Directional variations of fold axes in progressive deformation, an example from a Betic fold nappe in S Spain.

Singly folded and 'coaxially' refolded rocks generally have axial directions that plot as unimodal clusters and either small or great circle girdles. Cluster distributions of axes are characteristic of either cylindrical folds or sheath folds developed at high shear strains. Girdle distributions may occur due to conical folding or sheath folds developed at intermediate shear strains. A variety of axial distributions can occur in 'non-coaxially' refolded regions, since the variation of the axes of folds due to refolding depends on the initial interlimb angle and the angle between the older and younger axial planes. Characteristic fold axial distributions are summarised in a practical chart for general application. This study is illustated by the analysis of axial directions of isoclinal minor folds cogenetic with the final emplacement of the Aguilon fold nappe in S Spain.-from Author Dept. of Mineral. & Petrol., Inst. of Geol., Univ. of Uppsala, Box 555, S-75122 Uppsala, Sweden.