Strain reorientation of hematite and its bearing upon remanent magnetization

Using a computer‐based numerical model, we suggest that passive reorientation of hematite planar particles induced by strain perturb significantly the remanent magnetization carried by the hematite. The evolution of its direction and dispersion in a progressively increasing strain can be correlated with the evolution of passive lines. In real rocks, the amount of reorientation can be approched by measuring one of the three following tensors: strain tensor, orientation tensor of some hematite cristallographic axes, or magnetic susceptibility tensor. From examples taken in the Maritime Alps and in Brittany, we emphasize the possible correlations between these tensors (orientation of principal axes and axial ratios). Finally, we use our best estimate of strain to correct the paleomagnetic results by the strain removal technique we have already used in the Maritime Alps (Cogné and Perroud, 1985). When applied to the preliminary paleomagnetic results obtained from Ordovician deformed redbeds of Brittany, it allows to recover a conclusive primary magnetization direction, although the characteristic magnetizations appear post‐folding with respect to the classical fold test.