Previous studies have described an unusual inclusion pattern that consists of outwardly opening pairs of concave microfolds, termed ‘clamshells’ by Rosenfeld (1970), ‘millipede microstructure’ by Bell & Rubenach (1980) and ‘oppositely concave microfolds’ by Johnson & Moore (1996). Bell & Johnson (1989) interpreted these microstructures to indicate conditions of progressive inhomogeneous shortening, but subsequent studies have shown that the microstructures can form by any deformation path between bulk coaxial shortening and bulk simple shearing (Gray & Busa, 1994; Johnson & Moore, 1996; Johnson & Bell, 1996). Examples of millipede microstructures can be seen in sections through both the non-rotation and rotation spirals presented in this study (e.g. Fig. 4c,e,f; see also fig. 13 from Gray & Busa, 1994). This further supports the proposal that millipede microstructures represent a peculiar 2D slice through a 3D spiral, and are not an indication of the deformation path or mode of spiral formation.

Another inclusion pattern commonly observed in 2D sections through spirals is one that consists of closed loops of inclusion trails (e.g. Fig. 6b). These have been previously described in both real rocks (e.g. fig. 4 of Johnson, 1993a) and numerical simulations (Gray & Busa, 1994). The shapes of the loops vary from sub-circular (e.g. Fig. 6c,e) to elliptical (Fig. 6b) and crescentic (Fig. 6e), and the loops may be located entirely within the porphyroblast, or straddle the porphyroblast margin. The closed loops result from the non-cylindrical geometry of the spirals, and represent a section through either the ‘sheath fold’ part of the spiral (see above), or alternatively, a section through that part of the matrix foliation that had wrapped around the sphere prior to being overgrown. These closed loops are observed in any section oriented away from the XZ plane, but are best observed in sections cut parallel to the spiral axis.

The changing 2D inclusion trail geometries encountered with sections cut at progressively greater distances from the spiral centre can be seen in Fig. 6. In the XZ sections (Fig. 6a,d), the amount of apparent relative rotation between sphere and matrix decreases steadily in sections cut further from the centre of the sphere. This is of significance for studies that seek to measure the orientation of inclusion trails within a population of porphyroblasts, or correlate sections of inclusion trail between different porphyroblasts on the basis of orientation. In XY sections (Fig. 6c,f), the central section contains elliptical and crescent-shaped closed loop geometries, and these loops change in size as sections are cut at progressively greater distances from the sphere centre. Close to the sphere margin, the closed loops form circular shapes, representing a section through matrix foliation (and equivalent foliation included within the sphere) that has wrapped around the growing sphere. In YZ sections (Fig. 6g,e), the central sections contain near-symmetrical closed loops. With increasing distance from the sphere centre (toward positive X values), the upper loops change shape from elliptical to circular, while the lower loops remain elliptical. With increasing distance toward negative X values, the lower loops change shape from elliptical to circular, while the upper loops remain elliptical. The circular loops represent intersection of the narrow ‘sheath fold’ parts of the spiral, while the elliptical loops represent intersection of the more open parts of the spiral, further from the leading tip of the sheath fold.