This document defines folds and describes their key elements and classification. Folds are shapes or volume changes in rock layers caused by deformation. They indicate stress and can be described geometrically. Folds are classified based on attributes like their interlimb angle, symmetry, position, and isogon characteristics. Gentle folds have interlimb angles from 180 to 120 degrees, while tight folds are from 30 to 0 degrees. Symmetric folds have symmetry planes, while asymmetric folds are monoclinal. Anticlines fold upwards and synclines fold downwards. The document also discusses methods for constructing fold profiles.

1. Lecture #14

2. Definition
Fold is the shape or volume changes from one material
(bedding), which show as an arc or group of arc on the line
or plane structure on that material.
Fold is an important phenomena, that indicate
deformation characteristic especially the geometry
description with distortion and rotation.
An inhomogeneous deformation that occurred on
material/rock that has line and plane structure.
Need to consider that an element which is before has
curve form can change become plane or straight line or
an element can remain as plane structure or straight line
after deformation.

3. Fold Geometry Description
By geometry fold can be descript as single curve plane
Fold shape has varieties, from simple until complex
For simplification purpose, fold can be assume as an
cylinders surface with fold axis as the surface framework
and the element can be shown at the fold profile.

5. Elements of fold:
i. Hinge point; the maximum point on the curve of the folded
bed
ii. Crest; the highest point of the curve
iii. Trough; the lowest point of the curve
iv. Inflection point; the boundary point between two different
type of curve
v. Fold axis (hinge line); the maximum curve line on the surface
of folded plane
vi. Axial plane; plane that form through axis line on fold. This
plane not always a planar plane, but can be curved also known
as axial surface.
vii. Fold limb; generally is the side of the folded plane, which is
located between the hinge zone and inflection line.

9. Basic Fold Classification
Generally fold being classified based on the characteristic
which can descript the element geometrically
Classification and fold naming, normally not directly
represent the occurrence characteristic.
By indirect fold forming can represent the occurrence
characteristic or fold forming and type or material
Ex: tight fold represent major deformation, parallel fold
represent on the component bedding.
Based on interlimb angle; symmetry characteristic; fold
position; isogon.

11. Interlimb Angle
Is the smallest angle that form between fold limb
and being measured on the profile of fold.
This angle represent the tightness of fold
Classification based on Fleuty (1964);
Interlimb Angle Fold Description
180° – 120° Gentle
120° – 70° Open
70° – 30° Close
30° – 0° Tight
0° Isoclinals

13. Symmetry Characteristic
Symmetry is one of the criteria that represent form of a cylindrical
surface.
Symmetry characteristic being determined by the plane that through the
hinge line and divided the interlimb angle equally, which known as
symmetry characteristic, and this fold known as symmetry fold, and has
orthorhombic symmetric characteristic.
Series of fold known as symmetric if each of the fold has symmetry
characteristic and pattern periodically .
In symmetrical fold, amplitude and wave length, the comparison of this
element will be easy to descript.
If the plane trace through the hinge line and not as symmetry plane, the
fold known as asymmetry fold, which has monocline symmetry
characteristic.
Therefore it is necessary to add asymmetry characteristic such as the
nature of slope plane direction (vergence) or relative direction of anti-form
hill with sin-form hill such as wind direction, left right or clockwise
rotation for fold with plunge axis.

16. Fold Position
Folding that the limb dip goes to different direction known as anticline
Folding that the limb dip goes to the same direction known as syncline
Fold position can get from the fold axis and axial plane, surface.
Fleuty (1964) based on the dip tendency of axial plane and trend of fold
axis :
Angle Term Axial Plane Dip Axis Line Trend

18. Rickard (1971) classification based on the combination
of trend and pitch magnitude, and the dipping of the
axial plane.
To give more consistent position on the inclined fold
Example:
1. Upright fold: dip 85, trend 25: upright, gently,
plungging fold.
2. Inclined fold: dip 70, trend 45; steeply, inclines,
moderately plungging fold.
3. Reclined fold: dip 56, trend 55; moderately inclined
fold

20. Isogon
Lines that connected points from two surface fold
which the dip or apparent dip are same.
Isogon can be used as a basic fold classification based
on the parallel nature of the isogon and the direction
of the convergent and divergent (the inside part of the
isogon used to determined the direction of isogon
convergency- Ramsay 1967).
Fold with heavy isogon convergence: Parallel fold
Fold with weak isogon convergence: Similarity fold
or Divergent fold

22. Construction of parallel fold profile
Method that can be used to construct the parallel fold:
Arc method
Higgins method (1962)
Bust method (1929)

23. Arc-method

24. Higgins, 1962

25. Busk, 1929

26. Construction of the unparallel fold
profile use Boundary Ray
Bedgley, 1965