Viscoplastic modeling of texture development in quartzite

Polycrystal plasticity theory has been successfully used to simulate development of preferred orientation in rocks. In particular, G. Lister and coworkers have done a comprehensive study, applying the Taylor theory to quartzite. In these calculations it was assumed that deformation is homogeneous; that is, all grains deform at the same rate, that critical resolved shear stresses (CRSS) of slip systems remain constant, and that deformation is rigid‐plastic; that is, dislocations move only when the CRSS has been reached and then with indeterminate velocity. We have been investigating the influence of work hardening and of strain rate dependence on texture development. In particular, modification of the viscoplastic Taylor theory suggests that textures are greatly dependent on the rate sensitivity of flow stress when this stress exponent is small, such as in quartz where it is near 3. The influence of work hardening is less critical in the case of quartz. Results from Taylor simulations are also compared with those from a self‐consistent theory. The latter sacrifices local strain continuity for better stress equilibrium. In the self‐consistent scheme, grains which are favorably oriented for slip are allowed to deform at a faster rate. Texture patterns obtained with the two theories are moderately different. In self‐consistent deformation a C axis maximum in the intermediate strain direction (Y) is generated which is absent in Taylor deformation but is a common feature of many natural quartz fabrics. Grains associated with this maximum are most strongly deformed. Deformation modeling with more realistic boundary conditions adds complexities but appears to be necessary in the case of anisotropic and rate sensitive rocks.