Secondary structures due to brittle deformation: Part I

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Secondary structures due to brittle deformation
Part I

• Joints and shear fractures (chapter 5)

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• FRACTURE
• General term for a surface across which material
  continuity is lost.
• Can be microscopic to continental in scale
• Fractures can be divided into two broad types
  depending on whether there was any PERCEIVABLE
  relative movement along the surface

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• When there is definite (measurable) movement
  along a fracture surface, it is called a FAULT

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• When there is imperceptible movement along
  fracture surface, it can either be
• JOINTS (fracture walls pull apart, page 204)
• SHEAR FRACTURES (imperceptible shear along
  fracture walls, page 206)

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Joints Why bother? (pages 210-214)

• Channels for fluid circulation
• Rainwater and/or groundwater movement
• Pollutant transport
• Hydrothermal fluids containing economic minerals
• Oil and natural gas

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Controls rock strength

• Controls weathering/erosion patterns
• Helps quarrying
• May cause rockfalls or landslides

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Formation of joints

• Joints are MODE I or opening fractures (page 221)
• Walls move away perpendicular to the direction of
  maximum compression, s1
• Mode I fractures are oriented perpendicular to
  the direction of maximum tension, s3

FYI s1, s2 and s3 define the STRESS FIELD at
any point and they are mutually perpendicular to
each other (more later)
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s1
Mode I fracture
s1
s3
s3
Fracture opens
s1
s1
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Joint sets and joint systems (pages 205-206)

• Systematic joints
• Planar joints that comprise a family in which
• all the joints are parallel or subparallel to one
  another, and
• maintain roughly the same average spacing over
  the region of observation

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• Nonsystematic joints (page 206)
• tend to be nonplanar
• they do not parallel neighboring joints, and
• they have an irregular spatial distribution

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• A joint set (page 205) is a group of systematic
  joints
• Joint system (pages 205-206) Happens when two or
  more joint sets intersect at fairly constant
  angles (dont confuse it with systematic joints)
• The angle between two joint sets in a joint
  system is called dihedral angle.

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Joint data from the Hausz Bros. Quarry Collected
by Ryan F. and Shane D. Number of data points
40 Outer circle 10
Is this an example of a single joint set or joint
system?
What is the approximate dihedral angle?
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Joint data from the Hausz Bros. Quarry Collected
by Derek W. and Kristin A. Number of data points
40 Outer circle 20
How is this dataset different from the previous
one?
Is this a significant difference?
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• If the dihedral angle is 90 between two joint
  sets, the pair is called an orthogonal system
• If the two joint sets intersect with a dihedral
  angle significantly less than 90 (e.g., a
  dihedral angle of 30 to 60), the pair is called
  a conjugate system

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Joint data from the Hausz Bros. Quarry Collected
by Ryan F. and Shane D. Number of data points
40 Outer circle 10
Joint data from the Hausz Bros. Quarry Collected
by Derek W. and Kristin A. Number of data points
40 Outer circle 20
Is there a significant difference between the
dihedral angles of these datasets?