Transected folds: A study illustrated with examples from Canada and Scotland

Transected folds have contemporaneous cleavage that is not parallel to their axial surface, but cuts through the axial surface and both limbs with the same sense. Such folds can develop with perfectly synchronous formation of folds and cleavage. This may occur in rocks undergoing a bulk coaxial strain history or in an approximately coaxial phase of a more complex strain history in which the strain axes do rotate relative to the deforming rocks. In a noncoaxial strain history, transected folds may also arise by a different mechanism, in which cleavage formation is slightly delayed relative to fold formation. This may be achieved where the grain-shaping (cleavage-forming), intragranular deformation mechanisms are initially suppressed because rock flow is more easily accommodated by intergranular movement. The temporary local volume increases required by this “grain-boundary sliding” may occur during de-watering.

Transected folds may readily occur and have already been described in many areas. Therefore, field mapping techniques assuming a special geometrical relationship between folds and cleavage should be used only where the degree, or absence, of transection of folds by their contemporaneous cleavage has been established.