The myth of the Pacific dipole window

Abstract

The angular dispersion of virtual geomagnetic poles (VGP) from lava flows is often cited as having been anomalously low in the Pacific during the Brunhes epoch because the dispersion from Hawaiian lavas is said to be much lower than measured elsewhere. This led to the concept of the Pacific dipole window or Pacific non-dipole low. Because lavas tend to be erupted in bursts of activity, many of the Hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations. Geochronological controls, as are available for Hawaii, have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field. Unfortunately, in that analysis the angular dispersion of VGPs was calculated about the mean VGP instead of about the spin axis, and thus underestimated the dispersion. We have therefore reanalyzed the relevant data using the following criteria. (1) Only those lavas with α 95 < 10° have been considered. (2) A latitude-dependent cut-off angle is used to eliminate those vectors that are not part of the normal secular variation. (3) We have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector, because suitable geochronological controls are rarely available. Application of this new method to the Hawaiian data shows excellent agreement with the results from using geochronological controls. Where possible we have therefore included this method in our overall analysis. As expected, the resulting global data are compatible with a Fisher distribution about the spin axis. Brunhes age data from Hawaii ( N = 96 independent measurements) give a VGP angular dispersion about the spin axis of S F = 12.4°, and for the Pacific region as a whole S F = 12.5° ( N = 190) between latitudes 15° and 30° (north or south). These values are the same as S F = 12.4° ( N = 160) calculated for the rest of the world in the same latitude range. This clearly demonstrates that the hypothesis of the Pacific dipole window may confidently be rejected.