Origin of Cleavage and Schistosity 1. Slaty cleavage and Schistosity Cleavage is the result of rock flowage. Rock is shortened at right angles to the cleavage but lengthened parallel to it. Cleavage results from parallelism of platy and ellipsoidal grains. Cleavage forms perpendicular to the least strain axis that is, at right angles to the greatest principal stress axis. Rotation of platy and ellipsoidal grains. The greater the deformation, the more such grains will tend to lie perpendicular to least strain axis. Flattening of grains. A spherical quartz grain will be flattened, partly by granulation, partly by recrystallization. Any new platy minerals will crystallize with their flat faces perpendicular to the greatest principal stress axis.

Non foliated – no preferred direction Foliated – preferred direction of minerals or banding

High Pressure

Foliation resulting from directed stress

Mudstone devoid of cleavage from Wales; Clay flakes mainly chlorite with random arrangement Ill-cleaved mudstone from Wales; Planar structure developing Highly cleaved slate from Wales; Closely spaced layers with thin lenticels of randomly arranged flakes Cleavage in Slate

Origin of Cleavage and Schistosity 2. Fracture cleavage Fracture cleavage is a shear phenomenon that obeys the laws of shear fracture. Consequently, fracture cleavage is inclined to the greatest principal stress axis at an angle of about 30 degrees.

Origin of Cleavage and Schistosity 3. Shear cleavage Shear cleavage is a fracture cleavage along which displacements have taken place.

Origin of Cleavage and Schistosity 4. Slip cleavage Slip cleavage is associated with small crinkles. The shorter limbs of the crinkles become so stretched that they become planes of weakness. A compressive force perpendicular to the plane normal to the schistosity may cause the crinkles. In most cases a couple acting parallel to the schistosity causes asymmetrical crinkles.

Development of crenulation cleavage if enough shortening occurs, fold limbs are completely removed

Progressive development (a to c) of a crenulation cleavage for both asymmetric (top) and symmetric (bottom) situations. From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Symmetric crenulation cleavage Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag.

Asymmetric crenulation cleavage Asymmetric crenulation cleavages in mica-quartz-rich schist. Note horizontal compositional layering (relict bedding) and preferential dissolution of quartz from one limb of the folds. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag.

Origin of Cleavage and Schistosity 5. Bedding cleavage Isoclinal folding. Mimetic recrystallization. Flow parallel to bedding. Load metamorphism.