The role of pressure solution and intermicrolithon-slip in the development of disjunctive cleavage domains: a study from Helvick Head in the Irish Variscides

Abstract

The offset of sedimentary laminae, bedding surfaces, veins and other passive markers across disjunctive cleavage domains has commonly been attributed solely to pressure solution. A detailed study of disjunctive cleavage within siltstones and sandstones from Helvick Head, Co. Waterford, on the eastern margin of the Irish Variscides indicates that the cleavage domains were initially formed by pressure solution processes. However, the study also revealed features which cannot be explained by pressure solution alone. The cleavage domains represent planes of weakness within otherwise relatively homogeneous rock. During fold tightening they were rotated out of parallelism with the principal ( XY) plane of the finite strain ellipsoid, thus enabling shear movement between the microlithons. It is considered that this intermicrolithon-slip caused a rotation of the elongate quartz grains within the cleavage domains resulting in an asymmetry of their long axes with respect to the cleavage domain margin, grain microfracturing and occasionally slickensides on cleavage surfaces. Intermicrolithon-slip is unlikely to be a localized phenomenon but is probably widespread throughout the Irish Variscides and other orogenic belts.