Week 3: Folds and Structural Geology

• Structural Features of Folds

  hinge, limbs, axial plane, interlimb angle and fold axis.

  DRAW AND LABEL
• Why are structural features important

  • These features help determine the nature of the fold and the tectonic history of the rock, such as how much stress it experienced to be able to form a fold.
  • Folded rocks are most commonly produced in high pressure and temperature environments, such as in orogenic zones.
  • E.g., isoclinal folds require more compressional stress than an open fold
• Cleavage and Folds

  • Cleavage refers to the planar alignment of minerals within a rock, in which the minerals align themselves perpendicular to the direction of principal stress
• What does the form of the fold depend on

  • There are many different types of folds, resulting fold structures depend on characteristics of the formational environment, rock type, rock competency.
• Cylindrical vs non-cylindrical folds

  • Non cylindrical folds do not possess a uniform fold axis unlike cylindrical folds which do
• Parallel Limbs vs Similar Limbs


  • Parallel limbs retain thickness, similar do not
• Interlimb Angle Classifications

  • Folds can also be classified via their interlimb angle. DRAW
  • Isoclinal – 0˚-3˚
  • Tight – 3˚-30˚
  • Close –30˚-70˚
  • Open – 70˚-120˚
  • Gentle – 120˚-180˚
• Basic forms of folds

  • The age of the rocks must be known for anticlines and synclines to be determined
  • Anticline – draw with direction of younging
  • Syncline – draw with direction of younging
  • Monocline – draw, include that it is a step like fold
  • Chevron folds – draw, include that chevron folds require high amount of slip to form and therefore are most commonly seen in micas and clays
  • Disharmonic folds – draw, include that they include sharp changes in wavelength and shape, formed due to alternating layers of different competency of rocks
• Geothermal gradient


  • Geothermal gradients are the change in temperatures experienced as you travel deep into the crust
  • On average this is 20˚-30˚/km
  • As you increase the temperature in the crust, the response of rocks is based in the competency and mineralogy
  • E.g., Rocks with high amounts of feldspar are more competent than rocks with high amounts of quartz
  • This also leads to a change in seismic behaviour
• Conceptual Crustal Strength Models

  • There are 2 main conceptual models that provide simplified representations of seismic wave behaviour through the Earth’s interior. They are used as aids in understanding the geophysical behaviour and structure of the Earth as inferred from seismic wave behaviour
• The Crème Brulée Model

  • Used for oceanic lithosphere
  • the brittle ductile transition is deeper, meaning that the change to plastic flow of rocks doesn’t occur until below the MOHO
• Jelly Sandwich Model

  • Used for continental lithosphere
  • Which there are two zones of brittle-ductile transition, one above and one below the MOHO. Showing how rocks in both the upper and lower mantle experience changes in rock behaviour