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The palaeomagnetism of the giles complex, Central Australia
170
t’ALAEOMAC;NEl’lC
STUDIES
The non-dipole field is a number from a normal distribution with mean I* = if cos(%rlp, + $,f and standard deviation given by u = p { (ct~(Znt/~~ + ~~)l }. With appropriate values of the parartleters, the model duplicates all of the known facts of the magnetic reversal history, namely, periods of high reversal rate, interspersed by long periods (circa 15 m-y.) of quiescence, and very-long-period oscillations. In addition, observed and predicted histograms of polarity interval durations are in good agreement. Predictions based on the model are that: (1) the overall mean length of polarity intervals will be found to be 1.6 m.y.; (2) long periods of constant polarity will be found in the Silurian (normal) and Middle Cambrian (reversed); and (3) periods of frequent reversals will be found in the Jurassic, Carboniferous, Ordovician and Lower Cambrian.
THE PALAEOMAGNETISM CENTRAL AUSTRALIA
OF THE GILES COMPLEX,
R.A. FA<‘ER
Wollongong University College, Wollongong, N.S. W. (Australia) (Received February 9, 197 1)
540 Specimens have been prepared from samples taken at sixty-three sites in the Precambrian Giles Complex of central Australia. The Giles Complex is a series of coarsegrained layered mafic and ultramafic intrusions, the nine bodies sampled covering an area of 8,000 km2. The NRM of all specimens was measured, as well as the magnetic susceptibility of the samples. As expected the NRM directions were of limited value, and hence AF demagnetization was carried out. The correlation between both the Koenigsberger ratio Q and the “proportional Q-value” of Zijderveld, and the stability of magnetization as indicated by the AF demagnetization was very noticeable. The samples not rejected by these tests gave an overall mean direction which is statistically non-random at the 9976 level using ~~ats~~rl’s&i-squared test. and which is strongly divergent (angular difference I? 1”) from the present t~eld-direction. Hence it seems that the lnagtletization is stable. and is a TRM. llsing these results it appears as if the layering of the Giles Complex was not horizontal (or near horizontal) at the time of acquisition of the magnetization. The mean direction from most bodies samples indicate that the Giles Complex was in, approximately, its present position at that time. This would imply that the Giles Complex was intruded during a relatively short period (perhaps over only a few million years). AlternativeIy, the rocks may have been reheated above their Curie-ten~perature subsequent to initial cooling and tilting. Radiometric dating indicates that the Giles Complex is about 1,100 million years old. Assuming that the stable direction of magnetization is a TRM that represents a geometric dipole direction, the position of the south magnetic pole (relative to central Australia) at the time of cooling of the Giles Complex was latitude h9”S, longitude 161%. Using Irving’s convention, the palae~~-ge(~magnetic field at this time can be considered normal, or of the same sense as the present geomagnetic field.
t’ALAEOMAC;NEl’lC
STUDIES
The non-dipole field is a number from a normal distribution with mean I* = if cos(%rlp, + $,f and standard deviation given by u = p { (ct~(Znt/~~ + ~~)l }. With appropriate values of the parartleters, the model duplicates all of the known facts of the magnetic reversal history, namely, periods of high reversal rate, interspersed by long periods (circa 15 m-y.) of quiescence, and very-long-period oscillations. In addition, observed and predicted histograms of polarity interval durations are in good agreement. Predictions based on the model are that: (1) the overall mean length of polarity intervals will be found to be 1.6 m.y.; (2) long periods of constant polarity will be found in the Silurian (normal) and Middle Cambrian (reversed); and (3) periods of frequent reversals will be found in the Jurassic, Carboniferous, Ordovician and Lower Cambrian.
THE PALAEOMAGNETISM CENTRAL AUSTRALIA
OF THE GILES COMPLEX,
R.A. FA<‘ER
Wollongong University College, Wollongong, N.S. W. (Australia) (Received February 9, 197 1)
540 Specimens have been prepared from samples taken at sixty-three sites in the Precambrian Giles Complex of central Australia. The Giles Complex is a series of coarsegrained layered mafic and ultramafic intrusions, the nine bodies sampled covering an area of 8,000 km2. The NRM of all specimens was measured, as well as the magnetic susceptibility of the samples. As expected the NRM directions were of limited value, and hence AF demagnetization was carried out. The correlation between both the Koenigsberger ratio Q and the “proportional Q-value” of Zijderveld, and the stability of magnetization as indicated by the AF demagnetization was very noticeable. The samples not rejected by these tests gave an overall mean direction which is statistically non-random at the 9976 level using ~~ats~~rl’s&i-squared test. and which is strongly divergent (angular difference I? 1”) from the present t~eld-direction. Hence it seems that the lnagtletization is stable. and is a TRM. llsing these results it appears as if the layering of the Giles Complex was not horizontal (or near horizontal) at the time of acquisition of the magnetization. The mean direction from most bodies samples indicate that the Giles Complex was in, approximately, its present position at that time. This would imply that the Giles Complex was intruded during a relatively short period (perhaps over only a few million years). AlternativeIy, the rocks may have been reheated above their Curie-ten~perature subsequent to initial cooling and tilting. Radiometric dating indicates that the Giles Complex is about 1,100 million years old. Assuming that the stable direction of magnetization is a TRM that represents a geometric dipole direction, the position of the south magnetic pole (relative to central Australia) at the time of cooling of the Giles Complex was latitude h9”S, longitude 161%. Using Irving’s convention, the palae~~-ge(~magnetic field at this time can be considered normal, or of the same sense as the present geomagnetic field.