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