Strain and magnetic fabric in the Santa Catalina and Pinaleno Mountains Metamorphic Core Complex Mylonite Zones, Arizona

Abstract

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains.