No abstract

We have tested the hypothesis that the apparent increase in the northward offset of the axial dipole (i.e. the ratio g:/gy) with age for the last 25 Ma (Wilson & McElhinny) 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! maintained a value of about 0.05 gy throughout the last 35 Ma, and that the axial octupole had a small value of about 0.02 gy, 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 t o 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:, 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: 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.

The analysis of the time-averaged geomagnetic field is extended back to 200Ma. Palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region. These were returned, with their corresponding sampling sites, to their locations at the estimated dates of magnetization, in a fixed-hotspots framework. The corrected results were then grouped into 20Ma windows at intervals of 10Ma representing the past lOOMa, and 40Ma windows at 30Ma intervals for the more sparse 100-200Ma data. Global means and Fisher statistics were calculated for each window having included the axial quadrupole in the calculation. The value of this coefficient which gave the maximum value for the Fisher precision parameter (tightest grouping of poles) was taken as representative of each interval. The results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the Cenozoic. Tlus is equivalent to a northward offset axial dipole field (far-sided effect). During the late Cretaceous, this component appears to have changed sign with respect to the dipole. Negative values seem to have obtained throughout the Cretaceous long normal polarity interval, corresponding to a southward offset dipole (near-sided effect). The data distribution is inadequate for the resolution of the quadrupole at earlier times, and zero values cannot be discounted. Little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 Ma, the global mean polar path curving around the predicted fixed-hotspots pole at a distance of typically 5" latitude with little sign of rapid Tertiary polar wander as implied by studies of Pacific data alone. Between 100 and 200 Ma, however, there is a clear difference between the two reference frames, amounting to 17-19" in the Jurassic. This may reflect motion of the mantle relative to the geomagnetic axis, but may also include errors due to inaccurate determination of hotspot tracks and inter-hotspot motion.

No abstract

No abstract