Paleosecular variation of lavas from the Marianas in the Western Pacific Ocean.

Abstract

A paleomagnetic study of 24 lava flows from Pagan Island (18.1°N, 145.7°E) in the Mariana Islands was conducted to determine the geomagnetic paleosecular variation in the Western Pacific. All flows are normally magnetized, and attempts to obtain radiometric ages for these flows were unsuccessful, and paleointensity determinations by the ‘Thellier method’ were unreliable. The mean virtual geomagnetic pole, (88°N, 110°W) is not significantly different from the rotation axis, suggesting that the lavas erupted over a period that spans at least several thousand years. The between-site angular dispersion SB, is used as a measure of the secular variation. SB=6.0° with respect to the mean VGP, and SB=6.2° with respect to the axis of rotation with a 95 percent confidence interval for the latter of 5.2°<SB<7.8°. Possible interpretations of the data include: (1) the sites studied represent an inadequate coverage in time; (2) the Marianas should be included in the Pacific dipole window; (3) there is a belt of average low angular dispersion encircling the earth between (roughly) 0° and 40°N.