Fold Geometry, Classification, and Mechanics

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Chapter 14, 15, 16

• Fold Geometry, Classification, and Mechanics

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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.

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Scale of Folds

• Microscopic Need magnification
• Mesoscopic Occur from hand specimens to outcrop
  scales
• Macroscopic Occur on the map scale or larger

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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

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Anatomy of Folds
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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

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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

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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

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Folds and Stereographic Projection
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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

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Kinds of Folds
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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

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Fold Classification
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Sheath Fold in Calcsilicate in Canada
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Fold Classification
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Fold Classification
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Isoclinal-Recumbant Folds in Amphibollite and
Granitic Gneiss near Clemson South Carolina
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Isoclinal-Recumbant Folds near Walhalla South
Carolina with subsequent open folding
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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

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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

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Fold Classification
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Examples of some Folds from Tennessee and North
Carolina
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Map-scale parallel folds in the Valley and Ridge
Province of Pennsylvania
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