On the Behavior of the Geomagnetic Field During the Miocene

Abstract

This study comprises 118 discrete volcanic flows from the Miocene in the eastern Columbia River Basalt Group (CRBG) with the aim of testing the distributions of geomagnetic field directions as predicted by several geomagnetic field models. This is important because little is known empiri- cally about the behavior of the geomagnetic field prior to the last 5 million years. The results of this study will allow us not only to extend the temporal coverage of the geomagnetic field models, but also point to potential improvements to these models. The rock magnetic analysis of the samples for this collection shows that the primary magnetic directions are uniformly carried by titano-magnetite. Thus, the magnetic behavior of the samples reflects the magnetic field and not aspects of the mag- netic mineralogy.

To reduce systematic bias, the sites were analyzed as ungrouped and grouped site directions, where flows with similar directions within the 95% confidence interval were combined. Further- more, they were analyzed with visual inspection, 45°, and Vandamme (1994) cutoff criteria. The overall formation mean of these six scenarios shows that the paleomagnetic directions of the flows are southerly and up and northerly and down. For example, in the case of grouped sites with the Vandamme cutoff, the normal (Dec: 7.1, Inc: 60.5, k: 29.4, 95: 4.1), and reverse (Dec: 177.6, Inc: -59.6, k: 15.7, 95: 5.6) means depict this general behavior. For reference, the local inclination of the geocentric co-axial dipole field is about 64. Furthermore, deviating directions are also pre- sent, possibly due to excursions as part of secular variation, or as intermediate transitions between reversed and normal polarity intervals. No new rock ages were necessary for this study since the CRBG has undergone extensive geochronological analysis with high-quality, reproducible results. Additionally, integrating the Miocene ages of the flows and their corresponding paleomagnetic di- rections allows us to observe the evolution of the paleosecular variation (PSV) through the Neogene,

because it can be compared with the published characteristics of the geomagnetic field in the last 5 million years. Our elongation results coincide with those predicted from the TK03.GAD (Tauxe & Kent, 2004) model, but not the CJ98.GAD (Constable & Johnson, 1998) model. However, when comparing our dispersion results, we find that they do not conform to the TK03.GAD, CJ98.GAD, and CP88.GAD (Constable & Parker, 1988) models, which underpredict the dispersion values. Thus, this suggests that, although the geomagnetic parameters in general agree with these models, they require some refinement to accommodate the differences during the Miocene.