Excursions and secular variation of the Brunhes epoch geomagnetic field in the Indian Ocean region

Abstract

A total of 237 separate oriented samples have been taken from 33 separate lavas on Amsterdam Island, southeastern Indian Ocean. Six of these lavas, which are all of Brunhes age, record a 50°–80° latitudinal departure of the virtual geomagnetic pole from the present geographic pole. It appears that two departures, separate in time, recorded very similar virtual geomagnetic poles. Diminished geomagnetic intensities may have accompanied these departures, which could most readily be explained by a minor inclined dipole or nondipole feature persisting during a period of dipole collapse. The virtual geomagnetic poles for the remaining 27 lavas provide an angular standard deviation of 14.6° (with 95% confidence limits at 12.5° and 17.7°), which is consistent with present ratios of nondipole to main dipole fields in the latitudes involved. Comparison of the result with data from the Comore Islands (11.9°S) and Crozet (46.5°S) show that the Brunhes epoch geomagnetic secular variation in the Indian Ocean region was similar to the present geomagnetic field behavior.