Non‐axial planar cleavage and Caledonian sinistral transpression in eastern Ireland

Transected F 1 fold structures in eastern Ireland are associated with subhorizontal stretching in the S 1 , cleavage whereas axial planar cleavage contains a vertical elongation direction. This suggests that the non‐axial planar cleavage was influenced by a distributed strike‐slip ductile shear. A major NE‐SW trending F 1 syncline is described in which the minor F 1 folds show systematic variations in cleavage transection parameters. On the steep limb of the major syncline the cleavage transects the minor F 1 folds in a consistently clockwise sense, whereas on the normal limb anticlockwise transected folds are seen. Axial planar cleavage occurs at the core of the major syncline. Fold profile analysis indicates that the buckling of the layers began before the initiation of the cleavage. Open, parallel folds at the major synclinal hinge zone are progressively ‘flattened’ on the steep limb towards a major D 1 sinistral transcurrent fault. The angular transection, A, attains a maximum of 15° clockwise which diminishes to <5° at higher strains adjacent to the major fault. Incremental fibre growth in pressure shadows show a two‐stage tectonic strain superposition of vertical pure shear followed by sinistral transcurrent simple shear during the development of the clockwise transecting cleavage. Anticlockwise transected folds were influenced by local dextral strike‐slip on the southern margins of a rigid terrane. As a regional feature, the clockwise transection is explained by a sinistral transpressive deformation of end‐Silurian age.