Rock Folds: Formation, Types and Geometry

• Folds are bends/flexures in layered rock, responding to motion along faults, diapirism, compaction, ductile flow, and/or regional subsidence/uplift.
• Folds are expressed as dipping features corresponding to inclined foliations, such as stratigraphic bedding.
• Ductile folds form under high temperature and pressure conditions in meta-greywacke areas like New Zealand. 


Fold Terminology

• Fold Limb (dip domain): Area where rocks have a relatively constant dip.
• Fold Axis/Hinge: Location of maximum curvature that separates limbs.
• Hinge Line: Line of maximum curvature on a fold, parallel to the fold axis in cylindrical folds.
• Axial Surface: Plane connecting hinge lines for different layers, separating regions of different dip.

Cylindrical vs. Non-Cylindrical Folds

• Cylindrical Fold: Describable geometrically by passing a line (fold hinge line) parallel to itself through space without rotation/distortion.
• Non-Cylindrical Fold: Rotation/distortion of a line is necessary to trace the fold's form.

Fold Tightness

• Unfolded: 180° interlimb angle
• Gentle: 170-180° interlimb angle
• Open: 90-170° interlimb angle
• Tight: 10-90° interlimb angle
• Isoclinal: 0-10° interlimb angle

Fold Curvature

• Curved-Hinge Folds include Circular and Elliptical types.
• Angular-Hinge Folds include Chevron and Box types.
• An example of folding under moderately-low pressure and temperature conditions in interbedded shales and limestones can be seen in Kaikoura, New Zealand, demonstrating Angular Chevron Folds.
• Kink Bands: Parallel folding with parallel axial surfaces in well-bedded rocks which are thin with high contrast layers.

Intersection of Axial Surfaces

• Axial surfaces converge, forming a new axial surface orientation that bisects the angle when layer thickness is maintained, typically in map-scale structures under low to moderate pressure/temperature.

Characterizing Folds

• Characterization requires defining axial surface and hinge line orientation to uniquely describe folds.
• Classification by Dip: Folds are classified by the dip of the hinge and axial surface.
• Fold are classified as Upright, Inclined, or Recumbent, and Horizontal or Plunging.

Ramsay Classification

• Dip Isogons: Lines connecting equal dip points between top and bottom layers, used to classify fold type.
• Class 1: Inner curvature is greater than outer curvature.
• Class 1A folds are rare.
• Class 1B folds are common in low-temperature folding dominated by flexural slip-parallel layers.
• Class 1C folds are common in competent layers deformed at high temperatures.
• Class 2: Inner curvature equals outer curvature.
• Class 2 folds are common in rocks with low ductility contrasts folded by passive amplification-parallel isogons.
• Class 3: Outer curvature is greater than inner curvature, common in weak layers deformed at high temperature.

Fold Mechanics

• Layer-Active Folding includes buckling and bending.
• Layer-Passive Folding includes passive amplification.

Harmonic vs. Disharmonic Folding

• Harmonic Folding: No (or very gradual) changes in fold shape between successive layers, "in phase".
• Disharmonic Folding: Pronounced changes in fold shape between successive layers.
• The Canadian Rockies exhibit an example of harmonic folding.
• The Barbastro Anticline in Aragon, Spain shows disharmonic folds in layered carbonates and evaporites in its core.

Parasitic Folds

• Parasitic/Subsidiary Folds: Minor folds developing on a larger fold's flank, reflecting the fold limb's overall shear sense.
• Dextral "Z" folds and Sinistral "S" folds are examples of Parasitic folds.
• The asymmetry of parasitic folds points toward the crest of the larger fold.

Anticlines and Synclines

• Anticline: Concave downward, with the oldest rocks in the center.
• Syncline: Concave upward, with the youngest rocks in the center.

Explore the processes, conditions, and classifications related to folds in rock layers. Learn about axial surfaces, fold limbs, cylindrical vs. non-cylindrical folds and Ramsay classification. Understand the geometry of folds.