Abstract

Finite strain measurements obtained from formerly spherical natural strain indicators in slate are compared to the degree of preferred orientation of mica, as revealed by x-ray pole figure goniometry. The pole figures obtained for (002) are all elongated point maxima normal to the slaty cleavage; the short axes of these elongate maxima trend toward the direction of the linear “grain” in the plane of cleavage. The deformation ellipsoid has its shortest axis precisely perpendicular to the cleavage and its longest axis parallel to the linear grain in the cleavage. The geometric relations are consistent with the model proposed by March (1932) that assumes passive rotation of tabular markers and thereby allows strains to be predicted from observed preferred orientations and vice versa. The measured deformation ellipsoids reveal a compressional strain across the plane of cleavage of 63 percent, 66 percent, and 68 percent. The corresponding concentrations of poles to (002) predicted by the March model from these strains are 20.6, 25.6, and 31.6 times uniform, respectively, whereas pole concentrations of 16.3, 17.9, and 18.3 times uniform were actually measured. Hence the measured strains are more than adequate to account for the formation of the observed preferred orientations by a mechanism of rotation. This and other studies suggest that quantitative preferred orientation studies may enable both the magnitudes and directions of the principal finite strains to be estimated in deformed micaceous rocks that do not contain independent indicators of strain.

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