DEFINITION OF FOLD

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

steeply plunging.

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.

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