chapter5.pptx

Structuralgeology
•
•
•
•
•
Rock deformation and reasons
Study of folds, faults, and joints cleavage
Introduction to dip, strike and outcrop
Unconformity
Orientation of geological strata using geological maps, plans and cross sectionss

Structuralgeology
• Structural geology is the branch of geology that is concerned with the three
dimensional distribution of rock units and their deformation histories in both large
and small scale.
It infers all the structures present in rock that are formed during or after the
formation of the rock layers.
It aims to characterize deformation structures in order to characterize flow paths
followed by particles in rock during deformation due to tectonic forces
•
•

Rock Structures
•
1.
2.
•
Rock structures may be broadly divided as;
Primary Structures and
Secondary structures
Primary structures are the structures developed in rock body during or shortly after
it’s formation stage. It is generally found in sedimentary rock hence are also called
Primary sedimentary structures.
Beddings, lamination, cross beddings, graded beddings, ripple marks, sole marks, mud
cracks etc. are the examples of primary structures which have been discussed in
previous chapter
.
•

Rock Structures
• Secondary structures are the structures formed after the formation of rock layers
due to the different tectonic forces acting upon the rock.
Secondary structures are also called deformational structures as they are formed by
breaking, bending, stretching and compression of the original structures in the rock.
Lineation, foliation, boundinage, crenulation cleavage, fault, folds, joints, thrusts are
the examples of secondary structures.
•
•

• Deformation in rock may result due to different stresses acting upon the lithospheric
plates or temperature. Stresses are not acting equally on all the direction so giving
result to differential stress.
Differential Stresses acting upon the rock can be of three types;
Compressional stress
Tensional stress
Shear stress
Compressional stress may result due to the converging processes at the convergent
plate boundaries and cause the rock to squeeze, bend, fold and upthrust.
•
1.
2.
3.
•

• Tensional stress may arise as a result of stretching
and thinning of the crust rock causing the rock to
break and normal faulting.
Shear stress may arise where the two stresses are
acting on a same plane but in opposite direction
causing the blocks to slide past each other
. Shear
stress can be encountered at the transform plate
boundaries giving transform faults.
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Responseof rock
• The response of rock depends upon the rock type, surrounding temperature and the
pressure conditions, type of stress and length of time the rock is under stress.
Rock may show three types of behavior to stress;
Brittle deformation: rock breaks down into pieces on applying any kind of stress
Ductile deformation : rock initially bends on applying stress and ultimately breaks
with increase in amount of stress. Ductile deformation experiences through three
different phases of deformation;
•
1.
2.
a.
b.
c.
Elastic deformation
Plastic deformation and
Fracture or fail

Responseof rock
• At first, the deformation in rock is elastic, that means the body returns to it’s original
shape and size on removal of stress.
In the figure, the deformation stages at point A and B are elastic deformation and
can return to its original state.
At the point C, the rock crosses elastic limit and can not return to its original state
and enters the plastic region. Now the rock obeys, Hooke’s law which state that
strain is directly proportional to stress.
The body starts thinning and necking with increase of stress and ultimately fails after
point F causing rupture.
Rock behave differently at different depth. Rock at surface may be brittle but at the
depth ductility increases.
•
•
•
•

Attitudeof geologicalfeatures
• Attitude means the three dimensional orientation of some geological features such
as bed, fold , fault or joints and foliation.
The attitude of planar feature is defined by the strike, dip and dip direction.
Strike: Strike is the imaginary horizontal line on the bedding plane or any geological
plane representing intersection of that feature with horizontal plane. It is expressed
as compass direction of that line with respect to geographical north. For example,
strike is measured as N30°E, that means the strike line is oriented 30° from North
towards east.
Dip: Dip or dip amount is the acute angle made by the rock surface with the
geographical horizontal plane.
•
•
•

Attitudeof geological features
• Dip direction is the direction of the
inclined geological plane represented in
the azimuth number followed by a
degree sign. The relation of dip
direction is perpendicular to the strike
direction.
In map, the attitude of bed is
represented by a symbol resembling
capital letter ‘T’; where the relative
length is reversed. The longer line is
plotted parallel to strike of the bed and
the perpendicular shorter line shows
the dip direction, whereas dip amount
is noted near the shorter line.
•
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Measurement of the attitude
•
•
The attitude of any geological plane can be measured in field using Brunton compass.
A brunton compass consists of a magnetic needle that oscillates freely inside the
case for the reading, two bubble levels for horizontal leveling; one for dip amount
and other for the circular reading.
The bubble is centered in the red circle of bulls eye level for the horizontal reading.
The reading can be taken either in azimuth or in the quadrant.
•
•

lift pin Index pin
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Measurement of the
Brunton compass can be used to;
attitude
•
•
•
•
•
Locate North and set local declination.
Measure bearings
Measure strike and dip of planes
Measure trend and plunge of lines
Measure vertical angles.

Folds
• Folds are the geological features, size ranging from meters to hundreds of kilometers,
formed by the ductile deformation of rock strata by bending and curving without
breaking and fracturing due to compressional stress.
• Folds can be observed in sedimentary, metamorphic or igneous rock.

Parts of fold
• Axis or axial line is the straight line parallel to the hinge. The lines parallel to the
axis generates the fold itself
Hinge is the line of maximum curvature in the folded bed.
Axial plane is the plane parallel to axis of the fold that connects all the hinges
and cuts the fold into two equal halves.
Core is the in lying part between the limbs of the fold.
Limb: The side flanks of the fold are termed as limb. The limb extends from the
axial plane in one fold to the axial plane in next of an adjacent fold.
•
•
•
•
Crest and trough: These are concave and convex portions of wavy undulation.

Axial
Na.11e
Points of
:rt'J,a~immn
Curvature • trillte t.ine
~
Figure: Parts of Fold

Types of fold
i. On the basis of the shape
into two:
Ð Anticline fold
Ð Syncline fold
of the fold and the relative age of rocks, folds are classified
In anticline fold, the shape of the fold is convex upward, the limbs dip away from each
other and the older rocks occupy the core of the fold (Fig. 9.3). In the case of syncline,
the shape of the fold is convex downward, the limbs dip towards each other and the
younger rocks occupy the core of the fold.

Figure: a)Anticline b) Syncline

b) Classification based upon orientation
plane
of axial
A) Symmetrical:
•
•
•
B)
When both limbs form mirror image of each other
Limbs of both sides dip at equal angle of opposite direction
Axial plane is vertical
Asymmetrical:
• One limb dips steeper than the other
• Axial plane is inclined
c) Recumbent:
Both limbs are horizontal
Axial plane is horizontal
D) Overturned:
Such folds are characterized by inclined axial plane with the limbs
dipping essentially in the same general direction

Anticlines
c =
Symmetrical Asymmetrical Overturned Recumbent
Synclines
(--
Recumbent
Symmetrical Asymmetrical Overturned

C) Classification based upon interlimb
angle
a.Isoclinal fold: Folds with parallel limb, dipping in the same general
direction with equal angle is called isoclinal fold.
b. Tight fold: Folds with less than 30˚ interlimb angle
c. Close fold: Folds with 30-70˚ interlimb angle
d. Open fold: Folds with 70-120˚ interlimb angle
e. Gentle fold: Folds with 120-180˚ interlimb angle

D) Classification based on shape of Fold
• Similar fold: In such fold, the degree of folding is observed to be similar for indefinite
depth. The axial planes are thicker than limbs.
Parallel fold: Such fold is characterized by equal thickness of beds throughout the
sequence. Parallel folds are also called concentric folds and are characterized by an
equal thickness of the beds throughout the folded sequence. This feature results in a
change in the shape of the surface and a corresponding change in the form of a fold
both in
•
an upward and downwards.
• Drag folds are formed when a competent bed slides past through an incompetent
bed causing the incompetent bed to drag and deform into small folds.
Chevron folds: The hinge in this fold is sharp and angular
.
Box fold: The crest and trough are broad and flat and two hinge clues present.
•
•

c
Similar fold Parallel fold
l
b
Box fold

E) Classification based on plunge
of hinge line: Plunging Fold
s
a) Plunging fold: Hinge line is
horizontal, inclined or vertical
Non-plunging fold: The beds
on the opposite limbs strike
parallel to each other. They
don’t converge.
b)
Fold axis

3 A horizontal fold
.............._
axis is horizontal ... ~
Horizontal
fold
Figur~ 7-10b
Und~rstandlngEarth,FifthEdition
0 2007 W. H.Freeman and Company

Classificatio
n
and Types of
Folds

Causesof folding
•
•
•
•
Folding due to lateral compression
Folding due to intrusion
Folding due to overburden
Folding due to differential compression.

Recognition of fold in field
• Local or small scale folds can be directly observed in the fields in cut slopes during
construction and excavation for special purpose. But the large scale folds in the field
are difficult to recognize.
For such conditions the following techniques are followed:
Direct observation of folds is an effective way to recognize folds. But sometimes it
may not be easy in large scales due to erosion and removal of the crest part.
•
•
vDipping of the same rock strata in different direction indicates the presence of fold.
In such case, plotting the attitude in map and inferring the feature can be helpful for
the recognition of folds.

Recognition of fold in field
vEncounter of the certain ore minerals or salt deposits in a specified locality can also
indicate the fold. Since most of the ore, metals and mineral deposits are formed at
the core part of the fold.
vRepetition of strata: In regional mapping, if there exist repetition of strata in cyclic
order then the presence of fold can be predicted.
vGeophysical survey can be the most reliable technique for the recognition of the
folds.

Engineering significance of fold
• Folds can be an important feature for the mining engineers since most of the
petroleum deposits are encountered at the core of the domes or folds.
Folds may be formed due to the intrusion of magma deposits beneath the rock strata
which may contain economic minerals and precious gemstones.
Folds may be problematic in any engineering construction projects. Since folds are
the deformational structures, the rock at the hinge area of the fold are highly
strained and cause collapse of the structure if encountered as site rocks.
At the reservoir site, the curvature of the rock strata in synclines causes seepage of
the water in reservoir through the planes of discontinuities and ultimately increases
the risk of dam collapse.
•
•
•

Engineering significance
• Folds, if encountered at the tunnels can complicate the structure and can create the
stability problem if found at roof or floor. Axial region in the folded rocks should
either be avoided if possible or be treated well.
Folded rocks are strained and when subjected to small effort by nature or engineer
to disturb the adjustment, the rock may respond by release of energy. The release of
energy in tunnels during excavation is evident where huge blocks of rock starts
caving and falling with great force called rock bursts.
•

Study of Fault
Hanging wall
Footwall

Faults
• Faults are planar fracture or discontinuity in the rock mass along which
either side of the walls have moved past each other parallel to the plane.
The relative movement of the blocks along the fracture plane is essential
to be fault.
Faults are the deformational structures formed by the breaking of rock
due to brittle deformation followed by the relative movement along the
fault plane.
•
•

Parts of Fault
• Foot wall: The block below the fault
plane is termed as foot wall.
Hanging wall: the block above the
fault plane is termed as hanging wall
Fault plane: The plane surface along
which the block moves is termed as
fault plane.
Fault line or fault trace: is the line of
intersection of fault with surface of
earth.
Fault scarp: The exposed inner
surface of the fault plane due to
slipping down of the block.
•
•
•
•

Terminologies related to fault
• Generally, we cannot determine which side of the fault has moved, it could be either
both or we can only determine the relative motion between them.
The total displacement of two opposite blocks of the fault is termed as net slip.
If the slip of the block is along the dip direction of the fault, it is termed dip slip fault.
If the slip of the block is along the strike direction of the fault, it is termed as strike
slip fault.
In fault, the vertical displacement is termed as throw and the horizontal
displacement is termed as heave.
•
•
•
•

Types of Fault
•
1.
2.
3.
On the basis of the relative movement, faults are classified as;
Normal Fault:
Reverse Fault:
Strike slip fault:

Types of fault
1.
•
Normal fault:
The normal fault is the fault where the hanging wall moves down relative to the
footwall.
It results due to the tensional stress in the rock where lengthening and extension of
the crust occurs.
Mostly occurs in small scale with the displacement not more than 1m.
Larger scale normal faults are associated with the formation of block mountains.
Normal faults occur in the areas of regional crustal extension.
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•
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•

Types of fault
2.
•
•
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Reverse fault:
The fault in which the hanging wall block moves up relative to the foot wall block.
It results due to compressional stress and cause shortening of the crust.
When the reverse fault plane has the dip of less than 45 degrees, it is called thrust.
By definition, Thrusts are the low angle(<45°) reverse fault.
Thrust fault bring the older rock sequence over the younger rock.

Types of fault
3.
•
Strike slip fault:
If the slip is parallel to the strike direction of fault plane and the displacement is
horizontal, the fault is termed as Strike slip fault.
Strike slip fault results due to the shear stress and cause the distortion of any
horizontal feature.
•

Types of fault
• Oblique fault: The fault that have combination of both movement, dip slip and strike
slip, is termed as oblique fault.
• The displacement is in two direction vertical and horizontal.

Effects of faulting
•
•
•
Fault is the feature associated with rock deformation and rupturing.
Active faults may be the primary reason for the earthquakes.
Displacement and disruption of the ground and the structures lying over
the fault zone.
Offset of the river course due to faulting causes bending and meandering
of river
.
Development of the hot springs.
•
•

Field identification of faults
• Fault have distinguishable line of discontinuity however the line may not be clean
and smooth as expected. In such condition, the fault zone is termed for the zone of
complex deformation associated with the fault plane.
For identification of such fault zones, following features can be observed.
Presence of slickensides, grooves and striations. The covered or exposed rock with
such polished slickenside surface with striations in the direction of movement and
grooves are the evidences produced due to friction along the plane of blocks moved.
Presence of brecciated zone and sheared fault gauze provides the witness for the
fault.
•
•
•

Field identification of faults
• Encounter of the textures in the rock such as augen structure, porphyroblasts etc.
can also serve for the evidence of faulting.
Disturbance in the normal sequence of rock strata.
Repetition and omission of the rock layers is common in faulting.
In some regions, numbers of springs may occur along almost same line in the slope.
Interruption or offset of streams causing break in their profile can also indicate the
fault in past history.
Geophysical survey for various application may indicate faults.
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•
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• Faults can only be problematic feature for the civil engineers as they are the
deformational structures resulted due to breaking and dislocation of rock beneath.
Faulted rock are weak at foundation and may result to the damage and collapse of
the engineering structures.
The probability of further slippage in the fault in future is unknown. So an engineer
should consider whether the fault is active and can slip again within the lifespan of
the project.
As faults are the planes of discontinuity, it can form an easy pathway for the leakage
of water in the dams and reservoirs and ultimately result to the failure of the
structure.
•
•
•

• If the fault is encountered along the alignment of tunnels, it forms an outlet for the
leakage of groundwater and may lead to debris flow in the tunnel. If the water table
is above the tunnel alignment, the fault can serve for the collapse of roof and tunnel
itself.
Water also work as a lubricant in the fault plane and cause further slippage in the
fault.
The major fault zones are prone to earthquakes, landslides and fragile geologically.
So major consideration should be given while building any structure in such zones.
Fault gauge and breccia may create additional problems and they have to be cleared
to the sound bedrock for the construction in site.
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Fracture
• Fractures are any local separation or discontinuity plane in rock that breaks the
continuity in rock and divides into two or more pieces.
Fractures develops as a result of stress exceeding rock strength and sometimes
develop deep fissure or crevices in the rock.
Fractures provide permeability for free movement for fluids and hydrocarbons within
the rock. Highly fractured rock can form good aquifers and hydrocarbon reservoirs
since they posses both permeability and fracture porosity.
Fracture may be resulted by contraction due to fall in temperature or different
stresses acting on the rock.
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Joints
• Joints are the brittle-fracture surface in rocks along in which little or no displacement
has occurred.
The displacement might be towards perpendicular direction to the plane of
discontinuity but essentially not parallel to the discontinuity plane.
Present in nearly all surface of rocks and extend in various directions, generally more
towards the vertical than to the horizontal.
Joints may occur in well defined sets whose the members are oriented essentially
parallel to the each other
.
Since joints are the planar feature, it’s orientation can be measured similar to the
faults.
•
•
•
•

Joints- Classification
•
•
•
A. Based on opening and filling
Joints may be open ( jt 1 in fig) or closed ( jt 2 in fig)
Joints may open as a consequence of denudation( erosive process of breaking of
rock) , especially weathering or dissipation of residual stress.
All joints are initially tight and closed. Due to gradual weathering process, the tight
fractures are enlarged and opened to wide fissures, separating the blocks apart in
the direction perpendicular to the plane.
Open joints are commonly filled with minerals which reduces the shearing resistance
along the joint surface.
Closed joints are tight enough and posses no separation but allows fluids to pass
through the rock.
•
•
•

B. Based on origin or mode
( Genetic classification)
of formation of rocks
• Tension joint: developed due to
tensile force acting on the rock.
Common location for such
joints are the outer margin of
the folded sequence.
Shear joint: developed at the
vicinity of the fault planes and
shear zones .
Compression joint: developed
due to compression stress
acting on the rock. Common
location of such joints are at
the core of the folded
sequence.
•
•
r
~

C. Based on geometry
• Strike joint: joint developed parallel to the
strike of the bed.
Dip joint: joint developed parallel to the dip
direction of the bed.
Oblique joint: joint developed at the angle
between the dip and strike direction of the
associated bed.
Bedding joint: the joint developed parallel to
the bedding plane or the bedding itself is a
joint.
• /oblique joints
,,,.-}"7 (conjugate
.,/·"/
/
shearjoints)
•
dip
joint
• maximu
m
principa
l stress
directio
n
Fig: Joint classification based on geometry

D. Classification based
relationship
a) Regular joints:
on spatial
•
•
•
b)
•
•
•
Occur in parallel or sub- parallel joint sets
Repeated at regular intervals
Show a distinct regularity in occurrence Eg: Columnar,mural and sheet joints
Irregular joints
Occur randomly between systematic joints
Do not show any regularity in occurrence and distribution
Have curved and rough surfaces

Fig. 7.47 Mural Joint ~ f
Fig. 7.48 Columnar
Joint·
Fig. 7.49 Sheet Joint Fig. 7.50 Tension Joim
.~
.-'

• Mural joint: usually in granitic rock. It occurs with three joint sets in such a way that
one set is horizontal and other two are vertical, all three sets being mutually at right
angles to each other. This geometrical distribution of joint divides rock mass into
cubical blocks or murals called mural jointing.
Columnar jointing:
This is typical for volcanic igneous rocks. These are also called prismatic joints.
•
•
Sheet joints: In granites, and other igneous rocks, a horizontal set of joint often divides
rock mass in such a way that it gives appearance of a layered sedimentary structure
called sheeting structure or sheet joints.

Causesof jointing
• Contraction during rock formation: In the igneous rock, the contraction due to
cooling develops tensile stress in rock giving rise to jointed blocks.
Cyclic Expansion and contraction: The repeated expansion and contraction in rock
due to change in temperature also cause the development of prominent joints in
rock. This is most common in mountainous region.
Crustal disturbance: Due to lateral stresses(tensional, compressional or shear) acting
on the crust due to tectonic activities, joints are developed in rock.
Release of load: When the stress is removed from the rock lying under load are
released, the joints are developed parallel to the direction of load application.
•
•
•

Joint spacing
•
•
•
•
Joint spacing is the distance between any two adjacent joint planes of same joint set.
Joint spacing is often consistent within a specific rock in a specific environment.
The closer the joint spacing, lesser will be the strength of rock.
The fine grained rocks tend to have close spaced joint sets whereas the coarse
grained rock tend to have wide spaced joint sets.
W DELY SP CED CLOSELY SPACED
JOINTS
JOINTS

• Joints are the most prominent feature found in rock and has great influence in the
strength of rock which directly affects the stability of any engineering structure.
Joints are the source of weakness through which easy pathway for the leakage of
water in rock form.
For the site selection of the any civil structure such as dam, reservoir, tunnel
alignment or highway, a detail investigation about the joints should be done for the
economic and safe design.
If the rock in the foundation of the reservoir is heavily jointed and the water table is
low, there is high risk of water leakage through these joints under the dam and can
cause the failure.
•
•
•

• If the roof and side walls of the tunnel is highly fractured, the risk of rock slippage
and water leakage through these joint sets can cause great trouble and ultimately
resulting to collapse of tunnel.
If large joint set is dipping across the alignment of highway cut, the site is prone to
potential landslide.
Most of the landslides and slope failures occurs in the heavily jointed rock.
By understanding the nature and distribution of the joints, they can be treated by
grouting, bolting and shortcreting for the safety of the civil structure.
•
•
•

Unconformity
• Unconformity are the surface of
erosion or non deposition that
separates younger strata from the
older rock sequences.
Unconformities give the tectonic
history that had happened in the past.
Upper rocks are usually much younger
than lower rocks and there is a break
in geological record.
•
•

Types of Unconformity
• Angular unconformity: Those unconformities in which the strata in opposite side of
the erosional surface are not parallel and makes certain angle.
• The older strata of the rock sequences are tilted due to some tectonic cause and
eroded due to depositional gap. After some time, the new sequence of rock were
deposited giving the angular relation with the older sequence.

• Disconformity: The unconformity in which the rock strata of older sequence below
the erosional surface is parallel to the younger sequence.
Here no any tectonic upliftment has occurred.
The older sequence is separated from the younger sequence by the layer of
conglomerate, gravel or mixed rock.
•
•

•
•
•
Local unconformity: the erosional surface that extends only to local extent.
The time involved is short.
This results due to scouring action of stream in back and froth movement during the
flood.
Nonconformity: the unconformity in which the older rock is of plutonic origin.
•

Nonconformity
1. Granite formed.
2. Granite exposed by erosion.
3. Beds 1-3 deposited.
Angular
1 .
Beds
Unconformity
1-6 deposited.
Disconformity
1. Beds 1. 2
deposjted
2. Erosion
2. Beds 1-6 tilted.
J. Erosion
a, Beds 5-7 de
posited
4. Beds 9~ 1O
deposited.

Engineering significance and Field
identification
• As it involves erosional, depositional as well as tectonic processes, proper
considerations need to be taken in construction sites with unconformities.
• It gives the record about the geological processes that happened in the past.
Identification:
Can be easily recognized in field as there is a sharp contrast in color and rock type
between the rocks above and below a certain layer
.

Some more deformation structures
• Lineation:
Linear structure within rocks
Any linear arrangement of rocks or minerals such as parallel arrangement of elongated
mineral grains
Occur due to tectonic, mineralogical, sedimentary or geomorphic process.
• Foliation:
Structural feature formed by preferred orientation of minerals
It is a repititive layering in metamorphic rocks caused by shearing forces or differential
pressure during metamorphism.
• Crenulation clevage:
It is a special case of foliation. It is a microstructure created in metamorphic rocks. It is
formed when an early formed planar rock microstructure is again stressed to form
another planar structure.

Boudinage:
•
•
They are also called sausage structure.
When a tubular rigid body of competent bed rock is stretched, deformed it forms
boudins. These are ductile deformational structures.
They are typical features in shear zones.
•

Fabrics: Foliation and Lineations
There are bo h tectonic and primary fabrics
RandomFabrics PreferredOrientation
Folia ·on Lin a ion

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