Non-Dipole Behaviour During an Upper Miocene Geomagnetic Polarity Transition in Oregon

The palaeomagnetism of a section of 71 successive lavas from southeastern Oregon is presented. A geomagnetic polarity change from reversed to normal polarity which took place during the extrusion of the section 15 million years ago has allowed a palaeomagnetic examination of the non-dipole field during a period of diminished dipole intensity. During the polarity transition, there existed an apparent easterlydrifting non-dipole component. Together with an independent determination of the palaeo-intensity on one lava (Coe 1967) and other recently presented data (Goldstein et al. 1968) an application of Irving & Ward's(1964) geomagnetic field model to the data suggests a maximum dipole intensity as low as two per cent of the present value for a period of not less than 100 years during the transition, outside of which the data are consistent with some present-day ratios of non-dipole to dipole field intensity. A very short period of diminished relative dipole intensity occurred during the final stages of the polarity change, when the virtual geomagnetic pole returned to very low latitudes, without any polarity change actually occurring. Because of a great similarity with an independently presented result, it is suggested that this 'rebound effect' may reflect a fundamental aspect of the mechanism of at least some polarity changes. Comparison with estimates of the duration of some more recent geomagnetic polarity transitions suggests that the section may have accumulated in less than 50000 years. The lavas of normal polarity, which comprise most of the section, do not show any preferred association with lower or higher oxidation states.