Structural Geology Geol 305 Semester 071

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Structural Geology(Geol 305)Semester (071)

• Dr. Mustafa M. Hariri

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FRACTURES AND FAULTS
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Objectives

• This unit of the course discusses Fractures and
  Faults
• By the end of this unit you will be able to
• 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

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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..

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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

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Types of Fractures

• Extensional FractureIn extensional fractures the
  Fracture plane is oriented parallel to s1 and s 2
  and perpendicular to s 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.

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Importance of studying joints and shear fractures

• 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.

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Importance of studying joints and shear fractures

• 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.

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TYPES OF JOINT

• 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.

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Type of Fractures

• 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.

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Type of Fractures

• 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.

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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.

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Importance of Fracture Orientation

• Study of orientation of systematic fractures
  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.

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

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Fold and Joints

• Joints may form during brittle folding in a
  position related to the fold axis and axial
  surface as follows
• parallel
• normal
• oblique
• depending on stress condition.

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Fault Related Joints

• 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.
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Factors Affecting the Formation of Joints

• Rock type
• Fluid pressure
• Strain rate
• Stress difference at a particular time

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Characteristics 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 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.

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Characteristics 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 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.

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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.
• 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.
• 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.

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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)

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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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FAULTS
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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.

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Faults Types
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Features on the fault surface

• Grooves (parallel to the movement direction)
• Growth of fibrous minerals (parallel to the
  movement direction)
• Slickensides are the polished fault surfaces.
• 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)

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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 (concave-up) geometry so that they
  exhibit steep dip near surface and flatten with
  depth.
• Normal faults usually found in areas where
  extensional regime is present.

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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.

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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.

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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

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Other types of fault

• 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.

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CRITERIA FOR FAULTING

• 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.

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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 s1gt s 2gt s 3
• Thrust fault s 1 and s 2 are horizontal and s 3
  is vertical. Thus a state of horizontal
  compression is defined for thrust faults. Shear
  plane is oriented to s 1 with angle or lt 45º
  and // s 2.
• Strike-Slip faults s 1 and s 3 are horizontal
  and s 2 is vertical. Shear plane is oriented to s
  1 with angle or 45º and // s 3. Form also due
  to horizontal compression.
• Normal faults s 1 is vertical and s 2 and s 3
  are horizontal. Shear plane is oriented 45º or
  less to s 1 and // s 2. Form due to horizontal
  extension or vertical compression.

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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.

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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.

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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.

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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.

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BRITTLE AND DUCTILE FAULTS

• Brittle faults occur in the upper 5 to 10 km of
  the Earths 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

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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

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Characteristics 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.

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INDICATORS OF SHEAR SENSE OF MOVEMENT

• Rotated porphyroblasts and porphyroclasts.
• Pressure shadows
• Fractured grains.
• Boudins
• Presence of C- and S- surfaces (parallel
  alignment of platy mineral)
• Riedel shears.