Notes on fold and its type

1. Folds
• Undulation or bends or curvatures developed
in the rocks of the crust.
• Parts of a folds

2. The different parts or elements of a
fold
Fold morphology comprises of components/
elements like:
• Wavelength of fold: The folds are the wave like
undulations found in the layered rocks.
Wavelength of the fold is the minimum distance
between its two successive points of same phase.
It can also be defined as the distance between two
alternating inflection points. For practical
purposes the distance between two successive
inflection points is considered as half wavelength
of the fold

4. • Amplitude of fold: It is the length of perpendicular
drawn from hinge point of the fold on the line joining
the two successive inflection points of the fold.
• Hinge point: It is the point of maximum curvature
on the profile section of a fold. The profile of the fold
is a cross section or transverse section across the
hinge line of the fold.
• Hinge zone: Sometimes the maximum curvature of
the fold is not at a point but in a zone called hinge
zone. A hinge zone is a region where the dip of the
folded surface changes over small distance. Such a
region is also called a fold closure.

5. • Hinge line: It is the locus of hinge points of a
particular bedding plane. Hinge line of a fold is the
line of maximum curvature in the folded bed In other
words, we can say that hinge line is a line along
which change in the direction of dip takes place, and
on many folds this coincides with the amount of
maximum curvature.
• Fold axis: It is an imaginary line which by moving
parallel to itself generates the fold. The hinge line of a
fold is equivalent to ‘fold axis’ if it is straight and the
fold is cylindrical in nature.

6. • Inflexion point: It is a point of the fold with zero
curvature where its form changes. That is to say, it is
the point where an antiform changes into synform on
cross section of the fold or vice- versa.
• Inflexion line: This line is obtained by joining the
inflexion points of a folded layer. In other words, the
inflexion line is that line from where an antiform
changes into a synform or vice-versa.
• Limb: It is a portion of a fold between inflexion
point and hinge.
• Axial surface/ Plane: It is a surface formed by
joining of fold hinge lines of successive beds. When
the axial surface forms a plane it is called axial plane.

8. • Crest: It is that point which is located highest in the
profile section of the fold.
• Trough: It is that point which is located lowest in the
profile section of a fold.
• Crestal line: This is located highest in the fold and is
obtained by joining the crestal points of its layer.
• Trough line: This is located lowest in the fold and is
obtained by joining the trough points its folded layer.

9. Fold hinge
• hinge- part of a folded layer where there is
maximum curvature
Fold limb
• limb- part of a folded layer between hinges
Inflection point
• point- maximum slope on a limb (of an upright
fold), often midway between hinges; it is a point
separating concave curvature in on one direction
from concave curvature in the opposite direction.

10. Interlimb angle
• angle- the smallest angle between two limbs
Fold axis
• axis- an imaginary line along the fold hinge
Fold axial plane
• plane- an imaginary plane that bisects the
angle between limbs
Fold axial surface
• surface- a plane that passes through the hinge
lines of successive Layers

11. Fold median surface
• surface- a surface defined by joining adjacent
inflection points
Fold enveloping surface
• surface- a surface which is either tangential to
the fold hinges of small (second second-order
parasitic) folds or passes through the inflection
points of the folded layer
Fold profile
• profile- the shape of a fold projected on a
plane normal to a fold axis

12. Fold amplitude
• amplitude- the height measured from the point
to the median surface
Fold wavelength
• wavelength- the distance between adjacent
hinge lines
Fold crest
• crest- the highest point of a fold
Fold trough
• trough- the lowest point of a fold

13. Geometric elements of folds
• Fold hinge
• Fold profile
plane: Plane
perpendicular to
hinge
• Hinge zone
• Fold limb
• Fold axial plane

14. • Classification of Folds
Anticlines – Strata uparched, convex upwards
- older rocks occupy a position in the interior
or core
- limb dip away from each other
Synclines – downarched
- youngest rock occupy in the interior or core
- limb dip towards each other
Anticlines Synclines

16. Position of Axial Plane
• Symmetrical Folds – Axial Plane in vertical, limbs
are equal in length & dip equally
• Asymmetrical Folds – Unequal in length
• Overturned Folds – fold with inclined axial planes,
limbs are dipping essentially in same direction
Symmetrical
Asymmetrical

18. • Fan Fold - A fold of strata in which both
limbs are overturned, In anticlinal fan fold two limbs
dip towards each other – In syncline fan fold two limbs
dip away from each other

19. • Isoclinal folds – limbs are dipping in the same
direction at equal angles
• Recumbent Folds – lower limb is termed inverted or
reversed limb, core is the inner part

21. • Conjugate Folds – pair of fold / mutually inclined
axial plane
• Box Folds – flat top & steeply inclined limbs
Based on Degree of Compression
• Open Fold – thickness of rock is
• Closed Fold – thick crest & thin limb

23. • Mode of Occurrence
• Anticlinorium – large size fold running for
several hundred KM. It has numerous small
scale fold of various type

25. • Synclinorium
• Dome and Basins – group of strata centrally
uplifted from below. The strata dip away in all
direction. Basin : all layers dip towards a
common centre

26. Position of Axis
• Plunging Folds – axis makes an angle with the
horizontal plane
• Non Plunging Folds – Axis horizontal

27. Behaviour with depth
• Concentric Folds
• Similar Folds – axial region thick than limb
• Supratenous Fold – formed by erosion & deposition
various thickness
• Monocline – local warping – shows an abrupt local
steep inclination

28. • Homocline – strata dipping in the same
direction at uniform angle
• Drag Fold – minor fold within the body
weaker layer

29. • Chevron fold: The fold in which both the crest and
trough are sharp and angular such a zig zag folds are
called chevron fold.

31. Controlling Factors of Fold
Geometry
1. Composition and rheological properties of the
competent layer
2. Change in rheological properties of the layer as P
P-T change during folding
3. Nature of the less competent matrix
4. Competence contrast (viscosity ratio) between
the layer and the matrix
5. Mechanical properties of the interface between
competent layers (welded or detached)

32. 6. Thickness of the competent layer
7. Thickness contrast between the competent
and the incompetent lay layers
8. Presence (or absence) of multi multi-layer
sequence
9. Orientation of the layer (with respect to the
strain ellipse)
10.Magnitude of stress and scale of folding