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Folds Kinematics & Mechanics
Geol342J. Toro
Many diagrams are from Earth Structure, van der Pluijm and Marshak, 2004
Outline• Fold Geometry
– Axial surface– Hinge line
• Map patterns of folds• Fold Attitude• Fold Asymmetry- vergence• Kinematic Models
– Flexural folding• Buckling• Bending• Flexural Slip Folding• Forced folds
– Shear folding• Ductile flow
• Controls on fold wavelength– Thickness– Mechanical contrast
Why do we care about folds? Folded RockSyncline Anticline
3
Fold Attitude•Axial plane
•Hinge
Parallel fold Similar fold
Similar Folds Parallel Folds
Kink Folds (Chevron Folds)
Chert
Fold inter-limb Angle• Isoclinal= 0-10o
• Tight= 10-60o
• Open= 60-120o
• Gentle= 120-179o
STRA
IN
4
Fold Asymmetry-VergenceRelated to sense of shear Vergence
Fold sets: Parasitic folds
Re-folded Folds1
2
5
Origin of Folds
• Usually form by horizontal shortening (but not always!)
• Two main categories– Passive Folds (flow folds)– Flexural Folds (bending and buckling)
Passive Folds
Pollen on water
Passive Folds•No mechanical contrast between layers
•Amplification of irregularities due to differential flow
•Common in rocks at high T
•They can yield useful kinematic information (sense of shear, flow direction, etc)
Shear folds
• Found in metamorphic and igneous rocks• Require the slip of layers parallel to each
other• Like pushing your finger into a deck of
cards• Produce similar folds
Shear folding
Cleavage planes allow fold development
Shear foldingCrystallina fold- Alps
J. Amato©
6
Flexural Folds• Bending: require opposite torques
•Buckling: Compressive stress acting parallel to the layer
Bending- Colorado Plateau Monocline
Detached folds- Jura Mts.
Buckling(actually buckling and bending are both present in most cases)
Strain in a buckled single layer
Flexural Slip•Strata are stacks of multiple layers
•Mechanical properties vary from layer to layer
•Layers are not glued to each other
•Layers can slip relative to each other
•Flexural Slip Folding- > like folding a phone book
•Produces Parallel folds, kink folds
Flexural Slip Folding- multilayer
7
Folded RockKink Folds
Chert
Fault-Bend Folds can only form through flexural slip Fault-
propagation fold
The fault grows as displacement grows
Folding in viscous materialstiff layer in weak matrix
Fold wavelegth vs. viscosity(Biot-Ranberg eq.)
L= 2Πt (η/6 η0)1/3
Wave length
Layer Thickness
Viscosity (layer)
Viscosity (matrix)
8
Single layer folding•Controlled by the viscosity contrast between the layer and the matrix
•Layer more viscous than matrix
η/ η0
42
17.5
5.2Dietrich (1970)
Wavelength-Stiffness Relationship
L= 2Πt (B/6 B0)1/3
B= Elastic Modulus= E/(1-ν)
Stiff layers in weak matrix
No interferenceThick layer dominatesInterference
Wavelength-Thickness relationship
L/t= 27
η/ η0=476
Thickness-Wavelength relationship