweenawan rocks have been compiled to generate an apparent polar wander (APW) path for North America, known as the Logan Loop4–6. Comparisons of the Logan Loop with APW paths from other continents have been used to reconstruct the supercontinent Rodinia7,8. Palaeomagnetic directions from MCR rocks consistently reveal normal and reversed directions that are not antiparallel (with inclination differences of 20–30; Fig. 1)4. This pattern has been interpreted as reversal asymmetry, leading to speculation that significant non-dipole components may have contributed to the surface geomagnetic field1,2. The well-known problem of Keweenawan reversal asymmetry is often cited as an uncertainty in palaeogeographic reconstructions7–9, as the introduction of quadrupole and octupole components to the geomagnetic field can lead to significant discrepancies between true palaeolatitude and palaeomagnetically derived palaeolatitude10. Much of the movement of the late Mesoproterozoic APW path for Laurentia occurs across what has been interpreted as an ‘asymmetric’ reversal. The large magnitude of this apparent motion makes it important to understand whether the difference in inclination between normal and reversed directions is an artefact of a significant non-dipole