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Fracture orientations relative to the principal stress orientations
  Earth stress.   Stress is defined as the force per unit area acting on a plane. Any stress state at a point in a solid body can be described completely by the orientations and magnitudes of three stresses called principal stresses. The principal stresses are oriented perpendicular to each other and to the three planes of no resolved shear stress at the point. The drawing shows a block of rock having constant stress throughout. The symbol (sigma) designates compressive or tensile stress. Subscripts identify specific stresses. The principal stresses are defined: 1 > 2 > 3. Compressive stress and shortening strain are considered positive in rock mechanics and structural geology because in the earth all three principal stresses are always compressive (except around underground voids such as caves or very near to the earth's surface). Joints (extensional fractures) seem paradoxical because they are one of the most common types of natural rock fracture even though they require an effectively tensile driving stress. Pore-fluid pressure drives most joints by producing tensile effective stress through poroelastic loading of flaws that are orders of magnitude larger than typical pores. See Mechanics of jointing for a detailed discussion.

Figure 1.

  Joints (GREEN).
  • The tip of a growing joint is always perpendicular to 3 at the joint tip during propagation.
  • Curved joints indicate temporal and/or spatial variations in the orientation of 3 during joint growth.
Faults (RED).
  • New faults in virgin rock form in each lithology with an approximately constant acute angle between 1 and the two conjugate fault orientations. This angle ranges from 25 to 40 but is normally about 30.
  • Pre-existing discontinuities with a wide range of orientations can be activated as faults, provided they are not oriented perpendicular to a principal stress.
  • Deformation bands (faults with an anti-mode I component) also tend to form at 30 to 1 but can form at any angle <90.
Stylolites (BLUE) and compaction bands (not shown) form perpendicular to 1.



 

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