Plate motions and the geomagnetic field — I. Quaternary and late Tertiary

We have tested the hypothesis that the apparent increase in the northward offset of the axial dipole (i.e. the ratio g ° 2 / g ° 1 ) with age for the last 25 Ma (Wilson & M c Elhinny) is due to the failure to correct for plate motions. Spherical harmonic analyses were performed on two types of data, palaeomagnetic poles from continents, islands and seamounts, and magnetic inclinations from deep sea cores, after returning the sampling sites to their predrift locations in fixed hotspots coordinates. The results show that the quadrupole term g ° 2 maintained a value of about 0.05 g ° 1 throughout the last 35 Ma, and that the axial octupole had a small value of about 0.02 g ° 1 for the last 5 Ma. Sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years. Independent data sets for the past 2 Ma and for the period 2–6 Ma gave nearly identical solutions, showing that the persistence of the axial terms is not a phenomenon of the more densely sampled Quaternary field alone. Nor is it an artifice of predominantly northward motion of the plates: returning all 0–5 Ma data to a 5 Ma reconstruction failed to eliminate the quadrupole. The non‐zonal coefficients for the 0–5 Ma field are typically an order of magnitude less than the zonal terms, with the exception of h 1 / 2 , and are probably insignificant, suggesting that longitudinal drift is generally effective in averaging out these components. The present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 Ma, but there is evidence that g ° 2 may have changed little during the last 35 Ma. In addition, there has been very little relative motion between the palaeomagnetic dipole axis, as defined by the first degree terms, and the axis of the hotspots reference frame.