Duration and Structure of the Matuyama-Brunhes Geomagnetic Polarity Reversal from 40Ar/39Ar Dated Lava Flows

A fundamental property of the geomagnetic reversal process that remains poorly understood is the duration. Radioisotopic ages determined from transitionally magnetized lava flows erupted at four globally-separated sites constrain the Matuyama-Brunhes (M-B) transition to have lasted no less than 14 kyrs, and perhaps as long as 28 kyrs--that is, several times longer than widely thought. The paths of the virtual geomagnetic pole (VGP) contain a quasi-stationary clustering in or about Australia preceded and succeeded by apparently rapid movement away from and toward the geographic poles. These results mirror records from sediments that imply alternating steady-rapid directional movement to be an important characteristic of the dynamo reversal process. Further, however, our findings suggest that during the interval of the last reversal the geocentric dipole may have crossed the equator on five occasions. Most transitional VGPs overlie regions of faster seismic velocity and possibly lower temperature in the lowermost mantle. The long duration of the M-B reversal and the complex, yet geographically-restricted positions and movements of the lava-derived VGPs match a numerical dynamo simulation that imposes lateral variability in lowermost mantle temperature. Hence, more than reinforcing the hypothesis that the thermal state of the lower mantle modulates the geodynamo during reversals, the M-B lava record can be used to quantitatively constrain future attempts to numerically model the process.