Palaeomagnetism and Palaeosecular Variation on Lavas from Norfolk and Philip Islands, South-west Pacific Ocean

Abstract

Summary Norfolk Island and the adjacent Philip Island in the South-west Pacific Ocean are built of basaltic lavas and tuffs that were erupted between 3.1 and 2-35My ago during the Gauss epoch and the early part of the Matuyama epoch. Eruptions occurred at about 3.05 My ago during the Mammoth event, and between 2.8 and 2.35 My ago with one or possibly two intervals of quiescence within this period. Palaeomagnetic studies on over 200 samples from more than 80 sites, covering the whole time range of lavas found on the islands, yields an angular standard deviation of 7.5 for the directions as well as for the virtual geomagnetic poles. This value is very low compared with that expected from current models for secular variation. The deviation of the mean virtual geomagnetic pole from the Earth's rotation axis is 14. We conclude that only a partial palaeosecular variation has been recorded in the Norfolk Island lavas because insufficient time is represented by the rocks to determine it fully, despite the fact that results on lavas of both normal and reversed polarity are included. A time span of more than 0.5 My needs to be sampled to obtain an estimate of the palaeosecular variation. The small dispersion of palaeomagnetic results from lavas in the Island of Hawaii, reported by earlier workers, may have a similar explanation to that advanced for the Norfolk Island data. If this is so then the idea that the non-dipole field under the Central Pacific has been suppressed during the Brunhes epoch needs to be re-examined. The palaeomagnetic study of a number of lava flows erupted over a period of time shows a dispersion of the measured directions of magnetization. This dispersion at least in part reflects fluctuations in the direction of the geomagnetic field, and provided that a sufficiently long period of time has been sampled an estimate of the palaeosecular variation (PSV) can be obtained from these data. Studies of the PSV for different geological times and at different localities throughout the world provides valuable information concerning the behaviour of the geomagnetic field in the past. In this paper we present the results of a palaeomagnetic study of basaltic lavas of