Abstract

Dislocation glide is an important mechanism allowing plastic deformation of quartzites. A computer program using the Taylor-Bishop-Hill analysis (Taylor 1938, Bishop & Hill 1951) has been used to simulate quartz fabrics. This method of analysis requires a knowledge of a) the possible dislocation systems that can operate, b) the critical resolved shear stresses on each system, and c) the deformation path. All of these factors determine the type of lattice rotations in the grains of the rock and hence the types of fabric produced. If basal a glide is the dominant slip system then the c-axes tend to lie near the shortening axis, while if prism c glide operates the reverse applies. If, however, prism a glide operates then it is difficult to predict the e-axis fabric.

It has been found that elements of an orien-pattern tation appear after only 30% shortening in specific angular relations to the kinematic stretching axes. With an increase in strain the patterns become more intense. Hence the fabric of superposed deformations reflect only the orientation of the kinematic stretching axes of the last stage of deformation with early patterns being rapidly obscured.

Through an intensive study of fabric transitions an ordering of deformation environments according to the c-axes pattern developed is possible. Thus it has been possible to predict the orientation patterns found experimentally by Tullis (1970) and Tullis et al. 1973.

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