EVIDENCE FOR LATE PALEOZOIC AND MESOZOIC NON-DIPOLE FIELDS PROVIDES AN EXPLANATION FOR THE PANGAEA RECONSTRUCTION PROBLEMS

Abstract

Paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole, but in recent years it has become a concern that this assumption is not always valid because of long-term contributions from non-dipole fields, in particular octupole fields. We tested for the presence of a zonal octupole field in the Late Carboniferous, Permian, Mesozoic and Early Tertiary, by comparing the observed paleomagnetic paleolatitude distributions for the Laurentian (North America and Greenland) and European landmasses with those predicted from the mean paleopoles. Such a test is possible for a good-sized continent (not subsequently deformed) when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes. If the field were purely dipolar, coeval results should form a coherent dataset, such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1. If the slopes of regression lines are systematically different from 1.0 for successive time intervals, and granted that the Earth’s radius stayed the same, then it is likely that non-dipole fields can be held responsible. Regression line slopes (±standard errors) for three intervals analyzed are 0.78±0.04 (200–300 Ma), 0.93±0.11 (120–200 Ma) and 0.82±0.06 (40–120 Ma). As might be expected with a non-dipole contribution to the total geomagnetic field, paleomagnetic results from the most northerly areas of Laurentia and Europe (i.e. Ellesmere, Greenland and Svalbard) show clearly anomalous paleolatitudes. Because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain-forest occurrences [D.V. Kent, P.E. Olsen, Earth Planet. Sci. Lett. 179 (2000) 311–324], we argue that long-term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies. Estimates of the magnitude of the octupole/dipole field ratio are not very precise, but center around 0.1, which could cause errors in conventional paleopole determinations (using the dipole formula) of 7.5°; because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid-northern and mid-southern hemisphere locations could thus be off by as much as 15°. The well-known misfit between the paleomagnetic results from the Laurentia–European and Gondwana continents in a classical Pangea A configuration could be explained by such errors due to octupole fields. This explanation would negate the need to seek tectonic (Pangea B-type) solutions for the misfit. These solutions have generally remained unacceptable in terms of the geology of the Gondwana–Laurussia borderzone.