1. 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”

2. Nisqually Earthquake, Feb 28, 2001
6.8 Mw
52 km deep
No deaths
~400 injuries

3. Fault strength paradox:
San Andreas Fault and Pore Fluid Pressure

4. 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 + glide=creep
twinning
Diffusional mass transfer
Solid State Mass diffusion
Grain Boundary mass diffusion

5. 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
FYI, what is considered high-temp behavior for one mineral is low-temp. behavior for another mineral.
Normalized paramater, homologous temperature, Th
Th = T/Tm
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

6. 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)
FYI, what is considered high-temp behavior for one mineral is low-temp. behavior for another mineral.
Normalized paramater, homologous temperature, Th
Th = T/Tm
Beanbag experiment

7. 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
Vacancies – unoccupied siter in the crystal lattice and replaced by different atom (b)
(c) Or by interstitails, atom at a non-lattice site
Vacancies can migrate by exchange with atoms at neighboring sites (d) – also called diffusion
Motion of defects causes permanent strain while the material remains solid

8. Ductile deformational processes
Crystal defects
Point defects:
vacancy,
substitution impurity
Interstitial impurity
Motion of defects gives rise to permanent strain with the material losing cohesion (no fracturing)
Vacancies can migrate by exchange with atoms at neighboring sites– also called diffusion

9. Ductile deformational processes
Crystal defects- line defects
Two end-member configurations.
Edge dislocation: extra half-plane of atoms in the lattice
An area of the crystal that has slipped relative to the rest of the crystal.

10. Ductile deformational processes
Crystal defects
Two end-member configurations.
A) Screw dislocation: lattice is deformed in a screw-like fashion
An area of the crystal that has slipped relative to the rest of the crystal.

11. Ductile deformational processes
Crystal defects
Burgers vector b:
The vector that represents the magnitude and direction of the lattice distortion

12. 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)

13. 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:

14. Ductile deformational processes
Crystal defects and stress

15. Ductile deformational processes
Crystal defects

16. 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

17. 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 + glide=creep
twinning
Diffusional mass transfer
solid state mass transfer
pressure solution mass transfer
Constitutive Equations (flow laws)

18. Crystal defect movies
Bubble raft example

19. 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

20. 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

21. Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation
Dislocations can move by glide,

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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 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)
T is absolute temperature (in K)

36. 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

37. Outline:
Ductile Deformation
Three main mechanisms
Cataclastic flow-
Crystal plasticity
crystal plasticity mechanisms
dislocation glid
Dislocation climb
Dislocation climb + glide=creep
twinning
Diffusional mass transfer
solid state mass transfer
pressure solution mass transfer

38. 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 + 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

39. 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

40. 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.

44. 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
Temperature
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

45. 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) = =

46. 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
An area of the crystal that has slipped relative to the rest of the crystal.
diffusion, grain size (d)

47. Deformation Mechanisms
Important relations
Differential stress
versus
Temperature
An area of the crystal that has slipped relative to the rest of the crystal.

48. 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.

49. 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.

50. 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