Friday 12:00 Geology Seminar

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Friday 1200 Geology Seminar Dr. Lucy Flesch,
Purdue University Integration of Plate Boundary
Observatory and USArray Data to Quantify the
Forces Driving Deformation in the Western United
States
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Nisqually Earthquake, Feb 28, 2001
6.8 Mw 52 km deep No deaths 400 injuries
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Fault strength paradox San Andreas Fault and
Pore Fluid Pressure
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Outline Ductile Deformation Three main
mechanisms Cataclastic flow- Crystal
plasticity kinds of crystal defects point
defects line defects crystal plasticity
mechanisms dislocation glide Dislocation
climb Dislocation climb glidecreep twinning Di
ffusional mass transfer Solid State Mass
diffusion Grain Boundary mass diffusion
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Ductile deformational processes
Introduction how can rocks bend, distort, or
flow while remaining a solid? Non-recoverable
deformation versus elastic deformation Ductile
behavior weve used the words viscous and
plastic to describe the deformation- now well
talk about the actual physical processes Three
mechanisms 1) Catalclastic flow 2) Crystal
plasticity 3) Diffusional mass transfer Which
process dominates controlled by
temperature stress strain rate grain size
composition fluid content
Different rocks/minerals behave ductily at
different temperatures Homologous temperature
ThT/Tm Low temperature Thlt0.3 medium
temperature 0.3ltThlt0.7 High temperature Thgt0.7
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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 Remains cohesive (vs
gouge or breccia) Relatively shallow crustal
deformation (fault zones)
Beanbag experiment
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Ductile deformational processes

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

Crystal defects
Motion of defects causes permanent strain while
the material remains solid
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Ductile deformational processes
Crystal defects
Point defects vacancy, substitution
impurity Interstitial impurity
Vacancies can migrate by exchange with atoms at
neighboring sites also called diffusion
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Ductile deformational processes
Crystal defects- line 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 lattice is deformed in a screw-like
fashion
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Ductile deformational processes
Crystal defects
Burgers vector b The vector that represents the
magnitude and direction of the lattice distortion
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Ductile deformational processes
Crystal defects
Burgers vector b The vector that represents the
magnitude and direction of the lattice distortion
Magnitude of Burgers vector commonly on the order
of nanometers (1 x 10-9 m)
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Ductile deformational processes
Crystal defects
Mixed dislocations combination of edge and screw
Defects cause internal stress, can affect the way
the mineral responds to external stress
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Ductile deformational processes
Crystal defects and stress
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Ductile deformational processes
Crystal defects
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P.S. 4 coming soon Mid Term this Thursday,
review sheet on course website Folds and
Stereonets Lab- solutions of contoured fold data
on east wall
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Outline Ductile Deformation Three main
mechanisms Cataclastic flow- Crystal
plasticity kinds of crystal defects point
defects line defects crystal plasticity
mechanisms dislocation glide Dislocation
climb Dislocation climb glidecreep twinning Di
ffusional mass transfer solid state mass
transfer pressure solution mass
transfer Constitutive Equations (flow laws)
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Crystal defect movies
Bubble raft example
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Ductile deformational processes
Crystal Plasticity The migration of crystal
dislocations causes permanent deformation
Once activation energy is achieved, dislocations
can migrate Energy sources distortion of
lattice due to dislocation heat differential
stress
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations (line defects) can move by glide,
climb or cross slip Glide climb (creep)
Another crystal-plastic behavior is twinning
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations can move by glide,
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations can move by glide,
The glide plane contains the Burgers vector and
the dislocation line
Edge dislocation burger vector and dislocation
line are perpendicular Therefore, any edge
dislocation has only one plane orientation to
glide on
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations can move by glide,
The glide plane contains the Burgers vector and
the dislocation line
Screw dislocation Burgers vector and dislocation
line are parallel Therefore, any screw
dislocation has many plane orientations to glide
on
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations can move by glide,
Dislocations glide to the edge of the grain, can
produce stair-step structure called slip bands
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations (line defects) can move by glide,
climb or cross slip, Glide climb (creep)
Another crystal-plastic behavior is twinning
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocation by climb or cross slip,
Sometimes glide planes are blocked by point
defects. Edge Dislocation cant continue on
that plane, so dislocation climbs to new glide
plane
Requires significant energy for edge dislocation
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocation by climb or cross slip,
Requires significant energy for edge dislocation
Since screw dislocations have many possible glide
planes, easily cross-slip to another plane
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocation by climb or cross slip,
Requires significant energy for edge dislocation
Since screw dislocations have many possible glide
planes, easily cross-slip to another plane
Both Climb and Cross slip do need extra energy gt
typicaly occur at deeper (hotter) levels in the
earth. gt300 for quartz rich rocks gt500
feldspar, olivine
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations (line defects) can move by glide,
climb or cross slip Glide climb/cross slip
is often called dislocation creep
Another crystal-plastic behavior is twinning
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
Dislocations (line defects) can move by glide,
climb or cross slip creep
Another crystal-plastic behavior is twinning
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
twinning
twins that develop during growth of mineral
(Growth twins), have little to nothing to say
about conditions of deformation
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
twinning
Mechanical twins twins formed in response to an
applied stress. Common in calcite
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
twinning
Starting mineral
Apply differential stress
Dislocation boundary forms
Twinning plane
Partial dislocations glide, form twin
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Ductile deformational processes
Crystal Plasticity migration of crystal
dislocations causes permanent deformation
twinning
Mechanical twinning crystal plastic process that
involves glide of partial dislocation- atoms move
a fraction of a lattice distance Favored under
faster strain rates, lower temperatures
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Ductile deformational processes
Diffusional mass transfer occurs when an atom
(or point defect) migrates through a crystal
Easier for atoms to move around at higher
temperatures gt Diffusion rate faster at higher
temperatures
D is diffusivity D0 is a diffusion constant for a
given material (i.e., calcite, quartz, etc) E is
the activation energy (kJ/mol) R is the gas
constant (8.31 J/molK) T is absolute temperature
(in K)
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Ductile deformational processes
Diffusional mass transfer occurs when an atom
(or point defect) migrates through a crystal
Solid State Diffusion volume diffusion,
grain-boundary diffusion
Grains change shape to adjust to stress field
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Outline Ductile Deformation Three main mechanisms
Cataclastic flow-
Crystal plasticity
crystal plasticity mechanisms dislocation
glid Dislocation climb Dislocation climb
glidecreep twinning
Diffusional mass transfer
solid state mass transfer pressure solution mass
transfer
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Outline Ductile Deformation Three main
mechanisms Cataclastic flow- Crystal
plasticity kinds of crystal defects point
defects line defects crystal plasticity
mechanisms dislocation glide Dislocation
climb Dislocation climb glidecreep twinning Di
ffusional mass transfer solid state mass
transfer pressure solution mass
transfer Constitutive Equations (flow
laws) Mechanism Maps Not sects 9.7, 9.8, 9.9
just overview of 9.10
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Ductile deformational processes
Diffusional mass transfer occurs when an atom
(or point defect) migrates through a crystal
Pressure Solution At areas of high stress,
grains dissolve into fluid film, then migrate to
region of low stress, and recrystalize
Occurs at relatively low temperatures gt
Important deformation mechanism in the upper crust
Pressure Solution Video
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Ductile deformational processes
Diffusional mass transfer occurs when an atom
(or point defect) migrates through a crystal
Pressure Solution
Stylolites (pressure solution seams) in limestone
of Mississippian age, exposed on the side of a
rounded boulder in Hyalite Canyon, Gallatin
Range, Montana. These stylolites, like most, are
bedding-parallel, and thus most likely formed due
to the weight of the overlying rock. Calcite, the
dominant mineral, goes into solution under
pressure, and insoluble material, like organic
matter and clay, accumulates along the
dissolution surface, producing a dark, wiggly
line. Here, multiple stylolites have converged
and overprinted one another, resulting in a
mutli-level oscilloscope look.
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Ductile deformational processes
Constitutive Equations or Flow laws Relating
strain (or strain rate) to stress
Strain rate Stress function material
constant Activation energy Gas constant Temperatur
e
is strain rate (s-1) A is a
material constant E is the activation
energy R is the gas constant T is the
absolute temperature is a function of
differential stress
Remember the diffusion equation?
A E R T
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Ductile deformational processes
Constitutive Equations or Flow laws Relating
strain (or strain rate) to stress
For dislocation glide, the function of stress is
exponential
exp(sd)
exp(sd)
For dislocation glide and climb (creep), the
function of stress is raised to the power n
(sd)n
sdn
For diffusion, the stress function is stress and
the grain size (d)


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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)
dislocation glide exponential
dislocation creep, power law
diffusion, grain size (d)
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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