Preferred orientation of quartz produced by slip during plane strain

Abstract

The c axis preferred orientations developed in the original grains of quartzites experimentally and naturally deformed in plane strain have been measured, in order to extend previous results from axial compression experiments and to furnish comparisons with theoretical predictions. The preferred orientations developed in pure shear are similar to those found previously for axial compression: at lower temperatures or faster strain rates where basal and rhombohedral slip predominate, there is a maximum of c axes parallel to the compression direction, and at somewhat higher temperatures or slower strain rates where basal and prismatic slip predominate there is a small circle girdle of c axes about the shortening direction. However, the patterns developed in plane strain all exhibit, in addition, a connecting girdle through the intermediate principal strain axis. These preferred orientations represent one type of crossed girdle pattern, and are consistent with theoretical predictions made by Lister (1974) of preferred orientations resulting from slip. Quartzites experimentally deformed in a geometry close to simple shear develop preferred orientations indistinguishable from those due to axial compression, although Lister's (1974) calculated preferred orientations show some asymmetry. Several naturally deformed quartzites having the same microstructures as the experimental samples and exhibiting plane strain show the same patterns of preferred orientation as found in the pure shear experiments.