The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor

Abstract-Plane strain simple shearing of norcamphor (C,HiuO) in a see-through deformation rig to a shear strain of y = 10.5 at a homologous temperature of Ts = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c-axis girdle reflects localized pure shear extension along the main microshear set (S,) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the S, microshears. Grain-boundary migration recrystalliza-tion favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c-axis point maxima whose skeletal outline is oblique with respect to the S, microshears and the SZB; and (3) at high shear strains (y> 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains