1. Announcements Draft #1 of fault paper due next Wed.! Late papers: lose 5% of grade per day.

2. Today: Cleavages and Passive Folding (Davis and Reynolds, pp. 424-456)

3. Cleavage: closely spaced, aligned, planar surfaces; associated with folds- impart a splitting property to the rock- often cuts bedding

4. Cleavages: commonly parallel to axial planes of folds. They cut bedding! "Axial planar cleavage"

5. Cleavages: may fan slightly, but still close approximation for axial surface

7. Sometimes cleavages interact with fissility (tendency for rock to split along bedding plane) to form "pencil" structures

8. Cleavage can be very closely spaced- this is called a continuous cleavage (vs. a spaced cleavage)

9. Cleavage is an expression of variations in mineralogy and fabric (grain shape, size, and configuration)- these variations formed during deformation (secondary) "domainal" nature of cleavage

10. continuous cleavage under a microscope, showing domainal nature

11. Types of continuous cleavage: Purely based on grain size (which increases with higher T) (1) Slaty cleavage (fine-grained) (2) Phyllitic cleavage (med. grained- shiny) (3) Schistosity (coarse grained- can see micas)

12. Slaty cleavage

13. microfolds in mica layers "sheen" appearance phyllitic structure

14. Schistosity: coarser grained fabric

15. older layering can be folded + cut by younger cleavage to form Crenulation Cleavage

16. more... Crenulation Cleavage

17. Crenulation Cleavage at the microscopic scale

18. Strain Significance of Cleavage Relationship with folding: suggests cleavages form as a response to shortening and flattening Form perpendicular to  1 and S3 Confirmed in field and laboratory

19. How does cleavage form?? (1) Mechanical rotation of minerals (contributes a bit) (3) Pressure solution (dominant!!) 2) Directional Recrystallization (contributes a bit)

20. pressure solution "dimples" in conglomerate clasts

21. Formation of crenulation cleavage

22. stylolites- results of pressure solution in limestones

23. Passive Folds: wavelength shortened by pressure dissolution along cleavage planes

24. Passive Folds: generally similar folds- thickening in hinge zone. Intersection line between cleavage and bedding yields trend and plunge of hinge line!

25. Origin of Passive folds

26. TRANSPOSITION Lithologic layering is often mistaken for coherent bedding- it's not! It's layering due to major deformation cleavage/foliation is parallel to lithologic layering; lensoidal layers

27. How bedding gets transposed:

28. the beginning of limb destruction

29. destroy those limbs!

30. A lost limb due to transposition

31. transposition to the max!

32. Cleavage is a "flattening" fabric Pure shear (coaxial) vs. Simple shear (noncoaxial)? What about loss of volume?

33. What is it? What is the closely spaced fabric? Are these similar or parallel folds? Free folds, forced folds, or passive folds? What is direction of S1 (maximum stretch)? axial planar cleavage similar folds passive folds S1 is horizontal

34. Next Lecture: Foliations, lineations, and tectonites Please read (D&R, pp. 456-479; 485-492)

35. Important terminology/concepts cleavage axial planar cleavage continuous cleavage slaty cleavage phyllitic cleavage schistosity crenulation cleavage strain significance of cleavage pressure solution stylolites passive folds transposition flattening (pure shear) fabric volume loss (dilational strain)