ISOLATING COMPONENTS OF COMPLEX MAGNETIC FABRICS TO ENHANCE THE ANISOTROPY CORRECTION OF PALEOMAGNETIC DIRECTIONS AND INTENSITIES (Invited Presentation)

Magnetic fabrics reflect the crystallographic and shape preferred orientations of minerals in a rock, and thus record a rock’s deformation history. In orogenic settings, this history may be complicated, with multiple phases of metamorphism and tectonic overprints. If individual minerals form or alter at different stages during this process, their preferred orientation can record different strain histories. For example, secondary oxide minerals may form during a later stage, and thus exhibit a fabric different from the primary minerals. Similarly, multiple grain sizes of the same mineral may record different strain fields or stages of deformation. The whole-rock magnetic fabric will then represent a superposition of all the individual sub-fabrics developed over time. Due to their shape or magnetocrystalline anisotropy, preferred orientation of remanence-carrying grains will lead to an anisotropy of remanence. This in turn causes magnetization deflection, and affects the magnetization intensity, posing major challenges to paleomagnetic interpretation.

Whole-rock magnetic fabrics, described by anisotropy of magnetic susceptibility (AMS), may not be representative of the anisotropy of remanence acquisition. Remanence anisotropy, e.g. the anisotropy of anhysteretic remanent magnetization (AARM), is more appropriate to correct paleomagnetic data. However, if several remanence carriers are present, they may display different fabrics. Here, we show how the AARM within the same sample changes depending on the coercivity window over which it is imparted. In some samples, the highest coercivity grains show the strongest anisotropy, whereas in others it is the lowest or intermediate coercivity grains that are most anisotropic. The strongest AARM is not necessarily carried by the same grains that carry the highest portion of the remanence, or the characteristic remanence. Principal directions of the AARM can be similar for all coercivity windows in the same rock, but they can also display significantly different orientations. This will affect the corrections for both magnetization directions and paleointensities, especially in rocks with strong dependence of AARM on grain size. We propose that the variation of AARM with coercivity is investigated prior to anisotropy-correcting paleomagnetic data.