Development of early composite cleavage in pelites from West Donegal

Abstract

In the Portnoo-Rosbeg area of west Donegal the main penetrative cleavage, S 2, generally dips to the south with F 2 folds facing up to the north. In places the S 2 cleavage is cut by a gently SW-dipping crenulation cleavage ( S 3) verging and facing south on the long limbs of F 2 folds. A series of structural domains have been mapped in which the relationship of S 2 and S 3 changes from cross-cutting at a large angle (Rosbeg domain) to the development of a composite S 2 3 cleavage (Portnoo domain). The relationship between the two phases and the composite cleavage was investigated by mapping out cleavages (megascopic scale), detailed mesoscopic field observations and on a microscopic scale using textural relationships to widespread post D 2-pre D 3 garnet porphyroblasts. In addition to demonstrating the composite nature of the cleavage, the examples of D 2/ D 3 interference and the rotation of, and drag patterns around, the garnet porphyroblasts allow discussion of the kinematics of D 3. D 3 appears to have involved either bulk pure shear or north-directed bulk simple shear, or any intermediate type of deformation history, and was promoted by southerly directed active slip parallel to S 2.