Palaeomagnetic record of a late Tertiary field reversal

Abstract

summary. A palaeomagnetic record of a field reversal has been obtained from the Tatoosh intrusion in Mount Rainier National Park (latitude 46° 50° N, longitude 121° 45′W). fie VGP before and after the reversal is conformable with the North American polar wander curve. The reversal in direction is preceded by a decrease in intensity of the NRM of more than an order of magnitude. Immediately prior to the reversal the dispersion of the directions of magnetization increases. The reversal in direction is accomplished by a convoluted VGP path which traverses the Pacific and is somewhat confined in longitude. Subsequent to the completion of the change in direction the field intensity recovers, so that the intensity change takes at least twice as long as the directional change. The definition of the reversal plane in the Tatoosh intrusion gives the orientation of the cooling front at the time the reversal took place. An analysis of the cooling history of the body, in conjunction with the magnetic results, suggests that the body did not cool by simple conduction, and that convective heat transfer by meteoric water is important. This defines a two-stage cooling of the margin. At high temperature rapid cooling takes place as the water chills the hot rock. Later the circulating water in the cracked permeable margin maintains a moderate temperature for long periods. Long vertical cores from the intrusion have revealed a palaeomagnetic record similar to present secular variation records and suggests that some tens of metres of core from intrusive similar to the Tatoosh could give important resolution records of the geomagnetic field. A second reversal record was obtained from the Laurel Hill intrusion near Mount Hood, Oregon (latitude 45° 20′N, longitude 121° 4′W) which is approximately 10 Myr younger. The smoothed VGP paths for the two reversals are essentially identical. Other RN paths from North America appear to favour the Pacific, although the Lake Tecopa record of the last reversal is a notable exception. In comparing all available reversal paths, one finds that although individual paths are confined in longitude, no longitude is uniquely preferred nor does the site uniquely determine a path longitude. There is some preference amongst RN paths to favour the Pacific and the NR the Euroasian landmass, but the limited distribution of observation sites precludes a satisfactory distinction between site control or absolute control in geographical coordinates. Dominantly quadrupolar transitional fields are consistent with the records. Such fields are predicted by a variant of the Steenbeck Krause α2 DC dynamo, but αω dynamos may also exhibit quadrupolar transitional fields as indeed does the Sun. Some stationary nonaxisymmetric component is required to explain non-zero declination of the observed transitional fields.