Ductile deformational processes

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Ductile deformational processes de
Introduction how can rocks bend, distort, or
flow while remaining a solid? Non-recoverable
deformation versus elastic deformation Three
mechanisms 1) Catalclastic flow 2) Diffusional
mass transfer 3) Crystal plasticity Controlled
by temperature stress strain rate grain size
composition fluid content
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Ductile deformational processes
Catalclastic flow
Cataclastic flow rock fractured into smaller
particles that slide/flow past one another Large
grain microfracture at grain boundary scale or
within individual grains Shallow-crustal
deformation (fault zones)
Beanbag experiment
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Ductile deformational processes
Crystal defects

• Ductile behavior at elevated temperatures
• Achieved by motion of crystal defects (error in
  crystal lattice)
• Point defects
• Line defects or dislocations
• Planar defects

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Ductile deformational processes
Crystal defects

• Point defects
• Two types
• Vacancies Impurities

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Ductile deformational processes
Crystal defects
2) Line defects Also called a dislocation a
linear array of lattice imperfections. Two
end-member configurations. Difficult concept
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Ductile deformational processes
Crystal defects

• Two end-member configurations.
• Edge dislocation extra half-plane of atoms in
  the lattice

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Ductile deformational processes
Crystal defects
Two end-member configurations. A) Screw
dislocation atoms are deformed in a crew-like
fashion
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Deformation Mechanisms
Important relations
Normalized stress (normalized to shear modulus of
the material versus normalized temperature
(normalized to absolute melting temperature of
the material)
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Deformation Mechanisms
Important relations
Differential stress versus Temperature
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Deformation Mechanisms

• Crystalline structures and defects within rocks
  can deform by a variety of deformation
  mechanisms. The mechanism or combination of
  mechanisms in operation depends on a number of
  factors
• Mineralogy grain size
• Temperature
• Confining and fluid pressure
• Differential stress (s1 - s3)
• Strain rate
• In most polymineralic rocks, a number of
  different defm. mechanisms will be at work
  simultaneously.
• If conditions change during the deformation so
  will the mechanisms.

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The Main Deformation Mechanisms
5 General Catagories 1) Microfracturing,
cataclastic flow, and frictional sliding. 2)
Mechanical twinning and kinking. 3) Diffusion
creep. 4) Dissolution creep. 5) Dislocation creep.
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Deformation Mechanism Map
Cataclasis Dissolution creep Dislocation
creep Diffusion creep Pressure solution Each of
these mechanisms can be dominant in the creep
of rocks, depending on the temperature
and differential stress conditions.
Depth / Temperature
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• Fine-scale fracturing, movement along fractures
  and frictional grain-boundary sliding.
• Favoured by low-confining pressures
• Causes decrease in porosity and rock volume.

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Microfracturing, Cataclasis Frictional Sliding

• In response to stress, microcracks form,
  propagate and link up with others to form
  microfractures and fractures.
• Individual microcracks are quite often
  tensional.
• Continued development of microcracks results in
  progressive fracturing of grains, reducing the
  grain size .
• Motion by this mechanism is called cataclastic
  flow.
• Many of the fractures in granite are the result
  of differential thermal expansion - quartz
  indents weaker feldspar.

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Microcrack in Feldspar
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Microcracks break individual atomic bonds
Crack tips have nearly infinitesimally small
areas, which makes the stresses there HUGE!
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Mechanical Twinning and Kinking

• Occurs when the crystal lattice is bent rather
  than broken.
• The crystal lattice is bent symmetrically about
  the twin plane, at angles that are dependent on
  the mineral.
• Common in calcite and plagioclase.

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Kinking commonly occurs in micas and other platy
minerals that are susceptible to end loading.
The amount of kinking is not limited to a
specified angle as in twinning.
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Diffusion Dissolution Dislocation Diffusion atom
jump from site to site through a mineral. It is
thermally activated (higher T faster). Slow
and inefficient. Faster in the presence of
fluids. Requires vacancies. Most efficient in
fine grained rocks.
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Volume-Diffusion Creep

• Works at high T, in the presence of direct
  stress - diffusion allows minerals to change
  shape.
• Atoms systematically swap places with vacancies
  (like checkers).
• Vacancies move toward high stress and atoms
  toward low stress.
• Vacancies are destroyed when they move to the
  edge of the grain.