CLEAVAGE, AXIAL PLANAR
The fabric that develops axial planar to folds lies parallel to the XY principal plane of the finite strain ellipsoid. Normally, the axial planar cleavage is parallel to the hinge (which is generally parallel to Y of fse -but not at all necessarily ) and axial plane in which the direction normal to fold hinge and parallel to the fold amplification is the X of fse. The departures from this do exist. In wrench faulted regions, the Y is normally parallel to fold amplification direction and X parallel to fold hinges. As Flinn has pointed out, the fold hinges gradually rotate towards X of fse during progressive deformation. The folds are developed in Mahakoshal group rocks of quartzites, phyllites, cherts, metabasic rocks,metagraywackes and pelitic schists near Barmhan, Central India. The principal strains are generally systematically related to the fold elements and hence the cleavage which is axial planar to folds also bears a systematic relationship. Unless there is time lag between the development of folds and the development of cleavage, or the rock layers are quite heterogeneous, the cleavage that forms is generally parallel to the axial planes of folds. This implis that the axial planes of folds are parallel to the l1l2 plane of the finite strain ellipsoid. The fold hinges generally parallel the l2 principal strain and therefore folds are always accompanied by a stretching lineation perpendicular or sub perpendicular to fold hinges or that the stretching lineation is contained in the l1l 3 plane.
The cleavage may be a regular schistosity, a slaty cleavage, or a fracture cleavage or in the form of pressure solution spaced cleavage under low grade metamorphic rocks. If the fold hinges rotate towards l1 during late stage of fold development then the lineation may be developed parallel to fold hinges. But generally hinge parallel lineations associated with folds are morphologic types.
The inequant grains may get deformed parallel to l2 strain since this might occur with the expenditure of least work and hence some of the apparently developed mineral lineations could be parallel to fold hinge. THE CLEAVAGE AXIAL PLANAR TO FOLDS IS NOT PARALLEL TO SURFACES OF SHEAR AS GENERALLY BELIEVED. If dislocation of bedding is observed along axial planar cleavage, it only implies that the cleavage may not strictly parallel the l1l2 plane of the finite deformation ellipsoid and therefore some shear may occur along such planes.