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

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chapter 14 15 16

Chapter 14, 15, &16

Fold Geometry, Classification, and Mechanics

fold definition
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.
scale of folds
Scale of Folds
  • Microscopic – Need magnification
  • Mesoscopic – Occur from hand specimens to outcrop scales
  • Macroscopic – Occur on the map scale or larger
anatomy of folds
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
anatomy of folds2
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
anatomy of folds3
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
anatomy of folds4
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
kinds of folds
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
kinds of folds2
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
fold classification3
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
fold classification4
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