Relationship between non-cylindrical fold geometry and the shear direction in monoclinic and triclinic shear zones

We investigate the relationship between non-cylindrical fold geometry and the shear direction of the hosting high-strain zone by numerical modeling, and show that apical axes of non-cylindrical folds may develop into directions highly oblique to the shear direction if the zone has a pure shear component. The common practice of using well-developed sheath folds as indicators for the shear direction is not reliable and the use of immature non-cylindrical or sheath folds appears more reliable. Hinge lines of mature sheath folds approach parallelism with the fabric attractor (to which all material lines rotate), which can have variable angles with respect to the shear direction. In thinning zones, the fabric attractor is the direction of the maximum principal strain rate of the pure shear component and is parallel to the shear zone boundary. In thickening zones, it lies somewhere in the quadrant between the direction of the maximum principal strain rate of the pure shear component (perpendicular to the shear zone boundary) and the simple shear direction, and is generally oblique to the shear zone boundary. The exact location depends on the flow geometry of the shear zone.