4. Structural Analysis

In this section we illustrate a method to interpret displacement-controlled fibre patterns in antitaxial strain fringes put forward by Aerden (1996) and Koehn et al. (2000, 2001). In section 3 in this paper we argued that a single fibre should not be used for structural analysis since not all displacement-controlled fibres in a fringe grow in the same direction due to relative rotation of core-object and fringes. Therefore we use the "object-centre path method" to determine a fringe opening path that is independent of relative rotation and uses all fibres in a fringe. This method is illustrated in fig. 11. To determine an object-centre path the core-object is moved relative to each fringe and rotated around its centre in a way that displacement-controlled fibres are always fixed to the same point on the core-object surface. The connection of the position of the core-object centre during this procedure defines the object-centre path. This path can now be used to calculate bulk shear strain of the matrix (Koehn et al., 2001). To illustrate fringe growth we can reorientate fringes and core-object with respect to the external reference frame (shear zone boundary) and develop a movie as shown in fig. 12. In contrast to movies shown in section 3 of this paper where we assumed that fringes did always open parallel to extensional ISA of flow we now assume that they have a constant rotation rate between an inital position at the site of the extensional ISA and their final position. This produces a more realistic looking progressive development of the fringe structure where the opening direction varies relative to the extensional ISA. That this is a more realistic assumption has been shown by Koehn et al. (2000) with analog experiments of progressive fringe development. In their experiments, rotation rates did not change suddenly and the opening direction of fringes was not necessarily parallel to extensional ISA during progressive deformation.