Fract&fault

Structural GeologyStructural Geology
(Geol(Geol 305)305)
Semester (071)Semester (071)
Dr. Mustafa M. HaririDr. Mustafa M. Hariri

FRACTURES AND FAULTSFRACTURES AND FAULTS

ObjectivesObjectives
This unit of the course discusses Fractures and FaultsThis unit of the course discusses Fractures and Faults
By the end of this unit you will be able to:By the end of this unit you will be able to:
 Differentiate between the differentdifferent type of fracturestype of fractures
 Differentiate between the different type of faultsdifferent type of faults
 Understand the relationship between the different type ofdifferent type of
stresses and faultsstresses and faults
 Where faults form and how?
 Faults mechanicsFaults mechanics
 Role ofRole of fluid in faultingfluid in faulting
 Faults movement mechanismsFaults movement mechanisms
 Shear, Shear zones and different type of shearsShear, Shear zones and different type of shears

FRACTUREFRACTURE
FRACTURE:FRACTURE: is defined by Twiss andis defined by Twiss and
Moores (1992)Moores (1992) asas “..surfaces along“..surfaces along
which rocks or minerals have broken;which rocks or minerals have broken;
they are therefore surfaces acrossthey are therefore surfaces across
which the material has lost cohesion”which the material has lost cohesion”
Characteristics of fractures according toCharacteristics of fractures according to
Pollard and Aydin (1988)Pollard and Aydin (1988)
 fractures have two parallel surfaces thatfractures have two parallel surfaces that
meet at the fracture frontmeet at the fracture front
 these surfaces are approximately planarthese surfaces are approximately planar
 the relative displacement of originallythe relative displacement of originally
adjacent points across the fractures isadjacent points across the fractures is
small compared to the fracture length..small compared to the fracture length..

Fracture, Joint and FaultFracture, Joint and Fault
The term fracture encompasses both jointsThe term fracture encompasses both joints
and faults.and faults.
JOINTS:JOINTS: are fractures along which there hasare fractures along which there has
been no appreciable displacement parallel tobeen no appreciable displacement parallel to
the fracture and only slight movement normalthe fracture and only slight movement normal
to the fracture plane.to the fracture plane.
Joints are most common of all structures present in all settings in all kind of rocks as wellJoints are most common of all structures present in all settings in all kind of rocks as well
as consolidated and unconsolidated sedimentas consolidated and unconsolidated sediment

Types of FracturesTypes of Fractures
Extensional FractureExtensional 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:Three types of fractures have been identified:
 Mode IMode I fractures (joints)fractures (joints) it is the extensional fractures and formed
by opening with no displacement parallel to the fracture surface (see
above figure).
 Mode IIMode II andand Mode IIIMode III are shear fracturesare shear fractures. These are faults like
fractures one of them is strike -slip and the other is dip-slipis strike -slip and the other is dip-slip
Same fracture can exhibitSame fracture can exhibit both mode II and mode IIIboth mode II and mode III in different parts of thein different parts of the regionregion..

Importance of studying joints andImportance of studying joints and
shear fracturesshear fractures
 To understand the nature and sequence ofTo understand the nature and sequence of
deformation in an area.deformation in an area.
 To find out relationship between joints andTo find out relationship between joints and
faults and or folds.faults and or folds.
 Help to find out the brittle deformation in anHelp to find out the brittle deformation in an
area of construction (dams, bridges, and powerarea of construction (dams, bridges, and power
plantsplants..
 In mineral explorationIn mineral exploration to find out the trend andto find out the trend and
type of fractures and joints that hosttype of fractures and joints that host
mineralization which will help in exploration.mineralization which will help in exploration.

Importance of studying joints andImportance of studying joints and
shear fracturesshear fractures
 Joints and fractures serve as the plumping system forJoints and fractures serve as the plumping system for
ground water flow in many area and they are the onlyground water flow in many area and they are the only
routes by which ground water can move through igneousroutes by which ground water can move through igneous
and metamorphic rocks.and metamorphic rocks.
 Joints and fractures porosity and permeability is veryJoints and fractures porosity and permeability is very
important for water supplies and hydrocarbon reservoirs.important for water supplies and hydrocarbon reservoirs.
 Joints orientations in road cuts greatly affect bothJoints orientations in road cuts greatly affect both
construction and maintenance. Those oriented parallel toconstruction and maintenance. Those oriented parallel to
or dip into a highway cut become hazardous duringor dip into a highway cut become hazardous during
construction and later because they provide potentialconstruction and later because they provide potential
movement surfaces.movement surfaces.

TYPES OF JOINTTYPES OF JOINT
 Systematic joints:Systematic joints: have a
subparallel orientation and
regular spacing.
 Joint set:Joint set: joints that share a
similar orientation in same area.
 Joint system:Joint system: two or more joints
sets in the same area
 Nonsystematic joints: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 FracturesType of Fractures
 Plumose joints:Plumose joints: joints thatjoints that
have feathered texture onhave feathered texture on
their surfaces, and from thistheir surfaces, and from this
texture the direction oftexture the direction of
propagation of joints can bepropagation of joints can be
determined.determined.
 Veins:Veins: are filled joints andare filled joints and
shear fractures and theshear fractures and the
filling range from quartz andfilling range from quartz and
feldspar (pegmatite andfeldspar (pegmatite and
aplite) to quartz, calcite andaplite) to quartz, calcite and
dolomite.dolomite.

Type of FracturesType of Fractures
 Conjugate fractures:Conjugate fractures: pairedpaired
fracture systems, formed in thefracture systems, formed in the
same time, and produced bysame time, and produced by
tension or shear. Many of themtension or shear. Many of them
intersect at an acute angle whichintersect at an acute angle which
will be bisected by thewill be bisected by the
 Curved fractures:Curved fractures: occuroccur
frequently and may be caused byfrequently and may be caused by
the textural and compositionalthe textural and compositional
differences within a thick bed ordifferences within a thick bed or
large rock mass or they may alarge rock mass or they may a
result of changes in stressresult of changes in stress
direction or analysis.direction or analysis.
Cross cutting relationship and materialCross cutting relationship and material
filling the fractures can help in resolvingfilling the fractures can help in resolving
the chronological order of deformation.the chronological order of deformation.

FRACTURE ANALYSISFRACTURE ANALYSIS
Study of joints in an area will give information about theStudy of joints in an area will give information about the
sequence and timing of formation. It will also providesequence and timing of formation. It will also provide
information on the timing and geometry of the brittleinformation on the timing and geometry of the brittle
deformation of the crust and the way fractures propagatedeformation of the crust and the way fractures propagate
through the rocks.through the rocks.

Importance of Fracture OrientationImportance of Fracture Orientation
Study of orientation of systematic fracturesStudy of orientation of systematic fractures
provides information about theprovides information about the
orientation of one or more principleorientation of one or more principle
stress directions involved in the brittle.stress directions involved in the brittle.
Parameters measured for fractures are strikeParameters measured for fractures are strike
and dip.and dip.
Or strike of linear features from aerial photosOr strike of linear features from aerial photos
and landsat images.and landsat images.
Data obtained from fractures is plotted inData obtained from fractures is plotted in
rose diagram or equal area net. Equalrose diagram or equal area net. Equal
area net for strike and dip and rosearea net for strike and dip and rose
diagram for strike only.diagram for strike only.
Studies of joint and fracture orientationStudies of joint and fracture orientation
from LANDSAT and other satellitefrom LANDSAT and other satellite
imagery and photographs have a varietyimagery and photographs have a variety
of structural, geomorphic, andof structural, geomorphic, and
engineering applicationsengineering applications..

Strain -ellipsoid analysis
of joints in area may
help to determine
dominant crystal
extension directions

Fold and JointsFold and Joints
Joints may form duringJoints may form during
brittle folding in abrittle folding in a
position related to theposition related to the
fold axis and axialfold axis and axial
surface as followssurface as follows
 parallelparallel
 normalnormal
 obliqueoblique
depending on stressdepending on stress
condition.condition.

Fault Related JointsFault Related Joints
 Joints are also formedJoints are also formed
adjacent to brittle faults, andadjacent to brittle faults, and
movement along faultsmovement along faults
usually produces a series ofusually produces a series of
systematic fractures.systematic fractures.
Most joints form by extensional fracturing of rockMost joints form by extensional fracturing of rock
in the upper few kilometers of the Earth's crustin the upper few kilometers of the Earth's crust..
The limiting depth formation of extension fracturesThe limiting depth formation of extension fractures
should be the ductile-brittle transitionshould be the ductile-brittle transition..

Factors Affecting the Formation of JointsFactors Affecting the Formation of Joints
 Rock typeRock type
 Fluid pressureFluid pressure
 Strain rateStrain rate
 Stress difference at a particularStress difference at a particular
timetime

Characteristics of FracturesCharacteristics of Fractures
 Plumose structure: is the
structures formed on the
joint surface during its
propagation and provides
information about the joint
propagation direction.
 Hackle marks:Hackle marks: indicate
zones where the joint
propagate rapidly.
 Arrest line:Arrest line: forms
perpendicular to the
direction of propagation
and is parallel to the
advancing edge of
fractures.

Characteristics of FracturesCharacteristics of Fractures
 Bedding and foliation planes in coarse-
grained rocks constitute barriers to join
propagation. Bedding in uniformly fine-
grained 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 shalefrom sandstone into shale
if the least principle horizontal stressif the least principle horizontal stress
in shale is greater than that inin shale is greater than that in
sandstone.sandstone.
 Fractures will be terminated at theFractures will be terminated at the
contact between the two rocks.contact between the two rocks.

Joints Classified According to their EnvironmentJoints Classified According to their Environment
and Mechanism of Formationsand Mechanism of Formations (Engelder, 1985(
 Tectonic fractureTectonic fracture
 Hydraulic fractureHydraulic fracture
 Unloading fractureUnloading fracture
 Loading fractureLoading fracture
All of these types are based on theAll of these types are based on the
assumption that failure mechanism isassumption that failure mechanism is
tensile.tensile.

 Tectonic fractures:
Form at depth in response toForm at depth in response to abnormal fluid pressureabnormal fluid pressure andand
involve hydrofracturing. They form mainly byinvolve hydrofracturing. They form mainly by tectonic stresstectonic stress
and theand the horizontal compaction of sedimenthorizontal compaction of sediment at depthat depth lessless
than 3 kmthan 3 km, where the escape of fluid is hindered by, where the escape of fluid is hindered by lowlow
permeability and abnormally high pore pressurepermeability and abnormally high pore pressure is created.is created.
 Hydraulic fractures:
Form as tectonic fractures by theForm as tectonic fractures by the pore pressurepore pressure created due tocreated due to
thethe confined pressed fluid during burial and verticalconfined pressed fluid during burial and vertical
compaction of sediment at depth greatercompaction of sediment at depth greater than 5 kmthan 5 km. Filled. Filled
veins in low metamorphic rocks are one of the best ofveins in low metamorphic rocks are one of the best of
examples of hydraulic fractures.examples of hydraulic fractures.

 Unloading fractures:Unloading fractures:
Form nearForm near surface as erosion removes overburdensurface as erosion removes overburden
and thermalelastic contraction occurs.and thermalelastic contraction occurs. They formThey form
when morewhen more than half of the original overburdenthan half of the original overburden
has been removedhas been removed. The present stress and tectonic. The present stress and tectonic
activity may serve toactivity may serve to orient these jointsorient these joints. Vertical. Vertical
unloading fractures occurunloading fractures occur during cooling andduring cooling and
elastic contraction of rock mass and may occur atelastic contraction of rock mass and may occur at
depths of 200 to 500 m.depths of 200 to 500 m.
 Release fractures:Release fractures:
Similar to unloading fractures but they form bySimilar to unloading fractures but they form by
release of stressrelease of stress. Orientation of release joints is. Orientation of release joints is
controlled by the rock fabric. Released joints formcontrolled by the rock fabric. Released joints form
late in the history of an area and are orientedlate in the history of an area and are oriented
perpendicular to the original tectonic compressionperpendicular to the original tectonic compression
that formed the dominant fabric in the rock.that formed the dominant fabric in the rock.
Release joints may also develop parallel to the foldRelease joints may also develop parallel to the fold
axes whenaxes when erosion begins and rock mass that waserosion begins and rock mass that was
under burial depth and lithification begins to coolunder burial depth and lithification begins to cool
and contract, these jointsand contract, these joints start to propagatestart to propagate
parallel to an existing tectonic fabricparallel to an existing tectonic fabric..
Sheared fractures may be straight or curved butSheared fractures may be straight or curved but
usually can't be traced for long distance.usually can't be traced for long distance.

Joints within Plutons
Fractures form in pluton in response toFractures form in pluton in response to
cooling and later tectonic stress. Many ofcooling and later tectonic stress. Many of
these joints are filled with hydrothermalthese joints are filled with hydrothermal
minerals as late stage products. Differentminerals as late stage products. Different
types of joints are present with plutontypes of joints are present with pluton
(i.e. longitudinal, and cross joints((i.e. longitudinal, and cross joints(

NONTECONIC FRACTURESNONTECONIC FRACTURES
 Sheeting joints:Sheeting joints:
Those joints form subparallel toThose joints form subparallel to
the surface topography.the surface topography.
These joints mayThese joints may be morebe more
observed in igneous rocksobserved in igneous rocks..
Pacing within these fracturesPacing within these fractures
increases downward. Theseincreases downward. These
fractures thought that theyfractures thought that they
form by unloading overlongform by unloading overlong
time when erosion removestime when erosion removes
large quantities of thelarge quantities of the
overburden rocks.overburden rocks.
 Columnar joints and MudColumnar joints and Mud
Cracks:Cracks:
Columnar joints form in flows,Columnar joints form in flows,
dikes, sills and volcanic necksdikes, sills and volcanic necks
in response to cooling andin response to cooling and
shrinking of the magma.shrinking of the magma.

FAULT CLASSIFICATION ANDFAULT CLASSIFICATION AND
TERMINALOGYTERMINALOGY
Faults:Faults: Are fractures that haveAre fractures that have
appreciable movement parallel toappreciable movement parallel to
their plane. They produced usuallytheir plane. They produced usually
be seismic activity.be seismic activity.
Understanding faults is useful inUnderstanding faults is useful in
design for long-term stability ofdesign for long-term stability of
dams, bridges, buildings and powerdams, bridges, buildings and power
plants. The study of fault helpsplants. The study of fault helps
understand mountain building.understand mountain building.
Faults may be hundred of meters or aFaults may be hundred of meters or a
few centimeters in length.few centimeters in length. TheirTheir
outcrop may have as knife-sharpoutcrop may have as knife-sharp
edges or fault shear zoneedges or fault shear zone. Fault. Fault
shear zones may consist of ashear zones may consist of a
serious of interleavingserious of interleaving
anastomosing brittle faults andanastomosing brittle faults and
crushed rock or of ductile shearcrushed rock or of ductile shear
zones composed of mylonitic rocks.zones composed of mylonitic rocks.

Parts of the FaultParts of the Fault Fault planeFault plane: Surface that the movement hasSurface that the movement has
taken place within the fault.On this surfacetaken place within the fault.On this surface
the dip and strike of the fault is measured.the dip and strike of the fault is measured.
 Hanging wall:Hanging wall: The rock mass resting on theThe rock mass resting on the
fault plane.fault plane.
 Footwall:Footwall: The rock mass beneath the faultThe rock mass beneath the fault
plane.plane.
 Slip:Slip: Describes the movement parallel to theDescribes the movement parallel to the
fault plane.fault plane.
 Dip slip: Describes the up and downDip slip: Describes the up and down
movement parallel to the dip direction of themovement parallel to the dip direction of the
fault.fault.
 Strike slip:Strike slip: Applies where movement isApplies where movement is
parallel to strike of the fault plane.parallel to strike of the fault plane.
 Oblique slip:Oblique slip: Is a combination of strike slipIs a combination of strike slip
and dip slip.and dip slip.
 Net slip (true displacement): Is the totalNet slip (true displacement): Is the total
amount of motion measured parallel to theamount of motion measured parallel to the
direction of motiondirection of motion

 Separation:Separation: The amount opThe amount op
apparent offset of a faultedapparent offset of a faulted
surface, measured in specifiedsurface, measured in specified
direction. There are strikedirection. There are strike
separation, dip separation, andseparation, dip separation, and
net separation.net separation.
 Heave:Heave: The horizontalThe horizontal
component of dip separationcomponent of dip separation
measured perpendicular to strikemeasured perpendicular to strike
of the fault.of the fault.
 Throw:Throw: The vertical componentThe vertical component
measured in vertical planemeasured in vertical plane
containing the dip.containing the dip.

Features on the fault surfaceFeatures on the fault surface
 GroovesGrooves (parallel to the(parallel to the
movement direction)movement direction)
 Growth of fibrous mineralsGrowth of fibrous minerals
(parallel to the movement(parallel to the movement
direction)direction)
 SlickensidesSlickensides are the polishedare the polished
fault surfaces.fault surfaces.
 Small steps.Small steps.
All are considered a kind ofAll are considered a kind of
lineation. They indicate thelineation. They indicate the
movement relative trend NW,movement relative trend NW,
NE … etc.NE … etc.
Small steps may also be used toSmall steps may also be used to
determine the movementdetermine the movement
direction and direction ofdirection and direction of
movement of the opposingmovement of the opposing
wall. Slicklines usuallywall. Slicklines usually
record only the last momentrecord only the last moment
event on the fault.event on the fault.

ANDERSON FAULTS CLASSIFICATIONANDERSON FAULTS CLASSIFICATION
Anderson (1942) definedAnderson (1942) defined
three types of faults:three types of faults:
 Normal FaultsNormal Faults
 Thrust FaultsThrust Faults
 Wrench FaultsWrench Faults
(strike slip)(strike slip)

Normal FaultNormal Fault
Normal Fault: The hanging wall has moved down
relative to the footwall.
Graben:Graben: consists of a block that has dropped downconsists of a block that has dropped down
between two subparllel normal faults that dip towardsbetween two subparllel normal faults that dip towards
each other.each other.
HorstHorst :: consists of two subparallel normal faults that dipconsists of two subparallel normal faults that dip
away from each other so that the block between the twoaway from each other so that the block between the two
faults remains high.faults remains high.
Listric:Listric: are normal faults that frequently exhibit (concave-are normal faults that frequently exhibit (concave-
up) geometry so that they exhibit steep dip near surfaceup) geometry so that they exhibit steep dip near surface
and flatten with depth.and flatten with depth.
Normal faults usually found in areas where extensional regimeNormal faults usually found in areas where extensional regime
is presentis present..

Thrust FaultThrust 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 usuallyThrust faults usually
formed in areas offormed in areas of
comperssional regime.comperssional regime.
Thrust FaultThrust Fault

Strike-Slip FaultStrike-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 ofNote that the same sense of
movement will also bemovement will also be
observed from the other sideobserved from the other side
of the fault.of the fault.
Strike-SlipStrike-Slip
FaultsFaults

Transform FaultsTransform FaultsTransform Faults:Transform Faults: Are aAre a
type of strike-slip faulttype of strike-slip fault
(defined by Wilson 1965).(defined by Wilson 1965).
They form due toThey form due to thethe
differences in motiondifferences in motion
between lithosphericbetween lithospheric
plates.plates. They areThey are
basically occur wherebasically occur where
type of plate boundarytype of plate boundary
is transformed intois transformed into
anotheranother..
Main types of transformMain types of transform
faults arefaults are::
 Ridge-RidgeRidge-Ridge
 Ridge-ArcRidge-Arc
 Arc-ArcArc-Arc

Other types of faultOther types of fault
 en-echelon faults:en-echelon faults: Faults thatFaults that
are approximately parallel oneare approximately parallel one
another but occur in shortanother but occur in short
unconnected segments, andunconnected segments, and
sometimes overlapping.sometimes overlapping.
 Radial faults:Radial faults: faults that arefaults that are
converge toward one pointconverge toward one point
 Concentric faults:Concentric faults: faults that arefaults that are
concentric to a point.concentric to a point.
 Bedding faults (bedding planeBedding faults (bedding plane
faults):faults): follow bedding or occurfollow bedding or occur
parallel to the orientation ofparallel to the orientation of
bedding planes.bedding planes.

CRITERIA FOR FAULTINGCRITERIA FOR FAULTING
 Repetition or omissionRepetition or omission of stratigraphic units asymmetricalof stratigraphic units asymmetrical
repetitionrepetition
 Displacement of recognizable markerDisplacement of recognizable marker such as fossils,such as fossils,
color, composition, texture ..etc.)color, composition, texture ..etc.)..
 Truncation of structuresTruncation of structures, beds or rock units., beds or rock units.
 Occurrence of fault rocksOccurrence of fault rocks (mylonite or cataclastic or both)(mylonite or cataclastic or both)
 Presence of S or C structuresPresence of S or C structures or both, rotated porphyryor both, rotated porphyry
clasts and other evidence of shear zone.clasts and other evidence of shear zone.
 Abundant veinsAbundant veins, silicification or other mineralization along, silicification or other mineralization along
fracture may indicate faulting.fracture may indicate faulting.
 Drag UnitsDrag Units appear to be pulled into a fault duringappear to be pulled into a fault during
movement (usually within the drag fold and the result ismovement (usually within the drag fold and the result is
thrust fault)thrust fault)
 Reverse dragReverse drag occurs along listric normal faults.occurs along listric normal faults.
 SlickensidesSlickensides and slickenlines along a fault surfaceand slickenlines along a fault surface
 Topographic characteristicsTopographic characteristics such as drainges that aresuch as drainges that are
controlled by faults and fault scarps.controlled by faults and fault scarps.

FAULTS MECHANICSFAULTS MECHANICS
Anderson 1942 defined three fundamental possibilities of stress regimes and stressAnderson 1942 defined three fundamental possibilities of stress regimes and stress
orientation that produce the three types of faults (Normal, thrust, and strike-sliporientation that produce the three types of faults (Normal, thrust, and strike-slip)
note thatnote that σσ1>1> σσ 2>2> σσ 33
 Thrust fault: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 faultsStrike-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 faultsNormal 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 faultingRole of fluids in faulting
Fluids plays an important role in faulting.Fluids plays an important role in faulting.
They have aThey have a lubricating effect in the faultlubricating effect in the fault
zone as buoyancy that reduces the shearzone as buoyancy that reduces the shear
stress necessary to permit the fault tostress necessary to permit the fault to
slip. The effect of fluid on movement isslip. The effect of fluid on movement is
represented as in landslide and snowrepresented as in landslide and snow
avalanches.avalanches.

Faults movement mechanismsFaults movement mechanisms
Movement on faults occurs in two different ways:Movement on faults occurs in two different ways:
 Stick slip: (unstable frictional sliding)(unstable frictional sliding) involvesinvolves
sudden movement on the fault after a long-termsudden movement on the fault after a long-term
accumulation of stress. This stress probably the causeaccumulation of stress. This stress probably the cause
of earthquakes.of earthquakes.
 Stable sliding: involves uninterrupted motion along ainvolves uninterrupted motion along a
fault, so stress is relieved continuouslyfault, so stress is relieved continuously and does notand does not
accumulate.accumulate.
The two types of movement may be produced along theThe two types of movement may be produced along the
segments of the same fault. Stable sliding wheresegments of the same fault. Stable sliding where
ground water is abundant, whereas, stick-slip occurground water is abundant, whereas, stick-slip occur
with less ground waterwith less ground water

Other factor that control the type of movement isOther factor that control the type of movement is
the curvature of the fault surface.the curvature of the fault surface.
 Withdrawal of ground waterWithdrawal 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 faultPumping 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 slidingFault Surfaces and Frictional sliding
Fault surfaces between twoFault surfaces between two
large blockslarge blocks are alwaysare always
not planar especially onnot planar especially on
thethe microscopic scale. Thismicroscopic scale. This
irregularities andirregularities and
imperfections are calledimperfections are called
asperitiesasperities increase theincrease the
resistance to frictionalresistance to frictional
sliding. They also reducesliding. They also reduce
the surface area actually inthe surface area actually in
contactcontact.. The initial contactThe initial contact
area may be as little asarea may be as little as
10%, but as movement10%, but as movement
started the asperities willstarted the asperities will
break and contact will bebreak and contact will be
more.more.

Shear (frictional) Heating in Fault zonesShear (frictional) Heating in Fault zones
During movement of faults frictionalDuring movement of faults frictional heatheat
is generated due to the mechanicalis generated due to the mechanical
work. The heat generated can bework. The heat generated can be
related to an increase in temperature.related to an increase in temperature.
This friction heat is indicted by theThis friction heat is indicted by the
formation of veins pseudotachyliteformation of veins pseudotachylite
(false glass) in many deep seated fault(false glass) in many deep seated fault
zones and the metamorphism alongzones and the metamorphism along
subduction zones (greenschist andsubduction zones (greenschist and
blueschist facies).blueschist facies).
In some areas there is indication ofIn some areas there is indication of
temperature of 800ºc and 18 to 19 kbtemperature of 800ºc and 18 to 19 kb
(60km depth). This indicate that they can(60km depth). This indicate that they can
form in the lower crust or upper mantleform in the lower crust or upper mantle.
Fault zones may also serve as conduit forFault zones may also serve as conduit for
rapid fluxing of large amounts of waterrapid fluxing of large amounts of water
and dissipation of heat duringand dissipation of heat during
deformationdeformation.
Generally friction-related heating alongGenerally friction-related heating along
faults is a process that clearly occurs infaults is a process that clearly occurs in
the Earth, but difficult to demonstrate.the Earth, but difficult to demonstrate.

BRITTLE AND DUCTILE FAULTSBRITTLE AND DUCTILE FAULTS
Brittle faults occur in the upper 5 to 10 kmBrittle faults occur in the upper 5 to 10 km
of the Earthof the Earth’’s crust. In the upper crusts crust. In the upper crust
consist ofconsist of :
Single movementSingle movement
Anastomosing complex of fractureAnastomosing complex of fracture
surfacessurfaces.
The individual fault may have knife-sharpThe individual fault may have knife-sharp
contacts or it may consist of zone ofcontacts or it may consist of zone of
cataclasitecataclasite.
At ductile-brittle zone 10-15km deep inAt ductile-brittle zone 10-15km deep in
continental crust, faults arecontinental crust, faults are
characterized by mylonite. At surfacecharacterized by mylonite. At surface
of the crust mylonite may also occurof the crust mylonite may also occur
locally where the combination oflocally where the combination of
available water and increased heatavailable water and increased heat
permits the transition.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 ZONESHEAR ZONE
Shear zones are produced by bothShear zones are produced by both
homogeneous andhomogeneous and
inhomogenous simple shear, orinhomogenous simple shear, or
oblique motion and are thoughtoblique motion and are thought
of as zones of ductile shear.of as zones of ductile shear.
Shear zones are classified byShear zones are classified by
Ramsay (1980) as:Ramsay (1980) as:
1) brittle1) brittle
2) brittle-ductile2) brittle-ductile
3) ductile3) ductile

Characteristics of Shear ZonesCharacteristics of Shear Zones
Shear zones on all scales are zones
of weakness.
 Associate with the formation of
mylonite.
 Presence of sheath folds.
 Shear zones may act both as
closed and open geochemical
systems with respect to fluids
and elements.
 Shear zones generally have
parallel sides.
 Displacement profiles along
any cross section through
shear zone should be identical.

INDICATORS OF SHEAR SENSE OF MOVEMENTINDICATORS OF SHEAR SENSE OF MOVEMENT
1.1. Rotated porphyroblastsRotated porphyroblasts
and porphyroclasts.and porphyroclasts.
2.2. Pressure shadowsPressure shadows
3.3. Fractured grains.Fractured grains.
4.4. BoudinsBoudins
5.5. Presence of C- and S-Presence of C- and S-
surfaces (parallelsurfaces (parallel
alignment of platyalignment of platy
mineral)mineral)
6.6. Riedel shears.Riedel shears.

Fract&fault

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