chapter 14, 15, &16 fold geometry, classification, and mechanics
TRANSCRIPT
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Chapter 14, 15, Chapter 14, 15, &16&16
Fold Geometry, Fold Geometry, Classification, and MechanicsClassification, and Mechanics
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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.
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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
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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
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Anatomy of FoldsAnatomy of Folds
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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
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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
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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
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Folds and Stereographic ProjectionFolds and Stereographic Projection
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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
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Kinds of FoldsKinds of Folds
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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
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Fold ClassificationFold Classification
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Sheath Fold in Calcsilicate in CanadaSheath Fold in Calcsilicate in Canada
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Fold ClassificationFold Classification
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Fold ClassificationFold Classification
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Isoclinal-Recumbant Folds in Amphibollite and Isoclinal-Recumbant Folds in Amphibollite and Granitic Gneiss near Clemson South CarolinaGranitic Gneiss near Clemson South Carolina
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Isoclinal-Recumbant Folds near Walhalla Isoclinal-Recumbant Folds near Walhalla South Carolina with subsequent open South Carolina with subsequent open
foldingfolding
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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
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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
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Fold ClassificationFold Classification
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Examples of some Folds from Tennessee and Examples of some Folds from Tennessee and North CarolinaNorth Carolina
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Map-scale parallel folds in the Valley and Map-scale parallel folds in the Valley and Ridge Province of PennsylvaniaRidge Province of Pennsylvania
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