Joints, Shear Fractures, & Veins

• Joints


  Continuous and through-going planar structures along which there has been imperceptible "pull-apart" movement more or less perpendicular to the fracture surface
• Joints


  • Products of brittle fracture and they form when the tensile strength of stressed rock is exceeded
  • Form as a result of minor readjustments of rock bodies due to burial and compaction, heating and expansion, uplift, cooling and contraction, tectonic loading
  • Commonly become sites where minerals are precipitated as VEINS
• Mineralizing solutions invading rock bodies along joints and fractures

  1. Precipitating minerals from solution in the open space when the cracks are forced open and the chemical conditions are right (favorable temperatures and/or pressure)

  2. Precipitation may also be triggered by mixing of the different fluids that meet at the fractures

  3. The precipitated minerals seal the fractures
• Joints


  • Among the most abundant geologic structures
  • Dimensions (length and spacing) depend on size, thickness and stiffness of the rock body
• Systematic joints


  Planar, parallel and evenly-spaced joints displaying systematic preferred orientations and striking symmetry
• Joint system


  • Two or more sets of joints with each set having its own distinctive orientation and spacing
  • Orthogonal - dihedral about 90 degrees
  • Conjugate - dihedral < 90 degrees
• Non-systematic joints


  Joints that are irregular in form, spacing and orientation
• Joint sets


  Systematic over large regions
• Cross joints


  Occur between two members of a joint set
• Joint zone


  Made of en echelon sets of Fractures common in shear zones
• Significance of Joints


  • Mining and Quarrying
  • Civil Engineering
  • Groundwater Circulation
  • Hydrothermal solution and Mineral Deposits
• Reasons Why Analyzing Joints is Hard
• Shear fractures


  Jointlike fractures that are formed in the same way as faults are formed (has shear parallel to the fracture surface) but offset by shear is not visible at the outcrop level
• Shear fractures

  • Same in size and scale as joints and occur in sets of parallel planar structures
  • Spacing and density vary in relation to the mechanical properties of the rocks and in the presence or absence of major structures like faults and folds
  • Distinguished from joints by the presence of slickenlines (fine scale ridge-in groove lineations) with coatings of crystal fibers that have grown in the direction of shear displacement
• Fractures

  Non-systematic and extremely closely spaced fractures that occur pervasively within fault zones
• Shattered rocks


  Pervasively fractured rocks within fault zones
• Importance of Joints, Shear and Other Fractures

  • Fracture-controlled planes of weakness are important in quarrying
  • Permits circulation of fluids, rainwater and groundwater, pollutants and contaminants, hydrothermal mineralizing solutions, geothermal waters and oil and natural gas
  • Has significant effects in weathering and erosion and in the formation of landscapes
• Joint surfaces


  • Planar to curviplanar surfaces that intersect the tops and flanks of outcrops as lines
  • Two-dimensional form strongly influenced by the three-dimensional shape of the rock body in which the joint is contained
  • Tend to be rectangular in layered sedimentary rocks and elliptical in massive rock bodies

  
• Joint face ornamentation


  • Main joint face - the smooth planar surface
  • Fringes - serrated and roughly hewn outermost margins of a joint surface, replacing the single joint face with a number of very closely spaced en echelon joints intersecting the main joint face at 20˚ to 25˚
  • En echelon joints - consists of 3 or more short joints that are parallel and overlapping and arranged in a line
  • Origin - site of initial propagation of the joint surface where the energy is first released to form the break; usually coincides with a mechanical defect, flaw or irregularity (cavity, fossil, inclusion)
  • Hackles - linear to systematically curved markings that converge toward the origin of the joint; tiny ridges and troughs; hackles collectively display featherlike plumose structure radiating from one or more axes
  • Ribs - parabolicallly curved surface markings similar to fracturing in obsidian and plexiglass; represent positions of the joint front in the past; fossil record of a propagating joint front
• Mode I (opening)


  Extension fractures that open perpendicular to the plane of the joint (e.g. Joints)
• Mode II (sliding)


  Characterized by a sliding movement parallel to the fracture surface and perpendicular to the fracture front (e.g. shear fracture)
• Mode III (scissor)


  Movements are characterized by a scissors movement parallel to the fracture surface and parallel to the fracture front
• En echelon trace


  Where the level of exposure intercepts only the fringe of the joint face
• Single line trace


  Level of exposure cuts a deeper level of the joint face
• 1. intersections


  Joints that intersect in Y-patterns meet at approximately 120˚ angles, which is a configuration requiring minimum energy to achieve; typical of discontinuous contraction joints like mud cracks patterns or columnar joints
• 1. intersections


  Pattern that forms when systematic continuous joints intersect at acute angles
• 1. intersections


  Common in orthogonal joint systems where individual joint traces meet at right angles
• Patterns of termination of joints

  • Hook
  • Hook with T-intersection - forms when the hook with adjacent intersects with a trace coming from the other direction
  • En echelon segments
• Cross-fold joints


  Formed as mode I tensile joints oriented vertically and generally parallel to the compressional direction responsible for the folding; formed under conditions of elevated pore fluid pressure and directed tectonic stress
• Tectonic joints


  Form when pore fluid pressure become elevated as a result of tectonic squeezing and compression
• Hydraulic joints

  Form when pore fluid pressure become elevated as a result of vertical gravitational loading
• Young joints


  Formed near the surface during uplift and erosion
• Release joints


  Formed from the release of compressive stress; near-vertical mode I tension joints perpendicular to the former direction of tectonic compression
• Unloading joints


  Mode I tensile joins that are not related to the geometry of the fold belt
• Cross joints


  Long, planar fractures, evenly spaced and often coated with minerals, which are perpendicular to lineation; well developed near the roof of the pluton
• Longitudinal joints


  Less significant than cross fractures and have thinner mineral coatings; steeply dipping and strike parallel to the trend of lineation
• Stretching surfaces


  Occur in the upper reaches of the pluton; low-dipping shear surfaces marked by striations that trend in the direction of lineation
• Butting relation


  Kind of morphology indicates that the fracture propagates very rapidly. Younger joints nearly always terminate against older joints at right angles.
• Sheet structure or Exfoliation

  Thin, curved, generally convex upward shells which parallel the local topography; related to gravitational unloading of the granitoid terrain.

  
• Spalling and rock bursts in mines and quarries

  Pieces of rock may literally "explode" off of the newly exposed wall or tunnel.