Folds.

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Presentation transcript:

Folds

Kink Folds Chert

Parasitic folds

Parasitic Folds s M

Sheet fold in high strain rocks

Faults, folds and shortening

Re-folded Folds

1 2

Fold Asymmetry- Vergence Related to sense of shear

Fold Asymmetry

Fold Asymmetry- Vergence Related to sense of shear

Vergence

Origin of Folds Usually under form by horizontal shortening (but not always!) Two main categories Passive Folds (result of flow) Flexural Folds (bending and buckling)

Passive Folds

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 (or flow) 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

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

Kink Folds Chert

Fault-Bend Folds can only form though flexural slip

Fault-propagation fold The fault grows as displacement grows

Folding in viscous material stiff layer in weak matrix

/ 0 Single layer buckling 42 17.5 5.2 Controlled by the viscosity contrast between the layer and the matrix Layer more viscous than matrix 42 17.5 5.2

Stiff layers in weak matrix Thick layer dominates No interference Interference

Wavelength-Thickness relationship L/t= 27 / 0=476

Thickness-Wavelength relationship