Discuss the geometric relationship between cleavage and folding.

Defining Cleavage and Folding

Cleavage refers to the tendency of a rock to split along closely spaced, parallel planes. These planes represent a preferred orientation of platy minerals (like mica) or the development of microfractures. Cleavage is a type of planar fabric developed due to differential stress. It’s distinct from schistosity, which involves visible, aligned minerals.

Folding is the bending of layered rocks due to compressive stress. Folds are characterized by their limbs (sides), hinge (point of maximum curvature), and axial plane (imaginary plane dividing the fold symmetrically). Folds can be symmetrical, asymmetrical, overturned, or recumbent, depending on the intensity of deformation and the properties of the rock.

Influence of Cleavage on Fold Geometry

Cleavage doesn’t simply *occur* in folded rocks; it often *accompanies* and is *influenced by* the folding process. During folding, rocks undergo both ductile and brittle deformation. Cleavage develops as a result of ductile deformation, specifically the reorientation of minerals and the formation of microfractures in response to stress. The orientation of cleavage planes is directly related to the stress field during folding.

Here’s how cleavage influences fold geometry:

• Axial Planar Cleavage: In many folds, cleavage planes are parallel to the axial plane of the fold. This is particularly common in competent layers surrounded by less competent layers. The axial planar cleavage helps define the fold’s geometry and symmetry.
• Fold Limb Dip: The dip of cleavage planes within the fold limbs often mirrors the dip of the fold limbs themselves. However, cleavage can also be steeper or shallower than the limb dip, depending on the stress regime.
• S-folds and Z-folds: The geometry of cleavage within folds can help determine the sense of shear during deformation. S-folds exhibit cleavage that curves symmetrically around the fold hinge, while Z-folds show a more angular relationship.

Cleavage Orientation and Fold Style

The relationship between cleavage and folding is not always straightforward. The orientation of cleavage relative to the fold axis significantly impacts the fold style:

• Parallel Cleavage: When cleavage is parallel to the axial plane, folds tend to be symmetrical and well-defined. This indicates a relatively homogeneous stress field.
• Oblique Cleavage: If cleavage is oblique to the axial plane, the folds may be asymmetrical or overturned. This suggests a more complex stress field, potentially involving rotation or shear.
• Cross-Cutting Cleavage: In some cases, cleavage can cross-cut the fold limbs, indicating a later phase of deformation that overprinted the earlier folding.

The development of axial planar cleavage is often linked to the presence of fluids during metamorphism. These fluids facilitate mineral reactions and promote the alignment of minerals, leading to the formation of cleavage. The intensity of cleavage development is also influenced by the rock’s composition and the degree of metamorphism.

Cleavage Orientation	Fold Style	Stress Regime
Parallel to Axial Plane	Symmetrical, Well-defined	Homogeneous Compression
Oblique to Axial Plane	Asymmetrical, Overturned	Complex Compression & Shear
Cross-Cutting	Overprinted Deformation	Multiple Deformation Events