Development of preferred orientation and microstructure in sheared quartzite: comparison of natural data and simulated results

摘要

c-axis fabric and microstructures in a quartzite sample, sheared and extensively recrystallized under greenschist facies conditions, have been analyzed and compared with theoretical predictions using a viscoplastic self-consistent model modified to incorporate the effects of dynamic recrystallization. An asymmetric small-circle c-axis fabric about the finite shortening z-axis with a small half opening angle (35 deg) is present in the sample; it consists of four orientation components which are represented by host grain c-axis orientations (referred to as A, B, C and D): A and B are at high angles to the foliation plane, displaced against and with the sense of shear, respectively; C is in an intermediate direction between the Y- and Z-axis of finite strain, and D forms a subsidiary concentration around the intermediate strain (Y-) axis. B- and C-grains are favorably oriented for basal (0001) and pyramid {1011} slip, respectively, and strongly deformed, while A-and D-grains are unfavorably oriented for the slip systems and little or moderately deformed. Some of A-grains are even fractured. The degree of dynamic recrystallization increases with increasing strain undergone by differently oriented grains (in the sequence of A-, D-, and B-grains). Microstructural evidence and theoretical predictions indicate that harder A-, C- and D-grains were significantly consumed by the grain boundary migration of the softer recrystallized B-component (although the consumption of A-grains was not really documented in the quartzite sample). The conclusion is supported by the fact that the B-component is much more dominant in the recrystallized than in the host c-axis fabric. Hence, the c-axis maximum nearly perpendicular to the shear plane and apparently displaced with the sense of shear commonly found in naturally sheared quartzites (correlated with the B-component) is presumably developed by the growth of soft orientations for basal (0001) slip by grain boundary migrations at large strains.