Fold Geometry, Classification, and Mechanics 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

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

Folds and Stereographic Projection

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

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

Fold Classification

Sheath Fold in Calcsilicate in Canada

Fold Classification

Fold Classification

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

Fold Classification

Examples of some Folds from Tennessee and North Carolina

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