Remanence stability and magnetic fabric development in synthetic shear zones deformed at 500°C

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Abstract

Shear experiments were performed on magnetite-bearing calcite aggregates to examine magnetic fabric development and remanence stability in a deforming system using elevated temperature and pressure to encourage deformation by crystal-plastic processes. Samples composed of 1 wt % pseudo-single-domain magnetite (1–2 μm) in a calcite matrix were created with either strong or weak initial fabrics and deformed in coaxial simple shear to strains up to γ = 1.5 at constant strain rates between 6 × 10−5 and 1 × 10−4 s−1 at 500°C and confining pressure of 300 MPa. Samples were given weak field thermal remanent magnetizations prior to deformation. Demagnetization of postdeformation remanence reveals that a primary remanent magnetization can withstand deformation at pressures and temperatures approximately equivalent to greenschist facies metamorphic conditions on laboratory time scales, but this stability is found to depend on the character of the predeformation fabric. The origin of secondary remanence components acquired during deformation is uncertain but is likely to partially result from thermal viscous remagnetization. Complete postdeformation remagnetization in initially anisotropic samples appears to involve a stress-softening or piezoremanent magnetization mechanism. Postdeformation anisotropy measurements show progressive changes in magnetic fabric strength with strain. In the absence of a strong initial magnetic anisotropy, magnetic fabric intensity increases linearly as a function of strain; however, deformation that overprinted an existing fabric results in an apparent decrease of the initial anisotropy at low strains followed by rapid increases in magnetic fabric strength with increasing strain. Our results underscore the important role that initial fabric can play in determining the character of deformation fabrics.

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