Sampling strategies and the anisotropy of magnetic susceptibility of dykes

Most studies of the anisotropy of magnetic susceptibility (AMS) of dykes have assumed that susceptibility axes should define a unique orientation relative to the dyke walls and magma flow direction. Theoretical considerations, however, predict systematic variations of AMS as a function of the amount of shear experienced by the flowing magma. Although this feature of AMS might seem undesirable at first sight, based in a theoretical model it has been suggested that actually we can take advantage of such variation in the orientation of the AMS axes to infer magma flow directions more confidently than until now if proper attention is given to the scheme followed during sample collection. In this work we report the results of a pilot study made to test those theoretical predictions. We resampled two dykes from the Ko'olau dyke complex in O'ahu and for which magma flow direction was previously inferred albeit with some uncertainty. The resampling was made following the guidelines suggested by the theoretical work as closely as possible (i.e., collecting more than one profile parallel to the dyke walls and across the dyke). The results of our measurements provide the first empirical evidence supporting the occurrence of a cyclic fabric acquisition during the emplacement of at least some dykes. The procedure described in this paper therefore constitutes a self-contained approach that can be followed to test in a case by case basis the occurrence of such fabric behavior. This in turn should provide enough elements to increase the reliability of the interpretation in the obtained results.