Curves or bends in a planar structure such as bedding,
foliation, or cleavage, and one of the most common tectonic
structures in the crust of the Earth. They generally form by
buckling or bending processes in lithified rocks, but some
may also form during slumping of soft or wet sediments
before they are lithified. The geometry of folds depends on
the rheology of the rock, rheological or competence contrasts
between layers, and the conditions of deformation, including
temperature, pressure, and strain rate.
Folds are described using their orientation, shape, and
type of layering being folded. Antiforms are downward closing
structures, and synforms are upward closing structures.
To portray the geometry of a folded sequence, the shape of a
single folded surface needs to be described, as does the form
of a sequence of folded layers, the form of series of folds
involving a single surface, and the shape of a single folded
layer in profile. The fold hinge line is the line of maximum
curvature, the limb is the area between hinges, and the fold
axis is the imaginary line that when moved parallel to itself
generates a cylindrical surface parallel to the folded layer. The
fold axial surface (or plane) is the surface containing the
hinge lines of successive folds in a folded series of layers.
Upright folds have steeply dipping axial surfaces, whereas
gently inclined to recumbent folds have shallowly dipping
axial surfaces. The plunge of the hinge line determines
whether folds are further classified as gently, moderately, or
A fold train is a group of linked folds of comparable
dimensions. It is described by imaginary surfaces called
enveloping surfaces that are the limiting surface between
which the fold oscillates. A median surface passes through
the inflection lines between successive folds. The fold order
describes the wavelengths of folds, with the largest being first
order, second largest being the second order, etc. A fold system
may contain several superimposed fold trains of different
dimensions, resulting in a complex pattern of upward closing
and downward closing fold structures.
Historically, folds have been described around two geometric
models, the parallel fold and the similar fold. In a parallel
fold the thickness measured orthogonally across the
layers is constant throughout the fold, whereas the similar
fold shows considerable variation in layer thickness, with
thinning on the limbs. Concentric folds are parallel folds with
nearly constant curvature and circular boundary layers, and
these are common in high structural levels of fold belts. Tightness
is a measure of the interlimb angle between the tangents
of folded surfaces measured at the inflexion points. Gentle
folds have interlimb angles of 180–120°, open folds have
angles of 120–70°, close folds have 70–30°, and tight and isoclinal
folds have interlimb angles between 30–10–0°. Elasticas
are unusual folds with wide hinge zones and interlimb angles
of less than 0°. The fold shape can be further described by
characterizing the hinge shape as rounded, angular, or very
angular, and the limb shape as planar or curved.
Many folds are asymmetric and appear to be tilted in
one direction. Symmetric folds have their axial planes as axes
of symmetry and have axial surfaces perpendicular to
enveloping surfaces, whereas asymmetric folds do not. The
asymmetry can appear to be either “S” or “Z” shaped when
viewed in the direction of plunge. A fold’s vergence is the
direction toward which it is overturned and can be sinistral
(S) or dextral (Z), or overturned toward a specific geographic
direction. Asymmetric folds form in several ways. They often
form as minor folds on the limbs of major folds and verge
toward the antiformal axial surface, and away from synformal
axes. Knowing a fold’s vergence can help mapping locations
of larger antiforms and synforms. Asymmetric folds
also form in shear zones and during regional tectonic transport,
and their vergence can often be used to determine the
direction of tectonic movement.
Fold facing is the direction perpendicular to the fold axis
along the axial plane, toward the younger beds. Cleavage facing
is the direction normal to the bedding plane intersection
along the cleavage plane and toward the younger beds, and
fault facing is the direction normal to the bedding plane intersection
along the fault plane and toward the younger beds.
Proper analysis and use of facing data in the field can help
locate regions of major fold culminations, determine whether
specific units were inverted or right way up prior to a late
folding event, aid in the description and analysis of fold
geometry, locate regions of refolding, and indicate the direction
of tectonic transport.
Many geologic terrains have experienced multiple folding
events, and the rocks show complex patterns reflecting
this complex refolding. A generation is a group of structures
believed to occupy the same position on a relative timescale.
The correlation of generations is similar to stratigraphy, in
that small-scale structures are used to indicate large-scale
structures. Fold style and orientation are not particularly reliable
criteria for identifying folds of common origin, and their
use may lead to misinterpretation of both geometry and history.
Fold generations can be meaningfully defined in terms
of overprinting where one fold can be observed to fold geometric
elements of earlier fold generations. Where overprinting
relationships are not observable, folds should be grouped
into style groups instead of generations.
Two main kinds of fold generations are distinguished.
The first may be from two differently oriented strains, separated
widely or closely in time, and the second may form during
one continuous deformation but reflect a change in the
material properties through deformation and metamorphism.
In the absence of overprinting relationships, fold generations
can be correlated using the trend and plunge of hinge lines
and the strike and dip of axial surfaces. However, the map
projection of hinge lines (fold axes) should not be used for
correlations because two very different fold groups can have
the same axis orientation. In addition, second generation
folds can have different orientations depending on which
limb of the original folds they developed on, due to original
asymmetry. Furthermore, variations can develop depending
on the rock type the folds are developed in.
The correlation of different generations by fold style is
often done using the fold profile shape, details of the shape
(such as limb tightness), and the type of any axial planar fabric
such as cleavage. In many orogenic belts worldwide, fold
style changes regularly with different generations. First-generation
structures include primary layering (sedimentary beds,
igneous layers). Second-generation structures include early
folds that are typically tight to isoclinal and exhibit a slaty
cleavage or schistosity. Third and later generations are typically
more open, have more pointed hinges, a spaced cleavage,
or an axial planar cleavage. Crenulations are almost
never first-generation structures, because to get crenulations a
pre-fold schistosity needs to be present. Kink folds, or
chevron folds, have no axial planar foliation.
Folds of similar wavelength and different orientations
may interfere in complex ways, forming fold interference patterns.
Interpretation of fold interference patterns is typically
based on two-dimensional outcrop patterns of refolded units,
but accurate interpretation of the geometries of all fold generations
requires a three-dimensional analysis. The style of
early folds and the angular relationships between fold hinge
lines and axial surfaces determines the outcrop patterns of
the superimposed folds. Fold interference patterns are classified
into several types, which form through different angular
relationships between axes of first folds, poles to axial planes
of first and second folds, and the “shearing direction” of second
folds. Type I interference produces basins and domes,
Type II patterns form crescents, and Type III patterns are represented
by hooks. To properly interpret refold patterns, it is
necessary to measure and analyze a large number of orientation
data (fold hinges, axial surfaces) from carefully selected
map domains. A caution, however, is needed, as in constric
tional strain fields (such as those found in regions between
several diapirs) shortening may occur in all directions, and
structural forms resembling those produced by successive
overprinting may be produced.
See also CLEAVAGE; DEFORMATION OF ROCKS; OROGENY;
STRAIN ANALYSIS; STRUCTURAL GEOLOGY.
footwall See FAULT.