SHALLOW BIAS OF PALEOMAGNETIC INCLINATIONS IN THE PALEOZOIC AND PRECAMBRIAN

An updated analysis of the global paleomagnetic database shows that th e frequency distributions of paleomagnetic inclinations for the Cenozo ic and Mesozoic eras (younger than 250 Ma) are compatible with a rando m geographical sampling of a time-averaged geomagnetic field that clos ely resembles that of a geocentric axial dipole. In contrast, the freq uency distributions of paleomagnetic inclinations for the Paleozoic an d Precambrian eras (prior to 250 Ma) are over-represented by shallow i nclinations. After discounting obvious secondary causes for the bias, such as from data averaging, sedimentary inclination error, inhomogene ous lithological distributions, and tropical remagnetization, we show that the anomalous inclination distributions for the Paleozoic and Pre cambrian can be explained by a geomagnetic field source model which in cludes a relatively modest (similar to 25%) contribution to the axial dipole from a zonal octupole field and an arbitrary zonal quadrupolar contribution. The apparent change by around 250 Ma to a much more axia l dipolar field geometry might be due to the stabilization of the geod ynamo from growth of the inner core to some critical threshold size, a gross speculation which would imply that either the threshold size wa s rather large or the inner core nucleated rather late in Earth histor y. Alternatively, if a geocentric axial dipole model is assumed or can eventually be demonstrated independently, the anomalous inclination d istributions for the Paleozoic and Precambrian may reflect a tendency of continental lithosphere to be cycled into the equatorial belt, perh aps because geoid highs associated with long-term continental aggregat es influence true polar wander. (C) 1998 Elsevier Science B.V. All rig hts reserved.