Paleosecular variation models for ancient times: Clues from Keweenawan lava flows

Abstract

Statistical paleosecular variation models predict distributions of paleomagnetic vectors as a function of geographic position. Such models have been used in a variety of applications that test whether a given data set fairly represents the variability and average properties of the geomagnetic field. The simple relationship between inclination of the geomagnetic field and latitude predicted by geocentric axial dipole (GAD) models has been a cornerstone for plate reconstructions for decades, yet many data sets exhibit a tendency to be shallower than expected for a dominantly axial geocentric magnetic field. Too shallow inclinations have variously been interpreted as plate motion, permanent non-dipole field components or bias in inclination from sedimentary processes. Statistical PSV models could in principle be used to resolve the cause of inclination anomalies because there is a simple relationship between the elongation of the distribution of directions in the vertical plane and the average inclination. Shallowing of inclinations from sedimentary processes results in a progressive transformation of the elongation direction in the vertical plane containing the average direction into a pronounced elongation in the plane perpendicular to that. However, the applicability of statistical models based on the last 5 million years for more ancient times is an open question. Here we present new data from the Keeweenawan North Shore Volcanics ( ∼ 1.1 Ga). These data are consistent with statistical PSV model predictions and are less well fit by models that include a 20% axial octupole component. We also find evidence for a pervasive overprinting by hematite in a shallower direction and find support for the contention that the asymmetric reversal(s) observed in Keweenawan aged rocks along the North shore of Lake Superior can be explained as an age progression, with the reverse directions being older than the normal directions. Finally, we re-consider implications from an analysis of inclinations from the Global Paleomagnetic Database for the Paleozoic and Pre-Cambrian. We find that the data are inconsistent with a random sampling of any simple geomagnetic field model and conclude that the data set under-samples the field in a spatial sense.