1. Geology 3120 - Folding Mechanisms

2. Outline Decollement Buckle Folds Passive (a.k.a. Free) Folds
Kink Folds
Shear Folds
Flexural Slip
Sheath Folds

3. Decollement Buckle Fold
Zagros salt folds
Zagros salt folds
Zagros salt folds

4. Passive-Free-Buckle Foldsm2 m2 m1
If m1 > m2

5. Passive, or Free Folds

6. Mullions

7. Mullions

8. Multi-Layer Folds

9. Layer thickness and rheology define fold wavelength
Layer thickness and rheology both define fold wavelength
What color strata are weakest? Light or Dark?

10. Multi-Layer Folds

11. Parasitic Folds

12. Parasitic Folds

13. Shear Folds

14. Shear Folds

15. Kink Folds (photograph Taiwan kink-bands)

16. Kink Folds (common in shales, slates)

17. Flexural Slip

18. Vergence

19. Flexural Flow

20. Flexural Flow

21. Flexural Flow (aka Karl’s favorite rock!)
Fractures open outward
Extensional fractures end at sliding surface
Offset fracture shows vergence

22. Tangential Longitudinal Strain

23. Tangential Longitudinal Strain

24. Tangential Longitudinal Strain

27. Sheath Folds High Shear Strain Folds (i.e. shear zones)
Sheath, or Tongue Shaped
Stretching Lineations parallel to fold axis’

28. Sheath Folds Sheath Fold in Calcite Marble
High Shear Strain Folds (i.e. shear zones)
Sheath, or Tongue Shaped
Stretching Lineations parallel to fold axis’
Sheath Fold in Calcite Marble

29. Examples
This is a parasitic fold. Probably a felsic intrusive in a intermediate igneous rock. Think about viscosity and how many deformation events took place.

30. Examples
This is a folded vein o f granite in gneiss from scotland. Fold hinges are not cyllidndrical as the fold hinges die off toward the sea.

31. Examples
This is a chevron fold, very tight hinge and straight limbs. Layer parallel slip occurs probably.

32. In class stunts
Use Fig Newtons as an analgue to illustrate extension on outer arc of fold;
Wear two old shirts, wrap with rubber band and cut off sheath fold