G2 ROCK DEFORMATION

• Factors affecting rock deformation

  • Temperature
  • Pressure
  • Time
  • Rock type
• Temperature


  At high temperatures molecules and their bonds can stretch and move causing them to behave in a more plastic manner. At low temperatures materials are more brittle.
• Analogy to understand temperature effects

  • Chocolate left out in the warmth and heated close to its melting point will bend easily, whereas chocolate left in the fridge will be hard and brittle.
• Pressure


  A higher confining pressure materials are less likely to fracture- the pressure of the surroundings tends to hinder the formation of fractures. At low confining pressures, material will be brittle and will tend to fracture sooner.
• Time


  If stress is applied rapidly then the rock will tend to behave in a brittle manner and fault. If stress is applied slowly (over millions of years) then the rock will tend to behave plastically and fold.
• Competent rock


  • Hard, brittle, often crystalline e.g. granite, limestone, sandstone. They tend to joint and fault.
• Incompetent rock


  • Soft, plastic e.g. clay, shale. They tend to fold easily, show cleavage and plastic flow.
• Competent rocks behave britally and are more likely to fault

  • Incompetent rocks behave in a more ductile way and are more likely to fold
• Competent rocks:

  • Tension joints. Longitudinal joints normally form on the fold crest (area of maximum tension) or sometimes at one of the hinges. These form V shapes. the width of the joint decreases towards the core, further away from max tension.

  • Longitudinal joints (strike). Joints occurring perpendicularly to the stress, parallel to the fold axis and due to folding of competent beds.

  • Cross joints (dip). Parallel to the stress direction following the dip of the limbs.
• Incompetent rocks:

  • Plastic deformation is observed in the shale in the diagram. It is thicker at the hinge, where it has started to flow. Competent rocks will not show a change in thickness.

  • Where flat and platy minerals occur slaty cleavage can form during folding. The folding of shale/mudstones will often result in the formation of a slate.
• The distance between two successive anticlinal (or synclinal) hinges seen in profile is the wavelength.

  The amplitude is measured by taking half the distance along the axial plane from the median surface to the crest or trough
• Axial plane orientation


  Expressed as dip and strike or dip and direction of dip
• Axial planes


  Cut the hinge zone of a folded surface along the fold axis
• Fold axis orientation


  Expressed by its plunge and trend (plunge azimuth)
• Plunge


  Inclination measured from the horizontal in the imaginary vertical plane containing the line
• Trend


  Plunge direction
• Non-plunging folds


  • Have horizontal hinges or plunges of less than 10°
• Vertical folds


  • Plunge at 80-90 degrees
• Recumbent


  When the axial plane dips less than 10°
• Plunging folds

  Where the axes of folds are tilted so they dip away from the horizontal. The angle of plunge is the angle between the axis and the horizontal. Plunging folds form the same repeating pattern as non plunging folds, except their limbs converge around the axial traces
• The limbs of anticlines close in the direction they plunge. Points in direction of plunge
• P max means the most compressive force (but not necessarily the highest overall force) and P min is always the least compressive force (but not necessarily the lowest overall force). As one stress is more compressive than the others, the rock will at first be squashed until its elastic limit is reached and then it will fracture. We know from experiments that under unequal compressive stress like this a fracture will always form at an angle to P max (usually 20-40 degrees)