Flow cleavage in folded beds
Flow cleavage is distinguished from certain allied structures by its lack of banding and the characteristic development of new minerals that exhibit parallel crystallographic orientation. These petrographic features show that flow cleavage is due to recrystallization under differential stress rather than to orienting mechanical movements. Supporting this conclusion are the relations shown by simple folds in originally sandy and shaly beds, the argument being that the deformation of the slaty beds could not create orienting movements of such character and intensity as to produce flow cleavage which is uniform across the crests of the folds and is well developed even, in open folds. In field work where the relations between bedding and flow cleavage or allied structures are used to determine the character of the major folds, the principle that flow cleavage is parallel to the axial planes of the folds is of proven value. Field relations do not accord with certain other principles that have been advocated, such as the principle that foliation is parallel to the axial plane only in slip folds or the principle that axial plane foliation can be distinguished from bedding foliation by means of certain criteria derived from a statistical analysis of the fabric. In complex structures the relations between flow cleavage and bedding may not everywhere agree with the principle stated above. However, where abnormal relations occur they are believed to be due to complicating factors such as the presence of two periods of deformation, and several examples are given to support this viewpoint.