Magnetic fabrics are powerful tools in structural geology and tectonic studies, because they provide a fast and efficient measurement of mineral alignment, which helps interpret a rock's (de)formation history. The magnetic fabric of remanence-carrying minerals provides useful information when these grains record different deformation stages than the bulk minerals in a rock. When rocks contain several subpopulations of remanence-carrying minerals, each of these potentially displays a distinct fabric. This can lead to complex remanence anisotropies, being a superposition of all subpopulations' individual anisotropies. Characterization of partial remanence anisotropies has been used to investigate changes in fabric with grain size. However, most studies still report one bulk remanence anisotropy tensor per sample, and it remains to be determined how commonly different populations of remanence-carrying grains reflect different subfabrics. Based on a large sample collection including 93 specimens from different lithologies, we have investigated the coercivity dependence of anisotropy of (partial) anhysteretic remanent magnetization A(p)ARM. We find that the principal directions, degree, and shape of A(p)ARM are generally dependent on the coercivity window used to impart the anhysteretic remanent magnetizations (ARMs). Depending on the carrier minerals and their fabrics, ARM anisotropy can either increase or decrease when the ARMs are applied over larger coercivity windows. Additionally, the coercivity fraction that dominates the ARM anisotropy is not always the coercivity fraction that acquires the strongest mean ARM. This illustrates the complexity of characterizing remanence anisotropy, and highlights the importance of carefully choosing experimental parameters in A(p)ARM determination for both magnetic fabric and anisotropy correction studies.
Bibliographical noteFunding Information:
The authors wish to thank the community for answering our AARM experimental parameter survey. Anette von der Handt is thanked for the help with the microprobe measurements. We are grateful to Maxwell Brown, Suzanne McEnroe, Peter Hudleston, and Lisa Tauxe for the discussions on magnetic fabrics related to different sample sets included in this study. We also appreciate the efforts of Franti?ek Hrouda and an anonymous reviewer, as well as the Editor and Associate Editor, in helping to improve the initial version of this paper. This study was conducted under Swiss National Science Foundation project 167608, and measurements were performed at the Institute for Rock Magnetism (IRM) at the University of Minnesota. The IRM is a U.S. National Multi-user Facility supported through the Instrumentation and Facilities program of the National Science Foundation (Earth Sciences Division), and funding from the University of Minnesota. This is IRM publication 1810. The full set of data can be downloaded from the supporting information, or the MagIC database, https://doi.org/10.7288/V4/MAGIC/16724.
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- coercivity dependence
- magnetic fabric
- remanence anisotropy