Quartz of C-Axis Fabric Variation at the Margins of a Shear Zone Developed in Schists from Cap de Creus (Spain)

摘要

By means of geometrical analysis it is established that the studied shear zone, although approaching closely the simple shear model, does not satisfy it completely. Slight departures must be taken into account, i.e. there is an heterogeneity of strain along the shear zone on decimetric to milimetric domains, this caused by the non isotropic character of the original rock. Furthermore, a slight flattening component, besides the dominant simple shearing has to be taken into account when considering the bulk type of strain acting on the outer margins of the shear zone. In the outer margin of the studied shear zone two incipient sets of extensional crenulations, nearly symmetnc about the regional schistosity, appear. In such low strain domains, a weak caxis preferred orientation is already present consisting in a pseudo-two-girdle pattern. Stili at the marginal domains, but approaching the inner part of the shear zone, the regional schistosity progressively rotates into a mylonitic foliation. At the same time one set of the crenulations tends to vanish, while the other becomes better developed but conserves about the same angular relationship with the foliation. This fact coincides with the rotation and strengthening of the preferred orientation c-axis fabric pattern and its progressively more asymmetnc character with regard to the foliation. The final fabric is a nearly single girdle which tends to remain perpendicular to the persisting set of extensional crendations or shear bands if present. The obliquity of the girdle with respect to the mylonitic foliation is by no means related to the amount of strain.