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

c-axis fabric and microstructures in a quartzite sample, sheared and extens ively recrystallized under greenschist facies conditions, have been analyze d and compared with theoretical predictions using a viscoplastic self-consi stent model modified to incorporate the effects of dynamic recrystallizatio n. An asymmetric small-circle c-axis fabric about the finite shortening z-a xis with a small half opening angle (35 degrees) is present in the sample; it consists of four orientation components which are represented by host gr ain 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 she ar, respectively; C is in an intermediate direction between the Y- and Z-ax is of finite strain, and D forms a subsidiary concentration around the inte rmediate strain (Y-) axis. B- and C-grains are favorably oriented for basal (0001) and pyramid {10(1) over bar 1}[a] slip, respectively, and strongly deformed, while A- and D-grains are unfavorably oriented for the slip syste ms and little or moderately deformed. Some of A-grains are even fractured. The degree of dynamic recrystallization increases with increasing strain un dergone by differently oriented grains (in the sequence of A-, D-, C- and B -grains). Microstructural evidence and theoretical predictions indicate tha t harder A-, C- and D-grains were significantly consumed by the grain bound ary migration of the softer recrystallized B-component (although the consum ption of A-grains was not really documented in the quartzite sample). The c onclusion is supported by the fact that the B-component is much more domina nt in the recrystallized than in the host c-axis fabric. Hence, the c-axis maximum nearly perpendicular to the shear plane and apparently displaced wi th the sense of shear commonly found in naturally sheared quartzites (corre lated with the B-component) is presumably developed by the growth of soft o rientations for basal (0001) slip by grain boundary migration at large stra ins. (C) 1999 Elsevier Science B.V. All rights reserved.