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

Abstract

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.

Some global features of palaeointensity in geological time

“…On the other hand, the present-day geomagnetic field is about 90 per cent dipolar and dominance of the dipole term, 10 times stronger than other terms, is also true when the power of the field is interpolated to the surface of the core (Langel & Estes 1982). The dipole nature of the palaeomagnetic field is well established from the direction data for the past 200 Ma (Briden, Hurley & Smith 1981;Livermore, Vine & Smith 1984), and it is natural to suppose that a similar scheme governed the geomagnetic field for the earlier time. In view of these facts, palaeointensity is a good indicator of core dynamics even though little is known about the energy partitioning between toroidal and poloidai fields (the latter is the field observed by palaeomagnetism).…”

Changes in earth’s dipole

“…It is estimated that the 5 Ma time-averaged field is about 95% GAD (Johnson and Constable 1997;Carlut and Courtillot 1998), and the 120 Ma average field is comparably dipolar (Livermore et al 1984). Paleomagnetic evidence indicates that the main departure from the GAD configuration in the time-averaged field consists of a small axial quadrupole.…”

Late Cretaceous Polar Wander of the Pacific Plate: Evidence of a Rapid True Polar Wander Event

“…shows past locations of Earth's spin axis, Although it is widely accepted that plate modefined as the time-averaged paleomagnetic tion is the main cause of APW, drift of the spin axis relative to the mantle (termed "true ~o l a r wander" or TPW) is also a contributor. (My) [for example, (2)(3)(4)], but short-lived TPW events may have occurred with rates exceeding plate velocities, leading to large shifts in paleogeographic zones (5). Numerous hot spots and rapid tectonic motions have left an unparalleled, 140-My record of plate motion relative to the mantle on the Pacific plate (6, 7).…”