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- Sur le mécanisme de la schistosité
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Sur le mécanisme de la schistosité
Abstract
Previous theories on the mechanism of schistosity are recorded. The present authors emphasize the importance of primary strain-slip cleavage directly produced from sedimentary structures in the low-grade schists (epizone) of many "flyschoid" series exhibiting axial plane or fan-like cleavage. They describe schistosities on a microscopic scale—non penetrative schistosity associated with microlithons—as well as the mode by which they grade into the slaty-cleavage defined as a penetrative schistosity. A local mechanism is proposed for the origin of this strain-slip cleavage. Cleavage planes are derived from aligned microfolds (crenulations) by shearing of their limbs. The crenulations developed by deformation of the initial anisotropy planes. The limbs act as planes of weakness. The shearing strength is very low along the schistosity planes as compared with its value in the microlithons. Tangential stress along these planes is very low at one time and causes the gliding to be produced. The principal maximum stress is then quasi perpendicular to the schistosity plane which is the shear plane. On the other hand the mechanism of slaty-cleavage is discussed and it is shown that in this case the strain is also heterogeneous. The interpretation is discussed in relation with the problem of rotated garnets. Correlation between the development of axial plane schistosity and the similar folding of incompetent beds permits to discuss the origin of the shearing component along the cleavage planes. The folding of the competent beds in the "flyschoid" series partly following a concentric folding model is believed to be the driving mechanism of the shearing and also its brake. Relations between the various local mechanisms of schistosity are examined.
In a second part the evolution of schistosity with increasing metamorphic grade is reviewed, especially the cases of the schistosity in granulitic facies rocks and granitic ortho-gneisses. The significance of schistosity fronts and of the basement nappes with respect to schistosity is considered.