Use of magnetic fabrics and X-ray diffraction to reveal low strains in experimentally deformed Maggia gneiss

In this pilot study, we analyzed the response of magnetic fabrics and X-ray diffraction to low strains (≤ 0.0103) applied during quasi-static and dynamic deformation. Four cylindrical samples of Maggia gneiss were deformed under uniaxial compression oriented parallel to the foliation of the gneiss. Out of these four, two cylinders were deformed at dynamic strain rates (> 50/s) with a split Hopkinson pressure bar and two at quasi-static strain rates (< 10 –4 /s) using a hydraulic press. From each deformed cylinder, five to six specimens were retrieved. Under the polarizing microscope, the original samples and those deformed in the laboratory do not show any difference in microstructures. However, X-ray diffraction reveals gradual straining of the quartz and biotite lattice with increasing experimental strain. Moreover, after the experimental deformation, the maximum and intermediate principal magnetic susceptibility axes ( K 1 and K 2 ) form a girdle, which is clearly different from the triaxial distribution in the undeformed samples. The corrected degree of anisotropy and the oblateness of the magnetic fabrics increase. The magnitude of the intermediate susceptibility axis ( K 2 ) increases, and of the minimum susceptibility axes ( K 3 ) decreases. In this pilot study, we are able to show that XRD and magnetic fabrics are very sensitive to strain and capable of recording even low strains which do not present any other apparent evidence of deformation. With further studies, XRD can be established as a practical technique for measuring low mechanical strain in rocks.