Grain‐scale deformation and the fold test ‐ evaluation of syn‐folding remagnetization

Abstract

The paleomagnetic fold test examines for pre‐, syn‐ or post‐folding (re)magnetization. Generally, passive limb rotation is used to restore folded units to their original position, and the position of the Fisherian maximum determines the relationship between folding and magnetization. In a more complete analysis, the reorientation of material lines in different fold models is used. However, the magnetic vector should not be considered a material line as it represents the sum of vectors in individual magnetic carriers and does not cross grain boundaries. Therefore, the role of grain‐scale deformation mechanisms was examined, using two end members: rigid‐body rotation (spin) and homogeneous strain.From this study it is concluded that (1) the Fisherian maximum will in general occur before complete unfolding, (2) the sign of the inclination may change during unfolding and, consequently, ’cross‐over’ unfolding is not representative of syn‐folding remagnetization, (3) statistical analysis will not necessarily produce significant results. A number of recommendations are given, which include: stepwise unfolding (and unplunging) should be carried out routinely; local, rather than regional, fold tests should be performed; both limbs and the hinge of a single folded layer should be examined in order to determine the active deformation mechanisms that will enable corrections to be carried out; in addition to Fisherian statistics, the angle between magnetic vector and bedding across a fold should be measured and used for correction.