Flexural shear in a periclinal fold from the Irish Variscides

Abstract

Periclinal folds occur in a structural culmination in the centre of the Irish Variscan fold belt. The culmination marks a zone of enhanced shortening which is associated with a locally attenuated stratigraphy and an anomalous deformation sequence with respect to the remainder of the orogen. Originally vertical burrows have been used, along with cleavage-fold relations, to study both the three-dimensional evolution of the periclinal folding and the temporal relationship between folding and cleavage. As well as cleavage-forming flattening strains, the burrows record along-strike shear related to the propagation of the periclinal folds. A comparison with strain measurements made outside the culmination zone shows that the Galley Head area suffered higher strains. Integration of field data with published analyses of plasticine modelling of periclinal folds yields models for the propagation of a pericline and for the geometry of its lateral terminations.