SGES 3371 STRUCTURAL GEOLOGY II

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SGES 3371STRUCTURAL GEOLOGY II

• Lecture 7
• Ductile Deformation

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Ductile Deformation

• Eg. Layers of rocks permanently bent into a fold,
  while it remains a solid.
• How? Due to bending and stretching of atomic
  bonds in the crystal lattice?
• No. Such bending and stretching is elastic
  deformation, which is recoverable.
• Ductile deformation is not recoverable - due to
  permanent change in material.
• Main difference between ductile and brittle
  deformation Strain is distributed throughout the
  rock material undergoing ductile deformation
  (penetrative)
• Scale of observation mesoscopic

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Ductile Deformation

• Fundamental ductile deformation mechanism
• Cataclastic flow
• Diffusional mass transfer
• Crystal plasticity
• Which of the above mechanism actually occurs in
  the rock depends on Temperature, stress s, strain
  rate é, grain size, composition of rock fluid
  content.
• T is the most important factor.
• However, different mineral behave ductilely at
  different T

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Ductile Deformation

• Cataclastic flow
• Mesoscopically ductile but microscopically
  brittle
• Analogy bean bag experiment bag (mesoscopic)
  can change shape by sliding and/or breaking of
  the beans (microscopic)
• Frictional sliding and microfracturing is
  strongly P dependant.
• Cataclastic flow can only occur at shallow
  crustal levels (upper few km)

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Ductile Deformation

• Crystal defects
• Most common mechanism of ductile deformation at
  high T
• It is an error in the crystal lattice. 3 basic
  types
• Point defect
• Line defect (dislocation)
• Planar defect (stacking fault)
• The movement of the defect under differential
  stress causes permanent strain without lost of
  cohesion
• Point and line defects are most important in
  rocks
• Point defects ? difussional mass transfer
• Line defect ? crystal plasticity

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Crystal defects

• Point defects
• 2 types of point defects vacancies and impurity
  atoms.
• Vacancies are empty sites in the crystal lattice.
• Vacancies can migrate by exchange of atom at
  neighbouting sites. The process is called
  diffusion.
• Impurity atoms are (a) substitutional in which
  an atom is replaced by a different atom (b)
  interstitional in which an atom in at a
  non-lattice site of the crystal.

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Crystal defects

• Line defects (dislocation)
• It is a linear array of lattice defects
• 2 end-members edge dislocation screw
  dislocation
• Edge dislocation there is an extra half-plane
  of atom in the crystal lattice
• End of the extra half-plane is the dislocation,
  and it extends into the crystal lattice as a
  dislocation line (l).
• Screw dislocation the atoms are arranged in a
  cork screw like manner, the axis of the screw
  marks the dislocation line.

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Edge dislocation
Edge dislocation
Screw dislocation
Screw dislocation
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Crystal defects

• Line defects (dislocation)
• The presence of dislocation distorts a crystal
  which gives rise to a local stress around the
  dislocation.
• Edge dislocation compressive tensional
  stresses
• Screw dislocation shear stress

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Diffusional mass transfer

• Transfer of material through diffusion can cause
  flow of rocks.
• 3 mechanisms important in rocks
• Pressure solution in fluid phase
• Grain boundary diffusion (Coble creep) in solid
  state
• Volume diffusion (Nabarro-Herring creep) in
  solid state
• It occur when an atom or a point defect migrates
  through the crystal.
• It is T dependant (thermal energy causes atoms to
  vibrates, bonds break and reattach more easily)

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Diffusional mass transfer

• Grain boundary diffusion volume diffusion
• Given enough time or subjected to stress,
  vacancies will migrate to crystal surface and
  disappears.
• When a crystal is subjected to differential
  stress, atoms move to the sides where stress is
  least and vacancies migrates to the site where
  stress is greatest.
• Results in a change in the distribution of mass
  and crystal shape.
• The diffusion can occur through the entire
  crystal (volume diffusion) or concentrated along
  a narrow region at the grain boundary (grain
  boundary diffusion).

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Diffusional mass transfer

• Grain boundary diffusion volume diffusion
• Strain rate for both diffusions is a also
  function of grain size finer grain size ? higher
  strain rate.
• Activation energy for grain boundary diffusion is
  less than that for volume diffusion.
• Grain boundary diffusion is more significant for
  crustal rocks.
• Volume diffusion restricted to very high T (ie.
  in mantle) and/or very fine grain size.

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Diffusional mass transfer

• Pressure solution
• It requires the presence of a layer of fluid film
  on grain boundaries.
• Important for crustal rocks as mass transfer can
  occurs at low T, compared to earlier mechanisms.
• Transfer through a chemically active fluid that
  dissolves the crystal.
• The dissolved ions then move along a chemical
  gradient to regions of new growth.
• Chemical gradient caused by differential
  solubility in the presence of differential stress
  (surfaces perpendicular to s1 show enhanced
  solubility, the dissolved material transported to
  area of low stress, perpendicular to s3).

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Diffusional mass transfer

• Pressure solution
• Indicated by stylolites in limestone, grain
  overgrowths in sst, pressure shadows, etc.
• Also may results in differentiation formation
  of alternating layers of different composition.
• Dissolved ions may be transported away or may
  precipitate as veins
• Can results in substantial volume loss in rocks
  if dissolved ions migrates out of the rock

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Crystal plasticity / Intracrystalline deformation

• Crystal plasticity is strain produced by movement
  of dislocation through the crystal lattice.
• Dislocation movement can occurs by glide, a
  combination of glide and climb and mechanical
  twinning.

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Crystal plasticity

• Dislocation glide
• Movement of dislocation along a glide plane due
  to introduction of energy to the crystal through
  deformation temperature
• Edge dislocation to minimise the energy
  requirement, movement by successive breaking and
  reattachment of one bond at a time.
• Screw dislocation moves by shearing one atomic
  bond at a time

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Crystal plasticity

• Cross-slip climb (or dislocation creep)
• It is not always possible to propagate a
  dislocation to the edge of crystal.
• Obstacle due to presence of immobile
  dislocations.
• To overcome the obstacles, dislocation must move
  out of their current glide plane (cross-slip for
  screw dislocation, and climb for edge
  dislocation).

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Crystal plasticity

• Mechanical twinning
• Twins formed in response to applied stress.
• A surface imperfection, the twin plane, separates
  two regions of a twinned crystal.