Rock Deformation

• Rock deformation - the movement of tectonic plates creates stresses which cause rocks to deform → change in volume and/or shape
• Stress - the force acting on a body of rock per unit area
• Uniform/confining - stress is equal in all directions
• Hydrostatic stress - equal stress underwater
• Differential - stress is not equal in all directions
• Types of differential stress:

  1. Compressional
  2. Tensional
  3. Shear
• Compressional - push against each other (causes shortening or flattening)
• Tensional - pull against each other (cases stretching or extension)
• Shear - slide against one another (cause a shear strain)
• Stages of deformation:

  1. Elastic deformation
  2. Ductile deformation
  3. Fracturing or brittle deformation
• Strain - amount of deformation
• Elastic deformation -Reversible, non-permanent (it goes back after stretching it)
• Elastic deformation

  • Occurs at very low stresses
• Ductile Deformation

  • Irreversible, permanent change in volume or shape
  • The rock will bend or fold
• Fracturing or Brittle Deformation

  • Permanent strain where the rock breaks or fracture
• Brittle material:

  • low temperature
  • low confining pressure
  • high strain rate
• Ductile deformation

  • Irreversible changes that involve deformation of rocks into a series of wrinkles or folds (small or large scale)
• Anatomy of a fold

  1. Axial plane
  2. Fold Axis
  3. Limbs
• Axial plane - imaginary surface that divides a fold as symmetrically as possible
• Fold axis - line made by the length-wise intersection of the axial plane with fold
• Limbs - Correspond to the 2 sides of a fold
• Types of fold

  1. Symmetric Fold
  2. Asymmetric fold
  3. Horizontal fold
  4. Plunging fold
  5. Anticline
  6. Syncline
  7. Dome
  8. Basin
• Symmetric Folds

  • Axial plane is vertical
  • Limbs on either side of the axial plane are “mirror image” of each other
• Asymmetric folds

  • Axial plane is tilted
  • Limbs are not symmetrical about the axial plane
• Horizontal fold

  • Fold axis is horizontal
• Plunging fold

  • Fold axis is not horizontal
  • The elevation of the crest of the fold is not the same all throughout, it plunges into the ground
• Anticline

  • Oldest at the center
  • Upfolding
  • A shaped
• Syncline

  • Downfolding
  • Youngest at the center
• Antiform syncline

  • If its a syncline with the oldest at the center
• Dome

  • Central uplift
  • Youngest at the sides, oldest the center
• Dome

  • a circular or elliptical uplifted geologic feature on which the rock layers slope gently downward in all directions from a central high point.
• Fracture is an umbrella term for all types of cracks
• Fault- A fracture showing displacement of rock masses on either side of the trace
• Types of fault: based on type of displacement involved
• Types of fault:

  1. Dip-slip - vertical displacement
  2. Oppositely dipping normal faults
  3. Strike-slip - lateral displacement/horizontal
  4. Oblique - combination
• Normal Dip-slip:

  • the downthrown block is the hanging wall; tensional stress involved
• Reverse Dip-slip - the downthrown block is the footwall; compression stress involved; steeply dipping fault (>45 degrees)
• Thrust - similar to a Reverse fault but the fault dip is less steep (Less than 45 degrees)
• Horst: upthrown (foot wall) block
• Graben: downthrown (hanging wall) block