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Abstract

The virtual geomagnetic pole (VGP) trajectories during some geomagnetic polarity reversals of different ages are marked by anisotropic behaviour. This recurrent phenomenon may be reflected in the paleomagnetic data, even if the transitional field was not completely recorded. As the long-scale geomagnetic variations have a confined oscillatory character, the VGP paths from stratigraphically controlled sequences may be described on the basis of sine and cosine functions, even if time is not the independent variable. Here we considered longitude (or space) as the independent variable which had to be ‘unrolled’ to overcome the 360° repetitions as the VGPs moved around the geographic pole.

Sixteen VGP series from the Early Cretaceous Serra Geral lava flows of southern Brazil were analysed using a modified version of the periodogram for uneven data series, and a combination of information approach. The combination of all the spectra, as in a stacking procedure, reduces noise and results in a smooth curve highlighting features of interest. We found a set of highest correlation wavelengths of approximately 167, 190, 209, 257, 277 and 368°. Phase analyses using two different methods revealed strikingly good coherence for some of these wavelengths, indicating that they are not only artefacts of the spectral analysis. Similar analysis of magnetostratigraphic data from the Icelandic Magmatic Province indicated that the two datasets may have wavelengths of approximately 165 and 270° in common. These results suggest quasi-periodic behaviour, possibly with sub-harmonic instabilities owing to the modulating effect of inner Earth’s anisotropies influencing the pole trajectory.

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