Plate motions and the geomagnetic field — II. Jurassic to Tertiary

• Published in: Geophysical Journal of the Royal Astronomical Society Vol. 79; no. 3; pp. 939 - 961
• Main Authors: Livermore, R. A., Vine, F. J., Smith, A. G.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.12.1984
• ISSN: 0956-540X; 0016-8009; 1365-246X
• DOI: 10.1111/j.1365-246X.1984.tb02878.x

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 100Ma, 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. This 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.