Magnetic Fabrics

Anisotropy of Magnetic Susceptibility (AMS) is a measure of the inhomogeneity and alignment of magnetic grains within rocks that is often used as a proxy for mineral texture in geologic applications, (Biedermann, et al., 2015). AMS fabrics have been related to numerous events through a range of temperature-pressure conditions, from viscous flow in magmas (e.g., Knight and Walker, 1988; Ferré et al. 2002) through to folding and ductile-brittle shearing during relatively late stages of orogenesis (e.g., Torsvik et al., 1992; Greiling and Verma, 2001, Austin et al., 2019b). AMS is commonly used in structural geology (e.g., Bascou et al., 2013) to map strain fabrics in metamorphic terranes. It has also been used to map settling fabrics (e.g., Ferre et al., 1999), flow fabrics (e.g., Wiegand et al., 2017) and deformation fabrics in felsic (e.g., Glodji, 2014) through to ultramafic intrusions (e.g., Hrouda, et al., 2013; Biedermann, et al., 2015). It is increasingly finding application in economic geology, for understanding the structural controls on ore deposition (e.g., Sandrin, 2003; Austin et al., 2013; 2019b) and in a small number of cases, researchers have attempted to integrate the findings from palaeomagnetic methods with AMS data (e.g., Schmidt and Clark, 1994; Lamali et al., 2013; Austin and Patterson, 2021).

Measurement of AMS vectors is performed in a laboratory using an AGICO MFK1-A (or MFK2) Kappabridge Magnetometer coupled with a 3-D rotator. The technique requires oriented samples, either from oriented Diamond Core, or from Sun-compass oriented surface sampling and we aim quantify the consistency of the results via measurement of at least three specimen per sample. Processing of AMS vectors includes assessing the quality of the clustering, and calculation of key parameters (Jelinek, 1981) that describe the type of magnetic fabrics present. Such parameters include the degree of anisotropy (P),  the degree of magnetic lineation (L=K1/K2) and the degree of magnetic foliation (F=K2/K3) describe the style of fabric present whether the fabric is more planar (oblate) or more linear (prolate). The information derived can provide valuable insights into whether fabrics are the results of sedimentary, magmatic or tectonic processes and provide insights into the orientation of the the gravity field (during settling) or deformation vectors (during deformation).

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