Grain boundary hierarchy development in a quartz mylonite

Orientation contrast imaging using forescatter detectors and backscattered electron diffraction techniques in the scanning electron microscope have been used to investigate the boundary hierarchy characteristics of a quartz mylonite shear zone from Torridon, NW Scotland. The boundary hierarchy is defined as the relationship between the boundary misorientations, their enclosed domain size and their frequency distribution. By measuring the misorientation across every grain and subgrain boundary, the characteristics of the boundary hierarchy can be found. Two microstructural domains were studied: one partially recrystallized low strain domain with large relict grains and one fully recrystallized high strain mylonitic domain. Our results indicate that the processes of recovery, subgrain rotation and grain boundary migration recrystallization each produce identifiable boundary hierarchy signatures. In the relict quartz grains we have identified the processes of recovery and subgrain rotation; in the recrystallized quartz a cyclic steady state exists between these and other processes. Coupling these data with traditional microstructural observations allows a more rigorous investigation into the development of a high strain, fine grained mylonite from a coarse grained, undeformed protolith. We suggest that this type of detailed crystallographic microstructural analysis can greatly further our understanding of microstructural development in shear zones and may have implications for the effective use of (sub)grain size palaeopiezometers.