rock deformation

• Isoclinal folds


  • Angles of inclination on both limbs are the same
  • Axial plane is equidistant from the limbs
• Chevron folds


  • Angles of inclination on both limbs are the same
  • Axial plane is equidistant from the limbs
• Asymmetrical folds


  • Angle of inclination of one limb is greater than the other
  • Axial plane is not equidistant from limbs
• Asymmetrical folds


  • Overturned folds
  • Recumbent folds
• Parallel fold (concentric fold)


  • Equal distance between subsequent beds
  • Constant thickness of bedding plane
  • Radius of fold decreases towards the core
• Similar fold


  • Constant shape among subsequent beds
  • No equal distance
  • Bedding plane thickness is much larger in the hinge zone than in the limbs
• Disharmonic fold


  • No relation of shape or distance between subsequent beds (bedding thickness)
• Fold size classification


  • Major fold (megascopic) - mappable fold structure
  • Minor fold (mesoscopic) - fold the size of an outcrop
  • Microfold (microscopic) - fold in hand specimen and thin section size
• Fault


  Deformation on planar surfaces along which blocks on each side have moved relative to each other
• Hanging wall


  Rock above the fault
• Footwall


  Rock below the fault
• Strike


  Direction of the line of intersection between an imaginary horizontal surface and the fault plane
• Dip


  Angle that a fault plane makes with the horizontal plane
• Slip


  Amount of movement
• Strike-slip


  Component of slip parallel to the strike
• Dip-slip


  Component of slip parallel to the dip
• Net-slip


  Combination of strike-slip and dip-slip
• Separation


  Distance separating comparable beds or lobes in a plane perpendicular to the fault plane
• Strike separation


  Block separation in the fault plane along the strike direction
• Dip separation


  Block separation in the fault plane along the dip direction
• Heave


  Horizontal component of the dip separation
• Throw


  Vertical component of the dip separation
• Strike-slip fault


  • Slip movement is parallel to the strike of the fault
• Right-lateral fault (dextral fault)


  Ground opposite the observer appears to have moved to the right
• Left-lateral fault (sinistral fault)


  Ground opposite the observer appears to have moved to the left
• Dip-slip fault


  • Displacement is in the direction of the dip creating a hanging wall and a footwall
• Normal fault


  • Hanging wall appears to have moved downward relative to the footwall
  • Occurs where maximum stress is vertical and minimum stress is horizontal (tensional)
• Thrust fault


  • Hanging wall appears to have moved upwards relative to the footwall
  • Angle of the fault plane with the horizontal is less than 45°
• Reverse fault


  • Hanging wall appears to have moved upwards relative to the footwall
  • Angle of the fault plane with the horizontal is greater than 45°
• Oblique-slip fault


  • Both strike-slip and dip-slip components of a fault are observed
• Thrust and wrench faulting occur when the maximum stress is along the horizontal (active compressional forces)
• Thrust faults will occur relatively shallow in the earth's crust, while wrench faulting with horizontal extensions will occur at depths
• Thrust faulting and folding are often connected
• Hypocenter (focus)


  Points of movement along fault ruptures
• Epicenter


  Directly above the hypocenter
• Earthquake Focal Mechanism (EFM)


  Graphical summary of the strike, dip, and dip direction of the fault causing the earthquake
• The Philippine Fault is a left-lateral or sinistral strike-slip fault that extends from the tip of the Central Cordillera to the Southeastern part of Mindanao
• Joint


  Break in the rock with no relative movement on opposite sides
• Joints are mostly caused by the expansion of rocks as the weight of the overlying material is removed
• Columnar joints are the result of the cooling of lava