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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

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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. • They may occur as isolated folds or extensive fold trains of different sizes.**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**• 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 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 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 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**• 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 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**Isoclinal-Recumbant Folds in Amphibollite and Granitic**Gneiss near Clemson South Carolina**Isoclinal-Recumbant Folds near Walhalla South Carolina with**subsequent open folding**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 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**Map-scale parallel folds in the Valley and Ridge Province of**Pennsylvania

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