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

Abstract

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 (Table 1). This study is illustrated by the analysis of axial directions of isoclinal minor folds cogenetic with the final emplacement of the Aguilón fold nappe in S Spain. The axial directions of these (Ds) folds show a bimodal girdle distribution which is ascribed to their superposition on opposing limbs of the preceding (Dr) folds. Directional statistics suggests that D, folds were of chevron or similar type with interlimb angles of about 65 degrees before they all but disappeared after non-coaxial isoclinal D2 refolding.