Composite magnetic anisotropy fabrics: experiments, numerical models and implications for the quantification of rock fabrics

Abstract

Magnetic fabrics from rocks with multiple mineral-preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics. Our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and/or orientations of the individual mineral fabrics, and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks. These criteria include: (1) the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics, and (2) the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases, as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases.Composite magnetic fabrics are observed in the shales and slates of the Martinsburg Formation, Lehigh Gap, Pennsylvania. Modeling of the AMS (anisotropy of magnetic susceptibility) and ARMA (anhysteretic remanent magnetization anisotropy) behavior constrains the relative degree of anisotropy of the bedding-parallel and cleavage-parallel fabrics. In particular, ARMA model results allow a good estimate of magnetite fabric strength.We conclude that, in the presence of composite magnetic fabrics, quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric. Such determinations require knowledge of the mineral(s) that are responsible for the measured magnetic fabric and their behavior during deformation.

Complexity of the Anisotropy of Magnetic Susceptibility in Single-Phase Deformed, Low-Grade, Cleaved Mudstone

“…Recently, similar trends were observed on the basis of natural samples [5]. In the present paper, we present new data that confirm earlier conclusions [4,5] and further emphasise the complexity of AMS.…”

Magnetic fabrics indicating Late Quaternary seismicity in the Himalayan foothills

“…The relationship between finite strain and magnetic fabric is complex, but this can be understood by studying deformation environment and origin of AMS using appropriate instruments (Borradaile and Henry 1997). The transition from compaction fabrics to strain fabrics shows large changes (Housen et al 1993;Parés et al 1999) that have to be critically evaluated for reliable application of magnetic analyses to deformed rocks. Nevertheless, the consensus has been that magnetic lineation is parallel to extension direction (magnetic lineation = K max ), and orientation of K min axis is parallel to shortening direction (Borradaile and Henry 1997;Housen et al 1996;Aubourg et al 1999;Sagnotti et al 1994;Parés et al 1999).…”

Some remarks on fabric overprints and constrictional AMS fabrics in igneous rocks

“…Numerical modeling of magnetic fabrics is based on summing grain susceptibilities of a multi-grain system that is reorienting in a deforming matrix (Housen et al 1993;Benn 1994; Ž ák and Kabele 2011). The basic parameters are the shape and the magnetic susceptibility of the carriers, which are biotite and amphibole in our study.…”