Abstract

In deformation experiments at 5 kb confining pressure and room temperature, quartz-rich layers or veins in phyllite develop various structures such as folds, faults, and boudins during the growth of a pattern of intersecting kinks in the phyllitic matrix. The layers containing appreciable amounts of mica and having a foliation like that in the matrix follow the deformation of the matrix fairly closely. In other layers of massive pure quartz, folding appears to be initiated by the kinking in the matrix, but it develops in a more nearly independent way, giving rise to a change in style of the folds with progressive deformation. Greater competence would normally be attributed to these layers than to the matrix. However, the relation of the structures to kinking does not conform to the picture of a competent isotropic layer in an incompetent isotropic matrix, in which folding and boudinage are taken to be local instabilities in deformation of the layers, reflecting their shortening or lengthening, respectively. Apparently the inhomogeneous progression of deformation in the highly anisotropic matrix and the isotropic cataclastic behavior of the quartz, at least in the early stages, are associated with more or less passive response of the quartz-rich layers to the dominant role of the matrix instability, rather than a primary role of the layer instability.

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