College of
Science
College of
Science
Main Subjects:
➢ Introduction to Crustal Deformation
➢ Deformation (Stress & Strain)
➢ Types of Deformation (Strain)
➢ Folds Loading…
➢ Faults
➢ Joints
What is Crustal Deformation?
Rocks in the earth's crust have been subjected to severe
deformations resulting from tectonic forces.
These tectonic forces produced bending, rumpling, overturning
and fracturing of rocks.
What is Crustal Deformation?
➢ Sometimes, crustal deformation is
too large and causes movement of
rocks for hundreds of kilometers
(Thrusting).
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➢ Crustal movement can also be only
a few meters during earthquakes.
➢ Rifting (Spreading) and extension of
the crust produce elongated
depressions (e. g. The Red Sea) and
create ocean basins.
Deformation
➢ Deformation results in one or all of the following...
- Translation – Change in location.
- Rotation – Change in orientation.
- Distortion – Change in shape.
Deformation
● Deformation means all
changes in volume and or
shape of a rock body.
● Most crustal deformation
occurs along plates margins.
● As the plates interact along
their boundaries, tectonic forces
deform the involved rock units.
Causes of Deformation (Stress)
- Types of stress:
- Compressional – Squeezing.
- Tensional – Pulling-apart.
- Shear – Sliding past.
Deformation (Strain)
● Elastic strain – Reversible change in shape.
● Permanent strain – Irreversible change in shape.
● Two types of permanent strain: ductile and brittle.
● Changes in shape caused by deformation.
- Stretching – Pulling apart.
- Shortening – Compressing, squeezing.
- Shear – Sliding past.
Deformation (Strain)
- Ductile deformation: Rocks deform by flow and folding.
- Ductile deformation occurs in the deeper crust.
Plastic
S
tr Brittle
e Elastic
s
s
Strain
Deformation (Strain)
- Brittle deformation: Rocks break by fracturing
- Brittle deformation occurs in the shallow crust.
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Plastic
tr Brittle
e Elastic
s
s
Strain
Geologic Structures
● Geologic structures are
those geologic features
such as folds, faults, and
joints which result from
crustal deformation.
● They are called secondary
structures because they are
formed after the formation
of the rock containing them.
Significance of Studying Geologic Structures
• Most occurrence of oil and natural gas are associated
with geologic structures that act to trap these fluids in
valuable "reservoirs“.
• Rock fractures can also be the site of hydrothermal
mineralization, which is a major source of metallic ores.
• The orientation and characteristics of rock structures
must be considered when selecting sites for major
construction projects such as bridges, hydroelectric
dams and nuclear power plants.
A- Folds
● Layered rocks may be
deformed into curves called folds.
● Folds occur in a variety of
shapes, sizes and geometries.
●Folds often occur in a series.
● Orogenic settings produce large
volumes of folded rock.
● Deformed rock often
experiences multiple events.
A- Folds: Parts of the Fold
● Limbs: are the two sides of the fold.
● Fold Axis: is a line drawn along the
points of maximum curvature of each
layer.
● The fold axis is usually horizontal
(parallel to the surface). However, in
more complex folding, it is often
inclined at an angle known as the
plunge.
● Axial Plane: is an imaginary surface
that divides a fold as symmetrically
as possible.
A- Folds: Types of Folds
● 1- Anticline:
is a structure formed by
the up folding or
arching of rock layers.
The oldest strata are
found in the center
and the two limbs
usually dip away from
each other. Anticline Syncline Anticline
A- Folds: Types of Folds
● 2- Syncline:
is a structure formed by
the down folding of
rock layers. The
youngest strata are
found in the center
and the two limbs dip
towards each other.
Anticline Syncline Anticline
A- Folds: Types of Folds
Anticlines and synclines are usually found in
associations where the limb of an anticline is also
a limb of the adjacent syncline.
Anticline Syncline Anticline
A- Folds: Types of Folds
● A symmetrical fold is one in which the axial plane is vertical.
● An asymmetrical fold is one in which the axial plane is inclined.
● An overturned fold, or overfold, has the axial plane inclined to such
an extent that the strata on one limb are overturned.
● A recumbent fold has an essentially horizontal axial plane.
A- Folds: Types of Folds
● A plunging fold is one that has its axis tilted from the horizontal
so that the fold dips into the earth.
A- Folds: Types of Folds
Only one limb dipping
● Monoclines are broad flexures
horizontal horizontal
(bending) that have only one limb.
They usually result from vertical
displacement and not from
Basement rocks
compressional stresses as the other
folds.
● Homocline is a group of strata that
have a consistent dip over large
area.
A- Folds: Types of Folds
➢ Domes and basins are circular to oval structures which have
rock layers occurring in age-position contexts which are the same
as anticlines and synclines.
B- Faults
● Faults are fractures along which the opposite sides
have moved relative to one another and parallel to the
fracture surface.
● Faults are abundant and occur at a variety of scales.
● Faults may be active or inactive.
● Sudden movements along faults cause earthquakes.
● Faults may offset large blocks of Earth.
● The amount of offset is a measure called
displacement.
B- Faults: Parts of Fault
● Fault Plane: Is a flat surface
along which displacement
takes place.
● The hanging wall lies above
the fault plane, and the
footwall lies below.
● Throw: Is the amount of
relative displacement across
a fault plane.
B- Faults: Parts of Fault
Hanging wall
throw
Foot wall
Fault plane
B- Faults: Types of Faults
● Normal Faults: when the hanging
wall moves down relative to the
footwall.
● Reverse Fault: when the hanging
wall moves up relative to the footwall.
Reverse faults having a very low
angle are also referred to as Thrust
faults; in such case the fault plane is
often parallel to the bedding planes.
B- Faults: Types of Faults
● Strike-slip fault: in which the
dominant displacement is
horizontal and parallel to the strike
of the fault surface.
● The two blocks of rock simply slide
past each other with no
compression or extension.
● If the motion is to the right, it is
called a Right-lateral fault. If the
motion is to the left, it is called a
left-lateral fault.
B- Faults: Types of Faults
Strike Slip Fault are associated with transform plate boundaries
B- Faults: Types of Faults
Transform Faults and their occurrences
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B- Faults: Types of Faults
● The faulting can be
combinations of strike-slip
and normal or reverse fault,
in such case the fault is
called Oblique slip Fault.
● Strike-slip faults are the
most common style of
faulting at transform plate
margins.
B- Faults: Types of Faults
● Graben: when one block of the rock subsides relative
to the adjacent blocks.
● Horst: when one block of a rock rises relative to the
adjacent blocks.
C- Joints
● Planar rock fractures without offset.
● Result from tensional tectonic stresses.
● Systematic joints occur in parallel sets.
● Minerals can fill joints to form veins.
● Joints control weathering of rock.
C- Joints
Joints can also occur in
igneous rocks as a result
of cooling and are called
"Columnar Jointing" in
which the rock cracks into
columns with hexagonal
cross section.
C- Joints
Most Joints are the result of brittle failure when rocks in
the outermost crust are deformed.
End of Chapter 8
Chapter 9: Earthquakes
