Non-antipodal directions in magnetostratigraphy: an overprint bias?

Paleomagnetic records show very often that the directions of normal and reverse polarity are not quite antipodal, as one would expect for an axial geocentric dipole model. Specifically, in magnetostratigraphic studies of Tertiary sediments, which include large data sets of paleomagnetic directions of both polarities, it is commonly observed that reverse polarity inclinations are shallower than those of normal polarity. Such an inclination anomaly has important implications for both paleomagnetism and paleogeographic reconstructions. We investigate the conventional explanation for such asymmetry, which involves the presence of a persistent, partially unremoved present day field magnetization in the studied rocks. Both plate motion history and inclination shallowing, which we evaluate in detail, can play an important role in the paleomagnetic record. Our analysis shows that alternatively, it is plausible that the inclination anomaly is due to a contribution of an axial octupole, in accordance with observations of the recent (0–5 Ma) geomagnetic field recorded in lava flows.