THE USEFULNESS OF NON-CYLINDRICAL (SHEATH) FOLDS IN MONOCLINIC AND TRICLINIC SHEAR ZONES

The relationship between non-cylindrical fold geometry and the shear direction of the hosting zone is investigated by numerical modeling. The results show that non-cylindrical folds may develop into directions highly oblique to the shear direction if the zone has a pure shear component. The general practice of using well developed sheath folds as indicators for the shear direction is often 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, it is the direction of the maximum principal strain rate of the pure shear component (c-direction) and is parallel to the shear zone boundary. Apical axes of mature sheath folds and tubular folds are not parallel to the shear direction unless the shear direction is parallel to the c-direction. In thickening shear zones, the fabric attractor 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 pure shear strain rates along the principal stretching directions, the simple shear strain rate and the kinematic framework of the shear zone.

Two more practical applications of sheath folds in shear zones were found in this study: (1) When the orientations of the apical axes of individual non-cylindrical folds with various hinge angles are consistent, they are likely to be a reliable indicator of the shear direction. If they are inconsistent, no shear direction can be estimated. (2) If hinge lines of non-cylindrical folds, or of folds of opposite asymmetries, lie within a plane that is parallel to the shear zone boundaries, the shear zone is a thinning or simple shear zone. If the hinge lines lie within a plane that is oblique to the shear zone boundaries, or hinge lines seem to have rotated in a complex pattern, then the shear zone is probably a thickening zone. Therefore, non-cylindrical folds may help to determine whether a shear zone is thinning or thickening.