where At = cmax/cmin and is the degree of anisotropy of TRM. The TRM will be deflected through the angle (9 - <j>), which will have its maximum value when 0 = tan"W^t so that


For multidomain magnetite and other strongly magnetic minerals A, « An2 from both theory (Dunlop and Özdemir, 1997) and experiment (Cogné, 1987) so that, from (2.3.42)

7 2Av

where Ax = (xmax - Xmi„) and xav = (Xmax + Xmin)/2-anisotropy of TRM is approximately twice the susceptibility. Also, from (2.3.45)

Therefore, the percentage percentage anisotropy of

For an anisotropy of susceptibility of 5%, An = 1.05 and the maximum TRM deflection is 2.7°, for 10% it is 5.2° and for 20% it is 10.0°. Thus 10% anisotropy can be tolerated without seriously deflecting the TRM. The rocks most commonly used in paleomagnetism, such as basic igneous rocks, redbeds and unmetamorphosed sediments, rarely have anisotropies exceeding a few percent, so the effect is only likely to be significant in some metamorphic or strongly foliated rocks.

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Chapter Three

Methods and Techniques

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