Transcript Structural Geology (Geol 305) Semester (071)

Structural Geology
(Geol 305)
Semester (071)
Dr. Mustafa M. Hariri
FRACTURES AND
FAULTS
Objectives
This unit of the course discusses Fractures and Faults
By the end of this unit you will be able to:
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Differentiate between the different type of fractures
Differentiate between the different type of faults
Understand the relationship between the different type of
stresses and faults
Where faults form and how?
Faults mechanics
Role of fluid in faulting
Faults movement mechanisms
Shear, Shear zones and different type of shears
FRACTURE
FRACTURE: is defined by Twiss and
Moores (1992) as “..surfaces along
which rocks or minerals have broken;
they are therefore surfaces across
which the material has lost cohesion”
Characteristics of fractures according to
Pollard and Aydin (1988)
 fractures have two parallel surfaces that
meet at the fracture front
 these surfaces are approximately planar
 the relative displacement of originally
adjacent points across the fractures is
small compared to the fracture length..
Fracture, Joint and Fault
The term fracture encompasses both joints
and faults.
JOINTS: are fractures along which there has
been no appreciable displacement parallel to
the fracture and only slight movement normal
to the fracture plane.
Joints are most common of all structures present in all settings in all kind of rocks as well
as consolidated and unconsolidated sediment
Types of Fractures
Extensional Fracture
In extensional fractures the Fracture plane is oriented parallel to σ1
and σ 2 and perpendicular to σ 3.
Three types of fractures have been identified:
 Mode I fractures (joints) it is the extensional fractures and formed by
opening with no displacement parallel to the fracture surface (see
above figure).
 Mode II and Mode III are shear fractures. These are faults like
fractures one of them is strike -slip and the other is dip-slip
Same fracture can exhibit both mode II and mode III in different parts of the region.
Importance of studying joints and
shear fractures
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To understand the nature and sequence of
deformation in an area.
To find out relationship between joints and
faults and or folds.
Help to find out the brittle deformation in an
area of construction (dams, bridges, and power
plants.
In mineral exploration to find out the trend and
type of fractures and joints that host
mineralization which will help in exploration.
Importance of studying joints and
shear fractures
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Joints and fractures serve as the plumping system for
ground water flow in many area and they are the only
routes by which ground water can move through igneous
and metamorphic rocks.
Joints and fractures porosity and permeability is very
important for water supplies and hydrocarbon reservoirs.
Joints orientations in road cuts greatly affect both
construction and maintenance. Those oriented parallel to
or dip into a highway cut become hazardous during
construction and later because they provide potential
movement surfaces.
TYPES OF JOINT
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Systematic joints: have a
subparallel orientation and
regular spacing.
Joint set: joints that share a
similar orientation in same area.
Joint system: two or more joints
sets in the same area
Nonsystematic joints: joints
that do not share a common
orientation and those highly
curved and irregular fracture
surfaces. They occur in most
area but are not easily related to
a recognizable stress.
Some times both systematic and nonsystematic joints
formed in the same area at the same time but
nonsystematic joints usually terminate at
systematic joints which indicates that
nonsystematic joints formed later.
Type of Fractures
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Plumose joints: joints that
have feathered texture on
their surfaces, and from this
texture the direction of
propagation of joints can be
determined.
Veins: are filled joints and
shear fractures and the
filling range from quartz and
feldspar (pegmatite and
aplite) to quartz, calcite and
dolomite.
Type of Fractures
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Conjugate fractures: paired
fracture systems, formed in the
same time, and produced by
tension or shear. Many of them
intersect at an acute angle which
will be bisected by the
Curved fractures: occur
frequently and may be caused by
the textural and compositional
differences within a thick bed or
large rock mass or they may a
result of changes in stress
direction or analysis.
Cross cutting relationship and material
filling the fractures can help in resolving
the chronological order of deformation.
FRACTURE ANALYSIS
Study of joints in an area will give information about the
sequence and timing of formation. It will also provide
information on the timing and geometry of the brittle
deformation of the crust and the way fractures propagate
through the rocks.
Importance of Fracture
Study of orientation of systematic fractures
Orientation
provides information about the
orientation of one or more principle
stress directions involved in the brittle.
Parameters measured for fractures are strike
and dip.
Or strike of linear features from aerial photos
and landsat images.
Data obtained from fractures is plotted in
rose diagram or equal area net. Equal
area net for strike and dip and rose
diagram for strike only.
Studies of joint and fracture orientation
from LANDSAT and other satellite
imagery and photographs have a variety
of structural, geomorphic, and
engineering applications.
Strain -ellipsoid analysis
of joints in area may
help to determine
dominant crystal
extension directions
Fold and Joints
Joints may form during
brittle folding in a
position related to the
fold axis and axial
surface as follows
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parallel
normal
oblique
depending on stress
condition.
Fault Related Joints
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Joints are also formed
adjacent to brittle faults, and
movement along faults
usually produces a series of
systematic fractures.
Most joints form by extensional fracturing of rock
in the upper few kilometers of the Earth's crust.
The limiting depth formation of extension fractures
should be the ductile-brittle transition.
Factors Affecting the Formation of Joints
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Rock type
Fluid pressure
Strain rate
Stress difference at a particular
time
Characteristics of Fractures
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Plumose structure: is the
structures formed on the
joint surface during its
propagation and provides
information about the joint
propagation direction.
Hackle marks: indicate
zones where the joint
propagate rapidly.
Arrest line: forms
perpendicular to the
direction of propagation
and is parallel to the
advancing edge of
fractures.
Characteristics of Fractures
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Bedding and foliation planes in coarsegrained rocks constitute barriers to join
propagation. Bedding in uniformly finegrained rocks, such as shales and
volcanicalstic rocks, appears to be less
of barriers.
In sandstone bed propagation of
joints through the bed is slightly
offset from the layers above or
below.
Variation in bed thickness also affects
propagation direction.
In horizontal layering joints will not
propagate from sandstone into shale
if the least principle horizontal stress
in shale is greater than that in
sandstone.
Fractures will be terminated at the
contact between the two rocks.
Joints Classified According to their Environment
and Mechanism of Formations (Engelder, 1985)
Tectonic fracture
 Hydraulic fracture
 Unloading fracture
 Loading fracture
All of these types are based on the
assumption that failure mechanism is
tensile.
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Tectonic fractures:
Form at depth in response to abnormal fluid pressure and
involve hydrofracturing. They form mainly by tectonic stress
and the horizontal compaction of sediment at depth less
than 3 km, where the escape of fluid is hindered by low
permeability and abnormally high pore pressure is created.
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Hydraulic fractures:
Form as tectonic fractures by the pore pressure created due to
the confined pressed fluid during burial and vertical
compaction of sediment at depth greater than 5 km. Filled
veins in low metamorphic rocks are one of the best of
examples of hydraulic fractures.
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Unloading fractures:
Form near surface as erosion removes overburden
and thermalelastic contraction occurs. They form
when more than half of the original overburden
has been removed. The present stress and tectonic
activity may serve to orient these joints. Vertical
unloading fractures occur during cooling and
elastic contraction of rock mass and may occur at
depths of 200 to 500 m.
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Release fractures:
Similar to unloading fractures but they form by
release of stress. Orientation of release joints is
controlled by the rock fabric. Released joints form
late in the history of an area and are oriented
perpendicular to the original tectonic compression
that formed the dominant fabric in the rock.
Release joints may also develop parallel to the fold
axes when erosion begins and rock mass that was
under burial depth and lithification begins to cool
and contract, these joints start to propagate
parallel to an existing tectonic fabric.
Sheared fractures may be straight or curved but
usually can't be traced for long distance.
Joints within Plutons
Fractures form in pluton in response to
cooling and later tectonic stress. Many of
these joints are filled with hydrothermal
minerals as late stage products. Different
types of joints are present with pluton
(i.e. longitudinal, and cross joints)
NONTECONIC FRACTURES
Sheeting joints:
Those joints form subparallel to
the surface topography.
These joints may be more
observed in igneous rocks.
Pacing within these fractures
increases downward. These
fractures thought that they
form by unloading overlong
time when erosion removes
large quantities of the
overburden rocks.
 Columnar joints and Mud
Cracks:
Columnar joints form in flows,
dikes, sills and volcanic necks
in response to cooling and
shrinking of the magma.
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FAULT CLASSIFICATION AND
TERMINALOGY
Faults: Are fractures that have
appreciable movement parallel to
their plane. They produced usually
be seismic activity.
Understanding faults is useful in
design for long-term stability of
dams, bridges, buildings and power
plants. The study of fault helps
understand mountain building.
Faults may be hundred of meters or a
few centimeters in length. Their
outcrop may have as knife-sharp
edges or fault shear zone. Fault
shear zones may consist of a
serious of interleaving
anastomosing brittle faults and
crushed rock or of ductile shear
zones composed of mylonitic rocks.
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Parts of the Fault
Fault plane: Surface that the movement has
taken place within the fault.On this surface
the dip and strike of the fault is measured.
Hanging wall: The rock mass resting on the
fault plane.
Footwall: The rock mass beneath the fault
plane.
Slip: Describes the movement parallel to the
fault plane.
Dip slip: Describes the up and down
movement parallel to the dip direction of the
fault.
Strike slip: Applies where movement is
parallel to strike of the fault plane.
Oblique slip: Is a combination of strike slip
and dip slip.
Net slip (true displacement): Is the total
amount of motion measured parallel to the
direction of motion
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Separation: The amount op
apparent offset of a faulted
surface, measured in specified
direction. There are strike
separation, dip separation, and
net separation.
Heave: The horizontal
component of dip separation
measured perpendicular to strike
of the fault.
Throw: The vertical component
measured in vertical plane
containing the dip.
Features on the fault surface
Grooves (parallel to the
movement direction)
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Growth of fibrous minerals
(parallel to the movement
direction)
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Slickensides are the polished
fault surfaces.
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Small steps.
All are considered a kind of
lineation. They indicate the
movement relative trend NW,
NE … etc.
Small steps may also be used to
determine the movement
direction and direction of
movement of the opposing
wall. Slicklines usually
record only the last moment
event on the fault.
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ANDERSON FAULTS CLASSIFICATION
Anderson (1942) defined
three types of faults:
 Normal
Faults
 Thrust Faults
 Wrench Faults
(strike slip)
Different Type of Faults
Normal Fault
Normal Fault: The hanging wall has moved down
relative to the footwall.
Graben: consists of a block that has dropped down
between two subparllel normal faults that dip towards
each other.
Horst : consists of two subparallel normal faults that dip
away from each other so that the block between the
two faults remains high.
Listric: are normal faults that frequently exhibit (concaveup) geometry so that they exhibit steep dip near surface
and flatten with depth.
Normal faults usually found in areas where extensional regime
is present.
Normal Faults
Thrust Fault
Thrust Faults: In the thrust
faults the hanging wall
has moved up relative to
the footwall (dip angle
30º or less)
Reverse Faults: Are similar
to the thrust faults
regarding the sense of
motion but the dip angle
of the fault plane is 45º
or more
Thrust faults usually
formed in areas of
comperssional regime.
Thrust
Fault
Thrust Faults
Strike-Slip Fault
Strike-slip Faults: Are faults
that have movement along
strikes.
There are two types of strike
slip faults:
A] Right lateral strike-slip fault
(dextral): Where the side
opposite the observer
moves to the right.
B] Left lateral strike-slip fault
(sinistral): Where the side
opposite the observer
moves to the left.
Note that the same sense of
movement will also be
observed from the other side
of the fault.
Strike-Slip
Faults
Transform Faults
Transform Faults: Are a
type of strike-slip fault
(defined by Wilson 1965).
They form due to the
differences in motion
between lithospheric
plates. They are
basically occur where
type of plate boundary
is transformed into
another.
Main types of transform
faults are:
 Ridge-Ridge
 Ridge-Arc
 Arc-Arc
Other types of fault
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en-echelon faults: Faults that
are approximately parallel one
another but occur in short
unconnected segments, and
sometimes overlapping.
Radial faults: faults that are
converge toward one point
Concentric faults: faults that are
concentric to a point.
Bedding faults (bedding plane
faults): follow bedding or occur
parallel to the orientation of
bedding planes.
CRITERIA FOR FAULTING
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Repetition or omission of stratigraphic units asymmetrical
repetition
Displacement of recognizable marker such as fossils,
color, composition, texture ..etc.).
Truncation of structures, beds or rock units.
Occurrence of fault rocks (mylonite or cataclastic or both)
Presence of S or C structures or both, rotated porphyry
clasts and other evidence of shear zone.
Abundant veins, silicification or other mineralization along
fracture may indicate faulting.
Drag Units appear to be pulled into a fault during
movement (usually within the drag fold and the result is
thrust fault)
Reverse drag occurs along listric normal faults.
Slickensides and slickenlines along a fault surface
Topographic characteristics such as drainges that are
controlled by faults and fault scarps.
FAULTS MECHANICS
Anderson 1942 defined three fundamental possibilities of stress regimes and stress
orientation that produce the three types of faults (Normal, thrust, and strike-slip)
note that σ1> σ 2> σ 3
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Thrust fault: σ 1 and σ 2 are horizontal and σ 3 is vertical. Thus a state of
horizontal compression is defined for thrust faults. Shear plane is oriented to σ 1
with angle = or < 45º and // σ 2.
Strike-Slip faults: σ 1 and σ 3 are horizontal and σ 2 is vertical. Shear plane is
oriented to σ 1 with angle = or 45º and // σ 3. Form also due to horizontal
compression.
Normal faults: σ 1 is vertical and σ 2 and σ 3 are horizontal. Shear plane is
oriented 45º or less to σ 1 and // σ 2. Form due to horizontal extension or vertical
compression.
Role of fluids in faulting
Fluids plays an important role in faulting.
They have a lubricating effect in the fault
zone as buoyancy that reduces the shear
stress necessary to permit the fault to
slip. The effect of fluid on movement is
represented as in landslide and snow
avalanches.
Faults movement mechanisms
Movement on faults occurs in two different ways:
 Stick slip: (unstable frictional sliding) involves
sudden movement on the fault after a long-term
accumulation of stress. This stress probably the cause
of earthquakes.
Stable sliding: involves uninterrupted motion along a
fault, so stress is relieved continuously and does not
accumulate.
The two types of movement may be produced along the
segments of the same fault. Stable sliding where
ground water is abundant, whereas, stick-slip occur
with less ground water
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Other factor that control the type of movement is
the curvature of the fault surface.
 Withdrawal of ground water may cause near
surface segments of active faults to switch
mechanisms from stable sliding to stick slip, thereby
increasing the earthquake hazard.
 Pumping fluid into a fault zone has been proposed
as a way to relieve accumulated elastic strain
energy and reduce the likelihood of large
earthquake, but the rate at which fluid should be
pumped into fault zone remains unknown.
Fault Surfaces and Frictional sliding
Fault surfaces between two
large blocks are always
not planar especially on
the microscopic scale. This
irregularities and
imperfections are called
asperities increase the
resistance to frictional
sliding. They also reduce
the surface area actually in
contact. The initial contact
area may be as little as
10%, but as movement
started the asperities will
break and contact will be
more.
Shear (frictional) Heating in Fault zones
During movement of faults frictional heat
is generated due to the mechanical
work. The heat generated can be
related to an increase in temperature.
This friction heat is indicted by the
formation of veins pseudotachylite
(false glass) in many deep seated fault
zones and the metamorphism along
subduction zones (greenschist and
blueschist facies).
In some areas there is indication of
temperature of 800ºc and 18 to 19 kb
(60km depth). This indicate that they can
form in the lower crust or upper mantle.
Fault zones may also serve as conduit for
rapid fluxing of large amounts of water
and dissipation of heat during
deformation.
Generally friction-related heating along
faults is a process that clearly occurs in
the Earth, but difficult to demonstrate.
BRITTLE AND DUCTILE FAULTS
Brittle faults occur in the upper 5 to 10 km
of the Earth’s crust. In the upper crust
consist of :
Single movement
Anastomosing complex of fracture
surfaces.
The individual fault may have knife-sharp
contacts or it may consist of zone of
cataclasite.
At ductile-brittle zone 10-15km deep in
continental crust, faults are
characterized by mylonite. At surface
of the crust mylonite may also occur
locally where the combination of
available water and increased heat
permits the transition.
The two types of fault may occur within one
fault where close and at the surface
brittle the associated rocks are cataclasts
and at deep where ductile and brittle
zone mylonite is present
SHEAR ZONE
Shear zones are produced by both
homogeneous and
inhomogenous simple shear, or
oblique motion and are thought
of as zones of ductile shear.
Shear zones are classified by
Ramsay (1980) as:
1) brittle
2) brittle-ductile
3) ductile
Characteristics of Shear Zones
Shear zones on all scales are zones
of weakness.
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Associate with the formation of
mylonite.
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Presence of sheath folds.
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Shear zones may act both as
closed and open geochemical
systems with respect to fluids
and elements.
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Shear zones generally have
parallel sides.
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Displacement profiles along
any cross section through
shear zone should be identical.
INDICATORS OF SHEAR SENSE OF MOVEMENT
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Rotated porphyroblasts
and porphyroclasts.
Pressure shadows
Fractured grains.
Boudins
Presence of C- and Ssurfaces (parallel
alignment of platy
mineral)
Riedel shears.