1. Ductile deformational processes
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
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

2. 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)
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

3. Ductile deformational processes
Crystal defects
Motion of defects gives rise to permanent strain with the material losing cohesion (no fracturing)
Ductile behavior at elevated temperatures
Achieved by motion of crystal defects (error in crystal lattice)
Point defects
Line defects or dislocations
Planar defects

4. Ductile deformational processes
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
Point defects
Two types:
Vacancies & Impurities

5. Ductile deformational processes
Crystal defects
2) Line defects
Also called a dislocation – a linear array of lattice imperfections.
Two end-member configurations.
Difficult concept
An area of the crystal that has slipped relative to the rest of the crystal.

6. Ductile deformational processes
Crystal 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.

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

8. Deformation Mechanisms
Important relations
Normalized stress (normalized to shear modulus of the material
versus
normalized temperature (normalized to absolute melting temperature of the material)
An area of the crystal that has slipped relative to the rest of the crystal.

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

10. 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.

11. 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.

12. 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

13. Fine-scale fracturing, movement along fractures and frictional grain-boundary sliding.
Favoured by low-confining pressures
Causes decrease in porosity and rock volume.

14. 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.
These are brittle deformation mechanisms that operate on the grain and sub-grain scales.

15. Microcrack in Feldspar

16. Microcracks break individual atomic bonds
Crack tips have nearly infinitesimally small areas, which makes the stresses there HUGE!

17. 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.
To occur: 1) there must be a vulnerable twin plane across which shearing can occur and
2) The twin plane must be favorable oriented.

19. 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.

20. Diffusion: atom jump from site to site through a mineral.
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.
Diffuions creep is a slow, time-dependent strain that occurs at stresses well below the rupture strength of the rock.

21. 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.