Brunhes epoch geomagnetic secular variation on Terceira Island, central North Atlantic

Abstract

A paleomagnetic survey has been made of Brunhes epoch lavas and ignimbrites on Terceira in the Azores. At least six widely spaced samples were taken from each of 33 separate bodies. The corrected dispersion of virtual geomagnetic poles (SF) is 13.9 (with upper and lower 95% confidence limits of 16.3 and 12.2, respectively), consistent with the present geomagnetic field behavior in the same latitudes. Analysis of the data as nine independent surveys involving from two to seven separate specimens per body provides values of SF that are not significantly different. Thus an equally accurate value of SF could have been obtained with less than half the samples collected. Analysis of the effect of Kw (the within-body precision parameter) and other factors involved in the computation of SF suggests that the largest number of separate cores per separate body required for defining SF is four. Two separate lavas recorded a possible single departure of the geomagnetic field, and thus evidence of instability of the geocentric dipole during the Brunhes epoch is provided.