Oblique-cleavage folds: A critical discussion with examples from the Canadian Appalachians

Abstract

Examples of “transected folds” from three localities in Notre Dame Bay, north-central Newfoundland are described and analyzed. The oblique relationship of cleavage and folds is the result of overprinting of the folds by the cleavage in two of the three localities, and the result of overprinting of the cleavage by the folds in the third locality. Thus, it is argued that “transected folds” need not form synchronously with the oblique cleavage. An axial plane cleavage associated with the folds may be completely obliterated by the oblique cleavage or may never have existed. The term “oblique-cleavage folds” is introduced as a geometric term to describe folds with a cleavage oblique to their hinges. Transected fold models are generally based on the orientation of cleavage and folds with respect to the strain ellipsoids on the scale of the deforming zones or on the scale of layers of variable competence. Here we follow a different approach: transected fold models and the geometry of oblique-cleavage folds are discussed in terms of the progressive development of the folds and cleavage. We assume no special relationships to strain. Oblique-cleavage folds are most commonly reported from shear zones where the strain path is non-coaxial and the deformation is progressive. Such zones produce overprinting relationships even where only one deformation event is involved and commonly have a long history of reactivation. We suggest that the significance of this correlation is that “transected folds” are generally and perhaps always a product of overprinting. The age of folds and cleavage, however, may be very similar and both may be a product of the same protracted deformation event. Thus, in studying oblique-cleavage folds, the possibility that the cleavage is pre-, syn- or post-folding should be considered even where there is only one recognisable generation of both folds and cleavage.