Development of early composite cleavage in pelites from West Donegal

In the Portnoo-Rosbeg area of west Donegal the main penetrative cleavage, S₂, generally dips to the south with F₂ folds facing up to the north. In places the S₂ cleavage is cut by a gently SW-dipping crenulation cleavage (S₃) verging and facing south on the long limbs of F₂ folds.A series of structural domains have been mapped in which the relationship of S₂ and S₃ changes from cross-cutting at a large angle (Rosbeg domain) to the development of a composite $\text{S}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ 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 D₂-pre D₃ garnet porphyroblasts.In addition to demonstrating the composite nature of the cleavage, the examples of D₂/D₃ interference and the rotation of, and drag patterns around, the garnet porphyroblasts allow discussion of the kinematics of D₃. D₃ 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 S₂.