The rotation of magnetic grains

Abstract

It has been questioned whether magnetic remanence rotates as a rigid marker or as a passive marker (with no material properties) during tectonic strain. The remanence of a rock is actually the sum of the moments of individual grains, so we must first understand their rotation. Simple shear provides a simple strain history which may be used to distinguish between the two extreme possibilities. A passive marker cannot rotate through the shear plane but a rigid marker can: this is a useful criterion to distinguish between the two extreme models. However, for reasonable strains ( γ < 4 or Rs < 18), it is only possible to distinguish between rigid marker and passive marker behaviour for grains of low aspect ratio ( R < 5), preferably making a low initial angle with the shear direction. For these conditions, rigid grains would rotate through the shear plane. Because natural hematite usually has high aspect ratios ( R > 10) the passive model is successful in explaining the rotation of these grains, even though their behaviour is mechanistically closer to that of a rigid marker. This explains the success of field studies in which the remanence of redbeds has been de-strained using the hypothesis of passive behaviour, notwithstanding the reality that the natural iron oxide grains do not rotate in that manner.