A long-term octupolar component in the geomagnetic field ?

The hypothesis of a geocentric axial dipole (GAD), which is fundamental to paleomagnetism and plate reconstructions, has recently been somewhat challenged by suggestions that significant long-term octupolar contributions up to 10% (and even 25%) of the GAD may have existed, mainly in the Paleozoic but also since then. For instance, Si and Van der Voo (2001) propose that a value of 6% would account for the low inclinations observed in central Asia in the Cretaceous and early Tertiary. Following and updating our previous analysis of the global paleomagnetic data base Besse and Courtillot, 2002), we attempt to find evidence for octupolar contributions in the 0-200Ma period. An important component of our analysis is the inclusion of data from sites believed to have possibly undergone a tectonic rotation about a local vertical axis (contributing 174 out of 465 data). We analyze the positions of mean poles in 20-Ma windows in common-site longitude, respectively for the northern mid-latitudes, equatorial and southern mid-latitudes, searching for the distinctive antisymmetrical pattern expected for a dipole plus octupole (without quadrupole) field. We next analyze the distribution of "latitude anomaly" (derived from the inclination anomaly, i.e. observed minus expected in case of a pure dipole) versus dipole latitude. Based on these various data manipulations, we find no robust evidence for an octupole and estimate that values on the order of 3% are unlikely to have been exceeded in the last 200 Ma. A preliminary 200 Ma overall mean field has a quadrupole component on the order of 3ñ2% (i.e. significant) and an octupole of 3ñ8% (i.e. not significant). A by-product of the analysis using poles from "free to rotate" sites is clear confirmation of the amplitude of rotations undergone by for instance parts of the Adriatic promontory of Africa or the Colorado plateau. Refs.: Besse, J., and V. Courtillot, J. Geophys. Res., 107, doi:10.1029/2000JB000050, 2002; Si, J., and R. Van der Voo, Terra Nova, 13, 471-478, 2001.