Fault their geometry and classification

Prepared by
Azhar Mahmood
Topic
Fault their geometry and
classification

FAULTSFAULTS
In geology, a fault is a planar fracture or discontinuity in a
volume of rock, across which there has been significant
displacement along the fractures as a result of earth movement

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 different type of faultsdifferent type of faults
 Where faults form and how?
 Faults mechanicsFaults mechanics
 Role ofRole of fluid in faultingfluid in faulting
 Faults movement mechanismsFaults movement mechanisms

Fault Related JointsFault Related Joints Joints are also formed adjacentJoints are also formed adjacent
to brittle faults, and movementto brittle faults, and movement
along faults usually produces aalong faults usually produces a
series of systematic fractures.series of 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..

FAULT CLASSIFICATION AND TERMINALOGYFAULT CLASSIFICATION AND TERMINALOGY
..
Understanding faults is useful in design for long-term stability of dams,Understanding faults is useful in design for long-term stability of dams,
bridges, buildings and power plants. The study of fault helpsbridges, buildings and power plants. The study of fault helps
understand mountain building.understand mountain building.
Faults may be hundred of meters or a few centimeters in length.Faults may be hundred of meters or a few centimeters in length. TheirTheir
outcrop may have as knife-sharp edges or fault shear zoneoutcrop may have as knife-sharp edges or fault shear zone..

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

Fault their geometry and classification

FAULTS CLASSIFICATIONFAULTS 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)
 Oblique FaultOblique Fault

Normal FaultNormal Fault
Normal Fault: The hanging wall has moved down relative to
the footwall.
Normal faults usually found in areas where extensional regime is presentNormal faults usually found in areas where extensional regime is 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 usually formed inThrust faults usually formed in
areas of comperssionalareas of comperssional
regime.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 be observedmovement will also be observed
from the other side of the fault.from the other side of the fault.

Transform FaultsTransform FaultsTransform Faults:Transform Faults: Are a type ofAre a type of
strike-slip faultstrike-slip fault (defined by(defined by
Wilson 1965). They formWilson 1965). They form
due todue to the differences inthe differences in
motion betweenmotion between
lithospheric plates.lithospheric plates. TheyThey
are basically occur whereare basically occur where
type of plate boundary istype of plate boundary is
transformed into anothertransformed into another..
Main types of transform faultsMain types of transform faults
areare::
 Ridge-RidgeRidge-Ridge
 Ridge-ArcRidge-Arc
 Arc-ArcArc-Arc

ObliqueFaultsObliqueFaults
Oblique faults are faults on
which two directions of
displacement occur. It is a
combination of dip-slip motion and
strike-slip motion. Therefore, there
is a space between the faces of the
fault. And one side is higher,
vertically, than the other

Rift ValleyRift Valley
• A rift valley is when two normal
faults occur parallel to each other
and the land sinks between the
faults.
• There are two major examples of
this. One being the Great Rift
Valley in North Africa and the
other, the San Andreas Fault in
California.
• The top right picture is San
Andreas Fault and on the bottom
right is a diagram of what a rift
valley looks like.

Horst Fault
• A Horst is the opposite of a
rift valley. The land
between the parallel faults
is forced upward because
the two faults are being
pushed together.
• This process can take a long
time to occur because the
average plate movement is
one inch per year.
• There are examples of horst
faults on the left.

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, color,such as fossils, color,
composition, texture ..etc.)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 porphyry clasts andor both, rotated porphyry clasts and
other evidence of shear zone.other evidence of shear zone.
 Abundant veinsAbundant veins, silicification or other mineralization along fracture, silicification or other mineralization along fracture
may indicate faulting.may indicate faulting.
 Drag UnitsDrag Units appear to be pulled into a fault during movementappear to be pulled into a fault during movement
(usually within the drag fold and the result is thrust fault)(usually within the drag fold and the result is 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 are controlled bysuch as drainges that are controlled by
faults and fault scarps.faults and fault scarps.

Role of fluids in faultingRole of fluids in faulting
Fluids plays an important role in faulting. TheyFluids plays an important role in faulting. They
have ahave a lubricating effect in the faultlubricating effect in the fault zone aszone as
buoyancy that reduces the shear stressbuoyancy that reduces the shear stress
necessary to permit the fault to slip. Thenecessary to permit the fault to slip. The
effect of fluid on movement is represented aseffect of fluid on movement is represented as
in landslide and snow avalanches.in landslide and snow avalanches.

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

Fault their geometry and classification

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Fault their geometry and classification