Fabric development in a quartz-feldspar mylonite

Abstract

Variations in quartz c-axis fabrics in a mylonite hand specimen correlate with variations of microstructure. Large feldspar clasts in the quartz-rich groundmass have caused heterogeneous deformation on the microscale and different deformation paths undergone by the different regions are postulated to have caused the variations in c-axis fabrics. The different microstructures found in the regions are believed to be related to sympathetic variation in strain rate and the degree of local strain heterogeneity. The microstructures reflect a combination of dynamic recovery and dynamic recrystallization processes. Although the importance of these processes is acknowledged it is thought that fabrics have developed in relation to the deformation process, rather than from the effects of recrystallization modifying pre-existing or co-developing deformation fabrics. Although diffusional processes were important, deformation may have taken place primarily as the result of the conservative component of dislocation motion within the subgrain interiors. Simulations of fabric development were attempted using a program based on the Taylor—Bishop—Hill analysis. The observed asymmetric c-axis fabrics could arise in several ways. Only the implications of two-stage histories are explored in these simulations. It is suggested that fabrics develop in relation to the kinematic framework and that the orientation of the elements of a pattern of preferred orientation are sensitive to the last stages of deformation. Observations from the mylonite strongly support the idea of a late stage comparatively homogeneous deformation that has imprinted itself on the developed fabrics.