Unravelling patterns of folding in high-strain zones

Abstract

The careful recording of fold and fabric geometries permits evaluation of shear zone kinematics. Directions of shearing may be calculated via the Axial-planar Intersection Method (AIM) which incorporates analysis of minor fold vergence with axial-planar rotations. The geometry of sheath fold pairs may also be used to determine shear sense with sheaths closing in the transport direction typically displaying greater hinge-line curvature. Cross sections through the noses of sheath folds display catseye folds where the ellipticity of closures increases towards the centre of the eye, and bulls-eye folds where closures become less elliptical towards the centre, and reflect variations in bulk strain type from simple shear to constriction respectively.