8/13/2019 Fold - Geometry, Classification & Mechanics
1/24
Chapter 14, 15, &16
Fold Geometry, Classification,
and Mechanics
8/13/2019 Fold - Geometry, Classification & Mechanics
2/24
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.
8/13/2019 Fold - Geometry, Classification & Mechanics
3/24
Scale of Folds Microscopic Need magnification Mesoscopic Occur from hand
specimens to outcrop scales
Macroscopic
Occur on the map scaleor larger
8/13/2019 Fold - Geometry, Classification & Mechanics
4/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
5/24
Anatomy of Folds
8/13/2019 Fold - Geometry, Classification & Mechanics
6/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
7/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
8/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
9/24
Folds and Stereographic Projection
8/13/2019 Fold - Geometry, Classification & Mechanics
10/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
11/24
Kinds of Folds
8/13/2019 Fold - Geometry, Classification & Mechanics
12/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
13/24
Fold Classification
8/13/2019 Fold - Geometry, Classification & Mechanics
14/24
Sheath Fold in Calcsilicate in Canada
8/13/2019 Fold - Geometry, Classification & Mechanics
15/24
Fold Classification
8/13/2019 Fold - Geometry, Classification & Mechanics
16/24
Fold Classification
8/13/2019 Fold - Geometry, Classification & Mechanics
17/24
Isoclinal-Recumbant Folds in Amphibollite andGranitic Gneiss near Clemson South Carolina
8/13/2019 Fold - Geometry, Classification & Mechanics
18/24
Isoclinal-Recumbant Folds near Walhalla SouthCarolina with subsequent open folding
8/13/2019 Fold - Geometry, Classification & Mechanics
19/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
20/24
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
8/13/2019 Fold - Geometry, Classification & Mechanics
21/24
Fold Classification
8/13/2019 Fold - Geometry, Classification & Mechanics
22/24
Examples of some Folds from Tennessee and NorthCarolina
8/13/2019 Fold - Geometry, Classification & Mechanics
23/24
Map-scale parallel folds in the Valley and RidgeProvince of Pennsylvania
8/13/2019 Fold - Geometry, Classification & Mechanics
24/24