This lecture includes the fold terminology and classification of folds based of different criteria. Classification of folds based on: Direction of closing Attitude of axial surface Size of interlimb angle Profile Ramsay Classification of folds

2. Folds
Plastic deformation: continuity of the material is maintained
after the deformation.
Folds are the systematically curved layers of rocks.
A fold is a structure produced when a originally planer
surface becomes bent or curved as a result of
deformation.
Folds are (undulations or corrugations) expression of
plastic type of deformation which produce more gradual
but more continuous change in rock.

3.  Hinge(s): It is the area of maximum curvature. It is closure of
fold sometimes.
 Limb(s): these are the areas between the hinges or in other words
these are sides of the fold.
 Hinge line: it is possible to define a hinge line as line of
maximum curvature.The hinge line may be horizontal, inclined
or vertical.
 A single fold is comprised of a hinge and 2-limbs.
 In a series of folds,each limb is shared between adjacent folds.

4.  Axial plane: The imaginary
plane which is equidistant
from each limb of a fold and
which also bisects the angle
between the two limbs, is
called axial plane.
 The axial plane actually cuts
the hinge zone of the fold
along a line, which is termed
as fold axis or axis.
 Fold axial surface: It is that surface which passes through the
hinge line of the successive surfaces, such as is in general not a
plane is known as axial surface.

6.  Interlimb angle: The smaller
angle (acute angle) made by the
limbs of fold is known as the
interlimb angle.
 The size of interlimb angle can be
measure for the intensity of
deformation.
 Plunge: The attitude of the fold
axis is measured as the angle
between the axis and horizontal.
 This angle which must be measured in
vertical plane (like the dip angle), is
termed as the plunge (has both the
amount and direction.)

7.  Crest and trough: In
inclined fold the highest and
lowest points on the fold
surface don’t in general
corresponds to the with the
hinges, it is sometimes
advantageous to use the term
‘crest and trough’ for these
points.

8.  Amplitude and wavelength: These two elements are helpful in
measuring the size of the fold. If the hinges are not visible.
 λ/2 may be measured, instead by taking the distance between the two
inflection points on the either sides of the fold hinge.
 ‘inflection point are points where,the sense of curvature changes from one fold to other.
 The amplitude or height of a fold may be measured by taking half of the perpendicular
distance from the hinge to the line between the two adjacent hinges, or the
perpendicular distance from the hinge to the line between two inflection points.’

9. There are four main ways to classify the folds-
1. Direction of closing
2. Attitude of axial surface
3. Size of interlimb angle
4. Profile

10. 1. Classification on the basis of
direction of closing/closure
 folds that closed upward and the two limbs are
dipping away from hinge, these are termed as
antiform or convex upward fold, and those
closed downwards and two limbs are dipping
toward each other arte called synform or
convex downward fold.
• Folds that close sideways are called Neutral folds.
• In contrast to Antiform or synform, sideways closing folds. From
axial plane dip=0 (recumbent) to axial plane vertical (vertical
folds) through reclined (plunge of fold hinge down the true dip
of the axial plane of fold) .
• A sideways closing fold whose hinge line is parallel to direction
of dip of the axial plane is called a neutral or a reclined fold.

11.  Under normal conditions when
bedding becomes younger upward, an
antiform contain older rocks in the
core and known as anticline.
 Thus the term anticline should be
strictly applied only to a fold with older
rocks in the core conversely a syncline
is a fold which contains younger rocks
in it core.
 In area of more complex folding where
strata are commonly inverted, it is
possible to find the downward closing
anticline(synformal anticline) and
upward closing syncline(antiformal
syncline).

13. 2. Classification based on attitude of
axial surface
i. Upright
ii. Inclined
iii. Recumbent
iv. Inverted or overturned
v. Isoclinal
 Folds may be divided into three broad types based on the dip of
axial plane/axial surface.

14. i. Upright fold: Vertically or steeply dipping axial plane (80°-
90°).
ii. Incline fold: Moderately dipping axial plane (10°-80°)
iii. Recumbent fold: Horizontal to sub-horizontal axial plane
(0°-10°)

15. iv. Inverted or overturned fold: In inclined fold two limbs are
dipping in different directions (may be with the different
angle) but if the two limbs are dipping in the same direction
(with different angles), these folds are termed as inverted
folds.
One limb in such folds is normal and other is inverted.
The inverted limb is one which is rotated more than 90°.
V. Isoclinal folds- Both the limbs are parallel.

16. 3. Classification based on interlimb
angle
Problems arise in
measuring the
interlimb angle
where the limbs
are not straight.
In such cases
tangents are
drawn from
inflection points.

17. a. Parallel fold: The simplest is parallel fold in which
the adjacent fold surface or layer are parallel
i.e. the thickness of folded layer measured
perpendicular to the fold surface (i.e. orthogonal
thickness) remains constant.
 Special case of parallel fold is the concentric fold
where adjacent fold surfaces are arcs are a circle
with a common center known as the center of
curvature.
 Parallel fold die outward and downward as a
consequence of their own geometry and it is more
common in case of concentric folds.
 This is the most widely used classification of the folds.
 The fold profile is the shape of folded layer observed in the plane perpendicular
to the fold axis. Folds display considerable variation in style. Hence, further
classified on the basis of fold profile.

18. b. Similar folds: in these type of fold the orthogonal thickness of a fold
layer changes in systematic manner that the fold maintains uniform
thickness measured parallel to the axial surface.
The folded layers tend to be thicker in the hinge of the fold and
thinner along the limbs of the fold.

19. C. Chevron fold or Accordian fold
 Kink folds are small folds (less than a
meter) that are characterized by straight
limbs and sharp hinges. Typically they
occur in finely laminated (that is,
strongly anisotropic) rocks, such as
shales and slates
 Sharply bending a deck of cards is a
good analogy for the kinking process,
because kink folds are formed by
displacements between individual
laminae (individual cards in the
analogy). Chevron folds are the
larger-scale equivalent of kink folds.

20. d. Kink bands
 These are the narrow band only a few cm wide in which beds
assumed a dip which is steeper or gentler than in adjacent parts

22. e. Symmetric and asymmetric folds
 It both the limbs are equal-symmetrical, if unequal-
asymmetrical.

23. f. Parasite fold
 Whenever a package of alternating hard (competent) and
incompetent layers of rock undergo folding, small folds are
developed on the limbs and hinge of the layer known as
parasite fold.

26. g. Harmonic and disharmonic fold
 If in a thick succession, folds almost follow the same style, they
are called harmonic folds.
 If they don’t not follow the same style as other layers the fold is called as
disharmonic fold.

27. h. Conjugate fold
 These folds are found in pairs whose axial surfaces
are dipping towards each other.
 A box fold is a special type of conjugate fold having
almost a rectangular outcrop pattern.

28. Dome and basin
 A dome is a structural
feature where dips are
radial and away from the
center.
 Structural basin is the
counter part of dome
where dips are towards the
center.

29.  This classification scheme is
based on dip of isogons.
 Some folds have layers that
maintain their thickness
through the fold, while others
show thickened limbs or hinges.
These, and related features,
were explored by the British
geologist John Ramsay, who
classified folds geometrically by
means of dip isogons.

31.  By orienting the fold so that its axial trace
becomes vertical, lines or dip isogons can be
drawn between points of equal dip on the
outer and inner boundaries of a folded layer.
Dip isogons portray the difference between the
two boundaries and thus the changes in layer
thickness.
 Based on dip isogons, folds can be classified
into the three main types shown in Figure in
previous slide.

32.  Class 1: Dip isogons converge toward the inner arc, which is
tighter than the outer arc.
 Class 2 (similar folds, also called shear folds): Dip isogons
parallel the axial trace. The shapes of the inner and outer arcs are
identical.
 Class 3: Dip isogons diverge toward the inner arc, which is more
open than the outer arc.

33.  Class 1 folds are further subdivided into classes 1A, 1B
and 1C.
 1A folds are characterized by thinned hinge zones, while 1B folds,
also called parallel folds and, if circle shaped, concentric folds, have
constant layer thickness.
 Class 1C folds have slightly thinned limbs.
 Class 2 and, particularly, Class 3 folds have even thinner limbs and
more thickened hinges.
 Among these classes, Class 1B (parallel) and 2 (similar) geometries
stand out because they are easy to construct and easy to identify in
the field.

34. Thank you.