Upper Miocene and Pliocene geomagnetic secular variation in the Borgarfjördur area of Western Iceland

Abstract

A total of 362 successive lava flows, which were extruded at a regular rate between t= 6.7 and 1.6 Myr in the Borgarfjördur area of Western Iceland, have been subjected to palaeomagnetic study. In contrast to the result of a study by Wilson & McElhinny of palaeo‐magnetic data from a long sequence of lava flows in Eastern Iceland, there does not exist any long‐term increase of geomagnetic inclination in Western Iceland between t= 7 and 3 Myr which, as they show, would produce a change in the distance to the associated virtual geomagnetic poles (VGP's) from greater than the geographic co‐latitude to less than the co‐latitude (or, with respect to the site, from ‘far side’ of the geographic pole to ‘near‐side’ of the geographic pole). Instead the geomagnetic inclination is less than that required for an axial dipole field, providing ‘far‐side’ VGP positions for all data groups. The mean VGP positions are almost identical for four successive polarity epochs (two of reversed polarity, and two of normal polarity), consistent with reversal of the main dipole being accompanied by reversal of the non‐dipole field. S F the angular standard deviation of groups of VGP positions, is used as an expression of palaeosecular variation of the magnetic field. It is shown to be almost constant throughout the 5‐Myr period, suggesting that standing and drifting non‐dipole fields have not combined to produce strong secular variation which is in principle possible in high latitudes. This conclusion is weakened by the suspicion that the conventional exclusion of data from lavas with low‐latitude VGP positions has discriminated against the discovery of high palaeosecular variation rates.