Chapter 14, 15, &16

1. 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