G2 - Rock Deformation

• • Deformation - change in state/shape of rock under stress.
  • Stress - force of deformation.
  • Strain - the change in shape that results when stress is applied.
• Types of Stress:

  • Compression - rocks squeezed along same line; shortens rock layers by folding/faulting.
  • Tension - forces along same line in opposite directions; lengthens the rock (fractures/faults).
  • Shear - forces act parallel (different directions/rates).
• Strain:

  • rocks deform elastically until elastic limit reached + elastic deformation if rocks return to original shape when stress released
  • elastic limit exceeded, rocks deform plastically until fracture (brittle deformation; don't revert to original shape)
  • folding at greater depths (high temp./more elastic) + faulting nearer surface (rocks more brittle)
• Factors Affecting Deformation:

  • Temperature - higher temp. means rocks more ductile (rocks will fold) + cold rocks behave more brittle (rocks will fracture) + temp. increases with depth (more ductile deformation at depth).
  • Confining Pressure (from mass of overlying rock) - strength of rock increases with pressure (depth); rocks easier to deform (fault) near surface where confining pressure is low.
• Factors Affecting Deformation:

  • Strain Rate - if pressure applied for short period of time, rocks undergo brittle deformation + pressure applied over extended period of time creating plastic deformation.
  • Competence/Composition - mica/calcite more ductile but quartz and olivine more brittle + wet rocks show ductile deformation (water reduces friction/weakens chemical bonds); eg. sandstone is competent (brittle)/shale is incompetent (ductile).
• Normal Fault - maximum stress is vertical and minimum is horizontal + tensional forces (lengthening) - σint always parallel to fault plane.

  
• Reverse Fault - maximum stress is horizontal and minimum is vertical + compressional forces (crustal shortening) - σint always parallel to fault plane.

  
• Strike Slip Fault - shear planes develop 30° to maximum stress (conjugate near shear fractures) and are separated about 60° from each other + maximum/minimum stresses both horizontal.

  
• Fault Surface Ornamentation:

  • displacement scratches fault surface (direction of movement)
  • slickenslides (striated fault surface)
  • fibre growth (direction of fault displacement; latest movement)
  • fault rock types; fault gouge (fine/unconsolidated), fault breccia (angular, >1mm), and mylonite (cohesive, foliated, planar mineral orientation and ductile)
• Structural Reactivation

  • earlier-formed faults can be reactivated/folds refolded + structural inversion; reactivation of normal faults in compression and reverse faults in extension
  • faults reactivate when movement long formerly inactive fault helps alleviate strain within crust (faults change as stresses change)
  • reactivated faults have slickenslides (last direction of movement)