Recognition of structures in mid-crustal shear zones and how to discern between them

This chapter deals with the recognition and interpretation of mesoscopic structural features such as folds, foliations, and lineations, as produced by plastic mechanisms in high strain zones at midcrustal levels. Regardless of composition, rocks dominantly deformed by plastic deformation typically show flow structures and are classified as mylonites. After a brief historical introduction to the terminology adopted for mylonitic shear zones, linear and planar fabrics typical of this context are described. Some of these structures are investigated here because their correct interpretation is crucial to infer the kinematic picture of the entire shear zone. For example, in cases where opposite senses of shear are observed within the same high strain zone, it is imperative to discern between real and apparent kinematic indicators. In particular, sheath folds and lineations are discussed. Sheath folds may be misinterpreted as oblique folds, whereas stretching lineation can be confused with intersection lineation. Some examples given here can aid field recognition of such structures.