Structural Geology

1. Structural Geology
Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D.
Department of Geology
Acharya Nagarjuna University

2.  The major structures are
1. FOLDS
2. FAULTS
3. UNCONFORMITIES
4. JOINTS

4.  Structural geology is a branch of
geology dealing with the study of
structures found in rocks.
 It is also known as “Tectonic
Geology” or simply Tectonics.
 Structure is a term used to describe
the overall relationship of rock
masses, eg: folding, faulting, jointing
and unconformities.

5. Folds: Folds are one of the most common
geological structures found in rocks. When a
set of horizontal layers are subjected to
compressive forces, they bend either upward or
downwards.
The bends noticed in rocks are called folds.
Folds are described variously as wavy or arch-
like or curved or undulatory or wraping
appearances found in rocks.

6. Crest, Trough, Limbs, Anticline,
Syncline, Hinge, Axis of fold, Axial plane,
Plunge, Dome, Basin.
Anatomy of Folds

7. Crest and Trough :The curved portions of the
fold at the top and bottom are called crest and
trough.
Limbs or Flanks : These are the sides of a
fold. There two limbs for every fold and one
lime common to the adjacent folds.
Anticline and Syncline : When the beds are
bent upwards, the resulting fold is called
anticline (anti-opposite, cline-inclination).
Syncline is just opposite to anticline in its
nature.

9. Hinge :The axial plane divides a fold into its
two limbs
The surface trace of an axial plane is
called the hinge line (or axis) of the fold.
Axis of Fold : An axis of fold is defined as
the line of intersection between the axial
plane and the surface of any of the
constituent rock bed.
Axial Plane : It is an imaginary plane or
surface which divides a fold into two
equal halves.

10. Plunge of Fold: Folds having inclined axes are
called plunging fold.

11. Dome: Three-dimensonal anticline
resembling inverted cereal bowl
Basin : Three-dimensional syncline
resembling upright cereal bowl

12. Classification and types of folds
Based on different principles, the folds are
variously classified. Usually, folds are
classified on the basis of
i) Symmetrical character
ii) Upward or downward bend
iii) Occurrence of plunge
iv) Uniformity of bed thickness
v) Behaviour of the fold pattern with depth

13. Types of folds :
1. Symmetrical fold
2. Asymmetrical fold
3. Isoclinal fold
4. Overturned fold
5. Recumbent fold
6. Chevron fold
7. Plunging and Non plunging fold
8. Similar and Parallel fold
9. Drag fold
10. Fan fold

16. Symmetrical and Asymmetrical folds
when the axial plane divides a fold into
two equal halves in such a way that one
half is the mirror image of another,
then such a fold is called symmetrical
fold.
If the two halves are not mirror
images, then the fold is called as
asymmetrical fold

17. Isoclinal fold :Folds that have parallel limbs
are called Isoclinal folds.
Overturned fold :In this fold the two limbs
dip in the same direction but at different
angles. The axial plane is inclined.
Recumbent fold : A fold in which the axial
plane is absolutely horizontal and the limbs
are also more or less horizontal is called
Recumbent Fold.
Chevron fold : The fold, where the crest and
trough are sharp and angular are called
Chevron fold.

18. Plunging and Non-plunging folds: Folds having
inclined axes are called the ‘plunging fold.
Folds having horizontal axes are called non-plunging
folds.
Similar and Parallel fold : In similar folding the
bedding planes are similar having the same shape
downwards or upwards so the beds near the crest are
thicker and the beds at the limbs are thinner
Drag fold : These are may be defined as minor folds
developed within the body of incompetent beds during
the process of major folding.
Fan fold : It is an upright fold in which both the limbs
are overturned. In anticlines limbs dip towards the axial
plane and synclines they dip away from it.

20. Folds; Engineering Considerations
1. For major project like a dam, tunnel, railway
stations, etc., a site which is highly folded should be
avoided because the engineer may have to face much
troubles sooner or later as folds are easily fractured
even due to a slight disturbance.
2. If the project is of a scattered nature like electric or
telephonic poles the work can be carried out without
much of a risk.
3. Synclinal folded rocks may yield hard and tough
quality stones; where as anticlinal folded rocks will
yield weaker stones.
4. The anticlinal folds provide good prospects for stored
petroleum and hence in oil exploration, folds must not
be overlooked.