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 sl ate during non-coaxial progressive deformation. Development of these fringe s was modelled using a computer program 'Fringe Growth 2.0' which can simul ate incremental growth of crystal fibres around core-objects of variable sh ape. It uses object-centre paths as input, which are obtained from fibre pa tterns in thin section. The numerical experiments produced fibre patterns t hat show complex intergrowth of displacement-controlled, face-controlled an d intermediate fibres similar to those in the natural examples. The directi on of displacement-controlled growth is only dependent on the relative move ment between core-object and fringe, so that core-object rotation with resp ect to the fringe influences the fibre patterns and produces characteristic asymmetric fibre curvature. Object-centre paths should be used for kinemat ic analysis of strain fringes instead of single fibres since these paths re present 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 rotati on of fringes with respect to an external reference frame.