Chapter 14, 15, Chapter 14, 15, &16&16

Fold Geometry, Fold Geometry, Classification, and MechanicsClassification, and Mechanics

Fold DefinitionFold Definition

►FoldsFolds are wave-like structures that are wave-like structures that result from deformation of bedding, result from deformation of bedding, foliation, or other originally planar foliation, or other originally planar surfaces in rocks.surfaces in rocks.

►Fold can be very broad to tightly Fold can be very broad to tightly compressed.compressed.

►They may occur as isolated folds or They may occur as isolated folds or extensive extensive fold trainsfold trains of different sizes. of different sizes.

Scale of FoldsScale of Folds►MicroscopicMicroscopic – Need magnification – Need magnification►MesoscopicMesoscopic – Occur from hand – Occur from hand

specimens to outcrop scalesspecimens to outcrop scales►MacroscopicMacroscopic – Occur on the map – Occur on the map

scale or largerscale or larger

Anatomy of FoldsAnatomy of Folds

►CrestCrest – The highest point on the – The highest point on the cross section of a foldcross section of a fold

►TroughTrough – The lowest point on the – The lowest point on the cross section of a foldcross section of a fold

►LimbsLimbs – The straighter portion of – The straighter portion of the fold connecting the hinge the fold connecting the hinge zoneszones

►Hinge ZonesHinge Zones – Exhibit the greatest – Exhibit the greatest curvature of a foldcurvature of a fold

Anatomy of FoldsAnatomy of Folds

Anatomy of FoldsAnatomy of Folds

►Hinge LineHinge Line – The line joining – The line joining points of greatest curvature on a points of greatest curvature on a folded surfacefolded surface

►Axial SurfaceAxial Surface – When many hinge – When many hinge lines (from multiple folded lines (from multiple folded surfaces) connect forming a planesurfaces) connect forming a plane

►PlungePlunge – When the fold hinge is – When the fold hinge is inclined to the horizontalinclined to the horizontal

Anatomy of FoldsAnatomy of Folds►WavelengthWavelength – The distance – The distance

between subsequent crests or between subsequent crests or troughstroughs

►AmplitudeAmplitude – Half the distance from – Half the distance from a crest to the subsequent trougha crest to the subsequent trough

►VergenceVergence – The direction of leaning – The direction of leaning of the axial surfaceof the axial surface

Anatomy of FoldsAnatomy of Folds

►First-Order FoldsFirst-Order Folds – The master – The master (largest) fold(largest) fold

►Second-Order FoldsSecond-Order Folds – Smaller folds – Smaller folds on the flanks of first-order foldson the flanks of first-order folds

►Enveloping SurfaceEnveloping Surface – Enables the – Enables the relation of small to large scale foldsrelation of small to large scale folds

Folds and Stereographic ProjectionFolds and Stereographic Projection

Kinds of FoldsKinds of Folds►AnticlineAnticline – A folds that is concave – A folds that is concave

towards older rocks in its centertowards older rocks in its center►AntiformAntiform – A fold that is concave – A fold that is concave

downwarddownward►SynclineSyncline – A folds that is concave – A folds that is concave

towards younger rocks in its centertowards younger rocks in its center►SynformSynform – A folds that is concave – A folds that is concave

upwardupward►DomeDome – An antiform where bedding dips – An antiform where bedding dips

away from the central pointaway from the central point►BasinBasin – A synform where bedding dips – A synform where bedding dips

towards a central pointtowards a central point

Kinds of FoldsKinds of Folds

Kinds of FoldsKinds of Folds

►Cylindrical FoldsCylindrical Folds – Folds where a – Folds where a cylinder can be inscribed inside the cylinder can be inscribed inside the hingehinge

►Noncylindrical FoldsNoncylindrical Folds – Folds where the – Folds where the hinge is not parallelhinge is not parallel

►Sheath FoldsSheath Folds – Where the fold hinge – Where the fold hinge curves within the axial surface; curves within the axial surface; Normally occur in shear zonesNormally occur in shear zones

Fold ClassificationFold Classification

Sheath Fold in Calcsilicate in CanadaSheath Fold in Calcsilicate in Canada

Fold ClassificationFold Classification

Fold ClassificationFold Classification

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

Isoclinal-Recumbant Folds near Walhalla Isoclinal-Recumbant Folds near Walhalla South Carolina with subsequent open South Carolina with subsequent open

foldingfolding

Fold ClassificationFold Classification

►Parallel FoldsParallel Folds – Maintain constant – Maintain constant layer thicknesslayer thickness

►Concentric FoldsConcentric Folds – Parallel folds in – Parallel folds in which the folded surfaces define which the folded surfaces define circular arcscircular arcs

►Ptygmatic FoldsPtygmatic Folds – Have a lobate – Have a lobate shape and look like intestinesshape and look like intestines

►Similar FoldsSimilar Folds – Maintain their shape – Maintain their shape throughout the section and do not throughout the section and do not die out verticallydie out vertically

Fold ClassificationFold Classification

►ChevronChevron and and Kink FoldsKink Folds – Have straight – Have straight limbs and sharp angular hingeslimbs and sharp angular hinges

►Disharmonic FoldsDisharmonic Folds – Shape or – Shape or wavelength changes from one layer to wavelength changes from one layer to anotheranother

►Supratenuous FoldsSupratenuous Folds – The synclines – The synclines are thickened and the anticlines are are thickened and the anticlines are thinnedthinned

Fold ClassificationFold Classification

Examples of some Folds from Tennessee and Examples of some Folds from Tennessee and North CarolinaNorth Carolina

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