The origin of shape preferred orientations in mylonite: inferences from in-situ experiments on polycrystalline norcamphor

Abstract

Polycrystalline norcamphor (C 7H 10O) undergoing plane strain simple shear in a see-through deformation rig develops intergranular microshears whose activity and orientation is closely related to shape preferred orientation (SPO) of dynamically recrystallized grains and grain aggregates. Intergranular microshears nucleate at 80–90° to the sample’s shear zone boundary (SZB) and rotate synthetically toward this boundary. They accommodate increasing amounts of incremental shear strain at angles ranging from 40° to 60° ( S b orientation) with respect to the SZB. With progressive simple shear, both the rotation rate and the amount of strain accommodated by these microshears decrease, but strain accommodation increases as the microshears attain a lower angle (10–30°, S a orientation) to the SZB. The microshears gradually deactivate as they acquire inclination angles to the SZB of less than 10°. The deactivation of such microshears is accompanied by the nucleation of fresh, high angle microshears. This heterogeneous deformation is associated with two shape preferred orientations in the norcamphor mylonite: A steep, oblique grain SPO comprising the long axes of dynamically recrystallized grains (40–60°) subparallel to the S b-oriented microshears and gently inclined domainal SPO (20°) subparallel to the S a-oriented microshears. The domain SPO is defined by the length axes of grain aggregates with a uniform crystallographic preferred orientation. The angle between domainal and grain SPOs is a potential measure of the bulk vorticity during mylonitization.