1. Structural geology is the study of secondary geological structures like folds, faults, joints that form in rocks after their initial formation. 2. Folds form as a result of stresses that cause bending or undulations of layered rocks. Folds can be classified based on their geometry and orientation. 3. Faults form when rocks fracture and move relative to each other along a fracture surface due to stresses. Faults are classified based on the type and direction of movement between rock blocks. 4. Joints form when rocks fracture but there is no relative movement between the fractured pieces. Joints often form extensive fracture systems.

1. Department of Mining Engineering
•Subject : Mining Geology
• Faculty : B.Rakesh (M.Tech)
• Designation : Assistant Professor
• JB INSTITUTE OF ENGINEERING & TECHNOLOGY UGC AUTONOMOUS Bhaskar
Nagar, Yenkapally (V), Moinabad (M), R.R. Dist. 500075 – Telengana

2. Unit - I
STRUCTURAL GEOLOGY

3. STRUCTURAL GEOLOGY
• Structural geology is the study of factors such as origin, occurrence,
classification, type and effects of various secondary structures like
folds, faults, joints, rock cleavage and are different from those
primary structures such as bedding and vesicular structure, which
develop in rocks at the time of their formation.
Definition

4. Stress:
• This is a force acting on a body,
or rock unit that tends to
change the size or shape of that
body, or rock unit. Force per
unit area within a body.
• Stress brings about permanent
deformation if the strength of
the body is exceeded.

5. Strain:
• Change in size (volume) or shape of
a body (or rock unit) in response to
stress.
• Strain is the result of the application
of the stress. The stress that causes
the deformation of a rock is not
present any more but the strain is;
and so we can work backwards to
determine the stress.
• The strain tells you the kind of force
that acted on bedrock.

6. Ductile deformation
• A rock that behaves in a ductile or
plastic manner will bend while
under stress and does not return to
its original shape after relaxation
of the stress.
• Ductile behavior results in rocks
that are permanently deformed
mainly by folding or bending of
rock layer.

7. Brittle deformation
• Rocks exhibiting brittle behavior will fractural/break at stresses higher
than its elastic limit.
• Faults and joints are examples of structures that are formed by brittle
behavior of the crust.

8. Strike
• Strike describes the direction of surfaces along their line of
intersection with a horizontal plane.

9. Dip
• Dip is the angle at which a planar feature is inclined to the
horizontal plane; it is measured in a vertical plane
perpendicular to the strike of the feature.

10. Geological Compass

11. Structural Geology, the following are the major concerned:
1. The force acting on the rock.
2. The response of the rock.
3. The geometrical features of the rock.
Due to a force acting on a rock it may undergo deformation.
Topic:
 Folds
 Faults
 Joints

12. Folds
Folds are bends or undulations or wavelike features in layered rocks of the Earth’s
crust, as a result of the stresses (commonly lateral compression) to which these rocks
have been subjected to, from time to time in the past history of the Earth.
Folded rocks can be compared to several layers of rugs or blankets that have been
pushed into a series of arches and troughs.
The fact that rock is folded shows that it was strained in a ductile way rather than by
elastic or brittle strain.

13. Causes of Folding
• Folding of rocks may be caused by numerous factors or causes, which may be divided into two
main types:
• Tectonic causes.
• None-Tectonic causes of folding.

14. Tectonic causes.
• These are causes which are produced due to the forces operating
within the Earth’s crust, such as lateral compression caused by
shrinkage.

15. Non-Tectonic causes of folding.
• These include all those rock folding effects which are effective over
the ground surface, resulting, mainly under the influence of
gravitational force, such as
• a) Land sliding

16. b) Creeping (a gradual movement of rock and
debris down a slope/ a slow deformation of rocks
and minerals in response to prolonged stress).

17. Terminology related to STRUCTURAL GEOLOGY
• Crest: Crest is the line running through the highest points in an up-
arched fold.
• Trough: Trough is the line running though the lowest points in a
down-arched fold.
• Anticline: Is when the beds are up folded into an arch-like structure.
In such folds, the beds on either sides are inclined away from the crest
and that is why the name anti-cline.
• Syncline: Is when the beds are down folded into a trough like form. In
this case the bends on either side incline together towards the keel.

19. Axial plane
• Axial plane: This is the imaginary plane bisecting between the two
limbs of a folder, thus dividing the fold into two parts, as
symmetrically as possible.

20. Axis of the fold
• Axis of the fold: The line of intersection
of the axial plane with any bed of the
fold is termed as the plunge or pitch of
the fold. When the fold axis is inclined,
the angle which it makes with the
horizontal, as measured in a vertical
plane, is called the angle of plunge, or
plunge of the fold. Such folds with
inclined fold are known as plunging
folds or pitching folds.

21. Types of Folds
• Open Fold.
• Isocline Fold.
• Recumbent Fold.
• Overturned Fold.
• Upright Fold.
• Asymmetric Fold.

22. Classification Of Fold
Trishnangini Deka

23. Classification of Folds Based on
• Position of axial plane
• Degree of compression
• Behavior of depth
• Position of axis
• Profile of the outer surface
• Mode of occurrence
• Miscellaneous type

24. Position Of Axial Plane
Symmetrical folds
Asymmetrical fold
Recumbent fold
Isoclinal fold
Conjugate fold
Box folds

25. A. SYM METR ICALFOLD
A f o l d e d s u r f a c e f o r m s a s y m m e t r i c a l fold
i f i n p r o f i l e t h e s h a p e o n o n e s id e o f a f ol d i s
t h e m i r r o r i m a g e o f t h e s h a p e o n t h e o t h e r si de
a n d i f t h e a d j a c e n t l i m b s a r e i d e n t i c a li n
l e n g t h .
Ba s i c a l l y i t m a y b e a n t i c l i n e o r s y n c l i n e
a n d w he n c l a s s i f i e d a s s y m m e t r i c a l a n t i c l i n e or
s y n c l i n e .

26. SYMMETRICAL
FOLD

27. B. ASYMMETRICAL
FOLD
Asymmetrical folds in
profile have no mirror plane in
symmetry and the limbs are
unequal in length. In
asymmetrical fold the axial
plane is inclined.

28. B. ASYMMETRICAL
FOLD
A s y m m e t r i c a l folds in profile have no
m i r r o r p l a n e i n s y m m e t r y a n d t h e l i m b s a r e
u n e q u a l i n l e n g t h . I n a s y m m e t r i c a l f o l d t h e a
x i a l p l a n e i s i n c l i n e d .

29. C.Overturned Fold
It is an asymmetrical fold
with inclined axial plane in which
both the limbs are dipping
essentially in the same general
direction usually at different
angles.
Eg- Overfolds of krol
belt, The Himalaya. Andaman
Ridge.

30. OVERTURNED FOLD
A s y m m e t r i c a l folds in profile have no
m i r r o r p l a n e i n s y m m e t r y a n d t h e l i m b s a r e
u n e q u a l i n l e n g t h . I n a s y m m e t r i c a l f o l d t h e
a x i a l p l a n e i s i n c l i n e d .

31. D. Recumbent
Fold
This is
types
extreme
overturned
the
of
in which
folds
the axial plane becomes
nearly horizontal. In such
fold the upper limb is called
normal limb and the lower
limb is called inverted or
reversed limb.
Eg -Recumbent folds of the
Alps.

32. D.Recumbent Fold

33. E. Isoclinal
Fold
Folds having
parallel limbs are called
Isoclinal folds. In such
fold, the limbs dip at same
angle and same direction.
It is of 3 types-
1)Inclined Isoclinal fold
2) Vertical Isoclinal fold
3)Recumbent Isoclinal
fold.

34. IsoclinalFold

35. F. Conjugate Fold
In certain cases,
a pair of folds
that have
apparent mutual
inclined axial
plane. Such
folds are
described as
conjugate folds.

36. Conjugate Fold

37. G. Box Folds
In such type of folds the crest is broad and
flat and two hinges are present.

38. Degree Of
Compression
A) Open fold
In such type the
folding is mild, limbs meet at
the bends at an obtuse angle
and the thickness of the
rocks remains unchanged.

39. OPEN FOLD

40. CLOSE FOLD
Thinning at the
flanks and thickening at
the crest

41. Behaviour With Depth
A)Parallel fold
In such type the
folded layers are parallel
and the curvature is
greatest at the centre
reduces upwards in case
of upfold and downwards
in case of downfold.

42. B) Similar fold
Curvature of the
folded surface are similar and
it is thickened at the hinge and
thin along the limbs.

43. Position of axis
A) Plunging folds- when the axis makes an angle with the
horizontal plane.
B) Non-plunging folds- When the fold axis is horizontal, then it is
described as Non-plunging folds.

44. Profile Of The Outer Surface
A) Chevron fold- In this fold the hinges are sharp and
angular.
B) Fan fold- In this fold both the limbs are overturned.

45. Mode Of Occurence
A) Domes- It is an anticlinal structure which
plunges in all directions.
B) Basin- It is an synclinal depression having
controversial dip.

47. Miscellaneous Type
A) Monocline-It is due to the local
steepening of belt whereby their occurs
a sudden increase in the dip of the
bed.

48. B. Homocline
This can be well said as tilted beds
where all the beds have same amount of dip in the same
direction.

49. What isfault?
• It may be defined as a fracture surface in rock across
which there is relative motion parallel to the surface
between the adjacent blocks of the rock.

50. Fault terminology
• Fault plane: it is the planer surface along
which relative displacement of blocks takes
place during the process of faulting.

51. • The walls:
1. Hanging wall: it is the term used for
the faulted block which lies on the
upper surface of the fault plane.
2. Foot wall: it is the term used for the
faulted block which lies on the under
surface of the fault plane.

52. TYPES OF MOVEMENTS:-
There are two types of movements

53. Transitional Rotational

54. Relative movements:-
Strike slip Dip slip
Net slip
a
b a
b
a
b
d
c

55. Causes of faults:-
1. Faults are essentially the shear or sliding failures, resulting from tensional,
compressional, rotational stress acting the crustal rock masses.
2. They may be due to the shrinking Earth; or due to the convection
currents produced in the Earth.
3. Normal faults are assumed to have been formed under the horizontal
tension.
4. Thrust faults may be assumed to be originating from compressive stress,
which may throw the rocks into folds, and these intensely folded getting
fractured and faulted under shear.

56. Classificationof faults
• Following are the factors commonly considered
important :
1. The apparent movement of the rock blocks along the fault
plane.
2. The direction of the slip.
3. The relation of the fault altitude with the altitude of the
displaced bed

57. Apparent movement asbasis
• Normal faults: faults in which the hanging wall is moved
down withrespect tothe foot wall.

58. Cont….
• Horst: When two normal faults are on the either side of a
central wedge shaped block such that it appears high up
with respect to either blocks.

59. Cont….
• Graben:
• when two normal faults are on the either side of a wedge
shaped block such that it appears downwards with
respect to either blocks.

60. b. Reverse faults:
• This type of fault in which the hanging wall appears to
have moved up with respect to the foot wall and dips at
the angle more than 45 degrees

61. Thrust fault:
• These are variety of reverse faults in which the hanging
wall has moved up with respect to footwall and the fault dip
at the angle below 45 degrees.

62. c. Strike – slip faults:
It may be defined faults in which the faulted
blocks have been moved against each in the
horizontal direction along strikes.
There are two types of strike – slip
faults:
1.Right lateral strike-slip fault (dextral):
Where the side opposite the observer
moves to the right.
2.Left lateral strike-slip fault (sinistral):
Where the side opposite the observer
moves to the left.

63. On the basis of altitude (dip and strike)
• Strike faults: faults that develop parallel to the strike of strata.
In other words, the strike of the fault and that of disrupted
layers are essentially parallel.(figure a)
• Dip faults: faults that develop parallel to the dip of the
strata. In other words, the fault strike is parallel to the dip of
layers disrupted by faults. (figure b)
• Oblique faults: faults whose strike makes an oblique angle
with the strike of the rock in which it has caused the
displacement. (figure c)

66. Mode of occurrences as basis
a. Parallel faults: a group of normal faults occurring in the close
proximities having same dips and strikes. when this group gives step
like stricture in the structure then it is called step faults. (figure a).
b. Enechelon faults: Faults that are approximately parallel one
another but occur in short unconnected segments, and
sometimes overlapping. (figure b)
c. Peripheral faults: when in any region the majority of faults are
concentrated along the border or margin of the area. (figure c)
d. Radial faults: the group of fault that appear emerging outward
from a common point is called radial faults. (figure d)

68. On the basisof slip
• Slip and separation: slip may be defined as the relative
motion of rock blocks along fault planes.
1. Strike slip: the slip that occurs along the direction of
the strike of fault plane.
2. Dip slip : the slip that occurs along the direction of
the dip of the fault plane.
3. Oblique slip: the slip that occurs both in dip and strike
direction i.e. it is combination of strike and dip direction.

69. • Separation:
• The amount of apparent offset of a
faulted surface, measured in specified
direction. There are strike separation,
dip separation, and net separation.
1. Heave: The horizontal componentof
dip separation measured
perpendicular to strike of the fault.
2.Throw: The vertical component
measured in vertical plane containing

70. What are joints?
• Joints may be defined as the fractures that divide the rocks
into parts or blocks which have not been suffered any
relative motion along that fracture.

71. Cont…
• Joint set: it may be defined as group of or more joint surfaces
trending in the same direction with almost the same dip.
• Joint system: A joint system is a group of two or more joint
sets. (May have two or three intersecting sets)

73. Formation of Joints
• Joints results from brittle facture of rock body as the result of tensile
stresses(more chances) and compression stresses(less chances)
• When this happens the rock factures in a plane parallel to the
maximum principal stress and perpendicular to the the minimum
principal stress.

74. Terminology related to joints
• Nature: joints may be open or closed.
• Open joints: open joints are the joints in which the blocks have been separated for
the small distance to right angles to the fracture surface.
• Closed joints: closed joints are the joints in which the blocks are not separated from
each other but allow the fluids(water and gases) to pass through the rocks.
• Veins: generally open joints gets filled up by secondary materials which crystallize or
precipitate there forming thin or thick steaks of material. These steaks are called veins
when thin and fissure veins when there thickness is greater than 20 cm.

75. Classification
• Joints are classified on the basis of
I. Spatial relationship
II. Geometry
III. Genesis(origin)

76. On the basis of spatial relation
• Systematic joints: joints that shows distinct regularity in the
occurrences which can be easily measured or mapped. Such
joints occur in parallel joint set that are repeated in the rocks at
the regular intervals.
• Non-systematic joints: the joints that don’t posses any
regularity in their occurrence and distribution.

77. On the basis of geometry
• Strike joints: the joints which are parallel to the strike of the
rock.
• Dip joint: the joints which are parallel to the dip of the rock.
• Oblique joint: joints which are neither parallel to the strike nor to
the dip of the layer in which they occur.
• In stratified rocks some joints may develop essentially parallel to
the bedding planes. These are called bedding joints.

79. On the basis of origin of joints
1. Tension joints: joints which are developed due to the tensile force acting on the rocks.
The ,most common location of such joints are the outer margins of crests and troughs
in the folding.
1. Shear joints: these joints are commonly observed in the vicinity fault planes and
shear zones where shearing stress prevails. In folds they occur in axial regions.
2. Compression joints: rocks may be compressed too crushing and numerous joints
may result due to compressive forces. These occur in the compressive regions of the
folds like on in innermost margin of the axis of folds.

81. Engineering considerations
• DISAVANTAGES
• a. Joints are important because they split the rocks into a number
of pieces which, in turn, reduce the competence of rock mass,
increase the porosity and permeability and make them (rocks)
susceptible to quick decay and Weathering.
• b. Joints become avenues for the leakage of water in case of
reservoirs. If they are closely spaced in the upstream side, silting
problems also arise in reservoirs.

82. Cont..
c.The incompetence, leakage and other effects introduced by
joints in rocks may create foundation problems at dam sites.
d.Joints may pose ground water problems in tunneling.
e.Depending on the relation of the dip of joints in rocks with
reference to the surface slope, they may pose problems in
laying roads and railways along hill slopes.

83. ADVANTAGES
1. Increase the ground water potentiality in any place.
2. Suitable spaced joints (i.e., with neither very close nor very wide joint
intervals) not only facilitate the quarrying process or tunnelling process but
also reduce the cost by decreasing the use of explosives.

84. Unconformity
• It is one of the most common geological feature
found in rocks or in succession.
• It is different then all other geological structures
viz. the fold, joints and faults
• Unconformities are resulted due to tectonic activity
in form of uplift or subsidence of land
• It is referred to a period of non-deposition

85. • The fundamental "laws" of stratigraphy, formulated in
the 17th Century by Nicolas Steno, is the law of
Original Horizontality, which is known as Conformity
• That is, any deposition when takes place is totally in
horizontal fashion
• Later due to tectonic movement the layers or beds are
tilted (except in case of cross-bedding- which are formed
under fluvial (riverine) or aeolian (wind) environment.

86. Reasons for Unconformities
• Formation of unconformity involves:
• Horizontal or conformable strata or beds are formed
• Break in sedimentation or deposition
• Happens due to tectonic movements, that causes uplift or
subsidence of land surface
• Next phase of Deposition or sedimentation cycle, where
new sedimentation produce another set of conformable
beds

87. Types of Unconformities
• Non-conformity
• When the underlying rocks are
Igneous or Metamorphic (i.e.
unstratified) and the overlying
younger rocks are sedimentary
(stratified) = Non-conformity
• Disconformity
• When the underlying (older)
and overlying (younger)
sedimentary rock strata are
parallel and the contact plane
is an erosional surface=
Disconformity

88. • Angular unconformity
• When the underlying (older)
rocks and overlying (younger)
rock strata show some angle
w.r.t one another=Angular
unconformity

89. Angular unconformity • This sub-area in northern Chile
Showing a geological angular
unconformity: a contact
between layers of rock at
different angles.
• On the right side of the image,
Cretaceous sediments were
tilted upward to an angle of
about 50 degrees, then eroded.
On this surface volcanic
pyroclastic deposits were
deposited as a flat sheet.
• The section of rocks has been
eroding from the east, exposing
the tilted and flat rock layers.