The “van Zijl” Jurassic geomagnetic reversal revisited

Abstract : We have collected two new detailed records of what could be the second oldest well documented reversal, the "van Zijl" Jurassic (~180 Ma) reversal recorded in the thick basalt sequences of the Karoo large igneous province in Lesotho and South Africa. Sections yielded 10 and 8 independent transitional paleomagnetic directions respectively over two continuous 130 m and 160 m sequences. The corresponding VGP reversing paths share a number of consistent features and are integrated with a third record at Bushmen's Pass from Prévot et al. (2003) to yield a single reversal path with: 1-a large directional jump with no transitional poles between 45°S and 45°N in the first, main phase of the reversal, 2-a large elongated loop (multiple hairpin) to the SE with poles going down to ~20° latitudes in the second phase of the reversal, and 3-a rebound before finally settling in normal polarity. We have measured relative paleo-intensities on a set of carefully selected samples. Intensity was low prior to the directional changes and recovery to the following normal polarity was progressive. Transitional intensity is 80 to 90% lower than full polarity values. There is no significant increase in intensity at the time of transitional directional clusters. Our results are consistent with a weak non-dipolar reversing field, with faster and larger directional secular variation as the equator is crossed by VGPs. They tend to vindicate the notion of a short duration of the directional reversal, though one cannot draw global conclusions from a record limited to essentially a single location.
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Maud Moulin, Vincent Courtillot, Frédéric Fluteau, Jean-Pierre Valet. The “van Zijl” Jurassic geomagnetic reversal revisited. Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2012, 13 (3), ⟨10.1029/2011GC003910⟩. ⟨hal-02139522⟩

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