fold - geometry, classification & mechanics

8/13/2019 Fold - Geometry, Classification & 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 resultfrom deformation of bedding, foliation, orother originally planar surfaces in rocks.

Fold can be very broad to tightlycompressed.

They may occur as isolated folds orextensive 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 scaleor 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 pointsof greatest curvature on a foldedsurface

Axial Surface When many hingelines (from multiple foldedsurfaces) connect forming a plane

Plunge

When the fold hinge isinclined 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 concavetowards 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 cylindercan be inscribed inside the hinge

Noncylindrical Folds Folds where the hingeis not parallel

Sheath Folds Where the fold hinge curveswithin the axial surface; Normally occur inshear 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 andGranitic Gneiss near Clemson South Carolina


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Isoclinal-Recumbant Folds near Walhalla SouthCarolina 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 circulararcs

Ptygmatic Folds Have a lobate shapeand look like intestines

Similar Folds Maintain their shapethroughout the section and do not dieout vertically


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Fold Classification

Chevron and Kink Folds Have straightlimbs and sharp angular hinges

Disharmonic Folds Shape or wavelengthchanges from one layer to another

Supratenuous Folds The synclines arethickened and the anticlines are thinned


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Fold Classification


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Examples of some Folds from Tennessee and NorthCarolina


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Map-scale parallel folds in the Valley and RidgeProvince of Pennsylvania


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fold - geometry, classification & mechanics

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