Quantification of deformation fabrics in various rocks using magnetic anisotropy.

Abstract

Accurate quantification of preferred orientation fabrics in deformed rocks with magnetic anisotropy requires an understanding of the behavior of the mineralogical carrier(s) of magnetic anisotropy. Rock magnetic and petrologic studies were conducted to identify which mineral(s) carry the anisotropy in slates and deformed anorthosites. In the Martinsburg Formation, Lehigh Gap, Pennsylvania, anisotropy of magnetic susceptibility (AMS) is controlled by chlorite. Characteristics of the AMS ellipsoids are consistent with numerical model results of composite magnetic fabrics arising from the addition of two orthogonal, oblate component fabrics. The composite AMS fabrics in the Martinsburg Formation result from progressive dissolution/new-growth of chlorite during cleavage formation. The composite nature of the AMS fabrics shows that the shape of the AMS ellipsoid cannot be used to quantify the mineral fabrics of these slates. The combined results of anhysteretic remanent magnetization anisotropy (ARMA), numerical models, and paleomagnetism indicate that magnetite in the Martinsburg also dissolved and recrystallized as cleavage formed. The shales have bedding-parallel magnetite fabrics, and preserve a primary (Ordovician) natural remanence. Magnetite dissolution/new-growth remagnetized the slates, and produced composite ARMA fabrics in the pencil slates. In contrast to AMS, the shape and orientation of the end-member shales and slates are accurately recorded by the ARMA fabrics. The AMS fabrics of mylonites in the Parry Sound shear zone, Ontario, Canada, are controlled by magnetite. Based on a temperature of 630$\sp\circ \pm$ 50 C from geothermometry and extrapolation of experimentally-derived constitutive equations, the deformation mechanism of magnetite in this shear zone was dislocation creep (crystal plasticity). An empirical correlation between shear strain and AMS ellipsoid shape $(\rm k\sb{max}/k\sb{min}) = (X/Z)\sp{0.14}$ was obtained for these rocks. Such high shear strains ($\gamma$ = 9 to 12) cannot be obtained from rigid rotation of magnetite, but are consistent with plastic deformation of magnetite. Thus, the AMS fabrics accurately quantify both the magnitude and orientation of the tectonic fabric and finite strain in the Parry Sound shear zone.