Computer experiments to investigate complex fibre patterns in natural antitaxial strain fringes
Antitaxial non-deforming strain fringes from Lourdes, France, show complex quartz, calcite and chlorite fibre patterns that grew around pyrite in a slate during non-coaxial progressive deformation. Development of these fringes was modelled using a computer program ‘Fringe Growth 2.0’ which can simulate incremental growth of crystal fibres around core-objects of variable shape. It uses object-centre paths as input, which are obtained from fibre patterns in thin section. The numerical experiments produced fibre patterns that show complex intergrowth of displacement-controlled, face-controlled and intermediate fibres similar to those in the natural examples. The direction of displacement-controlled growth is only dependent on the relative movement between core-object and fringe, so that core-object rotation with respect to the fringe influences the fibre patterns and produces characteristic asymmetric fibre curvature. Object-centre paths should be used for kinematic analysis of strain fringes instead of single fibres since these paths represent the fringe as a whole. The length along the path can be interpreted in terms of finite strain and path curvature in terms of rigid body rotation of fringes with respect to an external reference frame.