A geological fold occurs when one or a
stack of originally flat and planar
surfaces, such as sedimentary strata,
are bent or curved as a result of
permanent deformation. Synsedimentary
folds are those due to slumping of
sedimentary material before it is
lithified. Folds in rocks vary in size
from microscopic crinkles to
mountain-sized folds. They occur singly
as isolated folds and in extensive fold
trains of different sizes, on a variety
of scales.
Folds form under varied conditions of
stress, hydrostatic pressure, pore
pressure, and temperature gradient, as
evidenced by their presence in soft
sediments, the full spectrum of
metamorphic rocks, and even as primary
flow structures in some igneous rocks. A
set of folds distributed on a regional
scale constitutes a fold belt, a common
feature of orogenic zones. Folds are
commonly formed by shortening of
existing layers, but may also be formed
as a result of displacement on a
non-planar fault, at the tip of a
propagating fault, by differential
compaction or due to the effects of a
high-level igneous intrusion e.g. above
a laccolith.
Describing folds
Folds are classified by their size, fold
shape, tightness, and dip of the axial
plane.
= Fold terminology in two dimensions=
Looking at a fold surface in profile the
fold can be divided into hinge and limb
portions. The limbs are the flanks of
the fold and the hinge is where the
flanks join together. The hinge point is
the point of minimum radius of curvature
for a fold. The crest of the fold is the
highest point of the fold surface, and
the trough is the lowest point. The
inflection point of a fold is the point
on a limb at which the concavity
reverses; on regular folds, this is the
midpoint of the limb.
= Fold terminology in three dimensions=
The hinge points along an entire folded
surface form a hinge line, which can be
either a crest line or a trough line.
The trend and plunge of a linear hinge
line gives you information about the
orientation of the fold. To more
completely describe the orientation of a
fold, one must describe the axial
surface. The axial surface is the
surface defined by connecting all the
hinge lines of stacked folding surfaces.
If the axial surface is a planar surface
then it is called the axial plane and
can be described by the strike and dip
of the plane. An axial trace is the line
of intersection of the axial surface
with any other surface.
Finally, folds can have, but don't
necessarily have a fold axis. A fold
axis, “is the closest approximation to a
straight line that when moved parallel
to itself, generates the form of the
fold.”. A fold that can be generated by
a fold axis is called a cylindrical
fold. This term has been broadened to
include near-cylindrical folds. Often,
the fold axis is the same as the hinge
line.
= Fold shape=
A fold can be shaped as a chevron, with
planar limbs meeting at an angular axis,
as cuspate with curved limbs, as
circular with a curved axis, or as
elliptical with unequal wavelength.
= Fold tightness=
Fold tightness is defined by the size of
the angle between the fold's limbs,
called the interlimb angle. Gentle folds
have an interlimb angle of between 180°
and 120°, open folds range from 120° to
70°, close folds from 70° to 30°, and
tight folds from 30° to 0°. Isoclines,
or isoclinal folds, have an interlimb
angle of between 10° and zero, with
essentially parallel limbs.
= Fold symmetry=
Not all folds are equal on both sides of
the axis of the fold. Those with limbs
of relatively equal length are termed
symmetrical, and those with highly
unequal limbs are asymmetrical.
Asymmetrical folds generally have an
axis at an angle to the original
unfolded surface they formed on.
= Deformation style classes=
Folds that maintain uniform layer
thickness are classed as concentric
folds. Those that do not are called
similar folds. Similar folds tend to
display thinning of the limbs and
thickening of the hinge zone. Concentric
folds are caused by warping from active
buckling of the layers, whereas similar
folds usually form by some form of shear
flow where the layers are not
mechanically active. Ramsay has proposed
a classification scheme for folds that
often is used to describe folds in
profile based upon curvature of the
inner and outer lines of a fold, and the
behavior of dip isogons. that is, lines
connecting points of equal dip:
Fold types
Anticline: linear, strata normally dip
away from axial center, oldest strata in
center.
Syncline: linear, strata normally dip
toward axial center, youngest strata in
center.
Antiform: linear, strata dip away from
axial center, age unknown, or inverted.
Synform: linear, strata dip toward axial
centre, age unknown, or inverted.
Dome: nonlinear, strata dip away from
center in all directions, oldest strata
in center.
Basin: nonlinear, strata dip toward
center in all directions, youngest
strata in center.
Monocline: linear, strata dip in one
direction between horizontal layers on
each side.
Chevron: angular fold with straight
limbs and small hinges
Recumbent: linear, fold axial plane
oriented at low angle resulting in
overturned strata in one limb of the
fold.
Slump: typically monoclinal, result of
differential compaction or dissolution
during sedimentation and lithification.
Ptygmatic: Folds are chaotic, random and
disconnected. Typical of sedimentary
slump folding, migmatites and
decollement detachment zones.
Parasitic: short wavelength folds formed
within a larger wavelength fold
structure - normally associated with
differences in bed thickness
Disharmonic: Folds in adjacent layers
with different wavelengths and shapes
(A homocline involves strata dipping in
the same direction, though not
necessarily any folding.)
Causes of folding
Folds appear on all scales, in all rock
types, at all levels in the crust and
arise from a variety of causes.
= Layer-parallel shortening=
When a sequence of layered rocks is
shortened parallel to its layering, this
deformation may be accommodated in a
number of ways, homogeneous shortening,
reverse faulting or folding. The
response depends on the thickness of the
mechanical layering and the contrast in
properties between the layers. If the
layering does begin to fold, the fold
style is also dependent on these
properties. Isolated thick competent
layers in a less competent matrix
control the folding and typically
generate classic rounded buckle folds
accommodated by deformation in the
matrix. In the case of regular
alternations of layers of contrasting
properties, such as sandstone-shale
sequences, kink-bands, box-folds and
chevron folds are normally produced.
= Fault-related folding=
Many folds are directly related to
faults, associate with their
propagation, displacement and the
accommodation of strains between
neighbouring faults.
Fault bend folding
Fault bend folds are caused by
displacement along a non-planar fault.
In non-vertical faults, the hanging-wall
deforms to accommodate the mismatch
across the fault as displacement
progresses. Fault bend folds occur in
both extensional and thrust faulting. In
extension, listric faults form rollover
anticlines in their hanging walls. In
thrusting, ramp anticlines are formed
whenever a thrust fault cuts up section
from one detachment level to another.
Displacement over this higher-angle ramp
generates the folding.
Fault propagation folding
Fault propagation folds or tip-line
folds are caused when displacement
occurs on an existing fault without
further propagation. In both reverse and
normal faults this leads to folding of
the overlying sequence, often in the
form of a monocline.
Detachment folding
When a thrust fault continues to
displace above a planar detachment
without further fault propagation,
detachment folds may form, typically of
box-fold style. These generally occur
above a good detachment such as in the
Jura Mountains, where the detachment
occurs on middle Triassic evaporites.
= Folding in shear zones=
Shear zones that approximate to simple
shear typically contain minor asymmetric
folds, with the direction of overturning
consistent with the overall shear sense.
Some of these folds have highly curved
hinge lines and are referred to as
sheath folds. Folds in shear zones can
be inherited, formed due to the
orientation of pre-shearing layering or
formed due to instability within the
shear flow.
= Folding in sediments=
Recently deposited sediments are
normally mechanically weak and prone to
remobilisation before they become
lithified, leading to folding. To
distinguish them from folds of tectonic
origin such structures are called
synsedimentary.
Slump folding: When slumps form in
poorly consolidated sediments they
commonly undergo folding, particularly
at their leading edges, during their
emplacement. The asymmetry of the slump
folds can be used to determine
paleoslope directions in sequences of
sedimentary rocks.
Dewatering: Rapid dewatering of sandy
sediments, possibly triggered by seismic
activity can cause convolute bedding.
Compaction: Folds can be generated in a
younger sequence by differential
compaction over older structures such as
fault blocks and reefs.
= Igneous intrusion=
The emplacement of igneous intrusions
tends to deform the surrounding country
rock. In the case of high-level
intrusions, near the Earth's surface,
this deformation is concentrated above
the intrusion and often takes the form
of folding, as with the upper surface of
a laccolith.
= Flow folding=
The compliance of rock layers is
referred to as competence: a competent
layer or bed of rock can withstand an
applied load without collapsing and is
relatively strong, while an incompetent
layer is relatively weak. When rock
behaves as a fluid, as in the case of
very weak rock such as rock salt, or any
rock that is buried deeply enough, they
typically show flow folding: the strata
appear shifted undistorted, assuming any
shape impressed upon them by surrounding
more rigid rocks. The strata simply
serve as markers of the folding. Such
folding is also a feature of many
igneous intrusions and glacier ice.
Folding mechanisms
Folding of rocks must balance the
deformation of layers with the
conservation of volume in a rock mass.
This occurs by several mechanisms.
= Flexural slip=
Flexural slip allows folding by creating
layer-parallel slip between the layers
of the folded strata, which, altogether,
result in deformation. A good analogy is
bending a phone book, where volume
preservation is accommodated by slip
between the pages of the book.
The fold formed by the compression of
competent rock beds are called "flexure
fold"
= Buckling=
Typically, folding is thought to occur
by simple buckling of a planar surface
and its confining volume. The volume
change is accommodated by layer parallel
shortening the volume, which grows in
thickness. Folding under this mechanism
is typically of the similar fold style,
as thinned limbs are shortened
horizontally and thickened hinges do so
vertically.
= Mass displacement=
If the folding deformation cannot be
accommodated by flexural slip or
volume-change shortening, the rocks are
generally removed from the path of the
stress. This is achieved by pressure
dissolution, a form of metamorphic
process, in which rocks shorten by
dissolving constituents in areas of high
strain and redepositing them in areas of
lower strain. Folds created in this way
include examples in migmatites, and
areas with a strong axial planar
cleavage.
Mechanics of folding
Folds in rock are formed in relation to
the stress field in which the rocks are
located and the rheology, or method of
response to stress, of the rock at the
time at which the stress is applied.
The rheology of the layers being folded
determines characteristic features of
the folds that are measured in the
field. Rocks that deform more easily
form many short-wavelength,
high-amplitude folds. Rocks that do not
deform as easily form long-wavelength,
low-amplitude folds.
See also
Orogeny
Mountain building
Thrust fault
Notes
General references
David D. Pollard, Raymond C. Fletcher.
Fundamentals of structural geology.
Cambridge University Press. ISBN
0-521-83927-0. 
Davis, George H.; Reynolds, Stephen J..
"Folds". Structural Geology of Rocks and
Regions. New York, John Wiley & Sons.
pp. 372–424. ISBN 0-471-52621-5. 
Donath, F.A., and Parker, R.B., 1964,
Folds and Folding: Geological Society of
America Bulletin, v. 75, p. 45-62
McKnight, Tom L; Hess, Darrel. "The
Internal Processes: Folding". Physical
Geography: A Landscape Appreciation.
Upper Saddle River, NJ: Prentice Hall.
pp. 409–14. ISBN 0-13-020263-0. 
Ramsay, J.G., 1967, Folding and
fracturing of rocks: McGraw-Hill Book
Company, New York, 560p.
Lisle, Richard J. "Folding". Geological
Structures and Maps: 3rd Edition.
Elsevier. p. 33. ISBN 0-7506-5780-4.
