Deformation of rocks

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					Crustal Deformation

  Stress and Strain

Types of Deformation

 Geologic Structures
    Folds and Faults

 Geologic Structures
      on Maps
    Orientation of deformed rocks
We need some way to describe the
 distribution of geologic structures.
Strike: bearing of a line defined by the
  intersection of the plane in question and
  the horizontal
Dip: angle between the plane and the
 horizontal, measured perpendicular to
Fig. 10.4
 Arrows indicates the                The length of the arrow
 force on 1 side of the              indicates how strong the
 block                               force is

Confining stress - equal     Differential stress - unequal
forces on all sides of the   forces on the sides of the
block (yellow arrows)        block (red extension of arrow)
(force per unit area)
. the ability of an object to resist

• compressive strength

• tensile strength

• shear strength
  Change in shape or size of an object in
     response to an applied stress.
               = Deformation
Three Types of Strain
                Ductile (Plastic)
                Brittle (Rupture)
  Elastic Deformation

a temporary change in shape
or size that is recovered when
the applied stress is removed.
Ductile (Plastic) Deformation
  • A permanent change in shape
     or size that is not recovered
     when the stress is removed.

      • i.e. it flows or bends
   Brittle Deformation
• the loss of cohesion of a body
under the influence of deforming

       • i.e. “it breaks”
        Factors Determining the
Style (ductile or brittle) of Deformation

       • Pressure
       • Temperature
       • Strain rate
       • Rock composition
• Pressure:
 –High P - Ductile Deformation
 –Low P - Brittle Deformation
• Temperature:
 –High Temp - Ductile Deformation
 –Low Temp - Brittle Deformation
• Strain Rate:
 –High Strain Rate - Brittle Deformation
 –Low Strain Rate - Ductile Deformation
• Composition:
 –“Hard”(e.g. granite) - Brittle Deformation
 –“Soft” (e.g. marble) - Ductile Deformation
           Types of folds
      (bent planar structures)
anticline: Flanks bent downwards, older
 rocks on the inside
syncline: Flanks bent upwardsolder rocks
  on the outside

     (scale - from mm to tens of km)
              Fold terms
• Axial plane: the plane of mirror symmetry
  dividing the fold into two limbs
• Axis: line formed by the intersection of
  the axial plane and a bedding plane
• Horizontal fold: where the fold axis is
• Plunging fold: where the fold axis is not
      Strike and Dip Symbols
          you should know.

Inclined beds             Horizontal beds

          Vertical beds
More Symbols you should know.

Horizontal Folds          Plunging Folds
 Horizontal Anticline   Plunging Anticline

Horizontal Syncline     Plunging Syncline
Fault Symbols you should know.
               Normal Fault

        D      Reverse Fault

Left-lateral   Right-lateral
Why is understanding deformation
     of the Earth important?

• Major part of earth history
• Explains origins of mountain belts
• Trapping hydro-carbons and the
  concentration of ore deposits
• Faulting causes earthquakes
• In Houston, growth faults and subsidence
     Faults may be "reactivated"
History of a fault may be very long
Previously developed weakness is the
 most likely place to break
  reactivation may have opposite sense as
'Active' = 10,000 to 100,000 years
  very important for dams and reactors
     Dip-Slip Faults

Faults where the movement
  (slip) between adjacent
   blocks was in the dip
  Types of Dip-Slip Faults:

Normal Faults: a dip-slip fault
 where the hanging wall moved
 down relative to the footwall.
A fracture along
  which some
movement has
 An example of
 brittle failure.
  Types of Dip-Slip Faults:

Reverse Faults: a dip-slip fault
 where the hanging wall block
 moved up relative to the
 footwall block.
   Strike-Slip Faults

Faults where the movement
  (slip) between adjacent
  blocks was in the strike
Types of Strike-Slip Faults:
• Left-lateral: where the opposite
  block moved to the left, relative to
  the observer.
• Right-lateral: where the opposite
  block moved to the right, relative
  to the observer.
        Fault Scarp
A scarp formed by the offset
 of adjacent blocks along a
 fault. It is like an exposed
  fault plane which may be
    modified by erosion.

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