Transcript Week 8 ductile deformation continued

Friday 12:00 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”
Nisqually Earthquake, Feb 28, 2001
6.8 Mw
52 km deep
No deaths
~400 injuries
Fault strength paradox:
San Andreas Fault and Pore Fluid Pressure
Outline:
Ductile Deformation
Three main mechanisms
Cataclastic flowCrystal plasticity
kinds of crystal defects
point defects
line defects
crystal plasticity mechanisms
dislocation glide
Dislocation climb
Dislocation climb + glide=creep
twinning
Diffusional mass transfer
Solid State Mass diffusion
Grain Boundary mass diffusion
Ductile deformational processes
Introduction: how can rocks bend, distort, or flow while remaining a solid?
Non-recoverable deformation versus elastic deformation
Ductile behavior – we’ve used the words viscous and plastic to describe the
deformation- now we’ll 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: Th=T/Tm
Low temperature~ Th<0.3
medium temperature~ 0.3<Th<0.7
High temperature~ Th>0.7
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)
Beanbag experiment
Relatively shallow crustal
deformation (fault zones)
Ductile deformational processes
Ductile behavior at elevated temperatures
Achieved by motion of crystal defects (error in crystal lattice)
1)Point defects2)Line defects or dislocations
3)Planar defects
Crystal defects
Motion of defects causes permanent strain while the material
remains solid
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
Ductile deformational processes
Crystal defects- line defects
Two end-member configurations.
A) Edge dislocation: extra half-plane of atoms in the lattice
Ductile deformational processes
Crystal defects
Two end-member configurations.
A) Screw dislocation: lattice is deformed in a
screw-like fashion
Ductile deformational processes
Crystal defects
Burgers vector b:
The vector that represents the magnitude and
direction of the lattice distortion
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)
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:
Ductile deformational processes
Crystal defects and stress
Ductile deformational processes
Crystal defects
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
Outline:
Ductile Deformation
Three main mechanisms
Cataclastic flowCrystal plasticity
kinds of crystal defects
point defects
line defects
crystal plasticity mechanisms
dislocation glide
Dislocation climb
Dislocation climb + glide=creep
twinning
Diffusional mass transfer
solid state mass transfer
pressure solution mass transfer
Constitutive Equations (flow laws)
Crystal defect movies
Bubble raft example
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
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
Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations
causes permanent deformation
Dislocations can move by glide,
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
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
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
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
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: can’t continue on that plane, so dislocation
climbs to new glide plane
Requires significant energy for
edge dislocation
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
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 => typicaly occur at deeper
(hotter) levels in the earth.
>300° for quartz rich rocks
>500° feldspar, olivine
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
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
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
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
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
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
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
=> Diffusion rate faster at higher temperatures
D  D0 exp(E * /RT )
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/mol*K)
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
Outline:
Ductile Deformation
Three main mechanisms
Cataclastic flowCrystal plasticity
crystal plasticity mechanisms
dislocation glid
Dislocation climb
Dislocation climb + glide=creep
twinning
Diffusional mass transfer
solid state mass transfer
pressure solution mass transfer
Outline:
Ductile Deformation
Three main mechanisms
Cataclastic flowCrystal plasticity
kinds of crystal defects
point defects
line defects
crystal plasticity mechanisms
dislocation glide
Dislocation climb
Dislocation climb + glide=creep
twinning
Diffusional 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
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
=> Important
deformation
mechanism in the
upper crust
Pressure Solution Video
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 beddingparallel, 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.
Ductile deformational processes
Constitutive Equations or Flow laws
Relating strain (or strain rate) to stress
eÝ A f (d )exp(E * /RT)
eÝ Strain rate

is strain rate (s-1)
f (d ) Stress function
A
is a material constant

E*
is the activation
energy
material
constant
A
R
is the gas constant
T E is
the absolute
temperature
Activation
energy
is a function of differential
stress
R Gas constant
T Temperature
Remember the diffusion equation?

D  D0 exp(E * /RT )


Ductile deformational processes
Constitutive Equations or Flow laws
Relating strain (or strain rate) to stress
eÝ A f (d )exp(E * /RT)
For dislocation glide, the function
of stress is exponential

f (d ) =exp(
= exp(dd))
eÝ A exp( )exp(E * /RT)
d
For dislocation glide and climb (creep), the function of stress is

raised to the power n
n
*
d
f ( ) =(
=  n)n

dd
d
eÝ A ( ) exp(E /RT)
 For diffusion, the stress function is stress and the grain size (d)
f (d ) = (
d
d r )

eÝ A (d d r )exp( E * /RT )
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)
Deformation Mechanisms
Important relations
Differential stress
versus
Temperature
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 (1 - 3)
• 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.
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.
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
Deformation Mechanism Map