Secular Variation Rates Research Articles

The Palaeomagnetism of the Samoan and Tongan Islands

Summary The remanent magnetization of the Samoan rocks is of varying stability. Samples from sixty sites are regarded as reliable indicators of the polarity of remanence but only sixteen sites are regarded as reliable indicators of the actual direction of the past geomagnetic field. Normal and reversed directions occur, the mean direction of which (So, - 327 is similar to the axial dipole field in Samoa. The scatter of Samoan site directions about the mean is greater than would be expected from secular variation rates during the last 200 years and suggests that secular variation in the Samoan area has not been abnormal when averaged over the time taken for the rocks to be laid down (2-2.5 m.y.). Samples from Tonga are stable but are insufficient in number for detailed interpretation, but results from 'Eua suggest tectonic instability on the edge of the Tonga Trench before the Upper Eocene. 1. Introdaction The palaeomagnetic properties of igneous rock samples from Samoa and Tonga were examined as part of an investigation of the variation of the geomagnetic field during the last few million years in islands in and around the margin of the Pacific Ocean. In particular it was hoped (a) to estimate the magnitude of secular variation in this region of slow change during historic times, (b) to test the hypothesis of world-wide reversals of the geomagnetic field and to use such reversals for geological correlation in an area where standard stratigraphical techniques are difficult to apply, and (c) to test the axial dipole hypothesis of the average geomagnetic field during the late Cainozoic. Palaeomagnetic results from the Hawaiian Islands have been described earlier (Doell & Cox 1961,1963, Tarling 1965) and the presence of reversals of magnetization in Samoa has been noted previously (Tarling 1962). The Samoan and Tongan Islands are in close proximity but these islands are representative of two distinct geological environments. The Samoan volcanics form part of the oceanic basalt suite of rocks while the Tongan Islands, lying along the Andesite Line, are representative of a 'continental' or 'subcontinental' environment. Therefore each island group is described separately following an outline of the methods used.

The accuracy of the paleomagnetic method as evaluated from historic hawaiian lava flows

An analysis was made of the various levels at which errors enter in paleomagnetic studies of laval flows, using as controls lava flows from Hawaii which cooled in known fields. The largest intraflow angular variations occur between different individually oriented cores and are due to rotation of blocks after cooling, to intraflow magnetic field variations at the time of cooling, and to orientation errors; other sources of angular error have much smaller effects. The ultimate accuracy of a paleomagnetic study is limited by the magnetic anomalies existing over the old terrain where the lava flows cooled. Because of these ancient anomalies, little additional accuracy can be gained in volcanic regions like Hawaii by collecting more than six oriented samples per flow, a number which results in an accuracy at the 95 per cent confidence level of 3° to 5°. If these results are applied to older lavas on Hawaii, the low rates of secular variation previously noted from observatory data are found to persist back to 1750.

A relationship between the geomagnetic secular variation rates

A relationship between the geomagnetic secular variation rates