Detecting cleavage-transected folds using cleavage-bedding intersections

Cleavage-transected folds, those with a non-axial planar cleavage, often have a cleavage—bedding intersection (cbi) that does not parallel the fold axis. The variation in attitude of the cbi around a fold is shown to be a sensitive indicator of even small amounts of axial transection in folds. The change in cbi plunge across a fold hinge is a particularly useful criterion for field recognition of transection in folds on a scale larger than the available outcrop. The patterns of cbi are first investigated theoretically for cylindrical folds showing only axial transection, that is with no apparent transection in the profile plane. More complicated folds involving cleavage fanning and refraction are then considered. The use of cbi variation is especially sensitive for tight folds and for those having divergent cleavage fans. The technique is valid only where cleavage faces sympathetically with the fold-facing direction throughout the fold.