Sheath fold development in monoclinic shear zones

We use numerical simulations to investigate the evolution of sheath folds around slip surfaces in simple-shear-dominated monoclinic shear zones. A variety of sheath fold shapes develops under general shear, including tubular folds with low aspect ratio eye patterns and tongue-like structures showing bivergent flanking structures in sections normal to the sheath elongation, which may potentially lead to confusing shear sense interpretations. Not all investigated monoclinic flow end-members lead to the development of sheath folds sensu stricto (folds with apical angle <90 degrees). The aspect ratio of the eye patterns, R-yz, correlates with the ratio between the principal strain in the Y-direction and the smaller of the principal strains in the X-Z plane, and thus it could be used in strain analysis.