The fold test as an analytical tool

Two fold tests for palaeomagnetism have recently been proposed that rely on the assumption that the total population of magnetic vectors is most highly concentrated with the rocks in the orientation they had at the time of acquisition of the magnetization. This leads to appealing, simple tests based on parameter estimation. However, it is shown that the underlying assumption is flawed and can lead to incorrect conclusions. McFadden & Jones (1981 ) previously developed an inference test based on the concept that the between-group dispersion of magnetization should be consistent with the within-group dispersion when the rocks are in the orientation they had at the time of magnetic acquisition. That test made unrealistic demands upon the sampling scheme for typical, realistic folding geometries and so it has been under-utilized. The McFadden & Jones test is extended by recognizing that it is sufficient to use groups with similar bedding corrections and that it is not necessary to insist on groups with common bedding corrections. These groups may easily be determined with a clustering algorithm. The point is that with the rocks in the orientation at which the magnetization was acquired, it should be immaterial how the groups are chosen.     

Thermochronometry and palaeomagnetism of the Archaean Nelshoogte Pluton, South Africa

²⁰⁷Pb/²⁰⁶Pb single-grain zircon, ⁴⁰Ar/³⁹Ar single-grain hornblende and biotite, and ⁴⁰Ar/³⁹Ar bulk-sample muscovite and biotite ages from the Nelshoogte trondhjemite pluton located in eastern Transvaal, South Africa, show that this granitoid had a protracted thermal history spanning 3213±4  Ma to about 3000  Ma. Whole-rock ⁴⁰Ar/³⁹Ar ages from cross-cutting dolerite dykes indicate that these were intruded at about 1900  Ma. There is no evidence of this or other, later events significantly affecting the argon systematics of the minerals from the pluton dated by the ⁴⁰Ar/³⁹Ar method.
  The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by ⁴⁰Ar/³⁹Ar dating of hornblende. This pole (18°N, 310°E, A ₉₅=9°) yields a palaeolatitude of 0°, significantly different from other Archaean poles from the Kaapvaal Craton. The palaeolatitude difference implies that there was significant apparent polar wander during the Archaean. A second, overprinting magnetization seen in the pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150–1950  Ma) poles for southern Africa.