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