1. Folds con’t

2. Review session for final exam
Thursday afternoon?
Thursday evening? 7:30 -???
Friday morning?

3. Kinematics of folding- outline
Active folds:
Flexural folds
Neutral surface folds
Passive folds:
Shear folds
Interpreting folds:

4. Kinematics of folding Inner workings of folds and strain patterns:
Three main models:
-Flexural folding
-Neutral-surface folding
-Shear folding (passive)

5. Kinematics of folding Two end-member ways to produce active folds:
Both produce parallel folds

6. Kinematics of folding- Flexural slip
Flexural folding occurs where there is significant mechanical influence of layering in the rock.
Flexural slip folding accommodates folding by layer-parallel slip between layers.
The layers actively participate in folding by bending and flexing.

7. Kinematics of folding- Flexural slip
slip between layers.
No strain (shear) at fold hinge
Strain (shear) occurs on fold limbs

8. (individual layers do not thin or thicken)
Kinematics of folding- Flexural slip
Inner workings of folds and strain patterns:
Parallel fold
(individual layers do not thin or thicken)
Are flexural slip folds Cylindrical?
What class of fold are flexural slip folds?
What kind of rocks do you think make flexural slip folds?

9. Kinematics of folding- Flexural slip
If all of the strain is in the limbs, what would strain pattern look like?
Kink and Chevron folds form via flexural slip

10. Kinematics of folding- Neutral Surface folding
Neutral surface folds:
Strain within hinge zone-
extension in outer arc
contraction in inner arc
neutral surface in between
Note: Neutral surface does not have to be in the middle of the layer
Are Neutral surface folds Cylindrical?
What class of fold are neutral surface folds?
What kind of rocks do you think make neutral surface folds?

11. Kinematics of folding- Neutral Surface folding
Inner workings of folds and strain patterns:
What type of structures would you expect to see on the outer arc?
On the inner arc?

12. Inner-arc compression
Kinematics of folding- Neutral Surface folding
Outer-arc extension
&
Inner-arc compression

13. Kinematics of folding: Outer- arc extension
Possible structures w/in outer-arc extension.
Boudinage: Pinch-n-swell structure
Folding of layers of different stiffness or competency, the competent layers will responding differently to the soft layers. Stiff layers resist deflection.
Layer parallel stretching

14. Kinematics of folding: Outer- arc extension
Boudinaged stiff-layer that is subsequently rotated & extended.
Common stiff-soft pairs
sandstone-shale
limestone-shale
chert-shale
sandstone-limestone
dolomite-limestone
gneiss-schist
calcsilicate-marble

15. More boudinage examples

16. Review session: Thursday, Dec. 1, 7:30 pm Lind 103
Final, Thursday Dec. 8, 10:00 am

17. Kinematics of folding- outline
Active folds: What do we mean by active folds?
-What are the two kinematic styles of active folds?
-Where is the strain (shear) accomodated in each style?
Passive folds:
Shear folds
Interpreting folds:

18. Kinematics of folding-Shear folding (Passive folds)
Layer has no mechanical significance=> passive fold

19. Kinematics of folding-Shear folding (Passive folds)
Common in rocks at high temperatures
Can yield useful kinematic information (sense of shear, flow direction, etc)

20. Kinematics of folding-Shear folding (Passive folds)
Passive shear folding is favored when the mechanical influence of layering in a rock is very weak.
Little to no mechanical contrast between the layers.
The layer is passively carried along.

21. Kinematics of folding-Shear folding (Passive folds)
Found in igneous and metamorphic rocks
Requires slip (or flow) along planes parallel to axial plane
Like pushing your finger into a deck of cards
Produce similar folds

22. Kinematics of folding-Shear folding (Passive folds)
Produce similar folds (thinning and thickening of individual layers)

23. Combinations of fold kinematics
What is the evidence of flexural folding?
What is the evidence of neutral surface folding?

24. Combinations of fold kinematics
Fractures open outward at fold hinge
Extensional fractures end at sliding surface on fold limb

25. Interpreting folds
Layer thickness and rheology: both define fold wavelength
What color strata are weakest? Light or Dark?

26. Interpreting folds Parasitic fold:
Felsic dike within diorite. Which is more “competent” (viscous)
How many deformation “events” occurred?

27. Interpreting folds Chevron fold:
Where did slip occur? (hinge or limb?)

28. Interpreting folds
Flexural-slip folding is enhanced by low cohesion between bedding layers – e.g., well bedded strata such as cherts & limestones.

29. Interpreting folds If there is no fault on this mountain:
How was shortening accomodated
-in rocks on the left side of the mountain?
- In rocks on the right side of the mountain?

30. Interpreting folds If there is no fault on this mountain,
What can you say about
-the relative competence of the cliff-forming layers on the left?
-the relative competence of the thinly bedded units on the right