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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

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Map Patterns of Folded rocks Sheep Mountain-Wyoming

Western PA

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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

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Fold Asymmetry-VergenceRelated to sense of shear Vergence

Fold sets: Parasitic folds

Re-folded Folds1

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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©

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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

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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)

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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