Plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals

The effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a Griggs apparatus. Experiments were conducted at shear strains (gamma) up to 5 at a temperature of 900 degrees C, confining pressure of 1.5 GPa, and shear strain rate of 10(-5) s(-1). Three starting orientations of crystal were used, to activate three slip systems: basal < a >, prism < a >, and prism hai, although slip-induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems. For crystals with higher water contents, basal hai and prism hai orientations are relatively weak and prism [c] orientations are stronger. All three initial crystal orientations undergo dynamic recrystallization with increasing shear strain, although the strain required for 100% recrystallization varies: gamma approximate to 2 for prism < a > slip, gamma approximate to 3.8 for basal hai slip, and gamma approximate to 5 for prism hai slip. For all three starting orientations, distinct domains of recrystallized grains develop with c axes parallel to Y of the strain ellipsoid (Ymax), replacing recrystallized grains of other orientations; the Ymax domains increase in size with increasing strain. In addition, strain markers show that strain is highly localized within the Ymax domains, indicating geometrical softening of up to an order of magnitude in effective viscosity.