Fold Classification and Mechanisms

Prepared by- PRAPHULLA SONOWAL, M. Tech 1
st
Sem, Dr. Harisingh Gour University, Sagar (M.P.) 1 | P a g e
A Seminar on
FOLD – ITS CLASSIFICATION AND
MECHANISM OF FOLDING
Department of Applied Geology
Dr. Harisingh Gour University, Sagar (M. P.)
Submitted by –
Praphulla Sonowal
Reg. No. Y17251016
M. Tech 1st
Semester
Dept. of Applied Geology,
Dr. Harisingh Gour University,
Sagar
Submitted to –
Prof. A. K. Shandilya
Dept. of Applied Geology,
Dr. Harisingh Gour University,
Sagar

st
ACKNOWLEDGEMENT
I am grateful to Prof. R.K. Rawat (H.O.D), Department of Applied
Geology, Dr. Hari Singh Gour University, Sagar for arranging this seminar,
there by offering us an opportunity perform our presentation.
I am highly indebted to Prof. A.K.Shandilya, Department of
Applied Geology, Dr. Harisingh Gour University, Sagar, for his guidance,
representation and sustained efforts in making us understand the Structural
Geology.
Without the participation of our teachers and non-teaching staff
we could not have conduct all work successfully.
Praphulla Sonowal
(M. Tech 1st sem.)

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CONTENTS
1. Introduction --------------------------------------- 4
2. Elements of fold --------------------------------------- 5
3. Classification of fold-------------------------------------- 8
4. Mechanism of folding ----------------------------------- 21
5. Conclusion --------------------------------------- 27
6. Bibliography --------------------------------------- 28

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1. INTRODUCTION:
Fold is a wave like structure which is formed by bending or
flexuring of any type of planes or layers in igneous, sedimentary and
metamorphic rocks due to compressional force.
Folds are best displayed by stratified formations. They may be
inferred from various kinds of data. The size of the exposures determines the
size of the folds that may be observed. Folds many thousands of feet across
may be observed in regions of high relief. Conversely, where the exposures are
small, only folds a few feet or tens of feet may be observed.
1.1 HISTORICAL DEVELOPMENT:
The word fold was used by Hall (1815) for the first time to explain
the rock structures, which were experimentally simulated by compressing cloth
within two board. Van Hise (1894) contributed on the geometry of folds, while
Willis (1891) worked out their mechanics. Early geometrical description of the
folds can be read in various text books by Leith (1923), Nevin (1931), Hills
(1963), de Sitter (1964), Whitten (1965), Billing (1975) and many more. In the
later part of the 20th
century, much work was directed towards developments
of fold theory, modeling, fold mechanism, both experimentally and
mathematically and now by computer simulation, beginning with Biot (1961).
He developed theories for single and multilayer folding in viscoelastic and
viscous media and application to rocks. On the basis of theory and model
experiments, Ramberg (1959 and other publications) made significant
contributions to the modern understanding of fold mechanism. Ramsay (1967)
has presented a simple classification of the parallel and similar folds, wherein
details and modification on each limbs can be compared.

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On the the most elaborate review on folds is now available by
Hudleston and Treagus (2010) wherein they have summarized information on
theory, experiment and nature of folds and their development.
2. ELEMENTS OF FOLD:
The elements of fold are as follows:
a) Hinge point: point located at the maximum curvature.
b) Hinge line: a line joining hinge point.
c) Hinge zone or hinge area: region on the folded surface near hinge line.
d) Enveloping surface: a surface joining successive hinge lines, located on the
same folded later.
e) Crest point: point located at maximum height of the folded layer with
reference to a horizontal reference plane.
f) Crest line: a line joining successive crest point.
g) Crestal plane/surface: plane/surface joining all the crest lines in a fold.
h) Trough: a point located at minimum height with reference to a horizontal
reference plane.

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i) Trough line: a line joining point of minimum height.
j) Trough plane/surface: plane/surface joining all the trough line in a fold.
k) Culmination point: point where crest line reaches its maximum elevation.
l) Depression point: point where the trough line reaches its minimum
elevation.
m) Core: the inner part of the fold.
n) Envelop: outer part of the fold.
o) Wavelength of fold: Distance between two continuous anticline or syncline.
p) Amplitude of fold: Distance between maximum or minimum height to
median surface.
q) Inflection point: point of minimum or zero curvature in a single layer fold.
r) Inflection line: a line joining points of minimum or zero curvature in a single
fold on the same surface. This line may also be curved.
s) Median surface: a surface passing through successive inflection lines.
t) Inflection surface: a surface joining successive inflection lines in different
layers within a single fold and defines the limit of a fold in 3 – dimensions.,
u) Fold domains: part of folded layer between two between two successive
inflection lines.
v) Fold limb: portion of the folded surface between hinge and inflection line.
w) Interlimb angle: angle between two tangents drawn at the inflection points
of a fold.

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x) Bisecting plane: a plane dividing the interlimb angle into two equal parts.
y) Fold axis: an imaginary line in space along which the fold is generated. It
does not have a fixed position on the folded surface unlike the hinge.
z) Axial plane or axial surface: an imaginary plane which divide the fold almost
at two equal half. Or it can be defined as an imaginary surface or plane
joining multiple hinge lines in a single fold affecting various layers.
aa)Axial trace: the exposure of the axial surface on the ground is called as axial
trace.
bb) Axial trend: azimuth of the hinge line. It is different from the axial trace.

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3. CLASSIFICATION OF FOLD:
Folds have been classified into various type on the following basis:
a) Based on fold closure
b) Based on symmetry
c) Based on plunge of fold axis
d) Fluety’s classification based on amount of plunge of fold axis
e) Based on orientation of axial plane
f) Fluety’s classification based on amount of dip of axial plane
g) Based on direction of younging relative to fold fold closure
h) Based on nature of hinge line
i) Fluety’s classification based on interlimb angle
j) Based on shape of hinge
k) Based on no of hinges
l) Based on geometrical elements (Dip isogone, axial plane thickness,
Orthogonal thickness) of fold by Ramsay
m) Based on superposition of fold
a) Classification of fold based on fold closure: On the basis of
fold closure, folds are classified as:
1. Antiform: fold domains having upward closure or negative curvature.
2. Synform: fold domains having downward closure or positive curvature.
3. Neutral fold: fold closing sideways.
4. Vertical fold: all surfaces in a neutral fold vertically dipping.

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b) Classification of fold based on symmetry: On the basis of
symmetry, folds are classified as:
1. Symmetrical fold: a fold in which the axial plane is a plane of symmetry and
the two limbs dip at the same angle but in opposite direction.
2. Asymmetrical fold: a fold in which the axial plane is not a plane of symmetry
and the two limbs dip at unequal angles in opposite direction.
c) Classification of fold based on plunge of fold axis: On the
basis of plunge of fold axis, folds are classified as:
1. Horizontal fold: a fold whose axis is horizontal.
2. Plunging fold: a fold whose axis is inclined.
3. Vertical fold: a fold whose axis is vertical.
d) Fluety’s classification based on amount of plunge of fold
axis: Fluety (1964) suggested the following classification on the basis of the
ampunt of plunge of the fold axis:

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1. Subhorizontal fold: plunge between 0 and 10o
.
2. Gently plunging fold: plunge between 10 and 30o
.
3. Moderately plunging fold: plunge between 30 and 60o
.
2. Steeply plunging fold: plunge between 60 and 90o
.
3. Sub-verticalfold: plunge between 80 and 90o
.
e) Classification of fold based on orientation of axial plane:
On the basis of orientation of axial plane, folds are classified as:
1. Upright fold (Willis & Willis 1929): with vertical or nearly vertical axial plane.
2. Recumbent fold: with axial plane dipping at an angle of 10o
or less.
3. Inclined fold (Willis & Willis 1929): With inclined axial plane.
4. Reclined fold (Sutton 1960): inclined fold in which the pitch of the fold axis
on the axial plane is between 80 and 100o
.

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5. Overturned fold: inclined fold in which both the limbs have the same sense
of inclination.
f) Fluety’s classification based on amount of dip of axial
plane: Fluety (1964) suggested the following definitions based on the
amount of dip of the axial plane:
1. Upright fold: dip 90-80o
.
2. Steeply inclined fold: dip 80-60o
.
3. Moderately inclined fold: dip 60-30o
.
4. Gently inclined fold: dip 30-10o
.
1. Recumbent fold: dip 10-0o
.
g) Classification of fold based on direction of younging
relative to fold fold closure: On the basis of direction of younging
relative to fold closure, folds are classified as:
1. Anticline: a fold in which direction of younging is away from the fold core.
2. Syncline: a fold in which direction of younging is towards the fold core.
3. Anticlinorium: a large anticline with many smaller folds on its back.
4. Synclinorium: a large syncline with many smaller folds on its back.

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5. Synformal anticline: a fold that closes downward but with direction of
younging away from the fold core.
6. Antiformal syncline: a fold that closes upward but in which the younging is
towards the fold core.
h) Classification of fold based on nature of hinge line: On the
basis of direction of nature of hinge line, folds are classified as:
1. Cylindrical fold: a fold which can be generated by moving a line parallel to
itself. A cylindrical fold has a rectilinear hinge line parallel to the fold axis.
2. Non-cylindrical fold: a fold which cannot be generated by moving a line
parallel to itself. The hinge line is either curved or the fold is conical.
3. Conical fold: a type of non-cylindrical fold whose shape approximates a part
of a cone.

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i) Fluety’s classification based on interlimb angle: On the basis
of interlimb angle, Fluety (1964) classified fold as:
1. Gentle fold: with interlimb angle between 180 and 120o
.
2. Open fold: with interlimb angle between 120 and 70o
.
3. Close fold: with interlimb angle between 70 and 30o
.
4. Tight fold: with interlimb angle less than 30 o
and greater than 120o
.
5. Isoclinical fold: with sub-parallel limb.
6. Fan fold: with negative interlimb. The term elastic is sometimes uncritically
used to describe a fan fold.

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j) Classification of fold based on shape of hinge: On the basis of
shape of hinge, Folds are classified as:
1. Round-hinged or broad hinged fold: a fold with a broad hinge zone
compared to the limb.
2. Chevron fold: a fold with straight limbs and with a sharp hinge.
3. Arrow-head fold: a fold with a sharp hinge and with distinctly curved limbs.
4. Cuspate fold: a train of fold with sharp hinge on one set of closures and with
rounded hinges on the oppositely directed closures.

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k) Classification of fold based on number of hinges: On the
basis of number of hinges, Folds are classified as:
1. Single-hinged fold: a fold with a single hinge between two points of
inflection.
2. Conjugate fold: a double hinged fold with sharp hinges.
3. Box fold: a double hinged fold with more or less rounded hinges. The term is
generally restricted to folds with flat tops and steeply dipping limbs.
l) Geometrical classification of fold: Geometrical classification of
fold is given by John G. Ramsay in 1967. It is based on dip isogons, axial plane
thickness and orthogonal thickness. Dip isogons are lines joining point of equal
dip on either side of the
folded layer. The orthogonal
thickness (tα) corresponding
to the dip angle α is defined
by Ramsay (1967) as the
distance between two
tangents at outer surface and
inner surface. The axial plane
thickness (Tα) is the intercept
between the two tangents, of
a line parallel to the axial surface trace on the profile plane. Ramsay classified
fold into the following class:

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1. Class 1 folds: In class 1 folds the dip isogons are convergent i.e. the radius of
the curvature of the outer arc (say at the hinge zone) is larger than that of the
inner arc. Ramsay (1967) has recognized three sub-classes under this heading:
1.1. Class 1A folds: Folds in which the orthogonal thickness is minimum at
the hinge, i.e. tα
’
> 1. Such a geometry implies that dip isogons are strongly
convergent. These were earlier described as supratenuous folds (Nevin
1931).
1.2. Class 1B folds: Folds in which the orthogonal thickness is constant
along the layer, tα
’
= 1. These are the parallel folds of Van Hise (1896) or
concentric folds of Leith (1923).
1.2. Class 1C folds: Folds in which the orthogonal thickness is maximum at
the fold hinge and decreases away from it. In class 1C folds, Cos α < tα
’
< 1.
2. Class 2 folds: Folds in which the dip isogons are parallel. This geometry can
develop only if the axial plane thickness is constant all along the fold or in
other words, the outer and the inner arcs have exactly the same shape. These
are the similar folds of Van Hise (1896). In Class 2 folds tα
’
= Cos α and Tα
’
= 1.
3. Class 3 folds: Folds in which the dip isogons diverge towards the fold core.
For such folds radius of the curvature of the outer arc (say at hinge) is smaller
than that of the inner arc. The orthogonal thickness is also maximum at the
fold hinges. In class 3 folds 0 > tα
’
> Cos α.

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Properties of folds (Ramsay, 1967)
Parameter Type of folds
Class 1 A Class 1 B Class C Class 2 Class 3
Dip isogons Strongly
Convergent
Convergent Weakly
Converget
Parallel Divergent
tα
’
> 1 = 1 Cos α < tα
’
< 1 Cos α 0 > tα
’
> Cos α
Tα
’
> Sec α Sec α Sec α > Tα
’
> 1 1 < 1
Comparison of
curvature of inner
and outer arcs
i > o i > o i > o i = o o > i
m) Classification of fold based on superposition of folds:
Ramsay (1967) and Hubber (1987) have geometrically classified interference
pattern of superposed folds on the basis of mutual relations between two sets
of folds, i.e. attitude of two folds axes and axial planes and on the flow
direction of the second deformation. Upright F2 fold (fold hinge f2) with a

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vertical flow direction a2 is superposed upon earlier F1 fold (fold hinge f1).
Four basic fold geometries are seen in the outcrops:

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a) Superimposed fold Type 0: It develops when a2 lies in the axial plane of F1
folds and the two fold hinges are sub-parallel to each other. It is
indistinguished from the single phase folds because the first fold gets amplified
after the superposition. If there is a dyke intrusion between the two folding at
high or 90o
angle to the axial plane of the F1 fold, dyke will be folded and take
shape of the second fold, while the layer affected by the F1 fold will be
amplified.
b) Superimposed fold Type 1 – Dome and Basin: Upright F1 folds with axis and
axial plane oriented at large angle to the F2 folds, while the angle between the
dip of axial plane of F1 fold and flow direction a2 is low it produces a dome and
basin pattern with the development of culminations and depressions in the
first fold and in combination with similar pattern in second fold, the geometry
looks like an egg carton. Each dome is surrounded by four basin and vice versa.
c) Superimposed fold Type 2 – Crescent and mushroom pattern: In this type of
fold pattern, hinges of the second fold are oriented at high angles to the axial
planes of the first folds, while the fold hinges of both the fold system are
oriented at high angle. When superposition takes place, axial surface and limbs
of the first fold are folded, while its hinges are bowed upwards and
downwards. Limbs of the first fold are refolded into a common antiform and
synform. Upon erosion, these folds will have variable geometry from near
circular shape at shallow level to crescent shape in deeper level in culmination
of the first fold. In still deeper parts it will lead to the mushroom geometry.
d) Superimposed fold Type 3 – doubled zigzag or hook shaped folds: In these
folds, differential movement direction a2 of the second phase lies at higher
angle to the axial planes of F1 folds, while f2 direction (hinge line of second
fold) lies very close to the hinge lines of F1 folds. In such cases, axial surface of
the first folds become curved, but the second fold hinges are not deflected,

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hence both the F1 and F2 fold hinges are sub-parallel to each other and are
called as co-axial.
4. Mechanism of folding:
Three factors control the formation of folds in rocks:
a. Orientation of force or stress
b. Deformation of folded layers
c. Geometry of folded layers.
Depending on the mechanism of folding the natural folds can be
classified into three broad categories (Ramberg 1963, Hudleston 1986):
a. Buckle folds, where viscosity between the layers is an important factor.
b. Bending folds, where stress is applied across the layers and
c. Passive folds (Flexural slip and Flexural flow or flexural shear).
Depending on the mechanism of folds can also be classified into
two broad groups:
a. Flexure folds and
b. Shear folds
c. Slip folds
d. Kink folding

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a. Buckling of layers: A
layer undergoes buckling
when it is subjected to
shortening parallel to the
layering, i.e. parallel
shortening. Buckle folds
are developed when
there is a viscosity
contrast between the
layer and the matrix in
which it is embedded.
The instability in the
material leads to the
production of folds of
various wavelengths.

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b. Bending of layers: Bending
of layers is developed when
stress acts across the layers at
high angles unlike the buckle
folds. Geometry of bounding
structures forces the bending
of rocks and thus causes the
development of bending folds.
Fig. Bending of layers (a) Boudins, (b) Basement extensional faults, (c) pluton or
salt dome.

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c. Passive folding (Flexural slip and flexural flow or flexural shear): Passive
folding occurs where the layering exerts no mechanical influence on the
folding. In this case
the layering only
serves as a visual
expression of strain
with no mechanical
or competence
contrast to
neighbouring layers.
Flexural
slip implies slip along
layers or very thin
layers during folding.
It maintains bed thickness and thus produces class 1B or parallel folds.
In cases where strain is more evenly distributed in the limbs in the
form of shear strain as is more commonly the case in the plastic regime,
flexural slip turn into the closely related term flexural shear or flexural flow.
Pure flexural flow can be visualized by straining the inscribed
circles and orthogonal lines in a layer. In this case strain dies out near the hinge
line, which in contrast with buckling/orthogonal flexure where strain pattern is
separated by a neutral surface. Pure flexural flow produces perfect Class 1B.

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d. Slip folding: Slip folding involves shearing, slip or flow along closely-space
planes which are oblique to the folded layer. This process produces ideal
similar folds or Class 2 folds. In this case, shear surface are commonly parallel
to the axial surfaces of folds and parallel to the lines of no finite elongation.
e. Kink folding: Angular kink folds are commonly developed in well-developed
layered sequences like thinly-bedded sediments or slates. Class 2 type
geometry characterizes the folds under this category, which includes kink
bands, chevron folds or conjugate folds of either dextral (z-shape) or sinistral
(s-shape).
Ramsay and Huber (1987) correlated types of kinks with different
orientation of strain ellipse: equally developed dextral and sinistral kinks with

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σ1 parallel to foliation and obliquely oriented σ1 in case of dextral or sinistral
kinks.

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5. CONCLUSION:
Folds are very important structure. Folding brings minerals like
copper and lead closer to the surface making their extraction easy. Folds may
also act as a structural trap in from which oil can be extracted. Eg. In Digboi oil
field oil is extracted from narrow faulted anticline. Features formed like fold
mountains attracts tourists who fetch the country foreign exchange. Fold
mountains receive heavy rainfall on the windward sides which encourage
growth of dense forests which provide variable timber. Heavy rainfall and
windward side enable agriculture. Fold mountains receives high rainfall giving
rise to rivers which are use for HEP generation. During folding faults develops
leading to earthquakes. Lee ward sides of fold mountains receives low rainfall
which discourage agriculture and settlement. Features formed during such fold
mountains hinder construction of roads and railway lines. It makes difficult to
use aircrafts due to poor visibility. Low areas adjacent to fold mountains
experience temperature inversion. This causes frost or coldness which
sometimes harms crops such as grapes.

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6. BIBLIOGRAPHY:
1. Ramsay, J. G., Huber M. I. : The techniques of modern structural geology
– Volume 2 : Folds and Fractures.
2. Jain, A. K. : An Introduction to Structural Geology, Geological society of
India.
3. Ghosh S. K. : Structural Geology - Fundamentals and Modern
Developments.
4. Billings, M. P. : Structural Geology, 3/E.
5. Fossen, H. : Structural Geology.

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