structural folds

Fold axis

the same as the hinge line—the line of maximum curvature on a folded layer

Cylindrical Fold v. Non-Cylindrical Fold

Cylindrical Fold v. Non-Cylindrical Fold

Hinge Line: Straight v. Curved/Bowed

Profile: Constant v. Changes along length

Looks: Like a pipe v.Like a canoe or dome

Common:Ideal, often approximated v. Very common

Anticline, syncline, antiform, synform

axial surface/plane

imaginary plane that passes through the sharpest part of the bend (the "hinge") in all stacked layers.

Axial trace

2D line formed by the intersection of a fold's axial surface with any other surface

Upright v inclined recumbent folds

Type Axial Plane Orientation Symmetry Key Characteristic

Upright Vertical (90–80°) Symmetrical Limbs dip away/toward each other equally

Inclined Tilted (80–10°) Asymmetrical Limbs have different dip angles

Recumbent Horizontal (10–0°) N/A One limb is upside down (inverted)

Over turned fold

Fold wavelength v amplitude

Wavelength Horizontal distance

How "squished" or wide the folds are.

Amplitude Vertical height

How "tall" or intense the deformation is.

Interlimb angle

the angle measured between the two limbs of a fold

Gentle v open v tight v isoclinal folds

Gentle 180° to 120° Very slight bending; limbs are nearly horizontal.

Open 120° to 70° Clear bend, but the limbs are still wide apart.

Tight 30° to 0° Limbs are nearly parallel; indicates intense deformation.

Isoclinal ~0° The limbs are parallel to each other

chevron v concentric folds

Feature Chevron Fold Concentric Fold

Hinge Sharp / Pointed Rounded / Curved

Limbs Straight Curved

Visual Zigzag / Sawtooth Smooth Arch

Parallel v similar folds

Feature: Parallel Fold v Similar Fold

Hinge: Usually rounded v Can be sharp or rounded

Limbs: Same thickness as hinge v Thinner than the hinge

Layer Shape: Top and bottom curves differ v Top and bottom curves are identical

Rock Type: Hard, brittle v Soft, plastic/metamorphic

Harmonic v disharmonic folds

Feature: Harmonic Folds v Disharmonic Folds

Wavelength: Identical across layers v Varies between layers

Appearance: Symmetrical and "neat" v Irregular and "messy"

Rock Consistency: Similar stiffness (competency) v Varying stiffness (competency)

Axial Surface: Continuous through layers v Broken or shifted between layers

fold symmetry/ asymmetry, vergence

Feature: Symmetrical Fold v Asymmetrical / Overturned

Limb Length: Equal v Unequal

Limb Dip: Equal angles v Different angles

Axial Plane: Vertical (90°) v Inclined (Tilted)

Vergence: Neutral (None) v Directed toward the steep limb

paracidic folds, S- and Z- folds

S-Fold v Z-Fold

Visual Shape Resembles the letter "S" Resembles the letter "Z"

Position (Anticline): Found on the Left Limb v Found on the Right Limb

Position (Syncline): Found on the Right Limb v Found on the Left Limb

Rotation Sense: Counter-clockwise (Sinistral) v Clockwise (Dextral)

Vergence: toward the Right v toward the Left

Hinge: main hinge→ Right v main hinge→Left

Sheath, box, kink, ptygmatic folds

Feature	Sheath Fold	Box Fold	Kink Fold	Ptygmatic Fold
Hinge Shape	Rounded & highly curved (cone)	Flat top, angular hinges	Sharp, angular (zig-zag)	Irregular, chaotic, lobate

Monocline

a simple, "one-step" bend in otherwise horizontal or gently dipping rock layers

Buckling (active) v bending (passive)

Feature: Buckling (Active) v Bending (Passive)

Force Direction: Horizontal v Vertical

Layer Role: Layers are active (stiff layers control shape) v Layers are passive (drape over moving blocks)

Common Result: Series of anticlines and synclines v Monoclines or drape folds

Passive folding

the rock layers have no mechanical strength of their own and do not resist the deformation

Flexural Slip v Flexural Flow

Feature: Flexural Slip v Flexural Flow

Mechanism: Sliding along discrete surfaces (bedding planes) v Continuous ductile flow (internal shear) within layers.

Layer Thickness: Remains constant (Parallel/Concentric Folds) v Varies (Thicker hinges, thinner limbs).

Conditions: Lower temperatures and pressures (Brittle-Ductile) v Higher temperatures and pressures (Ductile/Plastic)

Orthogonal flexure

Outer Arc (Top): Stretches (Tension) Cracks or "tension gashes" open up.

Neutral Surface (Middle): No change stays the same length; no stress.

Inner Arc (Bottom): Squashes (Compression) Rock becomes crowded or develops small ripples

Refolded fold

occurs when an existing fold is subjected to a later, second deformation event—usually from a different direction