1. Folds

2. Kink Folds
Chert

5. Parasitic folds

7. Parasitic Folds s M

9. Sheet fold in high strain rocks

10. Faults, folds and shortening

11. Re-folded Folds

13. 1 2

15. Fold Asymmetry- Vergence
Related to sense of shear

16. Fold Asymmetry

19. Fold Asymmetry- Vergence
Related to sense of shear

20. Vergence

21. 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)

22. Passive Folds

23. 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)

24. 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

25. Shear folding
Cleavage planes allow fold development

27. Flexural Folds Bending: require opposite torques
Buckling: Compressive stress acting parallel to the layer

28. Bending- Colorado Plateau Monocline

29. Detached folds- Jura Mts.
Buckling
(actually buckling and bending are both present in most cases)

30. Strain in a buckled single layer

31. 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

32. Flexural Slip Folding- multilayer

33. Kink Folds
Chert

34. Fault-Bend Folds can only form though flexural slip

35. Fault-propagation fold
The fault grows as displacement grows

36. Folding in viscous material stiff layer in weak matrix

37. / 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

38. Stiff layers in weak matrix
Thick layer dominates
No interference
Interference

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

40. Thickness-Wavelength relationship