Analog Modeling of Anisotropy of Magnetic Susceptibility as Affected by Pure Shear Strain on Original Magnetic Fabrics of Sedimentary Rock

Analysis of the anisotropy of magnetic susceptibility (AMS) is an easy, non-destructive method to determine the preferred orientations of minerals in rocks and rock analogs. The orientations of the principal susceptibility axes (Kmax≥Kint≥Kmin) of the AMS ellipsoid are generally parallel to the principal axes of the strain ellipsoid (X≥Y≥Z). The orientations of the AMS axes as well as the magnitudes change in response to strain, though a generally accepted correlation between the magnitudes of the principal axes of AMS and strain has not yet been established. A successful correlation of the magnitudes of the principal susceptibility axes of AMS and strain would allow an easy and non-destructive method of quantitative strain analysis. This would also allow quantitative strain analyses of rocks where traditional methods using strain markers fail. In this study, the quantitative aspect of the relationship between strain and AMS is investigated experimentally using artificial mineral mixtures with a sedimentary initial magnetic fabric in an attempt to correlate strain to AMS. Mineral mixtures of magnetite, biotite, and specular hematite with a matrix of Art Time DoughRTM (similar to PlaydohRTM) were mixed separately. To create a random magnetic fabric, the samples were kneaded by hand for approximately 15 minutes. The samples were then strained to 70% strain in one direction to create a sedimentary initial magnetic fabric. The orientation of this strain became the Z axis of the strain ellipsoid. The samples were then strained perpendicular to this axis incrementally from 0% to 40% strain in 5% increments, with the AMS measured at each interval. The orientation of this strain became the Y axis of the strain ellipsoid. The data from these experiments resulted in the quantitative correlation of strain and AMS for the magnetite mineral mixtures. The biotite and specular hematite mixtures contain enough magnetite inclusions and magnetite conversion respectively to dominate the AMS. This creates a more complex relationship that is not easily correlated quantitatively to strain. The experiments demonstrate that a strong qualitative relationship exists between both the orientations and the magnitudes of the axes of the strain and AMS ellipsoids.