Theoretical models of intermediate and inverse ferromagnetic, low-field AMS fabrics

One of the most common cases of mixing of normal and inverse magnetic fabrics and intermediate fabrics originates from the coexistence of SD and MD magnetite grains. Examples of such abnormal behavior occur a wide range of geological material including limestones, volcanic tuffs, ignimbrites, anorthosites, granites, mafic dikes and oceanic basalts. The theoretical model of Rochette et al. (1999) for intermediate fabrics is expanded by considering new combinations of normal and inverse fabrics. The models consider a situation where the normal and the inverse fabrics are co-axial. This assumption is realistic from a geological point of view as long as fabrics (tectonic or flow) would result from a single deformation of homogeneous nature at the scale of the specimen. Intermediate fabrics occur over a wide range of mixing proportions. This implies that if the proportions between normal and inverse components were randomly determined, intermediate fabrics would be likely to occur. The minimum amount of inverse component required for intermediate fabrics to form is about 20 % in the case of prolate normal (T = -0.50) and prolate inverse (T = -0.42) components. Such a small amount of inverse component may not be noticed where other geological factors, such as a complex flow pattern affect fabric variations. The identification of the inverse component of the fabric would require using techniques such as the anisotropy of anhysteretic remanent magnetization (AARM) or partial AARM which isolates the contribution of specific magnetic grain sizes. The degree of anisotropy resulting from intermediate fabrics is significantly lower than that of the end members. The shape factor of intermediate low-field AMS potentially varies considerably along a complete mixing line. This suggests that whenever intermediate fabrics occur neither the shape factor nor the degree of magnetic anisotropy relate to strain in a simple way. Reference : Rochette et al.1999. Tectonophysics , 307 : 219-234.