Estimates of Direction and Intensity of the Palaeomagnetic Field from the Mugga Mugga Porphyry, Australia

Abstract

The mean direction of NRM in Mugga Mugga Porphyry (late Silurian or early Devonian intrusion near Canberra) is calculated to have declination 16° E and inclination ‐40° with Fisher precision estimate (k) equal to 43. This corresponds to a palaeomagnetic pole at 158° W, 71° N. Alternating field and thermal demagnetization, as well as divergence of the mean from the present field direction, indicate stability and that the NRM is primary thermoremanence. The intensity (M) of NRM which is destroyed by heating in zero field through a particular temperature range is compared with the intensity (M′) of TRM acquired by cooling in the Earth's field through the same temperature range. By studying different temperature intervals, several estimates of M/M′ were obtained from each specimen. If arbitrary correction is made for the effects of alteration above 550°C, M/M′ was found to be constant within a factor of about 2 over all temperature intervals in four samples. This suggests that serious decay of NRM has not occurred, and that M may be close to the original NRM intensity. Assuming intensity to be proportional to applied field, it is estimated that the Earth's field at the time when the Mugga Mugga Porphyry was intruded was approximately a quarter of its present intensity. A large number of estimates of M/M′ is required before any estimate of ancient field intensity can be made; this is likely to apply to future studies of palaeointensity. Copyright © 1966, Wiley Blackwell. All rights reserved