1. Fold Geometry, Classification, and Mechanics
Chapter 14, 15, &16
Fold Geometry, Classification, and Mechanics

2. Fold Definition
Folds are wave-like structures that result from deformation of bedding, foliation, or other originally planar surfaces in rocks.
Fold can be very broad to tightly compressed.
They may occur as isolated folds or extensive fold trains of different sizes.

3. Scale of Folds Microscopic – Need magnification
Mesoscopic – Occur from hand specimens to outcrop scales
Macroscopic – Occur on the map scale or larger

4. Anatomy of Folds
Crest – The highest point on the cross section of a fold
Trough – The lowest point on the cross section of a fold
Limbs – The straighter portion of the fold connecting the hinge zones
Hinge Zones – Exhibit the greatest curvature of a fold

5. Anatomy of Folds

6. Anatomy of Folds
Hinge Line – The line joining points of greatest curvature on a folded surface
Axial Surface – When many hinge lines (from multiple folded surfaces) connect forming a plane
Plunge – When the fold hinge is inclined to the horizontal

7. Anatomy of Folds
Wavelength – The distance between subsequent crests or troughs
Amplitude – Half the distance from a crest to the subsequent trough
Vergence – The direction of leaning of the axial surface

8. Anatomy of Folds First-Order Folds – The master (largest) fold
Second-Order Folds – Smaller folds on the flanks of first-order folds
Enveloping Surface – Enables the relation of small to large scale folds

9. Folds and Stereographic Projection

10. Kinds of Folds
Anticline – A folds that is concave towards older rocks in its center
Antiform – A fold that is concave downward
Syncline – A folds that is concave towards younger rocks in its center
Synform – A folds that is concave upward
Dome – An antiform where bedding dips away from the central point
Basin – A synform where bedding dips towards a central point

11. Kinds of Folds

12. Kinds of Folds
Cylindrical Folds – Folds where a cylinder can be inscribed inside the hinge
Noncylindrical Folds – Folds where the hinge is not parallel
Sheath Folds – Where the fold hinge curves within the axial surface; Normally occur in shear zones

13. Fold Classification

14. Sheath Fold in Calcsilicate in Canada

15. Fold Classification

16. Fold Classification

17. Isoclinal-Recumbant Folds in Amphibollite and Granitic Gneiss near Clemson South Carolina

18. Isoclinal-Recumbant Folds near Walhalla South Carolina with subsequent open folding

19. Fold Classification Parallel Folds – Maintain constant layer thickness
Concentric Folds – Parallel folds in which the folded surfaces define circular arcs
Ptygmatic Folds – Have a lobate shape and look like intestines
Similar Folds – Maintain their shape throughout the section and do not die out vertically

20. Fold Classification
Chevron and Kink Folds – Have straight limbs and sharp angular hinges
Disharmonic Folds – Shape or wavelength changes from one layer to another
Supratenuous Folds – The synclines are thickened and the anticlines are thinned

21. Fold Classification

22. Examples of some Folds from Tennessee and North Carolina

23. Map-scale parallel folds in the Valley and Ridge Province of Pennsylvania