Reassessing the geocentric-axial-dipole (GAD) model for Proterozoic time with paleomagnetic directions from dike swarms

Abstract

The structure of the Proterozoic geomagnetic field is debated, hampering our understanding of ancient plate paleogeography and core evolution. We reassess the geocentric-axial-dipole (GAD) model for Proterozoic time using improved statistical methodologies and an expanded dataset of paleomagnetic directions from mafic dike swarms covering larger areas than previously considered. In addition to commonly used Fisher statistics, we employ Bingham and Kent statistics to evaluate both the tightness and elongation of virtual geomagnetic pole (VGP) distributions. Our results indicate that the Proterozoic field was predominantly a GAD, with a potential axial octupole (G₃) component of ∼10–15% in certain time intervals. Our findings do not support the previous assertions of significantly larger (25–29%) non-dipolar contributions in the Proterozoic. The identified Proterozoic non-dipolar component could result in apparent paleolatitude shifts of ∼10° shallowing, insufficient to explain low-latitude glaciations but potentially accounting for some offset paleomagnetic latitudes of Proterozoic paleoclimate records. Our study also shows that expanding the spatial distribution of VGPs using paleomagnetically independent paleogeographic reconstructions can improve the test’s sensitivity and provide tighter constraints on the structure of the geomagnetic field.