The development of a single cleavage in an area of repeated folding

Structural investigations in the Variscan fold interference pattern of SW Sardinia (Italy) document an intriguing example of the relationship between cleavage, folding and finite strain. Although Palaeozoic sediments were affected by two major folding events, both under identical low-grade metamorphic conditions, only a single penetrative cleavage formed, which is not unambiguously correlated with either of these events. Cleavage orientation changes smoothly along sigmoidally curving trajectories throughout the interference pattern. Thus, the same cleavage can be axial planar to either the first or the second phase folds, or transect both when curving from one set into the other. The singularity of the cleavage is supported by microscopic studies, which show no evidence of a pre-existing or overprinting tectonic cleavage. Finite strain determinations and numerous field observations demonstrate that cleavage always parallels the principal plane of finite strain, even though local strain varies according to the relative position in the fold interference pattern. Similar strain patterns were achieved by forward modelling of superposed buckle folds. Based on our findings we propose (i) that the finite strain pattern is due to the superposition of the heterogeneous strain fields associated with the individual folding events and (ii) that cleavage reflects the cumulative deformations that effected the rock. This has consequences for the regional interpretation of cleavage in our study area and elsewhere, and for the distinction between deformation phases in general.