Magnetization of dikes: A study of the paleomagnetism of the Widgiemooltha Dike Suite, western Australia

Oriented hand samples have been collected at twelve sites in five members of the Widgiemooltha dike suite of Western Australia. These dikes form a major tectonic lineament striking N 80°E, and they have a Rb : Sr isochron age of 2420 ± 30 m.y. At eleven sites, initially scattered directions became well grouped after magnetic cleaning, with a mean direction of D = 242°, I = −67°. Further magnetic stability is demonstrated by baked-contact tests, and the cleaned results yield a paleomagnetic pole situated in the southwest Pacific Ocean at 157°E, 8½°S with A95 = 8°. The mean direction lies close to the dike planes. A previous suggestion that by a suitable physical mechanism dikes tend to be magnetized in their plane is investigated. If it is assumed that dikes record the direction of the geomagnetic field at the time of intrusion, a simple geometrical analysis shows that for vertical dikes there is a 50% chance that this direction will lie within 22° of the dike plane. A compilation of seventy-six examples from the literature shows good agreement with the expectations of the analysis. It is suggested, therefore, that the data available at present provide no reason for supposing that dikes do not become magnetized parallel to the ambient field.