Module 10: Rock Deformation

• Deformation - is any change in shape, size, or orientation of a rock mass.
• Rocks may deform in many ways: they may deform in a brittle manner and develop cracks or fractures or they may also deform ductilely, bending due to stress leading to folding of rock layers
• Stress - is the force acting on a body of rock per unit area.
• Strain - the resulting deformation (change in shape, size, or both) from the applied stress.
• Uniform stress (confining stress) - stress acting on a rock mass is equal in all directions

  the resulting deformation from this stress only lead to a change in size with no change in shape
• Differential stress - stress acting on a rock mass is not equal in all directions which can further be classified into:

  Compressive stress

  Tensional Stress

  Shear Stress
• compressive stress - Forces that squeeze or push toward one another from opposite directions (cause shortening or flattening).
• Tensional Stress - Forces that pull away from one another in opposite directions (cause stretching or extension).
• Shear stress – Forces that are offset from one another and operate in opposite directions (cause a shear strain).
• When a rock is subject to increasing stress, it undergoes 3 stages of deformation:

  Elastic deformation, and pass the elastic limit, an object will undergo:

  Ductile Deformation, and finally

  Fracturing or Brittle Deformation
• Elastic Deformation - reversible, non-permanent and the rock will return to its original shape when the stress is removed and occurs at very low stresses
• Ductile Deformation - irreversible, permanent change in volume or shape and the rock will bend or fold
• Fracture/Brittle Deformation - permanent strain where the rock breaks or fracture
• Ductile deformation is characterized by irreversible changes that involve deformation of rocks into a series of wrinkles or folds
• Axial plane - The imaginary surface that divides a fold as symmetrically as possible

  Fold axis - the line made by the length-wise intersection of the axial plane with beds in the fold

  Limbs - correspond to the two sides of a fold

  
• The types of folds based on (vertical tilt) orientation of axial plane include symmetric and asymmetric folds
• The types of folds based on (horizontal tilt) orientation of fold axis are horizontal fold and plunging fold
• Symmetric folds - Axial plane is vertical and Limbs on either side of the axial plane are “mirror image” of each other
• Asymmetric folds - Axial plane is tilted and Limbs are not symmetrical about the axial plane
• Horizontal fold - Fold axis is horizontal
• Plunging fold - Fold axis is not horizontal and the elevation of the crest of the fold is not the same all throughout It “plunges” into the ground
• Anticline - an ‘arch’ in a fold, where the limbs dip (tilt downwards) away from the axis “/ \”

  “upfolding” , the parts of the originally horizontal strata that are pushed up

  The oldest rock/strata is at the center
• Syncline - the “U”-shaped part of the fold; limbs dip (tilt downwards) towards the axis “\ /”

  “downfolding”, the parts of the originally horizontal strata that are pushed down

  the youngest rock/strata is at the center
• Domes - uplifted at the center, and from there, strata dip down & outward (from the center) in all directions
• Basins - have a depressed center, and strata from all directions dip towards the center
• Joints - are roughly parallel, repetitive, fractures that do not show any displacement.
• Fault - a fracture with a discernible displacement of rock masses on either side of the trace
• Dip-slip faults - These are faults where blocks on either side of the fault move vertically.
• Footwall - The block where your feet are in contact with
• Hanging wall - the block where you can reach up with your arms
• The three types of faults that fall under dip-slip faults are:

  Normal fault

  Reverse fault

  Thrust fault
• Normal fault - Footwall moves up with respect to the hanging wall and is formed from extensional stress
• Reverse Fault - Hanging wall moves up with respect to the footwall and is formed from compressional stress
• Thrust fault - A specific type of reverse fault where the fault plane dips (inclined) at a gentle angle
• Strike-slip faults - where blocks on either side of the fault move horizontally and slide in opposite directions along the fault and can have either of two possible senses of movements: left-lateral or right lateral.

  
• Oblique-slip faults - These are faults where blocks on either side of the fault have both dip-slip (vertical movement) and strike-slip (horizontal movement) components.