Joints and Shear Fractures

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Chapter 8

• Joints and Shear Fractures

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Joints

• Joints are the most ubiquitous structure in the
  Earths crust, occurring in a wide variety of
  rock types and tectonic environmentsThey control
  the physiography of many spectacular landforms
  and play an important role in the transport of
  fluids Pollard and Aydin 1988

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Joints

• Joints Fractures along which there is no
  appreciable displacement parallel to the fracture
  and only slight movement normal to the fracture
  plane.

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Importance of Joints

• Economically important minerals can be found in
  joints
• Joints act as the plumbing system for
  ground-water
• Help to demonstrate the tectonic history of an
  area which is important for the construction of
  dams, bridges, power plants, and buildings.

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Three modes of fractures

• Three types of fractures have been identified
  with each one formed by a separate kind of
  motion.
• Mode I Opening of fractures
• Mode II Sliding of fractures
• Mode III Tearing of fractures

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Joint Systems

• Systematic Joints Parallel joints with regular
  spacing
• Joint Set Joints that share a similar
  orientation
• Joint System Two or more joint sets in the same
  area
• Nonsystematic Joints Do not share a common
  orientation, can be curved, and can form
  irregular fracture surfaces
• They occur in many areas but do not appear to be
  related to a recognizable stress field

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Systematic and Nonsystematic Joints
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Joint Appearance

• Joints
• Unfilled Generally recent
• May have smooth surfaces
• May have irregular surfaces
• May have concentric ridges
• May have a feathered texture called Plumose
  Joints

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Joint Appearance

• Joints
• Filled Also called Veins
• Feldspar or Aplite High temperature
• Quartz, Calcite, Chlorite, and Epidote Low
  temperatures
• Ore Minerals Low temperatures

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Calcite filled joints in Vermont
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Joint Sets

• Conjugate Joint Systems Paired joint sets that
  form at acute angles and are thus shear
  fractures.
• Difficult to make certain that the acute joints
  formed at the same time
• If you can prove they are conjugate then s1
  bisects the acute angle

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Fracture Analysis

• Study of joint systems in an area reveals the
  sequence and timing of tectonic events.
• The orientations of systematic fractures provides
  information about the orientation of the
  principal stress directions involved in brittle
  deformation.

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Joints and Principle Stress Axes
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Regional Tectonics

• Regional joint-orientation patterns may be
  determined by measuring strike and dip of
  mesoscopic-scale joints over a wide area.
• Bearing of linear stream systems
• Satellite imagery
• Topographic maps
• Aerial photos

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Aerial photo of Precambrian granite in Wyoming
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Graphical depictions of joint sets in Pigeon
Forge, TN
Lower-hemisphere equal-area plot
Rose diagram
Contour diagrams of a
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Jointing related to the strain ellipsoid
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Fracture and Lineament Orientation in Italy
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The Anatomy of Joint Surfaces

• Various features provide information on the rate
  and direction of propagation of joints.
• Hackle Marks Form in the zone where the joint
  traveled rapidly
• Arrest Lines For parallel to the advancing edge
  of the fracture and perpendicular to direction of
  propagation.
• Origin Can often determine the initial site of
  the joint. Joints always begin at a preexisting
  flaw in the rock such as a grain of atypical size
  or hardness, fossil, or concretion.

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Plumose joint surface showing primary surface
features.
1. Main joint face 2. Twist hackle fringe 3.
Origin 4. Hackle plume 5. Inclusion hackle
6. Plume axis 7. Twist-hackle face 8.
Twist-hackle step 9. Arrest lines 10. Constructed
fracture-front lines
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Features associates with the propagation of a
joint
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Plumose Joint Face, Ontario
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Glacially produced joint surface in Killarney
Granite, Ontario
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Controlling Factors of Joint Propagation

• Bedding and foliation planes in coarse-grained
  rocks act as barriers to joint propagation.
• Bedding in fine-grained rocks are often not
  barriers.
• Variation in bed thickness also affects
  propagation direction.

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Four Categories of Joints

• Tectonic Form at depth and are driven by
  tectonic forces.
• Hydraulic Form at depth during burial and
  compaction.
• Unloading Form near the surface when ½ of the
  overlaying sediment is removed by erosion.
• Release Joints Form after the release of
  horizontal stress and are controlled by existing
  rock fabric.

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Joints in Plutons

• Joints form in plutons in response to cooling and
  later tectonic stress
• Orientations of joints may be influenced by the
  boundary of the pluton

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Sheeting

• Sheeting (Same as unloading joints) Form
  parallel to surface topography in massive rocks.
  
• Spacing between sheets increases with depth.
• Can be used in quarrying stone.

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Columnar Joints

• Columnar Joints Form in response to cooling and
  shrinkage in magma.
• Form in Flows, Dikes, Sills, and Volcanic Necks
• Hexagonal prisms are the most efficient geometric
  shape.

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Contraction to Form Columnar Joints or Mud Cracks
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